<=0 O: 1 I! r- i 3- : r-R \° • i r=\ i a : m I a S a MANUAL OF ZOOLOGY BY RICHARD HERTWIG PROFESSOR OF ZOOLOGY IN THE UNIVERSITY AT MUNICH THIRD AMERICAN FROM THE NINTH GERMAN EDITION TRANSLATED AND EDITED BY J. S. KINGSLEY PROFESSOR OF ZOOLOGY IN TUFTS COLLEGE NEW YORK HENRY HOLT AND COMPANY 1912 COPYRIGHT, 1902 BY HENRY HOLT AND CO. COPYRIGHT, 1912 BY HENRY HOLT AND COMPANY THE- MAPLE . PRESS. YORK. PA PREFACE TO THE THIRD EDITION. THE favor with which the first and second American editions of Hertwig's Zoology have been received has led to a thorough revision of the whole with a close comparison with the latest German edition. In this there have been introduced many new features bringing the work up to date. These include a discussion of Mendelian inherit- ance, many modifications in the account of the theory of evolution, and a considerable enlargement of the Protozoa and especially of the pathogenic forms, making the volume of more value to the student of medicine. To have included these without changes elsewhere would have resulted in a much larger volume. But the demand in American colleges has been for a smaller work and so a reduction has been made in two ways. There has been a condensation by the elimination of unnecessary words and phrases and by the omission of considerable matter of minor importance. Then there has been the recognition of the fact that the book has two uses, one in the class room the other as a reference work. The two classes of matter have been distinguished by differences of type. No attempt has been made to bring the systematic names into accord with the latest vagaries of the systematists. No useful and could be served by changing or transferring the well-known names of Echidna, Coluber, Amia, Homarus, Unio, Holothuria, Amoeba, etc., while the confusion this would introduce would be enormous. It should be understood that while the revision is based upon the German edition of Professor Hertwig, he should not be held responsible for any changes introduced. The whole responsibility for these rests upon the American reviser. J. S. K. TUFTS COLLEGE, MASS., June, 1912. 111 PREFACE TO THE FIRST EDITIO ON account of its clearness and breadth of view, its comparatively simple character and moderate size, Professor Richard Hertwig's 'Lehrbuch der Zoologie' has for ten years held the foremost place in German schools. The first or general part of the work was trans- lated in 1896 by Dr. George W. Field, and the cordial reception which this has had in America has led to the present reproduction of the whole. This American edition is not an exact translation. With the consent of the author the whole text has been edited and modified in places to accord with American usage. For these changes the translator alone can be held responsible. Some of the alterations are slight, but others are very considerable. Thus the group of Vermes of the original has been broken up and its members distributed among several phyla; the account of the Arthropoda has been largely rewritten and the classification materially altered; while the Tunicata and the Enteropneusti have been removed from their position as appendices to the Vermes and united with the Vertebrata to form the phylum Chordata. Other changes, like those in the classification of the Reptilia and the nephridial system of the vertebrates, are of less importance. A large number of illustrations have been added, either to make clearer points of structure or to aid in the identification of American forms. Except in the Protozoa, American genera have in most cases been indicated by an asterisk. Numerous genera have been mentioned so that the student may see the relationships of forms described in morphological literature. In the translation the word Anlage, meaning the embryonic material from which an organ or a part is developed, has been transferred directly. As our language is Germanic in its genius, there can be no valid objection to the adoption of the word. As this work is intended for beginners, no bibliography has been given. A list of literature to be of much value would have been so large as materially to increase the size of the volume. Experience has shown that beginners rarely go to the original sources. This omission it the less important since in all schools where the book is likely to be used other works containing good bibliographies are accessible. Refer- vi PREFACE TO THE FIRST EDITION ence might here be made to those in the Anatomies of Lang and Wieder- sheim, the Embryologies of Balfour, Korschelt and Heider, Minot, and Hertwig, and Wilson's work on The Cell. The editor must here return his thanks to Dr. George W. Field for his kindness in allowing the use of his translation of the first part of the book as the basis of the present edition. J. S. KlNGSLEY. TUFTS COLLEGE, MASS., Sept. 19, 1902 TABLE OF CONTENTS. PAGE INTRODUCTION... i HISTORY OF ZOOLOGY 5 DEVELOPMENT OF SYSTEMATIC ZOOLOGY . . 6 DEVELOPMENT OF MORPHOLOGY 9 HISTORY OF THE THEORY OF EVOLUTION 14 THEORY OF THE ORIGIN OF SPECIES .... 18 GENERAL MORPHOLOGY AND PHYSIOLOGY. .. 50 GENERAL ANATOMY 50 The Morphological Units of the Animal Body 51 The Tissues of the Animal Body 63 Epithelial Tissues 64 Connective Tissues 74 Muscular Tissues 80 Nervous Tissues 83 Summary 87 The Combination of Tissues into Organs Vegetative Organs 91 Organs of Assimilation 91 Digestive Tract 93 Respiratory Organs 96 Circulatory Apparatus 99 Excretory Organs 104 Sexual Organs 107 Animal Organs no Organs of Locomotion no Nervous System 111 Sense Organs 114 Summary . . 122 Promorphology 123 GENERAL EMBRYOLOGY.... 1-37 Spontaneous Generation 127 Generation by Parents 128 Asexual Reproduction 128 Sexual Reproduction 129 Combined Methods of Reproduction 13° General Phenomena of Sexual Reproduction. . . 132 Maturation of the Egg ... 132 Fertilization ' 134 Cleavage Processes Heredity Mendel's Law Formation of the Germ Layers 145 Different Forms of Sexual Development 149 Summary 1 51 vii viii TABLE OF CONTENTS PAGE RELATION OF ANIMALS TO ONE ANOTHER 153 Relations between Individuals of the Same Species i ^3 Relations between Individuals of Different Species 156 ANIMAL AND PLANT 159 GEOGRAPHICAL DISTRIBUTION OF ANIMALS... 160 DISTRIBUTION OF ANIMALS IN TIME 164 SPECIAL ZOOLOGY 166 Phylum I. PROTOZOA .... 166 Class I. Rhizopoda J.-JQ Order I. Monera .... 172 Order II. Lobosa 172 Order III. Heliozoa 173 Order IV. Radiolaria .... 174 Order V. Foraminifera .178 Order VI. Mycetozoa . . iSo Class II. Flagellata . . . . 181 Order I. Autoflagellata 181 Order II. Dinoflagellata . . 184 Order III. Cystoflagellata ... 184 Class III. Sporozoa 185 Order I. Gregarinida .... 186 Order II. Coccidise . . . 188 Order III. Haemosporida .... 188 Order IV. Myxosporida . . 190 Order V. Sarcosporida . . 191 Class IV. Ciliata 191 Order I. Holotricha 195 Order II. Heterotricha 196 Order III. Peritricha 196 Order IV. Hypotricha 197 Order V. Suctoria 197 SUMMARY .... 198 METAZOA 201 Phylum II. PORIFERA 201 SUMMARY 206 Phylum III . CCELENTERATA 206 Class I. Hydrozoa 208 Order I. Hydraria 217 Order II. Hydrocorallinse 217 Order III. Tubulariae = Anthomedusae 217 Order IV. Campanulariae = Lepto medusas 217 Order V. Trachymedusse 218 Order VI. Narcomedusae 218 Order VII. Siphonophora 218 Class II. Scyphozoa 220 Order I. Discomedusae 223 Order II. Stauromedusae 224 Order III. Peromedusae 224 Order IV. Cubomedusae 224 TABLE OF CONTENTS ix PAGE Class III. Anthozoa ---- 224 Order I. Tetracoralla ......... 230 Order II. Octocoralla . 230 Order III. Hexacoralla. . 230 Class IV. Ctenophora. 232 SUMMARY .................. 235 VERMES .................. -237 Phylum IV. PLATHELMINTHES . . . 238 Class I. Turbellaria ............. . 240 Class II. Trematoda .......... 242 Order I. Polystomiae ...... 244 Order II. Distorrme ....... - 245 Class III. Cestoda ........ .247 Class IV. Nemertini ............. • 255 SUMMARY .................... -258 Phylum V. ROTIFERA .................. • 259 Phvllim VI. CCELHELMINTHES ............ ................................ 260 Class I. Chastognalhi .......... ................. 262 Class II. Nemathelminthes ........................... . 263 Order I. Nematoda ................... .263 Order II. Gordiacea ............... . 268 Order III. Acanthocephala ......... . 268 Class III. Annelida ......... .269 Sub Class I. Chaetopoda. . . .270 Order I. Polychastae .......... .276 Order II. OligochaetEe ..... ........ 278 Sub Class II. Gephyraa. . . .279 Order I. Chaniferi ....... . 281 Order II. Inermes ....... ........ 281 Order III. Priapuloidea. . .... - 281 Sub Class III. Hirudinei ..... .281 Class IV. Polyzoa ............... .284 Class V. Phoronida ................ - 287 Class VI. Brachiopoda .............. . 287 SUMMARY ........................ • 29° Phylum VII. ECHINODERMA ...................... • 291 Class I. Asteroidea ........... 295 Class II. Ophiuroidea ................. • 298 Class III. Crinoidea ............ .299 Sub Class I. Eucrinoidea ................ • 3°2 Sub Class II. Edrioasteroidea .......... . 3°2 Sub Class III. Cystidea ............. 3°3 Sub Class IV. Blastoidea .......... • 3°3 Class IV. Echinoidea ................ • 3°3 Class V. Holothuroidea .......... • 3°6 SUMMARY ................................ • Phylum. VIII. MOLLUSCA ................. • 3IQ Class I. Amphineura .................. • 3T5 Class II. Acephala ................... 3J7 Order I. Protoconchise .......................... • 323 Order II. Heteroconchia? ........................................ 324 x TABLE OF CONTENTS PAGE Class III. Scaphopoda 325 Class IV. Gasteropoda 325 Order I. Prosobranchiata 333 Order II. Opisthobranchiata 334 Order III. Pulmonata 335 Class V. Cephalopoda - 336 Order I. Tetrabranchia 346 Order II. Dibranchia 347 SUMMARY 347 Phylum IX. ARTHROPODA 349 Class I. Crustacea 359 Sub Class I. Trilobitas 363 Sub Class II. Phyllopoda 365 Order I. Branchiopoda 366 Order II. Cladocera 366 Sub Class III. Copepoda 366 Order I. Eucopepoda 369 Order II. Siphonostomata 369 Sub Class IV. Ostracoda 371 Sub Class V. Cirripedia 371 Sub Class VI. Malacostraca 374 Legion I. Leptostraca 375 Legion II. Thoracostraca 375 Order I. Schizopoda 375 Order II. Stomatopoda 376 Order III. Decapoda 377 Order IV. Cumacea 382 Order V. Syncarida • 382 Legion III. Arthrocostraca 383 Order I. Amphipoda 383 Order II. Isopoda 385 Class II. Acerata 387 Sub Class I. Gigantostraca 388 Order I. Xiphosura 388 Order II. Eurypterida 389 Sub Class II. Arachnida 389 Legion I. Arthrogastrida 391 Order I. Scorpionida 391 Order II. Phrynoidea 392 Order III. Microthelyphonida 393 Order IV. • Solpugida 393 Order V. Pseudoscorpii 394 Order VI. Phalangida 394 Legion II. Sphuaerogastrida * 395 Order I. Araneina 395 Order II. Acarina 396 Order III. Linguatulida 397 Tardigrada 398 Pycnogonida 399 Class III. Malacopoda 309 Class IV. Insecta JQO Sub Class I. Chilopoda 402 TABLE OF CONTENTS xi PAGE Sub Class II. Hexapoda 403 Order I. Apterygota . 418 Order II. Archiptera . . 419 Order III. Orthoptera .421 Order IV. Neuroptera . 42 1 Order V. Strepsiptera .... 422 Order VI. Coleoptera 423 Order VII. Hymenoptera . . 424 Order VIII. Rhynchota .. 427 Order IX. Diptera . . 430 Order X. Aphaniptera . . 431 Order XI. Lepidoptera . . 432 Class V. Diplopoda 433 SUMMARY . . 434 Phylum X. CHORDATA 438 Sub Phylum I. Leptocardii 439 Sub Phylum II. Tunicata 441 Order I. Copelata; . . 443 Order II. Tethyoidea 443 Order III. Thaliacea 447 Sub Phylum III. Enteropneusta 448 Sub Phylum IV. Vertebrata 449 Series I. Ichthyopsida 491 Class I. Cyclostomata 491 Sub Class I. Myzonles 493 Sub Class II. Petromyzonlei 493 Class II. Pisces 493 Sub Class I. Elasmobranchii 504 Order I. Selachii 506 Order II. Holocephali 507 Sub Class II. Ganoidei 507 Sub Class III. Teleostei 508 Order I. Physostomi 510 Order II. Pharyngognalhi 510 Order III. Acanthopteri 511 Order IV. Anacanthini 511 Order V. Lophobranchii 511 Order VI. Plectognathi 512 Sub Class IV. Dipnoi 512 Class III. Amphibia 513 Order I. Stegocephali 519 Order II. Gymnophiona 519 Order III. Urodela 519 Order IV. Anura 520 Series II. Amniota 520 Class I. Reptilia x 521 Order I. Theromorpha 526 Order II. Plesiosauria 526 Order III. Ichthyosauria 526 Order IV. Chelonia 526 Order V. Rhynchocephalia 527 Order VI. Dinosauria 528 Order VII. Squamata 529 xii TABLE OF CONTENTS PAGE Order VIII. Crocodilia. . 531 Order IX. Pterodactylia . . ... 532 Class II. Aves Order I. Saururae... ... Order II. Odontornithes . . 541 Order III. Ratitse. . . 54! Order IV. Carinatce 542 Class III. Mammalia 545 Sub Class I. Monotremata 558 SubC lass II. Marsupialia -559 Sub Class III. Placentalia. . . 561 Order I. Edentata . Order II. Insectivora. ... Order III. Chiroptera . . Order I\". Carnivora. ... ... 565 Order V. Rodentia 566 Order VI. Ungulata . . Order VII. Proboscidia Otder VIII. Hyracoidea 57° Order IX. Sirenia 57° Order X. Cetacea ^-i Order XI. Prosimiie ^72 Order XII. Primates.. .. 573 SUMMARY 575 GENERAL PRINCIPLES OF ZOOLOGY. INTRODUCTION. Man's Relation to Other Animals. — The observant man sees him- self in the midst of a manifold variety of organisms, which in their struc- ture, and even more in their vital phenomena, exhibit a similarity to his own being. This similarity, with many of the mammals, especially the anthropoid apes, has the sharpness of a caricature. In the inverte- brate animals it is softened; yet even in the lowest organisms it is still to be found: although here the vital processes which have reached such complexity and perfection in ourselves can only be recognized in their simplest outlines. Man is part of a great whole, the Animal Kingdom, one form among the many thousand forms in which animal organiza- tion has found expression. Purpose of Zoological Study. — If we would, therefore, fully under- stand the structure of man, we must, as it were, look at it upon the back- ground which is formed by the other animals, and for this purpose we must investigate their conditions. Apart from its relations to man, zoology has to explain the organization of animals and their relations to one another. This is a rich field for scientific activity; its enormous range is a consequence, on the one hand, of the well-nigh exhaustless variety of animal organization, and, on the other, of the different points of view from which the zoologist attacks his problem. In the first half of the last century the conception was prevalent that the aim of zoology is to furnish every animal with a name, to characterize it according to some easily recognizable features, and to classify it in a way to facilitate quick identification. By Natural History was under- stood the classification of animals or systematic zoology; that is to say, only one part of zoology, which can pretend to scientific value only when it is brought into relation with other problems (geographical distribution, variation, evolution). This conception has become more and more subordinated. The ambition to describe the largest possible number of new forms belongs to the past. In fact there is a tendency to undue neglect of classification. Morphology and Physiology to-day dominate the sphere of the zoologist's work. I 2 GENERAL PRINCIPLES OF ZOOLOGY Morphology, or the study of form, has first to describe all which can be seen externally, as size, color, proportion of parts. But since the external appearance cannot be understood without knowledge of the internal organs which condition the external form, the morphologist must make use of dissection, of Anatomy, and describe their forms and methods of combination. In his investigation he only stops when he has arrived at the morphological elements of the animal body, the cells. Therefore we cannot contrast Morphology and Anatomy, and ascribe to the former the description of only the external, and to the latter of only the internal parts; the kind of knowledge and the mental processes are the same in both cases. The distinction, too, is unnatural, since in many instances organs which usually lie in the interior of the body, belong in other cases to the surface of the body, and are accessible for direct observation. Comparative Anatomy. — For morphology, as for every science, the mere accumulation of facts is not sufficient to give the subject the character of a science; an additional mental elaboration of this material is necessary. Such a result is reached by comparison. The morphologist compares animals with each other according to their structure, in order to ascertain what parts of the organization recur everywhere, what only within narrow limits. He thus gains a double advantage: (i) an insight into the relationships of animals, and hence the foundation for a Natural System; (2) the evidence of the laws which govern organisms. Any organism is not a structure which has arisen independently and which is hence intelligible by itself: it stands in relation to the other members of the animal kingdom. We can only understand its structure when we compare it with the closely and the more distantly related animals, as when we compare man with the other vertebrates and with many lower invertebrates. We have to consider one of the most mysterious phenom- ena of the organic world, the path to the full explanation of which was first outlined by the Theory of Evolution, as will be shown in another chapter. Ontogeny. — To morphology belongs, as an important integral part, Ontogeny or Embryology. Only a few animals are completely formed in all their parts at the beginning of their individual existence; most of them arise from the egg, a relatively simple body, and then step by step attain their permanent form by complicated changes. The morphologist must determine by observation the different stages, compare them with the mature animals, and with the structure and developmental stages of other animals. Then there is revealed to him the same conformity to law which dominates the mature animals, and a knowledge of this conformity INTRODUCTION 3 is of fundamental importance as well for classification as for the causal explanation of the animal form. The developmental stages of man show definite regular agreements, not only with the structure of the adult human being, which in and of itself would be intelligible, but also with the struc- ture of lower vertebrates, and even with many of the still lower inverte- brate groups. Physiology. — In the same way as the morphologist studies the structure, the physiologist studies the vital phenomena of animals and the functions of their organs. Formerly life was regarded as the expression of a special vital force peculiar to organisms, and any attempt at a logical explanation of the vital processes was thereby renounced. Most modern physiologists have abandoned this theory of a vital force; they have attempted to explain life as the summation of extremely complicated chemico-physical processes, and thus to apply to the organic world those principles which prevail in the inorganic realm. Developmental Physiology ("Entwicklungsmechanik"). — Since each organism is the product of its development; since, further, the development represents the summation of most complicated vital processes, the explanation of the organic bodily form is, therefore, in ultimate anal- ysis a physiological problem; a problem whose solution lies still in the in- definitely distant future. It has to explain how the apparently simple fertilized egg is converted into the complicated adult organism with its many organs regularly arranged. The potentiality of the complexity of the adult must be contained in the egg. But it is still a matter of dispute as to how this potentiality is conditioned: whether as a mosaic of minute particles, each corresponding to a peculiarity of the adult organism, or as a substance of simpler structure, in which complexity only appears in the course of development. We can proceed experimentally in such a way that the conditions of development are artifically altered, and the results may be compared with the normal processes. It is also possible to study the modifications which one and the same developmental process under- goes in different species, modifications which are dependent upon the life conditions of the animals and of their young. Then, too, there are ex- periments of the same kind performed by nature and which have the same informing value as the artificially arranged conditions. Such researches have accomplished much in the last decade and have resulted in a deeper understanding of the developmental processes. The potentialities contained in the fertilized egg are the hereditary elements which are transmitted from the parent to the next generation and which result in the resemblance of the offspring to the parents. The study of these elements and the way in which they are transmitted from 4 GENERAL PRINCIPLES OF ZOOLOGY one generation to another — in other words, the laws of heredity — has recently made a great advance by investigations in two directions: (i) through the biometric method or the statistics of variation, and (2) by the so-called 'Mendelian analysis' of the hereditary potentialities. Both lines of investigation (to be considered more in detail later) have opened up in an unexpected way the possibility of submitting to exact research the questions of variation and heredity, fundamentally important for the understanding of the living world. Biology. — According as the relations of each organism to the external world are brought about through its vital phenomena, there belongs to physiology, or at least is connected with it, the study of the conditions of animal existence, (Ecology, often called Biology in the narrower sense, the broader meaning being the science of all living things, both animals and plants. This branch has of late attained considerable importance. How animals- are distributed over the globe, how climate and condi- tions influence their distribution, how by known factors the structure and the mode of life become changed, are questions which are to-day discussed more than ever before. Paleontology. — Finally to the realm of zoology belongs also Paleozo- ology or Paleontology, the study of the extinct animals. For between the extinct and the living animals there exists a genetic connection: the former are the precursors of the latter, and their fossil remains are the most trust- worthy records of the history of the race, or Phylogeny. HISTORY OF ZOOLOGY. Methods of Zoological Study. — In the history of zoology we can distinguish two great currents, which have been united in a few men, but which on the whole have developed independently, nay, more often in pronounced opposition to each other; these are on the one side the system- atic, on the other the morphologico-physiological mode of studying animals. In this brief historical summary they will be kept distinct from one another, although in the commencement of zoological investigation there was no opposition between the two points of view, and even later this has in many instances disappeared. Aristotle, the great Greek philosopher, has been called the Father of Natural History. Equipped with the literary aid of an extensive library and pecuniary means, he pursued the inductive method, the only one capable of furnishing secure foundations in the realm of natural science. There have been preserved only parts of his three most important zoological works, "Historia animalium," "De partibus," and "De generatione," works in which zoology is founded as a universal science, since anatomy and embryology, physiology and classification find equal consideration. How far Aristotle, notwithstanding many errors, had a correct knowledge of the structure and embryology of animals, is shown by the fact that many of his discoveries have been confirmed only within a cenlury. Thus Aristotle knew, though only lately rediscovered by Johannes Mtiller, that many sharks are not only viviparous, but that also the embryo becomes fixed to the maternal uterus and there is formed a contrivance for its nutrition resembling the mammalian placenta; he knew the diii'erence between male and female cephalopods, and that the young cuttlefish has a preoral yolk-sac. The position which Aristotle took in reference to the classification of animals is of great interest; he mentions in his writings about live hundred species. Since he does not mention very well-known forms, like the badger, dragon-fly, etc., we can assume that he knew many more, but did not regard it necessary to give a catalogue of all the forms known to him, and that he mentioned them only if it was necessary to refer to certain physiological or morphological conditions found in them. 5 6 GENERAL PRINCIPLES OF ZOOLOGY This neglect of the systematic side is further shown in the fact that the great philosopher is satisfied with two systematic categories, with etSos, species or kind, and yeVos or group. His eight yevrj //.eyio-ra would about correspond with the Classes of modern zoology; they have been the start- ing-point for all later attempts at classification, and may therefore be enumerated here: i. Mammals (£OJOTOKOVVTO. ev auroTs) . 2. Birds (opvi0es). 3. Oviparous quadrupeds (rerpaTroSa woroKoiWa) . 4. Fishes (t^ves) . ^. Molluscs (/xaAaKia). 6. Crustaceans (/mAa/co'crT-para) . 7. Insects (evro/Aa). 8. Animals with shells (oo-TpaKoSep/iaTo). Aristotle also noticed the close connection of the first four groups, since he, without actually carrying out the division, contrasted the animals with blood, erat^o. (better, red blood), with the bloodless, avai/u.a (better, colorless blood or no blood at all). DEVELOPEMENT OF SYSTEMATIC ZOOLOGY. Pliny. — It is a remarkable fact that after Aristotle, an exclusively systematic direction should have been taken. This is explicable only when we consider that the continuity of investigation was interrupted by the decline and ultimate complete collapse of ancient civilization, and by the triumphant advance of Christianity. The decay is seen in the writings of Pliny. Although this Roman scholar was long lauded as the foremost zoologist of antiquity, he is now given the place of a not even fortunate compiler, who collected from the writings of others the accurate and the fabulous indiscriminately, and replaced the natural classification according to structure by the unnatural division according to the place of abode (flying animals, land animals, water animals). Zoology of the Middle Ages. — The rise of Christianity resulted in the complete annihilation of science and investigation. Then came a time when answers to questions capable of solution by the simplest obser- vation were sought by rummaging of the works of standard authors. How many teeth the horse has, was debated in many polemics, which would have led to bloodshed if one of the authors had not thought to look into a horse's mouth. Significant of this mental bias which prevailed throughout the entire Middle Ages is the 'Physiologus' or 'Bestiarius,' from which the zoological authors of the Middle Ages drew much material. The book in its various editions names about seventy animals, among them many creatures of -fable: the dragon, the unicorn, the phoenix, etc. Most of the accounts given of various animals are fables intended to illustrate religious or ethical teachings. There are indeed, exceptions to this general characteristic of HISTORY OF ZOOLOGY 7 the Middle Ages, notably in the writings of the Dominican, Albertus Magnus and the Augustinian, Thomas Cantipratensis. In so far as he had opportunity, Albertus Magnus, endeavored to support his statements by personal observation. But that this beginning of the scientific method raised hardly an echo only emphasizes the general spirit of the time. At the close of the Middle Ages, when the interest in science awoke anew, Aristotle's conceptions were taken up and elaborated by the Eng- lishman Wotton. In 1552 he published his work "De differentiis animal- ium," in which he essentially copied the system of Aristotle, except that he admitted the new group of plant-animals or zoophytes. However, the title, 'On the Distinguishing Characters of Animals,' shows that of Aristotelian knowledge the systematic side obtained the chief recognition, and thus Wotton's work inaugurated the period of systematic zoology, which in Ray, but even more in Linnasus, found its most brilliant exponents. Linnaeus, the son of a Swedish clergman, was born in Rashult in 1707. Pronounced by his teachers to be good for nothing at study, he was saved from learning the cobbler's trade through the influence of a physician, who recognized his fine abilities and turned him to medical studies. He studied at Lund and LTpsala; at the age of twenty-eight he made ex- tended tours on the Continent, and at that time gained recognition from the foremost men in his profession. In 1741 he became professor of medicine in Upsala, some years later professor of natural history. He died in 1778. Improvement of Zoological Nomenclature by Linnaeus. — Linnaeus's most important work is his "Systema Naturae," which, first appearing in 1735, up to 1766-68 passed through twelve editions. This has become the foundation for systematic zoology, since it introduces for the first time (i) sharper divisions, (2) a definite scientific terminology, the binomial nomenclature, and (3) brief, comprehensive, clear diagnoses. Linnaeus divided the entire Animal Kindom into Classes, the Classes into Orders; these into Genera, the Genera into Species. The term Family was not employed. Still more important was the binomial nomenclature. Hitherto the common names were in use and led to much confusion; the same animals had different names, and different animals had the same names; in the naming of newly discovered animals there was no generally accepted principle. This inconvenience was entirely obviated by Linnaeus in the tenth edition of his Systema by the introduction of a scientific nomenclature. The first word, a noun, designates the genus to which the animal belongs, the following word, usually an adjectve, the species within the genus. The names Canis famttiaris, Canis lupus, Canis vulpes, indicate that the dog, 8 GENERAL PRINCIPLES OF ZOOLOGY wolf, and fox are related to one another, since they belong to the same genus, the genus of doglike animals, of which they are different species. Linnaeus's method was particularly valuable in the description of new species, inasmuch as it at the ouset informed the reader of the relationships of the new species. In his characterization of the various groups Linnaeus broke with the prevailing custom. His predecessors (as Gessner, Aldrovandus) had given a verbose and detailed description of each animal, from which the beginner was scarcely able to see what was specially characteristic for that animal, a matter which should have been emphasized in the definition. Linmeus, on the other hand, introduced brief diagnoses, which in a few words, never in sentence form, gave only what was necessary for recog- nition, a matter of great importance, in view of the enormously increasing number of known animals. Influence -of the Linnean System. — But in the great superiority of the Linnean System lay at the same time the germ of the one-sided development which zoology came to take. The perfecting of the system, which undoubtedly had become necessary, gave that a brilliant aspect, and hid the fact that classification is not the ultimate purpose of investi- gation, but only an important and indispensable aid to it. In the zeal for naming and classifying animals, the higher goal, knowledge of the nature of animals, was lost sight of, and the interest in anatomy, physiology, and embryology flagged. From these reproaches we can scarcely spare Linnaeus himself, for while in his "Systema Natura?" he treated of a much larger number of animals than any earlier zoologist, he brought about no deepening of our knowledge.' The manner in which he divided the animal kingdom is rather a retrogression than an advance. He recognized six classes: Mammalia, Aves, Amphibia, Pisces, Insecta, Vermes. The first four classes correspond to Aristotle's four groups of animals with blood. In the division of the invertebrated animals into Insecta and Vermes Linmcus stands undoubtedly below Aristotle, who set up a larger number of groups. But in his successors, we see the damage wrought by the systematic method. The diagnoses of Linnaeus were for the most part models, which, mutatis mutandis, could be employed for new species with little trouble. There was needed only some exchanging of adjectives to express the differences. With the hundreds of thousands of different species of animals there \vas no lack of material, and so the way was opened for that spiritless species-making which in the first half of the last century brought zoology into such discredit. HISTORY OF ZOOLOGY 9 DEVELOPMENT OF MORPHOLOGY. Anatomists of Classic Antiquity. — Comparative anatomy — for this chiefly concerns us here — for a long time owed its development to the students of human anatomy. The disciples of Hippocrates studied animal anatomy for the purpose of obtaining an idea of human organiza- tion from the structure of other mammals. The work of classical antiquity most prominent in this respect, the Human Anatomy of Galen (131-201 A.D.), is based chiefly upon observations upon dogs, monkeys, etc.; for in ancient times, and even in the Middle Ages, there was repugnance to making the human cadaver a subject of study. Middle Ages. — The first thousand years, in which Christianity ruled the mental life of the people, held to the writings of Galen and the works of his commentators, and seldom took occasion to prove their correctness by observations. With the ending of the Middle Ages the interest in scientific research first made its way. Vesal (1514-1564), the creator of modern anatomy, investigated the human cadaver and pointed out numerous errors in Galen's writings which had arisen through the extension to human anatomy of the dis- coveries made upon other animals. By his corrections of Galen, Yesal was drawn into controversy with his teacher, Sylvius, and with his renowned contemporary Eustachius, which did much for the development of comparative anatomy. At first animals were dissected only for the purpose of disclosing the cause of Galen's mistakes, but later through a «zeal for facts. It was natural that vertebrates were first studied, since they stand next to man in structure. Thus there appeared in the same century with Yesal drawings of skeletons by Coiter; the zootomical writings of Fabricius ab Aquapendente, etc. Beginning of Zootomy. — But later attention was turned to insects and molluscs, even to the echinoderms, ccelenterates, and Protozoa. Here three men who lived at the end of the seventeenth century deserve mention, the Italian Malpighi and the Dutchmen Swammerdam and Leeuwenhoek. The former's "Dissertatio de bombyce" was the pioneer for insect anatomy, since by the discovery of the vasa Malpighii, the heart, the nervous system, the trachea?, etc., an extraordinary extension of our knowledge was brought about. Of Swammerdam's writings attention should be called to "The Bible of Nature," a work to which no other of that time is comparable, since it contains discoveries of great accuracy on the structure of bees, Mayflies, snails, etc. Leeuwenhoek, finally, was most fortunate in the field of microscopic research. Besides other things he studied especially the minute inhabitants of the fresh waters, the 'infusion-animalcules.' 10 GENERAL PRINICPLES OF ZOOLOGY The great service of the men named above consists chiefly in that they broke away from the thraldom of book-learning and, relying alone upon their own eyes and their own judgment, regained the blessing of inde- pendent and unbiased observation. They spread the interest in obser- vation of nature so that in the eighteenth century the number of natural- history writings had increased enormously. There were busy with the study of insect structure and development, de Geer in Sweden, Reaumur in France, Lyonet in Belgium, Rosel von Rosenhof in Germany; the latter besides wrote a monograph on the indigenous batrachia, which is still worth reading. The investigation of the infusoria formed a favorite occupation for Wrisberg, von Gleichen-Russwurm, Schiiffer, Eichhorn, and O. F. Miiller. As a criterion of the progress made, a mere glance at the illustrations is sufficient. Any one will at a glance recognize the difference between the shabby drawings of an Aldrovandus and the masterly figures of a Lyonet or a Rosel von Rr.senhof. Peroid oif Comparative Anatomy. — Thus through the zeal of numerous men a store of anatomical facts was collected, which needed only a mental reworking; and this was brought about, or at least entered upon, by the great comparative anatomists who lived at the end of the eighteenth and the beginning of the nineteenth century. Among these the French zoologists Lamarck, Savigny, Geoffrey St. Hilaire, Cuvier, and the Germans Meckel and Goethe are especially to be named. Correlation of Parts. — When the various animals were compared with one another with reference to their structure there was obtained a series of fundamental laws, particularly the law of the Correlation of Parts and the law of the Homology of Organs. The former showed that there exists a dependent relation between the organs of the same animal ; that local changes of one organ lead to corresponding changes at some distant part of the body, and that therefore from the structure of certain parts an inference can be drawn as to the constitution of another part of the body. Cuvier particularly made use of this principle in reconstructing extinct animals. Homology and Analogy. — Still more important was the idea of the Homology of Organs. In the organs of animals a distinction was drawn between an anatomical and a physiological character; the anatomical character is the sum of form, structure, position, and mode of connection of organs; the physiological character is their function. Anatomically similar organs in closely related animals will usually have the same functions, as, for example, the liver of all vertebrates produces gall; here anatomical and physiological characteristics go hand in hand. But this need not be the case; very often it may happen that the same function HISTORY OF ZOOLOGY 11 is possessed by organs anatomically different; as, for example, the res- piration is carried on in fishes by gills, in mammals by lungs. Con- versely, anatomically similar organs may have different functions, as the lungs of mammals and the swim-bladder of fishes; similar organs may also undergo a change of function from one group to another; the hydrostatic apparatus of fishes has come to be the seat of respiration in the mammals. Organs with like functions — physiologically equivalent organs — are called 'analogous'; organs of like anatomical constitution — anatomically equivalent organs — are called 'homologous.' It is the task of comparative anatomy to discover in the various parts of animals those which are homologous, and to follow the changes in them conditioned by a change of function. Cuvier. — The foremost representative of comparative anatomy was Georges Dagobert Cuvier. His investigations extended to* the coelenter- ates, molluscs, arthropods, and vertebrates, living and fossil. He collected his extensive observations into his two chief works "Le regne animal distribue d'apres son organization" and "Lefons d'anatomie comparee." Of epoch-making importance was his little pamphlet "Sur un rapprochement a etablir entre les differentes classes des animaux," in which he founded his celebrated type theory, and which introduced a reform of classification. The Cuvierian division, the starting-point for all later classifications, differed from all the earlier systems in that the classes of mammals, birds, reptiles, and fishes were united in a higher grade under the name, introduced by Lamarck, of 'vertebrate animals'; and the so-called 'invertebrate animals' were divided into three similar grades, each equal to that of the vertebrate animals, viz., Mollusca, Articulata, and Radiata. Cuvier called these grades standing above the classes, provinces or chief branches (embranchements). But still more important are the differences which appear in the structural basis of the system. Instead of, like the earlier systematists, using a few ex- ternal characteristics for the division, Cuvier built upon the totality of internal organization, as expressed in the relative positions of the most important organs, especially the position of the nervous system, as determining the arrangement of the other organs. Thus for the first time comparative anatomy was employed in the formation of a natural system of animals. Cuvier found prevalent the theory that all animals formed a single connected series ascending from the lowest infusorian to man ; within this series the position of each animal was determined by the degree of its organization. On the other hand Cuvier taught that the animal kingdom consisted of several co-ordinated unities, the types, which exist inde- 12 GENERAL PRINCIPLES OF ZOOLOGY pcndently side by side, within which again there are higher and lower forms. The position of an animal is determined by two factors: first, by its conformity to a type, by the structural plan which it represents; second, by its degree of organization, by the stage to which it attains within its type. Comparative Embryology. — Evolution vs. Epigenesis. — The same results which Cuvier reached by the way of comparative anatomy were attained two decades later by C. E. von Baer by the aid of embry- ology. Embryology is the youngest branch of zoology. The difficulties of observation, due to the delicacy and the minuteness of the develop- mental stages, were lessened by the invention of the microscope and microscopical technique. Further, there was no idea of developmental history in the present sense of the word ; each organism was thought to be laid down from the first complete in all its parts, and only needed growth to unfold its organs (evolutid) ; either the spermatozoon must be the young creature which found favorable conditions for growth in the store of food in the egg, or the egg represents the individual and was stimulated to the 'evolutio' by the spermatozoon. This theory led to that of inclusion, which taught that in the ovary of Eve were included the germs of all human beings who have lived or ever will live. Caspar Friedrich Wolff combated this idea (1759) ; he sought to prove by observation that the hen's egg at the beginning is without organization, and that gradually the various organs appear in it. In the embryo there is a new formation of all parts, an epigenesis. This assault upon the evolutionist school was without result, chiefly because Albrecht von Haller, the most celebrated physiologist of the eighteenth century, sup- pressed the idea of epigenesis. Von Baer. — Carl Ernst von Baer in his classic work, "Die Entwick- lung des Hlihnchens, Beobachtung und Reflexion" (1832), established embryology as an independent study. Baer confirmed Wolff's doctrine of the appearance of layerlike anlagen, from which the organs arose; and on account of the accuracy with which he proved this he is considered the founder of the germ-layer theory. Further, he came to the conclusion that each type had not only its peculiar structural plan, but also its peculiar course of development. Here we meet for the first time the idea that for the solution of the questions of relationship of animals, and therefore a basis for a natural classification, comparative embryology is indispensable; an idea which in recent years has proved exceedingly fruitful. Cell Theory. — Of fundamental importance for the further growth of comparative anatomy and embryology was the proof that all organisms, as well as their embryonic forms, were composed of the same elements, the cells. This cell theory, was advanced by Schleiden and Schwann HISTORY OF ZOOLOGY 13 during the third decade of the last century and three decades later was completely remodelled by the protoplasm theory of Max Schultze. In the cell theory a simple principle of organization was found for all living creatures, and the wide ream of histology was open for scientific treat- ment. But the theory was of the greatest importance for developmental physiology, for only with the recognition of egg, spermatozoon, and cleavage spheres as nucleated cells, was there a sound basis for the solution of the problems of fertilization, heredity, and embryonic differentiation and for the experimental proof of hypotheses. With the establishment and systematic use of comparative anatomy, the cell theory and histology, the ground was prepared for the series of researches which characterized the second half of the nineteenth century. Great advances were made in vertebrate anatomy by the classic researches of Owen, Johannes Miiller, Rathke, Huxley, Gegenbaur and others; our conceptions of organization have been completely altered by the work of Dujardin, Max Schultze, Haeckel, and others, who have proved the unicellularity of the lowest animals. The germ-layer theory was further developed by Remak and Kolliker; and applied to the invertebrates by Kowalewsky, Haeckel, and Huxley. It is beyond the limits of this brief historical summary to go into what has been accomplished in other branches of the animal kingdom; it must here suffice to mention the most important changes which the Cuvierian system has undergone under the influence of increasing knowledge. Changes in the System. — Of the four types of Cuvier the branch Radiata was the one of which he had the least knowledge; it was also the least natural, since it comprised, besides the radially sym- metrical ccelenterates and echinoderms, other forms, which, like the worms, were bilaterally symmetrical, or, like many infusorians, were asymmetrical. C. Th. von Siebold introduced the first important change. He limited the type Radiata, or, as he termed them, the Zoophytes, to those animals with radially symmetrical structure (Echinoderms and the Plant-animals); separating all the others, he formed of the unicellular organisms the branch of 'primitive animals' or Protozoa; the higher organized animals he grouped together as worms or Vermes; at the same time he transferred a part of the Articulata, the annelids, to the worm group, and proposed for the other articulates, crabs, millipedes, spiders, and insects, the term Arthropoda. Leuckart, about the same time (1848), divided the Radiata into two branches differing greatly in structure. The lower forms, in which no separate body-cavity is present, the interior of the body consisting only of a system of cavities serving for digestion, he called the Ccelenterata 14 GENERAL PRINCIPLES OF ZOOLOGY (essentially the Zoophyta of older zoologists) ; to the rest, in which the alimentary canal and the body-cavity occur as two separate cavities, he gave the name Echinoderma. Thus there resulted seven classes: Protozoa, Ccelentera, Echinoderma, Vermes, Arthropoda, Mollusca, and Vertebrata. Still this arrangement does not meet the requirements of a natural system and is more or less unsatisfactory. Upon anatomical and embryological characters the Brachiopoda, the Bryozoa, and the Tunicata have been separated from the Mollusca; they form the subject of diverse opinions. The relation- ships of the first two groups have not yet been settled: of the Tunicata we know indeed that they are related to the Vertebrata, but the differences are such that they cannot be included in that group. The only way out of the difficulty is to unite vertebrates, tunicates, and some other forms in a larger division, Chordata. The Vermes, too, must be divided, as will appear in the .second part of this volume. In the last decade of the nineteenth century and the beginning of the present, physiological investigation has taken a place beside morphology. Its most important tool is experiment. While experiments have long been invoked to settle biological problems, they are now used in the most extensive and systematic manner; especially are methodical breeding and crossing experiments employed to solve the problems of variation and heredity. There are also investigations into the laws which regulate the animal form, in which the separate stages of embryonic and post-embry- onic development are exposed to modifying influences (removal or trans- plantation of blastomeres or parts of the body, employment of different temperatures, chemical, mechanical, electrical stimuli), and the results are compared with those of normal conditions. An important aid in these studies is the mathematical statistical method by which the value of the results of observation and experiment is tested. The second half of the century just closed was especially characterized by the development of the theory of evolution, the history of which is given in a separate section. HISTORY OF THE THEORY OF EVOLUTION. The theory of evolution has developed into a question whose impor- tance might, on a superficial examination, be underrated, but which has grown into a problem completely dominating zoological research, and has occupied not only zoologists, but all interested in science generally. This is the question of the logical value of the conceptions species, genus, family, etc. HISTORY OF ZOOLOGY 15 The Nature of Species. — In nature we find only separate animals: how comes it that we classify them into larger and smaller groups? Are the single species, genera, and the other divisions fixed quantities, as it were fundamental conceptions of nature, or a Creator's thoughts, which find expression in the single forms? Or are they abstractions which man has introduced into nature for the purpose of making it comprehensible to his mental capabilities? Are the specific and generic names only expressions which have become necessary, from the nature of our mental capacity, for the expression of relationships in nature, which in and for themselves are not immutable, and hence can undergo a gradual change? Practically speaking, the problem reads: are species constant or change- able? What is true for species must necessarily be true for all other categories of the system, all of which in the ultimate analysis rest upon the conception of species. Ray's Conception of Species. — One of the first to consider the con- ception of species was John Ray. In the attempt to define a species he encountered difficulties. In practice, animals which differ little in structure and appearance from one another are ascribed to the same species; this practical procedure cannot be carried out theoretically; for there are males and females of the same species which differ more from one another than do the representatives of different species. Ray reached the genetic definition when he said: for plants there is no more certain criterion of specific unity than their origin from the seeds of specifically or individually like plants; that is to say, generalized for all organisms: to one and the same species belong individuals which spring from similar ancestors. The 'Cataclysm Theory.' — With Ray's definition an uncontrollable element was brought into the conception of species, since no systematist usually could know anything, as to whether the representatives of the species before him sprang from similar parents. It was therefore only natural that the conception of species put on a religious garb, since by resting upon theological ideas it found a firmer support. Linmeus said: "Tot sunt species quot ab initio creavit infinitum Ens." Linmeus's definition showed itself untenable, as soon as paleontology began to make accessible a vast quantity of extinct animals preserved as fossils. Cuvier proved that these fossils were the remains of animals of a previous time. Just as the formation of the earth's crust by successive layers made possible the recognition of different periods in the earth's history, so paleontology showed how to recognize different periods in the vegetable and animal world of life on our globe. Each geological age was characterized by a special world of animals; and these animal worlds differed the more from 16 GENERAL PRINCIPLES OF ZOOLOGY the present, the older the period of the earth to which they belonged. All these generalizations led Cuvier to his cataclysm theory: that a great revolution brought each period of the earth's history to an end, destroy- ing all life, and that upon the newly formed virgin earth a new organic world of immutable species sprang up. As a believer in the immuta- bility of species, Cuvier was forced to combat the idea of any genetic connection between the living and the fossil forms. Cuvier's theory of cataclysms gave no scientific explanation of ihe origin of the successive populations of the earth. Such an explanation is only possible by the hypothesis that the later animal worlds have descended from the earlier. So it happened that the idea of the fixity of species was given up and replaced by that of mutability and descent — the theory of evolution. Darwin's Predecessors. — Even in Cuvier's time there prevailed a strong current in favor of this theory. It found expression in England in the writings of Erasmus Darwin (grandfather of Charles Darwin) ; in Germany in the works of Goethe, Oken, and the disciples of the 'natural philosophical' school; in France the genealogical theory was developed by Buffon, Geoffrey St. Hilaire, and Lamarck. Its clearest expression was found in Lamarck's "Philosophic zoologique" (1809); its arguments will be considered in the following paragraphs. Lamarck (born, 1744, died, 1829) taught that at first organisms of the simplest structure arose through spontaneous generation from non- living matter. From these simplest living creatures have developed, by gradual changes in the course of an immeasurably vast space of time, the present species of plants and animals, without any break in the continuity of life upon our globe; the terminal point of this series is man; the other animals are the descendants of those forms from which man has developed. Lamarck regarded the animal kingdom as a single series grading from the lowest animal up to man. Among the causes which may influence the change of organisms, Lamarck emphasized particularly use and dimse; the giraffe has obtained a long neck because he was compelled to stretch, in order to browse the leaves on high trees; conversely, the eyes of animals which live in the dark have degenerated from lack of use. The direct influence of the external world must be unimportant; the changes in the surroundings must for the most part act indirectly upon animals by altering the conditions for the use of organs. Evolution vs. Creation. — Lamarck's work remained almost unno- ticed by his contemporaries. Later there arose a violent controversy between the defenders and the opponents of the evolution theory when [1830] Geoff roy St. Hilaire in a debate defended against Cuvier the thesis HISTORY OF ZOOLOGY 17 of a near relationship of the vertebrates and the insects. The conflict ended in the complete overthrow of the theory of evolution; the defeat was so complete that the problem vanished for a long time, and the theory of the fixity of species again became dominant. This was occa- sioned by many causes. The theory of Geoff roy St. Hilaire and Lamarck was rather a clever conception than founded on abundant facts; besides, it had in it as a fundamental error the doctrine of the serial arrangement of the animal world. Opposed to this stood Cuvier's extensive knowledge, making it easy for him to show that the animal kingdom was made up of separate co-ordinated groups, the types. Lyell. — In the same year in which Cuvier obtained his victory, his theory of the succession of numerous animal worlds upon the globe received its first blow. Cuvier's cataclysm theory had two sides, a geo- logical and a biological. Cuvier denied the continuity of the various terrestial periods, as well as the continuity of the fauna and flora belonging to them. In 1830-32 appeared the "Principles of Geology" by Lyell, which, in the realm of geology, completely set aside the cataclysm theory. Lyell proved that the supposition of violent revolutions was not necessary to explain the changes of the earth's surface and the superposition of its strata; that rather the constantly acting forces, elevations and depressions, the erosive action of water, are sufficient to furnish a complete explanation. Very gradually in the course of time the earth's surface has changed, and passed from one period into the next, and still at the present day constant change is going on. The continuity in the history of the earth, here postulated for the first time, has since then become one of the fundamental axioms of Geology; on the other hand the discontinuity of living creatures, was for a long time regarded as correct. Darwin. — It is the great merit of Charles Darwin that he took up the theory of descent anew after it had rested a decade, and later brought it into general recognition. With this began the most important period in the history of zoology, a period in which the science not only made such an advance as never before, but also began to obtain a permanent influ- ence upon the general views of men. Charles Darwin was born in 1809. After studying at the universities of Edinburgh and Cambridge, he joined the English war-ship "Beagle," as naturalist. In its voyage from 1831 to 36 around the globe, Darwin recognized the peculiar character of island faunas, particularly of the Galapagos Islands, and the remarkable geological succession of edentates in South America; these facts formed the germ of his epoch-making theory. After his return to England Darwin lived, entirely devoted to scientific work, up to his death in 1882. He was incessantly busy in 2 18 GENERAL PRINCIPLES OF ZOOLOGY developing his conception of the orgin of species, the fundamental ideas of which he communicated to friends, but not until 1858 did Darwin decide to make them public. In this year he received an essay by Wallace which in its most important points coincided with his own views. At the same time with Wallace's manuscript an abstract of Darwin's theory was published. In the next year (1859) appeared the most important of his writings, "On the Origin of Species by means of Natural Selection," and in rapid succession a splendid series of works, the most important of which are: (i) " Upon the Variation of Plants and Animals under Domes- tication," (2) on "The Descent of Man." No scientific work of that century has attracted so much attention in the whole educated world, as "The Origin of Species." It was generally received as something entirely new, so completely had the scientific tradition been lost. In professional circles it was stoutly combated by one faction, with- another it found hesitating acceptance. Only a few men placed themselves from the beginning in a decided manner on the side of the great British investigator. There was a lively scientific battle, which ended in a brilliant victory for the theory of evolution. At the present time all our scientific thoughts are permeated with the idea of evolution. Post-Darwinian Writers. — Among the men who have most influ- enced this rapid advance is to be mentioned, besides A. R. WTallace, the co-founder of Darwinism, above all Ernst Haeckel, who in his "General Morphology" and his "Natural History of Creation" has done much towards the extension of the theory. Among other energetic defenders of the theory in Germany should be mentioned Fritz Miiller, Carl Vogt, Weismann, Moritz Wagner, and Nageli. Among the English naturalists are to be named particularly Huxley, Hooker, and Lyell. In America Gray, Cope, Morse, and Hyatt were early supporters. Darwinism was long in obtaining an entrance into France. DARWIN'S THEORY OF THE ORIGIN OF SPECIES. Before Darwin wrote the idea of fixity of species prevailed. It was recognized that all the individuals of a species are not alike, and that more or less variability occurs, so that it was possible to distinguish races and varieties within the species, but it was believed that the variations never transcended specific bounds. Darwin begins with a criticism of the term species. Are the concep- tions of species on the one side and that of race and variety on the other something entirely different? Are there special criteria for determining HISTORY OF ZOOLOGY 19 beyond the possibility of a doubt whether in a definite case we have to do with a variety of a species or with a different species? or do the con- ceptions pass into one another in nature? Are species varieties which have become constant, and are varieties species in the process of formation? Morphological Characters. — A. Distinction bet-ween Species and Variety. For the settlement of these fundamental questions morphological and FIG. i A. — English carrier-pigeon (after Darwin). FIG. IB. — English tumbler-pigeon (after Darwin). physiological characters can be considered. In the practice of the system- atists usually the morphological characters prevail exclusively; and hence will be here considered first. If, among a great number of forms 20 GENERAL PRINCIPLES OF ZOOLOGY similar to one another, two groups can be recognized which differ consider- ably from each other, if the differences between them be obliterated by no intermediate forms, and if in several successive generations they remain constant, then the systematist speaks of a 'good species;' on the other hand. he speaks of varieties of the same species when the differences are slight and inconstant, and when they lose their importance through the existence of intermediate forms. A definite application of this rule dis- closes great incongruities, many groups being regarded by one set of systematists as good species, by another only as 'sports,' i.e., as varieties of the same species. The differences between the 'races' of our domestic animals are often so considerable that formerly they were regarded not only as sufficient for the foundation of good species, but even of genera and families. In the fantail pigeon the number of tail-feathers, originally only 12-14, has increased to 30-42 (fig. ic) ; among the other races of FIG. ic. — English fantail pigeon (after Darwin). pigeons enormous variations are found in the size of the beak and feet in comparison with the rest of the body (figs. IA, IB) ; even the skeleton itself participates in this variation, as is shown by the fact that the total number of vertebrae varies from 38 to 43, the number of sacral vertebrae from 14 to n. B. Variation within the Species. — Now in respect to the occurrence of transitional forms and the constancy of differences, there is within one and the same 'good species' the greatest conceivable difference. In many very variable species the extremes are united by many transitions; HISTORY OF ZOOLOdV 21 in other cases sharply circumscribed groups of forms, or races, can he distinguished within the same species. In the race, the peculiar character- istics are inherited from generation to generation with the same constancy as in good species. This is shown in man, and in many pure races of domesticated animals. Physiological Characters. — A. Crossing of Species and Varieties — A critical examination leads to the conclusion that morphology is indeed useful for grouping animals into species and varieties, but it leaves us in the lurch when called upon to show the disinctions between a specie? and a variety. Therefore there remains open to the systematist only one resource, i.e., to summon physiology to his aid. This has disclosed considerable distinctions in reproduction. We should expect a priori that the individuals of different species would not reproduce with each other; on the other hand the individuals of one and the same species, even though of different varieties or races, should be entirely fertile. One must beware of arguing in a circle in proof of these two propositions; so the question must read: does physiological experiment lead to the same distinctions as does the common systematic experience? B. The Intercrossing of Species. — This field is as yet far from being sufficiently investigated; yet some general propositions can be set up: (1) that not a few so-called 'good species' can be crossed with one another; (2) that in general the difficulty of crossing increases, the more distant the systematic relationship of the species used; (3) that these difficulties are by no means directly proportional to the systematic divergence of the species. Thus hybrids have been obtained from species which belong to quite different genera, while very often nearly-related species will not cross. Among fishes we know hybrids of A bramis brama and Blicca bjorkna, of Trutta salar (salmon) and Tmtta fario (trout); among sea-urchins the spermatozoa of Strongylocentrotus lividus fertilize with great readiness the eggs of Echinus microtiiberculatus, but only rarely the eggs of Sphcerechinus granularis, which is nearer in the system. It also happens that crossing in one direction (male of A and female of B) is easily accomplished, but in the other direction (male of B and female of A) it completely fails; as, for example, the sperm of Strongylocentrotus lividus fertilizes well the eggs of Echinus microtuberculatus, but, conversely, the sperm of E. microttiber- culatus does not fertilize the eggs of S. lividus; salmon eggs are fertilized by trout sperm but not trout eggs by salmon sperm. Eggs have been fertilized by sperm belonging to different families, orders, and possibly classes — eggs of Pleuronectes platessa and Labrus rupestris by sperm of the cod, frogs' eggs by sperm of two species of Triton, eggs of a starfish (Aster ias forbesii) by milt from a sea-urchin, Arbacia pusttilosa. In 22 GENERAL PRINCIPLES OF ZOOLOGY these extreme cases, it is true, the hydrids die during or at the close of segmentation, before the embryo is outlined. In the case of animals where copulation is necessary the difficulties of experimentation increase, since often there exists an aversion between males and females of different species which prevents any union of the sexes. Yet we know crosses of different species; e.g., between the horse and the ass; our domestic cattle and the zebu; ibex (or wild buck) and she-goat; sheep and goats; dog and jackal; dog and wolf; hare and rabbit (Lepus darwini); American bison and domestic cattle; etc.; among birds, between different species of finches and of grouse; mallard and the pintail duck; the European and the Chinese goose (Anser ferns and A. cygnoides). Among the insects, especially the Lepidoptera, the cases are many, but the resulting eggs usually produce larvae of slight vital force. C. Fertility of Hybrids and Mongrels. — Since many hybrids, as the mule, have been known for thousands of years, the criterion is, as it were, pushed back one stage; if the infertility in cases of crosses in many species is not immediately noticeable, yet it may be apparent in the products of the cross. While the products of the crossing of varieties, the 'mongrels,' always have a normal, often an increased, fertility, the products of the crossing of species, the hybrids, should always be sterile. But even this is a rule, not a law. The mule (which only very rarely reproduces) and many other hybrids are indeed sterile, but there are not a few exceptions, although the number of experiments in reference to this point is very small. Hybrids of hares and rabbits have continued fruitful for genera- tions; the same is true of hybrids obtained from the wild buck and the domesticated she-goat; from Anser cygnoides and A . domesticus; f rom Salmo salvelinus and S. font mails; Cyprinits carpio and Carassius vulgar is; Bombyx cynthia and B. arrindia. Difficulties in Classification. — The final result of all this discussion may be summed up as follows: up to the present time, neither by physio- logical nor by morphological evidence has there been found a criterion which can guide the systematist in deciding whether certain series of forms are to be regarded as good species or as varieties of a species. Zoologists are guided rather in practice by a certain tact for classification, which, however, in difficult cases leaves them in the lurch, and thus the opinions of various investigators vary. Change of Varieties into Species. — The conditions above discussed find their natural explanation in the assumption that sharp distinctions between species and variety do not exist; that species are varieties which have become constant, and "varieties are incipient species. The meaning of the above can be made clear by a concrete case. Individuals of a HISTORY OF ZOOLOGY 23 species vary, i.e., compared with one another they attain greater or less differences. So long as the extreme differences are bridged by transitional forms we speak of varieties of a species; if, on the other hand, the inter- mediate forms have died out, or were not present in the beginning, and the differences have in the course of time become fixed, and so intensified that a sexual union of the extreme forms results either in complete sterility or at least in a marked tendency towards sterility, then we speak of different species. Species may be Related to each other in Unequal Degrees. — In favor of this view, that varieties will in longer time become species, is the great agreement which usually exists between the two. In genera which comprise a larger number of species, the species usually show also many varieties; the species are then usually grouped in sub-genera, i.e., they are related to each other in unequal degrees, since they form small groups arranged around certain species. With varieties the case is similar. In such genera the formation of species is in active progress; but each species formation presupposes a high degree of variability. Phylogeny. — It is clear that what has here been worked out for the species must also apply to the other categories of the system. Just as by divergent development varieties become species, so must species by con- tinued divergence become so far removed from one another that we dis- tinguish them as genera. It is only a question of time when these differ- ences will become still greater, and give rise to orders, classes, and branches, just as the tender shoots of the young plant become in the tree the chief branches from which spring lateral branches and twigs. If we pursue this train of thought we reach the conception that all the animals and plants living at present have arisen by means of variation from a few primitive organisms. Inasmuch as thousands of years are required for the forma- tion of new species through the variability of one, there must have been necessary for this historical development of the animal and vegetable kingdoms a space of time greater than our mental capacity can grasp. Since for the individual development (embryology) of an animal the term ontogeny has been chosen, it has also proved convenient to apply to the historical development of animals the term History of the Race or Phylogeny. Spontaneous Generation. — If we attempt to derive all living animals from a common ancestor, we must assume that this was extremely simple, that it was unicellular; for the simpler, the less specialized, the organiza- tion, so much the greater is its capacity for modification. Only from simple organisms can the unicellular organisms, the Protozoa, be derived. Finally, for the simple organisms alone can we conceive a natural origin. 24 GENERAL PRINCIPLES OF ZOOLOGY Since there was undoubtedly a time upon our earth when temperatures prevailed which made life impossible, life must have arisen at some time, either through an act of creation or through spontaneous generation. If, in agreement with the spirit of natural science, we invoke for the explana- tion of natural facts only the forces of nature, we are driven to the hypothe- sis of spontaneous generation, namely, that by a peculiar combination of materials without life, the complicated mechanism which we call 'life' has arisen. Starting from a basis of facts, by generalization we reach a simple conception of the origin of the animal kingdom, but we have in equal measure departed from the results of direct observation. Observations only show that species are capable of modifications. That this capacity for variation is a principle which explains to us the origin of the animal world, needs further demonstration. Proofs of Phylogeny. — The evolution of the existing animal world has taken place in the thousands of years long past, but is no longer acces- sible for direct observation, and therefore it can never be followed in the sense that we follow the individual development of an organism. In regard to the conception of the evolution of animals we can merely prove the probability; yet all our observations of facts not only agree with this conception, but find in it their only simple explanation. Such facts are furnished to us by the classification of animals, paleontology, geographical distribution, comparative anatomy, and comparative embryology. (1) Proofs from Classification. — It has long been recognized, that if we wish to express graphically the relationships of animals, their classes, orders, genera, and species, simple co-ordination and subordination are not sufficient, but we must have a treelike arrangement, in which the principal divisions, more closely or distantly related to one another — the branches, phyla, or types — represent the main limbs, while the smaller branches and twigs correspond to the several classes, orders, etc. This is, in fact, the arrangement to which the theory of evolution necessarily leads. (2) Paleontological Demonstration approaches nearest to direct proof; for paleontology gives the only traces of existence which the predecessors of the present animal world have left. Even here a hypothetical element creeps in. We can only observe that various grades of forms of an animal group are found in successive strata; if we unite these into a develop- mental series, and regard the younger as derived from the older by varia- tion, we connect the single observations by a very probable hypothesis. But the value of paleontological evidence is weakened much more by its extreme incompleteness. In fossils only the hard parts are generally HISTORY OF ZOOLOGY 25 preserved; the soft parts, which alone are present, or at least make up the most important portions of many animals, are almost always lost. Only rarely are they (muscle of fishes, ink-bag of cephalopods, outlines of medusae) preserved in the rocks. Even the hard parts remain connected only under exceptionally favorable conditions. If further we take into consideration the fact that these treasures are buried in the earth, and are usually obtained only by accident, in quarrying and road-building, it becomes clear how little of the racial history is to be expected from paleontology. FIG. 2. Archvopteryx lUkographica (after Steinmann-Doderlein). d, clavicle; co, coracoid; h, humerus; r, radius; u, ulna; c, carpus; I-IV, digits; sc, scapula. Examples of Paleontological Proof. — Yet paleontology has already furnished many important proofs of the theory of descent. It has shown that the lower forms appeared first, and the more highly organized later. Among animals in general the latest to appear were the vertebrates, and 26 GENERAL PRINCIPLES OF ZOOLOGY of these the mammals; among the mammals man. For smaller groups genealogical material has fortunately been found. Transitional forms connect the single-toed horse of the present with the four-toed Eohippos of the eocene; for all the hoofed animals a common ancestral form has been found in the Condylarthra. Transitional forms have also been found between the greater divisions, as, e.g., between reptiles and birds, the remarkable toothed birds, and the Archeeopteryx (fig. 2), a bird with a long, feathered, lizard-like tail. (3) Morphological Proofs. — When we employ comparative anatomy and embryology in support of evolution, we find that the two have so much in common that they can best be treated together. Cuvier and von Baer taught that the separate types of the animal kingdom are units, each with a special structure and plan of development peculiar to it; farther, that there are no similarities in structure or develop- ment forming a bridge from type to type. The first of these propositions is still regarded as correct, but the second, which alone is important for the theory of evolution, has become untenable. All animals have a common organic basis in the cell and are thereby brought close to one another; all multicellular animals agree in the principal points during the first stages of their development, during the fertilization, cleavage of the egg, and the formation of the first two germ-layers, and vary- from one another from this time on only in such differences as may occur within one and the same type. Also the peculiarities which distinguish each type in structure and in the mode of development are not without intermediate phases. In some representatives of each type the structure and the mode of development are simpler, thereby approaching to the conditions which obtain in the other types. The existence of such transitional forms is one of the most important proofs in favor of the theory of evolution. Fundamental Law of Biogenesis. — A fact that weighs heavily in favor of the theory of evolution is that the structure and mode of develop- ment of animals is ruled by a law which at present can only be explained by the assumption of a common ancestry. Each animal during its develop- ment passes through essentially the stages which remain permanent in the case of lower or better, more primitive animals of the same branch, as the following examples show: (i) In the early stages of development the human embryo (figs. 3, 609) possesses remarkable resemblances to the lowest vertebrates, the fishes. Like these it has gill-slits, a simple heart with auricle and ventricle; instead of a separation of aorta and pulmonary arteries (body and lung arteries) a single arterial trunk going from the heart; and aortic arches connecting this trunk with the descend- ing aorta. All of these are structure adapted for branchiate respiration HISTORY OF ZOOLOGY 27 and are functionally intelligible in fishes hut they are not compatable with a lung-breathing mammal and must undergo fundamental changes to become of use. They are, like so many other structures in the human being, not to be understood from present functions, but must have an historical meaning. (2) Frogs in their tadpole stage have an organiza- tion similar to that which is permanent in certain Amphibia, the Per- 3 FIG. 3. FIG. 4. FIG. 3. — Human embryo, 4.2 mm. long (after His). Pericardium and lateral body wall opened, yolk-sac and allantois cut away, course of blood-vessels shown; a, arterial trunk; a/, allantois stalk; c, precava, uniting with yolk and umbilical veins; d, intestine; do, yolk stalk; h, ear vesicle; A", ventricle of heart; o, upper jaw; r, olfac tory groove; s, tail; n, lower jaw; us, somites; V, atrium of heart; 1-5, the five arterial arches. FIG. 4. — Tadpoles of Rana temporaria. in, mouth; g, upper jaw; z, lower jaw; s, sucking-disc; kb, external gills; ik, region of the internal gills; n, nose; a, eyes; o, auditory vesicle; h, cardiac region; d, operculum. ennibranchiata (fig. 5), which stand lower in the system; they have a swimming tail and tuft-like gills, which are lacking in the adult frog. (3) There are certain parasitic Crustacea, which live upon the gills of fishes, and seem not at all like their relatives. They are shapeless masses which were formerly regarded as parasitic worms. Their systematic position was only determined by their embryology (fig. 6). Here it is 28 GENERAL PRINCIPLES OF ZOOLOGY shown that they pass through a nauplius-stage (fig. 6a), characteristic of most Crustacea, and that they then assume the shape of small Crustacea (fig. 6, b), like Cyclops (fig. 7, A), so widely distributed in fresh waters. Very often the males make a halt in the cyclops-stage while the female FIG. 5. — Siredon pisciformis (larva of Amblystoma tigrinum) (after Dumeril and Bibron). develops farther and assumes a shapeless form, so that there arises a very remarkable sexual dimorphism (fig. 8). All these examples, which can be multiplied by hundreds, can be explained in the same way. The higher FIG. 6.— \ I •Ichtheres pcrcarum. a, nauplius-, /;, cyclops-stage; c, adult female (after Claus). forms pass through the stage of the lower, because they spring from an- cestors which were more or less similar to the latter. Man in his em- bryological development passes through the fish stage, the frog the perennibranchiate stage, the parasitic crustacean first the nauplius- HISTORY OF ZOOLOGY 29 and then the cy clops-stage, because their ancestors were once fish-like, perennibranchiate-like, nauplius- and cyclops-like. Here is expressed a general phenomenon which Haeckel has stated under the name of 'the Fundamental Law of Biogenesis.' "The development history (ontogeny) m. A FlG. 7. — C\'djps coronatus (.4) and also its nauplius in lateral (5) and in ventral view (C). 7, 'head; II-V, the five thoracic, and behind these the five abdominal segments; F, furca; i, the first, 2, the second, antenna;; 3, mandibles; 4, maxilla-; 5, maxillipeds; 6-9, the first four pairs of biramous feet, while the rudimentary fifth pair are hidden; au, eye; o, upper Up; e, egg-sacs; d, gut; m, muscle. of an individual animal briefly recapitulates the history of the race (phylog- eny); i.e., the most important stages of organization which its ancestors have passed through appear again, even if somewhat modified, in the development of individual animals." 30 GENERAL PRINCIPLES OF ZOOLOGY FIG. 8. — Philicthys xiphice. a, female (after Claus), X4; b, male (after Bergsoe), Xi3. The Nervous System. — This law applies as well to single organs as to entire animals. The central nervous system of many lower animals (echinoderms, coelenterates, many worms) forms part of the skin; in its first appearance it belongs to the surface of the body, because it has to mediate the relations with the external world. In the case of higher animals, e.g., the vertebrates, the brain and spinal cord lie deeply imbed- ded in the interior of the body; but in the embryo they are likewise laid down as a part of the skin (medullary plate) which gradually through infolding and cutting off from this comes to lie internally (fig. 9) . The Skeletal System. — The skeleton of vertebrates is a further example. In the lowest chordates, amphioxus and the cyclo- stomes, the vertebra are lacking, and in their place we find a cylindrical cord of tissue, the notochord. In the fishes and Amphibia the notochord usually persists; but it is partially reduced and constricted by the vertebrae, which in the lower forms consist of cartilage, and in the higher of bone. Mature birds and mammals finally have a com- pletely ossified vertebral column; their embryos, on the other hand, have in the early stages only the notochord (amphioxus stage) ; later this notochord becomes constricted by the vertebrae (fish-amphibian stage) and finally entirely replaced; the vertebral column is in the beginning cartilaginous, only later becoming ossified. Comparative anatomy and embryology thus give the same developmental stages of the axial skeleton: (i) notochord, (2) notochord and vertebral column, (3) vertebral column alone, the latter at first formed of cartilage, then of bone. We have spoken of a parallelism between the facts of comparative anatomy and of embryology. But we should expect a threefold parallel- ism; for according to the theory of evolution the systematic arrangement of animals is based upon a third factor — phylogeny. The fossils, should give the same progressive series in the successive geological strata as the stages of forms found by comparative anatomy and embryology. We know instances of such threefold parallelisms. Comparative anatomy teaches that the lowest developed form of a fish's tail is the diphyceral (fig. 10, A); that from this the heterocercal (B), and from the hetero- cercal the homocercal form of tail-fin (C, D] can be derived. Embryo- HISTORY OF ZOOLOGY mf mf mk2 ik mf mp III. FIG. g. — Cross-sections through the dorsal region of Triton embryos at different ages (from O. Hertwig). In / the medullary plate (anlage of spinal cord) mp is marked otf from the skin (epidermis, ep) by the medullary folds (mf). In // the medullary plate, by inrolling of the medullary folds, is converted into a groove. In III the groove has closed into a tube (»), the spinal cord, which has separated from the rest of the ectoderm (epidermis), c, body cavity (ccelom); ch, notochord; rf>. cavity of primitive somite (myotome); r/z, yolk-cells; ik, entodenn; /<[, lumen of gut; ink1, ink-, somatic and splanchnic layers of mesoderm; n, spinal cord. 32 GENERAL PRINCIPLES OF ZOOLOGY logically the most highly developed fishes are first diphycercal, later heterocercal, and finally become homocercal. Last of all, paleonto- logically the oldest fishes are diphycercal or heterocercal, and only later do homocercal forms appear. FIG. 10. — Tail-fins of various fishes (from Zittel). A, Diphycercal fin of Polypterus bichir . (Vertebral column and notochord divide the tail into symmetrical dorsal and ventral portions.) B, Heterocercal tail of the sturgeon. (As a result cf an upward bending of the notochord and vertebral column the fin has become asymmetrical, the ventral portion much larger than the dorsal.) C, D, Homocercal fins, C, of Amia calva; D, of Trutta salar. (By a still greater upward bending of the notochord and vertebral column the dorsal portion has almost entirely disappeared and the ventral portion almost alone forms the fin, externally apparently symmetrical, but in its internal structure very asymmetrical.) ch, chorda; a, b, c, cover-plates. What has here been referred to is only a small fraction of the proofs which morphology offers in favor of evolution; it can only serve to show how morphological observations can be employed. For the reflecting naturalist the facts of morphology are a great inductive proof in favor of the theory of evolution. HISTORY OF ZOOLOGY 33 Distribution of Animals. — From Animal Geography we learn that the present distribution of animals is the product of the past. It will therefore be possible from this to figure out many of the earlier conditions of things. If we assume that from the beginning all animal species were consti- tuted as they now are, they would then have been placed by the purposeful Creator in the regions best suited to them; their distribution would there- fore have been determined by favorable or unfavorable conditions of life prevailing in the various regions, as the climate, food-supply, etc. If, on the other hand, we assume that the animal species have arisen from one another through variation, then there must have been, as an influence determining the manner of distribution, besides the conditions of exis- tence, still a second factor, which we will call the geological. We know that the configuration of the earth's surface has changed in many respects in the course of the enormous time of the geological periods; that land areas which earlier were united, have become separated by the encroach- ments of the sea; that by the upheaval of mountains important barriers to the distribution of animals were also formed. On the other hand regions which were formerly separated have become connected; islands, for example, being united by the emergence of land from the sea. From the fact that these two changes — the changes in the earth's surface and in the animal world established upon it — have gone on hand in hand there follows necessarily the consequence that greater differences in the faunal character of two lands must result, the longer the inhabitants have been separated by impassable barriers. For the various groups the character of the barriers is different; terrestrial animals, which cannot fly, are hindered in their distribution by the sea; marine forms by land barriers; for terrestrial molluscs mountain ranges, which are dry and barren, or con- stantly snow-capped, are effectual. Instances of Proofs. — Since attention has been called to these conditions, many facts favorable to the theory of evolution have been ascertained: (i) Of the various continents Australia has faunally an in- dependent character; when discovered it contained almost none of the higher (placental) mammals, except such as can fly (Chiroptera), or marine forms (Cetacea), or such as are easily transported by floating wood (small rodents), or such as could be introduced by man (dingo, the Australian dog) ; instead, it had remarkable birdlike animals (with beak and cloaca), and the marsupials, which have become extinct in the Old World and, opossums excepted, in America as well. The phenomenon is explained by the geological fact that in the earth's history Australia, with its surrounding islands, was certainly the earliest to lose its connec- 3 34 GENERAL PRINCIPLES OF ZOOLOGY tion with the other continents. While in the other parts of the earth the higher vertebrates, which were developed from the marsupials and their lower contemporaries, came, by way of the lands connecting the various continents, to have a wide distribution, in isolated Australia this process of evolution did not go on, and its ancient faunal character was preserved. (2) As Wallace has shown, the Malay Archipelago is divided faunally into an eastern and a western half. The fauna of the first has a thoroughly Australian character; that of the latter recalls Farther India and the Oriental Province. Differences in climate and vegetation cannot be the cause of this, since in both there are islands with dry and others with moist climates, with sparse and with luxuriant vegetation. The only ex- planation is that the eastern Malay Islands have developed geologically in connection with Australia, the western with India. Wallace tried to draw a sharp line ('Wallace's line') between the two regions, passing between the islands of Bali and Lombok. Later studies have not confirmed this, but have shown that between the two regions is a zone of islands (including Celebes) in which a mixture of faunas occurs. (3) Long before Darwin, the geologist von Buch, from the distribution of plants on the Canary Islands, came to the conclusion of a change of species into new species; viz., on islands peculiar species develop in secluded valleys, be- cause high mountain-chains isolate plants more effectually than do wide areas of water. Moritz Wagner has collected many instances which prove that localities inhabited by certain species of beetles and snails have been sharply divided by wide rivers or by mountain-chains, while in neighbor- ing regions related so-called 'vicarious species' are found. The peculiar character of the fauna and flora of isolated island groups also needs mention. The Hawaiian Islands have no less than 70 endemic birds out of a total of 116, the Galapagos 84 out of 108. Causal Foundation of the Theory of Evolution. — The Darwinian theory, so far as the above exposition shows, is fundamentally like the theories of descent advocated at the beginning of the last century by Lamarck and other zoologists; it is distinguished from these only by its much more extensive foundation of facts, and further in that it abandoned the successional arrangement and replaced it by the branched, tree-like mode of arrangement — the genealogical tree. But still more important are those advances which relate to the causal foundation of the descent theory. The doctrine of causes which has brought about the change of species forms the nucleus of the Darwinian theory, by which it is especially distinguished from Lamarckism. In order to substantiate causally the change of species, Darwin proposed his highly important principle of 'Natural Selection by means of the Struggle for Existence.' HISTORY OF ZOOLOGY 35 Artificial Selection. — In the development of this principle Darwin started with the limited and hence easily comprehended subject, the arti- fical breeding of domesticated animals. Whether these be the descendants of a single species or have arisen from crosses of two or more species (authorities are not in agreement in all cases) they behave like repre- sentatives of a single species. How have the various races and sub- race of pigeons, horses, cattle, dogs, etc. arisen? Darwin finds the causes of these great differences in artificial selection, practised by man for thousands of years. The method is to choose from the stock individuals showing the tendency toward the desired ideal in even the slightest degree more than their fellows, and then pairing these. By repetitions of this selection and breeding, the desired goal is slowly reached. This artificial selection depends upon three factors: (i) Variabilitv; the descendants of one pair of parents have the capability of developing new characteristics, thereby differing from their parents. (2) Hered- Uability of newly-acquired characters; the tendency of the daughter- generation to transmit the newly-developed characteristic to the succeeding generation. (3) Artificial selection; man selects for breeding suitable individuals, and prevents a new character which has arisen through variation from disappearing by crossing with animals of the opposite variational tendencies. If we compare with the facts of domestication the conditions of animals living in the state of nature, we find again variability and heredity, as effici- ent forces, inherent in all organisms, though the former is not everywhere of the same intensity. There are many species which vary only slightly or not at all, and therefore have remained unchanged for thousands of years. But contrasted with these conservative species are in every group plastic species, which are in the process of rapid change, and these alone are of importance in causing the appearance of new forms. Since heredity is present in all organisms, there is only lacking a factor corre- sponding to artificial selection, and this Darwin discovered in the so-called 'natural selection.' Natural Selection: Struggle for Existence. — Natural selection finds its basis in the enormous number of descendants which every animal produces. There are animals (e.g., most fishes) which produce many thousands of young in the course of their lives; not to mention parasites, whose eggs are numbered by millions. For the development of this multitude of germs there is no room on the earth. In order to preserve the balance of nature great numbers of unfertilized and fertilized eggs, as well as young animals and many that are mature but have not yet attained their physiological destiny, must perish. Many individuals will 36 GENERAL PRINCIPLES OF ZOOLOGY be blotted out by accidental causes; yet on the whole those individuals which are best protected will best withstand adverse conditions. Slight superiority in structure will be of importance in this struggle for existence, and the possessors of this will gain an advantage over their companions of the same species, just as in domestication each character which is useful to man is of advantage to the possessor. Among the numerous varieties that appear the fittest will survive, and in the course of many generations the fortunate variations will increase by summation, while destruction overtakes the unfit. Thus will arise new forms, which owe their existence to 'natural selection in the struggle for existence.' The 'Struggle for Existence.' — The expression 'struggle for exis- tence' is figurative, for only rarely does a conscious struggle decide the question of an animal's existence; for example, in the case of the beasts of prey, that one which by means of his bodily strength is best able to struggle with Ids competitors for his prey is best provided in times of limited food-supply. Much more common is the unconscious struggle: each man who attains a more favorable position by special intelligence and energy, limits to an equal degree the conditions of life for many of his fellow men, however much he may interest himself in humanity. The prey, which by special craft' or swiftness escapes the pursuer, turns the enemy upon the less favored of its companions. It is noticeable that in severe epidemics certain men do not fall victims to the disease, because their organization better withstands infection. Here the term 'survival of the fittest,' which Spencer has adopted in preference to 'struggle for existence,' is better. Instances of the Struggle for Existence. — Although the foregoing suffices to show that the struggle for existence plays a very prominent role, yet on account of the importance of this feature it will be illustrated by a few concrete examples. The brown rat (Mus decumanus) , which swarmed out from Asia at the beginning of the eighteenth century, has almost completely exterminated the black or house-rat (Mus rattus) in Europe, and has made existence impossible for it in other parts of the world. Several European species of thistle have increased so enormously in the La Plata states that they have in places completely crowded out the native plants. Another European plant (Hypochoeris radicata) has become a weed, overrunning everything in New Zealand. Certain races of men, like the Dravidian and Indian, die off to the same degree that other races of men, like the Caucasian, Mongolian, and Negro, spread. The more one attempts to explain that endlessly complicated web of the relations of animals to one another, the relations of animals to plants and to climatic conditions, as Darwin has done, so much the more does he HISTORY OF ZOOLOGY 37 appreciate the methods and effects of the struggle for existence. Islands in the midst of the ocean have a disproportionately large number of species ©f wingless insects, because the flying forms are easily carried out to sea. For example, on the Kerguelen Islands, remarkably exposed to storms, the insects are wingless; among them one species of butterfly, several flies, and numerous beetles. Sympathetic Coloration. — Very often, in regions which have a pre- vailing uniform color, the coat of the animals is distinguished by a similar hue; this phenomenon is called sympatlietic coloration. Inhabitants of regions of snow are white, desert animals have the pale yellow color of the desert, animals which live at the surface of the sea are transparent; representatives of the most diverse animal branches show the same phe- nomenon. The advantages connected therewith scarcely need an expla- nation. Every animal may have occasion to conceal himself from his pursuers; or it may be his lot to approach his prey by stealth: he is much better adapted for this the closer he resembles his surroundings. Natural selection fixes every advantage in either of these directions, and in the course of many generations these advantages increase. Among the most interesting are the cases of sympathetic coloration, of mimicry and of the development of secondary sexual characters as a result of sexual selection. Mimicry is referable to the same principle, except that the imitation is not here limited to the color, but also influences form and marking. Frequently parts of plants are imitated, sometimes leaves, sometimes stems. Certain butterflies with the upper surfaces of the wings beauti- fully colored escape their pursuers by the rapidity of their flight; if they alight to rest, they are protected by their great similarity to the leaves of the plants around which they chiefly fly. When the wings are folded over the back, the dark coloring of the under sides comes into sight and the color on the upper side is concealed. The parts are so. arranged that the whole takes on a leaf-like form, and certain markings heighten the imita- tion of the leaf (fig. n). Among the numerous species of leaf-butterflies there are different grades of completeness of mimicry; in many even the depredations of insects are imitated; in others the form and marking are still incompletely leaf-like, the marking being the first to come into exis- tence. Among the grasshoppers also there are imitations of leaves, like the 'walking-leaf,' PliyUium sice (folium, P. scythe, while other nearly related forms more or less completely approach the appearance of dried, sometimes of thorny twigs (fig. 12, a and b). Very often insects are copied by other animals. Certain butterflies, the Heliconias of warmer America, the Danaids of the Old World, fly heavily in large swarms, clumsy and yet are unmolested by birds, because 38 GENERAL PRINCIPLES OF ZOOLOGY they contain bad-tasting fat bodies. Another species of butterfly accom- panies them (Fieri dae), which does not taste bad, and yet is not eaten, because in flight, in cut, and marking of the wings it imitates the Helic- onix so closely that even a systematist might easily be confused (fig. 13). In a similar way bees and wasps, feared on account of their sting, are imitated by other insects. In Borneo there is a large black wasp, whose FIG. ii. — Leaf-butterflies. A, Kallima paralecta, flying: a, at rest (after Wallace). 5, Siderone strigosus, flying; b, at rest (after C. Sterne). wings have a broad white spot near the tip (Mygnimia aviculus). Its imitator is a heteromerous beetle (Coloborlwmbus fasciatipennis), which, contrary to the habit of beetles, keeps its hinder wings extended, showing the white spot at their tips, while the wing-covers have become small oval scales (fig. 14). With many species the mimicry occurs only in the HISTORY OF ZOOLOGY 39 a FIG. 12. — Grasshopper mimicry, a, Acanthoderus wattacei + . b, Phyllium scytlie FIG. 13. — Methona psidii, a bad-tasting Heliconiid, copied by theT?iend,LeptaliSGrise. (after Wallace.) 40 GENERAL PRINCIPLES OF ZOOLOGY females, since these are less numerous and have heavier bodies than the males. So there arises a sexual dimorphism. If the mimicing species have a wide distribution, different bad flavored species may be mimicked in different parts of the range. The females of Papilio merops mimic, in different regions, Danais chrysippus, Amanris eclieria and A. mavias, while the males have the same appearance throughout the whole habitat. FIG. 14. — a, Mygnimia aviculus, a wasp imitated by a beetle, b, Coloborhombus'fascia- tipennis (after Wallace). Three-fourths natural size. Sexual Selection is a special phase of natural selection, chiefly observed in birds and hoofed animals. For the fulfilment of his sexual instincts the male seeks to drive his competitors from the field, either in battle or by impressing the female by his charms. With strong wings and with spurs the cock maintains possession of his flock, the stag by means of his antlers, the bull with his horns. The male birds of paradise win the favor of the females by means of beautiful coloring; most singing-birds, by means of song; many species of fowl, by peculiar love-dances. Since all these characters belong chiefly to the male, and since only exception- ally are they inherited by the female (and even then are less pronounced), HISTORY OF ZOOLOGY -11 it is almost certain that in a great measure they have been acquired by the males through the struggle for the female. In the case of birds a second factor has undoubtedly co-operated to impress distinctly the often enormous difference between the feathers of the male and of the female —as is shown, for example, in the case of the birds of paradise (fig. 15); \ FIG. i5A. — Paradisea apoda, male (after Levaillant). for the nesting female inconspicuous colors and a close-lying coat of feathers are necessary in order that, undisturbed by enemies, she may devote herself to incubation. On the Efficiency of Natural Selection. — In the course of the last twenty-five years there has been much controversy as to how far natural selection alone is a species-forming factor. A number of objectors dispute the possibility of fortuitous variations being utilized in the struggle for 42 GENERAL PRINCIPLES OF ZOOLOGY existence and thus fixed as permanent characters. It is not easy to see how many characters, especially those used in classification, can be of use to their owners. It can only be said that they have developed in correlation with other important characters. But useful characters must be considerable in order to be seized upon by natural selection. Fortuitous variations with which Darwinism deals are too inconsiderable to be utilized by the organism and so to be of value in the struggle for existence. In most cases, too, alteration in one organ alone is not enough to be of value; usually a whole series of accessory structures must be Fig. 15!}. — Paradisea apoda, female (after Levaillant). modified. In short, there must exist a harmonious co-adaptation of parts, which presupposes a progressive and well-regulated development extend- ing through a long space of time, during which the struggle for existence could have exerted no directing influence. Thus, the wing of a bird in order to be used for flight must have already reached a considerable size; the muscles for moving it, the supporting skeletal parts, the nerves running to it must have a definite formation and arrangement. Then there are difficulties in that most animals are bilaterally or radially sym- metrical, many in addition segmented. In all these cases the same organ is repeated two or more times. Organs which are repeated symmetric- ally and usually those which are segmental agree in general in structure. One must therefore admit that the alterations of chance must have oc- curred at at least two points simultaneously and in exactly the same way. A further objection is that the action of natural selection would, under ordinary conditions, be negatived by unhindred crossing of the varying forms. If, for example, we do not isolate fantails from other pigeons, they will cross with these, and their descendants will soon resume the HISTORY OF ZOOLOOY 43 character of common pigeons. Finally, it has been claimed that for the formation of new species a simple variation of form is not sufficient; it must reach still farther: (i) a variation in different directions, a divergent development of the individual members of a species; (2) the disappearance of the transitional forms which unite the divergent forms. The objection that the struggle for existence cannot bring about the divergent development of individuals necessary for improvement is of least importance. Of the many variations appearing at the same time in a species two or more may be equally useful; then one set of individuals will seize upon one, another set upon the other advantage, and that in consequence of this both sets will develop in different directions. Conse- quently the intermediate forms which are not pronounced in the one or the other direction will be in an unfavorable position, and must carry on the struggle for existence with both groups of partially differentiated com- panions of their species, and, being less completely adapted, must fall. More important are the first two objections; they have led to the idea that the principle of selection alone is insufficient to explain the origin and adaptive modification of new forms. So new theories have been advanced, older ones revamped, sometimes to replace that of natural selection, sometimes to strengthen certain links of its chain. Limited space permits only an outline of the more prominent of these and that without any attempt at a decision as to how far they complete the Darwin- ian hypothesis, are compatible with it, or replace it. Germinal Selection. — According to its author, Weismann's 'germinal selection ' is only a completion of natural selection, the 'individual selection.' It presupposes a detailed knowledge of modern investigations in the lines of fertilization and heredity (see chapter on Fertilization) and hence can only be outlined here. Weismann believes that all variations which are selected in the struggle for existence and are fixed in the successive generations must have their sources in the germ cells and since these arise from the fusion of male and female sex cells, they must, in the first in- stance, have been contained in these. Each germinal anlage consists of extremely numerous elements, the 'determinants' of the peculiarities of the organism. Accordingly as certain of these determinants develop at the expense of the others or are weakened or modified, the organism arising from the germ has special peculiarities or variations. If certain determinants tend constantly in a certain direction and these be present in large numbers, these will persist so that individual selection can have its influence. Mutations. — The Mutation theory of de Vries is a considerable modification of the Darwinian theory. In rearing large numbers of the 44 GENERAL PRINCIPLES OF ZOOLOGY evening primrose, CEnothera lamarckiana, besides plants of the true lamarckiana type there also arise a not inconsiderable number of others which are distinguished from the mother plant in noticeable ways and these may be arranged in sharply defined groups of forms which de Vries has named Oe. gigas, Oe. nanella, Oe. scintillans, etc. These groups of forms resemble 'small species' to the extent that, from the first, inter- mediate forms are lacking and, prevented from crossing, they produce only individuals with the same characteristics. These suddenly appearing and sharply marked and hereditary variations de Vries calls Mutations. They have long been known in English as 'sports' and are Darwin's 'single variations.' While formerly these were regarded as merely special instances of general variation, de Vries regards them as different. The slight variations with which Darwinism had previously dealt, oscillate- like a pendulum about its point of rest — around a central point of greatest frequency and.result in no permanent modifications. Even in domestica- tion it is not possible to advance by the continued selection of the slight differences of these 'fluctuating variations/ and to fix them as inheritances. On the other hand stable forms are produced by mutations and these per- sist if the mutants are better adapted to the conditions of life than is the parent stock. Just so far as the struggle for existence here plays a deciding role the mutation theory is a basis for the selection theory. It differs from Darwinism to the extent of being an 'explosive' method of species formation, by which several kinds, and these relatively constant, suddenly come into existence. For the proper valuation of the mutation theory it will be necessary in the future to separate two questions: (i) Whether the sharp distinctions postulated by de Vries between mutation and variation actually exist; (2) W7hether the mutation theory is able to explain the numerous adaptations of organisms to their -environment. The mutation theory has shown anew how necessary it is to subject the phenomena of heredity and variation to exact examination. Thus there has developed in the field of botany an experimental direction which has contributed much to the solution of the problems and promises rich results for the future. Here come the studies of the statistics of variation, established by the mathe- maticians Galton and Pearson upon the old foundation of Quetelet, which more recently had received considerable modifications at the hands of the botanist Johannsen. This strives to show by the statistical method whether the charac- ters arising by fluctuating variations can be made hereditary by selection. If a study be made of the modifications of a single character in a 'population' (that is a large number of men, animals or plants living under similar conditions, but springing from different ancestors), it is seen — especially clearly if char- acters such as length, breadth, weight, number, capable of accurate quantitative statement be chosen — that the majority of individuals assume a middle point with regard to the development of this character, and that the variations from this mean, on either the positive or negative side (plus and minus variants) are HISTORY OF ZOOLOGY 45 the fewer, the farther they be removed from the middle value. From the figures of the statistics which express the relative frequency of each grade of peculiarity there can usually be constructed a curve (Gallon's curve) with regular ascending and descending limbs. If now the extreme plus or minus variants of such an animal or plant popula- tion be bred and the peculiarities under investigation be studied in the descend- ants, there is found a 'regression,' the descendants of the plus or minus variants approaching the mean fixed for the population. For example, very large parents have on the average, offspring smaller than themselves, diminutive ancestors children larger than they. But since the return to mean is not complete, there appears a possibility, by continued selection to establish the variation from the mean as a permanent character. Again there are the cases where 'pure strains' are employed in breeding, where the descendants from one and the same parents, or better, from a her- maphrodite plant, are used and the plus and minus variants are continually inbred. There then follows a complete reversion; the descendants of the plus variants give the same curve, with the same mean and the same limbs as the minus variants and the same as the pure strain. "Within the pure strain there is therefore a certain 'genotypic character,' revealed by Gallon's curve, equally ap- plicable, whether plus or minus variants or mean forms be employed for breeding; against which selection is powerless. From these results, drawn certainly from insufficient empiric material, many have concluded that the fluctuating variations, on which Darwin's theory lays such stress, cannot be taken into account in the origin of new species. The breeding of pure strains has given another important conclusion regard- ing the nature of variations: that mutations, that is pure-breeding, sharply cir- cumscribed variations, are much more common than had been thought. There are mutations, which on account of the inconspicuousness of the character are easily to be regarded as fluctuating variations, and very likely some of these were regarded by Darwin as such. If different field crops (wheat, oats, vetches, alfalfa) or many meadow grasses be cultivated in pure strains, there are found among the descendants of the same ancestors not a few varieties, which differ by such slight characters that the eye of the trained breeder is necessary to recognize them; yet, by prolonged pure cultures, they show themselves constant and furnish most favorable material for artificial and natural selection. Mendelism. — In a third way these studies of pure strains have been remark- ably fruitful. These are the researches which have followed the experiments of the Abbe Mendel upon inheritance, by crossing varieties, races and species. Since the explanation of the complicated phenomena involved will be given in a later section, it need only be said here that if in certain cases races be crossed and inbred for several generations, there arises a multiplicity of forms which have the appearance of newly arisen varieties. But more accurate study shows that these are not new forms but are only the grouping of manifold characters which were indeed present in the parents, often in a latent state. These 'analytic varieties' called forth by crossing are in part constant and can furnish material for new varieties. The importance of all of these researches cannot be estimated too highly; not because they have already given a final decision on the significance ot the various kinds of variability but because they have brought the problem of species formation from the region of much sterile theoretical speculation into the clear light of exact experimental investigation. Migration Theory.— To explain how characters formed by variation become fixed, and do not disappear again through crossing with differently 46 GENERAL PRINCIPLES OF ZOOLOGY modified individuals, Moritz Wagner has proposed the Theory of Geo- graphical Isolation, or the Migration Theory. New species may arise if a part of the individuals of a species should wander to a new place, in which crossing with the others of their species who were left behind is not possible. The same might occur, if geological changes should divide the region inhabited by a species into two parts, between which interchange of forms would no longer be possible. The animals remaining under the old conditions would retain the original characteristics; the wanderers, on the other hand, would change into a new species. Direct observations support this. A litter of rabbits placed at the beginning of the fifteenth century on the island of Porto Santo has increased enormously and the descendants have taken on the characteristics of a new species. They have become smaller and fiercer, have acquired a uniformly reddish color, and no longer pair with the European rabbit. A further proof of the theory of geographical isolation is the peculiar faunal character of regions separated from adjacent lands by impassable barriers, broad rivers or straits, or high mountains (comp. p. 34) ; especially instructive is the pecul- iar faunal character of almost every island. The fauna of an island resembles in general the fauna of the mainland from which the island has become separated by geological changes; it usually has not only these but also 'vicarious species,' i.e., species which in certain characteristics closely resemble the species of the mainland. Such vicarious species have plainly arisen from the fact that isolated groups of individuals, scattered over the island, have taken on a development divergent from the form from which they started. With all due recognition of the migration theory, it will never be possible to explain the multiformity of the organic world by it alone. It must be assumed that formerly the earth's surface possessed an enormous capacity for change; but the more rescent investi- gations make it probable that the distribution of land and water has not varied to the degree that was formerly believed. The experience of botanists, too, teaches that several varieties can arise in the same locality and become constant. Lamarckism. — \Vhile the migration theory agrees with Darwinism in this, that the new characters appearing through variation are to be regarded as the products of chance, yet it is just this part of the theory which has been subjected to searching criticism. Many zoologists have again adopted the causal foundation of the descent theory proposed by Lamarck and believe that the cause of species formation is to be found in part in the immediate influence of changing environment, in part in the varying use and disuse of organs, brought about by alterations in the con- ditions of life. Both principles, they say, are sufficient, even without the HISTORY OF ZOOLOGY 47 help of the struggle for existence, to explain the phylogenesis of organ- isms. (Neo-Lamarckism.) Influence of Environment. — To what extent can the environment directly bring about a permanent change in the structure of plants and animals? To decide this is no simple problem, on account of the com- plexity of the factors entering into the question. In cases where the food-supply is altered, organisms change in a very remarkable manner and within a short time; but these changes (Nageli's ' Modifications through Nutrition') seem to have no permanence. Plants which, found in nature in poor soil, are transplanted into rich soil, or vice versa, soon acquire quite a different appearance, and preserve this through the following generations, so long as they remain in the rich soil; but the plant quickly returns to its former appearance when replaced in its previous surroundings. In general, a change seems to be the more permanent the more slowly it has developed. In researches upon the influence of environment, it is better to experiment with slowly-working factors, such as light and heat, dry or moist air, different intensities of gravitation, of stimuli, etc., which can be excluded from the environment of the organism. In this way positive results seem to have been attained. When pupre of Vanessa •urtica and Arclia caja are reared in the cold (down to — 8°C.) the butterfly or moth which escapes is more or less conspicuously modified, the male the more. If these altered males and females be used for further breeding, a part of the male offspring will have the cold markings, even if reared in the normal conditions. It is however probable that in these cases the cold had a direct effect upon the germ cells from which the aberrant indi- viduals arose. The results of Tower in breeding potato-beetles are in the same direction and are even more conclusive as to the modification of the germ cells. Use and Disuse. — Regarding the efficiency of use and disuse, there is no doubt that an animal is influenced to a great extent by the manner in which the organs are used. The organs which are much used become strong and those which are not used become weak. The only question is whether these, in the strict sense of the word, newly-acquired character- istics are transmitted to the offspring, or whether the descendants, in order to attain to the same condition, must repeat in the same way use and disuse. In the latter case the cumulation of characteristics, and with it the possibility that these may become permanent, is excluded. It is to be regretted that accurate results are still lacking on a point so well adapted for experimental treatment. At this time rudimentary organs strongly favor the Lamarckian principle; for we see that cave animals, which for 48 GENERAL PRINCIPLES OF ZOOLOGY many generations have lived in darkness, are blind, either having no eyes, or only vestiges of them, incapable of function. This seems to justify the view that this condition is attributable to lack of use, since it has brought about a functional and anatomical incapacity, which has increased from generation to generation. Now we must believe that what is true for disuse must express itself in the reverse sense in the case of use. Nageli's Principle of Progression. — In conclusion, there is still to be considered the change of species from internal causes, which Nageli has termed the 'perfecting principle,' or the 'principle of progression.' It cannot be denied that each species is compelled, by some peculiar internal cause, to develop into new forms, up to a certain degree inde- pendently of the environment and of the struggle for existence. In all branches of animals we see the progress from lower to higher going on, very often in a quite similar way, in spite of the fact that the plan of organization is- so different in the various phyla. We see how the nervous system, lying near the surface in the lower animals, becomes in the higher animals internal; how the eye, at first a simple pigment-spot, becomes in worms, arthropods, molluscs, and vertebrates, provided with accessory apparatus, as lens, vitreous body, iris, chorioid. Here we see an energy for perfection which, since it occurs everywhere, must be independent of the individual conditions of life, and must have its special explanation in the reaction of the living substance to light. It is by no means justifiable to call an assumption, as here expressed, teleological, and to reject it as unscientific; rather the organism seems to be just as mechanically conditioned as a billiard-ball, whose course is deter- mined not only by contact with the cushions of the billiard-table, but also in a large measure by its indwelling force, imparted to it by the stroke of the cue. An organism, too, is a store of energy which it must necessarily have developed from itself, but it is of more extraordinary complexity, and to an equal degree also is independent of the external world. A complete independence naturally never occurs. Instead there is always an 'action' of the external world, a modifying influence which is carried on by the external conditions of existence, either directly or by the mediation of use and disuse. This outline of evolution has been given in a rather detailed way, because in the history of zoology it is the most important feature. No other theory has gained such a hold, none has propounded so many new problems and opened so many new fields for research. There is no other zoological theory which compares with it in value as a working hypothesis. To the objection that the theory is insufficiently grounded, HISTORY OF ZOOLOGY 49 it can only be replied that it is the only theory which agrees with our ex- periences and explains these in a simple way and on a scientific basis. In this sentence is given the merit of the theory, and also a limitation of its applicability. For on the one side the statement attributes the merit in the applicability of the system to the necessity of the human mind for simple explanations of the facts of natural science, and on the other hand it makes the degree of correctness dependent upon the state, whatever it may be, of our knowledge. Many investigators see no necessity of reconciling paleontology and our knowledge of plants and animals. To such the Darwinian theory proves just as little as any opposing theory. Meanwhile thoughtful naturalists will keep in mind that our knowledge of nature is making considerable advances, and is visibly becoming wider and deeper. It is possible, even probable, that these advances will lead to many modi- fications of the theory. The conception of the way in which forms have developed from one another admits, as the mutation theory shows, of very different expressions. On the other hand, we can affirm with great certainty that the principle of descent, which first obtained cre- dence through Darwinism, will be a permanent landmark of zoological investigation. GENERAL MORPHOLOGY AND PHYSIOLOGY. General Zoology: Animal Morphology. — In vital phenomena a certain degree of similarity can be followed through the animal kingdom; the way in which animals are nourished and reproduce their kind, move, and gain experience, is essentially the same in great groups. Correspond- ing to this, the apparatus concerned with the above-mentioned functions, the organs of nutrition and reproduction, of motion and sensation in their grosser and finer structure, and in their ontogeny, must be similar to one another and show evidence of some fundamental characters which always or frequently recur. (Ecology or Biology. — When by means of anatomy and embryology we have learned the general character of the organism, we must then study its relations to the environment. In this study of the conditions of animal life, cecology or biology, we consider the geographical range of animals, their distribution over the surface of the earth and in the different depths of the sea; further, the reciprocal relations of animals and plants, and of beast to beast, as these find special expression in colony-building, symbiosis, parasitism, etc. General Anatomy. — The synthesis of an organism, of which we can only gain an idea by general anatomy, actually takes place in nature during the development of every animal. Embryologically every organism is at some time a simple element, a cell; this divides and gives rise to tissues; from the tissues are formed organs, and from the organs the regularly membered whole of the animal body is composed. I. GENERAL ANATOMY. The Morphological Units.— The expression 'constituent parts of the animal body' can be used in a double sense. We can speak of the chemical units, the chemical combinations, which form the tissues; these are the subject of animal chemistry, and may therefore be passed over here. But we may also speak of the units of form (morphological units) of the animal body; these are the cells. These and their transformation into tissues, organs, and entire animals are for us of vastly greater importance. 50 GENERAL ANATOMY 51 I. THE MORPHOLOGICAL UNITS OF THE ANIMAL BODY. The Cell. — The study of the morphological units of the organic body first found a firm foundation in the cell theory. Every scientific study of the anatomy of plants and animals must therefore take the cell as its starting-point. The Cell Theory. — In order clearly to understand the conception of the cell and its name it is necessary to follow a little of the history of the theory of the cell. When the name was first given to the structures in plants it implied small chambers with firm walls and filled with air or fluid. Then came the discovery of a small body, the nucleus, inside the cell. Next Schleiden made the generalization that the cell was the ana- tomical and physiological unit from which all plants are formed, but he held the erroneous view that in the development of cells, the nucleus was formed in a sort of matrix, then around it a nuclear membrane arose by precipitation, and then a larger membrane, the cell wall, around the whole. Then Schwann extended the generalization to animals, thus giving it an extension to all organisms. In this Schleiden- Schwann cell theory the cell wall was all important, as through it diffusion currents must pass between the contents and the surrounding medium. Hence the wall and the contents must determine the character of the cell according to physical laws. Since the life of an organism is but the totality of the life of the cells of which it is composed, it was thought that the theory was a great advance in the problem of the physical explanation of the phenomena of life, and the origin of the cells themselves was as well explained as the formation of a crystal. Our conception of a cell has completely changed. We know that they do not arise like crystals, but from preexisting cells. The cell is not merely a part of an organism; it is a physiological whole, which shows us all the enigmas of life. The membrane and cell sap, so important in the Schleiden-Schwann theory, have but a subordinate place, but all impor- tant is the previously disregarded substance, protoplasm. Now a cell may be defined as a small mass of protoplasm with one or more nuclei. This change in the conception came so gradually, that the name cell has persisted, although it is an evident contradiction to call a solid lump without a membrane a cell. These changes were due to many different lines of investigation. Thus the early discovery that the chlorophyl bodies in plant cells move and, later, that the motion is due, not to the bodies, but to the substance in which they are imbedded. This substance, to which the name protoplasm was given, acquired prominence when it was found that in the simplest 52 GENERAL PRINCIPLES OF ZOOLOGY algae, it, together with the chlorophyl, could leave the cell wall and swim freely in the water, eventually giving rise to a new plant, while the cell wall no longer showed signs of life. Then it was discovered that many animal cells had no cell membrane. These observations at once placed the cell wall in the background, while the protoplasm was recognized as all important. Here, too, should be mentioned the researches on the Pro- tozoa, by which it was recognised that these organisms had no true organs, but carried on all of the functions of life by means of a granular substance, at first called sarcode. Then followed the recognition of the identity of the protoplasm of plants, the sarcode of the protozoa and the cell substance of animal cells. Equally important was the new idea as to the modifications of cells and the differentiation of tissues. These are not so much modifications of form and the like, based on osmosis and other physical phenomena as upon chemical changes. By means of its formative potentiality the protoplasm gives rise to non-protoplasmic structural parts, as, for ex- ample, -connective-tissue fibrils, muscle fibrils, nerve fibres, etc. These give the various tissues their specific character and perform their functions. The tissues also retain as the source of life and formation the unemployed remnants of cells, the connective-tissue corpuscles, muscle corpuscles, etc. Nature of the Cell. — The size of the animal cell varies; the smallest are the male sexual cells, the spermatozoa, whose bodies, in case of the mammals, may measure only 0.003 mm.; the largest, with the exception of the giant plasmodia of some Mycetozoa, are the egg cells. The yolk of the bird's egg, which alone forms the egg in the narrower sense, has for a time the value of a cell, and in the ostrich egg may reach a diameter of several inches. The form is likewise variable. Free cells, are usually spherical or oval as the egg cell shows; united into tissues, the cells, on the contrary, may be pressed together into polygonal or prismatic bodies, or may send out branching processes. Protoplasm. — So there is left to characterize the cell only its sub- stance: the cell is a mass of protoplasm with one or more nuclei. It is not known whether protoplasm is a definite chemical body, capable of infinite variation, or is a varying mixture of different chemical substances. So, also, we are not certain whether or not these substances (as one is inclined to believe) belong to the proteids. We can only say that the con- stitution of protoplasm must, with a certain degree of homogeneity, have a very extraordinary diversity. For if from the egg of a dog there comes always and only a dog with all his individual peculiarities; that a sea- urchin's egg, under the most diverse conditions, produces always a sea- GENERAL ANATOMY urchin; that a species of amoeba always performs only the movements characteristic of that species, we must assume that the functioning part of this cell, the protoplasm, has in each case its peculiarities. We are driven to the assumption of an almost unlimited diversity of protoplasm, even if we concede an important share in the prominent differences to the nucleus, of which we shall speak later. General Properties of Protoplasm. — The similarity of protoplasm expresses itself in its appearance and in its vital phenomena. Under slight magnification, protoplasm appears as a faintly gray substance (sometimes colored yellowish, reddish, etc., by pigments) in which numerous strongly- refracting granules are imbedded. The vital characteristics of this sub- stance are movement, irritability, power of assimilation and of reproduction. By using higher powers a finer structure can be seen in the 'homogeneous protoplasm' of earlier writers. It looks like a fine-meshed framework (filar substance, spongioplasm, cell reticuluin) the interstices of which are filled with other material (interfilar substance, enchylcma, ground substance}. The question whether this framework is formed of threads and trabeculae or whether the appearance is not formed by small cham- bers, bounded by fine partition-walls (foam structure of protoplasm), such as results when two fluids which do not mix (like olive oil and soda solution) are shaken together until a very fine froth is produced. This view that protoplasm has a foam structure explains how it can be a fluid aggregate with a fine structure. Regarding the fluid aggregate condition cf protoplasm (long called a 'solid-fluid') there has been much dispute. Exact re- searches regarding its physical condition show that it behaves like a fluid. Movement of Protoplasm. — Move- ment expresses itself first in changes of form of the whole body — amoeboid move- ment— and secondly in the change of position of the small granules in the interior of the protoplasm — streaming of granules. Examples of amceboid movement (fig. 16) are found in many Protozoa, and the colorless blood-cells (leucocytes) of multicellular ani- mals; here the protoplasmic body sends out coarser and finer processes, which may be again withdrawn, serving for locomotion and hence called pseudopodia. The streaming of granules can be observed in the interior of the cell-body, as well as in the pseudopodia extending from this. The pseudopodia may even be so fine as to be at the limits of visibility with our FIG. 16. — Amccba proteus (after Leidy). ek, ectosarc; en, entosarc; cv, contractile vacuole; n, nucleus; N, food-vacuoles. 54 GENERAL PRINCIPLES OF ZOOLOGY NT FIG. 17.^ — Gromia oi'iformis (from Lang, after M. Schultze). GENERAL ANATOMY 55 microscopes (fig. 17), yet in them the granules wander hither and thither like people on a promenade, simultaneously centripetally and centrifugally, some with greater, others with less speed. The granules are moved by the protoplasm, for if we feed the creature with finely-pulverized car- mine, these granules show the same remarkable streaming. Indeed, nothing better illustrates the great complexity of protoplasm than these phenomena of motion in such narrow limits as pseudopodia in general. Irritability of Protoplasm. — That amoeboid movements and streaming of granules can be induced, brought to a standstill, and modified by mechanical, chemical, and thermal stimuli, is a proof of the irritability of protoplasm. Most important are the thermal stimuli; if the surround- ing medium rise above the ordinary temperature, the movements at first become more rapid up to a maximum: from that point begins a slowing, finally coming to a standstill — heat-rigor. If the high temperature continue much longer, or if it rise still higher, death results. The fatal temperature for most animals is between 40° and 50° C. (io4°-i22° F.); its influence explains a part of the injurious effects which high- fever temperatures have upon the human organism. Like the heat- rigor, there is a cold-rigor, induced by a marked sinking of the tempera- ture below the normal. This is accompanied by a gradual diminution of mobility ; it results in death by freezing, which is, however, not so easily produced as death by heat. It is a remarkable fact that many animals, consequently their cells, may be frozen; and in this condition can endure still severer cold without dying. (For example: goldfish, a temperature of -- 8° to -- 15° C.; frogs, to - 28°; 'blind worms', to -- 25°). Nutrition and Reproduction. — Irritability and power of motion are necessary for assimilation. Most animal cells, for example almost all the tissue cells, are not suitabe for studying assimilation, because they live upon liquid nourishment. But certain cells of higher animals, the leucocytes, and most unicellular animals can be fed with solid substances; they take the food-particles into the midst of the protoplasm by flowing around them with the pseudopodia. They extract all the assimilable and reject the indigestible portions (fig. 16). In the case of assimilation it is to be noted not only that the cells use the food which they have taken for their own growth and for replacing worn-out parts, but also that most of them have the power of producing substances other than protoplasm; for example, many Protozoa form shells or skeletons which are hardened with silica or lime. This formative power, the building of plasm ic products, is the starting-point for tissue- formation. Cell Nucleus. — The reproduction of protoplasmic bodies is synony- 50 GENERAL PRINCIPLES OF ZOOLOGY mous with the division of the cell; but to understand this we must first consider the nucleus. This is a body enclosed in the protoplasm, whose form, though definite for each kind of cell, shows in general wide varia- tions. Usually it is spherical or oval, but it may be elongated or rod- shaped, bent into a horseshoe, with constrictions like a rosary, or even be branched or treelike (fig. 18) ; in many living cells it is but little different FIG. 18. — Various forms of nuclei, a, horseshoe-shaped nucleus of an Acinete; b, branching nucleus from the Malpighian vessel of a Sphingid larva; c, rosary-shaped nucleus of Stentor cceruleus. in appearance from the protoplasm and can only be seen with care and by employment of a special technique based upon the microchemical reaction of the nuclear substance. The Nuclear Substance. — The nuclear substance is distinguished from protoplasm, among other ways, by its greater coagulability in certain acids, e.g., acetic and chromic, which therefore are often used for demon- strating the nucleus. In its minute structure the nucleus affords a wonder- ful variety of pictures varying according to the objects chosen. Accord- ing to their reactions to stains two substances in particular are distin- guished: chromatin or nude in (fig. 19, ch), which is easily stained by certain staining-fluids (carmine, haematoxylon, saffranin), and the achromatin or linin, which stains only under special conditions. The achromatin forms a network or reticulum (according to another view a honeycomb structure) filled with a nuclear fluid, bounded exter- nally by a nuclear membrane. If little nuclear fluid be present, and the reticulum consequently be narrow-meshed, the nucleus seems compact. GENERAL ANATOMY 57 If the fluid be abundant, the nucleus appears vesicular. This is espe- cially the case when the lines of the framework are separated \)y consider- able amounts of nuclear fluid (fig. 19, 4). The chromatin enters into close relations with a less stainable element, also distinct from, achromatin, the plastin, (paranudein, p). In the protozoan nuclei plastin and chromatin are usually intimately united, the first forming a substratum in which the latter is imbedded (clip). The united substances are most frequently closely and regularly dis- tributed as fine granules on the reticulum, so that the entire nucleus ch p 4 5 6 FIG. iq. — Vesicular nuclei with achromatic reticulum and different arrangements of the chromitin and nucleolar substance: p, plastin (nucleolar substance); ch, chromatin; chf, chromatin plus plastin. i and 2, nuclei of Actinosphcerium; 3, of Ceratium hintndclla (after Lauterborn); 4, germinal vesicle of Unio (after Flemming); 5, nucleus with many chromatin nucleoli. appears uniformly chromatic (fig. 18). More rarely the mixture collects into one or more special bodies, the chromatic nucleoli (amphinucleoli, caryosomes, fig. 19, i, 2). The nucleolus is ordinarily a rounded body, more rarely branched (fig. 19, i). In the nuclei of the Metazoa there may occur the same intimate mix- ture of plastin and chromatin (6). As a rule, however, the plastin (apparently not the whole, but a surplus) is separate from the chromatin. Thus there occur in the nuclei of many eggs nucleoli which consist of two distinguishable parts, the one containing chromatin, the other chromatin free (4). Usually in tissue cells only the plastin has the form of 58 GENERAL PRINCIPLES OF ZOOLOGY nucleoli (true or chromatin-free nucleoli), while the chromatin is dis- tributed on the nuclear reticulum (chromatin reticulum). Much the same may occur in ihe Protozoa (fig. 19, 3). Beside and outside of the nucleus there occurs in many Protozoa a 'chromid- ial apparatus, ' a substance agreeing in its staining properties with the nuclear substance. Its pertinence to the nucleus is also shown by the fact that repeatedly it has been observed to arise from the nucleus (ActinosphcFriutn), as well as to be transformed into nuclei (Radiolaria, Monothalamia). The chromidial mass may surround the nucleus like a cortical layer (Euglypha, fig. 20, III, Radiolaria), or penetrate the protoplasm as a loose network (II), or form lumps or coiled threads. In this last shape the chromidial mass seems to be widely distributed in strongly functioning cells of Metazoa (I). Possibly the structures described as ' mitochondria' are identical with it. II. TIL I. ch FIG. 20. — Cells with chromidial apparatus. I, muscle cell of Ascaris (after Goldschmidt). II, Arcella, with two nuclei and loose chromidial net. Ill, Euglypha with compact chromidial envelope of the nucleus, ch, chromidial mass;/, food body; m, mouth of shell; n, nucleus; r, reserve material for new shell. Function of the Nucleus. — For a long time the function of the nucleus in the cell was shrouded in complete darkness, so that it was regarded, in comparison with the protoplasm, as of little importance. The evidence that the nucleus plays the most prominent role in fertilization has altered this conception. Then arose the view that the nucleus deter- mines the character of the cell ; that the potentiality of the protoplasm is influenced by the nucleus. If from the egg a definite kind of animal develop, if a cell in the animal's body assume a definite histological character, we are, at the present time, inclined to ascribe this to the nu- cleus. From this it follows that the nucleus is also the bearer of heredity; for the transmission of the parental characteristics to the children can only be accomplished through the sexual cells of the parents, the egg and sperm cells. Again, since the character of the sexual cells is determined by the nucleus, the transmission in its ultimate analysis is by the nucleus. This idea has a further support in experiments on Protozoa. If one of GENERAL ANATOMY 59 these animals be cut into nucleate and anucleate halves, the first lives and regenerates the lost parts; the anucleate portion moves about for a time, apparently as long as the stored energy lasts, but it cannot assimilate or reproduce the missing portions and so sooner or later it dies. The Centrosome. — Besides the nucleus there frequently occurs a special body in the protoplasm, the centrosome, which, on account of its small size and a behavior similar to achromatin with reference to staining- fluids, was long overlooked. It is well distributed among the Metazoa, but is absent from most Protozoa, in many of which it appears only at certain times and then disappears. It is probable that it is a derivative of the nucleus, a part of the achromatin which has left the nucleus; in other cases possibly a second nucleus which by degeneration has lost the chro- matin and retained only the motor nuclear substance, the achromatin. In its function the centrosome is a specific organ of cell division which controls both the division of the nucleus and that of the cell itself. Multiplication of Cells. — Increase in cells occurs exclusively by division or by budding (gemmation). Most common is binary division in which a circular furrow appears on the surface of the cell, deepens and cuts the cell into two equal parts. Mutiple division is more rare and can only occur in multi- nucleate cells. Here the cell divides simultaneously into as many (sometimes hundreds) daughter-cells as there were nuclei present. In all forms of division the similarity of the products is characteristic, while in budding the resulting parts are unequal, one or more smaller daughter- cells, the buds, being constricted from a large mother-cell (fig. 21). Direct Cell Division. — Every cell division is accompanied by nuclear division or nuclear division has previously occurred. Direct and indirect division are recognized. Direct division is most common in Protozoa, especially in nuclei with abundant chromatin (figs. 21, 120, 150, 155). The nucleus elongates and is divided by constriction, in the same way that the cell itself constricts. Since the protoplasm has no special arrange- ment for dividing the nucleus (the latter besides protected by its mem- brane), we must conclude that the nucleus divides itself. The dividing force resides in the achromatic framework, which correspondingly often FIG. 21. — Cell budding. Pcdjphrya geni- mipara with buds (a) which separate and form free young (b). N, nucleus. 60 GENERAL PRINCIPLES OF ZOOLOGY FIG. 22. — Spindle formation and division of the centrosomes in As- caris megalocephala (after Brauer). c, centrosomes; ch, chromosomes. exhibits a certain arrangement, a fibrous structure in the direction of the elongating nucleus. Indirect Cell Division, Karyokinesis. — Indirect cell division, karyokinesis or mitosis,' is most beautifully shown in cells, poor in chro- matin, which possess a centrosome. The process is introduced by a division of the centrosome (fig. 22). The daughter centrosomes migrate to two opposite poles of the nucleus, which now loses its membrane and becomes the nuclear spindle. The characteristics of the spindle are that it is drawn out into points at two poles which are indicated by the position of the centrosomes, while from these poles fine threads, the spindle-fibres, run to the centre or equator of the nucleus. These fibres are in many cases certainly derived from the achromatic nuclear reticulum, while in others a greater or less part in their formation is taken by the protoplasm (fig. 22.) A debated point is the rela- tions of the fibres in the equatorial plane of the spindle. Do all the fibres extend from pole to pole ? Do all of them end in the equatorial plane, so that the spindle consists of two cones of fibres separated at the equator? Or, lastly, are fibres of both kinds present in the same spindle ? It would appear that differences exist in these respects in different cells. All of the chromatin of the nucleus lies in the equator, united in the 'equatorial plate,' but by this must not be understood a connected mass but a layer of separate bodies, the chromosomes (fig. 23, a). These develop at the beginning of nuclear division by the union of the chro- matin granules (which are distributed diffusely over the reticulum of the resting nucleus) to strongly staining bodies, which are rarely spherical or rodlike, but usually have the shape of U-shaped loops. It is of the greatest theoretical significance, that their number is identical in all the cells of all the tissues of one and the same species. The first step in the mitotic formation of the daughter nuclei is the division of the chromosomes, which is usually completed in the equatorial plate (division of the equatorial plate), but may be completed earlier. The division is an accurate halving (fig. 23, b). The two halves of a mother-chromosome, the daughter chromosomes, now travel, under the influence of the spindle-fibres, towards the poles of the spindle. In this way, by a splitting of the equatorial plate, the lateral plates arise, the GENERAL ANATOMY 61 elements of each uniting and producing the daughter nuclei. The centro- somes remain separate as division organs for the next nuclear division (fig. 23, c, d, e). What further distinguishes the indirect from the direct cell division is the active participation of the protoplasm. The centrosome is the centre of a marked radiation (aster) of the protoplasmic reticulum (fig. 22). When the centrosome divides a double radiation appears, the monaster becomes an amphiaster. Not only the spindle-fibres but the protoplasmic d e f FIG. 23. — Cell division in the skin of SalamanJra maculosa (after Rabl). rays extend from the daughter chromosomes. Since the arrangement and degree of development of the protoplasmic radiations stand in definite relation to the different phases of cell division we must recognize in them the expression of the forces (apparently contractile) in the protoplasm which cause cell division. Between these two extremes of direct and indirect division are transitions which show how the mechanism of nuclear division has been completed step by step, first, by the fibrous arrangement of the nuclear reticulum (spindle structure : second, through the development of the centrosome by which the division ob- tains an influence on the protoplasm; and third, by the organization of the chromosomes. The irregular division of the chromatin mass in direct division is relatively crude in comparison with the complicated processes involved in the formation and division of the chromosomes. These become intelligible it we regard the chromatin as the controller of the cellular processes and the bearer of heredity (cf. fertilization, infra). The more highly organized the animal, the more its cells have to inherit and the more important it is that the physical basis of heredity should be accurately divided in amount and in quality be- tween the daughter cells. This is accomplished by mitosis. 62 GENERAL PRINCIPLES OF ZOOLOGY Connected with this great functional importance of the chromosomes as the bearers of characteristics are two much disputed problems, (i) The 'individual- ity of the chromosomes.' This sees the persistent organization of the cell in the chromosomes, which persist between two cell divisions, but are not recog- nizable as such because their substance is vacuolated and distributed through greater space. Of course this view does not conflict with the fact that they, like all living substance, undergo a gradual renewal, in which effete parts are replaced by new and there is an increase of its substance without which a repro- duction of chromosomes by division would be impossible. (2) The theory of the functional diversity of the chromosomes. If the chromosomes carry the characteristics, it is more probable that each one does not contain the germs of all the peculiarities of the organism; rather there is a division of labor by which the separate peculiarities are distributed among the different chromosomes. This view is supported by the fact that there is, in numerous instances, a morpho- logical differentiation between them (differences in size, shape, staining qualities). That in the last analysis each category of characters consists of at least two lines (male and female) is shown by the fact that, as is ex- plained in the section on fertilization, half of each nucleus is derived from the father, half from the mother. Nuclear Fragmentation is to be distinguished from direct division; by it the nucleus becomes broken up into a few or numerous parts. Such nuclear fragmentation is not rare in the Infusoria but it occurs occasionally in Metazoa (giant cells of bone marrow — fig. 24 — osteoblasts, certain stages of the genital cells). It is explained as follows. There normally exists a certain size relation between nuclear mass and protoplasmic mass. With greater cell activity, as with Infusoria which have long been well fed, the nucleus grows 24 Giant- at *ne exPense of the protoplasm until it reaches a size which cell with many makes further assimilation and increase impossible. This nuclei. kind of animal (or cell) can return to the normal vital activities if the nuclear mass be reduced. This is begun by the fragmentation and is completed by the resorption of the nuclear substance. Many cases of nuclear fragmentation, formerly regarded as amitoses are really functional conditions of actively functioning cells and have erroneously led to the view that amitosis is beginning cell degeneration. Multinuclearity, Multicellularity. — Nuclear division and cell division commonly constitute a well-arranged mechanical process, the separate phases of which follow one another according to a definite law. The plane of division is perpendicular to the long axis uniting the two poles of the spindle; usually also each phase of division of the nucleus corresponds to a certain phase of the protoplasmic division. But the interrelation of cytoplasm and nucleus is by no means an unchangeable and indissoluble one, for very often nuclear division takes place without participation of the cytoplasm. If this process be repeated several times, there results a mass of protoplasm with many nuclei (fig. 24), which now may become many cells, if subsequently the protoplasm divide according to the number of nuclei. Hence multinucleated protoplasmic masses are transitional stages between the simple mononucleated cell and a collection of several GENERAL HISTOLOGY 63 mononucleated cells, and in consequence of this are sometimes regarded as the equivalent of one cell, sometimes as equivalent to many cells, and are called sometimes multinucleated giant-cells, sometimes syncytia. In the following pages a multinucleated mass of protoplasm will he consid- ered as a single cell, because a cell is a vital unit, has a physiological individuality, and in this respect a multinucleated mass of protoplasm behaves like a mononucleated. As the tissue cells and the Protozoa show, the plane of organization is not raised in the least by the multinuclearity. A change only begins at the moment when many particles of protoplasm are separated from one another, and many vital units are formed, i.e., when in place of multinuclearity a true multicellularity appears. II. THE TISSUES OF THE ANIMAL BODY. Definition of Tissue. — In the formation of tissues two processes are operative: (i) the multiplication of cells into cell-complexes, and (2) the histological differentiation of cells. A tissue, therefore, can be denned as a complex of differen- tiated cells histologically similar. Histological Differentiation. — The chief result of the histological differentiation is that the cells have a definite form and definite relations to neigh- boring cells. In addition, there almost always occurs, as a more important feature, the histological modification of the cell. The fact has already been mentioned that the cell uses its food-material, no', only for its own growth, for increase of its proto- plasm, but also for forming substances, plasmic products, either in its interior (internal plasmic procl- - , 11- FIG. 2v — Forma- ucts), or more often on its surface (external plasmic tion of muscle t-lbriis products). The histological differentiation is the in the frog (diagram). ._ , . ,, • a, formative cell; b, formation of specific plasmic products. A cell in format}ve cell with becoming a muscle fibre (fig. 25), continually secretes upon its surface new fibrillae of specific muscle sub- stance until finally the remnant of the formative cell, the 'muscle corpuscle,' is contained in a mantle of muscle fibrillae. In the same way, each tissue, upon histological ex- amination, is seen to be composed of cells and plasmic products. The former control the formation, the renewal, and the sustenance of the tissue; the latter are the agents of its physiological function. The advantages of tissue formation are far-reaching, since in general they are connected with division of labor. So long as the cell unites in itself t\vo transversely stri- ated muscle fibrils; <-, formative cell with numerous muscle fibrils. 64 GENERAL PRINCIPLES OF ZOOLOGY all the vital functions, these are incomplete because they mutually hinder each other in their free development; the plasmic product, on the other hand, has only the single function peculiar to it and can therefore per- form this with greater completeness. Muscle fibrilke, the characteristic elements formed by the muscle cells, have preserved of the various prop- erties of pro oplasm only the capability of contraction; but this contrac- tion is much more energetic and stronger than the mere movement of protoplasm. Nerve fibriliae only transmit stimuli, but in far more rapid and orderly manner than does simple protoplasm. Classification of Tissues. — Since in every tissue its function interests us most, it is natural to classify tissues by the function and the intimate structure connected therewith. The tissues are arranged in four groups: i. Epithelial tissue; 2. Supporting tissue; 3. Muscular tissue; 4. Nervous tissue. Within these, however, certain parts of the animal body, to which indeed the term- 'tissue' is scarcely applicable, find no shelter; these are the sexual cells, the blood, and the lymph. The first may be spoken of in connection with the epithelium, the others with the supporting substances. i. Epithelial Tissues. Morphology of Epithelial Tissues. — An epithelium is a layer of cells covering any free surface, external or internal, of the body. The epithelia must be considered first, because they are the oldest tissues; the first to appear in the animal kingdom, there being animals which consist only of epithelia. Further, every metazoan, during the first stages of embryonic life, consists only of epithelial layers, the germ- layers. With this is connected the fact that epithelial cells have under- gone the least degree of histological change, and that the formation of plasmic products is subordinated. Function of Epithelium. — Epithelium forms a protecting and ex- cluding covering over surfaces, equally valuable whether the surfaces are external (surface of the body) or in cavities in the interior of the body (the body cavity, lumen of the gut and blood-vessels). The importance of the epithelia in this respect is shown by the fact that if the protecting layer be removed, inflammation arises and continues until the epithelium is regenerated. Only exceptionally do areas occur which are free from epithelium; the teeth of vertebrates, the antlers of stags, on account of their hardness, can exist, at least for a time, without epithelial covering. Glandular and Sensory Epithelia. — By their position epithelia are suited for two other functions: all substances which ought to be removed from the body — some because they have become useless, and consequently GENERAL HISTOLOGY 65 injurious (excreta), and others, as, for example, the digestive fluids, because they have to perform important functions (seer eta, — must pass the surface and are therefore exuded by the epithelia; these are the glandular epithelia. Further, all external influences chiefly impress the surface of the body, causing sensations; hence certain epithelia are of the greatest importance for the reception of sensory stimuli, and serve for hearing, seeing, smelling, tasting, and touching. Such areas of epithelium are called sensory epithelia. Covering Epithelium. — The covering epithelium consists of cells which are united by a small quantity of cementing substance. We speak of simple or of stratified epithelia, according as we find, in sections running perpendicularly to the surface, one or several superimposed layers of cells (figs. 26, 27, 28). Simple Epithelium. — Only one-layered epithelia are found in all in- vertebrated animals and in Amphioxus; in the vertebrates, on the other hand, they are limited to the internal cavities of the body, and even here are occasionally, as always in the skin, replaced by a many-layered epithelium. According to the shape of the cells we distinguish cuboidal or pavement, flat, and columnar epithelium. In pavement epithelium (fig. 26. b) the cells are developed about equally in all directions of space, and because they have become compressed by lateral pressure have the appearance of cubical blocks or paving-stones. In columnar epithelium the long axis, the distance from the deeper to the peripheral end of the cell, is especially great (fig. 26, c); finally, in flat or squamons epithelium this is greatly shortened (fig. 26, a) and the separate cells form thin plates. Flagellated and Ciliated Epithelia. — Further differences in these three kinds of epithelium are caused by the presence or absence of pro- cesses (cilia, or flagella) on the peripheral end of the cells. These arc fine threads which arise from the body of the cell, extend above the surface and maintain an extremely lively motion. In flagellated epithelium (fig. 26, d) each cell has only one vibratile projection, but this is strongly developed; in ciliated epithelium (fig. 26, e), the surface of each cell is covered with a thick forest of shorter threads moving in unison. Cuticle. — The majority of the one-layered epithelia are covered by a cuticle, a membrane secreted by the epithelial cells which hence very fre- quently shows the impression of the cells as polygonal markings. In many cases thin and inconspicuous, it may in other instances become thickened into a very considerable layer, much thicker than the matrix layer of epithelium which secretes it. The cuticle is composed of layers parallel with the surface, and forms a more effective protection for the body than does epithelium; it becomes a protective armor, as shown, 5 66 GENERAL PRINCIPLES OF ZOOLOGY among other examples, by the calcareous shells of molluscs and the chit- inous integument of insects (fig. 26, f). Stratified Epithelia. — The protection furnished by the cuticle in the case of simple epithelium, may in the stratified be obtained immediately through a change of a part of the cells themselves. In (he stratified epithelia the cells of the various layers can be distinguished by their form. FIG. 26. — Various forms of epithelia. a, flattened epithelium oiSycandra raphanus, a' in cross-section, a" in surface view; b and c, cuboidal and columnar epithelium of a mollusc (Haliotis tuberculata); d, flagellated epithelium of an actinian (C alii act is parasitica); e, ciliated epithelium from the intestine of the fresh-water mussel;/, epithe- lium (e) with cuticle (c) of Cimbex coronjtus( a wasp). The deepest layer consists of cylindrical cells; the superficial, of more or less flattened elements; between lie several layers of transitional forms, so that starting from the cylindrical cells we gradually pass through cubical cells to the flat cells of the surface. As this arrangement shows, there exists a genetic connection between the layers of cells: the lower cylindrical cells are in a state of active multiplication; their descendants, with gradual GENERAL HISTOLOGY 07 changes of form, become the superficial layers, here to replace an equal quantity of worn-out cells (fig. 27). In the course of this change of position, the cells may undergo an altera- tion; in the reptiles, birds, and mammals (fig. 28) they became cornified, first the margins, then the inner part of the cell, changing into horn {^keratin1} . The nucleus remains for some time, but at length this vanishes, -Xo Co FIG. 27. — Section through the skin of Petro- myzon planeri. Ep, the many-layered epithelium of the epidermis, including B, goblet cells; Kd, granular cells; Ko, club-cells; Co, corium (with blood-vessels, G), consisting of bundles of fibrils running horizontally (W) and vertically (5) (from Wiedersheim). FIG. 28. — Stratified epithelium of man. 5.17, stratum Malpighi; sc, stratum corneum. FIG. 29. — Single-layered epithe- lium of a snail, c, cuticle; d, goblet cells. and then the cell becomes completely changed into a dead, horny scale. In the skin of the higher vertebrates the zones of the living protoplasmic, and the cornified cells, are sharply marked off from one another. In cross-section they are readily distinguished as the stratum corneum (sc) and the stratum Malpighi (sM) of the skin (fig. 28). In the many- layered epithelia the cuticle has lost its importance, and it is either an in- conspicuous boundary line or is entirely absent. 68 GENERAL PRINCIPLES OF ZOOLOGY Glandular Epithelium. — Glandular epithelium is distinguished physiologically from ordinary epithelium by the fact that it also produces secretions or excretions; anatomically it is recognizable by the presence of gland cells, which carry on the secretion and, to a greater or less extent, reveal their character by their structure. Characteristic glandular cells are, for example, the goblet cells; here the secretion, usually mucus, is collected as a clear mass in the interior of the cell, the cytoplasm being compressed into a thin external wall, reminding one of a goblet, contain- ing the nucleus at its base (fig. 27,5, 29, d). Other gland cells are granular cells, swollen bodies filled with secretory granules (fig. 27, A'o). Natu- rally all grades between pavement and glandular epithelium occur. Com- monly the latter name is only employed when the gland cells are especially numerous, thereby giving to the area a pre-eminently secretory character. This is especially the case with the structures which have the name of glands, among which we distinguish unicellular and multicellular glands. Unicellular Glands. — Unicellular and multicellular glands increase the secretory surface by invagination. Invagination of a single cell produces the unicellular glands which are chiefly found among the invertebrate animals (fig. 30) ; a gland cell here be- comes so enormous that there is no room for it in the epithelium, but it is pushed into the deeper, the subepithelial layers, the nucleated cell body, distended by secretion, sending up a slender process, the duct, to the epithelial surface. Multicellular Glands. — In the forma- tion of multicellular glands a considerable area of glandular epithelium grows as a tube or duct from the surface down into the deeper tissues; this seldom remains simple; it usually branches and forms the compound glands, which may consist of hundreds or thousands of glandular FIG. 30. — Unicellular glands from edge of the mantle of Helix pomatia. e, epithelium; d, unicel- lular glands; p, pigment cells. sacs, all emptying into a common duct. Among the multicellular glands are to be distinguished tubular and acinmis (racemose) forms. In tubular glands (fig. 31) the simple or branched glandular pouches preserve the same tubular diameter from beginning to end; in the acinous glands (fig. 32), on the contrary, the blind end of the glandular pouch widens into a sac (acinus'), largely composed of secretory cells, and related to the duct, as grapes to their stem. To the tubular glands belong the GENERAL HISTOLOGY 69 liver, kidney and sweat glands of man; to the acinous belong the salivary glands, not only of vertebrates, but also of arthropods and molluscs. FIG. 31. — Tubular glands (after Toldt). A, glands of Lieberkiihn from the human intestine; A', of the conjunctiva of the eye; B, of the cat's stomach; C, from the medullary pyramids of the dog's kidney; D, from the cortex of the rabbit's kidney. Sexual Epithelium. — The sexual cells may be considered in connec- tion with glandular epithelium. As the secretion of some glands must be expelled from the body, so the sexual cells are elements which must FIG. 32. — Acinous salivary gland of the aph'd Orthezia catapliractti ('after List). In some acini the nuclei and boundaries of the cells are shown. reach the exterior in order to perform their function. Just as the gland-cells are usually scattered among ordinary epithelial cells, so the sexual cells almost invariably lie imbedded in epithelium of the skin 70 GENERAL PRINCIPLES OF ZOOLOGY (fig. 33), of the gut, of the body cavity, or of parts cut off from this (fig. 34). This connection of the sexual cells with the epithelium has a deeper meaning since many organisms, particularly those of low structure, consist exclusively of epithelia and therefore must develop FIG. 33. — Germinal epithelium of a medusa, ek, ectoderm; en, entoderm; o, egg; e, epithelium. their sexual products in epithelium. In other words, sexual and epithelial cells are the oldest elements of the animal body, and hence very early came into rela.tion with one another. Sexual epithelium (or germinal epithelium} like glandular epithelium has a tendency to grow into the subepithelial tissues in the form of FIG. 34. — Section through the ovary of a new-born child (after Waldeyer). ge, germinal epithelium; pe, primitive egg in the germinal epithelium; p, egg-pouch; g, egg-nest constricted off from the pouchlike growth (p) ; /, single egg with follicle ; v, blood-vessel. isolated or branching tubes (figs. 34, />, 35), and thus in many groups of animals the sexual organs resemble branched glands; hence one speaks as often of sexual glands as of sexual organs (fig. 34). The male and female cells, the specific elements of the germinal epithelia and of the GENERAL HISTOLOGY 71 sexual glands, differ from each other in the fact that the eggs are generally the largest, the spermatozoa the smallest, cells of the animal body. Egg-cell. — The egg-cell or oo'cyte (fig. 36) as it is formed in the ovary varies in size according to the animal group: in case of the microscopic Gastrotricha it is less than 0.04 mm., in man about 0.2 mm., in the frog several millimetres, and in large birds often several inches; however, only the yolk of the bird's egg is the egg-cell, the white and the shell are structures formed outside of the ovary in the oviduct. These remarkable differ- ences in size are caused less by the quantity of the peculiar cell-substance, protoplasm (primary yolk), than by the accumulation of dcutoplasm (food or accessory yolk, also briefly called yolk). !f 'V5;j[ The deutoplasm is to nourish the embryo during development, and hence consists of substances rich in fat and proteid, arranged in oil-drops, or in fine granules or polygonal bodies, the yolk- granules. Its quantity, and therefore the size of the egg, is in part proportional to the length of time which the egg is cut off from any other supply of nourishment. In general we find the largest eggs in the case of the highly organized oviparous animals, where a long development uj FIG. 35. TIG. 36. pIG 35 —Ovarian tube of an insect, T",;»r.^-; wtira: a, formative cell; />, follinilnr epithelium; c, nutritive cells; d, egg-cells; /, fibrous covering extending out into the terminal fibres (?) (after Waldeyer). JTIG> 26. — Immature egg-cell of Strongylocentrotus lividus. inside of the shell is necessary to lay the foundation of the manifold organs. Besides the protoplasm and deutoplasm, a cell nucleus or 72 GENERAL PRINCIPLES OF ZOOLOGY germinal vesicle always occurs in the egg. Its contents are mainly the nuclear fluid, through which is distributed an achromatic network, and in addition the nucleolus (germinal spot). The Spermatozoa, the morphological elements of the male reproductive product, are so small that their finer structure can be studied only with the strongest powers of the microscope (fig. 37, I and II). Easiest to rec- ognize is the head, which from its form — spherical, oval, sickle-shaped, etc. — often renders possible the specific determination of the spermatozoa. The head (2) is the closely compacted chromatic part of the nucleus, and hence colors very deeply in staining fluids. Often the head is continued in front as a sharp point, the perforatorium (i), which is apparently adapted to aid in the penetration of the egg in fertilization. Next comes GENERAL HISTOLOGY 73 an unstaining second part, the middle piece (4), and then the tail (5), a long flagellum, which causes the active motility of the ripe spermatozoon. Cytoplasm is present only in an extremely thin layer surrounding the nucleus. A centrosome (3) is nearly always present in the middle piece. With the exceptions of the Crustacea, nematodes and many myriapods, the spermatozoa are usually constructed after this type, often, with complicated modifications. In the groups just mentioned the spermatozoa are large and immobile and contain a homogeneous body (b) which is strongly refractive; its functions are uncertain. The spermatozoa of Ascaris (V) are shaped like a sugar loaf, the broad, rounded end containing the nucleus. The spermatozoa of the decapod Crustacea (III, IV) have three or more stiff processes arising from the periphery of the cake-like or cylindrical body which contains the refractile body, and in this again a rod (III, i), possibly to be compared to the perforatorium. In other Crustacea the spermatozoa are threads, often of extreme length (7 mm. in many ostracoda). It is noticeable that in some animals there are dimorphic spermatozoa. In Paludina vivipara (the same is true of other Prosobranchs), there arise in the same individual hair- formed spermatozoa with cork-screw shaped heads (Ila) and others, worm-like and with a bundle of flagella at the hinder end (lib). The first of these contains the normal chro- matin mass (eupyreme spermatozoa); the others have very little chromatin (oligopyreme spermatozoa). In many spiders where a similar dimorphism occurs, the second type of spermatozoa is chromatin free (apyremc}. The supposition that the dimorphism of the spermatozoa is connected with sex determi- nation receives support in the study of the spermatozoa of some Hemiptera. Here half of the spermatozoa have one chromosome ('accessory chromosome') more than the other half. Eggs which are fertilized by the relatively oligopyreme spermatozoon proba- bly produce male animals. ,1 Sensory Epithelium. — The last modification of epithelium is sensory epithelium, characterized by the connec- tion of certain of its cells, the sensory cells, with twigs of branching nerves which arise in the central nervous system. This connection may be of two kinds. In the first the cell is slender and filiform, the position of the nucleus being indicated by a swelling. The peripheral end is concerned with the reception of sensory stimuli, while the deeper end is continued directly into the nerve ends and correspondingly is branched into two or more extremely fine processes which take on the character of nerve fibrillae (fig. 38). In the second type the sensory nerve ends in a ganglion cell beneath the epithe- lium, sending processes into the latter, the ends of these being applied to FIG. 38. — Sensory epithelium. . 1 , of an Actinian; B, from the olfactory • epithelium of man; d, supporting cells; s, sensory cells. 74 GENERAL PRINCIPLES OF ZOOLOGY the sensory cell, the connection being one of contact, not of continuity. In both the peripheral end of the cell bears appendages for sense perception; auditory and tactile hairs, stronger processes in olfactory and taste cells, conspicuous rods in visual cells. Almost without exception the sensory cells are part of the skin (ectoderm), or arise from it in develop- ment. This is true for sense organs like the eye and ear of vertebrates, which are separated from the skin by thick intermediate tissue, for in these the sensory epithelium (retina, crista acustica) is derived from the ecto_- derm. Recent studies seem to show that the taste organs in some forms are entodermal in origin. Supporting Cells. — In sensory epithelium between the sensory cells are found still other epithelial cells, which are not connected with nerves, but have accessory functions: they serve as supports for the sensory cells; in the eyes they contain pigment; in the auditory organs they often bear the otoliths, etc. They have the general name of supporting cells. 2. Supporting or Connective Tissues. From a histological point of view there is no greater difference than that between epithelium and connective tissue; the former belongs to the surface, the latter to the interior of the body; in the former the cells play the chief role, in the latter their importance is subordinate to the plasmic products, the intercellular substances which chiefly determine the character of the various kinds of connective tissue. In spite of this contrast the connective tissues are genetically connected with epithelium. In embryos, which at first consist only of epithelia, the connection can be directly seen. The epithelia secrete a gelatinous substance from their deeper surfaces into which separate cells migrate. Thus arises the embryonic connective tissue, the mesenchyme (fig. 108). Function of Connective Tissue. — The primary function of con- nective tissue is to fill the spaces between the various organs in the interior of the body, thus connecting not only the single parts of the organs, but also the various organs themselves. In consequence of this the con- nective tissues contribute to the firmness of body, and are frequently employed in building up a skeleton. To accomplish this, substances which are usually firmer than protoplasm are formed on the surface of the cells, and, since they lie between the cells, these are called intercellular substances. In proportion as the intercellular substance increases in volume the cells themselves diminish and become inconspicuous connective-tissue cor- puscles, or, as seldom happens, entirely disappear. Since in connective tissues, the intercellular substances are most important, it follows that the GENERAL HISTOLOGY 75 distinctions between the various kinds rest chiefly upon the differences of this intercellular substance. The following forms are distinguished: (i) cellular connective tissue; (2) homogeneous connective tissue; (3) fibrous connective tissue; (4) cartilage; (5) bone. Cellular Connective Tissue (which, strictly speaking, does not belong here, since it does not arise from mesenchyme but directly from the metamorphosis of epithelium) shows the characteristics of the group least distinctly. It owes its name to the fact that the cells make up the chief FIG. 39. — Cellular connective sub- stance. Cross-section through the notochord of a newly hatched trout. FIG. 40. — Homogeneous connective sub- stance of Sycandra raphanus (after F. E. Schulze). mass, while the cell-products are inconsiderable. The cells are large, vesicular bodies which are closely pressed together and are consequently polygonal (fig. 39). They have between them a firm but thin layer of intercellular substance. Homogeneous Connective Tissue. — In homogeneous connective substance the intercellular substance (or matrix] is usually present in considerable quantity as a transparent mass, sometimes soft like jelly, often firmer (fig. 40). The cells lying in it are either spherical or send branching processes into the matrix. Such processes may unite to form meshes which, like a pseudopodial network, unite cell to cell. Frequently the matrix contains, in addition, isolated firm fibres or threads, which, on account of their physical characteristics, are called elastic fibres, and consist of a substance (elastiri) exceedingly resistant to all reagent-. Finally, in the matrix there may develop the finer connective-tissue fibrils characteristic of the next group; they may become so increased in number as to determine the character of the tissue. Fibrous Connective Tissue is characterized by the rich supply of connective-tissue fibrillae; these are fibres of extraordinary fineness, lying in a homogeneous matrix, which is the less evident the richer it is in 76 GENERAL PRINCIPLES OF ZOOLOGY fibres. The fibres may either cross in all directions, or may run essen- tially parallel and in a definite direction. Between them are found the rounded, spindle-shaped or branched connective-tissue corpuscles (fig. 41). It is characteristic of vertebrates that the fibres are grouped into bundles, each bundle generally surrounded by connective-tissue corpuscles, FIG. 41. — Fibrous connective tissue of an Actinian. •;? FIG. 42. — Areolar fibrous connective tissue (after Gegenbaur). metamorphosed into flat cells. The bundles, loosely interwoven, run in all directions (areolar connective tissue, fig. 42), or they may be parallel, forming a compact mass of fibres (tendinous tissue fig. 43). The fibrils of the fibrous connective tissue cf the vertebrates have the peculiarity not met with elsewhere, they are composed of glut-in, and upon boiling become gelatine or glue. FIG. 43. — Tendinous tissue (after Gegenbaur). i FIG. 44. — Cartilage (after Gegen- baur). r, perichondrium; b, transition into typical cartilage (a). Elastic Tissue. — In all fibroas connective tissue there may appear, as a further constituent, elastic fibres; they may indeed supplant the ordi- nary connective-tissue fibrils and become the predominant element of the tissue, which is then called elastic tissue. GENERAL HISTOLOGY 77 Cartilage. — Cartilage and bone are likewise tissues which find their charac- teristic development only in the vertebrates. In appearance cartilage is similar to the homogeneous connective tissue of many invertebrates; the matrix is homogeneous and, at first glance, appears structureless (fig. 44), but, under the action of certain reagents, assumes a fibrous condition. This, as well as the fact that the cartilage grows through changes of the perichondrium — a thin, fibrillar membrane covering its surface- makes it more evident that it is homo- geneously fibrillar; and it is thereby dis- tinguished from homogeneous connective tissue since it is not, like the latter, a lower but a higher stage of tissue forma- tion. The matrix of cartilage (chc.ndrin} by cooking produces a kind of glue which differs from true or glulin glue in that it is precipitated by acetic acid. The car- tilage cells lie in the matrix united in groups and nests, a mods of grouping pointing to their origin, since each group has arisen from a single mother-cell by successive divisions. In cartilage also, c elastic fibres are found; if present in great number, these change the bluish, shiny, hyaline cartilage into the yellow-colored elastic cartilage. The 'head cartilages' of the cepha- lopoda differ from vertebrate cartilage in that the cartilage corpuscles have e branched processes. Bone is the most complicated struc- ture in the series of connective tissues. It consists of a matrix (ossein), closely allied to glutin, so intimately combined with inorganic constituents that it appears under the microscope as a homogeneous mass. The proportion of organic and inorganic substances varies according to the age and species of animal: in man there is 65% inorganic to 35% organic <*,- rt^H '^J*M* "i, !*. '' -;•*-- -^ '- *•*-* substance; in the turtle, 63% to 37' , Of the inorganic constituents, the most important is calcic phosphate, 84'^; in smaller quantities, combinations of fluorine, chlorine, carbonic acid and mag- nesia. In compact bone the matrix is composed of the bone lamella: (fig. 45), whose arrangement is determined by ' '•v-fn_ '- J •-• '.S^*? • •** J - -" - i".i-i'^i MS^&£&'f^& FIG. 45. — Cross-section through the human metacarpus (after Frey). , of the came!; c, of the adder; ), the pericardium. The division of the heart into a part which receives the blood, the atrium or auricle (//), and a part which drives the blood onward, the ventricle (k), is of less functional importance; hence is not carried out in all cases. There are also valves (kl), which, by closing, prevent the blood from flowing back when the walls relax at the end of the contraction. Blood-vessels. — In order that the blood system may properly perform its function, in addition to circulation, it is necessary that the nutritive substances be readily taken up and given out again to the tissues. The part of the course of circulation concerned in this must have easily permeable walls, must be widely distributed in the body, and have a large superficial area. These demands are met by the capillaries (r), extremely fine, thin-walled and permeable epithelial tubes, which surround and penetrate all organs. Between the heart and the capillaries there exists, corresponding to their different functions, great differences in structure; they must therefore be united by special transitional vessels (i) vessels which begin large and thick-walled at the heart, and by branching, and thinning of their walls, pass gradually into the capillaries, the arteries (a) and (2) vessels (veins} which start from the capillaries and lead back to the heart, uniting to form larger and stronger vessels (v). Correlation of Respiratory Organs and Blood System. — It is a law that in all animals the blood-vascular system has been influenced in its arrangement and structure more by respiration than by nutrition in the narrower sense; there exists a correlation between the organs of respira- tion and of circulation. A double capillary region must be distinguished; besides the body capillary system already mentioned there is the respiratory capillary region, whose exclusive office is to remove the carbon dioxide from the blood and to furnish oxygen to it (gill and lung capillaries). A twofold capillary region makes necessary also a twofold system of arteries and veins (systemic arteries and systemic veins, respiratory arteries and respiratory veins'). The accompanying diagram (fig. 66) of the blood circulation of fishes illustrates this. Veins lead from the capillary region of the tissues of the body to the auricle of the heart. The contraction or systole of the auricle drives the blood into the ventricle. While the auricle enlarges (diastole) and refills with blood from the veins, the systole of the ventricle forces the blood through the gill arteries to the gill capillaries. Since systole and diastole of a heart chamber alternate, the heart acts as a suction and force pump, and the systole of auricle and ventricle must alternate in time. From the gill capillaries the blood goes to the 'gill- veins' ^efferent gill arteries), which unite into a single large trunk: this again gives off lateral branches passing into the capillary region of the 102 GENERAL PRINCIPLES OF ZOOLOGY body. Since the branches of the main trunk formed by the 'gill-veins' lead again into a capillary region they must, like the main stem, be called arteries. Arterial and Venous Blood. — During its course through the body the blood twice changes its chemical character and correspondingly its color. The blood which flows from the body capillary region has given up FIG. 66. — Scheme of circulation in a fish. a', ascending (Ventral) oarta; a~, descend- ing (dorsal) aorta; c, carotid; da, intestinal arteries; dc, intestinal capillaries; dv, intestinal veins; h, auricle; k, ventricle; ka, afferent gill-arteries; kc, gill-capillaries; kv, efferent gill-arteries; Ic, liver-capillaries; sc, body-capillaries; vc, cardinal veins; i, artrri.il arch; al, lateral artery; /")(/, alary muscles (alae corclis); lik, chambers of heart; o, ostia. 104 GENERAL PRINCIPLES OF ZOOLOGY cannot re-enter the blood-vessels in the same way, on account of the higher pressure in the capillaries. This overflow is conducted back to the veins by the lymph-vessels. These begin with lacunae in the tissues, and gradually pass into vessels with definite walls. The lymph-vessels of the digestive tract are particularly important since, during digestion, they become filled with the proteid and fatty constituents of the digested food; they are called chyle-vessels, because they contain the chyle, dis- tinguished from ordinary lymph by its milky color. Cold- and Warm-blooded Animals. — In connection with the blood- vascular system, two expressions are much used but not generally correctly understood, viz., cold-blooded and warm-blooded — or, more correctly, animals with variable and animals with constant temperatures. Under the head of animals with varying temperature (poikilothermal} or cold blood are placed forms whose temperature is largely dependent upon the tem- perature of the- environment, rising and falling with it, but usually a few degrees above it. In our climate, where the atmospheric temperature is considerably lower than the temperature of the human body , such animals, for example the frog, feel cold to our touch, since they have a much lower temperature than we. Such creatures as maintain about the same temperature, under any thermal condition are termed warm-blooded or constant temperatured, (idiothermal, homoiothermal) animals. Man in summer and winter under the equator and at the north pole, has approximately a temperature of 36° C. (98!° F.), showing higher temperatures only in fever. In order to maintain a constant temperature during the varying conditions, the animal must have the power to regulate the warmth of its body, either by limiting the production of heat, or by controlling its loss. If the en- vironment be warmer than is suitable for the body temperature, then the production of heat must be limited to the smallest quantity compatible with the vital processes; but, if this does not suffice, the loss of heat must be increased by evaporation from the surface, usually accomplished by active perspiration. If, on the contrary, the environment be cold, then every unnecessary loss of heat must be avoided, while the production of heat must be increased. It is clear that idiothermy, since it requires complicated apparatus, can occur only in the highly organized animals. IV. Excretory Organs. The excretory organs are tubes or glandular canals which open to the exterior either directly or by way of an end-gut (cloaca), and conduct substances which have become useless to the body to the outside. The presence of a blood-vascular system or a ccelom or both exercises an GENERAL ORGANOLOGY 105 important influence on their structure. When neither are developed the excretory tubules, in order to remove the excreta from the tissues, must branch and penetrate the body in all directions like a drainage system, being frequently connected in a network recalling the blood-capillaries (protonephridia or water-vascular system of parenchymatous worms, fig. 68). The canals begin with closed tubes, which are provided internally at the end with a bundle of actively vibrating cilia, the 'flame' (fig. 70). These flame cells are replaced in many proto- nephridia ('head kidneys' of many annelid larvae) by solcnocytes (fig. 69), cells with a flagellum enclosed in a tube. One or more main trunks lead from the canal system to the exterior. A little before the external opening (excretory pore) there is frequently a contractile enlargement, the urinary bladder. With the appearance of a ccelom there is a central place for the collection of excreta. The nephridia or segmcntal organs are usually simple (rarely branched) tubes, open at both ends. One opening is external (fig. 71), the other communicates with the ccelom by means of a ciliated funnel, the ncphrostome, a wide mouth with active cilia which connects with the canal of the tube. Through this the excre- tion is carried to the outside. The excretory organs (kidneys) of verte- brates are derived from such nephridia. The fact that in the embryos (and frequently in the adults) these open into the ccelom by nephro- stomes makes it probable that also in the vertebrates the ccelom was once important in excretion (fig. 72). The increasing import- ance of the blood-vessels which envelop the nephridial canals and l>ring to them the waste matter taken from the tissues is probably the cause of the loss of connection of the kidneys with the ccelom by degeneration of the nephrostomata. The relation of the blood vessels to the nephridial tubes becomes specially close by the development of the glomendi (Malpighian corpuscles'), bundles of capillaries carrying the walls of the canal before them and so projecting into the lumen of the tube. Since the nephridial tubules of the vertebrates open into a com- Fi.; 68 — Distomum hrp- aticum with water-vascular system (Jrom Hatschck). />, porous excre tori us; o, mouth. 10G GENERAL PRINCIPLES OF ZOOLOGY n FIG. 69. FIG. 70. FIG. 69. — Blind end of an annelid protonephridium with two connected soleno- cytes which open with their flagellate tubes into the excretory duct (after Goodrich) FIG. 70. — Blind end of one of the finest water-vascular canals (£) of a Turbellarian (from Lang). «, nucleus; /, processes of the terminal cell; u'f, 'flame' of the terminal cell; v, vacuole. n FIG. 71. FIG. 72. FlG. 71.- — Segmental organ of an oligocha^te (from Lang), fz, ciliated funnel; dis, septum; ngl, non-glandular, ng-, glandular, part of the canal; eb, terminal vesicle; In, body-wall. FIG. 72. — Scheme of a mesonephros of a vertebrate, h, nephridial tubules; m, Malpighian tubules with afferent and efferent blood vessels; n, nephrostomes; u, urinary duct. GENERAL ORGANOLOGY 107 mon canal leading to the exterior (ureter) they are commonly aggregated into a compact mass, the 'kidney.' B. Sexual Organs. Sexual Glands and Ducts. — In the sexual apparatus are distinguished the areas where the germ cells are produced, the sexual glands or gonads, and the ducts for these. The former are present, temporarily or perma- nently, in all multicellular animals; the latter may he absent. If the sexual products arise in the skin or in the walls of the digestive tract, as is usual in the ccelenterates, then special outlets are superfluous, since the ripe elements can reach the exterior directly by rupture of their covering or by means of the digestive tract. Germinal Epithelium and Germinal Glands. — Male and female sexual cells, as we have seen, originate from an undifferentiated incipient FlG. 73. — Sexual organs of Lnmbricu<; agricala (from Lang, after Yogt and Yung). The seminal vesicles of the right side are removed, but, ventral nerve cord; bv and bl, ventral and lateral ro\vs of sets; st, recepUcula semi n is, ,s/>, seminal vesicles of the left side, connected with a median unpaired seminal capsule (v/>;<). Enclosed in the latter are the te?tes (/;), and the seminal funnels (/), which lead into the vas deferens (rd); o, ovaries; -w, ciliated funnels leading to oviducts with egg capsule ((); di, dissepiments; 8-15, eighth to fifteeeth segment-. organ, or anlage, which is called the germinal epithelium. Usually it forms a part of the epithelial lining of the body cavity, in many animals per- manently, in others only temporarily; in the former case it separates, usually by constriction, and forms gland-like bodies, the gonads or sexual glands. Gonochorisrn and Hermaphroditism. — In n^ost animals the ger- minal epithelium produces either only female or only mak- sexual cells; 108 GENERAL PRINCIPLES OF ZOOLOGY such animals are called separate-sexed, diivcimis or gonochoristic, in opposition to the hermaphroditic forms, in which both kinds of sexual glands are contained in one and the same individual. Different degrees of hermaphroditism can be distinguished; commonly testes and ovary are contained in the same animal, some distance apart, as in the earth- worm, in which two segments are male, while a third segment is female (fig. 73). More rarely there is a union of testes and ovary into a single hermaphroditic gland; land-snails have an hermaphroditic gland, which' produces spermatozoa and eggs in the same follicle. Occurrence of Hermaphroditism. — Hermaphroditism is, in general, of more frequent occurrence in the lower than in the higher animals. Insects and vertebrates are, almost without exception, dioecious; only among the teleosts is hermaphroditism not rare. It also occurs in the myxinoids. More commonly FIG. 74. — Lateral hermaphroditism of a gipsy moth (Ocnrria dispar). Left female, right male (after Taschenberg). hermaphroditism occurs as an abnormality; a striking form is lateral her- maphroditism, in which one half of the animal has only male, the other half only female, gonads. If the males and females of a species be distinguishable by their appearance, then lateral hermaphroditism is expressed in their external form, since one half of the animal has the characteristic marks of the male, the other half those of the female (fig. 74). Still it must be noted that, in many instances where the external appearance pointed toward hermaphroditism anatomical investigation has disclosed either only male or only female sexual glands in a rudimentary condition (gynandromorphism). True hermaphroditism (the presence of both kinds of sexual glands in the same animal) is extremely rare in mammals and in man. What is described as hermaphroditism is usually gynandromorphism; rarely are both kinds of gonads present in the same indi- vidual, and then not in a functional condition. The wide distribution of hermaphroditism among the lower Metazoa has led to the erroneous view that this was the primitive condition, from which the gonochoristic condition has been evolved. Studies on nematodes, Crustacea and possibly molluscs have shown that, on the contrary, hermaphroditism has fol- lowed a dioecious condition, since with the disappearance of males, the female animals may develop male sexual cells before the ovaries are mature. Con- trasted to this ' protandry'1 a ' protogyncccy' is rare. Genital Ducts. — Very frequently the excretory apparatus furnishes outlets for the sexual products. In the annelids and vertebrates portions GENERAL ORGANOLOGY of the nephridial system, either exclusively or in addition to their excretory function, become genital ducts. Hence we speak of a urogenital system. This connection of genital and excretory organs has a double cause. Physiologically important is the fact that eggs and spermatozoa behave like excreta; substances which are no longer needed by the individual, but must reach the exterior in order to be- come of use. The morphological cause is the relation to the coelom. A urogen- ital system occurs only in animals in which the germinal epithelium arises from the epithelium of the ccelom, and in which the kidneys or their rudiments are in connection with the body cavity and thus form the natural outlet for its products. Whether the accessory sexual parts are portions of the excretory organs or are independent structures, they have in the animal series a definite arrange- ment adapted to their function (figs. 73 and 75). Canals lead from the gonads to the exterior, oviducts in the female, vasa defercntia in the male (and the hermaphroditic duct from the hermaph- roditic gland). Accessory Sexual Apparatus. — The terminal portion of the vas deferens is often very muscular and is called the ductus ejaculatorius; it may be evaginated or project permanently beyond the sur- face of the body as a penis or cirrus. The terminal portion of the oviduct is often widened so that two portions may be distinguished, the uterus, which har- bors the eggs during their development, and the vagina, which serves for copulation. In addition there may occur in both sexes other accessory glands of the most diverse character. Occasionally, in the animal kingdom, a part of the eggs degenerate and are used for the nourishment of the others. This degeneration may take place the uterus (Salamandra), in the egg cocoons (annelids), or in the ovary ( arthropoda, fig. 35, <•). In some cases a definite part of the ovary produ these 'yolk cells,' a condition that explains the fact that in many animal.-, (Fla- P\e — — \- FiG. 75. — Vortex rinlis (after Schultze and von GratT): b, brain with eyes; be, bursa copulatrix; , pigment; r, rhabdoms of the retina; s, visual cells. collecting in or shunning the lighted spot (positive and negative phototaxis). Phototaxis occurs, even when there are no special organs for the recog- nition of light (Infusoria, Hydra, many worms). It is increased when there are visual cells, that is light percipient spots connected with nerves. These may be on the surface, or deeper in position, if the overlying layers be translucent (earthworms, Amphioxus}. If numerous visual cells be united into a layer this is called a retina. In the lowest developmental GENERAL ORGANOLOGY 119 stages the visual cells are closely related to accumulations of pigment which occur either in or surrounding the cells. That this pigment is not absolutely essential for light perception is shown by the. visual powers of albinos which are free from pigment, but it clearly must increase the sensitivity of the cells, for pigmentation is so common that the simplest eyes may be defined as sharply denned pigment spots, to which there is frequently added a lens to concentrate the light (fig. 84, III). Eyes. — From such beginnings, which are evidently only intended to recognize light and darkness, there are all transitions to the image-forming eyes of the vertebrates and apparently the cephalopods. The retina is rendered more efficient by the development of r/iabdoms on the peripheral ends of the visual cells, rod-like processes which aid in light perception, and in the vertebrates usually divided into rods and cones (fig. 85, 9). P E G FIG. 85. — Human retina (after Gegenbaur). P, pigment- layer; E, layer of sensory cells; G, optic ganglion; i, limitans interna; 2, nerve-fibre layers; 3, ganglion cells; 4, inner reticular layer; 5, inner granular layer; 6, outer reticular layer; 7, outer granular layer; 8, limitans externa; 9, rods and cones; 10, tapetum nigrum; .17, Mtiller's fibres. In the vertebrates and manv invertebrates the retina contains a j reddish pigment, the 'visual purple,' which is quickly bleached in the light and as quickly regenerated in darkness, and which apparently plays an important part in vision. In the course of the optic nerve there are numerous ganglion cells which form an optic ganglion (figs. 85 and 356), lying outside the eye in the invertebrates, in die vertebrates forming a number of layers (G, fig. 85), inside the retina proper (K), which is formed of the visual cells (outer granular layer) with the fibres of the rods and cones and the rhabdoms themselves. Accessory Structures. — If a sharp image is to be cast on the retina, the light rays coming from a point without the eye must be brought again to a point on the retina by refractive substances (lens, cornea) ; therefore there must be a space between the dioptric apparatus and the retina. The 120 GENERAL PRINCIPLES OF ZOOLOGY eye is therefore developed as a camera obscura, the space between retina and lens being filled by the vitreous body (transparent cells or jelly — fig. 84). The amount of light is regulated by an iris, a pigmented membrane with circular opening, the pupil, the width of which is enlarged or contracted in accordance with the intensity of the light. Then nutrition is provided by a richly vascular coat, the chorioidea, and for protection there is a firm outer coat, the sclera. These accessory structures are developed and combined in the most diverse ways in the different classes of animals ;- eyes which are very similar in structure, like those of vertebrates and cephalopods (figs. 86, 349), have developed along entirely different ontogenetic and phylogenetlc lines. CNO VO FIG. 86. — Horizontal section through the human eye (after Arlt, from Hatschek). E, epithelium of the cornea (conjunctiva); C, cornea; vA, anterior chamber of the eye; /, iris; hA, posterior chamber of the eye; Z, zonula Zinnii; Os, ora serrata; Sc, sclerotic coat; Ch, choroidea; R, retina; />, papilla of optic nerve; m, macula lu'tea' area of most distinct vision; VO, sheath of the optic nerve; NO, optic nerve; C, arteria centralis; Cc, corpus ciliare; L, lens; Cv, vitreous body. The Eye of the Vertebrates.— The eye of the vertebrates usually is an approximately spherical body. Over the greater part of the circumference there is an opaque, fibrous or cartilaginous sclera, or sderotica, transparent only in the most anterior part, where it forms a projecting portion like a watch-glass, GENERAL ORGANOLOGY 121 the cornea. Internally to the sclera lies the chorioidea, which, at the junction of sclera and cornea, is changed into the iris. The iris, the seat of the color of the eye, is pierced by the pupil, which regulates the amount of light. Next internal to the chorioid follows a layer of black cells, the tapctum nigrum (pig- mented epithelium), and finally the retina itself, the expansion of the optic nerve which enters the eye at the hinder part. The tapetum nigrum and the retina arise together, and hence both end at the edge of the pupil, although the retina loses its nervous character at the or a serrata, some distance from the outer edge of the iris. The cavity of the eye is completely filled by the vitreous body, aqueous humor, and the lens. For vision the lens is the most important, since, next to the cornea, it influences most the course of the rays of light. It lies behind the iris, fixed to the anterior wall of the chorioid, which here is changed into the ciliary process. In front of it is a serous fluid, the aqueous humor, partly in the so-called posterior chamber of the eye, between the lens and iris, partly in the anterior chamber, between the iris and cornea. The single, larger cavity behind the lens is filled up by a jelly-like vitreous body. The image formed on the retina is in- verted. Shining of Eyes. — In many verte- brates there is a tapctum lucidum inside the chorioid which causes the so-called shining of eyes (cats). This is a layer, which reflects light so strongly that only a little light from the outside is necessary to illumine the back of the eye. There is no real production of light. The tapctum nigrum must be free from pigment in order that the tapetum lucidum may act. In many insects and spiders light is sim- ilarly reflected from the back of the eye. Phosphorescent Organs. — For a long time eye-like organs have been known, especially in animals from the deep seas (fishes, cephalopods, Crustacea). Many of these have been proved to be organs for the production of light, and the same is probably true of the others. Each is a spherical, eye-like body, arranged in a definite manner in the skin and of very varying structure. Many have a great resemblance to glands (fig. 87). The cells of the gland" follicle are apparently to secrete the phosphorescent substance, its light being made more effective by a lens-shaped body of transparent cells and by a reflector (not always present) consisting of strongly iridescent cells, and all surrounded by a pigment layer, the whole being so eye-like that they were at first taken for visual organs. We have in these to do with highly specialized structures differing from the phosphorescent apparatus so common in marine animals of all classes, where (Noctilnca, medusa;, corals, etc.), the phosphorescent sub- stance is widely distributed through the body. Perhaps the concentration of the phosphorescence in definite organs may serve to light the surroundings, to attract the prey and perhaps as an attraction between the sexes. In the hitler case there would be an analogy with the phosphorescent organs of insects, which are formed in a totally different way. FIG. 87. — Phosphorescent organ of dus (after Brauer). c, cutis; /, lens; p, pigment layer; s, phosphorescent secretion cells; t, reflecting tajn-tuni. 122 GENERAL PRINCIPLES OF ZOOLOGY SUMMARY OF THE MOST IMPORTANT POINTS OF ORGANOLOGY. 1. Organs are tissue complexes, differentiated from the surrounding structures by a definite form and adapted to the performance of a peculiar function; consequently each organ can be classified morphologically (according to structure and relations) and physiologically (according to function). 2. Organs of different animals may be physiologically equivalent, analogous organs (i.e., with similar functions). 3. Organs of different animals may be morphologically equivalent, homologous (developing in similar relations). 4. In the comparison of the organs of two animals three possibilities become evident, a. They may be at the same time homologous and analogous, b. They may be homologous, but not analogous (swim- bladder of fishes, lungs of mammals), r. They may be analogous, but not homologous (gills of fishes, lungs of mammals). 5. Organs are divided into animal and vegetative according to function. 6. Animal functions are those which are only slightly developed in plants ; in the animal kingdom, on the contrary, they undergo an increase and become characteristic. 7. Vegetative functions are developed with equal completeness, though in a different manner, in plants and animals. 8. Animal organs include the organs of motion and sensation, such as muscles, sense-organs, nervous system. 9. To the vegetative organs belong the organs of nutrition and re- production. 10. Under nutrition, in the widest sense, are included not only the taking in and digestion of food and drink, but also the taking in of oxygen (respiration), the distribution of food to the parts of the body, and the removal of matter which has become useless. 11. With nutrition, therefore, are concerned not only the digestive tract and its accessory glands, but also the organs of respiration, the blood-vascular system, and the excretory organs (kidneys). 12. The male and female sexual organs serve for reproduction. 13. The male and female organs may occur in different individuals (dicecious], or both may be found in one and the same animal (hermapliro- ditic) . 14. The highest degree of hermaphroditism is attained when one and the same gland (the hermaphroditic gland) gives rise to both eggs and spermatozoa. 15. Very often the sexual organs and the ducts from the kidneys are closely united; we then speak of a urogenital system. PROMORPHOLOGY 123 IV. Promorphology ; the Fundamental Forms. The structure of the individual animal depends uj on the definite arrange- ments of its organs, which are definite or only vary slightly in each group. Comparison shows that there are a few fundamental forms which play the same role in morphology as the crystal forms in mineralogy. But there is an important difference. A crystal is made up of similar parts, its form is the result of its physico-chemical composition. This condition cannot exist in animals as each organ is a complex of many chemical compounds. Nor, even where the symmetry is the most pronounced is there that mathematical accuracy found in crystals. The form of an animal depends upon its extension in space, and accordingly we may pass through it three axes at right angles to each other, these defining the position of three planes. According to the relations of the body to these we may define five fundamental forms. FIG. 88. — Sponge, Lophocalyx philippensis, with buds (after F. E. Schulze). Asymmetrical (anaxial) animals (fig. 88) are such as have no definite arrange- ment of parts with regard to axes or planes, the body growing irregularly in any direction as in many sponges and protozoa. Spherical (homaxial) animals have the parts arranged around a central point, through which innumerable axes and planes may be passed, each plane dividing the whole symmetrically as in some spherical protozoa, chiefly radiolaria (fig. 89). In radial (monaxial) symmetry there is a main or longitudinal axis which lies in the direction of growth. It may be longer, shorter or of the same length of the other axes, but it may be distinguished by the fact that it passes through parts, as the mouth, which are not found in other axes. Around this main axis the parts of the body are symmetrically arranged, like the spokes of a wheel, so that any plane passing through the main axis will divide the body into symmetrical parts. Most ccelenterates and echinoderms are more or les: completely radially symmetrical (fig. 90). 124 GENERAL PRINCIPLES OF ZOOLOGY In the case of biradial symmetry (fig. 91) there is the main axis, as in the last, and two other unequal axes at right angles to this, the inequality consisting in that organs occur in the line of the one that are not found in the other. One of these is called the sagittal axis, the other the transverse. Planes passing FIG. 89. — Haliomma erinaceus, a radiolarian. a, external, i, internal, latticed spherical skeleton; ck, central capsule; wk, extra-capsular soft parts; n, nucleus. FIG. 90. — Young Chrysaora (after Claus). I, perradii; II, interradii; gf, gastral filaments; sk, sensory pedicels. through the main axis and either of the others will divide the animal symmet- rically. Corals, sea anemones and ctenophores belong here. Bilateral symmetry has the same three axes, the two ends of the main or longitudinal axis being dissimilar, as well as those of the sagittal axis. These axes define three planes which have received names. That passing through the PROMORPHOLOGY 125 main and the sagittal axis is the sagittal or median plane and it divides the animal into symmetrical halves. A frontal or horizontal plane passes through the longitudinal and transverse axis, separating dorsal and ventral halves. FIG. 91. — Section of a young sea anemone (after Boveri). ss, sagittal plane; tt, transverse axis; I, II, III, septa of first, second and third orders; ek, ectoderm; en, entoderm;/, mesenterial filament; m, muscles; r, directive septa. D —rn .. ' FIG. 92 — Cross- section of a fish passing through the fore limbs. DV, sagittal axis; RL, transverse axis; a, dorsal aorta; c, body cavity; d, gut; ch, notochord; g, shoulder- girdle; h, heart; m, muscles; n, anterior end of the kidneys; p, pericardium; oh, neural arch; lib, haemal arch; r, spinal cord. A transverse plane passes through transverse and sagittal axes, separating ante- rior and posterior parts of the body. The great majority of animals belong here (fig. 92). 126 GENERAL PRINCIPLES OF ZOOLOGY Antimeres and Metameres. — The symmetrical parts of an animal are called antimcrcs; each antimere has organs which occur likewise in its adjacent antimere. The right arm of man is the antimere of the left, the right eye of the left, etc. Frequently there is also a repetition of organs in the direction of the long axis. Thus the body is made up not only of symmetrical parts, the antimeres, but also of similar parts placed one behind the other, the metameres. Metamerism or segmentation is spoken of when the body consists of numerous segments or metameres (consult fig. 60). Very often it is recognizable externally — when, for instance, the limits of the segments are marked on the surface by constrictions (arthropods and annelids). But this external metamerism may be entirely lacking, and the metamerism find expression only internally in the serial succession of organs. Man, for example, is segmented only internally; in his skeleton there are numer- ous similar parts, the vertebras, which follow one another in the long axis. In fishes the musculature also is made up of numerous muscle segments, as any one can readily see by examining a cooked fish. In the case of the externally segmented earthworm also, the ganglia of the nervous system, the vascular arches, the nephridia or segmental organs, the setae, and the septa of the body cavity are repeated metamerically. Homonomous and Heteronomous Metamerism. — The examples mentioned are well adapted for illustrating homonomous and heteronomous metamerism. The earthworm is homonomously metameric, because the single segments are much alike in structure, and only slight differences exist between them. Man and all vertebrates, on the contrary, are hetero- nomously metameric, because the successive segments, in spite of many points of agreement, have become very unlike. The segments of the head have an importance, for the organism as a whole, quite different from those of the neck, the thorax, or the tail. A division of labor has taken place among the segments of an heteronomous animal. Heteronomy and Homonomy. — The distinction between heteronomy and homonomy is of great physiological interest. 1 he more different the segments of an animal become the more dependent they are upon each other; so much has the whole become unified that the single parts can live only while the continuity is maintained. On the contrary, if the connection between the parts be less intimate, they are more similar, and the more able to exist after separation from one another. This is well shown in instances of mutilation. When many species of Lumbricicke are cut in two each part not only lives, but it even regenerates the part which is lacking; if, on the other hand, the same thing is done to a heteronomously segmented animal, either death immediately ensues, as in the case of the higher vertebrates, or the parts live for a short time a hopeless existence, as can be seen in the case of frogs, snakes, insects, etc. There is always a certain capacity for regeneration, which is the more restricted, the more complete the organization. While Crustacea, amphibia and reptiles GENERAL EMBRYOLOGY 127 can, for instance, regenerate lost appendages, the mammals have the regenera- tive powers reduced to the healing of wounds. In metamerism a phenomenon is repeated which obtains widely in the animal kingdom, and contributes towards its higher development; first there is a reduplication of part (here the segments), then a division of labor, so that the final result is a whole comj>osed of many parts, but a singly organized whole. II. GENERAL EMBRYOLOGY. Origin of Organisms. — Since the development of every individual begins with an act of generation, the ways by which new organisms may arise comes first. Admitting only that which has been actually observed, we must cling to the aphorism of Harvey, "Omne vivum ex ovo," altering it to Omne vivum e vivo: every living organism is derived from another living organism. We must limit ourselves to the mode of origin which has been termed tocogony, or generation by parents. The great importance which the question of generation without parents, or spon- taneous generation, has obtained through the evolution theory renders a consideration of this question necessary at this point. I. GENERATIO SPONTANEA (ARCHEGONY, ABIOGENESIS). The old zoologists, including Aristotle, believed that many animals, even frogs and insects arose spontaneously from the mud. This was not disproved until the seventeenth and eighteenth centuries, and even then the idea of spontaneous generation still held, especially for parasites, for in the history of each animal there was a time when it contained none of these, and they were supposed to arise from the superfluous plastic mate- rial of the host. Later it was found how the eggs obtained entrance, and then the idea of abiogenesis persisted only for microscopic organisms. Water, which contained no living thing, after standing a while, was found to contain organisms. Lastly it was discovered that these 'do not arise dc noi'o, but come from minute germs, carried by the winds, or distributed in other ways. If the fluids and the utensils are heated, and germs are prevented from entrance by proper means, no life will appear, even if the medium be kept for years. So it may be said, as the result of all recent experiment, that the present occurrence of spontaneous generation is not proved. First Origin of Life. — If we adopt the view that our earth was at one time in a molten condition and has gradually cooled, we must assume that life has not existed on the earth from eternity, but at some time has had its beginning. If we wish to base our explanation, not upon a supernatural act of creation, nor upon hypotheses like that of the transference of living germs from other worlds by meteors, there is left only the hypothesis that compounds of carbon, 128 GENERAL PRINCIPLES OE ZOOLOGY oxygen, hydrogen, nitrogen, and sulphur have been brought together to produce living substance. This process is called sptmtaneans generation. If the carbon, oxygen, nitrogen, etc., which are now combined in a stable manner in organisms were formerly unstable, the conditions for the origin of compounds, through whose wider combination life would be possible, may have been more favorable. Thus the hypothesis of the first origin of life through spontaneous generation is carried to a logical postulate. II. GENERATION BY PARENTS, OR TOCOGONY. We deal here only with those methods of reproduction which have actually been observed, i.e., generation by parents. These methods fall mainly into two great groups, asexual and sexual generation, monogony and amplngony, to which may be added a third group, a combination of the two. a. Asexual Reproduction. Monogony. Monogony Defined. — The chief characteristic of asexual reproduction is the fact that only a single organism is necessary. But since, in certain modes of sexual reproduction (hermaphroditism, parthenogenesis), this also holds true, further explanation is necessary. Asexual reproduc- tion must be a result of the growth of the organism, which has the peculiar- ity that it is not growth for an existing individual, but leads to the forma- tion of new individuals. It is noteworthy in this connection that many animals can reproduce asexually before they have reached the normal size (budding in embryo and larval polyzoa and tunicates). This growth may be general and result in an equal growth of all parts; or it may be local and consequently lead to the formation of an outgrowth in the region of greatest increase. In the first case division takes place, in the latter budding. Division. — In the case of division (cf. figs. 120, 123, 150) an animal separates into two or more equivalent parts, so that it is not possible to distinguish the mother and the daughter animal, for the original animal has completely disappeared in the young generation. The division is commonly a transverse one, the plane of division being perpendicular to the long axis of the animal; less common is longitudinal division, rarest is oblique. Budding. — In budding (fig. 93), the products are unequal. One animal maintains the identity of the mother, while the bud, the out- growth caused by local increase, appears as a new formation, as the daughter individual. Yet the difference between division and budding is bridged by intermediate conditions. GENERAL EMBRYOLOGY 129 b. Sexual Reproduction: Amphigony. Amphigony. — For sexual reproduction two animals are commonly necessary, a female and a male; the reproductive cells — the eggs — of one must be fertilized by the reproductive cells — the spermatozoa — of the other, and thus acquire the capacity of giving rise to a new organism. Now, since there are hermaphroditic animals and since with many of them the possibility of self-fertilization has been demonstrated, it becomes clear FIG. 93. — .4 , Hydra grisea with a bud ; B, first stage of bud. en, entoderm ; ec, ectoderm ; s, supporting lamella; /, tentacle of mother and bud; m, stomach; o, mouth. that the emphasis in the definition of sexual reproduction must be laid, not upon the individual, but upon the sexual products. Consequently the essential point of sexual reproduction is to be sought in the union of male and female sexual cells. Parthenogenesis and Paedogenesis. — This explanation is applicable to by far the greater majority of cases, namely, to all cases where the term sexual reproduction can be applied. Still, it has been demonstrated in many instances that two modes of reproduction formerly considered as monogony — parthenogenesis and paedogenesis — must be regarded as modifications of sexual reproduction, although the conditions mentioned above are not strictly satisfied. In both cases the eggs develop be- cause of some peculiar internal stimulus, without the occurrence of fertilization by spermatozoa. In case of padogenesis there is the addi- 9 130 GENERAL PRINCIPLES OF ZOOLOGY tional circumstance that reproduction is accomplished by animals which have not completed their normal development; for example, the larvae of certain flies reproduce before they have passed through the pupal stage. Paedogenesis consequently is parthenogenesis in an immature organism. Parthenogenesis and Typical Amphigony. — There is no absolute distinction between parthenogenetic eggs and those needing fertilization. On the other hand, their equivalency is fully shown in cases as the bees, where the queen decides at the moment of oviposition whether the egg shall receive a spermatozoan or not, this decision determining further whether the egg shall develop into a female (fertilized) or a male (unfer- tilized). Parthenogenesis is, therefore, not an asexual reproduction which was antecedent to sexual reproduction, but rather one which must have been derived from the sexual; it is a sexual reproduction in ivJiicJi a degeneration of fertilization has taken place. It is, therefore, more in accord with the natural relations to contrast reproduction by sex-cells with vegetative or growth reproduction (division, budding) rather than asexual with sexual reproduction. Sexual and Somatic Cells. — The distinction of sexual cells from the asexual reproductive bodies, the parts arising by division and budding, is shown by their relations to the vital processes of animals. The cells of a bud had a share in the vital processes of the animal before the beginning of reproduction; they were functional or somatic cells. In the fresh-water polyp (fig. 93), when a bud arises, the cellular material employed is that which was previously related to the mother animal in exactly the same manner as the other parts of the body wall. The sexual cells of an animal, on the contrary, are excluded from the vital processes, remaining in a resting condition, and con- serving their vital energies. Asexual reproduction is closely related to growth; sexual reproduction is not even a special form of it, but a complete renewal of the organism, a rejuvenescence of it. This explains the fact that asexual repro- duction is most common in the lower animals (coelenterates, worms), but is lacking from vertebrates, molluscs, and arthropods. The higher the organiza- tion of the animal the more the energies of its cells must be employed to meet the increasing demands upon their functional capacity, and so the more necessary is sexual reproduction. It is farther noteworthy that fission and budding occur most frequently in attached, sessile, or slightly moving animals (ccelenterates, polyzoa, ascidians, oligochaates), an indication that the distribution of asexual reproduction may be determined by the method of life. c. Combined Modes of Reproduction. Very often two modes of reproduction occur in the same species side by side. Many corals and worms have the power of multiplying by division or budding, and also of forming sex cells; other animals have no asexual reproduction, but their eggs develop according to circumstances, either parthenogenetically or after fertilization. The appearance of two kinds of reproduction is very often governed by the fact that individuals with GENERAL EMBRYOLOGY 131 different modes of reproduction alternate with each other. This is called alternation of generations in the wider sense, and of this two special forms are distinguished: metagenesis (progressive alternation of generations), and heterogony (regressive alternation of generations). Metagenesis. — Alternation of generations in the narrower sense, or •metagenesis, is the alternation of at least two generations, one reproducing only asexually, by division or budding, the other exclusively, or at least to a great extent sexually. The first generation is called the nurse, the second the sexual animal. The reproduction of hydromedusae furnishes the best examples (fig. 94). The nurses here are the polyps, which Fir,. 94. — Bougainvillea ramosa (from Lang). /;, hydranths (nurse) which have given rise to medusa-buds (mk) ; m, separated medusa, Margelis ramosa (sexual animal). usually united into a colony, never produce sexual organs, but bud sexual animals, the medusce. The medusae are unlike the polyps, being much more highly organized, and freely motile; only very rarely do they repro- duce asexually; on the other hand, they develop eggs and spermatozoa, from which the non-motile nurses, the polyps, develop. This example shows that, in alternation of generations, there is not only a difference in the mode of reproduction, but usually in addition, a difference in form 132 GENERAL PRINCIPLES OF ZOOLOGY and organization. Between polyp and medusa the difference is so great that for a long time these two, though stages of the same species, were referred to different classes of the animal kingdom. In many cases the alternation of generations may be still further complicated by two asexual generations following each other, before the return to the sexual genera- tion takes place. Heterogony is distinguished from metagenesis by the fact that the asexual generation is replaced by parthenogenesis. Consequently there alternate animals of sometimes quite different structure, one arising from fertilized, the other from unfertilized, eggs. Certain Crustacea, the Daphnidoe, show heterogony in a typical manner. During a large part of the year only females are found ; these increase parthenogenetically by 'summer eggs'; then males appear for a short time; they fertilize the ' winter eggs, ' which now are formed, from which again parthenogenetic generations arise. Very often heterogony has been insufficiently distin- guished from metagenesis, parthenogenetic reproduction being regarded as an asexual mode, as was the case in the trematodes. The sexually ripe Distomum produces very peculiar sporocysts; these again give rise parthenogenetically to the larvae of Distomum, the cercaria?. For a long time the erroneous view was held that the cells from which the cercariae arose were not eggs, but 'internal buds'. On the other hand there have been included under heterogony modes of reproduction in which no parthenogenesis whatever occurs, but in which only different forms and organization alternate. A hermaphroditic worm, formerly called Ascaris nigrovenosa, lives in the frog's lungs; it produces the separate- sexed Rhabdonema nigrovenosum living in mud, from whose eggs the ascarid of the frog is again produced. GENERAL PHENOMENA OF SEXUAL REPRODUCTION.. In sexual reproduction a series of developmental processes is observed which is repeated in an essentially similar manner in all multicellular animals. They are: (i) the maturation of the egg; (2) the process of fertilization; (3) the process of cleavage; (4) the formation of the germ- layers. i. Maturation. The egg (oocyte) with the large vesicular nucleus cannot yet be fertilized ; it must undergo a series of changes — the process of maturation, which consists in the replacement of the germinal vesicle by a much smaller egg-nucleus, and the formation at one pole of the egg of the 'directive corpuscles' or 'polar bodies ' GENERAL EMBRYOLOGY 133 r B C Formation of the Polar Bodies. — The germinal vesicle initiates these changes, its walls disappearing, its contents in part mingling with the cytoplasm of the egg, in part being employed in the formation of a nuclear spindle (directive spindle). The latter places itself with its axis in a radius of the egg so that one pole is turned towards the centre, the other being in the superficial layer of the egg (fig. 95, A). Now begins a regular cell-division, but the products of the division are of very unequal size; the larger part is the egg, the smaller quite insignificant part is the polar body (fig. 95, C). The latter projects above the surface carry- ing with it one half of the spindle, and when the globule is cut off half of the spindle is included in it. The Second Polar Body. — The part of the directive spindle remaining in the egg immediately forms a new spindle; the cell-budding is repeated (C, D) and leads to the formation of the second polar body. As a result two small cells (fig. 95, £) lie at one pole of the egg, in many cases even three, since during the formation of the second polar body the first may divide. The part of the directive spindle remaining after the second division becomes a vesicular resting nucleus, the egg- nucleus or female pronuclcns, the characteristic feature of the ripe egg capable of fertilization. In other words, by a double division there have been formed from the immature egg four (sometimes three) cells, of which one has retained by far the greatest part of the original mass of the cell and constitutes the ripe egg, while the others are small bodies like rudimentary eggs. The name directive corpuscles was given to them because in very many cases their position renders possible a definite orientation of the egg; i.e., a diameter, the main axis, can be passed through the egg, one end of which is marked by the directive corpuscles. With reference to later processes of development this end is called the animal pole of the egg, the opposite end the vegetative pole. D FIG. 95- — Formation of polar globules in AM iirix megalocephala (dia- grammatic, after Boveri). .-1. first directive spindle; B, cutting off of first polar body; C and /', two stairs of the second spin- dle; E, separation of second polar body. Relation between Maturation and Fertilization.— In many cases the maturation takes place before the entrance of the sperm, either in the ovary or at the beginning of the oviduct; in many animals, on 134 GENERAL PRINCIPLES OF ZOOLOGY the contrary, there ensues a pause after the first polar body has been formed, or the egg may remain in the oocyte stage; the egg then requires the entrance of a spermatozoon in order to complete the further changes, i.e., the formation of the second polar body and reconstruction of the egg-nucleus. This dependence of the last phenomena of maturation upon the beginning of fertilization led for a long time to the error that the formation of the polar bodies was a part of the fertilization process itself. Spermatogenesis. — The maturation of the egg has its counterpart in the formation (maturation) of the spermatozoa — spermatogenesis. As the oocyte, by division, gives rise to four cells (the polar globules and the ripe egg), so the spermatocyte, a cell comparable to the oocyte, divides into four spennatids. Yet the two differ in that usually all four spermatids become spermatozoa. That three of the four sex-cells of the female remain rudimentary (polar globules) the fourth alone forming an egg, is explained by the need of the egg to con- tain all possible material for use in development. Reduction Division. — In the maturation division of both male and female sex-cells agree in the chromosome reduction. This is due to the fact that the maturation spindles have but half the number of chromosomes characteristic of the species. Usually these chromosomes are distributed in four groups (tetrads] in the preparatory stages of the egg, the tetrads later being distributed among the four products of the divisions (fig. 95). The significance of this will be shown in connection with the phenomena of fertilization (p. 138). 2. Fertilization. Copulation and Fecundation. — The term fertilization refers to the internal processes which, after the meeting of the egg and spermatozoon, go on in the interior of the former and end with a complete fusion of the two sexual cells; on the other hand, special expressions are necessary for those preparatory processes whose purpose is to render fertilization possible. Very often, but not in all cases, there is an active transfer of the sperm from the male to the female, a copulation. In many marine animals (most fishes, echinoderms, ccelenterates) the eggs and the spermatozoa are discharged into the water, and the union of these (impregnation or fecundation) depends upon chance. Fertilization. — The process of fertilization begins with the entrance of the spermatozoon into the egg. Usually the egg is surrounded by a gelatinous envelope, the chorion, to which the spermatozoa adhere, and through which they bore until they reach the surface of the egg (fig. 96). But since the chorion, particularly in eggs laid in the air, may be hard and resisting, there exists very often a special arrangement, the micro pylar apparatus, for the entrance of the spermatozoon; this may be a single canal extending through the chorion, as in the eggs of fishes, or a group of such canals, as in most insects. Monospermy and Polyspermy. — Many spermatozoa may reach the egg but normally only one serves for fertilization. The spermatozoon which is in the slightest degree ahead of the others is met by a process of GENERAL EMBRYOLOGY 135 the protoplasm (fig. 96, A) by means of which it enters the egg. The egg is now impervious to all others. Only in pathological eggs can two or more spermatozoa enter and then multiple impregnation (di- or polyspermy) occurs. There are means of protection against this abnormal fertilization ; one, though not the only one, is the formation of the yolk-membrane, an impermeable envelope which is suddenly secreted from the surface of the egg, as soon as the spermatozoon has entered. Within the yolk-membrane FIG. 96. — Egg of Asterias glacialis during fecundation (after Fol). A, entrance of the spermatozoon; B, the spermatozoon has entered; the yolk-membrane has formed. the body of the egg contracts by discharging some of the more fluid con- stituents, so that between the yolk-membrane and the surface of the egg a cavity is formed, easily recognized in smaller fertilized eggs (fig. 96, B). In the large yolk-laden eggs of many insects and vertebrates several sper- matozoa may normally enter; but only one fuses with the egg-nucleus, the others degenerating sooner or later. Essential Feature of Fertilization. — After the spermatozoon has penetrated into the egg, the head and the middle piece containing the centrosome can still be recognized, as the chromatic and achromatic parts of the spermatozoon or sperm-nucleus (male proniicleus), while the tail and the slight amount of protoplasm disappear in the yolk. The centro- some of the sperm-nucleus gives rise to rays in the cytoplasm of the egg, like those observed during division. Preceded by these rays the sperm- nucleus travels towards the egg-nucleus until it reaches (fig. 97), ami fuses with it to form a single cleavage nucleus. The centrosome n<>\\- divides into two daughter centrosomes, which migrate to opposite poles of the cleavage nucleus and control its division. The cleavage nucleus changes to a cleavage spindle, which divides and thus initiates the em- bryonic development, the successive divisions being known as the cleavage or segmentation of the egg. Since not until this point is fertilisation complete, we arrive at the fundamentally important proposition that the essential feature of fertilization consists in the union of egg and sperm nuclei. 136 GENERAL PRINCIPLES OF ZOOLOGY Part Played by the Two Nuclei.— In many cases an abbreviation of development may take place, the stage of the cleavage nucleus being omit- ted, and the egg and sperm nuclei, without uniting, pass directly into the cleavage spindle. This in no wise alters the above-mentioned proposition, but yet it is important, because it shows more plainly how the two nuclei D A FIG. 07.— Four stages in the fertilization of Strongylocentrohts Uvidus (after L'n c v i (./(. c f 1 1 ' ( '( n .1 f i t £ (j {•Co ^ ill IC1 Kostanecki). .4, entrance of spermatozoon; B, turning cf sperm nucleus; C, approach and D, fusion of egg and sperm nuclei. (In A and B only a part of the egg is shown.) participate in the formation of the cleavage spindle. It shows that of the chromosomes which form the equatorial plate exactly one-half are fur- nished by the egg-nucleus, the other by the sperm-nucleus. For, even be- fore the spindle has been formed and the contour of the two nuclei has disappeared, the chromosomes destined for the spindle are completely developed in exactly the same number in each of these (fig. 98). A B •FiG. q8. — Fertilization of A scar is megnloccphala (after Boveri). A, the ends (centrosomes) of the spindle formed; B, the spindle completed; sp, sperm-nucleus with its chromosomes; ei, egg-nucleus; p, polar bodies. Heredity. — Recent observations have furnished a certain basis for the theory of heredity, the transmission of parental characteristics to the offspring. This transmission, on the whole, takes place with equal effect from the father's and from the mother's side; if we take the average of GENERAL EMBRYOLOGY 137 numerous cases, the child's peculiarities hold the mean between those of father and mother; or, in other words, male and female individuals on the average have an equal power of transmitting characteristics. Physical Basis of Heredity. — Since in all animals with external fertilization a material connection between parents and offspring can exist only through the sexual cells, these latter must contain the substances which render heredity possible; further, the two hereditary substances, in cases of equal capacity for transmission, must be present in the egg and in the spermatozoon in equal quantity. By this course of reasoning, the chro- matin which forms the chromosomes has come to be regarded as the bearer of heredity; for we know that the egg contains a great quantity of cytoplasm, but the spermatozoon only the slightest trace of it; that, on the other hand, egg-nucleus and sperm-nucleus furnish equivalent substances, and espe- cially the same number of chromosomes, to the cleavage spindles; hence only the chromatin can be regarded as the hereditary substance (idio- plasm'). This supports the view expressed before (p. 58) that the nucleus is the bearer of hereditary qualities and determines the character of the cell. Theory of Determinants.- — These facts of the maturation of the egg and spermatozoa and of fertilization have become the starting point for further investigations and associated theories, which in the last few years have acquired great significance. Their relations have also been shown to the extremely im- portant but long forgotten experiments on inheritance in plants by Mendel. If we accept the sexual nuclei or their chomosomes as the bearers of heredity, it follows that certain constituents of the chromosomes must contain the anlagen of the characteristics, partly male, partly female, which later develop in the offspring. The simplest setting forth of the connection between the anlagen and the developmental product is Weismann's 'theory of determinants.' This may be taken as a basis for the following discussion, although objections are brought against it. It represents an organism as a complex of innumerable peculiarities, as a sort of mosaic; and in a corresponding way, the anlagal sub- stance, the chromosome mass (idioplasm) as a similar mosaic of anlagal particles, the determinants. There is a determinant for every paternal or maternal char- acteristic in the offspring, be it prominent or be it latent. The chromosomes must therefore consist of orderly groupings of innumerable determinants. Merogony, Artificial Parthenogenesis. — Parthenogenesis shows that since an egg may develop, without the entrance of a spermatozoon, into a complete organism, the chromosomes of the egg are sufficient to produce all of the features necessary for life. This is more clearly shown by artificial parthenogenesis. Many eggs, which normally need fertilization for development, may be stimu- lated to develop by number of chemicals. Similarly the male chromosomes ;ne sufficient for normal development, for if an egg from which the nucleus has been removed be fertilized by a single sperm, it likewise forms an organism with all the necessary features (merogony). A fertilized egg must therefore possess duplicate determinants, male and female, for each elementary character, a double assortment of chromosomes. Whether a certain characteristic shall appear purely maternal, or purely paternal in form, or in varying compromises between the two would depend upon the energy of the determinants concerned. 138 GENERAL PRINCIPLES OF ZOOLOGY If a determinant be so strong that its fellow cannot come to expression it is spoken of as dominant, the one that succumbs is recessive. Mendel's Law.— Long before the phenomena of fertilization were known, Mendel had shown by experiments on plants, that if two individuals, easily distinguished by some peculiarity like color, be crossed, the cross so produced was, in many cases, not a blend of the two, but resembled exclusively one of the two parents. On crossing red and white flowered peas, no matter in which direction, the crosses were invariably red. The determinant for red was so dominant that the recessive determinant for white is powerless. But that the white determinant was present in the cross (according to the morphological con- clusions regarding fertilization this must be so) was shown when the red-flowered crosses were self fertilized (fig. 99). Then one fourth of the offspring were o 0=O 3=0 3 • O } white, J red, ± red, breeding true, capable of splitting. breeding true. FIG. 99. — Diagram of Mendelian inheritance. Black and white representing red and white flowering kinds. white and bred true; that is the white color, the recessive character, persisted in all the succeeding generations. Of the remaining three fourths, one fourth also bred true, but red, a sign that the recessive character was lost. Such pure breeding animals are called homozygous because the sygote, the fertilized cell from which they arise, contains, with reference to the peculiarity under investi- gation, only similar determinants. The remaining two fourths are heteroz\nk\ somatic layer; mk~, splanchnic layer; Ih, body-cavity. Mesothelium. — In the second case the mesoderm may preserve the epithelial character' of the two primary germ-layers, and is called m< thdiitm. It is cut off from the entoderm, the mode of development being shown in the worm Sagitta (fig. 109). When the gastrula of Sii:^itta has been formed two folds arise from the archenteric walls opposite the 148 GENERAL PRINCIPLES OF ZOOLOGY blastopore (A), thus partially separating a pair of lateral chambers from the rest. The process continues; the blastopore closes, while the entodermal folds extend to the opposite side, where they fuse with the walls (B). In this way a pair of ccvlomic pouches are cut off from the rest of the archenteron which forms the lumen of the digestive tract and its derivatives, while the walls of the pouches form the mesothelium, that of the digestive region the secondary entoderm. In each ccelomic pouch two walls are recognizable, an inner or splanchnic layer which unites with the entoderm to form the wall of the digestive tract, the splanchnopleure, while the somatic layer unites similarly with ectoderm to form an outer body wall, the soma- topleure. From the foregoing it is evident that the mesothelium is strictly not a single layer, but consists of two layers which pass into each other, and that its origin is closely connected with the formation of the body cavity. Occurrence of Mesenchyme and Mesothelium. — There are purely mesenchymatous animals, like the flat-worms, and purely mesothelial, like Sagitta, many annelids, and Amphioocus; there are also animals in which the mesoderm consists of mesenchyme and mesothelium: either the mesenchyme arises first and later the mesothelium, as in the echinoderms, or in the reverse order, as in most vertebrates. Histological and Organological Differentiation. — All the organs of an animal arise from the three germ-layers. The details differ in the various groups; the following is the most general: from the ectoderm arise the skin with its glands and appendages, the nervous system, and the sensory epithelium; the entoderm gives rise to the most important part of the digestive tract with its glands; while muscles, blood, supporting and connective substances, excretory organs, in whole or in part, arise in the mesoderm; the sexual organs are also usually mesodermal. Relations of the Germ-layers in Budding.— The question has been raised as to how far the germ-layer theory is applicable to the occurrences in asexual reproduction. At first one would expect that each organ of the daughter would arise from the corresponding organ of the maternal animal, or at least from a mass of tissue belonging to one of the same germ-layers. In many instances this is the case; in the budding of hydroids the entoderm and ectoderm of the bud arise from the corresponding layers of the maternarbody (fig. 93). But exceptions are known. In polyzoans and tunicates there are undifferentiated cells which are employed in cases of budding. In the regeneration of lost parts it is not necessary that the missing structure should be re-formed by the same layer from which it originally arose. The lens of Triton arises ontogenetically from the epithelium of the skin. If extirpated, it is regenerated from the pig- mented epithelium of the iris. Review of the Different Kinds of Reproduction. — The foregoing outline of reproduction is in accordance with the prevailing ideas. Although these are justified theoretically, they do not correspond to the actual relations, since the GENERAL EMBRYOLOGY 149 divisions of the Protozoa — cell divisions, and those of the Metazoa — divisions of cell complexes, are brought into the same category, in spite of their different morphological value. Further, they accept a causal connection with fertiliza- tion in a form that does not agree with the actual conditions. To make this clear we start with the fact that every reproduction depends upon an increase of cells. In unicellular organisms reproduction and multi- plication of cells are one and the same thing. In multicellular organisms, on the other hand, two kinds of cell division may be distinguished, which may be called somatic and propagating cell divisions. Both are continuations of the cleavage process. The first is concerned with the growth of the individual, since it increases the formative material for the functioning organs; the other renders reproduction possible, since it furnishes the sex cells from which the new individual develops. Fertilization enters in the course of the cell divisions which occur during life, in the Protozoa as the fusion of two individuals, in the Metazoa as the union - viviparous animals, for their eggs are fertilized before they are laid, and ha\c already completed the formation of the blastoderm. In the case of many snakes the egg-shell may contain, even at the time of laying, an animal all ready for hatching. Transitional forms of this kind show that no sharp liiu- can be drawn between 'egg-laying' and 'bearing living young' and one must guard against attributing too much importance to the apparent distinctions. In many divisions of animals oviparous as well as viviparous forms are found. The majority of sharks are viviparous, but some lay eggs; in most bony fishes the eggs are laid before fertilization. Exceptions are the viviparous surf perches of the Pacific and many Cryprinodonts of fresh water. Most of the Amphibia, reptiles, and insects are egg-layers, but not a few forms are viviparous and even in the same genus (Lacerta and Chamivlemi) there may be viviparous and ovo-viviparous species. Even among the mammals, for which for a long time the 'bearing young alive' was regarded as diagnostic, it has been discovered that the monotremes lay eggs. Finally, exceptions to the rule occur in one and the same species. Adders are ovo-viviparous, but under unfavorable conditions they retain the eggs inside their body until ready to hatch. SUMMARY OF THE FACTS OF ONTOGKXV. 1. The development of an animal begins with an act of generation; spontaneous generation and generation by parents are to be distinguished. 2. Spontaneous generation (abiogenesis) is the origin of living beings from lifeless matter (without pre-existing organisms). 3. The present existence of spontaneous generation is neither known nor is it, on the whole, probable; yet it is a logical postulate, in order to explain the origin of life on our globe. 4. Generation by parents, derivation of an animal from another of similar structure, can take place either by the sexual or the asexual mode. 5. Asexual generation may be either by division or by budding. 152 GENERAL PRINCIPLES OF ZOOLOGY 6. In division an organism grows regularly in all its parts, and by constriction falls into two or more equivalent new pieces. 7. According to the direction of the plane of division in reference to the long axis of the animal we speak of longitudinal, transverse, and oblique division. 8. In case of budding a local growth occurs ; the local outgrowth, the bud, separates from the mother as a smaller, usually incompletely formed animal. 9. According to the position and number of the buds we distinguish lateral, terminal, and multiple budding. 10. Sexual reproduction occurs by means of special sexual cells, which have no part in the ordinary functions of the body. 11. In sexual reproduction two kinds of cells unite, the female egg and the male spermatozoon (fertilization). 12. In rare 'cases the egg develops without fertilization: partheno- genesis; this is a sexual reproduction with degenerated fertilization. 13. P ado genes is is parthenogenetic reproduction by a young (i.e., incompletely developed) animal. 14. Different modes of reproduction (asexual, sexual, parthenogenic, predogenic) may occur in the same species; then these often occur in a regular order, and in such a way that individuals with different modes of reproduction alternate with one another: alternation of generations in the wider sense. 15. Alternation of generations in the strict sense (metagenesis} is the alternation of two generations, one reproducing by division or budding, the other sexually. The former is called the nurse, the latter the sexual animal. 1 6. The alternation of parthenogenesis or pasdogenesis with pro- nounced sexual reproduction is called heterogony. 17. Development which is inaugurated by sexual reproduction shows in nearly all multicellular animals a general agreement in the incipient stages: fertilization, cleavage, formation of germ-layers. 18. The essential point of fertilization lies in the complete fusion of egg and spermatozoon, particularly in the fusion of the nuclei, egg and sperm nuclei, to form the cleavage nucleus. 19. The cleavage of the egg is a cell division, a division of the fertilized egg into the cleavage spheres (blastomeres). The cleavage may be total (holoblastic egg) or partial (meroblastic egg) ; total cleavage is either equal or unequal, the partial either discoidal or superficial. 20. By repeated divisions of the cleavage spheres, and by the forma- tion of a cleavage cavity, there arises a one-layered embryo, the blastula. (ECOLOGY 153 21. By the invagination of the blastula the gastrula or two-layered embryo arises. 22. The gastrula contains the primitive digestive tract or archenteron, opening to the exterior through the blastopore; it consists of two epithelial layers, the entodenn or the inner germ-layer, lining the archenteron, and the ectoderm or outer germ-layer. 23. Between the inner and the outer germ-layer still a third, the middle germ-layer, mesoderm, may be formed. 24. The middle germ-layer arises either by an infolding or a cutting off of a part of the entodermal epithelium: epithelial mesoderm, mesotlie- lium; or by the migration of separate cells to form a gelatinous tissue (mesenchyme). 25. Many animals deposit their eggs before or shortly after fertilization (oviparous}; others lay eggs which have already been fertilized in the maternal body, and at the time of laying have passed through some of the stages of development (ovo-viviparous) . A third series of animals give birth to living young (viviparous}. 26. The development of an animal is either direct or indirect (meta- morphosis). 27. Indirect development or •metamorphosis is where the young animal, as it comes from the egg, differs from the sexually mature animal in two points: (a) by the lack of certain organs which occur in the sexually mature animals; (b) by the appearance of organs, larva.! organs, which are lacking in the sexually mature animals. III. RELATION OF ANIMALS TO ONE ANOTHER. Just as between the organs of an animal there exists a regular relation which is termed correlation of parts, so also the different individual animals stand in intimate reciprocal relations to one another. The con- ditions of existence of many animal species are altered, if other forms appear or disappear, or undergo an extraordinary reduction or increase in number of individuals. Such reciprocal effects are usually of a special nature and can be understood only by individual study; a few are of wide occurrence and are hence suitable for a general consideration; to such belong colony and society formation, parasitism, and symbiosis. I. RELATIONS BETWEEN INDIVIDUALS OF THE SAME SPECIES Colony Formation.— Colony and society formation are relation-; which exist between individuals of the same species An animal colon is a union of individual animals by an organic bodily connection; the 154 GENERAL PRINCIPLES OF ZOOLOGY latter may arise in. two ways: first, by animals, originally separate, par- tially fusing together; secondly, by individuals, formed by division and budding, remaining united with one another instead of separating. The first is extremely rare. Colony Formation by Fusion. — Many Protozoa fuse with one another and form larger bodies in which the individual animals can still be recognized. Among the metazoa, Diplozoon paradoxum (fig. no) is the only case known where two animals (Diporpa?), arising from different" eggs, normally unite into a double animal, which recalls certain double monsters, as the Siamese twins. FIG. no. — Development of Diplozoon paradoxum (from Boas), (i) Larva, from which comes (2) 'Diporpa.' (3) Two Diporpa; uniting. (4) The Diporpae have united into Diplozoon. m, mouth; d, digestive tract; h, posterior adhering apparatus; b, ventral sucking-disc, which serves for attachment to the dorsal cone, r. Colony Formation by Incomplete Division and Budding. — In general it can be said that colony formation rests upon incomplete asexual reproduction, since the new generation does not separate from the parent. The colonies of marine hydroids and corals (figs. 94, 205) may consist of thousands of individuals which, by repeated incomplete budding or division, have sprung from a single sexually produced mother animal. Community of Functions. — In the majority of cases the connection results in a considerable degree of community of functions. Stimuli which affect one individual are transmitted to the others of the colony; thus movements in common are rendered possible. In a similar way the food captured and digested by one animal serves for all. On account of the community of its functions, a colony appears like a unified whole, like an individual of a higher order; the same process which led to the formation of multicellular organisms is repeated. Just as there the elementary organisms, the cells (individuals of the first order) are united into a single animal (individual of the second order), so here the multi- cellular animals are united into a colony (individual of the third order). Polymorphism. — When a whole is made up of numerous equivalent (ECOLOGY 155 parts, the conditions for division of labor are present. Instead of the functions of the entire organism being distributed equally to the in- dividual parts, many of the latter become employed more for this, others again more for that function, and acquire a corresponding structure. In such colonies one speaks of polymorphism. Polymorphism appears oftenest in connection with the vegetative functions, leading to a dis- tinction between sexual animals and nutritive animals, as in the case of most Hydrozoa, where often nutrition is accomplished by animals B A FIG. in. — Praya diphyes (after Gegenbaur). .-1, the entire animal; B, a single group of individuals greatly magnified (Eitiloxid). i, covering scale; 2, nutritive polyp; 3, nettle-threads; 4, sexual bell. without sexual organs, and reproduction is carried on by animals without a mouth. But other functions may also become specialized. Siphon. >- phores are the classical examples of polymorphism (fig. in). Here united into a single body are locomotor animals, the swimming-bells, for locomotion only; covering scales, which serve only to protect the otli.-r- nutritive polyps, which alone take in and digest food; sexual animals and tactile polyps, concerned only with reproduction and with srn-ntum. In regard to the other functions each animal is related to its br«.tlu-rs and 156 GENERAL PRINCIPLES OF ZOOLOGY sisters; its very existence therefore has become dependent upon these; the single individual can live only while a part of a whole. Thus divi- sion of labor leads to greater centralization ; the more polymorphic a colony becomes, the more unified it is, the more it gives the impression of.being a single animal instead of an aggregation of single animals. In Social Animals the reciprocal dependence of the individuals is much less, since there is no organic connection, only a voluntary com- munal life. As asexual reproduction is of importance in colonies, so here the sexual plays a prominent role. Under the influence of the sexual impulse, many animals, even some of the lowest organisms, flock together, either permanently or periodically; sea-urchins, sea-cucumbers, many fishes, collect near the coast at the time of egg-laying; it draws together herds of deer, elephants, etc. The care of the young further leads to a closer organization, to a society. All insect societies are built up on this basis. Consequently, since the sexual life is the starting-point of social life, in the different groups of individuals forming the community, the sexual organs may be influenced in their development. Besides males and females (kings and queens) there are other animals with degenerated sexual organs incapable of function, the workers; the latter are either only rudimentary females (bees and ants) or rudimentary females and males (termites). While the kings and queens give rise to the next generation, the workers care for the young, look after the hive, provide food, pro- tection, and defence, if the latter be not delegated to a special class, the soldiers (termites). II. RELATIONS BETWEEN INDIVIDUALS OF DIFFERENT SPECIES. Where individuals of different species stand in close reciprocal rela- tions to each other the cause is in the advantages which the one derives from the other, or which both furnish reciprocally; the former condition is called parasitism, the latter symbiosis. Parasitism. — Parasites are organisms which dwell upon or in another organism, the host, and obtain nourishment from it. They have consequently come into a dependent condition and have undergone a more or less extensive change in their organization. Degeneration Caused by Parasitism. — The degree to which a parasite has become dependent upon its host is determined by the extent to which the parasite has adapted itself to the organization of its host. Therefore it is necessary, in speaking of parasitism, to consider the modi- fications which the parasitic life has caused in the structure. These concern most immediately the organs of locomotion and nutrition. Since (ECOLOGY 157 •oe a parasite needs to fix itself as firmly as possible to the host, the locomotor apparatus more or less completely disappears and an apparatus for fixation becomes necessary; parasites of different groups are provided with hooks, sucking-discs, etc. The fluids of the host furnish nourish- ment to the parasite: these are substances in solution which scarcely need digestion. Usually, therefore, the digestive canal is simplified or dis- appears; among the parasites there are gutless worms as well as gutless Crustacea. Very frequently the intestinal parasites live without oxygen; they are anaerobic (p. 92). The mode of life of a parasite is also simplified, since it is no longer compelled to seek its food; the nervous system and sense-organs undergo great degeneration; the former becomes limited usually to the most indispensable portion; the latter, except those of touch, may entirely disappear. Modification of the Sexual Apparatus by Parasitism. — The sexual apparatus, on the contrary, undergoes a strong develop- ment. While it becomes easier for the parasite to maintain itself, the existence of the species is more precarious. If a man die, then most of his parasites die with him, especially those in the interior of his body. In order that a parasitic species may not become extinct, it is necessary that the eggs be introduced into a new host. Since this is attended with difficulties, the parasites must produce an enormous number of eggs. The eggs, too, are distinguished by great resisting power and well-developed protec- tive organs, such as strong shells, etc.; the eggs of Ascarids continue to develop for some time in alcohol, being protected by their impermeable shell. Ectoparasites and Entoparasites.— All the above-mentioned phenomena are more conspicuous in the case of parasites which live inside of 112. Fir.. IT.}. FIG. 112. — Ta-nia muni (after Leuckart). FlG. 113. — Pentastonnim tirnioides female (after Li-u. - kart). //., hooks right ami left of mouth; or, unpaired ovary, I.:, i .rliing into two oviduct-, which unite into the unpaired vagina (;•,;); the latter receives the outlets of two rcceptacula semiiiis (rs), and winds around the digestive tract (rf); n; ceso- phagus. other animals, entoparasites, than in the case of the dwellers upon the skin or other superficial organs, the ectoparasites. In case of ento- parasites the transforming influence of parasitism is so considerable 158 GENERAL PRINCIPLES OF ZOOLOGY that representatives of the most diverse groups take on a remarkable similarity of appearance and structure. Pcntastomum (fig. -113), for example, belongs in the same class with the spiders, the Arachnida, but in external appearance it is entirely unlike them, resembling the tape-worms (fig. 112). Hence for a long time all entoparasites, on account of their similarity, were united into a single systematic group under the name of 'Helminthes,' comprising members of the Crustacea, worms, and spiders. Only by embryology was the unnaturalness of this grouping recognized. Entoparasitism therefore is one of the best examples for illustrating convergent development, i.e., animals of different systematic position acquiring, under similar conditions of life, a great similarity of structure and appearance. Symbiosis. — Less frequent than parasitism is symbiosis, or the association of animals for reciprocal advantages. Social animals frequently not only hold certain animals in bondage, but even seek to protect and serve them; as, for example, certain blind beetles, like Claviger or some species of plant-lice, (myrmecophiles) or even ants of other species and genera. But such cases of association correspond in part to the domestication of animals, or to slavery, as carried on by man. The ants keep the plant-lice in order to lick the sweet juice ('honey dew') which is secreted in their honey-tubes; they steal the pupas of other ants and rear them, to use them later as slaves. This state of things rests, consequently, not upon equal rights, since the one animal, in the present example the ant, brings about the association, while the other is passively led into it. Symphyly is close to true symbiosis. Besides Claviger, mentioned above, many other insects, mostly beetles, which are cared for by the ants, are found in colonies of ants and termites, since they have a sweet secretion on special bundles of hairs, which the ants lick off. Frequently the beetles eat the younger stages of the ants. An instance of most complete equal rights and true symbiosis is furnished us, however, by a hermit-crab and an actinian (fig. 114), Eupagiirus pubescens and Epizoantlius attieri- (-(iiiiis. Like all hermit-crabs this also inhabits a snail-shell from the opening of which only his legs and pincers are protruded. Upon this shell an Epizoantlius becomes attached and by budding soon covers it with a colony of polyps. The advantage which the actinian derives from this symbiosis is clear: it gains a share of the food which the crab obtains. It is less clear what the crab gains; however, the polyp is perhaps a protection to him by its nettle cells, while by growth it increases the size of the 'house' occupied by the hermit and thus saves him periodic changes of abode. Occurrence of Symbiosis. — That animals rarely live symbiotically with one another rests largely because the conditions of life of all animals to a certain point are similar or identical. They take in compounds rich in carbon and nitrogen, decompose them into carbon dioxide, water, and oxidation products containing nitrogen. All animals consequently are competitors in the struggle for food. For the same reason, conversely, symbiosis between plants and animals is not so uncommon. There are certain lower algae, the Zooxanthellae, FIG. 114. — A colony of Epizo- anthus americanus on the shell occupied by a hermit-crab (from Yerrill). (ECOLOGY 159 which often live in animals. The radiolarians so constantly contain green- or yellow-colored cells that for a long time these were regarded as constituent parts of the animal. Similar yellow and green cells inhabit the stomach epithe- lium of many actinians, corals, and even of many worms. The ZooxanthelUe are nourished by the carbon dioxide formed by the animal tissues, and breathe out oxygen, which in turn is used by the animal; further, they form starch and other carbohydrates, and any surplus thus formed may be food for the animal. Thus there is on a small scale that cycle which exists on a grand scale in nature between the animal and vegetable kingdoms. By aid of chlorophyl and sunlight plants decompose water and carbon dioxide and form from them oxygen, which they respire, and compounds rich in carbon, which they store in their tissues: they are reducing organisms. On the contrary, animals give off carbon dioxide and water, but take their oxygen from the air, and carbon compounds in their food; they use oxygen to break down the chemical combinations, to oxidize: they are oxidizing organisms. This explains why the favorable influence of plants upon animals ceases immediately when they change the character of their metabolism. With the disappearance of their' chlorophyl fungi and bacteria have lost the power of reducing carbon dioxide; they derive their food from other organisms and decompose this into carbon dioxide, water, etc.; like animals, they are oxi- dizing organisms, and consequently dangerous competitors and are the cause of many serious ailments. IV. ANIMAL AXD PLAXT. Distinction between Animal and Plant. — The consideration of symbiosis leads to the fact that a distinction exists between plants and animals in^the mode of metabolism, which may be expressed thus: plants usually take in carl urn dioxide and give off oxygen, while animals absorb oxygen and give out carbon dioxide. Hence it might be concluded that it is easy to discover universal distinctions between plants and animals. But the more one studies this ques- tion, the more difficult becomes its solution. The old zoologists believed that there are organisms which stand on the limits between the animal kingdom and the vegetable, and named these zoophytes or plant-ani- mals. Now we know that these are true animals with but a superficial similarity to plants; but, by means of the microscope, we have become acquainted with numerous lower organisms, and it is still doubtful in which of the two realms some of these, like the Myx<>- mycetes and many Flagellata, belong. ' Physiological Distinctions. — In the search for distinctions both physiological and morphological characters may be considered. Starting from the / physiological side Linnaeus ascribed to plants only the ^ ^_ _Lff)ilSt]miti_ capacity of reproduction and nutrition, but to animals ''^fter Schmarda). the power of motion and sensation in addition. Now ^ carina; /, tcrgum; .•>-, we know that vegetable, like animal, protoplasm is s'cutum. irritable and is capable of motion, as is shown by the active movements of the lower Algae, the great sensitiveness of the Mm, other plants; but further, we know that even many highly organized i e.g., Crustacea (fig. 115), lose the power of locomotion and become many fixed forms, e.g., the sponges (fig. 88), appear immovable and unatt by stimulation; thus the so-called animal functions cannot be re; affording accurate distinctions. 160 GENERAL PRINCIPLES OF ZOOLOGY Even the difference in metabolism is by no means sufficient. Every plant has a double exchange of material. In its movements and other vital functions the vegetable protoplasm produces carbon dioxide and consumes oxygen; at the same time there goes on, under the influence of sunlight and of chlorophyl, the reduction of carbon dioxide and the giving off of oxygen. In chlorophyl- containing plants the reducing process preponderates so considerably during the day that they give off a quantity of oxygen, and only at night, when the reducing process becomes interrupted on account of the lack of sunlight, does the pro- duction of carbonic-acid compounds become perceptible. But if the chlorophyl be absent the reducing processes disappear; chlorophylless fungi and bacteria, have, therefore, the same metabolism, so far as carbon dioxide is concerned, as animals. So also it is incorrect to say that only plants have the power to make cellulose, for cellulose is found in many lower animals, the rhizopods, in the highly organized group of tunicates and even among the arthropods. Morphological Distinctions. — Turning to the morphological characters, multicellular animals and multicellular plants are readily distinguished, since the former in the germ-layers have a principle peculiar to them; with the appear- ance of the gastrula each organism is undoubtedly an animal. But in unicellular animals the arrangement of the cells is lacking, and only the constitution of the single cell can guide us. Now are there unmistakable morphological differences between the animal and the vegetable cell ? Plant-cells have a Cellulose Membrane. — In the structure of plant and animal cells an important distinction is found in the fact that the former has a cellulose membrane, but the latter is usually membrane- less. To this distinction must be referred in the last analysis the widely different appearance of the two realms. Since the plant-cell is early surrounded with a firm coat, it loses a large part of its power of further changing its form; hence vegetable tissues and organs, in spite of the manifold' intracellular differentiations, like the chlorophyl granules, are uniform in comparison w^ith the inconceivable multiformity which animal structures disclose. The higher stages of organization which the animal kingdom reaches, even in its lower classes, is in great part the result of the fact that the cells of animals do not become encapsuled, but have preserved the capacity for more varied and higher de- velopment. But even here transitions are found be- tween the lower plants and animals. In the lower Algae the cells can leave their cellulose membrane, and swim about freely (fig. 116), before they enclose them- selves anew. On the other hand, most unicellular animals encyst; they pause in their ordinary functions, become spherical, and surround themselves with a firm membrane, sometimes even of cellulose. Since in both cases an alternation between the encapsuled and the free condition occurs, only the longer duration of the one or of the other can lead to a distinction. But here occurs the possibility that indifferent intermediate forms exist; their actual existence prevents, even yet, a sharp distinction between the animal and vegetable kingdoms. V. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. The Different Faunal Regions. — Even a superficial knowledge cf the distribution of animals shows that the animal population, the fauna, in different FIG. 1 1 6 . — CEdogo- nium in spore-formation (after Sachs) . A , a piece of the alga with escap- ing cell-contents: B, zoospore formed from the contents; C, zoospore fixed and germinating. DISTRIBUTION 161 reckons of the earth has an essentially different character. In part this is the immediate result of climatic differences. The polar bear, arctic fox, eider- ducks, are restricted to the polar zones, because they cannot endure more than a certain degree of warmth; on the other hand, the larger species of cats, the apes, the humming-birds, etc., occur only in warmer regions, because they are not sufficiently protected against cooler weather. If climate were the sole factor determining distribution, the faunal character of two lands which have similar climatic conditions would be essentially the same; conversely, the separate regions of a continuous territory extending through several climatic zones must have different faunas, according as they are nearer the equator or the poles. But such is not the fact; two tropical countries may differ more widely in their fauna than the hot and cold regions of one and the same country. Factors in Distribution. — Modern zoology endeavors to explain these conditions by regarding the present distribution of animals as the product of two factors: the gradual changes of the animal world, and the gradual changes of the earth's surface on which the animals are distributed (p. 33). The history of the earth as disclosed by geology shows two facts: (i) that the connections between parts of the earth have varied greatly; that, for example, at a time when the Mediterranean had not yet reached its present extent, Morocco, Algiers, Tunis, and Egypt were more closely united with the European coast of the Mediterranean than with the southern part of the African continent separated from them by the Sahara; (2) that considerable variations of climate have taken place: there prevailed in Europe in the tertiary period a subtropical climate which rendered possible the existence of animals which now occur in Algeria (lions). But later a glacial period began, which introduced over a large part of Europe arctic conditions, and consequently a fauna of northern animals (reindeer) which has left a few traces (Alpine hare) in the glacier regions. Hand in hand with the geological changes went changes in the animal world, the then existing species dying out under the change of conditions, or perhaps forming new species through gradual variations. Thus the distribution of animals constitutes an extremely complicated problem, for the solution of \\ InYh it must be known how the climates and the connections between the continents have changed, particularly in the later geological periods; further, not only how animals are distributed at present over the earth's surface but also how they were distributed in earlier times. Finally, we must have clear and detailed idea;- of the relationships and interrelationships of animals. It will be a long task to solve all the problems. What has been accomplished so far can only be regarded as showing that zoology, with its prevailing views of the changes of animals and of the earth, is on the right track. Two region-,, separated early in the earth's history and never again connected, must have greater differences in faunal characters than two lands still coniuvied or only recently separated. We travel in the northern hemisphere and find in widely separated regions strikingly similar fauna?, while under the equator or in the southern hemisphere, under the same conditions for each region, Mriking differences are seen. This is explained on the hypothesis that in all past periods as now the land masses of the northern hemisjxhere have hem closely connected, while the parts of the continents extending to the south have been separated through most of the earth's history. Students of distribution have' attempted to define the great faunal areas of the earth, the faunal provinces or regions, and within these the subregions These provinces have been based chiefly upon the distribution of mammals less upon that of birds and other animals; for the distribution of mammals chiefly determined by those changes of the earth's surface which are best known geologically and possess most interest. Elevation or depression of the earth'? ll 162 GENERAL PRINCIPLES OF ZOOLOGY surface often opposes impassable barriers to most mammals: rising, if it lead to the formation of glaciered mountain-chains; sinking, when arms of the sea are formed, which interpose straits or broader bodies, impassable to most mammals, between two land areas. Birds and strong-flying insects are less affected by all such changes; the majority of them can fly over arms of the sea and moun- tain-chains; there are birds which can even cross the Atlantic. The Six Primary Regions. — Of the systems of animal geography proposed that advocated by Sclater and Wallace finds most favor. They distinguish six primary regions: (i) the pakcarctic, comprising all Europe, northern Africa as far as the Sahara, and northern Asia to the Himalayas; (2) the Ethiopian, Africa south of the Sahara; (3) the oriental, including upper and farther India, southern China, and the western Malay Islands; (4) and (5) the nearctic and the neo- tropical regions, which make up the American continent and are divided at about the northern border of Mexico; (6) the Australian, with, besides Australia itself, the larger and smaller islands of the Pacific Ocean and the Malay Islands, east of Celebes and Lombok. (1) The Australian region is most sharply distinguished and by many is set apart as a distinct division called 'Notogaea.' Its isolated geographical position together with the fact that it has long been separated from other countries (apparently since the beginning of the tertiary) explains why only the oldest mammals, the monotremes and marsupials, have entered the region, while the placental mammals have not been able to follow. The marsupials, which in the secondary period also inhabited the northern hemisphere, and were replaced there in tertiary times by the placentals, were able to develop farther in the Australian region. Australia and the adjacent islands are thus the land of marsupials, which have persisted elsewhere only in South America, the opossum ranging north into the United States. On the other hand, at the time of dis- covery Australia lacked all terrestrial placental mammals except those (bats) which were not restricted by water and the Muridae, easily transported on floating wood. Two larger mammals, the wild dog (Canis dingo} and the pig of New Guinea (Sns papitanus), may have accompanied man, this being probable for the dingo, although his remains occur in the pleistocene along with those of the giant marsupials. Further peculiarities of the Australian region are the birds- of-paradise in New Guinea, the egg-laying mammals (monotremes), and the cassowaries and the Australian ostrich (DromcBus novcchollanditf}. It is easily understood that the island groups of the South Sea (Polynesia) have developed many faunistic peculiarities, as well as that an exchange of forms may have taken place between the islands of the oriental province and those faunally related to Australia, and that 'Wallace's Line' (p. 34) is not so sharp a boundary as was once thought (extension of marsupials into Celebes, of pla- centals into the Moluccas). On the other hand the distinctness of New Zealand needs mention. It is distinguished from Australia by a large number of peculiar birds (Apteryx and the extinct Dinornithidas), reptiles (the ancient Sphenodcni), and molluscs. If the bats and mice be excepted, New Zealand lacks all native mammals, even marsupials. (2) The Neotropical province (South and Central America) is, next to Australia, the most sharply characterized, and has also been set aside as a special division 'Neogaea,' partly in view of its geological history; during the cretaceous and early tertiary time it was separated from North America by the sea and had developed a peculiar fauna (e.g., gigantic edentates, no carnivores). These peculiarities disappeared towards the end of the tertiary by the entrance of carnivores and ungulates from the north and an extension of the edentates, marsupials, humming birds, etc., to the northern hemisphere. To Neogaea belong the platyrhine apes, the catarrhine to the Old World. Characteristic edentates are trie armadillos, sloths, and ant-eaters; the marsupials are repre- DISTRIBUTION 163 sented by the opossums and Ccrnolestcs, nearly related to the Australian Di- protodonts; among birds the humming-birds, toucans, Cotingidae, Tanagrichc, Tinamous, Palamedidae, Rhea, etc. The almost entire absence of insect! vores and the considerable development of rodents (cavies, agoutis, chinchillas) are noteworthy. The four remaining provinces are still closely connected geographically and form a third great division, 'Arctoga-a,' characterized by the entire absence of platyrhine apes, monotremes, and, except the North American opossum, of marsupials. In the secondary and tertiary times the northern parts of these lands were connected and an interchange of faunas occurred, this being the easier on account of the extension of the warm climate to the far north. Hence many unite the palasarctic and nearctic provinces into a 'holarctic' province, but when existing conditions are concerned it is better to retain them as distinct. (3) The Nearctic region has three mammalian families, the prong-horned antelope, the opossums, and the Haplodontae, peculiar to it; of the group of Amphibia, the Sirenidae and Amphiumidae. The Nearctic is distinguished from the nearly related palasarctic region through the crowding in of neotropical forms like the raccoon, opossum, humming-birds, etc. The absence of the stag, badger, wild swine and all true mice is noticeable. (4) The Pahearctic region covers the greatest area and consequently touches several others; hence climate and great distances have caused important differ- ences between the local faunas, but its contact with other regions explains the fact that it has no peculiar families. Deer, cattle, sheep, and camels have reached a great development; especially conspicuous genera are the chamois, squirrel, badger, and marmot. (5) The Ethiopian region has many animals found only there; among li the hippopotamus, giraffe, the recently discovered Oeapia, the aardvark, and, if we include Madagascar, the lemurs are characteristic. To these are added a rich development of antelopes and zebras and the gorilla and chimpanzee. Equally noteworthy is the entire absence of striking families and genera, such as the bears, moles, deer, goats, tapirs, sheep, and the true swine. Within the region the island of Madagascar occupies a remarkable position. This is the land of lemurs and Insectivora, the majority of the genera of lemurs living exclusively in Madagascar. On the other hand, the large beasts of prey, all the true apes, antelopes, elephants, and the various species of rhinoccro- arc absent. Consequently, since Madagascar is conspicuously distinguished from Africa, many zoologists separate the island from the Ethiopian region as an independent Malagassy province. (6) The Oriental region contains, next to Madagascar, the most lemurs among which the Tarsidae and Galeopithecidae arc exclusively oriental. Re- markable inhabitants are the gibbons and orang-utans, musk-deer, numerous families and genera of birds. Arctic and Antarctic Provinces. — Of late the view has gained ground that, besides these six, two other, circumpolar, provinces must he distinguished, the Arctic and the Antarctic. Both have a fauna consisting of few specie.- but numerous individuals, of which the auks, polar bear, reindeer, and arctic foxes are characteristic of the northern or arctic region, the penguins and the entire absence of land mammals of the antarctic. The Distribution of Aquatic Animals. — Since most seas are connected, the faunal regions cannot be distinguished so sharply as in the case oi the land faunas; conspicuous differences are present only when two oceans are separated by continents extending far to the north and south; such, for example, i-xi-t between the Red Sea and the Mediterranean, between the east and west coa of North America, even where they are separated only by the isthmus of Panama. 164 GENERAL PRINCIPLES OF ZOOLOGY Then, too, considerable differences may exist where currents of greatly different temperatures meet. Much more remarkable in the marine fauna are differences caused by the conditions of life in the different depths of the sea. A deep-sea fauna, a coast ''niiia, and a pelagic fauna can be distinguished. The coast fauna embraces the animals which inhabit the plant-covered rocky or sandy shore to a depth of a few hundred feet. The deep-sea fauna swims, creeps, or is fixed at the bottom of the ocean at depths of 1000 to the greatest depth yet known, 9430 meters, 5156 fathoms; it is distinguished from the coast fauna in part by its archaic character, for here very often genera and entire groups of animals exist, like the Hexactinellidas, crinoids, etc., which long were chiefly known through fossils from earlier geological ages. The Plankton. — The pelagic fauna comprises all forms which swim freely in the water, the plankton; here belong many ccelenterates, medusae, and cteno- phores, entire groups of Protozoa, like the radiolarians, many Crustacea, the heteropods and pteropods. These animals live either at the surface of the sea itself or floating at greater or lesser depths, to 8000 meters or even more. Usually they are gelatinous and of glasslike transparency; this must be regarded as sympathetic coloring and adaptation to the transparency of the water. The plankton of the deep seas, extending up to about 800 meters, forms a special fauna characterized by the brownish-red color, which is also found in the bottom animals. Distribution of Fresh-water Animals. — In fresh water two groups of animals must be distinguished, of which the one comprises mainly the more highly organized forms, the molluscs, fishes, and Crustacea, the other the lower animal world. The distribution of the former is mainly determined by the same factors which influence terrestrial forms; they are therefore of great impor- tance in matters of geographical distribution, yet it must be remarked that many fish at the breeding season ascend from the seas to the rivers (salmon, alewives, etc.) and on the other hand, others like the eels go from the rivers to the seas, so that the sea is not that sharp boundary for these animals that it is for land ani- mals. The distribution of the lower fresh-water animals, however, is cosmo- politan. The same infusorians and rhizopods, copepods, fresh-water sponges and polyps which occur in America seem to be distributed over nearly the entire earth. This is connected with the fact that all these animals have resting stages in which they endure desiccation. The resting stage, be it as a hard- shelled egg or as an encysted animal, may be borne about by the wind, or may be carried with the mud by aquatic birds, and upon again reaching the water resume its active state. VI. DISTRIBUTION OF ANIMALS IN TIME. It is the province of paleontology or paleozoology, to treat of animals in the earlier periods of the earth's history, but since it is necessary to draw upon paleontological facts to understand the living forms, and especially the verte- brates, an outline of the geological periods with the characteristic animals may be given here. I. Azoic OR ARCHEAN ERA. No organisms are certainly known from this age. The animal nature of Eozoon canadense of the Laurentian beds, once referred to the Foraminifera, is more than doubtful. DISTRIBUTION 105 II. PALEOZOIC ERA. 1. Cambrian. 4. Carboniferous. 2. Silurian. 5. Permian. 3. Devonian. The oldest paleozoic period, the Cambrian, contains only invertebrate fossils: silicious sponges, the problematical graptolites, medusa-, trilobites, gigantostraca, cystoids, holothurians, brachiopods, nautiloids, gasteropods, and a few lamellibranchs. Trilobites, cystoids, gigantostraca, and the blastoids and tetracoralla, which appear in the Silurian, reach their culmination and become extinct in the paleozoic. Fishes appear in the Silurian, and acquire a great development in the Devonian. The earliest Amphibia and reptiles come from the carboniferous. III. MESOZOIC ERA. i. Triassic. 2. Jurassic. 3. Cretaceous. The mesozoic era was the age of reptiles, which were represented by numer- ous forms, some of gigantic size; most of them becoming extinct in the creta- ceous. The first mammals appear in the triassic, the birds in the Jurassic. Among the invertebrates the ammonites, which appeared in the Devonian, reached their greatest development and became extinct in this era. IV. CENOZOIC ERA. (a) Tertiary. 1. Eocene. 3. Miocene. 2. Oligocene. 4- Pliocene. (b) Quaternary. 5. Pleistocene (Ice Age, Diluvium). 6. Recent. In the tertiary all of the now living orders of mammals and birds appeared, among them probably man, whose remains have been traced with certainty to the pleistocene. SPECIAL ZOOLOGY. SINCE comparative anatomy and the theory of evolution have made their impress upon systematic zoology one recognizes in classification not only a means of arranging the species, but also the possibility of expressing the relations which the larger and smaller groups bear to each other. The solution of these problems demands an accurate knowledge of compara- tive anatomy and embryology and a complete knowledge of animal forms based upon them. We are yet far from such a knowledge, farther with regard to some groups than others, and as a consequence systematic problems are not all equally advanced towards solution. In general it may be said that certain natural groups are recognized: (i) Chordata; (2) Mollusca (after the elimination of the Brachiopoda); (3) Arthropoda; (4) Echinoderma; (5) Ccelenterata (after the separation of sponges) ; (6) Protozoa. On the other hand, it is yet uncertain exactly how to regard the worms, brachiopods, polyzoa, and a few other forms. The general tendency is to distribute the worms into at least three branches (flat worms, round worms, and annelids) and to unite the Polyzoa and Brachiopoda in a branch of Molluscoida. In this way groups poor in species and of little importance in a general account of the animal kindom are placed on the same basis as the large and exceedingly important groups of vertebrates, arthropods, and molluscs, and thus obtain, espe- cially in the eyes of the beginner, an importance which does not belong to them. It therefore seems better in an elementary work to pursue a rather conservative course. PHYLUM I— PROTOZOA. All of the Protozoa are small ; some may be seen by a sharp eye as mere specks, but the majority are so minute as to be invisible except with a micro- scope. On the other hand, a few have a diameter to be measured by milli- meters, especially where hundreds of individuals are united in colonies. This small size is a result of the fact that the Protozoa are single-celled animals. Like all cells they consist of protoplasm, and they have the further cell attribute, one or more nuclei. Being unicellular, it follows that they lack true tissues and true organs; alimentary canal, nervous system, sexual organs, etc. The functions of nourishment, sensation, movement, and reproduction are performed more or less directly by the protoplasm. 106 PROTOZOA 167 In nutrition, in so far as it is not produced by substances in solution, foreign particles pass into the protoplasm and are digested by it. They usually lie during digestion in special collections of fluid, the food vacuoles (figs. 121, 150, etc., na), more rarely in the protoplasm itself. All in- digestible portions are cast out after a time. This taking in and casting out of foreign matter can take place in the naked Protozoa at any point of the surface, while in the more highly organized species when the outer surface is hardened by a pellicle or a thin c-uticula, there are definite open- ings which according to analogy with many-celled animals are spoken of as mouth and anus, or more precisely, cytostome and cytopyge. The mouth may connect with a tube, the oesophagus or cytopharynx, which ends free in the protoplasm. Structures may occur within the protozoan cell which recall the organs of higher animals, and which are called cell organs. \Yhile motion is usually produced by the protoplasm and its processes — pseudopodia, flagella, and cilia — there are Protozoa, like Stcntor and the Yorticellida- which have muscular fibrilke. The sensitiveness to light is often increased by an eye spot, a small pigment body in which even a lens may occur. More constant of cell organs are the contractile vacuoles (tig. 117, etc., rr), rarely absent from fresh-water species, but commonly lacking from marine forms. These have a definite place in the cell; their number is approxi- mately constant in most species; they exhibit extremely constant phe- nomena. The walls contract and empty the fluid contents to the exterior, often through a special duct. When one empties it completely disap- pears and is formed again anew in a short time, and is filled with fluid from the surrounding protoplasm. It thus resembles the contractile vacuoles in the water vascular system (excretory organs) of the worms to be described later. Apparently the contractile vacuoles are for the elimination of injurious substances in solution produced by the vital pro- cesses, among them possibly carbon dioxide, like a respiratory organ. The occurrence of such diverse differentiations, recalling organs and tissues, gives such a complicated appearance and such a decree of spi-ciali/ation t<> tin- protozoan body, that it was questioned for a time whether all could belong t" a single cell. Yet it was a mistake to doubt the unicellularity of the I'roto/oa. l»r according to our conceptions of the cell, there is the capacity to develop in many directions, to produce a kind of stomach, muscle fibres, sense apparatu-, skeleton and the like; although in the organization of the higher animals it produces only a specific product (muscle cells, contractile substance, gland cells, secretion). The vital phenomena of the Protozoa proceed from the protoplasm, but with a certain dependence upon the nucleus. If an infusorian or an Amoeba be cut into nucleate and anucleate portions, only the lirst can 168 PROTOZOA live. The part without the nucleus loses the capacity for assimilation, for growth, and for regenerating the lost parts. For a time it can react to stimuli, move about. Sensibility and contractility persist only so long as the necessary elements, formed under the influence of the nucleus, are present. When they are used up the last manifestations of life are lost and death ensues. So it may be said that the chemism of the cell needs the participation of the nucleus. The nucleus is also concerned in reproduction, of which the most primitive type is binary division (figs. 120, 150, 151). Budding is rarer, its character being most evident when several buds are separated simul- taneously from the mother animal (fig. 21). The nuclear division occurs in different ways. Like the cell body, it may divide amitotically, but it can present the complicated phenomena of mitosis (formation of spindle and chromosomes). In not a few instances the specific organ of division, the centrosome, "appears, so that all transitions from direct to extremely complicated division are present in the phylum. Very frequently the nuclei multiply without a corresponding division of the protoplasm, so that large masses of protoplasm, with hundreds or even thousands of nuclei arise (multinucleate cells, syncytia); or both nucleus and protoplasm may grow, without division, to extraordinary size. In both instances, after an interval of time, there is a simultaneous division into hundreds or thousands of reproductive particles; the pro- toplasm, in the first case, dividing in accordance with the number of nuclei present ; in the other following the division of the mother nucleus into a multitude of daughter nuclei. Many Protozoa divide in the free state while swimming or creeping about; others first encyst, that is, assume a spherical shape and secrete a protective envelope. In the Protozoa may occur a fusion of individuals — conjugation' — which in many respects has much similarity to the process of fertilization in Metazoa and in plants. In some (conjugation of many Rhkcpods) this does not correspond to true fertilization, since only the protoplasm unites (plasmogamy), while the fusion of nuclei (car yoga my) necessary to fertilization does not occur. In others a fusion of nuclei takes place. In the cases which have been accurately studied there has been seen, before the fusion of the nuclei, a process comparable to the formation of the polar globules in the egg, to this extent, that in each of the conjugating individ- uals the nucleus divides twice and of the products of division only one, the nucleus intended for caryogamy, persists, while the others (polar globules) degenerate. These cases of true fertilization may differ greatly. The conjugating individuals may be equal in size, iso gametes (most Infusoria, many Rhizo- PROTOZOA 169 poda), or there is a disparity in size (sexual dimorphism), in which smaller and consequently more mobile 'males' (microgamctes, zoosporcs) fertilize the larger fixed or slowly moving 'females' (macro gametes, oospores) as in Yorticellidce, most Sporozoa, and flagellates, forming with them a permanent zygote (copulation). The formation of a zygote can also occur by the permanent fusion of two isogametes, but usually the union of isogametes is transitory (conjugation) and lasts only long enough for cross fertilization, gamete A fertilizing B, and in turn bring fertilized by B, after which the two separate. A striking phenomenon is the not very rare 'autogamy' in which the mother animal divides into two daughter animals, which form polar globules and fuse to a zygote, an extreme case of inbreeding. Thirty years ago it could be laid down as a universal fact that the Protozoa in contrast to the Metazoa lacked sexuality. Since then observations have so increased that the conclusion is that fertilization occurs in all Protozoa, although the rarity of the process in many species renders the demonstration difficult. Perhaps also in many groups fertilization has been lost through degeneration (similar to the apogamy of plants). Still there remain certain interesting differences from the Metazoa. The Protozoa lack special sexual cells — eggs and spermatozoa. On the contrary, the whole body functions as a sexual cell. Further, the relations of fertilization to reproduction are not the same as in the Metazoa. (i) Protozoa may increa e in the same way before and after fertilization, indeed somewhat more slowly after (Infusoria). (2) Sometimes fertilization brings nourishment and repro- duction to a standstill, in which case encystment appears (many rhizopods and flagellates). (3) A third case is where division follows fertilization, occurring more rapidly and having another character (sexual reproduction, better 'nutu- gamic division,' 'sporogamy') than the pre-fertilization divisions (asexual repro- duction, 'schizogony,' better, 'metagamic reproduction'). These alternating pro- and metagamic reproductions have been called alternations of generations (most Sporozoa, many rhizopods). Analysis of these phenomena leads to the conclusion that we may speak ef fertilization but not of sexual reproduction in the Protozoa. As was said pre- viously (p. 149) these facts have great importance in the explanation of the existence of fertilization, since they show that it has not always the purpose c f stimulating the reproductive processes and thus leading to the formation of a new individual. Fertilization has to accomplish other things for the organism; they must be of great importance, since they are so widely spread; but as yet their significance is not clea1' The Protozoa, with small and soft protoplasmic bodies, are but slightly protected against drying up, and therefore they are aquatic. Nmie, like Amoeba terricola, are terrestrial, but these only occur in moist places. Salt and fresh water, of the latter stagnant pools rich in vegetation, are the favorite places. The fresh- water forms are cosmopolitan, so that the species in all lands are very similar. This depends upon certain peculiar- ities. The fresh-water Protozoa can become encysted and in the encysted 170 PROTOZOA stage can endure unfavorable conditions such as lack of food, freezing, or complete evaporation of the water. When thus protected they may be blown about by the wind or carried far on the feet of birds. Hence one group bears the name Infusoria, for if dry earth or dry plants (e.g., hay) be soaked in water and this infusion allowed to stand for some time, a Protozoan fauna will develop in it. The encysted animals in the earth or on the plants are awakened by the moisture to new life and leave the cyst. Spontaneous generation, as was once believed, does not occur here, for if one sterilize the materials and prevent the entrance of germs the water will remain uninhabited. The protozoa are very important from the pathological standpoint. Each of the four classes includes numerous parasites, the Sporozoa being exclusively parasitic. Many cause severe infective diseases (malaria, relapsing typhus, sleeping sickness, etc.) especially in the warmer climates, while in the north, at least as far as man is concerned, the bacterial diseases predominate. Many protozoan diseases are 'inherited,' that is the egg cells are infected by the parasites. This is the case with the pebrine disease of silkworms, the Texas fever of cattle and others. Historical. — On account of their invisibility the Protozoa were unknown until 1675; they were discovered in infusions by Leeuwenhoek, the discoverer of the microscope. Wrisberg called them Animalcula infusoria — infusion animals, and Siebold gave them the name Protozoa. Ehrenberg maintained that the Protozoa, like all animals, possessed alimentary canal, nervous system, muscles, excretory and sexual organs. Dujardin denied all this and recognized in them only a single homogeneous substance as sufficient to produce all vital phe- nomena. Siebold discovered that the Protozoa were unicellular. The fact that there are unicellular animals without organs and yet capable of existence was an extremely valuable addition to knowledge, for it not only broadens our conception of animal life, but it furnishes for the theory of evolution from simple organisms the most important link, the first of the chain. The different appearances of Protozoa depend upon the degree of organ- ological and histological differentiation. Since these are most prominent in the nourishing and locomotor structures, these become important in subdividing the group. In accordance with the motion and taking of food by pseudopodia, flagella or cilia, there are three classes: Rhizopoda, Flagellata and Ciliata (Infusoria, s. str.). To these are added the Sporozoa, modified in motions and mode of feeding by parasitism. Undoubtedly Rhizopoda, Flagellata and Spo- rozoa are much closer to each other than are the Ciliata; hence they are grouped as Plasmodroma or Cytomorpha in contrast to Ciliomorpha or Cytoida. Class I. Rhizopoda. First of the Protozoa are those organisms which lack permanent struc- tures for locomotion and nourishment, the protoplasm of the body per- forming these functions. The term Rhizopoda refers to the fact that the protoplasm sends out root-like processes or pseudopodia for locomotion and I. RHIZOPODA 171 for taking nourishment. These differ from true appendages in that they are not constant, but are formed according to demand and again disappear. A pseudopodium arises when the protoplasm streams to one point of the body and extends as a process beyond the surface. Since the proces becomes attached and draws the body after it, or since the protoplasm of the body may flow into it, a slow change of place occurs. In either case the process disappears in the organism, and new pseudopodia are formed at other places which are retracted in turn. This type of locomotion is called amoeboid after the Ama'ba, in which it was first studied. When the Rhizopoda in their wanderings meet particles of nourishment, they enclose them with their protoplasm and take them into the interior of the body (fig. 117, A"). 1 1 > ,!l/f/ FIG. 117. FIG. 117. — Aaiirlni proteus (after Leidy). ectosarc; n, nucleus; N, food-body. FIG. 118. — Rotalia freyeri (from Lang, after M. Schultze). FiG. 118 cv, contractile vacuole; en, entosarc; ek, The shape of the pseudopodia is approximately constant for each species, but it varies so with different forms that it may be used not only for separating species but groups. On the one hand, there are finger-liki- pseudopodia (fig. 117), on the other, those of such delicai -y that even under strong magnification they appear like fine threads (tig. i iS); and between these extremes many intermediate forms. Thread-like pseudo- podia usually branch, and when the branches meet they may fust' and form anastomoses, from which it follows that the pseudopodia are not 172 PROTOZOA covered by a membrane. The fine granules of the protoplasm usually enter the pseudopodia and produce here, as they move back and forth, the phenomenon of 'streaming.' Since foreign particles can participate in this streaming, it follows that the movements depend on the protoplasm itself. We have already used this fact (p. 55) to demonstrate the extraordi- nary complexity of protoplasm. When Rhizopoda increase by division, the division products frequently become flagellate spores or zoospores. The body becomes oval and develops, on the end which contains the nucleus, one or more rlagella, which move more ener- getically than pseudopodia, and are permanent as long as the zoospore stage persists (fig. 122). Since many Protozoa possess flagella along with pseudopodia, the boundary between Rhizopods and Flagellates is not distinct (fig. 119). The Rhizopoda form an ascending series in which the systematic characters become more and more pronounced; such are the assumption of a definite form, as in the Radiolaria and Heliozoa, the forma- tion of a skeleton of regular character, as in the Thalamophora, or the development of a peculiar re- production, as in the Mycetozoa. At the bottom FIG. 1 19. — Mastigamceba stand the Monera and the Lobosa whose characters aspera (after F. E. Schulze). are mostly negative, for neither form, skeleton, nor reproduction affords systematic distinctions. Order I. Monera. The most important character of the Monera is the lack of a nucleus. As with other negative characters this is somewhat uncertain. In many cases, especially when the protoplasm is filled with pigment granules, the nucleus is recognized with difficulty, and hence animals have been described as anucleate in which the nucleus was overlooked. So it is possible that, in the few forms now remaining in the group, the nucleus has merely escaped observation; possibly it is functionally replaced by the chromidia (p. 58). There are several theoretical reasons favoring the idea of anucleate organisms. It is easier to suppose that with the appearance of life there were organisms consisting of but a single substance than that these organisms had nucleus and protoplasm al- ready differentiated. Several species of Protamccba are placed in the Monera. Order II. Lobosa (Amcebina). Lobosa are primitive Rhizopoda with one or several nuclei. The species of Anuvba, forms which owe their name to their constant change of shape, are typical (figs. 117, 120). This change of form is due to the formation and disappearance of a few finger-like (lobose) pseudopodia. Body and pseudopodia consist of two layers, a soft granular inner entosarc (en) and a firmer, clear, outer ectosarc (ek). In the entosarc is usually I. RHIZOPODA: HELIOZOA 173 a single (sometimes several) nucleus (n), which is vesicular, and contains cither one large or several smaller nucleoli. A contractile vacuole is usually present. Reproduction occurs by binary or multiple division (fig. 120), and encystment has been observed, the protoplasm dividing into many hundred small amoeba?, a phenomenon always connected with fertilization processes (au- togamy?). Most Lobosa occur in fresh water; A. terricola in moist earth. There are also parasites like A. coli, rare in colder climates, frequently observed i n the tropics. According to recent researches two forms have been included under the name .4. coll, one innocuous, Entamceba coli, and another (possibly several) pathogenic forms, including E. histolytica, which appears in enormous numbers in abscesses of the liver and ulcers of the colon of men ill with tropical dysentery For the first of these it is certain, for the other probable, that infection is caused by encysted forms, which arise as a consequence of fertilization and are passed out with the feces. FlG. 1 20. — A mtrba poly podia in division (-^/v/ Crx FIG._ 124. — Thalassicolla pelagica. In centre the nucleus with coiled nucleolus, around it central capsule with oil globules; still outside the extracapsulum with vacuoles (extracapsular alveoli), yellow cells (black) and pseudopodia. contains numerous central capsules, bound together by protoplasmic threads, which form the pseudopodia on the surface (fig. 128). A second type is repro- duction by swarm or zoospores, which begins when the nucleus has divided into hundreds or thousands of daughter nuclei. The contents of the central capsule then divides into as many portions as there are nuclei, these become oval and develop two flagella (fig. 126), which soon begin to vibrate so that the central capsule is filled with a tumultuous crowd. With the breaking of the capsular membrane these swarm spores escape; here our knowledge of this type ceases. Since in many species there are macrospores and microspores it is probable that a copulation is necessary. I. RHIZOPODA: RADIOLARIA FIG. 125. FIG. 126. FIG. 125. — Acanthometra elastics, ck, central capsule; n, nuclei; />, pseudopodia; St, spines; Wk, extracapsulura. FIG. 126. — Zoospores of Collozoum inerme. a, niicrospore; b, zoospore with fusi- form body; c, macrospore. - •-:,,:- • ' • d \ I. - b c FIG. 127. FIG. 128. FIG. 127. — Eucvrtidium cranioides (after Haeckel). FIG. 128. — Collozoum inerme. a, jelly; b, oil globules in the central capsule; c, d, yellow cells; e, vacuoles. 12 178 PROTOZOA Common, if not constant, in the Radiolaria are the yellow cells, unicellular algae (Zooxanthellcr), which are also present in other animals. (Thalamophora, actinians, sponges, etc.). They afford an instance of symbiosis, or the living together of different organisms for mutual good. The Radiolaria are exclusively marine. In fair weather they float at the surface, but sink in times of storm. Certain species and even large groups (Phaeodaria) occur only at great depths (1500-4000 fathoms); several thousand species known. Order V. Thalamophora (Foraminifera, Reticularia). The Foraminifera, though not equalling the Radiolaria in beauty and variety of forms, exceed them in numbers of individuals, and have a great importance in the history of the earth. No other group of animals has had so great a part in the formation of beds of rock. The most prominent characteristic is afforded by the shell, which is closed at one pole, and usually open at the other, the pseudopodia passing through the aperture (fig. 129). Accordingly as the axis connecting these poles is altered, the shell becomes disc-like, spherical, flask formed or even coiled in a spiral. The interior of the shell is frequently divided by transverse partitions into numerous chambers (fig. 131). Such many-chambered shells (Polythalamia) are at first small, and consist of one or few chambers, but as the animal grows new chambers are added at the mouth of the shell. Openings (foramina') in the walls connect the adjacent chambers. The spiral shells with many chambers have a striking resemblance to the shells of the Nautilus (%• 352)- In the fresh-water forms the shell is built of an organic substance which may be strengthened by silica or the incorpora- tion of foreign particles. The more typical members, exclusively marine, have cal- careous shells with but the slighest trace of organic matter. The presence of minute pores in the shell is of systematic importance, the group of Perforata (fig. 118) being characterized by them. The animal portions form a cast of the inside of the shell (fig. 130), and consist of as many pieces as there are chambers in the shell, connected by plasma bridges passing through the foramina of the partitions. In the protoplasm there is a large nucleus (figs. 129, 130, «), which in some cases is early replaced by daughter nuclei. Contractile vacuoles usually occur only in the fresh-water forms. The pseudopodia project through FlG. 129. — Quadrula sym- metric a (after F. E. Schulze). cv, contractile vacuole; n, nu- cleus; TV, food-body. I. RHIZOPODA: THALAMOPHORA 179 the chief opening of the shell and in the Perforata probably through the pores in the shell wall. They are rarely finger-like (fig. 129); usually they are thread-like, branched and anastomosing (tigs. 17, 118). Reproduction is generally by fission, but presents many variations. Only rarely (fresh water Monothalamia) do both animal and shell divide; frequently the protoplasm protrudes from the mouth of the shell, a new shell is formed on the outgrowth and the protoplasm then divides, one of the resulting individuals retaining the old shell. In the marine Polythalamia the following process is general: The polynucleate protoplasm divides into numerous uninucleate 'embryos' which frequently, while still within the mother, develop a shell. FIG. 130. — Protoplasm of Globigerina after solution of the shell, n. nucleus. FIG. 131. — Young Milinla \vith several nuclei (from Lang). A second kind of reproduction leading to a fertilization process appears to be common. Many swarm spores arise in the shells of the Polythalamia. These fuse in pairs with each other. Both of these reproductions alternate with e;u h other, and with them is often connected a dimorphism of the individuals. The progamic generation, leading to the formation of gametes is distinguished by the long persistence of the chief nucleus and often by the structure of the shell (large central chamber, megasporic generation) of the metagametes arising from fertilization (polynucleate, microspheric gametes). A corresponding alternation has been observed in the Monothalamia. Sub Order I. MONOTHALAMIA. Mostly fresh-water. They have one- chambered shells of chitin or silica, often strengthened by foreign bodies. Con- tractile vacuole usually present. Pseudopodia lobose or filiform, branched or simple. A. Forms with finger-form pseudopodia: Arcclla,* (Jiiadrulu* (fig. 129); Difflttgia* These forms are merely shelled Amoeba: and are frequently referred to the Lobosa. B. Forms with branching and anastomosing filiform pseudopodia. Euglypha,* Groinia (fig. 17). Sub Order II. POLYTHAL- AMIA. Exclusively marine; many-chambered shells. Thick beds of rock like the chalk, nummulitic limestone, and green-sand are largely foraminiferal in origin. The living species have an average diameter of about i mm. Rarely species have a diameter of several centimeters (Psammonyx i'ulf where the body is covered with a cuticle. As a rule, there is also a lack of locomotor structures; but the occasional presence of amoeboid motion or flagella indicates a near relationship with rhizopods and flagellates, so that it is difficult to draw sharp lines between the three classes. There are very close relations between the parasitic flagellates (Trypanosomes) and the Hrcmosporida, and the Sporozoa must be regarded as rhizopods or flagellates modified by parasitism. It is characteristic of the reproduction that the developmental stages before and after fertilization — pro- and metagamic — have different characters. The 186 PROTOZOA progamic development (schizogony) leads as a rule to autoinfection, to increase of the parasite in the tissues of the host. The parasite increases in size and breaks up into numerous young (merozoites), which grow in turn and divide. This process may continue many times until a sexually ripe form appears. Only in the gregarines is this replaced by a single large growth, which, in the period of preparation for fertilization, is divided into many parts. Fertilization usually precedes encystment, only rarely occurring in the cyst. Occasionally there is a fusion of isogametes; usually there are non-motile' macrogametes fertilized by extremely active microgametes. In this the sexual dimorphism may be pre- pared long before and only be expressed in the generation (gametocytes) from which the macro- and microgametes arise. The metagamic development (sporogony) requires encystment and serves to introduce the germs into a new- host. Inside of the cyst spores are formed, which are rarely naked, usually surrounded with a firm envelope. In the spore the sporozoite arises, the starting point for the progamic development. In all of the divisions which precede fertilization or immediately follow it, a part of the protoplasm, containing degen- erating nuclei, remains behind as the residual body. From the development thus outlined the Myxosporida and Sarcosporida differ in some points, though they form spores .and sporozoites for new infections. Order I. Gregarina. The typical and longest known sporozoa are the Gregarines, parasites of oval or thread-like form (recalling round worms), usually somewhat flattened, which have only been found in the intestine or gonads, more rarely in the body cavity, of invertebrates. The protoplasm (fig. 145, A ) is separated sharply into a clear ectosarc (ek) and a granular entosarc (en). The ectosarc is covered by a cuticle («/), permeable by fluid food, for no cytostome exists. In many (perhaps all) there is a double striping of the body, a longitudinal recognizable by furrows on the surface and hence cuticular, and a transverse marking in the ectosarc, produced by circular or spiral muscle fibrilke. These muscles explain the peristaltic motion and the occasional bending of the body, but not the peculiar gliding motion by which locomotion is usually effected. It may be that the greg- arines secrete stiff gelatinous threads from the posterior end, and the elongation of these forces the body forward. In many gregarines (Polycystidae) the body is divided into a smaller anterior part^ the protomerite, and a larger deutomerite (fig. 145, A). Internally this division is marked by a bridge of ectosarc across the entosarc. The vesicular nucleus (there is but one in any gregarine) lies in the deutomerite. All gregarines are parasitic in youth wholly inside of cells or with the anterior end imbedded in the^host cell, which they leave in the developed stage. Many remain for a long time with a process of the protomerite in the cells. This process— the epimerite— is provided with threads or hooks for anchorage, and is usually lost when the animal gives up its connection with the host cell. Among the intestinal gregarines frequently occur 'associations' where two or more animals are fast- ened together head to tail in a row (fig. 145, A). Perhaps these associations are preparations for conjugation which occurs in development. III. SPOROZOA: GREGARIXA Reproduction typically occurs in an encysted condition (fig. 145, II). Usually two animals occur in a cyst. After each individual has become polynucleate by division of its nucleus, it divides at first superficially, T. FIG. 145. — Different Gregarina. I-VII, development of Stylorhynckus; I, S. longicollis (after Schneider); II, encysted 5. oblongatus (two animals) beginning gamete formation; III, same in later stage, the sexually differentiated gametes in copulation; IV-VII, formation and development of the zygote of S. l/»i^icollis more enlarged; IV, copulation of gametes; V, fusion; VI, beginning division; VII, 8, sporozoites formed. A. Clepsidrina Uattarum. 1-4, Monocystis magna (after Cuenot). i, two individuals in copulo in the spermatheca of an earthworm, sur- rounded by its spermatozoa; 2, encysted; 3 and 4, parts of cysts, formation and con- jugation of the gametes, more enlarged (according to Brasil the gametes are slightly differentiated sexually), cu, cuticula; dm, deutomerite; ek, ectosarc; en, entosarc; g, gametes; gl, zoospores, g-, oospore; pm, protomerite; n, nucleus; r, residual body: x, sperm of earthworm; z, zygote. later internally into small spheres, the gametes (III) . The gametes fuse in pairs to bodies which take a spindle shape and become enclosed in a firm envelope, the spores, zygotes or 'pseudonavicellae' (fig. 145, 4, ^ ' 188 PROTOZOA That always gametes of different origin fuse is shown by Stylorhynchus where the gametes of one animal are flagellate, those of another are station- ary. This dimorphism is so great in the Aggregatae that filiform spermato- zoids occur, as in the Coccidia. After the formation of the gametes the movements of the residual body bring about the expulsion of the pseudo- navicelke; and in many Gregarines sporoducts are present for their escape. With repeated formation of a residual body, the contents of the pseudonavicella divides into (usually eight) sporozoites or falciform spores, which must leave the spores and pass anew into the tissue cells in order to form gregarines. This escape of the sporozoites depends upon entrance into the proper host. Often the transformation of the contents of the cysts into pseudonavicellce takes place when the cysts have left the original host. Best known are the Monocystis tenax of the spermatheca of earthworms, and Clepsidrina blattarum of the cockroach. The American species have scarcely been touched. Order II. Coccidiae. Of all Sporozoa the gregarines are nearest the Coccidiae, which are also cell parasites with a single nucleus, but without either cell membrane or division into protomerite and deutomerite. Best known is Eimeria stieda; (also called Coccidium cuniculi and oviforme), parasitic in the liver and intestinal epithelium of mammals. In the progamic development the fully grown parasite (fig. 146, 2) divides inside the infected cell into many cells (3, 4, 8); these separate, infect other cells and begin growth and division anew (autoinfection). After this is repeated several times fertilization appears (5), some parasites giving rise to macrogametes, others by division forming small, actively swimming micro- gametes with one or two flagella. The fertilized macrogamete or zygote (6, 7, 9, 10) encysts, passes out and serves to infect a new animal. Beginning earlier or later, but only concluded in a new host, the contents of the cyst divide into several fin Eimeria, four) sporoblast-containing spores. Each spore (7, n) forms one or several (Eimeria, two) sporozoites, a residual body being left behind (r). Eimeria stieda produces cheesy granules in the liver of mammals. It is common in rabbits, rare in man. In cattle it is the cause of red dysentery. Order III. Haemosporida. The Haemosporida are very similar in structure and development to the Coccidia. They live in blood corpuscles, and on this account and from some analogies not sufficiently understood, they are regarded as related to the Try- panosomes. The Haemamcebal forms parasitic in man cause malaria, there being in these a progamic reproduction with autoinfection and a metagamic in which the disease is transferred to another host. The parasites in the blood corpuscles (fig. 147, 1-3) grow and divide (daisy form, 2), characterized by little accumulations of pigment derived from the haemoglobin of the corpuscle. These division products are set free by a breaking down of the corpuscle (period of chill) and infect other corpuscles. This autoinfection can continue a long time, until the Haemamoebae in the corpuscles grow, without dividing, to 'half moons' (4); these either become round and form macrogametes (5) or divide into eight microgametes (6). The conjugation of these seems only to take place III. SPOROZOA: ILEMOSPIIORIDA 189 FIG. 146. — 1-7. Developmentof Coccidium schubergi (after Schaudinn). i, entrance of sporozoites in cell; 2, its growth; 3, nuclear multiplication; 4, division into mero- zoites; 5, macro- and microgametes; 6, zygote divided into four sporozoiles. 8-u, Emeria stiedce (after Wasiele\vsky und Mctzner). 8, autoinfection (progamic increase) ; 9, formation of sporobUsts, 10, change of spores into sporozoites; u, spore with two sporozoites, more enlarged; c, z, sporozoite; e, epithelial cell; k, n, nucleus; mi. micro- gamete; o, macrogamete; r residual body; sp, spore; sp', sporoblasts. FIG. 147. — Development of Plasmodium pracox (pernicious malaria) (after Grassi). i, blood corpuscle with ncsvly t-nti-red Ha-mama-ba; 2, multiplication of parasite- 3 young before the breaking down of corpuscle; 4-5, formation of ma< i metes; 6 formation of microgametes; 7, fertilization; 8, fertili/.ed motile macro.ua m (ookinet)-o digestive tract of mosquito, in front salivary glands, stomach coverec encysted parasites; 10-12, division of cyst; 10, formation of sporoblast formation of sporozoites from sporoblast with the residual body; i.;. part of mosquito infected with sporozoites. All figures except 9 greatly enla I'.H) PROTOZOA when the gametes are taken into the digestive tract of a blood-sucking mosquito. The fertilized macrogamete, the ookinete, wanders into the intestinal wall, en- larges enormously, encysts and produces numerous naked sporoblasts. Each sporoblast gives rise to numerous sporozoites (u, 12) which wander into the salivary glands of the mosquito (13) and are transferred to the blood of man by the bite of the insect. For the transfer of human malaria apparently only mosquitos of the genus Anopheles will serve, not the more common Citlex. Since a temperature above 20° C. (68° F.) is best for the development of mos- quitos, and water is necessary for their development, the prevalence of malaria in warm climates is easily understood. The different kinds of malaria are caused by different parasites, the quartan fever being caused by Plasmodium (Hccma- mceba malaria, pernicious malaria by P. prcecox. Allied to the Haemamcebas and possibly also to the Trypanosomes, is Babesia (Piroplasma) bigemina, the cause of Texas fever in cattle. The tick, Boophilus bovis, serves as the interme- diate host, the parasites being passed by the eggs ('inherited') to the next genera- tion. Babesia bovis, intermediate host Ixodes rediwius, causes haemoglobinuria in cattle. Order IV. Myxosporida. The Myxosporida (fig. 148) are mostly large (sometimes visible to the naked eye) and occur especially in fish and arthropods. When they occur in hollow organs they are naked and have pseudopodia, but in parenchymatous organs like the heart, liver, brain, kidney, etc., they are usually en- closed in a membrane, and here they produce the greatest injury. At first binucleate, they soon become polynucleate, and apparently they can reproduce by fission. Even before the growth is ended they begin the process of sporu- lation, hence the name 'neosporida.' Repro- ductive bodies with one or two nuclei, the anlagen of the pansporoblasts, are differentiated in the protoplasm. In the best known forms each pansporoblast gives rise to two spores. By division there arise in all fourteen nuclei, two of which (fig. 148, r), with the surrounding protoplasm, form the envelope of the pansporo- blast. The others separate into two groups of six each. One pair in each group with their protoplasm form an 'amoeboid germ;' they ap- pear to be separated from each other early, but, though long separated, they at last unite (II, III, g), a case of caryogamy. Two other nuclei and protoplasm form the two-valved spore case, and the remaining pair furnish the 'pole cap- sules,' these being oval, and containing threads which under proper conditions, are protruded (III), the whole resembling a ccelenterate nettle cell. The threads are for attaching the 'psoro- sperms' (as the spores were formerly called). FIG. 148. — Development of Myxobolus pfeifferi, schematized (after Keisselitz). 7, pansporo- blast with envelope and residual nuclei, r, divided into two sporo- blasts; II, sporoblast developing into spores; s, envelope cells; p, pole cells with pole capsule; g, amceboid germs with two nuclei ; III, developed spore with ex- truded threads of the pole cap- sule, both nuclei of the amceboid germs fused. The amceboid germs are set free, as experi- ments on fishes show, by the digestive fluids, when they crawl into the tissues of the host. The number of pole capsules and of spores differs with the species. IV. CILIATA 191 The Myxosporida cause serious epidemics among fishes. The pebrine of the silkworm (the eggs are also infected) is caused by Nosema (Glugea) bom- bycis. Khinosporidium hominis, a parasite of the nasal mucous membrane of man in the tropics, is nearly related to the Myxosporidia. Order V. Sarcosporidia. The Sarcosporida (fig. 149) — also called Rainey's or Miescher's corpuscles- occur in the voluntary muscles of vertebrates, especially mammals. They are oval cysts lying in sarcolemma sacs between the fibrillae. They have a cyst, the wall of which is radially striped, and inside this, in the ripe condition, are snores, imbedded in a stroma, each spore containing numerous reniform or falciform sporozoites. Sarcocystis miescheriana in muscles of pig; S. nmris in the mouse; 5. lindemanni rare in human muscle. At the end of the Sporozoa may be mentioned some much disputed bodies of very minute size, which are found in several infectious diseases (variola, trachoma, hydrophobia, etc.) and are regarded as their cause. They have been united under the common head of CHLAMYDOZOA. .It 1. § ,': -: nk ' '.— -|- k -o — na FIG. 149. FIG. 150. FIG. 149. — Sarcocystis miescheriana, from diaphragm of pig (after Butschli). 65, cyst; sp, spheres of spores. Fig. 150. — Paramcecium aurcl la in division; 2, separation of cytostome of new indi- vidual from old cytostome, at an earlier stage ; 3, P. camlntum, flattened and schematic; cv, contractile vacuole, expanded and contracted; k, nucleus; na, nn' food vacuole and one forming; nk, micronucleus; o, cytostome; /, t', trichocysts, t', discharged. Class IV. Ciliata. The Ciliata rival the Rhizopoda in numbers and variety of form. They are so complicated in structure that they were long he'd as multicel- lular. The form is definite for the species; and in the 'ametabolous' forms is unalterable, the 'metabola' can be temporarily pressed out of shape in passing through a narrow space. This constancy of form is due to a cu- 192 PROTOZOA tide on the outside of the body, which in the 'ametabola' is firm; in the others very flexible. The cuticle is covered with cilia — small vibrating processes which move together, and serve not only as organs of loco- motion, but by creating vortices in the water bring food to the organism. They furnish the most important characteristic of the class (fig. 150). The presence of a cuticle necessitates a cytostome, except in the para- sitic species, since food particles cannot be taken in at every point. At the cytostome the cuticle with its cilia forms a funnel-like food tube (cyto- pharynx) into the protoplasm. At the bottom the cuticle is interrupted so that water and protoplasm are in contact. By the action of the cilia food particles are taken into the cytopharynx and pressed against the protoplasm, forming a small enlargement which finally sinks into the substance as a, food vacuole (na) which, by the streaming of the protoplasm, is carried about in the body. The digestible portions are absorbed, and those not -capable of digestion are cast out of the body at a fixed point (cytopyge) usually not recognizable at other times (fig. 150.3). Contractile vacuoles (cv) are lacking only in parasites and marine species. They are constant in number and position, and frequently have afferent ducts which empty into the vacuole, the vacuole in turn forcing the fluid to the exterior. Trichocysts, nettle bodies, and muscular fibrillas occur in some species. Trichocysts are minute rods vertical to the surface in the cortical layer, which under the influence of reagents (chromic acid) elongate into threads penetrating the cuticula. To these have been ascribed defensive functions; others regard them as tactile structures. They have no connection with the cilia. Nettle bodies are extremely rare. Muscle fibres lie between ectosarc and cuticle, and cause quick convulsive motions of the animal. There are two nuclei physiologically unlike. The larger of these (nucleus of older writers, macronucleus) is a large oval, rod-like, or spiral body, deeply staining with microscopic stains, and surrounded with a membrane. It controls all the common vital functions of the animal (motion, feeding, etc.). Beside it or in a depression in it is the much smaller micronudeus (nucleolus or paranucleus of older authors) which stains less deeply. In all sexual processes it comes to the front and can be called the sexual nucleus. Multiplication of Ciliata occurs by binary fission (fig. 150); more rarely, and then only in the encysted condition, by division into numerous parts. Budding is known in the Peritricha and Suctoria. In fission first the micronucleus divides mitotically, and then the macronucleus separates by elongation and constriction. The old cytostome persists in the an- terior offspring, but often an outgrowth from it (2, o') passes into the posterior half and develops into a new mouth. FIG. 151. — Conjugation in Param&cium. k, macronucleus: nk, micronucleus; o, cyto- stomes. I. Changes of micronucleus; left sickle stage, right spindle stage. II. Second division of micronucleus; into primary spindles (i, 5) and secondary spindles (2, 3, 4; 6, 7, 8). III. Degeneration of secondary spindles (2, 3, 4; 6, 7, 8); division of primary spindle nto male (im, $m) and female spindles (rz#, yu-'). IV. Exchange of male spindles nearly complete (fertilization), one end still in the parent animal, the other united with the female spindle, im, with 5^- and $>n with i-ui1; macronucleus broken up. V. The cleavage spindle t formed by male and female spindles dividing into the secondary cleavage spindles t', t". VI. VII. End of conjugation. The secondary cleavage spindle dividing into the anlage of the new micronucleus (nk'\ and that of the new micronucleus, pt (placenta). The fragments of the old macronucleus begin to degenerate. Since P. caudatum shows the earlier and P. aurclia the later stages better, thcx- forms have been used, P. caudatum for I-III, P. aurclia for the rest. The differences consist in the existence of one micronucleus in P. caudatum, two in P. aurclia and that in the latter the nuclear degeneration begins in I. 13 194 PROTOZOA The periods of fission are interrupted from time to time by the sexual process of conjugation, which will be described as it occurs in Paramcecium (fig. 151). Two individuals touch by their whole ventral surfaces, so that their cytostomes come together. In the neighborhood of the latter a bridge of protoplasm connects the two animals. Later the individuals separate. While these easily observable external processes are occurring there is a complete modification of the nuclear apparatus in the interior. The macronucleus increases in size, and breaks into small portions which disappear within the first week after copulation (probably by absorption), and give place to a new nucleus derived from the micronucleus. At the beginning of copulation the micronucleus becomes spindle-shaped, divides .and repeats the process, the result being the formation of four spindles in each animal, three of which break down, thus recalling the polar globules in the maturation of the egg (p. 133). The fourth or principal spindle places itself in the neighborhood of the cytostome at right angles to the surface and divides into two nuclei, the superficial being called the wandering or male nucleus, the deeper, the stationary or female nucleus. The male nuclei of the two copulating animals are exchanged, traversing the proto- plasmic bridge in their course (III). Both male and female nuclei usually become spindle-shaped, and the im- migrant male spindle fuses with the female spindle, forming a single spindle of division. At last, after processes which differ in the various genera, the division spindle pro- duces (usually by indirect means) two nuclei, one of which becomes the new macronucleus, the other the new micronucleus. In a comparison of the fertiliza- tion of the Metazoa, the female nucleus corresponds to the egg nucleus, the male nucleus to that of the spermatozoa. As the fusion of egg and sperm nuclei forms a segmentation nucleus, so here the divi- sion nucleus is formed in a similar manner. As the egg cell through fertilization acquires the capacity not only to produce sex cells but somatic cells — cells which carry on the common functions of the body — the fertilized micronucleus forms not only the new micronucleus, but FIG. 152. — Epistylis umbellaria (after Greeff). Part of a colony in 'bud-like' conjugation; r, microspores arising by division; k, microspore conjugating with a macrospore. IV. CILIATA: HOLOTRICHA 19.5 also the macronucleus which controls the body processes, and hence is the somatic nucleus. In other words, fertilization in the Ciliates leads to a complete new formation of the nucleus and thus to a new organization of the organism. In most Ciliata the conjugating individuals are similar, the fertiliza- tion is mutual, and the individuals separate later. In the Peritricha (mostly sessile forms, fig. 152), on the contrary, the resemblance to fertilization in the Metazoa is strengthened in that there is a sexual differ- entiation and a permanent fusion of the con- jugating individuals. Some animals — the macrogametes — retain their size and sessile habits; others by rapid division produce groups of markedly smaller microgametes. The latter separate and fuse completely with the macrogametes. The nuclear phenomena are much the same as' with Paramoscium, allowance being made for the permanence of the fusion. _, """ """ a... r. t FIG. 153. FIG. 154. FIG. 153. — Stentor polvmorphus (after Stein), a, peristomial area; b, roof of hypostome; g, contractile vacuole; n, nucleus; o, cytostome; r, adoral ciliated spiral; /, hypostome (excavation for mouth). FIG. 154. — Balantiaium coll (after Leuckart). Order I. Holotricha. The Holotricha are the most primitive Ciliates, since the cilia on all parts of the body are similar; being at most slightly stronger at one end of the body or inside of the cytostome. Best known are the species of Paramoecium* (\'\^. 150) occurring in stagnant water. Opalina ranarum* lives in the intestine of the frog. It lacks mouth, has numerous similar nuclei, no micronucleus and no conjuga- tion. The small encysted Opalina' pass out with the faeces, and are eaten by the tadpoles, which thus become infected. 196 PROTOZOA Order II. Heterotricha. Like the Holotricha the Heterotricha are everywhere ciliated, but they have a tract of stronger cilia, the adoral ciliated spiral, beginning at some distance from the cytostome and leading in a spiral course into the mouth. It consists of rows of cilia united into membranellce placed at right angles to the course of the spiral. In the Stentors* (fig. 153), the peristomial area, surrounded by the spiral, forms the broader end of the body, which tapers toward the other end, by which the animal may attach itself. Muscle fibres running lengthwise immediately under the cuticle produce energetic movements. Balantidiiim coli (fig. 154) appears in the large intestine of men ill with diarrhoea, it also occurs in swine without causing sickness. Other parasites of man are B. minutnm and Nyctotherus faba. Order III. Peritricha. The Peritricha have a broad peristome area around the cytostome; the oppo- site end has a corresponding pedal disc or is narrowed like a goblet and ends in a stalk (fig. 155). Only the adoral ciliated spiral is constant. It arises from the swollen margin T)f the peristomial area, and continues on the 'operculum,' a FIG. 155. — Carehesium polypinum (after Butschli). Left, a single animal; right, three stages of division, cv, contractile vacuole; n, macronucleus; n', micronucleus ; Nv, food vacuoles; os, cytopharynx; per, peristome; 7-5, reservoir of contractile vacuole; urn, undulating membrane; vst~ vestibule; wk, ring on which a posterior circle of cilia may develop. disc which projects free from the peristomial area, but in contraction is close against it, the peristome lips folding over all. Besides, there may ciliated drawn close be a temporary or permanent circle of cilia near the hinder end. The nucleus is usually sausage-shaped, much bent, and with the small micronucleus in its hinder angle (fig. 155, n'). The VoRTiCELLiD.«(figs. 152, 155), are attached by a long stalk which contains a slightly spiral muscle, dividing in the body into fine fibrillae which extend under the cuticle to the peristome. When the muscle in the stalk con- tracts it becomes coiled into a corkscrew spiral, drawing back the animal, and IV. CILIATA: HYPOTRICIIA, SUCTORIA 197 folding in the anterior end. Vorticrlla* is solitary; Carchesium* forms colonies with branched stalks; Zoothamnion,* colonies imbedded in a common jelly; Epistylis* (fig. 52), branched colonies with rigid stalks. The fantastic Ophryoscolex, Cycloposthinm, etc., are parasites in the stomach of ruminants. Order IV. Hypotricha. In this order the body is more or less flattened and ventral and dorsal sur- faces are differentiated. The back lacks cilia, but often bears spines and bristles. On the ventral side are several longitudinal rows of cilia, and also straight spines and hooked cirri composed of united cilia, of use in creeping. The cilia are $ w ~~ rif-/- , FIG. 156. FIG. 157. FIG. 156. — Stylonychia wytilns (after Stein), a, anal hooks; b, ventral hooks; c, contractile vacuole; d, frontal ridge; g, canal leading to contractile vacuole; /, upper lip; n, nucleus with micronucleus; f>, adoral ciliated spiral; r, marginal cilia; s, caudal cilia; 5^, frontal spines; z, anus (cytopyge). FIG. 157. — Division of Stylonychia invlilus (after Stein), c, c', contractile vacuoles of the two individuals; n, nucleus and micronucleus; />, />', adoral ciliated spiral; r, r', marginal cilia; w, w', ciliated ridges. used in locomotion and producing vortices which bring food. The macro- nucleus is often divided into two oval bodies connected by a thread; the micro- nuclei vary in number from 2 to 4 in the same species. These are the best forms for studying the micronuclei. Stylonychia* (figs. 156, 157). Order V. Suctoria (Acinetaria). The Suctoria differ from other Infusoria in the absence of cilia from the adult and consequently have no means of locomotion. They are fixed to some support either by the base or by a slender stalk. The body is usually spherical 198 PROTOZOA and is covered with a cuticle, which in Acineta is produced into a cup-like lorica. There is no mouth, but in its place tentacles, very fine tubes with contractile walls which begin in the protoplasm and protrude through the cuticle (fig. 158, F}. The Acinetaria kill other animals, especially infusoria, with their tentacles, and then suck the substance through these tubes. The contractile vacuole, rarely lacking, lies near the compact macronucleus; micronuclei are generally present. The ciliated young (fig. 158, E) are good swimmers. They arise either as buds from the surface of the mother (fig. 20) or as 'embryos' in her interior. This latter condition is only a modification of the other, part of the FIG. 158. — Suctoria (after various writers). A, Dendrosoma; B, Rhyncheta; C, Ophryodendron; D, Tokophrya; E, ciliated young of Sphcerophrya; F, diagram of capitate and styliform tentacles arising from ectosarc and canals in entosarc. outer surface being pushed into the interior to form a brood cavity in which the embryos arise. After swimming for a while the young come to rest, lose the cilia, and develop the tentacles. Some species of Podophrya in fresh water, also Sphcerophrya, parasitic in Infusoria. Acineta and Podophrya gemmipara (fig. 20) are marine. Summary of Important Facts. 1. The Protozoa are unicellular organisms without true organs or true tissues. 2. All vital processes are accomplished by the protoplasm, digestion directly by its substance, locomotion and the taking of food by means of protoplasmic processes (pseudopodia) or by appendages (cilia and flagella). 3. Excretion takes place by special accumulations of fluid, the con- tractile vacuoles. 4. Reproduction is by budding or by fission. At intervals there is a true fertilization (caryogamy) sharply distinct from mere fusion of plasma (plasmogamy). Fertilization may be accomplished by a permanent fusion (copulation) or a transitory union (conjugation) ; it may be isogamic, anisogamic or autogamic. 5. Protozoa are aquatic, a few living in moist earth; they can only exist in dry air, surrounded by a capsule (encysted) which prevents desiccation. 6. Since encysted Protozoa are easily carried by the wind, the occur- PROTOZOA: SUMMARY OF IMPORTANT FACTS 109 rence of these animals in water which originally contained none is easily explained. 7. The mode of locomotion serves for division of the Protozoa into the classes Rhizopoda, Flagellata, Ciliata, and Sporozoa. 8. The RHIZOPODA have temporary protoplasmic processes, the pseudopodia. 9. The Rhizopoda are subdivided into Monera, Lobosa, Heliozoa, Radiolaria, Foraminifera, and Mycetozoa. 10. The Lobosa and Monera have no definite shape. The Lobosa have a nucleus, the Monera are anucleate. 11. Heliozoa and Radiolaria are spherical and have fine radiating pseudopodia and frequently silicious skeletons. They are distinguished by a central capsule in the Radiolaria which is lacking in the Heliozoa. 12. The Thalamophora (Foraminifera) have a shell, closed at one end, the other open for the extension of pseudopodia. The shell is chitinous or calcareous, one or several chambered, straight or spiral; the pseudo- podia are occasionally lobular, but usually filiform, branching and anastomosing. 13. The Foraminifera are of great geological importance on account of their numbers and their shells, which have built and are still building extensive beds of rock (chalk, nummulitic limestone). The silicious skeletons of the Radiolaria are less important. 14. Mycetozoa (Myxomycetes) are mostly enormous Amoebae with reticulate protoplasm (plasmodium). They form complex reproductive structures (sporangia), recalling those of the fungi. 15. FLAGELLATA have one or a few long vibratile processes — flagella— which serve for locomotion and for the taking of food. 16. The Autoflagellata have only flagella; they feed like plants by means of chlorophyl (Volvocinae) , or upon fluid food (parasites), or upon solid food, either by pseudopodia, by a mouth (cytostome), or by a collar. 17. Several are parasitic in man (Trichomonas vaginalis, Lamblia intcsfinalis, and especially prominent Trypanosoma gambiense (cause of sleeping sickness). Perhaps Spirochccte pallida (cause of syphilis) belongs here. 1 8. The Dinoflagellata have two kinds of flagella and usually an armor of cellulose. 19. The Cystoflagellata have a gelatinous body enclosed in a firm membrane (Noctiluca). 20. SPOROZOA are parasitic Protozoa, usually without organs of loco- motion or mouth. They take no solid food, but live by osmosis on tissue fluids. The encysted animals produce spores (beginning with fecunda- 200 PROTOZOA tion and accompanied by a change of host). The spores divide into sporozoites. Multiplication without change of host (autoinfection) can occur. 21. The Gregarinida are temporary or permanent parasites in cells. CoccidlcB, Htzmosporida (cause of malaria, parasitic in blood corpuscles). 22. The Sarcosporida (Rainey's or Miescher's corpuscles of mamma- lian muscles) and Myxosporida (psorosperm capsules of fishes, psorosperm = spore) live in tissues or hollow organs. 23. The CILIATA have numerous vibrating processes, the cilia, a cuticle, and hence fixed openings for the ingestion of food (cytostome) and for extrusion of indigestible matter (cytopyge). 24. Of great interest is the occurrence of two kinds of nuclei, a func- tional macronucleus and a sexual micronucleus. 25. In conjugation portions of the micronucleus are exchanged and accomplish impregnation. The macronucleus degenerates and is replaced by part of the fecundated micronucleus. 26. The classification of the Ciliata is based on the structure and arrangement of the cilia. 27. The Holotricha have similar cilia over the whole body. The Heterotricha have, besides the total ciliation, stronger cilia in the neigh- borhood of the mouth (adoral ciliary spiral). The Peritricha have only adoral ciliation. The Hypotricha have the ciliary spiral and rows of cilia and coalesced cilia on the ventral surface. The Suctoria have cilia only in the young, later they become attached and feed through suctorial tentacles. APPENDIX. According to the evolution theory one should expect forms between (he Protozoa and Metazoa. The CATALLACTA — spheres of ciliated cells which in reproduction break up into single cells — have been described as such. Other peculiar many-celled animals whose position in the system is difficult to decide a,re,Salinella salve, LohmaneUa catenula, the ORTHONECTIDA and the DICYEMIDA. The Orthonectida and Dicyemida have a many-celled ectoderm, enclosing a solid mass of cells in the Orthonectida, a single giant cell in the Dicyemida. Salwrlla and Lohmanella consist of a single layer of cells enclosing a central digestive space. Since the Dicyemida live as parasites in the nephridia of cephalopods, the Orthonectida in worms and echinoderms, it is possible that their low organi- zation is the result of degeneration. Trichoplax adhtrrens, formerly placed here, is discoid, consisting of two epithelial layers separated by gelatinous tissue. It has recently been shown to be the larva of a medusa, Eleutheria. PORIFKRA 201 METAZOA. Excluding the Protozoa, all the phyla of the animal kingdom are included under the Metazoa, i.e., higher animals. The point of union is that they consist of numerous distinct cells, arranged in several layers. At least two layers are present; one — the ectoderm — bounding the body externally, and a second — the entoderm — lining the digestive tract. Between these two a third may occur, frequently separated by a body cavity into an outer or somatic layer forming part of the body wall, and an inner or splancJinic layer forming part of the intestinal wall. This middle layer is called mesoderm, no matter whether there be a body cavity or not. The multicellular condition allows a higher organization, which ap- pears in varying grades in the specialization of tissues and organs. Xo metazoan lacks a true sexual reproduction, that is one by sexual cells, but the possibility must not be overlooked that some species may have lost fertilization and may reproduce exclusively by unfertilized eggs in a par- thenogenetic manner. Many species, especially the lower worms and ccelenterates, also reproduce by budding and fission. The segmentation of the egg is characteristic of all Metazoa. The fecundated egg divides into numerous cells which, as blastomeres, remain united and form the germ. No Protozoan has a true segmentation, division producing new individuals which either separate completely or remain in slight connection as a colony. PHYLUM II. PORIFERA (SPONGIDA). The Porifera, or sponges, the most familiar representative of which is the bath sponge (Euspongia officinalis), are, with few exceptions, marine. In fresh water occur but a few species of Spongilla. The animals have no powers of locomotion, but are attached to stones or plants, along the shores or at depths up to 4000 fathoms. They form spherical masses, thin crusts, small cylinders, or upright branching forms. Frequently the shape varies so that there is no typical form. Striking motions are rare ; only with the microscope can one see the opening and closing of the pores and the currents of the gastrovascular system. The simplest sponges, the Ascons (fig. 159), are thin-walled sacs, fixed at one end, and with an opening, the osciilum (functional anus), at the other. The cavity of the sac, the 'stomach,' is a wide digestive cavity into which water bearing food enters through numerous small pores in the body wall. The basis of the body is a connective tissue permeated with branching cells (fig. 160) covered externally by a thin 202 PORIFERA layer of pavement epithelium which is easily destroyed. This epithelium (earlier called ectoderm) and the connective tissue (mesoderm) are now regarded as a common layer, 'mesectoderm,' since the pavement epithelium is often genetically only connective -tissue cells which have spread over FIG. 159. FIG. 160. FIG. 159. — Ascon stage of Sycandra (after Maas). e, entoderm; m, mesectoderm; o, osculum; p, pores. FIG. 160. — Section of wall of Sycandra raphanus (after Schulze). e, epithelium; en, collared flagellate cells; m, mesoderm with connective-tissue cells; o, eggs; st, calcareous spicules. <* d FIG. 161. — Section of Plakiua (after F. E. Schulze). c, canals leading from ampullae to cloacal tubes; e, ampullae; d, afferent canals; o, osculum. the surface. On the other hand, there is a distinctly differentiated ento- j derm in the shape of a one-layered flagellate epithelium lining the stomach, the cells of which (en) recall the Choanoflagellata (p. 184), since they have collars surrounding the flagella. The taking of food is accom- plished by the collared cells, its distribution by the amoeboid cells. PORIFERA 203 Sponges of this simple ascon type are few. As a rule sponges are more massive and have a more complicated canal system (iigs. 161, 162). The first step towards complication is seen in the Sycon type, in which the gas- tral cavity consists of numerous radial chambers or ampulla: which alone FIG. 162. — Section of cortex of Chondrillanucula, the skeleton omitted (after Schuize). cl, afferent canals; c2, efferent canals; g, ampullae; m, cloaca; o, osculum. contain the collared cells, while the central cavity, now called cloaca, is lined with pavement epithelium. By increase of mesoderm and corre- sponding thickening of the body wall the ampulla become separated from external and cloacal surfaces (Leucon type). They nevertheless retain their connection with both surfaces by means of cavities which may sSSfr A rf« FIG. 163. FIG. 164. FIG. 165. FIG. 163. — Surface view of dermal pores of Aplysina aerophoba (after Schuize). FIG. 164. — Ascyssa acufera (after Haeckel). FIG. 165. — Leucetta sagittata (after Haeckel). be lacunar (fig. 161) or consist of a system of canals. The canal system is double; one part is incurrent and leads from the dermal pores to the ampulke; the other or excurrent, from the ampulla- to the cloaca, the two being connected by the ampulke alone (fig. 162), the canals from the pores 204 PORIFERA uniting in trunks and these in turn branching to go to the ampullae. The excurrent canals also show a similar tree-like arrangement. Not infrequently extensive subdermal or subcloacal spaces occur. The relations may be more complicated by the development of several cloacae, or by the branching of the sponge (fig. 164), while still further the branches may anastomose (fig. 165), giving rise to a netwoik. Sponges may reproduce asexually, small portions separating as buds and producing new animals (fig. 88). Usually sexual reproduction prevails. The eggs, which like the spermatozoa arise from mesoderm cells (fig. 160), undergo segmentation and leave the parent as flagellate larvae (fig. 166, A). At fixation a kind of gas- trulation takes place, the blastopore (B) closes, and the osculum, an entirely new formation, arises at the opposite pole. FIG. 166. — Development of Sycandra raphainis (after Schulze). .4 blastula; B, gastrula at the moment of fixation; ek, ectomesoderm; en, entoderm. The sponges are frequently regarded as Coe- lenterata, but scarcely a single homology can l;e drawn between the two. The ccelenterate mouth is different from either pores or oscula. Indeed, 't is disputed whether the collared cells are entoderm. Most sponges possess a skeleton secreted by special mesoderm cells, and this skeleton affords the means, according as it is composed of calcic car- bonate or of silica, of dividing the sponges into two classes. Besides, there are two groups, Ceraospongiae and Myxospongiae, in which the skeleton is respec- tively of horny substance (spongin) or is lacking entirely. These seem to be •descendants of the silicious forms. Order I. Calcispongiae. The calc sponges are exclusively marine and mostly live in shallow water. They are grayish or white in color, of small size, rarely exceeding an inch in length. The skeletal spicules usually project through the epithelium, forming silky crowns in the neighborhood of the osculum. One-, three-, and four-rayed spicules are recognized, these ground forms presenting by unequal development a great variety of shapes. Sub Order I. ASCONES. Thin porose walls and central 'stomach.' Leucosolenia* Sub Order II. SYCONES. Cloaca present surrounded by ampulL'e radially arranged. Grantia,* Sycon,* Sycandra * Sub Order III. LEUCONES. A complicated system of branching canals in thick walls connects the ampullae with outer surface and cloacal cavity. Leucetta, Leucortis. Order II. Silicispongiae. The siliceous sponges are richest in species and occur at all depths of the sea, being frequently noticeable from their size and bright colors. They are subdivided into Triaxonia and Tetraxonia. In the Triaxonia the spicules composing the skeleton — appearing as if of spun glass (hence Hyalospongia, or SILICISPONGLE 205 glass sponges) — have three crossed axes (six threads radiating from a common point) — hence Hexactinellidte. The mesoderm is scanty and in consequence the canals are loose-meshed, lacunar spaces and the ampullae large and barrel- formed. In the Tetraxonia the mesoderm is usually abundant and the canal system well developed. The four-axial spicules of the Tetractinellidae must be regarded as the fundamental skeletal type. From this are derived the compact frameworks of the Lithistidas and the monaxial spicules of the Monactinellidae. In both groups the spicules may be united by secondary deposits of silica to an extensive framework; or the union is affected by spongin, which, if the spicules disappear, forms the whole skeleton (horny sponges); or, as in slime-sponges, the whole skeleton may be lost. Sub Order I. TRIAXONIA. HEXACTINELLID^;, chiefly deep teas; Eiiplectella aspergillum, Venus' flower-basket. Hyahmema. Sub Order II. TETRAXONIA. Typical are the largely extinct LITHISTID^E (some genera— Discodcnnia — persist in deep seas) and TETRACTINELLIDJS: Geodia.* Near here apparently belongs Oscar ella* without a skeleton (MYXOSPONGIA). MON- ACTINELLID.E, spicules united by spongin (Cornacuspongia); can even be entirely replaced by that substance. Numerous marine forms, and the fresh- water SPONGlLLlDjE (Spongilla* Ephydatia*), usually colored green by algae. They are distinguished by formation of gemmulce or statoblasts. At times the FIG. 167. — Skeletal structures of sponges (after Schulze and Maas). i, Horn fibre of bath sponge with spongiob lasts; 2-7, spicules of, 2, Esfxriti; 3, 4, ( 'orlicum; 5, Mysilla; 6, Tethya; 7, Farrea. protoplasm divides into round bodies, as large as the head of a pin and these become surrounded by a firm membrane often strengthened by collar-button-like spicules, the amphidiscs. These statoblasts survive times of freezing or drought On return of good conditions the contents escape and form small Spongillce, often utilizing the old skeleton. The spicules entirely disappear and nothing but the spongin fibres remain in the horny sponges, CERAOSPONGLE. The skeleton consists of an organic substance, spongin, which differs chemically from true horn — keratin. This spongin is laid down by peculiar cells, the spongioblasts (fig. 167, i), and it always consists of concentric layers. The fibres interlace, branch, and unite. Best known are the bath sponges; Eitspimgia ofjinnalis,'"'- occurring in the Mediterranean, West Indies, Florida, and other seas in many varieties. Best are the Levant sponges (var. inoUissiiim). Sponges of com- merce consist only of the skeleton, the animal parts being washed away.- Less valuable are Euspongia zimocca and Hippospongia cquina,* the horse-sponge. 206 CCELENTERATA Summary of Important Facts. 1. The sponge body is largely a mass of connective tissue covered externally with pavement epithelium (mesectoderm) and penetrated by canals. 2. An entoderm of collared flagellate cells occurs only in the ampullae or flagellate chambers which are intercalated between incurrent and ex- current canals (in ascons in the central cavity). 3. The animals receive food through fine pores in the body wall; indigestible matter is cast out through one or more oscula. 4. Since nerves, muscles, and sense organs are lacking or very weakly developed, only inconspicuous movements occur. 5. Sponges are divided into Calcispongue and Silicispongiae according to the character of the skeleton. f PHYLUM III. CCELENTERATA (CNIDARIA). The ccelenterates, formerly called Zoophyta (plant-animals), were united by Cuvier with the Echinoderma to form the type Radiata, a union which Leuckart, the father of the name Ccelenterata, set aside because separate intestinal and body cavities occur in the Echinoderma, while in the Ccelenterata' there is but a single cavity in the body. Each name indicates certain important characters of the group. (1) The name Zoophyta referred to the general appearance. Most ccelenterates, like plants, are fixed and by incomplete budding form bush- like or mossy colonies. This resemblance is but superficial, for there is not the slightest doubt of the animal nature of any ccelenterate. The name therefore does not imply that these are doubtful forms on the border between plants and animals. Besides, there are free-moving forms which swim with great ease. (2) Most Coelenterata are radially symmetrical. There is a main body axis, one end of which passes through the mouth and the other through the blind end of the digestive tract, and the organs of the body are radially arranged around this so that the body may be divided into symmetrical halves by numerous planes. In the higher Ccelenterata this may be replaced by a biradial symmetry or even by bilaterality (Cteno- phora, many Anthozoa). (3) The term Coelenterata is given because these animals contain a single continuous ca'lenteron or gastrovascular cavity. In its simplest form this is a wide-mouthed sac into which food passes for digestion. The single opening into it serves for both mouth and anus; the sac itself is the alimentary tract. Frequently lateral diverticula or branched canals CGELENTERATA 20- are given off from the central sac which distribute the nourishment to the peripheral parts of the body, and thus functionally replace the vascular system of higher forms. Since this gastrovascular system is primarily for nourishment, it is not a body cavity and one cannot say that the ccelen- terates are stomachless. On the other hand, the term 'ccelentcron,' that is, a cavity at once gastric and ccelomic (p. 148), is perfectly defensible, since in many higher animals which possess a true body cavity (coelom) this arises in development as diverticula from the primitive stomach (enteron) . Since such diverticula occur in ccelenterates without becoming independent, one can say that the gastrovascular system consists not only of intestinal portions but, in potent; a, of the ccelom as well. To even a superficial observation the Ccelenterata are more clearly animals than are the sponges. The single animals, though often united in colonies and fixed to some support, are capable of quick and energetic motion. These movements are most striking in the tentacles — long tactile processes in the neighborhood of the mouth, which feel for food, grasp it, and convey it to the mouth. The means of killing the prey are the cnidce (whence the name Cnidaria for the phylum), nematocysts, or nettle cells (fig. 1 68). These structures, of great systematic importance, are oval or elongate vesicles with fluid con- tents and firm membrane. Each is drawn out at one end into a long thread-like tube (hence an additional name, thread cells). In the resting stage the thread is spirally coiled inside the cell. On stimulation the thread is quickly extended ('explosion of cell') and produces a wound into which passes the irritating fluid con- tents. Some ccelenterates (e.g., Physalid) can produce in this way very painful nettling even in man. The nettle capsule arises as a plasma product inside a cell. When fully developed the nettle cell extends to the surface and ends with a tactile process (cnidocill) which, upon contact, stimulates the protoplasm and causes the explosion, the thread being everted like the finger of a glove. The cell itself is frequently enclosed by a muscular sheath or a network of muscle fibres. Among the coelenterates both sexual and asexual reproduction may occur, the latter usually by budding, more rarely by division. Sexual and asexual reproduction can be combined in the same species, producing an alternation of generations. FIG. 1 68. — Nettle cells of Coelen- terata (after Hertwig, Lendenfcld, ami Hamann). 208 CCELEXTERATA In comparison with the sponges the Ccelenterata may be called epi- thelial organisms. A mesoderm (mesogla-a) may be entirely lacking or may have but a subordinate development. The ectoderm and entoderm, on the other hand, are the important tissues — producing muscles, nerves, sense organs, sexual products and cnidae. Hence the group is often called Diploblastica — two-layered animals. Class I. Hydrozoa (Hydromedusae) . According to varying standpoints the Hydrozoa can be placed either higher or lower than the Anthozoa in the system, since in the former group two forms frequently occur in the life history, one agreeing well in struc- ture with the Anthozoa, the other standing on a higher grade. The first is the sessile and usually colonial polyp, the second the free-swimming medusa, well provided with sense organs. These are usually related to each other by an alternation of generations. The polyp is asexual and by budding produces medusae;. the medusa, on the other hand, is the sexual stage, and from its eggs polyps arise. The polyp of the Hydrozoa is the Jiydropolyp, forming an important archetype from which all other conditions— medusas, scyphopolyp, and coral polyp— may be derived. Our best example of this is the fresh-water Hydra. The body (fig. 169) is a sac, the closed end of which, the pedal disc, is used for attachment. The other end bears the mouth which leads to the gastrovascular (digestive) cavity. Around the mouth is a circle of tentacles used in capturing food. These are outgrowths of the body wall; the circle dividing the body into a peristome inside the circle and a column constituting the rest of the outer wall. Hydra has but two body layers (fig. 170), an entoderm of flagellate cells lining the gastrovascular space, and the ectoderm covering the outer sur- face. Between the two is the supporting layer (mesoglcea), a membrane without cells and hence not a body layer. Both layers consist of epithelial muscular cells (cf. p. Si), the basal ends of which are produced into smooth muscle fibres, those of the ectoderm running lengthwise, those of the entoderm around the body. The ectoderm further contains ganglion, nettle and sex cells. The nettle cells on the tentacles are crowded into small ridges or ' batteries.' The sex cells (at certain times) produce swell- ings on the column; a circle of male swellings close beneath the tentacles, the female cells farther down the column (fig. 169). Individuals reprodu- cing by budding are more common than the sexually mature (fig. 93). Small elevations appear on the column, enlarge, form tentacles, and at last a mouth, after which they may separate from the parent. HYDROZOA 209 In the sea are numerous hydroid polyps which, while agreeing in the main with Hydra, are distinguished from it in two important respects: (i) they do not directly produce sexual organs; (2) they reproduce asexu- ally, and by incomplete budding form persistent colonies. In this a series of parts have arisen which require special designations (fig. 171). The separate animals, liydrant/is, are connected by a system of tubes, the cocno- sarc, which, like the hydranths, consist of ectoderm, entoderm, and mesoglcea, and since the gastrovascular space continues in them, these distribute food throughout the colony. The ccenosarc may creep over en s ek c FIG. 169. FIG. 170. FIG. 169. — -Hydra viridis* testes above; ovarian enlargement and escaping egg below. FIG. 170. — Body layers of Hydra (after Schulze, from Hatschek). c, cuticula; en, nettle cells; ek, ectoderm; en, entoderm; s, supporting layer. some support (stone, alga, snail-shell, etc.) and form a network, the hydrorhiza, or it may stand erect and free, forming a Jiydrocaulus. Usually both hydrorhiza and hydrocaulus occur in the same colony. Usually the colony is strengthened and protected by the perisarc , a cuticular secretion of the ectoderm. In some (fig. 172) the perisarc stops at the base of the hydranth; in others (fig. 173) it expands distally into a wide-mouthed bell, the hydrot/ieca, into which the hydranth may retract. In rare cases this perisarc may be greatly increased and calcified, forming large coral-like masses with openings from which the hydranths may protrude (fig. 174). 14 210 CCELENTERATA FIG. 171. — Campanularia Johnston! (after Allman). a, hydranth with hydro- theca; b, retracted"; d, hydrocaulus; /, gonotheca, with blastostyle and medusa buds; g, free medusa. The hydrorhiza is shown as the creeping portion from which the hydrocauli and gonothecse arise. FIG. 172. — Section of Eudendrium ramosum. ek, ectoderm; en, entoderm; p, perisarc; s, supporting layer. HYDROZOA I'll The lack of sexual organs, which distinguishes most marine species from Hydra, is due to the fact that sexual individuals of special form are produced from the colony by budding. These, the medusae, may separate early from the colony and swim freely. A medusa (figs. 175, 176) has the form of a dome-like or disc-like bell and consists chiefly of very watery jelly. The bell or umbrella of the medusa is covered on both its surfaces- the concave or subumbrclla, the convex or exumbrella — with ectodermal epithelium. At the margin of the bell the ectoderm is produced into a FIG. 173. FIG. 174. FlG. 173. — Campanularla geniculata. ek, ectoderm; en, entoderm; />, perisarr, ex- panded around hydranth to a hydrotheca; s, supporting layer. FIG. 174. — A bit of Millepara alcicornis*, enlarged (after Agassiz). two-layered sheet with a central opening, the velum or craspedon (fig. 175, B, v) of systematic importance, since these medusa? are often called ("ras- pedota. Tentacles (usually 4, 8, or multiples in number) also arise from the edge of the bell just outside the velum. Comparable to the tongue of the bell or the handle of the umbrella is the manubrium, hanging from the highest point of the subumbrella and bearing the mouth at its tip. It contains the chief digestive space, front which radial canals run on the subumbrellar surface to a ring canal in the margin of the umbrella. The radial canals are usually four in nuinl >er, but in some species the number is increased during growth even to a hundred or more. Manubrium and canals are lined by entoderm, which also extends into the tentacles and forms their axes. 212 CCELENTERATA All other important organs arise from the ectoderm. Gonads arise in many species (fig. 176 from the ectoderm of the manubrium; in others A Fir,. 175. — Rhopalonema velatum. c, ring canal; e, exumbrella; £, gonads; 7z, oto- cysts; m, stomach; n, nerve ring; o, mouth; s, subumbrella; tr, /', tentacles of first and second order; v, velum. from the same layer covering the subumbrellar surface of the radial canals (fig. 175), forming in either case conspicuous, often orange or red, thickenings. Longitudinal ectodermal muscles move the tentacles in a HYDROZOA 213 snaky fashion, whence the name medusa. Circular striped muscles run on the subumbrellar side of bell and velum, causing the characteristic motion. By their contraction the bell becomes more arched and narrowed, while the velum (which hangs down when at rest — fig. 175, A) contracts like a diaphragm across the mouth of the bell (B). Since water is thus forced out through the opening the medusa is forced forward by the reac- tion. The circular muscles of the umbrella and velum are separated FIG. 176. — Tiara pilcata (after Haeckel, from Hatschek). by two nerve rings, one subumbrclhv, the other cxumbrellar in position (fig. 177, n1, ir), the first supplying the muscle plexus, the other the sensory organs— eyes of the simplest type, red pigment spots with or with- out a lens; and open or closed statocysts ('ears'). Tactile hairs are abundant on the tentacles. The statocysts are of two types, both beginning as open organs and reaching their highest development as closed vesicles. One type, the tentacular organs, occurs in the Trachymedusae (fig. 177, 1-4) the other, or velar organ, in the Leptomedusaj (5-6).' The tentacular organs are modified tentacles, the ento- 214 CGELENTERATA dermal axis forming the statoliths and the ectodermal covering the sense cells. In the yEginidae (i and 3) the club-like tentacles, seated on an auditory cushion, project freely into the water; in the Trachynemidae (2) they are partially trans- formed into vesicles, and in the Geryonidse they are completely enclosed and are sunk in the jelly of the bell. The velar organs of the Leptomedusae are placed on the subumbrellar surface of the velum. They may be either simple pits, or the mouths of the pits may close. In these both sense cells and stato- liths are ectodermal. Eyes and statocysts occur in different forms, a fact which formerly led to a division of medusae into ocellate and vesiculate groups. 1. ea.- m $06. m.— - FIG. 177. — Statocysts (ear vesicles) of medusae. 1-4, tentacular statocysts of Trachymedusse; 5, 6, velar of Leptomedusae; 7, marginal body of Acraspedia. i and 3, auditory clubs of Aeginopsis; 2, same of Rhopalonema, with beginning of ear vesicle; 4, statocyst of Gerycmia; 5, of Aequoria; 6, auditory pit of Mitrocoma annir; 7, marginal body of Aurelia. ek, ectoderm; en, entoderm; g, mesogloea; h, auditory hairs; m, cir- cular muscles cut across; «l, n~, upper and lower nerve ring; r, ring canal; s, statolith. While polyps and medusae apparently differ so greatly from each other, the medusae are only highly modified polyps adapted to a swimming life. The long axis of the polyp has been greatly shortened (fig. 178) and the cylindrical body developed into a disc; the mesoglcea of column and disc thickened to a thick layer of jelly; while manubrial cavity, radial and ring canals are remnants of the large gastrovascular space of the polyp, obliterated in the other regions by the pressure of the mesoglcea. To the parts thus formed only the velum and sense organs are added. This comparison of medusa with polyp is important in understanding HYDROZOA 215 the development, which usually includes an alternation of generations. From an egg of a medusa a small ciliated embryo (planula) escapes, which becomes attached, develops mouth and tentacles, and, by budding, produces a hydroid colony. This colony lacks sexual organs. By budding tr FIG. 178. — Diagram of sections of (A} a polyp and (B) a medusa, ek, ectoderm; ek', of exumbrella; ek-, of subumbrella; ek3, of manubrium; el, endoderm (cathamnal) layer arising from obliteration of digestive space; en, entoderm; r, ring canal; s, sub- umbrella; t, tentacles; v, velum; x, supporting layer (gelatinous in E). it produces sexual individuals (medusae) which separate and swim away. Since polyp and medusae are morphologically comparable, before the escape of the medusae the colony is polymorphic, consisting of individuals (hydranths) which reproduce only asexually and of others which have taken over the sexual reproduction (medusae). Hence alternation of FIG. 179. — Comparison of a medusa and a sporosac forig.X .1, fully developed medusa; B, medusa with the manubrium closed, still attached to the blastostyle; < ', medusa reduced to a simple manubrium (sporosac); D, last stage, eggs being produced in the body wall (Hydra). generations has arisen here from a division of labor or polymorphism of individuals originally of equivalent value, in which some individuals (the sexual) have separated and acquired a peculiar structure. While alternation of generation has arisen from polymorphism, it ran again produce it. This occurs when the medusae, instead of separating, 216 CCELENTERATA remain permanently attached to the colony. They then degenerate into 'sporosacs,' which always lack mouth, tentacles, and velum (fig. 179), often also radial and ring canals, so that at last there remains only the manubrium (spadix) and the sexual organs, the latter enveloped by the rudiments of the umbrella. Since medusae and sporosac replace each other in closely allied species, a common name, gonophore, has been applied to both. This developmental history may be modified in two ways: either the polypoid or the medusan generation may be suppressed. In the first case we have polyps which reproduce both sexually and asexually, in the other medusae whose eggs develop directly into other medusae. (A few medusae may bud new medusa?.) Thus we can have four conditions: (i) Polyps which produce sometimes asexually, sometimes sexually, but always polyps; (2) Medusae which always produce medusae; (3) Polyps and medusae in alternating generations; (4) Polyps and sessile medusae (sporo- sacs) united in a" polymorphic colony. FIG. 180. — American Trachy- and Narcomedusre. A, Liriope scutigera (after Fewkes). B, Cunocantha octon^ria (after Brooks). The Hydrozoa are almost exclusively marine. The colonial forms occur •mostly on rocky coasts down to a depth of 100 fathoms, but have been found in water 4000 fathoms deep. The medusas belong to the pelagic fauna. For a long time the only fresh-water species known belonged to the cosmopolitan genus Hydra, but more recently both hydroid and medusan forms have been found in various parts of the world. The Hydrozoa may be classified according to characters, derived either from the hydroid or the medusan stage. The former gives four groups: (i) HYDRARIA. Polyps with asexual and sexual reproduction; no persistent colonies, no perisarc, no gonophores (fig. 169). (2) TUBULARLY. Mostly colonial, with perisarc but without hydrothecae. Reproduction by gonophores (medusas or sporosacs, figs. 94, 172). (3) CAMPANULARI^;. Colonial, with perisarc and hydrotheca. Reproduction by gonophores arising in special perisarcal en- velopes, the gonotheca (figs. 171, 173). (4) HYDROCORALLINA. Colonial, with massive, calcified perisarc, resembling coral. Reproduction by sporosacs or rudimentary short-lived medusas (fig. 174). The characters derived from the medusas give five groups: (i) ANTHOMEDUS^E (Ocellatas). Gonads on the manubrium; no statocysts; eyes usually present; polyp generation present. (2) LEPTOMEDUS.E. Gonads on radial canals; usually velar statocysts; polyp generation present. (3) TRACHYMEDUS^:. I. HYDROZOA: HYDRARIA, CAMPANULARL£ 217 Gonads on the radial canals; tentacular statocysts; develop directly to medusae (rig. 180, A). (4) NARCOMEDUS^;. Gonads on the manubrium or gastral pouches; tentacular statocysts; no polypoid stage (fig. 180, B.) (5) SIPHONO- PHORA. Polymorphic, free-swimming colonies of Anthomedusas, no polyp generation. As there are medusae without polyp stages and polyps without medusae, a natural system must take into account both these features. When the life histories are traced it is seen that the Anthomedusa; and the Tubulariae are connected by an alternation of generations, as in Leptomedusas and Campanu- lariaa. There are three groups — Trachymedusae, Narcomeduste, and Sipho- nophora — without a hydroid stage, and two in which the polyp plays the chief role, the medusa being rudimentary _in the Hydrocorallinae, lacking in the Hy- draria. The hydroid polyps are usually but a few millimeters or fractions of a millimeter in size, but the huge Monocaulis iwperator, of the deep seas, two yards in length, forms an exception. The colonies are usually only a few inches in extent. The medusae have bells varying between a millimeter and a few inches in diameter (sEquoria forskalea sixteen inches). Order I. Hydraria. Until recently only the cosmopolitan species of Hydra were known. During most of the year they reproduce by budding (fig. 93), only occasionally develop- ing gonads (fig. 169). The eggs remain in connexion with the mother during segmentation, and later form an embryonal shell. In this 'encysted stage' they can be distributed by wind or water birds. These animals formed the basis of the celebrated researches of Trembley on regeneration. He showed that small portions w7hich included both body layers could regenerate the whole animal. His experiments upon turning the animals inside out have not been fully confirmed; for in such cases the layers resume their normal positions. Hydra grisea* (fusca), brown; H. viridis,* green, from the presence of symbiotic algae. Protohydra rydcri* without tentacles. Order II. Hydrocorallinae. Exclusively marine, forming colonies of thousands of polyps whose cal- careous skeletons so resemble true corals that they were associated with them until the animals were studied. Millepora alcicornis* (fig. 174), stag-horn coral, in Florida. The rosy Stylasters in tropical seas. Order III. Tubulariae =Anthomedusae (Gymnoblastea). As a rule these colonial forms with perisarc but without hydrotheca produce anthomedusse, but there are forms like Clava* and Hydractinia* which have sporosacs. Indeed, Coryuwrplia* and Monocaulis* differ only by medusa1 in the former and sporosacs in the latter. The medusae have the gonads on the manubrium, lack statoliths, and usually have a high-arched umbrella, and frequently eye spots. In the forms with alternation of generations different names are applied to the hydroid and medusan stages. Amon'T hydroids are Pennaria,* Syncoryne,* Endendri,* Tubularia,* among medusae Sarsia,* Turritopsis* Margclis* Nemopsis.* Order IV. Campanulariae = Leptomedusae (Calyptoblastea). These forms differ from the last in that they are always colonial and possess hydrothecaa, the medusae always being flattened Leptomedusae (p. 216). A peculiarity is the existence of gonothecas, closed perisarcal envelopes, inside 218 CCELENTERATA which the gonophores arise from the blastostyle, a specialized polyp, without mouth or tentacles (fig. 171, /). The typical Campanulariae produce medusae, while some forms, like Thaumantia* and sEquoria* have no hydroid stage; on the other hand, Sertularia* and Plunmlaria* have no medusa stage. Other common genera, Clytia* Diphasia* and Aglaophenia* among hydroids; Obelia* Tima* Rhegmatodes* among me- dusae. Possibly the fossil GRAPTOLITES be- •sb long near here. Only the perisarc is known; this has hydrothecae, in which it is supposed the hydranths occurred. Order V. Trachymedusae. These medusas, mostly from warmer seas, have no hydroid stage. The characters are given on p. 216, Trackynema,* Liriope* (fig. 1 80), and Campanella* in our waters, Geryonia, etc., in Europe. .fl Order VI. Narcomedusae. In addition to the characters on p. 217 may be added that the tentacles arise from the outside, above the rim of the bell. Cunocantha* (fig. 180), Cunina*, sEgina. The larvae frequently live as parasites on other medusae, and they may be able to re- produce asexually, forming sacs in which new medusae are budded. Order VII. Siphonophora. t FIG. 181. — Diagram of Siphono- phore (from Lang). A-H, groups of different individuals; ds, cover- ing scales; go, gonophores; hy, feed- ing polyps; p, 'feelers' (digestive); sb, float; sg, swimming bell (necto- calyx) ; st, stalk. The Siphonophora are among the most beautiful of pelagic animals, some transparent, some brightly colored. Each (fig. 181) consists of a colony of individuals springing from a common cosnosarcal tube which is strongly mus- cular and contains a central canal, lined with entoderm, by which the members of the colony receive their nourishment. At one end the tube is usually closed by a float of invaginated ectoderm, filled with air, the pneumataphore, which keeps the colony vertical in the water. The individuals, springing from the ccenosarcal axis, perform different functions and hence differ in structure. Close behind the float commonly come several swimming bells (nectocalyces) which retain only those medusan structures (bell, velum) necessary for swimming and those (ring and I. HYDROZOA: SIPHONOPHORA 219 radial canals) for the distribution of nourishment received from the common tube. Then come, scattered through the colony, the covering scales, for protection, firm gelatinous plates which have lost the ring canal, the muscles, and the bell shape of the medusa.'. Food is taken by wide- mouthed feeding tubes (liy) which may be compared to polyps (fig. 58) or the manubrium of a medusa. They digest the food by means of large masses of glands ('liver bands,') and send it by the central tube to all the members of the colony. At the base are long muscular tentacles (/) from which small lateral threads depend, each ending in a brightly colored swelling, the nettle head composed of large, closely packed nettle cells. These are the cause of the nettling, which in many species is so FIG. 182. — American siphonophores. A, Nanomia cara (after A. Agassiz). B, Velella meridionalis (.after Fewkes). C, Diph yes pray a (after Fewkes). severe as to be feared by man. The 'feelers' (/>) recall mouthless polyps and manubria; they are very sensitive and mobile and, while tactile, ap- parently in some cases are digestive organs. Latest to develop in the colony are the sexiial bells. They are usually brightly colored and re- semble small mouthless Anth.omed.usae without tentacles. They but rarely (Chrysomitra) separate from the colony, but usually persist as more or less reduced sporosacs. From this it follows that the Siphono- phora afford fine examples of division of labor and of the consequent polymorphism of individuals. This can indeed be carried so far that many convey the impression of being individuals with a multiplicity of organs. The Siphonophora are all marine, and occur most abundantly in tropical seas. Sub Order I. PHYSOPHOR/E (Physonectse). Float present, small; next a large series of swimming bells; then the other members of the colony, f'liy- sophora, Agalmia, Nanomia* (fig. 182). Sub Order II. CALYCOPHOR^E (Calyconectae) . Float lacking; one or two large swimming bells; the other in- 220 CCELENTERATA dividuals in groups which frequently separate before becoming mature, were once regarded as distinct animals. Praya, Diphyes* (fig. 182), in warmer seas. Sub Order III. CYSTONECT^E. Float greatly enlarged; the ccenosarcal tube reduced, the individuals (no covering scales nor swimming bells) attached to under side of the float. Physalia,* Portuguese man-of-war, stings severely. Sub Order IV. DISCONANTILE. Float a flattened disc; manubrium pro- jects from centre of lower surface. Par pita,* disc. Velella* (fig. 182). Class II. Scyphozoa (Scyphomedusae) . The Scyphozoa parallel the Hydrozoa in frequently having an alter- nation of generations; the asexual generation being the scyphopolyp or scyphostoma, the sexual an acraspedote medusa. In contrast to the Hydrozoa the asexual stage plays a subordinate role; it is closely similar in all species, and can even be lost (Pelagia), while the medusae are always well developed and present great variety of form. The scyphostoma (figs. 183, 184) recalls Hydra, but has a small perisarcal cup around the aboral end. Internally there are four longi- tudinal folds projecting into the gastral cavity and extending from the FIG. 18: FIG. 184. FIG. 183. — Scyphostoma of Aurelia aurita (from Korschelt-Heider). k, perisarc cup; pb, proboscis; s, stalk; t, gastral folds; tr, ectodermal funnels. FIG. 184. — Section of Scyphostoma (from Hatschek). gr, gastric pouches; s, gastric septa; sm, muscles. margin of the mouth to the opposite pole. These septa or taniola (fig. 184, s) appear in cross-section as small folds of entoderm supported by a process of the supporting layer containing a muscle band extending down from the peristome (fig. 184). They are important morpholog- ically, since in budding they produce the gastral tentacles (phaccllcz) of the medusa?. Further, they are the- first appearance of the septal system, so strongly developed in the Anthozoa. The medusas are large (four inches to four feet or more in diameter) with a slightly arched umbrella, often of almost cartilaginous consistency. II. SCYPHOZOA 221 A knowledge of the development is necessary in order to understand the medusa. The young medusa (Ephyra stage, fig. 185) is eight lobcd, each lobe with a sensory pedicel in a notch at the tip. These lobes indicate eight radii, the four passing through the angles of the mouth being the perradii, the others the interradii, the adradii being between the lobes (fig. 186). In most species the adradial regions increase with growth, and at last form a circular margin to the bell, divided by the notches of the original lobes (fig. 187), tentacles occurring only in the adradial regions. The medusae differ externally from those of the Hydrozoa in the absence of a velum (hence acra- spedote). Instead of a nerve ring there are eight nerve centres connected with the sensory pedicels. Each pedicel is a modified tentacle (fig. 177, 7) its en- todermal axis furnishing a statolith at the end, and usually a simple eyespot. The gastrovascular system begins with a quadrate or X-shaped mouth (fig. 1 86). The angles of the mouth are usually produced into long curtain- like oral tentacles of use in the capture of food. The 'stomach,' which begins just inside the mouth, gives off four interradial pouches, the gastro genital pockets, each containing a group of small gastral tentacles (phacelhc}, and the plaited folds of the gonads, these being, in contrast to the Hydrozoa, of entodermal origin. In this the Scyphomedusoe show relationships to the Anthozoa. From the central digestive sac arise in the Ephyra stage (fig. 185) eight radial canals to the sensory pedicels, and most adult medusa? have these same pouches and eight others, adradial in position. In some this primitive arrangement is complicated by a network of tubes (fig. 186). In the species with an alternation of generations the egg produces a ciliated larva, the planula (fig. iSS) which attaches itself and develops into a scyphostoma. This scyphostoma is capable of terminal, and often of lateral, budding. The lateral buds always produce new scyphostoma', the terminal, medusa?. In the latter the scyphostoma develops into a strobila, becoming divided by circular constrictions into a series of saucer- like discs, the young jelly-fish. As the successive discs become ready they separate from the pile and swim away as epliyrcc. At lirst the ephyra^ (fig. 185) have only four gastral tentacles, parts of the gastral FIG. 185. — Ephyra of Cotylorhi-,i (after Claus). gt, gastral tentacles (phacelke); rk, marginal (sensory) body. 222 CCELENTERATA / / II II FIG. 186. — Ulmaris prototypits (from Hatschek). /, radii of first order (perradii); 77, radii of second order (interradii) ; /, marginal lobes; o, oral lobes (cut away on right side) ; t, tentacles (adradial) ; the gonads (right side) are interradial. < FiG. 187. — Polydoniafrondosa* and one of its branching oral lobes, showing the closed grooves (.<;) (after Agassiz). II. SCYPHOZOA: DISCOMEDUS/E 223 septa of the scyphostoma (p. 220). Since the ephync differ markedly from the adult medusas and only gradually change into the sexual form, the alternation of generations is complicated by a metamorphosis. This metamorphosis persists in some cases (Pelagia noctilucd) where the alter- nation of generations is suppressed; the egg develops directly into an ephyra, which transforms into the adult jelly-fish. FIG. 188. — Development of Aurelia aurita (from Hatschek). First row, growth of planula to scyphostoma; below, strobilation (separation of ephyra?): left, oral view of scyphostoma; right, two ephyra:. Order I. Discomedusae. The foregoing account applies, as a whole to only the Discomedusa?, the widest distributed and most abundant of the Scyphomedusje. The order is divided into two suborders, I. SEM.*;OSTOME.E, mouth X-shaped with long fringed and very mobile arms at the corners of the mouth. Aurclid flavidula* and Cyanea arctica* common in north Atlantic waters, the latter large, exceptionally seven feet in diameter; Pelagia* Ulmaris (fig. 186). (2) RHIZOSTOMF/K, four oral arms which branch dichotomously; the mouth and grooves on the arms closed by union of their edges so that many small stomata remain through which food is taken. Stomolophus* Polydonia* (fig. 187). Certain Scyphomedusas are distinguished from the Discomedusae. Some of these are inhabitants of the deep seas and only recently known; others ditlrr so from the Discomedusae that the relationship was not seen at first. These have in common the rathannua, four partitions, homologous to the ta-niolir of the scyphostoma, which bear the phacellse and divide the peripheral part <>t the gastral cavity in such a way that the gonads are separated into eight groups. The marginal bodies vary in three ways. 224 CCELENTERATA Order II. Stauromedusae. Best known are the LUCERNARLE (fig. 189) which lack marginal bodies, but usually have four small tentacles in their place, while the adradial regions are drawn out into arms, bearing bundles of tentacles. The aboral surface of the bell is produced into a stalk by which the animals are attached. The TESSE- RUXE (unknown in America) are free-swimming. Order III. Peromedusae. Free-swimming, cup-shaped medusae, with four interradial sense bodies; mostly from the high seas. Pericolpa, Periphylla in Gulf Stream. Order IV. Cubomedusae. Differ in the four perradial sense bodies, ment unknown. Charybdea (fig. 190). Tropical and subtropical; develop- FIG. 189.- — Halyclystus auricularia* (after Clark). FIG. rgo. — Char \bdea marsiipialis (from Hatschek). Order V. Coronata. A coronal furrow on the exumbrella; four to sixteen marginal sense bodies as in Discomedusae, but eight gonads and presence of cathamma. Some of these formerly regarded as Discomedusae (under the name of Cannostomeas), because of eight sense bodies. Nausithoe albida arises by terminal budding from a scyphostoma (Stephanoscyphus mirabilis) parasitic in sponges. Atolla. Class III. Anthozoa (Actinozoa). The Actinozoa, including the sea anemones, sea pens, and corals, are exclusively marine. With few exceptions they are sessile and usually form colonies, often of enormous size. In this as in appearance (fig. 192) III. AXTHOZOA 225 they resemble the hydroid polyps. They have a pedal disc, column, tentacles, and peristome with central mouth. They are distinguished by their greater structural differentiation. The Anthozoan polyp has a well-developed mesoglcea, this being a layer of connective tissue with numerous cells, giving the animals a tough fleshy consistency. Still more important are the oesophagus and septa; bearing mesenterial filaments and gonads. The mouth, in the centre of the peristome, is usually oval or slit-like. Hence there is a biradial symmetry for there is a sagittal axis (fig. 191, s,s) -— m P FIG. 191. FIG. 192. FIG. 191. — Antheomorpha elegans. s, s, sagittal plane. FIG. 192. — Sagartia parasilica split lengthwise, a, acontia; c, septal canal; f, mesenterial filaments; g, gonads; m, sphincter muscle; o, oesophagus; p, peristome; r, septa of different orders; s, siphonoglyphe; ic, cut wall of column. passing in the long axis of the mouth, and a transverse axis at right angles to it. From the mouth the oesophagus hangs down into the body as a flattened tube and opens at its lower end into the wide gastro vascular cavity. This oesophagus is an inflected part of the peristome and hence lino I with ectoderm, and its lower end alone can be compared with the mouth of the hydrozoan (fig. 192). It usually bears at either end a specialized groove, the siphonoglyphe (s). The oesophagus is held in position by radial partitions, the septa (r), which stretch from base, column, and peristome to the oesophagus, dividing the peripheral part of the gastral space into small pockets, the radial chambers, connected below the end of the oesophagus with the central part. Above, these chambers continue into the tentacles. The tentacles therefore are outgrowths from the radial chambers and usually equal them in number. Besides the complete or primary septa which reach 15 226 CCELENTERATA the oesophagus, there may be others which do not reach the oesophagus and belonging to secondary, tertiary or other series (fig. 194). The septa support a number of important organs: the mesenterial filaments, gonads, and muscles. The mesenterial filaments are thick strands of epithelium, rich in glands and nettle cells, fastened like a hem on the edge of the septa. Since they are much longer than the peristomial- pedal length of the septa, they cause these latter to wrinkle and fold, thus strikingly resembling the mesenteries of the mammals. They envelope the food and press it in, thus aiding the succeeding intracellular digestion. Lower down, in some species, the filaments become free and form long threads, acontia, rich in nettle cells which are protruded for defence, either through the mouth or pores (cinclides) in the column. The gonads — only exceptionally hermaphroditic — lie beside the mesenterial threads as thickenings of the septum (fig. 192, g). The germ cells arise from the entoderm, but early migrate into the mesoglcea of the septum (193, 0). The eggs, when ripe, escape into the gastrovascular cavity. The young leave the parent at various stages of development, sometimes as planulae (fig. 197, A), sometimes as young with tentacles. The muscles are very important, morphologically. Muscles and nerves occur in both ectoderm and entoderm; but while the nerves are best developed in the ectoderm, especially around the mouth, and extend into the mesgolcea, the muscles of the ectoderm are weakly developed and are mostly confined to the peristome and the tentacles. The ento- dermal musculature is much stronger. Just outside of the tentacles is usually a strong circular (sphincter) muscle (m) which can close in the top of the column over the peristome. The septa also bear muscles, transverse on one side, longitudinal on the other, the latter producing ridges on the septa (fig. 193). FIG. 193. — Section of septum of Edwardsia tuberculata. ek, ectoderm; en, entoderm; me, supporting layer; mf, septal muscle; o, ovary; v, mesen- terial filament. III. ANTHOZOA 227 In the Hexacoralla (fig. 194) the septa are in pairs, with the muscle ridges facing each other, except at the ends of the sagittal axis, where they face out- wards. These are called the directives. Since the septa occur in' pairs, two kinds of radial chambers occur, those between the pairs being called inler- septal, those between the two of a pair being intrascptal. At first all Hexac- tinians have six pairs of septa — two pairs of directives, and four of lateral septa. With growth, septa of a secondary order may appear between these, giving twelve in all, then tertiary septa, the number of tentacles increasing with the septal chambers. The rule is not invariable, for some have modified the plan of six to four or ten, without altering the primitive condition. A B FIG. 194. FIG. 195. FIG. 194. — Transverse section of actinian (Adamsia diaphana) AB, plane of sym- metry, a second lies at right angles. I-IV, septa of four orders. FIG. 195. — Transverse section of an Octocorallan (Alcyonium}. x, siphonoglyphe; 1-4, septa of one side, with their muscles on one side, symmetrical with those of the other side. In the Octocoralla only eight septa are developed. These are disposed equally on either side of the oesophagus and may have (most octocorallans) all their muscles towards one end (fig. 195) or (Edwardsia, fig. 196, IV) have one pair reversed. It is to be noted that hexactinians pass through an Edwardsia stage. In Cerlanthus new septa are always added at one end of the sagittal axis (fig. 196, II), while in the extinct Tetracoralla (I), so far as one may judge from the hard parts, the septa have an arrangement with four as the basis. Most Anthozoa reproduce by division or budding as well as by eggs. Occasionally the buds separate; usually they remain connected with the mother, forming colonies of hundreds or thousands of individuals, con- nected by a cccnosarc, consisting largely of mesogloea with a covering 228 CCELENTERATA of ectoderm and penetrated by a network of entodermal canals. On disturbance the polyps retract into the coenosarc. Colonial Anthozoa, with few exceptions, have a skeleton (coral), secreted by the ectoderm, consisting of calcic carbonate or of an organic FIG. 196.— Arrangement of septa in various Actinozoa. I, Tetracoralla; II, Cerlan- thus; III, Octocoralla; IV, Edwardsia. horny substance, the two sometimes occurring together. The skeleton may be internal (axial) secreted by the coenosarc, or external (cortical), and formed by the polyps, repeating to a large extent their complicated struc- ture (figs. 199, 200). \i\Fimgia (mushroom corals) the cortical skeleton FIG. 197. — Corallium rubrum, red coral (after Lacaze Duthiers). A, ciliated young; B, young colony; C, part of colony with polyps in extension (a) and contraction (c); d, crenosarc; A, greatly, B, C, slightly enlarged. consists only of a base, with radiating ridges (sclerosepta} on the side to- wards the flesh. These alternate with the septa (sarcosepta) of the polyp. In most forms there is, in addition, a cup (theca) in the column of the polyp, the sclerosepta extending inward from this. III. ANTHOZOA 229 The theca arises by a fusion of sclerosepta. If this fusion takes place some distance inside the peripheral ends of the sclerosepta, the distal ends of these project on the outer surface as costce. Still outside these may be a second cup, the epitheca. In the centre may occur a large calcareous column or several smaller ones, the columella. As the polyps grow they build the thcca? higher and higher and consequently draw out from the deeper portions, which may become cut off by horizontal partitions, the tabulce. Such tabulae occur in some Madreporaria, Octocorallans, and Millepores (p. 217) which were formerly united in a group Tabu- latas. It was once thought that the coral was a cal- cified portion of the soft parts and hence that sclerosepta were hardened sarcosepta, etc. This has been disproved. The sclerosepta are formed in the radial chambers between the sarcosepta, and the theca inside and at some distance from the column, the outer surface of which secretes only the inconstant epitheca (fig. 199). From the above it would appear that the sclerosepta correspond in number to the sarcosepta, but this is not always the case. Thus the Helioporidae, which on the grounds of the skeleton were regarded as Hexa- coralla, are shown by the soft parts to be undoubted Octocoralla. By means of their skeletons the Anthozoa produce the well-known coral reefs. When the reef reaches the surface it produces an island, the most note- FIG. icj8. — Sderophyllia margariticula Rafter Klunz- inger). FIG. 199. FIG. 200. FIG. 199. — Diagrammatic section of the flesh and coral of a hexacorallan; above the line the section passes through the oesophagus, s; below the line it is lower down; r, directives; coral black. FIG. 200. — Diagram of relations of soft parts to coral (after Pfurtscheller). Shows beginning sclerosepta and theca. worthy form being the atoll, a ring-like structure with a central lagoon. The origin of these atolls, as well as that of fringing and barrier reefs, was for a long time explained by Darwin's and Dana's theory of coral reefs. Later invcsiiga- tions, notably those of Mr. Agassiz, afford another explanation. 230 CCELENTERATA Order I. Tetracoralla (Rugosa). Extinct forms from the paleozoic rocks with the parts arranged in fours (fig. 196, I). The present tendency is to regard them as modified Hexacoralla. Order II. Octocoralla (Alcyonaria) . These forms, which have eight single septa, are recognizable by their eight feathered tentacles (fig. 197). They occur in all seas from near the shore to great depths. In development there is a planula (fig. 201) in which the oesophagus arises as a solid ingrowth which becomes perforated later. The eight septa arise simultaneously. Usually colonies are formed by budding and a poly- morphism may occur, some individuals which have reduced septa and lack tentacles, taking in water for the colony. Many are phosphorescent. B V FIG. 201. FIG. 202. FIG. 201. — Three stages in development of Renilla reniformis (after Wilson). A, cleavage of egg; B, planula; C, development of oesophagus; ec, ectoderm; en, ento- derm; ;», mesoglcea; o, oesophagus. FIG. 202. — American sea-anemones. A, Edward sieUa sipunculoides (after Stimp- son). B, Bicidium parasiticum (after Verrill) . C, Bunodes stella (after Verrill). ALCYOXIID^; (Alcyonium*') , axial skeleton is lacking, the flesh contains numerous calcareous particles (sclerodcrmitcs). The sea pens, PENNATULID^:, have the basal part buried in the mud, the rest, expanded like a disc or feather, bears the polyps. An axial skeleton usually occurs in the stalk. Peimatnla* Renilla*. The GORGONIID.E (sea fans, sea whips) have an axis of more firm- ness, which may be calcareous, and the colony may branch and the branches anastomose. Here belongs, besides many tropical genera whose names end in 'gorgia,' the precious coral (Corallium rubrum, fig. 197), the fishing for which at Naples amounts yearly to half a million dollars. TUBIPORDXE, organ-pipe corals. The HELIOPORHXE were long regarded as Hexacoralla because of their massive skeletons with six sclerosepta. The paleozoic Syringopora belongs near Tubipora, while the FAVOSITID^E resemble the Alcyoniidae. Order III. Hexacoralla (Zoantharia). The simple tubular tentacles are highly characteristic of the Hexacoralla, as is the arrangement of the paired septa in sixes as described above. Yet there are exceptions to this rule. On the one hand is Edwardsia* with sixteen or III. ANTHOZOA: HEXACORALLA 231 more tentacles and only eight septa (fig. 202), but which exhibits a condition through which the young actinians pass; on the other hand, in the Zoantharia, Cerianthiae, and Antipatharia the rule of six has undergone extensive modifi- cation. Sub Order I. ACTINARIA (Malacoderma). The sea-anemones are mostly solitary, without skeleton; with numerous septa and tentacles. They occur in FlG. 203. — Astrangia dance*', five polyps in various stages of expansion. FlG. 204. — Cceloria arabica (after Klunzinger). all seas from tide marks to the greatest depth. A few are free, but most are sessile. Metridium,* Bunodes* Sagarlia,* Bicidium* (parasitic on Cyanea), Halcampa*. ZOANTHE^E have two kinds of alternating mesenteries, individuals of the colonies usually incrusted with foreign matter. Epizoanthus* lives symbi- otically with hermit crabs (fig. 114). Sub Order II. ANTIPATHARIA. Six pairs of septa and six (Antipathcs) or twenty- four (Gerardia) simple tentacles; colony with a black horny axis and no calcareous skele- ton. Simulate the Gorgonids. Sub Order III. MADREPORARIA (Scleroderma). This group, the richest in species of any, is characterized by the great development of the skeleton. Theca, septa, and usually columella are present, and fre- quently costse as well. Solitary forms are few. Usually they form colonies, frequently of thousands of individuals, bound together by a ccenosarc extending over the surface of the coral. A colony arises from a single animal by continued fission or budding. When the division is not complete the ani- mals may form long series with numerous mouths but with the other parts united, the result being that the surface of the coral is marked by long winding grooves — incompletely separated theca — with sclcro- septa, as in the brain corals (fig. 204). The fossil Tetracoralla (p. 230) are now regarded as modified Hexacorallans. (i) The APOROSA, a compact skel- eton, the gastral canals running outside of the skeleton. Some, like Sclerophylla (fig. 198), are solitary. Others, like Oculina* branch, and still others form FlG. 205. — Fai'ia carernosa (after Klunzinger). 232 CCELEXTERATA compact masses. Astrangia dance (fig. 203), only coral in New England; Astrcea; brain corals (Ccvloria, fig. 204, Mairicina); Favia (fig. 205). (2) FUNGIACEA, or mushroom corals, no theca. Some colonial, others (Fungia) solitary. A sort of strobilation in development. (3) POROSA, with skeleton porous like a fine sponge. Madrepora* deer's-horn coral (fig. 206), Poritcs, Astroides. FIG. 206. — Madrepora erythrcca (after Klunzinger). Class IV. Ctenophora. The Ctenophores excel all animals, even the medusa?, in transparency and delicacy of tissues; many are so soft that a strong current tears them, and no attempts to preserve them have been successful. The body is biradially symmetrical; i.e., is divided by both sagittal and transverse planes into symmetrical halves. Since the longitudinal axis is usually longer than the others, which are generally equal, the body is usually oval or pear-shaped. In Cesium the sagittal axis is greatly longer, giving the animal the form of a band, whence the name 'Venus girdle.' The bulk of the animal is composed of a soft jelly with connective- tissue cells, penetrated in every direction by polynucleate muscle cells (fig. 50) branched at their ends and apparently innervated by special nerve cells. On the outer surface is a layer of ectoderm, while in the in- terior is a system of branched entodermal canals. At the bottom of a depression (fig. 207, s) at the aboral pole is a thick- ened patch of ectoderm, the sense body, a typical statocyst (fig. 208). The thick sensory epithelium forms a shallow groove, strong hairs which rise from the edge of the groove arch over it, enclosing a space to be com- pared to an incomplete vesicle. In the centre is a spherical mass of stato- liths, supported on four bundles of S-shaped agglutinate cilia. From these bundles of cillia eight bands of thickened epithelium, at first in pairs (fig. 209, ws), later diverging, pass to the oral pole (fig. 207, ;-). These meridional bands (so called from their course) consist in part of ciliated epithelium, in part of the characteristic 'combs' which are the locomotor IV. CTENOPHORA FIG. 207. FIG. FIG. 207.— Diagram of Hormophora, cut in two. /, tentacle;/--3, root and sheath of tentacle; g, main perradial vessel which divides twice dichotomously to form the meridional vessels; m, stomach; mg, paragastric canals; f1-4, rows of combs overlying meridional canals; t, tl, funnel and funnel vessels; s, sense body. FIG. 207 A. — Swimming plate and epithelial cushion (after Chun). to ws ws L/t?>V -A \Myjgfi sk FIG. 208. FIG. 2oq. FIG. 208. — Section of sense body of Callianira. A. through the centre; B, excentric; d, roof of sensory groove;/, support of statoliths, o; p, pigment cell; sc, sensory cells. FIG. 2oq. — Aboral pole of Callianira (from Lang). /, supports of statoliths, o; />/>, pole plate; sk, sense body; to, openings of gastral funnels; ws, ciliated bands. 234 CCELENTERATA organs, and which must be regarded r.s transverse rows of long agglutinated cilia. The combs (tig. 2oyA) arise from thick epithelial ridges, transverse to the meridional bands, and are so far apart that the free edges of one comb overlap the base of the next like shingles. In consequence of their fibrous structure the combs are strongly iridescent and in motion cause a beautiful play of metallic red, blue, and green over the meridional bands These combs act like oars and row the body about. Since the combs begin some distance from the aboral pole, they are connected with it by means of ciliated grooves following the line of the meridional bands. Experiment shows that the sense body is an organ of equilibration and for correlating the action of the different rows of combs. The ectoderm gives origin to two other important organs, two pole fields and two tentacles. The pole fields (fig. 209, pp) are two epithelial patches extending a short distance in the sagittal axis from the sense body and possibly are olfactory or taste organs. The tentacles arise, in the trans- verse axis, from the bottom of deep tentacular sacs (fig. 207, f-) from which they project as long cords with numerous lateral branches, and into which they may be retracted. Tentacles and branches contain an axial muscle, while the ectodermal coating consists largely of adhesive cells. These are spherical bodies (fig. 210) covered with a very sticky granular secretion, and, like a Vorticella, supported on the end of a spiral stalk muscle. These are used in capturing prey, which adheres to them and is drawn inward by the muscles. The ectoderm also forms part of the gastrovascular system. It turns inward at the mouth — situated at the lower end of the chief axis — and lines the large space commonly called stomach (fig. 207, /»), but which corresponds to the oesophagus of the Actinozoa. At the aboral end of this stomach begin the true ento- dermal portions, the so-called funnels, and from them run canals distributed through the jelly to the various organs. Two (rarely four) funnel canals run to the aboral pole and empty (fig. 209, to) near the sense body; a second pair, the paragastric canals (fig. 207, mg), which run parallel to the oesophagus, end blindly. The perradial canals (g) proceed outward from the funnel, and besides giving off a canal to the base of the tentacle, each divides dichotomously Twice, first into interradial and then into adradial canals, each of these last connecting with a meridio- nal vessel running just beneath a row of combs, nourishing them as well as the gonads. The gonads consist of two bands, one male, the other female, running in that wall of the meridional vessel nearest to the combs FIG. 210. — Ad- hesive cells of Ctenophora (after Samassa). SUMMARY OF IMPORTANT FACTS 235 These gonads are regular in distribution, those of two vessels which are nearest each other being of the same sex. The eggs and sperm pass out through the gastrovascular system. The few species are divided into TENTACULATA, with tentacles, and XUDA, without. To the first belong the CYDIPPID^C, with pear-shaped bodies (Pleurobrachia*), Hormiphora (fig. 207); the LOBAT^E (Mnemiopsis,* Bolina*), with lobes; and the band-like CESTID/E (Cestum, Venus girdle). The BKROHXE (Beroe, Idyia*), with wide mouth, belong to the Xuda. The small creeping Cceloplana and Ctenoplana, are supposed by some to form a transition to the Turbellaria. Summary of Important Facts. 1. The CGELENTERATA and Echinoderma were formerly called Radiata because in most a radial structure is present; in the higher groups this is replaced by biradial or even bilateral symmetry. 2. The Coelenterata are sometimes called Zoophyta (animal plants), from their appearance and their attachment. In many the resemblance is heightened by their formation of plant-like colonies by fission and budding. 3. The name Coelenterata was chosen because they have but one cavity, a simple or ramified digestive sac, representing at once the ali- mentary tract and the as yet undifferentiated body cavity. 4. This coelenteric cavity is called the gastrovascular system because its branches distribute nourishment to all parts and so perform the func- tion of blood vessels. 5. The reproduction is either sexual or asexual, very frequently cyclical (alternation of generations). 6. The animals are provided with nerves, muscles, and sense organs and possess marked sensibility and mobility. 7. Especially characteristic are the tentacles and small nettling organs, the cnida?, in special cells. 8. Nearly all histological differentiation proceeds from ectoderm or entoderm, since the mesoderm (mesoglcea) plays but a subordinate role and usually functions only as a support (Diploblastica). 9. Four classes — Hydrozoa, Scyphozoa, Anthozoa, and Ctenophora are recognized. 10. In HYDROZOA and SCYPHOZOA there are usually two alternating generations, the sessile asexual polyp and the free-swimming sexual medusa. 1 1 . The hydroid polyp and the craspedote medusa are characteristic of the HYDROZOA. 12. The hydroid polyp is a two-layered sac of ectoderm and entoderm, 236 CCELENTERATA a supporting layer and a circle of tentacles. In the colonial forms there is usually a cuticular envelope, the perisarc, secreted by the ectoderm. 13. The craspedote medusa is bell-shaped, with smooth bell margin, its aperture partially closed by a diaphragm-like velum; the gonads are ectodermal. 14. The medusa1 arise by lateral budding from the hydroid. 15. If the medusa remain attached to the parent as a sporosac, alterna- tion of generations is replaced by polymorphism; it can entirely disappear with the total loss of either hydroid or medusa. 16. The scyphostoma and the acraspedote medusa are typical of the SCYPHOZOA. 17. The scyphostoma differs markedly from the hydroid polyp in the presence of four longitudinal gastric folds or septa (tocnioke). 18. The acraspedote medusa lacks a velum, has a lobed umbrella edge, gastral tentacles (phacelke), and entodermal gonads. 19. The medusa arises from the polyp by terminal budding (strobila- tion). 20. Alternation of generations rarely is lost, and then only by suppres- sion of the scyphostoma. 21. The ANTHOZOA have only one form, the coral polyp; it is distin- guished from the hydroid polyp by the ectodermal oesophagus, the radial septa reaching the oesophagus; the well-developed mesoglcea and the gonads which, arising from the entoderm, early migrate into the meso- gloea. 22. Most Anthozoa are colonial and produce a skeleton (coral) usu- ally of calcic carbonate, but sometimes of 'horny' substance. 23. The skeleton may be either axial or it may be outside the indi- vidual polyps (cortical skeleton). 24. The living Anthozoa are divided according to the number of septa into Octocoralla and Hexacoralla. With the latter the fossil Tetracoralla are allied. 25. The Hexacoralla have numerous tubular tentacles and six, or a multiple of six, pairs of septa. 26. The Octocoralla have eight single septa and eight feathered ten- tacles. 27. The CTENOPHORA are always free-swimming and have a large mesoderm with numerous muscle cells. 28. Nettle cells are absent, and are replaced by adhesive cells. 29. Most characteristic are the eight meridional rows of 'combs' whose motions are controlled by a common organ, the sense body, a statocyst. SUMMARY OF IMPORTANT FACTS L>:;7 30. The digestive tract consists of an ectodermal oesophagus (stomach) and a branching system of entodermal vessels. VERMES. A large number of forms above the Coelenterates are frequently grouped as Vermes, but there is little agreement as to what smaller divisions shall be included, some denying the existence of a natural group of worms, and separating the groups as phyla. Others include not only the flat-, round- and segmented worms, the Chaetognaths and rotifers, but also the brachiopods, Polyzoa and even the tunicates. Yet such is the variety of form, structure and development that, no matter what limitation be accepted, it is impossible to frame a definition which shall include all of the species commonly known as worms. In taking the step from the diploblastic Ccelenterates to even the lowest 'worms' such advances in structure are seen that a brief review of these is appropriate here. The worms are distinguished from the Ccelenterates by bilaterality, which is seen in the internal structure, in even those cases (round worms) where it is not visible on the exterior. There is also a higher degree of differentiation of organs — -the development of a ganglionic nervous system, excretory organs, and frequently of a blood-vascular system. This advance is correlated with the appearance of a true mesoderm, the layer from which (the nervous system excepted) these organs and the muscles arise. Then there is the dermo-muscular sac, the cause of the familiar 'worm-like' motions. This consists of an intimate connexion of the skin with the underlying muscles (figs. 212, 240, 241). The skin, a one-layered epithelium, ciliated or covered with a cuticle, rests on either a structureless membrana propria or on a cellular connective tissue, to which the muscles are attached. Longitudinal muscles are always present, and fre- quently circular as well, while in the parenchymatous worms diagonal, crossed and dorsoventral fibres may occur. While certain worms (cestodes) lack an alimentary canal, or like many nematodes, may have a blind, functionless gut, these conditions are the result of parasitic habits. In the lower worms the digestive tract is like that of the Coelenterates, consisting of archenteron and stomodeum, proctodeum and anus being absent. In most worms the tract is 'complete,' it being a tube with mouth and vent. The digestive tract is either imbedded in the parenchyma and cannot be dissected out (fig. 212), or it is surrounded by the body cavity (ccelom) which separates it from the body wall (figs. 238, 241), the muscles of which are meso- thelial in origin, in contrast to the mesenchymatous muscles of the flat worms. In the flat worms the excretory organs are protonephridia, and similar organs with solenocytes (p. 105) are common in the larvas of the higher worms, being often replaced in the adult by true nephridia. Here, too, a blood-vascular system first appears. Where it is lacking (flat worms) its place may be taken by the branches of the digestive tract, or in higher forms by the ccelom with its albuminous fluids. The phylogenctic origin of the circulatory system may be considered here. Two explanations of the circulatory vessels have been advanced, (i) They are canals separated from the alimentary canal and developed farther independently; therefore they have an entodermal epithelium. (2) They are spaces filled with albuminous fluid, arising between entoderm and mesoderm (a modified form of this is that . are remnants of the segmentation cavity); hence at first they had no epithelium and obtained it later from the mesoderm. Lately the latter view is regarded as the more probable, since most observers deny the existence of an epithelium in the blood vessels of worms and other invertebrates. Then in many worms 238 PLATHELMINTHES a perigastric sinus, arising by a separation of the intestinal layers, forms a part of the circulatory system. The nervous system of the 'worms' has in common a pair of supra-cesopha- geal ganglia ('brain') which sends out two strong longitudinal cords (to which others may be added) which must be regarded as part of the central nervous system since they bear ganglion cells. These cords may be lateral or on either side of the mid-ventral line. In the latter case those of the two sides may be united at regular intervals, thus giving the ladder type (p. 113), the ventral chain being connected with the brain by cords on either side of the oesophagus. This nervous system, always ectodermal in origin, may be epithelial, forming part of the skin, or it may sink to different depths in the other tissues. Mi FIG. 211. — Trochophore (Loven's larva) of Polygordius (from Hatschek). A. anus; dLM, dorsal muscles; ED, hind gut; J, stomach; J,, intestine; Mstr, mesodermal band; n, nerves; Neph, protonephridia; O, mouth; Oe, oesophagus; oeLM, oesophageal muscle; SP, apical plate; vLM, ventral muscle; ilum* (fig. 64), Planaria,* and Polyscelis;* 'Phagocata* with divided pharynx. The tropical land planarians (Bipalium,* 10 or 12 inches long) have been introduced into greenhouses. Order III. Rhabdoccelida. Small, even microscopic, recalling in habits and appearance the Infusoria; alimentary canal rod-like, without branches. Vortex* (fig. 75), fresh water; Mono ps* Monoscelis* marine. The fresh- water MiCROSTonnxE reproduce almost exclusively by fission. Class II. Trematoda. These are exclusively parasitic, some living on the skin or gills (ecto- parasites) or in the interior of other animals (entoparasites). In structure they are closest to the triclad Turbellaria, from which they differ by characters, the direct result of their parasitic life. Thus they have lost the cilia or have them only in the larva. On the other hand, they are covered with a cuticle often with spines and with suckers and hooks for adhesion to the host. The suckers are shallow pits of columnar epithelium lined with cuticle and furnished with a thick layer of radial and circular muscles, which by their contraction increase the lumen of the sucker, the edges of which are closely applied to the host. At least one such sucker II. TREMATODA 243 is present; if but one or two (entoparasites), one is at the anterior end (oral sucker) surrounding the mouth, while a second larger sucker may occur near the mouth (fig. 217), but may be (Ampliistonmm) at the ]><»ir- rior end. In the ectoparasites there are a pair of anterior suckers near the mouth; at the posterior end a single sucker, or a number of suckers or hooks or both on a sucking disc (fig. 219). Other results of parasitism are the weak development of sense organs and brain and development of accessory ganglia near the adhesive organs. Eye spots (two to four) occur occasionally in the ectoparasitic species and in the larvae of the entoparasitic, rarely in their adult condition. The alimentary tract is forked (fig. 218) and occasionally (fig. 217) has dendritic blind sacs. -4-U -CJ FIG. 217. FIG. 218. FlG. 217. — Distomum hepaticuni, liver fluke (from Boas), m, ca?ca of ta, limbs of digestive tract; s^.,, anterior and posterior suckers. FIG. 218. — Distomum lanccolatum. c, cirrus, beneath it the opening of the oviduct; d, vitellaria, the ducts leading to the shell gland; g, ganglion; h, testes with ducts to cirrus; /, Laurer's canal; o, ovary, the shell gland behind it; 5', s", anterior and median suckers, the pharynx and the bifurcated digestive tract leading from s'; u, uterus; IL', terminal vesicle of water-vascular (excretory) system. To parasitism may also be attributed the great development of the sexual organs, which at maturity fill a great part of the body. Their features may be seen in fig. 218. Two vasa deferentia pass forward from the testes (//), unite and form a seminal vesicle. The terminal portion of the united ducts can be protruded as a penis or cirrus (c) armed with retrorse hooks. It is usually enclosed in a 'cirrus pouch.' The unpaired ovary (o) is very small and pro- duces small eggs, deficient in yolk; hence the paired vitellaria ( —>- ~ ^ of genital duct, vitelline duct, mav occur in thousands and cause severe injury and uterus is lacking in the figure. This species may develop without an interme- diate host; the eggs taken into the stomach pass the cysticercoid stage in its walls and then pass to the intestine to become adult. T. diminuta* which has insects for its intermediate host, has been described from man. B. Forms passing the cysticercus stage in man. Besides the cysticercus IV. NEMERTINA 255 cellulose of T. solium, found in man, more frequent and of more important e i- the cysticercus of Tcenia cchinococcus (fig. 232), which lives as an adult in the dog, and is easily overlooked on account of its size. It is at most £ inch long and consists of a scolex and three or four proglottids. When the eggs are taken into the human stomach, as may easily happen by stroking and kissing infected dogs, the embryos are set free and wander into liver, lungs, brain, or other organs and produce here tumors which, in the case of the liver, may weigh ten or even thirty pounds. This extraordinary size is explained by the formation of daughter bladders (echinococcus) described above. FIG. 234. — Heads and proglottids of three tapeworms of man. Left, Ta-nia sagi- nala; middle, T. solium; right, Bothriocephalus latus, flat and side view of head. The heads enlarged about six times, the proglottids about ii (after Leuckart, Braun, and Schauinsland). Common Tee-nice of domestic animals are in the horse Anoplocephala plicata (4 to 30 inches), A. perfoliata (\ to 3 inches), A. mamillana (J to 2 inches); in ruminants, Moniezia,-* in the dog, T cent a marginata* (cysticercus in sheep and swine), T. serrata* (cysticercus in rabbits), T. echinococcus (above), T. ccenurus (cysticercus in brain of sheep, causing the disease called 'staggers'), Dipylidium cucumerina* (most common, larva in the flea and dog-louse); in the cat, Tcenia crassicollis* (cysticercus in mice). Several species occur in domestic birds, one (Drepanidotcenia infundibuliformis*), causing epidemics among chickens. Others in ducks and geese. Class IV. Nemertini. Most nemerteans are of appreciable size, some reaching a length of a yard or more (Linens longissimus 90 feet !), and yet they are so contractile that our Cerebratulus lacteus, which can extend itself to fifteen feet, can retract to two. Nemerteans are rare in fresh water or moist earth, but are most abundant in the sea, where they burrow through the mud or lie rolled up beneath stones. Many are noticeable for their bright colors. Their systematic position is a problem. 256 PLATHELMINTHES Like some flatworms they have a solid parenchyma bounded exter- nally by a ciliated ectoderm rich in mucus cells, and inside this at least two muscular layers, an outer circular and an inner longitudinal layer. They differ from all other Plath.elminth.es in having a complete alimentary tract, beginning with a ventral anterior mouth and continuing as a straight tube, with, usually, paired diverticula, to the vent at the posterior end of the body (fig. 235). Especially diagnostic is the proboscis, which lies dorsal to the alimen- tary tract and usually opens in front of the mouth. The proboscis is a p ps pm Iv di FIG. 235. — Diagram of Nemertean forig.). b, brain; c, ciliated pit; d, dorsal nerve trunk; di, dorsal blood-vessel; g, gastric caeca; i, intestine; /, lateral nerve trunk; h>, lateral blood-vessel; p, proboscis retracted; pm, proboscis muscles; pr, protonephridial tube; po, its opening; ps, cavity of proboscis sheath. muscular tube closed at one end and at rest is infolded like the finger of a glove inside a closed sac, the proboscis sheath, which extends far back in the body. Its tip is bound to the posterior end of the sheath by a retractor muscle. By contraction of the sheath the proboscis is everted, while it may be retracted again by the muscle. Nettle cells are not un- common in the proboscis wall, while in some- forms (the older Enopla) the effectiveness of the organ is increased by the presence of a dart-like stylet at the tip (reserve stylets occur on either side, fig. 236), and at the base of the stylet is a poison sac. The blood-vascular system consists of a pair of lateral tubes connected by transverse loops, and in most forms a third tube is present lying between the intestine and the proboscis sheath. The blood is colorless; it rarely contains red or green corpuscles. The central nervous system (in some forms still in the ectoderm) con- sists of a supracesophageal brain of a pair of ganglia, from which nerves run to the proboscis, and two lateral cords united on the ventral side by numerous transverse commissures. Connected with the brain, either directly or by means of a short nerve, are the cerebral organs or ciliated grooves, pits on the sides of the head, formerly regarded as respiratory, are now considered sense organs. Tactile organs and simple eyes are IV. NEMERTINI 257 widely distributed; statocysts are very rare. The excretory system consists of two tubes lying beside the lateral blood-vessels and connecting with branches terminating in flame cells, while they open separately to the exterior by one or several openings. 4 -• l> As a rule the nemertines are dioecious, the gonads forming a row of lateral sacs, alternat- ing with the intestinal blind sacs and opening pn dorsally. The development is sometimes direct, but usually a metamorphosis occurs in which a larva, the pilidium (or a reduced / form of it, Desor's larva), appears. The . „ pilidium is a helmet-shaped larva with, right and left below, a pair of lappets (fig. 237). • Ps The margins of lappets and helmet are ciliated, while at the top a bundle of longer cilia pro- - d ject from a thickened patch of ectoderm, the cs --- d ffi/C m FIG. 236. FIG. 237. FIG. 236. — Amphiporus pidcher (after Burger), a, alimentary canal; h, brain; c, ciliated groove; d, dorsal blood-vessel; /, lateral blood-vessel; m, retractor of proboscis; n, lateral nerve cord; o, ovary; p, poison sac; pn, protonephros; pr, proboscis; /».•?, pro- boscis sheath; r, rectum; 5, stylet of proboscis. FIG. 237. — Pilidium larva (from Lang, after Salensky.) es, invaginations which later give rise to the nemerrine skin; m, oral lobes; md, archenteron; rn, ring nerve; sp, apical plate; st, oesophagus; ink, ciliated band. apical plate, which functions as a central nervous organ. Inside is the simple oecal archenteron, the mouth (blastopore) opening between the lappets. By a complicated process of growth and infolding this mesenteron becomes enclosed in a separate skin, produced from four inpushings (es) ; an anus is formed, and at the time of metamorphosis 17 258 PLATHELMINTHES the worm thus produced escapes from the rest of the pilidium, which quickly dies. Order I. Protonemertini. Nervous system outside the muscles; no stylets in the proboscis; mouth behind brain. Carinella.* Order II. Mesonemertini. Nervous system in the muscles; mouth behind brain; no stylets. Cephalothrix.* Order III. Metanemertini. Nervous system inside the muscles, mouth in front of brain; proboscis as a rule with stylets. Geonemcrtes* and some species of Tetrastemn.a* terrestrial. Am phi poms* (fig. 236, fresh water), Nectonemertes* Malacobdclla* leech- like, with posterior sucker, parasitic in lamellibranchs. Order IV. Heteronemertini. Several muscular layers; nervous system in the muscles; mouth behind brain; proboscis unarmed. Linens* Micrura,* Cerebratulus* Zygeupolia* Summary of Important Facts. 1. The PLATHELMINTHES are flattened bilateral animals with- out coelom, whose nervous system consists of a supracesophageal ganglion and lateral nerve trunks; the excretory system of branched protonephridia. 2. The TURBELLARIA are the most primitive; the Trematoda and Cestoda have descended from them. 3. The Turbellaria are ciliated externally. They have no anus and no circulatory system. The digestive tract consists of ectodermal pharynx and entodermal stomach, the latter many-branched in the Polyclads, with three main branches in the Triclads, and rod-like in the Rhabdocccles. 4. Polyclads and Triclads are often united under the name Dendro- coela. 5. In the parasitic TREMATODA the cilia are entirely lost or confined to the larval stages. Hooks and suckers are present for attachment to the host; several in the ectoparasitic forms; only one or two suckers in the internal parasites. 6. In the Distomitv there occur heterogony and alternation of hosts. From the egg arises a sporocyst, always parasitic in molluscs, from the parthenogenetic eggs of which develop cercarias which become encysted Distomiae in the second host, sexual Distomue in the third. 7. Best known of the Distoma are D. hepaticurn and D. lanceolatum (rare in man, common in sheep) and D. hccmatobium in the portal vein of man in warm climates. ROTIFKRA 259 8. The CESTODA have no digestive tract; scolex and proglottids are usually developed. 9. The scolex is the organ of attachment, and as such is provided with suckers and frequently with hooks. It also produces the proglottids by terminal budding. 10. The proglottids contain an hermaphroditic sexual apparatus. 11. The eggs produce a six-hooked embryo which must pass into an intermediate host, either by taking the eggs with the food, or the embryo must pass into the water, where it infects fishes. 12. The embryo, in the intermediate host, becomes encysted and changes directly to a scolex (pleurocercoid) or into a bladder worm (cysticercus) which prcduces internally one or more scolices. 13. The scolex is freed from its cyst when taken into the stomach of the proper host, and then can develop into a tapeworm. 14. In man occur as cysticerci Tccnia echinococcus (adult in dog) and T. solium; as adults Tania solium (cysticercus in pigs), T. saginata (cysti- cercus in cattle), and Bothriocephalus latus (pleurocercoid in fish). 15. The NEMERTINI have a complete alimentary canal with anus, blood-vessels and a proboscis dorsal to the digestive tract. PHYLUM V. ROTIFERA (ROTATORIA). The aquatic rotifers or wheel animalcules are among the smallest Metazoa, and can be distinguished from the Infusoria, which they resemble in habits, only by the microscope. The body is divisible into three regions, head, trunk, and tail. The trunk is covered by a tough cuticle into which head and tail can FIG. 238. — Diagram of rotifer (after Delage et Herouard). b, brain; fc, flame cell; gg, gastric gland; i, intestine; m, mastax; ov, ovary; pg, pedal gland; pi', pulsating vesicle of excretory system; s, stomach. be retracted. The tail or 'foot' is often composed of rings which can be tele- scoped into each other. The last tail ring often bears a pair of pincer-like stylets by which, together with adhesive glands, the animal adheres to objects. The head is expanded in front to a trochal disc, an apparatus of varying shape, surrounded by a ring of cilia of use in swimming and in directing food to the ventral mouth. The alimentary canal consists of oesophagus, mastax (chewing 260 CCELHELMINTHES stomach), glandular stomach, and intestine; all except the mastax ciliated. The mastax bears two chitinous jaws (trophi), which in life are in constant motion and comminute the food. Above the oesophagus is the cerebral ganglion with which simple eyes and peculiar sense organs, the cervical tentacles, are frequently connected. The usually single ovary and the paired protonephridia empty into the posterior part of the alimentary canal, which thus becomes a cloaca. The males are much rarer and smaller and have a much simpler structure (fig. 239, 5). Usually the alimentary tract is reduced to a solid cord in which the testes are imbedded. PIG. 239. — Brachionus urceolaris. A, female with four eggs in various stages; B, male; C, 'flame' from protonephridia, greatly enlarged; b, urinary bladder; c, cloacal opening; (/, gastric glands; g, ganglion, with eye; /;, testis; k, mastax; m, stomach; o, ovary; p, penis; /, tentacle; iv, protonephridia. The Rotifers have large winter eggs enclosed in a thick shell and smaller thin-shelled summer eggs. The latter develop parthenogenetically and by their numbers and rapid growth aid in the distribution of the species. The winter eggs require fertilization, and have a long resting period, thus serving to tide over periods of cold or drought. The adults can withstand a certain amount of desiccation; and often occur in damp moss or in eave troughs in a sort of sleep from which they are awakened by water. In structure the Rotifiers are much like the trochophore larva; of annelids and molluscs to be described later. They are primitive forms, connected with the ancestors of these groups, and also, as sho\vn by nervous system and excretory organs, with the flatworms as well. Most species are cosmopolitan and in- habitants of fresh water. Near the Rotifera may be placed the fresh-water GASTROTRICHA (Ichtliydiiim* Clurttmotits*) and the marine ECHINO- DERID^E, forms which are little understood. PHYLUM VI. CCELHELMINTHES. The Ccelhelminthes are distinguished from all the forms which have gone before by a body cavity, separating the outer body wall from the OELHELMINTHES 201 intestine; but whether this coelom be homologous in different groups, e.g., ncmatodes and annelids, is not settled. The body muscles are developed from the outer (parietal) epithelial wall of the coelom and hence are 'epithelial muscle cells' (figs. 240, 241). The pro- FIG. 240. — -Section of A scaris lumbricoides through the pharyngeal bulb; beside it a bit of the body wall more enlarged, c, cuticle; d, dorsal line; h, hypodermis; ;«, long- itudinal muscle; n, nucleus of muscle cell; p, muscle cell; 5, lateral line; v, ventral line: w, excretory canal. 0... * -m : ^tlllJPt 5 more FIG. 241. — Transverse section of Sagitta bipuncta'.a and a bit of the body wall m^ic enlarged (after O. Hertwig). c, coelom; dd, entoderm; df, splanchnic mesothelium; e, epidermis; m, somatic mesoderm (muscles and epithelium); o, ovary. tonephridia of the larval stages are replaced by nephridia (fig. 71), connecting the body cavity with the outer world. Internally they begin with a ciliated funnel, the nephrostome, and continue as long coiled tubes, expanding just before the outer end to a kind of bladder. The gonads 262 CCELHELMINTHES (fig. 241, 0) are specialized parts of the coelomic epithelium and their prod- ucts are usually carried to the exterior by the nephridia (more rarely by special ducts), so that here, as in vertebrates, we can speak of a urogenital system. A closed blood system is now present, now absent. Little in general can be said of the nervous system; details will be given in connection with the separate classes. sc f \A ovd — -fl -ov Class I. Chaetognathi. These marine forms, a half to two inches long, per- fectly transparent, live at the surface of the sea, preying on other animals, and from their shapes and rapid motions deserve the name Sagitta — arrow — given some forms. The animals swim by means of horizontal fins, one sur- rounding the tail and one or two pairs on the sides of the trunk (fig. 42). On either side of the mouth are strong bristles used in seizing prey (Chaetognath, bristle-jaw). Internally the body is separated into head, trunk, and tail, by transverse septa which divide the ccelom into corresponding chambers. Each segment of the coelom again is divided into right and left halves by a mesen- tery (fig. 241), supporting the straight intestine, running lengthwise through it and terminating at the anus at the end of the trunk segment. The nervous system is entirely ectodermal. In the head is a pair of fused cerebral ganglia (fig. 243), in the trunk segment a large ventral ganglion, and these are connected by long oesophageal commissures. Of inter- est, because characteristic of nematodes and many an- nelids, are the relations of the musculature, which FIG. 242. FIG. 243. FlG. 242. — Sagitta hexaptera, ventral view (after O. Hertwig). a, anus; bg, ventral ganglion; d, intestine;//, fin; ho, testes; »;, mouth; oi1, ovary; m>d, oviduct; sb, seminal vesicle; sc, cesophageal commissure; sfl, tail fin; si, sperm; wo, female opening. FIG. 243. — Head of Sagitta bipunctaia, dorsal view (after O. Hertwig). an, nerve to au, eye; g, brain; gh, bristles; rn, nerves to ro, olfactory organs; sc, cesophageal commissure. consists of longitudinal fibres alone. The body cavity is lined with epi- thelium (fig. 241), which, where it abuts against the alimentary tract, is II. NEMATHELMINTHES: NEMATODA 263 called splanchnic mesoderm; that on the side of the coelom towards the ecto- derm is the somatic mesoderm. The muscles arise from the latter and are divided into four fields, right and left dorsal, right and left ventral. The sex cells also arise from the epithelium of the ccelom, the eggs in the trunk segment (fig. 241), the sperm in the tail. The eggs are carried to the exterior by special ducts. The sperm-forming cells early fall into the ccelom, where they develop the spermatozoa. These are carried out by canals which recall the nephridia of the annelids. The development of Sagitta is significant from two points of view. The archenteron (fig. 109) is divided by lateral folds into an unpaired middle portion and two paired lateral chambers; the first is the definitive digestive tract, the latter the anlagen of the coelomic diverticula. In other words, the ccelom is an outgrowth from the archenteron, i.e., is an enteroccele. Second: The gonads are derived from a pair of cells in the primitive entoderm, which later are carried into the coelomic walls. Here each divides into anterior and posterior cells, the anterior developing into the ovary, the posterior into testes. Hence here the male and female sex cells are beyond doubt descendants of a common mother cell. The few species are arranged in two or three genera, of which Sagitta, represented on our coasts by S. elegans,* is best known. Spadclla. Class II. Nemathelminthes. Like the flatworms, the roundworms are characterized by their shape, they being thread-like or cylindrical animals whose form is the result of the existence of a body cavity in which the viscera are so loosely held that on cutting through the muscular body wall they will fall out (fig. 244). Since the Nemathelminthes share this coelom with most annelids, the distinction between the two rests largely upon negative characters, the roundworms lacking the segmentation of the body cavity and the corre- sponding ringing or annulation of the body wall. The body cavity apparently is different since the splanchnic wall is lacking, the space lying between mesoderm and entoderm (pseudocccle). To the Nemathel- minthes belong three orders, much alike in habits and appearance but differing considerably in structure. Of these the most important are the nematodes. Order I. Nematoda. The nematoda contain numerous species of thread-shaped worms varying from o.ooi to i.o metre in length, many of which, through their wide distribution as parasites in plants, animals, and man, possess special interest. The outer surface is covered by a tough cuticle secreted by the subciiticiila, a fibrous ectodermal syncitium (fig. 240), which in cross- section shows four thickenings, the dorsal, ventral, and lateral lines. In the lateral lines run the excretory vessels, two longitudinal canals, united near the head by a transverse vessel opening on the ventral surface by an 264 CCELHELMINTHES unpaired pore to the exterior. They are related to the coelom by two giant cells on either side which send processes into the body cavity. These lateral and median lines divide the muscles (here only longitudinal) into four fields. These muscles are parts of the somatic epithelium, a layer of vesicular cells which by their size (fig. 240) so encroach upon the ccelom that scarce space is left for the alimentary canal and reproductive organs. \.n FIG. 244. FIG. 245. FIG. 244. — Structure of young female A scar is (based on a drawing by Leuckart). d, intestine; o, ovary; p, pharynx; s, lateral line; v, ventral line; va, vagina. FIG. 245. — Diagram of nervous system of a nematode (after Biitschli). c, com- missures; ii, dorsal nerve; i, infracesophageal, s, supracesophageal part of nerve ring; v, ventral nerve. The alimentary canal begins with a terminal mouth and ends with the ventral anus in front of the end of the body. The mouth connects with the muscular sucking oesophagus, which is expanded posteriorly to a pharyngeal bulb and is lined throughout with a cuticle. From this point to the anus the stomach-intestine is usually uniform (fig. 244). The oesoph- agus is surrounded by a nervous ring which sends forward and back a large II. NEMATHELMINTHES: NEMATODA :>»J.> number of nerves, those in the mid-dorsal and ventral lines being strongest. At points on these nerves are collections of ganglion cells, but a formation of ganglia, as in the annelids, does not occur (rig. 245). The only sense organs are tactile papilla; near mouth and genital opening, and eyespots in a few free living forms. The sexual organs of these rarely hermaphroditic forms are very simple. Males and females are easily distinguished, not only by the copulatory organs, but by the openings of the genital ducts. These, in the male (fig. 246), are in the end of the alimentary canal, which hence is a cloaca. In the female (fig. 244) there is a special genital opening on the ventral surface between mouth and anus, the position varying with the species. In general the structure of the reproductive organs is alike in both sexes. These are long tubes coiled forward and back and ending in line threads which produce eggs or sperm (ovaries, testes), while the rest serves as seminal vesicle, or receptaculum seminis, and ducts. In the male the genital tube is always single; in the female it is usually double, the right and left halves uniting a little before the external opening (fig. 244, va). Most common of copulatory organs in the male are spicula, bent spines, which lie in a sheath behind the vent and can be protruded through tfie cloacal opening. Besides there may be valves to right and left to clasp the female, or, as in Trichina, the whole cloaca is protrusible. Since there is copulation, the eggs are fertilized in the uterus, after which they are either laid or retained for more or less of their development, many, like Trichina, being viviparous. The postembryonic development depends largely upon the mode of life. Free-living species grow by repeated molts without much change of form. In many Anguillulidae, which show how free life can be transformed into parasitic, there is an alternation of generations (heterogony) from a protandric hermaphroditic entoparasitic to a free dioecious generation. The occasional suppression of the free generation which occurs in many Anguil- lulids leads to the Strongylidas, where the offspring of the parasitic generation can live free for a time (rhabditis larva?), but must return to parasitism to undergo a metamorphosis and become sexually mature. The free life is shortened again in the Ascaridas, where the eggs must pass to the exterior for a longer or shorter time, but the embryos only escape when the eggs are taken into another host. Lastly, there are species like Trichina where the free life is entirely suppressed and transportation from host to host takes place in the encysted condition pas- sively by food. This purely parasitic condition leads to species in which the rhabditis larva developed in water, enter a second host for encystment as the larvce of Filaria medinensis in the Cyclopicke. Family i. ANGUILLULIDAE; small thread-like nematodes which live in mud, organic fluids or plants, rarely in animals; male with two spicula. Angitillitla aceti* vinegar eel, in vinegar and stale paste. RliabJitis (Rhabdoncnia) nigro- venosa, lives in mud and stands in heterogony with a second form which lives in the lung of frogs. Strongyloides intestinalis, which has recently appeared in southern Europe, has a somewhat similar history, the adult stage being reached in the human intestine. In the tropics one stage of this is passed in moist earth, but in colder climates the free-living generation drops out. Here belong 286 CCELHELMINTHES Tylenchus tritici and Hcterodera schachti, the first doing great damage to wheat, the second to turnips in Europe. T. devastatrix attacks rye and hyacinths. Family 2. ASCARID^E. Mouth with three lips; males with two spicules. Nu- merous species in lower vertebrates, Ascaris lumbricoides,* the round worm of man (fig. 246), inhabits the small intestine, often in enormous numbers. The females are. about 5-6 inches, the males 4 inches in length. A female contains about 60,000,000 eggs. Shortly after fertilization the eggs pass out with the faeces, but develop without intermediate host if, in the course of two or three FIG. 246. — Dorsal, end, and ventral views of head and hinder end of male Ascaris lumbricoides (from Hatschek.) months, when . the embryo is formed, they are taken into the human intestine. The development of the pinworm, Oxyuris vermicular is*- is some- what similar except that the embryos are developed in the egg at the time of oviposition, and hence after a shorter stay outside the body are capable of in- fection. The white worm, not half an inch long, lives in the rectum, especially of children, and causes intolerable itching. Ascaris mystax* occurs in dogs and cats (occasionally in man). A. megalocephala* (a favorite animal for cytological researches) and Oxyuris equi in the horse, do little harm. Heterakis maculosa often destroys whole flocks of pigeons. Family 3. STRONGYLUXE. o FIG. 247. — Anterior end of hook worm, Ankvlostoniii duodenale (after Looss). di, lower teeth; g, lateral gland; m, oral capsule; p, dorsal tooth; o, oesophagus; v, ven- tral teeth. These are readily recognized by the bursa of the male, a broadening of the hinder end of the body by two wing-like processes, which contains two spicula. Frequent but not constant is a widened capsule surrounded by papillae at the mouth. Strongyltts* in domestic animals. Syngamus frachealis,* half to three quarters of an inch in length, the male and female always in pairs, cause the disease known as 'gapes' in fowl. Ankylostomum (Dochmius) duodenale* (fig. 247), about two fifths of an inch in length, lives in the small intestine of man, causing severe loss of blood. The eggs develop in moist earth, and hence people who II. NEMATHELMINTHES: NEMATODA L'i',7 drink muddy water (Fellahin of Egypt) or work much with clay (potters and brick-makers) are especially subject to infection. It was first known in Egypt; caused considerable trouble during the building of the St. Gotthard tunnel in Switzerland. More recently it has been recognized as frequent in our southern states, where it has become notorious under the common names of hook-worm and lazy worm. It has been thought that the Ankylostoma larvas obtain entrance to man through the skin, as in bathing, etc. Family 4. TRICHOTRACHELID^:. These are called 'hair necks' because that part of the body which contains the pharynx is hair-like and elongate. Trichocephalus dispar* of man (fig. 248, A), about an inch or an inch and a half in length, lives with its neck in the wall of the intestine near the caecum. Since it does not move, it causes little injury. ,->Sj'.-^*r '/.-- ip%>^ A C FIG. 248. — A, Trichocephalus ilispar, male with anterior end embedded in intestinal wall (from Leuckart). B, Trichina spiralis, male (.from Hatschek). cl, cloaca; t. testes. C, Trichina in muscle (from Boas). Trichina spiralis* (fig. 248, B, C), is much smaller, but much more dangerous. Two stages are to be distinguished, the encysted muscle Trichina and the sex- ually mature intestinal Trichina. The first was discovered in a human body in 1835; the latter was not known until much later. In the encysted stage it occurs in the muscles of pigs, rats, mice, man, rabbits, guinea pigs, dogs, etc. (never in birds), enclosed in an oval capsule about o. 4 to o. 6 mm. long and hence recognizable by a practised observer with the naked eye. Certainty in their recognition demands a low power of the microscope. Coiled up in the capsule is the worm, about i mm. long, which is not yet sexually mature. To attain this it must be transported into the intestine of another host. When, for instance, man eats trichinosed pork the worms are freed by the digestive fluids and, enter- ing the small intestine, become sexually mature in a few days. The female (3-4 mm. long, the male 1.5 mm.) penetrates the intestinal villi and in course of a month gives birth to 1500 (some say 10,000) living young, after which she dies. The young enter the lymph vessels, are carried by way of the thoracic duct into the blood-vessels, and wander into the muscles, especially those which are much worked, like the diaphragm, eye muscles, and muscles of the neck, and which consequently have a rich blood supply. They enter the sarcolemma of the muscle, destroy the muscle substance, and finally become enclosed by a capsule secreted by the host. The wandering takes place about the second or third week after infection, the encystment in about three months. A slight infection causes disagreeable symptoms; but where large numbers obtain entrance the cases are frequently fatal. The worst epidemic known was in 268 CCELHELMINTHES Emmersleben, Saxony, in 1884, where 57 died in four weeks from infection from one pig. Family 5. FILARIID.E. Extremely elongate, hair-like worms. Dracuncnliis medinensis, the guinea worm (the female about a yard long, and about as large as stout packing twine), produces abscesses beneath the skin in which the worm is coiled up. The embryos break through the wall of the mother and must enter the water and penetrate a small crustacean, Cyclops. It is appar- ently introduced into the human system by swallowing the Crustacea with drink- ing water. The worm occurs in tropical America. Filaria sanguinis hominis, 3 to 6 inches long, lives in the lymphatic glands of man, the young escaping into the blood, often in immense numbers. They often pass through the kidneys, where they produce serious disturbance. There is possibly a connection between them and elephantiasis. The intermediate host is apparently the mosquito. As yet they are known only in the tropics. Other species occur in man and other animals. Family 6. MERMITHID/E. Elongate nematodes in the body cavity of insects; they pass into damp earth, where they become sexually mature. They share with the Gordiacea the name 'hairworms.' Mermis* Order II. Gordiacea. The hairworms resemble the nematodes in general appearance, but differ greatly in structure. The body cavity has both splanchnic and somatic epithe- lium; the intestine is supported by mesenteries (fig. 249) ; there is an cesophageal FIG. 249. — Transverse section of young Gordius (after von Linstow). a, hypo- clermis; b, muscular layer; c, cuticle; d, parenchyma; e, f, muscles and mesenteries; g, alimentary canal; h, nervous system. nerve ring and unpaired ventral nerve cord, and the female genitalia open into the cloaca. The adults live in water, where they lay their eggs; the larva? live in insects, there being in some cases an alternation of hosts. They are popularly believed to be horse hairs changed into worms. Gordius* Chordodes* Near the Gordiacea must be mentioned the marine Nectanema,* young stages appar- ently passed in the mosquito. Order III. Acanthocephala. The adult spine-headed worms live in the alimentary canal of vertebrates. They resemble the Ascaridae (p. 266), but are easily distinguished by the pro- III. AXXKLIDA 260 r*K, nt-4 ••< ffi boscis, which may be retracted by muscles and exserted by contraction of the muscular body wall. This proboscis bores into the intestinal wall ami is held in place by numerous retrorse hooks (fig. 250). 1 he entire absence of an alimentary canal marks them off from Xematodes and Gordiacea, as also the peculiar structure of the reproductive organs and a closed vascular system in the body wall which extends into two sacs, the lemnisd, lying beside the proboscis sheath. The unpaired ganglion lies on the proboscis sheath between the lemnisci. An intermediate host occurs in development, the larva living in an arthropod. Thus the larva of Echinorhynchus gigas* of the pig lives in the larva of the 'June bug' (Melohntlui), that of E. proteus of European fresh-water fishes in Crusta- cea. E. hominis is extremely rare in man. The Acanthocephali are dioecious. The ovaries of the female early break up into groups of eggs which float in the body cavity. The ripe eggs have a peculiar method of escape from the body. There is a muscular uterus which connects by two narrow canals with the vagina and thus with the outer world. The uterus picks up immature and fertilized eggs indiscriminately by its wide mouth, but only those which are elongate, have a shell and contain embryos can pass the canals; the immature eggs are led through a ventral opening back to the ccelom. In E. gigas protonephridia open beside the genital opening. 7, Class III. Annelida. The metamerism, which occurs in a slight degree in the Chaetognathi, reaches its highest development in the Annelids, where it appears both in the outer ringing of the body and in the arrangement of the most important systems — excretory organs, nervous system, blood-vessels — internal segmentation. To this is added an extraordinary increase in number of body segments (somites, metameres), which can far exceed a hundred. The epithelial longitudinal muscles are reinforced by an outer layer of mesen- chymatous circular fibres. We can thus define the Annelids as worms with ccelom and with external and internal segmentation. In the development there frequently occurs a type of larva, the trocho- phore (p. 338). The above account applies most closely to the Chsetopoda and Archianellida. In other forms one may be lacking — in the Gephyrsea segmentation of Hirudinei most of the ccelom and the trochophore. -vet \--dr FIG. 250. — Male Echinorhynchus angu- status (from Hatsch- ek). b, penis sac; de, seminal vesicle; dr, glands; £, gan- glion; /, lemnisci; lig, ligament; >«,;«,,, re- tractors of proboscis and its sheath; />, pL-nis; r, proboscis; rs, proboscis sheath ; t, testes; ?•, nephridium; 07', ovary; rm, circular muscles; tm, transverse muscles; tr, nephrostome; vc, ventral cirrus; vd, vv, dorsal and ventral blood-vessels; vp, neuropodium. and retina (fig. 84, 1, II) . Statocysts are rare, but occur in diverse groups. Ciliated pits (olfactory ?) occur on the head, goblet organs (taste) on head and trunk, and lastly, lateral organs, sensory structures of unknown function, may be metamerically arranged. 272 CCELHELMINTHES The blood-vessels usually are represented by two main trunks which fre- quently (as in earthworms) contain blood colored red by haemoglobin. One trunk is dorsal, the other ventral, to the intestine, the two being connected by vessels (figs. 251, 255) in each segment. The blood passes forwards in the dorsal vessel, backwards in the ventral. It is propelled by con- tractile portions of the vessels; usually the dorsal vessel pulsates, but as in the earthworms, certain of the circular vessels in the anterior part of the body may function as hearts (fig. 255, c ). Rarely, as in the Capitellida?, circulatory organs may be lacking. dg Ig a oe ph st gc I b CO pt i-g p b < FIG. 255. — Anterior end of Pontodrilus marionis (after Perrier). a, vascular arches; b, ventral nerve chain; c, 'hearts'; co, oesophageal commissure; dg, dorsal blood-vessel; ds, septa; gc, cerebrum; I, retractors of pharynx; Ig, lateral blood-vessel; o, ovary; oe, oesophagus; p, receptacula seminis; ph, pharynx; pt, ciliated funnels of vas deferens; s, nephridia; 5/, pharyngeal ganglion; vd, vas deferens. The excretory organs (nephridia) were formerly known as 'segmental organs,' since they occur in pairs in each segment. Strictly speaking, each organ belongs to two segments. It usually begins in the anterior of the two with a ciliated funnel (nephrostome), passes through the septum, and, after convolutions, opens to the exterior in the second segment. The nephridial canal (usually lined with ciliated epithelium) often serves to carry off the sexual products, which in all chtetopods, arise in the ccelomic epithelium. The nephridia of Annelids seem to be derived from protonephridia (fig. 256, I, II), which finally opened into the ccelom (III, IV). In many species they are simple or branched tubes, closed internally by solenocytes, large cells drawn out into a tube which empties into the blind end of the excretory tubule and has a flagellum in the interior (fig. 69, p. 106). With the development of the nephrostome the branched condition and the solenocytes are usually lost. The relations of the nephridia to the sexual products appear to be secondary, and are brought about by large ciliated grooves of the peritoneal epithelium. There are three possibilities, (i) The sexual products are emptied by dehis- cence of the body wall; the ciliated organs are phagocytic. (2) The ciliated grooves at the time of sexual maturity open directly to the exterior and carry off the eggs and sperm (I and III). (3) They connect with the excretory tubules, be these nephridial or protonephridial (II or IV), the segmental III. ANNELIDA: CtL-ETOPODA 273 organs thus becoming sexual ducts. The second of these conditions explains the coexistence of reproductive funnels and nephridia in the genital segments of many oligochastes. In the oligochaites there are other modifications: Opening of the nephridia into fore or hind gut, connection of the tubules into a network with several openings to the exterior in each segment (Megascolicida,-). In many marine annelids there occurs a metamorphosis in which pelagic larvae occur. These, in spite of many modifications, are com- parable with 'Loven's larva,' the trochophore already described (p. 238). II. FIG. 256. — Different relations of nephridia and sexual ducts in chastopods (after Goodrich). I, hypothetical primitive condition; II, Phyllodoceids and Goniads; III, Dasybranchus; IV, Syllids, Spionids, etc. In I and III the ciliated grooves (sexual sacs, g) lead the sexual products (ei) direct to the exterior; in II and IV they empty into the nephridial canals, which are either protonephridia (I and II) covered with solenocytes, or (III and IV) are nephridia. The differences largely consist of modifications of the ciliary apparatus; the number of bands may be increased (polytroclie larvae), or a single band may occur at the middle (mesotroche) or at the end (telotroche) of the body. The larva becomes a segmented worm by the hinder end of the larva growing out and dividing into segments (fig. 257, B}. In this growth new mesoderm develops as a pair of bands (usually from a pair of cells at the hinder end, the teloblasts). This mesoderm divides, from in front back- ward, into the primitive segments. Each of these become hollow, forming a coelomic chamber. Since these, right and left, grow around the diges- tive tract, they give rise to the somatic and splanchnic mesothelium and form part of the digestive and body walls. Where they come in contact above and below the intestine they form the mesenteries which frequently is 274 CGELHELMINTHES disappear in the adult. In many worms the septa between the somites also breaks down and the coelomic cavities unite into one. The nephridia also arise independently of the protonephridial system, which is often called head kidney because the chief part of the trochophore forms the head of the adult. B kn mes mes FIG. 257. — A, larva of Polygordius; B, same changing to segmented worm (after Hatschek). a, anus; kn, excretory organ; mes, segmented mesoderm. The land and fresh-water annelids develop directly, but the embryos pos- sess a reminiscence of a larva in that the head lobes are very apparent and contain protonephridia, which leads to the conclusion that these animals earlier had a metamorphosis. From the resemblance of the trochophore to the Rotifera the farther conclusion is drawn that the annelids have descended from rotifer-like ancestors, the body cavity, nephridia, blood-vessels, and ventral nerve chain being new formations. Besides sexual reproduction many fresh-water and marine species reproduce asexually, this being possible from the great homonomy of the segmentation. By rapid growth at the hinder end as well as at a more anterior budding zone numerous somites are formed, which separate in groups from the parent to form young worms. In some cases the forma- tion of new somites may take place more rapidly than the separation, the result being chains of worms (fig. 258) which in some instances branch. By a combination of sexual and asexual reproduction a typical alternation of generations occurs, the origin of which receives light from the following facts: III. ANNELIDA: CH^TOPODA 275 In many polycha^tcs which reproduce exclusively by the sexual process the srx- less, slowly-moving young (a take) becomes so altered at sexual maturity as to have been described under another name. It becomes very active in its move- ments, and the hinder somites, which contain the gonads, develop special bristles and parapodia (fig. 263, A). Thus many species of Nereis pass into the ' Hcter- onereis' stage. In other Polychrctes the sexual part (epitoke) separates from the sexless atoke portion and swims freely, while the atoke produces new epitokes. In Samoa Eunice viridis reproduces in this way, the epitokes coming to the sur- face at certain times in incredible numbers, forming the 'palolo worm,' a delicacy in the Samoan diet. In still other species the epitoke regenerates the head and thus becomes an independent generation. Syllis and Heterosyllis are thus related. The Autolytidas are most complicated. Here the atoke, by FIG. 258. 259- FIG. 258. — Budding in Myrianida (after Milne-Edwards). The sequence of letters shows the ages of the individuals. FIG. 259. — Arrangement of a bristle in an Oligochaete (after Yejdowski). e, epithelium; rm, Im, circular and longitudinal muscles; m, muscle of the follicle; bl, chseta follicle, its chaeta in function; b2, follicle for replacement, the formative cell at its base. budding as in Myrianida (fig 258), forms chains of dimorphic individuals which later separate. The individuals of male chains (fig. 263) were formerly de- scribed as 'Polybostrichus,' the females as 'Sacconereis.' This same homonomy explains the regenerative powers of many worms. Thus if certain earthworms be cut in two, they will live and reproduce the lost parts. Another important character of the ChcTtopoda is the possession of bristles or c/nclcc. These arise in special follicles, singly or several in a group, there usually being four groups — right and left, dorsal or lateral and ventral — in each somite. Each follicle (fig. 259) is a sac of epithelium open- ing to the surface and having at the base a special cell for the development of each bristle. The developed chaetse project from the follicle and, moved by appropriate muscles, form small levers of use in locomotion. Their num- bers, shape, and support are of much systematic importance. 276 CCELHELMINTHES K Order I. Polychaetse. The Polychsctae owe their name to the fact that each group of bristles contains many chaetae; but more important is that the bristles of each side are supported by a fleshy outgrowth of the somite, the parapodium, in which two por- tions corresponding to the bunches of bristles — dorsal, notopod'mm; ventral, neuro- podium- — may be recognized (fig. 254). This is the first appearance of true appen- dages, but they differ from those of Arthro- poda in not being jointed to the body nor jointed in themselves. On the dorsal sur- face may occur diverse outgrowths, known, according to position or function, as cirri, elytra, gills, etc.; on the head, palpi and tentacles. The cirri are long processes on the parapodia, and like palpi are tactile (fig. 254). Elytra are thin lamelke which cover the back like shingles and thus protect the body (fig. 262). Nearly all Polychaetes are dioe- cious and undergo a more or less pronounced metamorphosis; with few exceptions (M anyimkia* from the Schuylkill, Nereis* in California) they are marine. They are usually FIG. 260. — Head with pro- truded pharynx of Nereis •versipedata (after Ehlers). c, cirri, k, jaws; I, head with eyes; p, palpi; t, tentacles. FIG. 261. FIG. 262. FIG. 261. — Amphitrite oriiata* (from Verrill). FIG. 262. — Head of Polynoe spinifera (after Ehlers). Back entirely covered with elytra; cirri and parapodia projecting at the sides. divided according to their habits into fixed (Sedentaria) and free forms (Er- rantia). The Sedentaria feed on vegetable matter, usually form leathery III. ANNELIDA: CrL£TOPODA .1 1 organic tubes in which foreign matter may be incorporated or which may be calcified. The worms project their anterior segments from the tubes. The Errantia often burrow, but from time to time swim about preying on other ani- mals. Correlated with habits are differences in structure. In the Errantia the head and trunk are not very different; the anterior part of the alimentary tract can be protruded as a proboscis, and, corresponding to their predaceous habits, is often armed with strong jaws (fig. 260). The Sedentaria have no such teeth, but there is a greater difference between anterior and posterior somites, In the latter the parapodia are weakly developed, and this part resembles the Oligocruetes in appearance. The head and beginning of the trunk (thorax) are richly provided with gills and tentacles for respiration and taking food (fig. 261). B FIG. 263. — New England Annelids (from Emerton and Verrill). A, male Autolytus; B, Sternaspis fossor; C, Cistenides gouldii; D, Clymene torquata. Sub Order I. ERRANTIA. Predaceous annelids with strongly armed pharynx. The EUNICIDJE, mostly represented on our shores by small species, contains some species a yard in length. Diopatra,* Nothria.* ALCIOPID^E, transparent, pelagic, with large, highly developed eyes (fig. 84). The SYLLID/E usually have three long tentacles; Autolytus* (fig. 263), Myrianidd* (p. 275). The POLYNOID/E* (Lepidonotus,* Polvnoe?' (fig. 262), are bottom forms with elytra. NEREIDS; Nereis virens* clam worm of all northern seas. Sub Order II. SEDENTARIA (Tubicola). These cannot wander, but live in tubes. SABELLID/E, tube is membranous and there is a crown of gills; Myxicola,* Chone,* Manyunkia* SERPULID^;, tube calcified and closed by an 278 CCELHELMINTHES operculum on one of the gills. Hydroidcs;* Proiula* ARENICOLID^E,* burrow in sand, have gills on the sides of body. MALDANID^E (Clymene,*fLg. 263) have extremely long segments and build tubes of sand. TEREBELLID^E (Tcrebella* Amphitrite (fig. 261), numerous filiform tentacles and branched gills on the anterior end. The ARCHIANELLID/E, which lack bristles and parapodia, must be placed near the Polychaetae and are usually regarded as very primitive forms which in structure and development (fig. 257) are of importance in the phylo- genesis of the Annelids. Polygordius* Order II. Oligochaetae. The Oligochsetes are almost as preeminently fresh-water and terrestrial forms as the Polychuetes are marine. They are in most respects simpler than their marine relatives, apparently the result of degeneration. Eyes are rudimentary or lacking, there are no palpi, cirri, or tentacles; gills are rare, but most striking FIG. 264. — Aulophonis vagus* in tube of Pectinatella statoblasts (after Leidy). FIG. 265. — Sexual organs of Lunihrlcus agricola (from Lang, after Vogt and Yung). The seminal vesicles of the right side are removed, bm, ventral nerve cord; bv and bl, ventral and lateral rows of setae; st, receptacula seminis; sb, seminal vesicles of .the left side, connected with a median unpaired seminal capsule (sbii). Enclosed in the latter are the testes (/z), and the seminal funnels (t), which lead into the vas deferens (rd). o, ovaries; iv, ciliated funnels leading to oviducts with egg capsule (e); di, dissepiments; 8-15, eighth to fifteenth segments. is the absence of parapodia, the bristles projecting directly from the body wall (fig. 259). The chaetas may be regularly distributed around each somite (Pcri- chceta) or gathered on the sides (Megascolex) or arranged in four groups so that in the animal four longitudinal rows occur. The animals are hermaphroditic, testes and ovaries lying in different somites. Usually the skin near the sexual openirrgs is thickened by numerous glands, forming a clitellum (fig. 252), which secretes the egg cocoons. and also functions in copulation, secreting bands which hold the animals together so that the sperm from one passes into the receptac- III. ANNELIDA: GEPHYR.^A 279 ulum seminis of the other. After impregnation the eggs are usually enclosed in cocoons. Sub Order I. LIMICOLA (Microdrili). Mostly fresh-water. The TUBIFICID.E, in consequence of the red blood, when present in large numbers color the bottom red. They form tubes in the mud. Tub if ex,* Clitellio irroratus* common on seashore. NAIDID/E, transparent forms living on water plants, reproduce asexually. Dero* and Aulophorus* (fig. 264) have gills around the anus. ENCHYTR.'EIDJ£; Pachydrilus. DISCODRILID/E (Myzobdclla), parasitic. Sub Order II. TERRI- COLA (Macrodrili). Terrestrial; the earthworms, our species of moderate size, in the tropics large (Megascolex australis four feet long). Lumbricus* Allobophora*; Diplocardia* with double dorsal blood-vessel; Pcricliccta* introduced from the — a / (I 1 d r * ( a ' pr g \... ...nc V FIG. 266. FIG. 267. FIG. 266. — Anatomy of Phascolosoma gonldi (orig.). a, anus; a, anterior retractors; d, digestive tract; g, gonads; m, mouth; n, nephridia; nc, ventral nerve cord; pr, posterior retractors. FIG. 267. — Larva (trochophore) of Echiurus (after Hatschek). a, anus; d, intestine; hw, postoral cilia; kn, protonephridia; m, mouth; mes, mesoderm bands with indication of segments; n, ventral nerve cord; sc, cesophageal commissure; sp, apical plate; vw, preoral ciliated band. tropics. Most species burrow through the earth, swallowing the humus and casting the indigestible portions on the surface. They loosen the soil and con- tinually bring the deeper parts to the surface. Details of the reproductive organs of one species in fig. 265. These vary and are used in classification. Sub Class II. Gephyrcca. The exclusively marine Gephyraea are distinguished at the first glance from the Chietopoda by the absence of segmentation. The body is oval 2SO ' CCELHELMINTHES or spindle-shaped, circular in section. The mouth, at the extreme anterior end, is either surrounded by a circle of tentacles (fig. 266), retracted together with the anterior end of the body by internal muscles, or is overhung by a dorsal preoral lobe or proboscis which may be several times the length of the body and forked at its tip (fig. 268). Internal segmentation is also lost, septa being entirely lacking. The nephridia are reduced in number, at most but three pairs being present, and in some but a single unpaired organ. They are the sexual ducts ; the duetiferi have special excretory organs (fig. 268, g) covered with branching canals opening to the body cavity by nephrostomes and ,4 B FIG. 268. — Bonellia viridis. A, female (after Huxley); B, male (after Spengel). c, cloaca; d, rudimentary intestine; g, excretory organ ; i, intestine; m, muscles supporting intestine; 5, balls of spermatozoa in B, in A, proboscis (preoral lobe); u, single segmental organ, functioning as oviduct ;vd, nephridium with ciliated funnel serving as vas deferens. emptying into the intestine. These resemble somewhat the branchial trees of the holothurians (infra), and hence the gephyrasa were formerly supposed to bridge the gap between holothurians and annelids, whence the name (ye'^vpa, bridge). The vascular and nervous systems are more like those of other annelids. The vascular system consists of a sinus around the digestive tract and a dorsal and usually a ventral longitudinal trunk; the nervous system of a brain, cesophageal collar, and ventral cord, III. ANNELIDA: HIRUDINEI 281 the latter without division into ganglia. The relations of the Gephyra?a to the Cha-topoda are shown by the development. In some (Ckctiferi) there is a trochophore (fig. 267) from which the worm arises, as in the Chastopoda, by growth at the hinder end; this at first has a segmented coelom and nervous system, the metamerism being lost later. Order I. Chaetiferi (Armata, Echiuroidea). With spatulate preoral lobe, often forked at the tip; at least a pair of ventral seUe; a trochophore in development. Echiurus* northern, T/ialassema* In Boncllia there is a marked sexual dimorphism (fig. 268); female, 2 to s inches, has a proboscis a yard long. The male, only i mm. long, totally different in form and color, lives parasitically in the oesophagus of the female (fig. 268, B). Order II. Inermes (Achaeta, Sipunculoidea). Distinguished by lack of chastas, the mouth surrounded by tentacles, and the dorsal and anterior position of the anus. The larva is a modified trocho- phore without preoral ciliated band and without segmentation; only two, sometimes but one, nephridia. The vascular ring around the mouth, with its dorsal, heart-like prolongation, is not circulatory. It is a special part of the coelom for the protrusion of the tentacles and has no connection with the in- testinal blood sinus. It is doubtful whether the Inermes are related to the Chuetopoda. Some unite them with Brachiopoda and Polyzoa in a group Prosopygii, so called in allusion to the dorsal position of the anus. Phascolosoina* (fig. 266). Phascolion* Sipiinculus.* Order III. Priapuloidea. No tentacles, mouth with teeth, terminal anus, two protonephridia united with sexual organs and opening either side of vent. Priapidus. Sub Class III. Himdinei (DiscopJiori). Three points of external structure distinguish the leeches from the chcetopods. First, the absence of bristles (except in Acant/iobdclla) and the presence of two suckers; the one on the posterior ventral surface is used only for attachment and locomotion, the other, sometimes scarcely differentiated, surrounds the mouth and is used in sucking the food. In locomotion anterior and posterior suckers are alternately attached, the body being looped up and extended like that of a 'span worm.' The animals can also swim by a snake-like motion of the whole body. A second point is the fine ringing of the body, there being usually many more rings than somites, the segments being divided by secondary con- strictions, there being three, five, or even eleven rings to a segment. The middle or one of the anterior rings often bears strongly developed sense organs. As in earthworms, certain of the somites may develop a clitellum which secretes the egg cocoons. 282 CCELHELMINTHES A third character is the marked dorsoventral flattening of the body (except in Ichthyobdellidie and a few other forms), the animals thus re- calling the flatworms. This may be the result of the very slight develop- ment of the ccelom. In most leeches there is a body parenchyma, traversed by muscles in which the organs are immediately im- bedded (fig. 269). The alimentary tract bears paired diverticula (fig. 270), varying in number in different species. Between the last and largest pair of these is the intestine, which opens dorsal to the posterior sucker. The jawed and jawless leeches show considerable differences in the pharyngeal region. In the first there are three semi- circular jaws in the pharynx, the free edge of each armed with teeth (fig. 271). To these are attached two muscles, one to'retract them, while the other exserts and rotates them, causing a triradiate wound from which the blood flows. This bleeding is difficult to staunch, since a secretion of glands on the lips and between the jaws f/^ Xf jf - ^ • SC *; — '•-»' •T c.y - - v«- m .• : :f"5 -a • L). In the jawless leeches the penis is lacking and the sperm, in pointed sprematophores, is inserted in the tissues of the leech. In the space between the epididymis and the first pair of testes are the ovaries (ov) and oviducts and an unpaired vagina («). The nephridia (17 pairs in this species) are complicated canals and are provided with bladder- like expansions. That the Hirudinei are true annelids and not segmented plathelminthes is based upon the view that their ccelom is reduced by ingrowth of parenchyma and altered to canals connected with the vascular system. At any rate the ven- tral and lateral vessels are to be regarded as remnants of a ccelom. In Clepsinc, etc., there are the dorsal and ventral blood-vessels of the Chastopoda and besides four longitudinal coelomic sinuses connected by lacunar spaces. The dorsal sinus encloses the dorsal blood-vessel, the ventral many of the viscera, among them the ventral nerve cord. These sinuses also have flagellated funnels which lead into lymphoid capsules, not, as was formerly thought, into nephridia. In the jawed leeches (apparently by degeneration as is the case in many poly- chaetes) the true blood-vessels are replaced by a canal system derived from the coelomic sinuses, which in Nephilis has in part a lacunar character. For the 284 COELHELMINTHES origin of these vessels from the ccelom the following points are in favor, (i) The ventral cord is enclosed in the ventral blood sinus; (2) the flagellate funnels, just alluded to, lie in the blood lacunas, usually in ampullar spaces between the ventral and lateral blood sinuses. Further relations are shown by Acanthobdella pelcdina, parasitic on fishes. This has both blood-vessels of the oligochaetes, a body cavity divided by septa, and chaetae. On the other hand, it is leechlike in other features; two suckers and sexual apparatus on the Hirudinean pattern. Branchiobdella, parasitic on the gills of the crayfish, is a chstopod devoid of bristles and furnished with a sucker in correlation with its habits. Order I. Gnathobdellidse. The jawed leeches include Hirudo medicinalis, once extensively used for blood-letting, now little employed. Hivmadipsa includes land leeches of the tropics. Nephelis* soft jaws. Macrobdella* includes our largest species. Order II. Rhynchobdellidae. Without jaws. CLEPSPINID^; mostly feed on snails and fishes. Clepsine* Hcementaria ghiliani of South America is poisonous. ICHTHYOBDELLID^E,* cylindrical, occur in salt and fresh water, parasitic on fishes. Ichthyobdella,* Pontobdella,* marine; Piscicola, fresh water. Class IV. Polyzoa (Bryozoa). In external appearance the Polyzoa closely resemble the hydroids, so that the inexperienced have difficulty in distin- guishing them. Like them by budding they form colonies which are either incrusting sheets or assume a more bush -like character. Further they have a crown of ciliated tentacles which can be spread out or quickly retracted. In internal characters the two groups are greatly different. The Polyzoa have a complete alimentary canal, with its three divisions, which is bent upon itself so that the anus lies near the mouth. The central nervous system lies be- tween mouth and anus, and the single pair of nephri- dia empty by a common opening. Beyond this it is difficult to go, since the two groups — Entoprocta and Ectoprocta — differ widely. The Entoprocta have no ccelom, resembling in this respect the Rotifera; the Ectoprocta are true Ccelhelminthes and by way of Phoronis show resemblances to the Sipunculoida ('Prosopygii,' p. 281) and also to the Annelida. Sub Class I. Entoprocta. The single individuals of the Entoprocta (fig. 273) are shaped like a wine- glass and are placed on stalks which rise from (usually) creeping stolons. The FIG. 273. — Loxosoum sin«idare (after Nit- sche) in optical sec- tion. A, rectum; Ga, ganglion; /, intes- tine; T, tentacles; V, stomach. IV. POLYZOA: ECTOPROCTA 285 circle of tentacles, arising from the edge of the cup, enclose the peristomial area in which are both mouth and anus, and between these the excretory and re- productive organs open. The space between the horseshoe-shaped intestine and the body surface is filled by a parenchyma containing muscle cells, and correspondingly the excretory organs are protonephridia. Urnatella* fresh- water. Pedicellina,* Loxosowa, marine. Sub Class II. Ectoprocta. The Ectoprocta have a spacious, often ciliated, ccelom between the alimentary canal and skin, so that these are separated and have a certain amount of independence (fig. 274). On this account has arisen a pecu- liar method (morphologically indefensible) of treating them as two in- dividuals, polypid, the intestine and tentacles; cyst id the rest, especially the body wall and skeleton. -en, FIG. 274. — Flustra membranacea (after Nitsche), a single animal, a, anus; ek, ectocyst; en, entocyst; /", funiculus; g, ganglion; k, collar which permits complete retrac- tion; m, stomach, also dermal muscular sac; o, cesophagus. A, avicularium; B, vibracu- larium of Bugula (after Claparede). The cystid is cup-shaped or saccular. It consists of an endocyst — the body wall — and an ectocyst — a cuticular skeleton, often strongly calcified, secreted by the ectoderm. The ectocyst covers only the base and side walls of the endocyst, leaving the outer end soft, thus forming a sort of collar into which the tentacles and adjacent parts of the cystid can be retracted. The opening thus formed in the ectocyst in many species (Chilostomata) can be closed by a lid (operculum). The circle of tentacles . surrounds the mouth alone, while the anus is outside near the collar. The 288 CCELHELMINTHES strongly bent alimentary canal extends into the ccelom and is bound at its hinder end by a cord, thefuniculus, to the base of the cystid. Gang- lion and nephridia lie between the mouth and anus. The gonads arise from the epithelium of the ccelom, the testes usually on the funiculus, the ovaries on the wall of the cystid. Hundreds and thousands of individuals form colonies (fig. 275) in which cystid abuts against cystid. The ccelom of adjacent cystids may be distinct or a wide communication may exist. The colonies grow by budding; in the Gymnokemata a part of a cystid becomes cut off as a FIG. 275. — -Small colony oiLophopus crystallinus Rafter Kraepelin), with young and old, some extended, others more or less retracted, o, statoblasts. daughter cystid in which the polypid — alimentary tract and tentacles- arises by new formation; or (Phylactolsemata) the bud anlage of the polypid arises before the first appearance of the cystid. Division of labor or polymorphism is common. Besides the animals already described, which are primarily for nourishment, three other indi- viduals may occur, ovicells, vibracularia, and avicularia. All three are cystids which have lost the polypid The ovicells are round capsules which serve as receptacles for the fertilized eggs. The vibracularia (fig. 274, B) are long tactile threads: the avicularia (fig. 274, A) are grasping structures which seize small animals and hold them until decay sets in ; the fragments serve as food for the polypids. The avicularia have the form of a bird's head, the movable lower jaw being a modified operculum. Under unfavorable conditions a polypid in a cystid may break down and be lacking for some time until better relations cause its new formation. Besides, in the depopulated cystids, there may appear statoblasts, internal buds en- veloped in a firm envelope which form a resting stage (fig. 275). Each stato- blast is surrounded by a girdle of chambers which by drying become filled with V. PHOROXIDEA. VI. BRACHIOPODA _>x; air, causing the statoblast to float when it again comes into water. From the statoblast a smaller polyzoon escapes which develops a new colony. The statoblasts are adaptations to the conditions of fresh-water life and occur only in the Phylactolaemata, where they arise as a sort of internal buds on the fu- niculus, just before the degeneration of the polypids. Order I. Gymnolaemata (Stelmatapoda). Tentacles in a ring around mouth. Numerous species, almost exclusively marine. CHILOSTOMATA, cystids can be closed by an operculum: Gemmel- laria* CelMaria* Bugida* Flustra* (fig. 274), Eschara* CYCLOSTOMATA, tubular cystids without operculum. Crisia* Tubulipora* Hornera* CTEN- OSTOMATA, cystid is more calcareous, closed by a folded membrane. Alcy- onidium,* Vcsicularia, Valkeria* marine; Paludicclla* (fresh-water). Order II. Phylactolaemata (Lophopoda). Tentacles borne on a horseshoe-shaped lophophore extending on either side of the mouth, the tentacles on its margins. All are fresh-water species. Pecti- natella,* Lophopus (fig. 275), Plumatdla.* Class V. Phoronidea. The single genus Phorcmis* was first called a chaetopod on account of its worm-like body situated in a chitinous tube like many sedentary annelids. Then it was placed in the Polyzoa, with which it is more nearly related. The young, described as Actinotrocha, is a modified trochophore with the mouth overhung by a large hood and with the postoral band of cilia drawn out into a series of fingers which become the tentacles of the adult; the anus is terminal. At the metamorphosis this larva becomes doubled on itself, so that the alimentary canal is U-shaped, the anus near the mouth, while the tentacles are borne on a horseshoe-shaped basis around the mouth. Class VI. Brachiopoda. On account of the bivalve calcareous shells the Brachiopoda were long regarded as molluscs, but later the fact that the valves are not paired as in the lamellibranchs, but are dorsal and ventral, that the nervous system, the excretory and reproductive organs, the body cavity, and the development resemble those of the annelids rather than those of the molluscs, led to their recognition as a distinct class allied to the former group. The body has a greatly shortened long axis (fig. 276) and in conse- quence a transversely oval visceral sac. In most a stalk (sf) for attach- ment arises from the posterior end. From the anterior side two folds, the mantle lobes (/>), extend forward, one ventral, the other dorsal, their free edges bearing bristles. Each mantle secretes a calcareous shell. In a few the dorsal and ventral shells are similar, but usually the ventral valve (in Crania attached directly without the intervention of a stalk) is more 288 CCELHELMINTHES strongly arched and has an opening at the posterior end for the passage of the stalk (figs. 277, 278). The flatter dorsal valve frequently bears a characteristic feature in the calcareous skeleton of the arms (fig. 278) which, when present, has very different forms. \\ hen closed the valves completely rf FIG. 276. — Anatomy of Rhynchonella psittacea (after Hancock), a', left, a~, right arm; «, opening into the cavity of the arm; d, intestine; e, blind end of the intestine; g, stomach with liver; m, adductors and divaricators of shell; o, cesophagus; />', /»'-'. dorsal and ventral mantle lobes; st, stalk; i, 2, first and second septum, on the second a nephrostome. enclose the body. When they open the gape is anterior, the posterior parts remaining in contact. Here, except in the Ecardines, a hinge is developed just in front of the posterior margin, consisting of teeth in the ventral valve which fit into corresponding grooves in the dorsal. Opening D h FIG. 277. — II aldheimia flavescens (from Zittel). Shell with arms and muscles, a, arm with fringed border (h); c, c', divaricators; d, adductors; D, hinge process (the vertical line shows position of hinge). and closing the valves are, contrary to what occurs in Lamellibranchs, active processes, accomplished by appropriate divaricator and adductor muscles (fig. 277). These produce scars on the shell, important in the study of fossil forms. VI. BRACHIOPODA The usually spirally coiled arms, which lie right and left of the mouth and which give the name to the class, fill most of the shell. On the outer side of each arm is a longitudinal groove, bounded by a row of small ten- tacles. By means of cilia on tentacles and groove food is brought to the mouth. These arms resemble the lophophore of a phylactoliemate Poly- zoan, which only needs extension and coiling to produce this condition. In development the arms of the Brachiopod pass through a lophophore stage. In the body there is a ciliated ccelom which extends into both arms and mantle folds. It encloses alimentary tract, gonads, and liver, and is FIG. 278. — Waldheimia flavescens (from Zittel). A, dorsal, B, ventral valve; a, b, c, impressions of muscular insertions; a, adductors; b", adjusters (stalk muscles); r, r', divaricators; s, hinge groove of upper valve in which the tooth (/) of the lower valve passes; /, support of arms; d, deltidium;/, foramen for stalk. divided into right and left halves by dorsal and ventral mesenteries sup- porting the intestine. Each half in turn is divided by incomplete septa into anterior, middle, and posterior divisions recalling those oiSagitta (p. 252). The arrangement of the septa is not so clear as in that form, the result of the shortening of the long axis and the twisting of the alimentary tract. This latter consists of cesophagus, stomach, which receives the liver ducts, and intestine, which in some species terminates blindly. The gonads are chiefly in the mantle lobes. The sexual cells pass out through the nephridia, which begin in one ccelomic pouch with wide nephrostomata, perforate the septum, and open to the exterior in the next somite. Since usually there are two septa, two pairs of nephridia may occur, but one is usually degenerate. The nervous system consists of an. cesophageal ring with weak dorsal ganglion, extending into the arms, and a stronger ventral mass representing the ventral chain. The heart lies dorsal to the stomach. 19 290 COELHELMINTHES In development the brachiopods recall both Sagitta and the Annelida. They resemble Sagitta in that in Argiopc the ccelom arises by outgrowths from the archenteron (fig. 279), divided later by septa into three pairs of pouches. They are annelid-like in the form of larva and in the presence of chaetse which are formed in separate follicles. Brachiopods were formerly so numerous that they are among the most important fossils in the determination of geologic horizons. Now there are but few species, some inhabitants of the greatest depths of the sea. FIG. 279. — Development of brachiopod (after Kowalevsky). A, gastrula with early enteroccelic pouches; B, closure of blastopore; C, ccelom. separated, body annu- lated; D, cephalic disc and mantle developing, the latter with long setae; E, attached embryo, the mantle lobes folded over cephalic disc (setae omitted), c, cephalic disc; d, dorsal lobe of mantle; e, enteroccele; m, mantle; v, ventral mantle lobe. IG' Order I. Ecardines. Hinge absent: valves similar, the stalk passing between them (Lingula*), or unequal, the ventral perforated by the stalk (Dis- cina), or the animal is directly attached by the shell (Crania). Order II. Testicardines. Hinge present, valves unequal, the ventral per- forated by the stalk; anus degenerate. Rhyn- chonclla* Terebratulina* in our colder waters. Spirifer, Orthis, Pentamenis, A try pa, important fossil genera. Summary of Important Facts. (1) The CCELHELMINTHES have a well-developed body cavity (ccelom). (2) The CEUETOGNATHI are hermaphroditic worms with three pairs of coelomic pouches, with fins, a'-d bristle-like jaws. (3) The NEMATODA are mostly dioecious, usually parasitic, elongate worms, with cylindrical unsegmented body, with nerve ring (no ganglia), paired excre- tory organs, and tubular gonads. (4) The most important species parasitic in man are Ascaris luvibricoides in the small intestine, O.vwm vcrmic-ularis in the large intestine, Ankylostoma duodenalis, and the notorious Trichina spiralis. In hot climates Filaria san- gulnis ho ni in is and Dracunculus medinensis. (5) The GORDIACEA have mesenteries and splanchnic mesoderm; they are parasitic in insects. ECHINODERMA 291 (6) The ACANTHOCEPHALI lack alimentary tract, have a spiny proboscis and a very complicated reproductive apparatus. The adults are parasitic in vertebrates, the young in arthropods. (7) The CH/ETOPOD ANXELIDS have segmented bodies, the segmentation showing itself in ringing of the body wall and in the separation of the ccelom into a series of pouches by transverse septa and the metameric arrangements of blood-vessels, ganglia, and excretory organs. (8) The CH^TOPODA are distinguished from other annelids by the chaetae (usually four groups in a somite) arising in special follicles. The cruets are few in the hermaphroditic Oiigochaeta?, numerous and borne on special parapodia in the Polychaette. (9) The GEPHYR^A are related to the Chaetopoda. They are saccular, with tentacles or well-developed preoral lobe. They have largely or entirely lost the segmentation. Evidence of segmentation appears in some cases in develop- ment and in the ventral nerve cord and nephridia. (10) The HIRUDIXEI are hermaphroditic Annelida which lack chata-, but have sucking discs. Their flattened bodies, rudimentary ccelom, and rich body parenchyma give them a certain similarity to the Plathelminthes. (n) The Hirudinei have either a protrusible pharynx (Rhynchobdella) or three toothed jaws (Gnathobdella). To the latter belongs the medicinal leech (Hirudo medicinalis}. (12) The POLYZOA are like the Hydrozoa in being colonial and having a circumoral crown of tentacles. They are distinguished by the complete ali- mentary canal, the large ccelom, and the ganglionic nervous system. (13) The PHORONIDEA are closely like the Polyzoa. (14) The BRACHIOPODA have a bivalve shell, the valves being dorsal and ventral. (15) The body cavity is divided by two septa into three (paired) chambers, of which one, rarely two, are provided with nephridia. (16) Most brachiopods are attached by means of a stalk. They are divided into Ecardines, without a hinge and with anus, and Testicardines, with a hinge and no anus. PHYLUM V. ECHINODERMA. The Echinoderms differ from most other animals by their radial symmetry, but recall in this respect the Ccelenterata, a fact which led to their inclusion in the 'Radiata' (p. 206), a view of their relationships which was set aside on account of their different structure, especially the pres- ence of a ccelom. In fact the radial symmetry of the echinoderms has a different value, for while in the Ccelenterata the number four or six is fundamental, Echinoderma are, with few exceptions, five-radiate. Further, the radial symmetry of the Ccelenterata is primitive, the echino- derms have descended from bilateral, possibly worm-like, ancestors, as is shown by the bilateral larrae and many indications of bilaterality in structure, especially in the more primitive forms (crinoids). This primitive bilaterality is to be sharply distinguished from that resulting from modification of radially symmetrical organs like the sexual and ambulacral systems of highly differentiated echinoderms (bivium of sea 292 ECHINODERMA urchins, trivium of holothurians). One pole of the axis of radial sym- metry is marked by the mouth, which in the echinoids, starfish and brittle stars, is turned downward; the other is frequently indicated by the anus. The structure of the integument gives these animals a characteristic appearance. Calcareous plates arise in the mesoderm, under the epithe- lium, which form a body armor or test, and since these are usually produced into spines, they give the name Echinoderma, spine skin, to the group. This skeleton at times becomes degenerate, as in the Holothurians (ft rarely entirely disappears as in Pelagothuria), but even then shows itself as 'anchors' and 'wheels' of lime. The sp/iccridia and pedicellaria, com- mon in echinoids and asteroids, are characteristic appendages of the in- tegument. The first are sense organs; the latter are usually stalked forceps-like grasping structures with calcareous skeleton. In life they are active and apparently either clean the skin or are defensive. They are oc- casionally provided with poison organs. Certain plates possess a morphological interest since they appear early in many larvae, and in the adults of different classes can be recognized in similar positions. In the neighborhood of the anus are five basalia, interradial in position, farther five radialia ('apical skeleton') and five inter- radial 'oralia' around the mouth. Not less characteristic than the skeleton is the ambulacral (or water- vascular) system (fig. 281). This is a system of ciliated tubes which begins FIG. 281. — Water- vascular system of starfish (orig.). a, ampullae; ab, \ O ' f IT — i ambulacra; c, radial canal; OT, madre- usually at the surface, ordinarily by a porite; n, radial nerve; p, Polian vesi- , ele;r, ring canal, beneath it the nerve calcareous plate, the madreponte, per- nng; s vesicle. stone canal; t, racemose f orated with fine pores for the entrance of sea water. The water passes into a tube which, on account of its calcified walls in the starfish is called the stone canal, and leads orally to a ring canal around the mouth. The ring canal bears usually several (up to five pairs) of Polian vesicles, which, with Tiedemann's vesicles of the starfishes, are now regarded as appendages which, like lymph glands, produce leucocytes. From the ring canal radiate five radial canals which give off, right and left in pairs, the ambulacral canals. These in turn connect with the ambulacra and ampullae, the highly characteristic locomotor organs of the echino- derms. An ambulacrum is a muscular sac which can be distended and lengthened by forcing in fluid from the ambulacral vessels, and is retracted ECHINODERMA 293 and shortened by its muscles. The ampulla is a reservoir connected with the ambulacrum and projecting into the body cavity. In locomotion the animal extends its ambulacra, anchors them by the sucking disc at the tips, and then pulls the body along by contraction of the ambulacral muscles. In the sessile crinoids and the ophiuroids (which move by their snake-like arms) the ambulacra lack suckers and ampulla?, and are not locomotor but tactile in function. So among the holothurians and sea urchins the ambulacra are often replaced by tentacles. Frequently each radial canal ends in an unpaired tentacle with olfactory functions. The arrangement of the ambulacral system influences that of other organs. Alongside the stone canal is an elongate organ formerly called the 'heart,' but now regarded as a lymphoid gland or a collection of excretory lymphoid cells (ovoid gland, paraxon gland, septal organ}. Ring and radial canals are accom- panied by corresponding blood canals, with which are often associated two vessels to the alimentary canal. The blood system is surrounded by a peri- haemal space, the ovoid gland and stone canal by the axial sinus, which in the starfishes and urchins passes into an ampulla close beneath the madreporite; this in turn connects with the lumen of the stone canal and also with the exterior through the pores of the madreporite. When the axial sinus is lacking (crinoids, holothurians) the stone canal may open into the body cavity, water entering this in the crinoids by pores in the theca. There is a nerve ring and radial nerve, frequently in the ectoderm, to which may be added an 'apical' nervous system. The courses of the radial vessels and nerves mark out five chief lines in the animal, the radii; between them come the inter radii. The stone canal, madreporite, and lymphoid gland are interradial in position, as are the gonads, usually five single or five pairs of racemose glands; in some cases but one is present. Echinoderms are rarely hermaphroditic. The gonads are supported in the spacious coelom by special bands, while mesenteries support the alimentary tract and its derivatives. The five gonads develop from a single anlage which is genetically con- nected with the lymphoid gland. Except in crinoids and holothurians this anlage becomes modified into a perianal ring (rhachis} from which the gonads bud. The so-called blood-vessels hardly deserve the name, since they are fibrous cords with lacunar spaces. The perihagmal space, like the axial sinus and the space, around the gonads and the genital rhachis, are derived from the coelom. Respiratory organs are represented by very various structures: branchicc, or thin-walled outpushings of the coelom, either around the mouth, as in Echin- oidea, or on the aboral surface, as in the Asteroidea, thebursce of the Ophiuroidea, the branchial trees of the Holothoroidea and the various parts of the ambulacral system. The Echinoderma are exclusively marine, occurring even in the deepest seas. Many groups, like the crinoids, are largely bathybial, others frequent rocky coasts. At the period of reproduction they pass their sexual cells into the sea, where fertilization occurs. In some, however, the young are carried about in brood cases until the earlier developmental stages are past. 294 ECHINODERMA Where there is no brood pouch the young escape from the egg as lame (fig. 282, I) which swim at the surface, and are distinguishable from the adults by their soft consistency, transparency, and bilateral symmetry. By the development of lobe-like processes and slender arms supported by calcareous rods the larvae assume the most different and bi- zarre shapes (plutei of echinoids and ophiuroids (VI), brack iolaria (VII) and bipinnaria (VI) of asteroids, auricularia (III) of holothurians), all FIG. 282.— Echinoderm larvae (after J. Miiller). a, anus; m, mouth; the black line, the course of the ciliated bands. 7, form common to all; II, III, developmental stages of auricularia (Holothurian); IV, V, stages of the Asteroid bipinnaria; VI pluteus of a spatangoid; VII, larva (brachiolaria) of Asterias (orig.). m. mouth- v vent. of which can be referred back to a common type with tri-regional alimen- tary tract and a ciliated band around the mouth, strikingly resembling tornaria, the larva of Balanoglossus. The different appearances of the larva? are due to the drawing out of the ciliated band into lobes and arms, and also to its becoming broken into parts which unite themselves into complete rings (}'). The metamorphosis of the bilateral larva into the radial adult is very compli- cated. It begins early with the formation of outgrowths from the archenteron (fig- 283), which become separated and form the anlagen of the coelom and ambulacral system. It is difficult to give a short summary of the development, partly from the differences in the separate groups, partly from the contradictions of authors. The following seems to be the most common. A vasoperiloneal diverticulum (fig. 283, he) arises from the bottom of the archenteron; this soon Iivides into right and left vesicles, the left acquiring a connection with the ex- terior Jmadreporic opening). Each vesicle separates into anterior (h) and posterior (c) parts, the anterior forming the anlage of the water-vascular (hydrocKle) system, the others the coelom. The two coelomic sacs expand and I. ASTEROIDEA 295 form the roomy body cavity of the adult, the membranes separating them furnish- ing the mesenteries. The right hydroccele remains rudimentary; the left, which has the external opening, separates into (i) a smaller anterior portion, the anlage of the ampulla and axial sinus; (2) the connecting duct or stone canal; and (3) a posterior cavity, the hydroccele in the narrower sense. The latter surrounds the oesophagus (ring canal) and sends off five radial diverticula, the anlagen of the radial canals, which form the basis of the conversion of the bilateral larva into the radially symmetrical adult (fig. 284). It is a question as to which group of Echinoderma is the most primitive, but ease of treatment makes it best to begin with the Asteroidea. FIG. 283. FIG. 284. FIG. 283. — Three stages in the development of the coelom and \vater-vascular system (after Bury and McBride). a, ampulla; b, stone canal; c1, c~, left and right coelom sacs; d, hind gut with anus; hl. Ir, left and right (rudimentary) hydroccele sac; he, common anlage of hydrocoele and coelom; m, stomach; s, stomocleum and mouth. FIG. 284. — Formation of Ophiuran from the pluteus larva uifter Miiller, from Korshelt-Heider) . Class I. Asteroidea (Starfish). Two parts can be recognized in the body of a starfish, a central disc and the arms, usually five in number, which radiate from it (fig. 290). The relations in which these stand to each other vary between two extremes. In many starfish the arms play the chief role and the disc appears as only their united proximal ends (fig. 285). On the other hand, the disc may increase at the expense of the arms, so that they form merely the angles of a pentagonal disc (fig. 286). In both arms and disc two surfaces are recognized, oral and aboral, which pass into each other, usually without a sharp margin. In the normal position the oral side is downwards and has the mouth in the centre and radiating from it to the tips of the arms the five ambulacral grooves. Near the centre of the aboral surface is the anus (when not degenerate) and excentric from it in an interradius is the madreporite (in many-armed species two to sixteen interradii may have madreporites). 296 ECHINODERMA A line passing through the madreporite and the opposite arm divides the body into symmetrical halves. This arm is called anterior, since in the irregular sea urchins (Spatangoids) the homologous area is clearly anterior, while the madreporic interradius is posterior. This plane of symmetry does not corre- spond with that of the larva. The two rays on either side of the madreporite form the bivium, the three others the trivium. FIG. 285. FIG. 286. FIG. 285. — Comet form of Linckia mult (flora (from Korschelt-Heider) One of the arms is producing'a new animal by budding. FIG. 286. — Culcita pent angular is, aboral view (from Ludwig). a, madreporite; b, reflexed end of ambulacral grooves. The skin is everywhere protected by large and small plates jointed together. In life it is extremely flexible, the arms can be bent in any direction, and the animal can work its way through narrow openings. Of the skeletal pieces the ambulacral plates need special mention. These FIG. 287. — A, cross-section of starfish arm (orig.). o, adambulacral plates; am, ambulacra; ap, ambulacral plates; b, branchiae; c, coelom; /?, hepatic caeca; i, inter- ambulacral plates; n, radial nerve; p, ampulla; r, radial canal; 7', radial blood vessel. B, ambulacral plates, ventral view, showing the ambulacral pores between. form the roofs of the ambulacral grooves, and between them are openings (fig. 287, B), the ambulacral pores, through which connection is made be- tween the ambulacra and ampullae. In each arm the pairs of ambulacral plates meet above the groove like the rafters of a roof. Laterally each I. ASTEROIDEA 297 ambulacral plate abuts against a small interambulacral plate, bearing usually movable spines. Beyond these come the less constant adambuln,- ral or marginal plates, and then those of the aboral surface. Each ambu- lacral area terminates at the tip of the arm with an unpaired ocular plate. FIG. 288. — Asterhcus verruciihilus, aboral surface removed (after Gegenbaur). g, gonads; h, hepatic caeca; i, stomach with anus. The organs lie in part in the ccelom, in part in the ambulacral grooves. The alimentary tract is in the ccelom and extends straight upward from the mouth to the aboral surface, where it ends with an anus or is entirely FIG. 28g. — Section through ray and opposite interradius of a starfish forig.). B branchiae; C, cardiac pouch of stomach; E, eye spot; G, gonad; H, 'liver'; .!/, mouth; N, radial nerve; P, pyloric part of stomach; RC, ring canal; RD, radial canal of water- vascular system; S, stone canal. closed (figs. 288, 289). By a constriction it is divided into a larger, lower cardiac portion and a smaller, upper pyloric division. From the latter arise five hepatic ducts which connect with five pairs of hepatic glands lying in the arms, while small ca-ca arise from the intestine in some 29? ECHINODERMA forms. The cardiac division gives origin to five gastric pouches which can be protruded from the mouth or retracted by appropriate muscles. The gonads are five pairs of racemose glands lying in the basis of the arms and opening interradially between the arms. Lastly, in the ccelom is the stone canal (accompanied by the lymphoid glands, and with it enclosed in the axial sinus) extending from the aboral madreporite to the ring canal near the mouth. The radial nerve, canal and blood-vessel, which start from the cir- cumoral rings, lie in the roof of the ambulacral groove between the ambulacra. The nerve, lying in the ecto- derm, ends at the tip of the arm in a compound eye spot colored with red or orange pigment which experiment shows is sensitive to light. A second nerve has been described lying in the ccelom of the arm. The ambulacral system corre- sponds with the foregoing description (p. 292), the ampulke as well as the five or more Polian and Tiedemann's (racemose} vesicles projecting into the ccelom. Since the arms contain nearly all im- portant organs, their physiological independ- ence is easily understood. Arms broken oQ. not only live, but regenerate first the disc and then new arms which appear at first like small buds (comet form, fig. 285). This separation of arms may occur through accident, or it not infrequently is produced by the animal itself. ASTERID.E, well developed arms, four rows of ambulacra; Asterias* Leptasterias* Heliaster* (numerous arms). SOLASTERID^E, two rows of ambulacra, arms sometimes numerous; Pythnnaster (fig. 290). ASTERINID.E, arms short or body is pentagonal, no large plates on the margins of the arms. Aster iscus (fig. 288). In other forms (Cukita* fig. 286, Hippasteria* Ctenodiscus*) the body is more or less pentangular, margin with large plates. Class II. Ophiuroidea (Brittle Stars). In these, as in the Asteroidea, there are disc and arms, the latter some- times branched, but the internal anatomy is different. The ambulacral plates have been drawn inside the arm and each pair fused to a large 'vertebra' (fig. 291). As a result the ccelom of the arms is greatly re- duced, the hepatic caeca are lacking, and the alimentary canal, which lacks an anus, is confined to the disc. By the ingrowth of ventral plates FIG. 2QO. — Pythonasler murrayi (after Sladen). Oral view show- ing ambulacral grooves. III. CRIXOIDEA 299 the ambulacral grooves are closed, and the ambulacra, which lack sucking discs, are tactile, locomotion being effected by the snake-like motion of the arms. The madreporite is on the ventral surface. Also on the ven- tral surface are five slits which con- nect with as many burscc, thin-walled respiratory sacs into which the sexua! organs open. The gonads are at- tached to a genital rhachis which coils through the disc. In many brittle stars, especially in young specimens, there is a kind of asexual generation (schizogony), the animal dividing through the disc, the halves regenerating the missing parts. OPHIURID.^, arms unbranched (Ophio- pholis* (fig. 292), Ophioglypha* Atnphhtra*); EURYALID.E, the arms branched (Astrophyton,* fig. 293). FIG. 291. — Section of Gphiuroid arm (orig.). a, ambulacrum; b, blood- vessel; c, ccelom; m, muscles of arm; n, nerve; r, radial water tube; v, 'ver- tebra' (coalesced ambulacral plates). FIG. 292. — Ophiopholis aculeata* (from Morse). FIG. 293. — Astrophyton arborescens, basket fish (from Ludwig). Class III. Crinoidea (Pelmatozoa). The crinoids or sea lilies are on the road to extinction. In early times, j especially in the paleozoic, they were very abundant, but to-day there are but few species, these mostly restricted to the greater depths of the ocean, only the Comatulidse occurring near the shore. The crinoids are attached to the sea bottom by a long stalk (fig. 294), composed of cylindrical discs which often bear five rows of outgrowths, the cirri. The young Coma- tulida? (fig. 295) are similarly attached, having a Pcntacrhuis stage, but later they separate and live a free life, a proof that the attached condition was primitive. When the separation takes place, one joint of ilu- sulk ECHINODERMA with its cirri remains attached to the animal, as the centr odor sal united with the lowest cup plates, the infra ba sals. On the upper joint of the stalk is a cup-shaped body (theca) the edges of which bear five or ten (usually branched) arms. The walls of the theca are covered with poly- gonal calcareous plates. FIG. 294. FIG. 295. FIG. 294. — Pent acr inns ma'-hayamts (after Wyviile Thompson), br, brachialia; c, cirri; d, distichalia; r, radialia; p, pinnuhf. FIG. 295. — Different Pentacrinus stages (a, b, c) of Antedon rosacea. I, arms; 2, cirri; 3, stalk. Usually the stalk bears five plates, the basalia, and then come five radialia, alternating in order with the basalia (fig. 296). In some there is a circle of infrabasalia in a line with the radialia. Frequently the elements of the arm, the brachialia, are directly attached to the radials (fig. 296). But often the arm branches once or several times dichotomously, and the first branching takes place at the base, so that the arms seem to spring from the theca. In these cases the first brachialia are considered as part of the theca and are called radialia distichalia (fig. 294). From the arms arise, right and left, a row of III. CRIXOID1.A 301 pinnules, lancet-shaped processes supported by calcareous bodies in which the sexual products ripen until freed by dehiscence (fig. 298). The mouth opening, in the middle of the oral disc which closes the theca, is frequently surrounded by five interradial plates, the oralia (fig. 205, B). The mouth, which in contrast to other echinoderms is directed upwards, connects with a spiral digestive tract in which oesophagus, stomach, and intestine can be distinguished. The anus is interradial and near the mouth (fig. 297). Five ambulacral grooves begin at the A B FIG. 296. — Hyocrinus bethleyainis. A, upper end of stalk with cup, and the bases of the arms; b, basalia; br, brachialia; r, radialia. B, oral surface of cup with mouth, five oralia, and the bases of the arms. mouth and extend out on the arms, branching with them and extending to the tips of the pinnuke. In the ten-armed species (fig. 297) the grooves fork near the mouth. These are ciliated and serve as conduits to bring food to the oral opening. Nervous, ambulacral, and blood systems begin with a circumoral ring. As in the asteroids, they follow the ambulacral grooves to the pinnuke, but the ambulacra have no suckers nor ampullae and are merely tactile tentacles. A typical stone canal is also lacking; in its place are five or several hundred tubules leading from the ring canal to the ccelom. Opposite their coelomic mouths are fine pores in the theca through which water enters to pass through the tubules into the ambulacral system. The ambulacral nervous system is weakly developed or may be absent. The apical system, on the other hand, is well developed and forms the axial cord running through the brachialia and 302 ECHINODERMA radialia to unite in a complicated plexus in the centrodorsal. A problematical so-called dorsal organ also begins in the centrodorsal and extends up through the axis of the theca to the oral disc. It is apparently homologous with the 'heart' of other echinoderms. Its upper end lies in a cell complex from which the reticulum of genital cords extends into the arms, swelling in the pinnuke to the gonads (fig. 298). The dorsal organ in the centrodorsal is enclosed in the chambered organ, a prolongation of the ccelom which extends into stalk ard cirri. Sub Class I. Eucrinoidea. The foregoing account applies entirely to the Eucrinoidea, which may be divided into two groups: Order I. TESSELLATA (Pateocrinoidea). Theca with its side walls composed of immovably united thin plates; the ambulacra! grooves usually completely covered by calcareous plates. Exclusively paleozoic. a Fio. 207. FIG. 298. FIG. 297. — Oral area of crinoid (Antedon), showing by dotted lines the course of the intestine from the mouth (m) to the anus (a) ; g, ciliated grooves leading from the arms to the mouth (prig.). FIG. 298. — Cross-section of pinnula of Antedon (after Ludwig). a, axial nerve cord; c, ciliated cups; cc, cceliac canal; g, gonad; s, sacculi; sc, subtentacular canal; t, tentacles. Order II. ARTTCULATA (Neocrinoidea). Ambulacra! grooves open, theca with compact, in part movably articulated, plates. This order left the other in the mesozoic age, and some families have persisted until now. Rhizo- crinus* (fig. 296) and Pentacrinus (fig. 294), deep seas; the COMATULID.*: of shallow water are fixed in the young, free in the adult. Antedon* (fig. 295). Sub Class II. Edrioasteroidea (Agelacrinoided). Theca of irregular plates; arms unbranched and lying on the theca. Possibly the ancestors of the noncrinoid echinoderms. Paleozoic. Agelacrinus. IV ECHINOIDEA 303 Sub Class III. Cystidea. Exclusively paleozoic; body spherical, composed of polygonal plates. Stalk and arm structures rudimentary, sometimes lacking. The AMPHORIDA are by some regarded as ancestral of all echinoderms. Holocystites, Echino- splucnles (fig. 299). V< FIG. 299. FIG. 300. FIG. 299. — Ecliinosplnrrites aurantium (from Zittel). FIG. 300. — Pentremites florealis (from Zittel). Lateral, oral, and aboral views. Sub Class IV. Bias to idea. Arms lacking; the mouth surrounded by five petal-like ambulacral areas. The group appears at end of Silurian and dies out with the carboniferous. Pentremites (fig. 300). Class IV. Echinoidea (Sea Urchins). The structure of the sea urchins is best understood in the spherical forms (figs. 301, 303). Mouth and anus lie at opposite poles of the main axis, each opening immediately sur- rounded by areas covered by calcare- ous plates, the arrangement of which varies with the family. Around the anus is the periproct, around the mouth the peristoine, the latter bearing sprue- ridia and in the Echinoids five pairs of interambulacral gills. Between peristome and periproct the body wall (coron'.i) is composed of calcareous plates, which, except in the Echinothu- ridce, are immovably united. Aside /IG .^.-C^oplcnrus ./?,„•/,/<„„,,* (after Agassiz). Aboral view, the from the extinct Pakechinoidea the spines removed to show the ambula- plates are arranged in ten double oral (a) and (6) interambulacral areas endme; respectively m the ocular and meridional TOWS, two rows being genital plates; in "the centre the four always intimately associated together. Plates of the periproct. Five of these double rows are ambulacral, the alternating five interam- bulacral. Both bear small hemispherical articular surfaces on which 304 ECHINODERMA are situated the spines, either long and pointed or swollen to spherical plates. These spines are moved by muscles so that they serve both as protecting and locomotor structures. The ambulacral plates are distin- guished from the interambulacral by the ambulacral pores by which the ambulacra on the surface are connected with the internal radial canals and ampulke. In most sea urchins the paired grouping of the pores results from the fact that a double canal extends from ampulla to ambulacrum. FIG. FIG. 303. Aboral view, showing the FIG. 302. — Clypeaster subdepressus (after Agassiz). petaloid ends of the ambulacral areas. FIG. 303. — Diagrammatic longitudinal section through a sea urchin. In the arrangement of the ambulacra two modifications, the band form and the petaloid, occur. In the first (Regularia) the ambulacra are equally developed from peristome to periproct (fig. 301). In the second oral and aboral regions may be distinguished (fig. 302). In the oral region alone are loco- motor feet always present, but these are irregularly arranged. In the aboral area the ambulacra are branchial or tentacular and are regularly arranged, their pores bounding five petal-like figures around the periproct, very distinct after removal of the spines (fig. 306). In the Regularia, the Cidaridae excepted, the interambulacral plates around the peristome show five pairs of notches for the gills, five pairs of thin -walled branching extensions of the body cavity. Ambulacral and interambulacral fields both end at the periproct with an unpaired plate, the five ambulacral plates (terminalia of morphology) being called ocular plates, since they often bear pigment spots formerly regarded as eyes. Each is perforated by the end of the radial canal and nerve. The five interambulacral plates (basalia*) are called genital plates, since they usually contain the openings of the genital ducts. One is often madreporite as well. Inside of the body is a spacious ccelom, to the walls of which the thin- walled alimentary tract is fastened by a mesentery. In the Clypeastroids this tract forms a simple spiral, but elsewhere it ascends from the mouth, turning once, and then, bending on itself, coils in the reverse direction to IV. ECHINOIDEA 305 the anus (fig. 304). Usually the first portion of the canal is accompanied by a siphon, an accessory tube opening into the main tube at either end. Except in the Spatangoids the mouth is surrounded by five sharp-pointed ed \ nd d A st FIG. 304. — Sea urchin opened around the equator. A, ambulacral area; 7. intrr- ambulacral area; L, lantern; d, intestine; ed, anal end of intestine; g, gonads; nd, siphon; oe, oesophagus; p, p', ring canal and Polian vesicles; st, stone canal. calcareous teeth, which in the Regularia are supported by a complicated system of levers, fulcra, and muscles, the 'lantern of Aristotle' (fig. 305). The ring canal and the ring of the blood system lie on the lantern, the stone canal and septal organ ('heart') extending upwards from them (fig. 303). The blood-vascular ring gives off two blood-vessels which run along the alimentary canal, while from the ring canal arise five ambu- lacral or radial canals which run on the inner side of the test, accompanied by nerves which, enclosed in a tube of infolded ectoderm, radiate, from a nerve ring. The gonads are five (rarely four or two) unpaired organs in the aboral half of the test, FIG. 305. — Aristo- tle's lantern of Stron- Ih'idus opening through the genital plates, that is, interra- ^^f ^aSli; I', dially as in the starfish. teeth. Order I. Palsechinoidea. Paleozoic forms with five ambulacral areas, the interambulacral areas con- taining more than two rows of plates. Melonites. Order II. Cidaridea (P^egularia). Ambuiacral areas band-like, body more or less spherical, mouth and anus polar. Common urchins; Toxopneustes* Strongylocentrotus* Arbacia* Ccelo- pleurus* (fig. 301). 20 306 ECHINODERMA Order III. Clypeastroidea. Flattened echinoids with central mouth and teeth; anus in the posterior inter- radius, sometimes marginal; five petaloid ambulacral areas. Clypeastcr (fig. 302), Echinarachnius* (sand dollar, fig. 306), Mellita,* with holes through the test. Order IV. Spatangoidea. Bilateral flattened forms more or less heart-shaped; mouth and anus ex- centric, no teeth; usually five petaloid ambulacral areas and four genital plates. From the forward position of the mouth it follows that only two ambulacral areas (bivium, p. 291) are upon the lower surface. Warmer seas. Spatangus* (fig. 307), Echinocardium, Brissus. a A FIG. 306. FIG. 307. FIG. 306. — Oral (A) and aboral (B) surfaces of the sand dollar, Echinarachnius par ma. a, anus; g, genital pores; /, ambulacral areas; m, madreporite; o, mouth. FIG. 307. — Young Spatangus purpureus (after Agassiz), the spines removed, oral surface. In front, the slit-like mouth; behind, the anus. The bivium without tubercles. Class V. Holothuroidea. The sea cucumbers are most removed of any group from the typical echinoderm appearance. At the first glance, except in Psolus, the skin appears naked and the characteristic plates absent. Yet these are im- bedded in the skin in the shape of plates, wheels, and anchors. The integument is leathery and muscular, with longitudinal and circular fibres. The saccular body gives these forms a worm-like appearance, strength- ened by its elongation in the main axis, and with the mouth and anus at the poles. Unlike other echinoderms these move with the main axis parallel to the ground, a condition which, to a greater or less extent, leads to a replacement of radial by bilateral symmetry. One surface (trivium) becomes ventral, the bivium dorsal, and in many the trivial ambulacra alone are locomotor, those of the bivium being tactile or wholly absent. The alimentary canal (fig. 308) (except in Synapta) is coiled in a uniform manner, although many minor convolutions may obscure this. V. HOLOTHUROIDKA It passes backwards in the median dorsal interradius, forward in the left ventral interradius, and then back in the right dorsal interradius to the anus. It is held in position by mesenteries (fig. 309), and near the anus by numerous muscular filaments. Into the terminal portion one or two FiG. 308. — Anatomy of Caudina arenata (after Kingsley). a, anastomoses of dorsal blood-vessel; b, branchial tree; d, dorsal blood-vessel;/, mesenterial filaments; g, genital opening; /, alimentary canal; /, longitudinal muscles; »t, mouth; <>, genital duct; p, pharyngeal ring; r, gonads, cut away on right side; /, ampulke of tentacles; •v, ventral blood-vessel. branchial trees may empty. These are tubular sacs with small branched outgrowths which are filled with water. They are respiratory, and are periodically filled with fresh water. In many species 'L'ui'icrian organs' occur; these are morphologically specially modified portions of the branchial tree and are either connected with them or separately with the 308 ECHINODERMA cloaca. Many zoologists regard them as defensive structures because of their sticky nature and because they can be cast out through the anus. The oesophagus is usually surrounded by a ring of five radial and five inter- radial plates which serve as points of attachment for the longitudinal muscles. Just behind it lie the ring canal, ring nerve, and the ring of the blood system, each giving off five radial branches which run inside the muscular sac of the body. From the beginning of the radial canals (rarely, as in Synapta, from the ring canal) tubes extend out- ward to form the extremely sensitive retractile tentacles which surround the mouth, and which either branch (Dendrochirotas) or bear frilled shield-shaped extremities (Aspidochirota;). A single Polian vesicle is usually present, and the occasionally branched stone canal connects (except in the Elasipoda) with the ccelom. FIG. 309. — Transverse section of Holutliuriatub'ulosa (after Lud- wig) . (/, digestive tract ; db, dorsal blood-vessel; g, gonad duct; h, skin; /;;/, longitudinal muscles; Iw, left branchial tree; m, mesen- Blood-vessels going from the vascular ring tenes; rl,r2, ambulacra! complex of bivium (ambulacral vessel) and nerve; r3-r3, same of trivium; rw, right branchial tree. form rich anastomoses on the alimentary canal. Only a single gonad (or a pair of united gonads) occurs. This consists of numerous tubules which open interradially, usually near the mouth. The regenerative powers of these animals are of interest. In unfavorable conditions they void the whole viscera and yet may live and reproduce the lost parts. In certain species are found a few parasites. One or two harbor a small fish (Fierasfer) in their cloaca and branchial trees, while parasitic snails (Entoconcha, Entocolax, Entcroxcnus) live in several species and a mussel, Entovalva mirabilis, in another. Order I. Actinopoda. Radial canals present, sending branches to the tentacles and ambulacra when present. Divided into Pedata, with ambulacra, and Apoda without. The PEDATA include the HOLOTHURID^E with peltate tentacles, (Aspido- chirotae). Holothuria* in warmer waters, one species forming the trepang of Chinese markets. Of the forms with branched tentacles (Dendrochirota?) are the CUCUMARIID.E, Citcumaria* Psolus* Thyonc* The deep sea ELASIPODA have statocysts and peculiar dorsal ambulacral processes. The APODA are represented by Caitdina* and Molpadia* Order II. Paractinopoda. No radial canals nor ambulacra. Tentacular canals arising from ring canal. Myriotrochw* Synapta* with statocysts, Oligotrochus* In Pelago- thuria the anterior end is expanded to a disc with tentacular processes, used in swimming, like the bell of a medusa. Summary of Important Facts. i. The ECHINODERMA share the radiate structure with the Coelenterata, but differ from them (a) in the numerical basis of the symmetry (five) ; (b) in that, as embryology shows, they have descended from bilateral forms. SUMMARY OF IMPORTANT FACTS 309 2. Farther characters are the existence of a ccelom, the ambulacral system, and the mesodermal spiny skeleton, which has given the name to the phylum. 3. The ambulacral system is locomotor and occurs nowhere else. It consists of a sieve-like madreporite (not always present), which passes water to the stone canal, and from this to the ring canal and the five radial canals to fill the ampulla and ambulacra. Lateral branches supply the tentacles and cause their extension. "%»"•' I- £. aN^tfvxt* s ^^^MV M^-^-i - FIG. 310.- "ucumaria frondosa, sea cucumber (from Emerton). 4. Blood-vessels and nerve cords run in the same radii as the radial canals of the ambulacral system; stone canal, madreporite, septal organ and genital ducts are interradial. 5. The Echinoderma are divided into five classes: (i) Asteroidea, (2) Ophiuroidea, (3) Crinoidea, (4) Echinoidea, and (5) Holothuroidea. 6. The ASTEROIDEA have a disc and (usually) five arms into which the gastric pouches and hepatic cssca extend. The ambulacral groove open. 310 MOLLUSCA 7. The OPHIUROIDEA also have disc and arms, but the ambulacral groove is closed and the hepatic caeca absent. 8. The CRINOIDEA have a cup-shaped body bearing arms, usually branching, with pinnuke, and a stalk, usually with cirri by which they are attached, either permanently or in the larval stages. In these latter free forms only the centrodorsal persists as the remains of the stalk. The Crinoidea are 'subdivided into Eucrinoidea, Edrioasteroidea, Cystidea, and Blastoidea. 9. The ECHINOIDEA are usually spherical or oval, armored with calcareous plates which extend as five pairs of ambulacral and five of inter- ambulacral meridional bands from peristome to periproct. 10. The ambulacral plates end at the periproct with an unpaired ocular plate; the interambulacral with a similar genital plate. The madre- porite is fused with one of the genital plates. 11. The regular sea urchins have the anus in the periproct, the mouth in the peristome; the ambulacral areas band-like. 12. The Clypeastroidea have a central mouth, the anus outside the periproct in the posterior interradius; the ambulacral areas petaloid. 13. The Spatangoidea are markedly bilateral, the mouth anterior, the anus posterior; ambulacral areas petaloid. 14. The HOLOTHUROIDEA are elongate and worm-like; the skeletal system greatly reduced; they are more or less bilaterally symmetrical and have usually a single gonad and one or two branchial trees. They are divided into Actinopoda, with radial canals, and Paractinopoda, without. PHYLUM VI. MOLLUSCA. At the first glance the molluscs, like the leeches and flatworms, appear like parenchymatous animals. A spacious coelom is absent; what was formerly regarded as such is a system of blood sinuses surrounding the viscera, and is especially well developed in the snails. More recently it has been shown that the molluscs have descended from ccelomate animals, in which, by encroachment of connective tissue and muscles, the coelom has been reduced to inconspicuous remnants, the pericardium and the lumen of the gonads^ Where the molluscan features are well developed, as in the snails, four parts may be recognised (fig. 311,5). The visceral sac forms most of the body; it is less muscular'than the rest and contains the alimentary tract, liver, nephridia, and gonads. In front it is continuous with the head, often separated by a neck, which bears the mouth and the most important sense organs, eyes and tentacles. Below, the visceral sac passes MOLLUSCA 311 into the foot, a muscular mass, usually used for locomotion. The mantle or pallium, a dermal fold, extends downward from the body and encloses a part of the body. In the Acephala (C) it has two halves, but in the snails (B) and cephalopods (A) it is unpaired, and either extends down on all sides (Chiton, Patella), or, like a cowl, covers the anterior side (most gasteropods), or envelops the posterior part of the body (pteropods, cephalopods). The mantle is important in two ways: its outer surface B FIG. 311. — Diagrams of three molluscan classes. A, a cephalopod (S?pia)\ B, a gasteropod (Helix); C, an acephal (Anodonta). a, anus; c, cerebral ganglion ;/M, foot; m, mantle chamber; sch, shell; t, siphon; v, visceral ganglion. Visceral sac dotted; mantle lined, shell black. is covered with epithelium which, like that of the adjacent surface, may secrete shell, a thick layer of organic matter (conchioliri) largely impregnated with calcic carbonate. The inner surface of the mantle, together with the outer surface of the body, bounds a space, the mantle cavity, which, from its most important function, is also called the respiratory chamber. Since most molluscs are aquatic, special vascular processes of the body, the gills or branchial, lie in this space; in the terrestrial forms it contains air and with a richly vascular dorsal wall, serves as a lung. 312 MOLLUSCA From the foregoing it will be seen that the mantle must exert an in- fluence on the shell and on the respiratory organs. Paired mantle folds form two valves, right and left, to the shell; a right and left branchial chamber, and right and left gills. With an unpaired mantle the shell is always unpaired, while the gills may retain their primitive paired condition. The gills in the mantle cavity are called ctenidia, from their resemblance to combs with two rows of teeth. Each consists of an axis (back of the comb), con- taining the chief blood-vessels, and two rows of branchial leaves. The whole is united to the wall of the branchial cavity by the axis (fig. 351). Many aquatic forms lack ctenidia, and then the respiration is either by the skin or by accessory gills which differ from ctenidia in structure and position (usually outside the mantle cavity). Those parts of the surface which are not covered by the shell have a columnar epithelium which is frequently ciliated and which contains large unicellular mucus glands (fig. 29), especially abundant on the edge of the mantle. These give these animals the soft slippery skin which is implied in the name Mollusca (moll is, soft). Although head, foot, and mantle are very characteristic of the molluscs, they are not always present. In the Acephala there is no distinct head region; many gasteropods lack the mantle and hence the shell and mantle cavity; in the Cephalopoda the foot is converted into the siphon and arms. rr. f\r — ^=-^_ i""-/ "• *ru>\ jm w B FIG. 312. — Nervous systems of Molluscs. A, most gasteropods; B, acephals; C, cephalopods and pulmonates. c, cerebral; pa, parietal, pe, pedal, pi, pleural, and v, visceral ganglia. The nervous system has some highly characteristic features. As a rule it consists of three pairs of ganglia associated with important sense organs and connected by nerve cords. One pair lies dorsal to the oesoph- agus and corresponds to the supracesophageal ganglion of the worms; it is the brain (cerebrum] and supplies the tentacles and eyes. A second pair lies ventral to the alimentary tract on the front part of the muscle mass of the foot ; these are the pedal ganglia with which the statocysts are connected. The third pair, the visceral ganglia, are also ventral, and near them are the third sense organs, widely distributed through the Mollusca, and from position and structure are regarded as organs of smell (osphra- dia). They are thickened patches of ciliated epithelium in the mantle MOLLUSC A 313 cavity. Pedal and visceral ganglia are united to the cerebrum by nerve cords, the cerebropedal and cerebramsceral connectives respectively. Ac- cordingly as these connectives are long or short the ganglia are wide apart or united into a nerve mass around the oesophagus. Primitive Mollusca (Amphineura) have a simpler condition. The cerebral ganglia are connected by a ring around the oesophagus (rig. 315, B). From it are given off two pairs of longitudinal nerve tracts, the ventral or pedal cords, and lateral or pleural cords, the latter united by a loop dorsal to the anus. By a concentration of ganglion cells in the higher molluscs the pedal cords give rise to the pedal ganglia, and similarly the pleural cords form three pairs of ganglia, the plcural and the parietal, as well as the visceral already mentioned, of the cere- brovisceral cord (fig. 312, A}. The pleural ganglia are connected with the pedal by nerve cords; the parietal innervates the osphradium. When farther concentration takes place the pleural may unite with the cerebral, and the par- ietal with the visceral (B), or both may fuse with the visceral (C). In the latter case (pulmonates, cephalopods) the visceral ganglion (in the wider sense) is associated with the pedal by the pleuropedal connective; while in the other (lamellibranchs, scaphopods) the connective is fused with the cerebropedal. Although the statocyst receives its nerve from the pedal ganglion, the centre of innervation lies in the cerebrum. In the Nuculidae the statocysts retain their connection with the parent ectoderm by means of a canal, which though closed, remains in part in the Cephalopods. Besides accessory eyes in various places, there are cephalic eyes, in general structure like those of the annelids. They are pits in the skin, the bottom differentiated to a retina. Usually they close to a vesicle, but only in the cephalopods do they reach a high development (fig. 349)- The heart, which lies dorsally, is arterial and consists of auriclap and ventricles. The ventricle is always unpaired; there are two auricles where two gills exist from which the blood flows to the heart, but with the loss of one gill one auricle may disappear. Distinct arteries and veins occur; capillaries are found only in the Cephalopoda, while in the lower molluscs (especially Acephala), the smaller arteries open into lacunar spaces which were formerly regarded as the body cavity. A completely closed vascular system does not exist even in the Cephalopoda. The heart is enclosed in a spacious sac or pericardium, which, with few exceptions, is connected with the nephridia by a ciliated canal (nephrostome), and in many molluscs (Cephalopoda, Solenogastres) is also related to the gonads. These facts support the view that the pericardium and the lumen of the gonads are the remnants of the ccelom; for here, as in the- annelids, the nephridia open by ciliated nephrostomes into the ccelom, and the sexual cells arise either from the ccelomic walls or from sacs cut off from them. Nephridia and sexual organs are primitively paired, but frequently are single by the degeneration of those of one side. The animals are either hermaphroditic or dioecious, but the gonads are always very large. 314 MOLLUSCA Even more room in the visceral sac is demanded by the digestive tract in which oesophagus, stomach, a coiled intestine, a voluminous liver, and usually salivary glands may be recognized. The liver is usually a paired tubular gland, emptying into the stomach. It not only digests fat and stores up glycogen, but forms an enzyme (cytase) which converts cellulose into sugar. The radula or lingual ribbon is also a characteristic organ, and its absence from the Acephala is probably the result of degeneration. It is a plate or band armed with teeth which lies on the floor of the pharynx- on a ventral ridge, the tongue, and is used for the comminution of food (figs- 334, 335)- Reproduction is exclusively sexual; budding, fission, or partheno- genesis being unknown. The eggs are usually united in large numbers, FIG. 313. — Veliger larva (trochophore) of Teredo navalh (from Hatschek). A, anus; /, stomach; ./,, intestine; I,, liver; LM.d, LAI.v, dorsal and ventral longitudinal muscles; Mes, primitive mesoderm cells; MP, teloblast; NepJi, protonephros ; O, mouth; Oe, oesophagus; R, rectum; 5, shell; Schl, hinge; SM.h, SM .v, posterior and anterior adductors; Sp, apical plate; Wkr, wkr, pre- and postoral ciliated bands; ws, cilia of apical plate. in a jelly and are either rich in deutopksm or are enveloped in a nourish- ing albumen. A few molluscs (e.g., Paludina vivipara) are viviparous. A metamorphosis is of wide occurrence, in which a veliger larva escapes from the egg (fig. 313); in this can be recognized head, foot, and mantle, even when one or the other of these is lacking in the adult. This shows that the frequent absence of mantle, shell, or head is not a primitive condition, but can only be explained by degeneration. The name I. AMPIIINEURA 315 veliger arises from the velum, a strong circle of cilia, which surrounds a velar field in front of the mouth, and which serves as a locomotor organ for the larva. In some cases (fig. 314, B) it is lobed like the trochus of a Rotifer. The veliger recalls the annelid trochophore and serves for the distribution of the species; it is therefore of great importance for animals A B FIG. 314. — Veliger stages, A, of a snail; B, of a Pteropod (from Gegenbaur). o, shell; op, operculum; p, foot; t, tentacle; v, velum. which, like most molluscs, are sedentary or slow-moving. In cases with- out metamorphosis (Cephalopoda, Pulmonata, etc.) the veliger stage is frequently indicated during embryonic development by a ridge of cells surrounding a preoral velar field Class I. Amphineura. These forms, some of which appear in the Silurian, are clearly the most primitive of molluscs, and are distinguished by a marked bilateral sym- metry. The nervous system already described (p. 313) consists of pleural and pedal cords with scattered ganglion cells and no ganglia, these cords being connected by numerous commissures (fig. 315, B) Sub Class I. Placophora (Chitonidce) . The chitons were formerly included among the gasteropods because of the presence of a creeping foot and a radula. They are at a glance distinguished from them by the rudimentary condition of the head (which lacks tentacles and eyes), and the peculiar shell. This last consists of eight transverse plates, overlapping like shingles which allows the animal to roll into a ball. The edge of the mantle extends beyond the shell and is covered with spines, while in the mantle cavity beneath are, right and left, a series of ctenidia. Nerves enter the shell and end with noticeable sense organs (aesthetes and, in some, eyes, fig. 316). There are no stato- cysts. The symmetry of the body is also expressed in the viscera. 316 MOLLUSCA The anus is medial, and right and left of it are the openings of the neph- ridia and sexual organs. The sexes are separate, the gonads unpaired, while, corresponding to the paired arrangement of the gills, there are two auricles to the heart. Trachydermon,* Amicula* Cryptochiton* B FIG. 315. — Lhiton squamosus, dorsal view (after Haller). A, the entire animal» B, after removal of shell and viscera; a, anus; C, brain; K, ctenidia; o, mouth; P, pedal nerve cord; pi. pleurovisceral nerve cord. a. .... FIG. 316. FIG. 317. FIG. 316. — Eye and aesthetes of Acanthopleura spiniger (after Moseley). 0, macrassthete; b, micraesthete;/, calcareous cornea; g, \ens;h, iris; k, pigmented capsule; n, p, nerves; r, retina. FIG. 317. — Neomenia carinata, ventral and side views (after Tulberg). a, anterior; b, posterior; c, ventral groove. Sub Class II. Solenogastres (Aplacophora). Wormlike forms without a shell (occurs in the larva of Dondersia) ; instead of a foot there is a longitudinal ventral ciliated groove; the radula may be lost; in Conchoderma it bears but a single tooth. The gills are either small or wanting. The usually hermaphrodite animals have the gonads emptying into the peri- cardium and thence by the paired nephridia ( ?). Marine, living in ooze or sand. Chatodcrma,* Neomenia, Dondersia. II. ACEPHALA 317 Class II. Acephala (Lamellibranchiata, Pelecypoda). These have, among the molluscs, the least powers of locomotion. Some are fixed, the majority burrow slowly through sand or mud; only a few spring by means of the foot or swim by strokes of the valves. Hence they need more protection than other species, and this is afforded by the strong shells in which the body can usually be completely enclosed. This shell recalls that of the brachiopod in that it consists of two halves or valves, but these valves are right and left and hence are usually similar in shape. Only when the animal rests permanently on one side is this symmetry lost (the extreme is reached in the fossil Ritdistes), and then the symmetry of the soft parts is affected. FIG. 318. a FIG. 319. FIG. 318. — Left valve of Crassatella plumbea, inner and outer surfaces (from Zittel). The outer surface showing lines of growth; no pallial sinus. FIG. 319. — Right valve of Mactra still tonim, with pallial sinus (from Ludwig- Leunis). Letters for both figures: a', anterior, a", posterior adductor scar; e, hinge; /, internal ligamental groove; in, pallial line; s, pallial sinus. The two lobes of the mantle which secrete the shell on their outer surface arise from the back of the animal and grow downwards, forwards, and backwards, so that they envelop the whole (fig. 322). Hence the oldest and thickest part of the shell, the wnbo, occurs near the back (fig. 318). Around this the lines of growth are arranged concentrically, lines which show how, by gradual growth of the mantle, the shell has in- creased in size. On the back the valves approach each other, and in the majority are movably connected by a hinge, which consists of projections or teeth on one valve fitting into depressions in the other. The valves 318 MOLLUSCA are opened by an elastic hinge ligament usually placed dorsal to and be- hind the hinge. The shell is closed by adductor muscles which extend through the body from shell to shell, leaving their impressions on the inner surface (fig. 319). Usually there occur an anterior and a posterior adductor equally well developed (Dimyaria) ; less frequently the anterior is rudimentary (Heteromyaria) or entirely disappears (Monomyaria). When the muscles are relaxed the elastic ligament opens the valves. The Jieterodont hinge is the typical form (fig. 319); each valve bears a group of teeth near the umbo, those of the left alternating with those of the right. Besides these cardinal teeth there are lateral teeth in front and behind, often pro- duced into ridges. The ligament lies behind the hinge and is usually visible from the outside (external ligament), but is occasionally transferred to the interior (internal ligament, fig. 318). The so-called schizodont and desmodont hinges are modifications of the heterodont. Then there are Acephala of appar- ently primitive character which either lack the hinge (dysodont), or have one composed of numerous teeth in a series symmetrical to the umbo (taxodont), or of two strong teeth likewise symmetrical to the umbo (isodont}. In these cases the ligament is developed in'front of as well as behind the umbo, and may be either external or internal. C FIG. 320. — Ventral views of siphonate and asiphonate acephals. .4, An^donta nea; B, Isocardia cor; C, Lutrana elliptica. a, anal siphon; b, branchial siphon; /, foot; k', outer, k", inner gill lamella; m, mantle; s, shell. Since the secretion of shell takes place most rapidly at the edge of the mantle, both are closely united, the union being strengthened by small muscles. So the edge of the shell has a different appearance from the rest, this part being marked off by a pallial line parallel to the margin (fig. 318). In many species (the Sinupalliata) the line at the hinder end makes a large bay (pallial sinus} (fig. 319, s}. Since the mantle folds are membranes with free margins, it follows that when the shell is closed these edges are pressed together, which would prevent the free entrance and exit of water. To accommodate this each mantle has its margin II. ACEPHALA 31 !> excavated at the posterior end, so that when brought together two open- ings, an upper and a lower, result (fig. 320, C). The lower of these is the branchial opening by which fresh water passes into the mantle cham- ber; it flows out after passing over the gills, along with the faeces, through the upper or cloacal opening. In many bivalves the free edges of the mantle grow together, leaving three openings (fig. 320, B), one for the protrusion of the foot, the others the two just described, now called the branchial and cloacal sip/ions. By further development the margins of these openings are drawn out into two long conjoined siphonal tubes (A), which for their retraction need special muscles; these are attached to the valves and thus cause the pallial sinus referred to above. In the shell there are three layers (fig. 321): on the outside a thin organic cuticula and below two layers largely of calcic carbonate. In many these two layers are distinguished as the prismatic layer and the nacreous laver, the first consisting of closely packed prisms; the nacreous layer of thin lamellae generally .-..- FIG. 321. — Section of shell of Anodonta. c, cuticula; p, prismatic layer; /, nacreous layer. parallel to the surface. These produce diffraction spectra and so the iridescent appearance of the shell; the finer the lines thus formed the more beautiful the play of colors. This is especially noticeable in the mother-of-pearl shell J/Y/, propodium; ps, penis; /, II, III, cerebral, pedal, and visceral ganglia. metapodium (fig. 342), the latter forming a tail-like elongation of the body. The propodium is vertically flattened and serves as a swimming organ. The Heteropoda are predaceous and extremely voracious. ATLANTIDVE and CARIN- ARIID^:, with shells; PTEROTRACHEID^;, without. Order II. Opisthobranchia. The Opisthobranchia have not varied from the primitive symmetry to such an extent as have Prosobranchs and Pulmonates. The anus is in or near the median line, although it may be far forwards. The nervous system is ortho- neurous, the twist being straightened (except in Actaeonida;). The heart also retains its primitive position, receiving blood from behind and forcing it forwards to the body through the aorta (fig. 337). In rare cases a (right) ctenidium, a poorly developed mantle, and a thin shell occur. Usually these have been lost and the place of the ctenidium is taken by accessor; gills of various forms or a dermal respiration occurs. The larvae have well-developed mantle and shell. Many of the Opisthobranchs afford fine examples, in form and coloration, of protective resemblance. All are hermaphroditic and marine. Sub Order I. TECTIBRANCHIA. Mantle and usually shell and cteni- dium present, Bulta* Philme* Aplysia. Sub Order II. PTEROPODA Transparent pelagic forms which in most points agree with the Tectibranchs. IV. GASTEROPODA: PULMONATA 335 The head and usually eyes and tentacles are lacking, while the fins (greatly developed parapodia) are highly characteristic, giving the name 'wing-footed' to these forms. They have rarely a single ctcnidium. The THECASOMATA have shells, LIMACINID^E, HYALEID.*: The shells of br FIG. 343. FIG. 344. FIG. 343.- — Hvaltfa complanata from above (after Gegenbaur). a, anus; br, gill; c, heart; g, gonad; h, liver; m, mantle; oe, oesophagus; re, nephridium; v, stomach; II, pedal ganglion and otocyst. FIG. 344. — A, Clione papilionacea.* CAVOLINID^ make the 'pterpod ooze' of the deep seas. GYMNOSOMATA; shell lacking. Pneumodermon, Clime* Sub Order III. NUDIBRANCHIA. Shell, ctenidia, and osphradia lacking; most possessing accessory gills (ccrata) of varying form and distribution. DORIDIID^; (Fig. 345;. TRITONIID^;, FIG. 345. — Doris bilamellata* FIG. 346. — .•Eolidia papillosa (from Lud \vig-Leunis). (Dendronolus*}; ELYSHD^E, cerata lacking. In ^olidae branches of the diges- tive tract enter the cerata, expand distally to small sacs filled with nettle cells (p. 207) used for defense,; they are derived from hydroids on which these animals feed Order III. Pulmonata. In several respects the Pulmonata are intermediate between the Proso- branchs and Opisthobranchs. Like the latter they are orthoneurous and her- maphroditic (fig. 339). On the other hand, the respiratory organ is far forwards 336 MOLLUSCA near the head, with the result that the auricle is forwards, the aorta behind. The Testacellidye have the lungs at the posterior end of the body. Occasionally streptoneurous conditions occur (Chilina). The lung, the most characteristic feature of the order, has already been mentioned (p. 326). Many pulmonates are aquatic, but since they have no gills they must occa- sionally come to the surface to fill the lung with air, but some, which live at great depths in the Swiss lakes and consequently cannot reach the surface, use the skin and to some extent the lung for water-breathing. Several genera (Planorbis, Pulmobranchia, Siphonaria) have formed secondary gills. Sub Order I. STYLOMMATOPHORA. Four retractile tentacles, eyes at the tips of the second pair. The HELICID^, a well-developed shell. Helix,* many hundred species. Pupa* Acliatina, Bulimus, many tropical species. LiMACiDyE. Shell reduced, completely concealed in the mantle. Li max,* Arion* Sub Order II. BASSOMATOPHORA. Only one pair of non- retractile tentacles, eyes at their base. LIMN^EHXE, pond snails. Limncea,* Planorbis.* Class V. Cephalopoda. The Cephalopoda are distinguished among the molluscs by their size and high organization. The majority measure, including the arms, from FIG. 347. FIG. 348. FIG. 347. — Octopus tonganus from the side (after Hoyle). Funnel and mantle fold to the right; back and eyes on the left. FIG. 348. — Loligo kobiensis, ventral view (after Hoyle). eight inches to three feet in length, a few are smaller (two to seven inches), while especially rare are the giants, some of which may be over forty feet long. For a long time these large species were only known from the tales of sailors. In the last half-century some of these forms, belonging to the genus Architeuthis, have been stranded on the coasts of Newfoundland V. CEPHALOPODA and Japan. One of the Newfoundland specimens had a body twenty feet long from head to tail, and one of the arms was thirty-five feet in length. The body of a cephalopod is divided by a constriction into head and trunk. At the extremity of the head is the mouth, and around this a circle of arms or tentacles. Each tentacle is tapering and bears on its oral surface rows of suckers (in some species altered to hooks). The Octopoda have eight of these arms, all equal in size (fig. 347), four on the right side, four on the left. The Decapoda (fig. 348) have in addition two longer arms between the third and fourth of the Octopoda, counting from the dorsal side. These Dear suckers only on the enlarged tips and can be retracted into special pouches. ae Tnf JV.Qp FIG. 349. FIG. 350. FIG. 349 — Diagrammatic section of Cephalopod eye (after Gegenbaur). ae, argentea (chorioid); C, cornea; ci, ciliary process; go, optic ganglion; ik, iris; k, carti- lages; L, lens; p, pigment layer; Re, cellular layer of retina; Ri, rod layer of retina; u1, white body. FIG. 350. — Schematic section of eye of Nautilus (from Balfour). .1, aperture <>f optic cup; hit, iris-like fold of integument, N op, optic nerve: R, retina. Behind the tentacles are the pair of large eyes which superficially closely resemble those of the vertebrates, since they have a transparent cornea and a large pupil surrounded by an iris. Internally the resem- blance is not less pronounced (fig. 349). Behind the iris is a lens and a vitreous body, the latter being bounded by the retina and this in turn by a pigmented silvery layer, the argentea or chorioid, which contains cartilages 338 MOLLUSCA recalling the sclerotic coat. Two striking peculiarities separate these eyes from those of the vertebrates and show that they have arisen inde- pendently and have an entirely different developmental history, (i) The cornea in most Decapoda has an opening by which water enters the anterior chamber; (2) the layer of rods in the retina abuts against the vitreous body and the ganglionic layer lies behind, while in the ver- tebrates the reverse is the case. FIG. 351. — Sepia officinal is, the mantle and left nephridial sac opened to show the vena cava leading to the branchial heart, ti, anus; b, d, lock of siphon and mantle; g, genital opening; A", head; k, ctenidium; n, nephridial sac; «', nephridial opening; sp, nephrostome; /, ink sac; Tr, siphon. The foregoing description applies to but part of the Cephalopoda. The very different Nautilidas have, instead of tentacles with suckers, numerous shorter tentacles on lobular appendages, which are developed differently in the two sexes. The eyes are deep pits, opening to the exterior by a small aperture, the base of the pit being occupied by the retina, while lens, vitreous body, iris, and cornea are lacking (fig. 350). It is to be noticed that the other cephalopod eyes pass through a Nautilus stage. In the trunk anterior and posterior regions are distinguishable, the two passing into each other on the sides. The anterior (which corre- sponds only in part to the ventral side of other molluscs) is wholly covered by the mantle, a strong muscular fold, which takes its origin from the periphery of the body, often encroaching upon the back and always ter- minating with free margins at the head. On opening the mantle by a V. CEPHALOPODA 339 ventral incision (tig. 351) the two ctenidia (four in Nautihis) are seen on the sides. Between them, in the middle line, is the anus, and right and left of this and a little behind are the nephridial openings (four in ATau- tilns, which also has osphradia). More lateral are the sexual openings of which one (usually the right) is commonly absent. At the head the mantle opens by a transverse slit to the exterior, but it can be closed and fastened by various locking contrivances (in Sepia, Loligo, etc., by button- like projections (, pedal ganglion; r, passage connecting with ovary; r', mouth of pericardial sac in nephridial sac; s, cesophagus with dorsal salivary gland; s/>, ventral salivary glands; st, stellate ganglion; sv, sympathetic ganglion; /, ink sac; v, visceral ganglion; rk, auricle of systemic heart; x, spiral blind sac. dorsal mass represents the cerebral ganglia; connected with this by broad commissures, the pedal and visceral (viscero-pleuro-parietal) ganglia lie close together ventrally. With these parts are associated V. CEPHALOPODA 343 upper and lower buccal ganglia. The large optic ganglia, in the optic nerve arising from the cerebrum and enclosed ventrally in the 'white- body,' a lymphoid mass, are especially characteristic, as are the gang- lia stellata, right and left at the anterior edge of the mantle (fig. 356, si), which owe their name to the radiation of fibres to innervate the mantle. An. unpaired sympathetic ganglion lies at the junction of stomach and intestine. Cerebral, pedal, visceral and optic ganglia are enclosed in the cephalic cartilage, which has the shape of a ring with wing-like processes. The complicated statocysts lie in the ventral arch of the ring. The sense of smell is highly developed. Apparently it resides in a pair of spots of skin between the eyes and the mantle which are richly supplied with nerves. In the Decapoda these are sunk in pits, in the Octopoda they form papillae. In Nautilus, which has also two pairs of osphradia, there is a papilla with a ciliated groove, beneath each eye, corresponding to the olfactory organ of the other groups. Most noticeable of the circulatory structures is the presence of two kinds of hearts (fig. 356). The systemic heart consists of two (four in Nau- tilus) auricles receiving arterial blood from the gills, and a median ventricle from which arise anterior and posterior aortae. Then there is a branchial heart at the base of each ctenidium which receives the blood from the vena cava and pumps it into the gill. Of venae cavae there are an anterior unpaired and two posterior paired op FIG. 357. — Nervous system oiSepia ofiicinalis from the side. gbi, in- trunks, the former dividing and sending a branch fe,rior buccal ganglion; gas, superior buccal to each branchial heart. These trunks are con- nected with the nephridia. The nephridial open- ganglion; gc, cerebral ganglion ; gp, pedal gan- glion; gv, visceral gan- ings (p. 339) lead to two spacious sacs through giion; n,bi buccal mass; which the veins pass obliquely. This part of the oe> oesophagus; op, optic ganglion. blood vessels bears venous diverticula which pro- ject into the nephridial sac and are covered with an epithelium of excretory cells. Near its mouth each nephridial sac communicates by a nephrostome with the (usually large) coelom. In the Octopoda the coelom is reduced to the gonads and narrow canals leading from the nephrostome to the gonads and branchial hearts, but else- where there is a well-developed system of connected cavities, consisting of the pericardium around the systemic and branchial hearts and the thin-walled genital sac, one wall of which bears the genital ducts, while on the other the sexual cells arise or the ducts of a separate sexual gland open (fig. 358). 344 MOLLUSCA - n The gonads of the always dioecious Cephalopoda are unpaired and lie far back in the visceral sac. The ducts in the female Octopoda (rarely in the males) and in some Decapoda (Oigopsida) are paired. In Nautilus only the right duct is functional in either sex, although the left is well developed. Elsewhere there is only the left duct. The oviducts are saccular with glandular walls; independently of them two pairs of glands open to the exterior, the accessory glands and the large nidamental glands. The vas deferens (fig. 358) is more complicated. It has swellings known as seminal vesicle, prostate, and Needham's sac, in which the spermatophores are stored. These latter have such a complicated structure and show such motions when swollen with k water that they were long regarded as para- sitic worms (fig. 359). The spermatophores are conveyed to the female by means of more or less modi- fied (hectocotylised) tentacles of the male. In a few genera the whole tentacle becomes a 'Hectocotylus' (fig. 360). It swells at its base to a sac in which the peripheral end is enclosed. This part, which contains a canal for the spermatophores, cuts loose from the male, and can creep about for days in the mantle chamber of the female. Since it appears like an independent animal, it was first described as a parasitic worm under the name Hectocotylus. In others the hectocotylization is not carried so far. t FIG. 358. — Male sexual organs of Sepia officinalis (after Grob- ben). a, ccelomic sac passing to the left and above into the pericardium; c, ccelomic canal to the vas deferens; d, vas def- erens; d', its opening to ccelom; /, portions of ccelom; n, Need- ham's pocket; n', its mouth; p', p2, prostates; t, testis; t', its opening to ccelom; t'5, seminal vesicle. FIG. 359. — Spermatophore of Sepia (from Hatschek, after Milne Edwards), a, dis- charging apparatus; b, packet of spermatozoa; c, envelope. The eggs are either fastened singly to aquatic plants or are laid in large gelatinous masses. They are rich in yolk, and in consequence undergo partial discoidal segmentation (fig. 105). The blastoderm, on the end of the oval egg, forms the anlagen of the separate organs (eyes, arms, siphon, and shell gland) as flattened projections. Later the embryonic body becomes distinct from the V. CEPHALOPODA 345 yolk, which, enclosed in a cellular envelope, remains attached to the rest, near the mouth, until it is absorbed in the growth of the young and the animal is ready for hatching (fig. 361). FIG. 360. — Male of Argonauta argo (after Muller, from Hatschek). 1-4, arms of right side; i.~4., arms of left side; 3, hectocotylised arm, at the left in its sac, at the right protruded. FIG. 361. — Embryos of Loligo pealei (orig.). a, arms; e, eyes;/, fin; g, ctenidia; h, statocyst; m, mantle; s, siphonal folds and siphon; v, anus; y, yolk sac. The Cephalopoda are exclusively marine. Some inhabit rocky shores, others the high seas. All are carnivorous and in turn are preyed upon by fishes, etc. Classification is based upon the number of gills and number and character of the arms. 346 MOLLUSCA Order I. Tetrabranchia. With four gills, four auricles, and four nephridia; numerous tentacles without suckers, a well-developed chambered shell (fig. 352), siphon of two separate epipodia, and simple eyes (fig. 350). Four living species, all belonging to Nautilus. FIG. 362. — Octopus bairdii* (from Verrill). A hectocotylised arm on the right side. 363. — Argonauta argo, paper sailor, female (after Rymer Jones). The animals, which live in the Malaysian regions, are rare, but their shells are abundant. In past time the tetrabranchs were very abundant; NAUTILID^, with straight (Orthoceras) or coiled shells (Goniatitcs, etc.), paleozoic. They V. MOLLUSCA: SUMMARY OF IMPORTANT FACTS 347 had simple septa. AMMOXITID/E, folded septa, largely mesozoic. Their pertinence to the tetrabranchiates is assumed from the character of the shell. Order II. Dibranchia. With two nephridia, two gills, and two auricles; eight or ten arms with suckers; highly organized eyes; shell rudimentary or absent. Sub Order I. DECAPODA. Ten arms, body with lateral fins. Rudimen- tary shell usually present. SPIRULID^;, with internal chambered loose-coiled shell. Spirula (fig. 353). OIGOPSIDA, with perforated cornea (p. 338) and two oviducts. Ommastrephes*; Architcntkis,* the giant squid (p. 336). MYOPSIDA. Oviduct single (left); cornea unperforated. Loligo* common squid; Rossia*; Sepia, cuttle fish, furnishing the 'cuttle bone' fed to cage birds, and the pigment sepia. Sub Order II. OCTOPODA. Eight arms, webbed at their base; shell very rudimentary, sometimes fragmentary or wanting; oviducts paired. Ocro- PODID^;, Octopus'^ (fig. 362), Alloposus.* ARGONAUTID.^, female with boat- like shell (fig. 363), males much smaller and without shell. In Argonautidse and PHILONEXID.E the hectocotylus separates. Summary of Important Facts. 1. The MOLLUSCA are parenchymatous animals with reduced coelom. They consist of head, visceral sac, mantle, and foot. 2. The head bears eyes and tentacles. 3. The foot is an unpaired muscular mass used in locomotion. 4. The mantle bounds the mantle cavity which is connected with respiration; it either functions as a lung or covers the gills (ctenidia). It secretes the shell from its outer surface. 5. Foot, head, mantle, and with the latter the shell, may be lost in many groups. 6. The molluscs agree in the nervous system. Three pairs of ganglia, connected with three pairs of sense organs, occur: a, cerebral ganglia and eyes; b, pedal ganglia and statocysts; c, visceral ganglia and osphradia (olfactory). 7. The heart is dorsal and arterial; it is enclosed in a pericardium (reduced ccelom) which connects with the nephridia by nephrostomes. 8. There is always a single ventricle and, according to the number of respiratory organs, one, two, or four auricles. 9. The alimentary canal is well developed; the liver large; salivary glands usually present. In most there is a pharynx or buccal mass with radula and jaws. 10. A veliger stage is common in development. 11. The Mollusca are divided according to the respiratory organs and appendages of the body into five classes: (i) Amphineura; (2) Acephala; (3) Scaphopoda; (4) Gasteropoda; (5) Cephalopoda. 12. The AMPHINEURA have an extremely simple nervous system in which the ganglia are replaced by nerve tracts. 34S MOLLUSCA 13. The ACEPHALA lack head and cephalic appendages. 14. They are bilaterally symmetrical and have paired organs: mantle folds, bivalve shell, gills, nephridia, and gonads. 15. In many Acephala (Asiphoma) the mantle folds are completely separated ventrally. 16. In theSiplionata the lower edges of the mantle are united, leaving three openings: (i) in front for the foot; (2) behind and below, the branchial siphon for the ingress of water and nourishment; (3) behind and above, the anal or excurrent siphon for the water used by the gills and the fa?ces. 17. There are two pairs of gills (ctenidia), which maybe comb-like, filiform, or most commonly lamellar (lamellibranchs). 1 8. Correspondingly the heart has two auricles; the unpaired ventricle is usually traversed by the rectum. 19. The foot is a compressed muscular mass frequently provided with a byssus gland. 20. The shell consists of cuticular, prismatic, and nacreous layers. It is closed by one or two adductors and opened by an elastic ligament. 21. Some Acephals (Protoconc/ia) have primitive gills and hinge; others (Heteroconcha) are more highly developed. 22. The SCAPHOPODA are primitive forms with tubular shells. 23. The GASTEROPODA (Cephalophora, or snails) have a distinct head bearing eyes and tentacles; a creeping foot, an unpaired mantle (occasionally absent), and a univalve shell or none. 24. The mantle cavity contains one or less frequently two ctenidia, or these may be degenerate and a lung may occur. 25. Nephridia and auricles are rarely paired (with paired gills); the gonads, always unpaired, are hermaphroditic or dioecious. 26. The shell is always unpaired; it is usually coiled in a (right-hand) spiral, and is frequently closed by an operculum. 27. According to character of nervous system, sexual organs, heart, and respiratory organs the Gasteropods are divided in to (i)Prosobranchia; (2) Opisthobranchia; and (3) Pulmonata. 28. The Opisthobranchia are hermaphroditic; orthoneurous; have secondary gills (or none), and have the auricle always behind the ventricle; shell and mantle reduced or absent. 29. The Pteropoda are pelagic Opisthobranchs with wing-like pro- cesses of the foot and frequently reduced shell or none. 30. The Prosobranchia have the gills (ctenidia — occasionally paired) far in front, and in consequence the auricle in front of the ventricle ; they are streptoneurous and dioecious; the mantle and shell well developed. ARTHROPODA 349 31. The Heteropoda are pelagic Prosobranchia with foot divided into fin and tail, shell rudimentary or absent. 32. The Pidmonata are in some respects (orthoneurous and her- maphroditic) Opisthobranch-like; in other respects — as in position of heart, development of shell and mantle — like the Prosobranchs; the mantle cavity connects with a lung. 33. The CEPHALOPODA have no true foot; but its homologues are to be found in the siphon and in the tentacles, usually provided with suckers, on the head ; they have an unpaired mantle and mantle cavity and a single shell or none. 34. The mantle cavity contains one or two pairs of ctenidia. Water is forced from the mantle cavity through the siphon. 35. The number of auricles and nephridia corresponds with the number of ctenidia; besides the systemic heart there are one or two pairs of branch- ial hearts, elsewhere unknown in molluscs. 36. The sexes are separate. 37. The ink sac is peculiar to Cephalopoda. 38. The eye is (usually) highly developed (with retina, chorioid, iris, cornea, vitreous body, and lens), as is the nervous system, which has, in addition to the usual centres, optic, sympathetic, and stellate ganglia. 39. The eggs have a discoidal segmentation. 40. The Cephalopoda are divided into Tetrabranchia and Dibranchia. 41. The Tetrabranchia (extinct save for Nautilus) have four gills, a chambered shell, primitive eyes, and finger-like cephalic lobes in place of tentacles. 42. The Dibranchia have two gills, eight or ten tentacles with suckers, and the shell is reduced or absent. PHYLUM VII. ARTHROPODA. Under the term Arthropoda are included the crabs, spiders, insects, and myriapods, which, together with the annelids, were united by Cuvirr in his sub-kingdom Articulata. Annelids and arthropods agree in many features. They are, as the term articulates implies, segmented animals, and they differ from the vertebrates, which are also segmented, in the extension of the segmentation, the ringing of the body, to the external surface. The boundaries between the successive segments, which cannot be recognized in the skin of the vertebrate, are marked in the articulates by constrictions of the body wall, whence the old names evro/xa and Insecta, applied to these forms. The articulates are further characterized by a ladder-like nervous system (fig. 78), in which the brain is supple- 350 ARTHROPODA merited by a ventral chain composed of ganglia metamerically arranged. The most evident distinctions between the annelids and the arthropods are (i) the character of the segmentation and (2) the presence of jointed appendages. In superficial appearance the lines between the segments are con- stricted more deeply in the arthropods than in the annelids. The cause of this lies in the character of the integument (fig. 26, f), which is developed as a hard armor, in which two layers are recognizable, the epidermis ('hypodermis') and the chitinous layer. The epidermis is a thin epithelium, while the chitinous layer is of greater thickness and, since it is secreted by the epidermis, is stratified parallel to the surface. Its firmness is due to chitin, which is unlike most organic substances in its resistance to acids and alkalis; only under the action of sulphuric acid and heat is it broken up into sugar and ammonia. Frequently (Myriapods, Crus- tacea) the chitinous armor is strengthened by the deposition of calcium carbonate and phosphate. A firm coat would render the animal incapable of motion were there not joints between the parts. While the segments themselves are heavily armored, the cuticle between them is reduced to a delicate articular skin, and this is so protected by a kind of telescoping of the segments that injury in these softer regions is nearly impossible (fig. 364). Since the ringing of the body is connected with this armoring, it disappears with the need for such protection. The hermit crabs (fig. 406) illustrate this. These animals live with the abdomen inserted in a snail shell. That part of the body which projects from the shell is armored, while the abdomen is soft- skinned and without traces of external ringing. The hardened cuticula causes the periodic molting (ccdysis or exuviation } . When once hardened it is incapable of distention and so would prevent farther growth. Hence when the body has completely filled the shell, the latter splits in definite places and the animal crawls out of the old 'skin' (exui'ia) and rapidly increases in size, while the new cuticula is yet soft and distensible. Another result of the cuticula is seen in the peculiar relations of both ordinary and sense hairs. These are cuticular structures, each usually secreted by a single epidermal cell and renewed after each molt. Each hair has a ball-like base situate in a socket in the surrounding chitin, and hence is movable; it is traversed by a canal in which is a process of the underlying matrix cell. In the case of sensory hairs these structures are connected with a nerve (fig. 80). The sense cell has. two processes; one peripheral, which enters the axis of the hair, the other central, which runs as a nerve fibre to the central nervous system. The cell itself may be in the epithelium or situated deeper and interpolated as a ganglion cell in the sensory nerve. The muscles which are inserted on the integument are sesrmental in character FIG. 364. — Dia- gram of Arthropod jointing; .4, in ex- panded, B, in con- tracted condition; 1-4, rings with con- necting membranes, the muscles indi- cated by dotted lines, (after Graber). ARTHROPODA ) l and are arranged in metameric muscle groups. Frequently they are inserted on the chitin by special tendons, portions of the chitin drawn inwards. Through such infoldings there arises in many arthropods an 'entoskeleton.' Another important character is the heteronomous segmentation, which, in the lowest forms (Peripatns and myriapods), is little pronounced, hut elsewhere leads to a marked inequality of the regions of the body. A few segments at the anterior end always fuse and form a head (fig. 365, C) ; behind this there is usually a second segment complex, the thorax (per don} (T), and then a third, the abdomen (pleori) (A). An apparent FIG. 365. FIG. 366. FIG. 365. — Campodea staphylinus (from Huxley). A, abdomen; C, head; T, thorax. FIG. 366. — Euscorpius italicus. a, abdomen; c, cephalothorax; />, post- abdomen; 5, sting; i, chelicera;; 2, pedipalpi; 3-6, walking legs. Below chelicera enlarged. reduction of regions occurs when the head and thorax unite (fig. 367, C/) to form a cephalothorax; or the number of regions may be increased (fig. 366) by a division of the abdomen into abdomen proper (a) and post- abdomen (/>). Finally, in many arthropods (e.g., the mites or acarina, fig. 368) it is impossible to recognize regions or somites because internal fusion of parts has obliterated the external evidences of segmentation. In order clearly to understand what is meant by head, thorax, etc., requires a consideration of the second character distinguishing the arthro- pods from the annelids, the jointed appendages, which give the name to the former group. The arthropodan appendages are highly developed 352 ARTHROPODA parapodia, differing in being jointed to the body, in consisting of a series of joints themselves, and in having their intrinsic musculature. There is but a pair of appendages to a somite, and this belongs to the ventral surface. Hence it follows (Savigny's law) that although a region may show no external signs of segmentation, if it bear more than one pair of appendages, we conclude that it is a complex of at least as many somites as there are pairs of appendages. Thus the unsegmented head of an insect consists of four somites, the cephalothorax of a lobster of thirteen, for the one bears four, the other thirteen, pairs of appendages. Ontogeny supports this, for in the embryo the somites are clearly visible. This statement is not ex- actly correct, for in certain insects and in the lobster there is one more FIG. 367. — Pal(pmon serratus (from Lud \vig-Leunis). A, abdomen; Ct, cephalo- thorax. FIG. 368. — Gamasus coleoptratorum (from Taschenberg). somite which is entirely lost in the adult. It is not necessary that each somite in the adult should bear appendages, since these may disappear in growth without leaving a trace. While originally all were locomotor, the appendages subserve many functions (fig. 369). Locomotor appendages (pereiopoda, feet or legs) are long and consist of a number of joints which may form flattened oars or may be provided with claws for creeping (8). Besides locomotor ap- pendages there are tactile appendages or antenna (i), chewing appendages (jaws, mandibles, maxilla, 2-4), false feet or pleopoda (9) of varying func- tions, and forms — maxillipeds (5-7) — transitional between jaws and legs. Aside from being tactile, antennae are characterized by position and in- nervation. They are always in front of the mouth and receive their nerve supply from the supracesophageal ganglion, while all other appendages ARTHROPODA 353 are innervated from the ventral chain. In their elongate shape antennae are not unlike legs, but they lack the terminal claws. The form of the jaws is strikingly modified. One or two basal joints serve for the comminution of food, and these parts are strong and are covered, especially on the medial side with a hard, toothed chitin (figs. 369, 2 ; 374, ///, I'). The other joints may entirely disappear, or may form a more or less leg-like appendage, the palpus. Since several appendages may be modified into jaws, the first are called mandi- bles, the next maxilla?, and second maxilla? may follow. The maxillipeds may have more the appearance of jaws, at other times are more leg-like (tig. 369, 5-7). The false feet (pleopoda) are small and inconspicuous ap- pendages which have various functions: they may serve as gills or supports for gills, places for the attachment of eggs, organs for the transfer of sperm, or as swimming or creep- ing organs. FIG. 369. — Appendages of the crayfish, i, first antenna; 2, mandible; 3, 4, first and second maxillce; 5, 6, 7, maxil- lipeds; 8, walking leg; 9, pleopod. These appendages have constant positions in the body. First on the head come the an- tenna? and then, in the region of the mouth, the jaws and, so far as they are present, the maxillipeds. Third come the true feet, and lastly, when they exist, the false feet. Those somites which bear antennae or jaws belong to the head, those bearing walking feet to the thorax, while the somites of the abdomen bear either false feet or lack appendages. As a sequence the cephalothorax is that region of the body which bears, besides antenna? and jaws, legs as well. The somites of Arthropoda have given rise to various disputes. Many zoolo- gists speak of a pre-antennal somite and a pre-antennal appendage, referring to the eye stalk of some Crustacea, which, however, differs markedly in its develop- ment from the true appendages. Those who accept an ocular somite must u\ Sub Class II. Phyllopoda. The Phyllopoda are the most primitive Crustacea. The name is derived from the leaf -like feet (fig. 375), which occur upon the thoracic region. The anterior appendages are schizopodal, the second pair of antennae often being efficient swimming organs. The number of somites varies between wide limits, there being less than a dozen in the Cladocera, while, if Savigny's law (p. 352) hold true, there are over sixty in seme Apodidae. Most forms (the Branchipodidae excepted) have a carapace. This forms a broad oval shell covering most of the body in the Apodidae (fig. 376); in the Estheriidae and Cladocera it is divided into right and left halves hinged together in the mid-dorsal line, thus giving these animals the appearance of bivalve molluscs. These forms have, besides the nauplius eye, a pair of compound eyes which in the compressed forms are frequently fused, although distinct in the young and retaining the double optic nerve throughout life. The liver is present in the shape of simple caeca; the heart, elongate, chambered, and with many ostia in the Branchiopoda, a short sac with only a pair of ostia in the Cladocera (fig. 383, //), lies dorsal to the intestine. The shell gland is well developed. In development summer and winter eggs are distinguished. The summer eggs form a single polar globule and develop parthenogenetically. The winter eggs form two polar globules and require fertilization. The thin-shelled summer eggs are carried by the mother in a brood pouch and soon hatch. The thick- FIG. 382. — Branchipus vernal is,* fairy shrimp (after Packard). shelled winter eggs fall to the bottom, where they require a long time for develop- ment. They may be dried or frozen without injury, and in some cases drying is necessary for development. This explains the appearance in early spring oi large numbers of Branchipus and Esther ia in pools which are dry in summer. The phyllopods are largely inhabitants of fresh water. The winter ri^s ; serve the species through times of drought and cold; the summer eggs are for the rapid increase of the species during the wet season. The same relations also explain the fact that males are rare and only appear at intervals, indeed are not known in some species. 366 ARTHROPODA Order I. Branchiopoda. The Branchiopoda are relatively large with numerous segments, leaf-like appendages, long, chambered heart, and lack swimming antennae. With few exceptions they are inhabitants of fresh water. According to the development of the carapace they are subdivided into three families. i. APODID.«. Body depressed, with large oval undivided carapace. Eggs carried in brood capsules formed by a pair of appendages. A pus* (fig. 376), Lcpidiirns* Protocaris of the Cambrian is apparently an Apodid. 2. BRAN- CHIPID.E. Body without carapace, the second antennae of the male large and modified for clasping the female. The female carries the summer eggs in a wide 'uterus' in the abdomen. Branchipus* (fig. 382), fresh water; Artemia* in brine; one has been transformed into the other by changing the water from fresh to salt or the reverse. 3. ESTHERIID/E. Body laterally compressed and enclosed in a bivalve shell, compound eyes fused; males very rare. Estheria* Litnnadia,* fresh water. Order II. Cladocera. Like the estheriids the small Cladocera have the body enclosed in a bivalve carapace, which in some is small and reaches back only over the first trunk seg- ments, in others is large, enclosing the body, with a notch for the protrusion of the head, while behind it terminates in a sharp spine. The head bears a pair of large swimming antennae and a much smaller first pair bearing olfactory bristles and, in the male, hooks for clasping the female. The body consists of few seg- ments, the heart is a simple sac, and the fused faceted eyes are capable of motion in a special optic capsule. The young eggs in the sexual organs always occur in groups of four (fig. 383). Of these but one grows into an egg, the others serving this as nourishment. Larger eggs with more yolk occur when several groups fuse to form a single egg. The summer eggs arise from a single group, the winter eggs from several groups of primordial ova. The space between the back of the animal and the shell serves as a brood pouch. The larger winter eggs — one or two in number — frequently remain for a while in the brood chamber and are there enveloped in a peculiar shell, the ephippium, consisting of two chitinous plates, like watch crystals, their edges closely appressed. DAPHNHXE. Shell well developed; Daphnia* (fig. 383), Bosmina* POLY- PHEMID.*:. Shell small, only functioning as a brood case; head with an enor- mous eye and large swimming antenna; no phyllopodous feet; marine and lacus- trine. Leptodora hyalina* appears at night, sometimes in great numbers, in some of our lakes. Evadne* (fig. 384), marine. Sub Class III. Copepoda. A general description of the copepods can only apply to the non- parasitic forms, since many of the parasites are so degenerate (figs. 6, 388) as to be recognized even as arthropods only by a knowledge of the develop- ment. The sixteen somites of the body are nearly equally divided among the three regions — head (6), thorax (5), and abdomen (5) — of the animal. (In Cyclops the first thoracic segment is fused with the head, the first two abdominal segments are fused — fig. 7.) The last abdominal segment is two-forked, forming ihefurca. While the abdomen lacks appendages, the thorax bears typical biramous appendages, consisting of a two-jointed I. CRUSTACEA: COPEPODA 307 FIG. 383. — Daphnia pulex. b, brood chambers with embryos; g, brain with nauplius eye; go, optic ganglion; h, heart; o, ovary; s, shell gland. The eggs arise at k, and separate, forming in groups of four, as at e, of which one becomes the egg, while the others abort (o) and form food. The egg then passes to the brood chamber, i, 2, first and second antennae; 3, mandible (maxilla rudimentary and invisible); 5-9, legs. Alimentary canal cross-lined. basiopodite, the basiopodites of a pair being frequently united for com- mon motion (fig. 374, /). Exopodite and endopodite, usually three-jointed, are fringed with bristles. Usually the fifth pair of thoracic appendages 368 ARTHROPODA is not so well developed, and in some cases is represented by two bunches of bristles. FIG. 384. — Evadne (orig.), showing the brood pouch filled with eggs and young. a2, second antenna; ao, adhesive organ; b, brain;/, furca; h, heart; i, intestine; I, liver; s, shell gland. FIG. 385. — Dioptonnts castor, b, ventral nerve cord; g, brain with nauplius eye; /;, heart, beneath it the ovary and digestive tract; sf>, spermatophores ; i, 2, first and second antenna-; 3, mandibles; 4, maxilla;; 5, maxilliped; 6-10, swimming feet. The first pair of antennae in the males may be hooked near the base for clasping; the second are sometimes biramous (fig. 374, //). The mandible (fig. 374, 777, V) is instructive, since a study of several species shows that it is derived from a schizopodal condition and that the first basal joint alone is used for chewing, the rest being reduced to a palpus of varying development. Both I. CRUSTACEA: COPEPODA 369 basal joints of the maxillae (fig. 374, 71') can be used in eating. Two maxillipeds (formerly regarded as the separated branches of an appendage) mark the termination of the head (fig. 385, 5). The internal anatomy is simple. There is no liver, and the straight intestine (fig. 385) runs without marked changes in size to the anus between the branches of the furca. The visual organ is the unpaired nauplius eye (which has given the name to one genus, Cyclops). It lies directly on the brain. The ventral chain has its ganglia irregularly dis- tributed. Gills are always absent, as are usually the heart and blood- vessels. The gonads are unpaired, but the sexual ducts, which open at the base of the abdomen, are paired. The females possess a receptaculum seminis distinct from the oviducts, to which the male attaches spermato- phores packed with sperm (fig. 385, sp). As the eggs leave the oviduct they are fertilized by the sperm issuing from the spermatophores, and are enclosed in a gelatinous substance, thus producing the so-called egg- sacs, attached to the abdomen, by which one can easily recognize the females (fig. 7). A nauplius hatches from the egg, and by budding seg- ments and appendages at the hinder end, and by a change of the nauplius appendages into antennas and mandibles, passes through a ' cyclops-stage ' into the adult. The Copepoda have descended from some phyllopod- like form. The poorly developed ventral chain, the loss, partial or com- plete, of a circulatory system, and the absence of gills are all against the view which would consider them primitive. Order I. Eucopepoda. The forms to which the foregoing description will apply are the Eucopepoda, and include many species, which often occur in enormous numbers in both fresh and salt water, forming the larger proportion of the plankton. They thus furnish the most important food supply not only for fishes but for the giant baleen whales. Celochilus septentrionalis occurs at times in such myriads that the sea for long distances is colored red. The CvcxopiDyE, no heart and paired egg sacs, fresh-water; Cyclops* (fig. 7). CALANID/E, fresh water and marine; heart present, single egg-sac. Diap- tomus* fresh water (fig. 385); Cetochilus* Pontilla* marine. HARPACTID^E, creeping forms, mostly marine; Canthocamptus* fresh water. The CORYC.^ID.E, half parasitic, include the wonderfully iridescent Sappliirhia* and the XOTODEL- PHID.E, parasitic in ascidians, form a transition to the next order. Order II. Siphonostomata (Parasita). There are aiso Copepoda to which the account in large type will not apply, animals of such strange appearance that many of them were long regarded as parasitic worms (figs. 6, 386, 388). Their mandibles are altered to piercing bristles and enclosed in a piercing proboscis formed of upper and lower lips. With this sucking organ they bore into the skin or gills of fishes. They have cylindrical forms or bodies of the most bizarre shapes, in which frequently no 24 370 ARTHROPODA segmentation is visible, while the appendages are rudimentary or even entirely lost. Indeed one would not recognize them as arthropods save for the following features: (i) Most of them have the typical Copepod egg-sacs attached to the hinder end. (2) A complete series of intermediate forms allows one to trace, step by step, the alterations of form from the free-living species to the most modified parasites. (3) Ontogeny is convincing. Most parasitic Copepoda leave the egg as a nauplius and pass through a Cyclops-stage before attaching themselves a. FIG. 386. FIG. 387. FIG. 388. FIG. 386. — Female Lernccocera esocina (from Lang, after Claus). A, armlike processes of anterior end; d, digestive tract; es, egg-sacs; od, oviduct; /r/4, rudimentary thoracic appendages. FlG. 387. — Argulus foliaceus (from Ludwig-Leunis). a, sting; a', antenna; b, mouth; c, intestine with liver; d, abdomen; pm1, pnr, first and second maxillipeds; pl-p*, biramous feet of thorax. FIG. 388. — Lerncea branchialis* (orig.). to fishes and becoming the highly degenerate parasites. These parasites are always females. The males scarcely pass the Cyclops-stage, copulate with the females and then die, or if they pass through the metamorphosis, they remain small and different in appearance (fig. 8). They occur attached to the female near the genital openings. There is thus here a marked sexual dimorphism. ARGULID^; (sometimes made a distinct order, Branchiura), fresh- water forms with compound eyes, liver lobes, and second maxillipeds metamorphosed into suckers. A r gains* (fig. 387). CALIGID.E (C aligns*), marine and brackish- water. LERN^OPODID^E. Fish parasites with maxillae united into an adhesive organ. Achthercs* (fig. 6), perch. LERN^HXE; worm-like parasites. Lerna'a* (fig. 388) ; Lenieeocera* (fig. 386) ; Pcnclla* I. CRUSTACEA: CIRRIPEDIA 371 Sub Class IV. Ostracoda. Like the Cladocera and the Estheriidae the Ostracoda are enclosed in a bivalve shell, which, when closed, includes not only the body but the head and appendages as well, these being protruded when the shell is opened. The valves are closed by an adductor muscle, opened by a hinge ligament like that of lamellibranchs. This resemblance to the molluscs is heightened by lines of growth upon the shell. The antennae, FIG. 389. — Cyprisfasciatiis, adult female (after Claus). I-IV, appendages; c, furca; e, eye; /, liver; m, adductor muscle of shell; o, ovary; 5, shell gland. the first simple, the second frequently two-branched, are used for swim- ming and creeping. The mandibles, maxillae, and three pairs of legs vary greatly from genus to genus. The internal structure is also variable. The Ostracoda are bottom forms and live in fresh and brackish water as well as in the sea. First two pairs of legs maxillary in character, the last de- veloped into a hook for cleansing the shell; heart present; marine; Cypridimi.* CYPRIDID.-K. First pair of legs maxillary in character; heart lacking; fresh water. Cypris,* Candona.* Sub Class V. Cirri pedia. The barnacles differ from all other Crustacea in that they have lost their locomotor powers and live attached to rocks, floating timber, and the like. In some cases they attach themselves to other animals, as crabs and molluscs, or, as in the case of Coronula and Tubiclnclla, to whales. This leads in Anelasma and the Rhizocephala to a true parasitism, the barnacle not only attaching itself to an animal but sucking its juices as food. The attachment is by the dorsal surface in the neighborhood of the head , and is initiated by the first antennae, in which is a cement gland secreting a rapidly hardening cement. The Hat region of fixation in the Balanidae 372 ARTHROPODA (fig. 390) is drawn out in the Lepadida? into a long muscular stalk (fig. 115). To this attached life are related all the peculiarities of structure. A fixed animal has greater need of protection than one which can flee from its enemies, therefore we find right and left mantle folds capable of com- plete closure, like those of an ostracode, each with two calcified plates the scuta and terga (figs. 115, 390, fy s, /), the first cephalic, the other pos- terior, in position. Between the pairs of these is the gap through cr which the feet are protruded. Besides there are other calcified por- tions, one of which, the carina (r), corresponds to the hinge-line of the ostracode and in some Lepads is sup- plemented by a farther unpaired piece, a -. t . .- p - c FIG. 390. FIG. 391. FIG. 390. — Balanits hameri* acorn barnacle (from Lang, after Darwin). Formed of rostrum, lateralia, and carina, the operculum of scuta (s) and terga (/). FIG. 391. — Anatomy of Lepas (after Claus). a, adductor muscle; c, carina; cr, cirri (feet); g, cement gland; I, liver; o, o', ovary and oviduct; p, penis; t, testis; tr, tergum; v, vas deferens. the rostrum. In the Balanidae the rostrum and carina are much stronger, while between them other paired pieces, the lateralia, are intercalated. Later- alia, rostrum, and carina arise from a base (usually calcareous) and form a capsule, closed above by a double valve formed of the paired scuta and terga, between which, when open, the animal can be seen (fig. 390). The body in both lepads and balanids has essentially the same struc- ture. It is flexed ventrally, so that mouth and vent are near each other, and bears six pairs of feathered feet, or cirri, which, when extended, become widely separated and form a most efficient means of straining small organisms from the water and conveying them to the mouth. These feet are biramous, with their branches ringed and thickly haired. Behind them is a rudimentary abdomen and an elongate penis; while the mouth is surrounded by a pair of mandibles and two pairs of maxillae. In internal structure the most noticeable feature is that the animals I. CRUSTACEA: CIRRIPEDIA 373 with few exceptions, in contrast to most other arthropods, are hermaphro- ditic, a condition possibly correlated with their sedentary life and the con- sequent need of self-impregnation. The testes lie in the sides of the body; the ovaries in the Lepadids are in the stalk, in the Balanids in the basal plate. In cases of several solitary hemaphrodite species complementary dwarf males occur. These are very small, purely male forms, with ex- tremely simple structure (fig. 392), which live inside the mantle cavity near the genital openings. The unsegmented body is enclosed in a sac (a B FIG. 392. FIG. 393. FIG. 392. — Male of ALcippe lampas. an, antenna; /, mantle lobes, m, muscles; oc, ocellus; p, penis; t, testis; vs, seminal vesicle. FIG. 393. — Nauplius (.4) and Cypris (B) stages of Sacculina carcini. (after Delage). i, 2, antennae; 3, mandible; /, cirrous feet; m, muscles; oc, nauplius eye; ov, anlage of ovary. soft-skinned shell), and anchored by the antenna?. The long penis pro- trudes from the mantle. In the genus Scalpellum there are purely her- maphroditic species, hermaphroditic species with complemental males, and purely dioecious species. Since the hard shells of the barnacles resemble those of the molluscs, it is not to be wondered that these forms were long regarded as belonging to that group. It was not until the development (fig. 393) was studied that the error was corrected. A large nauplius comes from the egg and later is metamorphosed into a second larval stage with bivalve shell which, from its appearance, is called the cypris-stage. This becomes fixed and develops into the adult, losing the compound eyes and retaining the nauplius eye. Order I. Lepadidae. Stalked cirripeds, with shell largely formed of scuta, terga, and carina; other parts may be added. Lepas anatifcra* (fig. 115), the goose barnacle, o\\rs its common name to a mediaeval myth which claimed that the Irish (or bernicle) goose developed from these animals. Anelasma squalicola, thin-skinned, parasitic on sharks, forms a transition to the Rhizocephala. 374 ARTHROPODA Order II. Balanidae. Sessile cirripeds with calcareous shell formed of carina, rostrum, and la'er- alia; scuta and terga forming the valves (fig. 390). Balanus* Coronula, attached to whales. Order III. Rhizocephala. These differ greatly from other cirripeds. They are parasitic on the abdo- mens of decapod crabs and consist of a stalk which penetrates the body of the host and a body which remains outside (fig. 394). The stalk branches in a root-like manner, penetrates the cephalothorax and absorbs its juices. Since the stalk furnishes the food, an alimentary canal is absent. The body lacks all appendages, is enclosed by a soft-skinned mantle, and is almost entirely "" "d FIG. 394. — Sacculina carcini attached to Carcinus mcenas, whose abdomen (d~) is ex- tended. »/, sex opening of Sacculina; r, network of roots ramifying the crab; 5, stalk. filled with the gonads. Since the adult parasites lack all arthropodan features, their position is only settled by their development (fig. 393), which is like that of other cirripeds. These forms are rare on the American coast. Sacculina, Peltogaster* Two more orders, ABDOMINALIA and APODA, parasitic in the mantle and shells of molluscs and other cirripeds, scarcely need mention. Sub Class V. Malacostraca. The Malacostraca are sharply marked off from the other Crustacea by having a body which consists of twenty segments, of which seven are abdominal (Nebalia has twenty-one, eight abdominal). The excretory organs are represented by the antennal glands, and shell glands are lacking except in the larvae and some Isopoda. The male genital ducts open on the thirteenth, the female on the eleventh, segment. I. CRUSTACEA: SCHIZOPODA Legion I. Leptoslraca. '.'> , ."> The Leptostraca connect the Phyllopoda with the higher groups. Their twenty-one somites (eight abdominal, eight thoracic, and five cephalic), and the openings of the genital ducts ally them to the Malacostraca. On the other hand, the bivalve carapace covering the cephalothorax and part of the abdomen, and the leaf-like thoracic feet, are phyllopoclan. They have an antennal gland and a rudimentary shell gland; an elongate heart which extends through cephalothorax and abdomen; and stalked compound eyes. The few species are all marine and belong to the genus Nebalia* (fig. 395). FIG. 395. — Nebalia bipes* (after Sars). /;, heart; 7°, intestine; o, ovary: a, adductor of carapace; b, brain; r, rostrum. Legion II. Thoracostraca (Podophthalmia) . The names given this division have reference, first, to the fact that the head and some of the thoracic segments are immovably united and covered by a firm carapace; second, that the compound eyes (except in Cumacea) are placed on movable eye stalks. The first five appendages are always two pairs of antennae, a pair of mandibles, and two pairs of maxilla1. The remaining pairs vary greatly and from one to three may be modified into maxillipeds, while the abdominal somites except the last (telson) usually bear appendages, at least in the female. There is usually a metamorphosis in which a nauplius stage may appear, most frequently in the lower forms (schizopods), but even in the decapods (Peneus). Order I. Schizopoda. These are small forms (fig. 396), mostly marine, in which the cephalothorax is covered by a carapace with which some or all of the thoracic somites are firmly united. The eight thoracic feet are biramous throughout life and are used in swimming. The posterior pair of abdominal feet together with the telson form a caudal 'fin' by means of which the animal can swim backwards. The delicate skin permits of diffuse respiration, and gills are frequently lacking. In some genera plates from the legs of the female enclose a brood case beneath the cephalothorax, thus giving these forms the common name of opossum shrimps. 376 ARTHROPODA The MYSIDID/E are widely distributed, several species of Mysis (fig. 396) occurring on our coasts and one in the Great Lakes. In these the endopodite of the sixth abdominal appendage contains a otocyst, with a calcic fluoride statolith. Other families are the EUPHAUSIID.E and LOPHOGASTRID^; of the high seas. FIG. 396. — Mysis elongata (from Gerstacker). «, J3, first and second antennae; a, expedite; au, eye; z, endopodite; o, otocyst; 1-7, abdominal somites. Order II. Stomatopoda. In structure of the cephalothorax these forms, known as mantis shrimps (from a resemblance to the insect, the praying mantis), are lower than the schizopods, since the last three or four thoracic somites are free and are not covered by the carapace. The appendages, however, are more developed, since only the three posterior of the thoracic feet are biramous and natatory. The four in front of these are prehensile and bear a pincer formed of the last two joints, the last being slender and usually toothed and closing in a groove of the penult joint like a knife blade in the handle. The first of" these raptorial sac. FIG. 39j.—Squilla mantis, at, at', first and second antenna?;/, sixth abdominal feet; k, gills; p, schizopodal thoracic feet; pr, pr', raptorial feet; />\, pleopoda; sa, telson. feet are the largest and are used in capturing fishes, etc. Since the thoracic feet are of little service for locomotion, the abdomen is long and stout, especially the caudal fin. The five anterior abdominal feet bear the gills, and correspondingly the elongate heart with many ostia extends into the abdomen. The transparent pelagic larvae were formerly regarded as adults and described as Alima and Enchthus. Squill a* Gonodactylus* They are burrowing animals and deposit their eggs in their holes. I. CRUSTACEA: DECAPODA :577 Order III. Decapoda. The Decapoda is the most important group of Crustacea, since it contains the shrimps, lobsters, crayfish, and crabs. It agrees with the Schizopoda in having a cephalothorax composed of thirteen fused somites, but differs in the structure and function of the thoracic extremities. Only the last five pairs (whence the name Decapoda) are locomotor. These lose the exopodite during development (Peneidai excepted) and become strong walking legs, terminated either with claws or pincers (chela). Usually the first pair is distinguished from the others by its size and by being chelate, and is a grasping organ. In the development of a chela the penult joint sends out a strong process, the 'thumb,' which extends as far as the last joint (the 'finger'), which closes against it. The mouth parts— a pair of mandibles, two pairs of maxillae, and three pairs of maxillipeds (fig. 369) — lie in front of the first pair of legs. The maxillipeds (7, 6, 5) show a biramous condition, while the maxillae (4, 3) retain considerable of the phyllopod character. In the mandibles (2) there is always a strong basal joint, which serves as a jaw, while this may bear additional joints, the palpus. Behind the mouth are a pair of scales, the paragnaths or metasfoma, which are not appendages. The antennae are usually distinguished as antennae (second pair) and aiitcnmila (first pair, fig. 369). They have large basal portions, which in the antennulae bear two many-jointed flagella, while the antennae proper have but a single though usually much larger flagellum. On the basal joint of the antennulae is the otocyst (p. 362), while the green gland opens on the basal joint of the antennae (fig. 400, gd). When the abdomen is not rudimentary (as in the crabs) the appendages of the sixth abdominal segment together with the telson form a strong caudal fin (fig. 400) ; the other appendages (fig. 369, /) are small, biramous organs to which, in the female, the eggs are attached. In the female the first pair is reduced, but in the male (except in Palinuridae) this pair is well developed, curiously modified, and serves as a copulatory (intromittent) organ. The shape of these appendages and the openings of the genital ducts — on the base of the third walking foot of the female, the fifth in the male — serve at once to distin- guish the sexes. Frequently also the males have the larger pincers. The thickness of the integument prevents diffuse respiration and accounts for the numerous gills (fig. 398) which are attached to the bases of the maxillipeds and walking feet or to the sides of the body near them. (In the Thalassinidas the gills are on the abdominal appendages). These gills are not visible externally, for the carapace extends down on the sides of the body as a fold (branch iostcgitc) over them, thus enclosing them in a branchial chamber. A process of the second maxilla? — the scaphog- natliite — plays in this branchial chamber and pumps the water over the gills, the water flowing out near the mouth. All decapods can live some time out of water; they retain some water in the gill chamber, which keeps the gills in a moist condition. In some tropical crabs which live almost exclusively on land there is a true aerial respiration, the lining of the 378 ARTHROPODA gill chamber being modified into a kind of lung traversed by numerous blood-vessels. In the palm crab (fig. 399) the gill chamber is divided into two portions, the upper part being pulmonary, the lower containing the reduced gills. Correlated to this localized respiration is the nearly closed circulatory system (fig. 400, A, B). The heart (//), a compact pentagonal organ, receives its blood from the pericardial sinus (pc) through three pairs of ostia, and forces it out through five arteries to the capillary regions of the body. The venous blood collects in a large venous sinus at the base of the gills (r), passes thence through gills, and is returned by several branchial veins (vbr) to the pericardium. FIG. 398. — Gills of Astacus exposed by cutting away the branchiostegite. pdb, plb, podo- and pleurobranchia of the corresponding segments; r, rostrum; i, stalked eyes; 2, 3, antennae; 4-6, mandibles and maxillae; 7-9, maxillipeds; 10, 14, bases of thoracic eet; 15, first pleopod. The alimentary canal is straight and has only one conspicuous enlarge- ment, the so-called stomach (fig. 400, A, m), divided into two portions, an anterior (cardiac) sac, lined with chitinous folds and teeth and serving to chew the food and bearing in its walls the 'crab-stones,' masses of calcic carbonate stored up to harden the armor rapidly after the molt. The second (pyloric) portion of the stomach is guarded by hairs and serves as a strainer, allowing only food sufficiently comminuted to pass. The two liver lobes — voluminous masses of branched glandular tubes (/) open just behind the stomach. The two antennal glands (C, gd), each provided with a large urinary bladder (&/), are dirty green in color, whence the name green glands often given them. The gonads (fig. 401) lie close beneath the heart, those of the two sides being united behind, while their ducts remain separate. The structure of the nervous system is in part dependent upon that of the abdomen. In the Macrura (fig. 400, C) the ventral chain consists of six ganglia in the thorax, six in the abdomen, but in the Brachyura I. CRUSTACEA: DECAPODA 379 ,cf, a. FIG. 399. — Diagrammatic section through Birgus Intro, showing lungs (from Lang, after Semper), a,, a4, afferent blood-vessels; a/z, pulmonary chamber; e&, e/, el', efferent blood-vessels; h, heart; k, gills; M, branchiostegite; />. pericardium. FIG. 400. — Anatomy of Crayfish (Antaeus']. A, dorsal surface removed; B, scheme of circulation; C, viscera removed, showing green gland and nervous system, a, anus; aa, hepatic artery; ae, antenna; ai, antennula, also sternal artery; am, muscles of stomach; ao, ophthalmic artery; ap, abdominal artery; av, ventral artery; bl, urinary bladder; br, gill arteries; c, cesophageal commissures; gd, green gland; gn', brain; gn2-13, ganglia of ventral chain; h, heart; hd, intestine; k, mandibular muscles; /, /', liver and its duct; m, stomach; o, otocyst; oes, cesophagus; on, oj>tic nerve; pc, pericardium; sgn, sympathetic nerve; t, t', unpaired and paired portions of testes; v, ventral blood sinus; vd, vas deferens; i'l>r, veins from gills to heart. 380 ARTHROPODA (fig. 402) these all flow together in a common mass, connected with the brain by two long oesophageal commissures. The development of most decapods is interesting from the number of larval forms. As a rule a zoea (fig. 379) is hatched from the egg; this passes next into a Mysis stage (fig. 403) in which head, thorax, and abdomen are distinct, the thorax bearing biramous feet like those of schizopods — a proof of the origin of the simple feet from the biramous type. In the crabs (Brachyura) the Mysis stage is replaced by a Megalops (fig. 404), in which the abdomen is well de- veloped, but the feet have lost their biramous character. In some prawns^ FIG. 401. FIG. 402. FIG. 401. — Reproductive organs of (.4) female and (B) male crayfish (from Huxley), od, oviduct; od', its opening on nth appendage;