iL. MARINE BIOLOGICAL LABORATORY. — *■■ * - Received ^.®JP.^.®?!.'P.®r....3p , . .1933 Accession No. .. .^^ .^.^ ^• Given by Dr., H.Q.b.t.. lU W.o.lco tjt Place, University of Nebraska *^:* No book or pamphlet is to be removed from the Lab- oratory without the permission of the Trustees. J McGRAW-HILL PUBLICATIONS IN THE ZOOLOGICAL SCIENCES A. FRANKLIN SHULL, Consulting Editok ANIMAL BIOLOGY McGRAW-HILL PUBLICATIONS IN THE ZOOLOGICAL SCIENCES A. Franklin Shull, Consulting Editor Chapman — Animal Ecology Fernald — Applied Entomology Graham — Principles of Forest Entomology Ilaupt — Fundamentals of Biology llaupt — Laboratory Directions for General Biology Metcalf and Flint — Destructive and Useful Insects Metcalf and Flint — Fundamentals of Insect Life Mitchell— General Physiology Noble — The Biology of the Am- phibia Pearse — Animal Ecology Reed and Young — Laboratory Studies in Zoology Riley and Christenson — Guide to the Study of Animal Parasites There is also a scries of McGraw-Hill Botanical Sciences, of which Edmund W Riley and Johannsen — Medical Entomology Rogers — Textbook of Comparative Physiology Rogers — Laboratory Outlines in Comparative Physiology Shull — Heredity Shull, LaRue and Ruthven — Princi- ples of Animal Biology Shull, LaRue and Ruthven — Labo- ratory Directions in Principles of Animal Biology Snodgrass — Anatomy and Physi- ology of the Honeybee Van Cleave — Invertebrate Zoology Wieman — General Zoology Wieman — An Introduction to Vertebrate Embryology Wieman and Weichcrt — A Labora- tory Manual for Vertebrate Embryology Wolcott — Animal Biology Publications in the Agricultural and Sinnott is Consulting Editor. ANIMAL BIOLOGY BY ROBERT H. WOLCOTT Professor of Zoology, Universiiy of Nfhraska First Edition McGRAW-HILL BOOK COMPANY, Inc. NEW YORK AND LONDON 1933 Copyright, 1913, 1919, 1928, 1933, by the McGraw-Hill Book Company, Inc. PHINTED IN THE UNITED STATES OF AMERICA All rights reserved. This book, or parts thereof, may not be reproduced in any form without permission of the publishers. THE MAPLE PRESS COMPANY, YORK, PA. PREFACE The fundamental propositions behind this text — the platform, so to speak, upon which it has been written — are as follows: 1. Life has a chemicophysical basis. 2. Life phenomena are the outgrowth of organization. 3. The central fact in life is metabolism. 4. Animals may be arranged in a progressive series with reference to organization. 5. The most complex animals are the most effective and also the most efficient from a metabolic standpoint. 6. Man, as the highest of animals, can learn by the study of animal life the principles of the most effective living. 7. He can also understand more fully his place in nature and can more justly judge the actions of his fellows; this in turn may contribute to his intellectual and spiritual development. 8. Every problem concerned with living is essentially a biological problem and capable of analysis and solution by the application of biological principles. The book has been prepared for use as a class textbook, not as a work of reference, and contains an amount of material which experience has shown can be covered in three recitation periods a week for one year. Since it will generally be used in beginning classes in which the majority of the students are freshmen and sophomores, an effort has been made to present the material in such a manner that it can be easily handled by such students with normal preparation. In other words the idea is to give the student an amount of material which he can cover in a way he can understand. Also since the majority of the individuals in such classes do not intend to specialize in the field of zoology, technicalities have been minimized and emphasis placed upon the broader aspects of the science and the general significance of the data presented, leaving to subsequent courses the filling in of details for students majoring in the subject. Feeling that the place to acquire a knowledge of the structure of animals is in the laboratory and not in the classroom, the author has reduced the amount of morphological material. In the case of those types handled in both class and laboratory, the facts given here are intended to tie the two together or to summarize the knowledge gained in the laboratory. In the University of Nebraska the "types method" vi PREFACE is followed in the laboratory work and two courses are offered, one carry- ing a credit of ten semester hours and the other six, and differing in the number of types covered. In the longer course, three recitation periods a week are required; in the shorter, two. It is suggested that in the selection of material for a shorter course the lightening of the load be done by taking in the classroom only a brief survey of Chaps. XV to XVII, XXV to XXXIX, and XLI to LX, inclusive, picking out sections here and there for the particular atten- tion of the students and letting the rest be merely read for the general impression gained. The numbering of the sections makes possible the assignment of certain ones for more intensive study and of others for consideration in connection with the laboratory work. In the topics handled in Part V, three aims have been in view: (1) To give a general survey of the field of zoology with a fairly even emphasis upon the various aspects; (2) to review many of the facts presented in previous parts, putting them in a different setting, and developing on the part of the student a broader view and a greater ability to apply these facts; and (3) to establish points of attachment to which advanced courses in the department may be articulated. It is felt by the author that these chapters afford a means for more ready correlation between the general subject and such special courses. Cross references facilitate the development of the habit of thoughtful reviewing and the perception of analogies and homologies, resemblances and differences, that form a part of the basis for true scholarship. Since correct spelling and exact pronunciation are among the clearest indications of careful training, the pronunciations of phylum and class names are given in the body of the text and the pronunciations of words in the Glossary are given. That the student may be led to observe the derivations of technical terms those of the phylum and class names are given and many common Greek and Latin roots are included in the Glos- sary. Italics are used in the text to indicate emphasis and also to call the attention of the student to words the definitions of which are to be learned. In the preparation of the book the author has made free use of other texts and of works of reference, particularly of Parker and Haswell's "Text-book of Zoology," a copy of which should be available to every teacher, and of the volumes of the Cambridge Natural History Series. In connection with illustrations borrowed from other books acknowledg- ment is made of their sources and of the courtesy of the different pub- lishers in granting permission. Of the figures, seventy-two are from borrowed engravings or were reproduced photographically by F. H. Shoemaker. Two of the original drawings (Figs. 108 and 115) were made by S. Fred Prince. With these exceptions all of the illustrations, either redrawn or original, are from drawings by the author's son, Robert A. Wolcott. PREFACE vii The author desires to acknowledge the help of many colleagues who have generously responded to requests for information and assistance, and, particularly, the advice and suggestions received from those asso- ciated with him in the zoological department at the University of Nebraska— D. D. Whitney, I. H. Blake, H. W. Manter, Otis Wade, E. F. Powell, H. E. Low, and G. E. Hudson. In the preparation of the manuscript he has profited by the intelligent cooperation of his assistant, Elmer Palmatier. The Author. Lincoln, Nebraska, August, 1933. CONTENTS Paoe Preface v PART I Fundamental Principles CHAPTER I The Field of Zoology ... 3 Appeal of zoology — Number of animals — Variety of animals — Distribution of animals — Relations of animals — Definition of zoology — Divisions of the subject — Scope of general zoology — Animal biology. CHAPTER II Matter 7 Definitions — Constitution of matter — Elements and compounds — Acids, bases, and salts — States of matter — Surface films — Mixtures — Ionization — Colloids — Colloidal emulsions — Reactions. CHAPTER III Energy 14 Forms of energy — Chemical energy — Laws of thermodynamics. CHAPTER IV Living and Nonliving Matter 16 Contrast between living and nonliving matter — Tests of life. CHAPTER V Protoplasm 18 Historical facts — Chemical character of protoplasm — Physical characteristics of protoplasm — Microscopical structure of protoplasm — Appropriateness of protoplasm as living substance — Life is a consequence or concomitant of organization. CHAPTER VI Life 24 Definition — Vital force — Vitalism and mechanism — Origin of life — Possi- bility of creating life. CHAPTER VII Cells 28 Definition — Sizes and shapes of cells — Numbers of cells — Structure of cells — General physiology of the cell — Development of knowledge of the cell — Cell theory and cell doctrine. ix 42591 X CONTENTS Page CHAPTER VIII Metabolism 32 Definition — Food — Steps in metabolism — Ingestion — Digestion — Absorp- tion — Circulation — Inspiration — Assimilation — Dissimilation — Secretion — Excretion — Expiration — Elimination — Egestion — Respiration — Anabolism and katabolism — Vitamins — Energy changes in metabolism — Uses of differ- ent foods — Storage — Metabolism the central fact in life. CHAPTER IX Plants and Animals 40. Comparison between plants and animals — Biology — Differences between plants and animals. CHAPTER X Growth and Reproduction. 43 Growth cycles — Limit of size — Reproduction. CHAPTER XI Mitosis 46 Normal cell division — Significance of mitosis — Amitosis — Continuity of cell life and chromatin — Growth of the cell. CHAPTER XII Forms of Animals 51 Asymmetry — Spherical, or universal, symmetry — Radial symmetry — Bilateral symmetry — Metamerism — Appendages — Homology and analogy. CHAPTER XIII Behavior ^^ Stimuli— Direct response— Conductivity— Part played by the nervous sys- tem — Physiological state. CHAPTER XIV Classification and Nomenclature 57 Definition — Arrangement of groups of animals — Nomenclature. PART II Protozoa CHAPTER XV Ameba 6^ Occurrence and appearance— Structure— Metabolism— Locomotion— Be- havior — Reproduction. CHAPTER XVI 6Q Paramecium Occurrence— Structure— Metabolism— Locomotion— Behavior— Reproduc- tion — Conjugation — Endomixis. CONTENTS xi CHAPTER XVII Protozoa in General 78 Classification — Mastigophora — Sarcodina — Sporozoa — Infusoria — General facts — Sexual reproduction in protozoa. CHAPTER XVIII Protozoa and Disease 87 Pathogenic protozoa — Malarial parasite. PART III Metazoa in General CHAPTER XIX Metazoa 93 Differentiation — Division of labor — Somatic and germ cells. CHAPTER XX Tissues 96 Definition — Epithelia — Supporting and connective tissues — Muscular tissues — Nervous tissues. CHAPTER XXI Organs and Systems 102 Definitions — Systems — Organs belonging to different systems — Other parts of the body. CHAPTER XXII Reproduction in the Metazoa 105 Methods of reproduction in metazoa — Sexual reproduction — Uniparental reproduction — Types of fertilization — Oviparity and viviparity — Metagene- sis. CHAPTER XXIII Origin of the Sex Cells 107 Gametogenesis — Spermatogenesis — Oogenesis — Comparison and contrast between spermatogenesis and oogenesis — Division of labor between the germ cells — Variations in gametogenesis. CHAPTER XXIV Fertilization 112 Steps in fertilization — Chromosome reduction — Significanc^e of synapsis. CHAPTER XXV Embryogeny 110 Types of egg cells — Forms of cleavage — Steps in embryogeny — Variations in embryogeny — Germ layers — Coelom. PART IV Metazoan Phyla CHAPTER XXVI Sponges 127 Relationship of sponges — Classification — Structure — Canal systems — Skele- xii CONTENTS Page ton — Histology — Metabolism — Behavior — Reproduction — Uses of sponges — Cultivation of sponges — Relations to other animals. CHAPTER XXVII Hydra I34 External features — Internal structure — Nematocysts — Neuromuscular mechanism — Metabolism — Behavior — Reproduction — Symbiosis — Regener- ation. CHAPTER XXVIII COELENTERATES IN GENERAL 142 Polyps and Medusae — Classification — Hydrozoa — Scyphozoa — Anthozoa — Color — Polymorphism — Metabolism — Behavior — Reproduction — Metagen- esis — Corals — Distribution and economic importance. CHAPTER XXIX Phylum Ctenophora 154 Structure — Advances in body plan — Activities. CHAPTER XXX Fresh-water Planarian 157 Structure — Internal structure — Metabolism — Reproduction — Behavior — Regeneration. CHAPTER XXXI Phylum Platyhelminthes 164 Classification — Turbellaria — Trematoda — Cestoda — Metabolism — R e p r o - duction — Occurrence and economic importance. CHAPTER XXXII Parasitism ....<• 1 70 Structure of parasites — Sheep liver fluke — Life history of the sheep liver fluke — Life history of a tapeworm — Behavior of parasites — Practical aspects. CHAPTER XXXIII Phylum Nemathelminthes 175 Structure of an ascaris — Characteristics and advances — Classification — Free-living nematodes — Metabolism — Reproduction — Life history of the pig ascaris — American hookworm — Trichinella — Filaria — Hairworms — Spiny-headed worms — Economic importance. CHAPTER XXXIV Other Unsegmented Worms 183 Phylum Nemertinea — Phylum Chaetognatha — Phylum Rotifera— Phylum Bryozoa — Phylum Brachiopoda. CHAPTER XXXV Starfish 191 External appearance — Skeleton and musculature — Water-vascular system — Internal organs — Feeding and metabolism — Nervous system and behavior — Reproduction — Regeneration and autotomy — Economic importance. CONTENTS xiii Page CHAPTER XXXVI Phylum Echinodermata 200 Retrogression — Specializations — Classification — Asteroidea — Ophiuroidea — Echinoidea — Holothurioidea— Crinoidea — Reproduction — Behavior — Color — Occurrence and economic importance. CHAPTER XXXVII Fresh-water Mussel 208 External appearance — Shell — Internal anatomy — Body mass and foot — Mantle — Gills — Digestive system and metabolism — Circulatory system — Excretory system — Musculature — Nervous system — Behavior — Reproduc- tion — Other fresh-water mussels. CHAPTER XXXVIII MoLLUsKS IN General 217 Classification — Amphineura — Gastropoda — Scaphopoda — P e 1 e c y p o d a — Cephalopoda — Metabolism — Behavior — Reproduction and regeneration — Economic importance. CHAPTER XXXIX Earthworm 229 External characteristics — Internal structure — Alimentary canal and metabo- lism — Circulatory system — Excretory system — Musculature and locomotion — Nervous system — Behavior — Reproductive system and reproduction — Regeneration — Economic importance. CHAPTER XL Reflex Action 238 Nervous functions — Reflex acts — Anterior ganglia. CHAPTER XLI Annelids in General.- 241 Classification — Archiannelida — Chaetopoda — H i r u d i n e a — G e p h y r e a — Metabolism — Behavior — Reproduction — Occurrence and economic impor- tance. CHAPTER XLII . Crayfish 250 External characteristics — Internal structure — Eyes and vision — Statocyst — Feeding habits — Behavior — Reproduction — Regeneration and autotomy — Economic importance. CHAPTER XLIII Crustacea 260 Malacostraca — Entomostraca — Behavior — Reproduction — Economic impor- tance — Biogenesis. CHAPTER XLIV Onychophora and Myriapoda 268 Onychophora — Myriapoda — Centipedes — Millipedes — R eproduction in myriapods. Xiv CONTENTS Page CHAPTER XLV Class Insecta • 271 External characteristics — Internal structures — Senses of insects — Reproduc- tion — Autotomy — Benefits from insects — Injurious types — Combating in- jurious insects — Beneficial insects — Social insects. CHAPTER XLVI Class Arachnida 296 External structure of spiders — Internal structures — Metabolism — Reproduc- tion — Spinning activities — Behavior — Economic importance — Scorpions — Mites — Other arachnids. CHAPTER XLVII Arthropods in General 304 Characteristics and advances — Classification — Behavior. CHAPTER XLVIII Phylum Chordata 306 Characteristics — Advances shown by the chordates — Classification. CHAPTER XLIX Lower Chordates 309 Hemichordata — Urochordata — Cephalochordata — Economic value. CHAPTER L Subphylum Vertebrata 316 Distinguishing characteristics — Body plan — Skin — Skeleton — Muscular sys- tem — Digestive system — Respiratory system — Circulatory system — Excre- tory system — Nervous system — Sense organs — Ear — Eye — Reproductive system — Advances shown by vertebrates — Classification. CHAPTER LI Class Cyclostomata 335 Classification — Myxinoids — Lampreys — Relationship of the cyclostomes — Economic importance. CHAPTER LII Class Elasmobranchii 338 Dogfish sharks — Other sharks — Skates and rays — Extinct elasmobranchs — Economic facts. CHAPTER LIII Class Pisces 344 Classification — Crossopterygii — Chondrostei — Holostei — Teleostei — Dipnoi — Body form — Scales — Fins — Locomotion — Air bladder — Forms of tails — Colors of fishes — Internal anatomy — Food of fishes — Respiration — Senses of fish — Behavior — Reproduction — Ages of fish — Deep-sea fishes — Remark- able fishes — Economic relations. CHAPTER LIV Terrestrial Vertebrates 359 Changes incident to the acquirement of a terrestrial mode of life — Origin of terrestrial adaptations. CONTENTS XV Page CHAPTER LV Class Amphibia 304 Classification — Urodela — Salieutia — A pod a — F o o d — C o 1 o r changes in amphibia — Nervous system and sense organs — Behavior — Reproduction and development — Neoteny and pedogenesis — Regeneration — Hibernation — Economic importance. CHAPTER LVI Reptiles and Birds 375 Structural characteristics — Embryonic modifications — Egg — Amnion — Allantois — Body coverings. CHAPTER LVII Class Reptilia 379 Classification — Internal structure— Squamata — Chameleons — Lizards — Snakes — Venomous snakes — Rhynchocephalia — Crocodilia — Testudinata — Economic importance. CHAPTER LVIII Class Ayes 390 External characteristics — Feathers — Internal structure — Classification — Origin of birds and of flight — Flight — The bird as a flying animal — Modifica- tions of l)irds — Plumage — Songs — Migration of birds — Reproduction — Economic importance. CHAPTER LIX Class Mammalia 406 External characteristics — Hair — Internal structure — Classification — Origin of mammals — Monotremes — Marsupials — Unguiculata — Primates — Ungulata — Cetacea — Hibernation — Reproduction — Economic importance. CHAPTER LX Anthropoid Apes and Man 422 Manlike apes — Erect position — Evidences in man of former arboreal life — Intermediate forms — Fossil men — Present-day man. PART V General Considerations CHAPTER LXI Animal Organisms 429 The organism — Definition — Income and outgo — Differentiation — Integra- tion — Centralization — Chemical control — Individuality — Life cycle in birds and mammals — Other life cycles — Practical considerations — Organismal concept. CHAPTER LXII Structure of Organisms 435 Grades of organization — Germ layers and tissues — Relationship of cells in metazoans — Organs and systems — Tegumentary system — Skeletal system — Digestive system — Glands — Respiratory system — Circulatory system — xvi CONTENTS Page Excretory system — Reproductive system — Muscular system — Nervous sys- tem — Convergence and divergence. CHAPTER LXIII Development of the Organism 447 Germ cells — Origin of germ cells — Maturation of the germ cells — Egg — Fertilization — Cleavage — Blastula — Gastrulation — Mesoderm formation — Tissue formation and organogeny — Postembryonic development — Potential immortality of germ cells. CHAPTER LXIV Energy Changes in Organisms 452 Chemical changes in the body — Organism compared to a fire — Organism compared to an engine — Organism more than a machine — Individuality — Rhythmicity — Uses of foods — Planes of metabolism — Body heat — Heat regulation — Warm-blooded and cold-blooded animals — Temperature of the human body. CHAPTER LXV Functions of Animal Organisms 458 Chemical cycles — Water — Digestion and absorption — Circulation — Respira- tion — Secretion — Internal secretions — Excretion and eUmination — Motor functions — Nervous activities. CHAPTER LXVI Behavior of Animal Organisms 469 Memory — Types of animal behavior — Direct response — Simple reflexes — Instincts — Habits — Learning — Intelhgence — Reasoning — Combinations of modes of behavior — Behavior of lower and of higher animals — Mind and consciousness. CHAPTER LXVII Animal Organisms in Relation to Their Environment 477 Facts of ecology — Relations of animals to plants — Physiological life histories — Habitat — Ecological factors — Reactions of the animal — Communities — Succession — Rhythms — Marine faunas — Fresh-water animals — Terrestrial faunas — Mimicry and protective resemblance. CHAPTER LXVIII Animal Organisms in Health and Disease 487 Definitions — Health in a protozoan — Comparison of protozoan and metazoan cells— Conditions of health— Causes of disease— Effect of individuality— Self-regulatory tendency in the bodj^ — Toxins and antitoxins — How the body fights disease — Immunity — Anaphylaxis and allergy — Maintenance of health in human beings. CHAPTER LXIX Relations between Animal Organisms 493 Solitary life — Associations of animals of the same species — Mating — Families — Colonies — Societies — Associations of animals of different species — Gregariousness — Epizoic associations — Commensalism — Mutualism — Sym- biosis — Parasitism — Predatism. CONTENTS xvii Page CHAPTER LXX Distribution of Animals 499 Present distribution — Past distribution — Place of origin— Dispersal of animals — Factors hindering dispersal — Modification of types — Periodic migration — Altitude — Oeeanit; distribution — Island faunas — Faunal divi- sions of the earth — North American life zones. CHAPTER LXXI Past Distribution of Animals 510 Fossils — Stages in fossilization — Geological ages — Geological time scale — Metamorphism — Animals of the past. CHAPTER LXXII Evolution of Animals 516 History of evolution — Evidences of evolution — Causes of evolution — Methods of evolution — Evolutionary series. CHAPTER LXXIII Inheritance in Organisms 527 Organisms from the genetic viewpoint — Determiners or genes — Behavior of chromosomes in maturation and fertilization — Effect of chromosome reduc- tion — Allelomorphs — Mendel — Mendelism — Hybrids — Distribution of char- acteristics in hybrids — Checkerboard diagrams — Multiple hybrids — Actual cases — Breeding the test for characters^ — Variations in inheritance — Breeding for certain characteristics — Inbreeding and crossbreeding — Inherit- ance of acquired characters — Inheritance of disease and abnormalities- Sex determination — Twins — Determination of sex in parthenogenesis — Sex- Unked characters — Linkage and crossing over — Eugenics. CHAPTER LXXIV Classification of Animals 541 History — Species — Polymorphism — Basis of classification — Basis of nomen- clature — Rules of nomenclature — Phyla — Phylogenetic tree. CHAPTER LXXV History of Zoology 549 Greeks — Dark ages — VesaUus — Harvey — Microscopists — Comparative anat- omy — Physiology — Cell theory — Embryology — Taxonomy — Evolution and genetics— Pasteur — Recent advances. Glossary 557 Index 587 PART I FUNDAMENTAL PRINCIPLES r<:3^ / CHAPTER I THE FIELD OF ZOOLOGY A wealth of animal life about us challenges our attention. Birds travel the highways of the air. Vegetation of all kinds swarms with animals. The surface of the earth is alive with crawling things, and under every object lying on the ground an animal community is hidden. The ground itself teems with life. And not only are fresh waters abun- dantly populated, but the sea, which has been supposed to mother all life, is occupied by a host of forms, greater in variety than those of any other environment. 1. Appeal of Zoology. — Among this vast assemblage are creatures which appeal to us because of their beauty or oddity of appearance; others to which we are attracted by their remarkable and interesting activities; still others whose varied and complex relationships to one another excite our wonder and suggest a multitude of questions; and, finally, many whose relations to ourselves, either beneficial or injurious, demand our serious consideration. 2. Number of Animals, — The number of individuals which at any given time is living in this world surpasses calculation and is beyond the power of the imagination to conceive. That large number, subject to the modifying influences of changing seasons and affected from time to time by an altering of the balance between animals of different kinds, is constantly maintained. Naturally great differences exist between differ- ent regions of the earth's surface, which, in very different degrees, offer the conditions favorable for animal life. 3. Variety of Animals. — The number of kinds of animals is not yet determined and probably never will be precisely known. Those living which have been previously described and named have been estimated at approximately 600,000; and there is no doubt that if all still unknown were added, the total would far exceed a million. The exact number, however, is subject to constant change, since some kinds of animals are continually becoming extinct and new ones are as continually being developed. This enumeration also takes no account of the millions of species which have lived in the past and have perished, some without leaving any trace, others represented more or less completely by fossils. Then, too, opinions differ greatly as to what constitutes the difference between two kinds or, in other words, what constitutes a species. The words type and form are frequently used in the same sense as species, or kind. 3 4 FUNDAMENTAL PRINCIPLES 4. Distribution of Animals. — Animals are found everywhere on the earth's surface, except perhaps on the glaciated tops of the highest mountains and at the poles. On these mountains creeping and flying forms pass the margins of the snow fields, and the areas of ice and snow at the poles are constantly invaded by such forms as are able to venture into them. Animal life is found throughout the waters of the sea and even penetrates to the deepest parts of the oceans. Animals burrow below the surface of the ground to considerable depths and also follow fissures still deeper to reach the farthest recesses of the most extensive caverns. Finally, myriads of living creatures live within the bodies of other living things, both plant and animal. 5. Relations of Animals. — Animals are related in various ways to other animals, to plants, and to their physical environment. Between parents and offspring the relation is that of descent. Between other animals the relation may be nutritive, one living upon the other; repro- ductive, where two join in the production of young; locomotor, where one attaches itself to another for the purpose of being transported from one place to another; or any one of many other relations which might also be named. Plants serve as food for animals, afford them conceal- ment, and are useful to them in other ways. A solitary existence, in which one animal lives without any relationship to any other, is possible but rarely occurs in nature. Animals possessing sex associate together for a longer or shorter time as mates. Many of the same kind live together, forming a colony or, as in the case of ants, bees, and wasps, are organized into a society. The relations of animals to their physical environments are manifold. Some are confined to the land, others to the water, and still others may be at home in both. Aquatic forms may be restricted to fresh waters, others may be only marine, while there are also those that pass from one to the other environment. Animals exist that spend all of their lives in the soil, and others that enjoy the power of flight pass much of their active existence in the air. 6. Definition of Zoology. — A study of animals from every aspect constitutes the science of zoology. This broad field is capable of being divided into many of less extent depending on the various aspects from which animals may be viewed, whether considered in whole or in part, as to structure or function, in relation to the inorganic environment or to other animals and the plants about them, or from the standpoint of the principles and laws which underhe and determine the phenomena exhibited by animal life. 7. Divisions of the Subject. — Zoology may be divided, in accordance with the manner in which animals are studied, into two great sub- sciences; these are morphology, which deals with animals as to form and structure, and physiology, which deals with them as to their functions. THE FIELD OF ZOOLOGY 5 Under the head of morphology are included anatomy, which is con- cerned with structure as made out by dissection; histology, which treats of structure as determined by the microscope; taxonomy, which is the study of the laws and principles of classification and which is based upon structure; geographical distribution, or zoogeography, the study of the geographic distribution of animals; and paleozoology, which deals with the fossil remains of animals. The reason for the placing of zoogeography under morphology is that in this field animals are treated as species or groups, a morphological basis. Physiology includes physiology in the narrow sense, which deals with the functions of the different parts of which the body is composed; ecology, which deals with the functional relations of animals to their environment; psychology, which is the study of the mental life of animals; and sociology, which is the study of animal societies. Three other fields belong to both morphology and physiology— embryology, which deals with the development of animals; pathology, which relates to the diseases that affect them; and parasitology, which is concerned with animals that live at the expense of their fellows. No effort has been made up to the present time, however, to separate the structural and the functional aspects in any of these three fields. Not only may zoology be divided into the two broad subsciences first named and their various divisions, based upon the manner of approach and method of investigation, but it also may be separated into many restricted sciences, each of which deals with a particular group. Among these are protozoology, which deals with the lowest, one-celled animals; helminthology, which is concerned with the worms; entomology, which is the study of insects; conchology, the study of moUusks; ichthyology, the study of fishes; herpetology, the study of reptiles and amphibians; orni- thology, the study of birds; and mammalogy, the study of mammals. Many other sciences, which concern less extensive groups and are less familiar, might be added to this Ust, but only one need be spoken of and that is anthropology, which is the study of man as to his physical nature. Other divisions of zoology are evolution, which seeks to explain the origin and modification of the different species, and genetics, which deals with the laws that underlie inheritance. It is evident from what has been said that the divisions of the subject cross one another. Anatomy, taxonomy, geographical distribution, ecology, and all of the other fields mentioned as differing in the point of approach or method will, for example, deal more or less with birds. Ornithology, on the other hand, may be considered from a morphological, physiological, taxonomic, distributional, or ecological aspect. The same is true of any other group of animals. In addition to the fields which have been mentioned, a long list of practical applications might be added which would greatly increase 6 FUNDAMENTAL PRINCIPLES the list of the divisions of zoology. Among such applications are animal husbandry, which deals with the cultivation of the domesticated higher animals, apiculture, with that of bees, and aquiculture, with that of fish and other aquatic forms; medicine, which is concerned with disease and the methods used in its treatment; and hygiene, which presents the principles involved in the maintenance of health. All of these involve the study of animal hfe and should really be included in zoology in its widest extent. 8. Scope of General Zoology. — Within the scope of a beginning course in zoology, it is impossible to handle more than the most general principles of the subject and the broader phenomena of animal life. None of the fields enumerated above can be more than barely introduced to the student; their further cultivation must be left for special courses. 9. Animal Biology. — To many persons the word zoology is associated with the structure and classification of animals, while the word biology conveys the implication of life and activity. This is an unwarranted connotation ; but because in this text the emphasis is on the latter aspect of the subject rather than on the former, it has been entitled "Animal Biology." Properly speaking, the term biology is applied to the combined sciences of botany and zoology. CHAPTER II MATTER Proficiency in all of the different divisions of zoology cannot be attained without considerable knowledge of physics and chemistry, though the different fields differ greatly in the demands they make upon such knowledge. An adequate grasp of even the most general and most fundamental zoological principles, however, requires a familiarity with the broad conceptions which underlie those sciences; and since many approach this subject lacking such acquaintance, it is necessary to review briefly these conceptions. Logically, the first subject to be considered is the nature of living matter. To understand this it becomes necessary to define what is meant by matter in general and to state some facts in regard to it. 10. Definitions. — For our purpose matter may be defined as all in this universe, of the existence of which we may be made aware through our senses, either directly, or indirectly by means of any kind of apparatus. This will include all that is revealed by the microscope, telescope, spectro- scope, or any other type of instrument. We commonly refer to all of our experiences as either material or spiritual. Those which are material presume the existence of matter; those which we term spiritual have no essential relation to it. 11. Constitution of Matter. — Matter differs in kind, exists in various forms, and exhibits a great variety of phenomena. The study of matter with respect to kind is in the field of chemistry; that of matter without regard to kind, including the phenomena of matter in general, belongs to physics. Most matter with which we are familiar does not consist simply of one kind of matter but is of the nature of a compound, consisting of two or more different kinds. A piece of any ordinary compound substance, as, for instance, a piece of chalk, is termed a mass and may by being broken into two parts be divided into two masses. These may be again broken, and the process may be continued, resulting in masses of smaller and smaller size, each still remaining chalk. This division may be carried beyond the limit of visibility by the unaided eye and even far beyond that by the microscope. The masses become smaller and smaller but each bit remains a mass. Finally a fragment may be conceived that can no longer be broken and the portions be alike. This smallest particle of any compound substance is termed a molecule. When molecules are 7 8 FUNDAMENTAL PRINCIPLES separated into smaller fragments these are unlike and are definite in number for every substance. These fragments are termed atoms. A molecule of chalk is divisible into five atoms — one of carbon, one of calcium, and three of oxygen. It has been found that an atom may be further subdivided into much smaller particles, one or more of which lie at the center and are termed protons, while the others, either associated with the protons in a nucleus or distributed at distances about it, are known as electrons. When, however, atoms are divided into these finer particles, they are found to be all of the same nature, and so all matter in this finely divided state becomes alike. Atoms of different kinds differ only in the arrangement of these component particles with respect to each other. 12. Elements and Compounds. — This division of matter into mole- cules, atoms, protons, and electrons belongs to physics. Chemistry, strictly speaking, deals only with atoms classified according to their kind and with molecules considered with respect to the kind and arrange- ment of the atoms of which they are composed. Each kind of atom is known as an element. Compounds are classified with respect to their composition in terms of elements and also with respect to the manner in which they react, or change, when brought in contact with other com- pounds or with elements. Chemists now recognize about 90 different elements, some of the most common of which are carbon, hydrogen, oxygen, nitrogen, iron, calcium, phosphorus, sodium, and potassium. To economize time and space in referring to these elements they are designated by symbols, which may be the initial letter of the name of the element, either in its English or in its Latin form, or two letters when it is necessary to distinguish between elements having the same initial. Thus, C represents carbon; Ca, calcium; H, hydrogen; N, nitrogen; and Fe (from the Latin /errMm), iron. 13. Acids, Bases, and Salts. — The elements are divided into two categories. Metals, which number more than three-fourths of the total, include gold (Au), silver (Ag), lead (Pb), copper (Cu), and iron (Fe), and also calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na). The nonmetals include oxygen (0), nitrogen (N), carbon (C), sulphur (S), silicon (Si), phosphorus (P), chlorine (CI), and iodine (I). Hydrogen (H) is not a metal but in chemical combinations acts like one. Metals combine with oxygen to form bases which, in solution in water, color litmus more or less strongly blue — that is, they are alkaline. Nonmetals, when combined with oxygen, yield acids which, in aqueous solutions, are sour to the taste and color litmus red. All acids contain hydrogen. A substance resulting from the union of a base and an acid is called a salt. Examples are table salt, or sodium chloride (NaCl) ; hme, or calcium carbonate (CaCOs); and blue vitriol, or copper sulphate (CUSO4). In all chemical combinations the number of atoms of each element in a MATTER 9 molecule of the compound is indicated by a figure written as a subscript. 14. States of Matter. — All molecules, and also smaller particles, are believed to be in continual motion, but this motion is restrained by the attraction which molecules or other particles exert upon each other. This attraction is proportional to the sizes of the particles and inversely proportional to the squares of the distances between them. Thus it follows that particles at a great distance exert an attraction which is practically negligible, but as they approach each other the attraction increases at a constantly accelerated rate. The relation between the molecules in a mass determines the character of the mass which they form, and thus we get the different states of matter. If the molecules are sufficiently close together that the attrac- tion between them holds them in the same relative position with respect to each other, the mass preserves constantly the same form and is termed a solid. If, however, the attraction is insufficient to preserve this form and the mass tends to change its shape, it is called a fluid. If the mole- cules of a fluid tend to remain together but there is so nearly a balance between the force of motion and the force of attraction that the mass easily changes shape, it is called a liquid. Under the influence of gravity the molecules in a liquid seek the lowest level and the upper surface of the mass becomes a plane surface. Finally, if the molecules of a fluid are so far apart that the attraction of one for another fails to keep them together and they tend to move in all directions, the mass expands and fills all available space and is termed a gas. Thus both liquids and gases are included under the term fluid. Some gases are heavy and expand, or diffuse, slowly; others are light and diffuse rapidly. Since there may be any degree of attraction, depending upon the sizes of molecules and the distances between them, these states of matter are not sharply defined but pass into one another through an indefinite number of gradations. A mass which is not a perfect solid but which may be made to change its shape gradually is termed viscous. Under the influence of varying degrees of heat and pressure, substances may be made to assume any desired degree of viscosity or to pass from one state to another. Tar may be heated until it flows readily, and a syrup cooled until it hardly flows at all. Under the influence of heat, liquids may be made to change to gases; and under the influence of increased pressure, gases may be liquefied. Under ordinary conditions, some liquids readily evaporate and change into gases, while some solids seem to pass directly into the gaseous state without appearing at any time as liquids. In a dry climate, snow may evaporate without wetting the surface on which it lies. It might be argued that a substance must pass through the liquid state in passing from the solid to the gaseous but there is difficulty in proving this. 10 FUNDAMENTAL PRINCIPLES 15. Surface Films. — In a liquid mass, as has just been said, the parti- cles are free to move; but if the mass is at rest, they do not do so, being equally attracted by other particles all about them and in a state of balance. This being true they are easily pushed aside by an object which passes through the liquid. The surface of the mass, however, is formed of a layer of molecules which are not in balance but which attract one another and are attracted to those below them to a degree which makes the penetration of this layer a matter of overcoming a certain amount of resistance (Fig. 1). This layer of molecules is called a surface film. ^^ It is th® presence of this film and r ^"-^f )*~*Cj the resistance it offers to penetration [j / \ which makes it possible, with suffi- (^ (^ cient care, to lay a dry needle upon the surface of a liquid and for it to CN /~\ remain there. The resistance which i^ V-^ this film offers to the penetration of OV-^ ^^ any object is also responsible for the V-^ ^-^ dimpling of such a surface when a C!^ V^ dry object is pressed down upon it. / V_y If the object is pressed with a con- FiQ. 1.— Diagram illustrating the bai- stantly increasing force the dimple anced attraction exerted upon a molecule , i n i , • i in a mass of liquid (a) , and the unbalanced becomes gradually deeper until attraction upon a molecule at the surface finally, when the forCC beCOmCS Suffi- (6). The two-pointed arrows (•^) indicate ■ , , , .1 ^i xi i • ^ mutual attraction between two molecules. ClCnt tO rupturC the film, the objCCt enters the liquid and is wet by it. Then the liquid, as a result of adhesion between the particles of the object and those of the liquid, rises on the surface of the object in a characteristic way which is familiar to everyone — that is, unless the liquid is a very heavy one, like mercury, in which case the surface film does not rise but is depressed next to the surface of the object, to which, in other words, the liquid does not adhere. Some animals, like the water striders, move freely about on water supported by the surface film, while others, like snails, cling to this film from below and move about hanging to it. The surface film thus serves as a highway which may be traveled on both its upper and its under surfaces. The strength of the surface film of a thick liquid causes a drop of it to stand up on a dry surface and assume the form of a flattened sphere, whereas the weakness of that of a thin liquid results in its spreading out over a considerable area in a thin sheet. 16. Mixtures. — Masses of different kinds may be associated in what may be termed, in general, mixtures. Two solids reduced to a state of fine division may be mixed; liquids also may be mixed, and gases as well; and any two of the three, or all three, may be mixed. If a solid is mixed with a liquid and remains in masses of greater size than molecules, the liquid is more or less turbid and the mixture is termed MATTER 11 a suspension. If, however, the sohd is reduced to particles of the size of molecules or of atoms, the mixture will become as clear as the liquid itself and such a mixture is termed a solution. The liquid is called the solvent and the dissolved solid the solute. In the same way one liquid may be mixed with another liquid and form a suspension or a solution, depending on the size of the particles. A suspension of one liquid in another is termed an emulsion. Gases, too, go into solution and also form suspensions, but such suspensions do not persist. A gas and a liquid may be shaken up together and a suspension created, the gas being distributed through the liquid in the form of bubbles, but the gas quickly escapes from the mixture except for what becomes dissolved. Gases also escape from solution unless there is just as much gas over the liquid as there is in an equal volume within it. This passage of gas either into solution or out of it, depending on whether the gas pressure is greater without or within, explains why animals take in oxygen and pass out carbon dioxide. This exchange, which is called respiration, takes place through extremely thin membranes which separate the air from the liquids in the body and which allow the gases to pass through freely. Differences in gas pressure also account for the constant entrance of oxygen into water to replace what aquatic animals have taken from it in respiration, and the constant escape into the water and then into the air of the carbon dioxide which they have produced. 17. Ionization. — Whenever acids, bases, or salts go into solution in water, there is a tendency for the molecules to separate into the compo- nent atoms or into radicals, which are groups of atoms. The atoms or radicals then exist free in the solution. These solutions conduct elec- tricity and are known as electrolytes. Free atoms or radicals in such a solution are found to carry minute electrical charges and are called ions. Those ions which are metallic in nature carry positive charges, and those which are nonmetallic carry negative charges; they are termed, therefore, positive or negative ions. Table salt (NaCl) in solution separates into sodium (Na) and chlorine (CI) ions; sodium sulphate (Na2S04), into Na and SO4 ions, SO4 being a radical. Na is a positive ion; CI and SO4 are negative. When, by evaporation of the solution or by precipitation, the substance which is in solution is made to reappear again in solid form, the ions combine, and the charges neutralize one another and disappear. This separation of ions in solution is known as electrolysis, or dissociation; different substances show great differences in the degree to which this occurs. Sugar and other substances which are non-conductors do not show much dissociation. Acids by dissociation produce H ions; bases, OH ions. 18. Colloids. — Many thin membranes occur in the bodies of animals in which openings exist of very minute size; similar membranes can be artificially produced. Whenever two different liquids are in contact 12 FUNDAMENTAL PRINCIPLES with the two sides of such a membrane, the hquids tend to mingle by the passage of molecules or atoms through the openings. When one or both of these two hquids contain sohds, other hquids, or gases in solution, the particles of these substances also may pass through the same "^openings. By the use of these membranes we can distinguish two categories of substances. Those substances which when they are in solution are capable of passing through such a membrane are said to be crystalloidal and if they normally exhibit this character ar- often called crystalloids, while those which will not pass through aie cermed colloidal kncl called colloids. But many crystalloids may be made to assume a colloidal condition. Of course it is a matter of relative size of particles and openings, but in general it is true that crystalloids are substances which exist in molecules of very small size or as atoms in a solution, while colloids are substances which exist as particles of larger size, are dispersed in hquids in the form of suspensions, and do not form true solutions. These colloidal suspensions are thicker or more hke glue than are crystal- loidal solutions. From crystahoidal solutions the substances are easily obtained in crystalline form, but this is not true of colloidal suspensions. Oils and fats and proteins, such as the albumen which forms the white of eggs, are colloidal. This separation of cohoidal from crystalloidal sub- stances is known as dialysis and the membranes which effect the separa- tion, as dialyzing membranes, or dialyzers. Other membranes are found in the body which under similar condi- tions permit some liquids or gases to pass through them and prevent others from so doing, but the passage takes place as a result of solution in the membrane and not through openings in it. Such a membrane is termed a semipermeaUe membrane. This process is known as osmosis. In this case the force behind the movement is called osmotic pressure. Osmotic pressure is subject to the same laws as is the pressure of a gas on the wahs of the container in which it is and against which it exerts pressure because of its tendency to expand. Whenever two hquids of different densities are in contact and do not mix, the plane of separation, which is in effect a surface film, acts hke a semipermeable membrane; some substances will pass through it, others will not. 19. Colloidal Emulsions.— In case the suspended droplets m an emulsion are colloidal, the mixture is termed a colloidal emulsion. The liquid in suspension is dispersed and is called the disperse phase, while the other hquid is called the dispersion medium, or the continuous phase. A colloidal emulsion is more or less jelly-like. It may at one time become thinner and assume the condition of a sol or at another time become thicker and assume the condition of a gel. This may be due to the transfer of liquid from the disperse phase into the dispersion medium, or vice versa, without the addition of more liquid from without. The MATTER 13 two phases, in other words, tend to change places, one being at one time dispersed in the other, at another time the other in the one. When the colloid droplets are scattered in the watery dispersion medium, it is a thin jelly; but when they swell, press upon each other, and the dispersion medium is restricted to the crevices between them, the whole becomes thick and tends to set or become firm (Fig. 2). The ability to change from one state to another and back again, over and over, causes a colloidal emulsion +o be called revemihle. A gelatin suspension in water forms such an emulsion. A B C D Fig. 2. — Diagrams to illustrate the change of a colloidal emulsion from sol to gel. In A the droplets of the disperse phase (not stippled) are shown scattered through the dis- persion medium (stippled) and the emulsion is a sol; in B the droplets are shown taking up liquid and swelling; in C this is continued until they press upon one another; in D the drop- lets are so crowded as to become continuous and to have become in fact the dispersion medium, while that which was the dispersion medium is now in droplets and has become the disperse phase. The emulsion has become a gel. 20. Reactions. — Whenever two substances are brought together and a change occurs which involves a recombination of the atoms in a manner different from that which previously existed, this change is termed a reaction. Reactions vary in speed and in the results attendant upon them with the character of the substances reacting. Some substances have the power to cause a reaction without themselves entering into it or being affected by it. Such substances are known as catalyzers, catalytic agents, ferments, or enzymes; the effect is called catalysis, ferment action, ox fermentation. A small amount of a digestive ferment is capable, if given time, of causing the digestion of any amount of the substance on which it acts and would itself be found undiminished in quantity and unchanged in character at the end of that time. Each ferment acts on a particular substance or on similar substances and is most active at a certain temperature or in a medium of a certain degree of acidity or alkalinity. CHAPTER III ENERGY Energy is usually defined in physics as the capacity to do work. It may be more simply expressed as that which is behind all action in this universe. Every change in the state of matter, in the form or position of a mass of matter, or in the chemical composition of matter involves a change in energy. 21. Forms of Energy.— Energy appears in two forms: potential, or fixed; and kinetic, or free. Potential energy is energy of position. Every particle of matter in this world possesses an amount of potential energy varying with its size and with its distance from the center of gravity of the earth. Gravitational attraction, if all restraint were removed, would cause it to fall to that center; in measure as it approached the center its potential energy would be changed to kinetic, and if it could be conceived as having arrived at that center it would possess no potential energy at all with respect to this terrestrial system. However, it would still have potential energy as a part of a larger system, the solar system, of which this terrestrial system is a part. It would also have potential energy as a part of the largest system, which is the universe. A molecule is an energy system, as is also an atom. In the free movement of a bit of matter its potential energy is changed into kinetic energy, or energy of motion. Any mass possesses potential energy because of the relation of its particles to one another. Energy of motion is manifested not only when masses change position but also when changes occur within them. It is also manifested when the atoms or molecules of which matter is composed cause by their movement certain characteristic phenomena such as heat, fight, and the passage of an electric current. These are all forms of kinetic energy. 22. Chemical Energy.— In many cases, when elements are made to combine to form a compound, the apphcation of kinetic energy is neces- sary to bring them into the proper relationship to each other. Part of this energy, at least, is represented by the potential energy which these particles possess by virtue of this relation. This form of potential energy is known as the energy of chemical union or simply as chemical energy. Every substance has an amount of chemical energy proportion- ate to the complexity of its structure— that is, to the number and variety of atoms which make it up. A very simple substance has a smaU amount of such energy; a very complex one may have a great deal. In general, 14 ENERGY 15 inorganic compounds have relatively small amounts of chemical energy, while organic compounds, on the other hand, usually have large amounts. When a compound is resolved into its component atoms, the chemical energy reappears as kinetic energy. This is the source of the charges carried by the ions in an ionized solution. Because of their greater complexity, a greater amount of kinetic energy can be produced in an animal body by the disintegration of organic compounds than by that of inorganic compounds. This kinetic energy may appear in various forms, as movement, light, heat, and electricity. 23. Laws of Thermodynamics. — Thermodynamics is a part of physics which deals with energy transformations. Two laws have long been con- sidered fundamental in this field: One is that of the conservation of energy, which states that the sum total of energy in this universe is the same at all times, being neither created nor destroyed but simply changed from one form to another. The other is the law of entropy, which states that energy tends to accumulate in potential form, or, in simpler language, that every- thing tends to run down. Associated with these is the law of conserva- tion of mass, which states that matter in this universe is neither created nor destroyed but is simply transferred from one place to another or changed in character. CHAPTER IV LIVING AND NONLIVING MATTER Living matter or matter which has been arranged under the influence of hfe is termed organic; other matter is inorganic. Any Hving thing may be called an organism. No element is found in living matter which is not found also in nonliving, but only a small part of those which are found in nonhving matter occur in Hving. As will be seen later, no more than about a dozen of the 92 elements known can be looked upon as normal constituents of living matter. Moreover, so far as is known, no forces operate in hving matter differing from those operating in nonlivmg mat- ter, but all of the phenomena of hving matter involving energy may be explained in terms of the same physical forces which operate in all inor- ganic matter throughout the universe. 24. Contrast between Living and Nonliving Matter.— Many pomts of contrast have been enumerated between hving and nonliving matter, some of which are more significant and others less so. None of them is capable of being applied in every case successfully and in such a manner as to yield an immediate result. It is frequently stated that both defimte- ness of size and definiteness of form distinguish hving things, but this distinction is only a very general one. Some crystals conform very closely to a certain size and approach with great mathematical exactness a typical form, but, generally speaking, masses of inorganic matter vary much more in size and shape than do masses of living matter. Con- formity to type form is one of the most effective means of distinguishing the species of hving organisms. The more important contrasts between the two kinds of matter are the following: 1 Chemical Composition.— Living matter, while varying in its precise chemical structure, approaches very closely, both in the number of elements contained and in the proportions between them, a certam definite composition. So close is this agreement that living matter is recognized as made up of a particular substance to which is given the name protoplasm. This does not occur in nonliving matter. 2 Or^am^a^ton.— Protoplasm possesses an internal organization, evidenced not only by its appearance under the microscope but also by certain chemical relationships, the presence of which is necessary in order that it may carry on the phenomena and exhibit the reactions which are associated with life. 3 Metabolism and Growth.— Troiopisism also possesses the power ot waste and repair and of growth. The carrying on of life activities 16 LIVING AND NONLIVING MATTER 17 involves the breaking down of living matter with the formation of wastes, such as urea, water, and carbon dioxide, and the liberation of kinetic energy, mainly evident as heat and movement. Living matter possesses the power to rid itself of this waste and also to take in new matter and, by adding it to its mass, to repair the loss which it has suffered. All these changes together constitute metabolism. If the material taken in and added to the mass is greater than the amount lost by waste, the result is an increase in the size of the living mass which is called growth. At the same time, mere increase in bulk does not necessarily imply growth. The taking up of water and the swelling that living matter undergoes under certain conditions is not growth. This word is properly applied only when new matter is added to the substance of protoplasm itself. This addition occurs in living matter in a manner different from that in which it occurs in inorganic. The latter usually increases in bulk by additions to the surface, or growth is by accretion; while in the former growth occurs by the introduction of new particles among those already present, which is growth by intussusception. 4. Reproduction. — Living things have the power to reproduce them- selves by the formation of other masses similar in every respect to the parent mass. Sometimes this similarity is perfect from the first. At other times a fragment from the parent mass gradually assumes the size and form of the parent. Nonliving things do not possess this power. 5. Irritability. — Living things, generally speaking, have the power of responding to changes in their environment, such changes acting as stimuli. This quality is termed irritability, or reactiveness. The response evidences itself in the movements of animals and results in the various ways in which animals adjust themselves to the conditions of their exist- ence. The organism is not itself modified in any essential respect by the reaction and may, under proper conditions, reassume precisely the character it possessed before the reaction occurred. Nonhving things may also be affected by changes in the environment, but the modification is not in the nature of adjustment, is destructive in its effect, and the thing cannot of itself regain its former character. 25. Tests of Life. — Living matter, however, is not always to be recog- nized by any characteristics which it possesses. A living seed may appear as inert as any bit of inorganic matter, and some animals may exist dried up and apparently without any of the attributes which belong ordinarily to living things. The test which may be applied in such cases to determine whether or not life is present, or is possible, is to place the object under such conditions of warmth and moisture as experience has shown tend to develop life activities and observe if under these conditions the distinctive phenomena of life are manifested. If they are, the infer- ence is that the object was alive or that the drying up occurred in such a way as not to destroy the organization that is behind all life phenomena. CHAPTER V PROTOPLASM Protoplasm is invariably associated with life, and so far as is now known life can exist in no other substance. When protoplasm ceases to be living, it quickly undergoes destructive chemical changes which reduce it to other and simpler compounds. 26. Historical Facts. — Protoplasm was first determined to be a particular substance, with characters of its own, in 1835, when it was described as existing in animals, by a Frenchman, Dujardin, who called it sarcode. The protoplasm of plants was first described in 1846 by a German, Von Mohl, who attached to it the name of protoplasm. This name had been used six years before by Purkinje but in a very restricted sense. That sarcode and protoplasm were one and the same substance was most thoroughly demonstrated by Max Schultze in 1861, and to this living substance, common to both plants and animals, is now applied the name given by Von Mohl. 27. Chemical Character of Protoplasm. — Protoplasm eludes exact chemical analysis. The chemical composition of an animal body may be determined, but this includes the skeleton, stored food, and other mate- rials which are nonliving. To secure a mass of perfectly pure protoplasm is difficult, and the exact analysis impossible. Nevertheless, certain general statements can be safely made. 1. It is almost entirely composed of 12 elements, including the following : Element Approximate percentage Oxygen Carbon Hydrogen Nitrogen Sulphur Iron Calcium Magnesium Sodium Potassium Phosphorus Chlorine 18 65 20 10 3 PROTOPLASM 19 There are very minute quantities of several other elements in the bodies of higher animals, including fluorine in the enamel of the teeth and also sihcon and iodine, but these are found only in certain tissues and are not considered normal constituents of protoplasm. Minute amounts of copper, manganese, zinc, and bromine have been found in marine inverte- brates. The first five elements named in the table above are always present and a certain number of the last group are also, but some of them may be lacking. Though some elements are present in small amount, this small amount is none the less vital to the performance of the functions of living matter. Certain of these elements are especially abundant in particular forms of protoplasm, such as iron in the protoplasm of red blood corpuscles and phosphorus in the protoplasm of the nerve and reproductive cells. 2. These elements are in combination in a variety of compounds which may be classified as follows: Organic Compounds Inorganic Compounds Proteins Salts Fats Water Carbohydrates Proteins are, vso to speak, foundation substances; about them is built up the complex aggregate called protoplasm. They contain four ele- ments — carbon, oxygen, hydrogen, and nitrogen — together with sulphur and in some cases phosphorus, and have certain pecuhar properties. They are colloidal and have a tendency to coagulate on heating. The protein molecule is very large and is built up of many amino acids, which are acids containing the amino group NH2. The number of different proteins in different forms of protoplasm is very great ; different species of animals have different types of protein, and particular tissues in one animal contain proteins which are not found in the other tissues of the same animal. Fats are also colloids and consist of carbon, hydrogen, and oxygen, the oxygen being very small in amount as compared with the carbon and hydrogen. This fact makes them susceptible of a great amount of oxidation, with the consequent production of a large amount of heat. There are three different types of fats: (1) true fats, which include com- pounds of glycerin and fatty acids, mostly oleic, palmitic, and stearic; (2) lipoids, including the phosphorus-containing fats, an example of which is lecithin, and the sugar-containing fats, or cerebrosides, found in nervous tissue; and (3) sterols, including cholesterol, which seems to occur in every animal cell. Carbohydrates, which include sugars and starches, may also be in a colloidal condition, although neither they nor fats show any tendency to coagulate with heat. They also consist of carbon, hydrogen, and oxygen, but there is relatively a much greater amount of oxygen than 20 FUNDAMENTAL PRINCIPLES in fats, the proportion of hydrogen and oxygen being the same as in water, or two to one. Only two kinds can be shown to exist as such in the tissues of the body, dextrose and glycogen. The water present in protoplasm maintains many substances in solu- tion. Water, however, is important as a constituent of protoplasm, not only as a very effective solvent but also because of its high specific heat, because of its comparatively high surface tension, and because of the fact that its presence gives to protoplasm the necessary consistency and enables it to vary this consistency. The high specific heat of water makes necessary the application of a large amount of heat to raise its temperature and allows it to give off a correspondingly large amount of heat in cooling, thus enabling it to exert a protective effect against sudden and extreme temperature changes in the living body. Salts are present in considerable number and are to a large degree ionized, though the degree varies with different salts. They aid in the maintenance of certain other substances in solution and take part in some of the reactions which are characteristic of living matter. Some of these salts are the chlorides, phosphates, and carbonates. 3. The substances enumerated above are associated together in a chemical aggregate which contains several thousands of atoms. Thus protoplasm is not so simple as other familiar substances but represents a complex of substances all associated together in a certain definite fashion. This definite arrangement of substances in protoplasm may be termed its chemical organization and is one of the most, striking of its characters. What this organization means may be illustrated in the following way: One might go to a jewelry store and ask the jeweler to give him every part which enters into the formation of a watch. The jeweler could heap in his hand the necessary number and kind of wheels, screws, pinions, and jewels as well as the hands, dial, case, and so on, so that within his hand he would hold everything necessary to make a complete watch. He would not, however, have a watch. This assemblage of parts does not become a watch capable of performing the service expected of it until these parts have all been arranged in a certain very definite relation of one to the other. So it is with the chemical substances which make up proto- plasm. Without the necessary organization the assemblage of parts named above is not a watch, and in the absence of the chemical organiza- tion which is a property of protoplasm that substance cannot be said to exist. This organization is made possible by the fact that protoplasm is a colloidal emulsion and the various constituents may be distributed among the droplets of the disperse phase and through the continuous phase. 4. Protoplasm is very unstable. It alters in composition in response to every change in the environment about it and when active remains for no two consecutive moments the same. PROTOPLASM 21 5. Protoplasm is also exceedingly variable. There is a difference chemically between the protoplasm of one species of animal or plant and that of another. Moreover, there is also a difference between the forms of protoplasm contained in the various structures found in the body of each animal and plant. With all the individuality that exists in living things it is conceivable that no two bits of living matter are ever precisely alike. This characteristic is back of all the adjustments of living matter to its environment. 6. Protoplasm also undergoes an orderly sequence of chemical reac- tions which we call metabolism, and as long as life is being manifested the cycle of such reactions is being repeated over and over. 28. Physical Characteristics of Protoplasm. — Protoplasm has the following physical characteristics : 1. It is viscid and gelatinous in consistency, differing in viscosity in various forms of life, in the various structures of the body, and under various conditions. 2. Its texture, generally speaking, is more or less granular. 3. It is colorless in pure form. All colors which it seems to possess are due to the presence of colored bodies within the living substance. 4. It is more or less translucent, being never perfectly transparent, and this translucency gives to it in mass a grayish appearance. 5. It is of the nature of an emulsoid, or colloidal emulsion, the various substances of which it is composed being distributed through the disper- sion medium and in the droplets of the disperse phase. Being colloidal in character and being reversible, it is possible for water and substances in solution to enter protoplasm from without, causing it to become more fluid, or to pass out from it, resulting in its becoming firmer in consistency. To the same fact and to the fact that the internal surface films in such an emulsion may act like semipermeable membranes is due the possibility of water and substances in solution passing in both directions through the walls of the droplets, causing them to swell or to shrink. As they swell and crowd together the whole tends to become a gel, and as they shrink and move with greater ease in the more fluid dispersion medium it tends to become a sol. This transfer of water may be the consequence of chemical changes taking place in either the substance of one phase or that of the other. This ability of protoplasm to change from sol to gel and back to sol over and over again is behind many vital activities, including all move- ment. The entrance of water and substances in solution into the mass at certain times and the giving up of water and other substances in solution at other times make possible the taking in of food and the giving out of waste. The passage of materials through the internal films, which are the walls of the droplets of the disperse phase, possibly plays a part in 22 FUNDAMENTAL PRINCIPLES the orderly sequence of reactions which takes place in living matter and makes possible growth by intussusception. 6. The salts in protoplasm are to a very great degree ionized, and the state of ionization contributes to the speed of chemical reactions within the mass. 29. Microscopical Structure of Protoplasm. — The droplets of the disperse phase in protoplasm are mostly too small to be within the range of microscopic vision, and those which are not too small are hardly visible. The structure of protoplasm as exhibited under the microscope, however, reveals the presence of firm granules of different sizes ; of fibers, which may form a network, or reticulum; and of droplets, or alveoli. None of these is evidence of the emulsoid nature of protoplasm but all may have a bear- ing on the function of the cell. Emphasis upon one or another of these different elements has formed the basis for three theories of the normal Fig. 3. — Semidiagrammatic sketches illustrating the different appearances exhibited by protoplasm. A, the granular type; represents cells from the liver of the mouse. B, the fibro-reticular type; represents a nerve cell with its fibers cut away. C, the alveolar type; represents a portion of alveolar protoplasm. (Figs. A and C modified from Wilson, The Cell, A, after Altmann, by the courtesy of The Macmillan Com,pany.) structure of protoplasm, which has been thought to be (1) granular, (2) fibrillar or reticular, or (3) alveolar. Though most protoplasm now seems to be alveolar in character, a granular appearance is often exhibited in gland cells, and fibers are prominent in nerve cells, muscle cells, and some epithehal cells (Fig. 3). 30. Appropriateness of Protoplasm as Living Substance. — It is evident from what has been said as to the chemical and physical char- acteristics of protoplasm that among these are many which contribute directly to the carrying out of life activities, and all are necessary in a substance in which life can manifest itself. Its physical characteristics, its chemical nature, its organization, its proneness to change, its ability to assume an almost infinite variety of forms, and its capacity constantly to carry on metabolism all make protoplasm the only appropriate living substance. Its exceedingly great complexity offers the possibility of almost infinite internal change and adjustment, while at the same time PROTOPLASM 23 the total chemical composition and the general character remain practi- cally the same. Life itself is ceaseless change. When this protoplasmic organization becomes fixed and no longer capable of change, it has suffered that which we call death. Very soon after death reanimation under any conditions becomes impossible because changes supervene which destroy the very organization itself. It is true that seeds and even some animals may be dried under proper conditions and exist for a long time in a dormant state. That the organi- zation remains intact and its capacity to undergo the changes which accompany life is unimpaired is shown by the fact that when placed under suitable conditions life activities are soon resumed. 31, Life Is a Consequence or Concomitant of Organization.— Living matter, then, is not living because it contains certain elements, for none of these elements is characteristic of life. All the chemical sub- stances which enter into protoplasm might be collected theoretically in proper proportions, but the mixture would not be protoplasm. Life is possible only when the organization which has been referred to above is effected; and when that organization is brought about, the other chemical and physical characteristics of protoplasm also become added to it. Protoplasm has been termed the "physical basis of life" and it is such in the sense that it furnishes the physical organization and the attendant conditions that make life possible. CHAPTER VI LIFE As indicated in the preceding topic, life is always associated with a certain type of organization of matter. It can be defined neither in terms of the chemical elements which enter into it, none of which is peculiar to it, nor in terms of the forces which act through it, since those forces are the same as those which also act through nonliving matter, the results being different only because of the organization. 32. Definition. — Life might be defined as the possession of a certain type of organization or as embracing certain phenomena. It might be conceived of as energy manifested in a manner made possible by its organi- zation. A precise definition is difficult to give in a form with which all would agree, but the following is suggested: Life is a continual series of reactions in a complexly organized substance known as protoplasm, by means of which the organization tends to adjust itself to a constantly varying environment. According to this definition a dormant mass of protoplasm, such as that in a seed, might possess the capacity to exhibit life but would demonstrate this only under certain favorable conditions. 33. Vital Force. — The theory has been held in the past that a mysteri- ous vital force acts through living matter and is responsible for the characteristic phenomena of life, but every attempt to demonstrate the existence of such a force has ended in failure. As knowledge of life phe- nomena has increased, it has constantly become more evident that all such phenomena can be explained by reference to the same forces which also operate in nonliving matter and, as far as is known, throughout the universe. 34. Vitalism and Mechanism. — Those who have believed in this vital force have been termed vitalists, and their view vitalism. Over against this is the conception that the body is like a machine, played upon by forces in its environment, and that life phenomena are mechanical responses to these forces. Those who have contended for this view have been termed mechanists, and their view mechanism. While vitalism is not a tenable conception today, the most extreme form of mechanism also does not appeal to the greater number of biologists, who observe phenomena which are distinguished as vital. The view of the majority might be stated as a modified form of mechanism. It is true that there is nothing peculiar in the chemical elements or the physical forces in living matter as distinguished from nonliving matter, but that does not mean 24 LIFE 25 that the chemical changes in protophism are precisely the same as those occurring in nonliving matter, nor does it mean that none of the phenom- ena associated with life is peculiar to living things. The differences, however, are the outgrowth of the organization and are not due to any- supernatural force which animates living bodies. 35. Origin of Life. — The question of the origin of life on this planet has been a source of speculation from early Greek times, if not before. Empedocles, a Greek, about 500 B.C. presented a theory of the origin of life which was that owing to attractive forces elements were combined into the parts of which plants were composed, and then under the influence of the same forces these parts were assembled in such a manner as to form whole plants. Animals were supposed to have originated in the same way as did plants, parts being formed first which later came together to form the animals. This theory, fantastic as it now seems, is the first definite theory of the origin of life and has earned for its author the title of father of evolution. Aristotle, another Greek who lived in the fourth century before Christ, had a theory more in harmony with present- day conceptions. He believed that living matter originated as a jelly formed at the shore of the sea and that out of this evolved first plants and then animals. The simplest forms developed first, followed in order by others of gradually increasing complexity up to man. The Mosaic, or special-creation, theory of the origin of life appears in the first chapter of Genesis and was the legendary explanation accepted by the Jews. According to this theory each kind of animal was created in the beginning with the same character it has today, or, in other words, each was the result of a special creative act. Because it is in the Bible this theory has been thought of as necessarily involving the idea of a divine providence and for that reason different from any other theory. As a matter of fact, however, the conception of a deity need not be associated with any one of the theories of the origin of life to the exclusion of its association with others. One who believes in a creative and ruhng spirit or force in the universe will attribute to it the creation of life no matter what his theory may be as to how creation actually occurred, while one who does not believe in such a force will leave it out of whatever scheme of creation he holds. Spontaneous generation implies the repeated creation of life whenever favorable conditions occur. A theory of spontaneous generation was held by the Greeks, who believed that various living forms found in fresh water died off each fall and were recreated each spring. The observations of Aristotle and others showed this belief to be incorrect in the case of many familiar forms. Gradually the number of animals thought to be spontaneously generated was reduced, until, in 1680, Redi, an Italian, effectively disproved the spontaneous-generation theory held at that time when he showed that fly maggots were not spontaneously generated in 26 FUNDAMENTAL PRINCIPLES decaying meat. This theory was again revived, however, when the microscope revealed the minute forms of plant and animal life which exist and which were immediately conceived by many to be spontaneously generated when the right conditions occur. The work of Pasteur in France and Tyndall in England during the latter part of the last century, however, disproved the possibility of spontaneous generation of even these minute forms. There are those today who entertain a belief in the possibility of the spontaneous generation of life, but there is no existing evidence to support their views. Another theory of the origin of life held by the physicists, Kelvin and Helmholtz, who also lived in the last century, explains the presence of life on the earth by stating that it was brought here on meteorites through the interstellar spaces from some other world. This meteoritic theory is frequently coupled with the conception that life has always existed in this universe and is simply passed from one world to another from time to time. This is unsatisfactory to biologists, because it puts the whole problem beyond the possibility of human explanation, and because the conditions which would have to be withstood by life coming to this planet in that way are apparently beyond the limits of endurance of living matter. For this reason the theory has been believed in by only a few, and these not biologists. Another theory of the origin of life, formulated by a German, Pfiuger, in the latter part of the last century, has been known as the cyanogen theory. According to this theory the earth was once exceedingly hot and the elements were in a free state. As it cooled a temperature and other conditions were reached which caused the union of carbon and nitrogen into a substance known as cyanogen, the formula for which is CN. As the earth cooled still more, water was formed by the union of hydrogen and oxygen; and then by the combination of cyanogen and water, cyanic acid (HCNO) was produced. The characteristics of cyanic acid resemble in many ways those of protoplasm: (1) It is a hquid which is transparent at low temperatures, but it tends to coagulate and become opaque at high temperatures. (2) It can increase in bulk by a process essentially like that of growth by intussusception. (3) Its molecules can be rearranged to form urea and it can be decomposed into carbon dioxide and ammonia. By the addition of sulphur and other elements to cyanic acid, proteins might have been formed and thus life might have developed. Another, known as the bacterial theory, is that of Osborn, who places the origin of life at a time when there was no soil on the surface of the earth, when all the water was fresh, and the air contained more carbon dioxide than at present. At that time the earth was shrouded in a dense cloud through which the sun never penetrated. This cloud was main- tained by constant evaporation from the heated surface of the earth. The air was warm and saturated with moisture. Lightning played LIFE 27 constantly through the clouds and rain descended in torrents. Under these conditions nitrates are conceived to have been produced in rain- water pools due to the discharges of electricity in the water-saturated atmosphere, and ammonia also appeared in volcanic waters. Such conditions favor the growth of bacteria, and Osborn has suggested that the first life may have developed in the form of these minute organisms. Able to make use of inorganic food, very resistant to destructive agencies, and capable of exceedingly rapid multiplication, they were able to main- tain existence and gradually evolved into higher but still simple forms from which both plants and animals have come. This theory suggests, the conditions under which life may have arisen and the nature of the earliest organisms but does not successfully solve the problem of the origin of life. Numerous other theories have been proposed, one involving the development first of ferments and then, under the influence of these ferments, the organization of living matter. Another theory is that an inherent tendency exists for simple compounds, under proper conditions, to unite themselves together and form more complex compounds and that as a consequence of this tendency, and in a favorable environment, protoplasm was gradually built up or synthesized out of the various com- pounds which it contains. None of these theories has proved satisfactory to biologists generally, and it must be confessed that at this time it is not possible to explain how life on this earth originated. That life must have appeared at a time when conditions were favorable goes without saying, but most biologists believe that at only one time in the history of the earth has there been such a fortunate concomitance of favorable conditions as to bring about this creation. From the life created at that time all living things which have ever existed on this earth have descended. 36. Possibility of Creating Life. — The creation of life by human agency has been the dream of men in the past, and the idea will surely continue to be entertained in ages still to come. In the present state of human knowledge, however, a realization of the dream seems to be out of the question. It appears hardly probable that the conditions which existed on the surface of the earth at the time when life first originated will ever be repeated in the laboratory. Theoretically it would be possible to assemble in the proper proportions those substances which exist in protoplasm, but the crucial thing— the bringing about of the organization which exists in living matter — seems beyond human power when the limitations under which men work are considered. Yet the idea is con- ceivable and efforts to bring it to fulfillment will probably never cease so long as the human race continues to exist. CHAPTER VII CELLS Living protoplasm always exists in the form of minute masses known as cells, which possess a characteristic structure. Organisms may consist of but one cell or of many, but in either case the cell may be considered the unit of structure, or the morphological unit. 37. Definition. — A cell may be defined as a mass of protoplasm in which can be distinguished a portion called the nucleus. A distinction may be drawn between the substance of the nucleus, which is termed nucleoplasm, and the protoplasm of the rest of the cell, which is called cytoplasm. 38. Sizes and Shapes of Cells. — Cells vary greatly in size. The most minute animal cells are one-celled blood parasites which are invisi- ble, or only barely visible, to the highest powers of the microscope. Most cells cannot be seen by the unaided eye. There are cells, however, which are relatively gigantic. A one-celled organism, parasitic in the alimentary canal of the lobster, reaches a length of two- thirds of an inch ; and egg cells, with the yolk which they contain, may even exceed this in diameter and contain a much greater amount of substance. Some nerve cells, the main cell body of which is not proportionately very large, possess fibers, which are parts of the cells, that may even reach a length of several feet. Cells also vary greatly in shape. The typical form, unaffected by environment or unmodified for the production of any particular function, is spherical, but the pressure of adjacent cells or from other structures may crowd the.se cells into a variety of shapes, such as polygonal, cubical, columnar, or flat and platelike. Other cells, particularly muscle cells, become greatly elongated and assume the form of fibers, while still others become very complexly branched (Fig. 4). 39. Numbers of Cells. — As has been previously stated, an organism may consist of but one cell; however, most organisms are made up of more, the numbers in the largest organisms running into the trillions. 40. Structure of Cells. — A cell (Fig. 5) consists of a mass of jelly- like cytoplasm inclosing a nucleus. The surface of this cytoplasm is covered by a plasvia jnemhrane, or cell membrane, which is living and semipermeable. Outside it may be a cell wall composed of material which is not protoplasmic and is nonliving, being a secretion formed by the cell. In animal cells this wall is often absent. 28 CELLS 29 The nucleus, which is set off from the cytoplasm by a nuclear mem- brane, shows a fine network of fibers known as linin fibers; and scattered throughout the nucleus, adhering to these hnin fibers, are masses of another substance known as chromafm. This name was given to this substance because it takes dyes or stains to a very high degree and when Fig. 4. — Various types of cells. A, epithelial cell shed from the lining of the human mouth; a is a side view of the cell. X 300. /J, human ovum, nearly mature. X 200. C, human sperm cell. X 1-300. D, diagram of a nerve cell. E, a bone cell; somewhat dia- grammatic. X 700. F, human blood corpuscles. X 1,000. G, nonstriated muscle cell from mammalian intestine. X 640. the cell is subjected to these, the chromatin stands out as scattered, deeply stained particles. A body which shows plainly in the nucleus is known as a nucleolus. Nucleoli, however, are of various kinds. Some are called plasmosomes or true nucleoh. Sometimes such a body is made up of granules of the chromatin massed together and is called a karyosome 30 FUNDAMENTAL PRINCIPLES or chromatin nucleolus. The more fluid portion of the nucleoplasm between these structures which have been enumerated is often called nuclear sap. In the cytoplasm appear several characteristic structures. A body appears, under certain conditions, near the nucleus, known as the central body, or centrosome, containing one or two granules called centrioles. More or less solid particles in the cell include living portions of the proto- plasm which have some particular function to perform, such as the chloro- phyll bodies which give the green color to plants. These have been given the general term plastids. Included in plastids are mitochondria, or chondriosomes, which are fiber-like and more compact structures, the Plasma membrane Central body con faining two centrioles Plasmosome Nucleus Chromatin Nuclear sap Vacuole Mefaplasm Cell wall Golgi bodies Nuclear membrane Linin Karyosome Cytoplasm Plastid Mitochondria Fig. 5. — Composite diagram of a cell having the form of a typical cell and containing all of the structures generally recognized as normal in cells not modified for any particular function. nature of which is in question; and Golgi bodies, which may be scattered through the cell or collected around the central body. Bits of food or waste particles which have collectively been called metaplasm may be present in the cytoplasm. Vacuoles are transparent droplets seen regu- larly in certain cells or at certain times in other cells. 41. General Physiology of the Cell. — There is a division of labor in the cell among the structures which have been named. The nucleus is, in a sense, the vital center. Cytoplasm alone is unable to carry on its activities and its life is brief after it is separated from the nucleus. Prob- ably under the influence of substances formed by the nucleus and passed out into the cytoplasm the latter does most of the ordinary work of the cell, including the taking in of food, the carrying on of many of the chemi- cal and physical changes associated with life, the passing out of waste, the reception of all stimuli, and the movements which occur in response to them. The chromatin is the medium by which hereditary characters are CELLS 31 transmitted, and therefore it determines the character of the cell. The central body with its centriole, or centrioles, is active in cell division. Plastids are living structures with active functions and are more numerous in plant than in animal cells; the chlorophyll bodies, which are one form of plastids, utilizing the energy of the sun's rays, build up carbohydrates from carbon dioxide and water. As indicated in the preceding topic some of the other structures play only a passive r61e, while the functions of others are not definitely known. 42. Development of Knowledge of the Cell. — The history of this development may be briefly summarized as follows: Hooke, an English microscopist, discovered in 1665 that cork was divided into little com- partments which, because they reminded him of the cells in a monastery, he called cells. In 1833, or 168 years later. Brown, also an Englishman, discovered the nucleus, and it was then supposed that the cell consisted of a living wall inclosing a nonliving, watery substance in which floated the nucleus, also living. It was not until 1835 that Dujardin, a French- man, as has already been stated (Sec. 26), discovered that this watery content of the cell was a substance of peculiar character and that it, too, was living. From this time the cell was believed to contain these three elements, which were found to be common to both plants and animals. It was discovered after a time, however, that cells existed which did not possess a cell wall. Thus the wall, which was at first supposed to be the essential part of the cell, was finally eliminated as a part of it and the word cell became really a misnomer. The most important contribution to the modern conception of the cell was that of Max Schultze, who, in 1861, showed that the substance of all cells, plant and animal, was similar, and who defined a cell as a "small mass of protoplasm endowed with the attributes of life. " 43. Cell Theory and Cell Doctrme. — The cell theory was due to the work of Schleiden, a botanist (1838), and of Schwann, a zoologist (1839). Each of these men had found cells in all hving matter which he had studied, and they presented, each in a pubhcation in his own field, a hypothesis which has been known as the cell theory, to the effect that living matter always exists in the form of cells. It was to them a theory, but in the time that has elapsed since the dates mentioned it has been found to hold good for all living substance which has been studied. Thus today we no longer consider it a theory but rather a fact, and so it has come to be known as the cell doctrine. This conception when first presented had a most profound effect upon biological thought, and its influence has been equaled only by that exerted by Darwin's theory of evolution. CHAPTER VIII METABOLISM Reference has been made previously to the fact that one of the char- acteristics of hving matter is its abiUty to carry on metabohsm — that is, its abihty to take material into the body and work it over in such a way as to make it a part of the Uving organization and from it to secure the energy with which to carry on the processes of hfe. 44. Definition. — All hving things, in the performance of their various activities, exhibit physical and chemical changes. A result of the former 5. Excrefion W.EIiminafion 10. EypJrafion Fig. 6. — Diagram showing the steps in metabolism as they occur in an ameba. is the hberation of the needed kinetic energy, and of the latter the forma- tion of waste materials which are thrown away. To replace the material so used and to provide a source for more kinetic energy, food must be taken into the body and incorporated in the organization. The sum total of all the chemical and physical processes involved is termed metah- olism. The discussion which follows applies particularly to animal organisms, but plants carry on metabohsm by a series of steps which, considering the difference in structure, parallel those in animals. 32 METABOLISM 33 45. Food. — The food of the organism, in the broadest sense, must inckide all of the compounds which enter into the chemical organization of protoplasm — that is, proteins, fats, carbohydrates, salts, and water. It must also include certain substances termed vitamins which seem to play a necessary part in the carrying on of metabolic activities. It is also essential that this food shall supply energy in such a form as to be available to the organism. \. Ingest/on 10. Eypi ration 5 . Inspiration 4. Circulation *). Excretion of nitrogenous waste 4. Circulation 3. Absorption 11. Elimination In every cell of the body 6. Assimilation 1. Dissimilation b. Secretion 9. Excretion 2. Digestion Q. Secretion of gastric Juice Bgesfi ion Fig. 7. — Diagram to suggest the steps in metabolism as they occur in the human body. For comparison with Fig. 6. 46. Steps in Metabolism. — Metabohsm takes place in the body by a series of very definite steps (Figs. 6 and 7), all of which are necessary in the metabolism of the higher animals, but certain ones of which are simplified or dispensed with in the case of the very simple animals. These steps are referred to in terms that are more or less in popular use with very loose and uncertain meanings. The words excretion, secretion, ehmination, and assimilation are frequently met but are usually used with an uncertain significance. It will be necessary, therefore, for the student 34 FUNDAMENTAL PRINCIPLES eiycerin /^^^^^^ Sugar (Carbohy draft mi no acids (Protein) to learn these words in this connection as scientific terms, each with a very precise meaning, and to keep this meaning separate from that which he may have hitherto attached to the word. 47. Ingestion. — The first step in metabohsm is the taking in of food — ingestion. This may occur in one-celled animals through any point on the surface of the body. While the same may be true to a certain degree in the case of higher animals, most of them, and some of the one-celled ones, take food through a particular opening on the surface of the body, called the mouth. Under the head of ingestion also occurs all mechanical processes such as chewing and swal- lowing which precede any chemical change. 48. Digestion. — As soon as the food is in position to be acted upon by digestive fluids, these are secreted into the cavity which contains it. By their action a series of chemical and physical changes is initiated, which results in reducing the solid food to liquid form and changing part of it chemically so as to render it capable of „ „ _. r •,, r being absorbed. This process is called JbiG. 8. — Diagram of a villus, one of the finger-like projections in the small digestion. In the higher animals digcs- intestine of a mammal to show how the ^ion may begin in the mouth and be absorption of organic foods takes place. The lymph vessels are in solid black, the Continued in the stomach and intes- blood vessels stippled. ^i^g Only organic foods need to be digested, the other foods being capable of absorption without undergoing this process. 49. Absorption. — The passage of the digested food from the food vacuole into the protoplasm of one-celled animals is termed absorption. In higher animals the same process takes place by the food entering the cells forming the lining of the alimentary canal and being then passed into the blood or lymph contained in blood vessels or lymphatics which lie behind these cells (Fig. 8). In vertebrates absorption occurs mostly in the small intestine. The digested food is not further changed during this process, though it may suffer a change as soon as the process is complete. In the process of digestion fats, for example, are broken down into fatty acids and glycerin but are changed back to fat in the cells into which they are absorbed. 50. Circulation. — Whether the animal is one- or many-celled, the food cannot be all utilized at the point of absorption but must be cir- culated throughout the living body for use in various parts. This circulation may take place within the cell, by osmosis from cell to cell, METABOLISM 35 or by means of a circulatory system, generally the blood circulatory system. 51. Inspiration. — Oxygen, as well as food, is constantly needed by the body. Its entrance into the body is termed inspiration. This may occur through all points on the surface of the body or may occur only through certain particular organs set aside for the purpose, such as lungs or gills. Upon entrance into the body oxygen is circulated in the same manner as food and taken up by the tissues as needed. This passage of oxygen into the tissues is termed internal inspiration; its entrance into the body, external inspiration (Fig. 9). Fig. 9. — Diagram to illustrate external and internal respiration. 52. Assimilation. — The food, having been brought to the point in the body where it is to be used, is taken up by the protoplasm and more or less intimately incorporated into the living mass, becoming, at least for the time, a part of the organization. This process of addition of new material to the existing material of the body is termed assimilation. This material, no longer food but a part of the protoplasm, may be soon used or it may remain for a greater or less length of time as a part of the cell before actually becoming involved in chemical changes. 53. Dissimilation. — Sooner or later chemical changes occur which collectively are called dissimilation, as a result of which protoplasm and the more complex food substances associated with it are broken down into simpler substances. Associated with these chemical changes is a transformation of part of the potential energy represented by these substances into kinetic energy, which appears mostly in the form of heat or movement. 54. Secretion. — If the substances produced in dissimilation can be utiUzed in any way by the body as a whole, they are termed secretions, and the process involved in their passing out of the cell which produces them is termed secretion. These may be passed out upon the surface of the body, into any cavity in the body, or into the blood and body fluids. Examples of such substances are the tears, which when poured 36 FUNDAMENTAL PRINCIPLES out upon the surface of the eyeball serve to keep it moist; other fluids, which also serve to moisten or lubricate internal surfaces; the digestive secretions, which when passed into the alimentary canal assist in the digestion of food; and also substances known as internal secretions. These internal secretions are carried over the body and perform various functions in connection with the carrying on of life activities, such as the regulation of metabolism and the control of growth processes. 55. Excretion. — Some products of dissimilation, such as urea, water, and carbon dioxide, seem to be of no use to the body and are termed excretions. The process by which they are passed out of the cell which forms them is termed excretion. In many cases the excretions are poured out directly upon the surface and are immediately disposed of; in other cases, however, they are formed in the body at some distance from the external surface and have to be transported to some particular part of the body before they can be passed out. Here again the circulation comes into play, it being as necessary for the carrying of waste matters to the point where they are passed out of the body as for the transporta- tion of food and oxygen to the cells. 56. Expiration. — The carbon dioxide formed in dissimilation is carried by the circulation to some particular part of the body where it is passed out. This part may be in the lower animals the general body surface or some particular structure within the body; in the higher ani- mals it is the gills or lungs. This process is expiration. Here also a distinction may be made between internal expiration, which is the passage of carbon dioxide out of the tissues into the blood, and external expiration, which is its passage from the body (Fig. 9). Expiration relieves the body of its gaseous waste. 57. Elimination. — Liquid waste may be eliminated from any point on the body surface, or it may be passed out by some particular structure. In the higher animals the kidneys and skin are the principal organs of elimination, though some elimination may occur through the walls of the alimentary canal toward its posterior end. In this last case elimina- tion should not be confused with egestion. As an example of the differ- ence between excretion and elimination may be mentioned the fact that urea is produced in the body in the liver, where excretion proper takes place, but it is very largely eliminated by the kidneys. 58. Egestion. — Egestion is the passing from the body of indigestible materials contained in the food, which are known collectively as feces. Feces might be referred to as solid waste, but they have not, properly speaking, been involved in the process of metabolism as have the sub- stances which are expired or eliminated. The material egested has been passed through the body but has at no time been a part of it. Egestion, again, may take place from any point on the surface of some of the ore- METABOLISM 37 celled animals or may take place through the posterior opening of the alimentary canal in higher forms. 59. Respiration. — The processes of inspiration and expiration taken together constitute respiration, which includes all gaseous interchanges in the body. 60. Anabolism and Katabolism. — The processes beginning with ingestion and ending with assimilation are collectively termed anabolism. Anabolism may be defined as the sum of all processes involved in the building up of the body. The processes beginning with dissimilation and ending with expiration and elimination are collectively termed katabolism. Kataholism may be defined as the sum of all processes having to do with the breaking down of the body and the getting rid of the waste matter resulting from it. Egestion, for reasons given in a preceding paragraph (Sec. 58), does not belong under either anabolism or katabolism. 61. Vitamins. — It has been found recently that providing the body with the necessary kinds and amounts of proteins, fats, and carbohy- drates or of salts and water is not sufficient. Something else is needed to enable it to assimilate the organic foods, and that is the presence of vitamins. These are organic substances of unknown composition that occur in certain natural foods. Vitamin A (fat-soluble) is present in many animal fats, milk, butter, and yolk of eggs but is deficient in vege- table substitutes such as oleomargarine. It promotes growth and perhaps protects the body against rickets. Vitamin B (water-soluble) is found in fruit juices, meat, milk, yolk of eggs, the coverings of grains and other seeds, yeast, and thin-leafed vegetables. It also promotes growth and guards the body from certain inflammatory conditions in nerves. Vitamin C (water-soluble) is contained in citrus fruits, raspberries, apples, beans, cabbages, carrots, turnips, and tomatoes and in liver. It prevents scurvy. Vitamin D (water-soluble and fat-soluble) occurs especially in cod-liver oil. It also prevents rickets, and a deficiency of it leads to an inability to form a properly calcified skeleton. 62. Energy Changes in Metabolism. — The food taken into the body represents a supply of potential energy. One object of dissimilation is to change part of this into kinetic form in order that the body can make use of it. This kinetic energy appears mostly as heat and as the mechan- ical energy exhibited in movement; a small part appears as electrical energy; and in some cases, in very small part, as light, shown in the luminescence of some organisms. Some of this kinetic energy is neces- sarily used in the securing of additional food, but some is also used in growth, in reproduction, and in carrying on other activities. Among the lower animals the portion of energy used in the securing of additional food is much larger than in the higher animals. The development of efficiency among the latter is, to a considerable degree, connected with 38 FUNDAMENTAL PRINCIPLES the possession of more effective food-securing devices, which leaves a proportionately larger part of the total energy of the body to be used in other ways. Man has solved this problem far more successfully than any animal below him, and the advance he has made to a dominating position in the animal kingdom may to a considerable degree be attrib- uted to this fact. 63. Uses of Different Foods. — The different foods serve different purposes in the body. The protein food is in part used to replace the protein of living tissue when that is used up. Carbohydrates furnish the mechanical energy expended in muscular movements. Fats are used chiefly as a source of heat. All dissimilative changes in the body liberate heat, but from fats, owing to the fact that they contain a very small amount of oxygen and are therefore susceptible of a great deal of oxidation, may be produced more heat than from any other food. Water must be maintained in large amount in the body, both because it is needed to give the required consistency to the protoplasm and because it serves as a vehicle for other substances in solution. Salts are essential constituents of protoplasm, also participating in the metabolic changes and exerting a regulatory effect upon them. Oxidation processes take place in all of the cells of the body, the extent of such processes in any given cell determining the amount of activity carried on by the cell. They do not occur in the blood except in the blood corpuscles, which are cells. 64. Storage. — The body does not in all cases make immediate use of the food absorbed, in which case it may be stored against future need. Fats are thus accumulated in the form of fat. Since carbohydrates are the chief sources of muscular energy and since the body must at all times have not only a ready supply but also a large volume in storage to be used as needed, there is in the liver an abundant supply of stored carbohydrate ready to be given out to the blood and circulated to all parts. An excess of carbohydrates may be changed to fats and stored as such. In the chemical changes in the body, carbohydrates may be derived from substances resulting from protein decomposition, and fats may in some cases be changed to sugar, but neither of them can be con- verted into proteins, since these contain nitrogen, which is lacking in carbohydrates and true fats. Proteins are not stored, but any excess is immediately broken down and the waste products eliminated. Storage should not be confused with growth, since the stored food is not a part of the protoplasmic organization. 65. Metabolism the Central Fact in Life. — All life activities result from metabolism in the living organism, and therefore life might be defined as the orderly series of metabolic changes which occur in matter possessing the necessary protoplasmic organization. In last analysis all of the functions of the living body may be described in terms of metabo- METABOLISM 39 lism. The animal organism may be conceived as an energy system, and it has also been likened to a chemical machine the product of which is kinetic energy. The plant organism, likewise, may be conceived as an energy system and as a chemical machine, but its product is largely the complex organic compounds which form the basis of the food of animals. CHAPTER IX PLANTS AND ANIMALS Two great groups of living things exist, plants and animals. The higher forms of the two are readily recognized, but the simpler ones lack the characteristics which serve to distinguish the higher types. Many simple living things cannot be satisfactorily assigned to either category. A German named Haeckel suggested as a way out of this difficulty that an intermediate group, which he termed Protista, be recognized. This sug- gestion, however, has not beenfollowed, because it would simply double the difficulty — instead of having to draw one line of demarcation which is very uncertain, it would be necessary to draw two lines, both as uncertain. 66. Comparison between Plants and Animals. — In many respects plants and animals agree. The protoplasm of which both are composed is, as far as can be seen, essentially the same. Although plant and animal cells have certain features which aid in their discrimination, those fea- tures are not essential characteristics of the protoplasm of which they are composed, and as far as present knowledge goes, the protoplasm of the two is indistinguishable. Indeed, it is generally assumed that there was but one creation of life on this earth and that from that first created life both plants and animals have sprung. This makes quite intelligible the difficulty in distinguishing between the simpler forms of the two. Metabolism is carried on essentially in the same manner in plants as in animals. Plants and animals have many activities in common and those in which they differ are developed gradually in passing from lower to higher forms. The lower plants are termed protophytes and the higher metaphytes, while the lower animals are called protozoans and the higher metazoans. The unit of plant structure is the cell, as is also the unit of animal structure, and plant cells present the same phenomena in connection with their multiplication as do animal cells. They are affected in a similar manner by various external forces. The higher forms of both possess sex. The mechanism of inheritance is the same and the phenomena connected with inheritance are also quite comparable in plants and animals. 67. Biology. — Because of the fact that there are many things in common between plants and animals, the subjects of botany and zoology are frequently considered as parts of one larger subject termed biology. It has to do with all that concerns living things in general and may be conceived of as divided into the two fields botany and zoology. In the further division of these two fields each can be divided into a series of 40 PLANTS AND ANIMALS 41 subsciences which in general correspond. Thus one can speak of plant morphology and animal morphology, of plant ecology and animal ecology, of plant physiology and animal physiology, and the same is true of taxonomy, pathology, embryology, and so on. 68. Differences between Plants and Animals. — The higher plants and higher animals, as has already been stated, present distinctions which are sufficient in all cases to enable us to assign a hving thing to either one category or the other. Among these distinctions are the following: 1. Movement. — Broadly speaking, the higher plants lack the power of movement and in all cases are without the power of locomotion. On the other hand, almost without exception, animals are possessed of both. 2. Manner of Growth. — In a general way it may be said that the plant grows by the addition of parts externally, such as the addition of leaves and twigs. There is also evidence of internal growth, as is seen in the gradually expanding trunk and constantly thickening branches, the new wood being added just underneath the bark. In animals, on the other hand, few parts are seen to be added externally, though some ani- mals show at times a gradual increase in size of wings, horns, and other visible parts ; growth is mostly internal and the body simply increases in size. 3. Cells. — Plant cells usually possess a distinct cell wall, composed of cellulose, which gives to the cell rigidity of form and to which is due the immobility of the plant body. Animal cells, on the contrary, often possess no wall of any kind, and the walls, when they are present, are generally thin and permit the cell to change shape. This fact contributes to the power of movement and of locomotion possessed by animals. 4. Food Securing and Metabolism. — There are minor differences between plants and animals connected with the metabolism of the two, but the steps are essentially the same (Fig. 10). However, the oxidation changes in the cells which are included in this text under the term dis- similation are by botanists termed respiration. Plants, in addition to carrying on the same type of metabolism as has been described for animals, have the power of manufacturing their own complex foods. By virtue of their possession of plastids and of their ability to utilize the energy of the sun's rays they can take simple substances from the earth and air and out of them synthesize complex substances such as proteins, fats, and carbohydrates. These processes are known as photosynthesis. After producing these substances plants make use of them in the same fashion as do animals, but animals being incapable of manufacturing such foods have to get them from plants or by eating other animals. Since plants take these simple substances in gaseous or liquid form there is no soHd waste left as a result of plant metabolism, and conse- quently egestion does not occur. Also, since plants build up proteins, which they add to their substance, and make immediate use of any nitrogenous matter liberated in protoplasmic activity, they do not 42 FUNDAMENTAL PRINCIPLES produce urea, a characteristic excretion of animals. In photosynthesis plants use up the available carbon dioxide and give off an excess of oxygen, while animals always utilize all the oxygen they can get and give off carbon dioxide as waste. Eli minoifion In spira fio n (oxygen) Organic food Expl ration ^ (COz) ■Eg est ion ^ (feces) a. Animcil and CO 2 Excess of oxygei •n b. Plant Fig. 10. — Diagrams contrasting the metabolic and food-manufacturing processes in plants with the metabolic processes in animals. Because of the difference in metabolism of plants and animals plants have often been referred to as predominantly anabolic in their activities, while animals are characteristically katabohc. Also, since in the presence of hght the processes concerned with photosynthesis outweigh those concerned in the metabolism of protoplasm, during the day plants use more carbon dioxide than they produce and produce more oxygen than they use. At night when photosynthesis is arrested they are, from a metabohc standpoint, on the same plane as animals. CHAPTER X GROWTH AND REPRODUCTION Whenever during the Hfetime of an animal assimilation exceeds dissimilation, there results an increase in the actual amount of protoplasm in the body; this increase is termed growth. When the reverse is true and dissimilation exceeds assimilation, the body shrinks in size; this process is known as emaciation. Many animals continue to grow throughout their lifetimes, although growth is more rapid at the beginning and slows up more and more with advancing age. This is true of many cold- blooded vertebrates, in the case of which size is a fairly clear index of age, other conditions being equal. Of course, in this case care must be taken to judge of the amount of available food, for in an environment in Birth Age 21 Age 45 Age 60 Death Fig. 11. — Diagram illustrating the growth cyde in man. This is intended to be typical, but individual growth cycles vary greatly, both as to the span of the whole and the propor- tionate lengths of different periods. which food is limited a limit is also set to the size of the animal, and no matter how old it may grow it will never equal in size an animal living under more advantageous conditions. 69. Growth Cycles. — The Ufe cycle of an animal comprises the whole series of phenomena from the time development begins to the death of the organism. Among the various aspects in which this can be studied is that which involves the growth cycle. This varies greatly with different animals. As has just been stated, some animals never cease to grow; others grow only during the early parts of their lives. The latter is true of insects, none of which ever grows at all after the adult condition is reached. The higher vertebrates, however, including man, have a regular growth cycle involving youth, maturity, and old age (Fig. 11). Growth is most rapid in this case at the beginning of life and remains still rapid until the end of the period of youth, when the individual has practically attained full stature. A very gradual growth still continues, 43 44 FUNDAMENTAL PRINCIPLES becoming constantly less rapid, until the maximum size is attained, which is usually somewhat beyond middle life. There is then a slow decrease in weight. Throughout the rest of the period of maturity this decrease continues, becoming gradually more pronounced; after the individual passes into old age, however, there is a more rapid emaciation, which ends with death. The youthful condition is termed adolescence; that accompanying old age, senescence. 70. Limit of Size. — No matter what the character of the life cycle may be or when growth takes place or for how long, there is in the case of all animals a size limit which is not surpassed. While in one-celled animals this varies somewhat in different lines of descent, in any one line it is rather closely approximated. In such forms there seem to be some metabolic relations in the cells, which, as this limit is approached, give rise to changes which automatically result in the division of the cell and the production of smaller organisms. The same thing is true of the individual cells which compose the bodies of higher animals, but the result is to produce a larger body and not new individuals. In some higher forms the process of cell multiplication practically ceases as the individual becomes adult; in other forms it stops in certain parts of the body but goes on in other parts. The organs of the central nervous system reach full size early in hfe. Bones and muscles continue to grow until the animal becomes adult. The skin, however, from the outer layer of which dead cells are continually shed, grows throughout Hfe by the multiplication of living cells in the deeper layers. The size of many- celled animals is also limited by various factors such as inheritance, the available food, and the activity of glands the secretions of which favor or hinder growth. 71. Reproduction. — Multiplication by cell division, which is the most common way among one-celled animals, is not possible to those which are many-celled, since the different cells which make up the latter became varied in form and structure and also become limited to the performance of one or a few functions out of the many that are pos- sessed by the body as a whole. Consequently such cells cannot reproduce the complete animal. Under these conditions certain cells are set aside for the purpose of reproduction and are relieved from the performance of any other duty. They serve as cells from which the development of another individual may be initiated, transmitting to that individual the characters of the parents in the bodies of which they have been produced. Such cells are termed sex cells or gametes. Of these there are two types which in the higher animals are known as egg cells and sperm cells, or sperms. The animal which produces egg cells is called female and the one which produces sperm cells, male. Generally speaking, egg cells are relatively large in size and sperm cells relatively very small, so that the former may be termed macrogametes and the latter, microgametes. These GROWTH AND REPRODUCTION 45 terms are common to both botany and zoology, the same being true of another term, zygote, which is apphed to the cell resulting from the union of a sperm cell and an egg cell in fertilization. This union of two sex cells is also known as syngnmy. This sexual type of reproduction is characteristic of higher animals, while asexual reproduction, which is any type not involving these sex cells, occurs chiefly in lower ones. CHAPTER XI MITOSIS The necessity of ultimate cell division in the cases of cells which continue to grow has been explained in the previous chapter (Sec. 70). If the cell thus dividing is itself a one-celled animal, then cell division and reproduction occur at the same time. If, however, the cell is only- one of the cells in a many-celled animal, then division does not in most cases result in reproduction, which is the formation of a new individual, but simply in an increase in size of the individual of which the cell is a part. It makes no difference, however, as to the precise manner in which the division is carried out except that in one-celled animals it is a rela- tively simple process and in many-celled animals it is more complex. 72. Normal Cell Division. — The ordinary way in which a cell divides is by a series of steps (Fig. 12) which do not occur in every case in precisely the same order, but all of which are passed through before the division is complete. These steps may be outlined as follows: First, the central body, if it has not before been visible, comes into view beside the nucleus and both it and its centriole divide into two. These two central bodies begin to separate, and as they do so fibers appear between them which form a spindle-shaped figure. At the same time the chromatin granules in the nucleus, which have been scattered irregularly upon the linin network, begin to collect together into a slender and very much tangled thread, or spire^ne, and the linin network as such begins to dis- appear. This thread shows itself early to be a double thread. Radiating rays, termed astral rays, appear about each central body, forming star- shaped figures known as asters. As the central bodies gradually separate each with an aster about it, the spindle fibers between them seem to press against the nuclear membrane. The chromatin thread shortens and thickens, forming a much less involved tangle, while the linin com- pletely disappears. This shortened, double chromatin thread breaks crosswise, producing a number of pieces which are known as chromosomes and which, because the spireme was double, are in pairs. During these changes, while the central bodies still continue to separate, the nuclear membrane disappears and the spindle swings into the area occupied hitherto by the nucleus. The central bodies come to lie on opposite sides of what was the nuclear area and the spindle stretches across this area from one central body to the other. The chromosomes arrange .themselves in a double row across the center of the spindle, forming in 46 MITOSIS 47 some cases what is known as an equatorial jdate. End views at this time show that the chromosomes may form a ring about the equator of the spindle but that more often they are distributed through it. The asters reach a maximum development, extending sometimes to the periphery of the cell. At this time the mitotic figure is called the amphiaster, and the stage the amphiastei- stage. The chromosomes are in pairs and the two of a pair are similar. Now the two of each pair of chromosomes Nucleus Centra/ body and cen- t Holes Linin Spindle Aster Spireme (Linin disappears) Spireme shor- tens and thickens Nuclear membrane disappears 7 Chromosomes Nuclear Asters membranes and Spindle appear disappear Fig. 12. — Diagrams representing the steps in a typical mitosis. The steps numbered 1 to 4 represent the prophase, 1 being a resting cell; 5, the metaphase; 6, the anaphase; and 7 to 9, the telophase, 9 showing the two daughter resting cells. begin to separate, one moving along the rays of the spindle toward one central body and the second moving toward the other central body. As the two separating groups of chromosomes approach the central bodies these chromosomes become scattered about the respective ends of the spindle in an irregular fashion. Then a series of steps occur which in a general way are the reverse of the steps occurring at the beginning of the process. The separate chromosomes become irregular in shape and fuse, 48 FUNDAMENTAL PRINCIPLES forming a meshwork, from which finally are produced granules of chro- matin scattered over a new network of linin, all formed out of the chro- matin. A new nuclear membrane appears about each group and thus two nuclei take form; this membrane is also formed from the chromatin, though the cytoplasm may assist in its formation. The spindle fibers gradually disappear, as do the astral rays. A constriction appears in the cytoplasm of the cell in the same plane as the equatorial plate, fre- quently as early as the stage in which the two groups of chromosomes begin to separate. This constriction grows deeper and deeper while the two new nuclei, or daughter nuclei, are being formed. Finally, as the spindle fibers disappear, the constriction cuts clear through the cell, which thus forms two new cells, each with a nucleus, and each half the size of the parent cell. The whole process of cell division which has been outlined is divided into four phases. All of the steps from the beginning to the time when the chromosomes line up in the equator of the spindle are termed collec- tively the prophase. The steps involving the arrangement of the daugh- ter chromosomes in pairs in the equatorial plane are termed the metaphase. The process of division may be arrested for a time at this point. The period of migration of the daughter chromosomes from the equator of the spindle to the poles is termed the anaphase, and the series of steps involved in the division of the cytoplasm of the cell and the construction of two separate nuclei is termed the telophase. These steps do not always occur in the same order and any general outline such as has been given will have to be modified in a great variety of ways to suit different cases. There may be two centrioles in the resting cell before mitosis begins. Variations occur in the time of the splitting of the chromatin thread and its division into chromosomes. In some cases the spireme is single and breaks transversely into chromo- somes during the prophase. These line up on the equator of the spindle and split longitudinally in the metaphase. The two of a pair so formed are similar to the parent chromosome, and it is assumed that each pair of chromosomes formed from the double thread is derived from a parent chromosome, to which they are similar. Sometimes the linin seems to disappear; at other times it seems to assist in the production of the spindle ; and in still other cases it seems to contribute to the formation of the chromosomes. In some cases, especially when the chromosomes form a ring around the spindle, a distinction may be drawn between the chro- mosomal or traction fibers which connect the chromosomes with the poles of the spindle, and which are also called mantle fibers because they are on the outside of the spindle, and the continuous fibers that run from one pole to the other. This mode of cell division has been called karyokinesis, meaning nuclear movement, or, more commonly, mitosis, from the Greek word for thread, referring to the chromatin thread. MITOSIS 49 The whole process may be conceived as a play in four acts in which there are no pauses between the acts. In the first act, the prophase, the characters are introduced and several scenes are presented which lead up to the second act, the metaphase. This is a grand tableau which shows the stage fully set and the characters in formal array. In the third act, the anaphase, a parade of the chromosomes results in their separation and the division of all the characters into two groups at the opposite sides of the stage. In the fourth act, the telophase, the char- acters in each group adjust themselves to the changed conditions and find their proper places in the new order. 73. Significance of Mitosis. — The universality of this process in the division of both animal and plant cells, and the regularity with which in every case the various steps occur suggest that the process is of vital importance. The great care with which the chromatin is divided between the two cells seems to show that this division is, of all these steps, the most significant. Recognizing in chromatin the substance which bears the hereditary qualities from cell to cell, and in the case of sex cells from animal to animal, this splitting has been conceived as having for its end the passing on of the hereditary qualities to each of the two daughter cells. Thus not only do these become structurally alike but each also possesses the same inherited characteristics as the other and as the parent cell. The various modifications of the process do not seem to affect this judgment. The equal division of characteristics is explained on the assumption that these characteristics correspond to units which are arranged in a longitudinal series from one end of the chromatin thread to the other, so that a longitudinal splitting of the thread involves the equal division of every unit and therefore the sharing of every characteristic. 74. Amitosis. — In contrast to the process just described, there has always been recognized another type of cell division known as amitosis, or direct cell division. Amitosis has been described as involving simply the constriction of the cytoplasm into two portions, this constriction also affecting the nucleus and dividing it into two portions, so that the whole cell becomes divided into two parts containing equal amounts of cyto- plasm and nucleus. It seems to occur only in cells which are highly speciahzed, lacking in vitality, or undergoing degeneration. 75. Continuity of Cell Life and Chromatin. — Two conceptions flow directly from a consideration of the phenomena of cell division. One is that all cells must be derived from previously existing cells, just as all living things receive their life from previously existing living things. This fact has been recognized for a long time and expressed in the apho- rism omnis cellula e cellula, or "every cell from a cell," which we owe to the German pathologist, Virchow. Another conception, based upon the equal division of chromatin qualitatively and quantitatively between the two daughter cells, is expressed in the phrase "continuity of chromatin" 50 FUNDAMENTAL PRINCIPLES and is that all chromatin has come from previously existing chromatin back to that of the first created protoplasm. This implies that in the first life created on this earth were inherent all the possibilities which have been realized in all living things that have since come from that life. 76. Growth of the Cell. — The two daughter cells resulting from cell division, each precisely similar to the parent cell except in size, grow and tend ultimately to reach the same size as the parent cell. This involves growth in all portions of the cell, and being growth by intussusception it is accomplished by the slipping in of particles of new material among particles already definitely arranged. Thus the organization, which is the central fact in protoplasm and is behind all life phenomena, is passed on unchanged. CHAPTER XII FORMS OF ANIMALS /: a% Animals of various kinds seem to present a great variety of forms, but when these are carefully studied it becomes possible to recognize a small number of distinct types. 77. Asymmetry. — Sijmmetry is regularity of form and involves the existence of corresponding parts. In the case of some animals, particu- larly the more simple ones, there seems to be no symmetry; this condition is termed asymmetry, and the animals are spoken of as being asymmetrical (Fig. 19). 78. Spherical, or Universal, Symmetry. — An ideal form of symmetry which is rarely approached in nature would be a form in which an indefi- nite number of planes might be passed through the center of the animal and each plane exhibit a structure precisely similar to that of every other, as weU as dividing the animal into symmetrical halves. This is termed spherical, or universal, symmetry. This is most nearly attained in some of the one-celled animals (Fig. 28 D). 79. Radial Symmetry. — Another form of symmetry which is presented by many of the lower many-celled animals is one in which the body can be divided by a number of radial planes into parts that are similar to each other. This type of symmetry is termed radial symmetry and the parts, since they are opposite around the center, are termed antimeres (Fig. 101). 80. Bilateral Symmetry. — Other animals are capable of being divided by a single median plane into similar right and left halves, the one being a mirror image of the other. This is termed bilateral symmetry and is characteristic of higher forms. 81. Metamerism. — Many bilaterally symmetrical animals have a body which is not divided into similar parts other than the right and left halves. Others, however, including the highest animals, are divided into a series of parts arranged in a linear series which, because they are placed one behind another, are termed metameres, or segments, the condition being termed metamerism. If these metameres are, generally speaking, similar to each other, it is known as homonomous metamerism, which is well illustrated by the common earthworm or angleworm (Fig. 134). If, however, these parts are dissimilar, the condition is termed heterono- mous metamerism; an example of this type is the crayfish (Fig. 152), in 61 52 FUNDAMENTAL PRINCIPLES Fig. 13. — Outlines of the fore limbs and their skeletons, of man (A), horse (B), bat (C), and bird (£)), to illustrate homology. The dotted lines pass through corresponding joints. FORMS OF ANIMALS 53 the case of which different metameres bear different types of appendages, such as feelers, or antennae, moiithparts, and legs. 82. Appendages. — If the body possesses such structures as have just been named or others which might be added, these are termed appendages. An appendage may be defined as a projecting part, capable of movement and performing some active function. Immovable horns, spines, hairs, and scales are not termed appendages, but movable spines, tentacles, legs, wings, and tails could all be recognized as such. 83. Homology and Analogy. — Whenever parts of the body, whether in the same or in different animals, are similar in plan of structure, they are termed homologous and the con- dition is referred to as homology. This usually involves similarity in origin and in mode of development. Whenever parts which are structur- ally or morphologically different in Fig. 14. — Outline of the wings of a 1 c ,1 r i- aU„ , butterfly and the veins in them, for compar- plan perform the same function, they .^^^ ^j^ ^^ ^.^ ^3 ^^ ^1^,^,^^, ^,„^io^gy. are analogous, and the phenomenon is referred to as analogy. Thus the foreleg of a horse, the arm of a man, the wing of a bat, and the wing of a bird are all homologous parts (Fig. 13). This homology concerns not only the division into segments which correspond to upper arm, forearm, wrist, and hand but also involves the skeleton, muscles, blood vessels, and nerves. The wing of a bird and the wing of a butterfly, however, possess nothing in common structurally but perform the same function, and the case is recognized as one of analogy. The wing of a butterfly (Fig. 14) is formed by an outfolding of the surface layers of the body wall, mostly of the skin, becomes jointed at the point of attachment, and is moved by muscles within the body. Since all likeness may be expressed in varying degrees, homology and analogy can be spoken of as being more or less complete or perfect. CHAPTER XIII BEHAVIOR By behavior is meant the sum total of an animal's movements. Noth- ing else can be included in behavior, since only by the movement of the body as a whole or of some part of it can an animal convey any indication of change within it. This also holds true with human behavior, since posture, facial expression, speaking, or any other mode of communication all involve movement. The movement of animals when involving no change in location of the animal is termed motion, but when change in location occurs it is locomotion. 84. Stimuli. — All movement is referred to some cause, either outside the animal or within it, which is termed a stimulus. If the cause is purely external the phenomenon is called external stimulation; if, on the other hand, the cause exists within the body of the animal, it is recognized as internal stimulation. The stimulus initiates a change, chemical and physical, within a part or whole of the animal which results in either motion or locomotion and which is called a response. External stimuli are either continuous or discontinuous. The first application of a con- tinuous stimulus produces a movement, while the continuation of that stimulus, if it is maintained at the same intensity, has no effect. When the operation of the stimulus ceases, movement is again observed. A discontinuous stimulus may be looked upon as a series of similar stimuli each of which produces the appropriate response on the part of the animal, both when it begins and when it ends. Since the response takes a certain length of time, discontinuous stimuli, if following each other with suffi- ciently brief intervals between them, may produce a continuous effect. An example is the production of a tetanic, or prolonged, muscular con- traction by a discontinuous electrical current. 85. Direct Response. — If the response which the animal gives immedi- ately follows the application of the stimulus and seems to be determined by the nature and force of the stimulus, the response is termed direct and is called a troy ism. Two words have been used in this connection: tropism, which means simply a turning, and taxis, which implies move- ment of the animal as a whole in response to the stimulus. Since the difference between these two is one determined by the locomotor ability of the organism, the word taxis has given way to the more general term tropism. 54 BEHA VIOR 55 Tropisms are named with respect to the stimulating agent. The following are generally recognized: 1. Thigmotropism, or response to contact. 2. Thermotropism, or response to temperature. 3. Phototropism, or response to light. 4. Chemotropism, or response to chemical stimulation. 5. Rheotropism, or response to mechanical currents. 6. Eledrotropism, or response to currents of electricity. 7. Geotropism, or response to the force of gravity. Animals in nature are subject to all of these forms of stimuli except that of the electric current, which is purely an artificial stimulus. If the response is such as to cause the animal either to turn toward the source of stimulation or to approach it, it is termed a positive response. If, however, it is such as to cause the animal to turn away from the source Fig. 15. — Sketch showing a long, rectangular glass aquarium partly covered by an opacjue hood, and containing organisms responding positively and negatively to light, and to an optimum. The direction of the light is shown by the arrows and the positions assumed by the animals at + (positive), — (negative), and opt. (optimum). of stimulation or to avoid it, it is termed a negative response. The mini- mum strength of the stimulus which is necessary to produce an effect is termed the threshold of stimulation. Some animals do not seem to respond positively either to the strength of the stimulus immediately above the threshold or to a maximum strength of the stimulus but do seem to be attracted to a position in which they receive the stimulus in a degree intermediate between the maximum and minimum. In this case they are said to exhibit a response to an optimum of the stimulus, corresponding to a certain strength expressed in degrees of temperature, intensity of light, or strength of chemical solution (Fig. 15). 86. Conductivity, — Though the stimulus may be received at a particu- lar point on the animal, the effect is not limited to that point but is con- ducted more or less throughout the body. This power of living matter to transmit the effect of the stimulus is termed conductivity. Irritability, or reactiveness, which is the power to respond to stimuli, as well as con- 56 FUNDAMENTAL PRINCIPLES ductivity, which is the power to transmit this effect, are both properties of Uving matter. 87. Part Played by the Nervous System. — In animals possessing no nervous system, behavior is summed up in the responses to stimuli or in the tropisms which the animal exhibits. In animals that possess a nervous system, the structure relations within the nervous system modify the responses in a variety of ways. Two or more cells are involved between the reception of the stimulus and the response, which accordingly is said to be indirect. The presence of the nervous system also makes possible more numerous and more varied effects due to internal stimuli. The result is the production of the complex forms of behavior character- istic of the higher animals. 88. Physiological State. — The character of the response which an animal will give to a stimulus is determined not only by the kind and strength of the stimulus but also by the condition of the animal and depends upon the state of the metabolic processes within its body. Thus a one-celled animal in the body of which there is no food, which is hun- gry, and which is at the end of a cycle of metabolism may give a response different from that of an animal which has recently fed and in the body of which the metabolic cycle has just begun. In higher animals different parts contribute to the physiological state of the whole. Animals which possess a nervous system exhibit physiological states dependent upon the varying conditions of that system, which in turn have a metabolic basis. Repeated and abnormal stimulation may throw an animal into a condition of excitement in which it acts in a manner quite unusual. The different feelings of which we are conscious at different times, the mental attitudes which dominate us, and our varying ability to carry on our different activities are all connected with different physiological states. Physio- logical states are back of what is called temperament, or mood, and explain one's ability to excel on one occasion and his inability to perform creditably on another. It is a certain physiological state resulting from a change in our ordinary routine which causes us to feel and act differently after a holiday or after an unusual experience. This explains "blue Mondays" and "off days." The word psychological is sometimes used instead of physiological when the nervous system is involved. CHAPTER XIV CLASSIFICATION AND NOMENCLATURE Whenever one has to deal with a great many objects of varied char- acter it becomes necessary to arrange them in such a manner that any additional object can at once be put in its proper place with respect to the others, that any particular object can at once be found, and that they may be referred to by groups. Such arrangement and grouping are called classification. It is imperative in the arrangement of a library and increasingly so as the library grows in size, is necessary in every mercantile establishment, and to a degree even desirable in the handling of objects in our homes. 89. Definition. — Zoological classification may be defined as the group- ing and arrangement of animals in such a way as to facilitate reference to them. If this grouping is based only upon the place where animals live or upon their form and structure, without regard to any relationships which may exist, it is termed artificial classification. If, however, an arrangement is secured such as to bring out the degree of relationship, assuming all animals to be related and to have evolved from the living matter first developed upon this earth, it could be called a natural classification. The basis of zoological classification is essentially artificial, but in so far as knowledge permits, zoologists endeavor to make it natural. 90. Arrangement of Groups of Animals. — The groups into which animals are arranged present a graded series, beginning with the whole, leading through those of gradually diminishing extent, and ending with each particular kind, which is a collection of like individuals. Generally speaking, any particular group will contain several groups of the next lower rank. It is evident that the characteristics of any one group will involve details of structure less fundamental than those of the next higher group and that, on the other hand, the characteristics of any group will be only those in which all the lower groups contained in it will agree. The names of the most widely used of these groups, in order of rank, illustrated by reference to a particular species of animal, are as shown on page 58. These group names in their plural form may be treated either as Latin or as English words, as, for example, phyla or phylums, and subphyla or subphylums; or they may be given only their Latin plural, as genera; or only their English plural, as in the case of all the other groups above. The word species is the same in both singular and plural forms. 57 58 FUNDAMENTAL PRINCIPLES Kingdom: Animalia (all animals) Subkingdom: Metazoa (many-celled animals). Phylum: Chordata (chordate animals). Subphylum: Vertebrata (vertebrates). Class: Mammalia (mammals). Order: Carnivora (carnivorous mammals). Family: Canidae (doglike carnivores). Subfamily: Caninae (dogs and their relatives). Genus: Canis (dogs). Species: familiaris (the domesticated dog). If the number to be handled in any group is very large, for convenience in classification groups are introduced between those given. A common method is by the addition of the prefix sub-, by which device, for instance, several subspecies may be included in a species. In a similar manner may be formed subgenera, subfamilies, suborders, subclasses, and, as given above, subkingdoms and subphyla. Another device consists of the intro- duction of words with the prefix super-, a superfamily being a group lower in rank than a subclass but one which includes several families. If these devices do not reduce the groups to convenient size, a variety of other words has been employed, such as series, divisioii, and legion. It has been found in nature that many widely distributed and variable species can be divided into smaller groups based upon geographical range, color, form, and other characteristics, and these may be called races and varieties. Various aspects of classification will appear as the different animals which make up the animal kingdom are treated more in detail, and the whole subject will later be reviewed. The names of the groups of animals above families vary in form, but the name of the family always ends in -idae and that of a subfamily in -inae. In each case the name of the family or subfamily is derived from that of the genus in that family or subfamily which is taken as the type genus, the names Canidae and Caninae, for example, being both derived from the name Cants. The names of groups higher than the genus are always written with a capital initial letter when used in a taxonomic sense but are not italicized. Generic names are capitalized and usually itali- cized, while specific and subspecific names are italicized but not capitalized. 91. Nomenclature. — In order that each animal shall have a distinctive appellation and that this may be the same throughout the world, it is necessary to avoid common names, which differ in different localities and often have a very uncertain application, and also to use a language which is the common language of scholars everywhere. For this reason each animal bears a scientific name which is Latin in form, though it may not be in origin, and which includes the names of the genus and species to which the animal belongs. For the purpose of exact reference, and since different authors may have referred to different species under the same name, to the generic and specific names is added the name of the CLASSIFICATION AND NOMENCLATURE 59 individual whom we recognize as the original authority for the name used. Thus the scientific name of the dog is written Canis familiar is Linnaeus, since Linnaeus bestowed this name upon this particular species. The name of the author is not italicized. If the form of the name is not the exact one the author used, his name is enclosed in parentheses. For example, the name Sorex aquaticus was given by Linnaeus to the common mole, but since it belongs to a more recently established genus, Scalopus, the name is written Scalopus aquaticus (Linn.). Authors' names are often abbreviated, as Linn, for Linnaeus. The name of the subspecies may be added to that of the genus and species, as Scalopus aquaticus machrinus (Rafinesque), the subspecies of the common mole found in the upper Mississippi valley. Sometimes the common name of an animal is also the scientific name of the genus, in which case the difference is shown by italicization and capitaHzation. For example, Paramecium and hydra are the common names of animals belonging respectively to the genera Paramecium and Hydra. PART II PROTOZOA CHAPTER XV AMEBA A SIMPLE PROTOZOAN This animal may serve as a type of the more simple one-celled animals, which are themselves the simplest forms in the animal kingdom. 92. Occurrence and Appearance. — Amebas occur commonly in fresh water. There are also numerous marine forms. The fresh-water species may be collected in a great variety of situations, such as watering troughs, spring pools, dams, stretches in streams where the water runs over rocky ledges, and wherever there is abundant aquatic vegetation. They are found gliding over the surfaces of submerged plants and of algae- covered mud, rock, or planking. One method which has been suggested for securing them is to collect a mass of pond weed, place it in a flat dish, and cover it with water, A brown scum which gathers on the water in a few days will be found to contain amebas if they were present in the pond from which the weeds were obtained. Not all appropriate habitats con- tain this animal, however, and one frequently has to search for a con- siderable time before coming upon a supply. When found, amebas are in the majority of cases only to be discovered by the use of the microscope. The very largest specimens of Amoeba proteus Leidy, however, are visible to the eye as minute whitish dots when seen against a dark background. Under the microscope an ameba appears hke a mass of colorless jelly, irregular in form, and, when active, con- stantly changing in outline. The generic name given to this animal is Amoeba, and under the rules of nomenclature it cannot be changed, but the common name is now quite generally spelled ameba. 93. Structure. — The animal (Fig. 16) owes the irregularity of its out- line to the fact that from the surface of the main mass extend numerous and variously shaped projections known as pseudopodia. These are constantly varying in length and thickness and may at any time be entirely withdrawn. An ameba has no cell wall but possesses an extremely delicate outer layer known as a plasmalemma. This is too thin to be seen even with higher powers of the microscope, but by the movement of particles on its surface it may be shown to exist. Below this is a clear layer of cytoplasm, the ectoplasm, much thicker than the plasmalemma but still forming a thin layer over the surface of the cell. Within this and forming the bulk of the body is the more granular endoplasm. The layer of ectoplasm is 63 64 PROTOZOA relatively thickest at the anterior end and at the tips of the projecting pseudopodia and is thinnest at the side away from the direction in which the animal may be advancing. Within the endoplasm are seen numerous vacuoles and a nucleus. The vacuoles are of three types: (1) Food vacuoles, which appear like colorless drops of water inclosing particles of food. They are spherical in form if the food particle is small and compact but assume somewhat the form of the particle if it is large and elongated in any dimension. Food vacuoles disappear with the egestion of the feces from the body. (2) Water vacuoles, which appear like perfectly transparent colorless drops Confracfile vacuole Ectoplasm Pse udopodium Food vacuoles Fig. 16. — Amoeba proteus Leidy. X 1000. of a spherical shape. They do not change under observation. (3) Con- tractile vacuoles, of which there may be one or more and which appear like the water vacuoles. However, they change in size, gradually growing larger until they reach a maximum, when they suddenly contract and disappear, the process being regularly repeated. Because of this alternate filling and contracting these are sometimes known as pulsating vacuoles. They may sometimes be recognized by the possession of a faint pinkish tint. Throughout the endoplasm may be seen numerous granules of varying shapes and sizes, including crystals, whose nature can be deter- mined by their crystalline form; and foreign substances, like bits of sand. Somewhere within the endoplasm, and usually nearer the end away from that which is advancing, is the nucleus. It is finely granular in texture, homogeneous in appearance, and refractive to light, which gives it a brighter or clearer appearance than the surrounding endoplasm. It also frequently has a faint bluish color. AMEBA 65 94. Metabolism. — The food of an ameba consists of minute aquatic plants and animals, though it will attempt to engulf any organism which it can surround. Small bits of organic matter are also taken in, but inor- ganic particles seem to be rejected unless they are accidentally taken in with a bit of food. Ingestion takes place (Fig. 6) with the formation of pseudopodia. These envelop the food, which thus becomes completely inclosed in the body. With the food is taken in a certain amount of water and so a food vacuole is formed. Into this vacuole is secreted hydrochloric acid, which gives to its contents an acid reaction. An enzyme which acts on proteins is then added and the reaction becomes alkaline. No enzyme acting on carbohydrates has been demonstrated, and the same must be said for one acting on fats. As digestion proceeds, the digested and dissolved food is passed into the fluid of the food vacuole, whence it is absorbed into the surrounding protoplasm. It is then circulated to all parts of the body and assimilated wherever needed. Dissimilation also takes place everywhere in the body, the digestive secretions being passed into the food vacuoles and the excretions into the contractile vacuole. This, contracting at intervals, expels the excretions from the body, thus accomplishing elimina- tion. After all that is digestible of the food has been removed from it and absorbed, the undigestible residue, or feces, is permitted to pass out through the wall of the body, the animal simply flowing away and leaving it behind. This is egestion. Respiration takes place everywhere through the body surface, but the carbon dioxide may also be expelled through the contractile vacuole. A continuous supply of oxygen is necessary for the life of the animal. If an excess of food above the immediate needs is taken in, growth occurs by intussusception and the animal gradually increases in bulk. Food may not be immediately assimilated and undoubtedly some of the granules present in an ameba which is well fed represent unassimilated or stored food. Some of the crystals seen in the animal appear to be excretions. 95. Locomotion. — ^Locomotion in the ameba is accomplished by means of the pseudopodia, which are temporary locomotor structures. Many theories have been given to account for the manner in which pseudopodia are formed and the way in which they effect locomotion. Their formation has been attributed to a lessened surface tension at the points where they are formed or to an increased surface tension elsewhere. The freshly formed pseudopodium has been described as adhering to the sub- stratum, the force of adhesion becoming a maximum at the extreme tip. However, the most careful and detailed description of locomotion in the ameba has recently been furnished by Mast. According to him the body of the moving ameba is divided into four parts: the plasmalemma, which is very thin and elastic; the clearer ectoplasm; the plasmagel, which is the 66 PROTOZOA outer motionless part of the endoplasm; and the plasmasol, which is the central moving portion of the endoplasm. This is shown in Fig. 17, which represents a simple type with one pseudopodium to which also the follow- ing description applies. The plasmalemma is quite permanent and is stationary over that part of the surface in contact with the substratum, to which it adheres. At the side of the animal away from the advancing pseudopodium, the plasmalemma is being carried upward and over upon the upper surface; on this surface it is being carried forward; at the tip of the advancing pseudopodium it is moving downward; and it is laid down against the substratum in advance of that portion which is temporarily stationary and of which it now becomes a part. Through the center of the animal is a forwardly directed flow of plasmasol, which as it approaches the advancing tip turns to the side and becomes plasmagel. > b a Fig. 17. — Diagrammatic representation of a simple ameba, such as Amoeba verrucosa Ehrenberg, viewed from the side and moving by the formation of a single pseudopodium. The arrows along the surface show the direction of movement of the plasmalemma, which is stationary against the substratum from a to b. The arrows within the body show the direction of flow of the plasmasol from the area of solution at the temporary posterior end to the area of gelation near the temporary anterior end. The arrow above the figure shows the direction in which the animal is moving. The onflow of plasmasol continues, serving to push this tip forward more and more. At the other side, the plasmagel is continually becoming plasmasol and thus providing the material for the continued flow. The forward movement of the ameba is accompanied by a continuous gelation and decrease in volume of the protoplasm toward the tip of the develop- ing pseudopodium and a continuous solution and increase in volume of that at the opposite side. In the case of an ameba with several pseudo- podia, one or more may be developing in the manner described above, while others may be disappearing. The latter will exhibit solution toward the tip and a flow of plasmasol back through the center into the main body. The elastic plasmalemma will show extension or contraction in different directions as it becomes adjusted to the changing outhne. There is a curious resemblance in the manner of functioning of the plasmalemma to the continuous metallic belt which forms the tread in a caterpillar tractor. 96. Behavior. — An ameba exhibits reactions to various stimuH. To contact it responds positively if the contact is a gentle one but negatively AMEBA 67 if it is forceful. When coming quietly in contact with food or with any indifferent object an ameba tends to increase the amount of body surface in contact, but when anything touches it at all violently it draws back and moves away. It avoids a strong light but does not seek darkness, so it selects an optimum. This reaction may be complicated by the effect of temperature when it is exposed to the rays of the sun. To chemicals in solution the response varies with the character of the chemical. To the normal constituents of the water in which the animal lives it is indifferent; Nucleus Fig. 18. — Fission and sporulation in Amoeba. Fission is illustrated in A to Z), which represent four stages in fission in a European species. {Taken from Doflein, " Lehrbuch der Protozoenkunde," after F. E. Schulze.) E and F show sporulation in Amoeba proteus. (Also from Doflein, by the courtesy of Gustav Fischer.) to substances which indicate the presence of food it responds positively, being thus brought to its food; to substances, however, which are not normal constituents of the water and which tend, therefore, to injure the ameba it responds negatively. Amebas have an optimum at a low temperature. Cold benumbs them as the temperature approaches the freezing point, while a temperature above 30°C. (86°F.) also retards their activities. 97. Reproduction. — At intervals amebas reproduce, doing so whenever they reach the hmit of size, which in Amoeba proteus is about 0.25 mm., or 0.01 inch. The ordinary form of reproduction is known as fission, or binary divi- sion. In this process the body elongates and a constriction appears in the 68 PROTOZOA middle. The constriction gradually gets deeper and deeper, cuts through the nucleus, and passes entirely through the body of the animal. The ameba thus becomes divided into two individuals, each half the size of the parent and each with a nucleus half the size of that of the parent. Fission involves a very simple mitosis (Fig. 18 A to D). Another form of reproduction is that of sporulation. This occurs only when conditions become unfavorable and is a means of carrying the animal over to a time when normal conditions are again reestablished. It occurs, therefore, when the body of water in which an ameba is living dries up and apparently comes about as a consequence of the increasing salt concentration of the water. It also occurs when the chemical com- position of the water changes in any way so as to be unfavorable or when other environmental conditions threaten the death of the animal. In sporulation, the pseudopodia are first drawn in, and the animal assumes a spherical form; a three-layered cyst is then formed about the surface, which serves for protection (Fig. 18 E). Within the cyst the nucleus undergoes division into a great many fragments, and the cytoplasm becomes divided in such a way that a small amount surrounds each fragment of the nucleus. Thus are formed what are termed pseudopodio- spores (Fig. 18 F). When the pond is again filled with water or when normal conditions are restored, the encysted mass takes up water, bursts its wall, and the liberated pseudopodiospores develop into Httle amebas. CHAPTER XVI PARAMECIUM A MORE COMPLEX PROTOZOAN The Paramecium may serve as an example of a one-celled animal in which there is a considerable degree of specialization. Certain parts of the body are permanently modified for the performance of particular functions, a process generally known as division of labor. 98. Occurrence. — Paramecia appear in abundance in any water in which there is a considerable amount of decaying organic matter. They abound in streams and all bodies of water polluted by the entrance of sewage, feeding upon the bacteria which swarm in such water, and frequently appear in ameba cultures in which there is an accumulation of decaying animal and plant matter. Since paramecia tend to gather at the surface and especially in contact with floating objects, they frequently form a white scum. They may, however, be found throughout the bodies of water in which they live. 99. Structure. — Paramecia (Fig. 19) are elongated in form and are frequently described as cigar-shaped, the anterior end being blunt and the posterior more pointed. Because the form is similar to that of a slipper, the anterior end representing the heel and the posterior the toe, they have been called slipper animalcules. A groove called the oral groove, or 'peristome, starts at the anterior end of the animal and runs obliquely backward from right to left or left to right, ending a little behind the middle of the body. If one were to conceive of himself looking at the Paramecium from in front, this could be expressed by saying that the groove may run in a clockwise direction, which would be the direction corresponding to the direction of motion of the hands of a clock, or in a counterclockwise direction, which would be the opposite. In Paramecium caudatum Ehrenberg the groove usually runs clockwise, but in cultures in which this is the prevailing type others may be found. Occasionally a culture will appear in which the majority of the individuals show a coun- terclockwise direction. The body is covered with fine hairlike cilia, which in the species referred to are longer at the posterior end. Near the end of the oral groove is an opening known as the mouth. This leads into a gullet, which is a cleft running a short distance back into the cytoplasm. Over the whole surface of the body is a colorless, elastic membrane known as the pellicle, or cuticle. This seems to be divided by raised lines 69 70 PROTOZOA into a great number of very small hexagonal areas. From the center of each of these areas arises a cilium. Just below the pellicle is the clear and relatively firm ectoplasm, often called the cortex, which contains a great many spindle-shaped cavities placed with their longer axes at right angles to the surface. These cavities are filled with a semifluid substance and are known as trichocysts. They open to the outside along the lines which bound the hexagonal areas. The cilia are described as having an axial thread continuous with the cortex and a covering continuous with Ectoplasm and trichocysts Endoplasm Macronucleus Food vacuole Pellicle Cilia Contractile vacuole Oral groove % — Micronucleus r ^ — Moutti Gullet Contractile vacuole Fig. 19. — Paramecium caudatum, Ehrenberg. X 400. the pellicle. The endoplasm contains food vacuoles, two contractile vacu- oles, and numerous granular masses various in size, shape, and character. The contractile vacuoles, one at each end of the body, lie between the ectoplasm and endoplasm and are made up of a central space sur- rounded by a system of radiating canals, from 6 to 10 in number. These radiating canals end blindly in the cytoplasm at the outer end and at the inner end empty into the central space, which in turn opens to the outside. After the central space becomes empty, the canals discharge their con- tents into it and then again gradually fill while the central space is being PARAMECIUM 71 A emptied. The two vacuoles empty alternately at intervals of about 10 to 20 seconds. Near the center of the animal or somewhat behind it, and not far from the mouth, is the macro nucleus, which has somewhat \^ the form of a lima bean, and a very much smaller micronu- cleus. The micronucleus is lodged in a depression on the surface of the macronucleus at a point which would corre- spond to the hilum of the bean, the place where the root comes out in germination. 100. Metabolism. — The food of paramecia consists of bacteria and minute forms of Protozoa. It is swept into the oral groove by the action of the cilia, carried back through the mouth, onward into the gullet, and finally passed into the endoplasm, where the food vacuole is formed. The passage through the gullet is effected by means of two or three undulating membranes, formed by rows of cilia placed side by side and fused. These food vacuoles are in constant circulation around the animal, following a definite course. Digestion takes place in the food vacuoles as it does in the ameba, and circulation, assimilation, and dis- similation are similar. Excretions are accumulated in the contractile vacuoles and eliminated through them to the outside. Expiration also takes place into the contractile vacuoles. Inspiration seems to be possible over the whole body surface. Egestion occurs at a particular point near the posterior end, where there is a potential opening through the ectoplasm known as the anus, or anal spot. 101. Locomotion. — Owing to the presence of the elastic pellicle, the body of the Paramecium exhibits elasticity, which is not observed in ameba. It can force its way through a narrow passage, the body contracting as it does so, but on its exit from the passage it immediately assumes its normal shape. Locomotion is effected by means of the cilia, which may beat either forward or backward and by means of which the animal swims in either direction through the water. Normally it moves forward. The cilia, however, do not beat directly backward but obliquely, so that the animal rotates on its long axis. The cilia in the oral groove also strike obliquely along the axis of the groove and Fig. 20. — The spiral path followed by a swimming Paramecium. Fia. 20. (Modified from Jennings, "Behavior of the Lower Organisms," by the courtesy of Columbia University Press.) The rotation of the animal on its axis is indicated by the position of the oral groove. The large arrow shows the direction of motion, and the axis of the spiral. 72 PROTOZOA produce a swerving. As a result of the combination of progression, rota- tion, and swerving, the animal follows a spiral path, the axis of the spiral being the direction of motion. This axis may be a straight line (Fig. 20). Thus the Paramecium, which is asymmetrical, is enabled to pursue a direct course though it is unable to swim directly forward or backward. 102. Behavior. — Paramecia react to various stimuli in a manner somewhat similar to amebas, though apparently they are not affected Fig. 21." — Diagram to illustrate the avoiding reaction given by a Paramecium to a solid object. {Modified from Jennings, "Behavior of the Lower Organisms," by the courtesy of Columbia University Press.) The arrows between the successive positions represented indi- cate the path followed, and those drawn within the outlines of the animal the direction in which the animal is moving. The direction of movement is also indicated by the position of the cilia; no attempt has been made to show rotation by any other structural features. by ordinary light. Their positive thigmotropism is seen in the tendency to come to rest against objects in the water. It is stated that for each chemical to which paramecia apparently give a positive reaction there is an optimum concentration in the region of which they tend to accumulate. The Paramecium possesses a temperature optimum at about 26°C. (69°F.), or about the temperature of ordinary pond water in summer, when para- mecia become most abundant. The animal possesses positive rheo- tropism, which leads it to swim against a current, or upstream. Paramecia also seem to respond negatively to gravity, tending to come to the top of the water in a vessel in which they are confined. The electric current affects the beating of the cilia, but this is not a natural stimulus, occurring only in the laboratory. One mode of response of the Paramecium is known as the "avoiding reaction, " which takes place in the following manner: When a Paramecium PARAMECIUM 73 comes in contact with a solid object, assuming it to be swimming at full speed forward, the force of the contact produces a negative response. The animal backs up, pivots upon its posterior end, and swims forward again. If it again strikes the object, it exhibits the same negative reac- tion, which is repeated until on swimming forward the animal passes the obstacle (Fig. 21). Though it is called an avoiding reaction, which would imply a consciousness on the part of the animal, all that it is doing is simply giving a series of negative responses. The term ' 'avoiding reac- tion" thus gives rise to impHcations which are unwarranted, a fact usually indicated by inclosing the words in Fig. 22. Fig. 23. Fig. 22. — A Paramecium shown pivoting upon its posterior end and sampling the water before starting off in a direction; which may be determined by the result of such sampling. {From Jennings, "Behavior of the Lower Organisms," by the courtesy of Colum.hia University Press.) The figures indicate successive positions and the arrows show the direction of movement. Fig. 2.3. — Diagram illustrating the method of trial and error in Paramecium. {Modified from Borradaile and Potts, " The Invertebrata," after Kiihn, by the courtesy of The Macmillan Company.) At the center is shown a soluble substance {C) which is dissolving and diffusing into the surrounding water. The circles represent zones of equal concentration, the zone 0, in which protozoans, which might bo paramecia, are gathered, being the zone of optimum concentration. The irregular line shows the path of a Paramecium which enters the area involved and after repeated stopping, sampling, and change in direction, comes to rest in the zone of optimum concentration at x. quotation marks. A slowly moving organism like an ameba would not exhibit such a response. Paramecia also show what is known as the method of "trial and error,'' which implies a series of experiments on the part of the animal. A Paramecium is constantly taking water into its oral groove with suffi- cient force to draw it from a little distance in front and to produce a cone of movement in the water. Thus it "samples" the water just ahead (Fig. 22). If the water is too hot or too cold or if it contains an injurious chemical substance, the animal gives an avoiding reaction. This may be repeated again and again. While paramecia seem to be swimming aimlessly through the water in all directions, the repetition of these avoiding reactions sooner or later brings them into that part of their environment which is most favorable (Fig. 23). The result 74 • PROTOZOA is the same as that seen in the "trial-and-error" mode of behavior of higher animals, and so this term is often apphed to the activity of this and similar Protozoa. Whenever a Paramecium is responding to one stimulus it often will not be affected by another stimulating agent unless the second one is very strong. It has been found, however, that the response to gravity is always set aside whenever the animal receives any other stimulus. The responses which paramecia give to stimuli are not always the same, the difference being due to the different physiological states of the animals. A Paramecium which is fully fed tends to come to the surface and remain quiet in contact with some object, though the sources of its food supply may be, in a laboratory culture, at the bottom of the jar. When it becomes hungry, however, it reverses its responses, swims to the bottom, secures its food, and then once more seeks the surface. Thus one physiological state gradually becomes changed into another, and a definite rhythm is established in the animal's movements. It has been found that a response becomes more pronounced after it has taken place a number of times. This indicates a change in physiological state and shows an effect of one response upon succeeding ones which has been termed summatiori oj stimuli. When violently stimulated either by a chemical or by contact, a Paramecium frequently responds by throwing out the contents of the trichocysts, which harden and form a barrier of fine threads. When the trichocysts are emptied, they are refilled by material which originates near the nucleus, probably from it, and passes through the endoplasm to the proper points in the ectoplasm. 103. Reproduction. — Paramecia reproduce only by fission, the animal being divided transversely into two. During this process both the macronucleus and the micronucleus divide, the old gullet divides into two, and two new contractile vacuoles arise by division of the old ones. The micronucleus divides mitotically, and perhaps the macronucleus does also. The entire process occupies from half an hour to two hours. Sub- sequent growth is rapid, and division occurs again after a number of hours. Paramecia multiply with great rapidity. It has been estimated that from one ancestor could be produced in one month, if all survived, a total of 265,000,000 individuals. 104. Conjugation. — At intervals occurs a phenomenon known as con- jugation (Fig. 24). When this occurs, two paramecia come together, attached by the surfaces on which the oral grooves are located. The pellicle breaks down at the point of contact, as does also the ectoplasm, and an endoplasmic bridge is formed between the two animals. During this time the micronucleus of each conjugant moves from the concavity in the macronucleus, where it has been lodged, grows larger, forms a spindle, and divides. A second division follows immediately. Of the PARAMECIUM 75 four micronuclei thus produced three degenerate and disappear, and the fourth divides again, this time unequally. The smaller of the two micronuclei of each animal now moves over the protoplasmic bridge into 777/5 animal not followed further; same as other 1. Two paramecia come together MicronucleiLs divides. Macronucleus begins to dis- integrate 3. Micronuclei divide and three of four disappear 4. Remaining micronucleus divides unequally 5. Smaller micronucleus crosses into other animal G. Each animal with its own larger micronucleus and smaller one from other animal 7. Two micronuclei fuse 8. Two animals separate. Each exconjugant with fusion micronucleus 9. Fusion micronucleus divides 10. Two micronuclei divide 11. Four micronuclei divide 12. Four micronuclei become macronuclei, three dis- o#o#««o» ? appear, one remains 13. Micronucleus divides and animal divides 14. In each of two micro- nucleus divides and ani- mals divide again Fig. 24. — Diagram illustrating conjugation in Paramecium caudatum Ehrenberg. (Siiahthj modified from Jennings, "Life and Death, Heredity and Evolution in Unicellular Organisms," by permission.) Macronucloi are indicated by large black bodies, micronuclei by the smaller ones, and those micronuclei which disappear by small circles. the other animal, passing the micronucleus from the other as it does so. Each of these micronuclei fuses with the larger micronucleus of the other animal, forming a fusion micronucleus. This process has been compared 76 PROTOZOA to fertilization in higher animals which possess sex, the smaller micronu- cleus being viewed as male and the larger one as female. However, it cannot, properly speaking, be called fertilization, since no gametes are involved. The macronucleus begins to degenerate soon after the micro- nucleus leaves it, breaking up into fragments. The two animals now separate and the fragments of the macronuclei slowly disappear, their substance being dissolved in the endoplasm. The fusion micronucleus in each of the exconjugants divides by mitosis into two, these into four, and these into eight, all equal in size. Four of 1. Animal before endomixis begins , . _;_i__-^ 3. Micronuc-lei again divide 4. Six out of eight micronuclei disappear. Animal divides 5. Each animal with one micronucleus 2. Micronuclei divide. Macronucleus disintegrates 6. Micronucleus of each divides 7. Micronuclei divide again. S. Two micronuclei in each animal become macronuclei 9. Micronuclei divide and animals divide 10. Four ordinary individuals Fig. 25. — Diagram illustrating endomixis in Paramecium aureha Mi'iller. (From Wood- ruff, "Animal Biology," by the courtesy of The Macmillan Company.) Large black bodies are macronuclei, small ones micronuclei, and small circles micronuclei which disappear. these then become larger and develop into macronuclei. Of the four remaining, three disappear. The fourth divides into two, and the animal divides, each of the two individuals formed having one micronucleus and two macronuclei. In each individual the micronucleus again divides and this is followed by fission, producing four animals each with one micro- nucleus and one macronucleus. This description applies to Paramecium caudatum; the steps in the process are variously modified in other species. The significance of conjugation is uncertain. Some investigators believe that after a long series of fissions the animals become senescent and conjugation serves as a process of rejuvenation which restores their vitality. Woodruff, however, has succeeded in maintaining a culture since May 1, 1907, without conjugation. On the twenty-fifth anniversary PARAMECIUM 77 — May 1, 1932 — the culture had attained the number of 15,300 genera- tions. The significance may be in the determining of inheritance. The scrapping of the old macronucleus and the development of a new one from the fusion micronucleus suggest the need of harmonious direction of activities in an animal differing from the conjugant parent by nuclear material received from the other conjugant. 105. Endomixis. — An interesting phenomenon analogous to conjuga- tion, but taking place within a single individual, has been observed in Paramecium aurelia INIiiller (Fig. 25). This species possesses two micronuclei, exhibits a definite rhythm in the rate of division, and periodi- cally undergoes what has been called endomixi}^. During the process the macronucleus breaks down and disappears and the micronuclei undergo two divisions, producing altogether eight micronuclei. Six of these disintegrate and disappear. The Paramecium then divides and each of the offspring receives one micronucleus. This micronucleus divides into two and these divide again, producing four. Two of these develop into macronuclei and two remain micronuclei. The mic- ronuclei divide again and the entire animal divides, resulting in two, each with two micronuclei and one macronucleus. Four individuals have thus been produced. This process also occurs in Paramecium caudatum and in other forms. The result of endomixis may be the same as that of conjugation. CHAPTER XVII PROTOZOA IN GENERAL The phylum Protozoa (pro to zo' a; G., protos, first, and zoon, animal)' includes all one-celled animals, the one cell which forms the body of the individual carrying on in simple fashion all of the general functions which are performed by the many-celled bodies of higher animals. This means that though the animals included in Protozoa are simple in that they are composed of only one cell, this cell is physiologically complex. Some of the Protozoa always exist as single cells. Others are associated in colonies in which they are all alike and each quite independent. In other proto- zoan colonies, however, certain functions, such as reproduction, are assumed by certain cells, which thus become reproductive individuals. 106. Classification. — Protozoa is the first phylum of the animal kingdom, but, since all other phyla have characteristics which they share and which distinguish them from Protozoa, it may be considered also as a group of higher rank than a phylum. In this case it becomes a subkingdom, with the same name, coordinate with the subkingdom Metazoa, which includes all the remaining phyla. Protozoa may be characterized as composed of animals existing as single cells. In the case of certain types these one-celled individuals are associated in colonies. The phylum is usually divided into four classes, each characterized by a distinctive locomotor structure or, in one class, by the absence of any such structure in the final stage of the animal. These classes are: 1. Mastigophora (mas ti gof o ra; G., niastix, whip, and phoros, bearing), or Flagellata (fla gel la' ta). — Have a limited number of long whiplike locomotor appendages known as flagella. 2. Sarcodina (sar ko di' na; G., sarkodes, fleshy), or Rhizopoda (rl zop' o da; G., i^hiza, root, and podos, foot). — Form pseudopodia, which are temporary structures developed from the surface of the body and which can be withdrawn. 3. Sporozoa (spo ro zo' a; G., spoi^a, seed, and zoo7i, animal). — Possess no locomotor structures in the final stage, though they have them in the earlier stages of their life histories. 1 The vowel sounds indicated in the pronunciation of this and other phjda and class names are described at the beginning of the Glossary (p. 557). In all cases where the nominative form of a Latin or Greek word does not contain the full root, the genitive is giv(>n, as, for instance, podoa, genitive, instead of pons, nominative, for the Greek word for foot. If a word comes from the Greek through the Latin, the Greek is given. 78 PROTOZOA IN GENERAL 79 Flagellum -Mouth Reservoir - 'Sfignipi ■Confrcfctile vacuole \Chromafophort 4. Infusoria (in fu so' ri a; L., infusus, poured into, crowded), or Ciliata (sil i a' ta).— Have a very large number of permanent small hair- like appendages known as cilia. By some authorities a fifth class is recognized which is called Swtoria. In the classification adopted here this is considered a subclass of the Infusoria. 107. Mastigophora. — A type of this class is the euglena (Fig. 26), a small greenish protozoan hving in bodies of fresh water. This animal agrees with the Paramecium in possessing, in addition to the ectoplasm and endoplasm, an elastic cuticle, which is striated. On the anterior end is a single long slender flagellum connected with a granule within the body known as a hleph- aroplast. This term is applied to any granule in a cell with which a cilium or flagellum is connected. The mouth is at the base of the flagellum. A permanent vesicle, the reservoir, into which several contractile vacuoles pour their contents, opens into the gullet. Close to the reservoir is a mass of red coloring matter called the stigma, or eyespot. It is, of course, not an organ of sight, though it is thought to be sensitive to light. Near the center of the body is a nucleus, and scattered through the protoplasm are many bodies of bright green color called chromatophores. Euglena is a type which possesses some of the characteristics of plants. Other members of this class show these to such a degree that they are by botanists considered plants and classified by them as such. A plantlike characteristic is the ability, by means of chlorophyll in the chromatophores, to manufacture part of its own food. This type of nutrition is known as holophytic, in contrast with the type which char- acterizes animals generally, which involves the ingestion of solid particles of organic food, and which is called holozoic. This resemblance to plants justifies their being considered the first class of the Protozoa. Many zoologists place Sarcodina first, believing them the simplest structurally. Euglena illustrates particularly well a reaction to an optimum stimu- lus. When placed in a vessel, one end of which is lighted and the other darkened, the animals gather neither at the light nor at the dark end but in a zone between the two where the optimum of light for this animal is found. Nucleus Fu;. 26. — Euglena viridis Ehrenberg. {Slightly modified, from Doflein, "Lehrhuch der Protozoenkunde,^' by the courtesy of Gustav Fischer.) X 800. 80 PROTOZOA The Mastigophora (Fig. 27) are divided into two groups: (1) Those which are animal-hke and which may be holozoic, saprophytic, or ento- zoic. Saprophijtic imphes the absorption of nonUving organic matter in solution directly through the surface of the body. Entozoic means living within the bodies of other animals. Such species live in the intestinal tract or blood stream of man or in the intestines of insects. (2) Those Fig. 27. — Several different species of Mastigophora. A, Proterospongia haeckeli Kent. {From Kent, "A Manual of the Infusoria.") X 530. B, Giardia I amblia Stiles. {After Wcnyon, in Archiv fur Protistcnkunde, Suppl. I.) X 2200. C, Trypanosoma gambiensc Button. {From Wcnyon, "Protozoology," by the courtesy of William Wood & Company.) X 1330. D, Noctiluca scintillans (Macartney). {From Kent.) X 40. E, Volvox aureus Ehrenberg. {From Dojlein, " Lchrbuch der Protozoenkunde," after Klein, by the courtesy of Gustav Fischer.) X 110. .\ colony containing six daughter colonies, developed from parthenogonidia. F, Uroglcnopsis amcricana (Calkins). X 350. which are more plantlike and which may be holophytic, saprophytic, or entozoic. An interesting form is the genus Proterospo?igia, which is a colony of individuals each bearing a flagellum and around it a protoplasmic collar. Another form, known as Giardia, the structure of which is quite complex, lives in the small intestine of man. These are both animal-like, as is also Euglena. PROTOZOA IN GENERAL 81 Among the plantlike types are several of interest, including Uroglena, found in reservoirs and imparting a peculiar oily odor and fishy taste to the water. Another form is Volvox, a very beautiful colonial animal — or plant — which lives in fresh water and which may consist of many thousands of cells. As it swims the spherical colony revolves, the motion M'/i ■/■■ / B Fig. 28. — Different types of Sarcodina. A, Rotalia freyeri. (From Doflein, " Lehrbuch der Protozoenkundc," after Max Schultze, by the courtesy of Gustav Fischer.) An example of the Foraminifera. B, Difiugia urceolata Carter. (From Leidy, " Fresh-water Rhizopods X 167. The shell is composed of sand grains. C, Actinosphaerium {From Kudo, ''Handbook of Protozoology," by pen-mission of the Thomas.) X 40. One of the Heliozoa. D, Hcliosphaera inermis Haeckel. {From Bronn, " Klassen und Ordnungen des Tierreichs," after Haeckel.) X 350. One of the Radiolaria. The skeleton forms a lattice work on the surface of the body. of North America.") eichhorni Ehrenberg. publisher, Charles C. being due to the combined action of all the flagella. Still another example is a marine form known as Nodiluca. This animal frequently collects on the surface of the sea in enormous numbers, the jelly-like bodies form- ing a thick scum which has the color and appearance of thick cream-of- tomato soup and which sometimes covers an area of many acres. At 82 PROTOZOA night, and when stimulated, these bodies are luminescent, giving to the water a pervading greenish white or bluish white light. 108. Sarcodina. — Sareodina (Fig. 28) include not only the ameba but also many other similar forms, some of which are parasitic. A number of them secrete an external shell of chitin, cellulose, lime, or silica, or Paras/fes Parasife. Nucleus of corpuscle ^m K);' Parasites Fig. 29. — Examples of Sporozoa. A, a hemogregarine in the red blood corpuscle of a frog. (From Hegner and Taliaferro, "Human Protozoology .'') X 550. B, section through the intestinal epithelium of a rabbit, showing infection with one of the Coccidia, Eimeria stiedae (Lindemann). {From Doflein, ''Lehrbiich der Protozoenkunde," after Thoma.) Highly magnified. C, Gregarina hlattarum Siebold, from the digestive tract of the cock- roach. {From Doflein, after Hertwig.) X 60. Shows an endwise union of two individuals, a union which occurs commonly and is known as syzygy. D, section through the intes- tinal wall of a meal worm (the larva of a beetle), infected with Gregarina polymorpha (Ham- merschmidt). {Also from Doflein, after Pfeiffer.) Highly magnified. Immature parasites in different stages of development are seen in the epithelium lining the intestine and one mature individual in the lumen of the canal. {A by the courtesy of The Macmillan Company; B, C, and D by that of Gustav Fischer.) they build one of particles of sand and other foreign objects held together by one of these substances. Interesting members of this class are the Foraminifera, which are mostly marine and which form shells of lime composed of numerous chambers united by openings called foramina, whence the name of the group. They occur in enormous numbers and exhibit great variety. When the shells of dead individuals sink to the sea bottom they form a soft mud or ooze, known as foraminiferous or Globigerina ooze, which, when solidified, becomes natural chalk. PROTOZOA IN GENERAL 83 Another f>;roup is Radiohiria. These have a central perforated cap- sule of chitin and a larji;cr inclosing shell of silica. They also are marine, existing in great numbers in the ocean, and when their shells sink to the bottom they form what is known as radiolarian ooze. When solidified this produces a rock of the nature of flint. These rocks occur in strata several hundreds of feet in thickness. Another group of Sarcodina found in fresh water have numerous slender, radiating pseudopodia containing axial threads of chitin. Because of the resemblance of the animal with those radiating pseudo- podia to the sun surrounded by its rays of light, they are frequently termed sun animalcules, and the order to which they belong is called Heliozoa. 109. Sporozoa. — The Sporozoa (Fig. 29) in their early stages fre- quently are ameboid but in their final stages they lack locomotor organs and form spores. They are parasitic in other animals and are generally transmitted to the host in the spore form. In some cases the life of the individual ends upon the formation of spores but in other cases spores are produced at intervals during the animal's lifetime. Among these forms are the gregarines, which exist within the cells of earthworms, cockroaches, and other insects as well as of other inverte- brates and which in their later stages become free in the body cavities of these animals. Those of an order known as Coccidia are found in the liver and intestine of man and other vertebrates as well as in some inverte- brates. Others are found in the blood or muscles of vertebrates or within the cells of fish. One form produces the silkworm disease known as pebrine. Pasteur discovered that this parasite is transmitted from the silkworm moth to the eggs before they are laid and that the caterpillars hatched from these eggs thus become infected. By showing how infec- tion can be avoided he saved the silkworm industry of France at a time when its existence was seriously threatened. 110. Infusoria. — Infusoria (Fig. 30) occur in both fresh and salt waters, while others are found parasitic in the bodies of higher animals. Para- meciu7n is an infusorian. Opalina is a form which lives in the intestine of the frog. In addition to cilia, infusoria frequently possess midulating mem- branes or cirri, formed by the fusion of numerous cilia. The body may be covered -all over with cilia of approximate!}^ equal length, or it may have the cilia distributed over certain portions. The cilia are upon the ventral surface in a form known as Stylonychia and in a circle around the blunt end of the trumpet- or bell-shaped body in the forms known as Stentor and Vorticella. These cilia are sometimes varied in size and shape in the different parts of the body. Several types of Infusoria form branching, treelike colonies. The Suctoria are attached animals the cilia of which are modified in such a way as to make tentacles of them. These have sucking discs at 84 PROTOZOA the tip by means of which the suctorian captures other protozoans and passes them back to the mouth to be taken into the body. 111. General Facts. — Protozoa vary in size from minute blood para- sites which are invisible or barely visible to the highest powers of the microscope to a gregarine, Porospora, which lives in the alimentary canal of the lobster and which may be 17 mm., or ^3 inch, in length. Most of Nucleus B Nucleus Nucleus Fig. 30. — Some forni.s of Infusoria. .4, a species of Podophyra. {After Butschli, "Klassen und Ordnunycii das Ticrreichs.") To illustrate the Suctoria. Highly magnified. B, Opalina ranarum Purkinje. {From Ke7it, "A Mamial of the Infusoria" after Zeller.) An infusorian parasite of frogs. X 80. C, a species of Vortictlla. {Modi- fied from Hegner, ''College Zoology," after Shipley and Machridc.) Showing a portion of the attachment stalk coiled. Highly magnified. D, Stcntor polymorphus Mi'illcr. {From Kent.) X 60. Attached individual. E, Balantidium coli Malmstcn. {From Thomson and Robertson, "Protozoology") X 400. F, Stylonychiamytil us Ehrenherg. {From Kent.) X 100. {C by the courtesy of The Macmillan Compa7iy; E by that of William Wood & Company.) them are not visible to the unaided eye. The shapes of Protozoa are also exceedingly varied. The cytoplasm of protozoans usually appears alveolar and is divided into ectoplasm and endoplasm. Sometimes the nucleus is scattered throughout the cell in small portions known as chromidia, when it is called a distributed nucleus. We have already noted that in certain cases there are two kinds of nuclei, the macronucleus and the micronu- PROTOZOA IN GENERAL 85 cleus. The former is believed to preside over the nutritive functions of the cell, the latter is active in cell division and transmits hereditary characters. This significance of the two seems borne out by the fact that after conjugation the old macronucleus disappears and a new one is formed from a micronucleus, which insures agreement in the hereditary Adoral membranel l_e ,Oral cilia Neuromotor center Dorsal membranelle Contractile vacuole Macronucleus Micronucleus Ectoplasm Contract! /e_ vacuole Cuticle Mouth opening Oral cilia Adoral membranelle 'Esophagus" Myonemes Skeletal lamina Endoplasm caecum Rectum ' Anus ' Fig. 31. — Diplodinium ecaudatum Fiorentini. {After Sharp, in Univ. Calif. Pub. ZooL, vol. 13, and by the coi/rtcsy of University of California Press.) An infusorian found in the stomachs of cattle, to illustrate the extreme of intracellular differentiation as exhibited by protozoans. Somewhat diagrammatic, and a composite based upon the study of actual longitudinal microscopical sections of preserved animals. X 750. The black ring around the esophagus, the connection from it to the neuromotor center, and the solid black areas at the bases of the membranelles form the neuromotor apparatus. character of the two nuclei. In many cases chromosome formation has been observed in the division of Protozoa, but in other cases it has not. Some Protozoa, both flagellates and infusorians, show great special- ization within the cell, and parts called cell organs, or organelles, are set aside for certain functions. This phenomenon is called in general, differentiation, and, since it occurs here within the cell, it is termed intracellular differentiation. Such parts are contractile strands of proto- plasm called myonemes, which correspond to muscles in higher animals; conducting strands and coordinating centers, which perform the func- 86 PROTOZOA tions of a nervous system; sensitive areas, which function as sense organs; and supporting parts, which form a sort of skeleton (Fig. 31). The food of protozoans consists of organic matter, both vegetable and animal, living and dead. Their metabolism is, in general, similar to that described for the ameba or the Paramecium. Because of the size of the animals the study of digestive enzymes is difficult, and there is little precise knowledge. Protozoans certainly digest proteins, have been shown to be able to use emulsified fats, and also are able to use certain starches. In addition to fission, or binary division, and sporulation, protozoans sometimes exhibit a third type of asexual reproduction known as gemma- tion, or budding. In this case individuals of smaller size than the parent grow out from it like buds and when developed break loose, later growing to the same size as the individual which produced them (Fig. 32). A B Fig. 32. — Gemmation, or budding, in Ephelota gemmipara (Hertwig). (After Hertwig, in Morphologisches Jahrhuch, vol. 1.) A, organism on stalk, showing two types of ten- tacles, suctorial and prehensile, the latter with spiral ridges on the surfaces. B, an indi- vidual showing the formation of buds, into each of which extends a portion of the nucleus. These buds become detached and free-swimming; they possess cilia on one side but later develop tentacles and become attached. X 120. 112. Sexual Reproduction in Protozoa. — Some colonial protozoans exhibit a simple form of sexual reproduction. The animals in the colony become divided into two types: the ordinary ones, known as nutritive individuals, or nutritive zooids, which reproduce by fission in the ordinary way; and a second type which is represented by reproductive individuals or gametes. These gametes exist in two forms : the larger macrogametes, which, like egg cells, are usually not active; and the smaller microgam- etes, which, like the sperm cells of higher animals, are active. When these two types of sex cells unite, a zygote is formed from which a new colony may arise. The macrogametes may also show a type of sexual reproduction without fertilization. When this occurs, they' remain within the colony, increase in size, divide into many cells, and finally escape to form new colonies. These groups of cells are known as parthe- nogonidia (Fig. 27 E). In many of the Sporozoa there are both sexual and asexual generations. The zygotes produce a number of spores which develop into sporozoites. These become nutritive individuals, or tro- phozoites, and these in turn may form another generation of gametes. CHAPTER XVIII PROTOZOA AND DISEASE Protozoa which when Uving in the bodies of other animals are capable of producing disease in those animals are termed pathogenic. Many such Protozoa are known. 113. Pathogenic Protozoa. — Among the Mastigophora are the try- panosomes (Fig. 27 C). One of these, found in certain parts of tropical Africa, produces a disease known as trypanosomiasis, or, because it is characterized by a loss of consciousness, sleeping sickness. These trypanosomes are transmitted from one person to another by the so-called tsetse flies. The sleeping sickness of Africa should not be confused with a disease in this country which exhibits the same symptom and which sometimes goes by the same name; in the latter case the loss of con- sciousness is not caused by an animal parasite but is due to congestion in the blood vessels of the brain. Among the Infusoria are forms belonging to the genus Balantidium (Fig. 30 E), which cause a type of dysentery known as balantidial dysen- tery. This is most common in the tropics. Among the Sarcodina are ameba-like parasites, usually acquired through drinking contaminated water, which are the cause of a serious and often fatal form of dysentery known as amebic dysentery. Here also belong the parasitic organisms found in the mouth which produce the condition known as pyorrhea. Sporozoa, however, includes a larger number of the protozoan parasites of man than do all the other classes together. Among these one of the most common is the malarial-fever parasite. The life history of this organism will be given in detail to illustrate the life cycle of a pathogenic protozoan, though it is more complex than that of many other types (Fig. 33). The spirochaetes, which cause syphilis and other diseases, are by some authorities considered as belonging to the Protozoa, while others consider them intermediate between the Protozoa and'the Bacteria and more closely related to the latter. 114. Malarial Parasite. — The malarial parasite may exist in the blood of man, where it undergoes a series of asexual generations which may continue for many years and even through the lifetime of the person. The individual parasite lives in a red blood corpuscle, into which it enters while in the spore stage. Then it changes to a form resembling a minute ameba. It feeds upon the contents of the corpuscle and when full grown 87 88 PROTOZOA nearly fills it. The parasite then sporulates. The rupture of the cyst formed in sporulation, accompanied by the rupture of the wall of the corpuscle, liberates numerous spores in the fluid of the blood. These enter other corpuscles and pass through a similar life history. The setting free of spores from many infected corpuscles corresponding to the starting ^f In reef ^/^o^ ^^^ ^o, ^-^ Asexual cycle in man Asexual spore Eni^r^ man Gamefo- cyfes Enfer_rnc>5^ifp_._, -Sexual spore Sexual cycle in mosquito Macro- |^^8 gamefe- Sporoblasf Ookinete 13 Oocyst Fig. 33. — Diagram of the life cycles of the malarial parasite of the tertian type, show- ing the asexual cycle in man and the sexual cycle in an anopheline mosquito. Stages 1 to 6 show the entrance of the spores into a red blood corpuscle and the growth and sporulation of the parasite; stage 7, the production of gametocytes; stage 8, their transformation into gametes; stages 9 and 10, fertilization and the zygote; stages 11 and 12, the change of the zygote to an active form, the ookinete, which penetrates the wall of the stomach and encysts; stage 13, forming an ooajst below the outer layer of the stomach wall; stages 14 and 15, the development of several sporoblasts in the oocyst, the development from each of many spores, and their dispersal into the body cavities; and stage 16, the entrance of these spores into the salivary gland. They are introduced with the saliva into a human being, stage 17, enter red blood corpuscles, and another cycle is begun. of a new generatfon, is accompanied by the liberation of poisons in the blood which cause an attack of chills and fever. The time between these attacks, therefore, indicates the period between generations of the parasite. These intervals are 24, 48, or 72 hours, corresponding to three forms of the disease known respectively as pernicious, tertian, and quartan malaria. In addition to these spores, there are also produced within the red corpuscles spores which become sexual in character and by means of which PROTOZOA AND DISEASE 89 the sexual cycle of the parasite may be initiated. This sexual cycle, however, does not occur in the body of man but must take place in a mosquito. If these sexual forms, which are the microgametocytes and macrogametocytes, do not enter the body of a mosquito, they do not further develop. The mosquito which is capable of transmitting the malarial parasite belongs to the genus Anopheles. It is distinguishable from the common mosquitoes, which belong to Culex, by the fact that it holds its body at an angle to the surface on which it rests. The body of a culicid mosquito is held parallel to such a surface. When the mosquito bites, it drills a hole through the epidermis with its proboscis and penetrates the vascular dermis. Then it injects into the wound saliva the effect of which is to prevent coagulation of the blood and thus permit the mosquito to suck until filled. It is the irritation caused by the saliva that produces the itching which is so often a feature of these bites. If in the blood sucked up by the mosquito there are only ordinary spores, the mosquito does not become infected and is not capable of transmitting the infection. If, however, there are microgametocytes and macrogametocytes, these give rise in the stomach of the mosquito, respectively, to microgametes and macrogametes, which unite to form zygotes. These zygotes become elongated, exhibit a gliding movement, penetrate the wall of the stomach, and encyst just beneath the outer layer. In these cysts are produced a great many spores, which, when they are set free, make their way through the body of the infected mos- quito to the salivary gland, in the cavity of which they accumulate. When this mosquito bites another person these spores are injected into the wound made by the proboscis, along with the saliva. In the blood they enter the red blood corpuscles, become ameboid, and thus another asexual cycle is begun. It is evident from this outline of the life cycle that after biting a marlarial individual and acquiring the infection, the mosquito cannot at once transmit the disease. It is necessary for such a transmission that there shall be sexual spores in the blood of the person bitten and that they shall be taken up by the mosquito. A sufficient length of time must also elapse for the sexual cycle to be completed and for spores to form from the zygote. This takes, on the average, about twelve days, though the time varies with the form of the disease and environmental conditions, such as temperature. In the absence of man the female mosquito feeds on the blood of other animals or upon the juices of plants. The male mosquito does not feed, nor does it bite, and therefore cannot become infected and transmit the disease. The sexes may be distinguished by their antennae; those of the male are complexly branched and have a feathery appearance, while those of the female are simple, straight, and hairlike. 90 PROTOZOA The malarial parasite belongs to the genus Plasmodium. Three species are recognized, corresponding to the three forms of the disease. Plasmodium vivax (Grassi and Feletti), producing tertian malaria, is the common one in the United States. The malarial-fever parasite was discovered in Algeria in 1880 by a French army doctor, Laveran, who found it in the blood of patients suffer- ing from malaria. In 1883 a Dr. King of Washington, D. C, presented evidence to show the transmission of the parasite by the mosquito; and this transmission was demonstrated experimentally by Sir Ronald Ross, an Englishman, in 1898. An Italian, Grassi, and his pupils worked out the complete life cycle. Previous to these discoveries it was generally believed that the disease could be acquired by the breathing of miasma rising from swamps and marshes, and the name, meaning literally bad air, was given because of this superstition. Owing to the work of the investi- gators named and others, it is now a fact of common knowledge that malaria can be conveyed to a person only through the bite of an infected mosquito of the proper kind. PART III METAZOA IN GENERAL CHAPTER XIX METAZOA All animals which are not protozoans are included in one subkingdom known as Metazoa. All such animals are similar in that they have a many-celled body in which the cells are not all alike but are varied in structure* and function, the activities of the whole being the result of their cooperative efforts. 115. Differentiation. — The modification of certain parts for the performance of corresponding functions is known as differentiation. In the Protozoa has been seen intracellular differentiation (Sec. Ill), as a result of which one particular structure within the cell comes to perform one function and the other structures other functions. In the highest of the Protozoa this results in an exceedingly complex cell. In the Metazoa, however, complexity does not result from the complexity of the individual cells which make up the animal but from differences between them. This intercellular differentiation results in a great variety of cells within one body. Differentiation which is concerned with struc- ture alone is morphological differentiation. Accompanying this, however, is differentiation in function, which results in cells of different structures having different functions, appropriate in each case to the structure. This may be termed 'physiological differentiation. All differentiation is based upon modification in the metabolic activities of the cells. As a result of differentiation the cells in the metazoan body become inter- dependent, in contrast to the independence which exists between protozoan cells, even in colonies. This interdependence, however, is fore- casted in the case of certain of the colonial Protozoa such as the volvox, in which protoplasmic bridges extend from cell to cell and in which certain reproductive cells are differentiated. It thus appears that the line of demarcation between the Metazoa and the Protozoa is not so sharp as might be supposed. It may, however, be drawn on the basis of the differentiation in the non-reproductive, or somatic, cells, which does not occur in the Protozoa but is characteristic of the Metazoa. 116. Division of Labor. — Another term for physiological differ- entiation is that of division of labor. The idea conveyed by this expres- sion involves an analogy between the development of the animal body and that of human society. Among very primitive peoples each indi- vidual is largely independent of his fellows, doing for himself all that he needs to have done. As society develops, certain individuals become 93 94 METAZOA IN GENERAL more proficient in the doing of certain kinds of work, and as a result a person skilled in one field exchanges the products of his labor for the products of the labor of another who is more proficient in some other field. This speciahzation in the work of the individual and the exchange of the results of that work develop in proportion as civilization advances. In the most highly civilized society the individual may spend his whole time doing simply one thing, as the making of a single part for a complex machine, most of the articles which he uses being secured by the purchase of the products of the labor of others. In the study of animals repre- senting various groups from the lowest to the highest a similar reduction in the degree of independence and increase of the interdependence which accompanies specialization may be observed. Developmen-f- of soma First generation Second 0|enerciti on Third generation Fig. 34. — Diagram to illustrate Weismann's conception of the continuity of the germ plasm and the development of the somatoplasm anew in each generation. Germ cells are black, somatic cells white. 117. Somatic and Germ Cells. — The earliest type of cells to become differentiated from the rest are the sex cells, or germ cells. It has been seen (Sec. 112) that they are set aside even among some of the colonial Protozoa. All the cells in the body other than sex cells are termed somatic cells. When a metazoan reproduces sexually, either the egg cell or the zygote which is to participate in the formation of the new indi- vidual separates from the body of the parent, and by differentiation the whole organization characteristic of the particular species of animal is developed. With the death of such an individual all the somatic cells perish. If we call the protoplasm of the somatic cells somato'plasm and that of the sex cells which transmit hereditary characters germ 'plasm, it may be said that the thread of life continues from generation to generation through the germ plasm alone, the somatoplasm being formed anew in each generation from the germ plasm. Only in the case of asexual reproduction in the Protozoa does the whole animal live on in the bodies of its descendants — that is potential immortality. The genetic continuity of the germ plasm was emphasized in the work of Weismann, whose conception is illustrated in the accompanying METAZOA 95 diagram (Fig. 34). The hereditary units, which determine the possi- biHties open to the animal, are passed on from generation to generation in the germ cells. In the various types of somatic cells, under the environmental conditions which surround each, are realized and mani- fested such of these possibilities as taken together equip the individual with its characteristic features. CHAPTER XX TISSUES As a result of differentiation a variety of cells is produced. These tend to be associated in groups of similar cells to which is applied, in general, the term tissues. 118. Definition. — A tissue is a group of somatic cells which are similarly differentiated — that is, which are similar in structure and which perform one or more functions in common — together with the structures produced by them. In some tissues is found intercellular material which is developed from the cells and which is very important in the performance of the particular function belonging to the tissue. Among the various tissues in the body are recognized four distinct types, classified on the basis of both structure and function: (1) epithelia, or epithelial tissues; (2) supporting and connective tissues; (3) muscular tissues; (4) nervous tissues. 119. Epithelia. — An epithelium, or an epithelial tissue, is the type of tissue which covers any free surface, either the outside of the body or the walls of cavities within it. In the simplest Metazoa this is the only kind of tissue present and there may be little differentiation in it in the differ- ent parts of the body. In the more complex animals, however, the epithelia found in various parts of the body become quite diversified and are named according to the shape of the cells or to the functions which they perform. For example, an epithelium which on its outer surface is made up of very flat cells is termed a ■pavement epithelium; one in which the surface cells are in the shape of long prisms, set at right angles to the surface, is called a columnar epithelium; and one in which the cells on the surface bear cilia is known as a ciliated epithelium. If an epithelium possesses only one layer of cells it is termed simple; if it has several layers, stratified (Fig. 35). Examples of epithelia named from their function are sensory, glandular, protective, and reproductive. The functions which epithelia perform are several. Some serve to protect the structures below them. Others contain sensory cells and serve to receive and transmit stimuli from the outside. Through epithelia all food has to enter the body, and also all waste matter has to leave. They also produce many of the secretions which, when poured out upon a surface, serve to moisten it, to lubricate it, or to digest food. Reproductive cells arise from what are called germinal epithelia. 96 TISSUES 97 All epithelia are similar in that their cells generally possess walls, they are relatively small and compact, are crowded closely together, and are usually cemented to one another by an intercellular cement secreted by the cells. The connective tissue underlying an epithelium often forms a thin sheet called a basement membrane (Fig. 35 C to E) to which the epithelial cells are attached. In some cases intercellular bridges of protoplasm connect one cell to the next, and in the absence of a basement membrane the deepest layer of cells in an epithelium may be anchored by rootlike projections which penetrate the tissue below them. Neither nerves nor blood vessels are ordinarily found in epithelia, though this does not apply to nerve terminals in sensory epithelia. \-4—-i L_J \ o o 1^ -- f ■ — ^-. oTo ° o o o B Basement membrane Fig. 35. — Semidiagrammatic sketches illustrating different types of epithelia. .4, simple pavement epithelium, seen in surface ^•ie\v and in section. B , section of simple short columnar, or cubical, epithelium, also seen in surface view and in section. C, sectional view of a simple columnar epithelium. D, section of simple ciliated epithelium. E, section of a stratified pavement epithelium. All highly magnified. Figure ^A also shows a single very thin pavement epithelial cell. When epithelial cells have a secretory function the secretion may be accumulated in droplets within the cells (Fig. 36). In the case of enzyme-secreting cells droplets or granules containing a zymogen are accumulated, and when secretion occurs the zymogen is transformed to the enzyme and passed out of the cell. These droplets or granules become massed at the outer end of the cell and the cells consequently become markedly granular in texture. The droplets or granules dis- appear when the enzyme is formed and passed out through the cell wall, to reappear during the time the gland is not secreting. Examples are the cells of the salivary glands and the pancreas. In some cases, how- ever, as in the case of cells which secrete mucus, the droplets flow together and form a great mass toward the outer end of the cell. The secretion is set free by the rupturing of the cell wall. Examples are the mucus-secreting goblet cells of the intestines of vertebrates. In still 98 METAZOA IN GENERAL other cases the secretion involves the destruction of the whole cell, which pours out its contents to form the secretion; examples are milk glands and sebaceous, or oil, glands. When epithelial cells undergo a change which makes them hard, the substance formed is horn, which chemically is a substance called keratin. In this fashion true horns, claws, nails, and tortoise shell are developed. In the case of teeth and some scales of vertebrates, however, enamel may be the substance produced. In some cases epithelia produce a hard covering by the hardening of a secretion; an example of such a hard covering is the cuticula of the bodies of insects, which contains ckitin. 120. Supporting and Connective Tissues. — These tissues are found in all parts of the body and differ from other tissues in the fact that the character of the tissue depends not so much upon the cells which it contains as upon nonliving intercellular materials formed by secretion from these cells. Examples of such materials are fibers, bone, and cartilage. Most of the embryonic connective tissue appears in the form of a network of branched cells and is known as mesenchyme. A prominent function of these tissues is support, either of the body as a whole or of some particular part. Among supporting tissues having this func- tion are fibrous tissues, which are characterized by theUai "ceils°" i" colls bundles of fibers or single fibers between the cells. of such a gland as the White, nonelastic fibers are usually collected in salivary glands or pan- , ,, , ., ,, i i- n-i ■ j. creas showing zymogen bundles; while ycllow, clastic fibers are,m most cases, granules accumulated single and, since they branch and anastomose, or Tdjlcenfto rhe^kimen! ^un together, tend to form a network. The fibrous or cayity, of the gland, tissues also serve to bind parts together and to hold fheTtSe ifl'lT them in place. Another type of supporting tissue is tebrate showing the cartilage, in which the space between the cells is rdT^'^e"™!""," occupied by a substance Icnown as chondrin, or the lumen of the intes- "gristle." Still another type is bone, in which there ^'"^" is laid down between the cells a deposit of salts of lime which makes the tissue very firm and capable of giving effective support to even a large body. Special types of fibrous tissue which also serve to bind parts together include the ligaments, which connect the parts other than muscles, and the tendons, which serve to connect muscles to other parts at their point of attachment (Fig. 37). An additional function which these tissues have, and which also is a passive function, is to store fat. Fat tissue is simply a connective tissue in which the cells, because they are filled with great globules of fat, have B Fig. 36. — Figures to illustrate the secre- TISSUES 99 become large and crowded upon each other, while the intercellular elements become the less conspicuous part of the tissue. The blood may also be considered as a connective tissue in which the intercellular elements are all in solution and form the blood fluid or plasma in which the cells float. 121. Muscular Tissues. — Muscular tissues have as their function motion and locomotion. As befitting cells set aside for this purpose, Haversicfncanaf Lacuna A Cana/icu/i 'Elastic fi^er Mast cell Fiber bundle Tendon cell ,<5g«!SJSJ;;^?i?^^ Connective tissue cor- puscle Leucocyte B White fiber bundle Chondrin Cartilage cell Fig. 37. — Different types of connective tissues; somewhat diagrammatic. A, bone, showing the haversian canals which transmit the blood vessels and nerves, and the lacunae, which lodge the bone cells, or bone corpuscles (refer to Fig. \ E). B, portion of subcuta- neous alveolar connective tissue, showing several tissue elements. C, fat. D, tendon in longitudinal section, showing longitudinal fiber bundles and rows of cells crowded into the space between them. E, section of cartilage with the cells lodged in spaces in the chondrin. All highly magnified. they become more or less elongated or fiber-like. In some cases, in order to secure a greater length of the contractile fiber, it is composed not of one cell but of many, all united into a single fiber, which gives evidence of its composite nature only by the fact that it contains many nuclei. The protoplasm within these cells becomes organized in a very complex manner and in such a way as to determine the direction in which con- traction shall take place. All muscle cells perform their function by 100 METAZOA IN GENERAL Cell body Dendrites virtue of their power to contract; their subsequent elongation is simply a matter of relaxation and returning to a normal shape. Three types of muscle tissue are recognized: (1) skeletal, striated and voluntary, found, generally speaking, in the muscles which are themselves organs under the control of the will and often attached to the skeleton; (2) visceral, nonstriated and involuntary, generally not under the control of the will and forming a part of other organs ; and (3) /iear^ muscZe, a type intermediate between the two others, found in the heart (Fig. 38). Skeletal muscles con- sist of large multinucleated fibers which show a very marked cross banding or, as it is termed, cross striation; visceral muscle fibers are individual cells and do not show this cross striation; heart Inuscle is made up of individual cells which are involuntary but cross-striated. Nucleus Sanooplasm Nucleus Confracf/le fibril A B Collateral Neurilemma Nucleus of neurilemma ■ Mo for -^■^^ ^ f/ber Fig. 38. Fig. 39. Fig. 38. — Different types of muscle cells. A, portion of a striated muscle fiber showing a section in which the contractile fibrils are divided into groups by partitions of semifluid sarcoplasm. Two nuclei are shown, surrounded by undifferentiated cytoplasm, and the whole fiber is surrounded with a delicate connective tissue sheath, or sarcolemma. B, three nonstriated muscle fibers, or cells. C, several cardiac muscle cells. All highly magnified. Fig. 39. — Diagram of a nerve cell, possessing a cell body and a meduUated motor nerve fiber, ending in a motor end plate. Such cells are characteristic of the spinal cord of vertebrates. The medullary sheath is acquired while the axon is in the outer layers of the cord, and the neurilemma as the fiber emerges from the cord. The nerve fiber is too long to be shown entirely, so a break is indicated. Of these types of muscle tissue, nonstriated muscles are found more generally in the lower animals and the striated muscles predominate in the higher forms. In the higher forms the nonstriated muscles are in TISSUES 101 the walls of the alimentary canal, the blood vessels, and the gland ducts. 122. Nervous Tissues. — Nervous tissues contain cell bodies from which extend processes or nerve fibeis which vary in length and in the degree to which they are branched (Fig. 39). The function of these tissues is to register the effect of stimuli and to conduct this efTect from cell to cell until it finally reaches a cell which gives the appropriate response. The nerve cells of a brain may themselves initiate impulses which stimulate another part of the body to action. Finally, nervous tissues have the power to conserve the effect of stimulation and to use it in modifying future action. The effect of a stimulus conducted along a fiber is known as an i7npulse. Irritability and conductivity are properties of all protoplasm but are developed to the highest degree in these tissues. A fiber which transmits an impulse to the cell body of which it is a part is known as a dendron or dendrite, while a fiber which transmits an impulse in the opposite direction is an axon, or axis cylinder process. Some nerve fibers have a fatty sheath and are said to be medullated; others, which lack this, are nonmedullated. CHAPTER XXI ORGANS AND SYSTEMS In the bodies of all Metazoa, except the lowest, tissues become associated together in such a fashion that several contribute to the per- formance of some function which belongs to the association as a whole. 123. Definitions. — An organ is a part of the body formed by an association of tissues all of which contribute to the performance of some function or functions. Many organs in the higher animals include repre- sentatives of all of the four different types of tissues. For instance, the heart is covered and lined with epithelium; the greater part of its wall is made up of muscular tissue; fibrous connective tissues serve to connect other tissues and give support; and nervous tissues receive the impulses from the central nervous system, coordinate them, and distribute them to the heart muscles. Not only are tissues associated in the body to form organs, but organs are associated to form systems. A system is a group of organs which collectively perform certain related functions. Thus the body might be conceived of as being built up by adding cells to cells to form tissues, tissues to tissues to form organs, organs to organs to form systems, and systems to systems to form the whole; or it could be analyzed in terms first of systems, then of organs, then of tissues, and finally of cells. 124. Systems. — Nine systems are recognized in higher animals. A list of these with the most prominent functions follows: 1. Tegumeiitary System. — Protection, temperature regulation, elimi- nation of a small amount of liquid waste, and external support. 2. Digestive System. — The ingestion, digestion, and absorption of food, the secretion of digestive ferments, egestion, and elimination of a small amount of liquid waste. 3. Circulatory System. — The transportation of food, oxygen, and the excretions of the body as well as the carrying about of certain internal secretions; also internal respiration. 4. Respiratory System. — The taking in of oxygen and giving off of carbon dioxide, or external respiration. 5. Excretory System. — The elimination of most of the hquid waste products derived from metabohsm (this would be more appropriately named the eliminative system, but the name excretory has been univer- sally used). 6. Skeletal System. — Protection and support. 102 ORGANS AND SYSTEMS 103 7. Muscular System. — Motion and locomotion. 8. Nervous System. — Reception of stimuli, sensation, coordination, and causation of muscular and secretory activity. 9. Reproductive System. — Reproduction. This enumeration does not cover all of the structures in the body. Such a tissue as fat, the function of which is storage, does not logically come under the head of any one of these systems. In the lower Metazoa many functions may be carried on in part by single cells and in part by tissues, and when finally organs and systems become clearly defined there is a gradual increase in the complexity of both, reaching its highest degree in the highest animals. 125. Organs Belonging to Different Systems. — Systems should always be analyzed in terms of the organs which compose them. The tegumentary system includes the skin and the structures contained in it, with the exception of sense organs, which are usually considered as part of the nervous system, and the skin muscles, which are generally referred to the muscular system. The skin occupies a somewhat equivocal position. It may play a part in absorption and in respiration as well as performing the functions already given. It has often been considered a single widely spread organ with smaller organs imbedded in it, such as various glands, and organs of attachment, like sucking discs. The more important regions of the digestive system in the vertebrates are the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and rectum, which may be considered, in a general sense, a series of tubular organs placed end to end making up the aHmentary canal. Some of these also contain other accessory organs, such as tongue, teeth, and certain glands. Other accessory organs lying outside the canal, as salivary glands, liver, and pancreas, also belong to this system. The organs of the circulatory system are the heart, blood vessels, lymph nodes, and spleen. The respiratory system includes, in various animals, gills, lungs, air passages, and tracheae, or breathing tubes, of insects. The pharynx, and in some cases the mouth, may be considered as belong- ing to this as well as to the digestive system. Nephridia in the lower forms and kidneys in the higher, the bladder, and the tubes which convey the urine are the organs of the excretory system. The muscles, individually, are the organs of the muscular system (Fig. 40) ; they contain muscle tissue and, in addition, fibrous connective tissue sheaths, tendons, and nerve endings. In a similar way bones and cartilages are examples of organs of the skeletal system; a bone, besides bone tissue, may contain marrow, a fibrous sheath, and cartilage, which coats certain areas on the bone, forming smooth surfaces for articulation with other bones (Fig. 40). One organ may be included within another and become a part of it, as blood vessels and nerves in muscles and bones. 104 METAZOA IN GENERAL The organs of the nervous system are the brain and spinal cord, nerves, and various sense organs. Nerves possess sheaths of connective tissue in addition to nervous tissue and are also supplied with blood vessels and smaller nerves. Of the reproductive system, the most important organs are the gonads, under which are included the testis of the male and the ovary of the female. There are also to be added the ducts which convey the sperm Blood -Nerve Articular cartilage Fibrous capsule of Joint Sarcolemma Muscle fibers Tendon Blood vessel Nerve Marrow Fig. 40. — A diagram to indicate the nature of bones and muscles as organs and the mode of attachment of a muscle to a bone. A bone as an organ consists of several tissues, such as bone, articular cartilage, marrow, and the fibrous periosteum, and into it enter blood vessels and nerves. A muscle contains muscle fibers, is covered with a fibrous sheath, ends in a tendon, and also is supplied with blood vessels and nerves. The fibers of the ten- don are interwoven with those of the periosteum, from which other fibers penetrate the bone, giving a firmer anchorage. cells and egg cells and a variety of other organs, such as yolk glands and shell glands in the female and copulatory organs in the male. These organs and systems will be described in greater detail when the animals which possess them are considered. 126. Other Parts of the Body. — Many divisions of the body are recognized which have no relation to organs and systems, such as the head, neck, trunk, and appendages. Appendages are usually not individual organs but often contain many organs belonging to several different systems. For instance the vertebrate limb contains organs of the tegumentary, circulatory, skeletal, muscular, and nervous systems. CHAPTER XXII REPRODUCTION IN THE METAZOA The general subject of reproduction was introduced in Chap. X, and reproduction in Protozoa has been considered especially in Sees. 97, 103, 111, and 112. 127. Methods of Reproduction in Metazoa. — In Metazoa the usual type of reproduction is sexual, although asexual reproduction is found in the lower forms. Fission occurs when the animal's body divides into two individuals equal in size. The process is called budding when an individual gives rise to another by the separation of a part smaller than that which remains and which is the parent. Both fission and budding occur in many of the lower metazoans. Some of the lower worms also undergo what is called fragmentation. Though not the same as sporu- lation, in the sense in which the word is used in connection with Protozoa, fragmentation is a mode of reproduction analogous to it and occurs when the body divides into a large number of fragments each one of which becomes a complete individual. 128. Sexual Reproduction. — Sexual reproduction in Metazoa usually involves two parents. It is then termed hi-parental. In this case the two parents usually differ from each other in their external appearance. The one which is termed the male produces sperm cells; and the other, called the female, produces egg cells. A species of which this is true is termed diecious, or bisexual, referring to its existence in the two sexes. On the other hand, particularly among lower Metazoa, there are those species in which one individual produces both egg cells and sperm cells and which therefore contains the organs of both sexes. Such species of animals are represented by only one type of individual and are called monecious, or hermaphroditic. Different species of hydra and of earth- worms are examples of monecious animals; almost without exception the vertebrates are diecious. 129. Uniparental Reproduction. — It is possible for an egg cell to develop without union with a sperm cell. When this takes place the phenomenon is termed parthenogenesis. It occurs in nature in a number of diecious animals in which exceedingly rapid reproduction contributes to the welfare of the race. Examples of such animals are plant lice, which are eaten by a vast number of other animals and which continue to exist only by virtue of exceedingly great powers of reproduction, and certain aquatic forms, like rotifers and water fleas, which are also eaten in great numbers by fish and other larger aquatic animals. 105 106 METAZOA IN GENERAL If rapidity of multiplication is the end reached in parthenogenesis, this is attained to a still greater degree if the animal does not wait to become mature before it becomes capable of reproducing. Reproduc- tion by an immature animal is known as pedogenesis and occurs in the case of a number of insects; for instance, the larvae of certain gall gnats and the pupae of some midges produce eggs which are capable of develop- ing without fertilization. 130. Types of Fertilization. — Animals which are monecious are capable of fertilizing their own egg cells, though actually in nature few such animals do so. When this occurs, the phenomenon is known as self-fertilization. In the case of animals which are diecious the fertiliza- tion of the egg cell of one individual must be by the sperm cell of another. This is known as cross-fertilization. Cross-fertilization is not the same as hybridization, the latter term being applied only when the two indi- viduals belong to different species. Cross-fertilization is a very general phenomenon and is practically universal among the higher animals; hybridization is much less frequent. 131. Oviparity and Viviparity. — Many animals retain for a time within their bodies their egg cells and the embryos which develop from them and give birth to living young. Such animals are termed vivi- parous and the phenomenon is viviparity. On the other hand, a great many pass the egg cell out of the body for development. These forms are termed oviparous and the phenomenon oviparity. In oviparous animals the egg cell when passed out of the body is usually provided with a greater or less number of protective envelopes various in character, and to the egg cell plus all of these envelopes is applied the term egg. In some cases fertilization takes place within the body before these enve- lopes are added, and here, as in viviparous animals, the phenomenon is referred to as internal fertilization. On the other hand, the egg may be of a character which permits fertilization after passage from the body. Such a type of fertilization is termed external fertilization. 132. Metagenesis. — There are animals in which both sexual and asexual types of reproduction occur, and these in alternate generations. One or more generations produced in one manner are followed by one or more produced in the other. This phenomenon is termed metagenesis, or alternation of generations. It is illustrated best among some marine hydroids and jellyfishes, in connection with the study of which it will be more fully described. CHAPTER XXIII ORIGIN OF THE SEX CELLS The first step in the production of a new individual sexually is the formation of sex cells. This takes place in gonads which arise from the germinal epithelium, which in turn is developed from the cells lining the coelom, or body cavity. 133. Gametogenesis. — The origin and development of the sex cells are termed gametogenesis. This may be divided into spermatogenesis, which deals with the male germ cell, called the sperm, sperm cell, or spermatozoon; and oogenesis, which deals with the female germ cell, called the egg cell, or ovum. In all references to the male germ cell in this text it will be called a sperm cell. The objection to the word spermatozoon is that it perpe- tuates an error; it means, literally, "sperm animal" and was proposed at a time when it was believed that these cells were themselves animals living in the bodies of higher animals. The term egg cell is preferred to ovum because the latter has been used both in this sense and also to apply to the whole egg. Both processes, spermatogenesis and oogenesis, begin (Figs. 41 and 42) quite early in the life of the embryo by the setting aside of a pri- mordial germ cell from which come all of the sex cells which will be devel- oped in that animal's body. This cell multiplies by repeated divisions until a very large number of cells is produced; the time during which this occurs is called the ynulti plication period. In spermatogenesis these cells are known as spermatogonia; and in oogenesis, oogonia. When the animal becomes sexually mature, these cells undergo the processes of growth and maturation, the growth period involving both an increase in the size of the cell and a union of hke chromosomes in pairs. This union of chromosomes is termed synapsis. At the end of the growth period the male cells are termed primary spermatocytes; and the female cells, primary oocytes. From this time on the processes of spermatogenesis and oogenesis differ. 134. Spermatogenesis. — The periods of multiplication and growth having been completed in spermatogenesis, the maturation period fol- lows (Fig. 41). The primary spermatocyte undergoes two maturation divisions. The first results in the formation of two secondary spermato- cytes, and the second in the formation of two spermatids from each of these secondary spermatocytes, making four spermatids altogether. The 107 108 METAZOA IN GENERAL chromosomes, which were brought together in pairs in synapsis, are separated again in one of these divisions, in which case, instead of each chromosome dividing, whole chromosomes pass to the poles of the spindle. Thus the number of chromosomes becomes reduced to half the number contained in the primordial germ cell. This peculiar type of cell division Multiplica+ion period Primordial germ cell and SO on for a very /arqe number of cell divis/ons QTaXJ any one spermatogonium ,. -Synapsis Primary spermatocyte Me/ otic division ^ I ] Secondary — J Spermatocyte /j \*^ [ / \v\ Spermatids Sperm cells Fig. 41. — Diagram illustrating spermatogenesis, the haploid number of chromosomes being four and the diploid eight. is known as a reduction division, or meiosis. The reduced number of chromosomes is known as the haploid number, while the larger number, found in all somatic cells and in all immature germ cells, is called the diploid number. Sometimes the reduction occurs in the first of these maturation divisions and sometimes in the second. The spermatids undergo a process of modification or ripening which involves a change in form and also the loss of a considerable amount of ORIGIN OF THE SEX CELLS 109 the cytoplasm. The nucleus becomes the larger part of the body, or head, of the mature sperm cell, which in many higher animals resem- bles in shape a tadpole with a very long tail. From a portion of the Multi plication period '*;•" \M Primordial germ cell Oogonict \^ v^ ^^£7 "^^yy "^^^ M^N /— \ / \ / \ / \ / \ I ancpl so on for oi large nurnber q-F cell divisions T\//^ (^ny one oocjoniuni Growth period Maturation period Secondary oocyte Matured egg cell ■Synapsis Primary oocyte Meiot/c division \ V^ Firs t polar b ody ® m Polar bodies ^Second polar '■^ody ■ '-Yolk accumulated in the egg cell Fig. 42. — Diagram illustrating oogenesis, the ohromosome number being the same as in Fig. 41. cytoplasm is formed the tail, and between the head and tail, connecting one to the other, is a middle piece which contains the centrioles. This tail, by rapid vibrating movements, can propel the sperm cell through a hquid medium at a relatively high rate of speed. 110 METAZOA IN GENERAL 135. Oogenesis. — In oogenesis, also, after the periods of multiplica- tion and growth have been completed, the primary oocyte undergoes a first maturation division in which, in contrast to what occurs in sperma- togenesis, it is very unequally divided (Fig. 42). One of the two daughter cells is small and, while it contains half the nuclear material, has prac- tically none of the cytoplasm. The other is much larger, receiving in addition to half the nuclear material practically all of the cytoplasm. The smaller daughter cell is termed the first polar body; and the larger one, the secondary oocyte. Following this division a second division of the first polar body may occur, giving rise to two smaller polar bodies each equal in size to half of the first polar body. The secondary oocyte undergoes another unequal division, a second polar body being formed as before with very little cytoplasm, while the larger cell is known as the egg cell. As in spermatogenesis one of these two divisions is a reduction division in which the number of the chromosomes is reduced to the hap- loid number. The result of oogenesis, therefore, is to produce one large, functional egg cell and either two or three polar bodies depending upon whether or not the first polar body undergoes division. These polar bodies die, disintegrate, and disappear. In effect, all of the cytoplasm which would have gone to four cells if the divisions of the cells had been equal has been accumulated in the one. This egg cell becomes still larger by the accumulation within it of yolk and thus becomes fully mature. This accumulation of yolk in the mature egg cell is to provide the necessary food supply for the embryo which will develop from it until the developing individual can secure food for itself. It should be observed that in both oogenesis and spermatogenesis all of the cell divisions except the reduction division are mitotic. 136. Comparison and Contrast between Spermatogenesis and Oogenesis. — It is clear from the description of the two processes that there are many ways in which they are ahke; the more important simi- larities may be enumerated as follows: 1. Both start with a primordial germ cell. 2. Both pass through three periods, namely, multiplication, growth, and maturation. 3. Both undergo a process known as synapsis in the growth period. 4. Both possess two maturation divisions. 5. Both exhibit a reduction in the number of chromosomes. On the other hand, the two processes are sharply contrasted in several ways: 1. Spermatogenesis results in the production from each spermato- gonium of four similar sperm cells, all of which are functional, while oogenesis results in the formation from each oogonium of only one large cell, the egg cell, and of three small nonfunctional cells or polar bodies. ORIGIN OF THE SEX CELLS 111 2. Mature sperm cells are very small and very active cells; mature egg cells, owing to their large size, are passive. 3. Because all of the sperm cells are functional and also because of a much greater number of multiplication divisions, the total number of sperm cells produced is enormously greater than the number of egg cells. The number of egg cells which become mature and may be fertilized is only a small fraction of the number actually produced. The rest act as nurse cells and contribute their substance to those which are to develop. A good example of this is seen in the fresh-water hydra, in the ovary of which a large number of egg cells are developed; only one of these, how- ever, becomes fully mature and capable of producing another individual. 137. Division of Labor between the Germ Cells. — It will be observed that in spermatogenesis and oogenesis there has been a division of labor between the two types of sex cells. In order that fertilization shall occur the two cells must come together, and to assure the development of the embryo a large store of food is provided. If both cells had a store of food neither would be able to move effectively and their union could not occur, but the accumulation of a sufficient store of food in one while the other becomes small and active makes it possible for the latter to seek out the former and to unite with it. That practically every mature egg cell will be fertilized is also insured by the enormous number of sperm cells produced as compared with the number of egg cells. This greater number, however, imposes no proportionately greater strain upon the energy of the individual, since frequently there is no more material in 100,000 sperm cells than in one egg cell. In a certain species of sea urchin the volume of the individual egg cell is equal to that of 500,000 sperm cells. 138. Variations in Gametogenesis. — In the details of this process many variations occur in different animals. All of the sex cells which the animal produces may mature at the same time; this is the rule in insects, which, after the maturation and fertihzation of the eggs, deposit them as rapidly as possible and soon die. Maturation periods may occur at intervals and the animal live through several breeding seasons. Many mammals exhibit this phenomenon. Birds have an annual breed- ing season. At other times than during the breeding season the sex organs in these animals are quiescent and the maturation of the sex cells is arrested. In still other animals, particularly in the male sex, matura- tion goes on continually and the animal can breed at any time. Under domestication the breeding season may be greatly extended. The domestic hen in the original state had a restricted breeding season and laid only a limited number of eggs. Under domestication the number has been increased until now (November, 1932) the record stands at 357 eggs in one year, which means practically continuous sexual activity. CHAPTER XXIV FERTILIZATION In a general sense fertilization may be defined as the union of the sperm cell with the egg cell, though, as will be seen, the process involves several steps, takes a certain length of time, and there may be a question B D E F Fig. 43. — Diagrams showing the successive steps in the fertilization of the egg cell of a sea urchin, which is mature when the sperm cell enters. {From Wilson, " The Cell," by the courtesy of The Macmillan Company.) A, the entrance of the sperm cell; the maturity of the egg cell is indicated by the two polar bodies. B, the approach of the two pronuclei, the centriole of the sperm cell and the aster developed about it preceding the male pro- nucleus. C, the meeting of the two pronuclei; the centriole has divided. D, the formation of two asters about the two centrioles, now on opposite sides of the two pronuclei, which are undergoing fusion. E, the fusion nucleus representing the two pronuclei during a period of pause, while the asters are reduced in size. Fertilization may now be said to be complete. F, the first cleavage division, which follows the pause, at the beginning of the anaphase. as to when the union is actually consummated. Two phenomena are involved: the activation of one cell by the other and the union of corre- sponding chromosomes from the two parents. The former effect is paralleled by artificial parthenogenesis. Loeb discovered in 1899 that the eggs of starfishes and sea urchins could be caused to develop by artificial 112 FERTILIZATION 113 stimulation, and echinoderms have since been the favored types in such experimentation. Since that time successful experiments have been carried out with invertebrates other than echinoderms, especially annelids and mollusks, and with fishes and frogs, none of which develop par- thenogenetically in nature. Several types of stimuli — mechanical, thermal, and chemical — have been found to be effective. The adult condition has been attained in but few cases. In an animal as high as the frog, however, tadpoles were reared which metamorphosed into adults. 139. Steps in Fertilization. — Usually the whole sperm cell enters the egg cell, but in some cases more or less of the tail is left outside and there enters only a nucleus the centrioles, and a very little cytoplasm. The B Fig. 44. — Diagrams showing the successive steps in the fertilization of the egg cell of a round worm, Ascaria, which matures after the entrance of the sperm cell. {From Wilson, "The Cell," by the courtesy of The Macmillan Coinpany.) A, the entrance of the sperm cell; the egg cell is in the condition of a primary oocyte. B, the formation of the first polar Ijody ; development of a sperm aster. C, the matured egg cell, with the polar bodies; the male pronucleus has increased in size; from the one centriole has developed two, each with an aster, and a spindle lies between them. D, the two pronuclei, now about equal in size and each containing chromosomes, meet on the spindle. E, a pause corresponding to that in Fig. 43iJ. F, first cleavage division. nuclei of the two cells, which are now called, respectively, the male and female -pronuclei, may, if both are mature, at once approach and fuse. In this case cell division follows after a time (Fig. 43). On the other hand (Fig. 44), the entrance of the sperm cell may take place before the egg cell has attained the necessary maturity, in which case the male pronucleus remains at one side until the maturation of the egg cell is complete and undergoes a slow growth in size by absorbing the fluid 114 METAZOA IN GENERAL from the cytoplasm of the egg cell. Then the two pronuclei approach each other. At the same time the centrioles which were brought in with the sperm cell become active and a spindle is produced near the center of the egg cell. The two pronuclei meet at the equator of this spindle. Chromosomes are formed in each, the nuclear membranes disappear as in an ordinary mitosis, and the two sets of chromosomes gather on the equator of the spindle, producing an amphiaster stage. Then the steps which are seen in ordinary mitosis occur in regular order, including metaphase, anaphase, and telophase, the final result being a division of the cell. This division initiates the development of the embryo. Fertilization may be said to be completed when the sperm cell enters the egg cell, when the two nuclei fuse, or, in the case last described, when the two nuclei cease to retain their identity and the chromosomes which develop from them come to lie in the equatorial plane of the spindle. In either of the cases described above, the chromosomes from the two parent cells appear clear and distinct and when they divide in the metaphase, each of the two groups of chromosomes which pass to the two poles of the spindle is half maternal and half paternal in origin. When at the end of the telophase the nuclei of the two daughter cells enter into a resting condition, these chromosomes lose their identity; but in each cell division which will follow in the development of the individual which is to be produced, the maternal and paternal chro- mosomes again appear. Thus the individual represents a minghng of the characteristics of the two parents, and each cell in the body has this mixed inheritance. As might be expected, the steps given above are varied in many ways in different types of animals but the essential facts remain the same. 140. Chromosome Reduction. — It now becomes evident why chromo- some reduction occurred in gametogenesis. Every species of animal has a characteristic number of chromosomes, a number which is found in every somatic cell in the body and remains constant generation after generation. This number seems to have no relationship to the structure of the animal or to its rank in the scale of animal life. For instance, there are 2 chromosomes in a parasitic worm (Ascaris) found in the horse, 8 in the fruit fly, 28 in the spotted salamander, 48 in man, and 208 in two species of crayfish. If the numbers of chromosomes were not reduced in the maturation of sex cells, the fertilized egg cells would contain twice the number possessed by the cells of the parents, and their number would continue to double with each succeeding generation. Chromosome reduction, however, results in passing on the same number from one generation to the next. In all references to chromosomes up to this point only those which act as mates in synapsis have been con- sidered. There are odd chromosomes which in meiosis pass to either FERTILIZATION 115 one or the other of the daughter cells. These are associated with the determination of sex and will not be considered until Chap. LXXIII is reached. 141, Significance of Synapsis. — In all of the somatic cells of the body the chromosomes of maternal and paternal origin remain separate in cell division, and since the corresponding ones from the two parents are similar, they appear in pairs of like chromosomes. In the process of synapsis in gametogenesis these like chromosomes unite, and this is followed by their separation again in chromosome reduction. In mitosis every chromosome is divided and the two daughter cells have the full, or diploid, number, but in meiosis whole chromosomes pass to the daughter cells, which thus acquire the haploid number. Since it is a matter of chance to which of the two poles of the spindle any particular chromosome goes, there is an assorting and chance distribution of chromosomes to the mature egg cell or sperm cell. Thus every individual not only represents a chance mingling of the chromosomes of its parents but also receives a chance selection from those of previous generations, the probable number received from each generation being progressively smaller in going back from one generation to the previous one. In one sense this union of the chromosomes in synapsis may be thought of as the final step in fertilization. CHAPTER XXV EMBRYOGENY The word embryogeny may be defined as the development of an animal from the time when the fertilized egg cell begins to divide until the organism has acquired an organization comparable to that of the adult. Until that time it is an embryo, but afterward it receives different names in the different types of animals. Examples are the larvae of many invertebrates, the tadpoles of amphibia, the chicks of birds, and the D E Fig. 45. — Diagrams of homolecithal egg cells and total cleavage. A, homolecithal egg cell with nearly uniform distribution of yolk. B, total equal cleavage resulting from the condition shown in A. C, homolecithal egg cell in which the yolk tends to accumulate toward the lower pole. D, total une(iual cleavage of the egg cell shown in C, when the first cleavage plane is horizontal, resulting in the production of two unequal cells. E, cross section of an eight-cell stage, resulting from cleavage in C in a case in which the first two cleavage planes were vertical, giving rise to four equal cells, and the third was horizontal, producing four smaller cells at the upper pole and four larger at the lower. The proto- plasm is stippled, the yolk indicated by the outlines of globules. fetuses of mammals. Generally speaking, the higher the place of the animal in the animal kingdom the longer will be the embryogeny. Any particular embryogeny is a part of a corresponding ontogeny, which covers the development of the animal from the beginning until it reaches full maturity. The term embryogeny should not be confused with embryology, which is a broad science covering not only the embryogenies 116 EMBRYOGENY 117 of all animals but also a large amount of detail and generalization which is outside all embryogenies. 142. Types of Egg Cells. — A part of the process of maturation in the egg cell consists in the accumulation of yolk, but the amount of yolk thus stored and its distribution in the cell are not the same in all egg cells. All egg cells show polarity. This is indicated by the polar bodies being formed at or near the upper pole and by the nucleus, which is always more or less excentric, being located nearer this pole. The opposite pole is called the lower pole. In some cases the yolk is not very great in amount and is scattered throughout the cytoplasm. Such egg cells are termed homolecithal (Fig. 45 A and C). Most of the lower invertebrates and almost all of the mammals possess this type of egg cell, though in mammals the condition is not primary but secondary and is due to adaptation to the peculiar mode of development. In some homolecithal egg cells there Fig. 46. — Diagrams of teloleeithal and centrolefithal egg cells, and discoidal and super- ficial cleavage. A, telolecithal egg cell, in which the protoplasm is all at the upper pole. B, the discoidal cleavage which occurs in a telolecithal egg cell. C, centrolecithal egg cell, with a superficial layer of protoplasm, protoplasm around the nucleus, and strands of pro- toplasm connecting the two. D, the superficial cleavage of a centrolecithal egg cell; an early stage, when the nucleus has divided into several nuclei and each, with a portion of protoplasm about it, is migrating toward the periphery. E, later stage in superficial cleavage showing the nuclei and cytoplasm at the periphery, forming a superficial layer of cells, and the yolk at the center. The protoplasm is stippled, the yolk indicated by the out- lines of globules. is somewhat more protoplasm toward the upper than toward the lower pole and somewhat more yolk toward the lower than toward the upper pole. In the egg cells of almost all vertebrates but the mammals, however, the yolk, which is present in very large amount, is massed toward the 118 METAZOA IN GENERAL lower pole, leaving the cytoplasm as a disc at the upper pole. In this case the egg cell is known as telolecithal (Fig. 46 A). In such egg cells the upper pole is called the animal pole; and the lower, the vegetal pole. Primitive mammals have telolecithal egg cells, and the ancestors of mammals doubtless had such egg cells. In the insects a third arrangement is presented. Here the yolk occupies the central portion of the egg cell, inclosing within it, at the very center, the nucleus, which is surrounded by some of the cytoplasm, while the greater part of the cytoplasm forms a layer over the whole surface. Such an egg cell is called centrolecithal (Fig. 46 C). Here there is a considerable amount of yolk, though relatively not so much as in telolecithal egg cells. 143. Forms of Cleavage. — The process which follows fertihzation and which results in the formation of a large number of cells from the fertilized egg cell is known as cleavage. The individual cells which are thus formed are termed blastomeres. The effect of the difference in the amount and distribution of the yolk is seen in the different ways in which egg cells cleave. In an ideal embryogeny, which may be accepted as that of a homolecithal egg cell, the plane in which the first cell division takes place is typically meridional, passing from one pole to the other. The second cleavage plane is also meridional, being at right angles to the first, and results in the formation of four similar cells. The third cleavage plane, however, passes at right angles to the two others, and thus eight cells are produced. Of these the upper four will be smaller than the lower four. Since the whole egg has been involved in the cleavage, the egg is sometimes termed holohlastic and the cleavage is called total. If the yolk is quite evenly distributed and the cells which result from the cleavage are all approximately the same size, it is termed equal cleavage (Fig. 45 B). If, however, the yolk is not evenly distributed but is greater toward the lower pole of the egg cell, the cells at the upper pole will be decidedly smaller than the other four. This is termed unequal cleavage (Fig. 45 D and E). In some cases the two cells first formed differ m size, the first cleavage plane being horizontal and the smaller cell being above the larger. In other cases the difference in the size of the cells does not develop until the third cleavage occurs, the upper four being smaller than the lower. There are many modifications of these different types In telolecithal and centrolecithal egg cells the cleavage planes do not pass entirely through the cell but only through the cytoplasmic portion, and thus the cleavage becomes partial; such egg cells are often termed 7neroblastic. In telolecithal egg cells the division of the cytoplasm results in an embryonic disc at the animal pole, and accordingly the cleavage is termed discoidal (Fig. 46 B). As development proceeds and the cells continue EMBRYOGENY ' 119 to multiply in number, this disc gradually surrounds the yolk, which is finally absorbed by the growing embryo. In centrolecithal egg cells the nucleus in the center of the cell divides repeatedly, each of the daughter nuclei being surrounded by a little mass of cytoplasm. As these nuclei increase in number they migrate toward the periphery, accompanied by the bits of cytoplasm, and enter the super- ficial cytoplasmic layer. Now division of the cytoplasm takes place by planes which cut it at right angles to the surface, and this for a time leaves each cell open toward the yolk in the center. A little later the walls of these cells become complete. Because a superficial layer of cells is in this way formed around the yolk it is termed superficial cleavage (Fig. 46 D and E). 144. Steps in Embryogeny. — In the ideal embryogeny (Fig. 47) pre- viously referred to as that of a homolecithal egg cell, cleavage may be conceived as resulting in the development of a compact mass of cells which, because of its general resemblance to the fruit of the mulberry tree, has been called a morula. As the multiplication of cells continues, a cavity begins to form in the mass. The embryo is then called a hlastula. This cavity increases in size until the blastula appears like a hollow rubber ball, the cells or blastomeres forming the wall, which is now called the blastoderm. The central cavity is variously known as the cleavage cavity, segmentation cavity, or blastula cavity and also as the hlastocoel. The blastoderm in a typical blastula is a single layer of cells, but in certain cases it is made up of more than one layer. As cell division is still going on, the blastula tends to increase in size with the increasing number of cells in the blastoderm, but these cells differ in size and also in the rapidity of their multiplication. Those toward the upper, or animal, pole are the smaller ones and are multi- plying more rapidly, whereas those toward the other pole are larger and are multiplying more slowly. This unequal growth causes an expansion of the upper wall of the blastula and leads to an invagination of the lower cells, the blastula thus becoming converted into a double-walled inverted cup. As soon as this invagination begins, the embryo is termed a gas- trula. As the gastrula develops further, the two walls come gradually closer together until finally the cleavage cavity becomes entirely oblit- erated. This process is called gastrulation, the cavity formed is known as the archenteron, or primitive digestive cavity, and the opening into it from the outside is termed the blastopore. The gastrula is thus made up of two layers of cells; the one forming the outer wall of the cup is called, because of its position, the ectoderm, and the one within, forming the lining, is known as the entoderm, or endoderm. Now a third layer of cells appears between the two others, being developed in some cases from the ectoderm and in others from the 120 METAZOA IN GENERAL entoderm. This third layer is termed mesoderm. If the mesoderm is composed of a meshwork of scattered cells which have passed from either of the other layers into the blastocoel, it is known as mesenchyme (Fig. 47 7). In the embryogenies of certain animals the mesoderm cells are formed by an outpocketing of entoderm cells, which pushes into the space between the entoderm and ectoderm (Fig. 48). In those of other animals they are separated from the wall of the archenteron as solid masses of cells, which later become hollow. In both of the latter cases the cells surrounding these cavities form mesotheliu7n (Fig. 48 H and 7). Ecfoderrn Blastocoel Blasfoderm G Blastopore H Archenteron I Fig. 47. — Diagrams illustrating the steps in an ideal embryogeny. A, the egg. B, the two-cell embryo. C, the four-cell stage. D, the eight-cell stage. E, the sixteen-cell stage. F , the morula, a solid mass. G, section of the blastula, with the blastocoel. H, section of the gastrula. /, gastrula in which the mesoderm cells are appearing in the blastocoel. These mesoderm cells will form mesenchyme. From the three germ layers tissues are developed. The tissues then become arranged to form organs, the process being termed organogeny. This series of stages and processes may be outlined in the following manner: First stage: The egg cell (normally previously fertilized). First process: Cleavage, or segmentation. Second stage: The morula. Second process: Formation of the cleavage, or segmentation, cavity. Third stage: The blastula {monohlastic, or one-layered, embryo). Third process: Development of the archenteron, or gastrulation. EMBRYOGENY 121 Fourth stage: The gastrula {diploblastic, or two-layered, embryo). Fourth process: Appearance of a third layer. Fifth stage: The triploblastic, or three-layered, embryo (not given a particular name). Fifth process: Tissue formation. Sixth process: Organogeny (development of organs). It should be remembered that the stages are not stopping points, that each of the processes lasts for a considerable time, and that the whole forms a continuous development. As the cleavage cavity first appears, the embryo is spoken of as an early blastula; as it increases in size, an older blastula; and just before invagination begins, a late blastula. In the same way reference may be made to an early and a late gastrula. 145. Variations in Embryogeny. — Since egg cells differ so much in the amount and distribution of the yolk it will be clear that many varia- tions in the course of embryogeny are bound to occur, and all the steps in the ideal embryogeny described cannot be expected to appear in any actual individual embryogeny. Different types of cleavage have been previously noticed. In total cleavage the blastomeres may be spirally instead of regularly arranged. It is then called spiral cleavage. When the yolk is reduced to a minimum and the blastomeres are in contact by only a small area, there may be no morula stage but cleavage maj^ result in the immediate development of a gastrula (Fig. 48). When the yolk is so abundant that the cleavage cavity is reduced to only a slit, invagination becomes impossible, and the resulting overdevelopment of the cells at the animal pole causes an outfolding. This is known as gastrulation by epihole (Fig. 253(t). The archenteron is formed under this fold, which may gradually grow around and envelop the whole embryo. In the case of the mammal, as will be seen later, a very marked change in the character of the embryonic stages results from the condition which involves the attachment of the embryo to the wall of the maternal uterus and its nourishment from the blood vessels of the mother. 146. Germ Layers. — Reference has been made to three germ layers. The blastoderm, appearing in the blastula, gives rise in the gastrula to the ectoderm and entoderm, and the mesoderm is added in the triplo- blastic embryo. These layers, in all Metazoa but the sponges, retain this relative position, and from each arise a certain number of tissues. The tissues derived from the ectoderm include the epithelial covering of the body, often known as the epidermis, which may extend inward a short distance at the external opening or openings of the digestive cavity or canal. They also include the epithelium lining all hollow organs the cavities of which open to a surface covered by epidermis. This includes such cavities as the external ear, the nasal chamber, and the spaces under the eyelids. All nervous tissues are also derived from the ectoderm. 122 METAZOA IN GENERAL From the entoderm is derived the epitheUum Uning the digestive cavity or caual, except at the open ends; also the epithehum hning all hollow structures formed as outpocketings of this cavity or canal. This latter category includes, in the air-breathing vertebrates, not only the ^ Nerve cord Mesoderm^ — / .chorda Archenferon Fig. 48. — Stages in the development of amphioxus, one of the lower chordates. (Drawn from Ziegler models, based on the ivork of Hatschek.) A, the four-cell stage, polar view, showing a cre\dce between the cells. B, cross section through the opposite cells of an eight-cell stage, showing a median space. C, a median section of the 32-cell stage; the median space i.s developing into a blastocoel and the embryo is becoming a blastula without passing through the morula stage. D, cross section of the blastula. E, invagination. F. the gastrula has become asymmetrical and has turned on its side; the dorsal surface is flattened, the ventral convex. G, the diploblastic embryo, showing the ventral ectoderm growing over the blastopore and upon the dorsal surface, covering in the dorsal ectodermal cells which will form the central nervous system. H, cross section of a later stage, showing the chorda, or notochord, arising as a median dorsal outpocketing of the wall of the archen- teron, and the mesoderm developing from dorsolateral outpocketings of the entoderm. / cross section of a later stage, showing the central nervous system, the chorda, and the enteron in the median line, and on each side a mesodermal pouch, containing a coelomic cavity. The walls of this pouch are mesothelium. linings of cavities of such organs as the liver and pancreas but also the lining of the so-called respiratory tract, consisting of the lungs and the passageways leading from the pharynx to them. From the mesoderm are developed all of the other tissues of the body, including muscles, connective and supporting tissues, the blood vessels ■ EMBRYOGENY 123 and the blood which they contain, and the epithehal Hning of all cavities developed in this layer. The mesodermal tissues in higher forms far exceed in bulk those from the ectoderm and entoderm combined. It will be observed from what has been said that an epithelium may be derived from any one of the three layers. The epidermis is ectodermal in origin, the epithelium hning the greater part of the digestive cavity is entodermal, and the epithelium lining the cavities within the mesoderm, including the lining of the heart and blood vessels, is mesodermal. Skele- tal parts may be formed not only from the mesoderm but sometimes from the ectoderm, and even in rare cases from the entoderm. 147. Coelom. — Any cavity formed in the mesoderm and surrounded by mesothelium is known as a coelom. When it is present the sex organs become developed from its wall and the excretory organs open into it. It is lacking in the lower Metazoa but it is present in most of the higher forms, in which it may be divided into several cavities. The layer of the mesoderm outside the coelom and lying against the body wall is called somatic; that inside the coelom and lying against the viscera, splanchnic. Since three cavities develop during the embryogenies of higher animals it is well to bring them into contrast, as may be done in the table which follows: Name of cavity Time of appearance Lining of wall Fate Segmentation or cleav- age cavity Blastula Blastoderm Disappears as the next is formed Archenteron Gastrula Entoderm Becomes the digestive cavity of the adult Coelom Triploblastic Mesoderm Becomes in the adult the embryo (mesothelium) body cavity and cavities derived from it PART IV METAZOAN PHYLA CHAPTER XXVI SPONGES Vacuole THE PHYLUM PORIFERA In the ocean are found many animals which would not be recognized as such by the ordinary observer, since they have neither power of move- ment nor power of locomotion, and since they form inert masses attached to various solid objects including the shells of other living animals. In many cases these are sponges, though some ascidians (Sec. 338) would fit the description. Sponges were long supposed to be plants and their animal nature was not fully established until about 1857, since which time they have been variously classified in the animal kingdom. 148. Relationship of Sponges. — In many respects sponges are like colonial protozoans. For instance, they possess collar cells (Fig. 49) which are similar to the collared cells of the colonial flagellate protozoan, Proterospongia (Fig. 27A).' On this account the sponges were for a time classified as colonial flagellate Pro- tozoa. They differ from them, how- ever, in the fact that the body is penetrated by a system of canals, whereas in colonial Protozoa the cells are upon the surface of the mass formed by the colony. They also differ in the fact that there are a number of different types of nonre- productive, or somatic, cells which perform different functions. In this respect sponges resemble higher animals. They have, therefore, been considered for some time to be Metazoa. Sponges differ fundamentally from other Metazoa by not having any digestive cavity, which is present in some form in all higher animals except where lost from degeneration. Instead, digestion is always intracellular, or wathin the cells, as in Protozoa. Neither do they have body layers corresponding exactly to those in other Metazoa, in which ectoderm, mesoderm, and entoderm retain from the beginning the same relative position in the body. In the sponges the layer which appears in the embryo as an ectodermal layer comes in the course of development to line central cavities and to have the function of circulating water 127 Nucleus Fig. 49. — A number of collar cells, or choanocytes, from one of the flagellated chambers of a fresh-water sponge, Spon- gilla lacustris (Linnaeus). {From Borra- dailc and Potts, " The Invertebrata," after Vosmaer, by the courtesy of The Macmillan Company.) 128 METAZOAN PHYLA through the body instead of carrying the animal about. These same cells are also digestive. In all other Metazoa digestion is carried on by cells of entodermal origin. In the sponges the layer which at the begin- ning seems to be entoderm comes to lie on the surface of the body and to perform the functions which we generally associate with ectoderm. The middle layer is not differentiated in the way the mesoderm is in all triploblastic animals and so is not recognized as a germ layer. The sponges are, if anything, diploblastic, but this term is not strictly apphca- ble because of the difference in the manner of development of the two body layers. In view of these facts it seems best to include sponges in the Metazoa but to separate them as a distinct group from all the rest and to call them Parazoa — literally, animals set off at one side. The rest of the Metazoa are called Enterozoa, or animals with a digestive cavity. The group Parazoa contains but the one phylum, the Porifera. 149. Classification. — Porifera (po rif er a; L., porus, pore, and ferre, to bear) is divided into three classes : 1. Calcarea (kal ka' re a; L., calcarius, limy). — Sponges which possess spicules of carbonate of lime; all marine. 2. Hexactinellida (hex ak ti nel' li da; G., hex, six, actinos, ray, ella, Latin diminutive, and eidos, form). — Sponges with siliceous spicules having three axes; confined to the deep sea. 3. Demospongiae (de mo spiin' gi e; G., demos, the people, and spongia, sponge). — Sponges with either spicules of silica, which are not triaxial, or a supporting framework of spongin, or both; mostly marine, but with a few fresh-water species. 150. Structure. — Various types of sponges differ greatly in their general form, in their size, and in their plan of structure (Fig. 50). Some are quite regular in shape, while others are irregular, being branched, often quite complexly so, fan-shaped, or cup-shaped. Some form raised masses, and others spread out like flat discs on the surface to which they are attached. Some are very small and are just visible to the naked eye, while others may be 5 feet in height. They are often brilliantly colored, and among the different species all colors may be seen. The shape of individuals of the same species is not always the same, though in a general way it conforms to a certain type; it may be much modified by environmental factors. On the surface of the sponge are very many small openings called ostia and a much smaller number of larger ones known as oscula (Fig. 51). Water enters through the ostia and leaves through the oscula. All openings are surrounded by spicules, which appear like spines, and these may form a barrier over the ostia, protecting them from objects which might do injury to the sponge. Within the body of simple sponges is a gastral cavity which opens by an osculum. In more complex sponges there may be many such cavities, each opening by an osculum. SPONGES 129 151. Canal Systems. — In sponges there are three principal types of canal systems, known as the ascon, sycon, and rhagon types (Fig. 51), of which the ascon is the simplest. The body of a sponge of this type has a thin wall which is penetrated by simple canals that run clear through to the gastral cavity. In this type flagellated cells line this cavity. Oscula fr 'Osculum Ostium Fig. 50. — Different types of sponges. A, Grardia ciliata (Fabricius), one of the Cal- carea, a simple sponge showing colony formation and budding. X 2. B, skeleton of Euplectella sp., a hexactinellid sponge known as Venus' flower-basket, showing the form and general structure; the spicules are white and like spun glass. X ^. C, Chalina oculata Pallas, one of the marine Demospongiae. (From Minchin, in Lankester's "A Treatise on Zoology," by the courtesy of A. and C. Black.) X %. D, Ephydatia fluviatilis (Linnaeus), a fresh-water sponge belonging to Demospongiae. {From Zacharias, "Die Tier- und Pflam- enwelt des Siisswassers.") X /^. In the sycon type a more complex plan is presented, with incurrent and radial canals. The ostia lead into incurrent canals which do not open into the gastral cavity; radial canals open into the gastral cavity but not to the outside. The two types of canals lie side by side and are connected by minute pores. The radial canals are lined with the flagel- lated cells. 130 METAZOAN PHYLA In the rhagon type the animal is much larger and the whole body forms a rather thick mass penetrated by a complexly branched canal system. In the fresh- water sponges, which may be taken to represent this type, the ostia lead into subdermal cavities. From these cavities incurrent canals run to chambers lined with flagellated cells. After the water has passed these flagellated cells it is carried by excurrent canals into a gastral cavity, which opens to the outside by an osculum. 152. Skeleton. — The classification of the sponges depends upon the character of the skeleton, which may be made up of spongin or of spicules. The spicules may be either calcareous or siliceous and differ in shape in the different forms. Spongin is a substance which chemically is similar to silk and which is formed by cells known as spongoblasts. Spicule- Oscu/a/^ Oscu/um Rac//(f/ ccrncf/ chamber Incurrent \/y^ '^ Oscu/un? ■^ ^ Gcistral cav/'-f-y A B C Fig. 51. — Diagrams of canal systems of sponges. ^4, ascon type. B, sycon type. C, rhagon type. (From Wieman, ''General Zoology," A and B after Minchin, and C modified from Parker and Haswell, by the courtesy of McGraw-Hill Book Company, Inc.) The gastral epithelium is shown by heavy black, the dermal epithelium by a light line. Arrows show water currents. forming cells are sderoblasts. The spicules may be straight rods with one axis, the monaxon type; or they may have three rays in one plane and be triradiate; or four rays lying in four planes, in which case they are known as tetraxon. They may have six rays, the ends of three axes, in which case they are triaxon; or they may have numerous rays and be polyaxon (Fig. 52). Many modifications of each type occur. 153. Histology. — There are in the bodies of sponges a number of different types of cells. In the outer, or so-called dermal layer, are flat epithelial cells, contractile cells, gland cells which secrete the material that attaches the animal to its support, and the cells which form the skeleton. In the middle layer are reproductive cells and wandering cells, the latter capable of ameboid movement. The cells of the gastral layer are flat epithelial cells or collar cells. These cells, however, do not work together to the same degree as do the cells in higher animals. The whole is really a great colony of semi-independent cells, and individuality is so little evident that zoologists have not agreed upon what constitutes an individual. H. V. Wilson has found it possible, by gently squeezing sponges through the meshes of fine silk cloth, to separate them into individual cells. These cells will then gather together in small groups SPONGES 131 and each group will grow into a sponge. This illustrates the semi- independent character of the cells. 154. Metabolism. — Metabolism is carried on practically in the same manner as it is in Protozoa. That there are, however, different enzymes acting on proteins, carbohydrates, and fats seems to be generally accepted. The food of sponges consists of minute plants and animals and also small particles of organic matter which are drawn into the ostia and through the canals by currents produced by the movement of the flagella of the collar cells. As this current sweeps these objects past the collar cells they are seized upon by the cells and ingested by means of pseudo- podia. The current of water pro- ceeds onward into the gastral cavity and out of the body through the osculum. The food which has been taken by the collar cells is digested in food vacuoles in the same manner as it would be digested by proto- zoans. Further steps in metabolism \j also occur like those in protozoans. Each cell excretes and respires for a Fig. 52. — Types of spicules. {From Sollas, "Cambridge Natural History," by the courtesy of The Macmillan Com- itself. The cells which are not collar pany.) a and b, monaxon; c, triradiate; cells receive their food d, tetraxon; e, triaxon;/, polyaxon. receive tneir lood more or less directly from the latter by absorption from cell to cell, aided by the ameboid wandering cells, which serve to carry both food and waste matter about the body. 155. Behavior. — Little is known of behavior in sponges generally. The larvae are ciliated and swim about, but the adults are attached and never move from their position. Some sponges possess fiber-like cells around the ostia and oscula which are capable of slowly contracting and closing these openings or of relaxing and permitting them to open. The opening and closing are so gradual, however, that they do not attract notice unless particular attention is given to them. These openings tend to open when the water is in motion but close when the water becomes quiet; they also open in fresh water and in weak solutions of atropin and close on exposure to the air or on injury to the animal or when the animal is subject to the experimental action of weak solutions of ether and cocaine. These fiber-like cells, since they have the function of both receiving stimuli and contracting in response to them, are termed neuro- muscuLar cells. Because groups of these cells surround openings which are closed by their contraction, the group is termed a sphincter. (This term is also applied to all muscles closing openings in the bodies of higher 132 METAZOAN PHYLA animals and man, such as the muscle which shuts off the stomach from the intestine, that which guards the exit from the bladder, and that which controls the passage of egested matter from the posterior end of the alimentary canal.) 156. Reproduction. — Reproduction is both sexual and asexual. The asexual mode of reproduction involves the gradual formation of buds which arise near the point of attachment of the parent. After growing for a time thus attached a bud may separate and begin an individual existence. If budding continues and the individuals remain together, a colony is produced (Fig. 50 ^). In addition to budding, some sponges have the ability to form gemmules. These are groups of cells which gather together in the middle layer and become surrounded by a siliceous shell. They are formed when living conditions become difficult and thus preserve the life of the organ- ism during such periods. In the fresh-water sponges gemmules are formed in the autumn, after which the adults die, and in the spring the gemmules develop into new sponges (Fig. 53). ^^i^fS^^^y:^ Sexual reproduction also occurs, both egg cells ^!^^'^<>:^: M -ir::^^ and sperm cells being formed in the same animal. 'Hi^^^^''^'^ki^^Mk '^^^^^ ^^^ ^^^^^ ^^^ ^"^ ^^^ jelly-like middle layer 'Wi^-^W-^^^^ where fertilization takes place. An embryo is i^:ty