MEDICAL Florence J. Chubb Memorial. A MANUAL OF ZOOLOGY BY RICHARD HERTWIG Professor of Zoology in the^University at Munich SECOND AMERICAN EDITION FROM THE FIFTH GERMAN EDITION TRANSLATED AND EDITED BY J. S. KINGSLEY Professor of Zoology in Tufts College NEW YORK HENRY HOLT AND COMPANY Copyright, 1902, BY HENRY HOLT & CO. ROBERT DRUMMOND, PRINTER, NEW YORK. PEEFACE. 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, translated in 1896 by Dr. George AY. Field, and the cordial recep- tion which this has had in America has led to the present reproduc- tion 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 Yermes of the original has been broken up and its members dis- tributed 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 Yerte- brata 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 to materially increase the size of the volume. Experience iii ~ II IV PREFACE. has shown that beginners rarely go to the original sources. This omission is the less important since in all schools where the book is likely to be used other works containing good bibliographies are accessible. Eeference might here be made to those in the Anat- omies of Lang and Wiedersheim, 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. KINGSLEY. TUFTS COLLEGE, MASS., Sept. 19, 1902. TABLE OF CONTENTS. PAGE INTRODUCTION x HISTORY OF ZOOLOGY 7 DEVELOPMENT OF SYSTEMATIC ZOOLOGY 8 DEVELOPMENT OF MORPHOLOGY 12 REFORM OF THE SYSTEM !8 HISTORY OF THE THEORY OF EVOLUTION 19 DARWIN'S THEORY OF THE ORIGIN OF SPECIES 25 GENERAL MORPHOLOGY AND PHYSIOLOGY 57 GENERAL ANATOMY 58 The Morphological Units of the Animal Body 58 The Tissues of the Animal Body 71 Epithelial Tissues 73 Connective Tissues. 83 Muscular Tissues 90 Nervous Tissues 94 Summary 97 The Combination of Tissues into Organs 99 Vegetative Organs 102 Organs of Assimilation 102 Digestive Tract 103 Respiratory Organs 107 Circulatory Apparatus 109 Excretory Organs 115 Sexual Organs 117 Animal Organs 121 Organs of Locomotion 121 Nervous System 122 Sense Organs 125 Summary 131 Promorphology 133 GENERAL EMBRYOLOGY 139 Spontaneous Generation 139 Generation by Parents 140 Asexual Reproduction 140 Sexual Reproduction 142 Combined Methods of Reproduction 143 General Phenomena of Sexual Reproduction 145 Maturation of the Egg 146 Fertilization 148 V vi TABLE OF CONTENTS. PAGE Cleavage Processes 151 Formation of the Germ Layers 156 Different Forms of Sexual Development 160 Summary 162 RELATION OF ANIMALS TO ONE ANOTHER 164 Relations between Individuals of the Same Species 164 Relations between Individuals of Different Species 167 ANIMAL AND PLANT 171 GEOGRAPHICAL DISTRIBUTION OF ANIMALS 174 DISTRIBUTION OF ANIMALS IN TIME 1 80 SPECIAL ZOOLOGY 182 Phylum I. PROTOZOA 183 Class I. Rhizopoda 187 Order I. Monera 189 Order II. Lobosa 189 Order III. Heliozoa 190 Order IV. Radiolaria 192 Order V. Foraminifera 196 Order VI. Mycetozoa '. 198 Class II. Flagellata 200 Order I. Autoflagellata 200 Order II. Dinoflagellata 203 Order III. Cystoflagellata 203 Class III. Ciliata 204 Order I. Holotricha 209 Order II. Heterotricha 209 Order III. Peritricha 210 Order IV. Hypotricha 211 Order V. Suctoria 212 Class IV. Sporozoa 213 Order I. Gregarinida 213 Order II. Coccidiae 215 Order III. Haemosporida 216 Order IV. Myxosporida 217 Order V. Sarcosporida 218 SUMMARY 218 METAZOA 221 Phylum II. PORIFERA 221 Order I. Calcispongiae 225 Order II. Silicispongiae 226 SUMMARY 227 Phylum III. CcELENTERATA 228 Class I. Hydrozoa 230 Order I. Hydraria 240 Order II. Hydrocorallinae 241 Order III. Tubulariae = Anthomedusae 241 Order IV. Campanulariae = Leptomedusae 242 Order V. Trachomedusae 242 Order VI. Narcomedusae 242 Order VII. Siphonophora 243 TABLE OF CONTENTS. vii PAGE Class II. Scyphozoa 245 Order I. Stauromedusae 250 Order II. Peromedusae 250 Order III. Cubomedusae 250 Order IV. Discomedusae 250 Class III. Anthozoa 251 Order I. Tetracoralla , 258 Order II. Octocoralla 258 Order III. Hexacoralla 259 Class IV. Ctenophora 261 SUMMARY 265 Phylum IV. PLATHELMINTHES 267 Class I. Turbellaria 268 Order I. Polycladidea 271 Order II. Tricladidea 271 Order III. Rhabdocoelida 271 Class II. Trematoda 271 Order I. Polystomiae 273 Order II. Distomiae 274 Class III. Cestoda 278 Class IV. Nemertini 289 SUMMARY 292 Phylum V. RoTiFERA 293 Phylum VI. CCELHELMINTHES 295 Class I. Chaetognathi 296 Class II. Nemathelminthes 298 Order I. Nematoda 298 Order II. Gordiacea 304 Order III. Acanthocephala 304 Class III. Annelida 305 Sub Class I. Chaetopoda 306 Order I. Polychaetae 311 Order II. Oligochaetae . 314 Sub Class II. Gephyraea 316 Order I. Chsetiferi 317 Order II. Inermes 317 Order III. Priapuloidea 317 Sub Class III. Hirudinei 318 Order I. Gnathobdellidae 321 Order II. Rhynchobdellidae 321 Class IV. Polyzoa 321 Sub Class I. Entoprocta 321 Sub Class II. Ectoprocta 322 Class V. Phoronida 325 Class VI. Brachiopoda 325 Order I. Ecardines 328 Order II. Testicardines 328 SUMMARY 328 Phylum VII. ECHINODERMA 329 Class I. Asteroidea 333 viii TABLE OF CONTENTS. PAGE Class II. Ophiuroidea 337 Class III. Crinoidea 338 Sub Class I. Eucrinoidea 342 Sub Class II. Edrioasteroidsa 342 Sub Class III. Cystidea 342 Sub Class IV. Blastoidea 342 Class IV. Echinoidea 343 Order I. Palechinoidea 345 Order II. Cidaridae 345 Order III. Clypeastroidea 346 Order IV. Spatangoidea 346 Class V. Holothuroidea 346 Order I. Actinopoda 349 Order II. Paractinopoda 349 SUMMARY 350 Phylum VIII. MOU.USCA 351 Class I. Ampliineura 356 Sub Class I. Placophora ' . . 356 Sub Class II. Solenogastres 358 Class II. Acephala 358 Order I. Protoconchiae 365 Order II. Heteroconchiae 367 Class III. Scaphopoda 369 Class IV. Gasteropoda 369 Order I. Prosobranchiata 378 Order II. Opisthobranchiata 381 Order III. Pulmonata 383 Class V. Cephalopoda 384 Order I. Tetrabranchia 394 Order II. Dibranchia 394 SUMMARY - 395 Phylum IX. ARTHROPODA 398 Qlass I. Crustacea 408 Sub Class I. Trilobitae 414 Sub Class II. Phyllopoda 415 Order I. Branchiopoda 416 Order II. Cladocera 417 Sub Class III. Copepoda 417 Order I. Eucopepoda 42 1 Order II. Siphonostomata 422 Sub Class IV. Ostracoda 422 Sub Class V. Cirripedia 423 Order I. Lepadidae 425 Order II. Balanidae 425 Order III. Rhizocephala 426 Sub Class VI. Malacostraca 426 Legion I. Leptostraca 427 Legion II. Thoracostraca 427 Order I. Schizopoda 428 Order II. Stomatopoda 429 TABLE OF CONTENTS. ix PAGE Order III. Decapoda 429 Order IV. Cumacia 437 Legion III. Arthrocostraca 438 Order I. Amphipoda 438 Order II. Isopoda 440 Class II. Acerata 442 Sub Class I. Gigantostraca 443 Order I. Xiphosura 444 Order II. Eurypterida 444 Sub Class II. Arachnida 444^ Legion I. Arthrogastrida 447 Order I. Scorpionida 447 Order II. Phrynoidea 448 Order III. Microthelyphorida 448 Order IV. Solpugida 449 Order V. Pseudoscorpii 450 Order VI. Phalangida 450 Legion II. Sphaerogastrida 451 Order I. Araneina 451 Order II. Acarina 453 Order III. Linguatulida 454 Tardigrada 455 Pycnogonida 456 Class III. Malacopoda 456 Class IV. Insecta 458 Sub Class I. Chilopoda 460 Sub Class II. Hexapoda 461 Order I. Apterygota 477 Order II. Archiptera 477 Order III. Orthoptera 480 Order IV. Neuroptera 481 Order V. Strepsiptera 483 Order VI. Coleoptera 483 Order VII. Hymenoptera 485 Order VIII. Rhynchota 489 Order IX. Diptera 491 Order X. Aphaniptera 493 Order XI. Lepidoptera 494 Class V. Diplopoda 496 SUMMARY 497 Phylum X. CHORDATA 501 Sub Phylum I. • Leptocardii 502 Sub Phylum II. Tunicata 505 Order I. Copelatae 506 O'rder II. Tethyoidea 508 Order III. Thaliacea 510 Sub Phylum III. Enteropneusta 512 Sub Phylum IV. Vertebrata 514 Series I. Ichthyopsida 555 Class I. Cyclostomata 555 TABLE OF CONTENTS. PAGE Sub Class I. Myzontes 556 Sub Class II. Petromyzontes 557 Class II. Pisces 557 Sub Class I. Elasmobranchii 569 Order I. Selachii 570 Order II. Holocephali 572 Sub Class II. Ganoidei 572 Order I. Crossopterygii 573 Order II. Chondrostei 573 Order III. Holostomi 573 Sub Class III. Teleostei 574 Order I. Physostomi 575 Order II. Pharyngognathi 576 Order III. Acanthopteri 577 Order IV. Anacanthini 577 Order V. Lophobranchii 578 Order VI. Plectognathi 578 Sub Class IV. Dipnoi 579 Class III. Amphibia 580 Order I. Stegocephali 586 Order II. Gymnophiona 587 Order III. Urodela 587 Order IV. Anura 588 Series II. Amniota 588 Class I. Reptilia 588 Order I. Theromorpha .. . . 594 Order II. Plesiosauria 594 Order III. Ichthyosauria 594 Order IV. Chelonia 594 Order V. Rhynchocephalia 595 Order VI. Dinosauria 595 Order VII. Squamata 596 Order VIII. Crocodilia 601 Order IX. Pterodactylia 602 Class II. Aves 603 Order I. Saururae 612 Order II. Odontornithes 612 Order III. Ratitae 612 Order IV. Carinatse 613 Class III. Mammalia 617 Sub Class I. Monotremata 631 Sub Class II. Marsupialia 632 Order I. Polyprotodonta 633 Order II. Diprotodonta 633 Sub Class III. Placentalia 634 Order I. Edentata 635 Order II. Insectivora 636 Order III. Chiroptera 637 Order IV. Rodentia 638 Order V Ungulata 639 TABLE OF CONTENTS. xi PAGE Order VI. Proboscidia 643 Order VII. Hyracoidea 644 Order VIII. Sirenia 644 Order IX. Cetacea 645 Order X. Carnivora 646 Order XI. Prosimiae 648 Order XII. Primates 649 SUMMARY 652 GENERAL PRINCIPLES OP ZOOLOGY. INTRODUCTION. Man's Relation to Other Animals. — The man who has learned to observe nature in a disinterested manner sees himself in the midst of a manifold variety of organisms, which in their structure, and even more in their vital phenomena, disclose to him a simi- larity to his own being. This similarity, with many of the mammals, especially the anthropoid apes, has the sharpness of a caricature. In the invertebrate animals it is softened; yet even in the lowest organisms, for our knowledge of which we are indebted to the microscope, it is still to be found : although here the vital processes which have reached such an astonishing com- plexity 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 organization has found expression. Purpose of Zoological Study. — If we would, therefore, fully understand the structure of man, we must, as it were, look at it upon the background which is formed by the conditions of organization of the other animals, and for this purpose we must investigate these conditions. To such endeavors the scientific knowledge of animal life, or Zoology, owes its origin and continued advancement. But meanwhile the subject of zoology has widened; for, 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 conse- quence, on the one hand, of the well-nigh exhaustless variety of animal organization, and, on the other hand, of the diiferent points of view from which the zoologist enters upon the solution of his problem. 2 GENERAL PRINCIPLES OF ZOOLOGY. In the first half of the last century the conception, which is still held by the public at large, was prevalent, if not quite uni- versal, in scientific circles, 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 understood the classification of animals, that is to say, only one .part of zoology, indeed a part of minor importance, which can pretend to scientific value only when it is brought into relation with other problems (geographical distribution, evolution). This conception has during the past five decades become more and more subordinated. The ambition to describe the largest possible number of new forms and to shine by means of an extensive knowledge of species 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. Morphology, or the study of form, begins with the appearances of animals, and has first to describe all which can be seen exter- nally, as size, color, proportion of parts. But since the external appearance of an animal cannot be understood without knowledge of the internal organs which condition the external form, the morphologist must make these accessible by the aid of dissection, of Anatomy, and likewise 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. Everywhere the morphologist has to do with conditions of form: the only difference lies in the instruments by means of which he obtains his insight, according to whether he gathers his knowledge through immediate observation, or after a previous dissection with scalpel and scissors, or by use of the micro- scope. 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 distinction is not logically correct, since 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 in some cases lie in the interior of the body, and must be dissected out, belong in other cases to the surface of the body, and are accessible for direct description. Further, on account of their transparency the in- ternal parts of many animals can be studied without dissection. Comparative Anatomy. — For morphology, as for every science, the proposition is true that the mere accumulation of facts is not INTRODUCTION. 3 sufficient to give the subject the character of a science ; an addi- tional 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, possibly restricted to the representatives of a single species. He thus gains a double advantage: (1) 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 rather in a regular dependent 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, e.g., when we compare man with the other vertebrates and with many lower invertebrate forms. Here we have to consider one of the most mysterious phenomena of the organic world, the path to the full explanation of which was first broken 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 com- pletely 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 com- plicated changes. The morphologist must, with the completest possible series, determine by observation the different stages, com- pare them with the mature animals, and with the structure and developmental stages of other animals. Here is revealed to him the same conformity to law which dominates the mature animals, and a knowledge of this conformity is of fundamental importance as well for classification as for the causal explanation of the animal form. The df3velopmental 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 structure of lower vertebrates, like the fishes, and even with many of the still lower animals of the invertebrate 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. Modern physiology has abandoned this theory 4 GENERAL PRINCIPLES OF ZOOLOGY. of vital force; it has begun the attempt to explain life as the summation of extremely complicated chemico-physical processes, and thus to apply to the organic world those explanatory princi- ples which prevail in the inorganic realm. The results obtained show that it is the correct method. Since each organic form is the product of its development, since, further, the development represents to us the summation of most complicated vital processes, the explanation of the organic bodily form is, therefore, in ultimate analysis a physiological problem; though of course a problem whose solution lies still in the indefinitely distant future. What has been actually accom- plished in this direction is only the smallest beginning, even in comparison with that which many falsely regard as already attained. 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 or Biology. This branch of the science has of late attained a very considerable importance. How animals are distributed over the globe, how climate and conditions 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 in the realm of zoology belongs also Paleozoology or Paleontology, the study of the extinct animals. For between the extinct and the living animals there exists a genetic relationship : the former are the precursors of the latter, and their fossil remains are the most trustworthy records of the history of the race, or Phylogeny. As in human affairs the present conditions can only be completely understood by the aid of history, so in many cases the zoologist must draw upon the results of paleontology for an explanation of the living animal world. The science of zoology would be subdivided in the above-men- tioned manner if we wished to proceed entirely on a scientific basis. Yet practical considerations have made many modifications neces- sary. On account of their paramount importance to the medical profession human anatomy and embryology have been raised to independent branches of science. In comparative physiology only the most general foundations have been laid; a more special* physiology exists only for man and the higher vertebrates; this, too, for the above-named reasons has been made a special branch of science. Paleontology also has, in addition to its specific INTRODUCTION. 5 zoological tasks, attained importance as a scientific aid to geology, since it furnishes the materials for characterizing and fixing the various geological ages and the earth's history during those ages. When, therefore, at the present day we speak of zoology, we usually refer to morphology and classification of living animals with consideration of their general vital phenomena. The views here given of the character of zoology have not been the same in all time. Like every science zoology has developed gradually; it has varied with each epoch and tendency, according as the systematic or the morphological or the physiological point of view was the prevailing one. It will now be interesting to take a hasty glance at the most important phases in the development of zoology. The reader will better understand the questions which now dominate zoological inquiry, if he know how these have arisen historically. 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 systematic, 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 oppo- sition between the two points of view, and even later this has in many instances disappeared. Aristotle, the great Greek philosopher, has been distinguished as the Father of Natural History, which means that his predeces- sors' fragmentary knowledge of zoology could not be compared with the well-arranged order in which Aristotle had brought together his own and the previously existing knowledge of the nature of animals. In Aristotle favorable external conditions were united with more favorable mental ability. Equipped with the literary aid of an extensive library and the pecuniary means then more indispensable than now for natural-history investigation, he pursued the inductive method, the only one which is capable of furnishing secure foundations in the realm of natural science. It is a matter for great regret that there have been preserved only parts of his three most important zoological works, " Kistoria 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, attained to 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 century. Thus it was known to Aristotle, though only lately rediscovered by Johannes Miiller, that many sharks are not only viviparous, but that also in their case the embryo becomes fixed to the maternal uterus and there is formed a contrivance for 7 8 GENERAL PRINCIPLES OF ZOO LOOT. nutrition resembling the mammalian and even the human pla- centa; he knew the difference between male and female cephalo- pods, and that the young cuttlefish has a preoral yolk-sac. The position which Aristotle took in reference to the classifica- tion of animals is of great interest; he mentions in his writings the very considerable number of about five 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. This neglect of the systematic side is further shown in the fact that the great philosopher is satisfied with two systematic cate- gories, with eidos, species or kind, and yeros or group. His eight yevrj juey terra would about correspond with the Classes of modern zoology; they have been the starting-point for all later attempts at classification, and may therefore be enumerated here : 1. Mammals (CcporoKOvvra ev avrois). , 2. Birds (opnOes). 3. Oviparous quadrupeds (rerpanodoi cooroKovvra). 4. Fishes 5. Molluscs 6. Crustaceans 7. Insects (errata). 8. Animals with shells Aristotle also noticed the close connexion of the first four groups, since he, without indeed actually carrying out the divi- sion, has contrasted the animals with blood, evai^a (better, animals with red blood), with the bloodless, avai}jia (better, animals with colorless blood or with no blood at all). DEVELOPMENT OF SYSTEMATIC ZOOLOGY. Pliny. — It is a remarkable fact that after the writings of Aristotle, in which classification is much subordinated and only serves to express the anatomical relationships in animals, an exclusively systematic direction should have been taken. This is explicable only when we consider that the mental continuity of investigation was completely broken on the one hand by the decline and ultimate complete collapse of ancient classic civiliza- tion, and on the other by the triumphant advance of Christianity. Ill S TORT OF ZOOLOGY. 9 The decay of zoological investigation, that had only just begun to bloom, begins in the writings of Pliny. Although this Roman general and scholar was long lauded as the foremost zoologist of antiquity, he is now given the place of a not even fortunate com- piler, who collected from the writings of others the accurate and the fabulous indiscriminately, and replaced the natural classifica- tion of animals according to structure by the unnatural, purely arbitrary division according to their place of abode (flying animals, land animals, water animals). Zoology of the Middle Ages. — The rise of Christianity resulted in the complete annihilation of natural science and investigation. The world-renouncing character, which originally was peculiar to the Christian conception, led naturally to a disposition hostile to any mental occupation with natural things. Then came a time when answers to questions capable of solution by the simplest observation were sought by painstaking learned 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 taken occasion to look into a horse's mouth. Significant of this mental bias which prevailed through- out the entire Middle Ages is the i Physiologus' or ' Bestiarius/ a book 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. In a similar way the religious element played an important role in the many-volumed Natural History of the Dominican Albertus Magnus, and Viiicentius Bellovacensis, and of the Augustine Thomas Cantimpratensis, although these used as a foundation for their expositions the Latin translation of Aristotle, the works of Pliny and other authors of antiquity. Wotton. — Under such conditions we must regard it as an im- portant advance that at the close of the Middle Ages, when the interest in scientific investigation awoke anew, Aristotle's concep- tions were taken up and elaborated from a scientific standpoint. In this sense we can call the Englishman Wotton the successor of Aristotle. In 1552 he published his work "De differentiis animalium," 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 the rich treasury of Aristotelian knowl- 10 GENERAL PRINCIPLES OF ZOOLOGY. edge the systematic results obtained the chief recognition, and thus Wotton's work inaugurated the period of systematic zoology, which in the Englishman Ray. but even more in Linnaeus, has found its most brilliant exponents. Linnaeus, the descendant of a Swedish clergyman, whose family name Ingemarsson had been changed after a linden-tree near the parsonage, to Lindelius, was born in Eashult in 1707. Pronounced by his teachers to be good for nothing at study, he was saved from the fate of learning the cobbler's trade through the influence of a physician, who recognized the fine abilities of the boy, and won him for medical studies. He studied at Lund and Upsala; at the age of twenty-eight he made extended tours on the Continent, and at that time gained recognition from the fore- most men in his profession. In 1741 he became professor of medi- cine 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; after his death there came out a thirteenth, edited by Gmelin. This has become the foundation for systematic zoology, since it introduces for the first time (1) a sharper division into the system, (2) a definite scientific terminology, the binomial nomen- clature, and (3) brief, comprehensive, clear diagnoses. In classi- fication Linnaeus employed four categories; he divided the entire Animal Kingdom into Classes, the Classes into Orders, these into Genera, the Genera finally into Species. The term Family was not employed in the " ; Sy sterna Naturae." Still more important was the binomial nomenclature. Hitherto the common names were in use in the scientific world, 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 prevailed 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 adjective, the species within the genus. The names Canis familiaris, Canis lupus, Canis vitlpes, indicate that the dog, 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 of naming was particularly valuable in the description of new species, inasmuch as it at the HISTORY OF ZOOLOGY. 11 outset informed the reader to what position of relationship the new species was to be assigned. In his characterization of the various systematic groups Linnaeus broke completely with the hitherto-prevailing custom. His predecessors (as Gessner, Aldrovandus) in their Natural Histories 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. Linnaeus, 011 the other hand, intro- duced brief diagnoses, which in a few words, never in sentence form, gave only what was necessary for recognition. Thus a way was found which insured comprehensibility in 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 under his influence. The logical 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 investigation, but only an important and indispensable aid to it. In the zeal for naming and classifying animals, the higher goal of investigation, 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, the father of this tendency. For while in his " Systema Naturae " 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, in comparison with the Aristotelian system, is rather a retrogression than an advance. Linnaeus divided the animal kingdom into six classes: Mammalia, Aves, Amphibia, Pisces, Insecta, Yermes. 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 Linnaeus stands undoubtedly behind Aristotle, who attempted, and in part successfully, to set up a larger number of groups. But in his successors, even more than in Linnaeus himself, 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 12 GENERAL PRINCIPLES OF ZOOLOGY. species of animals there was no lack of material, and so the arena was opened for that spiritless zoology of species-making which in the first half of the last century brought zoology into such discredit. Zoology would have been in danger of growing into a Tower of Babel of species-describing had not a counterpoise been created in the strengthening of the physiologi co-anatomical side. DEVELOPMENT OF MOKPHOLOGY. 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; this is due to the fact that even up to a recent date comparative anatomy was assigned to the medical faculty, while zoology belonged to the philosophical faculty, as if it were an entirely separate study. The disciples of Hippocrates had previously studied animal anatomy for the pur- pose of obtaining an idea of human organization, from the struc- ture of other mammals, and thus to gain a secure foundation for the diagnosis of human diseases. The work of classical antiquity most prominent in this respect, the celebrated Human Anatomy by Claudius Galenus (131-201 A.D.), is based chiefly upon obser- vations upon dogs, monkeys, etc. ; for in ancient times, and even in the Middle Ages, men showed considerable repugnance to making the human cadaver a subject of scientific investigation. Middle Ages. — The first thousand years in which Christianity formed the ruling power in the mental life of the people was quite fruitless for anatomy; in the main men held to the writings of Galen and the works of his commentators, and seldom took occa- sion to prove their correctness by their own observations. With the ending of the Middle Ages the interest in independent scien- tific research first broke its bounds. Vesal (1514-1564), the creator of modern anatomy, had the courage carefully to investigate the human cadaver and to point out numerous errors in Galen's writings which had arisen through the unwarranted application to human anatomy of the discoveries made upon other animals. By his corrections of Galen, Vesal was drawn into a violent controversy with his teacher, Sylvius, an energetic defender of Galen's authority, 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 and love for facts. It was natural that first of all HISTORY OF ZOOLOGY. 13 vertebrates found consideration, since they stand next to man in structure. Thus there appeared in the same century with VesaFs > Human Anatomy drawings of skeletons of vertebrates by the Nuremberg physician Goiter; the anatomical writings of Fabricius ab Aquapendente, etc. Beginning of Zootomy. — But later attention was turned also to insects and molluscs, indeed even to the marine echinoderms, ccelenterates, and Protozoa. Here, above all, 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 tracheae, etc., an extraordinary extension of our knowledge was brought about. Of Swammerdam/s writings attention should be called particularly to " The Bible of Nature," a work to which no other of that time is comparable, since it con- tains discoveries of great accuracy on the structure of bees, May- . flies, snails, etc. Leeuwenhoek, finally, was a most fortunate discoverer in the field of microscopic research, by him introduced into science. Besides other things he studied especially the minute inhabitants of the fresh waters, the ' infusion-animalcules/ a more careful investigation of which has led to a complete reversal of our conception of the essentials of animal organization. The Dawn of Independent Observation. — 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 what had been lost, the blessing of independent and unbiassed observation. They spread the interest in observation of nature over a wide circle so that in the eighteenth century the number of independent 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 the learned and the laity, as Wrisberg, von Gleichen-Russwurm, Schaffer, Eichhorn, and 0. F. Miiller. In most of the writings the religious character of the contemplations of nature are extraordinarily emphasized, and since we find that among these writers numerous clergymen (Eichhorn in Danzig, Goeze in Quedlinburg, Schaffer in Regens- burg) attained distinction, we have a sign that a reconciliation 14 GENERAL PRINCIPLES OF ZOOLOGY. had taken place between Christianity and natural science. As a criterion of the progress made in comparison with the earlier centuries, a mere glance at the illustrations is sufficient. Any one will at the first glance recognize the difference between the shabby drawings of an Aldrovandus and the masterly figures of a Lyonet or a Rosel von Rosenhof. Period of Comparative Anatomy. — Thus through the zeal of numerous men filled with a love of nature a store of anatomical facts was collected, which needed only a mental reworking ; and this mental reworking 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, Geoffroy St. Hilaire, Cuvier, and the Germans Meckel and Goethe are especially to be named. Correlation of Parts. — When the various animals were com- pared with one another with reference to their structure there was obtained a series of important fundamental laws, particularly the law of the Correlation of Parts and the law of the Homology of Organs. The former established the fact that there exists a dependent relation between the organs of the same animal, that local changes in one single organ also lead to corresponding changes at some distant part of the body, and that therefore from the constitution 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 the form of extinct animals. Homology and Analogy. — Still more important was the theory 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 all the anatomical features, as found in 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 has the function of producing gall; here anatomical and physiological characteristics go hand in hand. But this need not necessarily be the case; very often it may happen that one and the same function is possessed by organs anatomically different; as, for example, the respiration of vertebrates is carried on in fishes by gills, in mammals by lungs. Conversely, anatomically similar organs may have different functions, as the lungs of mammals and the swim-bladder HISTORY OF ZOOLOGY. 15 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, i.e. those anatomically equivalent, and to follow the changes in them conditioned by a change of function. Cuvier. — The foremost representative of comparative anatomy was Georges Dagobert Cuvier. He was born in 1769 in the town of Mompelgardt (Montbeillard), then belonging to Wiirtemberg, and obtained his early training in the Karlschule at Stuttgart. where, through the influence of his teacher Kielmeyer, he was led to the study of comparative anatomy. The opportunity of going to the seashore which was offered to him as private instructor to Count d'Hericy he employed for his epoch-making investigations upon the structure of molluscs. In 1794, upon the persuasion largely of the man who afterwards became his great opponent, Geoffroy St. Hilaire, he moved to Paris, where he was made at first Professor of Natural History in the central school and in the College of France, later Professor of Comparative Anatomy in the Jardin des Plantes. As a sign of the great regard in which Cuvier was held, it should be noticed that he was repeatedly intrusted with high educational positions and was made a French peer. As such he died in 1832, Type Theory. — Cuvier's investigations, apart from the mol- luscs, extended to the coelenterates, arthropods, and vertebrates, living and fossil. He collected his extensive observations into his two chief works " Le regne animal distribue d'*apres son organiza- tion " and " Lecons d'anatomie comparee." Of quite 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 in 1812 introduced a complete reform of classification. The Cuvierian division, which has become the starting-point for all later classifications, differed, broadly speaking, from all the earlier systems in this, that the classes of mammals, birds, reptiles, and fishes were brought together into a higher grade under the name, introduced by Lamarck, of ' verte- brate animals'; that further the so-called ' invertebrate animals ' were divided into three similar grades, each equal to that of the 16 GENERAL PRINCIPLES OF ZOOLOGY. vertebrate animals, viz., Mollusca, Articulata, and Radiata. Cuvier called these grades standing above the classes, provinces or chief branches (embranchements), for which later the name Types was introduced by Blainville. But still more important are the differ- ences which appear in the structural basis of the system. Instead of, like the earlier systematists, using a few external character- istics 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. " The type is the relative position of parts" (von Baer). Thus for the first time comparative anatomy was employed in the formation of a natural system of animals. Lastly the type theory established an entirely new conception of the arrangement 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 definitely determined by the degree of its organi- zation. On the other hand Cuvier taught that the animal kingdom consisted of several co-ordinated unities, the types, which exist quite independently 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 embryology. Embryology is the youngest branch of zoology. What Aristotle really knew, what was written by Fabricius ab Aquapendente and Malpighi upon the embryology of the chick, did not rise above the range of aphorisms, and were not of sufficient value to make a science. The difficulties of observa- tion, due to the delicacy and the minuteness of the developmental stages, were lessened by the invention of the microscope and microscopical technique. Further, the prevailing philosophical conceptions placed hindrances in the way; there was no belief in Embryology 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 (evolutio *) ; eithei the * This original meaning of ' evolution ' is different from that prevailing at present. BISTORT OF ZOOLOGY. 17 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 the doctrine 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 with his " Theoria geuerationis" (1759); he sought to prove by observation that the hen's egg at the beginning is without any organization, and that gradually the various organs appear in it. In the embryo there is a new formation of all parts, an Epigenesis. This first assault upon the evolutionist school was entirely without result, chiefly because Albrecht von Haller, the most celebrated physiologist of the eighteenth century, by his influence suppressed the idea of epigeuesis. Wolff was not able to establish himself in scientific circles in Germany, and was obliged to emigrate to Russia. Only after his death did his writings find, through Oken and Meckel, proper recognition. Von Baer. — Thus it remained for Carl Ernst von Baer in his classic work, "Die Entwicklung des Huhnchens, Beobachtung und Reflexion" (1832), to establish 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 con- clusion that each type had not only its peculiar structural plan, but also its peculiar course of development; that for vertebrates an evolutio bigemina was characteristic, for the articulates the evolutio gemina, for the molluscs the evolutio contorta, and for the radiates the evolutio radiata. Here we meet for the first time the idea that for the correct solution of the questions of relation- ship 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 com- posed of the same elements, the cells. This knowledge is the quintessence of the cell theory, which during the third decade of the last century was advanced by Schleiden and Schwann, and which two decades later was completely remodelled by the proto- plasm theory of Max Schultze. In the cell theory a simple prin- 18 GENERAL PRINCIPLES OF ZOOLOGY. ciple of organization was found for all living creatures, for highly and for lowly organized plants and animals, and the wide realm of histology was laid open for scientific treatment. KEFORM OF THE SYSTEM. Foundation of Modern Zoology. — With the establishment of comparative anatomy and embryology and the application of these to classification, and with the development of the cell theory and of histology, which is connected with it, we may say that the foundation of zoology was laid. Wonderful advances were made in vertebrate anatomy by the classic researches of Owen, Johannes Miiller, Rathke, Gegenbaur, and others; our conceptions of organ- ization have been completely altered by the work of Dujardin, Max Schultze, Haeckel, and others, who have proved the unicellu- larity of the lowest animals. The germ -layer theory was further elaborated by Remak and Kolliker; and applied to the invertebrate animals by Kowalewsky, Haeckel, and Huxley. It is beyond the limits of this brief historical summary to go into what has been accomplished in regard to the other branches of the animal king- dom; it must here be sufficient to mention the most important changes which the Cuvierian system has undergone under the influence of increasing knowledge. The Division of the Radiata. — Of the four types of Cuvier the branch Radiata was undoubtedly the one of whose representatives he had the least knowledge; it was therefore the least natural, since it comprised, besides the radially symmetrical coelenterates and echinoderms, other forms, which, like the worms, were bilaterally symmetrical, or, like many infusorians, were asym- metrical. Thus it came about that most reforms have here found their point of attack. C. Th. von Siebold was the originator of the first important reform. He limited the type Radiata, or, as ho 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 pro- posed for the other articulates, crabs, millipedes, spiders, and insects, the term Arthropoda. Leuckart, about the same time (1848), divided the branch HISTORY OF ZOOLOGY. 19 Eadiata into two branches differing greatly in structure. The lower forms, in which no special body-cavity is present, the interior of the body consisting only of a system of cavities serving for digestion, the alimentary canal, he called the Ccelentera (essentially the Zoophyta of the older zoologists) ; to the rest, in which the alimentary canal and the body-cavity occur as two separate cavities, he gave the name Echinoderma. The Present System. — Thus there resulted seven classes: i Protozoa, Coelentera, Echinoderma, Vermes, Arthropoda, Mol- lusca, and Vertebrata. Still this arrangement does not meet the requirements of a natural system and hence is more or less unsat- isfactory. Some zoologists are returning to the Cuvierian classifi- cation to the extent of uniting the segmented worms with the arthropods in a group Articulate. Upon the ground of important anatomical and embryological characters the Brachiopoda, the Bryozoa, and the Tunic^ita have been separated from the Mollusca; they form the subject of diverse opinions. The relationships 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 diiferences 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. HISTORY OF THE THEORY OF EVOLUTION. Importance of the Subject. — Before closing the historical intro- duction we must consider the historical development of a question whose importance might, on a superficial examination, be under- rated, but which from a small beginning 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 systematic conceptions species, genus, family, etc. 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 which the systematist distinguishes, 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 brought into nature for the purpose of making it 20 GENERAL PRINCIPLES OF ZOOLOO Y. 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 gradation of relation- ship 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 changeable? 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 conception of species was Linnaeus's predecessor, the Englishman John Ray. In the attempt to define what should be understood as 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 within the same species which differ more from one another than do the representatives of different species. Thus John 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.1— With Ray's definition an entirely uncontrollable element was brought into the conception of species, since no systematist usually knew anything, nor indeed could he know anything, as to whether the representatives of the species placed 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. Linnaeus said: "Tot sunt species quot ab initio creavit infinitum Ens"; with this he built up a conception of species upon the tradition cf the Mosaic history of creation, a procedure quite unjustified upon grounds of natural science, since it drew one of its iundamental ideas from transcendental conceptions, not from the experience of natural science. Linnaeus's definition showed itself untenable, as soon as paleontology began to make accessible a vast quantity of extinct animals deposited as fossils. With an odd fancy, the fossils, being inconvenient for study, were for a long time regarded as outside the pale of scientific research. They might be sports of nature, it was said, or remains of the Flood, or of the influence of the stars upon the earth, or products of an aura HISTORY OF ZOOLOGY. 21 seiniiialis, a fertilizing breath, which, if it fell upon organic bodies, led to the formation of animals and plants, but if it strayed upon inorganic materials gave rise to fossils. The foundation of scientific paleontology by Cuvier put an end to such empty specu- lations. Cuvier proved beyond a doubt that these fossils were the remains of animals of a previous time. Just as the formation of the earth's crust by successive overlying layers made possible the recognition of different periods in the earth's history, so paleon- tology taught how to recognize also the different periods in the vegetable and animal world of life on our globe. Each geological age was characterized by a special world of animals quite peculiar to it; and these animal worlds differed the more from 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, destroying all life, and that upon the newly formed virgin earth a new organic world of immutable species sprang up. Objections to the Cataclysm Theory. — By the supposition of numerous acts of creation the Linnean conception of species seemed to be rescued, though, to be sure, by summoning to its aid hypotheses which had neither foundation in science nor justifica- tion in theology. The logical results of Cuvier's cataclysm theory were conceptions of a Creator who built up an animal world only for the purpose of destroying it after a time as a troublesome toy; it has therefore at no time found warm supporters, at least among geologists, for whom it was intended. Of the prominent zoologists there is only to be mentioned Louis Agassiz, who till the end of his life remained faithful to this theory. Under these conditions it is readily understood how thinking naturalists, who felt the necessity of explaining the character of organic nature simply and by a natural law capable of general application, began to doubt the fixity of species, and were led to the theory of change of form, the Theory of Descent, or 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 the renowned 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 particularly by Buffon, Geoffroy St. Hilaire, and Lamarck. Its completest ex- pression was found in Lamarck's " Philosophic zoologique " (1809) ; its arguments will be considered in the following paragraphs. 22 GENERAL PRINCIPLES OF ZOOLOGY. Lamarck (Jean Baptiste de Monet, Chevalier de Lamarck, born in Picardy, 1744, died, Professor at the Jardin des Plantes, 1829) taught that on the earth at first organisms of the simplest structure arose in the natural way 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 oitr globe; the terminal point of this series is man; the other animals are the descendants of those forms from which man has developed. Lamarck, in accordance with the then prevailing conceptions, regarded the animal kingdom as a single series grading from the. lowest primitive animal up to man. Among the causes which may influence th6 change and perfecting of organisms, Lamarck emphasized particularly use and disuse; the giraffe has obtained a long neck because by a special condition of life 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 into functionless structures. The direct influence of the external world must be unimportant; the changes in the sur- roundings (Geoffrey St. Hilaire's le monde ambient) must for the most part act indirectly upon animals by altering the conditions for the use of organs. Evolution vs. Creation. — Lamarck's ingenious work remained almost unnoticed by his contemporaries. On the other hand there arose a violent controversy between the defenders and the opponents of the evolution theory when [1830] Geoffroy St. Hilaire in a debate in the Academy at Paris defended against Cuvier the thesis of a near relationship of the vertebrates and the insects, and set up the proposition that the latter were " vertebrates running on their backs." 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 from scientific discussion, and the theory of the fixity of species again became dominant. This error was occasioned by many causes. Above all, the theory of Geoffroy St. Hilaire and Lamarck was rather a clever conception than founded 011 abundant facts; besides, it had in it as a funda- mental error the doctrine of the serial arrangement of the animal world. * Opposed to this stood Cuvier's great authority and his extensive knowledge, the latter making it easy for him to show that the animal kingdom was made up of separate co-ordinated groups, the types. HISTORY OF ZOOLOGY. 23 Lyell. — In the same year in which Cuvier obtained his victory over Geoffroy St. Hilaire, his theory of the succession of numerous animal worlds upon the globe received its first destructive blow. Cuvier's cataclysm theory had two sides, a geological and a biological. Cuvier denied the continuity of the various terrestrial periods, as well as the continuity of the fauna and flora belonging to them. In 1830-32 appeared the "Principles of Geology" by Lyell, an epoch-making work, which, in the realm of geology, completely set aside the cataclysm theory. Lyell proved that the supposition of violent revolutions on the earth was not necessary in order 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, be it as ebb and flow of the tide, as rain, snow, or ice, or as the flow of rivers and brooks rushing as torrents towards the sea, are suffi- cient to furnish a complete explanation. Very gradually in the course of a vast space of time the earth's surface has changed, and passed from one period into the next, and still at the present day the constant process of change is going on. The continuity in the geological 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, although the geological support of this was frail, 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 influence upon the general views of men. Charles Darwin was born at Shrewsbury, Eng., in 1809. After studying at the universities of Edinburgh and Cambridge, he joined as naturalist the English war-ship " Beagle." 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 for him the germ of his epoch-making theory. Further results of this journey were his beautiful mono- graph on the Cirripedia, and the classic investigation of coral-reefs. After his return to England Darwin lived, entirely devoted to scientific work, chiefly in the hamlet of Down, county Kent, up 24 GENERAL PRINCIPLES OF ZOO LOOT. to the time of his death in 1882. He was incessantly busy in developing his conception of the origin of species, and in collect- ing for this a constantly increasing array of facts. The first written notes, the fundamental ideas of which he communicated to friends, particularly the geologist Lyell and the botanist Hooker, were made in 1844, but the author was not persuaded to give them publicity. Not until 1858 did Darwin decide to make his first contribution to science. In this year he received an essay sent by the traveller 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 fruit of many years, of preparatory labors. For the history of the theory the most important of these are: (1) "Upon the Variation of Plants and Animals under Domestication/' two volumes, which chiefly contain a collection of material for proofs; (2) on "The Descent of Man," a work which gives the application of the theory to man. No scientific work of this century has attracted so much atten- tion in the zoological, we may even say 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 well-wishing but hesitating accept- ance. 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 vic- tory for the theory of evolution. At the present time all our scientific thoughts are so permeated with the idea of evolution that we can scarcely speak of any considerable opposition to it. Post-Darwinian Writers. — Among the men who have most influenced this rapid advance is to be mentioned, besides A. K. Wallace, 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, even if they have taken special standpoints in refer- ence to the causes conditioning the changes of form. Among the English naturalists are to be named particularly Huxley, Hooker, and Lyell. In America Gray, Cope, "and Hyatt were early sup- HISTORY OF ZOOLOGY. 25 porters. 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 among systematists. It was recognized that all the individuals of a species are not alike, and that more or less pronounced 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. The Problem Stated. — Darwin begins with a criticism of the term species. Is the conception of species on the one side and that of race and variety on the other something entirely different ? Are there special criteria for determining 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 conceptions in nature pass into one another ? Are species varieties which have become constant, and precisely in the same manner are varieties species in the process of formation ? Morphological Characters. — A. Distinction between Species and Variety. — For the settlement of this fundamental question morpho- logical and physiological characters can be considered. In the practice of the systematists usually the morphological characters prevail exclusively; for that reason they will be here considered first. If, among a great number of forms similar to one another, two groups can be adduced which differ considerably from one another, if the difference between them be obliterated by no inter- mediate 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 discloses great incongruities, many animal and vegetable groups being regarded by one set of systema- tists as good species, by another only as 'sports/ i.e., as varieties of the same species. The differences between the ' sports ' of our domestic animals are in many instances so considerable that formerly they were regarded not only as sufficient for the founda- tion of good species, but even of genera and families. In the fantail pigeon the number of tail-feathers, formerly only 12-14, has increased to 30-42 (fig. Ic) ; among the other races of pigeons 26 GENERAL PRINCIPLES OF ZOOLOGY. 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 FIG. IA.— English, carrier-pigeon. (After Darwin.) FIG. IB.— English tumbler-pigeon. (After Darwin.) that the total number of vertebrae varies from 38 (in the carrier- pigeon) to 43 (in the pouter), the number of sacral vertebra from 14 to 11. B. Variation within the Species. — Now in respect to the occur- rence 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 HISTORY OF ZOOLOGY. 27 "by many transitions; in other cases sharply circumscribed groups of forms, or races, can be distinguished within the same species. In the race, the peculiar characteristics are inherited from genera- Fio. lc.— English fantail pigeon. (After Darwin.) tion to generation with the same constancy as in good species. This is shown in the human races, and in many pure, cultivated races of domesticated animals. Physiological Characters. — A. Crossing of Species and Varie- ties.— A critical examination leads to the conclusion that Mor- phology is indeed useful for grouping animals into species and varieties, but that it leaves us completely in the lurch when it is Called upon to show the distinctions between what should be called a species and what a variety. Therefore there remains open to the systematist only one resource, i.e., to summon Physiology to his aid. This has been done, and it 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 under normal conditions the individuals of one and the same species, even though they are of different varieties or races, should be entirely fertile. One must beware of arguing in a circle in proof of these two propositions; it would be an argument in a circle if an experimenter should regard two animals as representatives of one species only because they proved to be fertile together, while under their former relations he assigned them to different species. Bather the question for him must read: does physiological experiment lead to the same 28 GENERAL PRINCIPLES OF ZOOLOGY. systematic distinctions as does the common systematic experience, viz., the depreciation of constancy and the divergence of distin- guishing characters ? B. The Intercrossing of Species. — This field is as yet far from being sufficiently investigated experimentally; 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. The most favorable material for research is furnished by those animals in which artificial fertilization can be carried out, i.e., of which one can take the eggs and spermatozoa and mix them independently of the will of the animals. 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 Abramis brama and Blicca bjorkna, of Trutta solar (salmon) and Trutta far io (trout); among sea-urchins the spermatozoa of Strongylocentrotus lividus fertilize with great readiness the eggs of Echinus microtuberculatus, 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. microtuberculatus does not fertilize the eggs of S. lividus. Even better known is the fact that 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 rnpestris by sperm of the cod (Gadus morrhua), frogs' eggs (Rana arvalis) by sperm of two species of Triton, eggs of a starfish (Asterias forbesi) by milt from a sea-urchin, Arbacia pustulosa (??). In these extreme cases, it is true, the hybrids die during or at the close of segmentation, before the embryo is outlined. In the case of animals where copulation is necessary the diffi- culties of experimentation increase, since here often between males and females of different species there exists an aversion which prevents any union of the sexes. Yet in this case we know crosses of different species; among the vertebrates crossing takes place, e.g., between the horse and the ass; our domestic cattle and the HISTORY OF ZOOLOGY. 29 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 (Anas boschas) and the pintail duck (Dafila acutd)'3 the European goose and the Chinese goose (Anser ferus and A. cygnoides}. Among the insects, especially the Lepidoptera, the cases are many, but the resulting eggs produce larvae of slight vital force only in the case of Actias luna and A. isabellce. 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 con- tinued fruitful J:or generations; the same is true of hybrids obtained from the wild buck and the domesticated she-goat; from Anser cygnoides and A. domesticus; from Salmo salvelinus and S. fontinalis; Cyprinus carpio and Carassius vulgar is; Bombyx cynthia and B. arrindia. D. Inbreeding. — Even the second of the above statements, that individuals of a species, provided they are sound, always reproduce with one another, needs limitation. Breeders of animals have long known the disastrous consequences of inbreeding — that the repro- ductive power is reduced even to sterility if, for breeding, descendants of a single pair be continually chosen. Darwin has collected not a few cases where undoubted members of the same species have been completely sterile with one another; as certain forms of primrose and other di- and tri-morphic species. Exam- ples of the sterility of mongrels are known only in botany (certain varieties of maize and mullein). Conditions Governing Fertility in Sexual Reproduction.— When we look over these facts it would seem as if continued fertility in sexual reproduction were guaranteed by a not too con- siderable diiference in the sexual products. Too great similarities, as these exist in inbreeding, and too great differences, as in the 30 GENERAL PRINCIPLES OF ZOOLOGY. hybridization of different species, are injurious and are abhorred by Nature. Sexual reproduction possesses an optimum; if this be departed from in either direction, diminution gradually follows. But for that reason it has already been said that here gradual and not primary differences exist, and therefore this character cannot be employed as a primary distinction* bet ween species and varieties. Difficulties in Classification. — The final result of all this dis- cussion may be summed up as follows: up to the present time, neither by physiological nor by morphological evidence has there been successfully fixed in a clear and generally applicable way 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 investi- gators vary. Change of Varieties into Species. — The conditions above dis- cussed 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 explana- tion of a concrete case. Individuals of a species begin to vary, i.e., compared with one another they attain a greater or less- difference in character. So long as the extreme differences are bridged by transitional forms we speak of varieties of a species ; if, on the other hand, the intermediate transitions have died out, and the differences have in the course of a long space of time become fixed, and so very much 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 in the large majority of cases exists between the two. In genera which comprise a remark- able 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. In regard to the varieties also the case is similar. In such genera the formation of species is in active progress; but each species formation presup- poses a high degree of variability. HISTORY OF ZOOLOGY. 31 Phylogeny. — It is now clear that what has here been worked out in the case of the species must also apply to the other cate- gories of the system. Just as by divergent development varieties become species, so must species by continued divergence become so far removed from one another that we distinguish them as genera. It will be only a question of time when these differences will become still greater, and cause the establishment of orders, classes, and branches, just as the tender shoots of the young plantlet become in the strong tree the chief branches from which spring lateral branches and twigs. If we pursue this train of thought to its ultimate consequences, 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 at least many thousands of years are required for the formation of several new species through the variability of one, there must then 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 idea of the individual develop- ment (embryology) of an animal the term Ontogeny has been chosen, it has also proved convenient to apply to the historical development of animals — though this has not been observed, but only inferred — the term History of the Race or Phylogeny. Spontaneous Generation.— If we attempt to derive all living animals from a common primitive form, we are compelled to assume that this was of extremely simple organization, that it was unicellular; for the simpler, the less specialized, the organization, so - much the greater is its capacity for variation. Only from simple organisms can the lower unicellular organisms, the Protozoa, be derived. Finally, for the simple organisms alone can we conceive a natural origin. Since there was undoubtedly a time upon our earth when temperatures prevailed which made life impossible, life must at some time have arisen, either through an act of creation or in a natural way through spontaneous generation. If, in agreement with the spirit of natural science, we invoke for the explanation of natural facts only the forces of nature, we are driven to the hypothesis of spontaneous generation, namely, that by a peculiar combination of materials without life, the compli- cated mechanism which we call ' life ' has arisen. This hypothesis also supposes that the first organisms possessed the simplest con- ceivable structure. Variability not proven to be a Universal Principle. — Starting from a basis of facts, by generalization we reach a simple concep- 32 GENERAL PRINCIPLES OF ZOOLOGY. tion of the origin of the animal kingdom, but we have in equal measure departed from the results of direct observation. Observa- tions only show us that species are capable of changes and can from themselves produce new species. That this capacity for variation is a universal principle, a principle which explains to us the origin of the animal world, needs further demonstration. Proofs of Phylogeny. — The rise of the existing animal world is a process which has taken place in the thousands of years long past, but is no longer accessible for direct observation, and there- fore it can never be proved in the sense that we explain the indi- vidual development of an organism. In regard to the conception of the simple evolution of animals we can merely prove the probability; yet it is shown that all our observations of accessible facts not only agree with this conception, but find in it their only simple explanation. Such facts are furnished to us by the classi- fication of animals, paleontology, geographical distribution, com- parative anatomy, and comparative embryology. (1) Proofs from Classification. — For a long time it has been recognized, and in recent times finds ever-increasing confirmation, 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 one must select a treelike arrangement, in which the principal divisions, more closely or dis- tantly 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, as seen above, necessarily leads. (2) Paleontological Demonstration approaches nearest to what one might call direct proof; for paleontology gives us the only traces of existence which the predecessors of the present animal world have left. Even here a hypothetical element has crept into the demonstration. We can only observe that various grades of forms of an animal group are found in successive strata; if we .unite these into a developmental series, and regard the younger as derived from the older by variation, we depart, strictly speak- ing, from the basis of fact. But the value of paleontological evidence is weakened much more by its extreme incompleteness. In fossils only the hard parts are generally preserved; the soft parts, on the other hand, which alone are present, or at least make up the most important part of many animals, are almost always lost. Only rarely are the soft parts (muscle of fishes, ink-bag of HISTORY OF ZOOLOGY. 33 cephalopods, outlines of medusae) preserved in the rocks. Even the hard parts remain connected only under exceptionally favor- able conditions. If further we take into consideration the fact that these treasures are buried in the bosom of the earth, and are usually obtained only by accident, in quarrying and road-build- ing, and besides only extremely seldom excavated with scientific care, it becomes sufficiently clear how little is to be expected from the past and indeed future material of paleontology. Examples of Paleontological Proof. — Yet paleontology has already furnished many important proofs of the theory of descent. Fio. 2.— Archceopteryx lithnijraphica. (After Zittel.) cl, clavicle; co, coracoid; 7i, humerus ; r, radius; w, ulna ; c, carpus ; I-IV, digits ; sc, scapula. 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 of these the mammals; among the mammals man. For smaller groups genealogical material has 34: GENERAL PRINCIPLES OF ZOOLOGY. 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 starting-point or 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 Archceopteryx (fig. 2), a bird with a long, feathered, lizard-like tail. (8) Morphological Proofs. — When we employ comparative anatomy and embryology in support of evolution, we find that the two studies have so many points 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 similari- ties in structure and in the development forming a bridge from type to type. The first of these two propositions is still regarded as correct, but the second, which alone is important for the theory of evolution, has become quite 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 only in such differ- ences 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. Especially from the branch of the worms there lead off transitional forms to the other branches : Balanoglossus to both echinoderms and chordates, the annelids and Peripatus to the arthropods, the tunicates and Amphioxus to the vertebrates. In some representa- tives 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, and speaks against the assumption of a rigid unvarying type in Cuvier's sense. Fundamental Law of Biogenesis. — A fact that weighs heavily in the balance in favor of the theory of evolution is the fact that the structure and mode of development 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 development passes HISTORY OF ZOOLOGY. 35 through essentially the stages which remain permanent in the case of lower or at least more primitive animals of the same branch, as 432 1 ell FIG. 3.— Human Embryo, about third or fourth week. 1-4, visceral arches with gill- slits between them : 1, mandibular arch ; 2, hyoid arch ; 3 and 4, first and second gill-arches, a, eye ; n, nasal pit : ft, cardiac region ; e land e II, fore and hind extremities; rn, mesodermal somites. FIG. 4.— Tadpoles of Rana tempnraria. TM, mouth ; g, upper jaw; 2, lower ja^; «, sucking-disc ; hb, external gills ; ifc, region of the internal gills ; w, nose ; a, eye; o, auditory vesicle ; /i, cardiac region ; c/, operculum. the three following examples will show: (1) In the early stages of development the human embryo (fig. 3) possesses remarkable 36 GENERAL PRINCIPLES OF ZOOLOGY. resemblances to the lowest vertebrates, the fishes. Like these it has gill-slits, the same arrangement of the heart and of the arterial vessels, certain fundamental features in the development of the skeleton, etc. (2) Frogs in their tadpole stage have an organiza- tion similar to that which remains permanent in the case of certain Amphibia, the Perennibranchiata (fig. 5), which stand FIG. 5.— Siredon pisciformis (larva of Amblystoma tigrinum). (After Dum6ril and Bibron.) lower in the system; they have a swimming tail and tuft-like gills, which are lacking in the adult frog. (3) There are certain para- sitic Crustacea, which live upon the gills of fishes, and seem not FIG. 6. — Achtheres percarum. a, nauplius-, h, cyclops-stage ; c, adult female. (After Claus.) 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 HISTORY OF ZOO LOOT. 37 shown that they pass through a nauplius-stage (fig. 6«), charac- teristic of most Crustacea, and that they then assume the shape of small Crustacea (fig. 6, b), like Cyclops (fig. 7, A), so widely dis- ctu FIG. 7.— Cyclops coronatus (A) and also the nauplius in lateral (B) and in ventral view (G). I, head; II- F, the five thoracic, and behind these the five abdominal seg- ments ; F, furca ; 1, the first, 2, the second, antennae ; 3, mandibles ; 4, maxillae: 5, maxillipeds ; 6-9, the first four pairs of biramous feet, while the rudimentary fifth pair are hidden ; cm, eye ; o, upper lip ; e, egg-sacs ; (7, gut ; m, muscle. tributed in fresh waters. Very often the males make a halt in the cy clops-stage while the female 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 forms GENERAL PRINCIPLES OF ZOOLOGY. Flmai8eTaPf?ef ffius x (after Bergsoe), x 13. pass through the stage of organization of the lower, because they spring from ancestors which were more or less similar to the latter. Man in his embryological development passes through the fish stage, the frog the per- ennibranchiate stage, the parasitic crus- tacean first the nauplius- and then the cyclops-stage, because their ancestors were once fish-like, perennibranchiate- like, nauplius- and cyclops-like. Here is expressed a general phenomenon which Haeckel has stated in a general proposition under the name of 'the Fundamental Law of Biogenesis. ' ' ' The development history (ontogeny) of an individual animal briefly recapitulates the history of the race (phylogeny); i. e. , the most important stages of organi- zation which its ancestors have passed through appear again, even if somewhat modified, in the develop- ment of individual animals." Examples of the Application of this Law. — Hie Nervous System. — This law applies as well to single organs as to entire animals. The central nervous system of the 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 embedded in the interior of the body; but in the embryo it is laid down likewise as a part of the skin (medullary plate) and which gradually through infolding and cutting off from this comes to lie internally. One can demonstrate this change oi position by cross-sections through the dorsal region of embryos of different ages of any vertebrate (fig. 9). The Skeletal System. — The skeleton of vertebrates is a further example. In the lowest chordates, amphioxus and the cyclostomes, the vertebrae are lacking, and in their place we find a cylindrical cord of tissue, the chorda dorsalis (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 or a combination of bone and cartilage. Mature birds and mammals finally have a HISTORY OF ZOOLOGY. 39 FIG. ^.-Cross-sections through the dorsal region of Triton embryos at .different ages (from O. Hertwig) In T the medullary plate fanlage of spinal cord) rn» is off from the skin (epidermis, ep} bv the medullary folds (wf). In II the lary plate by inrolling of the medullary folds is converted into a groove In 11 the groove has closed into a tube n has previously oc- curred. Direct and indirect division are recognized. Direct division is most common in Protozoa, and especially in nuclei with abundant chromatin (fig. 20, 145). The nucleus is elongated and is divided by constriction, in the same way that the cell itself constricts. Since the proto- plasm has no special arrangement with regard to the dividing nucleus (the latter besides protected by its membrane), we must conclude that the nucleus divides itself and is not passively divided. The dividing force resides in the achromatic framework, which correspondingly often 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 chromatin, which possess a centrosome. The process is introduced by a division of the centrosome (fig. 21). The daughter centro- somes 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 centro- GENERAL ANATOMY. 69 somes, 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. A debated point is the relations of the fibres in the equa- torial plane of the spindle. Do all the fibres extend from pole to pole ? Do all Of them end in the equatorial FIG. 21.— Spindle formation imd divi- sion of the centrosomes in Ascaris plane, SO that the Spindle Consists of megalocephala. (After Brauer.) c, centrosomes ; c?i, chromosomes. two cones ot 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 objects. 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, for the chromatin of the nucleus divides early into particles which are rarely spherical or rodlike, but usually have the shape of U-shaped loops. These chromosomes are of equal size in the same FIG. 22.— Cell division in the skin of Salamandra maculosa. (After Rabl.) cell, and, what is of greater theoretical significance, their number is identical in all the cells of all the tissues of one and the same species. 70 GENERAL PRINCIPLES OF ZOOLOGY. The first step in the karyokinetic formation of the daughter nuclei is the division of the chromosomes, which is usually com- pleted in the equatorial plate (division of the equatorial plate), but occasionally may be completed at an earlier stage. The division is an accurate halving (fig. 22, b). The two halves of a mother- chromosome, the daughter chromosomes, now travel, under the influence of the spindle-fibres, towards the opposite poles of the spindle. In this way, by a splitting of the equatorial plate, the lateral plates arise, the elements of each uniting and producing the daughter nuclei. The centrosomes remain separate as division organs for the next nuclear division (fig. 22, 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 of the protoplasmic reticulum (fig. 21). When the centrosome divides a double radia- tion (amphiaster) appears. Not only the spindle-fibres but the protoplasmic rays extend from the daughter chromosomes. Since the arrangement and degree of development of the protoplasmic radiations stand in certain relation to the phases of cell division we must recognize in them the expression of the effective forces (apparently contractile) in the protoplasm which cause cell division. Between these two extreme cases of direct and indirect division are all possible transitions which show how the mechanism of nuclear divi- sion has been completed step by step, first, by the fibrous arrangement of the nuclear reticulum (spindle structure) ; second, through the develop- ment of the centrosome by which the division obtains an influence on the protoplasm ; and third, by the development of the chromosomes. In reference to the latter 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 if 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 between the daughter cells. This is accomplished by mitosis. Nuclear Fragmentation is to be distinguished from direct division ; by it the nucleus becomes broken up into numerous parts which alone cannot live and as a rule degenerate. A typical example is afforded by the breaking up of the macron ucleus during conjugation in the Infusoria (fig. 146). Multinuclearity, Multicellularity. — Nuclear division and cell division commonly constitute a well-arranged mechanical process, GENERAL HISTOLOGY. 71 the separate phases of which follow one another according to a definite law. The plane of division is per- pendicular to the long axis uniting the two poles of the spindle. But the interrelation of cytoplasm and nucleus is by no means an unchange- able 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. 23), which now on its part may become many cells, if subsequently the protoplasm divides according to the number of nuclei. Hence multi- Fc?ii^ith nucleated protoplasmic masses are transitional stages nuclei- between the simple mononucleated cell and a collection of several 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 cell-complexes or syncytia. In the following pages a multinucleated mass of protoplasm will be considered as a single cell, because the essential feature of the cell is that it constitutes a vital unit, it 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 ANTMAL BODY. Definition of Tissue. — In the formation of tissues two processes are operative: (1) the multiplication of cells by means of division into cell-complexes, and (2) the histological differentiation of cells. A tissue, therefore, can be defined as a complex of differentiated cells Mstologically similar. Nature of Histological Differentiation. — The histological differ- entiation consists chiefly in that the cells have definite form and definite position relative to the neighboring cells; in addition, there almost always occurs, as a second and more important feature, the histological modification of the cell. The fact has already been mentioned that the cell uses its food-material, not only for its own growth, for increase of its protoplasm, but also, in another manner, for forming substances, protoplasmic products, GENERAL PRINCIPLES OF ZOOLOGY. either in its interior (internal plasmic products), or more often on its surface (external plasmic products). The histological change is the formation of specifically functioning pla&mic products. If we ^ take as an example the manner in which a cell becomes a muscle fibre (fig. 24), we see that it continually secretes upon its surface new fibrillaa of specific muscle substance (in the case of the vertebrates, new cross-striated muscle fibrillae), until finally the remnant of the formative cell, the muscle corpuscle, is contained in a mantle of muscle fibrillae. In an analogous way, each tissue, upon histological examination, 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 advan- tages of tissue formation are far-reaching, since FIG. 24.— Formation & of muscle fibrils in m general they are connected with division of the frog. (Dia- & -L gram.) a, forma- labor (frequently referred to later). So long as tivecell; b, forma- .. .;, ., . ., ,,, ,, ,, ., , „ .. tive cell with two the cell unites in itself all the vital functions, atedSnmsciJfibrnsj these are incomplete because they mutually witn°rni?mVe r o^s 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 discharge its duties with greater completeness. The muscle fibrillae, the characteristic elements formed by the muscle cells, have preserved of the various properties of protoplasm only the capability of contraction; but this power of contraction is much more energetic and stronger than the mere movement of protoplasm. The nerve fibrillae serve only for the transmission of stimuli, but in an extraordinarily more rapid and orderly manner than does simple protoplasm. Classification of Tissues. — Since in every tissue its function interests us most, it would be natural to base the classification of tissues upon the function and the intimate structure connected therewith. For a long time the tissues have been arranged in four groups: 1. Epithelial tissue; 2. Supporting tissue; 3. Muscular tissue; 4. Nervous tissue. Within these, however, certain con- stituent 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 former may be spoken of in connexion with the epithelium, the latter in connexion with the supporting substances. GENERAL HISTOLOGY. 73 i. Epithelial Tissues. Morphology of Epithelial Tissues. — On several grounds the epithelia must be considered first. They are the oldest tissues; they are the first to appear in the animal kingdom, there being" animals which consist only of epithelia. Further, each separate organism during the first stages of embryonic life consists only of epithelia] layers, the germ -layers. "With this is also connected the fact that in epithelial tissues the cells have undergone the least degree of histological change, and that the formation of plasmic products is subordinated. Function of Epithelium, — The most important purpose of the epithelium is to form a protecting and excluding covering over surfaces, equally valuable whether the surfaces are external (surface of the body) or caused by 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, are parts of the body which, on account of their hardness, can exist, at least for a more or less considerable time, without epithelial covering. Glandular and Sensory Epithelia. — By their superficial posi- tion epithelia are suited for presiding over two other functions: all substances which ought to be removed from the body — some because they have become useless, and consequently injurious- (excreta), and others, as, for example, the digestive fluids, because they have to perform important functions (secreta) — must pass the surface, and are therefore separated by the epithelia ; these are the glandular epithelia. Further, all influences of the external world chiefly impress the surface of the body, causing sensations; hence also certain epithelia are of the greatest importance for the recep- tion 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, in order to serve the function of the tissue, 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 (figs. 25, 26, 27). 74 GENERAL PRINCIPLES OF ZOOLOGJ. Simple Epithelium. — Exclusively one-layered epithelia are found in all invertebrated 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 occasioDally, as always in the skin, replaced by a many-layered epithelium. According to TIG. 25 —Various forms of epithelia. a, flattened epithelium of Sycandra raphanw, a' in cross-section, a" in surface view; b and c, cuboidal and columnar epithe- lium of a mollusc (Haliotis tuberculata) ; d, flagellated epithelium of an actinian (Calliactis parasitical; e, ciliated epithelium from the intestine of the fresh- water mussel; /, epithelium (e) with cuticle (c) of Cimbex coronatus (a wasp). the shape of the cells we distinguish cuboidal or pavement, flat, and columnar epithelium. In the case of pavement epithelium (fig. 25, V) the cells are all developed about equally in all direc- tions 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. 25, c)', finally, in flat or squamous epithelium this is greatly GENERAL HISTOLOGY. 75 shortened (fig. 25, a) and the separate cells have become changed into thin plates. Flagellated and Ciliated Epithelia. — Further differences which obtain in the three kinds of epithelium mentioned above are caused by the presence or absence of processes (cilia, or flagella) on the peripheral end of the cells. Both are fine threads which arise from the body of the cell, extend above the surface and here maintain an extremely lively motion. In case of flagellated epithelium (fig. 25, d) each cell has only one vibratile projection, but this is strongly developed ; in the case of ciliated epithelium (fig. 24, e), on the other hand, the surface of the cell is covered with a thick forest of minute threads moving in unison. Cuticle. — The majority of the one-layered epithelia are covered by a cuticle, a membrane which is secreted by the epithelial cells in general, and hence very frequently shows the impression of the cells as polygonal markings. In many cases thin and inconspic- uous, it may in other instances become thickened into a very con- siderable layer, much thicker than the matrix layer of epithelium which secretes this cuticle. The cuticle is plainly composed of layers parallel with the surface, and forms a more effective protec- tion for the surface of the body than does the epithelium; it becomes a protective armor, as shown, among other examples, by the calcareous shells of molluscs and the chitinous integument of insects (fig. 25, /). Stratified Epithelia. — The protection furnished by the cuticle in the case of simple epithelium, may in the stratified be obtained immediately through a chemical change of a part of the cells themselves. In the stratified epithelia the cells of the various layers always can be distinguished by their form. The deepest layer consists of cylindrical cells; the superficial, on the other hand, of more or less flattened elements; between lie several layers of transitional forms, so that starting from the cylindrical cells we gradually pass through the cubical cells to the flat cells of the sur- face. As this arrangement shows, there exists a genetic con- nexion between the cell -layers: the lower cylindrical cells are in a state of active multiplication; their descendants, with gradual changes of form, become the superficial layers, here to replace an equal quantity of worn-out cells (fig. 26). In the course of this change of position, the protoplasmic bodies may undergo an alteration; in the reptiles, birds, and mammals (fig. 27) they became cornified, first the margins, then the inner part of the cell, changing into horn. Of the living cell 76 GENERAL PRINCIPLES OF ZOOLOGY. the nucleus remains for some time, until at length this vanishes, 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 no longer capable of life, are sharply marked off from one another. In cross-section they are readily distinguished as the stratum corneum (sc) and the stratum FIG. 27. - Stratified epithe- lium of man. sM, stratum. Malpighi; sc, stratum cor- neum. FIG. 26.— Section through the skin of Petromj/zon planeri. Ep, the many-layered epithelium of the epidermis, including B, goblet cells; Ko, granu- lar cells; Ko, Co, derma (with blood-vessels G), consisting of bundles of fibrils running hori- zontally ( W) and vertically (S). (From Wieders- heim.) FIG. 28.— Single-lay ere d epithelium of a snail. c, cuticle ; d, goblet cells. Malpighi (sM) of the skin (fig. 27). In the many-layered epithelia the cuticle has lost its importance, and it is either an inconspicu- ous boundary line or is entirely absent. Glandular Epithelium. — Glandular epithelium is distinguished physiologically from ordinary pavement epithelium by the fact that it also produces secretions or excretions; anatomically it is recognizable by the presence of t gland cells/ 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, GENERAL HISTOLOGY. 77 the goblet cells; here the secretion, usually mucus, is collected as a clear mass in the interior of the cell, the cytoplasm being com- pressed into a thin external wall, reminding one of a goblet c6n- taining the nucleus at its base (fig. 26, 28, d). Other gland cells are the granular cells, swollen bodies completely filled with secre- tory granules (fig. 26, K'6). Naturally all grades of transition between pavement and glandular epithelium occur. Commonly the latter name is only employed when the gland cells are especially numerous, thereby giving to the epithelial 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 invetebrate animals (fig. 29); a gland cell here becomes so '' enormous that there is no room for it d- in the epithelium, but it is pushed into the deeper, the subepithelial layers, the nucleated cell body, dis- tended by secretion, sending up a slender process, the duct, to the epithelial surface. Multicellular Glands. — In the for- mation of multicellular glands a con- siderable area of glandular epithelium grows as a cylindrical cord or tube from the surface down into the deeper tissues; this COrd of Cells seldom FIG. 29 -Unicellular glands from edge of the mantle of Helix po- remains simple; it usually branches matta. e, epithelium; d, uniceiiu- J lar glands; p, pigment cells. and forms the compound glands, which may consist of hundreds or thousands of glandular sacs, all emptying into a common duct. Among the multicellular glands are to be distinguished tubular and acinous (racemose) forms. In tubular glands (fig. 30) the simple or branched glandular pouches preserve the same tubular diameter from beginning to end; in the acinous glands (fig. 31), on the contrary, the blind end of the glandular pouch widens into a sac (acinus), largely composed of secretory cells, and related to the outer part of the glandular pouch, the duct, as grapes are to their stem. To the tubular glands belong the liver, kidney and sweat glands of man; to the 78 GENERAL PRINCIPLES OF ZOOLOGY. acinous belong the salivary glands, not only of the vertebrates, but also of the arthropods and molluscs. FIG. 30.— Tubular glands. (After Toldt.) A, glands of Lieberkuhn 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; U, from the cortex of the rabbit's kidney. Sexual Epithelium. — The sexual cells may be considered in connexion with glandular epithelium. As the secretion of some FIG. 31.— Acinous salivary gland of the aphid Orttiezia cataphracta. (After List.) In some acini the nuclei and boundaries of the cells are shown. glands must be expelled from the body, so the sexual cells are elements which differ from the rest of the organism, and must reach the exterior in order to perform their function. Just as the GENERAL HISTOLOGY. 79' gland-cells are usually scattered among ordinary epithelial cells, so the sexual cells, almost without exception, lie embedded in epithelium; it maybe in the epithelium of the skin (fig. 32), of the FIG. 32.— Germinal epithelium of a medusa, eh, ectoderm; en, entoderm ; o, egg; e, epithelium. gut, of the body cavity, or of parts cut off from this (fig. 33). This connexion of the sexual cells with the epithelium has a deeper meaning in the fact that many organisms, and particularly organisms of low structure, consist exclusively of epithelia and FIG. 33.— Section through the ovary of a new-born child. (After Waldeyer.) geT 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 fol- licle; r, blood-vessel. therefore must necessarily develop 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 relation with one another. Sexual epithelium (or, as it is often called, germinal epithe- lium) like glandular epithelium has a tendency to grow into the subepithelial tissues in the form of isolated or branching tubes 80 GENERAL PRINCIPLES OF ZOOLOGY. (figs. 33, p, 34), and thus in many groups of animals the sexual organs bear the character of branched glands; for this reason one speaks as often of sexual glands as of sexual organs (fig. 34). The male and female cells, the specific ele- ments of the germinal epithelia and of the sexual glands, differ in the fact that the eggs are generally the largest, the spermatczoa the smallest, cells of the animal body. Egg-cell. — The egg-cell (fig. 35) as it is formed in the ovary varies in size according to the animal group: in case of the micro- scopic Gastrotricha it is less than 0.04 mm., in man about 0.2 mm., in the frog several millimetres, and in the large birds often several inches ; however, only the yolk of the bird's egg is the egg-cell, the white of the egg and the shell are structures which are formed outside of the ovary in the oviduct. These remarkable differences in size are caused less by the quantity of the peculiar cell-substance, protoplasm (formative or primary yolk), than by the accumulation of deutoplasm (food or accessory yolk, also FIG. 34. FIG. 35. a, formative cell; 7), follicular epithelium; c, nutritive cells; d,J5gg-cells; /, fibrous covering extending out into aid FIG. 34. — Ovarian tube of an insect, Vanessa urticce. epithelium; c, nutritive cells; d, egg-cells; the terminal fibres (g). (After Waldeyer.) FIG. 35.— Egg-cell of Stronyylocentrotus Uvidus. briefly called yolk). The function of the deutoplasm is to nourish the embryo during development, and hence consists of substances rich in fat and proteid, arranged in spherical oil-drops, or in fine GENERAL HISTOLOGY. 81 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-continued course of development is necessary to lay the foundation of the manifold organs. Besides the protoplasm and deutoplasm, a cell nucleus or germinal vesicle (sometimes visible to the naked eye) surrounded by a membrane always occurs in the egg. Its contents are mainly the nuclear fluid, through which is distributed an achromatic network, and in addition the nucleolus, called also the germinal spot. Often there are multinucleolated germinal vesicles, especially in eggs which contain very much yolk. 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. 36, a and /?). Easiest to recognize in them is the head, which from a J5 FIG. 36 —Various spermatozoa, a, of the night-hawk; 0, of the green frog; >, of tho crayfish ; 6, of a crab ; e, of the round worm (Ascaris). n, nucleus ; w, middle piece ; », flagellum ; 7c, homogeneous body. its variety of form — spherical, oval, sickle-shaped, etc. — often renders possible the specific determination of the spermatozoa. The head is the closely compacted chromatic* part of the nucleus, and hence colors very deeply in staining fluids. Next comes an GENERAL PRINCIPLES OF ZOOLOGY. unstaining second part, the middle piece, and then the tail, a long flagellum, which causes the active motility of the ripe sperma- tozoon. Cytoplasm is usually present only in an extremely thin layer surrounding the nucleus. The spermatozoa of nearly all animals, except the nematodes and crustaceans, are constructed according to this type. In these two groups it is worthy of notice that the spermatozoa are remarkably large and incapable of motion, and that they enclose a homogeneous strongly refractive body (fig. 36, k), previously not found, the significance of which is not clear. The spermatozoon of Ascaris (fig. 36, e) has the form of a sugar-loaf with a broad rounded end, containing the nucleus; the spermatozoon of the crayfish (fig. 36, y), on the other hand, has the shape of a cake- pan, from whose periphery springs a circle of fine, stiff, and pointed fibres. The two kinds of spermatozoa found in a few animals are problem- atical. In the testis of one and the same individual of Paludina vivipara occur together hair-like spermatozoa with corkscrew heads and vermiform spermatozoa with a bunch of cilia on the hinder end. The first accomplish fertilization ; the physiological significance of the second is unknown. The last modification of epithelium of which we have to speak is sensory epithelium, characterized by the connexion of certain of its cells, the sensory cells, with the finest twigs of branching nerves which arise in the central nervous system. This connexion may be of two kinds. In the first the cell (primary sense cell) is slender and filiform, the position of the nucleus being indicated by a swelling. The peripheral end is concerned with the reception of sensatory stimuli, while the deeper end is continued directly into the nerve ends and correspondingly is branched into two or more extremely fine processes FIG. 37.— Sensory epithelium, a, of an which take on the character of Act in ian ; /3, from the olfactory epi- ,*•, •-,! /n ow\ theiium of man; d, supporting cells; nerve nbrillae (ng. 67). In the second type the sensory nerve ends in a ganglion cell beneath the epithelium, which sends processes into the latter, the ends of these being applied to the sensory cell (secondary sense cell), the connexion being one of contact, not of GENERAL HISTOLOGY. 83 continuity. In both the peripheral end of the cell bears appen- dages for sense perception; auditory and tactile hairs, stronger processes in the case of olfactory and taste cells, conspicuous rods in visual cells. Almost without exception the sensory cells are part of the skin (ectoderm), or at least arise from it in develop- ment. This is true for sense organs like the eye and ear of verte- brates, which are separated from the skin by thick intermediate tissue, for in these the sensory epithelium (retina, crista acustica) is derived from the ectoderm. Supporting Cells. — In the region of the sensory epithelium and 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 con- tain pigment; in the auditory organs they often bear the otoliths, etc. They have the general name of supporting or sustentative cells. 2. Connective Tissues. Contrast of Epithelium with Connective Tissue. — From a his- tological point of view there can be found no greater difference than exists 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, on the con- trary, 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 connexion 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. 107). 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 conse- quence of this the connective 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 proto- plasm are formed on the surface of the cells, and, since they lie between the cells, these are called intercellular substances. In GENERAL PRINCIPLES OF ZOOLOGY. proportion as the intercellular substance increases in volume the cells themselves diminish and become inconspicuous corpuscles, the connective-tissue corpuscles, or, as seldom happens, entirely disappear. Since, in the connective tissues, the intercellular sub- stances are most important, it is readily understood that the dis- tinctions between the various kinds of connective tissue rest chiefly upon the differences of this intercellular substance. The following forms are to be distinguished: (1) cellular connective tissue; (2) homogeneous connective tissue; (3) fibrous connective tissue; (4) cartilage; (5) bone. Cellular Connective Tissue shows the characteristics of the group least distinctly. It owes its name to the fact that the cells make up the chief mass, while the cell-products are inconsiderable. The cells are large and vesicular bodies which, like plant cells, are closely pressed together and are consequently polygonal (fig. 38). They have secreted between them a firm but thin layer of inter- cellular substance. FIG. 38.— Cellular connective substance. Cross-section through the notochord of a newly hatched Trout. FIG. 39.— Homogeneous connective sub- stance of Sycandra raphantu. (After F. E. Schulze.) Homogeneous Connective Tissue. — In the case of homogeneous connective substance the intercellular substance (or matrix) is usually present in considerable quantity as a transparent mass, nearly invisible under the microscope, sometimes soft like jelly, often firmer (fig. 39). The gelatinous 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 GENERAL HISTOLOGY. 85 their physical characteristics, are called elastic fibres, and consist of a substance (elastin) exceedingly resistant to all reagents. Finally, in the matrix there may develop the finer connective- tissue fibrils, the characteristic element of the next group; they may become so prominent by increase 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 fine- ness, lying in a homogeneous basal substance, which is the less evident the richer it is in fibres. The fibres may be either con- fusedly arranged, crossing in all directions, or may run essentially parallel and in a definite direction. Between them are found the rounded, spindle-shaped or branched connective-tissue corpuscles (fig. 40). It is characteristic of vertebrates that the fibres are grouped into bundles. Each bundle is generally surrounded by connective-tissue corpuscles, metamorphosed into flat cells. The FIG. 40.— Fibrous connective tissue of an Actinian. FIG. 41. — Areolar fibrous connective tissue. (After Gegenbaur.) bundles, loosely interwoven, run in all direction (areolar connec- tive tissue, < cellular tissue' of the earlier authors) (fig. 41), or they may be almost parallel, forming a compact mass of fibres (tendinous tissue) (fig. 42). Since the fibrils of the fibrous con- nective tissue of the vertebrates have another peculiarity not met with elsewhere, in that they are composed of glutin, and upon boiling become gelatine or glue, it is well to reserve for these forms of tissue the special name connective tissue. Elastic Tissue.— In all fibrous connective tissue there may appear, as a further constituent, elastic fibres; they may indeed 86 GENERAL PRINCIPLES OF ZOOLOGY. supplant the ordinary connective-tissue fibrils and become the predominant element of the connective tissue, which is then, spoken of as elastic tissue. // 1 FIG. 43. FIG. 42. FIG. 42.— Tendinous tissue. (After Gegenbaur.) FIG. 43.— Cartilage. (After Gegenbaur.) c, perichondrium ; b, transition into typical cartilage (a). Cartilage. — Cartilage and bone are likewise tissues which find their characteristic development only in the vertebrates. In its appearance cartilage is similar to the homogeneous connective substance of many invertebrated animals; the matrix is homo- geneous and, at first glance, appears quite structureless (fig. 43), but, under the action of certain reagents, assumes a fibrous condi- tion. This conduct, as well as the fact that the cartilage grows through changes of the perichondrium, — a thin, fibrillar skin covering its surface, — makes it more certainly evident that it is homogeneously fibrillar; and it is thereby distinguished from homogeneous connective substance since it is not, like the latter, .a lower but a higher stage of tissue formation. It is worthy of note that the matrix of cartilage (chondrin) by cooking produces a kind of glue which differs from true or glutin glue in that it is precipitated by acetic acid. In the matrix the cartilage cells lie united in groups and nests, a mode of grouping pointing to their origin, since each group of cells has arisen from a single mother- cell by successive divisions. In cartilage also, elastic fibres are found; if present in great number, these change the bluish shiny, hyaline cartilage into the yellow-colored elastic cartilage. Bone is the most complicated structure in the series of connec- tive tissues. It consists of a matrix (ossein), closely allied to GENERAL HISTOLOGY. 87 glutin, so intimately combined with inorganic constituents that it appears under the microscope as a homogeneous mass. The propor- tion of organic and inorganic sub- stances varies according to the age and species of animal : in man, for example, there is 65$ inorganic to 35$ organic substance; in the turtle, 63$ to 37$. Of the in- organic constituents, the most im- portant is calcic phosphate, 84$; in smaller quantities, combinations of fluoric, chloric, carbonic acids and magnesia. Morphologically the matrix is composed of the bone lamellae (fig. 44), whose arrange- ment is determined by the surfaces present in and upon the bone. In a hollow bone (like that of the upper arm or of the hand) there is an outer surface to which a fibrous skin, the bone-skin or periosteum, is closely applied; the presence of the marrow-cavity necessitates a second surface. Finally, the solid mass of the bone is permeated by the Haversian canals, which run chiefly in a longitudinal direction, united into a network by cross or oblique canals, and serve for the passage of blood-vessels. Since the bone lamellae arrange themselves parallel to the surfaces mentioned, two systems may be distin- guished in cross-section, the fundamental lamellae and the Haversian lamellae. The former are arranged parallel to the sur- face of the periosteum and of the marrow-cavity and form a mantle of concentric layers around the marrow-cavity. Into this groundwork the Haversian canals with their lamellae enter, destroying and superseding the fundamental lamellae coming in their way. The Haversian lamellae are concentrically arranged around the lumen of the Haversian canals just as the fundamental lamellae are around the marrow-cavity. FIG. 44. — Cross-section through the human metacarpus. (After Frey.) a, surface of the periosteum; 7>, surface of the marrow-cavity; c, cross-sec- tions of the Haversian canals and their system of lamellae; d, funda- mental lamellae; e, bone corpuscles. 88 GENERAL PRINCIPLES OF ZOOLOGY. Formation of Bone. — The stratification of bone is caused by its mode of origin. Where the bone borders upon the Haversian canals, the marrow-cavity, and the periosteum, there is transiently or permanently an epithelial-like layer of cells, osteoblasts, which secrete the bone-substance on their surface. Certain cells in the matrix participate in this secretion, and here give rise to the bone-corpuscles, which are distinguished from the cartilage -cells by their numerous processes ramifying through the matrix. The processes of a bone-corpuscle branch, and unite with the neighbor- ing cells through fusion of the processes, an arrangement most beautifully seen in dried bone, because here the cavities and the canals of the matrix are filled with air. Special modification of bony tissue, the substance of fish-scales and of the teeth, called also ivory or dentine, should be mentioned. Blood and Lymph, here treated in connexion with the connec- tive substances, are in reality not tissues at all, but nutritive fluids. Two kinds of nutritive fluids occur in the vertebrates, red blood and the colorless, weakly opalescent, or cloudy white lymph. The blood of man and other vertebrates, consists of a fluid and the organized constituents. The fluid or blood-plasma is, apart from inorganic constituents, specially rich in proteids; after the removal of the blood from the blood-vessels a part of these separate by coagulation and form the blood-clot, made up of fibrin, leaving a fluid poor in proteids, the blood-serum. The organized con- stituents, the blood-cells, are distin- guished as red and white blood-cor- puscles. The latter, the leucocytes, are present in smaller numbers and have great similarity to the amoebae found in water; they are particles of protoplasm, contain a nucleus, devour foreign bodies (for example, carmine granules injected into the blood), and move in the ' amoeboid ' manner by putting out pseudopodia (fig. 45). Red Blood-corpuscles. — In the mature condition > the red blood- corpuscles of vertebrates (fig. 46) are circular or oval discs, which by external influences (e.g., by pressure) may temporarily be bent, incised, or otherwise modified in form, but cannot actively change their shape, because they no FIG. 45 —White blood-corpuscles, o, of man; b, of the crab (n, the nu- cleus). GENERAL HISTOLOGY, 89 longer consist of protoplasm. Embryologically they arise from true, nucleated, protoplasmic cells; whether these cells are iden- Fio. 46.— Red blood-corpuscles, a, of man; />, of the camel; c, of the adder; d', of Proteus (seen from the edge); tl", surface view; e, of a ray; /, of Petromyzon; n, nucleus (all the blood-corpuscles are magnified 700 times, except d, which is mag- nified 350 times). tical with the leucocytes or are special < erythroblasts ' is still undertermined ; but gradually the protoplasmic cell-body changes completely into a plasmic product, the stroma of the blood- corpuscle. If the nucleus be retained in this metamorphosis, there is a slight swelling in the centre of the disc; if, however, the nucleus degenerate, the bilateral convexity is replaced by a shallow concavity. In the latter case, one has, in reality, no right longer to speak of blood-cells, since all the characteristic constituents of the cell — nucleus and protoplasm — have disappeared. Systemati- cally the red blood-corpuscles are of interest, since non-nucleate forms are found only in the mammals (fig. 46, a, b), nucleated ones in all the other vertebrates (c, d). The mammals also have circular, the other vertebrates oval, discs. To this, however, exceptions occur, since among the mammals the Typloda (camel, llama) have oval, the Cyclostomes have circular, blood-corpuscles. Haemoglobin. — The red blood-corpuscles are the cause of the color of the blood, as well as the agents of one of its most impor- tant functions, the interchange of gases; both are connected with the fact that the stroma contains the coloring matter of the blood or licemoglobin. Haemoglobin belongs to the few crystallizable proteids and is remarkable for the presence of a small, though extremely important, quantity of iron, and also for its affinity for oxygen. Haemoglobin containing oxygen, oxy-haemoglobin, causes the carmine-like color of the so-called arterial blood; oxygen-free, ' reduced ' haemoglobin causes the dark red, faintly bluish color of venous blood. 90 GENERAL PRINCIPLES OF ZOOLOGY. Lymph is distinguished from blood by the entire lack of red blood-corpuscles and the slight coagulability of its plasma. Lymph is accordingly a proteid-containing fluid with leucocytes, which are here called lymph-corpuscles. In the majority of invertebrated animals there is present only one kind of nutritive fluid, and not even this in every class; the fluid is called blood, although it is usually colorless. Where color is present, it is generally, if not always, a yellowish red or an intense red; this may, even as in the vertebrates, be caused by haemoglobin (among the molluscs in Planorbis, Area tetragona, A. now, Solen legumen, Tellina planata, Pectunculus glycimeris, and others; among the annelids in the Capitellidae, Glycera, Poly cirrus, Leprcea, leeches, and earthworms; among insects in Chironomus). Often other coloring matter occurs instead of haemoglobin : in the cuttlefish, many snails, and in the lobster and Limulus, the oxygen is taken up by the bluish haemocyanin, which contains a trace of copper; in the Sipunculids by haemoerythrin, etc. The blood-plasma, as a rule, is the seat of the color (Chiro- nomus, Hirudinea, earthworms, and most other annelids); only exceptionally do colored blood-corpuscles occur, as in the case of Area, Solen, and the other mussels mentioned above, and also in the genus Phoronis. Colored elements containing haemoglobin, identical with blood-corpuscles, are found besides in the ccelomic fluid of many annelids (Capitellidae, Glycera, Leprea, Polycirrus), and in the ambulacral vessels of echinoderms (Ophiactis virens, some Holothurians). Most widely distributed in the invertebrate animals are the leucocytes, which are distinguished by their active amoeboid movements; still even these may be absent, and then the blood is a fluid without any organized corpuscles. 3. Muscular Tissue. Characteristics of Muscular Tissue. — Most sharply character- ized functionally is the muscle- tissue, inasmuch as it is the agent of active movements in the animal body. Since active mobility occurs in protoplasm, it is important to notice the differences between the two kinds of movement. The distinctions lie in the direction and in the intensity of the movement. A mass of protoplasm has the capacity to move hither and thither in all directions, because in it there is a high degree of mobility be- tween the smallest particles. Muscles and hence their separate GENERAL HISTOLOGY. 91 elements, the muscle-fibres and muscle-fibrils, on the contrary, can shorten only by correspondingly increasing in diameter (fig. 47); they can therefore accomplish motion only in a definite direction, that of the axis of the muscle. The muscle-substance consequently is more limited in its movement than is protoplasm, but on the other hand it has the advantages of greater energy and greater rapidity. An observer conversant with the different kinds of motion is able to decide with considerable accuracy, from the intensity and rapidity, whether in a given case a movement has been brought about by the agency of protoplasm or by the contractile substance in the narrower sense (muscle-substance). tracted state. Formation of Muscle-substance. — These physiological con- siderations show that protoplasm and the contractile substance are morphologically different, and that therefore one must distinguish sharply between formative cells, or muscle-corpuscles, and the product of these cells, the contractile substance, just as in the case of connective tissue, between the connective-tissue corpuscles and the connective-tissue fibrils. This distinction actually occurs, but optically it is not equally demonstrable, for the reason that it is not prominent histologically. In animal histology there are recognized two kinds, it might even be said two stages, in the formation of muscle-substance, the homogeneous, or smooth, and the cross-striated. Since the former looks very similar to non- granular protoplasm, the boundary-line between it and the muscle-corpuscle is more difficult to recognize than in the case of the cross-striated muscle-substance, which in its minute structure is quite different in appearance from protoplasm. In cross-striated muscles the contractile portion consists of two substances regularly alternating with one another in the direction of the contraction of the muscle, of which the one is doubly, the other singly, refractive (figs. 24, 47, 50). Smooth and Cross-striated Muscle-fibres. — The smooth muscle- substance represents a lower stage of development than the cross- striated, since it chiefly occurs in the less highly organized and more inactive animals. Interesting in this respect is the fact that in the two stages of development of one and the same animal the simple and inert polyp has smooth muscles, while the more highly organized and actively motile medusa has cross-striated muscles (fig. 48). The difference in their action has led in the vertebrates 92 GENERAL PRINCIPLES OF ZOOLOGY. to a peculiar distribution of the muscle-substance, the smooth musculature being chiefly distributed to the internal organs, which are not under control of the will (involuntary muscles), while the musculature of the body, subject to the will and hence demanding more rapid action, is cross-striated (voluntary muscles). We must not conclude that the difference between smooth and cross-striated musculature coincides with the distinction between visceral and body musculature; it should be noticed that the body musculature of all molluscs is smooth, the visceral as well as the FIG. 48.— Epithelial muscle-cells, o, of a medusa; b, of an actinian. body muscles of many insects and Crustacea, and the muscles of the heart of vertebrates are cross-striated. It was pointed out above, in connexion with epithelia and connective tissue, that these tissues differed fundamentally. This contrast has its bearing in dealing with the muscles, for both epithelial and mesenchymatous cells may form contractile sub- stances and therefore there are two genetically different kinds of muscles, the epithelial and the mesenchymatous (contractile fibre- cell). Both kinds of muscle-cells can a priori form smooth as well as cross-striated muscle-substance; but the collection of con- nective (mesenchymatous) tissue around inner organs has caused most contractile fibre-cells to be smooth, while most of the epithelial muscle-cells are cross-striated. Epithelial muscle-cells are cells of which one end extends to the surface of the body or the surface of an internal cavity (body cavity, lumen of the gut, etc.), and may here have a cuticle, cilia, or flagella, while at the opposite end it has secreted contractile substance in the form of muscle-fibrils (fig. 48). They combine the double function of epithelial and muscle cells. Contractile fibre-cells, on the other hand, are connective-tissue cells, which usually have surrounded themselves with a layer of contractile substance; corresponding to their origin, they have the form of connective-tissue cells, and are spindle-formed or branched; the branches arising from the ends of the cells (fig. 49). The similarity of form renders the distinction between ordinary connective-tissue cells and fibre-cells difficult; if the contractile GENERAL HISTOLOGY. 93 layer on the surface be slightly developed, the distinction is im- possible. To recognize the character of the elements, therefore, we must choose well-defined examples, in which the uninucleated or the multinucleated mass, the < axial substance/ is sharply marked off from the muscle-mass, the ' cortical layer' (fig. 49, c, d, e). FIG. 49. FIG. 50. FIG. 49.— Contractile fibre-cells, a, of man; 1)-e, of Beroe (a Ctenophore); ft, young fibres ; c, branched ends ; d, middle portion of a fibre; e, cross- section. FIG. 50.— Cross-striated primary bundle. (After Gegenbaur.) w, nuclei ; s, a point where the sarcolemma is plainly shown on account of the tearing of the fibres. In vertebrates and arthropods the contractile fibre-cells occur in the vegetative organs as elements of the ' organic musculature ' ; on the other hand we find here the epithelial musculature in the cross-striated primary bundles, separated from the epithelium, and only developmentally referable to the epithelium of the body cavity (fig. 50). A primary bundle is a cylindrical mass, bounded externally by a structureless envelope, the sarcolemma. Its con- tents consist of fine fibrils, which, closely parallel to one another and pressed closely together, run from one end of the mass to the 94: GENERAL PRINCIPLES OF ZOOLOGY. other. Each fibril is formed of singly and doubly refractive parts, which alternate with one another in more or less compli- cated arrangement. Since now the doubly refracting parts of the fibrils within a bundle lie at about the same level, there is caused a cross-striation extending through the whole bundle. Finally, scattered here and there between the muscle-fibrils are the muscle- corpuscles, spindle-shaped protoplasmic bodies with a nucleus, the remnants of the cells which have formed the musculature. 4. Nervous Tissue. Function of Nervous Tissue. — As the muscular tissue brings about motion, so the nervous tissue serves for the transmission of stimuli. It communicates the stimulations of the sense-organs at the periphery to the central nervous system, the seat of conscious- ness, and here brings about perception (centripetal nerve tracts); further, it transmits the voluntary impulses to the periphery, par- ticularly to the musculature (centrifugal nerve tracts). By the nervous system, finally, the stimuli arising in various places are co-ordinated, thus furnishing the elements for that which we call independent psychic activity. Elements of Nervous Tissue. — The agent of the transmission of stimuli is undoubtedly a specific nerve-substance different from protoplasm. Hence we speak of nerve fibrillae as of muscle fibrillae, the product of the special nerve-cells, but the relations involved are not sufficiently understood. The elements of the nervous system are divided into ganglion cells and nerve-fibres, but it must be remembered that these are not independent of each other, but that the fibres are enormously elongated processes of the ganglion cells. In the vertebrates the ganglion cells vary greatly in size; besides small elements there are large cells, only exceeded by the eggs in size, which correspond- ingly have large nuclei recalling the germinal vesicles. Unipolar, bipolar, and multipolar ganglion cells are recognized, the differ- ences depending upon the number of processes (nerve-fibres) which arise. In multipolar cells the number is very large (fig. 51) and are of two kinds, dendrites and axons or neurites. Dendrites are so called because they branch again and again, not far from their origin from the cell. The axons (of which there is usually but one to a ganglion cell) can be followed to a long distance with- out giving off branches, except here and there lateral side twigs (collaterals) which arise at right angles to the main fibre; they GENERAL HISTOLOGY. often pass over into peripheral nerves, so the morphological distinc- tion from dendrites lies in the greater distance of the region of branching from the body of the ganglion cell. In bi- polar ganglion cells both pro- cesses are neu rites, the cell itself thus being an element intercalated in the course of a nerve-fibre, as also is a uni- polar ganglion cell. The single process of this divides near the cell in a T-shaped man- ner, so that the unipolar cell is to be regarded as a bipolar ganglion cell in which the two neurites are united for a short distance. This conception is intel- ligible in the light of recent researches on the structure of the ganglion cell and its pro- cesses (fig. 52). Both consist They branch at their tips, FIG. 51.— Multipolar ganglion cell of man, (After Gegenbaur.) a, axon. FIG. 52.— Motor ganglion cell from the thoracic region of the spinal cord of a dog, (After Bethe.) n, nucleus. of extremely fine fibrillae, and inter- and perifibrillar substances cementing them together. Each process brings a bundle of GENERAL PRINCIPLES OF ZOOLOGY. fibrillae to the ganglion cell, in which they spread out and pass over into other processes. The branching of neurites and dendrites is a separation of the contained fibrillse; the ganglion cell, the place of exchange of fibrillae between the various processes. Hence the ganglion cell is not a simple cell, but a cell plus plasma products. The similar fibrillar structure of nerve-fibres has long been known. In the central nervous system of vertebrates the most minute elements are the nerve fibrillae, distinguished from muscle fibrillae. by the absence of cross- striation ; from connective - tissue fibrillae by the ease with which they are injured; in preserved material they frequently swell and show vari- cosities (fig. 53). Many fibrillae united in a bundle form a nerve- fibre (fig. 54, A) which is called a gray nerve-fibre in distinction from the white or medullated fibres. In the latter the fibre or axis-cylinder is surrounded by a medullary sheath (fig. 54, B) composed of my elm, a fat- like substance, blackened by osmic acid and separated into variously shaped ( myelin drops. ' The medul- lary sheath appears to act as an FIG. 53. FIG. 64. F.G. 55. insulator. FIG. 53.— Nerve flbriiise with varicosi- Both medullated and non-med- ullated fibres can be enclosed in a 'sheath of Schwann.' This is a feature of the fibres composing the peripheral nervous system and is lacking in brain and spinal cord. It is a delicate envelope with nuclei here and there (fig. 55). At times it forms constructions which cut through the medullary sheath to the axis-cylinder (nodes of Ranvier). Multipolar and bipolar ganglion cells also occur in the inverte- brates, most commonly in the coelenterates (fig. 56), more rarely in worms (e.g., Lumbricus), arthropods, and molluscs, and then chiefly in the peripheral nervous system. In the ganglia (the nervous centres of the last three groups) the ganglion cell usually gives rise to a single strong process, which, however, is richly pro- Tided with lateral branches or dendrites (fig. 74). The medullary B ties. (From Hatsohek.) FiG.54.— Non-medullated (_Q_..Q fiv,^ FIG. 55.-Medullated j- nerve-fibres, A, without, B, with sheath of Schwann. (From Hatschek.) GENERAL HISTOLOGY. . 97 sheath and sheath of Schwann are usually absent in invertebrates even in the peripheral nerves. A thin myelin layer has been rarely observed in arthropods and annelids. On the other hand the true conducting elements, the nerve fibrillae, have been seen in inverte- FIG. 56. — Ganglion cells of an actinian. brate nerve-fibres, and these have been followed into the ganglion cell in which the afferent and efferent fibrillae are united in a lattice-like manner. SUMMARY OF HISTOLOGICAL FACTS. Cells. — 1. The most important morphological element of all tissues is the cell. 2. The cell is a mass of protoplasm which contains one or several nuclei (uninucleated, multinucleated cells). 3. The nucleus probably determines the specific character of the cell, since it influences its functions; accordingly it is also the bearer of heredity. 4. Cells and nuclei increase exclusively by division or budding. Tissues. — 5. Tissues are complexes of numerous similar his- tologically differentiated cells. 6. Histological differentiation rests in part upon the fact that the cells take on a definite form and arrangement, in part upon the formation of plasmic products, which determine the character of the tissue (muscle-fibres, connective-tissue fibrils). 98 GENERAL PRINCIPLES OF ZOOLOGY. Classification of Tissues. — 7. According to function and struc- ture (1) epithelia, (2) connective tissue, (3) muscular tissue, (4) nervous tissue are distinguished. 8. The physiological character of epithelia is determined by the fact that they cover the surfaces of the body, their morphological character in that they consist of closely compressed cells united only by a cementing substance. 9. According to their further functional character epithelia are divided into glandular epithelia (unicellular and multicellular glands), sensory, germinal, and protective epithelia. 10. According to the structure are distinguished simple (cubi- cal, cylindrical, squamous epithelia) and stratified epithelia, ciliated and flagellated epithelia, epithelia with or without cuticle. 11. The physiological characteristic of the connective tissues is that they fill up spaces between other tissues in the interior of the body. 12. The morphological distinction depends upon the presence of the intercellular substance. 13. According to the quantity and the structure of the inter- cellular substance the connective substances are divided into (1) cellular (scanty intercellular substance); (2) homogeneous; (3) fibrous connective tissue; (4) cartilage; (5) bone. 14. The physiological character of muscular tissue is its increased capacity for contraction. 15. The morphological characteristic is the fact that the cells have secreted muscle-substance. 16. According to the nature of the muscle-substance are dis- tinguished smooth and cross-striated muscle-fibres. 17. According to the character and origin of the cells (muscle- corpuscles) the muscles are divided into epithelial (epithelial muscle-cells, primary bundles) and connective-tisue muscle-cells (contractile fibre-cells). 18. The physiological distinction of nervous tissue rests upon the transmission of sensory stimuli and voluntary impulses, and upon the co-ordination of these into unified psychic activity. 19. The conduction takes place by means of nerve-fibres (non- medullated and medullated fibrils and bundles of fibrils); the co-ordination of stimuli by means of ganglion-cells (bipolar, multipolar ganglion-cells). 20. Blood and lymph are proteid-containing fluids; rarely without cells, they may contain only colorless amoeboid cells (white GENERAL ORGANOLOG7. 99 blood-corpuscles, leucocytes), or in addition to these also red blood-corpuscles. 21. Red blood-corpuscles occur, in the main, only in verte- brates and cause the redness of the blood ; they are absent in most invertebrate animals. 22. When invertebrate animals have colored blood (red, yellow), this is usually due to the color of the blood-plasma. 23. The red blood-corpuscles are nonnucleated in mammals, nucleated in all the other vertebrates. III. THE COMBINATION OF TISSUES INTO ORGANS. An Organ Defined. — Organs are formed from the tissues. An organ is a tissue complex, marked off from the other tissues, which has taken on a definite form for carrying on a special function. Thus a single muscle is an organ which consists of a certain amount of muscular tissue; with scalpel and scissors it can be removed from its environment as a connected whole and can still accomplish a definite movement. Principal and Accessory Tissues. — In each organ there is a tissue which determines the function of the organ, and therefore its physiological character. This may be called the principal tissue, for there may be other accessory tissues present, which merely support or render possible the function of the principal tissue. In the muscle of the vertebrates we find, besides the muscle-fibres, connective tissue which, like a kind of cement, unites the bundles of muscle; blood-vessels which provide nourish- ment; finally, nerves by which the muscles are aroused to action. In the human liver also, besides the functionally most important part, the liver-cells, blood-vessels, nervous and connective tissues are present. These accessory tissues are usually found only in the highly developed organs; in the case of the lower animals they may be absent ; thus the digestive tract of ccelenterates has only an epithelial lining; their nervous system consists merely of a cord of nerve-fibres and ganglion-cells. Effect of Use and Disuse. — It is of the greatest importance for the permanency of an organ that it be constantly in function. Living substance is distinguished from the non-living by the fact that, if it be destroyed by use, it is immediately replaced, often by more than sufficient to make good the loss. Functioning tissues and organs under favorable conditions increase in volume; on the 100 GENERAL PRINCIPLES OF ZOOLOGY. other hand, functionless parts undergo a gradual decrease, which finally leads to their disappearance. Change of Function of Organs. — The two factors mentioned, that the permanence of the tissues depends upon continued use, and that usually several tissues enter into the structure of an organ, are important for the understanding of the principle of change of function which plays a prominent role in the meta- morphosis of animal form. It may happen that an organ is brought under changed conditions and no longer has an oppor- tunity to function as before. In that case the functioning tissue, from lack of use, gradually degenerates, but the organ may persist by means of its accessory tissues if the new conditions make it possible for one of them to attain to functional activity, and to give the organ a new physiological character. Examples of Change of Function. — A muscle, for example, may become functionless from many causes. Should the muscle-tissue disappear there are still left the accessory tissues, particularly connective tissue permeated by blood-vessels; this may remain in- tact and form a protecting band, a tendon, or fascia. We have then, morphologically, the same organ, changed in its physio- logical character; the muscle has undergone a change of function, and has become a ligamentous band. The visceral arches of fishes afford another example; these primarily are supports for the gills; if now by the acquirement of terrestrial habits the gills be lost, the visceral arches become functionless and correspondingly under- go a partial degeneration; but a part persists by assuming a new function, and forms the jaws, the hyoid bone, and the small bones of the ear, 'which, in spite- of their quite different functions, are morphologically the same structures as the gill-arches. Homology and Analogy. — In the History of Zoology (page 14) it was shown that comparative anatomy has caused a discrimina- tion between homology or morphological equivalence, and analogy or physiological equivalence, i.e., between organs which appear in the same relative positions and relations, and organs which have the same function. What we have here learned of the structure of organs makes it evident that morphological and physiological characters do not necessarily coincide, that morphologically similar organs may have different functions, morphologically different organs the same functions. Systems of Organs. — Organs wholly identical, or, at least, functioning in an equivalent manner, may occur in considerable numbers in the same body. A man has many muscles, and many GENERAL ORGANOLOGT. 101 organs which carry on digestion. Hence we may group all organs which in the body have equivalent or similar functions, and speak of systems of organs. In all we recognize nine such systems: (1) skeletal, (2) digestive, (3) respiratory, (4) circulatory, (5) excre- tory, (6) genital, (7) muscular, (8) nervous, and (9) sensory systems. Not all are necessarily present; a skeleton, for instance, is frequently lacking. The most different functions which in man are divided among different complicated and specialized systems may be performed in a lower animal by one and the same apparatus. Yet according to the fundamental functions the fol- lowing groups of organs may be recognized : I. Organs of assimila- tion (2-5); II. Organs of reproduction (6); III. Organs of motion (7) ; IV. Organs of perception (8 and 9). Vegetative and Animal Organs.— The organs of assimilation and of repro- duction (I and II) are grouped together as vegetative, the others (III and IV) as animal organs. The older zoologists used to say that assimilation and reproduction are functions which are common to animals and plants ; that, on the contrary, sensation and motion are lacking in plants, and are exclusively characteristic of animals. The atom of truth in the funda- mental idea needs reconsideration in the light of our present knowledge. We have seen that the protoplasm of plants and animals has not only the power of assimilation and reproduction, but also power of motion and of irritability. The latter characteristics consequently cannot be entirely lacking in all the plants, for they are found in the most important. In fact many plants, as the mimosas, the compass-plants, insectivorous plants show great irritability ; many low plants, the reproductive states of algae, move quite as actively as, or even more actively than, many of the lower animals. On the other hand, there are many animals which in the mature condition are fixed in position like plants. Many Protozoa and worms, most of the zoophytes, some echinoderms like the Crinoids, even many Crustacea, the cirripedes (barnacles), can change their location only during the earlier stages of development, in later life being limited to movements of single parts of the body, the arms, tentacles, etc. In the sponges motions are so insignificant that they cannot be seen at all by the naked eye, and scarcely even with the aid of the microscope. Nevertheless the two terms, animal and vegetative, must be retained. For although motion and sensation occur in the vegetable kingdom, still they reach no high development ; indeed we may say they become more and more inconspicuous the higher the plants ; in the animal kingdom, on the contrary, they are unfolded in extraordinary perfection and lie at the basis of its most characteristic features. 102 GENERAL PRINCIPLES OF ZOOLOGY. Vegetative Organs. A. Organs of Assimilation. Assimilation Defined. — If the term assimilation be used in its widest sense, one must speak in this connection of all the con- trivances in the animal body which render growth possible during the period of progressive development, and, during mature life, compensate for the loss of energy connected with each period of functional activity, in order to preserve to the body its functional powers. In each period of functional activity organic compounds are oxidized. Compounds which are especially rich in carbon and hydrogen (as well as some nitrogen and sulphur) and are poor in oxygen are changed by oxidation into carbon dioxide, water, and various nitrogenous products, like urea, uric acid, etc. A com- pensation takes place, for not only is the useless substance removed, but also compounds of oxygen and materials rich in carbon are furnished to the tissues to replace the material oxidized. Assimilation in Animals. — In lowly organized animals all the processes connected with compensative assimilative changes take place through the agency of one and the same organ, the digestive tract; but in the higher animals, through specialization, normal assimilation is a definite series of separate phenomena. Between the lower and the higher animals there are evidently intermediate conditions where specialization has halted at an earlier or a later stage. Different Organs of Assimilation. — Assimilation begins with the presence of suitable food; the solid and liquid constituent parts of the body must digest and incorporate this, i.e., it must be altered so that it can be absorbed and distributed to the tissues. All this takes place through the agency of the digestive tract, which is provided with accessory organs, the digestive glands; the digestive tract likewise removes all matter remaining undigested (the faeces). The necessary oxygen, gaseous food, so to speak, is usually taken, however, by particular parts of the body, the respiratory organs, the gills or lungs. The oxygen and the digested (consequently liquefied) organic and inorganic compounds must further be distributed in the body to the organs and tissues according to their needs. Therefore there are usually blood- vessels or circulatory organs, which permeate the body in all directions. But the tissues need not only a means of obtaining but also of getting rid of certain compounds. The accumulation GENERAL ORGANOLOGY. 103 of the oxidation products arising from functional activity is injurious, to some extent even poisonous, to the organism; conse- quently they must be removed, and in a dissolved state they are taken up by the blood-vascular apparatus, and are brought to definite places for expulsion or excretion. Fluid wastes are expelled by the kidneys of vertebrates, the Malpighian vessels of insects, the water-vascular system of worms; these, together with their accessory apparatus, are embraced under the name ' excretory organs/ Excreta are to be distinguished from fceces; excreta are substances which have been a part of the tissues of the body itself, and, through oxidation, have become useless; while those sub- stances which constitute the faeces were useless from the beginning, and have never belonged to the body, but have remained separated from the tissues by the boundary of the epithelium of the digestive tract. The gaseous oxidation product of the animal body, carbon dioxide, is removed by the blood-vascular apparatus through the agency of the respiratory organs. Since in the respiratory organs there takes place an exchange of the useless carbon dioxide for the oxygen necessary to life, these organs have a double function, being, at the same time, excretory organs and organs for taking up food. After this general survey, we must enter somewhat mor& minutely into a discussion of the various systems of organs. I. The Digestive Tract. Archenteron or Primitive Digestive Tract. — Since the taking in of food and its assimilation are functions most important for the well-being of the animal, it is to be expected that of all the organs in the animal series the digestive tract should be formed first, and also in almost every case should be earliest established in the embryo. The fact that many worms (cestodes) and Crustacea (Rhizocephala) have no digestive tract does not alter this state- ment; for it can be definitely affirmed that, in adaptation to special conditions of life, particularly parasitism, the digestive tract has degenerated. The simplest multicellular, free-living animals are merely simple or branched digestive pouches which have only a single opening, functioning both as mouth and anus (fig. 57). Such an animal has necessarily two epithelial layers, one of which lines the digestive tract, the other covers the surface of the body. These two fundamental cell-layers are called ento- derm and ectoderm. In many coelenterates they are the only 104 GENERAL PRINCIPLES OF ZOOLOGY. layers of the body. In most animals they are separated by inter- mediate tissues, called collectively mesoderm. The higher the animal, the more differentiated is the mesodermal layer. The primitive digestive cavity lined by entoderm is called the archen- FIG. 57. FIG. 58. FIG. 57.— Longitudinal section through the nutritive polyp of a siphonophore. (After Haeckel.) o, mouth-opening; en, entoderm ; efc, ectoderm. FIG. 58.— Stenostoma leucops, in division, a, ectodermal fore-gut, at a' forming anew for the hinder animal; rn, the blindly ending entodermal mid-gut; e, ectodermal ciliated epithelium ; 0, ganglion with ciliated pit ; to, water-vascular canal ; y', ganglion of the hinder animal. teron. In the case of medusae and polyps it forms the entire diges- tive tract, but in most animals this is not sufficient for the needs of digestion and the alimentary tract is increased by invaginations of parts of the surface of the body. Stomodaeum and Proctodaeum. — Even in many coelenterates and lower worms an invagination arises at the anterior end of the digestive tract, forming the ectodermal fore-gut or stomodmim (fig. 58). From the higher worms onwards, it is accompanied by a second invagination at the hinder end, the ectodermal hind-gut, or proctodceum (fig. 59) ; embryologically, this is formed as a blind GENERAL ORGANOLOGT. 105 sac whose closed end unites with the likewise closed posterior part of the archenteron (now called also mesenteron or mid-gut) until the separating wall disappears, whereupon mid- and end-gut com- municate with each other, and the digestive tract becomes a canal extending through the entire body. Divisions and Appendages of the Digestive Tract. — The part which the archenteron takes in comparison with the ectodermal FIG. 59. FIG. 60. FIG. 59.— Bee-larva just after hatching : seen from the ventral surface. The diges- tive tract consists of three portions; a, fore-gut; m, mid-gut; e, hind-gut (not yet connected with the mid-gut) ; sg, limits of segments ; st, stigma ; t, trachea ; n, ventral nerve-cord. (After Butschli.) FIG. 60.— Digestive tract of the domestic fowl, a, oesophagus ; b, crop; c, glandular stomach; d, gizzard ; e, liver ; f, gall-bladder ; gr, pancreas ; ft, t, small intestine; fc, caeca ; J, large intestine ; m, ureters ; n, oviduct ; o, cloaca. proctodseum and stomodaeum in making up the completed diges- tive tract is very different in the various groups. On one side the Crustacea, on the other side the vertebrates, offer the strongest 106 GENERAL PRINCIPLES OF ZOOLOGY. contrast; the Crustacea have a very short mid-gut and consequently a long extent of fore- and hind-gut formed from the ectoderm; in vertebrates, on the contrary, the ectodermal portions are extremely short. The width of the lumen varies in the course of the alimentary canal and renders possible the distinction of different divisions, which, so far as possible, have been provided with uniform names. Fig. 60, drawn from a domestic fowl, illustrates the usual terms. The mouth-opening leads into a wider cavity, which is usually divided into an anterior division, the buccal cavity, and a posterior one, the pharynx. The narrow tube leading from this is the oesophagus (a) ; here and there it may widen, or bear a pouchlike pagination, the crop or ingluvies (#), for the temporary reception of food. From the oasophagus the food passes into a considerable enlargement, the stomach. Birds, like many other animals, have a double stomach, a thin-walled portion rich in glands, and a second part, the walls of which are remarkable for the thick masses of muscle; the former is the glandular stomach (c], the latter is the grinding stomach or gizzard (d), serving for comminu- tion of the food. Behind the stomach the digestive tube narrows into the small intestine (h), the hinder widened part of which is the large intestine (/), terminating in the anus. The limit of the small and large intestine is usually marked by blind pouches, the cceca (&). Connected with the anal gut also are the outlets of the kidneys (m) and of the sexual apparatus (n) ; hence the terminal portion, serving as the outlet for the urine and faeces, and also for the sexual products, is called the cloaca (o). In animals which require abundant food the area of the alimentary tract is not sufficient to furnish the digestive fluids, so that evaginations of the wall (glands) serve to increase this. Into the mouth empty the salivary glands; into the first part of the small intestine, close behind the stomach, the liver (e) and the pancreas (g) (or a single glandular apparatus, whose secretion combines the characters of gall and of pancreatic juice, the hepato- pancreas). Finally, in the hind-gut there sometimes occur glands which form a fetid secretion — the anal glands. The length of the digestive tract is chiefly influenced by the kind of food. In many groups of animals there is found a difference between herbivores and carnivores, the former having a very long and consequently convoluted digestive tract. That of a carnivore is about four or five times the length of the body, while in an herbivorous ungulate, on the other hand, it is twenty to twenty-eight times. Similar, GENERAL ORGANOLOQY. 107 though -not so great, are the differences between carnivorous and plant-eating beetles. II. Respiratory Organs. Sources of the Oxygen used in Breathing. — The oxygen which each animal must obtain in exchange for the carbon dioxide formed in the tissues is derived either from the air or from the water, according as the animal is terrestrial or aquatic. Less frequently it is the case that water-dwellers breathe air, and hence are com- pelled, from time to time, to rise to the surface of the water for a FIG. 61.— Left second foot o£ a crayfish with attached gill (br). (After Huxley.) czp, coxopodite; bp, oasipodite; ip, ischiopodite; mp, meropodite; cp, carpopodite; pp, propodite; dp, dactylopodite; cxs, bristles of the coxopodite; e, lamina of the gill. supply of air; this is true for the great marine mammals, and for many insects, spiders, and snails which are found in fresh water. Air- and water-breathing takes place exclusively through the skin, so long as this is delicate and readily permeable, and so long as no higher development of organization necessitates a more active interchange of material. If, on the other hand, the demand for oxygen be greater, other more special breathing-organs are found — gills for water-breathing, lungs and tracheae for air-breathing, in 108 GENERAL PRINCIPLES OF ZOOLOGY. addition to which the skin functions as an accessory organ of more or less importance. Gills. — The gills are usually thin-walled areas of the skin which are abundantly supplied with blood-vessels, and where richly branched tuftlike projections or broad leaves have grown out, thus furnishing the largest possible surface for the interchange of gases; these occur in such a position as to be most exposed to fresh water ; in the crayfish, for example, they are on the legs, where the motion drives fresh water constantly through them (fig. 61); in the swimming worms, on the back; in the tube-dwelling worms, at the anterior end, projecting out of the tube (fig. 62); in most FIG. 62.— Anterior end of Terebella nebulosa. (After Milne Edwards.) ph, pharynx ; -yd, dorsal, vv, ventral, blood-vessel ; br, gills ; t, tentacles. amphibians (fig. 4), on each side of the neck. More rarely the digestive tract functions for water-breathing; in the fishes, Enteropneusta, and tunicates gills have been formed in connection with the pharynx, its lateral walls being pierced by the gill-slits, which open to the exterior on the surface of the body. The water containing oxygen in solution passes out through the gill-slits, and bathes the gill-filaments, which are richly provided with blood- vessels. The hind-gut also in many fishes, insects, and worms GENERAL ORGANOLOGY. 109 may become an accessory respiratory organ, being filled from time to time with fresh water. Aerial Respiration. — In the air-breathing animals the respira- tory apparatus is derived either from the digestive canal or from the skin. With the vertebrates the former is the case, since the lungs, either directly or by the mediation of the trachea and bronchi, are in connexion with the lumen of the digestive tract. On the contrary, in the case of invertebrate animals (snails and spiders) when the term ' lung ' is used, it refers always to an invaginatioii or sac of the skin; of such a nature are the tracheae of insects, tubes containing air, beginning at the surface of the body with a hole or stigma, and branching internally (fig. 59, st). Distinctions between the Respiratory Systems of Chordates and Invertebrates. — In general, then, a distinction can be drawn between the respiratory systems of vertebrate and invertebrate animals : in the former, the digestive tract, or derivatives from it, are respiratory; in the latter, on the contrary, it is the skin. On the side of the vertebrates the only exceptions are most amphibians and a few fishes (Protopterus), in which the gills are tuf tlike pro- jections of the skin (figs. 4 and 5); while among the invertebrates some aquatic insects respire by the hinder end of the digestive tract. III. Circulatory Apparatus. In order that the oxygen, taken up by the respiratory organs? and the constituents of the food digested in the alimentary canal may reach the tissues, there is no need of special organs, so long as the body consists of only two thin epithelial layers, the ectoderm and entoderm. When, however, a third, a mesodermal, layer is interpolated between these, and the body consequently becomes more bulky, there is usually some apparatus for distributing .the food. The simplest is when the digestive tract departs from the character of a straight tube and branches, and by means of these branches extends into the various parts of the body. We speak then of a g astro-vascular system, because the alimentary canal itself takes on the function and the branching arrangement generally characteristic of the vessels or ' vascula' (fig. 63). Coelom. — The coslom or enterocoele is apparently derived from a pair of gastric diverticula which have become completely cut off from the archenteron (compare development of mesoderm, infra). It is a cavity pushed in between the intestinal tract and the body- wall, is lined by a special epithelium, the peritoneum, and encloses 110 GENERAL PRINCIPLES OF ZOOLOGY. most of the vegetative organs. If the two halves of the coelom approach, without uniting, dorsal and ventral to the gut, the result is dorsal and ventral membranes, the mesenteries, which support the alimentary canal. Of these the ventral is most fre- quently, the dorsal least often, degenerate. In many invertebrates the coelom plays an important role in nutrition since it contains a lymphoid fluid, rich in proteids and containing cellular corpuscles. It loses this significance the more the blood system is developed, FIG. 63. FIG. 64. FIG. (&.—Leptoplana tremellaris. a, mouth ; b, buccal cavity ; c, opening of the head of the pharynx into the buccal cavity ; d, central stomach ; e, branched ento- dermal gut; /, ganglion; 0, testicle; ft, seminal vesicle; /c, uterus; /, receptaculum seminis ; m, female sexual opening. FIG. 64.— Schema of circulation of the blood, a, arteries ; c, capillaries ; 7i, auricle; /c, ventricle ; fel, valves ; p, pericardium ; v, veins. and in the vertebrates, so far as nutrition is concerned, it is a rudimentary organ. A sharp distinction should be drawn between the ccelom and other cavities in the body. Not every ' body cavity ' is a coelom, but frequently there occur large spaces which are entirely different in origin and in relations. Frequently, as in arthropods, these 'body cavities' contain blood and are in reality but expansions of the vascular system. To such cavities the term hcemocode has been given. Heart, Arteries, Veins, Capillaries. — The most complete method of food distribution is accomplished by the Uood-vessels, GENERAL OROANOLOGY. Ill which, therefore, belong generally to the higher animals, and function whether a body cavity is present or not. Blood-vessels are tubes with fluid contents, the blood, which transports the oxygen received through the respiratory organs, as well as the food absorbed from the digestive tract, and later gives these up to the tissues. Since such an interchange of substances presupposes that the blood circulates in the vessels, definite parts in the course of the blood-vessels are contractile; they are covered by muscles which by their contraction narrow the tube and push the fluid forwards. In the lower forms wide areas in the course of the blood-vessels are contractile; in higher animals a greater regularity of circulation is reached ; a definite specialized muscular part of the course, the heart, alone propels the blood. The Higher Development of the Heart. — A free motion of the heart is only possible when it is separated from the contiguous tissues and enclosed in a special cavity (fig. 64). Hence we see that the heart always lies either free in the body cavity or enclosed in a special pouch ( p), the pericardium (in all cases a portion of the general body cavity, but not always of the coslom, which has become independent). The division of the heart into a part which receives the blood, the atrium or auricle (h), and a part which drives the blood onward, the ventricle (&), is of less functional importance; hence this division is not carried out in all cases. There are also special mechanisms within the heart, the 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 (c), extremely fine and thin-walled tubes, which surround and permeate all organs. Through their walls, usually formed of a thin epithelial layer alone, the proteid substances for nourishing the tissues can pass, and the oxygen can be exchanged for carbon dioxide. Between the heart and the capil- laries there exists, corresponding to their different functions, great differences in structure; they must therefore be united by special transitional vessels — vessels which begin large and thick-walled at the heart, and by branching, and thinning of their walls, pass gradually into the capillaries; of such vessels there are two kinds, 112 GENERAL PRINCIPLES OF ZOOLOGY, the firmer arteries (a) leading to the capillary region, and the thinner- walled veins (v) leading back to the heart. Correlation of Respiratory Organs and Blood System, — It is a law that in all animals the blood-vascular system has been influ- enced in its arrangement and structure more by respiration than FIG. 65.— Scheme of circulation in a fish, a', ascending (ventral) aorta; a2, descend- ing (dorsal) aorta ; c, carotid , da, intestinal arteries ; dc, intestinal capillaries , dv, intestinal veins; 7i, auricle; k, ventricle; Tea, afferent gill-a.rteries; kv, efferent gill-arteries ; Zc, liver-capillaries ; sc, body-capillaries ; i>c, cardinal veins ; vTi, hepatic vein ; ty', jugular vein. by nutrition in the narrower sense; there exists a correlation between the organs of respiration 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 GENERAL ORGANOLOGY. 113 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, respira- tory arteries and respiratory veins). The accompanying diagram (fig. G5) 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; from the auricle the blood flows into the ventricle, and through the afferent gill-arteries into the gill-capil- laries. Thence it is conducted through the 'gill-veins7 (eiferent arteries), which unite into a single large trunk; this again gives off lateral branches passing into the capillary region of the 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 corre- spondingly its color. The blood which flows from the body capillary region has given up its oxygen to the tissues, receiving in exchange carbon dioxide, and has become dark red. This character is maintained until, in the gill-capillaries, it again becomes oxygenated, giving up the carbon dioxide and becoming bright red. The different character of the blood can be recognized in the arteries and veins of the systemic circulatory system; the dark blood containing carbon dioxide is called venous, and the bright red, containing oxygen, arterial blood, since the former flows in the veins, the latter in the arteries. These terms are entirely unsuitable, as can readily be seen from the above diagram (fig. 65), because they easily lead to the false assumption that veins must always conduct blood containing carbon dioxide, and arteries always oxygenated blood. In opposition to this, the diagram shows that, in the respiratory circulation (the shorter course), the conditions must be the reverse of those in the systemic circula- tion, since here the arteries contain ( venous/ while the veins contain ' arterial/ blood. Closed and Lacunar Blood-vascular Systems. — Such a blood- vascular system as has here been described is called a closed one, because the blood always flows in special tubes provided with their own walls. Opposed to the closed stands the lacunar blood-vascular system; here the blood-vessels lose, after a time, the character of tubes and become wide cavities, or sinuses, which, without special walls, are enclosed between the intestines and other organs (hsemocoale, supra). 114 GENERAL PRINCIPLES OF ZOOLOGY. Example of Lacunar Blood-vascular System. — The best exam- ple of a lacnnar blood-vascular system is furnished by the insects and myriapods, which have only the heart and short arterial trunks; from the ends of the arteries the blood enters the haemocoele, and from this through lateral slits (ostia) again enters the heart (fig. 66). In the groups of arthropods and molluscs are found all transi- tions between so extreme a case of a lacunar blood -vascular system and the almost com- pletely closed one. Here appears again a close correlation of the circulatory and respir- atory organs, the latter determining the development of the former. If the respira- tion be diffusely distributed over or through the body, and the distribution of the oxygen goes on without special vessels, the circula- tory apparatus is very simple; on the other hand, if the respiration be connected with definitely restricted areas, and a regular dis- i«. uu. — .fi.ii bei-iui: eiiu \JL , ., . . " ,, , ., the heart of Scoiopen- tribution of oxygen be necessary, the appara- tus is differentiated into heart, arteries, veins, n!' alary and capillaries. Details may be found in the f heart j sections on crustaceans, spiders, and insects, ' FIG. 66.— Anterior end of o, ostia. f Lymph- vessels. — A special part of the vascular system is the lymph system, which is known only in vertebrates. In the capil- lary region of the body, it is true, proteids may pass over into the tissues, but it is evident that a possible overflow cannot re-enter the blood-vessels in the same way, on account of the higher pres- sure prevailing in the capillaries. This overflow is conducted back to the veins through the lymph-vessels. The lymph-vessels begin with lacunae in the tissues, and gradually pass into vessels with definite walls. The lymph-vessels of the digestive tract are par- ticularly important since, during digestion, they become filled with the proteid and fatty constituents of the digested food; they are called the chyle-vessels, because they contain the chyle, distin- guished from ordinary lymph by its milky color. Cold- and Warm-blooded Animals. — In connexion with the blood-vascular system, two expressions are much used but not generally correctly understood by the general public, viz., cold- blooded and warm-blooded — or, more correctly, animals with GENERAL ORGANOLOGT. 115 variable and animals with definite temperatures. Under the head of animals with varying temperature (poikilothermous) or cold blood are placed forms whose temperature is largely dependent upon the temperature 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 tempera- ture of the human body, such animals, for example the frog, would feel cold to our touch, since they, particularly in the cool season, have a much lower temperature than we. Such creatures as, living under any thermal condition, maintain about the same temperature, are termed warm-blooded or definite- temperatured (idiothermous, homoiothermous) 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 tem- perature during the varying external conditions, the animal must have a heat-regulator; it must have the power to regulate the warmth of its body, on the one hand by limiting the production of . heat, on the other by controlling its loss. If the environment be warmer than is suitable for the body temperature, then the pro- duction of heat must be limited to the smallest quantity com- patible 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, conversely, every unnecessary loss of heat must be avoided, while the production of heat must be increased. It is clear that idiothermy, since it requires compli- cated apparatus, can occur only in the highly organized animals. IV. Excretory Organs. Nature of the Organs of Excretion. — The excretory organs are tubes or glandular canals which open upon the surface of the body, either directly or by way of an end-gut (cloaca), and conduct sub- stances which have become useless to the body to the exterior. The presence of a blood-vascular system or a coelom or both together exercises an 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 (proto- nephridia or water-vascular st/stem of parenchymatous worms, 116 GENERAL PRINCIPLES OF ZOOLOGY. fig. 67). The canals begin with closed tubes, which are provided internally at the end with a bundle of actively vibrating cilia, the ' flame ' (fig. 68). 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 coelom there is a central place for the collection of excreta. The nephridia or segmental organs — usually simple tubes (rarely branched) open at both ends — lead from this to the exterior. One opening is ex- ternal (fig. 69), the other communicates with the co3lom by means of a ciliated n FIG. 67. FIG. 68. FIG. 67.— Distomum hepaticum with water-vascular system. (From Hatschek.) jp, porus excretorius ; o, mouth. FIG. 68.— Blind end of one of the finest water-vascular canals (k) of a Turbellarian. (From Lang.) n, nucleus ; /, processes of the terminal cell ; it1/, ' flame ' of the terminal cell ; u, vacuole. funnel, the nepJirostome, a wide mouth with active cilia which connects with the canal of the tube. Through this the ex- cretion (in annelids peritoneal cells laden with guanin — the dis- integrated ' chloragogue ' cells) is carried to the outside. The excretory organs (kidneys) of vertebrates are derived from such nephridia. The fact that in the embryos (and frequently in the adults) these open into the coelom by nephrostomes makes it probable that also in the vertebrates the coelom was once important in excretion (fig. 70). The increasing importance of the blood- vessels which envelop the nephridial canals and bring to them the waste matter taken from the tissues is probably the cause of the loss of connexion of the kidneys with the coelom by degenera- GENERAL ORGANOLOGY. 117 tion of the nephrostomata. The relation of the blood-vessels to the nephridial tubes becomes specially close by the development of the glomeruli (Malpighian corpuscles); bundles of capillaries »fe N* M FIG. 69. FIG. 69. — Segmental organ of an Oligochaete. (From Lang.) fz, ciliated funnel ; dis, septum ; ngl, non-glandular, tig'1, glandular, part of the canal ; eh, terminal ves- icle ; In, body- wall. FIG. 70.— Diagram of the primitive kidney of a vertebrate. (From Hatschek.) Dotted lines mark the limits of the segments. A, anal opening; P, mouth of the duct of the primitive kidney (W); Ns, nephrostome ; M, Malpighian bodies of the seg- mental tubules (S). carrying the walls of the canal before them and so projecting into the lumen of the tube. B. Sexual Organs. Sexual Glands and Ducts. — In the sexual apparatus of animals are distinguished the areas where the germinal cells are produced, the sexual glands or gonads, and the ducts for these. The former are present, temporarily or permanently, in all multicellular animals; the latter, on the contrary, may be completely absent. If the sexual products arise in the skin or in the walls of the digestive tract, as is usually the case in the coelenterates, then 118 GENERAL PRINCIPLES OF ZOOLOGY. 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 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 permanently, in others only temporarily; in the latter case it separates, usually by constriction, and forms gland- like bodies, the gonads or sexual glands. Gonochorism and Hermaphroditism. — In most animals the germinal epithelium produces either only female or only male sexual cells; such animals are called separate-sexed, dio&cioiis or di FIG. 71.— Sexual organs of Lumbricm agricola. (From Lang, after Vogt and Yung.) The seminal vesicles of the right side are removed, Zwi, ventral nerve- cord; IIP and of, ventral and lateral rows of setae; st1, st2, receptacula seminis; sb1, sb2, TO", the three seminal vesicles of the left side, which are connected with a median unpaired seminal capsule (sbw). Enclosed in the latter are the anterior and pos- terior testes (Ti1, ft2), and the anterior and posterior seminal funnels (t1, t2), which lead into the vas deferens (vd). o, ovaries; to, ciliated funnels leading into the oviducts (ov); di, dissepiments; VIII-XV, eighth to fifteenth segments. 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 distin- guished; commonly testes and ovary are contained in the same GENERAL ORGANOLOGT. 119 animal, some distance apart, as in the earthworm, in which two segments are male, while a third segment is female (fig. 71). More rarely there is a union of testes and ovary into a single glandular body or hermaphroditic gland; our 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 two cases of normal hermaphroditism are known among them, a sea-perch, Serranus scriba, a bony fish, and Myxine glutinosa, the hagfish. More commonly hermaph- roditism occurs as an abnormality; a striking form is lateral hermaphroditism, 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 FIG. 72.— Lateral hermaphroditism of a gipsy moth (Ocneria dispar). Left female, right male. (After Taschenherg.) of the animal has the characteristic marks of the male, the other half those of the female. Hermaphroditic lepidoptera and bees are known in which the male half bears the special form of the male antennas, eyes, and wings, and thus is essentially different from the female half (fig. 72). Still it must be noted here that, in many instances where the external appearance pointed towards 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 does not in the majority of cases deserve the name. 120 GENERAL PRINCIPLES OF ZOOLOGY. Genital Ducts. — Very frequently in the animal kingdom the excretory apparatus furnishes outlets for the sexual products. In the annelids and in the vertebrates portions of the nephridial system, either exclusively or in addition to their excretory function, become accessory sexual organs. Hence we speak of a urogenital system. This remark- able connexion of genital and excretory organs has a double cause, a physiological and an anatomical. Physiologically important is the fact that eggs and spermatozoa behave like excreta; being substances which are no longer destined for the use of the individual, but must reach the exterior in order ta become efficient. The morphological cause is the relation to the co3lom. A iirogenital system is formed only in animals in which the germinal epithelium arises from the epithe- lium of the ccelom, and in which the kidneys FIG. 73. — Sexual appara- ,-\ • j- j j_i • tus of Vortex vtridis. or their rudiments stand permanently in glands- connexion with the body cavity and thus form the natural outlet for its products. Whether the accessory sexual parts are por- tions of the excrefcorJ organs or are inde- pendent structures, they have in the animal series a definite arrangement adapted to their function (fig. 73). Canals lead from the sexual glands to the exterior, the oviducts in the female, the vasa defer entia in the male (and the herma- phroditic duct from the hermaphroditic gland). Accessory Sexual Apparatus. — The terminal portion of the vas deferens is often very muscular and is called the ductus ejacula- torius; it may be evaginated as a penis or cirrus, or project permanently beyond the surface of the body. The terminal portion of the oviduct is often widened so that two portions may be distinguished, the uterus, which harbors 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. Oviduct and vas deferens may be provided with sac-like evaginations which serve for the reception of the sperm. In the female these are called receptacula seminis, in the male vesiculce seminales; the former give lodgment to sperm which enters the female sexual passages during coition, the latter GENERAL ORGANOLOGY. 121 to sperm which has been formed in the testes of the same indi- vidual. Animal Organs. I. Organs of Locomotion. Voluntary Locomotion. — The power to change their location voluntarily is a peculiarity so prominent in animals that to the general public it is sufficient for deciding whether an organism belongs to the vegetable or to the animal kingdom. On this account it is necessary to call attention to the fact that numerous animals lose the power of locomotion, becoming fixed to the ground, to plants, or to other animals. All sponges and corals, most hydroid polyps, and the crinoids among the echinoderms, have actively swimming larvae, but become fixed in the adult and thus obtain such a marked similarity to plants that, although true animals, they were long regarded as plants. Further, many mol- luscs and worms are firmly fixed by their shells; indeed, many crustacean forms, the cirripeds, have completely lost their free motility. But a more careful investigation in all these cases will show that the power of moving the separate parts exists, for the corals can retract their tentacles, the cirripeds their featherlike feet, and the clam can close its shell. Locomotion among Lower Animals. — The lowest forms, the Protozoa, progress almost exclusively by processes of the cell: pseuclopodia, cilia, or flagella. In the metazoa this is rarely the case. Amoeboid movements of the epithelial cells, indeed, occur in the coelenterates and also in many worms, but do not suffice for change of position. More effective is the ciliated or flagellated epithelium, by which ctenophores, turbellarians, and rotifers swim; this occurs, besides, in many larvae of animals which, in the mature state, are unable to change their location or do so only by the aid of muscles. Nearly all coelenterates, echinoderms, molluscs, and the majority of the worms leave the egg-membranes in the form of the planula, i.e., as a larva swimming by means of cilia. Locomotion among Higher Animals. — The musculature is alone adapted for energetic motions. The arrangement of this varies with and depends upon the constitution of the skeleton. Forms without a skeleton have commonly the i dermo-muscular tunic/ a sac of circular and longitudinal muscle fibres which is firmly united 122 GENERAL PRINCIPLES OF ZOOLOGY. with the skin. If a skeleton be formed by the skin, as in the arthropods, then the sac breaks up into groups of muscles, which find points of attachment upon the dermal skeleton; if, on the other hand, as in the vertebrates, an axial skeleton be formed, a fixed point is furnished for muscular action, so that the muscula- ture obtains a quite new character, in particular a deeper position. A locomotor apparatus quite unique is the ambulacra! system of the echinoderms, a system of delicate little tubes with protrusible portions which function as feet, described in connexion with that group. II. Nervous System. Scarcely a system of organs in the animal series shows such a regular development as the nervous system. The different stages which can be grouped may be termed the diffuse, the linear, the ganglionic, and the tubular types. Diffuse Nervous System. — The diffuse type is certainly the most ancestral; it shows the two elements, nerve fibres and ganglion cells, regularly distributed through the whole body, or, at least, through certain layers of the body. The skin of the body, the ectoderm, is to be looked upon as one of the fundamental elements in the nervous system, since it is related to the external world, and hence receives the sensory impressions, so important for the development of nervous tissue. The corals and hydroid polyps are examples, since in them the ectoderm is permeated in all directions by a delicate, subepithelial spider-weblike network of nerve fibres and ganglion cells, which encroach even upon the entoderm. Linear Nervous System. — From the diffuse type the other chief types can be derived through concentration, which is chiefly conditioned by the fact that there are a few points which are most advantageously located for the reception of sensory stimuli, and hence for the development of nervous elements. In the medusae such a place is the rim of the bell; consequently a stronger nerve- cord remarkably rich in ganglion cells is found here. This, as well as the nerve-ring and the five ambulacral nerves of echino- derms, may be called a central system, thereby distinguishing the rest of the nervous network as the peripheral nervous system. Ganglionic Central Nervous System. — Numerous transitional forms lead to the ganglionic central nervous system of the worms, molluscs, and arthropods (fig. 74). The central nervous system here consists of two or more ganglia; each ganglion being a GENERAL OROANOLOOT. 123 rounded bunch of regularly arranged nerve-fibres and ganglion- cells. The former constitute the centre of the mass, and, since they cross in all directions, give the appearance of fine granula- tions; this fact has led to the unsuitable, because misleading, name of < Leydig's dotted substance/ The ganglion-cells, on the other FIG. 74.— Third abdominal ganglion of a crayfish. (After Retzius.) C, connective or longitudinal commissure; G, ganglion cell layer; g' ganglion cell whose neurites enter the connective; 02, ganglion cell whose neurites enter the peripheral nerve ; _L, Leydig's dotted substance; JV, peripheral nerve. hand, collect in a thick layer around the dotted substance. The peripheral nerves, and also the commissures, the cords connecting similar ganglionic masses, extend outwards from the ganglia. Supracesophageal (or Cerebral) Ganglia. — Since most animals are symmetrical, the ganglia occur in pairs; left and right ganglia correspond to one another and are connected simply by a cord of nerve-fibres, the transverse commissure. Of most constant occur- rence are two ganglia, which lie dorsally above the pharynx, and hence are called the supracesophageal or cerebral ganglia. If other ganglia occur, they lie ventrally and below the digestive tract (ventral nerve-cord). 124 GENERAL PRINCIPLES OF ZOO LOOT. Ladder Nervous System. — A widely recurring arrangement is that termed the ladder nervous system (of annelids and arthropods) (fig. 75). Numerous pairs of ganglia (in the example before us, nine) lie in serial order on the ventral side of the animal, and are connected by longitudinal commissures (connectives), and also by transverse com- missures connecting the left and right ganglia. The first pair of the series is. formed by the infra-oesophageal ganglion, which sends out commissures right and left, surrounding the pharynx, to the supra- O3sophageal ganglion. The supra- and infra- oesophageal ganglia together with the oesophageal commmissures form the cesopha- geal ring, a nerve-ring surrounding the oesophagus. Tubular System. — The tubular type of nervous system is found only in the chordates (fig. 76). The vertebrate brain and spinal cord may be regarded as parts of a tube with greatly thickened walls, developed in differ- ent ways. In the centre lies the extremely narrow central canal, which widens anteriorly into the several ventricles of the brain. In a transverse section the nervous elements FIG. 75. — Ladder nervous -i T , -, -> ^ . system of Porceiuo scatter are seen grouped around the central canal m a manner almost the reverse of that of the ionic type. On the periphery lies a °f nerve-fibres (the < white matter' of human anatomy) ; next is a central portion formed of ganglion-cells and nerve-fibres (the 'gray matter'), which is marked oil from the central canal by a special epithelium (ependyma). Relations between the Nervous System and the Skin. — For almost all animals it has been ascertained that the nervous system arises from the ectoderm. Therefore, in many animals, the nerve- cords and the ganglionic masses lie pemanently in the skin: in others only during the development, later becoming separated by splitting off or by infolding, and thus coming to lie in the deeper layers of tlje body (fig. 9). GENERAL ORGANOLOGT. 125 III. Sensory Organs. Sensations of the Lower Animals. — What we know of the •character of the external world is founded upon experiences gained through our sensory organs. We thus know the external world only in so far as it is accessible to the senses, controlled by the judgment. If things exist outside of ourselves which have no influence upon our senses, we can form no conception of them. It follows from this proposition that we can gain knowledge of the .ju w ,VH * A* FIG. 76.— Cross-section of the human spinal cord. (From Wiedersheim.) Black repre- sents the gray, white the white substance of the cord ; Cc, central canal, sur- rounded by the anterior and posterior commissures (C and C'); FIG. 81.— Ocellus (oc) of a medusa (Lizzia Koellikeri) with lens (0. FlGn!!fcI£lman-retinaV (After Gegenbaur.) P, pigment-layer; E, layer of sensory ; G, optic ganglion; 1, limitans mterna; 2, nerve-fibre layers; 3, ganglion- cells; 4, inner reticular layer; 5, inner granular layer; 6, outer reticular layer- 7. V Muller^^bres761'5 8' limitans externa? 9i r°ds and cones; 10, tapetum nigrum; optic ganglion, which either lies as a detached body outside of the eye, or is united with the retina into a connected whole. The 130 GENERAL PRINCIPLES OF ZOOLOGY. considerable thickness of the vertebrate retina is due to the fact that it includes the optic ganglion. The parts (fig. 82) called reticular layers, inner granular layer, ganglion cells, and nerve- fibre layer, constitute the optic ganglion; the layer of visual cells consists only of the outer granular layer and the connected rods and cones. The outer granules are the nuclei of the visual cells to which rods and cones belong. Accessory Structures. — The eye may be further complicated by special refractive bodies (cornea, lens, vitreous body) which G NO vo FIG. 83.— Horizontal section through the human eye. (After Arlt, from Hatschek.) .E, epithelium of the cornea (conjunctiva) ; C, cornea ; vA, anterior chamber of the eye; I, iris; hA, posterior chamber of the eye; Z, zonula Zinnii; Os, ora ser- rata ; Sc, sclerotic coat ; Ch, choroidea ; U, retina ; p, papilla of optic nerve ; m, macula lu tea, area of most distinct vision; VO, sheath of the optic nerve; NO, optic nerve; 0, arteria centralis; Cc, corpus ciliare; J/, lens; Or, vitreous body. concentrate the light in order to cast an image upon the retina; and an iris to regulate the amount of light. Then, too, means for nutrition (the choroid coat) and for protection (sclerotic coat) must be provided. If all these parts be present, a structure results such as is found in the squid and in the vertebrates (fig. 83). GENERAL OROANOLOGY. 131 The Eye of the Vertebrates. — The eye of the vertebrates usually is an approximately spherical body whose surface is formed by a firm membrane. Over the greater part of the circumference this is an opaque, fibrous or cartilaginous covering, called the sclera, or sclerotica; it is transparent only in the most anterior part, and here it forms by its greater convexity a projecting por- tion like a watch-glass, the cornea. Internally to the sclera lies the choroidea, an envelope of connective tissue, rich in pigment and blood-vessels, 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 in the centre by the pupil, an opening the varying size of which regulates the amount of light. Next internal to the choroid follows a layer of black cells, the tapetum 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 ora 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 choroidea, 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 mass of tissue, the vitreous body. The image formed on the retina is inverted. The Various Types of Eyes. — Between the simple pigment- spot and the highly organized vertebrate eye are many transitional stages: pigment-spots with lens and vitreous body, with enveloping and nourishing coverings, etc. The faceted eye of insects and Crustacea shows a special type of development, described later under the Arthropoda. SUMMARY OF THE MOST IMPORTANT POINTS OF ORGANOLOGY. 1. Organs are composed of tissues, and by their environment are led to the formation of a body of definite shape and to the performance of a single function; consequently every organ is 132 GENERAL PRINCIPLES OF ZOOLOGY. characterized morphologically (according to its structure and its relations) and physiologically (according to its function). 2.- Organs of different animals may be physiologically equiva- lent, analogous organs (i.e., with similar functions). 3. Organs of different animals may be morphologically equiva- lent, 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). c. They may be analogous, but not homologous (gills of fishes, lungs of mammals). 5. Organs are divided into animal and vegetative. 6. Animal functions are those which are not completely foreign to plants, but are only slightly developed in them; 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. To the animal organs belong the organs of motion and sensation, such as the muscles, the sense-organs, the nervous system. 9. To the vegetative organs belong the organs of nutrition and reproduction. 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 indi- viduals (diwcious), or both may be found in one and the same animal (hermaphroditic). 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. PROMORPHOL OOT. 133 IV. PROMORPHOLOGY, OR STUDY OF THE FUNDAMENTAL FORMS. Organic and Inorganic Bodies. — The structure of the individual animal rests upon the regular combination of differently-function- ing organs. The organs thus assume a relation to one another which is definite for each animal group, or varies only in subordi- nate ways. If the various groups be compared with reference to the principle of the arrangement of parts, there appear a few fundamental forms which play a role in morphology similar to that of the fundamental forms of crystals in mineralogy. But we must not carry this comparison too far, and attempt to compare the study of the fundamental forms, the promorphology, of animals FIG. 84.— Spongilla fluriatilis, fresh-water sponge. (After Huxle with dermal pores; be, region of the ampullae; c/, (After Huxley.) a, superficial layer osculum. with crystallography as of equal value. A crystal is a mass made up of similar parts; its form is the necessary and immediate result of the chemico-physical constitution of its molecules. A direct connection of this kind between molecular structure and funda- mental form does not, and cannot, exist in the organism, since each organ is composed of many chemical combinations. Conse- quently there is lacking also the mathematical regularity which occurs in crystals. Even in the case of animals which have the greatest regularity in the arrangement of their parts there is not an entire conformity to the demands of the fundamental form, so> that we are compelled to ignore certain greater or less variations. If, for example, we call man bilaterally symmetrical, we overlook not only the slight asymmetry of a nose awry, etc., but also what is more important — that the liver has been pushed to the right, GENERAL PRINCIPLES OF ZOOLOGY. the heart to the left; and that the digestive tract throughout its entire course runs asymmetrically. * FIG. 85.— Halinmma erinaceus, a radiolarian. «, external, i, internal, latticed spheri- cal skeleton ; cfr, central capsule; wk •, extra capsular soft parts ; ?», nucleus. JT Sfi FIG. 86. — Nausithoe^ an acraspedote medusa (after Lang), seen from the end of the greatly shortened main axis, pr, perradii; tr, interradii; ar, adradii (perradii and interradii mark the four planes of symmetry of the animal); *«r, subradii: rf, mantle-lobes; f, tentacles; s/f, sensory organs: g, sexual organs; gff, gastric fila- ments; rn, subumbrellar circular muscle; in the centre the cross-shaped mouth- opening. we Symmetry. — Now, according to the three dimensions of space, can pass three axes, perpendicular to each other, through the PROMORPHOLOGT. 135 body of an animal, and up to a certain degree may characterize it according to the nature of these axes; further, we may characterize it according to the planes by which it can be symmetrically halved, the planes of symmetry. Thus we find the following fundamental forms : 1. Anaxial, asymmetrical, irregular, or amorphous funda- mental form (fig. 84). 2. Homaxial, symmetrical in all directions, spherical funda- mental form (fig. 85). 3. Monaxial, radially symmetrical (fig. 86). 4. Simple heteraxial, biradially symmetrical (figs. 87, 88). 5. Double heteraxial, bilaterally symmetrical (fig. 89). 1. Anaxial or asymmetrical animals, so called, are those in which the arrangement of parts is not regularly de- fined in any direction or space, and they therefore may grow irregularly in any direc- tion. There is no fixed central point, and there is no possibility of running definite axes through the body or of dividing it into symmetrical parts. (Many sponges and many Protozoa.) 2. Homaxial or spherical animals have the sphere as their fundamental form ; the parts of the body are arranged concentri- cally around a fixed central point, so that any number of axes and planes of symmetry and the* th« macronucleus in division, ft, macronucleus; separates bv elongation and construction. n/r, micronuclei; o, cytostome J of the separating individuals. The old cytostome persists in the anterior At 2 an early stage of division of cytostome. offspring, but often an outgrowth from it (fig. 145, 2, of) passes into the posterior half and develops into a new mouth. 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. 146). Two individuals touch at first in front, and then by their whole ventral surfaces, so that their cytostomes come together. In the neighborhood of the latter a plasma bridge connects the two. Later the individuals separate. While these easily observable external processes are occurring there is a com- plete 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 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, three of which break down, thus recalling the polar globules in the maturation of the egg (p. 146). The fourth or primary spindle places itself in the neigh- III. CILIATA. 207 borhood of the cytostome at right angles to the surface and divides into two nuclei, the superficial being called the wandering or FIG. 146. — Conjugation in Paramcecium. fc, macronucleus; ?i stomes. micronucleus; o, cyto- I. Changes of micronucleus; left sickle stage, right spindle stage. II. Second division of micronucleus into primary spindles (1, 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 into male (1m, 5m) and female spindles (I?/;, 5w). IV. Exchange of male spindles nearly complete (fertilization), one end still in the parent animal, the other united with the female spindle, 1m with 5w and 5m with Ito; 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 (nfc')» and that of the new macronucleus, pt (placenta). The fragments of the old macronucleus begin to degenerate. Since P. caudatum shows the earlier and P. aurelia the later stages better, these forms have been used, P. caudatum for I-III, P. aurelia for the rest. The differences consist in the existence of one micronucleus in P. caudatum, two in P. aurelia^ and that in the latter the nuclear degeneration begins in I. male nucleus, the deeper, the stationary or female nucleus. The male nuclei of the two copulating animals are exchanged, travers- 208 PROTOZOA. ing the protoplasmic bridge in their course. Both male and female nuclei become spindle-shaped, and the immigrant male .spindle fuses with the female spindle, forming a single spindle of division. At last the division spindle produces (usually by indi- rect means) two nuclei, one of which becomes the new macronu- cleus, the other the new micronucleus. In a comparison of the fertilization 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 division 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 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 equivalent, the fertilization is mutual, and the individuals separate later. In the Peritricha (mostly sessile forms, fig. 147), on the contrary, the Fio. \it.-Epistylis umbellnria. (After Greeff.) Part of a colony in 'bud-like ' conju- gation r, microspores arising by division; fr, microspore conjugating with a macrospore. resemblance to fertilization in the Metazoa is strengthened in that there is a sexual differentiation and a permanent fusion of the conjugating individuals. Some animals — the macrospores — retain their size and sessile habits; others by rapid division produce III. CILIATA: HOLOTRICHA, HETEROTRICHA. 209 groups of markedly smaller microspores. The latter separate and fuse completely with the macrospores, only a small cuticular sac persisting to indicate the fusion. The nuclear phenomena are much the same as with Paramcecium, allowance being made for the permanance of the fusion. Order I. Holotricha. The Holotricha are doubtless 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 on the inside of the cytostome. Best known are the species of Paramcecium* (fig. 144) occurring in stagnant water. Opalina ranarum * lives in the intestine of the frog. It lacks mouth, has numerous similar nuclei, no micronu- oleus and no conjugation. The small encysted Opalines pass out with the faeces, and are eaten by the tadpoles, which thus become infected. Order II. Heterotricha. Like the Holotricha the Heterotricha are everywhere ciliated, but they have a tract of stronger cilia, the adoral ciliated spiral. This is a band of cilia beginning at some distance from the cytostome and leading in a spiral course into the mouth. It consists of rows of cilia united into ' membranellse ' placed at right angles to the course of the spiral. In the best- known heterotrichans, the Stentors * (fig. 148), the peristomial area, surrounded by FIG. 148. FIG. 149. FIG. 148. — Stentor polymorphus. (After Stein.) a, peristomial area; b, roof of hypo- storae; 0, contractile yacuole; n, nucleus; o, cytostome; r, adoral ciliated spiral; t, hypostome (excavation for mouth). FIG. UU.—Balantidiuni coli. (After Leuckart.) 210 PROTOZOA. the spiral, forms the broader end of the body, which gradually tapers toward the other end, by which the animal may attach itself by small plasma threads. Muscle fibres which run length- wise immediately under the cuticle produce energetic move- ments. Stentor polymorphus * when attached builds a gelatinous case. S. cceruleus.* Balantidium coli (fig. 149) appears in the large intestine of men ill with diarrhrea; it also occurs in swine without causing sickness. Other parasites of man are B. minu- tum and Nyctotherus faba. Order III. Peritricha. In the Peritricha there is always a broad peristome area with the cytostome; the opposite end has a corresponding pedal disc or is narrowed like a goblet and ends in a stalk (fig. 150). Only FIG. 150.— Carchesium polypinum. (After Biitschli.) Left, a single animal; right, three stages of division, cv, contractile vacuole; n, macronucleus: n', micronucleus; JVv, food vacuoles: os, cytopharynx; per, peristome; vs, reservoir of contractile vacuole; «»*, undulating membrane ; vst, vestibule; wk, ring on which a posterior circle of cilia may develop. the adoral ciliated spiral is constant. It arises from the swollen margin of the peristomial area, and continues on the ' operculum/ a ciliated disc which projects free from the peristomial area, but in contraction is drawn close against it, tliQ peristome lips folding over all. Besides, there may 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. 150, nf). The best known representatives are the VORTICELLID^E (figs. 147, 150), attached by a long stalk which is usually hollow and contains a slightly ///. CILIATA: HJPOTRICHA. 211 spiral muscle. This extends into the body and divides up into fine fibrilla3 which extend under the cuticle to the peristome. When the muscle in the stalk contracts it becomes coiled into a corkscrew spiral, drawing back the animal, and folding in the anterior end. Vorticella* is solitary; Carche- sium* forms colonies with dichotomously branched stalks; Zoothamnion* colonies imbedded in a common jelly ; Epistylis* (fig. 147), branched col- onies with rigid stalks, the muscle being confined to the base of the body. Order IV. Hypotricha. In this order the body is more or less flattened and a ventral and a weakly arched dorsal surface are differentiated. The back lacks cilia, but often bears spines and tactile bristles. On the ven- tral side are several longitudinal rows of cilia, and besides straight ID1.. FIG. 151. FIG 152. FIG. 151.— Stylonychfa mytilns. (After Stein.) «, anal hooks; 6, ventral hooks; c, con- tractile vacuole: rf, frontal ridge; g, canal leading to contractile vacuole; I, upper lip; 71, nucleus with micronucleus: p, adoral ciliated spiral; r, marginal cilia; s, caudal cilia; .sf, frontal spines; z, anus (cytopyge). FIG. 152.— Division of Sti/ionychia mytilns. (After Stein ) c, c', contractile vacuoles of the two individuals; n, ?V, nucleus and micronucleus; p, p', adoral ciliated spiral; r, »•', marginal cilia; w;, «;', ciliated ridges. spines and hooked cilia composed of united cilia. These latter are of use in creeping. The strongly developed adoral cilia are of use in locomotion and in producing vortices which bring food. The macronucleus is often divided into two oval bodies connected by a thread; the micronuclei vary in number from 2 to 4 in the same 212 PROTOZOA. species. These are the best forms for studying the micro nuclei. The species of Stylonychia* (figs. 151, 152) are best known. 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 and is covered with a cuticle, which in the genus Acineta is produced into a cup-like lorica. There is no mouth, but in its place tentacles or sucking feet, very fine tubes with contractile walls which begin in the protoplasm and protrude through the cuticle (fig. 153, F). The Acinetaria kill other animals, especially infusoria, with their tentacles, and then FIG. 153.— Forms of Suctoria. (After various writers.) A, Dendrosoma; B, Rhyncheta; C, Opliryodendron; D, Tokophrya; £", ciliated young of Sphceroplirya; F, diagram of structure showing capitate and styliform tentacles arising from the ectosarc and corresponding canals in the entosarc. suck the substance through these tubes. The contractile vacuole, rarely lacking, lies near the compact macronucleus; micronuclei are generally present. In contrast to the immobile adults the young which are ciliated (fig. 153, E) after the pattern of ciliates, are good swimmers. They arise either as buds from 'the surface of the mother or as ( embryos 9 in her interior. This latter condition is only a modifi- cation of the other, for parts of the outer surface become pushed into the interior, and there 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 Podoplirya are widely distributed in fresh water, also Sphcerophrya, parasitic in Infusoria. The species of Acineta as well as Podophrya gemmipara (fig. 20) are marine, living on hydroids and Poly- zoa. IV. SPOROZOA: OREGARINA. 213 Class IV. Sporozoa. Under the name Sporozoa are united several groups of Protozoa which, while they differ much in structure, have much in common in life and development. They are parasites in Metazoa, many of them in the cells themselves, causing their degeneration (Cytospo- FIG. 154. — Sporozoa. -4, cyst of Clepsidrina with sporoducts; B, Clepsidrina, two indi- viduals (after Schneider); C, Eimeriafalcifonnis, from mouse; £>, same, falciform embryos: JK, Hoplorhynchus dujnrdinii^ from Litkobius; F, Gregarina atgantea. from lobster; Cr, Sarcocystis miescheri, from pig; H, Myxidium (after Th6olan); I,Rhopa- locephalus, alleged cause of cancer (after Korotneff). ridae). They take no solid food, but are nourished by fluid mate- rial absorbed through the whole surface. In reproduction they form a large number of l sporoblasts/ which when enveloped with a membrane are called ' spores/ the contents of which usually break up into several small bodies or ' sporozoites/ The sporozo- ites for their development must leave the host. The resemblances to the Rhizopods (Mycetozoa) are unmistakable, especially those Sporozoa which have pseudopodia for much of their life. 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 so far have only been found in invertebrates, where they live in the intestine or gonads, more rarely in the body cavity. The protoplasm (fig. 155, /) is sepa- rated more sharply than in other Protozoa into a clear ectosarc (elc) and a granular entosarc (en). The ectosarc is covered by a cuticle (not always easily seen, but frequently with a double con- tour) (cu), which must be permeable by fluid food, for no cyto- stome exists. In many (perhaps all) there is a double striping of the body, a longitudinal recognizable by furrows on the outer surface and hence cuticular, and a transverse marking in the PROTOZOA. ectosarc, produced by circular or spiral muscle fibrillae. These muscles explain the peristaltic motion and the occasional sharp bending of the body, but not the peculiar gliding motion like that of diatoms by which locomotion is usually effected. This is pm.— I. en. FIG. 155.— Development of Gregarina blattarum. I, conjugation ; II, A-C, a cyst in transformation into pseudonavicellae; III, A, a pseudonavicella greatly enlarged; B, same with sickle-formed sporozoites; CM, cuticle; dm, deutomerite; ek, ecto- sarc; en, entosarc; ?i, nucleus; pm, protomerite; pn, pseudonavicellae; rJc, re- sidual body; sfc, sickle-form sporozoites. explained by the view that the gregarines secrete stiff gelatinous threads from the posterior end, and the elongation of these forces the body forward. In many gregarines (Poly cyst idae) the body is divided by a cir- cular incision into a smaller anterior part, the protomerite, and a larger deutomerite. 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. An epimerite — a structure connected with the peculiar type of parasitism — occurs in many species. All gregarines are parasitic in youth inside of cells. They later leave these, but many remain for a long time with a process of the protomerite imbedded in the cells. This process — the epimerite — is provided with threads or hooks for IV. SPOROZOA: COCCIDIAE. 215 anchorage, and is lost when the animal gives up its connexion with the host cell. Among the intestinal gregarines frequently occur l associations ' where two or more animals are fastened to- gether head to tail in a row. Perhaps these associations are prep- arations for conjugation which occurs in development. Eeproduction occurs exclusively in an encysted condition (fig. 155, II, ^4). Usually two animals (sometimes one, rarely more than two) occur in a cyst. A fusion of the two encysted animals does not take place, but it is probable that a nuclear exchange (recalling that of ciliates) takes place. After each individual has become polynucleate by division of its nucleus, it divides at first super- ficially, later internally into small particles, the sporoblasts (II, B), which change into spores, here called pseudonavicellae. The pseudonavicellae are inononucleate bodies with firm membrane and usually spindle form in shape (III, AA). In these processes a part of the gregarine takes no part. This residual body appears under proper conditions to swell up and rupture the cyst, thus freeing the pseudonavicellae. In many gregarines there are sporo- ducts for the escape of the pseudonavicellae (fig. 154, A). The contents of the pseudonavicellae divides into (usually eight) sporo- zoites or falciform spores which must pass out from the spores and into the cells of the host 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 pseudo- navicellae takes place when the cysts have left the original host. Best known are the Monocystis tenax of thespermatheca of earthworms, and Gregarina (Clepsidrina) Uattarum of the cockroach. The American species have scarcely been touched. One species is abundant in the intes- tine of Geophilus. Order II. Coccidiae. The gregarines of all Sporozoa are nearest the Coccidiae, which are also cell parasites with a single nucleus, but without either cell membrane or division into protomerite and deutomerite. In most species, as in Coccidium cuniculi, there are two types of reproduc- tion, an endogenous, leading to ' autoinfection/ and an exogenous, concerned in the transfer of the germs to other hosts. In the first (lacking in many species) the Coccidium divides into many falciform germs which separate from each other and, without alternation of hosts, enter other cells. The second type is begun by fertilization. Certain individuals, by rapid division form microgametes, small bodies swimming with serpentine motions or by one or two flagella. Other individuals do not divide, but form 216 PROTOZOA. macrogametes which are fertilized by the microgametes, and then encyst, pass to the outside, and serve for the infection of other animals. The contents of the cyst begin to divide, sooner or later, into sporoblasts (in Coccidium, four) containing spores, the FIG. 156.— Coccidium cuniculi, from the liver of the rabbit (from Wasielewski. a, &r young Coccidia in the epithelial cells of bile duct, the nucleus of the cell in the upper process; c, encysted; d, e, contraction of protoplasm; t, h, g, spore forma- tion; /, ripe spore with two germs and a residual body. process being completed only after entrance into a new host. Each spore forms one or more sporozoites, a portion of the sub- stance being left behind. Coccidium cuniculi (oviforme) in the liver of mammals, especially rabbits (rare in man), producing cheesy granules. C. perforans in the intestine of rabbits, rare in man. Order III. Haemosporida. In structure and development these are much like the Coccidiae ; they live in blood corpuscles. The forms occurring in man pro- duce malaria. Here, also, there are endogenous (autoinfecting) and exogenous generations transferring the parasites to other hosts. The parasites in the corpuscles (fig. 157, a to d) grow and abed e f g FIG. 157.— Plasmodium laverni, var. quartana (from Wasielewski, after Labb6), from the blood of a malarial man. a, newly infected blood corpuscle; b, somewhat larger germs; c, full-grown parasite with strong pigmentation; d, rounded form with large nucleus; e, beginning of germ formation; /, rosette of germs around a residual body; g, germs set free by degeneration of corpuscle. divide, producing ( daisy-like forms' characterized by little accu- .mulations of pigment derived from the haemoglobin of the blood. IV. SPOROZOA: MYXOSPORIDA. 217 These division products are set free by a breaking down of the corpuscle (period of chill) and infect other corpuscles. Thus autoinfection can continue until at length sexual forms appear — ' spheres ' or macrogametes, flagellate microgametes — incapable of infecting the corpuscles. The conjugation of these seems to take place when they are taken into the stomach of a blood-sucking mosquito. After fertilization, the oosphere wanders into the intestinal wall of the mosquito, grows larger, encysts, and produces many sporoblasts, which in time form many sporozoites. These migrate into the salivary glands of the mosquito and thence are transferred to man with the sting of these insects. Since a tem- perature above 20° C. (68° F.) is best for the development of the stages in the mosquito, and since mosquitos need water for their development, the prevalence of the disease in moist, warm regions is easily understood. For the transfer of human malaria 'not all mosquitos will serve, but apparently only those of the genus Anopheles. The species of Culex convey bird malaria. The differ- ent kinds of malaria seem to be produced by different parasites. Order IV. Myxosporida. The Myxosporida (fig. 158) 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 or- gans like the heart, liver, brain, kidney, etc. , they are usually enclosed in a mem- brane, and here they produce the great- est injury. At first binucleate, they soon become polynucleate, and it would appear that they can reproduce by fission. Even before the growth is ended they begin Flo 158_M the process of sporulation. In the in- terior single spherical protoplasmic bodies separate, these having at first a single nucleus, later more (as many as ten). From each of these bodies arise from two to many spores, the so-called psorosperms. These (fig. 158, 3) are enclosed in a bivalve shell which includes, besides a binucleate germ, one, two, or four polar capsules, these resembling somewhat the nettle organs of the coelenterates. They are oval and contain threads which, under certain conditions, are protruded (fig. 158, 2) MJIX- «««"* itAerkuhni ; fc, degen- eratmg nucleus; n, vacuole formerly regarded as cleus; p, body, in threads. nu- cnidocil-like pole s with exserted 218 PROTOZOA. and serve to fix the capsule, while the amoeboid germ creeps out and penetrates the tissues of the host. Experiment shows that fishes are infected through the alimentary canal. The Myxosporida frequently cause serious epidemics in fish. This was noticeably the case with the fish in the aquaria at the Chicago exposition. Myxobolus, Myxidium. Invertebrates may also be infected, the celebrated pe*brine of the silkworm being caused by Nosema (Glugea) bombycis. Order V. Sarcosporida. The Sarcosporida (fig. 159) — also called Rainey's or Miescher's corpuscles — occur in the voluntary muscles of vertebrates, especially mammals. They are oval cysts lying in sar- colemma sacs between the fibrillse. They have a cyst, the wall of which is radially striped, and inside this, in the ripe condition, are spores, imbedded in a stroma, each spore con- taining numerous renif orm or falciform sporo- zoites. Sarcocystis miescheriana in muscles of pig; S. muris in the mouse; 8. lindemanni rare in human muscle. 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 (sarcode), 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 contractile vacuoles. 4. Reproduction is by budding or by fission. Conjugation has been witnessed in many, and possibly occurs in all. True con- jugation is a process of fertilization (caryogamy), in contrast to fusion of plasma (plasmogamy). 5. Protozoa are aquatic, a few living in moist earth; they can only exist in dry air in the encysted condition, surrounded by a capsule which prevents desiccation. 6. Since encysted Protozoa are easily carried by the wind, the occurrence of these animals in water which originally contained none is easily explained. 7. The mode of locomotion serves as the basis for division of FlO. 159. — Sarcocystis miescheriana^ from diaphragm of pig. (After Biitschli.) 6s, cyst; sp, spheres of spores. IV. SPOROZOA: SUMMARY OF IMPORTANT FACTS. 219 the Protozoa into the classes Rhizopoda, Flagellata, Ciliata, and Sporozoa. 8. The RHIZOPODA have changeable protoplasmic processes, the pseudopodia. 9. The Rhizopoda are subdivided into Monera, Lobosa, Helio- zoa, 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 radiat- ing pseudopodia and frequently silicious skeletons. They are dis- tinguished by the occurrence of a central capsule in the Radiolaria which is lacking in the Heliozoa. 12. The Thalamophora (Foraminifera) have a shell, closed at one end, at the other with opening for the extension of pseudopodia. The shell is chitinous or calcareous, one or several chambered, straight or spiral, sometimes with close walls, sometimes perforated with pores; the pseudopodia 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 lime- stone). The silicious skeletons of the Radiolaria are less important. 14. Mycetozoa (Myxomycetes of botanists) are mostly enormous Amoebae with branched reticulate protoplasm (plasmodium). They form complex reproductive structures (sporangia, etc.), 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 (Volvocina) by means of chlorophyl, or have a mouth for the tak- ing of food, or a collar (Choanoflagellata). 17. The Dinoflagellata have two kinds of flagella and usually an armor of cellulose. 18. The Cystoflagellata have a gelatinous body enclosed in a firm membrane (Noctiluca). 19. The CILIATA (INFUSORIA in the narrower sense) have numerous slender vibrating processes, the cilia, a cuticle, and hence fixed openings for the ingestion of food (cytostome) and for extru- sion of indigestible matter (cytopyge). 21. Of great interest is the occurrence of two kinds of nuclei, a functional nucleus (macronucleus) and a sexual nucleus (micro- nucleus, paranucleus). PROTOZOA. 22. In conjugation portions of the micronucleus are exchanged and accomplish impregnation. The macronucleus degenerates and is replaced by part of the fecundated micronucleus. 22. The classification of the Ciliata is based on the structure and arrangement of the cilia. 23. The Holotriclia have similar cilia over the whole bod}^ The Heterotricha have besides the total ciliation stronger cilia in the neighborhood of the mouth (adoral ciliary spiral). The Peri- triclia have only adoral ciliation. The Hypotriclia have, on the ventral surface, the ciliary spiral and rows of cilia and coalesced cilia. The Suctoria have cilia only in the young, later they become attached and feed through suctorial tentacles. 24. SPOKOZOA are parasitic Protozoa, usually without organs of locomotion or mouth. They take no solid food, but live by osmosis on tissue fluids. In reproduction the encysted animals produce spores (apparently always beginning with fecundation and accompanied by a change of host). The spores divide again into sporozoites. Besides, multiplication without change of host (auto- infection) can occur. 25. The Gregarinida are temporary or permanent parasites in cells. (Spores = pseudonavicellae, sporozoite = falciform embryo). Coccidite, Hcemosporida (cause of malaria, parasitic in blood corpuscles). 26. The Sarcosporida (Rainey's -or Miescher's corpuscles of mammalian muscles) and Myxosporida (psorosperm capsules of fishes, psorosperm — spore) live in tissues or hollow organs. APPENDIX. According to the evolution theory one should expect forms between the Protozoa and Metazoa. The CATALLACTA — spheres of ciliated cells which in reproduction break up into single cells — have been described as such. FlQ. 160.— Section of half of Trichoplax adhcerens. (After Schulze.) Peculiar many-celled animals whose position in the system is difficult to decide are, further, Trichoplax adhcerens, Salinella salve, the ORTHO- NECTIDA and the DICYEMIDA. Trichoplax (fig. 160) is discoid and consists of twoepithelial-like cell layers separated by gelatinous tissue. The Ortho- PORIFERA. 221 nectida and Dicyemida have a many-celled ectoderm, enclosing a solid mass of cells in the Orthonectida, a single giant cell in the Dicyemida. Sali- wella consists of a single layer of cells enclosing a central digestive space. Since the Dicyemida live as parasites in the nephridia of cephalopoda, the Orthonectida in worms and echinoderrns, it is possible that their low or- ganization is the result of degeneration. METAZOA. Excluding the Protozoa, all the "branches of the animal kingdom may be included under the head Metazoa, i.e. higher animals. The point of union is that they consist of numerous distinct cells, and that these cells are arranged in several layers. At least two layers are present, a layer — the ectoderm — which bounds the body externally, and a second, lining the digestive tract — the entoderm. Between these two a third layer can occur, which frequently is separated by a body cavity into an outer or somatic layer forming part of the body wall, and an inner or splanchnic 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 development of the organization, which appears in varying grades in the specialization of tissues and organs. In no metazoan is there lacking a true sexual reproduction, that is one by sexual cells, but the fact must not be overlooked that many species reproduce (possibly exclusively) by unfertilized eggs in a parthenogenetic manner. Besides the sexual reproduction many species, especially the lower worms and coelen- terates, reproduce by budding and fission. For all the Metazoa the segmentation of the egg is characteristic to a high degree. The fecundated egg divides into numerous cells which, as segmentation cells (blastomeres), remain united and form the germ. .N~o Protozoan has a true segmentation. Division there produces new individuals which either separate completely or exceptionally remain in slight connexion 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 (recently subdivided into several subgenera). The ani- mals have no powers of locomotion, but are attached to stones or plants, along the shores or at depths up to 6000 metres (4000 222 PORIFERA. fathoms) . They form spherical masses, thin crusts, small cylinders, or upright branching forms. Frequently the shape varies so that one cannot speak of a typical form. It was also difficult to decide about the animal nature of the sponges. Striking movements of the body are rare ; only by aid of the microscope can one see motion — the opening and closing of the pores and the streaming of the gastrovascular system. The simplest sponges, the Ascons (fig. 161), are thin-walled sacs, fixed at one end, and with an opening, the osculum (func- tional anus), at the other. The cavity of the sac, the s stomach/ is a wide digestive cavity into which water bearing food obtains entrance through numerous small openings or pores in the body wall. The basis of the body is a homogeneous or fibrous connective en. ek.. FIG. 161. Fio. 162. FIG. 161.— Olynthus. (After Haeckel.) e, spicules; z, eggs; o, osculum; p, pores; tt,, 'stomach.' FIG. 162. — Section of wall of Sycmidra raphanus. (After Schulze.) e, ectodermal epithelium; e?i, collared flagellate cells; m, mesoderm with connective-tissue cells; o, eggs; s£, calcareous spicules. tissue permeated with branching cells (fig. 162) covered externally by a thin 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, ' meso- ectoderm/ since it has been shown that the pavement epithelium is often genetically only connective-tissue cells which have spread over the surface. On the other hand there is a distinctly differen- tiated entoderm in the shape of a one-layered flagellate epithelium lining the stomach, the cells of which (en) recall the Choanofl agellata PORIFERA. 223- (p. 202), since they have collars surrounding the flagella. It has therefore been attempted to regard each flagellate cell as an indi- vidual, and the whole sponge as a colony of Flagellata, a view which neglects the other tissues, not only the connective tissue and the epithelium already mentioned, but sex cells, amoeboid wandering cells, and contractile fibre cells which close the pores. The taking of food is accomplished by the collared cells. Sponges of this simple ascon type are few. As a rule the sponges are more massive and have a more complicated canal system (figs. 164-166). The first step towards complication is seen in the Sycon sponges (fig. 163), in which the gastral cavity/ FIG. 163. FIG. 164. FIG. 163.— Stereogram of Sycon sponge (orig.). a, ampullae with pores in their walls;. c, cloacal chamber, with the openings of excurrent canals; i, incurrent canals; o, osculum. FIG. 164. — Section of Leucortis pulvinar. (After Haeckel.) a, aboral pole ; c, efferent canals from the ampullae to the cloaca; e, ampullae; t, mesoderm; o, osculum; v, cloaca. consists of numerous radial outpushings (the flagellate chambers or ampulla) which alone contain the collared cells, while the cen- tral cavity, now called cloaca, is here lined with pavement epi- thelium. By increase of mesoderm and corresponding thickening of the body wall the ampullae become separated from external and cloacal surfaces by the ingrowth of tissue (Leucon type). The ampullae nevertheless retain their connexion with both surfaces: by means of a system of canals. . This canal system is double; one- part is incurrent and leads from the dermal pores to the ampullae;; 224 PORIFERA. the other or excurrent from the ampullae to the cloaca, the two being connected by the ampullae alone. Both may consist of lacunar spaces (fig. 164), or have a more regular arrangement (fig. 165), FIG. 165.— Section of cortex of Chondrilla nucula, the skeleton omitted. (After Schulze.) c1, afferent canals; c2, efferent canals; <;, ampullae; m, cloaca; o, osculum. the canals from the pores 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 these may be FIG. 166. FIG. 167. FIG. 166. — Surface view of dermal pores of Aplysina aerophobia, (After Schulze.) FIG. 167.— Ascyssa acufera.^ (After Haeckel.) repressed; again by the branching of the sponge (fig. 167), while still further the branches may anastomose (fig. 168), giving rise to a network. Sponges may reproduce asexually, small portions separating as buds and producing new animals. Usually sexual reproduction prevails. Eggs and spermatozoa arise from mesoderm cells (fig. 162), are fertilized and undergo segmentation at the point of origin, and leave the parent as flagellate larvae (fig. 169, A). At fixation CALCISPONGI^. 225 a kind of gastrulation takes place, the blastopore (fig. 169, B] closes, and the osculum, an entirely new formation, arises at the opposite pole. FIG. 168. FIG. 169. FIG. 168.— Leucetta sagittate*. (After Haeckel.) FIG. 169.— Development of Sycandra raphanus. (After Schulze.) A, blastula; B, gas trula at the moment of fixation; ek, ectomesoderm; en, entoderm. The sponges are frequently regarded as Ccelenterata, but scarcely a single homology can be drawn between the two. The ccelenterate mouth is different from either pores or oscula. Indeed it is disputed whether the collared cells are entoderm. Nearly all sponges possess a skeleton secreted by special mesoderm cells, and this skeleton affords the means, according as it is composed of calcic carbonate or of silica, of dividing the sponges into two classes. Besides, there are two groups, the Ceraospongiae and the Myxospongiae, in which the skeleton is respectively of horny substance or spongin or is lacking entirely. These, however, seem to be descendants of the silicious forms. Order I. Calcispongise. The calc sponges are exclusively marine and mostly live in shal- low water. They are grayish or white in color, of small size, rarely exceeding an inch or so in length. The skeletal spicules which arise in the mesoderm usually project through the epithelium and form, especially in the neighborhood of the osculum, silky crowns. One-, three-, and four-rayed spicules are recognized, these ground forms presenting by unequal development a great variety of shapes (fig. 170). Sub Order I. ASCONES. Sponges with thin porose walls and a cen- tral 'stomach' (figs. 161, 167). Leucosolenia* Sub Order II. SYCONES. A cloaca present surrounded by ampullae radially arranged (fig. 163). Grantia,* Syeon,* Sycandra* 226 PORIFERA. Sub Order III. LEUCONES. A complicated system of branching in- current and excurrent canals in the thick walls connects the ampulla with the outer surface and the cloacal cavity (figs. 164, 168). Leucetta, Leucortis. FIG. 170.— Sponge spicules. (From Lang.) Order II. Silicispongise. The siliceous sponges are richest in species and occur at all depths of the sea, being frequently noticeable from their size (up to a yard) and their bright colors. They are subdivided into Triaxonia and Tetraxonia. In the Triaxonia the spicules com- posing the skeleton — appearing as if of spun glass (hence Hyalospongia, or glass sponges) — have three crossed axes (six threads radiating from a common point) — hence Hexactinellidae. The mesoderm. is scanty and in consequence the afferent and efferent canals are loose-meshed lacunar spaces and the ampullaB large- and barrel-formed. In the Tetraxonia, on the other hand, the mesoderm is usually abundant and the canal system well developed. The four-axial spicules of the Tetractinellidse must be regarded as the fundamental skeletal type. From this are derived the compact agglutinated frameworks of the Lithistidae 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 effected 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. The HEXACTINELLID^: belonging here live chiefly in the deep sea, and for a long time only a few species were known : Euplectella aspergillum, Venus' flower-basket, tubular, consisting of fine spicules. Hyalonema. Apparently the horny sponges Aplysina and Aplysilla, as well as the slime-sponges, Halisarca,* have descended from this group. Sub Order II. TETRAXONIA. Typical representatives are the largely extinct LITHISTID.E (of which some genera — Discodermia — persist in deep SUMMARY OF IMPORTANT FACTS. 227 seas) and the TETRACTINELLID.E : Geodia.* Near here apparently belongs Oscarella,* without a skeleton (Myxospongia). In the MONACTINELLID^: the spicules are united by spongin (Corna- cuspongia), and can even be entirely replaced by that substance. Numer- ous marine forms, among them Chalina,* and also the fresh-water SPONGILLID.E (Spongilla* Ephydatia *), widely distributed as encrusting masses on submerged sticks and stones. The natural color is light grayv but they are usually colored green by Algae. They are distinguished! from most marine relatives by the formation of gemmulae or statoblasts. At times the protoplasm divides into round bodies, as large as the head of a pin and these become surrounded by a firm membrane strengthened in. many forms by collar-button-like spicules, the amphidiscs. These stato- blasts remain entangled in the skeleton and survive times of freezing or drought. On return of good conditions the contents escape and form- small Spongillce, often utilizing the old skeleton. This process recalls, encystment among the Protozoa. When the spicules entirely disappear and nothing but the spongiro fibres remain we have the horny sponges or CERAOSPONGLE. The skeleton consists of a horny substance, spongin, which differs chemically from the substances of true horn — keratin. This spongin is always laid down in long fibres by peculiar cells, the spongioblasts, and it always consists of concentric layers. The fibres interlace, branch, and unite into a skeleton. The best known horny sponges are the bath sponges, Euspongict officinalis* occurring in the Mediterranean, West Indies, Florida, and other seas in many varieties. Best of all are the Levant sponges (var. mollissima). Sponges of commerce consist only of the skeleton, the ani~ mal parts being killed and, after decay, washed away with fresh water. Less valuable are Euspongia zimocca and Hippospongia eqirina* the? horse-sponge, while the Cacospongice are useless. Summary of Important Facts. 1. The sponge body is largely a mass of connective tissue cov- ered externally with pavement epithelium (meso-ectoderm) 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 excurrent canals (in ascons in the central cavity). 3. The animals receive nourishment through fine pores in the body wall; indigestible bodies are cast out through one or several oscula. 4. Since nerves, muscles, and sense organs are lacking or very weakly developed, the animals show the most inconspicuous move- ments. 5. Sponges are divided into Calcispongiae and Silicispongiae according to the character of the skeleton. 228 CWLENTERATA. PHYLUM III. CCELENTERATA (CNIDARIA, JSTEMATO- PHOKA). The animals belonging to the coelenterates were formerly called Zoophyta (plant-animals). They 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 a special intes- tine and a special body cavity occur in the Echinoderma, while in the Coelenterata there is but a single system of cavities in the body. Each of the three names indicates certain important characters of the group. (1) The name Zoophyta was selected with regard to the gen- eral appearance. Most coelenterates, like the plants, are fixed and form bush-like or mossy colonies by incomplete budding. This resemblance, is but superficial, for in any accurate investigation there cannot be the slightest doubt of the animal nature of any Ccelenterate. The name therefore must not be understood to imply that these are doubtful forms which stand on the border between plants and animals. Besides, there are not only fixed but free-moving forms which swim in the water 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 Coelenterata this may be replaced by a biradial symmetry or even by bilaterality (Ctenophora, many Anthozoa). (3) The term Coelenterata is given these animals because they contain a single continuous coelenteron or gastrovascular cavity. In the simplest species this is a wide-mouthed sac into which food passes for digestion. The single opening into it serves at once as mouth and anus; the sac itself is the alimentary tract. Frequently lateral diverticula or branched canals 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 an error to call it a body cavity and to say that the coelenterates are stomachless. On the other hand, the term ' coelenteron, ' that is a cavity which is at once gastric and ccelomic (p. 158), is perfectly defensible, since in many higher animals which possess a true body CCELENTERATA. 229 cavity this is seen in development to arise as diverticula from the primitive stomach (enteron). Since such diverticula occur in coelenterates without becoming independent, one can say that the gastrovascular system consists not only of intestinal portions but, in potentia, of the coelom as well. To even a superficial observation the Coelenterata 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 threads, in the neighborhood of the mouth, which have the functions of feeling for food, grasping it, and conveying it to the mouth. The means of killing the prey are the cnidae, nematocysts, or nettle cells (fig. 171), which with rare FIG. 171.— Nettle cells of Coelenterata (After Hertwig, Lendenfeld, and Hamann.) exceptions in Protozoa, Turbellaria, and molluscs occur in no other group. These structures, of great systematic importance, are oval vesicles with fluid .contents and firm membrane. Each is drawn out at one end into a long tube, so delicate as to appear as a thread (hence an additional name, thread cells). This thread is sometimes armed throughout its length with retrorse hooks, or it may have only a few stronger hooks on its basal portion, which is thicker than the rest. 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 contents. Some coelenterates (e.g+ Physalia) 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 stimu- lates the protoplasm and causes the explosion. The cell itself is. frequently enclosed by a muscular sheath or a network of muscle fibres. Among the ccelenterates both sexual and asexual reproduction. 230 C(ELENTERATA. may occur, the latter usually by budding, more rarely by division. Sexual and asexual types of reproduction can be combined in the same species, producing an alternation of generations. In comparison with the sponges the Co3lenterata may be called epithelial organisms. A mesoderm (' mesogloaa') may be entirely lacking or may have but a subordinate development. The ectoderm and entoderm, on the other hand, are the important tissues — pro- ducing 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 are frequently introduced into the life Mstory, one agreeing well in structure 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 hydropolyp, forming in the branch of coelenterates an important archetype from which all other conditions — medusae, scyphopolyp, and even the coral polyp — may be derived. Our best example of this is the fresh-water Hydra, so common in pools and streams. The body (fig. 172) is a sac, the hinder closed end of which, the pedal disc, is used for attachment. The other end bears the mouth which leads to the internal gastrovascular (digestive) cavity. Around the mouth is a circle of tentacles used in capturing food (mostly small Crustacea). 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. 173), an entoderm of flagellate cells lining the gastrovascular space, and the ectoderm •covering the outer surface. Between the two is the supporting layer (mesogloea), a structureless membrane without cells and hence not a body layer. Both layers consist of epithelial muscular cells (cf. p. 92), 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 HYDROZOA. 231 are crowded into small ridges or batteries. The sex cells (at cer- tain times) produce swellings on the column; a circle of male swellings close beneath the tentacles, the female cells farther down the column (fig. 172). Individuals reproducing by budding are more common than the sexually mature (fig. 90). Small eleva- ek s ek c FIG. 172. FIG. 173. FIG. 172.— Hydra viridis,* testes above; ovarian enlargement below. FIG. 173.— Body layers of Hydra. (After Schulze, from Hatschek.) c, cuticula; en, nettle cells; eto, ectoderm; en, entoderm; s, supporting layer. tions appear on the column, enlarge, form tentacles, and at last a mouth, after which they may separate from the parent. In the sea are numerous hydroid polyps which, while agreeing in the main with Hydra, are distinguished from it in two important respects: (1) they do not directly produce sexual organs; (2) they reproduce asexually, and by incomplete budding form persistent colonies. In this formation of colonies a series of parts have arisen which require special designations (fig. 174). The separate animals are the hydranths, and are connected together by a system of tubes, the ccenosarc, which, like the hydranths, consist of ecto- derm, entoderm, and mesogloea, and since the gastro vascular space continues in them, these effect a distribution of food throughout the colony. The coenosarcal tubes may creep over some support (stone, alga, snail-shell, etc. ) and form a network, the hydrorhiza, or it may stand erect and tree-like, forming a liydrocaulus. Usually both hydrorhiza and hydrocaulus occur in the same colony. 232 CCELENTERA TA. FiQ.llL—Campanulnrfajohnstoni. (After Allman.) o, hydranth with hydrotheca; 6, retracted; rf, hydrocaulus; /, gonotheca, with blastostyle and medusa buds; g, free medusa. . —en FIG. 175.— Section of Eudendrium ramnsum. ek, ectoderm; en, entoderm; p, perisarc; ,s, supporting layer. HYDROZOA. 233 Usually the colony is strengthened and protected by the perisarc, a cuticular tubular secretion of the ectoderm. In some (fig. 175) the perisarc stops at the base of the hydranth; in others (fig. 176) it expands distally into a wide-mouthed bell, the hydrotheca, into which the hydranth may retract at times of danger. In rare cases FIG. 176.— Campanularia geniculata. eft, ectoderm; en, entoderm; p, perisarc, ex- panded around hydranth to a hydrotheca; s, supporting layer. this perisarc may be greatly increased and calcified, forming large coral-like masses with openings from which the hydranths may protrude (fig. 177). FIG. 177.— A bit of Millepora alcicornis, enlarged. (After Agassiz.) The lack of sexual organs, which distinguishes the marine species from the fresh-water Hydra, is explained by the fact that sexual individuals of special form are produced from the colony 234 CGELENTERATA. by budding. These, the medusae, may separate early from the colony and swim freely. A medusa (figs. 178, 179) has the form FIG. 178.— Rhopalonema velatum. c, ring canal; e, exumbrella; g, gonads; h, otocysts: m, stomach; n, nerve ring; o, mouth; s, subumbrella; t', t'\ tentacles of first and second order ; v, velum. of a dome-like or disc-like bell and consists chiefly of an ex- traordinarily watery jelly. The bell or umbrella of the medusa is covered on both its surfaces — the concave or subuinbrella, the con- HTDROZOA. 235 vex or exumbrella — with ectodermal epithelium. At the margin of the bell the sub- and exumbrellar ectoderm is produced into a two-layered sheet with a central opening, the velum or craspedon (fig. 178, B, v) of systematic importance, since these medusae are often spoken of as Craspedota. Tentacles (usually 4, 8, or multiples in number) also arise from the edge of the bell just outside the velum. FIG. 179.— Tiara pileata. (After Haeckel, from Hatschek.) Comparable to the tongue of the bell or the handle of the umbrella is the manufrrium, hanging from the highest point of the subumbrella and bearing the mouth at its tip. It contains the chief digestive space from which radial canals run on the sub- umbrellar surface to a ring canal in the margin of the umbrella. The radial canals are usually four in number, 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. 236 C(ELENTERATA. All other important organs arise from the ectoderm. Gonads arise in many species (fig. 179) from the ectoderm of the manu- brium; in others from the same layer covering the subumbrellar surface of the radial canals (fig. 178), forming in either case con- spicuous, often orange or red, thickenings. Longitudinal ectoder- mal muscles move the tentacles in a snaky fashion, whence the name medusa. Circular striped muscles run on the subumbrellar side of bell and velum, causing the characteristic motion. By this contraction the bell becomes more arched and narrowed, while the J B D FlG. 180.— Otocysts of Medusae.— A, Cunina ; B, Rhopalonema ; C, Carmarinn (Trachy- medusae) ; D. Octorchis (Leptomedusan). a, epithelium ; /i, auditory cells ; hfy origin of hairs ; hh, auditory hairs ; hp, auditory cushion ; o, otoliths ; ?i, audi- tory nerve ; nr, nerve ring. velum (which hangs down when at rest — fig. 178, A) contracts like a diaphragm across the mouth of the bell (fig. 178, B). Since water is thus forced out through the opening the medusa is forced forward by the reaction. The circular muscles of umbrella and velum are separated by the nerve ring, with which are connected the sensory organs — eyes of the simplest type; red pigment spots with or without a lens (fig. 81); and open or closed auditory vesicles (otocysts). Tactile hairs are abundant on the tentacles. HYDROZOA. 237 The auditory organs are of two types, both beginning as free organs and receiving their highest development as closed vesicles (otocysts). One type, the tentacular organs, occur in the Trachymedusae, the other, or velar organ, in the Leptomedusae. The tentacular organs are modified tentacles, the entodermal axis of which forms the otoliths and the ectodermal covering the sense cells. In the ^Eginidae (Fig. 180, A) the club-like tentacles, seated on an auditory cushion, project freely into the water; in the Trachynemidse (Fig. 180, B) they are partially transformed into vesicles by the upgrowth of epithelium, and in the Geryonidae (Fig. 180, C) they are completely enclosed arid are sunk in the jelly of the bell. The velar organs of the Leptomedusa3 are placed on the subumbrellar sur- face of the velum. They may be either simple pits (Fig. 180, J?), or the mouths of the pits may close (Fig. 180, Z>). In these both sense cells and otoliths are ectodermal. Eyes and otocysts occur in different forms, a fact which formerly lead to a division of medusae into ocellate and vesicu- late groups. While polyps and medusae apparently differ so greatly from each other, their morphology shows that the medusae are only highly modified polyps adapted to a swimming life. The long axis of the polyp has been greatly shortened (fig. 181) and the cylindrical FIG. 181.— Diagram of sections of (A) a polyp and (B) a medusa, ek, ectoderm; ck', of exumbrella; efc2, of subumbrella; ek\ of manubrium; e7, endoderm (cathamnal) layer arising from obliteration of digestive space ; en, entoderm ; r, ring canal ; s, subumbrella ; t, tentacles; V, velum ; x, supporting layer (gelatinous in B). body developed into a disc; the mesoglcea of column and disc thick- ened to a conspicuous layer of jelly; while manubrial cavity, radial and ring canals are to be interpreted as remnants of the large gastrovascular space of the polyp, obliterated in part by the pressure of the mesogloea. To the parts thus formed only the yelum and sense organs are added. This comparison of medusa with polyp is of importance in understanding the development, which usually is complicated by an alternation of generations. From the eggs of the medusae a small ciliated embryo (planula) escapes, which becomes attached, 238 C(ELENTERATA. develops mouth and tentacles, and, by budding, produces a. hydroid colony. This hydroid colony lacks sexual organs. It produces by budding the sexual individuals, the medusae, which separate and swim freely. Since polyp and medusas are morpho- logically comparable, there is a time before the escape of the medusae when the colony is polymorphic, consisting of asexual individuals (hydranths) which reproduce only asexually and of others which have taken over the sexual reproduction (medusas). Hence we conclude that the alternation of generations here has arisen 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 can again produce it. This occurs when the medusae, instead of separating, remain permanently attached to the colony. They then degenerate into ' sporosacs/ which always lack mouth, tentacles, and velum (fig. 182), often also radial and ring canals, so that at last FIG. 182.— Comparison of a medusa and a sporosac (orig.). A, fully developed medusa; B, medusa with the manubrium closed, still attached to the blastostyle ; C, ', last stage, eggs being pro- medusa reduced to a simple manubrium (sporosac) ; D duced in the body wall (Hydra). 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, gonopliore, 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 produce new medusae by budding.) Thus we can have four conditions: (1) Polyps which produce sometimes asexually, sometimes sexually, but always HYDROZOA. 239 polpys; (2) Medusae which always produce medusae; (3) Polyps and medusae in alternating generations; (4) Polyps and sessile medusae (sporosacs) united in a polymorphic colony. 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 medusae 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 (Proto- hydra ryderi,* America ; Polypodium hydriforme, Russia) and medusan forms (Limnocodium soivei'byi, Brazil ; Limnocnida tanganyicce, Africa ; Halomises lacustris, Trinidad) have been found. Cordylophora lucustris * occurs in the brackish waters of Europe and America. The Hydrozoa may be classified according to characters, derived either from the hydroid or the medusan stage. The former basis gives us four groups : (1) Hydraria. Polyps with asexual and sexual reproduction ; no per- sistent colonies, no perisarc, no gonophores (fig. 172). (2) Tubulariae. Mostly colonial, with perisarc but without hydrothecae. Reproduction by gonophores (medusae or sporosacs, figs. 91, 175). (3) Campanulariae. Colonial, with perisarc and hydrotheca. Repro- duction by gonophores arising in special perisarcal envelopes, the gonotheca (figs. 174, 176). (4) Hydrocorallina. Colonial, with massive, calcified perisarc, resem- bling coral. Reproduction by sporosacs or short-lived medusae. FIG. 183.— American Trachy and Narcomedusae. A, Liriope scutigera. (After Fewkes.) .B, Cunocantha octonaria. (After Brooks.) The characters derived from the medusae also give five groups : (1) Anthomedusae (Ocellatae). Gonads on the manubrium ; no audi- tory organs ; eyes usually present ; polyp generation present. (2) Leptomedusae. Gonads on radial canals ; usually velar auditory organs ; polyp generation present. (3) Trachymedusae. Gonads on the radial canals ; tentacular auditory organs ; develop directly to medusae (fig. 183, A.) (4) Narcomedusae. Gonads on the manubrium or gastral pouches ; tentacular auditory organs ; no polypoid stage (fig. 183, B.) 240 C(ELENTERATA. (5) Siphonophora. Polymorphic, free-swimming colonies of Anthome- , Opercularella pumila. Other common genera, Clytia,* Dipliasia* and Aglaophetiia* among hydroids; Obelia,* Tima* RUegmatodes* among medusae. Possibly the fossil group of GRAPTOLITES belongs near here. Only the perisarc is known, and this is composed of hydrothecae, in which it is supposed the hydranths occurred. Order V. Trachymedusae. These medusae, mostly from warmer seas, have no hydroid stage. The characters are given on p. 239, Trachynema, Liriope*(ftg. 183), and Cam- panella in our own waters, Geryonia, etc., in Europe. Order VI. Narcomedusae. In addition to the characters on p. 239 may be added the fact that the tentacles arise from the outside above the rim of the bell. Cunocantha * (fig. 183), and Cunina * in our warmer waters, jEgina in Europe. 7. HTDROZOA : SIPHONOPHORA. 243 si) Order VII. Siphonophora. The Siphonophora are among the most beautiful of pelagic animals, some transparent, some brightly colored. Each (fig,. 187) consists of a colony of individ- uals springing from a common C03- nosarcal 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 filled with air, the pneumataphore, which acts as a hydrostatic apparatus, and keeps the colony vertical in the water. The individuals, springing from the coenosarcal axis, perform differ- ent functions and hence have differ- ent structures. Close behind the float commonly come several swim- ming bells (nectocalyces) which re- tain of medusal structures only those (bell, velum) necessary for swimming and those (ring and radial canals) for the distribution of nourishment received from the common tube. Then come, scattered through the colony, the covering scales, for pro- tection, firm gelatinous plates which have lost the ring canal, the muscles, and the bell shape of the medusae. Food is taken by wide-mouthed feed- ing tubes (In/} which may be com- pared to polyps (fig. 57) or the m nubrium of a medusa. They digest the food by means of large masses of glands (' liver bands ') and convey it calyx); st' stalk' by the central tube to all the members of the colony. At the^ base are long muscular tentacles (t) 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 produced by the siphonophores, which, in many is so severe as to be feared by man. The ' feelers ' 244 CCELENTERATA. recall mouthless polyps and manubria; they are very sensitive and mobile and, while tactile, apparently in some cases are digestive organs. Latest to develop in the colony are the sexual bells. They are usually brightly colored and resemble small mouthless B3iM FIG. 188.— Stephalia coronata. (After Haeckel, from Lanp.) A, in section; au, canal to float; fca, canal system of stalk; o, mouth ; other letters as in fig. 188. Anthomedusae 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 Siphonophora afford fine examples of division of labor and of the consequent polymorphism of indi- viduals. 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 trop- ical seas. Sub Order I. PHYSOPHOR^E (Physonectee). Float present, but small ; next a large series of swimming bells, and then the other members •of the colony. Physopliora, Agalmia, Nanomia* (fig. 189). Sub Order II. CALYCOPHOR^E (Calyconectse). Float lacking ; one or two large swimming bells ; the other individuals in groups which fre- quently separate before becoming mature, and were once regarded, under the name Eudoxia, as distinct animals. Praya, Diphyes* (fig. 189), in warmer seas. Sub Order III. CYSTOKECT^E. Float greatly enlarged ; the creno- sarcal tube reduced, the individuals (no covering scales nor swimming II. SCTPHOZOA. 245 bells) being attached to the under side of the float. Physalia, the Portu- guese man-of-war, occurs as far north as New England. It is brightly colored, and, sitting high on the water, is driven about by the wind. It stings very severely. Sub Order IV. DISCONANTH^E. Float a flattened disc with con- centric air chambers; the manubrium projects from the centre of the lower FIG. 189.— American siphon ophores. A, Nanomia earn. (After A. Agassiz.) B. Velella meridionalis. (After Fewkes.) C, Diphyes praya. (After Fewkes.) surface of the float. Porpita* with circular disc. Velella* (fig. 189), the paper sailor, has a triangular ' sail ' on the disc. Both are tropical and subtropical. Class II. Scyphozoa (Scyphomedusae). The Scyphozoa parallel the Hydrozoa in that they frequently have an alternation of generations. The asexual generation is the FIG. 190. FIG. 191. FIG. 190.-Scyphostoma of Aurelia aurita. (From Korschelt-Heider.) fc, perisarc cup ; pb proboscis; s, stalk; t, gastral folds; tr. ectodermal funnels. Mo. 191.— bection of Scyphostoma. (From Hatschek.) gr, gastric pouches; s, gas- tric folds ; SOT, muscles. scyphopolyp or scyphostoma, the sexual an acraspedote medusa. In contrast to the Hydrozoa the asexual stage plays a subordinate 246 CCELENTERATA. role; it is closely similar, even in the most different species, and can even be lost (Pelagia), while the medusae are always well devel- oped and present great variety of form. The scyphostoma (figs. 190, 191) recalls superficially Hydra, ibut is distinguished externally by a small perisarcal cup in which the aboral end is placed. Internally there are four longitudinal folds projecting into the gastral cavity and extending from the margin of the mouth to the opposite pole. These septa or tceniola appear in cross-section as small folds of entoderm supported by a process of the supporting layer. They are important morpholog- ically, since in budding they produce the gastral tentacles ( pliacellce) of the medusae. Further, they are the first appearance of the septal system, so strongly developed in the Anthozoa. The acraspedote medusae are large forms (four inches to four feet or more in diameter) with an arched umbrella often of almost cartilaginous consistency. They are distinguished from the craspe- dotes externally by notches in the margin of the umbrella, dividing the periphery into lobes. In the common forms at least eight lobes occur (figs. 192, 193), each notched at its tip, and in the notch the sensory pedicels bearing both ears and eyes and covered by a lappet. FTG. 192. — Ephyra of Cotylorhiza. Ill SOHie (fig. 193, /, //) the Sen- (After Glaus.) gt, gastral tentacles i i <• 11 11- (phacellee); rfc, marginal (sensory) SOry lobes follow each other, but 111 others the intermediate region is also notched, the sensory pedicels then being found only on careful search (fig. 194). Tentacles, when present, spring from the notches of the intermediate region. The sensory pedicels predicate the position of eight principal radii, of which four are called the perradii, the four alternating with them the interradii. Adradii are radii lying between the principal radii. The lobing of the umbrella influences all other structures. There is no velum (hence these are called Acraspedia), its place being taken by a thick muscular mass (fig. 86, m) on the sub- umbrellar surface. Instead of a nerve ring there are eight nerve centres connected with the sensory pedicels. Each of these pedicels (fig. 195) is a modified tentacle with an entodermal 11. SCYPUOZOA. t I 11 247 — H FIG. 193.— Ulmaris prototypus. (From Hatschek.) 7, radii of first order (perradii); //, radii of second order (interradii); /, marginal lobes; n oral lobes (cut away on right side); ^, tentacles (adradial); the gonads (right side) are interradial. FIG. 194.— Polyclonia frondnsa * and one of its branching oral lobes, showing the closed grooves (s). (After Agassiz ) 248 CCELENTERATA. axis and an outer layer of ectoderm. The entoderm forms an otolith sac at the tip, while the ectoderm furnishes a nervous cushion of ganglion cells and fibres and usually a simple eye spot. Less evident is the effect of the lobing on the internal organs. The gastrovascular system begins with a quadrate or X-shaped mouth (fig. 193). The perradial angles of the mouth are usually produced into long curtain-like oral tentacles of great use in the capture of food. The ' stomach/ which begins just inside the mouth, gives off four interradial (i.e., alternating with the corners of the mouth) FIG. 195.— Sense organs of Aurelia aurita. (After Schewiakoff.) ec, ectoderm; en, • entoderm; gv, gastrovascular space ; w, supporting layer ; o, cup eye ; oc, pigment eye ; ot, otolith sac ; rg, olfactory groove. pouches, the gastrogenital pockets. The epithelium of these pouches produces on the one hand a group of small gastral tentacles (phacellse), each extremely mobile and supported by an axis of mesogloea; on the other plaited folds of the gonads, these being, in contrast to the Hydrozoa, of entodermal origin. In this the Scyphomedusae show relationships to the Anthozoa. From the central digestive sac arise the peripheral portions. These consist in the larval medusae (Ephyra stage, fig. 192) of eight radial canals to the sensory pedicels, and in most adult medusae of these same pouches and eight others, adradial in position, to the tentacles. In some this primitive arrangement is complicated by an extensive network of tubes (fig. 193). In the species with an alternation of generations the egg pro- duces a ciliated larva (fig. 196) which attaches itself and develops into a scyphostoma. This scyphostoma is always capable of ter- //. SCYPHOZOA. minal, and often of lateral, budding. The lateral buds always produce new scyphostomae, the terminal, medusae. 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 ' ephyrae/ At first the ephyrae (fig. 192) have only four gastral tentacles, parts of the gastral folds of the scyphostoma (p. 246); they lack marginal tentacleSj FIG. 196.— Development of Aurelia aurita, (From Hatschek.) First row, growth of planula to scyphostoma; below, strobilation (separation of ephyree): left, oral view of scyphostoma ; right, two ephyree. but have the eight lobes and the corresponding sense pedicels. Since the ephyrae differ markedly from the adult medusae and only gradually change into the sexual form, the alternation of genera- tions is complicated by a metamorphosis. This metamorphosis persists in some cases (Pelagia noctilu-ca) where the alternation of generations is suppressed; the egg develops directly into an ephyra, which becomes transformed into the adult jelly-fish. On the other hand no case is known where the medusa generation is dropped out and the scyphostoma give rise sexually to other scy- phostomae. 250 C(ELENTERATA. Some forms differ from the foregoing description in structure and ap- parently in development. Some have only four sen- sory bodies, the places of the other four being taken by tentacles. In these cases the sensory organs lie (Peromedusae) in the same radii (i.e., interradii) as the sexual organs or (Cubomedusae) the sense organs are perradial. Lastly, some have no sensory organs, their place being either taken by tentacles or left vacant (Stauromedusae). This shows that tenta- cles can replace sensory pedicels, and since they have essentially the same structure, they, like the cordylii •of the Trachymedusae, are modified tentacles. Order I. Stauromedusae (Calycozoa). The best known forms are the Lucernariae (fig. 197), whose exumbrellar surface is drawn out into a stalk by which the animals are attached. The disc is drawn out into eight lobes, each with a cluster of knobbed tentacles. .Several species, dark green in color, occur in New England waters. The Tesseridce (unknown in America) are free-swimming. Fia. 197. — Halyclystus auricularia.* (After Clark.) Order II. Peromedusae. Cup-shaped medusae with four interradial sense bodies. ,sea forms. Pericolpa, Periphylla in the Gulf Stream. Mostly high Order III. Cubomedusae. Sense organs perradial in position. Occurring in tropical and semi- tropical seas. Charybdea (fig. 198). Development unknown. Order IV. Discomedusae. These are the most abundant and richest in spe- cies of Scyphomedusae and hence have served as the basis of the foregoing account. The order is subdi- vided according to the characters of the mouth. (1) CANNOSTOMJS, mouth triangular without oral tentacles; shape and other features of the ephyra retained in the adult. Nausithoe albida (fig. 86) of Europe is noticeable because its scyphopolyp, de- scribed as Stephanocyplius mirabilis, is parasitic in sponges. Liner ges and Atolla in the Gulf Stream. (2) SEM^OSTOMJE, mouth X-shaped with long fringed and folded arms at the corners. Aurelia flamdula * and Cyanea arctica* common in New England, the latter, the 'blue jelly,' often very large; disc 7 feet in diameter, tentacles extending a hundred feet or more. Pelagia * in our warmer waters. (3) RHIZO- STOME^E.four oral arms, these brancheddichotomously. 19?-,~ cjiarybdea -phe !nouth and grooves on the arms closed by union ipialis.* (From * ihek.) of their edges so that many small sucking stomata FIG. marsi Hatsc III. ANTHOZOA. 251 remain through which nourishment is taken. Stomolophus* and Polyclo- -nia frondosa* (fig. 194) on coral banks in our warmer seas. Class III. Anthozoa (Actinozoa). The Actinozoa, including the sea anemones, sea pens, and corals, are exclusively marine. With, few exceptions they are sessile and form colonies, often of enormous size. In this as in appearance (fig. 199) they resemble the hydroid polyps. They have a pedal FIG. 199. — Antheomorpha elegans. s, s, sagittal plane. disc, column, tentacles, and peristome with central mouth. They are distinguished by their greater completeness in histological and organological differentiation. The Anthozoan polyp has a well- developed mesoglcea, the supporting layer of the hydroid being here a layer of connective tissue with numerous cells, giving the animals a tough fleshy consistency. Still more important as points of distinction are the presence of an oesophagus and septae bearing mesenterial filaments and gonads. The mouth lies in the centre of the peristome, and in shape is usually oval or slit-like. Hence there is a biradial symmetry — of importance in the architectonic of the polyp — for there is a sagittal axis (fig. 199, s, s) passing in the long axis of the mouth and a transverse axis at right angles to it. From the mouth the oesopha- gus hangs down into the body as a flattened tube and opens at its lower end into the wide gastro vascular cavity. In its development this oesophagus is an inflected part of the peristome and hence lined with ectoderm, and its lower end alone can be compared with the mouth of the hydrozoan (fig. 200). The 03sophagus is held in position by radial partitions, the septa, which stretch from base, column, and peristome to the 252 C(ELENTERA TA. 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 con- tinue into the tentacles. The tentacles therefore are outgrowths from the radial chambers and usually are equal in number to them. Besides the complete or primary septa which reach the oesoph- FlG. 200.— Stereogram of an Anthozoan (orig.). In the cut edges the ectoderm white, the entoderm blocked, the supporting layer black. The septa show the septal muscles, and the communication of the interseptal chambers with the oesophagus is seen. agus, there may be others incomplete in that they do not reach the oesophagus and belonging to secondary, tertiary or other series (fig. 203). The septa support a number of important organs : the mesen* terial 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 III. ANTHOZOA. 253 the mesenteries of the mammals. 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 inside the mesenterial threads as band-like folded thickenings (fig. 201, 7^3). They arise as in the Scyphomedusae from the entoderm, but early migrate into the FIG. 201. FIG. 202 FIG. 201.— Sections of Cereus spmostts, showing complete and incomplete septa. u, acontia; 6, mesenterial filament; c, septal stoma; g, gonads; ft1, septa of first order with gonads; /i2— /i4, incomplete septa of second to fourth order; t1— *4, corresponding tentacles. FIG. 202. — Section of septum of Edioardsia tuberculnta. ek, ectoderm; en, entoderm; me, supporting layer; mf, septal muscle; o, ovary ; u, mesenterial filament. mesoglcea of the septum (fig. 202, o). The eggs, when ripe, escape into the gastrovascular cavity by dehiscence. The young leave the parent at various stages of development, sometimes as planulae (fig. 206, 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, forming especially a CCELENTERA TA . thick subepithelial sheet of fibres and ganglion cells in the pori- stome, the muscles of the ectoderm are weakly developed and are confined to the peristome and the tentacles. The entodermal musculature is much stronger. At the oral end of the column is usually a strong circular (sphincter) muscle which by its contrac- tion can draw the top of the column over the peristome. The septa also bear muscles, on one side running transversely, on the other longitudinally, the latter alone being strongly developed and producing marked ridges (fig. 202) on the septa. In the Hexacoralla the septa are arranged in pairs, not only in being close to each other, but in having similar faces turned towards each other. The rule is (fig. 203) that in each pair the sides bearing muscle ridges are turned towards each other, but in two pairs lying m the sagittal axis these muscles are turned outward. From these relations these septa are called directives. It is however to be noted that in our common anemone, Me- tridium, occasionally three, more frequently but one pair of directives occur. The paired condition of the septa allows the recognition of two kinds of radial chambers; between the two of a pair is an intraseptal, FIG. 203. — Transverse section of actinian (Adamsia diaphana) AB, plane of symme- try, a second lies at right angles. I-IV, septa of four orders. between two pairs an interseptal chamber. New septa only appear in the interseptal chambers. At one time all Hexactiuians have but six septa, a pair of directives and, right and left, four lateral septa. With growth, other septa of a secondary order may appear in the interseptal areas, giving six of these. And so with septa of the tertiary order. Irregularities how- ever occur, and forms are found which have abandoned this sexfold plan 177. ANTHOZOA. 255 and have assumed a plan of four or ten, but without altering the primitive conditions. In the Octocoralla (fig. 204) the conditions are simpler, only eight septa being developed. These are disposed equally on either side of the oesophagus and may have (most octocorallans) all their muscles towards one end, or (Edwardsia, fig. 205, IV) may have the muscles of one pair reversed. It is to be noted that hexactinians pass through an Edwardsia stage. In Cerianthus new septa are always added at one end of the sag- ittal axis (fig 205, II), while in the extinct Tetra- coralla (fig. 205, I), so far as one may judge from the hard parts, the septa have an arrangement with four as the basis. FIG. 204.— Transverse sec- tion of an Octocprallan (Alcyonium). x, siphono- glyphe; 1-4, septa of one side, wi with their muscles on one side, symmetrical with those of the other side. IV FIG. 205. — Arrangement of septa in various Actinozoa. I, Tetracoralla ; II, Cerian- thus; III, Octocoralla; IV, Edwardsia. By far the greater part of the Anthozoa reproduce by budding as well as by eggs. Only rarely do the buds separate, but generally they remain connected with the mother to form a colony of hun- dreds or thousands of individuals. These are connected by an extensive coenenchym or coenosarc, consisting largely of mesoglcea, but having an outer coat of ectoderm and penetrated by a system of branching and anastomosing entodermal canals (fig. 206). On disturbance the polyps can quickly retract themselves into the coenosarc. The colonial Anthozoa have almost invariably a skeleton, secreted by the ectoderm and consisting either of calcic carbonate or of an organic horn-like substance. Sometimes the horn and lime alternate. One recognizes an axial and a cortical substance. The axial skeleton is confined to the deeper portions of the coenosarc, while the cortical portions are formed by the polyps themselves and to a large extent (figs. 207, 208) repeat their complicated structure. Except in a few forms (Fungia) a theca is present ; this is a calcareous cup, and from this usually extend inward calcareous partitions called, in distinction to the fleshy- or sarco- septa, the sclerosepta. 25C C(ELENTERATA. 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 e: FIG. 206.— Corallium rubrum, red coral. (After Lacaze Duthiers.) A, ciliated young; .B, young colony ; C, part of colony with polyps in extension (a) and contrac- tion (c); d, coenosarc; Z>, stereogram of a branch; 6, c, partly and completely re- tracted polyps; rf, coenosarc; e, skeletal axis exposed; /', /, larger and smaller coenosarcal canals; m, mesenterial filaments; s, oesophagus; <, retracted tentacles; A, greatly, #, C, D, slightly enlarged. of these project on the outer surface as costae. 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 (fig. 208). Pali are small III. ANTHOZOA. 257 free particles between the inner ends of the sclerosepta and the columella, while synapticulce are small projections connecting the septa. As the polyps grow they build the theca3 higher and higher and consequently draw Fio. 208. FIG. 207. — Sclerophyllia mnrgariticola. (After Klunzinger.) FIG. 208.— Section of coral of Caryophyllia cyathus. (After Koch.) Outside the theca, septa (I-XII) of first and second order, their pali and, in centre, columella. out from the deeper portions, which may become cut off by horizontal parti- tions, the tabulee. Such tabulae occur in some Madreporaria, Octocorallans, and Millepores (p. 241) which were formerly united in a group Tabulatae. FIG. 209. Fio. 210. FIG. 209.— 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 ; 7-, directives : coral black. FIG. 210.— Diagram of the relations of the coral to the polyp. (After Koch.) Ectoderm lined, mesogloea black, entoderm dotted, coral white, a, theca; b, mesenteries; c, costae ; d, basal plate ; e, external wall ; /, sclerosepta. It was once thought that the coral was a calcined 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 258 CCELENTERATA. the sarcosepta, and the theca inside and at some distance from the col- umn, the outer surface of which secretes only the inconstant epitheca (fig. 209). 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 Hex- acoralla, are shown by the soft parts to be undoubted Octocoralla. By means of their skeletons the Anthozoa produce large accumulations of carbonate of lime, the well-known coral reefs, on the bottom of the sea. These are formed by many species, the Madreporaria playing the most important role. When the reef reaches the surface it produces an island, the most noteworthy 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 investigations, notably those of Mr. Agassiz, afford another explanation. Order I. Tetracoralla (Rugosa). Extinct forms from the paleozoic rocks with the parts arranged in fours (fig. 211). The present tendency is to regard them as modi- fied Hexacoralla. Order II. Octocoralla (Alcyonaria). These forms, which have eight single septa, are externally re- cognizable by their feathered tentacles, eight in number (fig. 206). B FIG. 211. FIG. 212. FIG. 211.— Diagram of septa in a tetracorallan. (Orig.) FIG. 212.— Three stages in development of Renilla reniformis. (After Wilson.) A, cleavage of egg ; B, planula ; C, development of oesophagus ; ec, ectoderm ; en, entoderm; r/i, mesogkea ; o, oesophagus. They occur in all seas from near the shore to great depths. In development there is a planula (fig. 212) 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 polymorphism may occur, some individuals which have reduced septa and lack tentacles, taking in water for the colony. Many are phosphorescent. ///. ANTHOZOA: HEX A COR ALL A. 259 In the ALCYONIIDSE (Alcyonium,* Anthomastus) an axial skeleton is lacking, but the flesh contains numerous calcareous particles, the scleroder- mites. The sea pens, PENNATULID.E, 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. Pennatula,* colder waters ; Renilla,* warmer seas. The GORGONIID^E (sea fans, sea whips) have an axis of more firmness, which may be calcareous, and the colony may branch and the branches anastomose. Here belong, besides many tropical genera whose names end in <• gorgiaj Primnoa* of our colder waters; Isis of tropical seas, with skeleton of alternating calcareous and horny parts, and the pre- cious coral (Corallium rubrum, fig. 206) of the Mediterranean, the fishing for which at Naples amounts yearly to half a million dollars. In the TUBI- PORID^E, or organ-pipe corals, the separate polyps are enclosed in parallel tubes united at intervals by horizontal plates. The Helioporce were long regarded as Hexacoralla because of their massive skeletons with six sclero- septa. The paleozoic Syringopora belongs near Tubipora, while the FAVOSITIDJE resemble the Alcyoniidse. 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 (common in our colder waters), in which there are sixteen or more tentacles and only eight septa (fig. 205), but which exhibits a condition through which the young actinians pass ; on the other hand in the Zoantharia, Cerianthise, and Antipatharia the rule of six has undergone extensive modification. Sub Order I. ACTINARIA (Malacoderma). The sea-anemones are mostly solitary, without skeleton; with numerous septa and tentacles. They occur in all seas from tide marks to the greatest depth. A few are free, but most are sessile. Except the colonial Zoanthese all can creep by the pedal disc. Represented in our seas by Metridium, Bunodes, Sagar- tia, Biddium (parasitic on Cyanea) Halcampa, etc. The Zoantheae have two kinds of alternating mesenteries and the individuals of the colonies are usually incrusted with foreign matter. Epizoanthus lives symbi- otically with hermit crabs (fig. 113). Sub Order II. ANTIPATHARIA. Six pairs of septa and six (Antipathes) or twenty-four ( Gerardia) simple ten- FIG. 213.— American sea-anemones. A, ides (after stimp- tacles; colony with a black horny axis son, B, Biddium parasiticunt (after Verrill), C, Bunodes sttlla (after Ver- rill). 10 calcareous skeleton, late the Gorgonids. SimU- 260 CWLEXTERATA. Sub Order III. MADREPORARIA. This group, the richest in species of any, is characterized by the great development of the skeleton. Theca, septa, and usually columella and synapticuli are present, and frequently costaB as well. Solitary forms are few. Usually they form colonies, fre- quently of thousands of individuals, bound together by a coenenchym extending from polyp to polyp over the surface of the coral. A colony FIG. 214. Fio. 215. FIG. 214.— Astrmigia danae* ; five polyps in various stages of expansion. FIG. 215.— Coeloria arabica. (After Klunzinger.) FIG. 216. FIG. 217. TIG. 216.— Cladocora ccespitosa. (After Heider.) Relations of coral and flesh. FIG. 217.— Favia cavernosa. (After Klunzinger.) arises from a single animal by continued fission or budding. When the division is not complete the animals 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 sclerosepta, as in the brain corals (fig. 215). Since but little is known of the soft parts, the classification of the Mad- reporaria is based upon the coral. Three sections of the sub order are recog- nized. (1) APOROSA, with compact skeleton. Some, like Caryophyllia IV. CTENOPHORA. 261 (fig. 208) and Sclerophylla (fig. 207) are solitary. Others, like Oculina,* branch, and still others form compact masses. Astrangia danae (fig. 214), the only true coral in New England; Astrcea, the brain corals (Cceloria, fig. 215, Diploria, Manicina); Cladocora (fig. 216), Favia (fig. 217). FIG 218.— Madrepora erythrcea. (After Klunzinger.) (2) FUNGI ACEA, or mushroom corals, with no outer wall to the coral. Some are colonial, others (Fungia) are solitary. A sort of strobilation in de- velopment. (3) POROSA, with skeleton porous like a fine sponge. Madre- pora* deer's-horn coral (fig. 218), Porites, Astroides. Class IV. Ctenophora. The Ctenophores excel all marine animals, even the medusae, 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 almost always biradially symmetrical; i.e., is divided by both sagittal and transverse planes into sym- metrical 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 con- nective-tissue cells, penetrated in every direction by polynucleate muscle cells branched at their ends and apparently innervated by special nerve cells. On the outer surface is a layer of ectoderm, while in the interior is a system of branched entodermal canals. At the bottom of a depression (fig. 22 IB, p) at the aboral pole is a thickened patch of ectoderm, the sense body, which has considerable resemblance to an otocyst (fig. 222). 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 compared to an incomplete vesicle. In the centre is a spherical 262 C(ELENTERATA. FIG. 221B. FIG. 219.— Swimming plate and epithelial cushion. (After Chun.) FIG. 220. — Hormiphora plumosa. (After Chun.) FIG. %%,l.—-PleurobracMarho an(l from them the canals distributed phora°f (Aner through ^ne j6^ t° *ne various organs. Two Samassa.) (rarely four) funnel canals run to the aboral pole and empty (fig. 223, to) near the sense body; a second pair, the paragastric canals (fig. 221 B, mg), which run parallel to the 03sophagus, end blindly. The perradial canals (c.pr) proceed out- ward from the funnel, and besides giving off a canal to the tentacle (tg) each divides dichotomously twice, first into interradial and then into adradial canals, each of these last connecting with a meridional 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 ves- sel nearest to the combs. In spite of their position they are apparently ectodermal in origin. 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 of the group are divided into the TENTACULATA, with tentacles, and the NUDA, without. To the first belong the CYDIP- PID^E, with pear-shaped bodies (Pleurobrachia* on our coast, fig. 222), and Hormiphora (fig. 221); the LOBAT^E (Mnemiopsis,* Bolina*), with lobes; and the band-like CESTID^E (Cesium, the Venus girdle) of the warmer seas. The BEROID.E (Beroe, Idyia*}, with wide mouth, belong to the Nuda. The small creeping forms, Cceloplana and Ctenoplana, are supposed by some to form a transition to the Turbellaria. SUMMARY OF IMPORTANT FACTS. 265 Summary of Important Facts. 1. The CCELENTERATA (together with the Echinoderma) were formerly called Radiata because in most a radial form of structure is present; in the higher groups this can be transformed into 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 system of cavities, a simple or ramified digestive sac, repre- senting at once the alimentary tract and the as yet undifferen- tiated body cavity. 4. This ccelenteric apparatus is called the gastrovascular sys- tem because its branches distribute nourishment to all parts and so perform the function 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 cnidse, in special cells. 8. Nearly all histological differentiation proceeds from ectoderm or entoderm, since the mesoderm (mesoglcea) plays but a subordi- nate role and usually functions only as a support. 9. Four classes — Hydrozoa, Scyphozoa, Anthozoa, and Cteno- phora are recognized. 10. In HYDKOZOA and SCYPHOZOA there are usually two alternating generations, the sessile asexual polyp and the free- swimming sexual medusa. 11. The hydroid polyp and the craspedote medusa are charac- teristic of the HYDROZOA. 12. The hydroid polyp is a two-layered sac of ectoderm and entoderm, 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 medusae arise by lateral budding from the hydroid. 15. If the medusa remain attached to the parent as a sporosac, 266 C(ELENTERATA. alternation 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 (taeniolge). 18. The acraspedote medusa lacks a velum, has a lobed umbrella edge, gastral tentacles (phacellse), and entodermal gonads. 19. The medusa arises from the polyp by terminal budding (strobilation). 20. Alternation of generations rarely is lost, and then only by suppression of the scyphostoma. 21. The ANTHOZOA have only one form, the coral polyp; it is distinguished 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 mesoglcea. 22k. Most Anthozoa are colonial and produce a skeleton usually of calcic carbonate, but sometimes of l horny ' substance. 23. The skeleton may be either axial or it may extend over the individual polyps (cortical skeleton). 24. The living Anthozoa are divided according to the number of septa into Octocoralla and Hexacoralla. To these are added the fossil Tetracoralla. 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 tentacles. 27. The CTENOPHOKA 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, constructed like an otocyst. 30. The digestive tract consists of an ectodermal oesophagus and a branching system of entodermal vessels. PL A TUELMINTHES. 267 PHYLUM IV. PLATHELMINTHES (FLATWORMS). This group is well characterized by the name. AVith few exceptions (rhabdocceles, many trematodes) the nearly flat ventral surface and the slightly arched back are closely approximate and pass with a more or less sharp margin into each other. In many cases the ventral surface is distinguished by its lighter color. In all the bilaterally symmetrical body is composed of a solid paren- chyma, a mass of connective tissue traversed by muscle fibres, in which the various organs — alimentary tract, nerves, excretory and reproductive organs — are imbedded. In the lower forms the di- gestive system is markedly like that of the co3lenterates ( Actinozoa, Ctenophora) in that there is but a single opening and this leads by an ectodermal oesophagus (stomodaeum) to the interior. In para- sites the digestive tract may be lost. The skin is a one-layered epithelium, sometimes ciliated, sometimes protected by a thick cuticula. Inside this comes a muscular layer (fig. 225) in which FIG. 225.— Transverse section fright half) of a Planarian. ri, vitellaria; dv, dorso- ventral muscle fibres; e, ectodermal epithelium with cilia; {/, gastric diverticula; 7), testicular follicles ; lm, longitudinal muscles (dots, in section) ; n, lateral nerve cord. longitudinal muscles are always present, and in addition frequently circular and oblique muscles, as well as those passing from dorsal to ventral surfaces. The nervous system (fig. 228) consists of a pair of ganglia (' brain') in front of (i.e., above) the oesophagus and longitudinal nerves leading backwards from it. The excretory organs (fig. 226) are composed of a series of tubes, the protone- phridia or « water- vascular system/ which branch and ramify the parenchyma. In most the sexes are united in one individual and the reproductive organs take up considerable space. There is a small paired or unpaired ovary and vitellaria, usually paired and branched. The eggs arise in the ovary, and to these are added nourishment in the shape of cells (abortive ova) rich in yolk from 268 PL A THELMINTHES. the vitellaria. At the point where oviducts and yolk ducts unite a single egg cell together with several yolk cells are united into an oval body — the compound egg — protected "by a shell secreted by special glands (fig. 227, A). This forms only an apparent exception to the rule that the egg is but a single cell, for the development shows that only the egg cell takes a direct part in the FIG. 226. FIG. 227. FIG. 226.— Excretory system of Cercaria. (After Albert Lang.) ft, limb of bladder ; b', same with urinary concretions; cc, collecting canal; cs, canals of ventral sucker; cv, collecting vacuole; e, eye; ep, excretory pore; J, lumen of tail; os, oral sucker ; vs, ventral sucker. FIG. 227.— Eggs of Dtatomum nodulosum. (After Schauinsland.) A, before develop- ment; U, later, the yolk broken down, d, yolk cells ; ei, egg cell ; eh, ectoderm ; en, entoderm ; p, pigment spot. formation of the embryo and is the true ovum, while the yolk cells- break down and furnish food to the growing embryo (fig. 227, B). Class I. Turbellaria. The Turbellaria are small, only a few being measured by inches, while many are almost microscopic in size. The name Turbellaria has reference to the currents produced by the ciliated ectoderm which covers the body, the cilia arising from the single layer of columnar epithelial cells (fig. 58). This ectoderm serves at once for motion and for respiration. Most species are aquatic (fresh water or marine), only a few land planarians living in moist earth. In the water they either creep slowly over stones or plants on their ventral surface, or they swim freely. In swimming the larger species progress by undulations of the body, the smaller by means of the cilia. /. TURBELLAR1A. 269 The alimentary canal (fig. 228) consists only of oesophagus (pharynx) and mesenteron, the latter terminating blindly since no intestine or anus is present. The mouth is on the lower surface, at some distance from the anterior end, being occasionally in the middle or even behind the middle of the body (fig. 231). It leads into the muscular oesophagus, which is frequently enclosed in a special sheath and then can be protruded like a proboscis. FIG. 228. FIG. 229. FlG. 228.— Digestive and nervous systems of Syncoelidium pellucidum. (After Wheeler.) a, alimentary tract ; 6, brain ; In, longitudinal (ventral) nerves ; m, marginal nerve ; pi, longitudinal nerve of pharynx ; pi\ ring nerve of pharynx ; , redia which has produced rediae internally ; E, redia with cer- cariee; F, cercaria : G. encysted Distomum. ^j, eye spot; Z>, digestive tract; Z>r, open- and only rarely other attaching apparatus. They are markedly separated from the Polystomes by their life history. The alterna- tion of hosts necessitated by the endoparasitic life is complicated by an alternation of generations (better heterogony, p. 145) with 276 PLATHELMINTIIES. metamorphosis. To illustrate this the history of Distomum hepatwum of the sheep is chosen (fig. 236). The eggs leave the maternal uterus before embryonic develop- ment is begun, pass down the bile ducts and thence by the intes- tine to the exterior. They must come into water and remain here awhile before the ciliated larva (< miracidium/ A) escapes by a lifting of the lid of the shell. This larva bores its way into a small snail (sp. of Limncea), where it grows into a ' sporocyst' (B). The sporocyst, a muscular sac with protonephridia but lacking all other organs, produces in its interior eggs which develop into a second reproductive sac, the ' redia ' (D). These are distinguished from the sporocysts by the possession of pharynx and a tubular intestine as well as a birth-opening for the escape of the young pro- duced inside. According to the season these young are either 'cercariae' (F)t or several generations of rediae may follow before the cercariae appear. The cercarias are adapted for an aquatic life, since each has, besides the characteristic organs of a Distomum (genitalia excepted), a strongly vibratile tail. The cercariae escape from the snail, swim about in the water until the tail drops off, • when . they encyst on water plants. When these encysted young are eaten by sheep along with the vegetation, infection follows. In general it can only be said of the life history of other Trematoda that the miracidia must penetrate a mollusc, and that the different species have many modifications : (1) Ordinarily development begins in the ma- ternal uterus. (2) Many miracidia are naked or only partly ciliated. (3) In many species the miracidia only hatch when the egg is taken into the stomach of a snail along with food. (4) Very frequently the cercaria passes from the water into a new host (mollusc, arthropod, or vertebrate) and becomes encysted here. In such cases there are three hosts in the cycle. (5) On the other hand the history may be simplified, as when the sporocyst in the snail produces directly * cercariae without tails ' (i.e., small Distoma), which only need to be eaten by the definitive host to reach the sexually mature condition. (6) It is doubtful if the sporocyst may be omitted and the miracidia develop directly into redia. As the adjacent scheme shows, the typical development is distributed among three hosts by the intercalation of a second aquatic interval. It consists of two generations ; one extends from the fertilized egg to the sporocyst, the second begins with the unfertilized egg of the latter and de- velops, through the cercaria and the encysted Distomum, into the sexually mature individual. There is no sexual reproduction by fission or budding, rather an alternation of sexual and parthogenetic generations or heter- ogony. Columns a and c show how the history maybe simplified and com- plicated. Best known of the Distomeae are the following: Distomum (Fasciolaria) hepaticum, the liver fluke (fig. 232), about the size and shape of a pump- II. TREMATODA: DISTONE^I. 277 kin-seed. It lives in the bile-ducts of sheep, cows, pigs, etc., and rarely (twenty known cases) of man. It stops up the ducts and causes a disease DEVELOPMENT OF DISTOME^E. (a) Simple (ft) Ordinary (c) Complicated * Larva Water fl f Larva Water - I Larva Water o 1 .2 ! ta •{ Sporocyst Host I Mollusc V C Sporocyst, perhaps also redia Host I Mollusc Gener Sporocyst Host I Mollusc |^ Redia M «.2 I-H Cercaria Water _f Cercaria Water Encysted Distomum Hoet I d .2 g-t Encysted Diatomum Host II . Encysted Distomum. Host II Sexually Mature Host II 3 O Sexually Mature Host III 9 d Sexually Mature Host III Distomum Distomum 1 Distomum known as 'liver rot,' generally resulting in death. The history as described above shows why sheep pastured in moist places are subject to tho disease, and why wet seasons are times of epidemics. Thus in the rainy year of 1830 about one and a half mil- lions of sheep were killed in England ; in 1812, 300,000 in the neighborhood of Aries, France. This species is frequently accom- panied by D. lanceolatum, less than half an inch in length (fig. 233). Bilharziahcematobia is a human parasite, most common in hot climates, and especially so among the Fellahin of Egypt. The sexes are separate. The male, half an inch long, by inrolling of the ventral side (fig. 237) forms an incomplete canal (canalis gynsecophorus) in which the more slender female usually lies. These united worms occur in the portal vein and connected vessels. They follow these vessels in either direction and lay their eggs in the mucous membrane of the ureters and urinary bladder, as well as in liver and intes- tine. The suppurative sores of the urinary tract cause albuminuria or, by hemor- hage, haematuria. Diagnostic of the dis- ease is the presence of the eggs, each with a spine, in the urine. Several other species occur in man, among them D. carnosum* and Z>. westermanni* in Fio. 237. — Bilharzia hcematobia. Female in the gynaecophoral canal (c) of the male; s', s", anterior and posterior suckers. 278 PLATHELMINTHES. America. Other species occur encysted in man, two (D. ophthalmobius and Monostomum lentis) in the capsule of the lens and in the lens itself. The genus Amphistomum is common in the intestine of Ungulates, one species, A. hominis, occurring in man. With few exceptions the adult stages of all Distomes occur in vertebrates, the larval stages in molluscs. Aquatic birds are very apt to be infested with them, and " it may be of interest to gourmets to know that the trail of a woodcock largely consists of distomic Trematodes." Class III. Cestoda. The majority of the cestodes, and especially those of the human intestine, are distinguished from the similarly entoparasitic trema- todes in a striking manner. But the boundaries between the two groups disappear in certain forms like Archigetes, Caryophyllceus, and AmpUilina, parasitic in lower vertebrates or invertebrates and which are now assigned to the trematodes, now to the cestodes. The most important character of the cestodes is that as a result of their parasitic life they have lost the last traces of an alimentary canal, and are nourished by the juices or the partially digested food of the host, since the fluid nourishment is taken in through the skin into the body parenchyma. It is a disputed question whether the cuticula of the surface is penetrated with pores for this purpose. Two other characters are so striking that they are among the first thought of. (1) The differentiation of two developmental stages, the bladder worm, or cysticercus, living chiefly in paren- chymatous organs (muscles, liver, brain), and the sexually mature animal, living as a parasite in the alimentary tract; (2) the division of the body of the adult into different parts, the head or scolex, and following this a series of joints or proglottids. Since this last feature holds for all human tapeworms and hence for the best known species, the following description begins with these typical forms. The sexually mature tapeworm or strobila (fig. 238) consists of a single scolex in front, and behind this follow in a single row the proglottids. The number of these last varies from smaller forms (Tcenia ccliinococcus, fig. 252) with three or four to several hun- dreds or even several thousands, a fact which speaks for the enor- mous size of some species. The proglottids are derivatives of the scolex, from the hinder end of which they become separated by a kind of budding. This explains the well-known fact that the body is not rid of the tapeworm, so long as the head remains in the host. It also explains the peculiar shape of the worm, which is III. CESTODA. 279 almost thread-like in front, increasing posteriorly to a broad band, whence the common name. At first the proglottids are small; they increase by individual nourishment to considerable size, and FIG. 238. FIG. 239. FIG. 238.— Tcenia saginata. (From Boas, after Leuckart.) Head with series of pro- glottids taken from various regions of the strobila. FIG. 239. — Nervous system of Monezia. (After Tower.) a, suckers ; e, excretory tubes; p, cerebral ganglia. Nerves black. separate from the hinder end of the chain and live separately when a certain measure of development is reached. For example, the young proglottids of the human tapeworm, Tcenia solium, are 0.5 280 PLATHELMINTHES. mm. broad and 0.01 mm. long; the ripe proglottids at the end are elongate oval, 5 mm. broad and 12 mm. (half an inch) long. Head and proglottids have certain common characters. Their connective-tissue parenchyma contains numerous spherical con- cretions of lime, and consists of cortical and medullary sub- stance. The first contains to a marked degree the muscles, the latter the other organs. Nerves and water-vascular system extend through the whole length of the worm. In the head is the paired cerebral ganglion of the flatworms (fig. 239), sometimes fused to a single mass by the great development of the commissure or partially concealed by accessory parts connected with attachment (fig. 242). From the brain two principal nerves run backwards, usually near the edges of the proglottids (fig. 244, N). The water-vascu- lar (excretory) system begins with a capillary network richly provided with flame cells. It extends through head and proglottids ; usually four main trunks are present, two being less developed and it is possible are sometimes absent. The two chief trunks are fre- quently connected by a cross-trunk on the hinder margin of each proglottid (fig. 244). The system opens on the posterior edge of the last proglottid, but accessory mouths may occur on other proglottids. The scolex and proglottids are distinguished by the facts that the proglottids contain the sexual organs, while the scolex bears the anchoring apparatus, for the latter has, besides producing proglottids, to fasten the worm in the intestines. Most important of the adhesive organs are the suckers (acetabula) ; less important are the hooks, which, in numbers, are either arranged in a circle or are borne on protrusible and retractile probosces (fig. 240—242). When a circle of hooks is present it is on the anterior end and is moved by a special apparatus, the rostellum. This is a plug of complexly arranged muscles (fig. 242) which can arch and flatten the central area. In many species the arching is increased by a muscular sheath, the flattening by retractors. Each hook has its point outwards and its base with two roots, one of which rests on the rostellum; the protrusion of the rostellum forces the points outwards into the mucous membrane of the intestine. In some Tcenice without the circle of hooks (T. saginata) the rostellum is replaced by a sucker-like depression. Since the rostellum arises in development from a similar cup, it may be a modified apical sucker, but it is doubtful how far comparisons may be made with the oral sucker or the alimentary tract of the trematodes. The sexual organs are hermaphroditic and are present in num- bers equal to those of the proglottids, so that these were formerly III. CESTODA. 281 regarded as sexual individuals of a colony, each with its own reproductive apparatus. Two types must be recognized. In the one the presence of vitellaria and the separate openings of uterus and vagina recall the conditions in trematodes, while in the second FIG. 240. FIG. 242. FIG. 240.— Apical view of head of Tcenin solium. (From Hatschek.) FIG. 241.— Head of 'Jetmrhuuchus viridis. (After Wagner.) Dissected to snow the internal parts of the proboscides (<>) and the ganglion (a). FIG. 242.— Schema of action of rostellum. On the right the hooks are exserted for adhesion, on the left retracted, r, rostellum; s, sheath; /, longitudinal muscles. #2 od sd dg u ov FIG. 243.— Proglottis of Bothriocephalus latus. (After Sommer.) Right only vitel- larium, left only testes. shown, cb, cirrus sheath opening with the vagina; d<7, vitelline duct: dt, vitellarium: /i, testes; od, oviduct; ov, ovary; sd, shell gland;, tt, uterus; vci, vagina; vd, vas deferens (dark-lined); 10, excretory canal. the uterus ends blindly and the vitellaria are modified into a small albumen gland. Since vagina and vas deferens almost always open together, self -impregnation is possible. Besides cross-fertilization of separate proglottids has been seen. The general features of the two types may be made out from figures 243 and 244, reference 282 PL A THELMINTHES. Fia. 244.— Proglottid of Tceiiia sac/innta, near maturity. (After Sommer.) rfo, cirrus sheath; d£, vitellarium ; fc, genital pore; JV, nerve cord; Keph^ excretory canal ; ov, ovary; rs, receptaculum seminis; sdr, shell gland; £, testes; it, uterus; vd, vas deferens. FIG. 245.— Eggs of parasites from the human intestine, enlarged 400 diameters. (From Leuckart.) a, Ascaris lumbricoides; 6, c, Oxi/uris vermicularis ; d, Trichocephalus dispar; e, Dochmius duodenalis; f, Dislomum hepaticum; g, Dist. lanceolatum; h, Tcenia solium; i, T. saginata; k, Bothriocephalus latus. 777. CESTODA. 283 being made to the description of the organs in the trematodes (p. The difference in the sexual apparatus has its influence on the peculiarities of the egg (fig. 245). In BothriocepJialus it is large (&), has a tough shell with a lid, and encloses a small egg cell with numerous yolk cells. The eggs of Tcenia (h, i) are small, with a layer of albumen and a delicate shell which is lost early. It is replaced by an embryonic shell, a radially striped envelope secreted by the embryo in a somewhat advanced stage. It is in this condition that one usually sees Taenia eggs. A further consequence is a difference in development. In most Bothriocephalidse, as in the Trematoda, the egg must enter the water for its further development. Here a ciliated oval larva escapes which contains a six-hooked larva (oncosphcera, fig. 2 FIG. 246.— Development of Bothriocephalus latus (From Leuckart.) A, ciliated larva ; #, same with escaping six-hooked larva ; CY, young encysted Bothrio- Qephalus. The ciliated envelope is temporary and is cast off like the ciliated coat of the trematode larva. The six-hooked larva JD some unknown way enters a fish, becomes encysted (pleurocercoid) in muscles or viscera, and changes directly into the head of a Both- riocephalus. This on being taken, in feeding, into the intestine of the proper host develops into the adult. The longer and better known history of the Tmnias differs con- siderably. The distinctions are early recognizable, since the six- hooked larva lacks the ciliated envelope but is enclosed in its homologue, the embryonic shell already alluded to. Since this envelope cannot open of itself, the young must be freed from it by its digestion in the stomach of the proper intermediate host. Thus the eggs of Tcenia solium must pass into the stomach of the pig (they are taken by admixture of the food with embryos con- tained in faecal matter) and after being freed from their shell in 284 PL A THELMINTHES. the stomach the larvae with their six hooks bore through the intes- tinal wall and migrate, using the blood-vessels in their course, into the muscles, or more rarely other organs. Here they develop into bladder worms (cysticerci). In this they become oval and secrete a cyst to which, as a foreign body, the pig adds an envelope of connective tissue. The cysticercus blastema grows through Fio. 247.— Structure and development of the cysticercus (C. cellulosae of Tcenia solium). a, measly meat, natural size; below an escaped cysticercus; 6, cysticer- cus, with exserted scolex, enlarged; c-e, development of the scolex, more en- larged; c, young cysticercus with blastema of scolex (above) and water- vascular net ; d, e, different stages of scolex in receptaculum, the cysticercal wall mostly removed. increase of cells, but more by the infiltration of serous fluid, so that it becomes distended into a delicate translucent vesicle. So abundant can this be that in T. solium the microscopically small embryo can grow in three or four months to the size of a bean or pea; in other species as large as a hen's egg. By invagination the wall of the bladder produces the blastema of the scolex (fig. 247, c). This has at first a sac-like shape, but soon increases in length, its growth being confined by an envelope, the receptaculum (d), so that it is bent. At the apex of this blind sac arises the characteristic armature of the scolex which makes it possible to say what tapeworm will come from the cysticercus. Thus in T. solium there are four suckers and a crown of hooks. These parts are at first inverted III. CESTODA. 285 and only come to their definitive position on the outside of the scolex when the latter is protruded as one would turn out the finger of a glove. The further development follows when the cysticercus is taken into the stomach of the new host. When man, for instance, eats infected ('measly') pork, the cysticerci are freed by action of the digestive juices and later the scolex is everted. The embryo passes to the intestine, becomes attached and, surrounded by nourishment, begins to grow, the bladder remaining attached to the hinder end, and soon the formation of proglottids begins in the middle piece connecting the bladder with the scolex. So rapid is the growth that in ten or twelve weeks Tcenia solium begins to set proglottids free. In cases where the bladder reaches a considerable size it has the power •of producing more than a single scolex. The bladder of Coennms cerebralis, which lives in the brain of sheep, produces hundreds of scolices. The num- ber is even greater in Tcenia echinococcus, in which the bladder increases by budding for some time, and by the formation of numerous daughter bladders produces marked tumors in the liver of man and domestic animals, before the formation of scolices begins. In the interior of each daughter vesicle appear a number of brood vesicles, each of which produces numbers of scolices, so that from a single six-hooked embryo thousands of scolices can arise (fig. 253). This extreme case stands in contrast to others which connect with the development of Bothriocephalus, in which the cysticercus is replaced by a cysticercoid (fig. 248). Here there is no infiltration and the scolex is closely enclosed by an envelope comparable to the bladder wall. All of this is of importance in the correct conception of the development of a tapeworm, which was earlier believed to be a complicated alternation of generations; the bladder to be a stage which by endogenous budding produced scolices; the scolex, in turn, a stage which by terminal budding produced the sexual animals, the proglottids, and the tapeworm itself a •chain of individuals, a strobila. This view, so easy to learn, so easily ex- plaining the development, contains two errors. The bladder is not an inde- pendent generation, but only the precocious hinder end of the scolex. The tapeworm is not a colony, but a single animal; the proglottids are not in- dividuals, but specialized parts of a single whole. This view is confirmed by a comparison with other forms. The Caryophylla3ida3 (fig. 249) are single bodies, the anterior end elongate and taking the place of the scolex, while the broader hinder part contains a single hermaphroditic apparatus. In the Ligulida3 the body is still unjoiuted, but has increased in length and contains numerous sets of sexual organs. This duplication of the repro- ductive apparatus explains the appearance of proglottids. Family 1. CARYOPHYLL^ID^: (Cestodaria). Cestodes without ace- tabula, simple sexual apparatus, scolex and proglottis not differentiated. Distinguished from tremat9des by absence of digestive tract. Larval stages in invertebrates, adults nearly always in fishes. Caryophyllceus (fig. 249) 286 PL A TIIELMINTIIES. in the intestine of cyprinoids; AmpMlina in body cavity of sturgeon; Areliigetes in annelids (Samuris). Family 2. LIGULID.E. No acetabula; numerous sexua.l organs, but no proglottids. The immature stages in the body cavity of fishes, the adults, in the intestine of birds. Ligtila. FIG. 248 FIG. 350. FIG. FIG. 248 — Cysticercoid in invaginated and/extended condition from Arion ater* (From Hatschek.) FIG. 249.—Ca1ryophyllceusmutabilis. (After M. Schultze.) df, vasdeferens; dv, vitelline duct; fc, scolex; ov, ovaries; ps, penis; v.s, vagina with receptaculum seminis; f, testes; nt, uterus; vi\ yitellarium ; v.s, yesicula seminalis. The connexion of vagina with the crossing point of genital duct, vitelline duct, and uterus is lacking in the figure. FIG. 250. — Tapeworms of fishes. (After Linton.) A, Echinobothrium variabile *; B, Rynchobothrium bisulcatum * ; C\ Tetrabotkrium* Family 3. TETRARHYNCHID.E. With scolex and proglottids, the head with four protrusible hooked probosces (fig. 241). Immature and mature stages in fishes. Tetrarhynchus, Rynchobothrium.* Family 4. TETRAPHYLLID^E. Head with four very mobile suckers, often armed with hooks. Echinobothriiun * (fig. 250), Acanthobothrium.* III. CESTODA. 287 Family 5. BOTHRIOCEPHALID.E Scolex and proglottids present; head spatulate with two sucking groves on the narrower sides. Most interesting is Bothriocephalus latus (fig. 251), the largest tapeworm which occurs in the human intestine (also dogs and cats), and which may reach a length of forty feet and consist of over four thousand proglottids. As has been out- lined above, the pleurocercoid occurs in fishes, and man acquires the parasite by eating uncooked fish. It is especially abundant in Russia, the eastern provinces of Prussia, and in Switzerland. It is rare in America and occurs most frequently in immigrants. Other species occur in man in Greenland (R cordatus) and China (B. mansoni). FIG. 251.— Head and ripe proglottids of Bothriocephnlus Intus, the head showing the sucker at the angle, the proglottids the marking produced by the uterus. Family 6. T.ENIAD.E. With scolex and separable proglottids; thescolex always bears four suckers and in many a ro- stellum with a circle of hooks (fig. 252). In the proglottids the vitellarium is replaced by an albumen gland; the uterus is caecal, and the genital pore occurs usually laterally in the proglottids, alternating right and left, rarely only on one side (Hymenolepis, Anoplocepha- lus). It is rarely doubled in a proglottid (Dipylidium, Moniezia). Intermediate stage a cysticercus or cysticercoid. The human tape- worms are grouped here together, but are sub- divided accordingly as the sexual animal or the cysticercus has been found in man. A. Tcenice sexually mature in the human intestine. Most noticeable are Tcenia solium and T. saginata, the differences between which are shown in fig. 252 and the follow- ing table. It is to be noticed that, in spite of the lack of hooks, the stronger suckers render T. saginata more difficult to expel. Tcenia FIG. 252.— Head and ripe pro- solium, as the table shows, is not rare in the f ^ ?' ioiiunT™ S"ff"1" cysticercus stage in man and occurs sometimes in places, like the brain and eyes, where it causes severe injury. These cases are in part explained by lack of cleanliness in the food, which may contain eggs, but it is possible through internal infection ; pieces of the worm passing the pylorus and entering the stomach, where they are digested, setting the embryos free. 288 PL A THELMINTHES. *3 Length (n} of the worm Character Occurrence Head 2>£ Uterus and (h) of the of of ? S ripe proglottids Cysticercus Cysticercus § With rostel- lum and circle of hooks (26 in 2 rows); 4 weak 1 Each side with 7-9 large branched pouches a. 10 feet, h. 9-11 mm. long, 6-7 mm. broad 6-20 mm., with abundant fluid In pigs, occa- sionally in muscles, brain, and eyes of man, rarely in * suckers mammals Sterna saginata No rostellum ; no hooks ; 4 strong suckers i Each side with 20-30 delicate little branched pouches a. 20 to 25 feet and more, b. 18-20 mm. long, 5-7 mm. broad 4-8 mm., tough, with little fluid Cattle Many other Tcenice, which are common to other mammals, occur occa- sionally in the human intestine. In mice and rats occur T. (Hymenolepis} murina and T. diminuta (= leptocephala). The first (identical with T. nana) has recently been very abundant in human intestines in Italy. The worm, an inch or two long, may occur in thousands and cause severe in- jury. This species may develop without an intermediate host ; the eggs taken into the stomach pass the cysticercoid stage in its walls and thence to the intestine to become adult. T. diminuta (=flavopunctata), which has insects for its intermediate host, has been described from man. Other species occur in the tropics. B. Forms passing the Cysticercus stage in man. Besides the Cysticercus cellulosae of T. soliitm that of T. acanthotrias (possibly identical with T. solium) has been found in man. More frequent and of more im- portance to the physician is the Cysticercus of Tcenia echinococcus (fig. 253), which lives as an adult in the dog, and is easily overlooked on ac- count of its size. It is at most 5 mm. (£ inch) long and consists of a scolex and three or four pro- glottids. The scolex bears four suckers and hooks on the rostellura. 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, *n tne case °^ *ne ^ver' may weigh ten or even Right sexually mature; thirty pounds. This extraordinary size is ex- left a part of an echmo- • • « i_ .» .« j.- c j coccus with two brood plained by the formation of daughter bladders licesUleS and their 8C°" (echinococcus) described above. Echinococci are more common in cattle, sheep, and swine than in man. Common Tcenice of domestic animals are in the horse Anoplocephala plicata (4 to 30 inches), A. perfoliata (£ to 3 inches), A. mamillana (i to IV. NEMERTINI. 289 2 inches); in ruminants, Moniezia, expansa (usually 7 feet, sometimes 30 feet or more), often fatal, M. denticulata (1 to 5 feet), the most common tapeworm of cows; in dogs, Tcenia marginata (cysticercus in sheep and swine), T. serrata (cysticercus in rabbits), T. echinococcus (above), T. COB- nurus (cysticercus in brain of sheep, causing the disease called * stag- gers'), Dipylidium cucumerina (most common, larva in the dog-louse, Trichodectes); in the cat, Tcenia crassicollis (cysticercus in mice). Several species occur in domestic birds, one (Drepanidotcenia infundibuliformis), causing epidemics among chickens. 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 a specimen of 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 system- atic position is a problem. Frequently they are included in the Plat helm in thes, but the presence of an anus, of distinct vascular system, and the higher organization in other respects renders such a position doubtful. Like some flatworms they have a solid parenchyma bounded externally by a ciliated ectoderm rich in mucus cells, and inside this at least two muscular layers, which, when but two are pres- ent, are an outer circular and an inner longitudinal layer. They differ from all other Plathelminthes in having a complete ps pm FIG. 254. — Diagram of Nemertean (orig.). 5, brain; c, ciliated pit; d, dorsal nerve trunk ; d/, dorsal blood-vessel ; gc, gastric caeca ; ?, intestine ; Z, lateral nerve trunk; h\ lateral blood-vessel; p, proboscis retracted ; pm, proboscis muscles; pn, protonephridial tube ; po, its opening ; ps, cavity of proboscis sheath. 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. 254). Especially diagnostic is the proboscis, which lies dorsal to the alimentary tract and usually opens separate from the mouth. The 290 PL A THELMINTHES. f •nepA proboscis is a 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 uncom- mon in the proboscis wall, while in some forms (the older Enopla) the effective- ness of the organ is increased by the presence of a dart-like stylet at the tip (reserve stylets occur on either side, fig. 255), and at the base of the stylet is the opening of 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 excretory system consists of two tubes lying close beside the lateral blood- FIG. 255. 1 10. 256. FIG. 255.— Young Tetrastemma obscurum. (From Hatschek, after M. Schultze.) a, anus; cc, dorsal commissure; eg, cerebral ganglia; /, ciliated grooves; i, digestive tract: Iv, lateral, mt>, dorsal blood-vessel; nep/i, water-vascular tubes; nZ, lateral nerve; oc, eye spot; or, proboscis pore; r, proboscis; r,, glandular hinder portion of pro- boscis; rm, retractor of proboscis; sf, stylets: *, opening of excretory system. FIG. 256.— Pilidium larva. (From Lang, after Salensky.) eg, invaginations which later give rise to the nemertine skin; w, oral lobes; rad, archenteron; r?i, ring nerve; sp, apical plate; st, eesophagus; wTc, ciliated band. IV. NEMERTINL 291 vessels and connecting with branches terminating in flame cells, while they open separately to the exterior by one or several open- ings. The central nervous system (in some forms still in the ectoderm) consists of a supracesophageal brain of a paired 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 placed on the sides of the head. These, formerly regarded as respiratory, are now considered sense organs. Tactile organs and simple eyes are widely distrib- uted ; otocysts are very rare. As a rule the nemertines are dioecious, the gonads forming a row of lateral sacs, alternating with the intestinal blind sacs and open- ing 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 gelatinous helmet-shaped larva with right and left below a pair of lappets (fig. 256). The margins of lappets and helmet are ciliated, while at the top a bundle of longer cilia project from a thick- ened patch of ectoderm, the apical plate, which apparently func- tions as a central nervous organ. Inside is the simple caecal arc li- en teron, the mouth ( blast opore) opening between the lappets. By a complicated process of growth and infolding this mesenteron becomes enclosed in its own skin, produced from four inpushings (es)\ an anus is formed, and at the time of metamorphosis 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 in the parenchyma inside the muscles, mouth in front of brain; proboscis as a rule with stylets. Geonemertes* and some species of Tetrastemma* terrestrial. Amphiporus* (numerous eyes), Nectonemertes* Malacobdella,* leech-like with posterior sucker, parasitic in lamellibranchs. 292 PLATHELM1NTHES. Order IV. Heteronemertini . Body wall with several muscular layers, the nervous system in the muscles; mouth behind brain; proboscis unarmed. Linens,* Micrura* and Cerebratulus * (Meckelia) on our coast, with cerebral organs. Eupolia. Summary of Important Facts. 1. The PLATHELMINTHES are bilateral animals of flattened form whose nervous system consists of a supracesophageal ganglion and lateral nerve trunks; the excretory system of branched water- vascular tubes (protonephridia). 2. The TURBELLARIA are the most primitive; the Trematoda and Cestoda ha^e descended from them. 3. The Turbellaria are ciliated externally. They have no anus and no circulatory system. The digestive tract consists of cctoder- mal pharynx and entodermal stomach, the latter many-branched in the Polyclads, with three main branches in the Triclads, and rod-like in the Rhabdocceles. 4. Polyclads and Triclads are often united under the name Dendroccela. 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 DistomicB there occur heterogony and alternation of hosts. From the egg arises a sporocyst, always parasitic in mol- luscs, from the parthenogenetic eggs of .which develop cercarise which become encysted Distomia3 in the second host, sexual Di- stomiae in the third. 7. Best known of the Distoma are D. liepaticum and D. lanceolatum (rare in man, common in sheep) and D. hcematobium in the portal vein of man in warm climates. 8. The CESTODA are characterized by the entire absence of digestive tract, and usually by the existence of scolex and pro- glottids. 9. The scolex is the organ of attachment, and as such is pro- Tided 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 embyro which must pass into an intermediate host. This is accomplished either by taking the eggs in passively with the food, or the embryo must pass into the water, where it infects fishes. ROTIFERA. 293 12. The embryo, in the intermediate host, becomes encysted and changes directly to a scolex (plenrocercoid) or into a bladder worm (cysticercus) which produces internally one or more scolices. 13. The scolex is freed from its cyst when taken along with food into the stomach of the proper host, and then acquires the capacity of development into a tapeworm. 14. In man occur as cysticerci Tcenia echinococcus (adult in dog) and T. solium; as adults Tcenia solium (cysticercus in pigs), T. saginata (cysticercus in cattle), and Bothrioceplialus latus (pleurocercoid in fish). 15. The NEMERTINI are distinguished by a complete alimentary canal with anus, and a proboscis dorsal to the digestive tract. PHYLUM V. ROTIFERA (ROTATORIA). The aquatic wheel animalcules, or Rotatoria, 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 be f FIG. 257.— Diagram of rotifer. (After Delage et Herouard.) 7>, brain; /c, flame cell; gy, gastric gland; i, intestine; w, mastax; ov, ovary; pg^ pedal gland; pv, pulsat- ing vesicle of excretory system; g, stomach. retracted. The tail or < foot ' is often composed of rings which can be telescoped into each other and which by their superficial resem- blance to segmentation formerly led to the association of the roti- fers with the Arthropoda. The last tail ring often bears a pair of pincer-like stylets which together with adhesive glands enable the animal to adhere to objects. The head has the most delicate cuticle and is expanded in front to a trochal disc, an apparatus of varying appearance, which is surrounded by a ring of cilia of use in swimming and also in directing food to the ventral mouth. The 294 ROTIFER A. alimentary canal consists of oesophagus, mastax (chewing stomach), glandular stomach, and intestine; all except the mastax cilated. The mastax bears two chitinous jaws (trophi), which in life are in constant motion and comminute the food. The cerebral ganglion lies above the oesophagus, 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 cloacal in character. For a long time males were unknown until Dal- rymple discovered that these are much rarer and smaller ( dwarf Fio. 258.— Brachionus urceolaris. A, female with four eggs in various stages ; B, male ; C, 'flame' from protonephridia, greatly enlarged; b, urinary bladder; c, cloacal opening; d, gastric glands; 0, ganglion, with eye; ft, testis; fr, mastax; TO, stomach; o, ovary; p, penis; f, tentacle; 10, protonephridia. males/ and that they have a much simpler structure (fig. 258, #). Usually the alimentary tract is reduced to a solid cord in which the testes are imbedded. The Rotifers have two kinds of eggs, 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 subserve 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 adult animals can withstand a certain amount of desiccation; and they are often found in damp moss or in eave troughs in a sort of sleep from which they are awakened by water. CCELHELMINTHES. 295 In structure the Rotifiers are much like the trochophore type of embryo of annelids and molluscs to be described later. They must hence be regarded as extremely primitive forms, connected at once with the ancestors of these groups, and, as shown by nervous system and excre- tory organs, with the flatworms as well. Most species are cosmopolitan and inhabitants of fresh water. Many species in America. Near the Rotifera may be placed the fresh-water GASTROTRICHA (Ichthydium, Clmtonotus) and the marine ECHINODERID^, forms which are little understood. PHYLUM VI. CCELHELMINTHES. The Coelhelminthes are distinguished from all the forms which have gone before by the presence of a body cavity, separating the outer body wall from the intestine. This cavity is the coelom, but whether it be homologous in different groups, e.g. nematodes and annelids, is not settled. The body muscles are developed from the FIG. 259.— Section of Ascari* lumhricoides through the pharyngeal bulb; beside it a bit of the body wall more enlarged, c, cuticle ; d, dorsal line ; 7i, hypodermis ; m, longitudinal muscle; n, nucleus of muscle cell; p, muscle cell; s, lateral line; r, ventral line ; u\ excretory canal. outer (parietal) epithelial wall of the coelom and hence are < epi- thelial muscle cells ' (figs. 259, 260). The excretory organs con- nect the body cavity with the outer world and hence are nephridia (earlier called segm en tal organs, cf. fig. 69). 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 sexual apparatus is simple. The gonads (fig. 260, o) are 296 CCELHELMINTHES. specialized parts of the ccelomic epithelium and their products 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 FIG. 260.— Transverse section of Sagitta bipunctata and a bit of the body wall more enlarged. (After O. Hertwig.) c, coelonx; dd, entoderm; d/, splanchnic meso- thelium; e, epidermis; w, somatic mesoderm (muscles and epithelium); o, ovary. absent. Little in general can be said of the nervous system: details will be given in connexion with the separate classes. Class I. Chaetognathi. These marine forms, a half to two inches long, perfectly trans- parent, are well adapted to serve as an introduction to the ccelomate worms. They live at the surface of the sea, preying on other ani- mals, and from their shapes and rapid motions deserve the name Sagitta — arrow — given some forms. The animals swim by means of horizontal fins, one surrounding the tail and one or two pairs on the sides of the trunk (fig. 261). On either side of the mouth is a lobe bearing strong bristles used in seizing prey (Chaeto- gnathi, bristle-jaw). Internally the body is divided into three segments, head, trunk, and tail, by transverse septa which divide the coelom into corresponding parts. Each segment of the cce- lorn again is divided into right and left halves by a mesentery (fig. 260), supporting the straight intestine, running lengthwise through it. The intestine terminates at the anus at the end of the trunk segment. The nervous system is entirely ectodermal. In the head is a /. CH^TOGNATBI. 297 pair of fused cerebral ganglia (fig. 262), in the trunk segment a large ventral ganglion, and these are connected "m by long cesophageal commissures. Of interest, because characteristic of nematodes and many f annelids, are' the relations of the musculature, which consists of longitudinal fibres alone. The body cavity is lined with epithelium (fig. 260), which, so far as it abuts against the ali- mentary tract, is called splanchnic (or visceral) mesoderm; that on the side of the ccelom to- wards the ectoderm is the somatic mesoderm. " The muscles arise from the latter layer and are divided into four fields, right and left dorsal, right and left ventral. The sex cells also arise from the epithelium of the coalom, the eggs »r in the trunk segment, the sperm in the tail. The eggs are carried to the exterior by special ducts. The sperm-forming cells early lose their connexion with the epithelium, fall into , the ccelom, where they develop the spermato- zoa. These are carried out by canals which by their relations to the co3lom recall the ne- phridia of the annelids. 171 FIG. 261. FIG. 262. FIG. 261.— Sagitta hexaptera, ventral view. (After O. Hertwig.) a, anus; &, seminal vesicle; sc, cecophageal commissure; sfl, tail fin; s/, sperm; wo, female FIG. 262.— Head of Sagittn bipunctata, dorsal view. (After O. Hertwig.) an, nerve to ati, eye; g, brain; gh, bristles; ?-?i, nerves to ro, olfactory organs; sc, oesophageal commissure. The development of Sagitta is significant from two points of view. The archenteron (fig. 108) is divided by lateral folds into an unpaired middle portion and two paired lateral chambers ; the first is the defini- tive digestive tract, the latter the anlagen of the ccelomic diverticula. 298 CCELHELMINTHES. In other words the ccelom is an outgrowth from the archenteron, i.e. is an enterocoele. Second : The gonads are derived from a pair of cells in the primitive entoderm, which later are carried into the coelomic walls. Hence 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 of Chaetognathi are arranged in two or three genera, of which JSagitta, represented on our coasts by 8. elegans* is best known. Spadella. Class II. Nemathelminthes. Like the flat worms, 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. 259). 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 corresponding ringing or annulation of the body wall. To the Nemathelminthes 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 0.001 to 1.0 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 underlying hypodermis (fig. 259), a layer corresponding to epithelium and cutis, which in cross-section shows, median and lateral, four thickenings, the dorsal, ventral, and lateral lines. In the lateral lines run the excre- tory vessels, two longitudinal canals which are united near the head by a transverse vessel opening on the ventral surface by an unpaired porus excretorius 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, as in Chsetognaths. These muscles are parts of the somatic epithelium, a layer of vesicular cells which by their size (fig. 259) so encroach upon the ccelom that scarce space is left for the alimentary canal and reproductive organs. A splanchnic mesoderm is lacking. The alimentary canal begins with a terminal mouth and ends with the anus, which is ventral and in front of the end of the body. //. NEMATHELMINTHES : NEMATODA. 299 The mouth connects with the muscular oesophagus, which, for sucking purposes, 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. 263). The oesophagus is surrounded by a nervous ring which sends forward and back a FIG. 263. FIG. 264. FIG. 263.— Structure of young female Ascaris (based on a drawing by Leuckart). d, intestine; o, ovary; s, lateral line; v, ventral line: va, vagina. FIG. 264.— Diagram of nervous system of a nematode. (After Butscbli.) c, commis- sures; d, dorsal nerve; z, infraoesophageal, s, supraoesophageal part of nerve ring ; v, ventral nerve. large 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 (fig. 264). 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. 265), are in the end of the alimentary canal, which hence is a cloaca. In the female (fig. 263) there is a special genital opening on the ventral surface between mouth and anus, the position varying with the species. In general the struc- 300 CCELHELMINTHES. ture of the reproductive organs is alike in both sexes. In both, on account of the great fertility, these are long tubes coiled for- ward and back and ending in fine 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. 263, 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 the cloacal opening, appropriate muscles caus- ing them to retract. Besides there may be valves to right and left to clasp the male, or, as in Trichina, the whole cloaca is pro- trusible. 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 post- embryonic 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 an hermaphroditic entoparasitic to a free dioecious generation. The occasional suppression of the free gen- eration which occurs in many Anguillulids leads to the Strongylidse, where the offspring of the parasitic generation can live free for a time (rhabditis larvae), but must return to parasitism to undergo a metamorphosis and become sexually mature. The free life is shortened again in the Ascaridae, 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 con- dition passively by food. Family 1. ANGUILLULIDS ; small thread-like nematodes with double pharyngeal swelling which live in mud, organic fluids or plants, rarely in animals ; male with two spicula. Anguillula aceti, vinegar eel, 2 mm. long, in vinegar and stale paste. Rhabditis (Rhabdonema) nigrovenosar not 1 mm. long, lives in mud and stands in heterogony with a second form which lives in the lung of frogs. Strongyloides intestinalis of the tropics, but which has recently appeared in southern Europe, has a some- what similar history, the adult stage being reached in the human intes- tine. Here also belong numerous plant parasites of which Tylemhus tritici and Heterodera schachti demand notice, the first doing great dam- age to wheat, the second to turnips in Europe. Tylenchus devastatrix attacks rye and hyacinths. 77. NEMATHELMINTIIES: NEMATODA. 301 Family 2. ASCARHLE. Mouth surrounded by three lips, one dorsal, two ventral ; males with two spicules. Besides numerous species in lower vertebrates two of the most common parasites of man, the human round- worm and the pin worm, belong here. The former, Ascaris lumbricoides, * FIG. 265. — Dorsal, end, and ventral views of head and hinder end of male Ascaris lumbricoides. (From Hatschek.) inhabits the small intestine, often in enormous numbers. The females average about 5-6 inches, the males 4 inches, in length. The ani- mals are enormously prolific, a female containing about 60,000,000 eggs. The eggs (fig. 246, a) are easily recognized. Shortly after fertilization the eggs pass out of the intestine with the faeces, but develop without intermediate host if in the course of two or three months, when the embryo has developed, they are taken into the human intestine. The development of the pinworm, Oxyuris vermicular is,* is somewhat 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 infection. The white worm, not half an inch long, lives in the rectum, especially of children, and in crawling outside the anus causes intolerable itching. Ascaris mystax* occurs in dogs and cats (occasionally in man). A. megalo- cephala * (a favorite animal for cy tological researches) and Oxyuris equi in the horse. These do little harm. On the other hand Heterakis maculosa often destroys whole flocks of pigeons. Family 3. STRONGYLID.E. These are readily recognized by the bursa of the male, a broadening of the hinder end of the body by two ring-like FIG, 266.— Anterior end of AnTcylostomum duodenale, dorsal and side views, o, inner, h, outer ventral teeth ; c, dorsal tooth of m, mouth capsule ; d, stylet ; e, ventral ridge ; oe, oesophagus. processes, which contains two spicula. Frequent but not constant is a widened capsule surrounded by papillae at the mouth. A number of species of Strongyltts occur in domestic animals. Syngamus trachea- 302 CCELHELMINTHES. Us* half to three quarters of an inch in length, the male and female always in pairs, cause the disease known as 'gapes 'in fowl. Ankylo- stomum (Dochmius) duodenale*(ftg. 266), about two fifths of an inch in length, lives in the small intestine of man, causing severe loss of blood and the disease known as Egyptian chlorosis. The eggs develop in mud and moist earth, and hence people who 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, and now is common in Germany. Recently it has been thought that the Ankylostoma larvae obtain entrance to- man through the skin, as in bathing, etc. Family 4. TRICHOTRACHELID.E. These owe their common name of ' hair necks ' to the fact that the part of the body which contains the pharynx is hair-like and elongate, while the pharynx itself traverses a peculiar cord of cells. Long- est known of the family is Trichocepl talus dispar* of man (fig. 267), about an inch or an inch and a half in length, which lives with its neck burrowed like a cork- screw in the wall of the intestine near the caecum. Since it does not move, it causes little injury. Its presence can be recog- nized by the oval brown double-shelled eggs (fig. 246, d) in the faeces. A second species, Trichina spiralis * (figs. 268, 269), is much smaller, but much more dangerous. Two stages are to be distinguished, the encysted muscle Tri- china and the sexually mature intestinal Trichina. The first was discovered in a human body in 1835; the latter was not known until much later, its history being worked out by Leuckart, Virchow, and Zenker. In the encysted stage it occurs in the muscles of pigs, rats, mice, man, rabbits, guinea pigs, dogs, etc. (never in FIG. 267. FIG. 269. FIG. 268. intestinal wall. (From Leuck- birds), enclosed in an oval capsule about FiG.ai268. — Trichina spiralis, male. 0.4 to 0.6 mm. long and hence recogniz- tesTe™ HatSCbek)' ''*' cloaca: '' able by a practised observer with the FIG. 269.— Trichina in muscle. (From naked eye. They are more easily seen when they are partially calcified and have a whitish color. Certainty in their recognition demands a low power of the microscope. In the capsule is coiled up the worm, about 1 mm. long, which is not yet sexually mature, although furnished with the //. NEMATHELMINTHES : NEMATODA. anlagen of sexual organs. To attain this it must be transported into the intestine of another host, ^hen, for instance, man eats trichinosed pork the worms are freed from the muscle and capsules by the digestive fluids and, entering the small intestine, become sexually mature in a few days. The female (3-4 mm. long, the male 1.5 mm.) penetrates into the superficial layer of 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, on the other hand, penetrate the lymph vessels, and by way of the thoracic duct are carried into the blood-vessels, and wander from the capillaries into the muscles, especially those which are much worked, like the diaphragm, eye muscles, and muscles of the neck, and which conse- quently 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 Emmersleben, Saxony, in 1884, where 57 died in four weeks from infection from one pig. FIG. 270.— Transverse section of young Gordius. (After von Linstow.) a, hypoder. mis; b, muscular layer; c, cuticle; d, parenchyma; e,/, muscles and mesenteries; g, alimentary canal ; ft, nervous system. Family 5. FILARIID.E. These are extremely elongate, hair-like worms. Their best-known representative is Dracunculus medinensis, the guinea worm (the female about a yard long, and about as large as stout packing twine), which produces a sickness known to the Greeks as dracontiasis. It forms abscesses beneath the skin in which the worm is coiled up. The em- bryos break through the wall of the mother and must enter the water and penetrate a small crustacean, Cyclops. It is apparently introduced into the human system by swallowing the Crustacea with drinking water. The worm has recently been found in the tropics of America. A second species is Filaria sanguinis hominis, the adults of which — 3 to 6 inches long — live in the lymphatic glands of man, while the young escape into the blood, often in immense numbers. They often escape 304: C(ELIIELMTNTHES. through the kidneys, where they produce serious disturbance (albuminuria, haematuria). There is possibly a connexion between them and elephanti- asis. 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. MERMITHIDTE. Elongate nematodes with six oral papillae. They live in the body cavity of insects and pass into damp earth, where they become sexually mature. They share with the Gordiacea the common 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 epithelium ; the intestine is supported by mesenteries; there is an oesophageal nerve ring and unpaired ven- tral nerve cord, and the female genitalia enter the cloaca. The adults live in water, where they lay their eggs; the larvaa live in insects, there being in some cases at least an alterna- tion of hosts. These (and the Mermithida?) are popularly believed to be horse hairs changed into worms. Gordius,* Chordodes* Near the Gordiacea must be mentioned the marine Nectonema,* the young stages of which are apparently passed in the mosquito. Order III. Acanthocephala. The species of spine-headed worms live in the alimentary canal of vertebrates. In appearance they resemble the Ascaridae (p. 301), but are easily distinguished by the proboscis, which may be re- tracted by muscles and exserted by contraction of the muscular body wall. This proboscis bores into the intestinal wall and is held in place by numer- ous retrorse hooks (fig. 271). In internal anatomy the entire absence of an alimentary canal marks them off from Nematodes 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 lemnis.cS, 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 Ecliin- orhynchus (Gigantorhychus) gigas* of the pig lives in the larva of the ' .Tune bug' (Melolontlia), that of E. proteus of European fresh-water fishes in Crustacea. One species, E. hominis, is ex- tremely rare in man. .-<* FIG. 271.— Male Ectiinorhyn- ckus angustatus. (From Hatschek.) b, penis sac ; de, seminal vesicle; dr, glands; gr, ganglion; /Jem- nisei; lig, ligament; mim?, retractors of proboscis and its sheath ; p, penis ; r, proboscis ; rs, proboscis sheath; t, testes; vd, vas deferens. 777. ANNELIDA. 305 Class III. Annelida. The segmentation of the body, which was shown in a slight de- gree in the Chsetognathi, reaches its highest development in the Fio, 272.— Diagram of annelid somites (orig.). am, acicular muscles; c, coelom; cm, circular muscles ; ev, circular blood-vessels; d, dorsal blood-vessel; /, intestine ; Im, longitudinal muscles; rn, mesentery; ?i, nerve cord; no, nephridium; ne, nv% neuro- and notopodia, forming parapodium; s, septum; so, somatopleure ; sp, splanchnopleure ; £, typhlosole. Annelids, where it appears both in the outer ringing of the body and in the arrangement of the most important systems of organs — metameric arrangement of excretory organs, nervous system, blood- FIG. 273.— Trochophore (Loven's larva) of Polygordius. (From Hatschek.) A, anus; dLM, dorsal muscles; ED, hind gut: «/, stomach; J,, intestine; Mstr, mesodermal band; ?i, nerves; Neph, protonephridia; O, mouth; Oe, oesophagus; oeLM, oesopha- geal muscle; SP, apical plate; vLM, ventral muscle; vLN, lateral nerve; Wkr^ wkr, pre- and post-oral zones of cilia; WS, apical cilia; wz, adoral cilia. vessels — internal segmentation. To this is added an extraordinary increase in number of body segments (somites, metameres), which can far exceed a hundred. We can thus define the Annelids as 306 CCELHELMINTHES. worms with ccelom and with external and internal segmentation. In the development there frequently occurs a type of larva, the trochophore, which must be referred to here, since it is of great morphological significance, resembling in structure the rotifers and recalling the larva of the molluscs and to a less extent that of the echinoderms. It is (fig. 273) a gelatinous body traversed by an alimentary canal with fore-, mid-, and hind-gut regions. At first it is everywhere ciliated, but with advance of development the cilia become restricted to certain thickened tracts of epithelium, the ciliated bands. One of these, the preoral band, is especially constant. It runs circularly ( Wkr) around the body, surrounding a circular prestomial area, in the centre of which is the anlage of the cerebral ganglia, a thickened patch (apical plate) of ectoderm, often bearing a tuft of cilia. Other ciliated bands (post-oral, perianal) often occur. Of internal organs, besides numerous muscle fibres, the most noticeable are the excretory organs, true protonephridia, which open to the exterior either side below. The trochophore in some respects resembles the larvae of some Turbel- laria (fig. 231) and Nemertines (fig. 256), showing that the annelids are related to these groups. The above account applies most closely to the Chsetopoda and the closely related Archianellida. In other forms one or more features may be lacking — in the GephyraBa segmentation of the body; in the Hirudinei most of the coslom and the trochophore. Yet these are so closely related that they must be included under the common head; the missing characters have been lost during evolution. Sub Class I. Chcvtopoda. These, like the Nematoda, are cylindrical worms, but are at once distinguished by the segmenta- tion. Deep circular constrictions (fig. 274) bound the somites ex- ternally. Internally the coelom is divided by the septa — delicate double membranes which extend from the ectoderm to the alimen- tary canal — into as many cham- bers as there are metameres, while a longitudinal mesentery, also double, separates the coelomic FIG. 274.— Earthworm, side view and anterior end enlarged. (After Vogt and Jung.) 1, first segment with mouth and prostomium; 15, male sexual opening; 33-37, clitellum. ///. ANNELIDA: CH^ETOPODA. 307 pouches of the right side from those of the left (figs. 275 and 272). The alimentary canal also shows distinctions; for while it differs greatly in the various species, it has constantly a terminal anus, while the mouth is ventral and is overhung by the preoral segment, the prostomium. Nervous system, blood-vessels, and excretory organs are influ- enced by the segmentation. The nervous system is built on the ladder plan. It begins with a supraoesophageal ganglion (' brain ') lying in the prostomium, from which the cesophageal commissures pass around the oesophagus to form the ventral chain, which con- sists of as many pairs of ganglia, united by longitudinal commis- FIG. 275.— Anterior end of Nate elinguis. 7i, cerebrum, connected by commissure with ventral chain, n\ dy, contractile dorsal, vg, ventral blood-vessel; w, muscular layer of skin; clb, vb, dorsal and ventral chaetae; d, septa; 7f, prostomium; o, mouth. sures, as there are somites present. These ganglia of the ventral chain are closely similar, since the segmentation of the body is homonymous. There is but the slightest division of labor among the somites, and hence they differ but slightly among themselves. The prostomium always bears tactile organs and frequently eyes, which in many marine forms are highly developed, with lens, vitreous body, and retina. Otocysts are rare, but occur in diverse species. Ciliated pits (olfactory) occur on the head, goblet organs (taste) on head and trunk, and, lastly, lateral organs, sensory struc- tures of unknown function, may be metamerically arranged. The blood-vessels are most frequently represented by two main trunks which frequently (as in earthworms) contain blood colored red by haemoglobin. One trunk runs dorsal, the other ventral, to the intestine, the two being connected by vessels (figs. 272, 276) in each segment. The blood passes forward in the dorsal vessel, backwards in the ventral. It is propelled by contractile portions of the vessels; usually the dorsal vessel pulsates, but, as in the earth- worms, certain of the circular vessels in the anterior part of the 308 CCELHELMINTIIES. body may function as hearts (fig. 276, c). Rarely, as in the Capitellidse, circulatory organs may be lacking. The excretory organs (nephridia) were formerly known as * segmental organs/ since they occur in pairs in each segment. These supplant the embryonic protonephridia; each consists of an internal ciliated funnel, the nephrostome, a more or less convo- dg lg a oe St QC ds CO s o vd pt vg p FIG. 276.— Anterior end of Pontodrilus marionis. (After Perrier.) a, vascular arches; b, ventral nerve chain; c, 'hearts'; co, oasophageal commissure; dg, dorsal blood- vessel; ds, septa; gc, cerebrum; Z, retractors of pharynx; lg, lateral blood-vessel: o, ovary; oe, oesophagus; p, receptacula seminis; ph, pharynx; pt, ciliated funnels of vas deferens; 8, nephridia; vd, vas deferens. FIG. 277.— Schematic cross-section of an annelid. (After Lang.) etc, aciculum; b, chsetae; hrn, ventral nerve cord: dc, dorsal cirrus: dp, notopodium; /c, gill: In?, longitudinal muscles; rnd, digestive tract; ?ip, nephridium; oy, ovary; rm, circu- lar muscles; tm, transverse muscles; fr, nephrostome; uc, ventral cirrus; vd, uv, dorsal and ventral blood-vessels; vp< neuropodium. luted tube, and the external opening (fig. 69). In many instances (Oligochaetes, some Polychaetes) the nephrostome is in one somite, the external opening in the succeeding. The nephridia also usually serve as genital ducts, carrying away the reproductive cells, which HI. ANNELIDA: CH^TOPODA. 309 always arise from the ccelomic epithelium. In the Oligochaeta (fig. 286), besides the nephridia in the genital segments special oviducts and vasa deferentia occur which are usually regarded as modified nephridia. Of the many modifications of nephridia only a few can be noticed here. Occasionally there may be more than one pair in a somite, or they may have more than one nephrostome. Again, they may be lacking from more or fewer somites. In many Oligochsetes they may empty into the anterior or posterior ends of the digestive tract. In many (Glycera, Hesione, Nephthys, Goniadd) the internal ends of the nephridia are branched, the branches being closed by * solenocytes,' tubular cells bearing an internal bundle of cilia. In many marine annelids there occurs a metamorphosis in which pelagic larvae occur. These, in spite of their many modifications, are comparable with the < Loven's larva/ the trochophore already mes FIG. 278.— .4, larva of Polygordiu*; B, same changing to segmented worm. (After Hatscnek.) a, anus; Ten, excretory organ; mes, segmented mesoderm. described (p. 306). The differences largely consist of modifica- tions of the ciliary apparatus; the number of bands may be increased (polytroche 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. 278). In this 310 CCELIIELMINTHES. jointed portion the coelom arises as a new formation, divided from the first into separate chambers. The nephridia also arise de novo, independent of the protonephridial system, which is often called head kidney because the chief part of the trochophore forms the head of the adult. The fresh-water annelids develop directly, but the embryos pos- sess a reminiscence of an earlier larval life in that the head lobes are very apparent and contain protonephridia, which leads to the con- clusion that these animals earlier had a metamorphosis. From the resemblance of the trochophore to the Rotifera the farther conclu- sion 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 may reproduce asexually, this being rendered possible by the great homonymy of the segmentation. By rapid growth at the hinder end as well as at a more anterior budding zone numer- ous somites are formed which separate in groups from the parent to form young worms. In some cases the formation of new somites FIG. 279.— Budding in Myrianida. (After Milne-Edwards.) The sequence of letters shows the ages of the individuals. may take place more rapidly than the separation, the result being chains of worms (fig. 279). By a combination of sexual and asexual reproduction a typical alter- nation of generations occurs, the origin of which receives light from the following facts: In many polychaBtes which reproduce exclusively by the sexual process the sexless slowly-moving young (atoke) at sexual ma- turity becomes so altered in appearance as to have been described under another name. It becomes very active in its movements, and the hinder III. ANNELIDA: CH^ETOPODA. 311 somites, which contain the sexual organs, develop special bristles and para- podia (fig. 284, A). Thus many species of Nereis pass into the * Hetero- nereis ' stage. In other Polychaetes the sexual part (epitoke) separates from the sexless atoke portion and swims freely, while the atoke produces new epitokes. In the Samoaii Islands Eunice viridis reproduces in this way, the epitokes coming to the surface 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 AutolytidaB furnish the most complication. Here the atoke, by budding as in Myrianida, fig. 279) forms chains of dimorphic individuals which later separate. The individuals of male chains were formerly described as * Polybostriclms,'1 the females as ' Sacconereis."1 This same homonymy ex- plains the regenerative powers of many worms. Thus if certain earth- worms be cut in two, they will continue to live and will reproduce the lost parts. Another important character of the Chaetopoda is the posses- sion of bristles or chaetae. These arise in special follicles, singly or several in a bunch, of which usually there are four — right and left, dorsal or lateral and ventral — in each somite. Each follicle (fig. 280) is a sac of epithelium opening on the surface and having at the base a special cell for the development of each bristle. The developed bristles project from the follicle and, moved by appro- FIG. 280.— Arrangement of a bristle in an Oligochaete. (After Vejdowski.) e, epithe- lium; rm, iw, circular and longitudinal muscles; w, muscle of the follicle; b,, chaeta follicle, its chaeta in function ; ba, follicle for replacement, the formative cell at its base. priate muscles, form small levers of use in locomotion. Their numbers, shape, and support are of much systematic importance. Order I. Polychaetse. The Polychaetae owe their name to the fact that each group of "bristles contains many chaetae; but more important is that the 312 C(ELHELMINTHES. bristles of each side are supported by a fleshy outgrowth of the somite, the parapodium, in which two portions corresponding to the bunches of bristles — dorsal, notopodium; ventral, neuropodium — may be recognized (fig. 281). This is the first appearance of A K FIG. 281.— A, head with protruded phaiynx; B, parapodium of Nereis versipedata. (After Ehlers.) c, cirri; fc, jaws; I, head with eyes; p, palpi; *, tentacles. true appendages, but they differ from those of Arthropoda in that they are not jointed to the body nor jointed in themselves. On the dorsal surface may occur diverse outgrowths, known, accord- Fia. 282.— Ampkitrite ornata* (From Verrill.) ing 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. 281). Elytra are thin lamellse which cover the back like shingles and thus protect the body. ///. ANNELIDA: CH^ETOPODA. 313 Nearly all Polychsetes are dioecious and undergo a more or less pro- nounced metamorphosis ; with few exceptions (Manyunkia * from the Schuylkill, Nereis * in California) they are marine. They are usually divided according to their habits into fixed (Sedentaria) and free forms (Errantia), but this classification lacks a morphological basis. The Seden- taria feed on vegetable matter, usually form tubes of leathery organic substances, in which foreign matter may be incorporated or which may be calcified. The worms project their anterior segments from the tubes. The Errantia often secrete gelatinous tubes in which they can hide, but they never lose their powers of locomotion, and from time to time leave their retreats and swim about preying on other animals. Correlated with habits are differences in structure. In the Errautia 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. 281, A). In the Sedentaria there are no such pharyngeal teeth, but, on the other hand, there is a greater difference between anterior and posterior somites. In the latter the parapodia are weakly developed, and this part resembles the OligochaBtes in ap- pearance. The head and beginning of the trunk (thorax) are richly provided with gills and tentacles for respiration and the taking of food (fig. 282). Sub Order I. ERRANTIA. Predaceous annelids with strongly armed pharynx. The EUNICID^E, mostly represented on our shores by small species, contains some species a yard in length. Diopatra,* Nothria* The ALCIOPHXE are transparent pelagic forms with large, highly developed eyes. The SYLLIDJS usually have three long tentacles ; Autolytus* Myrianida* (p. 310). The POLYNOHXE * (Lepidonotus* Poly- projecting at the sides. noe* (fig. 283), Aphrodite aculeata,* the sea mouse, 6 inches long) are bottom forms with elytra covering the back. NEREIDS ; Nereis virens,* the clam worm of all northern seas. Sub Order II. SEDENTARIA (Tubicola, Cryptocephala). These forms cannot wander about at will, but live in their tubes. In the SABELLI- DJE the tube is membranous and there is a crown of gills ; Myxicola* Chone,* Manyurikia* In the SERPULHLE the tube is calcified and is closed by an operculum on one of the gills. Hydroides; * Spirorbis,* forming coiled tubes on seaweed ; Protula* The ARENICOLID2E,* which burrow in sand, have gills on the sides of the body. The MALDANID^E (Clymene* Axiothea*) have extremely long segments and build tubes of sand. The TEREBELLID^E (Terebella* Amphitrite (fig. 282), Thelepus*) have numerous filiform tentacles and branched gills on the anterior end. The AECHIANELLID^], which lack bristles and parapodia, must be placed near the Polychaetae and are usually regarded as very 314 CCELHELMINTHES. primitive forms which in structure and development (fig. 273) are of importance in the phylogenesis of the Annelids. Polygordius. * ABO FlG. 284.— New England Annelids. A, male Autolytvs ; B, Sternaspis fossor ; C, Cis- tenides gouldii ; Z>, Clymene torquata. (From Emerton and Verrill.) Order II. Oligochaetae. The Oligochaetes are almost as preeminently fresh-water and terrestrial forms as the Polychaetes are marine. They are in most respects simpler than their marine relatives, apparently the result of degeneration, which has followed from the more simple conditions of existence. Eyes are rudimentary or lacking, they have no palpi, cirri, or tentacles; gills are rare, but most striking is the absence of parapodia, the bristles projecting directly from the body wall (fig. 280). The chaetae may be regularly distributed around each somite (Pericliceta) or gathered on the sides (Megas- colex) or arranged in four bunches so that in the animal four longitudinal rows occur. The animals are hermaphroditic, testes and ovaries lying in different somites. Usually the integument in the neighborhood of the sexual openings is thickened by the ///. ANNELIDA: CH^TOPODA. 315 presence of numerous glands, forming a clitellum (fig. 274), which secretes the egg cocoons. The clitellum also functions in copula- tion, secreting bands \vhich hold the animals together so that the sperm from one passes into the receptaculum seminis of the other. After impregation the eggs are usually enclosed in cocoons. Sub Order I. LIMICOLA (Microdrili. ) Mostly fresh-water forms. The TUBIFICID.E, in consequence of the red blood, when present in large numbers color the bottom red. They quickly retract into the tubes which FIG. 285.— Aulophorus vagus, in tube of Pectinatella statoblasts. (After Leidy.) they form in the mud. Tubifex* Petoscolex ; Clitellio irroratus* common on our seashores. The NAIDID^E are transparent forms living on water plants which reproduce asexually throughout nearly the whole year. \r-vd cti FIG. 286.— Sexual organs of Lumbricus herculeus (after Vogt et Jung); the seminal vesicles of the right side cut away, hm, ventral nerve cord; foZ, bi\ lateral and ven- tral rows of setse; di, septa ; ftj, fts, testes, enclosed in sperm reservoir; o, ovaries ; ew, oviducts ; sbu, sperm reservoir ; sftj, 2, 3, sperm sacs (seminal vesicles) : *t „ seminal receptacles; t,, ts, seminal funnels connected with the vasa deferentia; to, funnels of oviducts; rd, vas def erens. Z>ero* and Aulophorus* have gills around the anus. ENCHYTILEIDJE ; Distichopus, Pachydrilus. The DISCODRILIDJE (Bdellodrilus, Nyzobdella) are parasitic. Sub Order II. TERRICOLA (Macrodrili). Here belong the terrestrial forms, the earthworms, our species of moderate size, in the tropics large 316 C(ELHELMINTHES. species (Megascolex australis four feet long). Our species belong to Lumbricus* Allobophora*; Perichceta* has been introduced from the tropics; Diplocardia * with double dorsal blood-vessel. Most species agree in habits; they burrow through the earth, swallowing the humus and casting the indigestible portions on the surface. They loosen the soil and are continually bringing the deeper parts to the surface, and thus do great good. Contrary to oft-repeated statements, earthworms occurred in our prairies and plains when first broken up by the plow. Details of the reproductive organs of one species are shown in fig. 286. These vary greatly and are largely used in classification. Sub Class II. Gephyrcea. The exclusively marine Gephyraea are distinguished at the first glance from the Chaetopoda by the entire absence of segmentation. The body is oval or spindle-shaped, circular in section. The mouth, at the ex- treme anterior end, is either surround- ed by a circle of tentacles (fig. 287} and is retracted together with the anterior end of the body by internal retractor muscles, or is overhung by a FIG. 287. FIG. 288. FIG. 287.— Anatomy of Phascalosoma gouldi (orig.). a, anus; a, anterior retractors d, digestive tract; g, gonads; m, mouth; n, nephridia; TIC, ventral nerve cord pr, posterior retractors. FIG. 288.— Larva (trochophore) of JSchiurus. (After Hatschek.) a, anus; d, intestine hw, postoral cilia; fcra, protonephridia; m, mouth; mes, mesoderm bands with indi- cation of segments; n, ventral nerve cord; sc, oesophageal commissure; sp, apical plate; vto, preoral ciliated band. ///. ANNELIDA: GEPHYR^A. 317 dorsal spatulate preoral lobe or * proboscis ' which may be several times the length of the body and forked at its tip (fig. 289). Internal segmentation is also lost, septa being entirely lacking. The nephridia are also reduced in number, at most but three pairs being present, and in some but a single unpaired organ. They are sexual ducts, and in the Chaetiferi there are special excretory organs (fig. 289, g] covered with branching canals opening to the body cavity by nephrostomes and emptying into the intestine. These resemble somewhat the branchial trees of the holothurians (infra), and hence these animals were formerly supposed to bridge the gap between holothurians and annelids, whence the name (yetyvpa, bridge) Gephyrsea. The vascular and nervous systems are more like those of other annelids. The vascular system consists of a dorsal and usually a ventral longitudinal trunk; the nervous system of a brain, oesophageal collar, and ventral cord, the latter without division into ganglia. The relations of the Gephyrsea to the Chaetopoda are shown by the development. In some (Chaetiferi) there is a trochophore (fig. 288) from which the worm arises, as in the Chaetopoda, by growth at the hinder end; this at first has a segmented coalom 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 setae; a trochophore in development. Ecliiurus pallasii* in our northern waters, Thalassema* farther south. In Bonellia there is a marked sexual dimorphism (fig. 289). The female is 2 to 3 inches long and has a proboscis 8 to 12 inches long. The male is only 1 mm. long, totally different in appearance, and lives parasitically in the oesophagus of the female (fig. 289, B). Order II. Inermes (Achaeta, Sipunculoidea). Distinguished by lack of chsetse, the mouth surrounded by tentacles, and the dorsal and anterior position of the anus. The larva is a modified trochophore without preoral ciliated band and without segmentation; only two, sometimes but one, nephridia. Phascalosoma* common on our shores. Phascolion strombi * builds tubes in deserted snail shells. Sipun- culus.* Order III. Priapuloidea. No tentacles, mouth with chitinous teeth, terminal anus, no nephridia; two protonephridia united with the sexual organs and opening either side of the vent. Development unknown. P?*iapulus, Halicryptus. 318 C(ELHELMINTHES. Sub Class III. Hirudinei (Discophori). Three points of external structure clearly distinguish the leeches from the chsetopods. First, the absence of bristles (except in Acanthobdella) and the presence of two suckers, one of which occurs on the posterior ventral surface and is used only for attachment and locomotion, the other, sometimes scarcely differentiated, A t FIG. 289.— Bonellia viridis. A, female (after Huxley); J?, male (after Spengel). c, cloaca; if, rudimentary intestine; 0, excretory organ; i, intestine; w, muscles sup- porting intestine; s, balls of spermatozoa in B, in A, proboscis (preoral lobe); w, single segmental organ, functioning as oviduct; vd, nephridium with ciliated funnel serving as vas deferens. surrounds the mouth and is used in sucking the food. In locomo- tion 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 well 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 primitive segment rings being divided by secondary constructions, there being three, five, or even eleven rings to a segment. The middle or one of the anterior rings is often distinguished by bearing strongly developed sense organs. As in the earthworms, certain of the somites at the time of reproduction may develop into a clitellum which secretes the egg cocoons. ///. ANNELIDA: HIEUDINEL 319 A third character is the marked flattening of the body in the dorso ventral direction (except in Ichthyobdellidae and a few other forms), the animals thus recalling the flatworms. This may be the result of the very slight development of the coelom. In most leeches there is a body (parenchyma, traversed by longi- tudinal,transverse and dorsoventral muscles in which the organs are immediately imbedded (fig. 290). The alimentary tract is provided with paired diverticula (fig. 291), varying in number in different species. Between the last and largest pair of these sacs is the intestine, which opens dorsal to the pos- terior sucker. The jawed and jawless leeches show considerable differences in the pharyngeal region. FIG. 290.— Transverse section of Hirudo medicinalis. (From Lang.) dm, Zm, rm, dorso- ventral, longitudinal, and circular muscles; vi, vd, w, lateral, dorsal, and ventral blood-vessels, the latter surrounding the ventral nerve cord, ni; h, testes; vd, vas de^erens; md, midgut; np, nephridial tubule; enp, urinary bladder. FIG. 291. — Digestive tract of Hirudo medicinalis. (From Lang.) a, oesophagus; 6, in- testine; d,, da, gastric diverticula. In the first there are three jaws in the phaynx, semicircular chitin- ous plates, the free edge of each armed with numerous calcified teeth (fig. 292). To these are attached two muscles, one to retract them, when not in use, into pockets, while the other exserts them and rotates them, causing a triradiate wound from which the blood flows. This bleeding is difficult to staunch, since glands on the lips and between the jaws produce a secretion which hinders the coagulation of the blood. In the jawless leeches a sharp conical process arising from the pharynx can be protruded from the mouth, and serves for wounding and sucking. The vascular system usually contains red blood, and consists, in the Gnatho- bdellidae, of four longitudinal trunks, a dorsal, two lateral, and a 320 C03LHELMINTHES. ventral, the latter surrounding the ventral nerve cord. These are connected by a complicated system of capillaries. The nervous system consists of brain and ventral cord, the lat- ter containing frequently twenty-three ganglia (the first of five fused, the last of seven). Nerves from the brain go to the eyes. Eight and left of the ventral cord are the hermaphroditic sexual organs. In Hirudo medicinalis (fig. 293) there are nine pairs of FIG. 292. FIG FIG. 292.— Hirudo medicinalis^ medicinal leech. (After Leuckart.) a, anterior end with three jaws (fc); 6, a single jaw with its muscles. FIG. 293.— Nervous system, blood-vessels, sexual organs, and nephridia of a leech, ventral view, ft, testes; hb, urinary bladder; ly, lateral blood-vessel; n, ventral nerve cord; n/i, epididymis; ov, ovary; p, penis; sc, iiephridia; it, uterus and vagina; vd, vas deferens; vg, ventral blood-vessel. (7i), the ducts of which unite to form a vas deferens on either side (vd). These pass forward, form by coiling a so-called epididymis (nh) and empty into the median unpaired penis (p). In the space between the epididymis and the first pair of testes are the ovaries (ov) and oviducts and an unpaired vagina (u). The nephridia (17 pairs in this species) are complicated and are pro- vided with bladder-like expansions. That the Hirudinei are true annelids and not segmented Plathelminthes is based upon the view that their coelom is reduced by ingrowth of paren- chyma and altered to canals connected with the vascular system. At any rate the ventral and lateral vessels are to be regarded as remnants of a coelom. In Clepsine there are the dorsal and ventral blood- vessels of the Chaetopoda and besides four longitudinal coelomic sinuses connected by transverse anastomoses. The dorsal sinus encloses the dorsal blood- vessel, the ventral many of the viscera, among them the ventral nerve cord. This is also to be regarded as ccelomic, since the nephrostomes con- nect with it. In most Hirudinei a canal system filled with blood has arisen from the coelom and blood-vessel, and in Neplielis is in part lacunar in character. Further relations are shown by Acanthobdella peledina, parasitic on fishes. This has both blood-vessels of the Oligochsetes, a IV. POLTZOA. 321 body cavity divided by septa and chsetsd. On the other hand it is leech- like in other features; two suckers and sexual apparatus on the Hiru- dinean pattern. Order I. Gnathobdellidae. The jawed leeches include Hirudo medicinalis, once extensively used for blood-letting but now little employed. Hcemadipsa includes land leeches, one of the terrors of travelers in the tropics. In Nephelis * the jaws are soft. Macrobdella * includes our largest native species. Order II. Rhynchobdellidae. Without jaws. The CLEPSINID^E mostly feed on snails and fishes. Clepsine * in our waters. Hcementaria officinalis of Mexico is used for blood-letting ; H. ghiliani of South America is poisonous. The ICHTHYO- BDELLID^E, cylindrical, occur in salt and fresh water, parasitic on fishes. Ichthyobdella,* Pontobdella* marine ; Piseicola, fresh water. Class IV. Polyzoa (Bryozoa). In external appearance the Polyzoa closely resemble the hydroids, so that the inexperienced have difficulty in distinguishing them. Like them by budding they form colonies which are either gelatinous or calcareous 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 charac- ters 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 nerv- ous system lies between mouth and anus, and the single pair of nephridia empty by a common opening. Beyond this it is diffi- cult to go, since the two groups of Polyzoa — Endoprocta and Ecto- procta — differ so widely that one may doubt whether they belong together. The Entoprocta have no coelom, and resemble in this respect the Rotifera; the Ectoprocta are true Coelhelminthes and by way of Phoronis show resemblances to the Sipunculoida and so to the Annelida. Sub Class I. Entoprocta. The single individuals of the Entoprocta (fig. 294) are shaped like a wine-glass and are placed on stalks which rise from (usually) creeping stolons. The 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 reproductive organs 322 C(ELHELMINTHES. open. The space between the horseshoe- shaped intestine and the body surface is completely filled by a pa- renchyma containing muscle cells, and correspondingly the excretory organs are proton ephridia. In our fresh-water Urna- tilla gracilis * these organs are branched and begin with flame cells. Pedicellina * and Loxosoma, marine. Sub Class II. Ectoprocta. In the Ectoprocta there is a spacious, often ciliated, coelom between the alimen- tary canal and skin, so that these are separated and have a certain amount of independence (fig. 295). On this account FlO. 294. — Loxosoma Rinqulare. -, . ,. j -i -i , -i n (After Nitsche.) single in- nas arisen a peculiar method (wholly dividual in optical section. • :j .c -ri 11 • 11 \ £ i i-" A, rectum; Ga, ganglion; incleiensiDle morphologically) oi treating 8tomaecshlne;r'tentacles;^them as two individuals, polypid, the in- testine and tentacles; cystid the rest, especially the body wall and skeleton. FIG. 295,— Flustra membranacea (after Nitsche), a single animal, a, anus; etc, ectocyst: en, entocyst; /, funiculus; 0, ganglion; fc, collar which permits complete retrac- tion; m, stomach, also dermal muscular sac; o, oesophagus. A^ avicularium; //' vibracularium of Bugula. (After Claparede.) The cystid is cup-shaped or saccular. It consists of an endo- cyst — the body wall — and an ectocyst — a cuticular skeleton, usually strongly calcified, secreted by the ectoderm. The surface of the IV. POLTZOA: ECTOPROCTA. 323 entocyst is always covered by the ectocyst only on the basis and side walls; the peripheral end remains soft and forms a sort of collar into which the tentacles and adjacent parts of the cystid can be retracted. In the ectocyst there is, as will be seen, a larger or smaller opening which 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 strongly bent alimentary canal extends into the cystid and is bound at its hinder end by a cord, the funiculus, to the base of the cystid. Ganglion and nephridia lie between the mouth and anus. The gonads arise from the epithelium of the coelom, the testes usually on the funiculus, the ovaries on the wall of the cystid. Hundreds and thousands of individuals form colonies (fig. 297) in which cystid abuts against cystid. The coelom of adjacent cystids may be distinct or a wide communication may exist. The colonies grow by budding; in the Gymnolaemata a part of a cystid becomes cut off as a daughter cystid in which the polypid — alimentary tract and tentacles — arises by new formation ; or (Phylactolaemata) 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 individuals may occur, ovicells, vibracularia, and avic- ularia. 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. 295, B) are long tactile threads; the avicularia (fig. 295, A) are grasping structures of uncertain function. They have been seen to seize small animals and hold them until decay set in. It is possible that 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 forma- tion. Besides in the depopulated cystids there may appear statoblasts, lens-shaped many-celled internal buds enveloped in a firm envelope which form a resting stage for the preservation and distribution of the species. Each statoblast is surrounded by a girdle of chambers which by drying become filled with air, causing the statoblast to float when it again comes, into water. From the statoblast a smaller polyzoon escapes which de- velops a new colony. The statoblasts are adaptations to the conditions of fresh-water life and occur only in the Phylactolaemata. Order I. Gymnolaemata (Stelmatapoda). The tentacles in a ring around the mouth. The numerous species are almost exclusively marine and are abundant on every coast. In the 324 CCELHSLMINTHES. •CHILOSTOMATA the cystids can be closed by an operculum : Gemmel- laria,* Cellularia* Buyula* Flustra* (fig. 295), Eschara* The €YCLOSTOMATA have tubular cystids without an operculum. Crisia* -TT FIG. 396.— American gymnolsematous Polyzoa. (After Busk, Hincks, Norman, and Verrill.) A, Tnbulipora flabellaris, young; B, Flustrella hispida ; C, Eucratea chelata ; D, Gemellaria loricata ; E, Kinetoskias tniitti; F, Membranipora spini- fera ; (?, Porella loevis ; H, Lepralia americana : /, Cribillina puiicturata. Ttibulipora,* Hornera* In the CTENOSTOMATA the cystid is more cal- careous and the opening is closed by a folded membrane. Alcyonidium,* Vesicularia, Valkeria.* Paludicella* (fresh- water). Order II. Phylactolaemata (Lophopoda). Tentacles borne on «, horseshoe-shaped lopliophore extending on either Fio. 297.— Small colony of Lnphopus crystallfnus (after Kraepelin), with young and old, some extended, others more or less retracted, o, statoblasts. side of the mouth, the tentacles on its margins. All are fresh-water species. Pectinatella,* Lopliopus (fig. 227), Plumatella* V. PHORONIDEA. VI. BRACHIOPODA. 325 Class V. Phoronidea. The single genus Phoronis* occurs on our eastern shores. The animal was first placed among the Chaetopoda 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 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 time of metamorphosis this larva becomes doubled on itself by a complicated process, so that the alimentary canal is U-shaped and the anus is 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 dis- tinct class allied to the former group. The body has a greatly shortened long axis (fig. 298) and in consequence a transversely oval visceral sac. In most a stalk (st) for FIG. 298.— Anatomy of Rhynchonella psittacea. (After Hancock.) a', left, «', right arm; a, opening into the cavity of the arm: d, intestine; e, blind end of the intes- tine ; 0, stomach with liver; m, adductors and divaricators of shell; o, oesophagus; p', p2, dorsal and ventral mantle lobes; sf, stalk; 1, 2, first and second septum, on the second a nephrostome. attachment arises from the posterior end. From the anterior side two folds, the mantle lobes, extend forwards (/?), one ventral, the 326 C(ELHELMINTHES. other dorsal, their free edges bearing bristles. Each mantle secretes a shell largely composed of carbonate and phosphate of lime. In a few the dorsal and ventral shells are similar, but usually the ventral valve (in Crania attached directly without the FIG. 299.— Waldheimia flavescens. (From Zittel.) Shell with arms and muscles, a, arm with fringed border (/i); c, c', divaricators; d, adductors; D, hinge process (the vertical line shows position of hinge). intervention of a stalk) is more strongly arched and has an opening at the posterior end for the passage of the stalk (figs. 299, 300). The flatter dorsal valve frequently bears a characteristic feature in the skeleton of the arms (fig. 300) which, when present, has greatly FIG. 300. — Waldheimia flavescens. (From Zittel.) A, dorsal, 5, ventral valve; a, b, c, impressions of muscular insertions; a, adductors; h", adjusters (stalk muscles) ; r, c', divaricators; s, hinge groove of upper valve in which the tooth (t) of the lower valve passes ; Z, support of arms; d, deltidium; /, foramen for stalk. different expression. Its basis consists of two calcareous rods which, bilaterally symmetrical, project downwards from the dorsal valve. These may be connected by a curved transverse band, and from their ends a spiral process may extend on either side. This apparatus supports the spiral arms. When closed the valves com- pletely enclose the body. When they open the gape is anterior, VI. BEACHIOPODA. 327 the posterior parts remaining in contact. At this part, except in the Ecardines, a hinge is developed just in front of the posterior margin, consisting of projections (teeth) in the ventral valve which fit into corresponding grooves in the dorsal. Opening and closing the valves are, contrary to what occurs in Lamellibranchs, active processes, accomplished by appropriate divaricator and adductor muscles (fig. 299). These produce scars on the shell, important in the study of fossil forms. 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 the spiral axis runs a longitudinal groove which reaches to the tip of the arms and is bounded by a row of small tentacles. By means of cilia on tentacles and groove food is brought to the mouth. These arms strongly resemble the lopho- phore of a phylactolaemate Polyzoan, 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 body cavity which extends into both, mantle folds. It encloses alimentary tract, gonads, and liver, and is divided into right and left halves by a dorsal mesentery support- ing the intestine. Each half in turn is divided by incomplete septa into anterior, middle, and posterior divisions recalling those of Sagitta (p. 296). If the arrangement of the septa is not so clear as in that form, it is to be explained by the shortening of the long axis and the twisting of the alimentary tract. This latter consists of oesophagus, 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 outwards through the nephridia, which begin in one coelomic pouch with a wide nophrostome, 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 degen- erate. The nervous system consists of an cesophageal ring with weak dorsal ganglion, which sends nerves into the arms, and a stronger ventral mass representing the ventral chain. The heart lies dorsal to the stomach. In development the brachiopods recall both Sagitta and the Annelida. They resemble Sagitta in that in Argiope the coelom arises by out- growths from the archenteron, divided by septa into three pairs of pouches. They are annelid-like in the form of larva and in the presence of chastae which are formed in separate follicles. In an earlier period of the earth brachiopods were so numerous in species and individuals that they are among the most important fossils in the determination of geologic horizons. 328 C(ELUELMINTHES. Now there are but few species, some inhabitants of the greatest depths of the sea. FIG. 301.— Development of brachiopod. (After Kowalevsky.) A, gastrula with early enterocrelic pouches; B, closure of blastopore; C, coelom separated, body annu- lated; D, cephalic disc and mantle developing, the latter with long setae; E, at- tached embryo, the mantle lobes folded over cephalic disc (setae omitted), c, cephalic disc; d, dorsal lobe of mantle; e, enterocoele; rn, mantle; v, ventral man- tle lobe. Order I. Ecar dines. Hinge absent; valves similar when the stalk passes out between them (Lingula *), or unequal when the ventral is perfo- rated by the stalk (Discina) or when the animal is directly attached by the shell (Crania}. Order II. Testicardines. Hinge present, valves unequal, the ventral perforated by the stalk; anus degenerate. Rhyn- chonella,* Terebratulina* in onr colder waters. Qpirtfer, Orthis, Pentamerus, Atrypa, important fossil genera. FIG. wrebratuiia sep tentrionalig* Summary of Important Facts. (1) The CCELHELMINTHES are characterized by a well-developed body cavity (coelom). (2) The CH.ETOGNATHI are hermaphroditic worms with three pairs of ccelemic pouches, with fins, and bristle-like jaws. (3) The NEMATODA are mostly dioecious, usually parasitic elongate worms, with cylindrical unsegmented body, with nerve ring (no ganglia), paired excretory organs, and tubular gonads. (4) The most important species parasitic in man are Ascaris lumbri- coides in the small intestine, Oxyuris vermicularis in the large intestine, the blood-sucking Ankylostoma duodenalis, and the notorious Trichina spiralis. In hot climates occur Filaria sanguinis Jwminis and Dracun- culns medinensis. (5) The GORDIACEA have mesenteries and splanchnic mesoderm; they are parasitic in insects. (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. ECHINODERMA. 329 (7) The CH^TOPOD ANNELIDS have segmented bodies, the segmentation showing itself in ringing of the body wall and in the separation of the coeloem into a series of pouches by transverse septa and the metameric arrangements of blood-vessels, ganglia, and excretory organs. (8) The CH^ETOPODA are distinguished from other annelids by the chaetaa (usually four bunches in a somite) arising in special follicles. The chaetae are few in the hermaphroditic Oligochaetae, numerous and borne on special parapodia in the Polychaetae. (9) The GEPHYR^A are closely related to the Chaetopoda. They are saccular, with a crown of tentacles or well-developed preoral lobe. They have largely or entirely lost the segmentation. Evidence of segmentation appears in some cases in development and in the ventral nerve cord and nephridia. (10) The HIRUDINEI are hermaphroditic Annelida which lack chaetae but have sucking discs. Their flattened bodies, rudimentary ccelom, and rich body parenchyma give them a certain similarity to the Plathelmin- thes. (11) The Hirudinei have either a protrusible pharynx (Rhynchobdella) or three toothed jaws (Gnathobdella). To the latter belongs the medici- nal 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 alimentary canal, the large coelom, 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) cham- bers, 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 Echinoderma are separated from most other animals by their radial symmetry, but recall in this respect the Ccelenterata, a fact which led to their inclusion by Cuvier in the group ' Radiata/ a view of their relationships which was set aside by Leuckart on account of their different structure, especially the presence of a coelom. In fact the radial symmetry of the echino- clerms has a different value, for while in the Coelenterata the number four or six (apparently derived from four) is fundamental, Echinoderma are, with few exceptions, five-radiate. Further, the radial symmetry of the Coelenterata is primitive, that of the Echinoderma, as development shows, is derived from the bilateral 330 ECHINODERMA type. In other words, the echinoderms have descended from bilateral, possibly worm-like, ancestors. The structure of the integument gives these animals a charac- teristic appearance. In the mesoderm under the epithelium calcareous plates arise, forming a body armor or test, and since these are usually pro- duced into spines, they have given the name Echinoderma, spine skin, to the group. This mesodermal skeleton at times becomes degenerate, as in the Holothurians (it rarely entirely disappears as in Pelagothurid), but even then shows itself as spicules and ' wheels ' of lime. The sphaeridia and pedicellaria (fig. 303) — not always present — Flia'ria!°3cioseddiand are characteristic appendages of the integument. °Pen 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. 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 arms are five basalia, inter- radial 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. 304). This begins usually externally and then ordinarily by a calcareous plate, the madreporite, which is perforated with fine pores and serves for the entrance of sea water. The water passes into a canal which, on account of its calcified walls in the starfish (fig. 305), is called the stone canal and leads FIG. 304. FIG. 305 FIG. 304.— Water- vascular system of starfish (orig.). a, ampullae ; a/>, ambulacra ; c, radial canal; rn, madreporite; n, radial nerve; p, Polian vesicle; r, ring canal, beneath it the nerve ring; s, stone canal; f, racemose vesicle. FIG. 305 —Transverse section of stone canal of Astropecten aurantiacus. (After Ludwig.) ECHINODERMA. 331 orally to a ring canal around the mouth. The ring canal bears usually several (up to five pairs) Polian vesicles, which, with Tiedemann's vesicles of the starfishes, are now regarded as appen- dages which, like lymph glands, produce the 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 echinoderms. An ambulacrum is a muscular sac which can be distended and lengthened by forcing in fluid from the ambulacral vessels, on the other hand can be retracted and shortened by its muscles. The ampulla is a sac connected with the ambulacrum and projecting into the body cavity. In locomo- tion the animal extends its ambulacra, anchors them by the suck- ing disc at the tips, and then pulls the body along by contraction of the ambulacral muscle. In the sessile crinoids and the ophiuroids (which move by their snake-like arms) the ambulacra are not locomotor but tactile in function, lacking suckers and ampullae. So among the holothurians and sea urchins the ambulacra are in many places replaced by tentacles. Frequently each radial canal ends in an unpaired tentacle with olfactory functions. The arrangement of the ambulacral system conditions the arrangement of other organs. Alongside the stone canal is a saccular organ formerly called the ( heart/ but now regarded as a lymphoid gland (ovoid gland, paraxon gland). Ring and radial canals are accompanied by corresponding blood canals, with which are often associated two vessels to the alimentary canal. There is a similar nerve ring and radial nerve, frequently in the ectoderm, to which may be added an enteroccelic or apical nervous system, possibly of peritoneal origin. The courses of the radial vessels and nerves mark out five chief lines in the animal, the radii; between them come the secondary radii or interradii. 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. The gonads are supported in the spacious ccelom by special bands, while mesenteries support the alimentary tract and its derivatives. Respiratory organs are represented by very various structures: branchiae, or thin-walled outpushings of the coelom, either around the mouth, as in Echinoidea, or on the aboral surface, as in the Asteroidea, the bursae of the Ophiuroidea, the branchial trees of the Holothuroidea and the various parts of the ambulacral system. 332 ECIIINODEUMA. The Echinoderma are exclusively marine, occurring in large numbers even in the deepest seas. Many groups, like the Crinoids, are largely bathybial, others frequent rocky coasts. At the period of reproduction the urchins, starfish, and holothurians frequent the shallow waters, passing 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. m y -\---ct FIG. 306.— Echinoderm larvae. (After J. Miiller.) a, anus; m, mouth; the black linei the course of the ciliated bands. /, form common to all ; //, ///, developmental stages of auricularia (Holothurian) ; 7F, V, stages of the Asteroid bipmnaria; FI, pluteus of a spatangoid; F//, larva (Brachiolaria) of Asterias (orig.). m» mouth ; v, vent. Where there is no brood pouch the young escape from the egg as larvas which swim at the surface, and are distinguishable from the adults (fig. 306, /) 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 bizarre shapes (plutei of echinoids and ophiuroids, brachiolaria and bipinnaria of asteroids, auricularia of holothurians), all of which can be referred back to a common type with tri-regional alimentary tract and a ciliated band around the mouth, strikingly resembling tornaria, the larva of Balanoglossus* The different appearances of the larvae 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 (fig. 306, F). The metamorphosis of the bilateral larva into the radial adult is very complicated. It begins early with the formation of outgrowths from the archenteron (fig. 307), which become separated and form the anlagen of I. ASTEROIDEA. 333 the ccelom and ambulacral system. This becomes divided, and one por- tion develops itself as a ring around the oesophagus, the future ring canal, and from this five outgrowths, the radial canals, arise. Since these canals, as they grow out, carry the body walls before them, the arms in the starfishes, which show the process most clearly, arise as outgrowths which recall buds (fig. 308). This has given rise to one view which regards the arms as individuals, the whole body (and hence that of all echinoderms) as a colony of five individuals. According to this the development would be a kind of alternation of generations, the larva being the asexual genera- FIG. 307. FIG. 308. FIG. 307.— Formation of the coelom in Echinus. (From Korschelt and Heider.) A, first anlage of coelom; B, later stage; C, complete constriction of coelom (vaso- peritoneal vesicle) from archenteron FIG. 308.— Formation of Ophiuran from the pluteus larva. (After Miiller, from Kor- shelt-Heider.) tion which by budding produces the colony. Yet this view does not agree with the actual relations, since it draws an untenable contrast between the larva and the perfect echinoderm. The most important organs of the former are carried over into the latter, and the echinoderm brings the anla- gen to further development. In the insects many features which are lack- ing or incompletely developed in the larva are developed in the course of the metamorphosis. There is a metamorphosis in the echinoderms as in insects. 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. 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. 316). The relations in which these parts 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 (figs. 309, 310). On the other hand the disc may 334 ECHINODERMA. increase at the expense of the arms, absorbing these in its growth so that they form merely the angles of a pentagonal disc (fig. 311). 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 in the FIG. 309.— Comet form of Linckia multiflora. (From Korschelt-Heider.) One of the arms is producing a new animal by budding. Fia. 310. FIG. 311. FlG. 310.— Ophidiaster ehrenbergi. (After Haeckel). Comet form: one of the original arms shown only in part. FIG. 311. — Culcita pentangularis^ aboral view. (From Ludwig.) a, madreporite; &, re- flexed end of ambulacral grooves. centre the mouth and radiating from it to the tips of the arms the five ambulacral grooves. On the aboral surface is the anus (when not degenerate) near the centre, and excentric from it in an inter- radius is the madreporite (in many armed species two to sixteen radii may have madreporites). A line passing through the madreporite and the opposite arm divides the body into symmetrical halves. This ray is frequently spoken of as anterior, since in the irregular sea urchins (Spatangoids) the homologous arm is clearly anterior, while the madreporic interradius is posterior. This plane of symmetry does not correspond with that of the larva. The two rays on either side of the madreporite form the bivium, the three others the trivium. The skin is everywhere protected by large and small plates jointed together. These make a dry starfish hard and stiff, but in life it is extremely flexible, the arms can be bent in any direc- /. A8TEROIDEA. 335 tion, and the animal can work its way through narrow openings. Of the skeletal pieces the ambulacral plates need special mention, A B FIG. 312.— -4, cross-section of starfish arm (orig.). a, adambulacral plates; am, ambu. lacra; op, ambulacral plates; 6, branchiae; c, coelom: /i, hepatic caeca; i, inter- ambulacral plates; n,, radial nerve; p, ampulla; r, radial canal; r, radial blood- vessel. B, ambulacral plates, ventral view, showing the ambulacral pores between. These form the roofs of the ambulacral grooves, and between them are openings, the ambulacral pores, through which connexion is made between the ambulacra and ampullae. In each arm the pairs of ambulacral plates meet above the groove like the rafters of a FIG. 313.— Asteriscus verruculatus, aboral surface removed. (After Gegenbaur.) gonads ; /i, hepatic caeca ; /, stomach with anus. roof. Laterally each ambulacral plate abuts against a small inter- ambulacral plate, bearing usually movable spines. Beyond these comes the adambulacral plates, and then those of the aboral sur- face. Each ambulacral area terminates at the tip of the arm with, an unpaired (ocular) plate. 336 ECIIINODERMA. The organs lie in part in the coelom, in part in the ambulacra! 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 closed (figs. 313, 314). By a FIG. 314.— Section through ray and opposite interradius of a starfish (qrig.). B, branchiae; (7, cardiac pouch of stomach; .E1, eye spot; G, gonad; //, 'liver'; M% mouth; N, radial nerve; P, pyloric part of stomach; RC, ring canal; RD, -radial canal of water-vascular system ; tf, stone canal. 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. The cardiac division gives origin to five gastric pouches which can be protruded from the mouth or retracted by appro- priate muscles. The gonads are five pairs of racemose glands lying in the basis of the arms and opening interradially between the arms. Lastly, the stone canal, extending from the aboral madre- porite to the ring canal, and the lymphoid gland lie in the coelom. The radial nerve, canal, and blood-vessel lie in the roof of the ambulacra! groove between the ambulacra. The nerve ends at the FIG. 315.— Longitudinal section of eye of Asterias. (Orig.) 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 corresponds with the foregoing description //. OPHIUROIDEA, 337 (p. 330), the ampullae as well as the five or more Polian and Tiedemann's (racemose) vesicles projecting into the co3lom. Since the arms contain nearly all important organs, the physiological independence of these is easily understood. Arms broken off not only live, but regenerate first the disc and then new arms which appear at first like small buds (comet form, figs. 309, 310). This separation of arms may occur through accident, or it may be, and not infre- quently is, produced by the animal itself. Examples of species with well-devel- oped arms and ambulacra in four rows are furnished by the ASTERID^E, repre- sented on our shores by the five-finger Aster ias * and Leptasterias,* and Heli- aster* with numerous arms. In the SOLASTERID^E the ambulacra are two- rowed; arms sometimes numerous. Py- tlionaster (fig. 316). In the ASTERINID^E the arms are short or the body is pentag- onal, no large plates on the margins of the arms. Asteriscus (fig. 313). In other forms (Culcita* fig. 317, Hippasteria* CtenodiSGUS*) the body is more Or less FIG. pentangular, the margin being covere.d with large plates. Class II. Ophiuroidea (Brittle Stars). In these the animal consists, as in the Asteroidea, of disc and arms, the latter sometimes 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. 317). As a result the co3lom of the arms is greatly reduced, the hepatic caeca are lack- ing, and the alimentary canal, which lacks an anus, is confined to the disc. By the ingrowth of ven- FIG. SIT. — Section of Ophiuroid arm tral plates the ambulacral grooves (ong.). a, ambulacrum; Z), blood ves- _ . G sei; c, coeiom; w, muscles of arm; n, are converted into tubes, and the nerve; ?•, radial water tube; v/verte- , , , . , , _ . . bra ' (coalesced ambulacral plates). ambulacra, which lack SUCking discs, are tactile organs, locomotion being effected by the snake- like motion of the arms. The madreporite is on the ventral sur- 316 — Pythonaster murrayf. 338 ECHINODERMA. face. Also on the ventral surface are five slits which connect with as many bursae, thin-walled respiratory sacs into which the sexual organs open. In many brittle stars (Ophiocnida, Opliiothelia, Ophiocoma), especially in young specimens, there is a kind of asexual generation (schizogony), the animal dividing through the disc, the halves regenerating the missing parts. The classification is based largely on small details. In the majority the arms are unbranched (Ophiopholis * (fig. 318), Opliwglypha* Amphiura *), but in the EURYALID^E, or basket fish, the arms are branched (Astrophy.ton,* fig. 319), but not, as usually stated, dichotomously. FIG. 318. FIG. 318.— Ophiopholis aculeata* FIG. 319. (From Morse.) FIG. 319.— Astrophyton arborescent, basket fish. (From Ludwig.) Class III. Crinoidea (Pelmatozoa). The crinoids or sea lilies are on the road to extinction. In early times, especially in the paleozoic, they were very abundant, but to-day there are but few genera and species, these mostly restricted to the greater depths of the ocean, only the Comatulidae occurring near the shore. The crinoids are attached to the sea bottom by a long stalk which contains a central canal (fig. 320). This stalk is composed of cylindrical discs and often bears five rows of outgrowths, the cirri. In the Comatulidse (fig. 321) the adult is not thus attached, swimming about in the water with the arms or moving about on the tang. In their earlier stages these animals have a stalk (fig. 322), passing through a Pentacrinus stage, a proof that the fixed condition was the primitive one. In these forms, when the separation takes place, one joint of the stalk with its cirri remains attached to the animal, as the centrodorsal united with the lowest cup plates, the infrabasals (fig. 321). 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 III. CRINOIDEA. 339 Fio. 320.— Pentacrinus macleayanus. (After Wyville Thompson.) FIG. 321. FIG. 322. FIG. 321.— Adult of Antedon macronema. (After Carpenter.) FIG. 322.— Different Pentacrinus stages (a, b, c) of Antedon rosacea. I, arms; 2, cim; 3, stalk. 340 ECHINODERMA. walls of the theca are covered with polygonal calcareous plates. Usually the stalk bears five plates, the basalia, and then come five radialia, alternating in order with the basalia (fig. 323). In some FIG. 323.— Hyocrinus bethleyanus. A, tipper end of stalk with cup, and the bases of the arms; b, basalia; ftr, brachialia; r, radialia. J?, oral surface of cup with mouth, five oralia, and the bases of the arms. there is a circle of infrabasalia in a line with the radialia. Fre- quently the elements of the arm, the brachialia, are directly attached to the radials (fig. 323). 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 (figs. 320, 321). From the arms arise, right and left, a row of pinnulae, lancet-shaped processes supported by calcareous bodies in which the sexual products ripen until freed by dehiscence (fig. 325). The mouth opening, in the middle of the oral disc which closes the theca, is frequently surrounded by five radial plates, the oralia. The mouth, which in contrast to other echinoderms is directed upwards, connects with a spacious digestive tract in which oesopha- gus, stomach, and intestine can be distinguished. The anus is interradial and near the mouth (fig. 324). Five ambulacral grooves begin at the mouth and extend out on the arms, branching with them and extending to the tips of the pinnulae. These are ///. CRINOIDEA. 341 ciliated and serve as conduits to bring food to the mouth. Nervous, ambulacral, and blood systems begin with a circumoral ring. They follow the ambulacral grooves as in the asteroids, but the ambulacra FIG. 334. t IG. 325. FIG. 324.— Oral area of criuoi&(Antedon\ showing by dotted lines the course of the in- testine from the mouth (m) to the anus (a) ; y, ciliated grooves leading from the arms to the mouth (orig.). FIG. 325 —Cross-section of pinnula of Antedon. (After Ludwig.) a, axial nerve cord- c, ciliated cups; c, c, coeliac canal; g, gonad; s, sacculi; sc, subtentacular canal ; t, tentacles. here 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. Oppo- site their ccelomic mouths are fine pores in the oral disc through which water enters to pass through the tubules into the ambulacral system. The ambulacral nervous system is weakly developed. The enterocoale system, on the other hand, is well developed and forms the axial cord running through the brachialia and radialia to unite in a ring in the centrodorsal. A problematical organ, the so-called dorsal organ, also begins in the centrodorsal and extends up through the axis of the theca to the oral disc. It is possibly a lymphoid gland, possibly a structure for the transfer of nutriment, and is apparently homologous with the ' heart ' of the starfish. 342 ECIHNODERMA. Sub Class I. Eucrinoidea. The foregoing account applies entirely to the Eucrinoidea, which may be divided into two groups : Order I. TESSELLATA (Palseocrinoidea). Theca with its side walls composed of immovably united thin plates ; the ambulacral grooves usu- ally completely covered by calcareous plates. Exclusively paleozoic. Order II. ARTICULATA (Neocrinoidea). Ambulacral grooves open, tneca with compact, in part movably articulated, plates. This order left the other in the mesozoic age, and some families have persisted until now. Rhizocrinus* (fig. 323) and Pentacrinus (fig. 320), deep seas ; the COMA- TULID^E are fixed in the young, free in the adult. Antedon* (fig. 321). Sub Class II. Edrioasteroidea (Agelacrinoidea). Theca of irregular plates ; arms unbranched and lying on the theca. Possibly the ancestors of the noncrinoid echinoderms. Paleozoic. Agela- crinus. Sub Class III. Cystidea. Exclusively paleozoic ; body spherical, composed of polygonal plates. Stalk and arm structures rudimentary, sometimes lacking. The AMPHO RIDA are by some regarded as ancestral of all echinoderms. Holocystites, Echinosphcerites (fig. 326). FlO. 326. Fio. 327. FlO. 328.—Echinosphceritesaurantium. (From Zittel ) FIG. 3SR.—Pentremitesflorealis. (From Zittel). Lateral, oral, and aboral views. Sub Class IV. Blastoidea. Arms lacking ; the mouth surrounded by five petal-like ambulacral areas. The group appears at end of Silurian and dies out with the carbon- iferous. Pentremites (fig. 327). IV. ECHINOIDEA. 343 Class IV. Echinoidea (Sea Urchins). The structure of the sea urchins is best understood in the spherical forms (figs. 328, 330). Mouth and anus lie at opposite poles of the main axis, each open- ing immediately surrounded by areas covered by calcareous plates, the arrangement of which varies with the family^ Around the anus is the periproct, around the mouth the peristome, the latter bearing sphasridia and in the Echinoids five pairs of interambulacral gills. Be- tween peristome and periproct the _ FIG. 328.— Ccelopleuriisflnridanus. body wall is composed ot calcareous plates, which, except in the Echino- thuridse, are immovably united. Aside from the extinct Palaachei- noidea the plates are arranged in twenty meridional rows, or, more accurately, in ten double rows, two rows being always intimately associated together. Five of these double rows are ambulacral, (After Agassiz.) Aboral "view, the spines removed to show the ambulacral (a) and (ib) interambulacral areas, end- ing respectively in the ocular and genital plates ; in the centre the four plates of the periproct. FIG. 329. FIG. 330. FIG. 329.— Clypeaster suhdepres&us. (After Agassiz.) Aboral view, showing the peta- loid ends of the ambulacral areas. FIG. 330.— Diagrammatic longitudinal section through a sea urchin. the alternating five interambulacral. Both bear small hemispheri- cal articular surfaces on which are situated the spines, either long and pointed or swollen to spherical plates. These spines are ex- tremely mobile and are moved by muscles so that they serve both as protecting and locomotor structures. The ambulacral plates are 34:4: ECHINODERMA. distinguished from the interambulacral by the ambulacral pores by which the ambulacra on the surface are connected with the internal ampullae. In most sea urchins the paired grouping of the pores results from the fact that a double canal extends from ampulla to ambulacrum. In the arrangement of the ambulacra two modifications, the band form and the petaloid, occur. In the first the ambulacra are equally developed from peristome to periproct (fig. 328). In. the second oral and aboral regions may be distinguished (fig. 329). In the oral region alone are loco- motor feet always present, but these are so irregularly arranged that no striking figure results. In the aboral area the ambulacra are tentacular in character and are regularly arranged, their pores bounding five petal- like figures around the periproct, very distinct after removal of the spines. In the Echinoids, the Cidarids 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 (radialia of mor- phology) being called ocular plates, since they often bear pigment spots formerly regarded as eyes. They are perforated by the end of the radial canal and nerve, the latter here uniting with the epi- thelium of the skin. The five interambulacral plates are called genital plates, since they usually contain the openings of the genital ducts. One is often madreporite as well. The interior of the body is occupied by a spacious coelom, to nd d FIG. 331. — Sea urchin opened around the equator. A, ambulacral area; J, interam- bulacral area; L, lantern; d, intestine; ed, anal end of intestine; g, gonads; nd, siphon; oe, oasophagus; p, p', ring canal and Polian vesicles; st, stone canal. the walls of which the thin-walled alimentary tract is fastened by a mesentery. In the Clypeastroids this tract forms a simple spiral, IV. ECHINOIDEA. 345 but elsewhere it is a double spiral. It ascends from the mouth* turning once, and then, bending on itself, coils in the reverse direction to the anus (fig. 331). Usually the first portion of the canal is accom- panied 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 calcareous plates, the teeth, FTG. 332. — Aristotle's which in the Echinoids are supported by a l^rofusiimd^°l(^t- complicated system of levers, fulcra, and mus- d?i«?h™a^iveoU; r«i cles, the 'lantern of Aristotle' (fig. 332). The ring canal and the ring of the blood system lie on the lantern, the stone canal and ovoid gland (' heart') extending upwards from them (fig. 330). The blood-vascular ring gives oft two blood-vessels which run along the alimentary canal, while from the ring canal arise five ambulacral or radial canals which run on A iurl a, anus ; g, genital pores ; i, ambulacral areas ; m, madreporite ; o, mout FIG. 333 — Oral (A) and aboral (B) surfaces of the sand dollar, Echinarachnius par ith. the inner side of the test accompanied by nerves which radiate from a nerve ring. The gonads are five (rarely four or two) unpaired organs in the aboral half of the test, opening through the genital plates, that is, interradially as in the starfish. Order I. Palaeechinoidea. Paleozoic forms with five ambulacral areas, the interambulacral areas containing more than two rows of plates. Melonites. Order II. Cidaridea (Regulares). Ambulacral areas band-like, body more or less spherical, mouth and anus polar. Here belong the common urchins, represented on our coasts, by Toxopneustes* Strongylocentrotus,* Arbacia* C&lopleurus* (fig. 328). 346 ECHINODERMA. Order III. Clypeastroidea. Irregular flattened echinoids with central mouth and teeth ; anus out- side the periproct in the posterior interradius, sometimes marginal ; five petaloid ambulacral areas. Clypeaster (tropical), Echinarachnius* (sand dollar, fig. 333), Mellita* with holes through the test. Order IV. Spatangoidea. Bilateral flattened forms more or less heart-shaped ; mouth and anus excentric, 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. 334) are upon the lower surface. Warmer seas. Spatangus* Echinocardium, Brissus. Class V. Holothuroidea. The sea cucumbers are most re- moved of any sroup from the to™1 echinoderm appearance. At the first anus. The bivium without tu- glance the skin appears naked and the characteristic plates absent. Yet these are imbedded in the skin in the shape of plates, wheels, and anchors (fig. 335). The integment is tough, leathery, and muscular, with FIG. 335.— Dermal plates of Holothurians. A, Myriotrochus rinkii. (After Daniels- sen.) _B, Thyone briareus ; C, Synapta girardii (orig.). longitudinal and circular fibres. The saccular body gives these forms a worm-like appearance, strengthened 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 ambu- HOLOTHUROIDEA. 347 , genital duct; p, pharyngeal ring ; ?•, gonads, cut away on right side; f, ampullae of tentacles ; v, ventral blood-vessel. 348 ECUINODERMA. lacra alone are locomotor, those of the bivium being tactile or wholly absent. In the body cavity (fig. 336) lies the alimentary canal, which (except in Synapta) is coiled in a uniform manner, although many minor convolutions may obscure this. It passes backwards in the median dorsal interradius,forwardin the left ventral interradius, and then back in the right dorsal interradius to the anus. It is held in position by mesenteries (fig. 337), and near the anus by numerous muscular filaments. Into the terminal portion one or two branchial trees may empty. These are tubular sacs with small branched outgrowths which are filled with water. The similarity of these to the excretory organs of some Gephy- raea (p. 317) was one ground for re- FIG. 337. — Transverse section of garding those forms as intermediate Hnlothuria tubulosa. (After Lud- f wig.) d, digestive tract ; db, dor- between worms ana ecnmoderms. sal blood-vessel; 0, gonad duct; „,, n n n, skin; tm, longitudinal muscles; They are to be regarded as respiratory, liv, left branchial tree ; wi, mesen- • i-i • j* n .en j -j.i teries ; r', r3, ambuiacrai complex since they are periodically mled with f resh water. In many species < Cuvie- ™, right branchial tree. rian organg , OC(Jur . thege are morpho. logically specially modified portions of the branchial tree and are either connected with them or separately with the 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 interradial 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 a radial branch which here runs inside the muscular sac of the body. From the beginning of the radial canals (rarely, as in Synapta, from the ring canal) tubes extend outward to form the extremely sensitive retractile tentacles which surround the mouth, and which either branch (Dendrochirotae) or bear frilled shield-shaped extremities (Aspidochirotae). A single Polian vesicle is usually present, and the stone canal (except in the Elasipoda) connects with the coelom. Blood-vessels going from the vascular ring 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 usually interradially near the mouth. F. HOLOTHUROIDEA. 349 The regenerative powers of these animals are of interest. In unfavor- able conditions (hence in preserving the animals in alcohol without nar- cotization with chloral) 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. A parasitic snail, Entoconcha mirabilis, lives in one species of Synapta, and a mussel, Entovalva mirabilis, in another. Order I. Actinopoda. Radial canals present, sending branches to the tentacles and am- bulacra when present. Divided into Pedata, with ambulacra, and Apoda. without. The PEDATA include the HolothuridsB with peltate tentacles. FIG. 338.— Cucumaria frondosa, sea cucumber. (From Emerton.) Holothuria * in warmer waters, one species furnishing the trepang of Chinese markets. The CUCUMARIID^E represented in our waters by Cucu- maria * (Pentacta) with regular rows of ambulacra, Thyone * with them scattered, and Psolus* scaly with a creeping disc. The deep-sea ELA- SIPODA belong to the Pedata. The APODA are represented by Gaudina * (fig. 336) and Molpadia* Order II. Paractinopoda. No radial canals nor ambulacra. Tentacular canals arising from ring canal. Myriotrochus* Synapla* Oligotrochus* (fig. 339). 350 ECHINODERMA. Summary of Important Facts. 1. 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. 2. Farther characters are the existence of a coelom, the ambulacral system, and the mesodermal spiny skeleton, which has given the name to the phylum. 3. The ambulacral S3^stem is locomotor and occurs nowhere else. It consists of a sieve-like plate, the madreporite (not always pres- ent), which passes water to the stone canal, and from this to the FIG 339,—Oligotrochus vitreus.* (After Danielssen and Koren.) ring canal and the radial canals to fill the ampullae and ambulacra. Lateral branches supply the tentacles and cause their extension. 4. Blood-vessels and nerve cords run in the same radii as the radial canals of the ambulacral system; stone canal, madreporite, ovoid gland, and genital ducts are interradial. 5. The Echinoderma are divided into five classes: (1) Aster- oidea, (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 caeca extend. The ambu- lacral groove open. 7. The OPHIUROIDEA also have disc and arms, but the ambu- lacral groove is closed and the hepatic caeca absent. 8. The CRINOIDEA have a cup-shaped body bearing arms, usually branching, with pinnulae, and a stalk, usually with cirri. They are either temporarily or permanently attached. 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 meridional bands from peristome to periproct, five pairs of ambulacral and five of interambulacral. 10. The ambulacral plates end at the periproct with a single ocular plate; the interambulacral with a similar genital plate. The madreporite is fused with one of the genital plates. MOLLUSC A. 351 11. The regular sea urchins have the anus in the centre of the periproct, the mouth in the peristome; the ambulacral areas are 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 an- terior, 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 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 flatworms and leeches, give the impression of parenchymatous animals. A spacious coelom is absent; what was formerly regarded as a body cavity is a system of sinuses surrounding the viscera and connected with the blood system, and is especially developed in the Acephala. More recently the view has gained ground that the molluscs have descended from ccelomate animals, and from forms in which, by encroach- ments of a connective tissue and muscular parenchyma, the coelom has been reduced to the inconspicuous remnants of the pericardium and the lumen of the gonads. Where the molluscan organization is well developed, as in the snails, four parts may be recognized in the body (fig. 340), The visceral sac forms the chief mass of the body; it is less rich in muscles than the rest because it is reduced to a thin peripheral layer by the alimentary canal, liver, nephridia, and gonads. In front it is continuous with the head, which, according to the group, is more or less marked off by a neck, and bears, besides the mouth, the tentacles and eyes, the most important sense organs. Below, the visceral sac passes into a muscular mass, usually used for loco- motion, the foot. From the back extends the pallium or mantle, a dermal fold which envelops a goodly part of the body. The Acephala (fig. 340, C) have a double mantle, right and left, both halves springing from the dorsal line and extending down over the visceral sac and foot. The cephalopods (fig. 340, A) and the snails (fig. 340, B), on the other hand, have an unpaired mantle which arises from about the central part of the back and either extends 352 MOLL U8C A. down on all sides or, like a cowl, covers either the anterior or posterior parts of the body. The mantle is of importance in two ways : its outer surface is covered with epithelium which, like that of the adjacent surface, has the power of secreting shell, a thick cuticular layer of organic matter (conchiolin) largely impregnated with calcic carbonate. The inner surface of the mantle, together SOU FIG. 340.— Diagrams of three molluscan classes. A. a cephalopod (Sepia) ; B, a gas- teropod (Helix); C, an acephal (Anodontd). a, anus; c, cerebral ganglion; /it, foot; m, mantle chamber; .sc/i, shell; £, siphon; v, visceral ganglion. Visceral sac dotted; mantle lined, shell black. with the outer surface of the body, bounds a space, the mantle cavity, which, from its most important function, is also called the bran- chial chamber. Since most molluscs are aquatic, special vascular processes of the body, the gills or branchiae, lie in this space; in the terrestrial forms its walls serve as lungs and thus are respiratory. From the foregoing it will be seen that the character of the mantle must exert an influence on the shape of the shell and on the respiratory organs. Paired mantle folds necessitate 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 MOLLUSCA. 353 always unpaired, while the gills may retain their primitive paired condition. The gills in the mantle cavity are called ctenidia, from their resem- blance to combs with two rows of teeth. Each consists of an axial portion (back of the comb), containing 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. 385). In many aquatic forms the ctenidia are lacking, and then the respiration is either diffuse by the skin or by accessory gills which by structure (usually outside the mantle cavity) are distinguished from the ctenidia. Those parts of the surface of the mollusc which are not covered by the shell have a columnar epithelium which is frequently ciliated and which contains unicellular mucus glands, especially abundant on the edge of the mantle. These give these animals the soft slip- pery skin which is implied in the name Mollusca (mollis, soft). Many-celled glands, like the byssus gland of the Acephala, the pedal gland of many snails, occur. Although the existence of head, foot, and mantle is very char- acteristic 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; in the Cephalopoda the foot is converted into other appendages, the siphon and arms. These modifications are to be explained by degeneration and evolution. In the nervous system are also 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 oesophagus and corresponds to the supraoesophageal ganglion of the worms; it is the brain (cerebrum) and supplies V^- Fia. 341.— Nervous systems of Molluscs. A, most gasteropods; J9, acephals; C, cepha- lopods and pulmonates. c, cerebral; pa, parietal, pe, pedal, pi, pleural, and v, visceral ganglia. 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 which are connected with the otocysts. The third pair, the visceral ganglia, are also ventral, and near them are the third sense organs, which are widely distributeed through the Mollusca, and which from position and structure are regarded 354 MOLLUSC A. as organs of smell (osphradia). They are thickened patches of ciliated epithelia extending into the mantle cavity. Pedal and visceral ganglia are united to the cerebrum by nerve cords, the cerebropedal and cerebrovisceral connectives respectively. Accord- ingly 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 lie dorsal to the oesophagus and are united by a cord around the oesophagus (fig. 344). From it are given off two pairs of lat- eral 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 the pedal cords give rise to the pedal ganglia, and similarly the pleural cords form three pairs of ganglia, the pleural and the parietal, as well as the visceral already mentioned, of the cerebrovisceral cord (fig. 341, 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 parietal with the visceral (fig. 341, B), or both may fuse with the visceral (C). In the latter case the visceral ganglion (in the wider sense) is associated with the pedal by the pleuropedal connective ; while in the other the connective is appa- rently absent because fused with the cerebropedal. Although the otocyst receives its nerve from the pedal ganglion, the centre of innervation lies in the cerebrum. The heart, which lies dorsally, consists of auricles and ven- tricles. The ventricle is always unpaired, but 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, and especially in the 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, and in many molluscs (Cephalopoda and some Acephala) is also related to the gonads. These facts support the view, already mentioned, that the pericardium and the lumen of the gonads are the remnants of the coelom; for here, as in the annelids, the nephridia open by ciliated nephrostomes into the ccelom, and the sexual cells arise either from the coelomic walls or from sacs cut off from them. Even more important for this view would be confirmation of the disputed statement that in Paludina vivipara the coelom (enteroccele) arises as diverticula from the archenteron. MOLLUSC A. 355 Nephridia and sexual organs are primitively paired, but fre- quently are single by the degeneration of the structures of one side. The animals are either hermaphroditic or dioecious, but the gonads are always very large. Even more room in the visceral sac is demanded by the digestive tract in which oesophagus, stomach, a coiled intestine, a voluminous liver, and frequently salivary glands may be recognized. The radula or lingual ribbon is also a char- acteristic organ, and its absence from the Acephala is probably to- be explained by 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 communication of food (figs. 366,. 367). Keproduction is exclusively sexual; budding, fission, or parthen- ogenesis have not yet been observed. The eggs, united in large numbers, are usually enveloped in jelly and are either rich in deutoplasm or are enveloped in a nourishing albumen. A few~ molluscs (e.g., Paludina vivipara) are viviparous. A metamor- ties FIG. 342.— Veliger larva (trochophore) of Teredo navalte. (From Hatschek.) A, anus: J, stomach ; J^ intestine; £, liver; LM.d, LM.v, dorsal and ventral longitudinal muscles ; Meg, primitive mesoderm cells ; JfP, teloblast ; Neph, protonephros ; O, mouth ; Oe, oesophagus ; R, rectum ; S, shell ; ScM, hinge ; SM.h, SM.v, posterior and anterior adductors; S», apical plate; Wkr, wkr. pre- and postoral ciliated bands ; uvs, cilia of apical plate. phosis is of wide occurrence. In such cases a 'veliger' larva escapes from the egg (fig. 342) ; in this can be recognized head, foot, and mantle, even in those cases where one or the other of these is lacking in the adult. This shows that the absence of 356 MOLL USCA. mantle, shell, or head, which occur in large groups of molluscs, is not a primitive condition, but can only be explained by degen- eration. The name veliger arises from the velum, a strong circle of cilia, which surrounds a frontal or velar field in front of the mouth, and which serves as a locomotor organ for the larva. In some cases (fig. 343 B} it is lobed like the trochus of a Rotifer. Fia. 343.— Veliger stages. A, of a snail; B, of a Pteropod. (From Gegenbaur.) o, shell; op, operculum ; p, foot ; t, tentacle; v, velum. The veliger recalls the annelid trochophore and serves for the distribution of the species; it is therefore of great importance for animals which, like most molluscs, are sedentary or slow-moving. In cases without metamorphosis (Cephalopoda, Pulmonata, etc.) the veliger stage is frequently indicated during embryonic devel- opment 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 symmetry. The nervous system already described (p. 354) consists of pleural and pedal cords with scattered ganglion cells and no ganglia, these cords being connected by numerous commissures (fig. 344, B}. Sub Class I. PlacopJiora (CMtonidce). 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 con- dition of the head and the shell. This last is unique among mol- luscs; it consists of eight transverse plates overlapping like shingles, which allows the animal to roll itself into a ball. The edge of the L AMPHINEURA. 357 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 (aes- FiG.344. — Chiton squamosus, dorsal view. (After Haller.) A, the entire animal; /?, after removal of shell and viscera, a, anus; C', brain; K, ctenidia; o, mouth; P, pedal nerve cord; pi, pleurovisceral nerve cord. thetes and, in some, eyes, fig. 345). The symmetry of the body is also expressed in the viscera. The anus is medial, and right and FIG. 345. — Eye and aesthetes of Acanthopleura spiniger. (After Moseley.) a, macrges- thete; b, micrassthete; /, calcareous cornea; gr, lens; /i, iris; fc, pigmented cap- sule ; n, p, nerves ; ?•, retina. left of it are the openings of the nephridia 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. The Chitons are represented on our northeastern coast by several small species (Trachydermon,* Auricula*}; farther south and on the Pacific shores are larger species (Cryptochiton *). 358 MOLLUSC A. Sub Class II. Solenogastres (Aplacophora). Worm-like forms without shell; the foot rudimentary and at the bottom of a ventral groove. The radula is also reduced; in Chcetoderma it bears but a single tooth. The gills are either small or wanting. The usually hermaphrodite animals have the gonads emptying into an unpaired chamber (pericardium?) and FIG. m.-Neomenia corinata, thence to the exterior by the paired BSS^IS^^SS nephridia. Clmtoderma in New Eng- terior;c, ventral groove. land; ^eomenia, Dondersia. Class II. Acephala (Lamellibranchiata, Pelecypoda). These have, among the molluscs, the least powers of locomo- tion. Some are fixed, the majority burrow slowly through sand or mud ; only a few spring by means of the foot or swim by open- ing or closing the shells. Hence it is that they need more pro- tection 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 rather than dorsal and ventral, and hence are usually symmetrical in shape. Only when the animal rests permanently on the right or left side is this sym- metry lost, and then the symmetry of the soft parts is affected. The two lobes of the mantle which secrete the shell on their outer surface arise from the back of the animal and grow down- wards, forwards, and backwards, so that they envelop the whole -(fig. 352). Hence the oldest and the most thickened part of the .shell, the umbo, occurs near the back (fig, 347). Around this the lines of growth are arranged concentrically, lines which show how, by gradual growth of the mantle, the shell has increased in .size. On the back the valves approach each other, and in the majority are movably connected by a hinge, which consists of projections (' teeth ') in one valve fitting into depressions in the •other. In the Brachiopoda the valves are opened by appropriate muscles; in the Acephala by an elastic hinge ligament usually placed dorsal to and behind the hinge. The shell is closed by adductor muscles which extend through the body from shell to shell, leav- ing their impressions or scars on the inner surface (fig. 347). //. ACEPHALA. 359 Usually there occur an anterior and a posterior adductor equally well developed (Dimyaria); less frequently the anterior is rudi- mentary (Heteromyaria) or entirely disappears (Monomyaria). FIG. 348. FIG. 347.— Left valve of Crassatella plumhea, inner and outer surfaces. (From Zittel.) The outer surface showing lines of growth ; no pallial sinus. FIG. 348. —Right valve of Mactra gtuitorwm, with pallial sinus. (From Lud wig-Leunis.) Letters for both figures: a', anterior; a", posterior adductor scar; e, hinge; I, internal ligamental groove ; m, pallial line ; s, pallial sinus. When the muscles are relaxed (as always occurs at death) the elastic ligament opens the valves. The heterodont hinge is the typical form (fig. 348); 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 in front and behind 'lateral teeth? often produced 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. 347). The so- called schizodont and desmodont hinges are modifications of the hetero- dont. Then there are Acephala of apparently 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 like- wise 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. Since the secretion of shell takes place most rapidly at the edge of the mantle, both are closely united, the union being strength- ened 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. 347). In many species, the Sinu- 360 MOLLUSC A. palliata, the line at the hinder end makes a large bay (pallial sinus) (fig. 348, 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 exca- vated at the posterior end, so that when brought together two openings, an upper and a lower, result (fig. 349, 0). The lower FlG. 349.— Ventral views of siphonate and asiphonate acephals. A, Anodonta cygnea ,' B, Isocardia cor ; (7, Lutraria elliptica. a, anal siphon ; />, branchial siphon ;/, foot; A;', outer, A.", inner gill lamella; wi, mantle; s, shell. of these is the branchial opening by which fresh water passes into the mantle (branchial) chamber; it flows out after passing over the gills, along with the faeces, through the upper or cloacal open- ing. In many bivalves the free edges of the mantle grow together, FIG. 350.— Section of shell of Aru ;. c, cuticula; p, prismatic layer; I, nacreous layer. leaving three openings, one for the protrusion of the foot, the others the two just described, which are now called the incurrent (branchial) and excurrent (cloacal) siphons (fig. 349, B). By further development the margins of these openings are drawn out //. ACEPHALA. 361 into two long conjoined tubes (fig. 349, A}, which for their retrac- tion need special muscles, which are attached to the valves and thus cause the pallial sinus referred to above (fig. 348). In the shell three layers may be distinguished (fig. 350) : 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 layer, the first consisting of closely packed prisms; the nacreous layer of thin lamellae generally parallel to the surface. These by their free edges 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 shells Meleagrina and Margaritina margaritifera. When foreign substances get between mantle and shell they stimulate a greater secretion of nacreous substance and become surrounded by layers of it. In this way pearls are formed. K? K3 FIG. 351.— Anatomy of Anodonta, the mantle, gill, and liver of the right side removed, the pericardium opened. 1, 2, anterior and posterior adductors; I, II, III, cerebral, pedal, and visceral ganglia; a, anus; 7.1, ib2, upper and lower limbs of organs of Bojanus ; frr, branchial siphon; d, intestine; e, nephridial opening; fu, foot ; 0, gonad ; Ti1, ft2, ventricle and auricle of heart ; 7c', insertion of both lamellae of right gill ; fc3, 7c4, inner and outer lamellae of left gill; Z, left liver ; I', its opening in ?n, stomach; rn7, pallial line; r1, anterior, ra, posterior retractor muscle: sp, nephrostome : v, labial palpus. The arrows show the planes of sec- tion of fig. 352. The gills lie between the mantle and the body and from their lamellar character have given rise to the name Lamellibranchiata. (figs. 351, 352). Two gill-leaves occur on either side, although occasionally the outer or both may degenerate. Frequently the gills of the two sides unite behind the body and produce a parti- tion which separates the mantle cavity into a small dorsal cloacal MOLLUSC A. chamber and the larger lower respiratory cavity. Into the cloaca empty the anus and the water which has passed over the gills; it opens to the exterior through the excurrent siphon. The incurrent siphon leads into the branchial chamber. In front of the gills are two more pairs of leaf -like lobes, the labial palpi, between which is the mouth. The gills are variously developed. The Nuculidse — the most primitive of living Acephala — have true ctenidia consisting of an axis grown to the body and an inner and an outer row of gill leaves (fig. 355). From this the filibranch type is easily derived. The gill leaves grow out into ...*' FIQ. 352.— Projection of sections shown by the arrows in fig. 351. 61, b2, upper and lower limbs of nephridium (organ of Bojanus) ; rf, intestine ; e, nephridiopore ; /M, foot ; g, gonad ; h1. ventricle surrounding the intestine ; h2, auricle : fc1, fc2, inner and outer gill lamellae ; J, hinge ligament ; m, mantle ; n, cerebro-visceral com- missure ; sp, nephrostome ; v, venous sinus. long filaments, each bent on itself so that it presents two limbs, a descend- ing and an ascending. These branchial threads are so matted together that they give the impression of a continuous leaf. In the true lamellar gill the threads of the filibranch grow together at intervals, leaving open- ings, the gill slits. Since there is an ascending and a descending limb, it follows that each gill consists of an inner and an outer leaf (fig. 352), leav- ing a space between into which the gill slits open. This internal space in some serves to contain the young. The complete enclosure of the body in the mantle folds has led to a degeneration of the head and its normal appendages //. ACEPHALA. 363 {Aeephala). Hence there are only two divisions in the body, dorsally the visceral sac and ventrally the foot. The foot, degener- ate in many, has a broad sole only in Pectunculus and the Nuculi- dre; usually it is hatchet-shaped (Pelecypoda), that is, compressed with a rounded ventral margin. It may be enormously expanded and contracted again. This expansion is often explained by the taking of water into the blood, but now it is generally accepted that it is accomplished by forcing blood from other regions into it. While the foot by this extensibility can serve as a locomotor organ, it also functions in many as an organ of attachment. Inside is a large byssus gland which can secrete silky threads, the fcyssus (fig. 353), one end of which is fastened to foreign objects by FIG. 353.— Mytilus edulis*. (After Blanchard.) a, edge of mantle ; b, spinning finger of foot ; c, byssus ; d, e, retractors of foot ; /, mouth ; 0, labial palpi ; 7t, mantle ; i, j, inner and outer gills. means of a finger-like process of the foot, while the other end remains in connection with the foot. Molluscs which have a byssal gland are found anchored by a thick bunch of byssal threads to stones, etc. The heart, surrounded by a pericardium, usually occupies the most dorsal part of the visceral sac. It consists of a ventricle and a pair of auricles (figs. 351, 352, 7*1, 7*2). The auricles receive the blood direct from the gills; the ventricle forces it out through anterior and posterior aortae (fig. 351), the latter lacking in many species. The excretory organs (organs of Bojanus) lie immediately below the pericardium. The organs of the two sides touch in the 364 MOLLUSCA. middle line. Each consists of a dorsal smooth-walled chamber and a lower portion traversed by threads, both connected behind but separated elsewhere by a thin partition. The lower chamber is connected in front with the pericardium by a ciliated canal, the nephrostome, while the upper opens to the outside by a short canal, the ureter, the external opening being in the region of the inner cavity of the inner gill. In this way a connexion is estab- lished from the pericardium to the exterior, the apparatus being apparently a true nephridium. In many it serves also as genital duct, but usually the genital and reproductive ducts are separate. The animals are usually dioecious, the gonads being acinose glands. The digestive tract (fig. 351) begins with a short oesophagus, widens out to a large stomach from which a slender intestine leads, with many convolutions, to the anus. In the majority of Acephals the terminal portion enters the pericardium in front and below, passes through the ventricle and out through the upper posterior wall of the pericardium. In its course the alimentary tract i& enveloped by the gonads and the voluminous liver, the secretion of the latter emptying by two ducts into the stomach. Usually the stomach has a blind sac, in which lies the 'crystalline style/ a rod-like structure of uncertain significance. The three typical molluscan ganglia (p. 353) are uncommonly wide apart. The two brain ganglia (cerebropleural ganglia) lie either side of the mouth at tho base of the labial palpi and central to the anterior adductor. They are very small, since cephalic sense organs are lacking, and are united by a transverse supra- oesophageal commissure. The posterior ganglia, composed of the united parietal and pedal ganglia, lie near the anus ventral to the posterior adductor. The pedal ganglia, rather far forward in the muscles of the foot, are closely approximate. Of the higher sense organs only the otocysts near the foot are constant. The labial palpi are also highly sensory, while two small osphradia occur at the basis of the gills. When eyes occur they are, as in the scal- lops (Pectinidae), arranged in a row like pearls on the margin of the mantle. Small tentacles with sensory powers may occur both on the margin of the mantle and on the tip of the siphon. Veligers (fig. 342) are very common in development. When this stage is lacking the history may contain a metamorphosis as in the fresh-water Anodonta. The young which grow in the maternal gills are known as Glochidia, which are distinguished from the adult by a byssus thread, by only a single adductor, and by a hook or tooth on the free margin of //. ACEPHALA: PRGTOCHONCHI^]. 305 the shell (fig. 354). After escape from the gills they swim about by opening and closing the shells, and by means of the hooks attach themseJves to passing fish. They produce an ulcer in the skin of the fish in which they grow, and by renewal of the shell and the adductor muscles attain the de- finitive condition. After this metamor- phosis they fall to the bottom, to live henceforth half buried in the mud. Structure of gills, hinge, edge of mantle, and adductor muscles have been used as basis of classification, the usual divisions being founded on characters derived from only one of these organs. Order I. Protochonchiae. The primitive character of these forms is shown by the struc- ture of the gills, which are either ctenidia (Protobranchiata) or FIG. 354.— Glochidium of Anodonta. (From Balfour.) ait, adductor ; by, byssus ; s, sense hairs ; sli, shell. FIG. 355.— Anatomy of Nucula. (After Drew.) oa, anterior adductor; ft, byssal gland; c, cerebral ganglion; ct, ctenidium; /, foot; 7?, heart; 7, labial palpus; o, otocyst; Pi pedal ganglion; pa, posterior adductor; «, stomach ; t, appendage of palpus; r, visceral ganglion. filamentary (Filibranchiata), yet here and there, as in the scal- lops and oysters (Pseudolamellibranchiata), the fusion of gill fila- ments is already begun. Hinge and ligament are symmetrical with regard to the umbo, or vary little from symmetry. The hinge may be lacking, and the ligament is wholly or in part internal. The mantle edges are free, and rarely is there the first trace of fusion. 366 MOLLUSC A. FIG. 356.— Yoldia limatula* (From Binney-Gould.) FlO. 357.— A, Modiolaplicatula*; B. Pecten irradians*; C, Mytilua edults.* (From Binney-Gould.) //. AGEPHALA: HETEROCONCHI^E. 367 Sub Order I. DIMYARIA. Two equally developed adductors. The taxodont NUCULID^E have ctenidia, a broad foot, pleural and cerebral gan- glia separate, and gonads emptying through the nephridia, all points which show them extremely primitive. Niicula* Leda* Yoldia.* The ARCID^E. are also taxadont, but filibranch. Scapharca* Argina* SOLEMYID^E. Sub Order II. ANISOMYARIA. Anterior adductor rudimentary (Heteromyaria) or wanting (Monomyaria). With the exception of the isodont SPONDYLID^E, all the families lack a hinge (dysodont). To the Heteromyaria belong the MYLILIDJE, or mussels, with strong byssus and shells pointed anteriorly. Modiola* Pinna* Mytilus edulis, abundant on our mud flats ; eaten in Europe, but occasionally poisonous. Dreissenia polymorpha, a brackish and fresh-water species, has spread from the Caspian through central Europe. Lithodomus * bores into stone. The AVICULID^E of warm seas have wing-like projections either side of the umbo. The pearl oysters of the East and West Indies (Meleagrina) belong here. The OSTR^EID^E and the PECTINID.E are monomyarian. The Ostrasidas, or oysters, usually become attached by the right valve. Our American Ostrcea mrginiana differs from the European species in having the sexes separate. The Pectin idas, or scallops, are free-swimming and are well known for their highly developed green eyes on the edge of the mantle. Order II. Heteroconchiae. Gills always lamellar, their outer surface frequently plaited. Hinge — in rare cases (Anodonta) lost by degeneration — is hetero- dont or modified from a heterodont condition. The mantle edges but rarely free in their whole extent ; siphons usually present, but in some so small (Integripalliata) as to cause no sinus in the pallial line; in others (Sinupalliata) large, the pallial line having a marked sinus. Anterior and posterior adductors equally developed. Sub Order I. INTEGRIPALLIATA. The UNIONID.E (Naiadee) include the fresh- water mussels, of which hundreds of species occur in the Missis- sippi basin, some of which are markedly iridescent and afford material for pearl buttons. In some pearls of value are occasionally found. Unio,* Anodonta* The tropical TRIDACNID.E, with small siphons, includes the FIG. 358.— .A, Saxicava arctica; B, Astarte sulcata ; C. Siliqua costata. (From Binney- Gould.) largest Acephala, Tridacna gigas, the shell of which may be four feet long and weigh three hundred pounds. The heart shells 368 MOLL USC A. Cardium*, Serripes*) and ASTARTID^E, marine, and the fresh-water CYCLA- DID.E (Cyclas, Pisidium *), about the size of peas, belong here, as probably do the extinct RUDISTID.E of the cretaceous. Sub Order II. SINUPALLIATA. The VENERID.E with swollen shells, represented by the quahog, Venus mercenaria on our east coast and by brightly colored species in the tropics; the MACTRID^E or hen clams, and the flattened delicate TELLINID^E (Tellina*, Macoma*), have short siphons. In others the siphons are so large that they cannot be entirely retracted within the shell. This is the case in the MYID^E, represented in all northern seas by the long clam, Mya arenaria, and in the razor clams (SOLENID^E; SolenEnsatella*). The allied SAXI- CAVID^ have burrowing species. These forms connect with others in which the united siphons far exceed the rest of the body in length, giving the animal a worm-like ap- pearance (fig. 359). Since the valves do not cover the whole shell, they are supplemented by accessory shells, or the worm-like body secretes a tube in which the rudimentary valves are imbedded (fig. 360). The PHO- LADID^E, some of which are phosphorescent, burrow in wood, clay, or stone. The shell is well developed. In the ship worms (TERE- DHXE) the shells, on the other hand, are small, while in some species the burrows made by these animals in wood are lined by calcareous deposits. The several species of Teredo* by their boring habits do much damage to wood in the sea, especially in the tropics. The GASTROCH^ENID^E also form tubular shells, the valves being imbedded in the tube (fig. 360); at the smaller anterior end the tube is open, but the other end is closed by a perforated plate, giving these animals the name of * sprinkling-pot ' shells. Lastly, there should be mentioned the Septibranchiata, in which the have the shaPe of a sePtum perforated LudWig-Leu- by gill slits separating the branchial and clo- chambers> siienia^ Cuspidaria. B FlG. 359. — Te- redo navalis, ship worm in its tube, the siphons (a, anal; b, bran- chial) drawn out of the tube (r); k, shell. B,teeth of the shell enlarged. Ispe5$? little-known nis.) a, shell. III. SCAPHOPODA. IV. GASTEROPODA. Class III. Scaphopoda (Solenoconchae). The tooth shells are primitive forms which have some resem- blances to the Acephala in the paired liver and iiephridia and in structure of the nervous system (with the exception that a buccal ganglion is present and the pleural ganglia are distinct from the cerebral). In some points they are primitive (persis- tence of jaws and radula), but in others they are considerably modified. They lack gills, have unpaired dioecious gonads, rudimentary heart (no auricle), and have two bunches of thread-like tentacles either side of the mouth. The mantle lobes, which are paired in the larva, unite below, forming a sac open at either end, and this secretes a shell shaped like the tusk of an elephant, from the larger end of which protrudes the long three-lobed foot used for boring in the sand. Dentalium (fig. 361), Entalis* FIG. 361.— Dentalium elephan- tinum, tooth shell ; left the animal, right the shell £foot ; ?, liver region; o, inder opening of mantle. Class IV. Gasteropoda. Although more highly organized than the Acephala, the snails are in some respects more primitive. The regions of the body- foot, visceral sac, head, and mantle — occur in all orders, although in each one or more forms may occur in which one or another part is lost. As a rule the foot is flattened ventrally to a creeping sole. In it may be distinguished anterior and posterior processes, the pro- podium and metapodium, a sharp lateral margin, the parapodium, and, above these, appendages or ridges, the epipodia. Inside the foot is usually a pedal gland. The head bears (1) the tentacles, a pair of muscular lobes or hollow retractile processes; (2) a pair of primitive vesicular eyes, which usually lie at the basis of the tentacles, but may rise even to their tips. In many snails the eyes are on special stalks which, as in the stylommatophorous Pulmonata, form a second pair of tentacles. The protrusion of the tentacles is caused by an inflow of blood, their retraction by muscles attached to the tip which draw them in like a finger of a glove. 370 MOLLUSC A. The mantle begins on the back and extends thence forward over the body to near the beginning of the head. It covers the mantle cavity, a spacious chamber, which in the water-breathing Prosobranchiata, etc., contains the gills (ctenidia) and opens outward by a large aperture under the margin of the mantle. The edge of the mantle may be produced into a long groove-like siphon, conveying water to and from the branchial chamber, which is of importance in determining the shape of the shell. When, by degeneration of the gill, the animals become air-breath- ing, the mantle cavity becomes a lung, and the opening, by growth of the mantle edges to the body, becomes a small spiraculum, closed by muscles. The visceral sac, by the great development of the gonads and liver, becomes very large. Since growth downwards is prevented by the muscular foot, the organs press towards the back, carrying before them the dorsal wall at the origin of the mantle folds, the line of least resistance. Some organs, like nephridia and heart, may be pressed into the mantle cavity. When the visceral sac, as often occurs, becomes enormous, it does not stand directly upwards, but coils from left to right in a spiral. The older the animal the more the spiral coils and the larger the last or body whorl. The visceral spiral therefore begins at the tip with narrow whorls which increase in size with approach to the rest of the body. From the foregoing the shape of the shell is easily understood. As a secretion of the mantle it takes the form which the mantle assumes under the influence of the visceral sac. With slight devel- opment of the visceral sac it forms a flattened cone (fig. 362, A), or is slightly coiled at the apex, as in the abalone (B). When the visceral sac is greatly elongate the shell is correspondingly an elongate cone. It is rarely irregularly coiled (Vermetidae, fig. 362, 0). It is usually coiled like a watch spring in one plane, or like a spiral staircase ; in the latter case the shell is more or less conical (fig. 362, D, E) and one can speak of its apex and base. In the middle of the base is usually a depression, the umbilicus. Sometimes the coils are loose and do not touch in the axis con- necting umbilicus and apex, so that one can look into the space, but usually the coils fuse together into a calcareous pillar, the columella, around which the whorls pass (fig. 362, E, c). The shell increases to a certain size by additions from the mantle edge; and since this determines the aperture, the shell is marked with parallel lines of growth. The pigment is elaborated on the edge of the mantle, and in the formation of the shell passes into IV. GASTEROPODA. 371 it, causing its color pattern. When the siphon is present the shell shows a corresponding process. Thus are distinguished holostomate shells with smooth mouths (fig. 362, D) and siphono- FIG. 362.— Various forms of shells. (After Schmarda, Bronn, and Clessin.) A, Patella costata: B, Haliotis tuberculata; C. Vermetus dentiferm ; D, Lithoglyphus naticoides; E, shell of Murex opened to show c, columella ; 8, siphon. stome shells, in which the anterior margin is drawn out in a groove (fig. 362, E). A simple conical shell without further evidence is not proof of primi- tive structure. It may arise from the spiral form by degeneration, if the visceral sac be reduced. Thus the shells of Fissurella and Patella are to be explained, for the viscera here show the results of an earlier spiral twist. In most places the union between shell and soft parts is not very firm, but the connexion at the aperture is more intimate, while a muscle is at- tached to the columella (musculus columellaris) at about the middle point of its height, the other end being inserted in the foot. It is for the retrac- tion of the animal within the shell, first the anterior part with the head and then the rest with the metapodiuin. In this the metapodium is folded so that its dorsal surface lies towards the aperture. Hence in many species- this surface secretes a door, or operculum, which closes the aperture when the body retracts. Since the aperture increases in size with growth, the- operculum must also enlarge, which is accomplished in a spiral manner (fig. 362, D), the process sometimes showing in a spiral line on the out- side. So-called eye stones are the opercula of small Trochidee and Tur- binida3. Land snails are usually without opercula, but at certain times, 372 MOLLUSC A. as in hibernation, they can close the shell by a calcareous plate, the epi- pJiragm. In the spring this separates from the shell and is lost. In most gasteropods the shell is coiled to the right, but in some species (fig. 363) the whorls are constantly turned to the left, while reversed specimens occasionally occur in many species which are normally dextral. In the shell there are at most two layers, an inner lamellar layer (not always present), which sometimes is highly iridescent, and an outer porcellanous layer, which is °Pa(lue and contains the pigment. In rare of Lanistes carinatus. cases the mantle and consequently the shell are lack- £sTn u'ing, or the mantle is present but the shell is rudi- mentary and not visible externally because the mantle folds have grown over it. In these cases the visceral sac is not prominent. Since the shell- less forms possess a mantle and shell in the young, the adult conditions are explained by degeneration. Only a few gasteropods are like the Amphineura and Acephala in being bilaterally symmetrical. Usually the spiral twist of the visceral sac has resulted in a torsion of other parts from left to right, in which alimentary tract, nephridia, gills, heart, and nerv- ous system take part. The intestine is bent in this way, the anus opening into the mantle chamber on the right side, or the twisting may be continued so far as to double the intestine on itself, the anus being in the middle line in front, near the head. Nephridia, TTlG. 364.— Three diagrams illustrating the torsion of the body and the twisting of the nervous system in gasteropods. (After Lang.) A, bilateral, B, asymmetrical, C, streptoneurous condition. The reference letters are placed upon the organs of the primitive left side, a, anus: c, cerebral ganglion: 0, ctenidium; 7, auri- cle; w, mouth; n, nephridial opening; o, osphradium; pa, parietal ganglion; pe, pedal ganglion; pi, pleural ganglion; v, ventricle. gills (with them the osphradia), and heart wander in company, so that the organs primitively belonging on the left side may be trans- IV. GASTEROPODA. 373 f erred to the right and vice versa. With this there is a tendency to asymmetry and the loss of the organs (usually of the primitively left side). When the nervous system takes part in the twisting a notable crossing of the cerebrovisceral commissures takes place, known as streptoneury or chiastoneury (fig. 364, c). The alimentary canal begins with a muscular region which in some groups is developed into a large protrusible proboscis (fig. 365). The pharynx, which follows, contains the tongue, a ventral ridge supported by one or more cartilages and covered by a cutic- ular layer, the radula or lingual ribbon (odondophore). The upper surface of the radula is armed with sharp, backwardly di- rected teeth (fig. 366) which, are usually arranged in trans- verse and longitudinal rows,, but which vary so in num- ber, form, size, and arrange- ment that they are of value in classification. Although the radula covers the tongue, it is x r FIG. 365. FIG. 366. FIG. 365.— Pyrula tuba, male. (After Souleyet.) The mantle has been cut on the right side and turned to the left, reversing the pallial organs, a, anus ; c, ctenid- ium ; cm, columellar muscle ; /, foot ; 7i, heart in pericardium ; i, intestine ; I, liver; 777, mantle; m/, floor of mantle cavity; ?7, nephridium; ns, opening of nephridium; o, osph radium ; p, proboscis; pe, penis; t, testes; v, vas deferens cut in two. FIG. 366.— Pharyngeal region of Helix pomatia. A, side view ; B, section, m, muscle; oe, oesophagus ; r, radula ; rs, radula sac ; sp, salivary duct ; z, lingual cartilage. formed in the radula sac, which lies behind the tongue. From this it grows forward like a nail over its bed as fast as it is worn out in front. It is opposed in eating by a single median or a pair of lateral jaws (lacking in carnivorous forms). The rest of the alimentary canal is convoluted, the anus being 374 MOLLUSC A. usually on the right side in front, in or beside the mantle chamber (figs. 365, 370, 371). Rarely it empties in the middle line behind. (Esophagus, stomach, and intestine are slightly marked off from each other. The convolutions of the intestine are enveloped FIG. 367.— Row of teeth from the radula of Trochus cinerartus. (After Schmarda.) by the liver, which by its large size forms the chief part of the visceral sac. A pair of salivary glands empty into the pharynx, these in the Doliidae secreting free sulphuric acid. The nervous system usually differs from that of other molluscs in that the pleural and parietal ganglia are free (p. 353). If the commissures be short, the ganglia are collected near the pharynx and, thus freed from the body torsion, are symmetrical (orthoneu- rous, fig. 368, //). If the cerebrovisceral commissure be longer, the result is almost always streptoneury (chiastoneury). Pleural and visceral ganglia hold their place, but the right parietal ganglion crosses above the intestine to the left side (hence called supra- intestinal), while the left passes under the intestine to the right side (subintestinal), the cerebrovisceral commissure being twisted like the figure 8. The strong development of the pharynx is ac- companied by buccal ganglia. The existence of streptoneurous forms among the orthoneurous Opisthobranchs (Actoson) and Pul- monata (Chilina) shows that orthoneury in these groups has arisen from streptoneury. Gills, heart, and nephridia are best treated together. Certain genera (Haliotis, Fissurella) recall the Acephala in having these organs in pairs, while the intestine passes through the heart. As a rule the asymmetry induced by the torsion of the body has resulted in the loss of the ctenidium, osphradium, nephridium, and auricle of one (the primitively left) side. Prosobranchs and Opisthobranchs are recognized accordingly as the gills are on the anterior or posterior part of the body. In the Opisthobranchs (fig. 369) the ctenidia have been lost and are replaced by secondary gills on the back. Here the heart is in front of the gills; it receives blood from behind and forces it forward to the head by an aorta. In the Prosobranchs the heart has been twisted about ninety IV. GASTEROPODA. 375 degrees, so that the auricle is in front and the ctenidium in front of this (fig. 370), while the aorta leads backwards. The nephrid- ium, which communicates with the pericardium by a nephro- stome, is rarely a racemose gland; usually it is saccular, the lumen /z. FIG. 368. FIG. 369. FIG. 368.— I, streptoneurous nervous system of Paludina. (After Hering, from Gegenbaur.) II, orthoneurous system of Linmcen. (After Lacaze-Duthiers.) A, visceral; B, buccal; C, cerebral; p, pedal; PI, pleural; s6, sp, s\ib- and supra- intestinal ganglia; n, olfactory nerve; p, otocyst. FIG. 369. — Diagram of circulation in Doris. (After Leuckart.) a, auricle; c, gills around anus; t, tentacle; v, ventricle; ar, vessels returning venous blood from the body. bearing gland cells and concretions; its duct either empties into the mantle cavity or beside the anus. The sexual organs in some forms (Cyclobranchs and many Zygobranchs) empty into the nephridia. They show two extremes. On the one hand are completely dioecious species, on the other there may be complete hermaphroditism (many Tectibranchs, Pteropoda), in which the male and female organs are united throughout their extent. Intermediate stages occur; those of the pulmonates are described below. 376 MOLLUSC A. In the Helicidae there is a hermaphrodite gonad which lies together with the liver in one of the first whorls of the shell (fig. 371, «). A coiled genital duct follows which widens to a thick-walled 'uterus' (u) along FIG. 370.— Anatomy of Cyprcea tigrfs. (After Quoy et Gaimard.) br, ctenidium; c, heart; d/, vas deferens; /t, liver ; m, stomach ; N, cerebral ganglion ; oc, eye ; pe> penis ; ph, pharynx, the radula drawn out; r, rectum ; re, nephridium; £, testes. which a second seminal canal appears to lie. Actually in the interior there is but a single lumen, the different appearances being due to glands in the walls. A separation into vas deferens and vagina occurs at the end of the uterus. The vas deferens (vd) proceeds as a small coiled canal to the genital pore. Here it enlarges to a protrusible penis (p) with which is connected a retractor muscle and an appendage, the flagel- lum (fl). The vagina is broader and goes straight to the genital pore, where it meets the penis. Connected with the female genitalia are the large albumen gland (ei) at the beginning of the uterus and a receptac- ulum seminis (r) ; a round vesicle connects with the vagina by a long duct, and (not always present) two ' finger-form glands.' Lastly, the dart sac (ps) of the vaginal wall, which secretes a calcareous stylet, the * love dart,' which in copulation acts as a stimulus to the male genitalia. In spite of hermaphroditism a copulation lasting for days may occur, con- nected with which is the fact that in many species the male cells are first matured, then the female (proterogyny) ; or the reverse may occur (proterandry). The sexual opening is almost always on the right side, beside the anus or in front of it on the head. Its position may be rec- ognized in hermaphroditic species and in dioecious males by the IV. GASTEROPODA. 377 grooved dermal fold, the penis (fig. 370, pe). Occasionally this is separated from the genital pore, but is connected with it by a cili- ated groove. The terrestrial snails lay their large tough-shelled eggs in damp earth; in the aquatic forms the eggs are laid in masses, usually FIG. 371.— Anatomy of Helix pomatia, the roof of the pulmonary sac cut at the left side and turned to the right; the pericardium and visceral sac opened and the viscera separated, a, anus; c, columellar muscle; d, intestine; ei, albumen gland ; /, finger-form gland; /Z, flagellum: fu, foot ; 0, cerebral ganglion ; h, heart ; Z, liver; ZM, lung ; m, stomach; w, nephriaium; n', its opening; p, penis; ps, dart sac; r, receptaculum seminis; s, pharynx; sp, salivary gland; M, uterus; t>, vagina ; vd, vas def erens ; 2, hermaphrodite gonad. gelatinous, each egg with a layer of albumen and a firm shell. Occasionally there is a kind of nest, as is the case with lanthina which carry the mass of eggs, attached to the foot, about with them. A few gasteropods are viviparous. In the development the great constancy with which the veliger stage (figs. 342, 343) appears is noticeable. Most marine larvae swim by their velum (often divided) at the surface before creeping at the bottom. But in those cases where the snail leaves the egg 378 MOLLUSC A. in the definitive condition the velum is usually developed in embryonic life, sometimes so strongly that the embryo rotates in the surrounding fluid. Order I. Prosobranchia. The Prosobranchs, like most gasteropods, have the twisting of the visceral complex from left posterior to right anterior, so that the anus lies on the right side near the head, the nervous com- missures are twisted into an 8, and the nephridia of the right side have been carried to the left, where they lie far forward. This has twisted the heart so that it receives branchial blood from in front and sends it backwards through the aorta. The sexes are separate and the shell and mantle are usually well developed. Accordingly as the mantle is drawn out in a siphon or not, the shells are siphonostomate or holostomate (p. 371). Certain Prosobranchs are near the primitive Amphineura in the reten- tion of both ctenidia, both auricles, and both nephridia, but in the great majority only one gill (the primitive right) is present and the corresponding auricle alone is well developed, although the other may exist in a rudimentary condition. Sub Order I. ASPIDOBRANCHIA (Diotocardia, Scutibranchia), Ctenidium bipectinate (fig. 372) or absent. There are usually two auricles and two nephridia. DOCOGLOSSA (limpets), auricle single ; FIG. 372. FIG. 373. FIG. 372.— Fissurella patagonica, ventral view. (From Bronn.) br, the paired gills; p, foot. Fio. 373.— Acmcea testudinalis* limpet. (From Binney-Gould.) one or no ctenidinm ; intestine not passing through heart, shell conical. ACM.EIDJE with ctenidium. Acmcza* (fig. 373). PATELLID.E, ctenidia lacking, replaced by a ring-like mantle gill. Patella (fig. 362, A). ZYGO- IV. GASTEROPODA: PROSOBRANCHIA. 379 BRANCHIA. Two ctenidia (fig. 372), shell with marginal slit or with holes corresponding to an anal notch in the mantle; auricles and ne- phridia paired; heart traversed by intestine. FISSURELLID^, keyhole lim- pets; shell conical, with apical opening. HALIOTID^E, abalones; shell weakly spiral, flat, with a series of holes. Haliotis* (fig. 362, B). AZYGO- Fio. 374.— American Pectinibranch gasteropoda. (From Binney-Gould.) A, Crepidula fornicata; B, Lacuna vincta; C, llyanassa obsolete, ; D, Littorina palliata; E, L. litorea ; F, Urosalpinx cinerea ; G, Purpura lapillus ; H. Buccinum undatum ; I, Lunatia heros. BRANCHIA. One ctenidium, but two auricles. TROCHID^E, operculum liorny ; Trochus, Margarita* TURBINID^E, top shells; operculum calca- reous. Turbo, Phasianella. Sub Order II. PECTINIBRANCHIA (Monotocardia, Ctenobranchia). ^Ctenidium unipectinate, osphradium well differentiated (fig 365), intestine 380 MOLLUSC A. not passing through the heart. Many groups are recognized, based upon the structure of the lingual ribbon. Of the thousands of species only a few groups can be included here. KHACIIIGLOSSA ; siphonostornate, predatory. MURICID.E (Murex, Purpura* Urosalpinx*) have an anal gland secreting a substance first colorless, turning to purple by exposure to air. The Tynan purple was produced by Murex trunculus. Urosalpinx cinereus* drills into oysters. BUCCINID.E, whelks, VOLUTID^E, and OLIVID^E belong here. TOXIGLOSSA ; CONID^E, with large oesophageal poison gland, some species producing severe wounds. Conus, tropical; Bele.* T^ENIOGLOSSA ; NATICID.E, Neverita* and Lunatia,* common snail of Atlantic coast, their egg-masses being the familiar sand saucers. LIT- TORINID.E ; periwinkles. CYPR^EID^B, cowries; Cyprcea moneta of India is used as money in Africa. AMPULLA RID^E; amphibious, part of branchial cavity acting as lung, part containing ctenidium. PALUDINID^E, fresh water. CYCLOSTOMID.E, tropical terrestrial forms, the mantle cavity a lung. HETEROPODA. In all details of gills, genitalia, heart, and nervous- system these are true Pectinibranchs, but from an exclusively pelagic life have acquired peculiar modifications. As in most pelagic animals the body is gelatinous and transparent. The head is elongate, and the body is- enlarged so that usually it cannot be retracted into the shell. Most char- acteristic is the division of the foot into pro- and metapodium (fig. 375), the FIG. 375.— Carinaria mediterranea (after Gegenbaur), shell removed. A, metapodium ; a, anus ; ar, aorta ; #, visceral sac ; ftr, branchiae, the heart above ; 'if, vas def- erens ; o, mouth ; oc, eye with tentacle ; 05, oasophagus : p. propodium : ns. penis ; I, II, III, cerebral, pedal, and visceral ganglia. latter forming a tail-like elongation of the body. The propodium is verti- cally flattened and by its undulations serves as a swimming organ. The Heteropoda are predaceous and extremely voracious ; they swim back downwards. The ATLANTID^ can completely withdraw into the shell and close it with an operculum ; the CARINARIHLE (fig. 375) have a shell which scarcely covers the visceral complex ; the PTEROTRACHEID.E have no shells. IV. GASTEROPODA: OPISTHOBRANCHIA. 381 Order II. Opisthobranchia. The Opisthobranchia have not varied from the primitive sym- metry to such an extent as have Prosobranchs and Pulmonates. The anus is in the plane of symmetry or only slightly removed from it, although it may be placed far forwards. The nervous system is orthoneurous, the twist being straightened (except in Actseonidae). The heart also retains its primitive position, receiv- ing blood from behind and forcing it forward to the body through the aorta (fig. 369). In rare cases a (right) ctenidium, a poorly developed mantle, and a thin shell enveloped in the latter occur. Usually these have been lost and the place of the ctenidium is taken by accessory gills of various forms or a dermal respiration FIG. ZlG.—Hyalcea complmata from above. (After Gegenbaur.) a, arms ; 6r, gill ; c, heart ; , Thelyphonid (after Blanchard). o, abdomen ; rig.) marine aud brackish-water CALIGID.E (Caligus *) have similar habits. LERN^EOPODID^:. Fish parasites with maxillae united into an adhesive organ. Achtheres * (fig. 6), parasitic on perch. LERN^EID^E ; worm-like parasites. Lerncea branchialis,* common on gills of cod ; Lernceocera* (fig. 423), on pike ; Penella.* Sub Class 1 V. Ostracoda. Like the Cladocera and the Estheriidae the Ostracoda are en- closed in a bivalve shell, which, when closed, includes not only the body but the head and appendages as well, these being pro- truded when the shell is opened. The valves are closed by an adductor muscle, opened by a hinge ligament like that of lamel- libranchs. This resemblance to the molluscs is heightened by lines of growth upon the shell. /. CRUSTACEA: CIRRIPEDIA. 423 The antennae, the first simple, the second frequently two- branched, are used for swimming and creeping, and are bent back- wards and provided with numerous joints and hairs. The following appendages (mandible, maxillae, and three pairs of legs) FIG. 426. — Cypris fa sciatus, adult female. (After Glaus.) I-IV, appendages; c, furca; e, eye; /, liver; m, adductor muscle of shell; o, ovary; s, shell gland. vary greatly from genus to genus. The internal structure is also variable. The Ostracoda are largely bottom forms and live in fresh and brackish water as well as in the sea. CYPRIDINID^E. First two pairs of legs maxillary in character, the last developed into a hook for cleansing the shell; heart v^esent; marine. Cypridina* CYPRIDID.E. First pair of legs maxillary in character ; heart lacking; fresh water. Cypris,* Candona.* Sub Class V. Cirripedia. The cirripeds, or 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, 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 sur- face in the neighborhood of the head, and is initiated by the first antennae, in which is a cement gland secreting a no. 427.-flaZajitw/iameri,* acorn rapidly hardening cement. The region Da™ne)' Formed of rosteum' nf fivatirm in +Vio "RalQnirJco (Grr /fO^N lateralia, and carina, the nxation in tne tfaiamdae (fig. 4xJ7) opercuium of scuta («) and lies in the plane of the head; in the terga (t)- Lepadidae it is drawn out into a long muscular stalk (fig. 114). To this attached life are related all the peculiarities of structure. 424 ARTEROPODA. It is clear that a fixed animal has greater need of protection than one which can flee from its enemies, therefore we find not only a right and left mantle and a shell capable of complete closure, like that of an ostracode, but also in this calcified plates, the scuta and terga (figs. 114, 427, s, t), the first cephalic, the other pos- terior, in position. Between the pairs of these is the gap through which the feet are protruded. Besides there are other calcified portions, one of which, the carina (fig. 114, c), corresponds to the hinge-line of the ostracode and in some Lepads is supplemented by a farther unpaired piece, the rostrum. In the Balanidae the rostrum and carina are much stronger, while between them other paired pieces, the lateralia, are intercalated. Lateralia, 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. 427). The body in both lepads and balanids has essentially the same structure. 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 abdo- men 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, in contrast to almost all other arthropods, are hermaphro- ditic, a condition possibly correlated with their sedentary life and the consequent need of self -impregnation. Yet it is to be remem- bered that the common forms have a long penis, so that these animals, crowded closely together, can fertilize each other. In cases of several species which live solitary complementary males occur. These are very small, purely male forms, with extremely simple structure (fig. 428), which live inside the mantle cavity near the genital openings. The un- Se^mented ^°^ ls enclosed in a sac (a lobes; m , muscles ;oc, ocellus: soft-skinned shell), and anchored by the p, penis; t, testis; vs, seminal n vesicle. antennas. The long penis protrudes from the mantle. In the genus Scalpellum there are purely hermaph- /. CRUSTACEA: CIRRIPEDIA, LEPADID^S, BALANID^E. roditic 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. 429) was studied that oe FIG. 429.— Nauplius (A) and Cypris (B) stages of Sacculina carcini. (After Delage.) 1, 2, antennae ; 5, mandible ; /, cirrhous foot ; m, muscles ; oc, nauplius eye ; ou, anlage of ovary. 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 nau- plius eye. Order I. Lepadidae. Stalked cirripeds, with shell largely formed of scuta, terga, and carina ; other parts may be added. Lepas anatifera* (fig. 114) is the goose barnacle, which owes its common name to a mediaeval myth which claimed that the Irish (or bernicle) goose developed from these animals. L. fascicularis,*(tig. 430), abundant floating on the eastern coast. Anelasma squalicola, a thin-skinned barnacle, is parasitic on sharks and forms a transition to the Rhizocephala. Order II. Balanidae. Sessile cirripeds with calcareous shell formed of carina, rostrum, and lateralia; scuta and terga forming the valves (fig. 427). Balanus ~balanoides* common on our coast. Coronula dwdemata, attached to the skin FlG of whales. fascicularis,* goose barnacle. (From Smith.) 426 ARTHROPOD A. Order III. Rhizocephala. These forms differ so much from the other cirripeds as to demand sepa- rate mention. They are parasitic on the abdomens of various decapod crabs and consist of a stalk which penetrates the body of the host and a body which remains outside. The stalk, which branches in a root-like man- Fio. 431.— Sacculina carcini attached to Carcinus mcenas, whose abdomen is extended, ru, shell opening; r, network of roots ramifying the crab; s, stalk ; a, o, d, anten- nula, eye and anus of the crab. ner, penetrates the cephalothorax and absorbs its juices. Since the stalk furnishes the food, an alimentary canal is absent. The body lacks all ap- pendages, is enclosed by a soft-skinned mantle, and is almost entirely filled with the gonads. Since these forms lack, as adults, all arthropodan features, their position is only settled by their development, which shows (fig. 429) no great difference from 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 Crus- tacea 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 some Isopoda. The male geni- tal ducts open on the thirteenth, the female on the eleventh, segment. /. CRUSTACEA: LEPTOSTRACA. 427 Legion I. Leptostraca. The Leptostraca connect the Phyllopoda with the higher groups. They have twenty-one somites, eight abdominal, eight thoracic, and five cephalic, and this 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 abdo- men, and the leaf-like thoracic feet, are phyllopodan. They have an antennal gland and a rudimentary shell gland ; an elongate heart which extends through cephalothorax and abdomen; and com- Fie. 432.— Nebalia Znpes.* (After Sars.) 7t, heart; i, intestine; o, ovary; a, adductor of carapace ; b, brain ; r, rostrum. pound stalked eyes. The few species are all marine and belong to the genus Nebalia. N. Mpes * (fig. 432). Legion II. Thoracostraca (Podophthalmia). The names given this division have reference, first, to the fact that the head and more or fewer of the thoracic segments are united into an immovable part, covered by a firm carapace; second, that the compound eyes (except in Cumacea) are placed at the ends of movable eye stalks, a condition which has possibly arisen from the inflexibility of the anterior part of the body. The first five appendages are always two pairs of antennae, a pair of mandibles, and two pairs of maxillae. The remaining pairs vary greatly in character 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 metamor- phosis in development in which a nauplius-stage may appear, most frequently in the lower forms (schizopods), but even in the deca- pods (Peneus). 428 ARTEROPODA. Order I. Schizopoda. These are small forms, mostly marine, in which the cephalo- thorax is covered by a carapace with which some or all of the FIG. 433.— Amphithoe. (From Gerstacker.) a", a2, first and second antennae ; aw, eye; VlI-XIIIi the seven free thoracic segments ; 1-7, abdominal segments. thoracic somites are firmly united. The eight thoracic feet retain throughout life a biramous condition and are used in swimming. The posterior pair of abdominal feet together with the telson form FIG. 434.— Mi/sis elonqata. (From Gerstacker.) a, /3, first and second antennae; a, ex- pedite; cm, eye; «, endopodite; o, otocyst ; 1-7, abdominal somites. a caudal ' fin ' by means of which the animal can swim backwards. The delicate skin permits of diffuse respiration, and gills are fre- quently 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. I. CRUSTACEA: STOMATOPODA. 429 The MYSIDID.E are the most widely distributed, several species of My sis (fig. 434) occurring on our coasts and one in the Great Lakes. In these the endopodite of the sixth abdominal appendage contains an otocyst, with a calcic fluoride otolith. Other families are the EUPHAUSIID^E and LOPHO- GASTRID.E of the deeper seas. Order II. Stomatopoda. In structure of the cephalothorax these forms, known as mantis shrimps (from a resemblance to the insect, the praying mantis), have not advanced as far as the schizopods, since the last three thoracic somites remain free and are not covered by the carapace. FIG. 435.— Squilla mantis, at, at', first and second antennae ; /, sixth abdominal feet; fc, gills ; p, schizopodal thoracic feet ; pr, pr', raptorial feet ; ps, pleopoda ; sa< telson. The appendages, however, are more differentiated, since only the three posterior 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 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 re- garded as adults and described as Alima and Erichthus. Squilla empusa lives on our east coast, Gonodactylus in Florida. They are burrowing animals and deposit their eggs in their holes. 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 de- 430 ARTHROPODA. velopment and become strong walking legs, terminated either with claws or pincers (chelae). Usually the first pair is distinguished from the others by its size and by being chelate, and becomes not locomotor but grasping in function. In the development of a chela the penult joint sends out a strong process, the ( thumb/ FIG. 436.— Erichthus stage of Squilla (orig.). which extends as far as the last joint (the e finger '), which closes against it. The mouth parts — a pair of mandibles, two pairs of maxillae, and three pairs of maxillipeds (fig. 404)— lie in front of the first pair of legs. The maxillipeds (7, 6, 5)show clearly a biramous condition, while the maxillae (4, 3) retain considerable of the original phyllopod character. In the man- dibles (2) there is always a strong basal joint, the edge of which serves as a jaw, while this may bear additional joints, the palpus. Behind the mouth are a pair of scales, the paragnaths or metastoma, formerly regarded as appendages. The antennae are usually distinguished from their size as antennae (second pair) and antennulce (first pair, fig. 404). They have large basal portions, which in the antennulae bear two many-jointed flagella, /. CRUSTACEA: DEC APOD A. 431 while the antennae proper have but a single though usually much larger flagellum. On the basal joint of the antennulae is the auditory organ (p. 412), while the green gland opens on the basal joint of the antennae ^flg. 439, 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. 439); the other appendages (fig. 404, 7) are small, bira- mous 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 (introm it- tent) organ. The condition of these appendages as well as the openings of the genital ducts — on the base of the third walking foot in the female, the fifth in the male— serve at once to distinguish 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. 437) which are attached pdb.Q pdfc.13 FIG 437.— Gills of Astacus exposed by cutting away the branchiostegite. pdb, plb. podo- and pleurobranchia of the corresponding segments; r, rostrum; 1, stalked eyes; £, 3, antennae ; A-e, mandibles and maxillae; 7-9, maxillipeds; 10, #, bases of thoracic feet; 15, first pleopod. to the bases of the appendages (maxillipeds and walking feet) or to the sides of the body near them. (In the Thalassinidse — forms near the Astacidae — 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 (branchiostegite) over them, thus enclosing them in a branchial chamber. A process of the second maxillae — the scapliognathite — 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, a fact readily explained when we remember that they retain some water in the gill chamber, which keeps the gills in a moist con- dition. In some of the tropical land crabs which live almost ex- clusively on land there is a true aerial respiration, the lining of the gill chamber becoming modified into a kind of lung traversed 432 ARTHROPODA. by numerous blood-vessels. In Birgus latro the gill chamber is divided into two portions (fig. 438), the upper part being pulmo- nary, the lower containing the reduced gills. FIG. 438.— Diagrammatic section through Birgus latro, showing lungs. (From Lang, after Semper.) a,, a4, afferent blood-vessels : ah, pulmonary chamber; ek, el, el'% efferent blood-vessels ; ft, heart; k, gills ; kd, branchiostegite ; p, pericardium. C bl... FIG. 439.— Anatomy of Crayfish (Astacus). A, dorsal surface removed ; B, scheme of circulation ; C, viscera removed, showing green gland and nervous system. a, anus ; oa, hepatic artery ; ae, antenna ; az, antennula, also sternal artery ; am, muscles of stomach ; «o, ophthalmic artery ; op, abdominal artery ; av, ventral artery; bl, urinary bladder; ftr, gill arteries; c, oesophageal commissures; gd, green gland; 0n', brain ; .qr?i2-13, ganglia of ventral chain; ft, heart; hd, intes- tine ; fc, mandibular muscles ; I, l\ liver and its duct ; w, stomach ; o, otocyst ; oes, oesophagus ; on, optic nerve ; pc, pericardium ; sgn, sympathetic nerve ; t, t', unpaired and paired portions of testes ; t?, ventral blood sinus ; vd, vas deferens ; vdr, vems from gills to heart. /. CRUSTACEA: DEC APOD A. 433 Correlated to this localized respiration is the nearly closed cir- culatory system (figs. 439, A, B). The heart (/*), a compact pentagonal organ, receives its blood from the uericardial 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 (v), passes thence through gills, and is returned by several branchial veins (vbr) to the pericardium. The alimentary canal is straight and has only one conspicuous enlargement, the so-called stomach (fig. 439, A9 m), divided into two portions, an anterior sac (cardiac pouch), lined with chitinous folds and teeth and serving to chew the food and bearing in its walls the so-called ' crab-stones/ which are masses of calcic carbon- ate stored up to harden the armor rapidly after the molt. The second or 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 (I) open just behind the stomach. The two antennal glands (fig. 439, C, gel), each provided with a large urinary ^bladder ( bl), are dirty green in color, whence the name green glands often applied to them. The gonads (figs. 440) lie close beneath the heart, those of the two sides FIG. 440. FIG. 441. FIG. 440.— Reproductive organs of (A) female and (B) male crayfish. (From Hux- ley.) od, oviduct; od', its opening on llth appendage; ou, ovary; f, testes; vd, vas deferens ; vd', its opening on 13th appendage FIG. 441.— Nervous system of crab, Carcinus. (From Gegenbaur.) a, antennal nerves ; c, cesophageal commissures; gi, fused ventral chain perforated for sternal artery ; gs, brain ; o, optic nerve. 434 ARTHROPODA. 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. 439, C) the ventral chain consists of six ganglia in the thorax, six in the abdomen, but in the Brachyura (fig. 441) these all flow together in a common mass, connected with the brain by two long oasophageal commissures. The development of most decapods is interesting from the number of larval forms. As a rule a zoea (fig. 415) is hatched from the egg ; this passes next into a Mysis-stage (fig. 442) in which head, thorax, and abdo- men are distinct, the thorax bearing biramous feet like those of schizo- pods— 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. 443), in which the abdomen is well developed but the feet have lost their FIG. 442. FIG. 443. FIG. 442.— Phyllosoma larva (Mysis-stage) of Palinurus. (After Gerstacker.) A, ab- domen ; C', head : T, thorax ; a and i, exopodites and endopodites of thoracic feet. FIG. 443.— Megalops larva of Portunus. (From Lang, after Glaus.) 2, antennae ; IV- VIIi\ thoracic appendages ; a2-a6, abdominal somites (a* is the seventh). biramous character. In some prawns (Peneus) the series is rendered more complete by the appearance of a nauplius and a metanauplius with many appendages, before the zoeal stage. In the crayfish and many land crabs the metamorphosis has been lost, but the lobster leaves the egg in the Mysis-stage. Differences may occur even in the same species; thus in the European Palcemonetes varians the embryo, in the sea, leaves the egg as a zoea ; in fresh water in the Mysis-stage. Sub Order I. MACRURA. Abdomen well developed ; antennae long ; ventral nerve chain elongate ; no megalops-stage in development. CARIDE A. Body compressed ; no sutures on carapace ; feet weak, ex- ternal maxillipeds pediform; a large scale on the second antennae. In the PENEID^E there are weak exopodites. Peneus* Sicyonia* PAL^EMO- NID^E, mandibles bifid at tip. Palcemon, Alpheus,* Hippolyte,* Panda- lus.* In the CRANGONID.E the mandible is simple. Crangon* Sabinea* /. CRUSTACEA: DEC APOD A. 435 ASTACOIDEA. Carapace crossed by a transverse groove. The ASTACID.E have well-developed chelae. Cambarus* includes the crayfish of the A B FIG. 444.— A, Crangon vulgaris *; B, Pandulus montagui.* FIG. 445.— Eupagurus bernhardus, hermit crab. (From Emerton.) eastern states; those of the Pacific coast and Europe belong to Astacus* The lobsters belong to Homarus* PALINURID^J (Loricata), no chelae, 436 ARTHROPODA. body with heavy armor; larva leaf -like and transparent 'glass crabs/ culled Phyllosomae (fig. 442). Palinurus* spiny lobster. PAGURIDEA, hermit crabs; abdomen reduced, soft-skinned, and hidden for protection in a snail shell which the animal carries about, which habit has resulted FIG. 446.— A, Platyonichus ocellatus,* lady crab ; B, Libinia emarginata* spider crab (From Emerton.) in a spiral twisting of the abdomen. Some hermits (Eupagurus) carry sea anemones or hydroids on their shell, cases of symbiosis (p. 170). Eupagurus* Clibanarius* Allied is Birgus, the palm crab of the East /. CRUSTACEA: CUMACEA. 437 Indies, which is said to climb palm trees for the cocoanuts, which it eats. Its respiratory organs have been referred to on p. 432. Sub Order II. BRACHYURA. Body depressed; abdomen rudimentary and folded in a groove under the cephalothorax; antenna short; never more than one pair of feet chelate; ventral nerve cord concentrated (fig. 441). Omitting some inconspicuous groups like the porcellain crabs (PoR- CELLANIDJ2), the HiPPiD^:, and the LITHODID.E, which are united as a group of Schizosomi from the fact that the last thoracic segment is free from the carapace and its appendages are rudimentary, the sub order is usually divided as follows: LEUCOSOIDEA (Oxystomata). Body oval or triangular, area of mouth parts triangular, the apex anterior. Calappa, Matuta* Hepatus * of warmer seas. OXYRHYNCHA (Maioidea). Cephalothorax triangular, narrowed in front; mouth area (as in the following tribes) quadrilateral. Mostly tropical. Hyas,* Libinia* Pugettia* spider crabs. CYCLOMETOPA. Body broader than long, regularly arcuate in front. CANCRID.E, with last pair of feet pointed. Cancer* shore crab; Pano- peus,* mud crab. PORTUNID^E, with last pair of feet flattened paddles. Platyonichus *; Neptunus liastatus* when thin-skinned after molting, is the 'soft-shell crab' of the markets. CATOMETOPA. Front of carapace nearly straight; body from above nearly quadrilateral; Gelasimus* the fiddler crabs of our warm shores; Pinnotheres ostreum* common in oysters; GECARCINID^E (Z7ca, etc.), land crabs of the tropics, which only go, to the sea at the reproductive season to lay their eggs. Order IV. Cumacea. Small marine forms with sessile eyes, three or four free thoracic somites; appendages biramous; a brood sac beneath the cephalothorax. Of interest because combining arthrostracan and thoracostracan features. Diastylis (fig. 447). FiG. 447.— Diastylis quadrispinosus. Especial interest also centres in the little known Anaspides tasmanice from lakes in Tasmania, which unites schizopod and amphipod characters. It has the stalked eyes, caudal fin, and biramous feet of a schizopod; otocysts in the antennulge like a decapod; but agrees with the amphipods in shape of body and in free thoracic segments. The epipodial plates are paralleled elsewhere only in carboniferous species, with which these forms apparently are closely allied. 438 ARTHROPODA. Legion III. Arthrostraca (EdriopUthalmata). Although the head of the Arthrostracan consists of six seg- ments, it is remarkably short. It bears six pairs of appendages, one of the normal thoracic pair being added to it as maxillipeds. Eyes, when present, are aggregates of ocelli situated on the sides of the head. There are seven thoracic segments, the appendages of which are walking feet which lack exopodites. The abdominal appendages, when present, are always biramous, the telson never bearing appendages, and in the Amphipods is greatly reduced, sometimes being split nearly its whole length. The nervous system (figs. 75, 448) is of the ladder type. The alimentary canal is straight and has an anterior enlargement, the FIG. 448.— Male Orchestia cavimana. (After Nebeski.) a', a2, antennae ; ao, aop, anterior and posterior aortse ; c, heart ; d, digestive tract ; 0, brain and eye ; ft, testes ; /c, gills; 7c/, maxilliped ; Z, liver; ra, excretory organ ; ?i, ventral nerve cord; o, rudimentary ovary; vd, vas deferens ; I- VII, thoracic feet; 1-3. anterior, A-6', posterior abdominal feet. chewing stomach, behind which empty one or more pairs of long liver tubes, while in a few Amphipods a pair of excretory tubes, the so-called Malpighian tubules, empty into the intestine near its end. Respiratory and circulatory systems vary so that they are best described in connexion with the two orders. Order I. Amphipoda. The Amphipods are almost exclusively aquatic, a few species living on the shore near high-tide mark. A few live in fresh water (Gammarus, Allorchestes), the majority being marine. On land they move by a leaping motion, whence the common name, L CRUSTACEA: AMPHIPODA. 439 beach fleas. In swimming the abdomen is alternately bent against the breast and then forcibly straightened. The body is usually strongly compressed from side to side. The thoracic feet generally bear large epineural plates (fig. 433), which extend the sides of the body downward, while on the inner side delicate gills or gill sacs (fig. 449, br) arise from their bases. In the female brood lamellae (brl) are added — broad chitinous plates which enclose a brood chamber beneath the body in which eggs or young are carried. 'The three an- terior pairs of abdominal feet are two-branched, richly haired, and serve to create currents of water Fl«- 449 -Cross-section of Amphipod (Corophium). (From Lang, after De- which pass forward over the gills. The remaining abdominal feet, though biramous, are short and stout and form springing organs explains why the abdominal part of the heart is degenerate and only the anterior thoracic portion with three pairs of ostia persists. thoracic leg; bm, ventral chtae; brl, brood lage.) b/, nerve cord; br, branc lamella; d< intestine; 7i, heart; I, liver: ov, eggs in brood chamber. The position of the gills Sub Order I. HYPERINA. Large head and eyes; strong prehensile feet. Live attached to other pelagic animals on which they feed. Hyperia medusarum * lives on the jelly fish Cyanea; Plironima,* warmer seas. Sub Order II. GAMMARINA. Head much smaller; abdomen well devel- oped; are mostly free swimmers. Numerous species in the sea. Cfam- Fig. 450.— Gammarus ornatus.* (From Smith.) marus * occurs in shallow water, some being fluviatile; Orchestia * above tide marks. Chelura terebrans * destroys piles and other submerged wood. Sub Order III. L^EMODIPODA. Parasitic or semi-parasitic forms in which the first (second) somite is fused to the head; appendages are lacking 440 ARTHROPODA. from some of the thoracic segments and the abdomen is reduced. Species of Caprella* are common on hydroids. Cyamus ceti is parasitic on whales. Order II. Isopoda. The Isopoda are readily distinguished from the Amphipoda by their depressed (i.e. horizontally flattened) bodies. The feet are adapted for creeping, and a brood pouch is formed as in the Am- phipoda, but gills are lacking here since some of the abdominal feet are modified for respiration (fig. 451, k). In the abdomen, the somites of which exhibit a great tendency to fusion, the telson, as in all Malacostraca, is without appendages; the sixth somite FIG. 451. FIG. 452. FIG. 451.— Asellus aquaticus. (From Ludwig-Leunis.) a1, a2, antennae; />r, brood pouch; fc, pleopoda modified to gills; md, mandibles- p1-/)7, thoracic feet; paHxt'i abdominal feet (pleopoda); I-V1, head; VII-XIII, thoracic segments; XIV- XX, abdominal segments, partly fused. FIG. 152,—Cymothoa emarginata. (After Gerstacker.) p8, sixth pleopod. bears, in the walking forms, long forked appendages (fig. 451); in the swimming species (fig. 452) they are flattened and, with the telson, make a swimming organ. The five anterior pairs of pleo- poda are modified for respiration, by the expansion of the endop- odites into thin-walled plates, while the exopodites and the whole first pair serve as opercula or gill covers. As a result of this posi- tion of the gills the heart (usually with two pairs of ostia) is ab- dominal in position. In the terrestrial species the gills are adapted for breathing damp air. In Porcellio and Armadillidum the first or first and second opercula are permeated with a system of air tubes, which physiologically, though not morphologically, are comparable to the tracheae of insects. In the Isopoda the tendency to parasitism is greater than in the Amphipoda. Many swimming forms attach themselves to fishes and feed by boring with their mouth parts, which are modified for the purpose, /. CRUSTACEA: 1SOPODA. 441 into the skin. The Bopyridae live in the branchial chamber of shrimps. Cryptoniscus is a shapeless sac which attaches itself to the stalk of Sacoii- lina (p. 426), and, after causing the death of this parasite, uses its network of « roots ' for its own nourishment. The Entoniscidae (fig. 453) attack FIG. 453.— Entoniscus porcellance. (From Gerstacker, after Miiller.) A, male; 2?, female; C, heart; he, liver; la, brood lamellae; ov, ovary. FIG. 454.— J, Idotea irrorata *; B, Limnoria lignorum *; <7, ^ga psora * (' salve bug '); D, Leptochela algivola* (After Harger.) Decapoda and, pressing the skin before them, penetrate the interior. Their strange shape is largely due to the lobe-like brood lamellae. They are 442 ARTHROPODA. usually hermaphroditic, but have besides complemental dwarf males (fig. 453, A). Sub Order I. ANISOPODA. Six free thoracic segments; heart tho- racic; first thoracic foot (on head) chelate; abdomen with swimming feet. A group intermediate between Amphipoda and other Isopoda. Tanais,* Leptochela * (fig. 454). Sub Order II. EUISOPODA. Seven free thoracic segments. ONISCID.E; terrestrial, familiarly known as sow bugs; Ligia, on seashore; Porcellio* Oniscus* ArmadilUdum* 'pill bug.' ASELLID.E (fig. 451), fresh water. SPILEROMID.E, head broad, body rounded and convex; Sphwroma* Lim- noria lignorum * (fig. 454), the gribble, attacks submerged wood and is nearly as destructive as Teredo. IDOTEIDJE, free-living, marine, with usually elongate bodies; Idotea,* Ccecidotea* BoPYRnxE, parasitic on Caridea; body of female disc-like, asymmetrical, without eyes; Bopyrus* CYMO- THOID^E, parasitic on fishes or in their mouths. Cymothoa* Mga* Cirolana* Sub Order III, ENTONISCIDA, parasites whose general features are described above. Entoniscus, Cryptoniscus. Class II. Acerata. The animals comprising this group were formerly divided among the tracheates (p. 408) and the Crustacea, but more recent studies show that, although differing widely in respiration, the forms included are closely allied in structure and development and present many differences from both Crustacea and from other tracheates (Insecta). The former views were based upon a con- fusion between analogy and homology, it being thought that tracheae wherever found were homologous structures. In the Acerata the body is usually divided into two regions, cephalothorax and abdomen, though in some cases (mites) the two regions become fused. The cephalothorax consists of six somites which always bear appendages, and these appendages are arranged in a circle around the mouth, the basal joints of one or more pairs frequently serving as jaws. None of these appendages are like antennae (whence the name of the group). The abdomen consists of a varying number of somites, all of which may be free, or, again, may be fused into a common mass. These abdominal somites bear appendages in the embryo, but in the adults (except the Xiphosura) these are usually lost or so modified that their existence is only recognized by a study of development. The alimentary canal is straight, without marked enlargements, and lacks a chewing stomach. The liver is large and opens into the intestine by two or more pairs of ducts. The nervous system has some or all of its ventral ganglia arranged in a ring around the //. ACER AT A: GIGANTOSTRACA. 443 oesophagus, and in many forms is enclosed in the ventral artery. Excretory organs, in the shape of neph- ridia, are frequently present and open to the exterior at the base of the second or the fifth pair of appendages. Malpighian tubes may occur, but these, unlike those of other tracheates, are entodermal in origin and hence not homologous with them. FIG. 455. FIG. 456. FIG. 455.— Digestive tract of Ctenida ccementaria. (From Lang, after Dug6s.) a, ab- domen ; an, anus; da, rff, diverticula ('liver') of midgut ; g, brain; v6, rectal bladder (stercoral pocket) ; vm, excretory tubules. FIG. 456.— Lung book of Zilla cadophyla. (After Bertkau.) a, a lung leaf separated from the other leaves, 6 ; st, spiracle. The respiratory organs are either gills, lungs, or tracheae. The gills are borne on some of the abdominal appendages. The lungs are sacs on the anterior abdominal somites opening by narrow slits (fig. 461) to the exterior. The anterior wall of each lung sac is made up of thin plates arranged like the leaves of a book, and em- bryology shows that these lung books are gill books drawn into the ventral surface of the abdomen. The tracheae in development pass through a gill-stage and a lung-stage, the tracheal tubes being outgrowths of the spaces between the lung leaves which penetrate all parts of the body. The reproductive openings are on the basal somite of the abdo- men. The spermatozoa are motile. The development is direct, there being no metamorphosis. Sub Class I. Gigantostraca. Marine forms with gills on the 2-6 abdominal appendages; bases of five pairs of cephalothoracic feet masticatory ; a pair of medinn ocelli and a pair of compound eyes on the cephalothorax. 444 ARTHKOPODA. Order I. Xiphosura. Cephalothorax large ; abdomen terminated by a long spiniform telson. Limulus polyphemus of our east coast, commonly known as king crab FIG. 457. FIG. 458. Fio. 457.— Limulus polyphemus.* horseshoe crab (orig.). FIG. 458.— Ventral surface of Limulus moluccanus. (From Ludwig-Leunis.) chelicerse ; 2-5, walking feet ; 6', pushing foot ; 6a, flabellum ; 7, genital" operculum 8, gills (there should be five) ; P, base ' ' of telson. or horseshoe crab. Other species on eastern shore of eastern continent. They burrow beneath the sand and mud of the bottom and feed on worms. In the spring they come to the shore to lay eggs. Order II. Eurypterida. Extinct Silurian and Devonian forms with small cephalothorax and large twelve-jointed abdomen. The animals are intermediate between the xiphosures and the scorpions. Eurypterus; Pterygotus. some species seven feet long. Sub Class II. Arachnida. Under this name are included a number of orders of greater or less extent which can be arranged around the spiders, or Aranea, as a centre. There is considerable modification of form, and the following account applies only to the more typical groups. In these the cephalothorax and abdomen are separated by a distinct line, and since the abdominal appendages almost entirely disappear in the adult, the number of somites can only be ascertained where their boundaries are evident. The number varies between six in the phalangids and thirteen in the scorpions. The cephalothorax is, except in the Solpugidae, a single piece- //. ACEEATA: ARACHNIDA. 445 which bears six pairs of appendages; the four posterior pairs, con- sisting typically of seven joints, are locomotor, so that the posses- sion of eight legs is as characteristic for an arachnid as ten for a decapod or six for a hexapod. The first pair of append- ages, the chelicerce (fig. 459), are preoral, the second, m pedipaipi, beside that open- ing. The chelicerae are short and con- sist of two or three joints, the terminal joint either folding back upon the other or, pincer-like, meeting an opposable thumb. In the spiders the last ioint or n ... , . . . . * , FIG. 459.— Mouth parts of Epeira. claw is forced into the prey, introducing i, cheiicera; *, pedipaipi; p » , , , -, . . ! palpus; J, basal plate. poison irom a sac in the basal joint. The pedipaipi are elongate, leg-like, their basal joints often form- ing a lip, the other joints forming the palpus, which may end with a claw or a pincer. The question has often been discussed as to whether the chelicerae are the homologues of the antennas of other arthropods. The embryological evidence, which cannot be detailed here, is in favor of their equivalence to the second antenna of the Crustacea, and to the mandibles of insects. Since the Arachnida usually suck their food, the oesophagus is frequently widened to a sucking stomach, behind which comes the true stomach, with which, as well as with the intestine, a number of so-called liver tubes may arise (fig. 455, da, dt). These may be restricted to the abdomen alone, as in the scorpions. The hinder part of the intestine is often enlarged into a rectal vesicle (stercoral pocket), just in front of which the excretory tubules (so- called Malpighian tubules) empty. These resemble the true Mal- pighian tubes of insects in function, but differ in being entodermal in origin. Besides there also occur, coxal glands (modified ne- phridia), of which only one pair comes to development, and this may lose its external opening on the base of the appendage. The oesophagus is always closely surrounded by a nerve ring composed of brain above and of part of the ventral chain on the sides and below, the thoracic and more or fewer of the abdominal ganglia entering into its composition (fig. 405, D). Of sense organs, besides tactile hairs, only the eyes (fig. 406), 2-12 in number, are well known. Hearing is well developed, but it is uncertain whether certain hairs on the legs and palpi are the seats of the recognition of sound. The function of the ' lyriform organs/ which occur in the skin of body and legs in several groups, is unknown. 446 ARTHROPODA. The respiratory organs already alluded to (p. 443) have their spiracles, always few in number, on the anterior ventral part of the abdomen and, it is stated, sometimes on the cephalothorax. The internal organs are the lungs and the tracheae. A lung is a rounded sac just inside the spiracle and consists of numerous leaves on the anterior wall of the lung sac. Each leaf is covered on each side by a thin layer of chitin and contains a blood space in its in- terior, while between the leaves are flattened spaces into which the air enters (fig. 456). The tracheae, on the other hand, are branched tubes arising from the abdominal spiracles and penetrating the abdomen (fig. 460). These are lined with chitin, and to strengthen them with- out undue thickness this lining is thrown into folds, usually arranged in a spiral. In the scorpions and tetrapneumonous Araneina only lungs occur. In other spiders one pair of lungs is replaced by tracheae, while in most other arachnids only tracheae occur. (The smaller mites and FIG. 460.— Beginning of paired parasites lack specialized respiratory or- trachese of Anyphozna uccen- , . , , ,, . taatn. (After Bertkau.) st, gans and circulatory organs as well.) These facts, aside from embryological con- ditions, show that lungs and tracheae are morphologically equiva- lent. The localization of respiration in the abdomen has resulted in having the heart in the same region. It is noticeable that, as the tracheae are developed, the circulatory vessels are reduced. In the scorpions, which have only lungs, the circulation is most nearly complete. In development the arachnidan tracheae arise from the abdominal appendages, as do the lungs. (In the Solpugidse and some mites cepha- lothoracic tracheae occur, but nothing is known of their development.) This fact shows that they are entirely different in origin from the tracheae of insects, while numberless details show that these structures are only to be compared with the gills of Limulus. The gonads (only the Tardigrades are hermaphroditic) are abdominal in position and open by paired ducts (sometimes with a single mouth) on the first abdominal somite. In most cases the animals are oviparous, but the scorpions and many mites bear liv- ing young. In many instances the mothers care for their eggs and young, the scorpions carrying their families on their bodies. Only rarely is there a metamorphosis, and then in the aberrant forms //. ACERATA: SCORPIONIDA. 447 like the Linguatulida and Acarina, where the young have but two or three pairs of appendages, acquiring the others later. Legion I. Arthrogastrida. Arachnida in which the abdominal somites are distinct. Order I. Scorpionida. The scorpions bear a superficial resemblance to crayfish and for a long time were associated with them, since (fig. 402) they have four pairs of walking feet (3-6), while the pedipalpi (2) are large and bear pincers. The chelicerae are also chelate. The pedipalpi and the two anterior pairs of legs have the basal joint expanded for chewing. The peculiarities of the abdomen mark the group off from all other arachnids. It consists of seven broader somites attached by their whole width to the cephalothorax and behind six narrower somites, forming a tail or postabdomen. The last somite is bent ventrally in a sharp spine and contains two large poison glands. It is the « sting ' of the animal, which, in the case of the small species, causes painful wounds in man ; and in the large tropical species is, perhaps, fatal. Usually scorpions feed upon insects, which they seize with the pincers, and, arching the FIG. 461.— Under surface of scorpion, showing the combs and the outlines of the lung sacs with their spiracles (orig.). tail over the back, kill with the sting. On the ventral surface of the second abdominal somite (fig. 461) are a pair of appendages, the combs or pectines; rods with teeth on one side of uncertain function. They are clearly appendages with modified gill leaves, and from their nearness to the sexual opening and their rich nerve supply are supposed to be stimulating organs in copulation. The 448 ARTHROPODA. next four segments bear spiracles which lead to four pairs of lung sacs. The heart is abdominal and the ' liver 9 diverticula are con- fined to the same region. The large number of abdominal ganglia distinct from the oesophageal ring is also characteristic. From three to six pairs of eyes occur. The scorpions are inhabitants of warm regions, ranging north with us to the Carolinas and Nebraska. Buthus* Centrums.* Order II. Phrynoidea (Pedipalpi, Thelyphonida). The thoracic segments are fused, and of the appendages only the last three are walking feet, the third pair having the last joint (tarsus) developed into a long many- jointed tactile flagel- FlO. 462.— Phrynus (Phrynichus) reniformis. (From Schmarda.) him. The chelicerae are strong and spined, but end in a claw, not in a pincer. The chelicerae are also clawed and are possibly poison organs, since the bite of these animals is feared. The abdomen consists of eleven or twelve somites and contains two pairs of lungs. There are eight eyes — two large ones in the middle of the cephalo- thorax, and three small ones on either side. The species are tropical. Phrynus (fig. 462) has a simple abdomen ; Thelyphonus* (fig. 405, D) has a short postabdomen which bears a long, many-jointed thread. One species in the southwestern United States. Order III. Microthelyphonida. Small animals as yet known only from Texas, Sicily, Paraguay, and Siam. They have a general resemblance to a scorpion, the chelicerae are three-jointed and chelate, the pedipalpi simple, neither these nor any of the legs having chewing lamellae. The head is distinct from two * thoracic segments/ the abdomen is eleven' jointed and is terminated by a long many-jointed caudal flagellum. //. ACES ATA: SOLPUGIDA. 449 Lung sacs, which are true appendages without lung leaves, occur on abdominal segments four to six, and are eversible. The ovary FIG. 463.— Koenenia wheeler i.* (From Wheeler.) is unpaired, the testes paired. There is a circumcesophageal nerve ring and a single abdominal ganglion. No Malpighian tubes occur. Kcenenia.* Order IV. Solpugida (Solifugae). In these the cephalothorax is broken up into a head bearing the chelicerae, pedipalpi, and the first pair of legs; and three posterior free somites, each bearing a pair of legs, thus giving these forms a certain resemblance to the Hexapoda (infra). The chelicerae are strong and chelate, the pedipalpi are simple and are used in walk- ing, while the first pair of legs are tactile. Respiration occurs by four pairs of tracheae, the first of which opens between the first and 450 ARTHKOPODA. second ( thoracic ' somites, a condition which deserves embryologi- cal investigation. The abdomen consists of nine or ten somites, and the head bears two ocelli. As the name implies, the Solpugidae are nocturnal, living by day in holes in the sand and searching for their prey at night. In the Old World they are reputed as poisonous, but no poison glands occur. "Warmer parts of U. S. Solpuga,* Galeodes* Datames * (fig. 464). FIG. 464. FIG. 465. FIG. 464.— Datames formidibilis* (After Putnam.) FIG. 465.— Chelifer bravaisi. (From Schmarda.) 1, cheliceree; 8, pedipalpi. Order V. Pseudoscorpii. These small forms resemble the true scorpions in the chelate cheliceraB and pedipalpi (fig. 465), and in the abdomen joined by its whole breadth to the thorax. They differ in the lack of post- abdomen and sting. They breathe by tracheae; have from two to four ocelli, and spinning glands opening on the second abdominal somite. These animals, 2-3 mm. long, live in moss, etc., and among old and dusty books, where they feed on mites and minute insects. Their bodies are flattened and they run side wise. Chelifer,* Obisium,* Chernes.* Order VI. Phalangida. The abdomen in the harvestman, or 'daddy long legs/ is less evidently segmented than in the forms already mentioned, nor is it sharply distinct from the cephalothorax. The small body bears four pairs of exceedingly long legs; the cheliceraB are drawn out //. ACERATA: AEANEINA. 451 in long horny processes; the pedipalpi are tactile organs as in the true spiders. The males possess a long penis, and the females a FIG. 466.— A phalangid laying eggs. (After Henking.) long ovipositor (fig. 466). They have two or four ocelli and breathe by tracheae. These largely nocturnal animals are predaceous, feeding upon small mites. In structure they fqrm in some ways an approach to the Acarina. Phalangium* Liobunum* Legion II. Splicer ogastrida. Arachnida with the abdominal somites fused so that no traces of segmentation remain. Order I. Araneina. In the spiders the soft-skinned body is divided by a deep con- striction into cephalothorax and abdomen (fig. 467). The four pairs of legs are adapted for springing or for walking, the hinder pair being also accessory to the spin- ning. It bears a comb-like claw with which several threads are combined into a stronger cable. The chelicera bears a sharp claw (fig. 459), traversed by the duct of the poison gland with which the prey is killed, although but few (species Of LatrodecteS, fig. 468, FlO. 467.— Epeira tnsularis* round- web the tarantula, and the bird spiders, Mygalidae) can injure man. The pedipalpi are used as feeling 452 ARTUKOPODA. organs and with the basal maxillary process to comminute the food. In the male the pedipalpi have the terminal joint swollen to a pear-shaped structure (fig. 469) by which the sexes are easily FIG. 468. FIG. 469 FIG. 470. FIG. 468.— Latrodectes macfmis,* poison spider. (After Marx.) FIG. 469.- Pedipalp of Pardosa uncnta. (After Emerton.) FIG. 470.— Spinnerets of Epeira diadema. (After War burton.) 1, 2, 3, first, second, and third spinnerets; /, threads. distinguished. This is used to convey the spermatozoa to the female, a rather dangerous process, as the male is apt to be killed by the much stronger mate. At the hinder end of the abdomen, just in front of the anus, are the spinnerets, which are reduced appendages, as is shown by their paired arrangement and their jointing (fig. 470), as well as by development. They are truncate and have at the tip a ' spin- ning field' from which numerous minute, two-jointed spinning tubes, resembling hairs, arise, each of which is the end of a duct of a silk gland. Different kinds of glands, producing silk for differ- ent purposes, occur. The number of spinnerets varies between two and three pairs, and in front of these may be an unpaired spinning region, the cribrellum, so that hundreds or even thousands (Epei- ridae) of glands may be present. The secretion of the glands hardens in contact with the air, and the single threads are united by the combs of the hinder feet, into a larger cord which can be regulated in size according to the number of glands which are active. Yet the largest cord is finer than the finest silkworm silk, hence it is often used for the cross-hairs of telescopes. The spider silk has many uses; it is used to line the nests, to form cocoons for the eggs, as a means of descent from high places, and to form the well-known webs. The nervous system consists of a brain and a circumoesophageal ring, and, in the Mygalidae, a single abdominal ganglion. The arrangement of the six or eight ocelli and the relative lengths of the legs are matters of systematic importance. Two pairs of respiratory organs occur. In the Tetrapneumones there are two pair of lungs, but in the Dipneumones the //. ACERATA: ACARINA. 453 hinder pair are replaced by tracheae, which may open by separate spiracles (Tetrasticta) or by a common opening (Tristicta, fig. 460). Sub Order I. TETRAPNEUMONES. Four lungs, four spinnents and eight eyes in two rows. The MYGALnxsare the most important group, large- forms which spring upon their prey, capturing even small birds and mice. To the genus Mygale* belong the spiders (commonly but erroneously called tarantulas) which occur in banana bunches. Here also belong the trap- door spiders, Cteniza,* of the southwest, which excavate burrows in the FIG. 471.— Cteniza ccementaria in its tube, closing the lid. o, eyes ; b, inside of lid! with places for the claws ; c, egg cocoon. soil, line them with silk, and close them with a hinged lid (fig. 471). Atypus.* Sub Order II. DIPNEUMONES. One pair of lungs, one of trachea; six spinnerets. Here belong most of the native and numerous tropical species. Some (VAGABUND^E) use their webs only to line the nests and enclose the eggs, which are either hidden away or carried about attached to the body, while they spring upon or chase their prey. SEDENTARIA are- the web builders, their webs varying widely in structure. Of the first group the SALTIGRADA include forms which jump upon their prey (Attus,* PhidippuS)* Habrocentrum*), and the CITIGRADA (Lycosa,* Dolomedes,* Trochosa *), which run their prey down. Among these is the true Taran- tula, T. apulice of Italy, whose bite was once believed to cause a frenzy only to be cured by peculiar music (' Tarantello '). The Sedentaria are divided, according to the web-building habits- The ORBITELARLE or orb weavers (Epeira* Argiope*) form vertical webs which in many instances are com- plete circles. The RETITELARLE (Theridium* Erigone *) build irregular webs. The species of Latrodectes * are reputed poisonous to man (fig. 468). The TUBITELARI.E build horizontal webs with a tube to the mar- gin in which they lay in wait for insects. Order II. Acarina. The mites, partly from parasitism, partly from other conditions of life, have become, in some instances, considerably modified. With the fusion of cephalothorax and abdomen the last traces of segmentation in the body are lost. Yet they retain the six pairs of appendages — four pairs of legs which at once distinguish them from the parasitic hexapods; and two pairs of mouth parts, modi- fied into a sucking beak. This consists of a tube formed by the 454 ABTHROPODA. basal joints of the pedipaJpi, in which the chelicerae, either chelate, clawed, or stylet-like, play. Since the mites are small and half or wholly parasitic, they are much simplified in structure. Frequently heart and tracheae are lacking. The larva as it escapes from the egg lacks the last pair of legs and then closely resembles certain imperfectly segmented parasitic insects like the lice. The red mites or TROMBIDIID^E and the water mites, HYDRACHNID.E (Hy- draclma* Atax *), are free-living in the adult condition, but parasitic as young. The IXODID^: or ticks (Ixodes*}, live in woods or on bushes, attack man and other mammals, burrowing beneath the skin, sucking the blood un- til they become enormously swollen and fall off. The much smaller males FIG. 472. Fm. 473. FIG. 472. — Sarcoptes scabei, female itch mite. (After Leuckart ) FIG. 473. — Demodex folticulorum, follicle mite. (From Ludwig-Leunis.) are attached to the females and take no food. Argas persicus, of eastern lands, with habits like a bedbug, is poisonous. The GAMASID^E are para- sitic, species of Gamasus * occurring on beetles and Dermanyss-us* on bats. The ACARID.E include permanent parasites like Sarcoptes scabei*' (fig. 472), the cause of the 'itch,' and the closely allied cheese mite. The follicle mite, Demodex folliculorum,* lives in the sebaceous glands of various mammals, including man (fig. 473). Order III. Linguatulida. Elongate mites like Demodex lead to the Linguatulida, which as adults live in the frontal sinuses of carnivorous mammals, as en- cysted young in the liver of herbivorous forms, especially rodents. The body is long, flattened and ringed, and hence somewhat tape- worm-like (fig. 112). The adults have the mouth at the base of a chitinous capsule, and on either side are two hooks regarded as the claws of the first and second legs. Inside the body is a spa- cious cavity traversed by the alimentary canal which is without appendages. The nervous system is largely a circumcesophageal //. ACERATA: LINGUATULIDA, TARDIGRADA. 455 ring; the sexual organs are very complicated, the males having the openings in front, the females at the hinder end. The presence of these parasites in animals causes a profuse catarrh, and the eggs pass out with the mucus. Falling on vegetation, these are FIG. 474. FIG. 475. FIG. 474.— Larva of Pentastomum proboscideum. (After Stiles.) rf, stomach; c, gland cells ; m, mouth ; st, stylet ; ?/, posterior larval hooks ; J, 2, legs. FIG. 475.— Macrobiotus hufelandi, water bear. (After drawings by Greef and Plate.) I-IV^ legs ; d, accessory glands ; m, stomach ; mfc, mouth capsule ; ov, ovary ; sp, salivary glands ; st, stylets ; vm, excretory tubules ; blood cells in the body. liable to be eaten by various animals. The larvae (tig. 474) have a boring apparatus in front and two pairs of legs, the latter being lost in the metamorphosis except for the hooks. It is by no means certain that these are degenerate arachnids. The points in favor of such a position are about equally balanced by those against. Pentastomum. Usually associated with the Arachnida are two other groups of very doubtful position, which until more definite knowledge is obtained, may remain near them. Tardigrada. These are minute fresh-water forms, known to microscopists as 4 water bears ' (fig. 475), which owe their name to their slow motions. They have four pairs of short, hooked legs, their sole Arachnidan charac- ter. The genital ducts empty into the rectum ; the nervous system has four ventral ganglia ; heart and respiratory organs are lacking. In de- velopment they are remarkable for the large ccelomic pouches. In the 456 ARTHROPODA. feet are glands recalling nephridia in their history. It is possible that these animals are to be placed among the Coelhelminthes. Macrobiotus* Pycnogonida (Pantopoda). These marine animals have a cylindrical body, with a tubular probos- cis in front and an abdominal appendage behind, and four pairs of very long legs. In front of the legs is a pair of small chelate appendages and usually a pair more like pedipalpi. In the male there is an additional pair of ' ovigerous ' legs to which the eggs are attached after being deposited by the female, thus giving a total of seven appendages, a num- FIG. 476.— Nymphon stroemii * (orig.). c, chelicerse ; o, ovigerous legs ; p, pedipalpi ? r, rostrum. ber not reached in any arachnid. Diverticula of the stomach extend into- the legs ; a heart is present, but respiratory organs are lacking. The- Pycnogonids, which creep slowly over seaweeds and hydroids, may be (1) a distinct group of arthroproda, or (2) modified arachnids, or (3), and less probable, Crustacea. Nymphon* Phoxichilidium* Colossendeis.* Class III. Malacopoda (Protracheata). These forms, including only a single family PERIPATID^E, show a strange mixture of annelid and arthropodan (or ' tracheate ') Fio. 477.— Peripatus capensis. (From Balfour, after Moseley.) characters, so that they are usually regarded as representatives of the stock, early separated from the annelids, from which the Insecta have descended. They recall the annelids by the presence of nephridia, so characteristic of that group, which begin by a closed vesicle (reduced coelom), pursue a short course, and expand into a urinary bladder before opening at the bases of the legs (fig. 478, so). On the other hand they possess tracheae, long unbranched ///. MALACOPODA. 457 tubes which arise in numbers from the spiracles, which are irregu- larly distributed in each somite (fig. 478, tr). FIG. 478.— Anatomy of female Peripatus opened dorsally. (From figures of Moseler and Balfour.) a, anus ; at, antennae ; 6m, ventral nerve cords; d, digestive tract; go, genital opening ; o, ovary ; ogr, brain ; p, pharynx ; sd, slime gland ; so, ne- phridia ; sp, salivary gland; tr, tracheae ; w, uterus. The soft-skinned body, which shows no external ringing, bears the legs, each terminated by claws. These legs somewhat resemble the annelidan parapodia in that they are not jointed and are not sharply separated from the trunk. Each segment bears legs, while the head is provided with three pairs of appendages: a pair of ringed antennae, a pair of mandibles, which lie in the oral cavity, and a pair of mouth papillae, at the tips of which are the openings of the slime glands, the sticky secretion of which is squirted out and serves to capture insects (fig. 478, sd). The nervous system consists of a pair of cerebral ganglia (og), supplying the antennae and a pair of very primitive eyes; and a pair of ventral cords (bin), swollen slightly in each segment, which 458 ARTHROPOD A. connect dorsal to the anus and are connected in the trunk by numerous non -segmental commissures. The description may be completed by saying that the straight aliment- ary canal (p and d) bears only salivary glands (sp) ; that it is accompanied throughout by a dorsal heart ; that the gonads (the sexes are separate) open just in front of the anus (#o), their ducts being modified nephridia. The animals are viviparous, live in decaying wood, hide by day and hunt their prey at night. The several species have a wide but discontinuous distribution (South America, Cape of Good Hope, New Zealand, etc.), an indication of great antiquity. Recently the forms have been divided into several genera, Peripatus, Peripatopsis, Opisthopatus^ etc. Class IV. Insecta. The Insecta is a distinct group marked off from all other arthropods by several important characters. The appendages show no signs of a schizo- podal condition. The head is always a distinct region, bearing a single pair of antennae, a pair of" mandibles, and two pairs of maxillae, the posterior pair often being fused into a lower lip or labium. The respiratory organs are trachea (figs. 479, 480), which resemble the trachea of FIG. 479. FIG. 480. FIG. 479 —Tracheal system of Machilis. (From Lang, after Oudemans.) fc, head; J-JJI, thoracic somites; s, spiracles; 1-10, abdominal somites. FIG. 480.— Portion of trachea of caterpillar. (From Gegenbaur.) A, mam trunk; B, C, D, branches; a, epithelium with nuclei, b; d, air in tracheal tube. IV. 1NSECTA. 459 man only in that they are tubes filled with air, and kept from collapse by firm walls. They open to the exterior by openings (spiracles, stigmata) on the sides of the body. They are inpushings of the skin and consequently have the same structure, an epithe- lium and an outer chitinous layer. The latter lines the lumen of the tubes, and since it must be thin to permit the passage •of gases (oxygen, carbon dioxide), and at the same time firm, to keep the tubes open, it is thrown into folds which usually pursue a spiral course. The turns of the spiral are so close that it gives the tubes a ringed appearance. Inside the spiracles the tracheae branch repeatedly until they end in the tissues in fine tracheal capillaries. In general it may be said that each segment has a right and a left spiracle and corresponding tracheal systems (fig. 59), but this scheme is complete in no known species, for there are always some segments (especially in the head) which lack these organs and are supplied from adjacent segments (fig. 479). Again, the tracheae may be connected by longitudinal trunks (fig. 494, ib), so that spiracles occur in only a part of the segments, these supplying the whole system. Although the tracheae are for aerial respiration, there are aquatic insects, but these also breathe air, since they carry air about with them entangled among the hairs which surround the spiracles. Then, too, aquatic larvae often have tracheal gills, thin-walled processes of the integument which project into the water and are penetrated by numerous tracheal twigs (fig. 495). The alimentary tract always has excretory organs, the Mal- pighian tubules, connected with it. These vary in number be- tween wide limits, but are always placed at the junction of the rectum with the rest of the track They diifer from the physiolog- ically similar tubes of the Arachnida in being of ectodermal origin, so that no homology can be traced between them. The gonads are always paired and placed dorsal to the intestine, while the ducts (at least in some cases modified nephridia) open ventrally at the hinder end of the body. The spermatozoa are motile. In the subdivision of the ' tracbeate ' arthropods a group of Myriapoda is usually recognized, containing forms known as centipedes and ' galley worms.' These two types are in reality very different. The centipedes (Chilopoda) show in all structural features close relationships to the Hex- apoda, while the other group, Diplopoda, differ in almost every respect, except the presence of numerous walking legs, from the Chilopoda. Hence, since the object of classification is to show resemblances and dif- ferences, the group of Myriapoda has been dismembered, the Chilopoda 460 ABTIIROPODA. being considered here, the Diplopoda as a distinct class at the end of the group of Arthropoda. Sub Class I. Chilopoda. The most striking characteristic of the chilopods is their long, flattened bodies, each of the numerous somites bearing a pair of FIG. 481.— Diagram of transverse section of a centipede (orig.). d, digestive tract; gonad; n, nerve cord; s, spiracle and tracheae. FIG. 482. FIG. 483. FIG. 482.— Mouth parts of Scolopendra morsitans. 1, antennae ; 2, mandibles; 3, max- illae ; A, second maxillae (labium) ; 5, poison feet. FIG. 483.— Scolopendra morsitans, centipede. (After Schmarda.) six- or seven-jointed limbs. The head bears a pair of long antennae and usually numerous ocelli, which only in Scutigera show a ten- IV. INSECTA: HEXAPODA. 461 dency to become compound. The mouth parts (fig. 482) are a pair of mandibles and two pairs of maxillae, both united in the median line. Besides, the first pair of legs (fig. 482, 5), with their fused bases, extend forward beneath the head and form the poison claws. Their terminal joints are sharp and contain the ducts of poison glands. The spiracles (at least a pair to every other somite except those of the head) are lateral in position in the soft integument between the dorsal and ventral plates (fig. 481). The heart is elongate, with chambers in each somite (fig. 66); there are two large Malpighian tubes, and the nervous system is elongate, with ganglia in each .somite. The gonads are dorsal to the intestine and are unpaired, while the single duct opens ventrally in the preanal somite. The LITHOBIID.E, with 15 leg-bearing somites, have certain dorsal plates enlarged and overlapping the succeeding somites ; Lithobius,* common under stones, etc. SCOLOPENDRID.E, centipedes; at least 17 legs and 5 ocelli ; Scolopendra* in warmer regions (fig. 483). GEOPHILHLE, not less than 30 pairs of legs, spiracles 2 less than legs. Geophilus* SCUTIGE- RiDyE, legs very long, 15 leg-bearing segments, but only 8 dorsal plates. jSoutigera.* Sub Class II. Hexapoda. The Hexapoda is by far the largest division of the Arthropods, since it contains at least ten times as many known species as all the rest. The number is so large that it cannot be given with accuracy; an estimate is 250,000. Since the tropics, which have not been exhaustively studied, are very rich in insects, it is con- ceivable that there are at least a million different species in the world. On the other hand great uniformity of structure exists, all adhering with great fidelity to plan of structure, regional divi- sions, and number of appendages under the most diverse conditions, so that the difference between the most extreme forms is far less than that in Crustacea or Arachnida. But while hexapods thus lose in morphological interest, they gain in their life relations, in the way that they are injurious or beneficial to man, in their breed- ing habits, and in their intellectual and social relations. From the evolutionary standpoint they show marked adaptations to environ- ment, and the large number of species is only possible by taking advantage of every opportunity in nature. Of systematic importance are the regional division of the body and the number and character of the appendages. In the body three regions are distinguished, often separated by marked con- 462 ARTHROPODA. strictions: head, thorax, and abdomen. The number of abdomi- nal somites, varies with the order and even with the family, FIG. 484.— Schematic section of a hexapod through the thorax (orig.). ex, coxa; d, digestive tract: /, femur; 7i, heart; n, notum; pi, pleuron ; st, sternum; (, tibia; (a, tarsus ; tr, trochanter. ranging between eleven (in some larvae and embryos twelve) in the Orthoptera and five in many Diptera. Each cuticular abdo- minal segment consists of two plates, tergite (dorsal) and sternite (ventral), united on the sides by a softer mem- brane which contains the spiracles. Head and thorax, on the other hand, have a constant number of somites. The thorax is plainly divided into three segments, pro-, meso- and met at h or ax , each composed of three elements, an unpaired dorsal portion, notum; a pair of lateral plates, pleura, and an unpaired ventral sternum (fig. 484). For sim- Fio.485.-Head of a grasshopper, plicity one speaks of pronotum, meso- ' ifabiasi; pafprrv'; sternum, etc., to indicate the portions of labrum; wid, mandible ; 'mpl fV, ~ e*vnarnfp apo-mpnt« TTif> Vi^nrl ia a maxillary palpi; mx, maxilla; tjl€ ts* o, occiput; v, vertex. continuous capsule in which the follow- ing parts are recognized: in front and dorsal clypeus and frons; dorsal and posterior a vertex and an occiput; laterally gence, ven- trally a gula. The appendages show that the head is composed of at least four somites. The view that the head consists of six somites is based on the existence of two more segments without appendages in the embryo, a preantennal and a postantennal (intercalary, premandibular), as well as the knowledge that the brain, in which formerly only antennal ganglia were recognized, consists of three pairs of ganglia (proto-, deuto-, and trito-cerebrum). IV. INSECTA: HEXAPODA. 463 The appendages (fig. 484), seven pairs, are confined to the head and thorax (see, however, infra). The three thoracic segments bear three pairs of legs, whence the name Hexapoda. The legs are inserted between pleura and sterna and begin with a short coxa (c), followed by a trochanter (tr), also short. The two following joints are long, the first, the femur (/ which, however, is not appendicular in character. Both labium and labrum may bear unpaired processes on their oral surfaces, an epipharynx above, a hypopharynx below the mouth, neither of them true appendages. The different kinds of food necessitate differences in the char- acter of the mouth parts, — chewing, licking, sucking, or piercing — all referable back to the chewing kind, and these in turn are modified legs. In the description of the chewing type it is well to begin with the maxillae (fig. 486), because of their easy com- parison with the other mouth parts and with the legs as well. These begin with a triangular joint, the cardo (c), which is fol- lowed by a larger stipes (st). The stipes in turn supports two chewing lobes, the inner, or lacinia (li), and an outer, or galea (le), these being processes segmented off from the stipes. In the Orthoptera and Coleoptera only the lacinia is sharp-pointed ; the galea may either form a sheath for the lacinia, or, as in many beetles (fig. 514), it may be tactile and jointed again. The stipes also bears the maxillary palpus (pm), consisting of from three to six similar joints, and is the mostly leg-like part of the appendage. 464 ARTHROPODA. The labium arises as a pair of processes which early approach each other and fuse behind the mouth. All the parts of the maxilla may be recognized, only it must be remembered that the basal parts of the two sides are fused. The united cardines form an under chin, the submentum, the stipites a chin or mentum, which in the Orthoptera is cleft, a result of incomplete fusion. This may bear inner and outer processes, the glosscv (gl) and the para- glosses (pg) respectively, and the labial palpus. The mandible con- FIG. 486. FIG. 487. Fio. 486. — Chewing mouth parts of cockroach (Periplaneta orientalis). The letter- ing is the same in figs. 486-489. c, cardo ; gl, glossa ; /it/, hypopharynx ; I, lobe ; le, li, external and internal lobes of maxilla; Zr, labrum ; m, memtum ; md, man- dible ; mx, maxilla ; p, pm, maxillary palpus ; pg, paraglossa ; pi, labial palpus ; sm, submentum; st, stipes. Fio. 487.— Licking mouth parts of bumble bee (Bombus terrestris). sists of merely the basal joint, altered for biting, while the rest of the appendage, common in Crustacea as the mandibular palpus, is lacking. The licking mouth parts, like those of the bees (fig. 487), stand next to those already described, there being many transitional stages. Labrum and mandibles retain their primitive condition, while maxillae and labium are greatly elongate, are connected at the bases, and can be folded away beneath the head or extended at will. The small submentum is followed by an elongate mentum IV. INSECTA: HEXAPODA. 465 which bears the unpaired tongue or glossa (gl), which corresponds to the fused glossae (or to the hypopharynx?) of the first type and which is used for sucking honey and hence has the form of a nearly closed tube. Beside it lie the rudimentary paraglossse (pg) and the well-developed palpi. Similarly the maxillae have small cardines and palpi, while the stipites and the undivided lobe (/) are long and well developed. The piercing mouth parts of the flies (Diptera) and bugs (Ehynchota) can be compared with those of the bees in so far as the labium forms the groundwork of the whole (fig. 488). The FIG. 488. FIG. 489. PIG. 488.— Sucking mouth parts of mosquito, Culex pipiens. (After Muhr.) The groove of labium opened by removing labrum; the stylets separated. FIG. 489.— Sucking mouth parts of a butterfly. (After Savigny.) ma:', ma;", shows how right and left maxillae unite into a tube; right labial palpus (pi) with hairs removed. beak (rostrum, haustellum) of these animals corresponds to the labium; it is a grooved structure, either fleshy and flexible, or stiff .and jointed. The edges of the groove are inrolled so that there remains a narrow dorsal slit, which can be closed by the slender upper lip (Ir). The tube formed of these parts contains four stylets, toothed or with retrorse hooks at the tip. These are the 466 ARTHROPODA. mandibles and maxillae, and a fifth stylet, the hypopharynx (liy) can be present. Palpi, which only occur in the Diptera, belong to the maxillae (p). Reduction in number of stylets to four or three, or their complete absence (some flies), is brought about by fusion or by degeneration. The haustellum serves as a case for the suck- ing tube, which in the Ehynchota is formed by the united maxillae, in the Diptera by labrum and hypopharynx. The proboscis, or haustellum (the so-called tongue), of the Lepidoptera (fig. 489) is a long tube coiled like a watch spring beneath the head. It consists of two long grooved maxillary galea firmly united by their edges. The maxillary palpi are well de- veloped in the moths; elsewhere they show all stages of reduction to complete disappearance. Labium and labrum are reduced to small triangular plates at the base of the proboscis, the labium bearing a pair of hairy palpi (pi). The mandibles are represented by small plates or bunches of hair. These conditions gain in in- terest when we remember that in the larva the mandibles are strong biting organs, while the maxillae are small hooks, and the labium is better developed only in those parts connected with the silk glands, a beautiful example of relations of structure to life conditions. In contrast to the other regions, the abdomen lacks appendages in the adults. Only in the lower group of Thysanura are small lobes present, behind and in the same line with the thoracic feet, which may be regarded as abdominal feet. Apparently, too, the appendages of the last segment, the stylets and cerci, are modified limbs, but the parts (gonapophyses) used in copulation and oviposition are different in character. False feet, or pro-feet, occur on the abdomen of the larvae of the Lepidoptera and the Tenthredinidae, but since these are fleshy unjointed processes, it is doubtful whether these are true abdominal limbs, like those of other Arthropoda, or are structures independently acquired. Besides ventral appendages the insects usually have two pairs of dorsal outgrowths upon the meso- and metathorax, the wings. They are lateral folds of the chitinous coat of the notum and con- tain on their interior extensions of the blood sinuses and of the tracheae, which are protected by thickenings of the chitin, causing the network of ' veins' or < nervures ' in the wing. Both wings may be elastic, flexible, and adapted for flight, or the hinder pair may alone partake of this character (true wings or alae), while the first pair may be thick and parchment-like wing covers, or elytra, under which the true wings are concealed when at rest. When only the base of the wing is thus thickened hemelytra result. Between the origins of the anterior wings is frequently a chitinous. IV. IN SECT A: HEX APOD A. 467 plate, the scutellum, while between the hinder wings is a similar postscutellum. In many insects one pair of wings is lacking, the anterior pair being retained in the Diptera, the posterior in the Strepsiptera; these are clearly cases of degeneration. The entire absence of wings may occur from two causes; wings have apparently never been developed in some (primary lack of wings of the Apterygota), while there are others in which we must believe that wings once present have been lost, because nearly related forms — bugs, lice, etc. — have wings, or because certain individuals (male cockroaches, sexual ants and termites) are winged (figs. 506, 528, 529). The prothorax of all recent insects is wingless, but in some of the Archiptera of the coal period wing rudiments occurred on this somite. As a result of differences in food the alimentary canal (figs. 490, 491) varies greatly. The ectodermal stomodaeum begins with a pharynx, which in the sucking insects is a sucking apparatus with radial mus- cles. The oesophagus, which follows, may be widened to a crop (ingluvies), or it may have a caecal outgrowth which in the butterflies may take the shape of a stalked vesicle (falsely ' sucking stomach'). Also ectoder- mal is the gizzard (km,pv), or pro- ventriculus, the chitinous lining of which is toothed for grinding the food. The true stomach, of ento- dermal origin (m, cd), frequently bears blind sacs or gastric caeca (ap) ; in general it is short and its junction with the hinder ectodermal portion, the proctodeum, is marked by the entrance of the Malpighian tubules (vasa Malpighii, vm). The latter, excretory in fuuction, arise from the proctodeal region. The latter is usually differentiated into a small in- testine and a two-regional (colon and rectum) large intestine. The rectum may have enlargements called rectal glands. True glands, however, occur only at the beginning and FIG. 490.— Alimentary tract of Card- bus auratus. (From Lang, after Dufour.) av, anal vesicle ; arf, anal gland ; cd, stomach with caeca ; ed^ hind gut; m, ingluvies (crop); fc, head; oe, oesophagus; pv, proven- triculu s (gizzard ): r, rectum; vm, Malpighian tubules. 468 ARTHEOPODA. end of the alimentary tract ; into the mouth empty from one to four pairs of salivary glands (sp) ; at the anus are defensive anal glands with their malodorous secretions of a protective character. The alimentary tract with the other viscera is enveloped in the fat body, a soft mass which contains, besides fat cells and connec- tive tissue, concretions of uric acid. The nervous system (fig. 405) has the ventral cord, especially in primitive forms (Apterygota, Archiptera, Orthoptera, fig. 491), rtg FIQ. 491.— Viscera of male cockroach (Perip/cmefa orientalis). (Partly after Huxley.) /-///, segments of thorax and corresponding legs; 1-10, abdominal segments; a, anus; <»(/, ventral ganglia; ap, gastric ceeca ; at, antenna; W, salivary bladder; ?/, sexual opening; h, heart; fc»\ crop; km, gizzard; /, labial palpus; m, stomach (the arrow shows the connexion between m and km}, also maxillary palpus ; mg, male genitalia ; oe, oesophagus ; oy, brain; 7-, rectum; sp, salivary gland; tg, thoracic ganglia; iig, infracesophageal ganglion ; inn, Malpighian tubules. nnd nearly all larvse (fig. 59), long and composed of numerous separate pairs of ganglia. In beetles, moths, bees (fig. 494), and flies the cord is shortened and the ganglia are in part fused. The brain arises by the fusion of three pairs of ganglia (proto-, deuto-, and tritocerebrum), and is, especially in the adult, very complex. It is connected on either side with a large optic ganglion the size •of which is correlated to that of the eyes. In the adult condition the Hexapoda have a single pair of highly developed compound eyes {figs. 407, 408), which not infrequently occupy nearly the whole •of the top of the head. Between and in front of these small and simple ocelli, usually three in number, frequently occur, especially in insects which are strong fliers. These are either lacking or poorly developed in the larvae, while the compound eyes are fre- quently replaced by groups of from two to six closely crowded ocelli. Of other sense organs only the tactile hairs of the skin are known with certainty, while similar hairs on the antennas and about the mouth are supposed to be organs of smell and taste, since- these senses are known to be well developed. The tympanal organs of the Orthoptera are the only structures which can be with IV. INSECT A: HEX APOD A. 469 much probability connected with hearing. These are thin drum- like parts of the chitin, framed in thicker portions (figs. 492, 493), beneath which is a tracheal vesicle, with a nerve ending in a ' crista acustica.' The power of producing sound is widely distributed and often highly developed, the organs for this purpose varying widely in character. Stridulating organs are formed by ridges on wings and legs, which are rubbed against each other or against similar ridges on the body. Humming is produced by the action of the. FIQ. 492. Fia. 493. FIG. 492.— Side view of grasshopper, s, spiracles ; f, tympanic organ. FIG. 493.— Anterior tibia of a Locustid with tympanum, t. (From Hatschek, after Fischer.) wings or by the passage of air through the spiracles, which are often provided with vibrating membranes which also serve to close these openings. The tracheae (figs. 479, 494) are usually united, just inside the spiracles, by longitudinal trunks from which fine branches extend, enveloping and penetrating all the organs with delicate silvery threads. This connexion of tracheae renders it possible for the spiracles of some segments to disappear. The spiracles of the abdomen are the most constant, usually occurring in the soft mem- brane between the sternites and tergites; the thorax at most has but two pairs, the head none. In insects with good powers of night many of the tracheal trunks are expanded to large air sacs, which may be of value as reservoirs of air, so that the ordinary respiratory motions are less necessary during flight. An interesting adaptation of the tracheal system to aquatic life occurs in the Iarva3 of many Archiptera (Odonata and Mayflies) and Neuroptera, and even among Lepidoptera (Paraponyx) and Coleoptera (Gyrinida3). The spiracles here are usually closed, and the taking of oxygen occurs either through the skin or by means of so-called tracheal gills — bushy or leaf-like appendages of the surface or the rectum, richly permeated by tracheal branches (fig. 495). In such cases the tracheal system has two portions, one which receives oxygen from and gives off carbon dioxide to the water ; the other which supplies the tissues with oxygen and receives carbon dioxide. 470 ARTHROPODA. Since the tracheae, with their fine branches, supply the tissues directly with oxygen, the blood-vascular system is rudimentary. Directly under the back lies the elongate tubular heart in a special FIG. 494. FIG. 495. FIG. 494.— Anatomy of honey bee. (From Lang, after Leuckart.) ", antennae: au, eye ; 6, legs; cm, chyle stomach; ed, rectum ; hm, honey stomach iproventriculus) , rd, rectal glands; s£, spiracles ; tb, tracheal chambers with tracheae ; v/n, Mal- pighian tubules. FIG. 495.— Abdomen of Ephemera larva (from Gegenbaur) with tracheal gills, c; a, tracheal trunks ; b, intestine ; d, caudal bristles (cerci). pericardial sinus. This is a part of the haemoccele cut off from the gastric portion of this space by an incomplete partition in which, right and left, are the wing muscles (alee cordis) of the heart. The heart receives its blood through lateral ostia (eight or fewer) from the pericardial sinus or (Orthoptera) through ventral openings from the large haemocoele. The blood passes forward through an anterior aorta into the haemocoele and thence back to the pericardial sinus. The arrangement of the viscera, fat bodies, and muscles gives a certain regularity to the circulation, especially in the appendages. Accessory pulsating ampullae in the bases of IV. INSECTA: HEXAPODA. 471 the antennae (Orthoptera) help in the flow of the blood. It is noteworthy that many beetles (Meloidae and Coccinellidae) squirt blood through the jointing membranes of the legs as a means of protection. The Hexapoda are dkscious. The gonads consist of a few or many ovarial or testicular tubules (figs. 496, 497), the latter some- times coiled into small oval bodies. Ovaries and testes are paired and lie, right and left, in the abdomen. Their paired ducts (ovi- FIG. 496. FIG. 497. FIG. 496.— Male genitalia of Melolontha. (From Gegenbaur, after Fabre.) gl, accessory glands; t, testes; vd, vas deferens; vs, seminal vesicles. FIG. 497.— Genitalia of female Hydrobius. (From Gegenbaur, after Stein.) be, bursa copulatrix; gl, tubular glands; o, ovarial tubes; ov, oviduct with glands; rs, re- ceptaculum seminis; v, vagina. ducts, vasa deferentia) open separately in the Ephemerida, but in all other Hexapoda there is a single ventral unpaired sexual open- ing just in front of the anus. This arises as a median invagination of the ectoderm (hence lined with chitin), which extends inwards and meets the genital ducts (modified nephridia), and forms the ductus ejaculatorius of the male, the vagina of the female. Aside from many accessory glands, the sexual apparatus shows the follow- ing differentiations: in the male vesiculaa seminales, as widenings or diverticula of the vasa deferentia ; in the female the receptaculum seminis and the bursa copulatrix. The latter may be either the vagina or a blind sac arising from it, or a special invagination of the ectoderm, emptying into the vagina by an internal canal. It receives the penis. The receptaculum seminis, a stalked vesicle connected with the vaginia or the bursa, has a special biological interest. In insects which copulate but once during life it retains the spermatozoa for a long time — four years in bees — in a living condition. As the eggs are laid they are impregnated by sperma- 472 AETHROPODA. tozoa from it. Since a firm shell or chorion is developed around the egg in the ovary, access of spermatozoa is only possible by the existence of a micropylar apparatus, a system of tubes penetrating the chorion at one end of the egg. Oviposition occurs in many insects by means of an ovipositor which may project free from the body (fig. 509) or may be re- tracted into it. It consists of four or (Orthoptera) six parts or gonapophyses developed from the eighth and ninth abdominal segment, which form a tube. In many Hymenoptera this struc- ture has become modified into a sting (aculeus), and is provided with poison glands, making it an efficient weapon of defence. From its nature the sting is of necessity confined to the females. In the males there is usually a protrusible penis which is frequently composed of the same parts as the ovipositor ; in others of metamor- phosed somites. Further sexual differences lie in the form of the antennae, shape and color of the wings, modifications of the eyes, etc. In many insects the eggs may develop parthenogenetically. Plant lice and scale insects reproduce for generations asexually, and parthenogenesis is widely distributed among Hymenoptera, Lepi- doptera, and Neuroptera. The conditions among the bees are especially interesting, since here the determination of sex rests with the existence or non-existence of fertilization (pp. 142, 487). Much rarer than the ordinary parthenogenesis is that special form, known as paedogenesis, which occurs only in certain Diptera like Miastor. In the female Miastor larva (fig. 498) the eggs develop FIQ. 498.— Larva of a Cecidomyid with psedogenetic daughter larvae. (From Hatschek, after Pagenstecher.) before the appearance of the ducts, so that the young can only escape by rupture of the mother. After several paedogenetic generations there appear at last larvae which pupate and produce adult male and female flies. With the exception of these paedogenetic forms, the Pupipara, many Aphidae and a few other viviparous species, the Hexapoda are oviparous. The development begins, after oviposition, by a superficial segmentation of the egg. Later there appear two em- bryonic structures, the yolk sac and the amnion; the first, in con- trast to the vertebrate structure with the same name, is dorsal. IV. INSECTA: HEX APOD A. The amnion is a thin layer of cells which covers the ventral surface and arises in a manner similar to the vertebrate amnion ; folds aris- ing from the blastoderm in front and behind, right and left of the embryo, fuse with one another and produce a double envelope, an inner amnion, an outer serosa. With the rupture of the amnion and egg shell, the postembry- onic development begins. This differs so in the different orders that ametabolous, hemimetabolous, and holometabolous insects are recognized, i.e., insects with direct development without meta- morphosis, those with partial and those with complete metamor- phosis. The ametabolous young is closely like the adult, so that it only has to grow, with periodic ecdyses, and to mature its re- productive organs. Since no insect has wings when it leaves the egg, this direct development is possible only in wingless forms like the Apterygota and Apt era. All winged insects, on the other hand have a more or less pro- nounced metamorphosis, the final cause of which is the necessity of developing wings. This view holds although there are wing- less insects with a complete metamorphosis. These forms (fleas, wingless moths, and ants) have undoubtedly sprung from winged species and have inherited from them the metamorphosis which has been retained after the wings were lost. In incomplete metamorphosis the differences between the newly hatched young and the adult, or imago, gradually disappear (fig. 499). At the second molt the wings often appear as small folds in the chitinous wall of meso- and metathorax ; they grow with each ecdysis, until at last, in size, form, and movability, they are functional wings. The chitinous coat of each wing pad (fig. 499, B, 1, 2} encloses the compressed and folded wing of the next stage. Since the larvae by _ J FIG. 499.— Hemimetabolous develop- their lack OI Wings are placed in ment of Perlanigra. (From Hux- different circumstances from the adult, the differences between the two may be increased by the development of special larval organs. Thus the aquatic larvae of the May flies and dragon flies differ from the adults not only in the absence of wings, but by the different form and the tracheal gills, which are almost always lost at the last molt (fig. 495). A, wingless larva; B, larva with wing pads, l. 2 ; C, adult ; /, II, III, thoracic segments. 474 ARTHROPODA. Increase in the differences of environment and the correlated increase in larval characters lead to complete metamorphosis. In order to profit as much as possible by its adaptation to its environ- ment the larva retains its shape as long as possible ; the gradual change is suppressed and the alteration in form necessary to the metamorphosis is postponed until the end of the larval life, to the period between the last two molts. In this interval there is such .an energetic transformation of the organism that the performance •of ordinary vital functions, especially motion and feeding, is in- terfered with or rendered impossible. This last stage therefore becomes a period of rest, the pupal stage, upon the existence of which great weight must be laid in the definition of complete met- amorphosis. The more complete the condition of rest the more pronounced is the holometabolous development. From this point of view different types of pupae are distinguished : pupae liberae, pupae obtectae, and pupae coarctatae. In a free pupa (pupa libera) the appendages stand out from the body (fig. 500), so that not FIG. 500.— Larva and pupa of May beetle, a', a", fore and hind wings ; an, anus ; at, antennae; o, eyes; p'-p'", legs ; s£, spiracles. only the segmentation of the body but the antennae, legs, wings, and often the mouth parts of the imago are visible. Such pupae have a certain power of motion, as, for instance, the pupae of many Neuroptera and mosquitos, the latter rising and falling in the water. The covered pupae (pupae obtectae) at the moment of pupation have free appendages which with the hardening of the chitin become closely appressed to the body, so that even by close inspection only indistinct contours can be seen (fig. 501). Motion is confined to bending of the whole body, as is familiar in the pupae of moths and butterflies. The pupae coarctatae are without motion because here the pupa (in structure a pupa libera) is en- closed in a larger coat, the last larval skin (Muscajia). The variations among larvae are even greater than with pupae. Here structure and jointing of the body are so completely under the influence of environment that with similar or different con- IV. INSECT A: HEXAPODA. 475 ditions larvae widely remote from the systematic standpoint may closely resemble each other, while those of closely related species may differ extremely. The leaf -feeding larvae of Lepidoptera (fig. FIG. 501. FIG. 502. FIG. 503. FIG. 501.— Pupa of Sphinx ligustri. (After Ludwig-Leunis.) i, eye; 2, head ; 3, an- tennae; £-6', thoracic somites; 7, hind, #, fore wing; &, legs; J0, proboscis; Ji, ab- dominal somites ; J2, spiracles. FIG. 502.— Larva of Sphinx liyustri. (After Ludwig-Leunis.) 71, caudal disc ; p, tho- racic feet ; ps, prolegs. FIG. 503.— Larva (maggot) of blowfly, Musca vomitoria. (After Leuckart.) 502) and Tenthreds are brightly colored, the thoracic appendages remaining small, and are reinforced by the fleshy ventral append- ages, the prolegs or pedes spurii. The predacious larvae of many beetles and Neuroptera have long thoracic legs, strong mandibles, and no prolegs. Other beetle larvae, which burrow in wood or live in the earth (fig. 500), have plump whitish bodies, with the legs rudimentary or wholly lacking. These lead to the maggot- like larvae, in which the mouth parts are inconspicuous and the distinction between head and thorax may vanish. Such soft-skinned annulated sacs occur in the bees (fig. 59) and other Hymenoptera, as well as in many flies (fig. 503) ; that is, in animals which live in an abundance of food either because of parasitism or because the mother has provided plenty. From the outer appearance one would gain the impression that these holometabolous larvae not only lacked the wings, but that the appendages of the imago were entirely absent or had an entirely different form; farther, that wings, and frequently antennae, legs, and mouth parts, come into existence at the moment of pupation, and then in remarkable size and completeness. A more accurate investigation shows that the anlagen of all these structures are formed long before pupation, often at the first molt. The wings 476 ARTHROPOD A. of a butterfly are present in the caterpillar as small folds or proc- esses of the surface which increase in size with each molt. That they are not visible externally is due to the fact that they are pushed FIG. 504.— Diagram of development of wings and legs from the imaginal discs of a. fly during metamorphosis. (After Lang.) /i, larval hypodermis; t, imaginal hyppdermis ; /, ?u, imagin&l discs and legs and wings formed from them; s, con- nexion of discs with hypodermis; x, chitinous larval skin. into the body and enclosed in sacs opening to the exterior. Such anlagen are called imaginal discs; with their recognition the dis- tinctions between complete and incomplete metamorphosis in part disappear, since in the first the structures of the imago, even if in a modified shape, are outlined very early. Still there remains much to be remodelled during the pupal rest. The muscles must be adapted to the new locomotor organs, the digestive tract to the altered food, the nervous system re-formed. Since a great part of the previous structures must be broken down to afford material for the reconstruction of the organs, the pulpy nature of the inside of the pupa is easily understood. In a rapid degeneration of the tissues the material, consisting of indistinctly separated cells, is- so homogeneous that it was formerly thought that the pupa re- turned to the indifferent condition of the egg (Histolysis of flies). In the classification four points are of special importance: (1) The segmentation of the body, in which it is to be noted whether the segments. of thorax and abdomen follow without change of form, or whether the thorax, by the closer union of its somites, is sharply marked off from both head and abdomen. (2) The character of the wings, which are either lacking in the lower forms or are delicate chitinous structures, with numerous veins, the wings of the two thoracic segments similar. In the higher forms a degeneration of the wing veins or a leathery consistence of the membrane, together with a divergent development, partial reduction of antennge and posterior wings may occur. (3) The structure of the mouth parts, and (4) the type of development, both described above. From these characters it is easy to differentiate six orders: Lepidoptera, Diptera, Aphaniptera, Rhynchota, Hymenoptera, and Coleoptera. The remaining forms were formerly divided among the Orthoptera and Neuroptera, but IV. INSECT A: HEX APOD A, APTERYGOTA. 477 these groups are not considered natural and the attempt has been made to divide them into more or fewer groups. Here the Pseudoneuroptera or Aphaniptera are separated from the Neuroptera, the wingless forms or Apterygota from the Orthoptera. Order I. Apterygota. At the bottom of the Hexapoda come forms which lack wings and which show no evidence of having descended from winged an- cestors. They are regarded as slightly modified descendants of the ancestral Hexapod. Besides the lack of wings they show many primitive characters; compound eyes are poorly developed or lack- ing; the tracheal system, when not degenerate, consists of isolated tracheal bushes, rarely connected by longitudinal trunks (fig. 479) ; the mouth parts, resembling somewhat those of Orthoptera, are for biting, though frequently rudimentary; the development is always ametabolous. Sub Order I. THYSANURA (Bristle-tails). Body elongate, with long bristles (cerci) at the hinder end. Lepisma saccharina,* silver fish, common among old books and papers, does considerable damage. It is covered with shining scales. Campodea * (fig. 400), with rudimentary abdominal appendages. Machilis* lapyx* with caudal forceps. Sub Order II. COLLEMBOLA (Spring-tails). Com- pressed forms in which the bristles bent under the body serve as a spring, throwing the animals (one to three mm. long) forwards. Podura *; Anurida maritima* in tide pools ; Entomobrya *; Lipura *; Achoreutes ni- valis* the snow flea. Order II. Archiptera (Pseudoneuroptera). These represent the primitive forms of winged insects. The elongate body consists of numerous Packard'.) (After segments and usually bears the cerci of the Thysanura. The wings are delicate and transparent, supported by a close net- work of nervures, both pairs being very closely alike. The mouth parts are of the typical biting kind; the maxillae have lacinia and galea; the labium, with glossa and paraglossa, is frequently deeply cleft. These points of primitive structure are correlated with a primitive, usually hemimetabolous de- velopment. The distinction between larva and imago is largely one of presence or absence of wings, although larval organs like gills (Amphibiotica) may occur. Frequently the development is direct when the imagines, as in some Termites and the Psocida?, are wingless. 478 ARTIIROPODA. The Archiptera were formerly united with the Neuroptera on account of similarities of wings. The separation is due to characters of mouth parts and development. Sub Order I. CORRODENTIA. Larvae distinguished from the im- agines by difference in size and, in the winged forms, by lack of wings. Best known are the TERMITID.E (Isoptera), or white ants, which must not be confused with the true ants (Hyrnenoptera), from which they are dis- tinguished by the similar body segments, the mouth parts, and the simple development. Like the true ants, they have a well-developed social state. A colony of termites, consisting usually of thousands of individuals, forms a nest with numerous chambers and passages. They are nocturnal, and they burrow, without coming to the surface, through old wood (timbers of houses, furniture, picture frames, dead wood in the forest, etc.). They line these chambers with a cement-like substance composed of refuse which has passed through the alimentary canal. Many species build dome-like nests, ten or fifteen feet high, fifteen to twenty or twenty-five feet across, of chewed earth. In a colony are winged and wingless individuals, the latter with ametabolous development (fig. 506). The wingless forms have the sexual organs rudimentary, but, in contrast to ants and bees, may belong to either sex. They are frequently blind, have strong mandibles, and are of two kinds, the workers (c) and the large-headed soldiers (d). The winged forms are sexually functional (6). Shortly after the metamor- phosis they swarm, and then the wings are bitten off at the base and 'king' and 'queen' either form a new colony or enter one already in existence. After copulation the abdomen of the queen, by the formation of numerous eggs, swells to an enormous size (e). Since the swarming Fio. 506.— Termes flavipes* white ant. (From Riley.) a, larva ; b, winged male ; c, worker ; d, soldier ; e, queen ; /, pupa. individuals form the prey of birds and other animals, it often happens that a colony is left without a royal couple. In such cases the line is perpet- uated by reserve males and females, sexual animals which have not com- IV. INSECTA: HEXAPODA, ARCHIPTERA. 470 pleted the metamorphosis but are in the wing-pad stage. The termites are able, by quantity and quality of food, to modify the development of the larvae and to determine which type of individual shall be produced. The termites are farther noticeable for the bitter wars they conduct against the true ants. Termes flavipes * in our northern states. T. fatalis, Africa. Allied to the Termites are the often wingless PSOCID^, or book lice. Trades divinatorius * is the book louse. Other species are winged and live in moss, etc. Near here also belong the MALLOPHAGA, which, like lice, live upon mammals and especially on birds. Like true lice they are wingless, but they have biting mouth parts. Trichodectes* on the dog, ox, etc.; Goniodes,* Docophorus* Nirmus,* etc., on birds. Sub Order II. AMPHIBIOTICA. The three families united here differ much in structure, but agree in having aquatic larvae with tracheal gills (fig. 495). These are ventral bushes in the Perlida3, wing-like or bushy appendages of the abdomen in the Ephemeridae, and three-leaved appendages in those Odonata which do not respire by tracheal branches in the rectum. All of these larvae are predaceous, especially the larvae as well as the adults of the Odonata. The Odonate larvae have a peculiar apparatus for the capture of prey. The mentum and sub- mentum of the labium are greatly elongate and when folded bring the tip like a mask beneath the mouth. The ^ structure can be suddenly extended (fig. 507) and grasps- FIG. 507. FIG. 508. FIG. 507.— Larva of ^schna grandis. (After Rosel von Rosenhof.) a1, aa, wing pads j m, mask ; st, spiracles. FIG. 508.— Ephemera vulgata. (From Schmarda.) The caudal bristles incomplete. the food. PERLID^E (Plecoptera) ; hind wings the larger. Perla* Ptero- narcys* stone flies. EPHEMERID^E, fore wings large, the hinder small or absent ; May flies, life very short in the adult state. Ephemera,* (fig. 508) Cleon* Beetisca.* ODONATA (Libellulidse), wings nearly equal, the hinder slightly larger ; Dragon flies, veritable insect hawks destroying numberless mosquitos. LibelLula* JEsclma,* Agrion* Gfomphus.* Sub Order III. PHYSOPODA (Thysanoptera). Wings slender, fringed with hairs ; tarsi bladder-like at tip ; mouth parts bristle-like, probably used for sucking. The position of this group is very uncertain. Thrips,* Limothrips* 480 ARTHROPODA. Order III. Orthoptera. Like the Archiptera these are hemimetabolous or in a few •cases ametabolous, and the mouth parts (fig. 486) are fitted for biting, the mentum being cleft. On the other hand the wings have lost the delicate membranous character and have become more parchment-like, the fore wings being smaller and serving as •covers for the larger, softer, and folded hind wings, which are the efficient organs of flight; the condition in these respects recalling somewhat the Coleoptera. The abdomen bears cerci and fre- quently stylets. In internal anatomy the large number of Malpi- ghian tubules is noticeable (fig. 491). Sub Order I. CURSORIA. With rather long legs fitted for rapid run- ning. Only the cockroaches (BLATTID.E) belong here. Wings may be absent, according to the species, in either sex, but more frequently in females. The more common cockroach, the ' Croton bug ' (Blatta ger- manica *), is a well-known pest in houses. The larger Periplaneta orien- talis* is common in ships and bakeries. Other species in our woods. Sub Order II. DEEMATOPTERA (Euplexoptera). Front wings short •elytra ; the hinder wings being folded crosswise and packed beneath them, or rudimentary ; cerci developed to a forceps-like structure terminating the body, whence the name Forficula* given one genus. Labidura* These forms are often called earwigs, from an erroneous belief that they enter the human ear and injure the drum. The group on account of its wing structure is often made a distinct order. Sub Order III. GKESSORIA. Legs long, slender, adapted to a slow •walking motion. In the MANTID.E the prothorax is very long and bears a pair of long raptorial feet which when at rest are held in a position which causes these insects to be known as 'praying Mantes.' Phasmomantis* warm countries. PHASMID^E, with short prothorax, almost exclusively tropical, represented throughout northeastern United States by Diaphero- mera femorata* the walking stick. The members of this family are noted for their mimicry of twigs and leaves (fig. 12.) Sub Order IV. SALTATORIA. Hinder legs long, strong, and for jumping ; the other pairs much smaller. Hinder femora large and muscu- lar, tibiae elongate and spined. Wings usually functional and in the migrating species capable of sustained flight. Produce sound (stridulate) by rubbing the anterior wings together (Locustidae, Gryllidae) or against the legs (Acridiidae). Tympanal apparatus (p. 468) on the anterior tibiae (Locustidae, fig. 493, and many Gryllidae) or on the first somite of the abdomen (fig. 492). Stridulation occurs only in males, and in our com- mon crickets the number of notes is directly dependent upon tempera- ture, which, on the Fahrenheit scale, may be determined by the formula, T = 50 + ?LZ! — , in which T stands for temperature and n for number IV. INSEGTA: HEXAPODA, NEUROPT£jRA. 481 of chirps per minute. The females may readily be recognized by the ovi- positor. ACRIDIID.E ; antennae and ovipositor short ; tympani abdomi- FIG. 509.— Locusta caudata. (After von Wattenwyll.) I, ovipositor. nal. Acridium* ; Melanoplus* (M. spretns, the 'grasshopper7 which did such damage in the Missouri River States in 1873-75) ; (Edipoda* ; Tettix* LOCUSTID.E ; antennae* long ; tympani on first tibiae ; ovipositor long, flattened ; tarsi four-jointed. Haden&cus* wingless, blind, in caves; Conocephalus * ; Cyrtoplnlus * and Microcentrum,* katydids ; Anabrus* wingless. GRYLLID.E, Crickets : antennae long ; ovipositor long, cylindri- cal; tarsi three-jointed; tympani on first tibia. Gryllus*; (Ecanthus* tree crickets ; Gryllotalpa* mole crickets, burrowing. OrderlV. Weuroptera, The Neuroptera closely parallel the Archiptera, and the two were formerly united, since they have the same wing structure and FIG. 510. — Myrmeleo formicarius. (From Schmarda.) 1, imago; 2, larva; 3, pupa in its cocoon. show in general appearance great similarities. Thus the ant lions (fig. 510) recall the dragon flies; the Chrysopinae, the Perlidae. The Neuroptera, however, are holometabolous and have a resting stage, although the pupae (pupae liberae) are capable of some mo- tion. The mouth parts are for biting, and in some the labium has no notch in the middle. 482 ARTHROPODA. Sub Order I. PLANIPENNIA. Biting mouth parts. SIALID.E, wings well developed, mouth not rostrate, larvae aquatic, commonly called dob- Fia. 511.— Corydalis cornutus,* hellgrammite, male. (From Riley.) FIG. 512,—Phryganea grandis. (From Schmarda.) sous. Corydalis,* hellgrammite ; Stalls* HEMEROBIID^, lace wings ; wings well developed, mouth not rostrate ; larvae with sucking mouth IV. INSECTA : HEXAPODA, COLEOPTEEA. 483 parts, predaceous. Chrysopa* feeds on plant lice ; Myrmeleo* ant lions; larvae dig funnel-like pits in sand and capture the ants, etc., which fall into them. PANORPIDJE (Mecoptera) ; mouth prolonged into a rostrum ; Panorpa, *Bittacus* Sub Order II. TRICHOPTERA (caddis flies). Wings usually large ; mouth parts rudimentary, forming a short sucking tube which, with the wings covered with hair-like scales, recalls the Lepidoptera ; larvae aquatic with tracheal gills ; build cases of foreign matter, stones, sticks, etc., in which, like a hermit crab, they live ; pupation occurs in these tubes. Phryganea * (fig. 512), Hydropsyche* Order V. Strepsiptera. These forms, comprised in a single family, STYLOPID.E, are parasitic on the Hymeiioptera. The six-legged larvae (fig. 513, 3} press in between the ventral abdominal plates of bees or wasps and pupate there. The quickly FIG. 513.— Xenos rossi. (After Boas.) 1, female : «, male ; 3, larva ; /-///» thoracic somites ; a1, rudimentary fore wing ; a2, hind wing. flying male (2) escapes from the pupal skin ; it recalls somewhat a beetle ; has small fore wings and large hinder ones. The wingless, legless female (1) remains in the pupal skin and is fertilized there ; she is viviparous. Insects infested with these parasites are ' stylopized.' The affinities of the order are doubtful. The forms are frequently included with the beetles. Stylops,* Xenos.* Order VI. Coleoptera. The beetles are the highest of the Hexapoda with biting moutb parts. They are closest to the Orthoptera, as is shown by the structure of mouth parts and wings. The mandibles are strong; the maxillae (fig. 514) have lacinia and galea; the labium consists, of a submentum (often called mentum), behind which the rudi- mentary mentum with its palpi, paraglossae, and glossae (the latter frequently fused to a ligula) are retracted. (In the genus Nemognatha the maxillary galea form a sucking organ.) The group is distinguished from the Orthoptera by the holometabolous development with pupae liberae, while the larvae (fig. 500) show many modifications corresponding to the mode of lite. Another character is afforded by the wings. The anterior pair, separated at the base by a scutellum, are hard elytra not fitted for flight, and 484 AETHROPODA. from these the order receives its name Coleoptera, sheath wings. Tinder the elytra are protected the delicate much folded hinder wings, the organs of flight (lacking in insects with fused elytra), the second and third thoracic wings and those of the abdo- FIG. 514. FIG. 515. FIG. 514.— Maxilla of Procrustes coriaceus. c, cardo ; le, galea ; IL lacinia • vm nalrnis- st, stipes. FIG. 515.— Calosoma sycophanta. (After Ludwig-Leunis.) men are covered by the elytra, these are soft above. Externally the relations of the elytra cause a regional division peculiar to the beetles (fig. 515) : head, prothorax, and a third division composed of meso- and metathorax plus abdomen covered by the elytra. The numerous species of beetles— over 100,000 are described— are sub- divided into normal forms and Rhynchophora, the normal forms being subdivided again upon characters derived from the tarsi as follows : Sub Order I. PENTAMERA. Tarsus five-jointed, the last club-shaped and bearing the claws, while the other four are short and somewhat heart- shaped (fig. 516, «). This is the largest division and contains the tiger beetles (CICINDELID^E) and the pre- daceous CARABIDJE (fig. 515) ; the water beetles, HYDRO- PHILID.E and DYTISCIDJE ; the LAMELLICORNIA or SCARA- BEID^, represented by the ' June bugs,' Melolontha* and the large tropical Dynastes ; the fire flies, LAMPYRIDTE ; the rove beetles, STAPHYLINIDJE, etc. Sub Order II. HETEROMERA. First and second legs pentamerous, third apparently four-jointed ; few forms belong here, among them the ' oil bottles ' (ME- LOID.E) and the blister beetles, CANTHARID.E, both of them containing a peculiar substance, cantharidin, which renders the Spanish flies, Lytta vesicatoria, an important ingedient of blistering plasters. Some of the TENEBRIONID^E live in the larval stages in flour, etc. Sub Order III. TETRAMERA (Cryptopentamera). Tarsi with the FIG. 516.— a, pen- tamerous tarsus of Dytisciis ; 6, crypto pentamer- ous tarsus of Cocci n el la ; t, tibia ; *, reduced tarsal joint. IV. INSECTA: HEXAPODA, HYMENOPTERA. 485 penult joint rudimentary, giving the impression of four joints. The families included here*- very numerous in species, are injurious to vegeta- tion. The larvae of the long-horned CERAMBYCID.E bore in wood. The CHRYSOMELID^E, of which the Colorado potato beetle (Dory- phora decemlineata) is the most notorious, feed on leaves. Sub Order IV. TRIMERA ; tarsi with penult and anti- penult joints rudimentary, so that they appear three- jointed. Best known are the COCCINELLID^E, or lady birds, whose larvae, because of their destruction of plant lice, etc., are of value to man. Sub Order V. RHYNCHOPHORA, snout beetles ; head produced into a long snout, at the apex of which are the mouth parts. Here belong several families of weevils, FIG 517.— some of which do damage to grain, nuts, timber, etc. U2Jel-nut**wee! Curculio* Conotrachelus * Calandra* Hylesinus* Bala- vil. ninus* (fig. 517). Order VII. Hymenoptera. The Hymenoptera, of which bees, wasps, and ants are well- known representatives, have biting jaws, while the other mouth parts are elongate and in a minority of the group converted into a sucking organ. In the bees (fig. 48?) the glossa unite, producing a nearly closed tube, which lies in a sheath formed by the other mouth parts, the mandibles alone retaining the primitive form. Since mouth parts vary, the structure of the wings and body seg- mentation have great value in defin- ing the order. The wings are mem- branous and are supported by few nervures (fig. 518), and in flight they act as one pair, since the two are usually connected by hooked bristles on the hind wing, which engage in a groove on the hinder margin of the front wing. The fore wings are the larger and, correspondingly, the mesothorax exceeds the other tho- racic somites, so that these, especially FiG.5i8.-sir*r0/0a«,sawfiy. (After the prothorax, seem but parts of the strong mesothorax. Besides, the first abdominal ring unites to the thorax so intimately in the En- tophaga and Aculeata as to seem part of it. The constriction which then separates thorax and abdomen comes between the first and second abdominal somites, and when the second (petiole) is elongate the stalked abdomen, familiar in the wasps, results. 486 AETHROPODA. The sexes are readily distinguished by the genital armature. The female is provided with the ovipositor already described, which when used for this purpose only, projects from the hinder end of the body (fig. 518), but when used as a sting (aculeus), can be drawn back in the body when at rest. The sting, naturally lacking in the male, is connected with a poison gland, the secretion of which owes its effect, not, as once believed, to formic acid, but to a little known basic substance. The distinction between ovipositor (terebra) and aculeus affords chp.rac- tersof systematicimportance; othersare furnished by the development, which is always holometabolous. The pupae, in all important points, are similar (pupae liberae), but two kinds of larvae are distinguished. Some have larvae with well-developed legs ; many of them are green in color and distin- guished from the larvae of Lepidoptera by the greater number of prolegs. Others have footless larvae (fig. 59). The first occur where the larva must shift for itself, the second where it is surrounded by an abundance of food, either provided by the parents or by the host in which it is parasitic. Sub Order I. TEREBRANTIA. Terebra present ; larvae with feet at least on the thorax; eggs laid on leaves or in wood, usually without gall formation; the larvae therefore must move in order to feed. The TEN- THREDINID^E, saw flies, feed on leaves and have caterpillar-like larvae. Cimbex* Nematusf SIRICHLE (Uroceridae), horn tails, the larvae bore in wood and are whitish. Sub Order II. ENTOPHAGA. Terebra present ; larvae legless, para- sitic in galls or in animals. Some use the ovipositor to lay their eggs in leaves, roots, or stems of plants. Galls are then produced, diseased struc- tures by which the larvae are nourished. Others use the ovipositor to lay their eggs on or in other insects and larvae. The young feed on the tissues of the host and at last cause its death, often before the completion of the metamorphosis. The gall-producing forms are the CYNIPHLE, some of which afford examples of heterogony (p. 145), in which the alternating generations are distinguished by different structure, by sexual and parthenogenetic reproduction, and by different kinds of galls. So different are the two generations that they have frequently been described as differ- ent genera. The inquilines do not form galls, but lay their eggs in the galls of other species. The insect parasites are divided among several families, the more prominent being the ICHNEUMONID^, BRACONID.E, and CHALCIDID^E, those of the first being large, the others FIG 519 — Chalets flavi- sma^ or minute. These forms are of immense value pes* (After Howard.) to agriculture, as they keep down, as no economic entomologists or insecticides can, injurious forms. Sub Order III. ACULEATA. Females with stings ; larvae footless, maggot-like. The digger wasps or FOSSORES excavate tubes in the earth in which they lay their eggs and then bring into the holes as nourishment IV. INSECTA: HEXAPODA, HTMENOPTERA. 487 other insects which they have paralyzed by a sting in the ventral cord. Some true wasps have similar habits. Most wasps (VESPARLE) and bees (APIARLE) have different habits. They bu/M wonderful homes of chewed wood (the first pulp paper) or skilfully trimmed leaves, earth, etc., or of wax which the animals (bees) secrete between the joints of the abdomen. The nests, which are to contain the young, are either small tubes or hexagonal cells which are united to horizontal or vertical combs; the food is either honey, pollen, or chewed fruits. The fact that the offspring are better protected when numerous individuals protect them has apparently led in the wasps and bees to different grades of social states. The honey bees (Apis mellifica*), which live in a colony, consist of three kinds of individuals distinguished by structure of the head (fig. 520) and other FIG. 520.— Heads of Apis mellifica. (After Boas.) a, queen; b, worker; c, drone with the compound eyes meeting above. features : a single queen, some hundred drones, and about ten thousand workers. These last are females and hence have stings, but have rudi- mentary functionless sexual organs; their work being to build the home, to protect the young, and provide food for the winter. The business of egg-laying belongs to the queen, who copulates but once, at the beginning of her reign, when she and a drone take a wedding flight. For the four years of her life the sperm retains its vitality in the receptaculum seminis. In laying the eggs she can permit entrance or not of the spermatozoa at will and thus produce males or females. A queen who has not been fer- tilized or who has exhausted her supply can only lay drone eggs. The further fate of the eggs depends upon the food of the larvae; with a small amount of bee bread (pollen) workers are produced, but the same larva placed in a larger cell and fed with the * royal jelly ' (much like blanc mange in appearance) will develop into a sexually mature queen. On escape from the queen-cell the young queen with a part of the colony swarm and start a new colony. This operation may be repeated once or twice, but if there be danger of depleting the hive the remaining queen Iarva3 are killed. Wasps and bumble-bee colonies last but a year and are reformed by a fertilized female which has lived through the winter. The ants (FORMICARY) have gone beyond the bees in the social organi- zation. They have also departed most from the other Hymenoptera in that the workers, sometimes the sexual individuals, are wingless and the sting is rudimentary or entirely lacking. Only the Poneridee sting like bees and wasps ; the others bite and squirt the secretion of the persistent poison gland (formic acid) into the wound. The homes of the ants are 488 ARTHROPODA. less wonderful than those of the bees, but their social organization is fre- quently more complicated. In the colony occur the wingless workers (rudimentary females with wing pads in larval life which are lost in pupa- FIG. 521. FIG. 522. FIG. 5Zl.—Myrmecocystus melliger* honey-sac ant. (Orig.) FIG. 522.— Plant of Hydnophyton- (After Forbes.) Showing the bulb occupied by ants. tion), and of these frequently there are different kinds, large-headed soldiers and small-headed workers ; * honey sacs' in Myrmecocystus ; and the sexual animals, queens and drones, which copulate in a marriage flight. The queen after the flight returns to the colony. Frequently other insects, like the Aphides, are con- nected with the colony, these being kept for the honey dew they produce. Many ants steal the pupae of others and keep the adults when they emerge as slaves. In Polyergus rufescens this has gone so far that the masters cannot care for themselves and must be fed by the slaves ; otherwise they die. The ants possess extreme interest on account of their care- fully planned wars (Ecitons) ; on account of their relations to plants, some species making nests in the growing plant and protecting it by their bites ; the leaf- cutting ants take leaves from trees and carry them into their underground nests for the cultivation of fungi on which they feed ; the agricultural ants from their plantations and stores of grain, and the honey ants from the fact that certain FIG. 523.— Head of Cicada septendedm, workers (fig. 521) act as reservoirs of the mouth parts separated (ori >nna; e, compound eye; /, bium ; wd, mandible ; nix, maxilla, enormous Size. a, antenna; e, compound eye; S& boney, these ' honey sacs' swelling up to IV. INSECTA: HEXAPODA, RHYNCUOTA. 489 Order VIII. Rhynchota. The Khynchota, or bugs, in their external appearance are nearest to the Archiptera and Orthoptera. The head, thorax, and abdomen are joined in the same way; the development is hemi- metabolous, and in the wingless species ametabolous. In some cases, as the Cicadas with their membranous wings, the confusion with the Orthoptera has led to these being called locusts; on the other hand the delicate-winged Aphides resemble the Archiptera. Yet all Rhynchota may be recognized by the sucking proboscis (fig. 523), consisting of the grooved labium in which the needle- like mandibles and maxillae play. The wing structures afford the basis of division into three sub orders. Sub Order I. HEMIPTERA (Heteroptera). Anterior wings hemelytra, i.e., leathery at the base, soft and elastic at the tip (fig. 524); between the 6 FIG. 524.— Pentatoma rufipes. (From Hajek.) s, scutellum. hemelytra is a conspicuous triangular scutellum (s) which covers more or less of the dorsal surface. Hemelytra and scutellum occasionally disap- pear. A further characteristic is the presence of stink glands, producing a most disgusting odor, which open in the adults ventrally on the rneta- thorax; in the larvae dorsally on the abdomen. According to habits the many families may be grouped into the aquatic HYDROCORES and the terrestrial GEOCORES. Of the first the BELOSTOMHXE are noticeable from their size, Belostoma americana* being nearly 2£ inches long and capable of inflicting severe wounds. Other families are NEPHXE (Ranatra, water scorpion), NOTONECTID.E, HYDROBATIDJS, etc. Of the Geocores the REDU- VIID.E, which feed on other insects; the ACANTHIIDJE (Acanthia lectuaria* the bed bug); the LYGJEHXE, containing the chinch bug, Blissus leucop- terus* so injurious to grain; and the PENTATOMID^E, or stink bugs, may be mentioned. Sub Order II. HOMOPTERA, Wings, when not degenerate, similar in texture throughout, although often differing in size. They are either parch- ment-like or delicate membranes. The CICADID^, represented by Cicada septendecim* the seventeen-year 'locust,' and C. tibicen* or dog-day har- vest fly, are noticeable from their shrill notes, produced by a stridulating drum on the abdomen. C. orni of the Old World fig. 526) punctures ash trees, causing the flow of manna. The CERCOPID.E contains the spittle bug (Apropliora *) which causes drops of foam on grass. The leaf hoppers, 490 ARTHROPOD A. FIG. 525.— Cicada septendecim* seventeen-year locust. (From Riley.) a, pupa; b, case from which the imago, c, has escaped; d, twig bored for oviposition. pupa FIG. 526.— Cicada orni. (From Schmarda.) or JASSHXE, contain some injurious forms, Erythronura vitis* damaging the grape, while the true hoppers, MEMBRACID.E (fig. 527), are scarcely less ^ injurious. None of these, however, are such serious pests as the plant lice and scale insects. In the COCCID.E, or scale insects, the wingless female dies after laying the eggs and covers them with her dead scale-like body. Here belong the cochineal insects, Coccus cacti,* the dried bodies of which furnish the pigment carmine, and the lac insects, Coccus lacca, as well as a host of injurious forms, like the orange scale, Aspidotus aurantii* and the worse San Jose scale, A. pernieiosus* which has recently been spread throughout the country. The APHID^E, or plant lice, are soft-skinned and with their honey-containing excrement form a substratum for the growth of FIG. "wt.-Ceresa buba- inJurious fungi- They reproduce largely by parthe- lus* buffalo leaf hop- nogenesis, a reason for their rapid multiplication ; Marlatt.) . IV. INSECTA: HEXAPODA, DIPTERA. 491 .arous females are wingless. At times winged females appear and spread 2 FIG. 528. — Phylloxera vastatrix. (From Ludwig - Leunis.) 1, winged generation; 2, grape root, with nodosities (a) caused by Phylloxera ; #, wingless root-generation. the pests. Winged males appear in the autumn, and the fertilized eggs endure the winter. Of all the species none is more injurious than the Phylloxera vasta- trix * of the grape, which with us does slight damage, but in Europe has destroyed whole vineyards. This is one of our returns for the many pests the Old World has sent us. Sub Order III. APTEKA. Wingless bugs with direct development, commonly known as lice, of which three species attack man, one living in the hair (Pediculus capitis*), the others (P. vestimentorum * and Phthirius FIG. 52d.—Phthirtus inguinalis*) upon the body, live on other mammals. Other species crab louse. (After Leuckart.) Order IX. Diptera. Like the Rhynchota, the Diptera, or flies, are sucking insects, but the sucking tube or haustellum is different, here consisting of a tube formed of both labium and labrum, and containing stylets which include, besides mandibles and maxillae (often rudi- mentary), the hypopharynx (fig. 488), the maxillary palpi being present. Only the anterior wings (hence Diptera) are well de- veloped, the hinder wings being replaced by the halteres or bal- ancers, small drumstick-like structures richly supplied with nerves and functioning as organs of equilibration. The thorax is, as in 492 ARTHROPODA. FIG. 530.— Musca, house fly (orig.). the Hymenoptera, sharply marked off from head and abdomen, its somites being frequently fused. The development is holometab- olous, two kinds of larvae and pupae occurring in its course. The larvae are always apodal, but have either a distinct head with biting mouth parts or they are headless and have a rudimen- tary sucking apparatus (fig. 531). The pupae are correspondingly either free with powers of motion, or are pupae coarctatae (p. 474). De- velopment thus affords characters of systematic importance, and these are supplemented by dif- ferences in length of legs, antennae, haustellum, FIG. 531. -Larva of and in body form. In number of species the : Diptera stand next to the Coleoptera; in num- ber of individuals they far exceed them. Sub Order I. NEMOCERA. Elongate with long, many-jointed antenna', long proboscis, long legs. The larvae live in damp'places or in water, where, lacking legs, they swim by movements of the body. The pupae can also swim well. Best known are the innocuous crane flies (TIPULID^E) and the mosquitos (CuLiciD^E) with their numerous species affecting man, among them the forms which carry yellow fever, and Anopheles,* which distribute malaria. The CECIDOMYID^E include the injurious Hessian fly, Cecidomyia destructor* and the paedogenetic Miastor (fig. 498). Sub Order II. TANYSTOMA. Resemble the Muscariae (with which IV. INSECTA: HEXAPODA, APHANIPTERA. 493 they were formerly united) in the short stout bodies, short antennae and legs. They are distinguished from them and resemble the Nemocera in their long proboscis and in development. The larvae and pupae live in damp places or in water and move rapidly, the larvae having biting mouth parts. Here belong the black flies, SIMULIID.E, which excel the mosquitos in their viciousness, and the horse flies, TABANHLE, the females of which attack cattle and men, as well as horses, with their painful bites. Sub Order III. MUSCAKL/E (Brachycera, after removal of Tanystoma). Body short, stout ; antennae three-jointed with a bristle (arista) (fig. 532) ; legs short, ending in an adhesive organ (pulvillus) ; larvae headless FIG. 532. FIG. 533. FIG. 532.— Left, Erax bast unit, robber fly ; right, antenna of Muscid showing arista at a. FIG. 533,—Gastrophilus equi,* bot fly. (From Hajek.) 7i, halteres. living in decaying substances or parasitic in other animals. The Mus- CID^E include the house ft\e&(Muscadomestica* and other species), the blow fly (Calliphora vomitoria *), and the flesh fly (Sarcophaga carnaria*), which is viviparous. The ASILID.E, or robber flies, prey on other insects, as do some of the SYRPHHLE : Eristfilis* of this family has an aquatic 'rat- tailed larva,' one end being drawn out into a long breathing tube. , peri- branchial space; c, notochord ; 0, gonads ; /, liver; m," muscles ; ?i, nephridia ; o, mouth ; p, atrial opening ; r, spinal cord ; sp, gill slits. allows the underlying muscle segments to show through. It differs from the fishes in lack of skull (Acrania), vertebrae, brain, heart, and kidneys, although the rudiments of brain and excretory organs are present. Connective tissue is almost entirely absent, the body consisting of much-folded epithelia separated by thin gelatinous layers. An axial skeleton is present in the notochord, which extends the whole length of the body (fig. 541, c). Above it lies the spinal cord, with a central canal, which expands in front into a rudi- mentary cerebral vesicle. A pigment spot in this brain is the /. LEPTOCARDIL 503 primitive eye, but other places are sensitive to light. The olfac- tory organ is an unpaired pit on the anterior end of the body; and at its bottom, in the young, is an opening, the anterior neu- ropore, which leads into the anterior end of the neural canal. It is a point of delayed closure of the embryonic medullary folds. Of the alimentary tract more than a third is occupied by the pharynx with the gill slits. It begins with an oval mouth, sur- rounded by cirri, and is perforated by numerous gill slits, be- tween which elastic gill arches form a support for the walls (fig. 542, kb). In the young the gill slits open directly to the anterior, but later, somewhat as in Tunicata, into a peribranchial chamber FIG. 542.— Section of the gill region of Amphioms. (After Lankester and Boveri.) a, aorta descendens; 6, peribranchial space ; r, notochord ; co, ccelom (branchial body cavity) ; e, hypobranchial groove, beneath it the aorta ascendens ; g, gonad; kb, gill arches: fcd, pharynx; I, liver ; m, muscles; n, nephridia, on the left with an arrow; ?•, spinal cord ; *n, spinal nerve ; sp, gill slit. {#) which allows the escape of the water through a porus branchialis (fig. 541, JP), behind the middle of the body. On the ventral floor of the pharynx is a ciliated hypobranchial groove (fig. 542, e), the homologue of the tunicate endostyle and of part of the vertebrate thyroid. It leads back to the straight digestive tract which opens on the left side near the end of the body, and bears in front a blind liver sac which extends forward into the gill region (figs. 504 CHOEDATA. 541, 542, I). The vascular system, with colorless blood, consists of a dorsal arterial (a) and a ventral venous trunk connected by ]ateral loops or arches. The ventral trunk begins as a subintestinal vein under the intestine, branches as a portal vein over the liver, and, reuniting again in a ventral vessel, continues forward, as the aorta ascendens, below the gills. From this the vascular arches — gill arteries — pass up between the gill slits and form the dorsal vessel, the aorta descendens. A true heart is lacking, but various parts of the vessels — a part of the ventral trunk and the bases 'of the gill arteries — are contractile, whence the name Leptocardii. As the pharynx lies in the peribranchial chamber, the digestive portion of the tract lies in a true body cavity or ccelom, which ex- tends forward (fig. 542, co) into the branchial region as well as into the gill-walls (branchial coalom) and into the outer walls of the peribranchial chamber (peribranchial coelom). In the peri- branchial ccelornare the gonads (g), a series of pouch-like cell fol- licles which, by dehiscence, allow their products to escape into the peribranchial chamber. Into this chamber also empty the excre- tory organs which were long sought for in vain. These are (n) a series, on right and left sides, of ciliated canals apparently cor- responding to the pronephros of the vertebrates. Each canal begins with at least one ciliated nephrostome in the coelom and opens separately like an annelid nephridium. Like the structure, the development is comparatively simple. The following points deserve special mention : (1) The eggs have a nearly equal segmentation (fig. 96). (2) A typical invaginate gastrula (fig. 105) occurs. (3) The mesoderm arises as a series of pouches, right and left, from the mesenteron, which later separate and represent the primitive segments. Hence these are clearly mesothelial in nature. From the cavi- ties of these arises the body cavity, which is consequently an enteroccele. (4) The dorsal surface of the entoderm between these coelomic pouches becomes folded off from the rest and forms the notochord, which lies between the digestive tract and the nervous system. (5) The nervous system arises from a longitudinal groove which becomes folded into a tube and is connected for a while with the digestive tract by a neuren- teric canal. Amphioxus * contains a few closely related species which occur on our southeastern coast, in Europe, Indian Ocean. Recently other genera have been described — Asymmetron * in America, Heteropleuron in the South Seas. The animals live in quiet bays and bury themselves in the sand, with only the mouth above the surface. Like all animals with rudimen- tary eyes, they shun the light and are greatly excited by strong illumi- nation. I/. TUNIC AT A. 505 SUB PHYLUM II. TUNICATA (UROCHORDA). In their adult condition the Tunicata, or sea-squirts, bear some resemblance to the siphonate mollusca, especially in the posses- sion of incurrent and excurrent orifices, usually close together, and a mantle. Hence these forms were long associated with the FIG. 543.— Diagram of a tunicate (orig.). a, atrium ; 6, nervous ganglion ; e, endo- style ; i, intestine; m, mouth; n, subneural gland; s, stomach; /, tunic. In the centre the branchial basket with the gill slits communicating with the peri- branchial space, and this in turn with the atrium. molluscs ; later they were associated with the worms, but their de- velopment shows them to be more nearly related to the vertebrates. The group owes its name to the tunic or mantle — lacking in the Copelatse — an envelope (fig. 543, t) which is formed like a cuticle by the epithelium of the skin, but which is distinguished from ordinary cuticula by its structure. It much resembles connective tissue in that cells from the mesoderm wander into the ground substance, which is sometimes fibrous, sometimes homogeneous, and has an interesting chemical nature. It consists of the same proportions of carbon, oxygen, and hydrogen (C6H1006) as cellulose and agrees with this substance, so characteristic of plants, in its reactions (blue color with iodine-iodide of potassium and sulphuric 506 CHORDATA. acid, violet with chloriodide of zinc). Nowhere else among ani- mals is there such a rich formation of cellulose. The anterior part of the digestive tract is modified into a pharynx or branchial chamber, the walls of which become per- forated with a varying number of gill slits, these leading either directly to the exterior or, more usually, into a peribranchial chamber, and from this to a cloaca or atrium (a), before reaching the outside world. While the respiratory water passes through the gill slits the food particles which it contains are received by a ring-shaped ciliated band (peripharyngeal band) and, enveloped by mucus, are ]ed to the oesophagus. This mucus is formed by a ciliated glandular groove, the endostyle (e), on the ventral surface of the pharynx. Between the gill region (end of the endostyle) and the stomach lies the ventral heart enclosed in a pericardium. It has the peculiarity, met nowhere else, of changing the direction of its contractions at frequent intervals; after the heart has driven the blood for a time to the gills it rests a while and then begins to force the blood in the opposite direction, pumping it from the gills and sending it towards the stomach. If we add to the foregoing that a dorsal ganglion and a her- maphroditic gonad are present, the striking features of the group are enumerated. The extreme forms, the Copelatae and the Thaliacea, are rather remote, but they are connected by interme- diate forms, the Ascidiae and Pyrosomas. Order I. Copelatae. These small forms — one or a few centimeters in length — are pelagic; they have the anterior end inserted in a gelatinous envelope or 'house' which replaces the lacking tunic. They swim like a tadpole by means of a tail which arises from the hinder end of the trunk. The alimentary canal (fig. 544) is bent on itself, and both it and the two large gill slits, in contrast to all other tuni- cates, open directly to the exterior. The heart (lacking only in the Kowalewskidae) is ventral and the hermaphroditic gonads and the nervous system dorsal. The latter consists of a cerebral ganglion, with beside it an extremely simple auditory organ and a ciliated groove, and farther a chain of ganglia extending into the tail. The notochord, a gelatinous structure enclosed by a sheath of cells, forms the skeletal axis of the tail ventral to the nerve cord and gives attachment to muscles. Oikopleura,* Appendicularia,* Fritillaria; Kowalewskia. II. TUNIC ATA: COPELAT^. 507 FIG. 544.— Oikopleura cophocerca. (After Fol.). A, the whole animal, removed from its 'house,' dorsal view; B, body, side view with base of tail. «, anus ; c, notochord; a', branchial region ; d", stomach ; en, endostyle ; /, ciliated peripharyngeal bands; 0, g', brain and first ganglion of tail; 7i, testis; m, mouth; o, ov, ovary; s, gill slits. B FIG. 545.— Ciona intestinalis. A, from the left side, the cellulose tunic and dermal muscular sac removed ; B, from the right side, the tunic entirely removed, pharynx opened from the mouth, a, anus ; c, cellulose tunic below with adhesive processes ; d, cloaca; d, rectum; e, atrial opening; en, endostyle ending above in the peri- pharyngeal band; g, ganglion; 7i, mouth of the 'hypophysis'; hr, heart, with peri- cardium ; ho, branched testes ; i. oral opening; /c, gill sac ; m, muscular sac ; oe, oesophagus ; orf, oviduct, the black line beside it the vas deferens ; ov, ovary ; s, partition between atrium and body cavity ; st, stomach ; t, crown of tentacles. 508 CHORDATA. Order II. Tethyoidea (Ascidiaeformes). With the exception of the pelagic Pyrosomidae all of the ascidi- ans are attached to rocks, etc., in the sea. The greater necessity for protection caused by this sedentary life has resulted in a great development of the cellulose tunic, which, enveloping the internal organs, gives these animals a swollen, somewhat shapeless appear- ance. Two openings, mouth and atrial opening, lead into the interior, and the water which issues from these, when the animals are taken from the ocean, has given them the common name of * sea-squirts/ On removing the tunic, which is but slightly attached to the other parts except at mouth and atrial opening, a muscular sac is seen (fig. 545), the fibres running circularly and longitudinally. In- side this sac are the viscera, the pharyngeal region by far the most conspicuous. The mouth leads to a short tube with tentacles (/), and then to the pharynx, a wide sac which lies in a large cavity, the peribranchial fl ^^f^WW^ "~\ chamDer> the walls of the pharynx and ! ' vliiff] M n fl ) ^^ ^e enclosing space uniting on the ventral U liMJlMMJI I side (fig. 543). The pharyngeal walls are perforated like a net by small ciliated gill slits, arranged in longitudinal and transverse rows (fig. 546), through which the water received from the mouth passes into the peribranchial chamber and thence FlG. 546.— dona intestinalis, a bit of the wall of the gill sac to tiie atrium, and so out to the external enlarged to show the gill , , slits. world. While the respiratory water thus passes out in a nearly direct course, the food particles which it contains pass into the digestive tract. By means of a ciliated tract (peripharyngeal band) just inside of the tentacles and surrounded by mucus secreted by the endostyle (or hypobranchial groove), the food is carried back to the oesophagus (oe) at the base of the gill chamber, and thence to the stomach (usually provided with liver glands), and on to the intestine. The anus is at the base of the special portion of the peribranchial chamber, which also receives the genital ducts and hence is known as the cloaca or atrium. In the body cavity, which is greatly reduced in the species with concentrated bodies, occur the digestive tract, the sexual organs, and the heart; the latter, frequently S-shaped, extends be- tween the stomach and the endostyle. Opposite to the endostyle //. TUNIC AT A: TETHTOIDEA. 509 is the ganglion in the dorsal wall between oral and atrial open- ings. Below it (rarely above it) is a branched subneural gland which, from its relations and its opening into the prebranchial part of the alimentary tract, has been compared to the vertebrate hypophysis. In many there exist special excretory organs, numer- ous blind vesicles filled with excreta. From the eggs are hatched small swimming tadpole-like larvae (fig. 547), resembling Appendicularia and, like it, consisting of FIG. 547.— Development of an Ascidian. (After Kupffer and Kowalevsky.) 1, larva, just hatched ; 2, cross-section through the tail of a slightly younger larva; 3, much younger stage, formation of notochord and nervous system ; U, anterior end of a larva just before attachment. (2, Phallusia mentula ; 8, A, Ph. mammillata.) cm, eye; c, notochord ; cZ, tunic: d, digestive tract; d', its nutritive, d", its respira- tory division; e, atrial vesicle; ek, ectoderm; en, entoderm; h, brain; t, oral in- vagination ; ni, muscles of tail; n, neural tube ; ne, neurenteric canal ; o, otocyst. trunk and tail, in which the chordate features are strongly marked. The digestive tract is confined to the trunk; dorsal to it lies the tubular nervous system in which three parts are recognizable: in front a vesicular brain with a simple eye and an otocyst imbedded in its walls; farther back a narrower portion (' medulla oblongata ') ; lastly, a spinal cord extending into the tail. In the axis of the tail is a notochord which extends forward a short distance into the trunk between digestive tract and nervous system. In the metamorphosis of the free larvae into the sessile ascid- ians four processes are important: (1) The larvae attach themselves by means of three ventral anterior papillae ; (2) The tail is retracted and, after preliminary fatty degeneration, is absorbed; (3) The body becomes more or less spherical by development of the tunic; 510 CHORD AT A. (4) Two dorsal invaginations are formed, these grow together, en- velop the pharyngeal region, and form the atrium and peribranchial chamber. It is to be noted that this arises from the dorsal sur- face and extends ventrally, while the peribranchial chamber of Amphioxus arises by folds which grow ventrally over the pharynx. Besides sexual reproduction many ascidians reproduce by bud- ding. Where this occurs it results in the formation of colonies, a matter of systematic importance. Sub Order I. MONASCIDI^E. Simple ascidians of considerable size ; sometimes with transparent, sometimes with thick opaque tunic. The CLAVELLINID.scis; g, sexual region; k, gill region; kr, collar; vb, ventral blood-vessel. The systematic position of the Enteropneusta is not settled beyond a doubt. In the possession of gill slits and in the formation of the dorsal nervous system it closely resembles the other chordates, and the resemblance is strengthened by similarities in de- tails of structure of the gills. The advocates of this view recognize the notochord in a blind tube, sur- rounded by tough membrane and thickened beneath, which extends from the pharynx into the proboscis. Embryology throws but little light on the problem. Some species have a direct development (fig. 553, B, C), while others have a larva (Tornaria, fig. 553, A) which so resembles the larvae of certain echinoderms that it was long held to belong to that phylum. The chief resemblances are in the relations of the ciliated bands to the alimentary tract and in the presence of the proboscis cavity, FIG. 553.— A, Tornaria larva of Balano- qlossus. (After Morgan.) n, apical plate ; ac, preoral part of ciliated band ; be1, fee2, btf, coelomic pouches ; m, mouth ; p, postpral part of ciliated band 5, C, twqF stages of Balano- ylossus with direct development. (Af- ter Bateson.) a+ anus ; be, branchial clefts ; c, collar ; dc, digestive part of alimentary «anal ; in, intestine ; ?ic, fc notochord '; p, proboscis. 514: CHORD ATA. which, like the ambulacral system, opens to the exterior. Two deep-sea forms, Cephalodiscus and Rhabdopleura, have the same type of ' noto- chord,' and the first has a pair of gill slits. In other respects these are strikingly Polyzoau in appearance. SUB PHYLUM IV. VERTEBRATA. In the vertebrates only the internal segmentation occurs. This- is shown, and most clearly, in the lower Vertebrata, in the muscles (myotomes, myomeres), the myocommata or myosepta which sep- arate them, and the protovertebrae from which they arise; in the nerves (neurotomes), the skeleton (sclerotomes), the blood-vessels,, and in the excretory organs (nephrotomes). In the higher verte- brates this metamerism is visible only in the embryonic stages. In part this absence of external segmentation has its cause in the heteronomy (p. 399) of the body and the obliteration of segmental boundaries, consequent upon the union of somites into body re- gions, of which at least three — head, trunk, and tail — at most six — head, neck, (cervical) thorax, lumbar, pelvic (sacral), and tail (caudal) — occur. Not less important in this respect is the charac- ter of the skeleton. The cuticular skeleton, which is the cause of the annulation of the arthropod, is entirely lacking. The skin remains soft, or contributes to a subordinate degree, more for pro- tection than for support, to the formation of a skeleton (dermal skeleton of fishes, alligators, turtles). On this account firmer tissue is formed in the axis of the body, which, in the lowest ver- tebrates and the embryos of the higher, appears as the notochord already mentioned, but in the higher is supplemented by the verte- bral column and skull. The skin of the vertebrates is distinguished from that of all invertebrates by two characters (figs. 26, 27): the many-layered condition of the epidermis, and the considerable thickness of the derma. The epidermis is but rarely covered by a delicate cuticle; usually such a protection is unnecessary since — and especially in the land forms — the superficial layers become cornified and hence furnish the necessary resistance without a cuticle. There are two layers to be distinguished, the deeper stratum Malpighii and the superficial stratum corneum (sM and sc\ see p. 76). The second constituent of the integument, the derma (cutis, corium), arises from the mesoderm (mesenchyme). It consists of many layers, often stratified, of close connective tissue, and is usually separated from the underlying structures, especially the muscles, by a loose tissue rich in lymph spaces, the subcutaneous IV. VERTEBRATA. 515 tissue. Both of these constituents of the skin, aside from their own firmness, can give rise to protective structures. The horny layer of the epidermis in places becomes greatly developed and thus forms the tortoise shell of the turtles, the scales, shields, and scutes of the snakes and lizards, the feathers of the birds, the hair and horns of the mammals. Other epidermal products are the claws, nails, and hoofs of the terrestrial vertebrates. The derma is often the seat of ossifications which, in contrast to the deeper bones, are called the dermal skeleton. First of the dermal skeletal parts are the scales of the fishes,, which, in spite of similarity of name, are different from the epidermal scales of the reptiles. They may be traced back to the primitive form, the pla- coid scales of the Elasmobranchs. These are rhombic plates, bearing in the middle pointed spines, which are called dermal teeth from similarity in structure and development to the teeth of the mouth cavity (fig. 554). They consist of dentine (d) and have a large pulp cavity (p), with numerous blood- vessels in the interior. Whether the thin layer (.sr/<) covering the tip can be called enamel is disputed. Der- mal teeth and true teeth are identical structures which, in consequence of different position and consequent dif- 1'erence of function, have developed differently. The scales of fishes have a wider anatomical interest, since from them have arisen, besides the bony plates which form the resistant armor of the turtles, alligators, and many mammals (Armadillos), important parts of the axial skeleton, the secondary or membrane bones. A membrane bone is a bony plate which has arisen from a fusion of dermal ossifications, becomes transferred to a deeper position, and contributes to the completion of the axial skeleton. After what was said above about the relations of dermal and true teeth it is readily seen that a further source of formation of mem- brane bones lies in the lining of the mouth cavity. In describing the axial skeleton, the notochord comes first. This has already been mentioned in connexion with the lower Chordates. It persists in the cyclostomes, but from them upwards 516 C HOED AT A. it is gradually replaced by the vertebrae arising around it. It is of 'cntodermal origin (fig. 9), arising as a longitudinal band of the epithelium of the archenteron (/, di), and, becoming cut off, •comes to lie in the long axis of the body between digestive tract and nervous system (//, ///) ; here it forms a cylindrical rod con- sisting of a connective tissue which, as already said, resembles plant tissues because of the vesicular nature of its cells (fig. 38). In transverse section (fig. 555) the chorda is surrounded by three layers, internally by a fibrous noto- chorda] sheath, then an elastic layer (not always present), the elastica externa, so called because an elastica interna is some- times present inside the notochordal sheath; and lastly a skeletogenous layer (SS), also called the outer notochordal sheath. This last is a mesodermal con- nective-tissue layer and is therefore con- nected with the other connective-tissue sheaths which surround muscles, nerves, etc., and deserves special mention because in it the cartilages and bones arise from which the vertebrae and skull are formed. Cells from it can penetrate the notochor- dal sheath, converting it into fibrous car- , ._ tilage, thus enabling it to participate in , to ' the formation of the vertebras. 0. ,n , n -, -. ., 1,1 Since the notochord and its sheaths are elastic and give under the strain of the muscles, they are unsegmented. The seg- mentation of the axial skeleton begins with the appearance of firmer tissue in cartilage and bone. Then there is a separation of successive parts, and with this the gradual forma- tion of vertebral column and skull. For both there is a con- nected series of developments, if studied with reference to the ontogenetic processes or in the comparative manner from the lower to the higher forms. The first parts of the vertebral column to appear are the upper and lower (figs. 555, 556), or neural and hcemal arches. These consist of paired parts in the skeletogenous layer which abut against the notochord, and which are usually a pair to the somite, although occasionally two or more pairs, the arches proper and the intercalaria^ may occur. The neural arches (arcus vertebras /z 555.-Transverse section of axial skeleton of Pe- tromyzou. (From wieders- heim.) C, notochord; Cs, notochordal sheath ; Ee, elastica externa : f, fatty tissue M, spinal cord ; P, tissue ; -SVS, skeletogenous tissue ; Ul>, lower process of skeletogenous tissue. IV. VERTEBRATA. 517 of human anatomy) enclose a spinal canal surrounding the spinal cord, the parts of the arch, neurapophyses, uniting above the cord to form the spinous process (frequently an independent part of the skeletal axis). In the caudal region, in the same way, hcemal arches may be formed of licemapopliyses and hcemal spine, the arches surrounding the blood-vessels of the tail (fig. 557). In the trunk region the ventral arch behaves differently. Since the large body n FIG. 556.— Vertebrae of sturgeon, ch, notochord; /, exit of nerve ; j, dorsal and ventral intercalaria ; n, neural canal: o7>, neural arch; s, chordal sheath; r, rib; ubf haemal arch. Bone white, cartilage dotted. A. FIG. 557. FIG. 558. FIG. 557.— Caudal vertebrae of a carp, section (.4) and nearly side view (B). cli, space filled by notochord; /t, haemal arch; ?i, neural arch; 06, neural spine ; ub, haemal FIG. 558. — Thoracic vertebra, ribs, and sternum of a mammal. (From Wiedersheim.)' Co, capitular head of rib ; Co, neck of rib; Cp, bony rib; Kn, cartilaginous rib : Ps, spinous process ; Pt. transverse process (diapophysis) ; St, sternum ; T6, tuber- cular head of rib; WK, vertebral centre. cavity with its viscera, varying in size (digestive and reproductive organs), is here, the haemapophyses extend outwards and down- wards and are divided into two parts, a basal apopliysis and a, lower movable portion, the rib (fig. 556). Also the lower union of haemapophyses with haemal spine does not occur; the ribs ar& either free (fishes) or are (at least in part) connected ventrally by 518 CHORD ATA. a breast bone or sternum (Amniotes, fig. 558). The sternum is a derivative of the ribs. In development the ventral ends of the ribs of a side fuse and then these fused tracts of the two sides unite to form the sternum. The haemal arches lie internal to the longitudinal muscles of the body, and in the trunk region they lie in the same position just beneath the peritoneum. These are hcemal ribs and are found only in teleosts and ganoids. The ribs of all other vertebrates (elasmobranchs, amphibia, amniotes) are morphologically different and are called lateral or pleural ribs. They develop independently of the vertebral column in a horizontal connective-tissue septum which extends out through the longitudinal mus- cles from the axial skeleton to the skin, dividing the musculature into dorsal (epaxial) and ventral (hypaxial) portions (fig. 89). In the elasmo- branchs these pleural ribs are attached to the haemapophyses, in the others to the transverse processes (diapophyses), which arise from the neurapophyses, and parapophyses, which arise from the vertebral centres. In the caudal region, often also in the cervical, lumbar, and sacral regions, the pleural ribs and dia- and parapophyses fuse to form lateral processes. These occur concurrently with haemal arches in the tails of many Am- phibia and reptiles and some mammals, forming the chevron bones which, as in fishes, enclose the caudal blood-vessels. The presence of intercalaria in cyclostomes, sharks, and ganoids indicates that primitively a double vertebra arose in each somite. Paleontological and embryological re- searches on reptiles support this view. In most vertebrates either the basal ends of the arches broaden •out around the notochord and fuse with one another, or perichordal cartilages arise independently, furnishing in either case firm sup- ports, the vertebral bodies, or centra, for the system of arches. These increase in size at the expense of the notochord on the in- side, sometimes leading to its almost complete obliteration, as in the mammals; in others, as the fishes, the reduction is less com- plete. The fishes have awphicoele vertebra} (fig. 557), that is, the centra are hollow at either end. In these cups the notochord exists even in the adult, and when small connecting portions ex- tend through the centra the notochord takes the form of a rosary with alternating enlargements and contractions. Histologically the vertebral column may be either cartilage or bone; usually it is first formed in cartilage, which is later replaced by bone. If the ossification does not occur, the column remains cartilaginous; if incomplete, cartilage and bone appear side by side. iSince these histological differences are combined with varying de- grees of persistence of the notochord and with modifications in the form of the vertebrae and their processes, there results an extraor- dinary variety in the appearance of the vertebral column. / V. VER TESSA TA. 519 In order to allow for bending where complete centra are present vari- ous conditions occur, (a) Opisthocode vertebrae have a socket on the hinder surface which receives the convex anterior end of the succeeding centrum, forming a ball-and-socket joint. (6) Precocious vertebrae have these relations reversed, the socket being in front, (c) The vertebrae may articulate with a 'saddle joint' (birds), (d) Between two successive vertebrae an elastic intervertebral ligament may occur (mammals). The neurapophyses may bear, in addition to the transverse processes, anterior and posterior articulating processes (zygapophyses) connecting the sepa- rate vertebrae. The skull, the anterior continuation of the axial skeleton, occurs in all vertebrates; it appears before the vertebrae, for it is found in the cyclostomes, which lack these. It surrounds the brain as the vertebrae do the spinal cord: and, like them, its first stages are formed in the skeletogenous layer surrounding the anterior end of the notochord. It is so related to the surrounding parts that it may in general be said to be equivalent or homodynamous with the vertebrae, although we cannot agree with Oken and Goethe, the founders of the vertebrate theory of the skull, that it has arisen by the fusion of vertebrae. On the other hand skull and vertebrae are parts arising in the common basis of the skeletogenous layer, but which have developed in different directions. Three stages are recognized in the development of the skull: the membranous, the cartilaginous cranium, and the bony skull. The first, which consists of connective tissue, occurs only in the early embryonic stages, scarcely a trace of it persisting in the adults. It is early replaced by the cartilaginous skull, which may persist unaltered throughout life in the lower fishes (elasmo- branchs, sturgeon). In most vertebrates, however, ossification sets in, embracing a part (fishes, amphibians) or the whole of the carti- lage (birds, mammals), converting it in the latter case into a bony capsule. In the bony skull two kinds of bone, primary and sec- ondary, are recognized, these varying in their origin. The pri- mary or cartilage bones develop from the cartilage itself, either in its interior (entochondrostoses) or in its enveloping perichondium (ectochondrostoses). The secondary or membrane bones are, in their origin, foreign to the axial skeleton and arise from the ossifi- cations in the skin (scales) or in the mouth (teeth), already re- ferred to (p. 515 ). They sink into the deeper portions and apply themselves to the axial skeleton, especially to those parts where, from lack of cartilage, no primary bones can be formed (parostoses). Still it is not settled how far these distinctions may be carried. According to Gegenbanr all ossifications arose primarily in the skin 520 CHORD AT A. or mucous membranes, and primary bones are merely membrane- bones which have entered the cartilages and replaced them. Ac- cording to this view it is conceivable that the same bone in one animal may arise as a membrane bone and in another as a primary bone, a view which is of importance in the homologies and no- menclature of many bones. It is but just to say that this view is not universally accepted. The cartilaginous cranium (chondrocranium) is most complete beneath the brain. This basal portion is a direct continuation of FIG. 559.— Chondrocranium of Amplduma. o?ip, antorbital process; r/p, ascending process of quadrate; c, cornu trabeculee; e, ethmoid plate; ef, endolymph fora- men; j, jugular foramen; I, lamina cribrosa; m. Mockers cartilage ; N, notocnorcr oc, oculomotor foramen; ocp, occipital process; o/, optic foramen ; p, parachor- dal; pal, palatine foramen ; p/, perilymphatic foramen; q quadrate; s, stapes; sp, stapedial process ; f, trabecula; trc, crest of trabecula; V, VII, VIII, foramina for V, VII, VIII nerves. the vertebral column, and a part of it (the paracliordals) embraces the anterior end of the notochord, while part (the trdbeculce) ex- tends in front of the end of the notochord. The side walls of the skull are increased by the cartilaginous envelopes of the two sense organs, the nasal and otic capsules, around the nose and ear. Be- tween these is a hollow for the eye which contributes nothing to the skull. In only a few forms is the chondrocranium completely IV. VERTEBEATA. 521 closed; usually gaps (fontanelles) occur in its roof, and frequently in its floor. The higher the animal intellectually and the larger its brain the more the connective tissue (primordial cranium) is called upon to roof in the chondrocranium. Hence it is that in the reptiles, birds, and mammals, where it is also confined to embryonic life, the chondrocranium is relatively the smallest. Since it only closes above in the occipital (hinder) region, while it gaps widely in front, it follows that the secondary bones play an important part in the completion of the skull. The bony skull presents great difficulties from the standpoint of comparative anatomy, in part from its varying appearance in the different groups, in part on account of the number and com- plicated arrangement of the constituent bones. It may be said in beginning that as a rule the same bone reappears in the separate classes, and that the difficulties are connected with the fact that certain bones may fail to develop (Amphibia), or they may fuse to larger elements (mammals). A further complication results from the intimate union with the cranium of bones of the visceral arches, which, strictly speaking, do not belong to it. me. fis firo as FIG. 560.— Skull of carp, the visceral skeleton removed. (A) Cartilage bones: ocb ocl, ocs, basi-, ex-, and supraoccipitals ; ego, epiotic ; pto, pterotic ; sp/io, sphe- notic; pro, prootic ; as, alisphenoid ; o.s, orbitosphenoid; me, mesethmoid ; ee, ect- ethmoid. (B) Ventral membrane bones : p.s, parasphenoid ; vo, vbmer. (C) Dorsal membrane bones : p, parietal ; /?-, frontal ; l-U, exits of nerves. The primary bones (preformed in cartilage) can be divided ac- cording to the cranial regions into four groups: (1) bones of the hinder part of the head — occipitalia; (2) bones of the ear region — otica; (3) bones near the eye — splienoidalia; and (4) of the nasal capsule — ethmoidalia. The occipitalia — four in number (figs. 560-562) — united in the higher mammals to a single occipital 522 CHORD AT A. bone, surround the foramen magnum, the opening through which the spinal cord passes to connect with the brain. These are & pair of exoccipitals, right and left, a supraoccipital above and .a basioccipital below. The otica depend in their development upon the extent of the otic region. In the fishes, where this part is large, several bones may be present : epiotic, pterotic, sphenotic, prootic, and often opisthotic. In the mammals, on the other hand, these are fused to a single petrosal bone (figs. 561, 562) of small size. Since the otic bones usually do not reach the middle line below, the sphenoidalia rest direct upon the basioccipital behind and in front upon a presphenoid bone, both unpaired but arising from P<* No, Jmt 01 FIG. 561. -Skull of goat. (From Clans.) Als, alisphenoid; Bs, basisphenoid; (7, occip- ital condyle; Eth, mesethmoid, covering the ectethmoid; Fo, optic foramen in orbitosphenoid; Fr, frontal; I»ix, premaxillary; 7p, interparietal ; Ju, jugal (malar); La, lachrymal ; MX, maxillary; JV«, nasal; O/>, basioccipital; Ol< exoc- cipital ; Ors, orbitosphenoid; Pa, parietal; Pal, palatine; Pe, petrosal; Pm. paramastoid process; Ps, presphenoid ; Pt, pterygoid ; S/, frontal sinus in frontal bone ; £>pb, basisphenoid ; ventral (s), and lateral (S) columns of the cord. Corresponding to each muscle segment two nerve roots arise- from the cord, a dorsal root, with a ganglion (spinal ganglion) at some distance from the cord, and a ventral root, without a ganglion. The dorsal root contains only sensory fibres — i.e., those carrying; nervous impulses to the cord — and is afferent, while the ventral roots are efferent and contain only motor elements (Bell's Law). These roots unite into a mixed root, which then divides into dorsal and ventral branches. The brain of vertebrates in general corresponds in its funda- mental plan (fig. 568), best seen in development, with the brain of man. At an early stage it consists of three vesicles, one after the other, a fore brain (prosencephalon), a mid brain (mesencepha- lon), and a hind brain (metencephalon). Usually this stage is reached before the closure of the medullary folds. Formerly it was stated that a condition with five vesicles -ir FIG. 568. FIG. 569. FIG. 568.— Diagram of a vertebrate brain. (From Wiedersheim.) Aq, aqueduct ; O, central canal ; FM, foramen of Monro (connexion of lateral ventricles with each other and with the third) ; HH: cerebellum ; MH, corpora bigemina (optic lobes) ; NHt medulla oblongata; J?, spinal cord; /, olfactory lobe ; oZ, optic lobes ; p, pinealis. followed upon this with three, the mid brain remaining undivided, while the hind brain divides into cerebellum (cb) and medulla oblongata (m) ; the fore brain into cerebrum and 'twixt brain. This is unnatural so far as the hind brain is concerned, for cere- 534 CHORD AT A. bellum and medulla are related to one another as roof and floor of one and the same cavity (fig. 569). The distinction between the first and second vesicles is problematical. The fore brain becomes divided into three parts by an inpushing at its anterior end: an impaired middle portion, and in front a right and a left diver- ticulum. These paired portions, increasing in size, form the cere- bral hemispheres, and together with a small connecting part represent the first cerebral vesicle, while the unpaired portion forms a second vesicle, the 'twixt brain. Introducing the terms of human anatomy for the separate parts of the brain, the first vesicle consists of the two cerebral hemi- spheres whose dorsal and lateral walls are usually thick and are called the pallium, while in the floor of each hemisphere is an enlargement, the corpus striatum (cs). The spaces in the hemi- spheres are the first and second ventricles (sv). From the front portion of each hemisphere arises a distinct region, the olfactory lobe (o/), which gives origin to the olfactory nerve. Since the organ of smell is frequently at some distance from the brain, the olfactory nerve must be elongate, as in the Amphibia (fig. 614), or the olfactory lobe must lengthen, as in many Elasmobranchs (fig, 592). In the latter case the swollen end of the lobe is close to the olfactory epithelium and is connected with the brain by a long stalk, the tractus, while the swelling is called the bulbus olfacto- rius. Both, as parts of the brain, must be distinguished from the olfactory nerve. In the region of the second vesicle only the lateral walls become thickened, producing the optic thalami, directly adjoining the corpora striata; the roof of this vesicle develops no nervous sub- stance, but remains a thin layer of epithelium closing in the third ventricle above (///)• The floor is also thin-walled between the thalami and is pushed downwards, forming a funnel-like pocket, the infundibulum (i). The third vesicle, as a rule, is divided by a deep longitudinal dorsal groove, dividing the cavity into a right and left ventricle, while the two halves of the roof are known as the optic lobes or corpora bigemini. In the mammals alone (in ^vhich there is also a transverse groove dividing the optic lobes into the corpora quadrigemini) the cavity of this mid brain is re- duced, by thickening of the walls, to a narrow canal, the iter or aqueduct of Sylvius, with the result that the term fourth ventricle is transferred to the cavity of the hind brain. This last region is called the medulla oblongata ; it is a prolonga- tion of the spinal cord, and in many respects shows a similar struc- IV. VEBTEBRATA. 535 ture. It is distinguished from the cord externally in that it gradually increases in size in front, while its roof is reduced to a thin epithelium, often torn away in dissection, leaving an opening, the fossa rhomboidalis, into the ventricle. In front of this fossa is the cerebellum, often a thin transverse nervous lamella, but usually is a considerable part of the brain, composed of a median ' vermis ' and two lateral cerebellar hemispheres. Although these five parts are present in all vertebrates, the appearance of the brain in the various classes is very different, because the relative size and form of the parts undergo great variations. In the lower vertebrates optic lobes and medulla oblongata are disproportionately large, while the cerebrum, and often the cerebellum, are insignificant in size; in the cerebrum, again, the hemispheres may be smaller than the corpora striata and the olfactory lobes. In the higher vertebrates, on the other hand, the cerebrum and cerebellum far surpass the other parts, the increase in size of the cerebrum being proportional to the in- crease in intelligence. The cerebral hemispheres grow backwards, in man and the apes covering the other parts, while in front the olfactory lobes are carried by a similar overgrowth to the lower surface. Since the capacity of the skull is limited, the cortex of the cerebrum, the seat of intelligence, is increased in amount by the development of folds, gyri, separated by sulci. Somewhat similar conditions exist in the cerebellum, which in mammals and birds is, next to the cerebrum, the largest part of the brain. Connected with the 'twixb brain are two problematical organs, one, the epiphysis (pinealis), being dorsal; the other, the hypophysis (pituitary body), ventral. The hypophysis arises like a gland by an outgrowth from the embryonic mouth. This hypophysial pocket cuts off from its source, increases by budding, and fuses with parts derived from the end of the infundibulum to a single two-lobed body. It has been compared with the subneural gland of the Tunicata (p. 509). The epiphysis is an outgrowth from the roof of the brain, from which develops in many vertebrates the parietal organ. In many reptiles this has the structure of an eye (pineal eye), and in these, separated from the brain, but connected with it by a nerve, it lies in a special cavity in the parietal bone, which occurs not only in recent but in fossil forms. Above the eye the skin may be transparent. The nerves which come from the brain mostly arise from the region between the mid brain and the spinal cord, especially from the medulla oblongata. The olfactory and optic nerves are an exception, the one arising from the cerebrum, the other from the 'twixt brain, but both, and especially the optic, differ so much from the peripheral nerves that they can hardly be classed with them. 536 CHORD AT A. Development shows that the optic nerve is a part of the brain. Following custom, however, and including these two, the pairs of cranial nerves may be enumerated in the terms of human anatomy as follows: I, N. olfactorius; II, N. opticus; III, N. oculomotoiius; IV, N. trochlearis (patheticus); V, N. trigeminus; VI, N. abducens; VII, N. facialis; VIII, N. acusticus; IX, N. glossopharyngeus; FIG. 570. — Diagram of cranial nerves (shark), a, alveolaris ; ft, buccalis ; c, cere- brum ; cb, cerebellum; ct, chorda tympani ; e, ear ; er, external rectus muscle ; /, inferior rectus muscle ; g, Gasserian ganglion ; h, hyoid cartilage; hm, hyoman- dibular ; i, internal rectus muscle ; 10, inferior oblique muscle ; j, Jacobson's commissure ; I, lateralis of vagus ; m, mouth ; me, Meckel's cartilage: md, mandi- bularis ; mar, maxillaris superior; n, nose ; o, optic lobes; op, ophthalmicus profun- dus; os, ophthalmicus superficialis; p, pinealis; pi, palatine ; po, posttrematic branches; j?»\ pretrematic branches; pn, pneumogastric (intestinal) of vagus; ptg, pterygoquadrate; s, spiracle; so, superior oblique muscle; sr, superior rectus muscle; f, 'twixt brain; I-X, cranial nerves: 1-5, gill clefts. X, N. vagus (pneumogastricus), XI, N. accessorius; XII, N. hypoglossus. The accessorius in fishes and amphibia is a part of the vagus; the hypoglossus, strictly speaking, belongs to the spinal nerves and only secondarily is associated with the cranial nerves, which explains its course, outside the skull, in cyclostomes and amphibia. Since the head undoubtedly consists of several coalesced body seg- ments (at least as many as there are visceral arches, and apparently more), the question arises whether the cranial nerves are as evidently seg- mental as are those of the trunk. To this is allied the further question whether Bell's Law that a mixed nerve consists of dorsal sensory, and ventral motor components is applicable here. Both problems have been much discussed in recent years, but as yet the final answers have not been given. It is probable that the present cranial nerves, the optic and olfac- tory excepted, have arisen by manifold rearrangements of segmental nerves. On the other hand it seems impossible to accept Bell's Law here without considerable modification, since many cranial nerves (facialis, trigemenus, etc.) contain motor fibres, although they are formed like dorsal roots. IV. VERTEBBATA. 53T Besides the nervous system of the body already outlined, the vertebrates have a special nervous system supplying the viscera, — the sympathetic system, — and in this a special central organ consisting of right and left cords beneath the vertebral column, in which ganglia are incorporated. The last of these ganglia lies at the base of the caudal vertebrae, the most anterior at the beginning of the neck. From the latter nerve corda extend into the head and are connected with ganglia (otic, sphenopalatine). This system sends out nerves in the form of delicate networks (plexus sympathetici) which usually accompany the blood-vessels to the vegeta- tive organs (intestine, sexual apparatus, etc.). It is also connected with the spinal nerves. Regarding the sense organs of the vertebrates we stand on firmer ground than with the invertebrates, since their great simi- larity to those of man supports the ideas of their functions derived from studies of their structure. The tactile organs make an ex- ception, since only in land animals, and not in fishes, do they resemble those of man. These organs, in all forms above fishes, have the peculiarity that the nerves do not end in epithelial cells, but in special tactile cells of the derma, which either lie isolated in the connective tissue (Amphibia, reptiles), or, grouped together,, produce tactile corpuscles (birds, mammals, fig. 571). These are oval bodies and are im- bedded in special papillae of the derma. In form and position they are much like the Vater-Pacinian corpuscles, which are distin- guished by their histological structure (fig. 78) and, since they also occur in internal organs (mesentery of cat), are of problematic function. v— >JV^T Besides these mesodermal nerve endings there FIG. 571.— Tactile cor- are present in all vertebrates intraepithelial tongife. nerve branchings which are best seen in the cfeff6) cornea of the eye and in animals, like pigs and Petitions, moles, with sensitive snouts. Even here the finest nerve twigs do not end in epithelial cells, but in small knobs between them. Fishes lack tactile cells, tactile corpuscles, and end bulbs; hence the skin is provided with sense organs in which a sensory epithelium occurs. The dermal nerves pass into the epidermis and end in oval corpuscles, which, while imbedded in a stratified epithelium, consist of a single layer of sense cells. According to structure, nerve hillocks and nerve-end buds are distinguished. The first are the specific organs of the lateral line, to be men- tioned later, of fishes and branchiate amphibians and amphibian larvae, and therefore appear to subserve special and important sensa- 538 CHORD AT A. tions connected with aquatic life; hence the idea of a 'sixth sense/ lacking to man (cf. p. 125). The end buds are especially collected in the neighborhood of the mouth, on the lips and bar- bels. Since they also occur in the mucous membrane of the mouth, especially in the palatal regions, they connect with the taste organs. The taste buds have the same structure as the end buds of fishes. They occur in all classes of vertebrates, and are most abundant in man in the walls of the circumvallate papillae at the base of the tongue; in rodents on the large foliate papillae, etc. The end buds also lead to the olfactory organs. The olfactory epithelium of many fishes and amphibia is a stratified epithelium with closely arranged end buds (fig. 572), By disappearance of FIG. 572.— Section of olfactory epithelium of a fish (Belone). (From O. Hertwig, after Blaue.) e, epithelium • fc, olfactory buds ; n, nerves. the isolating parts of the ordinary epithelium the end buds form a continuous sensory epithelium, which is the rule in most ver- tebrates. The olfactory organ, the nose, lined with its sensory epithelium, .acquires a special interest both from its grade of development and from the important systematic distinctions it affords. Except the cyclostomes, which have an unpaired nasal sac, all vertebrates have paired olfactory organs. In adult fishes and in the embryos of higher forms are two pits which lie in front of or dorsal to the mouth; they are either distinct from it or only connected with it by an oronasal groove in the skin (fig. 599). If the animal be terrestrial and replace branchial by pulmonary respiration, a respiratory canal is developed in connexion with the nose. The oronasal groove closes to a tube which begins with an opening (nostril) on the surface and ends with a second opening (choana) in the mouth cavity. The olfactory sac proper is included in the wall of this tube, usually on its dorsal surface (fig. 573). In Am- IV. VERTEBRATA. 539 phibia, lizards, snakes, and birds the choanais far forward, behind the upper jaw; in alligators, turtles, and mammals it is carried far back, in crocodiles and some mammals (edentates) nearly to the vertebral col- umn. This position is brought about by the development of the hard palate, a parting wall which divides the primitive mouth cavity into two portions, a lower, the persistent or secondary mouth cavity, and an upper, which, as secondary nasal Fl) has been closed in ; in B the optic vesicle (v) has reached the lens (?) and on the right is being converted into the double-walled optic cup with, as shown in C\ an outer tapetal (e) and an inner retinal layer (/). to an optic vesicle which is connected with the brain by an optic stalk. The vesicle extends out to the periphery and, coincidently with the de- velopment of the lens, is folded into a double-walled optic cup with outer or tapetal, inner or retinal layers. If the position of the epithelial cells be followed, it will be seen that the peripheral ends rest upon the tapetum, and when these ends develop the rhabdomes, these must grow into the tapetal layer. In contrast to the retina, the lens develops as an invagination from the epithelium of the body (fig. 574) ; sclera, cornea and vitreous body from connective tissue. Thus the important part of the eye arises from the brain and is later provided with accessory apparatus which arise from peripheral parts. The invertebrate eye, on the other hand, with all its parts arises from the skin. The vertebrate eye is furnished with secondary structures : with mus- cles which move it, with lids which protect the cornea from injury and drying. The lids are dermal folds which extend over the eyeball from above and below. To these a third lid, the nictitating membrane, may 542 CHORDATA. be added. It arises from the inner angle of the eye, and can extend over the cornea beneath the upper and lower lids. A special lachrymal gland, which occurs at the outer angle of the eye, provides the fluid to moisten, the cornea, while a second or Hurder's gland occurs at the inner angle^ when a nictitating membrane is present. Both are lacking in the An- ainnia. The ear, at the level of the medulla oblongata, rivals the eye in its complication of structure. In development it has one point in common with the invertebrate ofcocyst — it dermal pit ass ecu arises as an ecto- which is usually completely cut off from its par- ent layer, and only in elasmo- branchs remains connected with the exterior by a tube, the elsewhere closed endolymphatic duct. In the cyclostomes it con- sists of a single vesicle with a single macula acustica; from the fishes upwards the vesicle becomes divided by a constric- tion into an upper utriculus and a lower sacculus (fig. 575), the connecting utriculosaccular FIG. 575.— Diagram of membranous laby- duct being narrow ill the mam- rinth of a fish. (From Wiederaheim.) aa, ae, ap, anterior, external, and poste- maiS. rior ampullae ; ass, superior utricular sinus ; ca, ce, cp, anterior, external, and posterior semicircular canals ; cus, utri- culosaccular canal; de, ductus en- dolymphaticus ; 7, lagena ; rec, recessus utriculi ; se, sacculus utriculi ; ss, supe- rior utricular sinus ; sp, posterior utri- cular sinus; u, utriculus; t, origin of en- dolymph duct. culus macula from t Both utriculus and sac- receive a part of the a -i. acustica. Diverticula occur, giving name of labyrinth. From the utriculus arise three semicircular canals, connected at either end with this cavity, each swollen at one end to an ampulla, containing a special nerve termination, the crista acustica. These canals stand at right angles to each other in the three dimensions of space and with- out doubt subserve the sensation of equilibration (p. 128). They are an outer horizontal, an anterior vertical (nearly sagittal), and a posterior vertical (nearly transverse). The non-ampullar ends of the two vertical canals unite, a condition which is understood when it is recalled that in cyclostomes these canals alone are present, and in Myxine form a single canal with two ampullae. A later formation is a diverticulum from the sacculus, which IV. VERTEBRATA. 543- appears even in the fishes as a small pocket, the lagena, containing a part of the macula acustica; in the reptiles and birds the lagena becomes much larger, and in the mammals is spirally coiled and is known as the cochlea. A part of the macula acustica of the lagena develops into a special nerve-end apparatus, the organ of Corti. The membranous labyrinth described above is partially or en- tirely enclosed in the side wall of the skull in the otic capsule, which may ossify to the otic or petrosal bones. In the birds and mammals the enclosure is such that the structure is duplicated in bone, so that the membranous labyrinth lies in a bony labyrinth,. FIG. 576.— Diagram of human ear. (From Wiedersheim.) a, 7), vertical semicircular canals ; c, their upper connexion ; Co, the connexion in bony labyrinth ; Cow, ductus cochlearis; Con', cochlea; Cr, canalis reunions; Ct, tympanic cavity (left), cupula terminates (right); d, perilymph; De ductus endolymphaticus ; Dp, Dp', ductus perilymphaticus ; Kl, Kl\ bony labyrinth surrounding the mem- branous labyrinth, the perilymph space black ; M, conch of ear (left), membrane closing fenestra rotunda (right); Mae, external auditory meatus; Jffc, tympanic membrane; S, sacculus; SAp, ear bones (represented as a rod) ; Se, sacculus en- dolymphaticiis ; St, Sv, scalae tympani and vestibuli ; Tb, Tb\ Eustachian tube and its entrance into pharynx ; *, connexion between scalae tympani and vestibuli; t, insertion of ear bones in fenestra ovalis ; 2, utriculus. the two being separated by lymph spaces (fig. 576). These spaces- are developed in the cochlea into two tubes, the scala tympani and scala vestibuli, the two connecting only at the tip, being separated elsewhere in part by the membranous cochlea (the ductus cochlearis or scala media). The spaces of the bony labyrinth are filled by two different fluids: inside the membranous labyrinth an en- dolvmph, and between this and the walls of the bony labyrinth a perilymph. 54:4 CHORD AT A. Accessory structures may be added to this auditory apparatus proper, their purpose being to bring sound waves to it. Such .structures are but occasionally present in fishes (it is not certain that they hear), since the sound waves are easily carried by the water to the tissues and thence directly to the ears. On the other hand, with the change to terrestrial life such a sound-conducting apparatus is necessary on account of the differing densities of the air and the tissues. So we find from Amphibia onwards a vibrat- ing membrane — the tympanic membrane — which receives the sound vibrations from the air and carries them to a chain of ear bones (ossicula auditus), which in turn transmits them to the inner ear or labyrinth. These structures are not always functional (cetacea), and they may be wholly or in part rudimentary (urodeles, snakes, Amphisbaenids). To understand this apparatus it must be recalled that the ear lies between the hyoidand mandibular arches in the neighborhood of a canal which leads from the surface to the pharynx. In the fishes this canal is the spiracle, a reduced gill cleft. In the Anura and amniotes it consists of an air chamber closed exter- nally by the tympanic membrane, stretched on a tympanic an- nulus, while the opening to the pharynx is retained. The part next the membrane becomes expanded into the tympanic cavity, this with the membrane forming the tympanum or drum. The part connecting with the pharynx is usually narrowed and is called the Eustachian tube. The membranous labyrinth lies in the wall of the tympanic cavity and touches it at one or two points where the bony auditory capsule is interrupted, the always present fenestra ovalis, and the fenestra rotunda, lacking in Amphibia. When it is recalled that the mandibular arch lies just in front •of the spiracle, and the hyoid close behind it, it is readily under- stood how parts of these arches can enter the tympanum and produce the ear bones. In Anura, reptiles, and birds a columella has one end attached to the stapedial plate, which lies in the fenestra ovalis, while the other is in- serted in the drum membrane, the whole conveying the waves across the tympanum to the labyrinth. In the 'Fio. 577.— Ear bones of man. mammals the structure is different, since in™?; ^SSffiSSf; 1; the columella is replaced by two bones, stapes> the malleus, which is attached to the the £m arteries> Pass viding below into the two bronchi; fV,p mlla fhp dorsal trnnlr Z, position of diaphragm; I,*, 3, Sa, tn<3 £111S> l UnK> globes of right and left lungs. collects from these, must contain oxygenated blood, which is sent by the carotids to the head, and by the dorsal aorta and the vascular loops to the body. It thus becomes venous and flows back into the ventral trunk. This scheme of circulation in fishes needs further description. The heart, a strong muscular organ enclosed in a pericardium, con- sists of two parts, auricle and ventricle, separated by valves. The trunk (ventral aorta) arising from the auricle is arterial and cor- responds to the ascending aorta and pulmonary artery of man. The arterial arches of the gill region which arise from it pass di- rectly into the dorsal vessel only in young fishes (fig. 597); later they furnish the branchial circulation of gill arteries, gill capillaries, and gill veins (fig. 65). The dorsal trunk is the dorsal aorta (aorta descendens) ; the ventral trunk, which only occurs in the embryo, is the subintestinal vein, from which the portal vein arises. To this are added a system of paired veins, consisting of Cuvierian IV. VERTEBRATA. 549 ducts and jugular and cardinal veins, the latter with growth en- croaching more and more into the territory of the subintestinal vein. The circulation of the fish type undergoes a great modification with the loss of gills and the appearance of pulmonary respiration. Gills and gill capillaries disappear, and the branchial circulation is reduced to arterial arches leading direct from the ventral to the dorsal aorta. The swim bladder received its blood from the body (systemic) circulation, but with the functioning of the lungs pul- monary arteries and veins come into existence, while the arterial arches in part disappear, in part are divided between the pulmonary I II 111 IV FIG. 580. — Diagram of modification of arterial arches in various vertebrate classes. White, vessels which degenerate; cross-lined, vessels containing arterial blood; black, vessels containing venous blood. /, Dipnoi; //, Urodeles with pulmonary respiration; HI, Reptiles: IV, Birds (in mammals the left instead of the right aortic arch persists), ao1, venous aorta of reptiles; ao2, arterial aorta; ast, arterial trunk; a, b, arches which usually disappear: ad, dorsal aorta: d.B. ductus Botalli; fc, gill capillaries; pu, pul- monary artery; 1-k, persistent arterial arches. and systemic circulations (fig. 580). Of the six arches which usually appear in the embryo, the first and second, and the fifth in animals with lungs, usually disappear. The last arch (4), which even in the Dipnoi supplies the swim bladder, becomes a pulmonary artery, the other arches (1 and 2) furnish the systemic portions, the dorsal aorta (2) and the carotids supplying the head (1). Since special pulmonary veins, distinct from the systemic circula- tion, carry the blood from the lungs to the heart, the heart be- comes divided by a septum which separates it into right and left halves. The right half retains the venous character of the fish heart; since the right auricle receives the systemic veins, the right ventricle gives off the pulmonary artery. The left half is purely arterial, receiving arterial blood by the left auricle from the lungs and sending it out through the aorta ascendens to the body. A complete separation of pulmonary and systemic circulation, and a corresponding division of the heart, occurs only in birds and mam- 550 CUORDATA. -mals. Keptiles and amphibia show how the modification has been accomplished. In these the separation begins in the venous sys- tem and extends to the auricle, in the reptiles the septum arises in the ventricle. In the arterial system remnants may persist, such as a connexion (ductus Botalli) of the pulmonalis with the aorta (//, d.B), or an aortic arch may arise with the pulmonalis from the right side of the heart (III, ao). Besides blood-vessels, lymph vessels occur in the vertebrates as com- plements of the venous system. The fluids which collect in the spaces of the connective tissue are taken by them and carried into the large venous trunks. Usually the action of the heart and the movements of the body are sufficient to cause the flow of this lymph, but special lymph hearts may occur. The lymph vessels distributed to the digestive tract play an important role, since they serve in the resorbtion of digested food. They are called chyle ducts because their contents, the chyle, rendered white by oil globules at the time of digestion, distinguishes them from other lymphatics. The most important features of lymph and blood have already been noticed (p. 88). In special places small bodies, the lymph glands, are inserted in the course of the lymph vessels, in which lymph •corpuscles arise. Among these from its structure is to be enumerated the spleen, colored bright red by its rich blood supply. The sexual and excretory organs are so closely associated that they are generally united as the urogenital system. The sexual products are formed in the embryo from a special region of the peritoneal epithelium on either side of the vertebral column. These primordial cells early leave their primitive position, and sink into the underlying connective tissue (fig. 33), forming in the male glandular tubes, in the female cords which break up into numbers of round follicles, each containing a single larger cell, the ovum. In the male the gonads thus formed are compact and fre- •quentlv oval, the testes; in the female they are looser and follic- ular ovaries. The deposition of the sexual cells occurs in many fishes by way of the body cavity and the abdominal pores, and in this case a part of the ccelom may be cut off as a special vas deferens or oviduct. In most vertebrates the ducts are formed from a part of the nephridial system. Embryology shows that there are three kinds of nephridia in vertebrates: (1) the pronephros, or head kidney; (2) mesonephros, or Wolffian body; (3) metanephros, or kidney proper, with the corresponding pronephric, mesonephric (Wolf- fian), and metanephric (uretei) ducts. The first two of these ducts are genetically connected, since the development of the elasmobranchs shows that the pronephric duct, by splitting, gives IV. VERTEBRATA. 551 rise to two canals, the Wolffian (mesonephric), and the Miillerian ducts, the latter retaining its relation to the pronephros. The pronephros is usually functional only in embryonic life and then only in early stages, possibly in some cases not at all. Its relations to the other parts are yet in question. In most m/(0d) FIG. 581.— Scheme of urodele urogenital system based on Triton. (From Wieders- heim, after Spengel.) A., male; B, female, a, excretory ducts; gn, sexual part of mesonephros; Ho, testis; Zo, Leydig's duct (ureter i; wgr, Miillerian duct (oviduct) ; mg\ its vestigial end in male ; JV, functional part of mesonephros ; Or, ovary; Ot, ostium tubse; Ve, vasa efferentia ; *, collecting duct of vasa effer- entia (rudimentary in B). teleosts the mesonephros is equally developed in nearly the whole length of the body cavity, but in the Amphibia (fig. 581) and many elasmobranchs its anterior part is smaller than the rest, a condition which has its explanation in its relations to the sexual apparatus. 552 CHORDATA. In the males (excepting many fishes) the testes become con- nected with the anterior end of the Wolffian body (fig. 581, A), so that the urinary tubules of the latter come to be seminal ducts, while the hinder portion remains excretory, this condition being permanent in the Amphibia. In the amniotes the anterior meso- nephros retains its connexion with the testes, forming the vasa efferentia, while the Wolffian duct forms the vas deferens, a por- tion of it greatly coiled being the epididymis. The remainder of the Wolffian body degenerates, a portion only persisting as the paradidymis. In the females (fig. 581, B) the mesonephros is smaller in front, as in the males, but the connexion of this with the ovary does not exist, so here the Wolffian duct is solely excretory, and not, as in the males, excretory and seminal duct. In the female amniotes the Wolffian body almost entirely disappears, for in both sexes of the reptiles, birds, and mammals the metanephros or kidney proper is a new formation, growing forwards from the posterior end of the Wolffian duct. In the females of elasmobranchs, Amphibia, and Amniotes the Miillerian duct serves as an oviduct, its anterior end opening by the ostium tubae into the abdominal cavity and receiving the eggs as they escape from the ovary. In the male the Miillerian duct disappears early. The union of sexual and excretory organs to a urogenital system arises from the same relations as in the annelids ; both organs arise from the coelomic epithelium and have temporary or permanent connexion with the body cavity. This has already been described for the gonads. The urinary tubules of both pro- and mesonephros are derivatives of the coelomic epithelium and possess an arrangement recalling that of the annelids in a striking manner. As is shown (fig. 70) in the scheme of the embryo selachian, the nephridial system consists of numerous canals, segmeu tally arranged, connected by funnels (nephrostomes) with the body cavity; and differs from the segmental organs of the annelids in that they do not open singly to the exterior, but by a common duct. They also differ in their further development by increasing greatly in number and forming a compact organ, and, finally, by the formation in a certain part of a network of blood-vess'els, the glomerulus, which pushes into the lumen of the tube. The ducts of the urogenital system open behind the anus in most fishes on a urogenital papilla; in the elasmobranchs, amphib- ians, birds, and most reptiles dorsally into the hinder part of the digestive tract, which thus becomes a cloaca. In turtles and mammals the urogenital canal opens into the urinary bladder, a ventral diverticulum of the rectum which first appears in the IV. VEHTEBRATA. 553 Amphibia. Urinary and sexual ducts then either open into the urogenital sinus, the lowest part of the bladder leading to the cloaca (turtles, inonotremes), or this part receives only the geni- tal ducts, while the ureters enter the base of the bladder. The urogenital sinus remains in connexion with the cloaca in the turtles and monotremes; in the other mammals a cloaca occurs only in embryonic life. Later, by formation of the perineum, the cloaca is divided into a hinder digestive and an anterior urogenital canal. Step by step the stages may be followed from urogenital ducts opening behind to those opening in front of the anus. Asexual and parthenogenetic reproduction are unknown in the vertebrates. The impregnation of the eggs in the lower groups is usually external and occurs during oviposition; in the higher internal copulation is effected by opposition of the genital ori- fices or by the development of an intromittent organ, the penis. The fertilized egg can undergo a part or the whole of its devel- opment in specialized parts of the oviduct (uterus). Accordingly viviparous and oviparous forms are distinguished, and between these extremes those that are ovo viviparous (cf. p. 161). Most elasmobranchs are viviparous, but many are oviparous. In the teleosts oviparous forms predominate, but there are viviparous exceptions. So, too, among the reptiles and Amphibia there are some viviparous species among the egg-laying majority. The birds and mammals are most constant, the first being exclusively ovoviviparous, while all the mammals bring forth living young with the exception of the ovoviviparous monotremes. Three embryonal appendages may occur in the development, the yolk sac, the amnion, and the allantois. The yolk sac is small in those vertebrates which have some yolk, but not enough to cause meroblastic segmentation (Amphibia), yet it is everywhere present and is best developed in those groups (fishes, fig. 582, reptiles and birds) with discoidal segmentation, and is the result of the accumulation of food material in the digestive tract, which forces out its ventral wall like a hernia. Its presence in the mam- mals, which have small eggs lacking in yolk, is an indication that these have descended from large-yolked forms, such as the mono- tremes yet are. The embryo either lies directly on the yolk or is connected with it by a yolk stalk. While the yolk sac is widely distributed, the amnion and allan- tois are restricted to reptiles, birds, and mammals, which are con- sequently spoken of as Amniota or Allantoidea, in contrast to the fishes and Amphibia, which are frequently called Anamnia or Anal- 554 CHORD AT A. lantoidea, from the absence of these structures. The amnion is a sac which envelops the whole embryo and is connected with the rest only at the umbilicus, that is, the point where the yolk sac projects from the ventral wall. In this sac is an albuminous FIG. 582. Fm. 583. FIG. 582.— Shark embryo. (From Boas.) y, part of yolk sac ; y, external gills in front of pectoral fins. FIG. 583.— Embryonic envelopes of a mammal. (Diagram after Kolliker.) aft, amni- otic cavity ; al, allantois ; aw, amnion ; da, yolk stalk ; ds, yolk sac ; e, embryo : hh, ventral wall of embryo; r, extra-embryonic coelom; sft, serosa ; sz, serosal villi. amniotic fluid. The amnion is genetically a part of the ventral surface; it develops ventrally as folds — lateral, anterior, and pos- terior— which grow up over the back on all sides and unite above the embryo. The allantois is an enlargement of the urinary bladder. This grows out from the body cavity at the umbilicus and extends be- tween yolk sac and amnion and then grows in all directions until its folds meet above the back. The part of the allantois which re- ceives the urine may be enlarged or not. The rest of the out- growth consists of blood-vessels and connective tissue. The blood- vessels are the most important, for the allantois forms the respira- tory apparatus of the embryo, and in the mammals it develops the placenta, by which nourishment as well is conveyed to the young. Yolk sac, amnion, and allantois are enveloped in a common coat, the serosa. Aristotle and his followers recognized four divisions of vertebrates, and these were retained by Linne" and Cuvier under the names Pisces, Reptilia or Amphibia, Aves, and Mammalia. Blainville (1818) divided the second of these into two classes, retaining the name Reptilia for the one, Amphibia for the other. Milne Edwards showed that this division corresponded IV. VERTEBRATA: CTCLOSTOMATA. 555 with one between the higher and lower groups, the amniote and the anam- niote divisions. Later Haeckel divided the fishes, separating the Cyclo- stomes from the others as a distinct class, while Huxley pointed out the close resemblances between the reptiles and birds, grouping them as Sauropsida. Another division of convenience but not of much systematic importance contrasts the fishes with all other forms, the Tetrapoda, so called from the possession of legs rather than fins. SERIES I. ICHTHYOPSIDA (ANAMXIA, ANALLANTOIDA). Vertebrates respiring for a time or throughout life by means of gills ; neither amnion nor allantois present in the embryo. Class I. Cyclostomata (Marsipobranchii, Agnatha). The class of Cyclostomes contains but few species, among which the lamprey eels and the slime or hag fishes are best known. In shape they are eel-like. They are distinctly vertebrate in the possession of large liver and nephridia; of a muscular heart with auricle and ventricle, lying in a pericardium; olfactory lobes, epiphysis and hypophysis, and the higher sense organs. In the brain, cerebrum and cerebellum are not so prominent as are the optic lobes and medulla. The inner ear is not divided into utric- ulus and sacculus, and it has but one or two semicircular canals, but always two ampullae. The skin (fig. 26) consists of derma and a stratified epidermis. The cyclostomes are distinguished from the true fishes by the lack of a vertebral column. The axial skeleton of the trunk consists either of the notochord alone or of it and small neural arches. A cranium and a basket-like gill skeleton are present, but so different are these from those of other vertebrates that homologies are dif- ficult. The absence of paired fins is important. Since the median fins are supported by horny threads alone, the cartilaginous appen- dicular skeleton — alone of importance — is entirely wanting. Then the skin lacks scales, and the mouth, true dentine teeth, for the pointed brown teeth arranged in circles in the mouth of the lam- prey (fig. 584), and the fewer teeth of the myxinoids, are purely epidermal products and cannot be compared with the teeth of other vertebrates. Other important differences have given rise to names applied to the group. The name Cyclostomata refers to the circular mouth, an ex- ternal feature, which, however, rests on the important fact that the jaws are absent or extremely rudimentary, and do not close on each other as do the jaws of other vertebrates. This cyclostome •condition is of value to the animals, as it aids them in sucking on 556 CHORD AT A. to other animals. At the base of the dome-like mouth cavity is the so-called tongue, which is the sucking apparatus, since it can be drawn backwards like a piston (fig. 584). The name Marsipobranchs has been given on account of the form of the gills, which are usually six or seven in number, but in Bdellostoma may be twelve or fourteen on either side. Each gill cleft consists of three parts, the gill sac (marsupium), which alone contains gills, and the afferent and efferent ducts (fig. 585). These canals arise separately, and may continue so (Bdellostoma), but in Petromyzon the afferent ducts unite to a single tube which opens ventrally in the pharynx. In Myxine (fig. 585) the conditions are reversed, the efferent canals uniting to empty through a single external opening. A third name, Monorhina, has been given, since these forms, in contrast to all other vertebrates, have an unpaired olfactory organ. The single nostril, lying in the mid line of the head,. FIG. 584. FIG. 585. FIG. 584. — Mouth of Petromyzon marinus with horny teeth and tongue. (From Gegenbaur.)- FIG. 585.— Gill apparatus of Myxine glutinosa. (After J. Muller.) a, atrium; «6, gill artery and gill arch : 6r, gill sac (the lines show the gills) ; for', efferent canal; c, cesophageo- cutaneus duct; rf, skin turned away; t, afferent gill canal; o, oesophagus; s, mouth of atrium ; v, ventricle of heart. opens into a nasal sac, from the bottom of which a canal descends towards the roof of the mouth, ending blindly in Petromyzontes (Hyperoartia), or penetrating it in the Myzontes (Hyperotretia), so that an inner nasal opening (choana) occurs. A paired olfac- tory nerve supplies the organ. Sub Class I. Myzontes (Hyperotretia). Semiparasitic cyclostomes with cirri around the mouth, very primitive nephridia, right and left rows of slime sacs, eyes rudimentary (lens, sclera, IV. VERTEBRATA: PISCES. 557 and choroid lacking). From the large amount of mucus they are known as slime eels. They bore into fishes and eat the flesh. Myxine* on the east coast, Bdellostoma * (Polistotrenid) on the west. Sub Class II. Petromyzontes (Hyperoartia). Several American species of lampreys, all belonging to Petromyzon* (with sub genera), have well-developed dorsal fins, and seven branchial openings. They occur in salt and fresh water, some marine species FIG. 586.— Petromyzon marinus,* sea lamprey. (After Goode.) ascending streams to lay their eggs. The young pass through a larval (Ammoccetes) stage with rudimentary eyes and slit-like mouth. Many of the species live on the mucus and blood which they rasp from fishes. Here may be mentioned a group of fossils, the OSTRACODERMI, of uncertain position. They have fish-like bodies, but no skeleton or jaws are known. They flourished in paleozoic seas. Pteraspis, Cephalaspis, Ptericlitliys. Class II. Pisces (Fishes). The term fish is used in a wider and a narrower sense. In the first it includes any aquatic vertebrate swimming by means of fins and breathing by gills; in the more strict sense, as used here, it means aquatic branchiate forms with vertebral column, cranium, and well-developed visceral skeleton; with paired as well as unpaired fins, these supported by a cartilaginous or bony skeleton in addition to horny rays; with double nasal pits; with a skin and oral mucous membrane which can produce ossifications, the scales and teeth. The cyclostomes are thus excluded. The fishes are the best adapted of all vertebrates for an aquatic life, and their whole organization must therefore be considered from this stand- point. The epidermis consists of numerous layers of protoplasmic cells with an extremely thin external cuticle. Cornifications of this epidermis are lacking under ordinary conditions, with the excep- tion-of a thin portion of the external subcuticular layer. At the time of sexual maturity cornifications increase greatly in most Cyprinoids and many Salmonids, producing hard bodies in the skin, 558 CHORD AT A. the l pearl organs/ Enormous numbers of large slime cells give the fishes their well-known slippery skins. Since the epidermis contributes nothing to the firmness of the body walls, all protective structures arise from the derma, which is composed of many layers of dense connective tissue and furnishes the characteristic dermal skeleton, the scales. These lie at the boundary of epidermis and derma, commonly imbedded in pockets of the latter, and are, on account of their different structure, of systematic value, although the classification based entirely upon them is no longer retained. The placoid scales (fig. 554, 587, 4) nave already been men- tioned, because they form the starting point for dermal ossifica- tions and teeth (p. 515). They are rhombic bony plates, usually close together like a mosaic, but not overlapping. In the centre of each is a spine, directed back- wards, in which is a pulp cavity, while the tip of the spine is cov- ered with a cap of hard substance, variously called enamel or vitro- dentine. The ganoid scales (fig. 587, 3) are usually rhomboid and arranged like parquetry. In the early stages they may bear teeth, The outer surface is always covered with a thick layer of 'ganoin/ which gives, even in fossils, an iridescent effect, a most characteristic feature. The ganoin is no longer regarded as enamel, but the most superficial layer of dentine (vitrodentine). Cycloid and ctenoid scales are closely related. They are always more loosely placed in the pockets, from which they are easily with- drawn as in ' scaling ' a fish. They are arranged in oblique, trans- verse, and longitudinal rows, and overlap like shingles, one scale covering the parts of two scales behind. The cycloid scales (fig. 587, 1) are approximately circular with a middle point, surrounded by concentric lines, from which go radiating lines. The ctenoid scale (2} has the radial and concentric lines of the cycloid, but has the hinder edge truncate and the free portion bearing small spines or teeth, processes of the concentric ridges. Besides these types of scales many fishes bear considerable FIG. 587.— Scales of fishes. 1, cycloid; ctenoid; 3, ganoid; A, placoid. but these are lost in the adult. IV. VERTEBRATA: PISCES. 559 spines (strongly developed single scales) and larger bony plates, these last usually resulting from the fusion of numerous scales. The coloration of fishes is threefold in origin. The silvery lustre is due to crystals of guanin which occur not only in the skin but in the peri- toneum and pericardial walls. In some fishes from their iridescence (Alburnus lucidus) these crystals become of commercial value. They are freed from the skin by boiling with ammonia and, suspended in the fluid, form the important part of essence of pearl (essence d'orient) which is used in making artificial pearls, being either applied to the outside of ala- baster balls (Roman pearls) or as a coating to the inside of glass beads (Paris pearls). The other colors of fishes are due in part to the numerous strongly pigmented fat cells, in part to * chromatophores ' in the derma, which, under control of the nervous system, can alter their form and extent and thus produce color changes in the fish. It is by means of these chromatophores that fishes adapt themselves to their surroundings. It is of interest to note that destruction of the eyes results in loss of power to change color. The axial skeleton shows many conditions which are unknown outside the class, and varies in character from group to group, the most important differences consisting in its cartilaginous or bony character. The vertebrae are nearly always amphiccelous, the notochord persisting in the cavities between the successive centra (fig. 557). Neural and haemal arches occur, these having as key- stones the unpaired spinous processes. The neural arches extend throughout the columns; the haemal are complete only in the tail; in the trunk the haemal spines are absent and the haemal processes, divided into basal processes and ribs, surround the viscera. A sternum is everywhere lacking. When ossification is lacking or is incomplete, two pairs of arches may occu»r in each segment, the anterior being the stronger and alone persisting in fishes with ossi- fied vertebrae ; the second is much smaller, so that its elements are not called arches, but intercalaria (figs. 556, 588). The great number of visceral arches, and their independence from the cranium, are characteristic of fishes. After removal of these the cranium in all cartilaginous fishes is very simple (fig. 588), but in the teleosts, with the appearance of ossification, be- comes very complicated, since the bones are very numerous and are not, as in mammals, in part fused to larger bones. There are also great differences between the different families of fishes, some having bones which are lacking in others (figs. 560, 589). The large membrane bones of the cranial roof (parietals, p, frontals, /, and nasals, no) and the large ventral parasphenoid (ps) are especially constant. The vomer in front of the parasphenoid is 560 CHORD ATA. unpaired, while in all other vertebrates it is paired. Most con- stant of the cartilage bones are the ethmoids (the paired ecteth- moids, ee, and the sometimes paired mesethmoid), and the four occipitals. On the other hand the otic and optic regions vary considerably; the otic region, from its great size, has several bones, usually (fig. 589) five in number: pterotic, pto, often called ol ic. ns. < \ . v g/i.Jf. jio CO. S. 7. FIG. 588.— Cranium, visceral arches, and part of vertebral column of Mustelus vulgaris. ao» antqrbital process; co, copula; gp, foramen for glossopbaryngeal; H, otic capsule and hyoid; Hm, hyomandibular; ic, intercalare; Md, mandible (Meckel's cartilage); JV, nasal capsule; o, optic foramen; 06, neural arcb ; po, postorbital process; Pq, ptery goquadrate ; ps, spinous process ; J?, rostrum ; r, ribs ; tr, trigeminus foramen ; v, vagus foramen; 1-8, visceral arches: 1, labial; 2, mandibular; 3, byoid; U-8, gill arches. squamosal; sphenotic, spho, frequently called postfrontal; epiotic, epo; prootic, pro; and opisthotic, 00, the last sometimes lacking. In the region of the eye the cartilaginous sphenoids are rarely well developed, the large parasphenoid taking their place. The same is true of the ali- and orbitosphenoids, tliese sometimes form- ing an interorbital septum (fig. 560) or a more or less wide in- terorbital fenestra (fig. 589). The character of the visceral skeleton is related to the aquatic life. All fishes have numerous gill arches (five to seven, mostly five), which, since their function — gill supporting — is similar, are similar in structure. So far as they are not degenerate they con- sist each of four parts and are connected by unpaired copulas, these often being fused. The upper ends are frequently toothed and, in chewing, are opposed by the rudimentary last arch, on which account these are spoken of as the superior and inferior pharyngeal bones. The anterior visceral arches are greatly different in car- tilaginous and bony fishes. In the former (fig. 588) the pterygo- quadrate (pq) and the Meckelian cartilage bear teeth and oppose each other in biting. In the bony fishes (fig. 589) the teeth of IV. VERTEBRATA: PISCES. 561 the lower jaw oppose the tooth-bearing elements, premaxillary and maxillary, of the maxillary series, while the pterygoquadrate elements — the palatine and the series of pterygoids — are the an- tagonists of the hyoid. A second characteristic of the bony fishes is already outlined in the cartilaginous fishes : the modification of the hyomandibular to qio jiLo we. FIG. 589.— Skull of haddock. Infraorbital ring and operculum. outlined in red. n, angulare; ar, articulare ; as, alisphenoid; de, dentary; ee, ectethmoid; ekt, ectopterygoid; eng, os entoglossum ; ent} entopterygoid; epo, epiotic; /r, frontal; h'-ft3, hyoid elements: Jim, hyomandibular; ih, interhyal; ma, maxilla; me, mesethmoid; mt, metapterygoid; na, nasal; neb, ncl, ocs, basi-, ex-, and supra- occipital ; oo, opisthptic ; p, parietal ; pa, palatine ; y>rm, premaxillary ; pro, pro- otic ; ps, parasphenoid ; pto, pterotic ; qit, quadrate; rbr, branchiostegals ; spfto, sphenotic; xy, symplectic ; w, vomer; w, vertebra. Bones outlined in red: in/, inf raorbital ; to, interoperculum ; o, operculum ; pro, preoperculum ; .so, suboper- culum ; 1, 2, 3, axes of ..abial, mandibular, and hyoid arches. a suspensor of the jaws. In the elasmobranchs (especially the skates) the parallelism of hyoid and mandibular arches is lost, the hyomandibular separating from the hyoid and attaching itself to the hinge of the jaws. In the teleosts the hyomandibular is thus brought in connexion with the quadrate, and lies between it and the cranium, the joint being thus indirectly supported from the cranium, a bone, the symplectic (known only in fishes) helping out the suspensor, while another bone, the interhyal, connects this with the hyoid, which itself divides into two, so that the hyoid arch, like a gill arch, consists of four elements. 562 CHORDATA. The opercular apparatus does not occur in all fishes. It is a number of bony plates and processes which arise from the hyoid arch and extend backwards over the gills, protecting them. It arises in part (opercular bones — (9, Pro, So, lo, fig. 589) from the hyomandibular, in part (branchostegal rays) from the hyoid bone. The significance of this apparatus will be spoken of in con- nexion with the gills; it gives the fish head a definite character, but covers its structure, on which account it, like the infraorbital ring, is shown in red in the figure 589. The appendages are also influenced by the aquatic life. In contrast to the cyclostomes, there are two pairs of paired fins, the thoracic or pectoral, and the pelvic, ventral, or abdominal fins; in contrast with Amphibia, reptiles, and mammals, which occasionally have fin-like structures, the fishes have three unpaired fins, dorsal, caudal, and anal fins. Only rarely, as in the eels, the ventral fins are lacking; more rarely (Maraenidae) the pectorals are lost. The function of the fins in swimming and in balancing makes it neces- sary that they be broad and well-supported plates. Hence it is that numerous skeletal parts are present; besides those preformed in cartilage, numerous horny or bony rays; further, that all parts should be similar and closely, even if flexibly, bound to each other. Joints are unnecessary except at the base where the fins join the supports and move upon the body. The supports of the paired fins are the girdles, arched skeletal parts, which in the sharks are held only by muscles, a statement which is true for the pelvic girdle of all fishes. This is why the ventral fins so readily change their place. Their primitive position is at the hinder end of the body cavity (Pisces abdominales, figs. 598, 601). From this point they can move forward to beneath the pectorals (Pisces thoracici, fig. 602), or may even come to lie in front of them (Pisces jugu- lares) in the throat region (fig. 606). The pectoral arch is united to the vertebral column in the skates; to the skull by a series of bones in the teleosts. The dorsal and anal fins are supported by elements preformed in cartilages which rest upon the neural or haemal spines and in turn support the fin rays. In the caudal fin the rays rest directly upon the spinous processes. Three types of caudal fin are rec- ognized— diphycercal, heterocercal, and homocercal (fig. 10), distinctions of great importance. The primitive type is the diphy- cercal, in which the vertebral column extends directly into the middle of the fin, dividing it into symmetrical halves. In the heterocercal type the vertebral axis binds slightly upwards at the IV. VERTEBRATA: PISCES. 563 base of the fin, so that the dorsal part is reduced, the ventral greatly enlarged, the result being extremely asymmetrical, as seen from the exterior. The homocercal fin is symmetrical externally, but in reality is extremely asymmetrical. The end of the vertebral column, the unossified notochord, is bent abruptly upwards, and hence the fin is almost entirely formed of the ventral portion, which is usually divided by a terminal notch into upper and lower halves. The homocercal fin begins with a diphycercal and passes through a heterocercal stage in development. In correspondence with the simple motions the musculature is simple and consists largely of longitudinal muscles divided into myotomes, which are conical with the apex in front, and are so inserted in each other that a cross-sec- tion gives concentric circles. In a section there are at least two such systems, the muscles being divided by a lateral in- cision into dorsal and ventral halves. There are also smaller groups of muscles related to fins, gill arches, jaws, etc., but of much smaller size, derivatives from the larger mass. Electric and pseudelectric organs, which occur in different fishes, sometimes in the trunk, at others in the tail, are formed by the modification of mus- cles. Each organ consists of numerous closely packed vertical or horizontal col- umns, each column, like a Voltaic pile, consisting of layers of gelatinous plates (equivalents of muscle bundles) in which the nerves, with special end plates, termi- nate. The discharge is electronegative. The brain shows the low position of the class in the slight development of the cerebrum (fig. 591). This is especially true of the teleosts, in which, in place of a cortex, there is only a thin epithelial layer (Pall), what was formerly called cerebrum being only the corpora striata. The independent olfactory lobes lie either close to the cerebrum (most teleosts, Lol) or are separated from it by an olfactory tract (fig. 592, h). The optic thalami are small (d), but below them are enlargements characteristic of fishes, the lobi inferiores, and between them the sacculus vasculosus. Both optic lobes and cerebellum are greatly developed. The nose consists of two preoral pits, the opening being divided by a bridge of skin into anterior and posterior nostrils. In many selachians the nostrils are connected with the mouth by a groove covered by a fold of skin, and in the Dipnoi there is a choana. The eye has several peculiari- ties. The lens is very convex, almost conical, due to the slight refraction caused by the passage of light from the water into the cornea. Further, EP FIG. 590.— Driagrammatic section of electrical apparatus. (From Wiedersheim.) The arrow points dorsally or anteriorly. BG, con- nective-tissue framework; EP^ electrical plates; G, gelatin- ous tissue; N, nerves entering through the septa; NN, nerve terminations. 564 CHORD ATA. the eye is very short-sighted because light is so absorbed by water that objects forty feet away are invisible. With this is connected the cam- panula Halleri. The processus falciformis, a sickle-shaped outgrowth of the choroid, extends from the entrance of the optic nerve into the vitreous body as far as the lens, swelling out into the campanula ; this contains a muscle which draws back the lens and so is an apparatus of accommoda- tion. Near the entrance of the optic nerve is a problematic organ, the L.Oi: a FIG. 591. FIG. 592. FIG. 591.— Brain of trout. (After Wiedersheim.) BG, corpus striatum ; GP, pine- alis ; HH, cerebellum ; Lol, olfactory lobes ; MH, optic lobes ; NH, Medulla oblongata ; PalL pallium, in part cut away : VH, cerebrum ; I-XII^ nerves. (See p. 536.) FIG. 592. — Brain and nasal capsules of Scyllium catulus. (From Gegenbaur.) a, me- dulla ; 6, cerebellum ; c, optic lobes ; d, 'twixt brain ; 0, cerebrum ; /i, bulbus and tractus olf actorius ; o, nasal capsules. choroid gland, consisting largely of blood-vessels (rete mirabile). Chon- drifications and ossifications of the sclera are common. Lids are weakly developed or absent, and only some elasmobranchs have a nictitating mem- brane. The ear has a relative size found in no other vertebrates, the labyrinth corresponding well with fig. 575. The labyrinth contains in many teleosts two otoliths, the asteriscus and sagitta, the first being especially large. Experiments show that the ears are primarily for balance, and hearing is doubtful. Strychninized fish do not respond to sound, if in its production mechanical vibrations are avoided. Of all sense organs the most noticeable are those of the skin, especially those of the lateral line, which are nowhere else so well developed and which occur elsewhere only in cyclostomes and aquatic amphibia. In fishes a line on either side usually begins at the tail and extends to the head, where it divides into several curved lines (fig. 602, 81). Its position is marked by a groove or a canal in the scales which opens to the exterior by numerous canals through the scales. Branches of trigeminus, facialis. glossopharyngeus, and especially the lateral branch of the vagus (fig. 570) go to these organs, the latter extending back to the tail. These supply special IV. VERTEBRATA: PISCES. 565 sense organs, which may be grouped in several lines or occur in pits (am- pullae) in the skin in other places. Their function is obscure, since noth- ing of the sort occurs in man or mammals. They are specific organs of aquatic vertebrates and possibly have to do with the perception of water pressure. The alimentary tract is spacious only in the oropharyngeal region. Then it narrows to a tube in which the various regions are not sharply marked off from each other. Mouth and pharynx frequently bear teeth. In the teleosts the bones of the floor of the cranium and those of the visceral arches may be covered with coalesced heckel-like teeth. In the elasmobranchs the teeth are mostly confined to the lower jaw and the pterygoquadrate, but are in rows, the anterior row alone being functional; but as these are loosely held they are easily torn out, when they are replaced by the row behind. Liver and spleen are always present; pancreas and gall bladder usually occur. In many fishes blind sacs, the pyloric caeca, occur at the junction of stomach and intestine (fig. 593, B)', others have a spiral valve (A), a fold of mucous mem- brane, which extends like a spiral stairway into the lumen of the intestine, increasing the digestive surface. Caeca and spiral valve rarely occur in the same fish. B FIG. 593.— Digestive tracts of (A) Squatina vulgaris (partly opened) and (B) Tra- chinus radiatiis. (From Gegenbaur.) ap, pyloric caeca ; c, rectum ; d, bile duct ; dp, duct of air bladder; i, intestine ; oe, oesophagus ; p, pylorus ; i\ stomach ; vs, spiral gland ; #, rectal gland. Gills of two types occur (fig. 594, A and B). In both the gill clefts, which lie between successive branchial arches, begin by openings in the pharynx, but differ in their external openings. In the elasmobranch type (A) the external openings are a series of slits separated by broad dermal bridges which cover the gills and gill clefts (fig. 598). The gills are vascular folds of mucous mem- 566 CHORD AT A. brane with secondary folds which extend on anterior and posterior sides of the cleft. Each arch except the last, as the sections (fig. 594, A, and 595) show, bears two rows of gill folds (demi- FIG. 594.— Pharynges of (.4) Elasmobranch (Zygoma) and CB) Teleost (Gadus\ the skull removed and on the left the gill slits cut across, a, attachment of upper jaw to cranium; as, outer gill slit; Jb, gill arch; fol1, foZ2, anterior and posterior gills (demibranchs) ; ft, dermal projection ; tow, hyomandibular ; is, inner gill cleft ; m, mouth ; ma, maxillare ; o, oesophagus ; op, operculum ; ops, opercular opening; pa, palatine; phi, inferior pharyngeal bones; pq, pterygoquadrate ; pnn , premaxilla; s, shoulder girdle ; uk, lower jaw; z, tongue. branchs) which belong to different clefts and are separated from each other by tissue containing the cartilaginous gill rays. In the second type (B), which occurs in all other fishes, the dermal bridges are lacking, and the tissue between the demi- branchs has more or less completely disappeared, so that the demibranchs of one arch become connected^ their free ends pro- jecting into the water like the teeth of a double comb. Here, on account of their very delicate structure, they would be ex- posed to serious injury were they not protected by the operculum or gill cover. This is a fold of skin arising from the hyoid arch and extending back over the gill region. It is supported by two groups of bones, the opercular bones proper (fig. 589, 0, /Sfc, lo, Pro), attached to the hyomandibular, and the branchiostegals IV. VERTEBRATA: PISCES. 567 Fio. 595.— Sections of gill arches of Gadm (left) and Zygcena (right), slightly enlarged, a, artery ; ft, gill arch ; W*f bi2, demibranchs ; h, dermal projection ; r, cartilage ray; V, vein ; z, tooth. (r&r) from the hyoid, these latter supporting the branchiostegal membrane. Between the free edge of the operculum and the branchiostegal mem- brane and the skin of the body behind is the opercular cleft (fig. 594, ops), which is obviously not identical with a gill cleft, but leads into an atrium into which the gill clefts empty. In many elasmobranchs and ganoids there is a rudimentary cleft, the spiracle, between the pterygoquadrate and hyomandibular, in which a rudimentary gill, or pseudobranch, may occur, this often per- sisting when the spiracle is closed. Besides gills, fishes, with .the exception of elasmobranchs and some teleosts, have a swim bladder which is usually regarded as the homologue of the lungs. It is often shaped like an hour glass, filled with air, and may open into the oesophagus by a pneumatic duct (Physbstomi), or this, appearing in development, may be lost in the adult (Physoclisti). The air bladder serves for respiration in the Dipnoi and possibly in some ganoids (Lepidosteus and Amia), but is usually a hydrostatic apparatus, its enlargement or compression altering the specific gravity of the fish. In fishes brought up from great depths the expansion of air in the swim bladder frequently forces the viscera out through the mouth. The heart, enclosed in the pericardium, lies immediately behind, the gill region, and is protected by the shoulder girdle. It always consists of auricle and ventricle (fig. 596), separated by a pair of valves to prevent back-flow of the blood; it sends the blood to the gills by the arterial trunk (ventral aorta), and receives it from the body through a thin-walled sac, the venous sinus, in which the hepatic veins and the Cuvierian ducts (formed by union of jugular and cardinal veins) empty (figs. 65, 597). The most important differences lie in the development of conus and bulbus arteriosus. These are muscular accessory organs, the first arising from the heart, the other from the arterial trunk; and correspondingly the conus has striped, the bulbus smooth muscle fibres. The anterior end of the heart contains < semilunar ' valves, which, like the auriculo-ventricular vjilves, prevent the back-flow of the blood. When, by increase in the number of valves, this part becomes elongate, a conus arteriosus (fig. 596, A) is formed. The 568 CHORDATA. bulbus ( (?) is a muscular swelling in front of the conus, in the arterial trunk. The connexion of ventral and dorsal aortas is effected in young fishes (fig. 597) by the gill arteries directly; later by means of the complicated loops of the gill circulation. When these are de- FIG. 596,— Forms of hearts of fishes in schematic long section. (After Boas.) A, chian and most ganoids; B, Amia; C, Teleost. a, auricle; 6, bulbus arteriosus; c, conus arteriosus; fc, valves; s, sinus venosus; t, truncus aortse; v, ventricle. FIG. 597.— Head of embryo teleost. (Diagram from Gegenbaur.) a, auricle ; abr, ventral aorta with arterial arches; ad, dorsal aorta; c, carotid ; dc, Cuvierian duct, formed by union of jugular and posterior cardinal veins ; n, nostril ; *, gill clefts ; sv, sinus veno- sus; v, ventricle. veloped, afferent branchial arteries, gill capillaries, and efferent arteries can be recognized, the latter uniting to form the dorsal aorta and also giving off the arteries (carotids), which go to the head. The nephridia are a pair of large reddish-brown organs lying outside the body cavity to the right and left of the vertebral column, usually extending from heart to anus. Their ducts empty behind the anus or in the dorsal wall of the intestine and are often provided with enlargements called, from their functions, urinary bladders, although totally different morphologically from the urinary bladder of the higher vertebrates. The gonads, suspended IV. VERTEBRATA : PISCES. 569 by mesorchia or mesovaria, are large and project into the body cavity. They are rarely unpaired. In the elasmobranchs and most ganoids their products pass out by the urogenital system (p. 552), in other forms by the pori abdominales or by special ducts. Cuvier divided the fishes into cartilaginous and bony groups, an im- portant step so far as the extremes (elasmobranchs and teleosts) were concerned. Agassiz recognized a middle group which he named Ganoidei, from the character of the scales, but his account was modified and made more accurate by Johannes Miiller, who also included the Dipnoi among the fishes. At present the group of ganoids is retained largely as a matter of convenience. Its members are more closely related with the teleosts than with the elasmobranchs, and in America Ganoids and Teleosts are united under the head Teleostomi, the name alluding to the presence of a true upper jaw comparable to that found in higher vertebrates. Sub Class I. Elasmobranchii (Plagiostomi, Chondropterygii). The elasmobranchs, the shark-like fishes, are almost exclu- sively marine, varying in length from a foot and a half to sixty feet, living almost exclusively on other vertebrates, and noted for their voracity. Sometimes slender and cylindrical, as in the sharks (fig. 598), sometimes flattened dorsoventrally, as in the skates (fig. Spl FIG. 598.— Acanthias vulgaris* dogfish. (From Claus.)*"/?, ventral fin; Br, pectoral fin; A'.s, gill clefts ; n, nostril ; R', R*, dorsal fins ; 6', heterocercal caudal fin ; Spl, spiracle. 599), they agree in form in that the head is prolonged into a snout, which is usually supported by a cartilaginous prolongation of the cranium, the rostrum (fig. 588, R). The mouth lies ventrally, at more or less distance from the anterior end, and is transverse, whence the name Plagiostomi — transverse mouth. This position makes it necessary that a shark approaching its prey from below must turn on its back before biting. The tail is heterocercal or is drawn out in a long filament. The skin is covered with placoid scales, usually close together, these being so small in some cases that the skin — shagreen — is used instead of sandpaper for polish- ing. More rarely the scales are larger, and the spines, which project from the skin, justify in size and form the term dermal teeth. Such strong spines occur especially at the front of the 570 CHORD ATA. dorsal fins (ichthyodolurites of paleontologists). The skeleton is cartilaginous, frequently calcified on the outside. The calcifica- tion can also extend into the vertebrae, producing star-like figures. Since bone is lacking, the sharks have no upper jaws, but bite with the pterygoquadrate. The amphicoelous vertebras (lacking in the Holocephali and the extinct Cladoselachii, Ichthyotomi, and Acanthodidae), have neural arches, small ribs, and intercalaria. The number of gill arches and clefts varies between five and seven, the first cleft lying between the hyoid and the first branchial arch. Besides, most elasmobranchs have a spiracle and pseudobranch (fig. 598, Spl}. Except in the Holocephali the gill clefts open sep- arately, the hyoid arch being without an operculum. In the visceral anatomy these points are of importance as dis- tinguishing elasmobranchs from Teleostomes. (1) The heart has a large conus, with several rows of valves (fig. 596, A), but lacks a bulbus. (2) The alimentary tract (fig. 593, A) has a spiral valve, but lacks swim bladder and pyloric caeca. (3) The sexual products are carried to the exterior by the urogenital ducts. The eggs escape from the follicles of the ovary (occasionally unpaired) by dehiscence into the body cavity, and from thence by the unpaired ostium tubae and the paired Miillerian ducts to the exterior. The spermatozoa traverse the anterior part of the Wolffian body (' kid- ney'). Sexual and reproductive ducts open dorsally into the cloaca. Male elasmobranchs are distinguished by the presence of a copu- latory structure (mixipterygium) developed by enlargement of some radii of the ventral fin (fig. 599, c). The large eggs, rich in yolk, are fertilized in the oviducts and usually develop in uterine enlargements of the ducts. The embryos (fig. 582), with long gill filaments protruding from the gill slits, are nourished by the yolk in the yolk sac. In Mustelus and Car- charias, as Aristotle knew, there is the formation of a placenta, which differs from that of the mammals in that the embryonic blood supply arises from the blood-vessels of the yolk sac and are not allantoic. There are oviparous elasmobranchs, and in these the egg is surrounded by albumen and a shell, but these eggs differ from those of birds in that the skull is horny and is usually drawn out at the four corners, sometimes with threads for attaching the egg to plants, etc. Order I. Sjelachii. With the notochord more or less completely replaced by verte- bral centra; no dermal bones. Sub Order I. DIPLOSPONDYLI. Gill slits lateral, six or seven in number, a single dorsal fin. Chlamydoselaclms with terminal mouth. Hexanchus,* mouth normal, six gill slits ; Heptanchus, seven gill slits. IV. VERTEBRATA: PISCES, SELACHIL 571 Sub Order II. SQUALI (Euselachii). Normal sharks, with cylindrical bodies, free thoracic fins, heterocercal tail, lateral gill slits. Most of them are fast swimmers and are rapacious, the teeth being usually pointed, with sharp or toothed edges, but in some the teeth are pavement-like and are used for crushing shell fish. The numerous families are distinguished by vertebral characters, number of dorsal fins, presence of nictitating mem- brane, etc. In the GALEID.E, in which the nictitating membrane is present, belong, besides the dog-sharks (Mustelus * and Galeus*), the largest of all sharks, Carcharinus* some of which have man-eating reputations. The hammer heads (Zygcena *) are closely allied. The mackerel sharks Lamna*) and the great white * man-eater,' Carcharodon,* lack nictitating membranes. All of the foregoing have star-shaped figures in the verte- brae (p. 570). In the dog-fishes, represented by Acanthias vulgaris* (or Squalus acanthias, fig. 598), there is a spine in front of each dorsal fin. Sub Order III. RAI^E. In the skates the body is flattened horizontally fig. 599), and the pectoral fins, also flattened, are united to the sides of FIG. 599.— Bawi batw, male, ventral view. (After Mo'bius and Heincke ) B, ventral, Br, pectoral fin; B, rostrum; a, anus; c copulatory part of ventral: to sill clefts; wi, mouth; /i, nostril; between them the oronasal groove. the body, the union usually extending clear to the tip of the snout, and frequently back to the pelvis, giving the body a rhombic appearance from above. The animals swim by undulating motions of these fins. They mostly lie quiet on the bottom, and hence the lower surface is white, the upper colored. The union of the fins to the side has resulted in trans- 572 CHORD ATA. fer of the gill slits to the lower surface, the spiracles to the upper. The teeth are usually pavement-like. The PRISTINE, or sawfishes, are the most shark-like, but are readily recognized as belonging here by the position cf the gill slits. The common name is due to the fact that the snout is pro- longed into a paddle-shaped blade, the edges armed with teeth. Pristis* RAIID^E; the typical members of the group ; Raia.* Closely allied are the TRYGONID^:, or sting rays, with whip-like tail with one or two spines, the ' stings,' at the base ; Vasyatis* The torpedos (TORPEDINID^E) have smooth skins, and have electrical organs, kidney-shaped bodies, on either side between gill arches and pectoral skeleton. Torpedo* Order II. Holocephali. These forms, which have no common English names, differ from the selachii in having the pterygoquadrate arch, which bears a few large chisel teeth, fused with the cranium without a suspensor; in FIG. 600.— ChimoRra monstrosa. (From Kingsley.) having a dermal fold constituting an operculum, which covers the gill slits; and corresponding with this, the gills more on the teleost type (p. 566). Lastly, the vertebral centra are not developed. Chimcera.* Fossils appear in the Devonian. The CLADOSELACHII (Cladoselache), ICHTHYOTOMI (Pleur acanthus), and ACANTHODID^E are paleozoic forms in which vertebral centra were lacking. In Cladoselache the skeleton of the paired fin consisted of numerous simi- lar radii and was more primitive than the archipterygium; Pleuracanthus was diphycercal, and the head, as in Acanthodes, bore dermal bones. Sub Class //, Ganoidei. The ganoids form a transition group in which elasmobranch and teleost characters are mingled in a notable manner. They have the spiral valve of the sharks, the swim bladder of the telosts; the heart with the conus is selachian, the respiratory structures — the comb-like gills and the operculum — are as distinctly teleostean. The hyoid arch, with the development of the operculum, has not entirely lost its respiratory function, since in garpike and sturgeon it bears an opercular gill, and often there is a pseudobranch in the spiracle. The skeleton is always ossified in certain parts; large IV. VERTEBRATA: PISCES, GAN01DEI. 573 membrane bones lie on the shoulder girdle, on the roof and floor of the skull (parasphenoid) ; the horny threads of the fins are bony rays. In general the skeleton ranges between two extremes — an extremely primitive cartilaginous condition with persistent noto- chord, and one with a more than ordinary degree of ossification. It is important for the systematist to find characters in all ganoids which occur only in the group. The ganoid scales, used by Agassiz, are not sufficient, since the sturgeon has bony plates free from ganoin, while the paddle bill (Polyodon*) has almost no dermal skeleton, and Amia has cycloid scales. Most recent and fossil forms possess fulcra, bony plates with forked ends lying shingle- like in front of the fins (fig. 10, B), but these are not universal, and are absent, e.g., in Amia and Polypterus (fig. 10, A and C). The group is largely American. The few recent ganoids fall into three distinct groups. Order I. Crossopterygii. These are largely extinct, but two genera persisting to-day. The tails are diphycercal or heterocercal; the pectoral fins have the basal portion scaled ; broad gular plates beneath the jaws in place of branchiostegals; the skeleton well ossified. Polypterus and Calamoichthys from Africa. The order was probably ancestral to the Amphibia. Order II. Chondrostei. These forms resemble the sharks externally in the heterocercal tail, spiracle, ventral position of the mouth; internally in the cartilaginous skull and (except Polyodori) in the pterygoquadrate serving as upper jaw. In the vertebral column they are more primitive than most selachians, since centra are lacking, the neural and haemal arches and the intercalaria resting direct on the notochordal sheath (Sg. 556). ACIPENSERIIME, with FIG. 601.— Acipenser sturio* common sturgeon. (After Goode.) large bony dermal plates. Acipenser,* sturgeon. The swim bladder furnishes isinglass, the ovaries make caviare. POLYODONTIDJB, with naked skin and long paddle-like snout, toothed maxillaries present. Polyodon* paddle fish. Order III. Holostei. In these the skull is ossified as in teleosts; maxillary and premaxillary bones are present, the pterygoquadrates reduced and not meeting in front, and the mouth terminal. The body may be covered either with ganoid or 574 CHORD AT A. •cycloid scales. The living forms (the group appears in the trias) have ossi- fied opisthocoelous vertebrae and diphy- or homocercal tails. LEPIDOSTEULE. Scales rhomboid, branchiostegal rays present, a pseudo- branch, but no spiracle. Lepidosteus* garpike. AMIID.E, distinctly teleos- tean in appearance with cycloid scales, amphicoalous vertebras, and heart with reduced conus (fig. 596, B). Amia* bow fin. Sub Class III. Teleostei. The teleosts owe their name to the extensive ossification of the skeleton, which consists, in the trunk, of amphicoelous vertebrae, and in front a skull with numerous primary and secondary bones, already enumerated (p. 560, fig. 589). Maxillaries and premaxil- laries are present, but these are frequently without teeth, since other bones of the mouth (vomers, palatines, liyoid, gill arches, superior pharyngeals — the latter alone in Cyprinoids) may bear teeth. Frequently there are present small bones, usually forked, lying in the intermuscular septa above the ribs, which are not pre- formed in cartilage. These are the epipleurals, and are distinct from the ribs. In the fins both cartilage and dermal rays are ossi- fied, the former remaining small, the rays forming most of the support. These rays may either be soft and flexible (Malacopteri) or hard and spine-like (Acanthopteri), a matter of classificatory value. In the first case they consist of numerous small threads FIG. 602.— Perca fluviatilis. (From Ludwig-Leunis.) A, anal fin ; B, ventral fin; Br, pectoral fin , K, operculum ; JV, nostrils ; R\, R^, spinous and soft dorsal fins ; tf, caudal fin ; 67, lateral line. (fig. 602, Br, A, B, Rz), in the other the parts of a ray are fused to a spine which, sometimes provided with poison glands (Scorpcena, Amphacantlie, etc.), become good defensive weapons. The tail is usually homocercal; the diphycercy of eels and other fishes is sec- ondary. The dermal skeleton consists of ctenoid or cycloid scales, sometimes of spines or body plates. In rare instances the skin is naked. IV. VERTEBRATA: PISCES, TELEOSTEI. 575 The hyoid arch always bears an operculum and branchiostegal membrane, but there is no opercular gill. The gills of the comb-like type, are confined to the four anterior gill arches, but they may be reduced to even two and one-half pairs of demi- branchs. Instead of a conus (present in Butrinus), the bulbus arteriosus is well developed; a spiral valve is lacking, but pyloric appendages are common. A swim bladder is usually present, but its duct is frequently closed. The teleosts are distinguished from all vertebrates except the cyclo- stomes and perhaps some ganoids in that the nephridial system does not form part of the sexual ducts. The eggs and milt are deposited through the abdominal pores or by special canals developed from the body cavity. Copulation occurs in only a few viviparous forms (Embiotoeidae, Garribu- sia, etc.). The rule is that males and females deposit their reproductive products in the water at the same time, and this leads to the enormous schools of herring and other fishes which occur yearly at certain times. This also explains the ease with which artificial impregnation in fish culture is performed. In rare instances the males care for the young, as in the case of the sticklebacks ; more noticeable are the conditions in the lophobranchs (sea horses and pipe fish), where the males receive the eggs in a brood pouch on the ventral surface. A metamorphosis is known only in the eel-like fishes, the larvae of which — originally described as distinct under the name Lepto- cephalus — are flat, transparent forms with colorless blood, enormous tails, and extremely small trunk. These larvae normally occur in the sea at the depth of some hundred fathoms. The fresh-water eels go to the ocean for propagation. On the other hand many salt-water fish go to fresh water for reproduction. The classification of the fishes is yet in an unsettled state, partly owing to the large number of forms, partly to the fact that the groups intergrade. Most European writers recognize six divisions, Physostomi, Anacanthini, Pharyngognathi, Acanthopteri, Chaetognathi, and Lophobranchii. Our authorities separate the Ostariophysi from the Physostomi, the Pediculati and Hemibranchii from the Acanthopteri, and unite the Anacanthini and some of the Pharyngognathi with the Acanthopteri and make a distinct group, Synentognathi, of the others. The characters on which these divi- sions are based are less convenient for the tyro than those adopted here. Order I. Physostomi. The character to which this name refers is not readily seen without dissection, the persistence of the duct of the swim bladder. This is, however, correlated with the soft character of the fin rays (few exceptions) and the abdominal position of the ventral fins. The Ostariophysi are remarkable in having a chain of bones connecting the swim bladder with the ear. More than a third of the food fishes and nearly all of the fresh-water fishes belong here. 576 CHORD ATA. The Ostariophysial families are the SILURID^E (1000 species), or cat-fish, with barbies about the mouth, of which Malapterurus, FIG. 603.— Salmo solar,* Atlantic salmon. (After Goode.) the electric cat of Africa, is most noteworthy. The or carp (1000 species), and the suckers, CATOSTOMID^E, have little food Value. The electric eel of South America belongs to the GYMNONOTI. The other families are true Physostomes. The SAL- MONICA are easily recognized by the 'adipose dorsal/ a fin formed of a fold of skin without fin rays. The trout and salmon (Salmo *) belong here and are among the most important food fishes. Osmerusi* smelt; Coregonus,* white fish; CLUPEID^:, herring, shad; ANGUILLID^;, eels, the breeding habits referred to above. ESOCID^;, pike and pickerel. AMBLYOPSID^E, blind fish of Mam- moth Cave. Order II. Paryngognathi. In many fishes the inferior pharyngeal bones (i.e., the last rudimentary gill arch) fuse to form a single bone, and these forms are called Pharyngognathi. Some have spiny fins, among the ;, including Ctenolabrus,* the cunners, and Tautoga,* the FIG. 604.— Ctenolahrus cceruleus,* cunner. (After Goode.) tautog. These are placed among the Acanthopteri by American authors. Others have only soft fin rays. These are the Synento- gnathi and include the EXOCCETID^:, or some of the flying fishes, in which the pectoral fins are very large, acting as parachutes when the fish leap from the water. Exocc&tus.* IV. VERTEBRATA: PISCES, TELEOSTEL 57T Order III. Acanthopteri (Acanthopterygii). This is the largest group of fishes, its members usually having the ventral fins thoracic in position and more than three rays spiny in dorsal, anal, and ventral fins. The sticklebacks (GASTERO- STEID^E) and some other forms have the pharyngeal bones reduced, the ventral fins farther back, and form the group Hemibranchii. Gasterosteus.* The perch of fresh water (PERCID^E), Perca* and Micropterus* (black bass), and the marine SERRANID.E, some of which are hermaphroditic, have ctenoid scales. The SCOMBRID^E, with Scomber* the mackerel, and TJiynnus* the horse mackerel, and Fio. 605. — Scomber scomLrus, mackerel. the XIPHIID^E, or sword fishes, in which the snout is prolonged into a long sword, are the most important edible fishes of the group. The LORICATI, including the sculpins (Coitus,* Hemitripterus,*)trQ- quently have the body armored with bony plates. The EMBIOTOCID^E, or surf perches of the Pacific, are viviparous. The suck fishes, Remora,* Echeneis,* have the first dorsal modified into a sucker on the top of the head. Order IV. Anacanthini. These are soft-finned fishes in which the ventral fins lie in FIG. 606.— Gadus morrhua* cod. (After Storer.) front of the pectorals. Structure goes to show that these have descended from Acanthopteran forms. With few exceptions 578 CHORD AT A. (Lota,* burbot), all are marine. The GADID^E, with Gadus,* in- cluding the cod and haddock, and the PLEURONECTID^E, with Hippoglossus,* the halibut and other genera, the floun- ders,, turbot, and sole, make this the most important group of marine fishes. The Pleuronectidae, from their asym- metry, need a word. The young are perfectly symmetrical, but the animals turn on one side, the lower becoming white. The eye of this side gradually works over to the upper side, twisting the bones of the skull in its progress. Order V. Lophobranchii. A small group of marine species, having in common gills composed of small rounded tufts, the body covered with a segmented armor of bony plates and peculiar breeding habits, the male carrying the eggs and young in a brood pouch. The sea horses, Hippocampus,* with their horse-like heads, and the FIG. GOT. -Hippocampus hepta- slender pipe fishes, Syngnathus,* belong gon Goc us,* sea horse. (After code.) here. Order VI. Plectognathi. A small group of peculiar compact fishes, in which the bones in each jaw are coossified, the ventral fins reduced or absent. In the trunk fishes, Ostracodermi, the body is enclosed in a firm angu- FIG. 608.— Chilomycterus geometricus,* swell fish. (After Goode.) lar box of bony plates. The G-ymnodonta, or swell fishes (fig. 608), have the power of inflating the body to spherical sacs. The flesh is poisonous. IV. VERTEBRATA: DIPNOI. 579 Sul Class IV. Dipnoi (Dipneusti). The lung fishes have the form of true fishes, with scales and paired fins, supported by a single or a doubly pinnate archiptery- gium. The median fin is not separated into dorsals, caudal and ventral, and the caudal part is diphycercal. The skeleton is very primitive, consisting largely of cartilage, the notochord being re- tained to a great extent. The animals live in fresh water and, under ordinary conditions, breathe by gills which are covered by an operculum. In the gills there are some peculiarities recalling amphibian structures, Protopterus, and the young of Lepidosiren having external as well as internal gills. The resemblances are strengthened by the periodic appearance of pulmonary respira- tion. The lung fishes live in the tropics in pools and swamps which, during the hot season, may be more or less completely dried up. When the water becomes too foul for branchial respiration, the swim bladder is used. This is a paired or unpaired sac with a duct leading to the oesophagus, and the interior has its respira- tory surface increased by the development of air cells. Protopterus indeed can live out of water; it burrows in the mud at the dry season, and builds a cocoon lined with mucus in which it remains rifi. oiU. — Protojjterus annectens, lung fish. (From Boas.; quiescent until the wet season. The nose is respiratory, with a choana opening into the mouth cavity. The last gill vessels give off pulmonary arteries, and there are veins carrying the blood back to the heart. The heart itself shows the beginning of division into arterial and venous halves, especially in the regions of the conus and auricle. The few species now living have a wide and discontinuous distribution, and are the remnants of a much richer group which appeared in the paleozoic. MONOPNEUMONIA, with one swim bladder : Ceratodus of Aus^ tralia. D'IPNEUMONIA, with two bladders : Protopterus, Africa ; Lepido- siren, South America. Possibly the paleozoic ARTHRODIRA, some of gigantic size (Dinichthys), belong here. 580 CHORD ATA. Fe Class III. Amphibia. There are two views as to the origin of the Amphibia. Accord- ing to the one they have descended from Crossopterygian ganoids (and this seems the better supported); the other is that they have come from the Dipnoi. The group is distinguished at once from the fishes by the absence of fins. There is, it is true, a median fin in larval life, and this may persist (Peren- nibranchs, Triton], but it is never divided into dorsal, caudal, and anal, and it lacks any skeletal support (figs. 4, 5). The paired fins are replaced by pentadactyle feet (p. 529). These are often webbed and are used for swimming; they are also used for creeping and leaping, and are consequently jointed between the separate skeletal elements (fig. 610). Besides the shoulder and hip joints, which alone occur in fishes, there occur also elbow (knee), wrist (ankle), and finger joints. The number of digits is not always five, for a reduction to four, three, or eve a two occurs. The connexion of the girdles with parts of the axial skeleton (lacking in most fishes) is of importance. The pelvic girdle is con- c, oentrale; F, nected with the vertebral column by means ; i intermedium; f~ of the ilium, which articulates either directly metacarpals and digits. or by a sacral rib with the single sacral ver- tebpa> yentrall y the t WQ halveg of the gj rd](> fuse, and usually the limits of ischium and pubis cannot be traced. The attachment of the pectoral girdle is less firm (fig. 564, A}. The dorsal portion, the scapula, ends free in the muscles; the ventral, differentiated into coracoid and clavicle, is often connected with the sternum, but this is 'not connected with the vertebral column, since the ribs are too short to reach it. The sternum is frequently connected with an episternum. The vertebral column often (Perennibranchs, Derotremes, Caecilians, and many Stegocephali) resembles that of fishes in amphicoelous centra and persistence of notochord. The notochord may disappear, there then occurring opisthocoelous (Salamandrina) IV. VERTEBEATA: AMPHIBIA. 581 or procoelous centra (most Annra). There is also an articulation of sknll with vertebral column, rare in fishes but characteristic of land animals, by which the first vertebra (atlas) becomes distinct from the rest. The skull is remarkable for the extent to which the chondrocra- nium is retained and the consequent small number of primary bones (figs. 611, 612). The bones of the orbital region are repre- PP TIG. 611.— Frog skull from below. (From Wiedersheim.) For letters see fig. 612. sented by a pair each of ali- and orbitosphenoids in the urodeles, by a ring of bone, the sphenethmoid (os en ceinture), in the anura. The auditory region usually contains only prootics, the occipital only exoccipitals. The absence of other occipitals is often of value in distinguishing between amphibian and reptilian skulls, since in the former the articulation with the atlas is consequently by double occipital condvles. Of secondary cranial bones are to be men- tioned the nasals, frontals (in many pref rentals also), and parietals, the latter two fused in anura to f rontoparietals ; ventrally the large parasphenoids. The cranium is increased by the addition of the large quadrate cartilage, which becomes applied to the otic capsule and (Anura) fuses with it, while the rest of its arch (pterygoquadrate) extends forward in a more or less complete condition, reaching the nasal capsule in the Anura. The quadrate cartilage is covered externally by the squamosal (paraquadrate), and supports the lower jaw, com- posed of Meckel's cartilage surrounded by membrane bones 582 CHORD AT A. (dentary, splenial, angulare, etc.); its articular portion, like the quadrate, being rarely incompletely ossified. Vomers, palatines, and pterygoids appear in the base of the skull, all three forming a continuous arch in the Anura; in front of them lie the premaxil- fo. os. FIG. 612.— Lateral and hinder views of frog skull. (After Parker.) Letters for this and 611 : an, angulare ; As, alisphenoid cartilage; co (Cocc), occipital condyles; col, columella; d, dentary; E (e), sphenethmoid; fo, foramen magnum; FP, frontoparietal; Gk, otic capsule: h',h", hyoid and copula; jg, jugal; M (m), maxillary (in lower jaw mento-Meckelian) ; ink, Meckel's cartilage; N, Nl, nasal capsule; na, nasal; ofo, os, cartilages from which basi- and supraoccipitals arise elsewhere ; ol (Olcrt), exoccipital ; p/, frontoparietal ; Pal, palatine ; p (PP), palatine arch; Pmx, premaxillary ; Pro, prootic; Ps, parasphenoid: Pt, pterygoid; Qw, quadrate; Qjg, jugal; s, direct connexion between afferent and efferent arteries; c, carotid; I, afferent artery; p, pulmonary artery; v, ventricle; l~U, afferent arteries; l'-3\ gills. FIG. 616. — Heart and arches of frog (diagram), a/ a,,, right and left auricles ; aa, ventral aorta; ad, as, right and left aortic arches (radices aortse); c, carotid; cu, cutaneus; ?, lingualis; p, pulmonary artery; ss, subclavian; v, ventricle; ve, vertebralis; 1, 2, 4, persisting arches. pulmonary respiration receives arterial blood. There is, however, but a single ventricle, and the arterial trunk is, at least externally, single. The arterial arches show different relations and have different fates. With branchial respiration the first three afferent and efferent arteries are connected in two ways, the one by the IV. VERTEBRATA: AMPHIBIA. 585 capillaries of the gills, the other direct (fig. 615, b). In the fourth arch there is no gill system, but on the other hand this arch gives off the pulmonary arteries (p) to the lungs. With the loss of gills (fig. 616) the third arch frequently dis- appears entirely (Anura), as well as the gill circulation of the others, while the direct circulation persists, at least in part. The first arch gives rise to the carotids, supplying the head (c) ; the second unites with its fellow of the opposite side to form the dor- sal aorta; the fourth forms the pulmonary artery and, in the Anura, gives off a cutaneus artery (ou) to the skin. A longitu- dinal fold inside the arterial trunk is so arranged that the venous blood from the body coming to the heart through the right auricle is mostly sent out through the fourth arch to the lungs and the skin, while the blood returned from the lungs by the pulmonary vein passes through the left auricle and then through the first and second arches (carotid and aortic arches). So there is here a separation of pulmonary and systemic circulations, although the blood all passes through a common ventricle. The sexual organs (fig. 581) are similar to those of selachians. The eggs pass from the ovary to the oviducts (Miiller's duct), and in this are enveloped with a gelatinous layer. The spermatozoa, on the other hand, pass through the anterior part of the Wolffian body (' kidney') and thence out through the ureter. The distinc- tion from the selachians lies in the fact that a urinary bladder, lying ventrally to the rectum, is present, at some distance from the urinary ducts, which open dorsally into the cloaca. Besides sexual organs fat bodies frequently occur, lobed and often brightly col- ored structures, best developed between the reproductive seasons. A sort of copulation occurs, and internal impregnation is effected in many urodeles and in the Gymnophiona, but not in the Anura. The Anura and most other forms are oviparous, but occasionally, as Salamandra maculosa and S. atra of Europe, viviparous species occur. Many inter- esting brooding habits are known. The male of Alytes obstetricans wraps the cords of eggs about his legs and crawls into a hole until the young are hatched, while the females of Amphiuma and Ichthyophis watch over the eggs. The male of Rhinoderma darwinii has a large sac arising from the pharynx in which the eggs and young are cared for until the completion of the metamorphosis. In Pipa americana the male places the eggs on its back, the skin thickening around them so that each lies in a separate pocket, from which the young escape at length in nearly the adult form. In Nototrema and Notodelphys there are dermal sacs upon the back for the reception of the eggs. 586 CHORDATA. The development of the Amphibia possesses special interest, since it affords the only easily observable instances of a metamor- phosis among the vertebrates. This metamorphosis is the more marked the wider the adults are from the fishes. In the Anura a larva, the tadpole (fig. 4) escapes from the egg. It lacks lungs, but has three pairs of external gills, no legs, but a swimming tail with a fin-like fold. In the metamorphosis the gills and tail — larval organs — are lost, while lungs and legs are formed. A complica- tion is introduced into the metamorphosis in that, for a time after the loss of the external gills, internal branchiae, lying in gill slits, occur. These, however, are not visible from the exterior, since a fold of skin grows back over them, thus forming a cavity, the atrium, into which the gill slits open, and which in turn opens to the exterior by an opening (rarely paired), usually on the left side (fig. 617). In the tailed forms the metamorphosis is simplified, Fia. 617.— Side view of tadpole, e, eye; g, opening of atrium; I, hind leg; w, mouth; u, vent. usually consisting in the loss of the external gills and sometimes in the change of form of the tail, which may lose its fin fold and become cylindrical. The last traces of a metamorphosis disappear in the perennibranchs, where lungs occur and the gills persist (Siren is said to lose the external gills and then re-form them). In the Anura the metamorphosis is lost when, as in Hylodes mar- tinicensis, the whole development occurs in the egg, the young hatching in the adult form. Order I. Stegocephali. Extinct forms with well-developed tail, numerous membrane bones in the skull, and frequently a bony armor, at least on the ventral surface. Some were of gigantic size, and some from the folded condition of the enamel of the teeth are known as Laby- rinthodonta. The group appears in the carboniferous (footprints in the Devonian), and died out in the trias. IV. VERTEBRATA: AMPHIBIA. 587 Order II. Gymnophiona (Caecilise, Apoda). These are the nearest of living forms to the Stegocephali, but fossils are entirely unknown. The group is exclusively tropical, occurring in Ceylon, African islands, and America, a discontinuous distribution indicative of great age. They are burrowing ani- mals and feed on small invertebrates. As a result of this subterranean life the eyes are small and concealed under the skin, the legs are entirely lost, so that the animals are snake-like in appearance. In the skin there are usually small bony scales; the drum of the ear is lacking; the vertebrae are amphicrelous. Inside the egg many species have three pairs of beautifully feathered gills (fig. 618), a •nrrknf nf fVimr r»^H- inpnoA fn fVm Am tneir pertmenc< to tne Am- phibia. Later, for a time, there is an external gill opening which finally closes. ffypoffeophis, Seychelles; C'acilia, America. FIG. 618.— Larva of Ichthyophis uiutinfafu*. (From Boas, after Sarasins.) Ichihyophis, Ceylon; Order III. Urodela (Gradientia). Of recent forms of Amphibia the urodeles are the most fish- like. The vertebral column consists of numerous vertebrae, and of these a large part are behind the sacrum and consequently belong to the tail. Ribs are present, but so short that they do not reach the sternum, which is weakly developed or is entirely absent. Tympanum, and Eustachian tube are entirely lacking, as are the vocal chords and the production of sound. Sub Order I. PERENNIBRANCHIATA. Two or three gill slits, three bushy gills, and a swimming tail persist throughout life. Necturus,* mud puppy, with legs and two gill slits. Siren,* three gill slits, hind legs lack- ing. Proteus, of Austrian caves, much like Necturus, but nearly blind. Sub Order II. DEROTREMA. External gills lost, but an opening in the neck leading to the gill slits. Meiwpoma* (Cryptobranchus), hell- bender, legs strong ; Amphiuma* legs rudimentary. Sub Order III. SALAMANDRINA (Hyetodera). After the loss of gills the gill slits close. Amblystoma,* remarkable for the length of time the larvae retain their gills, A. tigrinum (fig. 5) and the Mexican axolotl even breeding in the larval stage. The adult of the true axolotl is unknown. Pletliodon* Spelerpes.* The European Salamandra atra and 8. macu- 588 CHORD AT A. lata are viviparous, the former undergoing its metamorphosis inside the mother. Order IV. Anura. The anura have the compact bodies familiar in frogs and toads, with a small number (7-9) of trunk vertebrae and complete absence of tail ; the caudal vertebras being represented by a long bone, the urostyle. Ribs are sometimes distinct, sometimes fused to the transverse processes; the limbs are larger than in other Amphibia, and are frequently used for leaping and climbing. Ear drum and tympanic membrane are lacking only in the Pelobatidae; their presence is correlated with the existence of vocal cords and the pro- duction of sound. The metamorphosis includes a tadpole stage. Sub Order I. AGLOSSA. Toad-like anura with degenerate tongue and unpaired opening of the Eustachian tube. Pipa (p. 585), South America ; Dactylethra, Africa. Sub Order II. ARCIFERA. Tongue present, Eustachian tubes widely separate, coracoids of the two sides overlapping. BUFONID^E, toads, tooth- less ; Bufo* the dermal glands poisonous. PELOBATIDAE, with teeth, usually no tympanum. Scaphiopus,* burrowing toad, with tympanum. HYLID^E, tree toads, toothed ; tips of toes with sucking discs ; Hyla,*Acris* Sub Order III. FIRMISTERNIA. Tongue present, Eustachian tubes distinct, coracoids firmly united in the middle line. RANMLE, frogs. Rana catesbiana,* bull frog, the largest frog known ; numerous other American species. SERIES II. AHNIOTA. Vertebrates with amnion and allantois (p. 554) in embryonic life; with the pro- and mesonephros functional only in the em- bryos, and replaced in the later stages by the true kidney (meta- nephros) ; ducts of the embryonic excretory system retained only so far as they have genital functions; gill slits appearing as trans- itory structures, but without gills .and never functional. There :are two great divisions of the Amniotes, the Sauropsida and the Mammalia. The Sauropsida include the Eeptilia and the Aves, which agree with each other and differ from the mammals in having a single occipital condyle, the quadrate acting as suspensor of the jaws ; ankle joint between the first and second rows of tarsals; the presence of epidermal scales, nucleated red blood corpuscles, and a cloaca. Class I. Reptilia, On account of similarity of form, the reptiles and Amphibia were long united. They form parallel groups: urodeles and liz- ards, frogs and turtles, csecilians and snakes. Hence the points IV. VERTEBRATA: REPTILIA. 589 of distinction must be emphasized. The most important are two : the reptiles belong to the Amniota and, as such, have the em- bryonal features of the group; second, although often aquatic, they are, in the entire absence of branchial respiration, in character of skin and skeleton, in their entire structure, like the true land animals. The skin, the better to withstand desiccation by the air, is strongly cornified, so that in the epidermis a many-layered stratum corneum and a many-layered stratum Malpighii can be distin- guished. At the tips of the toes the stratum corneum develops strong claws. Further protection is afforded by the thick derma, often capable of being tanned into leather, in which not infre- quently bony plates occur. Dermal glands are very rare, the femoral pores of the lizards (fig. 625, &), which appear like the ducts of glands, being produced by the ends of cornified epithelial cones. The axial skeleton, both skull and vertebral column, is nearly always ossified; only exceptionally (Splieno- don and the amphiccele Ascalabotae) are considerable parts of the noto- chord retained. The vertebrae are usually proccelous. In the skull of reptiles (as in the allied birds) are many characters which they share with Amphibia and which distinguish them from mam- mals. This is especially the case with the visceral skeleton. As in- the Amphibia, the hinder end of the pterygoquadrate is attached to the otic capsule; the quadrate is ossified and affords the articulation for the lower jaw, which is composed of many bones. The squamosal lies at the base of the quadrate and, in the Squamata, is intercalated between it and the cranium. Behind it is the columella, its inner end inserted in the fenestra ovalis. From the quadrate the palatine series of bones — pterygoid, palatine, vomer — extends forward, these being frequently toothed; and in front of and parallel to it the pre- FIG. 619.— Ventral view of skull of Tropidonotus. (From Wieders- heim.) Bp.basioccipital; B*, basi- sphenoid (in front also parasphe- noid); C7i, choana; Cocc, occipital condyles; Eth, ethmoid cartilage; F, frontal; Fo, fenestra ovalis; M, maxillary ; of, exoccipital; 7, parietal; P/, pref rental; P7, pal- atine ; Pmx, premaxillary ; Pf, pterygoid r Qu, quadrate; Squ, squamosal; Ts, transversum ; Vb, vomer ; II, optic foramen. 590 CHORDATA. maxillaries and maxillaries. Extremely characteristic of the reptiles, the turtles excepted, is an os transversum, which appears in no other vertebrates. It extends from the hinder end of the maxillary to the pterygoid (figs. 619, 626, 627, 630, Ts, tr). A jugal is also frequently present. Of the other visceral arches, since gills are lacking, only the hyoid bone and laryngeal carti- lages persist. In the cranium the complete ossification of the occipital region is noticeable, the four occipital bones being present. The basi- occipital forms the larger part of the single occipital condyle, in which parts of the exoccipitals participate, the single condyle being the sharpest distinction between the reptilian and amphibian skull. The basisphenoid, which lies in front of the basioccipital, has an anterior process or rostrum, representing the rudimentary parasphenoid (possibly presphenoid). Above, the skull is roofed in with membrane bones : parietals (frequently fused and perforated by the parietal foramen for the pineal eye), frontals, nasals, as well as pre- and postfrontals and postorbitals, and usually lachry- mals as well. The ethmoidal region is largely cartilaginous ; ali- and orbitosphenoids are small and variable. Only the prootic is constant in the otic region ; epiotic and opisthotic usually fusing with the occipitals, the opisthotic being large and distinct only in the turtles. The zygomatic arch (lost in snakes) is formed of jugal and quad rat ojugal, while above it may be a second arch formed of postorbital and squamosal. The convex occipital condyle forms, with the concave surface of the first vertebra (atlas), an articulation for motion in the ver- tical plane and lateral motions, while a twisting around the long axis of the body is permitted by the joint between the atlas and the second vertebra, the axis or epistropheus. The atlas is a bony ring, its centrum having separated and united with the body of the axis, forming a pivot around which the atlas turns. There are two sacral vertebrae, and the vertebras of the trunk are divided into thoracic and lumbar, the former bearing long ribs which reach to the sternum, while the shorter ribs of the neck end freely. Limbs are lacking in snakes and some lizards. When present the number of digits varies between three and five (usually four or five). In the pelvis ischium and pubis are separated by an obturator foramen and are united with the corresponding bones of the oppo- site side by a double symphysis. In the shoulder girdle scapula and coracoid alone are constant, a clavicle occurring in turtles and lizards, in the latter an episternum (fig. 564) as well. Of con- siderable systematic importance is the position of the ankle joint. IV. VERTEBRATA: REPT1LIA. 591 This is intertarsal in character, in that it occurs between the first and second rows of tarsal bones (fig. 636, C). MD FIG. 620.— Viscera of Alligator. (From Wiedersheim.) ED, rectum; If, heart; L, liver; Lg, lung ; M, stomach; MD, intestine; Oe, oesophagus; P, pylorus; Tr, trachea ; ZB, body of hyoid; ZH, its cornua ; *, perforations of hyoid. Since reptiles lack even transitory gills, the gill slits are com- pletely degenerate before the young escapes from the egg. Dermal 592 CHORD ATA. respiration is far less important than with the Amphibia, lungs, as in birds and mammals, being the respiratory organs, and in these a progressive development may be followed. The larynx is followed by a trachea with cartilage supports in its wall, and this either opens directly into the two lungs or divides into two bronchi, which, in Varanus, may divide again inside the lungs. The lungs in the more primitive forms are subdivided only peripherally, but in the higher groups the whole is chambered, partitions extending inwards to the intrapulmonary bronchus. Since the respiration is entirely pulmonary, the heart is divided into a left arterial and a right venous half, and a corresponding separation of systemic and pulmonary blood-vessels occurs (fig. 621). The two auricles are completely separated, while a septum extends into the ven- tricle, complete in the crocodiles, but not in turtles, lizards, and snakes. Yet even in the crocodiles a mixing of arterial and venous blood occurs since in the large aortic trunks which arise from both ventricles a commu- nication, the foramen Panizzae, per- sists. The arterial trunk is divided by internal partitions into three ves- sels, which are but rarely visible from the exterior. One of these arises from the right ventricle, carries venous blood, and takes over the fourth arterial arch, which gives off FIG. 621.— Heart of crocodile with ar- the pulmonary arteries (4, p). A second vessel arises from the right ventricle, is purely arterial and con- nects with most of the remaining arterial arches' the first> which vessels from the heart, .and the (aortic arch, ad) of the SCCOnd connexion (foramen Panizzae) be- V ' . ' tween the arterial trunk and the arch. The third Vessel Connects Oil left aortic arch, just in front of . . . the heart. the one hand with the remaining (left, second) arch and on the other with the right or venous half of the heart. The foramen Panizzse occurs between this and the right aortic arch. IV. VERTEBRATA: REPTILIA. 593 The venous character of the left aortic arch and the incomplete ventricular septum (or presence of foramen Panizzae) prevent a complete separation of systemic and pulmonary circulations. In the turtles a third element enters, the persistence of a ductus Botalli (as in Urodeles, fig. 580, II, dB). To the foregoing adaptations to a terrestrial life may be added indications of higher development. The brain shows two advances. The cerebellum, especially in turtles and alligators, has be- come large, and the cerebrum grows dorsally and backwards over the 'twixt brain and forms the temporal lobes of the hemispheres. The parietal organ is developed as nowhere else. In many lizards it forms an unpaired dorsal eye lying beneath the skin in the parietal foramen. The paired eyes possess lids (usually upper and lower as well as a nictitating membrane), and frequently (turtles, lizards, and many fossils) a ring of bony plates (sclerotic bones) in the sclera. A new opening in the petrosal, the f enestra rotunda, places the tympanic cavity and the labyrinth in close relations. In the excretory system amniote characters prevail. The Wolffian body with its duct is functional in the embryo. Later there arises behind it the permanent kidney (metanephros) with the ureter, while the embryonic structures disappear with the ex- ception of those retained as accessory to the genital apparatus. Thus in the male the vas deferens and epididymis are formed from the Wolffian duct; in the female the Mullerian duct (early lost in the male) becomes the oviduct. Usually the urogenital canals open dorsally in the cloaca, rarely in an elongation of the urinary bladder (Chelonia). This latter is lacking in snakes and crocodiles. Almost all reptiles lay eggs; only in the Squamata (some snakes and lizards) are viviparous or ovoviviparous forms present. The eggs much resemble those of birds, in that the large yolk is sur- rounded with a layer of albumen and enclosed in a fibrous, often calcified shell. To open the egg the embryo has an egg tooth on the tip of the snout ; this consists of dentine in the Squamata, but elsewhere, as in birds, is horny. From these relations it follows that internal impregnation must occur; the eggs undergo a discoidal (meroblastic) segmentation. Copulatory organs to accomplish this internal fertilization occur, and these are of classificatory im- portance, since they differ in character in the Squamata on the one- hand, the turtles and crocodiles on the other. These differences are correlated with differences in the form of cloacal opening and in structure of skull and skin, so that all living species may be- 594: CHORDATA. divided into two groups, the Lepidosauria, containing the lizards, snakes and Sphenodon, and the Hydrosauria with turtles and croc- odiles. This, however, ignores the fossil forms. When these are taken into consideration another grouping must be adopted. Order I. Theromorpha. Extinct reptiles from the Permian and triassic which are closely re- lated to the stegocephalous amphibia; with amphiccelous vertebrae, im- movable quadrate, and from two to six sacral vertebras. The ANOMODON- TLA, with partial or complete loss of teeth, stand near the turtles, while the THERIODONTA, in which a heterodont dentition is developed, resemble in this and some other respects the mammals, which, by many, are sup- posed to have descended from them. Order II. Plesiosauria. Extinct aquatic forms from the triassic to the cretaceous, some forty feet in length. They had long necks, and the limbs were modified into .swimming paddles recalling the flippers of the whales. The quadrate was immovable, and the jaws, with numerous teeth in sockets, were long. Order III. Ichthyosauria. These forms resembled the Plesiosaurs in skin, swimming feet, elongate jaws, and quadrate, but had the teeth (sometimes absent) in grooves rather ' FIG. 622.— Restoration of Plexiosaur. (After Dames.) than in sockets, and short necks. Some species at least were viviparous. Their range in time was like that of the preceding order. Order IV. Chelonia (Testudinata). The turtles form in external appearance a sharply circumscribed group, with the short and compact body enclosed in a bony case, from which only head, tail, and legs protrude (fig. 623). The case consists of a convex dorsal portion, the carapace and a flat- tened ventral plastron, the two being united in most forms at the margins. Each consists of bony plates, the positions and names of which may be learned from the adjacent cut. It only needs mention that the neural plates are united with the spinous pro- cesses, the costals with the ribs, and that the entoplastron is re- IV. VERTEBRATA: REPTILIA, CHBLONIA. 595 garded as an episternum. It is not connected with the internal skeleton, since the sternum is lacking. The pelvis is only rarely fused with the plastron. This bony case is usually covered with horny shields, their number and arrangement usually agreeing with the plates of the case, although without their contours exactly coinciding. More important are the great firmness of the skull and the immovable condition of the quadrate, the lack of an os transver- sum and of any but basisphenoid of the sphenoidal bones, and by A FIG. 623.— Carapace (A) and Plastron (B) of Testudo grceca. (From Wiedersheim.) C, costal plates; E, entoplastron ; Kp. epiplastron: H, posterior: Hp, hypoplastron; Hy, hyoplastron ; M, marginal plates ; J\T, neural plates ; JVp, nuchal plate ; Py, pygal plate ; .R, ribs ; V, anterior ; Xi, xiphisternum. growth forward, and backwards by which the girdles are brought inside the ribs. The teeth are entirely lost, and, as in birds, the jaws are enclosed in sharp horny beaks, in many cases efficient weapons against larger vertebrates. The cloacal opening is oval, its major axis corresponding to that of the body, and in its anterior end is an unpaired erectile penis used in copulation. Turtles appeared in the Permian, and the group has persisted until now. Characters of armor and legs serve to contrast sharply the land and sea turtles; the first with well-developed legs, five-toed in front, four- toed behind, the toes with claws; the carapace arched, into which legs, head, and tail may be retracted. In the sea turtles the feet are flipper- like (fig. 624), claws mostly absent, and the carapace weakly united to or free from the plastron, flat and incapable of covering head or appendages. The fresh-water species are intermediate in position. Sub Order I. ATHECA. Carapace of numerous mosaic scales and not connected with ribs and vertebrae; skin leathery. Dermochelys (Sphargis) coriacea,* the leather-back tortoise of warmer seas, reaches a weight of 1500 pounds. 596 CHORD AT A. Sub Order II. TRIONYCHIA. Fresh-water forms with poorly ossified carapace, but ribs and vertebrae connected with it. Our leather turtles. (Amyda*) and soft- shelled turtles (Aspidonectes*)ot savage habits belong here. Sub Order III. CRYPTODIRA. Carapace well developed and united with ribs and vertebrae, but the pelvic arch free. The species are numer- ous, including terrestrial, fresh-water, and marine forms. CHELYDRID^, fresh water, tail long. Chelydra serpentina, * snapping turtle ; Machrochelys FIG. &24.— Eretmochelys imbricata, tortoise-shell turtle. (From Hajek.) lacerti?ia,* alligator turtle. CHELONID^E, marine, paddle-like feet. Tha- lassoclielys caretta,* loggerhead; Chelone my das* green turtle, the favorite of epicures; Eretmoclielys imbricata, whose horny shields furnish tortoise shell. TESTUDINID.E, terrestrial, including Xerobates* the ' gopher turtle * of the South, the giant Testudoot the Galapagos Islands, and the enormous fossil Colossochelys atlas of India, 18-20 feet long, 8 feet high. Other families contain our mud turtles (Kinosternon *), box turtles (Cistudo*), and terrapins (Malaclemmys*). Sub Order IV. PLEURODIRA. Pelvis united to carapace and plastron. All belong to the southern hemisphere. Order V. Rhynchocephalia. These resemble the lizards not only in body form (four five- toed feet) and in scaly skin, but in certain anatomical matters as well: lack of hard palate, presence of epipterygoid, transverse cloacal opening, and heart, lungs, and brain. On the other hand they recall the crocodiles in having two postorbital arches and immovable quadrate. The large abdominal sternum and abdominal ribs are noticeable as well as the uncinate processes of the true ribs. The notochord is but incompletely replaced. The group appears in the Permian and is thus one of the oldest of reptilian types, and is usually regarded as ancestral to all the orders yet to be mentioned. The only living species, Sphenodon (Hatteria) punctata, belongs to the New Zealand region. Order VI. Dinosauria. This order included some of the largest land animals which have ever existed. Some of them were from forty to one hundred feet long and twelve to twenty feet high (Amphiccelias, Camarasaurus). In some there IV. VERTEBRATA: REPTILIA, S QUA MAT A. 597 •was an exoskeleton, some of the plates of which in the stegosaurs measured a yard across. Among the characters of the group are the fixed quadrate, jugal and postorbital arches, three to ten sacral vertebra, and ilium •elongate in front of and behind the acetabulum. Some of these forms (Orthopoda) in pneumaticity of bones, in having the pubic bones directed backwards, and in the formation of an intratarsal joint, resembled the birds, and have been regarded as the ancestors of that group. The Dino- saurs were confined to mesozoic time. Order VII. Squamata (Lepidosauria, Plagiotremata). One of the characters which unite lizards and snakes and which has given the name Plagiotremata is the transverse form of the cloacal opening (fig. 625), behind which, in the male, are the nS'Jr -na ' • fir ar FIG. 625. FIG. 626. FIG. 625.— Hinder trunk and hind limbs of a lizard. (From Ludwig-Leunis.) a, cloacal slit ; b, femoral pores ; sea, anal shield. FIG. 626.— Skull of Ameiva vulgaris. an, angulare ; ar, articulare ; co, epipterygoid ; cr, coronoid ; d, dentary ; /r, frontal ; j, jugal ; la, lachrymal ; m, maxillary ; na, nasal ; p, postorbital, above and behind it the parietal ; p/, pref rontal ; pr, pre- maxilla ; pt, pterygoid ; g, quadrate ; oj, quadratojugal ; sq, squamosal ; tr, trans- versum. paired copulatory organs, each lying in a sac from which they can be everted like the finger of a glove. The names Squamata and Lepidosauria refer to the scaly condition of the skin. These scales are horny structures and somewhat distinct from the bony scales of fishes. The derma forms flattened papilla which resemble the scales of fishes in that in many species they contain bony plates. These papillae determine the character of the epidermis. Since the stratum corneum is especially thick on the top of the papillae and thinner between them, rhomboid and oval plates occur, which either lie flush with each other (shields) or overlap like shingles (scales). The rule is that the head is covered with regu- larly arranged shields, each with its name, the trunk with scales in longitudinal, transverse, and oblique lines. Outside these is a layer of cornified cells, the pseudocuticula, and outside of all an inconspicuous true cuticle. Since all cornified cells are dead and 598 CHORDATA. require periodic removal, the horny layers are cast yearly and re- placed by new. During this periodic molting, which recalls that of arthropods, the animals are sickly and apt to die in captivity. All Squamata are characterized by the slenderness of the cranial bones (fig. 619, 626, 627), which, especially in the Lacertilia, incompletely close in the cranium. The quadrate is movable, and the squamosal is intercalated between it and the cranium. A hard palate is lacking, and the choanse, as in the amphibia, lie far forward (fig. 619, Ch). There is a wide gap in the partition between the two ventricles of the heart. Sub Order I. LACERTILIA (Saurii). The lizards are usually distin- guished from, the snakes by the possession of limbs, but a few forms, undoubted lizards, like the glass snakes and Amphisbsenaa. lack limbs. These are distinguished by the existence of the scapula and the iliac bone united to the vertebra, and especially by the presence of a sternum, which never occurs in snakes. In the skull is a peculiar bone (lacking only in Chameleons and Amphisbsenae), found nowhere else, the epipterygoid (fig. 626, co); it reaches from the pterygoid to the parietal, and from its fffrffr FIG 627.— Skull of rattlesnake. (From Boas.) F>; frontal; ft, hypmandibular (09111- mella); MX, maxillary: iV, nasal: Os, supraoccipital; Fa, parietal; Pal, palatine; P/, postfrontal; PC/, pref rontal ; Pt, pterygoid ; Px, premaxilla ; Q, quadrate; 3g, squamosal; 2V, transversum; 1, dentary; 3, articulare. slender shape is sometimes called columella, but is not to be confounded with the true columella of the ear. The bones of the jaws are firmly united, so that the mouth has no special capacity for opening widely. The jugal- quadratojugal arch is present. In external appearance the presence of eyelids, nictitating membrane, tympanic membrane, and Eustachian tube are noticeable, these being absent only in the Amphisba3na3. In the Ascalabota?, as in snakes, the lids grow together, forming a transparent covering over the eyes. Fossil lizards are rare, but the group dates back to the cretaceous. Section I. ASCALABOT^E (geckos). Skeleton incompletely ossified, noto- chord persistent, amphicoele vertebrae; skin granular rather than scaly, usually adhesive discs on the toes by which they climb vertical surfaces or can walk upon ceilings. Two hundred species. Phyllodactylus* IV. VERTEBRATA: REPTILIA, SQUAMATA. 599 Section II. CRASSILINGUIA. Tongue thick, fleshy, not protrusible from the mouth, or only slightly so. IGUANIOE ; American, often a comb of spines on the back, teeth pleurodont, i.e., firmly united to the inner side of the jaw. Three hundred species. Anolis,* Sceleporus* Phrynosoma* * horned toads.' AGAMID.E; Old World, teeth acrodont, i.e., seated on the angle of the jaw bones. One hundred and fifty species. Chlamydosaurus, Draco volans, with ribs greatly elongate and supporting a dermal fold which acts as a parachute. Section III. FISSILINGUIA. Tongue long and thin, divided at the tip, and capable of wide protrusion from the mouth, and in Varanus retractile into a sheath. TEJID^E ; American, teeth acrodont ; Cnemidophortts* Tejus. HELODERMATIDJS, pleurodont ; Heloderma,* the ' Gila monsters,' are the only poisonous lizards. LACERTILID^E (Lacerta) and VARANID^E (Vara- nus, the monitors) are Old World forms, Lacerta vivipara bringing forth living young. Section IV. BREVILINGUIA. Tongue short, slightly notched at the tip, slightly protrusible. Four hundred species. SCINCID^, with tendency to reduction of the limbs. Eumeces,* Oligosoma* In Anguis and Typhline the legs are absent. ZONURID.E, with a finely scaled groove along the side; all Old World except our Ophisaurus ventralis,* the glass snake, a limb- less form with brittle tail. Section V. ANNULATA. In many respects snake-like ; legs and epi- pterygoid, tympanum, and movable eyelids lacking and usually girdles ; tropical or subtropical. In Chirotes sternum and reduced fore legs retained. AmpliisbcBna. Section VI. VERMILINGUIA ; includes the Old World chameleons (our FlG.628. — Head of chameleon with tongue extended. ' chameleon ' is Anolis, — supra} with long fleshy tongue, lying rolled up in the mouth, but protrusible and used for catching insects, its end being covered with a sticky mucus. Other characteristics are the ring-like eye- lids functioning as an iris, the climbing feet in which the toes are united into two opposable groups; epipterygoids, clavicle, sternum, and tympanic membrane lacking. The chameleons are best known from their changes of color, produced by rapid alterations in the size and shapes of the chroinatophores. Color changes occur in other lizards, but not to such an extent as here. 600 CHORD ATA. Sub Order II. PYTHONOMORPHA. Large, extinct, extremely elon- gate reptiles with four flipper-like limbs and strong swimming tail. Flourished in the cretaceous. Mosasaurus, CUdastes. Sub Order III. OPHIDIA. The snakes are distinguished from most lizards by the absence of limbs, and connected with this the similar verte- brae in which only trunk and caudals can be distinguished. The caudals lack ribs, but these are present and long in the trunk region, serving for locomotion and supporting the body on their distal ends. . Since there are legless lizards, it is further necessary to say that in the Ophidia the girdles and sternum are lost, only the Peropoda having remnants of the hinder appendages and pelvis, but these not connected with the vertebral column. Further distinctions exist in sense organs and jaws. The columella is indeed present, but tympanum and Eustachian tube are lacking. The eye- lids also seem to be wanting, but examination shows, in front of the cornea and separated from it by a lachrymal sac, a transparent membrane, com- posed of the fused eyelids (outer cornea). The apparatus of the jaws (figs. 619, 627) is remarkable for its great extensibility, which enables snakes to swallow animals larger than themselves, after coiling around them and crushing them. This extensibility is in part due to the fact that the bones of the lower jaw are bound together at the symphysis by elastic ligaments, in part to the freedom of motion of the bones of the upper jaw (excepting the small premaxillaries) and the palate. Further, the sqtiamosal (&g), quadrate (Q), and transversum (Tr) are elongate and slender, the quadrate being widely separated by the squamosal from the skull, while the zygo- matic arch is entirely absent. The food is forced down the throat by hook-shaped bones on palatines and pterygoids. A wide distension of the stomach is rendered possible by the elasticity of its walls and the great mobility of the ribs, which are not united ventrally by a sternum. In the non-poisonous snakes the dentition is similar on jaws and palate bones (fig. 619). The vomer and, usually, the premaxilla are tooth- less. In the poisonous serpents poison fangs appear on the maxilla (fig. 627) and are distinguished from the other teeth by their greater size and connex- ion with a large poison ghuid. The duct of the gland opens at the base of the tooth ; the poison which is pressed out by the pressure of the jaw muscles is led to the tip of the tooth either by a groove (proteroglyphic tooth, fig. 629, A) or, when the groove is closed to a cnnal (solenoglyphic tooth, B), through this . pro- canal which opens at base and tip of teroglyphic (grooved) tooth of co- tl tnnth bra, and section of same; #, #,, so- tue 1 tn- The asymmetrical character of the lungs is interesting. In the Peropoda one lung (apparently the left) is much smaller than the other ; in the poison snakes and some others it is rudi- Fm. 629— Pison fangs. lenoglyphic tooth (tubular) of rattle- snake ; g, poison canal ; jo, pulp cavity. IV. VERTEBRATA: REPTILIA, CROCODILIA. 601 mentary or even absent. In the Typhlophidae, on the other hand, the right appears to be degenerate. 'The urinary bladder is always absent. The excreta, chiefly uric acid, accumulate as a solid mass in the cloaca and form the chief part of the excrement ; the faeces, on account of the extraordinary digestive powers, being small in amount. Section I. OPOTERODONTA (Angiostoma). Burrowing blind tropical snakes with the mouth incapable of distension, the animals living on small insects. Typhlops. Section II. PEROPODA. These large snakes have paired lungs and rudi- ments of hind extremities ; lack poison fangs, and kill their prey by mus- cular power. Python, Africa ; Boa and Eunectes (anaconda), South America. Section III. COLUBRIFORMIA. Ordinary snakes (over 500 species) with numerous teeth in the upper jaw, but with appendages entirely absent. Some are poisonous, some not, but no structural lines can be drawn be- tween them. The AGLYPHA have no grooved teeth. Tropidonotus,* water snakes; Bascanion,* black snakes; Eutainia,* garter snakes. The PRO- TEROGLYPHA, with grooved teeth, perma- nently erect, are poisonous. Most are brightly colored. Elaps,* the coral snake; Naja tripudians, the cobra of India ; N. haje, Cleopatra's asp. Here belong the pelagic sea snakes of the Indo-Pacific, which are viviparous. Section IV. SOLENOGLYPHA. With the maxilla reduced and serving as a socket for the single large tubular tooth with one or more reserve teeth (fig. 627). VIPERID^, Old World, no pit between nostril and eye. CROTALID.E, New World and Asia, with a pit between nose and eye. Crotalus* with the tail ending in a rattle formed by remnants of cast skins, is common throughout the United States. Agliistrodon contortrix* copperhead, and A. piscivorus, moccasin, lack the rattle. Bothrops lanceolatus of the An- tilles, possibly the most poisonous snake. Order VIII. Crocodilia (Loricate). The crocodiles, alligators, etc., ome of the forms already mentioned in the oval cloacal open- ing with single copulatory organ, immovable quadrate, and the bony plates in the skin. In shape they are lizard-like, but in structure they differ from all other living reptiles Coee FIG. 630.— Ventral surface of skull of crocodile. (From Wiedersheim.) Cocc, occipital condyle: C/i, cho- ana ; jg, jugal ; M, maxillary ; O/>, basioccipital ; Or/>, orbit ; Qi, quad- ; Pi, pala- ratojugal ; Qtt, quadrate tine ; Pmx, premaxilla; goid; Ts, transversum. ptery- 602 CHORD AT A. and approach most nearly to the Theromorphs. The maxillaries, palatines, and ptery golds have united in the living species in the middle line, forming a hard palate and forcing the vomers upwards into the nasal region. This same process has carried the choana (fig. 630, Cli) to the back of the skull. Some of the ribs have two heads; the ears and nostrils are provided with valves. A sternum is present and, farther back, abdominal ribs and an ab- dominal sternum. The jaws are extended into a long snout, and the teeth, which occur only on the margins, are placed in sockets (alveoli). The four-chambered heart has already been described (p. 592). The animals move slowly on land, but in the water, thanks to their strong, keeled tail, they are very active. They have a strong smell, owing to musk glands in the cloaca and on the under jaw. The group appeared in the trias, and of the three sub orders two, the Pseudosuchia and Parasuchia, are extinct. Sub Order EUSUCHIA. External nostrils united, choana posterior; five toes in front, four behind. Gavialis, India, snout long and slender. Alligator lucius* alligator ; Crocodilus,* most species Old World, one, C. americanus,* occurring in our southern waters. Order IX. Pterodactylia (Pterosauria). Extinct reptiles of the Jurassic and cretaceous, adapted for flight. The bones were hollow and the wings were broad membranes, supported, like those of a bat, by the body and the greatly elongated fifth digit of the FlG. 631. — Dimorpliodon, a pterodactyle. (After Woodward.) fore limbs. Some were sparrow-like in size and some, Pteranodon, had a wing expanse of twenty feet. Yet one of these large forms from Kansas had its pelvic opening so small that its eggs could not have been more than half an inch in diameter. IV. VERTET3RATA: AVES. 603 Class II. Aves. While structurally the birds stand very near the reptiles, yet by the development of wings and the feathering of the body the group is one strictly circumscribed. The skin is in some places, as the lower part of the legs, covered with horny scales and shields, on the toes are claws, but as a rule the fingers are feathered. On most places the skin is soft and thin, since the derma and stratum corneum are poorly developed. Periodic molts of the integument do not occur, since the horny layer, as in mammals, undergoes a constant renewal. These peculiarities of the skin are correlated with the appearance of the protecting plumage. The feather, like the hair of mammals, is exclusively epithelial in character, but of a much more complicated structure. The cor- nified epithelium forms a firm axis, the scape, from which, right and left, arise branches, or barbs. The scape is solid as far as the barbs extend (rachis, or shaft), while below it is hollow (quill, or calamus). The quill is inserted deep in the derma, in a follicle, and is provided with muscles for its movement. Its hollow in most fully developed feathers is empty save for the ' pith/ a small amount of dried tissue. In young growing feathers it is occupied by a richly vascular connective tissue, the feather papilla, which, for purposes of nourishment, extends inwards from the derma. The feather may therefore be regarded as a cornified outgrowth from the skin which has arisen on a papilla of the derma, a view which corresponds well with its development and shows its homology with the scales. In many birds (cassowaries) two well- developed feathers arise from the same follicle — a fact which explains the existence of a rudimentary feather, the hyporachis, or after-shaft, attached to the scape below. In contour feathers the barbs are, to a great extent, united into a vane. Right and left of the shaft they lie close together and parallel, each repeating in miniature the entire feather, the barb having branches or barbules, which, overlapping the barbules of adjacent barbs, give the vane its close texture. The vane is held together by minute hooks on the barbules of one barb interlocking with those of the next. Down feathers (plumes) differ from contour feathers in the absence of hooks and the loose arrangement of the barbs. Since feathers consist of cornified epithe- lium and these cells are held firmly (only in powder down is there a gradual loss), they, like the scaly coat of the snakes and lizards, must be molted yearly and replaced by new. Young birds or embryos have only down feathers. Later the contour feathers arise in regular order in the feather tracts, or pterylae, between 604: CHORD AT A. which are apteria in which no contour feathers appear (fig. 632). Since the contour feathers overlap like shingles, they form a firm coat of plumage beneath which the down and semiplumes form a warm coat. FIG. 632. FIG. 633. FIG. 632. — Feather tracts and apteria of pigeon, dorsal view. (From Ludwig-Leunis.) FIG. 633.— Regions and feathers of Falco lanarius. (From Schmarda.) As, secondaries ; Ba< belly ; Br, breast ; Bz< rump ; D'-D'", wing coverts ; Di, gonys of bill ; EF, alula ; F, culmen of bill ; H, occiput ; HS, primaries ; K", throat ; L, legs ; JV, neck ; Sch, crown ; SF, parapterium ; St, forehead, lower tail coverts ; Sz, rectrices ; W, cheek; WH, cere with nostril; Zh, toes. Besides these covering feathers (coverts, or tectrices, fig. 633, D) there are the longer feathers of the wing, the remiges, and the tail feathers, or rectrices (8z). The larger remiges form the chief part of the wing; they spring from the part of the limb corresponding to the hand (carpus, metacarpus, phalanges) and are known as primaries (HS), while the secondaries (As), arising from the forearm, are shorter. These are over- lapped at the base by the coverts (D, D', D'1) and by the parapterium (SF) FIG. 634.— Wing skeleton of stork. (From Gegenbaur.) c, c', carpalia of first row; ft, humerus ; wi, fused metacarpals and carpals of second row ; p-p'\ phalanges of first three fingers ; r, radius ; w, ulna. springing from the shoulder. A few feathers arising from the first finger remain distinct from the remiges and form the alula (EF). In the water birds especially the feathers are oiled by the secretion of a pair of glarrds at the base of the tail above the coccyx. Since the feathers are not only for protection, but give to most birds the power ol prolonged flight, they predicate a special mode IV. VERTEBRATA: AVES. 605 S/- of life, under the influence of which all of the other organs exist. The character of the skeleton, the respiratory organs, and in part the sense organs and brain, are connected with the powers of flight. As the feathers of the wings, like the fins, form what may be called a paddle working as a whole, the skeleton of these limbs is simplified (fig. 634), first, by the reduction of the fingers, of which only three with a small number of phalanges persist (j>,p',p")\ second, by fusion of the corresponding metacarpals (m) with each other and with the adjacent carpal bones. On the other hand, in order that there may be the necessary en- ergy and the most complete transfer of the same to the body, the con- nexion with the skeletal axis is strengthened by special development of the parts. In the shoulder girdle (fig. 635) all three elements are firm, a sword-shaped scapula (s), a colum- nar coracoid (c), and clavicles which are usually united to a i wish-bone/ or furcula (/). Clavicles and furcula are united directly or by ligaments to the broad sternum, the anterior face of which is developed into a strong keel, the carina, or crista sterni, in order to give the largest surface for attachment of the large muscles of flight. Usually the greater the powers of flight the more devel- oped the carina, yet in some cases (albatross) the weak carina is com- pensated for by the enormous width of the sternal plate. In running birds (ostriches, etc.) the carina is entirely gone. The thoracic framework is rendered more firm by the development of uncinate processes from the ver- tebral parts of the ribs (u) which overlap the succeeding ribs. Since the fore limbs are no longer used for walking, the sup- port of the body depends upon the hinder extremities, and this has brought about two striking characteristics — the broad union of the pelvis with the vertebral column, and the simplification of the leg skeleton. In the embryo the ilium (fig. 635, il) is connected only with the two sacral vertebrae present in most reptiles, but Fm. 635.— Trunk skeleton of stork. (From Gegenbaur.) as, sternal rrt of rib ; c, coracoid ; era, keel ; furcula (fused clavicles) ; /p, fused spinous processes of thoracic vertebrae; il, ilium; is, ischium; oc, vertebral part of ribs; p, pubis; 8, scapula; sp. spinous process; st,st', sternum and abdominal processes; ?t, uncinate processes; x, acetabu- lum. 606 CHOIWA TA. later it extends forward and back, uniting with at least nine ver- tebrae and sometimes with as many as twenty-three; while the iliac bones of the two sides meet dorsal to the vertebral column. This extensive union of pelvis and axial skeleton is understood when we recall that in walking or at rest the vertebral column is not vertical as in man, but is inclined. Ischium and pubis are peculiar in that they extend backwards, parallel to each other, from the acetabulum, and that only exceptionally (ostrich) are the bones of the two sides united by a symphysis. In the hind limbs occur conditions similar to those which will PIG. 636.—^!, leg of Buteo vulgarte. a, femur; 7>, tibio-tarsus; b', remains of fibula; c, tarso-metatarsus ; c', same, front view; dl-tia, toes. J3, lower leg of bird embryo; C, of lizard. /, femur; t, tibia; jj, fibula ; £x, tarsales of first row (talus); ti, tar- sales of second row; between these intertarsal joint; I-F, digits. (From Gegenbaur.) be repeated in the ungulates. The weight of the body makes it necessary that the simplification found in the wing should be re- peated in the lower leg and foot, and that the numerous bones usually occurring in these regions be replaced by one which shall support the pressure (fig. 636). Therefore the fibula, well de- veloped in the embryo (B), becomes reduced to an inconspicuous rudiment; the metatarsals, distinct in the embryo (B), fuse to a IV. VERTEBRATA: AVES. 607 single tarso- metatarsus (A, c), which has below as many articular surfaces as there are toes (since the fifth toe only appears in the embryo, at most four, in some three or even two, d-d'"). At the same time the tarsals disappear by fusion with adjacent parts. Even in reptiles (C) a part of the tarsals unite with the bones of the shank, and the remainder with the metatarsals; in the birds the union is completed, the proximal series fusing with the lower end of the tibia to form a tibio-tarsus, the distal with the metacar- pus to form the tarso-metatarsus, in this way producing the inter- tarsal joint so characteristic of birds. In respect to the vertebral column, it only needs mention that the vertebrae articulate with each other by a so-called saddle-joint, that (in living birds) only a few caudal vertebrae persist behind the pelvis, that these are partially fused to a single bone, the pygo- style, which supports the tail feathers, and that, corresponding to the well-developed neck, there are many cervical vertebras, among them an atlas and an axis, all except the last two fused with the corresponding cervical ribs. The skull (fig. 637) resembles closely that of the lizards in the presence of a single occipital condyle, in the movable condition of the quadrate upon the cranium, and in the presence of a slender columella. On the other hand an os transversum is lacking. The cranium, as a result of the increase in size of the brain, is more spacious; the bones of its walls fusing early so that the sutures Pal FIG. 637.— Skull of young bustartt. (From Glaus.) Als, alisphenoid ; Ang, angulare ; Art, articulare; £), dentary ; Et, mesethmoid ; Fr, frontal ; Jmx, premaxillary ; J, jugal ; L, lachrymal ; MX, maxillary ; JV, nasal ; 01, exoccipital ; Os, supra- occipital ; P tne ear is highly organized, z, pineaiis. the lagena of the labyrinth being greatly en- larged and the sound-conducting apparatus (columella, tympanum, 2 V. VERTEBRA TA : A VES. 611 etc.) well developed. The beginnings of an external ear are seen in the deeper position of the drum membrane. Since the power of flight necessitates vision at great distances, most birds have exceed- ingly sharp sight, and the eye itself (fig. 642) is in general con- re Op FIG. 642.— Eye of owl. (From Wiedersheim.) Oi, choroid; CM, ciliary muscle; Co, cornea; Cv, vitreous body ; Ir, iris : L, lens; Op, optic nerve; OS, sheath of nerve ; P, pec ten; Rt, retina; Sc, sclera; VK, anterior chamber; t, sclerotic bones. structed for distance. Peculiarities of the bird's eye, already weakly developed in the reptiles, are the pecten (P), a comb- shaped growth of the choroid into the vitreous body, and the scleral ring, a circle of bones developed in the sclera and support- ing the outer part of the eye. Among birds there is spirited rivalry for the females, especially among polygamous species. At the time of mating the males seek to win the favor of the females either through striking motions (dances), by singing, or by beauty of plumage. All of these peculiarities are confined to the male and frequently lead to a marked sexual dimorphism. The dis- tinction in plumage is commonly strengthened at this time, the male receiving the brilliant wedding dress. Thus we speak of the spring molt, although there is only a color change and only exceptionally a renewal of the feathers. The return to every-day clothes only occurs with a molt, and this comes at the close of the reproductive season. The reason for the dull plumage of the female is due to the fact that she usually sets on the nest, at which time inconspicuous colors protect her from destruction by enemies. In only a few instances is the heat neces- sary for incubation produced by other causes, such as the heat of the sun upon the sand in which the eggs are buried, or the increase of temperature caused by fermentation in decaying vegetation (Megapodes). The rule is 612 CHORD AT A. that both sexes build the nest, which with the weaver birds is most skil- fully constructed; occasionally among social species the nests are placed under a common roof. When the clutch of eggs is complete the female (rarely the male) begins the incubation, at this time in some instances losing the feathers from certain regions the better to warm the eggs. Many birds, like hens and ducks, are so far advanced when they leave the nest that they can follow the mother and feed themselves. Such birds are called Prsecoces — in contrast to the Altrices, which hatch with incomplete coat of feathers and therefore need the warmth of the nest and the pro- tection and care of the parents. The migrations of birds possess great interest. We distinguish among birds permanent residents and others which, in order to obtain food, take long journeys, the migratory species. At the approach of cold weather these seek the south, following regular paths in their travels. They can- not, like reptiles and amphibians, hibernate at the period when insects and fruit are scarce, because their greater intelligence and their more ener- getic vital processes demand a more rapid metabolism and a continuous food supply. Hence the birds, like the mammals, in contrast to the 'cold-blooded ' reptiles, amphibia, and fishes, maintain, under all extremes of external temperature, a body heat of 38-40° (44° ?) C. (100-104° F.). The classification of birds is in a state of change. The older system based upon adaptive characters is not in harmony with the results of care- ful anatomical study, which would divide the whole class into many small groups. For this reason it has been thought best to retain the older sys- tem of larger, easily recognized divisions, and to call attention, where necessary, to the contradictions with later results. Order I. Saururae. The view that birds are closely related to reptiles has received considerable support by the discovery of fossil birds with teeth. The most reptilian of these occur in the Jurassic of Bavaria, and only two specimens have been found. In these (Arch&opteryx lithographica) the carpals and metacarpals have not fused, the three fingers are well developed and clawed, and the caudal verte- brae, although bearing feathers, form a long slender tail like that of a lizard (fig. 2). Order II. Odontornithes. These forms, from the cretaceous of Kansas and Colorado, also had teeth. In the ODONTOEM^: (Ichthyomis) there was a keeled sternum and normal pygostyle. In the ODONTOHOLC^E (Hesper- ornis) the wings were reduced (only the humerus persisting), the sternum was without a keel, and the caudal vertebrae formed a broad paddle. Order III. Ratitae. Here are included several families, very different in structure, which agree in having the feathers not arranged in feather tracts;, IV. VERTEBRATA: AVE8, CARTNAT^E. 613 and in that, together with the lack of flight, many structures normally connected with it are absent. The bones are but slightly pneumatic, the sternum has no keel, and a furcula is not formed, the clavicles being rudimentary (Dromceus] or not present as dis- tinct bones. The wings are small and lack primaries and seconda- ries adapted for flight, for typical contour feathers with close vanes, as well as typical down feathers, are absent. Since several structures apparently adapted for flight occur here (fusion of hand bones and often of caudal vertebrae ; arrangement of wing muscles), it is probable that the Ratites have descended from carinate forms by loss of power of flight. The anatomical distinctions between the various families lead one to believe that they have arisen from different groups of carinates and hence do not form a natural assemblage. Section I. STRTJTHIONES, with long humerus, long legs and neck. STRUTHIONIDJE, two-toed ostriches of Africa, Struthio camelus. RHEID^E, South American three-toed ostriches, Rliea americana, nandu. Section II. CASUARINA ; three toes, humerus short. Dromceus, emus; Casu- arius, cassowaries. Section III. APTERYGES, bill long, nostrils near the tip, rudimentary wing skeleton; four toes. Apteryx, kiwi, of New Zealand. The DINORNITHID^E, three toes, wing skeleton absent ; giant birds (thirteen feet high) of New Zealand; now extinct, but apparently contemporaneous with man. The JEpiornis, a gigantic bird of Madagascar, possibly belonged near these. Skeletons and eggs holding two gallons found in alluvium. Order IV. Carinatae. The name refers to the presence of the keel to the sternum, which is correlated with the powers of flight possessed by most species. Other characters of the class are the presence of rectrices and remiges on tail and wings, and the fusion of clavicles to a furcula. There are strong fliers, like the raptores and albatrosses, which have but a small carina ; in many poor fliers the carina may be entirely absent. The furcula is not always present, the clavicles not uniting (many parrots and toucans) or being absent (Mesites). The remiges are also degenerate in some carinates, as in the pen- guins (which are flightless, although they have a strong carina), where they take the shape of small scales. Thus the distinctions between ratite and carinate birds vanish in places. Sub Order I. GALLINACEA. The hen-like birds are praecoces with compact bodies and well-developed wings and legs, so that they run and fly well without excelling in either direction. The feet have three toes in front, usually connected by a membrane at the base (fig. 643, c); the fourth toe is behind and at a higher level. Above this in the male is usually the 614 CHORD ATA. spur, a process of the tarso-metatarsus, covered with horn. The margins of the upper jaw overlap the lower; the beak is bent downward at the tip and is about as long as the head. Naked, richly vascular lobes form comb and wattles which are specially large in the more elegantly plumaged males. The PHASIANID^E are polygamous; Phasianus, with many species of pheasants; Gallus bankiva of the Sunda Islands, the ancestors of domestic FIG. 643.— Foot forms. (From Schmarda.) a, semi-palmate, wading of Ciconia ; ^perch- ing of Turdus ; c, rasorial of Pliasiu-uua ; d, raptorial of Falco ; e, adherent of Cypselus ; /, cursorial of strut hio ; 0, zygodactyl (scansorial) of Pious ; h, lobate of Pnrticcps ; i, lobate and scalloped of Fulica ; fc, palmate of Anas ; I, totipalmate of Phaethon. fowl. Meleagris* the turkeys. The TETRAONID^E are partly polygamous, partly monogamous. Coturnix,* quail ; Perdix* partridge ; Bonasa,* grouse. The incubation of the Megapodes has been referred to (p. 611). Sub Order II. COLUMBINE. The pigeons are distinguished from the Gallinacese by the more slender bodies, shorter legs, the toes free, and the longer wings capable of prolonged flight. They are altrical ; the crop produces a milky secretion used in feeding the young. The COLUMBID^E are the most widely distributed and are represented in the tropics by numerous beautifully colored species. Columba.* According to Darwin the domestic pigeons come from C. Uvia, the blue rock pigeon ; Ectopistes migratorius* passenger pigeon, practically exterminated. Allied was the dodo, Didus ineptus, of Madagascar, exterminated in the eighteenth century. Sub Order III. NATATORES. A number of families, while differing much in structure, are united by their inclination for an aquatic life. They are called swimming birds (Natatores) because, thanks to their IV. VERTEBRATA: AYES, CARINAT^E. (U5 webbed feet, they are excellent swimmers and divers. Either all four toes are connected by the web (totipalmate, fig. 643, Z), or only the three anterior toes are webbed (palmate, fig. 643, k), or the three toes are each bordered with a swimming membrane (lobate, fig. 643, h). Thus the foot struc- ture gives distinctions which forbid a closer association of the families, and this is strengthened by differences of wing and beak. On the other hand palatal structures show that here, as in the Grallatores, very diverse forms- are associated. Section I. LAMELLIROSTRES (Anseriformes), feet palmate; the beak soft- skinned up to the hard tip, its margins with transverse horny plates. Anas boschas* wild duck, source of domestic breeds. A. mollissima, eider; Anser* goose (domestic derived from A. ferus). Cygnus* swans. Sec- tion II. TUBINARES (Longipennes), predaceous birds with strong beak, tubular nostrils, palmate feet, and long wings capable of rapid and pro- longed flight. Diomedea, albatross; Larus,* gulls ; Sterna,* terns. Sec- tion III. URINATORES. Birds with small wings, sometimes reduced to- flippers, and upright position owing to position of the legs far back. The ALCID^E (Alca impennis* the great auk, exterminated in the nineteenth century), which are northern and are related to the gulls, and the antarctic IMPENNES (Aptenodytes — fig. 644, penguin) agree in having palmate feet, but otherwise differ greatly in structure. Some of the COLYMBID.E (Urinator* loons) have palmate feet, others (Colymbus,* grebes) have lobate feet. Section IV. STEGANO- PODES, with totipalmate feet. Pele- canus* pelicans; Phalarocorax,* cormorants; Phaethon,* tropic birds. Sub Order IV. GRALLATORES. The wading birds affect swampy lands and the shores of the sea, ponds and streams, their legs being lengthened, chiefly by elongation of the tarso-metatarsus, the feet semi- palmate (fig. 643, a), and the feath- ers only on the upper parts, the lower with horny plates, all feat- ures adapted to the wading life. FlG- &&•— Aptenodyt.es patagonica, penguin. Correlated is the striking length of (From Brehm-> neck and beak. These features have appeared in groups which are very different in anatomical characters. Section I. CICONIFORMES. Beak with a strong horny coat. Ardea* herons; Ibis ; Ciconia, storks ; Phcenicopterus,* flamingo. Section II. GRUIFORMES. Beak always with soft skin at the base, often extending to the tip. Grus,* cranes; Rallus* rails; Otis, bustards, terrestrial. Section III. CHARADRIFORMES. Allied to the auks and gulls. Scolopax* woodcock ; Charadrius,* plover. 616 CHORDATA. Sub Order V. SCANSORES. The climbing birds are readily recog- nized by their zygodactyle feet (fig. 643, gn, in which two toes (2 and 3) are directed forwards, the other two (1 and 4) backwards. The forms united under this head differ much in structure and their association does not rest on blood-relationship. Section I. CUCULIFORMES. The PSITTACI, or parrots, are brightly colored mostly tropical birds with short, high, compressed, and strongly bent beak .and fleshy tongue. But one species (Conurus carolinensis*) in the United States. Cacatua, Plictolophus, cockatoos; Mdopsittacus, Psittacus, parrots. CUCULI, bill slightly arched or straight ; outer toe usually versatile ; Cuculus, Coccygus,* cuckoos. Section II. PICARLE. The woodpeckers have a long, straight, conical beak and long, protrusible tongue; Picus* Nearly allied are the toucans (Rhamphastos) of the tropics. Sub Order VI. PASSERES. This is by far the richest in species of all the groups of birds. They are altrices of moderate size, with slender feath- ered tarsi and strong, horny beak without cere. Of the three anterior toes the two outer are either united or separated to the base (fig. 643, 6), while the hind toe is at a level with the rest. In some, which are usually "but not invariably noticeable for the powers of song of the males, there are special muscles to the syrinx which are lacking in other birds. These are called Oscines, in contrast to the other Passeres, the crying birds, or €lamatores. These groups are further distinguished by a large, freely movable hind toe in the Oscines, while in the Clamatores it is restricted in its motions. Section I. OSCINES. All our song birds belong here: FRINGILLID.E, finches; Passer domesticus* English sparrow; Loxia* crossbills '; ICTER- ID.E ; Icterus,* orioles ; Dolichonyx* bobolink; ALAUDID.E, Alatida* sky- lark ; SYLVICOLID^:, Dendrceca,* Helminthophaga* warblers; TURDED^B, Turdus* thrushes; Siala* bluebirds; HIRUNDINID.E, Hirundo* swallows; TROGLODYTID^S, wrens; CORVID^E, Corvus* crows; Cyanocitta* jays. The PARADISEID^E, or birds of paradise, with marked sexual dimorphism, are closely related to the crows (fig. 15). Section II. CLAMATORES. Here are frequently included a few groups (COTINGID^E, TYRANNID^E) best developed in South America and the lyre birds (MENURID.E) of Australia. Earlier other forms were regarded as allied, but now are separated as Cypselo- morphre, or Coraciformes, and united with the owls and Picarise. CYPSELE- D^E ; Chcetura* chimney 'swallow,' with adherent feet (fig. 643, c). TROCHILID.E, humming birds, best developed in tropical America; Trochi- lus* CAPRIMULGID.E, night hawks ; Antrostomns vociferus* whippoor- will. ALCEDINID.E, kingfishers, Ceryle* BUCERONTID^E, horn bills, tropical. Sub Order VII. RAPTORES. The birds of prey are strong birds of considerable size. They have the tarso-metatarsus feathered and four strongly clawed toes of what is termed the raptatorial type (fig. 643, d). The beak is strong, the upper half, strongly hooked at the tip, extending over the lower. There are two groups recognized which probably are not closely related. Section. I. FALCONIFORMES. Slender birds with close plumage and extraordinary sight; related structurally to the herons. CATHARTIDJE, IV. VEETEBRATA: MAMMALIA. 617 buzzards ; Cathartes aura* turkey buzzard. PANDIONID.E, Pandion halmtus* fish hawk; FALCONID^E : Aquila* Halicetus,* eagles ; Buteo* buzzards; Falco* falcons; Accipiter* hawks. Section II. STRIGES, owls; compact birds with loose, fluffy plumage, large eyes in a circle of feathers; more closely related structurally to the Caprimulgidae than to the Falconiformes. Bubo,* horned owls; Scops,* screech owls; Strix* gray and brown owls ; Speotyto* burrowing owls. Class III. Mammalia. The mammals occupy the highest place among the vertebrates, and consequently in the animal kingdom; they also possess a special interest for us, for man, in structure and development, belongs to the group, although separated in intelligence from the most highly organized of the members by a wide gap. The most striking characteristics of the mammals again are furnished by the skin. In fact one may, with Oken, call them hair-animals, since hair is as diagnostic as feathers are for birds. The hairs (fig. 645, H) are cuticular structures which are seated FIG. 645.— Section of skin of man. (From Wiedersheim.) Co, derma (corium); D, oil gland; F, fat; (?, blood-vessels; GP, vascular papilla; If, hair; JV, nerves; J!VP, nerve papilla; Sc, stratum corneum ; £D, SD\ sweat gland and duct; £M, stratum Malpighii. on papillae of the derma, and are nourished by blood-vessels in these. The lower end, the root of the hair, lies in a pit in the epidermis, the hair follicle, and is surrounded by a double envelope, the epithelial root sheath, formed by an inpushing of the epidermis and an outer connective-tissue follicular sheath. Small muscles attached to the base of the larger hairs serve for their erection. 618 CHORDATA. Since side branches are lacking, the structure of the hair is more simple than that of feathers, and the forms fewer. Wool is char- acterized by its spiral turns; then there is straight hair which, by increase in size, forms the * whiskers ' (vibrissse) on the upper lip of many mammals, bristles (swine), and lastly the spines of hedge- hogs and porcupines. In the pelts of many animals two kinds of hair may occur, wool below and straight hair outside. Histolog- ically hair consists of cornified cells, often arranged in medullary and cortical layers. On the outside there may be another layer recalling the pseudocuticula of reptiles. In most mammals there is a periodic shedding and renewal of the hair, the new hair aris- ing from the old follicle (? from the old papilla). Ordinarily this occurs only in spring. Besides hair some mammals have true scales. Constant horny structures are the armatures of the tips of the digits, which, according to form, are divided into claws (ungues), hoofs (ungulae), and nails (lamnae). The old view that the hair, like feathers, corresponds to the scales of reptiles has recently found both defenders and opponents, the latter think- ing it probable that the hair has arisen from the nerve-end structures of aquatic vertebrates. The claws, together with those of reptiles and birds, must have come from horny scales, which indeed occur in many amphibia as hollow cones capping the toes. The dorsal part of this scale, the claw plate, becomes especially strong, its formation taking place at the base, the root, from whence it is forced forward over the bed (in man the limit of nail formation is shown by the lunule). The ventral part of the scale, the subungua or solenhorn, is poorly developed in true claws because its region is restricted by the arching of the claw plate in both directions, but is more evident in hoofs, in which the plate is curved only horizontally. In the horse it forms the * sole,' lying between the frog and the hoof. It is rudimentary or entirely lost in the nails of apes and man. The skin of mammals is further characterized by its richness in glands, of which, with few exceptions, there are two kinds, sebaceous and sweat glands. The first are acinose glands, and usu- ally open in the hair follicles, giving the hair the required oiliness (fig. 645, D}. The sweat glands, except in the monotremes, are entirely independent of the hairs, and are simple tubes, coiled at their deeper ends (SD), secreting a fluid sweat which is of great value in the preservation of a constant temperature, its evapora- tion cooling the body. Under the influence of sexuality the glands in certain regions, and especially the sebaceous glands, develop great activity and form considerable glandular pouches or pockets : caudal and anal glands of many carnivores, hoof glands and sub- orbital glands of ruminants, musk and castor glands of musk deer IV. VERTEBRATA: MAMMALIA. 619 and beaver (fig. 652, a). More important than these are the modi- fications of dermal glands into mammary or in ilk glands, which, indeed are so characteristic that they have given rise to the name mammalia. These are almost invariably sebaceous glands (in the monotremes sweat glands) which empty in great numbers upon a restricted area of the skin, which, except in monotremes, is elevated into true nipple (fig. 646, A), or around which the adjacent skin B FIG. 646.—^!, true, B, false nipple. (After Gegenbaur.) becomes elevated in tubular form (B) as in the cows. The mam- mae are always symmetrically arranged upon the ventral surface, sometimes in the breast region, but more frequently in the inguinal region. There are at least two, usually more (22 in Centetes). In general the number corresponds to the maximal number of young at a birth. A dermal skeleton occurs in few species (e.g. , the firm bony plates of the armadillos) ; on the other hand the axial skeleton shows many features not occurring elsewhere. In the skull many ol the bones already referred to are evident only as centres of ossi- fication, fusing early with their neighbors to form larger bones. As the temporal bone shows, parts of diverse origin may fuse — parts of the visceral skeleton and parts of the cranium; membrane and cartilage bones — so that a sharp line between cranial and facial portions cannot be drawn. So it becomes necessary in describing the skull not to follow exactly the model adopted so far, but to take that of human anatomy. In the hinder region of the skull is a large occipital bone (figs. 561, 562), jointed to the atlas by double occipital condyles, and arising by the fusion of the four bones of the occipital region. Besides it includes usually a membrane bone, the interparietal, which occurs only in mammals. This is, strictly speaking, a paired bone, arising in the angle between the parietal and the supraoccipital and fusing with the latter. In front of it lie in the rool oi the cranium, as in other vertebrates, the parietals (fused with the interparietals in many ruminants and rodents), the fiontals and nasals, the lachrymals being always associated with 620 CHORDATA. them. In the floor of the cranium the sphenoid bone lies in front of the basioccipital portion of the occipital. In many mammals this consists of an anterior and a posterior portion throughout life ; in man this condition occurs at least in the embryo. Each of these parts in development consists of three elements, the posterior of the basisphenoid as the body, and the paired al [sphenoids (great wings); the anterior is similarly composed of the presphenoid and the paired orbitosphenoids (lesser wings) (fig. 562, Spb, Ps, Ah, Ors). In front of the sphenoid lies the ethmoid, Eth, likewise formed from three parts, the mesethmoid, which forms a partition between the two nasal cavities, and the paired ectethmoids, which form the lateral walls of the nasal cavities. These last have com- -fia. os. Fio. 647.— Skull of embryo Tatusia. (After Parker, from Wiedersheim.) Cartilage dotted, membrane and membrane bones lined, o, incus (quadrate); de, dentary; /r, frontal; h, (above) membrane over anterior fontanelle, (below) hyoid bones; iw, premaxillary: ju, jugal (malar) ; kb, remnants of gill arch; to, lachrymal; m/f, Meckel's cartilage; mx, maxillary; n, malleus (articulare); rw, nasal: <>, oc- cipital cartilage; os, supraoccipital; pa, parietal ; pe, petrosal ; sg, squamosal ; .sf, stapes; tj/, tympanic. plicated folds on their inner surface, the superior and middle turbinated bones, which support the olfactory membrane, thus greatly increasing its surface. With these is associated the os tur- binale, a distinct bone, the inferior turbinated bone of human anatomy. The temporal bone, which is intercalated between the roof and floor of the skull, can only be understood by its embryonic rela- tions and its connexion with the first and second visceral arches (fig. 647). Its centre is formed by the petrosal (pe), developed in the walls of the otic capsule, to which, as elsewhere in the vertebrates, are attached: (1) the cartilaginous jaw arches, the quadrate (a), and the mandibular (n and mlc} ; (2) the cartilaginous IV. VERTEBRATA: MAMMALIA. 621 hyoid arch, the stapes (in part equalling the hyomandibular, st), and the hyoid proper (h) (compare with the visceral skeleton of the selachian, fig. 588). To these are added the membrane bones, the squamosal (sq), at the base of the quadrate, which increases as the latter loses in size, and below the squamosal the tympanic (ty). With ossification of the cartilaginous parts several centres form the petrosum, which fuses with the squamosal, and frequently with the tympanic, which in some forms enlarges to a conspicuous bulla ossea. Petrosum and squamosal on the one side, tympanic on the other, enclose a space, the tympanic cavity, into which the upper parts of both visceral arches extend, ossifying into the ear bones, the quadrate to the incus, the hyomandibular possibly to stapes (fig. 577). The tympanic in uniting with the squamosal (forming Glaser's fissure) encroaches on the mandibular cartilage so that the upper end (n), which is homologous with the articulare of other ver- tebrates, is enclosed in the tympanic cavity and, along with a sec- ond bone, the angulare, ossifies to form the malleus, while the lower portion, Meckel's cartilage proper (mk), becomes pinched off. Meckel's cartilage gradually disappears; on the other hand the surrounding membrane bone, the dentary (de) increases and alone forms the lower jaw, which now forms a new articulation with the squamosal. It will be noticed that the old articulation was between cartilage bones, the new between membrane bones developing around the cartilages. (There is, however, some evi- dence to show that the mammalian lower jaw consists of several bones, some of them preformed in cartilage, and that one of these forms the articulation with the squamosal.) The lower part of the hyoid arch, the hyoid, remains outside the ear and often fuses with the petrosal. The upper end (styloid process) may then become entirely separate from the lower, which becomes attached to the copula (body of hyoid) as the anterior horn, the connecting cartilage being reduced to a stylohyoid liga- ment. In the hyoid of mammals there is also included a remnant of a gill arch as the posterior horn. As the quadrate, by its modification into the incus, becomes strikingly reduced, the rest of the arch — vomer, palatine, and pterygoid — is poorly developed in contrast to the large maxillary bones. Premaxillaries and maxillaries (fused in man to a single bone) form an important element in the face, and send backwards and inwards palatine processes into the roof of the mouth. These encroach upon the bones of the palatal series; the vomers of the 622 CHORDATA. two sides are pressed together to a single bone lying vertically entirely within the nasal partition; the palatine and pterygoid are forced backwards. The palatines contribute to the hard palate, the pterygoids only exceptionally (Cetacea, many edentates); the latter usually lose their independence and fuse with the nearest bone of the base of the cranium, the basisphenoid (more accurately with a process of the basisphenoid, the lamina externa of the pterygoid process, the pterygoid forming the lamina interna). Thus the hinder sphenoid, like the temporal, contains cranial and visceral elements. In the vertebral column the cervical and the rib-bearing thoracic vertebrae are always distinct, and the same, with the ex- ception of the Cetacea and Sirenia, is true of lumbar, sacral, and caudal vertebrae. Of sacrals there is one in all embryos, and throughout life in the marsupials, elsewhere from two to five, rarely, as in edentates, as many as thirteen. The number of ver- tebrae in each group is rather restricted. Thus, except in Brady- pus tridactylus (9), Cholcepus hoffmanni and Manatus (6), the number of cervicals is always seven. Of the appendicular skeleton the girdles are most interesting. The coracoid, which in mono- tremes reaches the sternum, is reduced in all other mammals to a small coracoid process of the scapula. More rarely the clavicle is lacking (rapid runners); in the monotremes it extends to the episternum (fig. 648, Cl, Ep)\ elsewhere it appears to articulate with the sternum, in reality by the intervention of interarticular cartilages (once regarded as a FIG. 648.— Sternum and shoulder girdle of Ornithorhynchus paradoxm. (From rudimentary episternum, now Wiedersheim.) Cl, clavicle; Co, Co', i • N T coracoid; Ep, episternum; G, glenoid Called preclaviae). In the pelvis fossa for humerus; S, scapula; St, ,, ,-, , , * . manubrium sterni (anterior element all three elements are lUSed to a of sternum). • i • • -i • single os mnommatum; pubis and ischium unite ventrally with each other, enclosing between them the obturator foramen (fig. 655). The pubes of the two sides unite by a symphysis which can extend back to the ischia. Since the mammals in general are distinguished from other vertebrates by their intelligence, the brain is characterized by the size of cerebrum and cerebellum (fig. 649). In contrast to birds IV. VERTEBRATA: MAMMALIA. 623 and fishes, the cerebellum (IV) is differentiated into a median verrnis and lateral cerebellar hemispheres. In the cerebrum the mantle comes first into consideration. Its frontal lobes grow for- wards over the olfactory lobes, which consequently lie farther and farther back on the lower surface. The temporal lobes extend right and left over the optic lobes and down to the floor of the cranium ; the occipital lobes cover successively the mid brain, cere- bellum, and medulla oblongata. Since the greatest increase of intelligence lies within the mammals, the cerebra may be arranged in an ascending series. In the monotremes, marsupials, insectivora, and rodents (fig. 649, A) the olfactory lobes are A B C -to x. Y-^a FIG. 649.— A, brain of rabbit (after Gegenbaur); J3, of fish otter. <7, of pavian monkey (after Leuret and Gratiolet). I, cerebrum; III, optic lobes; IT7, cerebel- lum; V, medulla oblongata; to, olfactory lobes. visible in front, usually the mid brain behind (/// ). In the lemurs, carnivores (fig. 649, B), and ungulates the olfactory lobes are completely, the cerebellum partly, covered. In man and the anthropoid apes, on removing the roof of the skull, only the two cerebral hemispheres are visible, all other parts being more or less completely covered. Further, it is to be noted that in the first group the surface of the cerebrum is smooth, while in the others the cortex is increased by infolding and the formation of convolutions (gyri and sulci) which reach their greatest complication in the anthropoid apes and especially in man. A consequence of the increase in size of the brain is the great development of the connecting nerve tracts, which become more and more prominent as parts of the brain. Thus the two halves of the cerebrum are connected by a large transverse tract, the corpus callosum; two solid cords, the crura cerebri, run back from the cerebrum to the other parts, while a transverse commissure, the pons Varolii, passes below, connecting 624 CHORDATA. the two sides of the cerebellum. These connexions in the other vertebrates are small, and even in the lower mammals, like mono- tremes and marsupials, are but slightly developed. The increase of cerebrum and cerebellum, which occurs chiefly in the dorsal portion, has resulted in flexures in the axis of the brain, already in- dicated in the reptiles, increased in the birds, and reaching their maximum in the mammals. Instead of continuing in the course of the spinal cord, the axis of the brain bends ventrally in the medullar region (cervical flexure), then in the region of the pons again dorsally (pontal flexure), an at the level of the optic lobes again ventrally (cephalic flexure). By its increase in size the brain has influenced the skull in an interesting way ; for, while even in birds the brain is almost entirely confined to the region behind the eyes, in the higher mammals it has extended forward to the olfactory region. Thus there comes an increase of the cranium at the ex- pense of the face. The relative sizes of the two were adopted by Camper as an index of intelligence, and were measured by l Camper's angle,' a method which has since undergone considerable improvements. Of the sense organs the nose is characterized by three features. An outer nose, supported by cartilage and often extended as a proboscis, has been formed. Its cavity has been increased, since by the formation of hard and soft palate a part of the primitive mouth cavity has been included in it. Its upper portion, the olfactory region, has been complicated by the formation of olfactory folds, supported by the turbinated bones already referred to (p. 620). To increase the mucous surface there are extensions of the nasal cavity, frontal, maxillary, and sphenoidal sinuses, into the corresponding bones. The eye has the upper and lower lids, besides the nictitating membrane in a more or less reduced condition. The ear, except in monotremes, Cetacea, Sirenia, and some seals, has a conch supported by cartilage, while the external auditory meatus is always present. Internally the ear is much modified, since the three bones, malleus, incus, and stapes (p. 544), occur nowhere else, while the lagena has been greatly lengthened, coiled into a spiral with two to four turns (figs. 80, 576), while inside the wonderful organ of Corti has been developed. Of digestive structures, the teeth — which are restricted to max- illary, premaxillary, and dentary bones — need special mention, because of the distinctions they afford from all other vertebrates, and because of their importance in differentiating the various orders. If we omit the monotremes, edentates, and whales, in which there is marked degeneration in the dentition, there are four particulars which show the dentition of mammals more de- veloped than that of other vertebrates. (1) The number of teeth IV. VERTEBRATA: MAMMALIA. 625 is constant for the species, usually for the genus, and often for the family. As man normally has thirty-two teeth, so the dog has forty-two, the anthropoid apes thirty-two, the platyrhine apes thirty-six, etc. (2) The teeth are firmer. The body of dentine is divided, by a slight constriction, into a crown covered with enamel, and a root enveloped in cement (bony tissue). The roots are placed in separate sockets (alveoli) in the jaws, and in those cases where continuous growth is necessary the pulp persists and the teeth, as in the incisors of rodents and the tusks of elephants and pigs, grow indefinitely. (3) In consequence of their greater firmness the teeth are not used up so fast and do not require rapid replacement. There occurs only one change, in which the denti- tion present at birth or developed soon after — the milk, or lacteal, dentition or, better, first dentition — is replaced by the second or permanent dentition (diphyodont mammals). In some cases (monophyodont mammals) there is no change, the first dentition being permanently retained (marsupials, perhaps toothed whales), or the first dentition is more or less rudimentary (edentates, many rodents, bats, seals, some insectivores). Besides the two typical dentitions traces of a third or even of a fourth may occur. A prelacteal dentition of calcified germs which are never functional is best seen in marsupials, and is rare in placental mammals. A dentition following the permanent one is outlined in many placen- talia, and some of its teeth may exceptionally come into function. (4) Among the teeth a division of labor has brought about change of form (heterodont dentition). The teeth of the premaxillaries and their antagonists in the lower jaw are single-rooted and usually have more or less a chisel shape, hence they are called incisors even when, as in in- sectivores, the crowns are needle-like (fig. 661). Behind the incisors (in the maxillary bone in the" upper jaw) is the canine tooth (fig. 650, c), which is single-rooted and has usually a conical crown (probably a modified premolar). Following the canine come the mo- lars, broad teeth mostly with two roots FIG. 650. — Permanent and milk , J _ _ ,. dentitions of the cat. (From ana tubercular crowns. Only the an- Boas.) c, canines; pa-p4, pre- , ., .,, _ molars; m', molar (the milk den- terior ones appear in the milk den- tition darker and each letter tition, while the others appear only in the permanent dentition and are not replaced. preceded by d). On this develop- 626 CHORD AT A. mental basis the molars are divided into premolars (bicuspids of dentists), which appear in both dentitions, and the true molars, which occur only in the last. From the foregoing it will be seen that every species of mam- mal is characterized by its dentition, and these features may be expressed by a short formula. It is only necessary to place the number of each of the four kinds of teeth mentioned in their regular order, those of the upper jaw separated from those of the lower by a horizontal line, to express this. Since the two sides of the body are symmetrical, only those of one side need be enumerated, and in case that one kind be absent the deficiency is indicated by a zero. The dental formula of man would thus be fff|; of the rein- deer, in which in the upper jaw incisors and canines are absent, •JJf-f. The different formulae, by comparison, give us a funda- mental formula from which they have been derived by reduction. This was probably The molars undergo, according to the food, the greatest modification of form. As a starting point the bunodont tooth may be taken which occurs in omnivorous mammals and which has the crown with several blunt projections or cones. With animal food (fig. 650, 657) the cones be- come sharper and cutting (secodont dentition of carnivores and insec- tivores), and when the cutting angle becomes very sharp, with a special prominence on the inner side, it is spoken of as a flesh or carnasial tooth. In vegetable feeders the cones become connected by crests (lophs) or are half-moon-shaped (lophodont or selenodont). Since the cones and lophs become in part worn away and the grooves between them are filled with cement, there arise broad grinding surfaces strengthened by the harder and more resistant enamel of the cones and lophs ; this extends inwards as folds from the outer enamel wall of the tooth ; the folds may become cut off and form islands of enamel on the grinding surface (dentes complicati of ungulates). When the folds extend in regular order from the outside and inside and meet in the middle they form numerous successive leaves, bound together by cement (compound teeth of elephants, fig. 667, and many rodents). Paleontological investigation, with which the more recent erabryologi- cal results are in accord, has shown that a great regularity prevails in the formation of the cones of the molars. Triconodont and tritubercular teeth are recognized, in which the three cones are either arranged in a line or in a triangle, as well as multitubercular teeth with more numerous cones irregularly arranged. The triconodont type develops farther by the formation of secondary cones. The development of these occurs in dif- ferent ways in molars and premolars. Since the latter are the more sim- ple, their distinction from the molars does not rest alone upon the existence of a milk dentition, but upon structure as well. This is important, because it happens that there are premolars which are not replaced (marsupials, IV. VERTEBRATA: MAMMALIA. 627 many insectivores and rodents) and, on the other hand, beneath the molars the anlagen of replacing teeth may be found. The latter fact shows that the molars, strictly speaking, belong not to the permanent but to the milk dentition. They are late in formation and are therefore parts of the first dentition carried over into the second. The mouth, which contains tongue and teeth, is separated from the next division of the alimentary tract, the pharynx, by the uvula. The pharynx narrows behind into the oesophagus, the en- trance of which into the stomach is marked by a constricting cardia. At its other end the stomach has a similar constrictor, the pylorus, separating it from the intestine. In the latter small and large intestines (the latter consisting of colon and rectum) are differentiated by the diameter of the lumen. The small in- testine opens laterally into the colon and at the junction arises a blind diverticulum, the caecum, which is small in mammals with animal food, but in herbivores (especially rodents) is always large and forms a conspicuous part of the alimentary tract. The ver- miform appendix (primates, rodents) is a narrower part of the caecum. Three pairs of salivary glands empty into the mouth, the liver and pancreas into the small intestine (duodenum). Most important of respiratory peculiarities is the diaphragm, which separates the body cavity into thoracic and abdominal cavi- ties. This occurs only in its beginnings in other vertebrates (perhaps even in Amphibia). In the thoracic cavity are the O3sophagus, heart with its pericardium, and especially the trachea, bronchi, and lungs; the remaining vegetative organs are in the abdominal cavity. The diaphragm is a muscular dome, its con- vex side towards the thoracic cavity; by contraction it flattens an increases the size of the cavity, in consequence of which air is drawn into the lungs (inspiration). On relaxation the lungs con- tract from their own elasticity and force out a part of the air (expiration). The intercostal muscles, which raise and lower the framework of the chest, also play a part, as in birds. The respira- tory ducts (fig. 579) begin with the larynx (with vocal cords), which can be closed from the pharynx by the epiglottis; this is followed by the trachea, which divides into right and left bronchi. Each bronchus divides again and again, and the finest of these divisions, the bronchioles, are continued as alveolar ducts to small chambers, the infundibula, both these and the alveolar ducts being lined with small respiratory pockets, the alveoli. The heart, with two auricles and two ventricles, is completely separated into systemic and pulmonary halves. In early embryonic 628 CHORD AT A. life the arterial trunk, which at first is simple, is divided into a pulmonary artery, arising from the right half of the heart and carrying venous blood, and an aorta ascendens, with arterial blood, connected with the left half. In contrast with the reptiles, the right aortic arch is entirely lost, the left persisting. The urogenital system is of great importance in the separation of the group into smaller divisions (fig. 651). In both sexes this consists of practically the same parts in early embryonic life. These are the early formed Wolffian body ( W ) ; the permanent kidneys, which appear later and are not shown in the diagram ;. FIG. 651. FIG. 65i. FIG. 651.— Diagram of embryonic mammalian urogenital system. (From Balfour,. after Thompson.) cl, cloaca; cj>, genital process: go, genital cord; i, rectum; te, ridge for formation of labia or scrotum ; rn, Mullerian duct ; ot, gonad ; mi, urogenital sinus ; W, Wolfflan body ; w, Wolffian duct; 3, ureter; A, urinary blad- der ; 5, continuation of latter to allantois (urachus). FIG. 652. — Urogenital system of male beaver. (From Blanchard.) a, castor eum sacs ; b, openings of their ducts into preputial canal; c, tip of penis ; d, preputial opening ; e, anal glands; f, their ducts; 0, anus ; 7i, base of tail; i, corpora caver- nosa ; 7c, Cowper's glands; /, seminal vesicles; 7n, vasa deferentia; n, testes; o, urinary bladder with ureters. the urinary bladder (4), a part of the allantois which extends (5) into the foetal appendages; the three ducts, the Mullerian (m), the Wolffian (w), and the ureter (3). These ducts no longer empty into the intestine, but into the allantoic structures, the ureters into the base of the urinary bladder, the Wolffian and IV. VERTEBRATA: MAMMALIA. 629 Miillerian ducts into the urogenital sinus (ug}, the lower continua- tion of the bladder. The gonad is connected with the Wolffian duct. In the anterior wall of the urogenital sinus is a mass of highly vascular tissue (cp), from which and a surrounding fold the external genitalia are developed. Since the urogenital sinus opens from in front into the intestine, there is always a claoca (cl) in the embryonic stages, which persists throughout life in the mono- tremes, and to a considerable extent in the female marsupials; in all other vertebrates it is divided by a partition, the perinaeum, into a urogenital opening in front and an anal opening behind. From this indifferent condition the male and female apparatus are derived, the structures being closely similar in most males (fig. 652). The Miillerian duct vanishes, while the Wolman duct be- comes the vas deferens and its accessories, serving as a canal for the genital products, while the external genitals arise from the other parts mentioned, these forming an intromittent organ (penis). In the female the Wolffian body and duct degenerate, the Miillerian ducts become the reproductive canals. The modi- fications of these become of great systematic importance. In the monotremes both ducts open separately and become differentiated into two parts (fig. 653, A), anterior oviducts with wide openings A. FIG. 653.— Female genitalia of (^4) Echidna aculeata; (B) of Didelphys dorsigera; (O Phascolom us wombat. (B and C, after Wiedersheim.) c/, cloaca; d, rectum; /*, urinary bladder; n, kidney; o, ovary; od, oviduct; pu, month of ureters; s?t, uro- genital sinus; f, ostium abdominale tubae; rt, uterus; it', opening into vagina; wr, ureter; r, vagina; vb, vaginal blind sac. into the body cavity (od, t) and the uterus («). The ureters open into the sinus (and not into the bladder) between the uterine openings. In the marsupials (B and C) there are three divisions, oviduct, uterus, and vagina; besides, the two Miillerian ducts may approach, near the uterus (B}, or fuse in this region (C') in some 630 CHORD ATA. species, forming an unpaired blind sac (vJ), which may even open into the urogenital sinus as a third vagina. This partial fusion of the vaginae of the marsupials is completed in the placental mam- mals, the single vagina and the sinus forming a single canal (fig. 654). Here the uterine portions may remain distinct (uterus A FIG. 651.— A, uterus duplex; B, uterus bicornis; C, uterus simplex. (From Gegen- baur.) od, oviduct; i/, uterus; v, vagina. duplex of rodents, A), or they may fuse partially (uterus bicornis of insectivores, whales, ungulates, and carnivores, J9), or they may be completely fused (uterus simplex of apes and man, C). Thus there are three different types of the female genitalia, in which the vagina is not differentiated (Ornithodelphia), or is double (Marsupialia), or is single and unpaired (Monodelphia). To these correspond three types of development. The Ornitho- delphia are oviparous, the others viviparous, but are distinguished by the duration of pregnancy. The eggs of the viviparous forms are so small (about .01 inch) that they have a total, nearly equal segmentation. Such eggs require nourishment from the mother in order to produce an animal with the complicated structure of a mammal. Since in the Didelphia the uterine nourishment is usually very incomplete, the period of pregnancy is very short, in comparison with the Monodelphia, in which a placenta, a com- plicated apparatus for the nourishment of the young, appears; hence the marsupials, with their small imperfectly formed young, are often called Aplacentalia; the Monodelphia, Placentalia. All mammals care for the young, this being chiefly or wholly done by the mother, who not only supplies them with milk but protects them in warm if rude nests. Most mammals are monogamous, some polygamous, while in others there is no permanent association of the sexes. The body temperature is constant and ranges from 36° to 41° C. (98° to 106° F.) ; in Echidna it is only 26° to 34° C. (79° to 83° F.). In most, continual feeding is necessary for existence; from this rule there are a few exceptions, like the bears, marmots, badgers, etc., which hibernate during the winter^ taking no food. At this time there is a fall in the temperature due to the diminished metabolism. IV. VERTEBRATA: MAMMALIA, MONOTREMATA. 631 Sub Class I. Monotremata (Ornithodelphia, Prototheria). A few mammals, confined to Australia and New Guinea, divided among the genera Echidna, Proechidna, and Ornitho- rhynchus, are the only living representatives of the group. They are distinguished from all other mammals by laying eggs about half an inch long, rich in yolk and with soft shells. These undergo in the uterus a discoidal (meroblastic) segmentation and are then incubated by Ornithorhynchus in a nest, by Echidna in a tempo- rary pouch (marsupium) on the ventral surface of the body. On hatching the young are nourished by the secretion of enormously enlarged sweat glands, which form two large masses to the right and left of the mid-ventral surface, and which must not be con- founded with the milk glands (sebaceous) of other mammals. Each opens on a special region of the ventral surface, which is slit-like in OrnithorUynchus, a flattened pocket in the others. Other distinctions from other mammals, which are also points of resemblance to reptiles and birds, are the strong development of the episternum and the extension of the coracoid to the ster- num (fig. 648), the termination of the ureters in the urogenital sinus and not in the fundus of the bladder (fig. 653), the exist- ence of a cloaca in both sexes, and the specifically bird-like char- acter of the female sexual organs, in which the large left ovary is alone functional, and uterus and vagina are not differentiated. But with all this it must not be forgot- ten that the monotremes have the hair, the skull, the urogenital sinus of true mammals, and in the pres- ence of marsupial bones (fig. 655, Om) show a close relationship with the marsupials. The upper end of the hyoid is connected directly or by a ligament with the cartilaginous auditory opening, while a scarcely /* visible external ear occurs. The jaws , i • i FIG. 655.— Pelvis (left side) of Ornith. are toothless and enclosed in horny orhynchus paradox™. (From Wie- sheaths, yet in the young of Orni- S*T Ilium;' r^SEIfem'SS thorhynchus there are in each jaw ^rsnpiai bone; p,osPui three pairs of multitubercular molars, which are later replaced by four broad horny plates. 632 CHORDATA. ECHIDNID.E. The spiny ant-eaters have the body covered with bristles, snout with a worm-shaped tongue used in catching insects; Echidna aculeata of Australia, feet five-toed, with digging claws ; Proechidna (Acanthoglossus) of New Guinea, three-toed. ORNITHORHYNCHID.E. The duckbills are toothless, close-haired animals with horny jaws which resemble those of a duck; the five-toed feet with a swimming web especially well developed on the fore feet. Ornithorhynchus paradoxus of Australia. Fio. 656.— Ornithorhynchus paradoxus, duckbill. (From Schmarda.) The male has a spine with a gland on the hind feet which fits in a corre- sponding pit on the thigh of the female and apparently plays a role in copulation. The oldest fossil mammals are possibly to be regarded as belonging to the monotremes. These appear in the trias and form a group, MULTITU- BERCULATA (Allotheria), which is but imperfectly known (Tritylodon, MicrolesteSj Plagiaulax). Their multitubercular teeth resemble the tempo- rary ones of Ornithorhynchus, while there are indications that the cora- coid existed as a distinct bone. Less certain are the PROTODONTA (Droma- therium, Microconodon) of the American Jurassic, of which only the lower jaws are known. Sub Class II. Marsupialia (Didelphia). These, like the remaining mammals, are viviparous. They have small eggs which undergo a total segmentation in most species, and develop in the maternal uterus, being nourished by a secre- tion from its walls. In a few species there is a placenta which, in Perameles, is allantoic in origin, in Dasyurus viverrinus possibly also from the yolk sac. In most species there is no placenta. In all there is insufficient nourishment and the young are born in a very immature condition. They are therefore carried a long time by the mother in the marsupium, a pouch formed by a fold of skin on the posterior ventral surface, into which the nipples open. The ventral surface is supported by the marsupial bones, slender rods articulated, right and left, at the pubic symphysis. Other characteristics of the marsupial skeleton are the inflected posterior angle of the lower jaw (fig. 657, a) and the rudimentary replace- ment of teeth. The milk teeth and molars (first dentition) are as a whole retained, only premolar 3 being replaced by another IV. VERTEBRATA: MAMMALIA, MARSUP1ALIA. 633 tooth; but it is in question whether this belongs to the second den- tition or is a belated member of the first. The sexual apparatus has already been described (p. 630). Marsupials are known from the secondary (Jurassic) and tertiary strata of Europe and both Americas. They were apparently then spread over the whole earth, but were crowded out by the placental mammals and persisted only as remnants (Ccenolestes and the opossums) in America, but as a richly developed fauna in Australia. In the latter region they con- FIG. 657.— Lower jaw of Thylacinus cynocephalus (from Flower), showing (a) the inflected angle chaiacteristic of marsupials; cd, articular surface. tinued because here, on account of the early separation of this continent from the rest of the world, no development of Placentalia occurred. The placentals are entirely lacking in Australia with the exception of those introduced by man and such (mice, bats, seals) as easily pass from island to island. In their present habitat, in adaptation to similar conditions they have undergone a development analogous to that of the placentals in other parts of the earth, so that they present groups parallel with the carnivores, rodents, insectivores, and ungulates. Order I. Polyprotodonta (Zoophaga). Many marsupials, among them the oldest, have a dentition adapted to animal food. They have numerous incisors (up to five in each half-jaw), strong canines, and sharp-pointed molars (fig. 657). Some in teeth, as well as in body form, resemble the Insec- tivora, others the carnivores. The Dasyuridse are carnivorous: Dasyurus ; Sarcophilus ursinus, the Tasmanian 'devil,' dangerous to larger mammals; Thytaeinus, pouched wolf. The PERAMELID.E are insectivorous ; Perameles, bandicoot. The DIDELPHYID^E, or opossums, which are confined to America (chiefly South) are more carnivorous in dentition and recall the apes with their opposable thumb. Didelphys virginiana* Order II. Diprotodonta (Phytophaga). The herbivorous habits are correlated with the degeneration of canines, which usually are lacking in the lower jaw and are at least very small in the upper. There are also only two incisors, of large size, in the lower jaw, while the middle two of the upper are much larger than the one or two lateral which may be present. The PHASCOLOMYID.E are the rodents of the marsupials with one chisel- like incisor in each half of each ja\v. Phascalomys, wombat. The MACRO- 63J: CHORD AT A. PODID.E, or kangaroos, resemble the ungulates in their large herds on the grassy places. The fore legs being very small, the animals leap with the strong hind legs and tail. Macropus giganteus. The PHALANGISTID.E have very variable teeth. They resemble in habits the squirrels, Petaurus having the same parachute folds as does our flying squirrel. The Dipro- todonta contain many fossil forms in Australia and a few in South America. Some of the Australian fossils were very large, Diprotodon australis larger than a rhinoceros. Sub. Class III. Placentalia (Monodelpliia). The first reason for associating the mammals of the Old World and most of those of the New together as Placentalia is an embry- ological one, the presence of a placenta. When serosa, amnion, and allantois (p, 553) have developed in the embryo, the vessels of the allantois spread out beneath the serosa and form with this the chorion, which sends small processes or villi into the now highly vascular mucous mem- brane of the uterus in order to obtain nourishment somewhat as a tree obtains food by its roots. These villi may be distributed over the greater part of the sur- face (fig. 658), producing the chorion frondosum, or diffuse placenta, which occurs in Cetacea, perissodactyles and many artio- dactyles (swine). On the other hand the vilH may be restricted to certain places, becoming very FIG. 658.— Diagram of mammalian em- . ., _,,, . biTO with chorion frondosum;afo,amni- Strong there. JLhlS glVCS rise to otic cavity; aZ, allantois; am, amnion; , -i j ->• -j i as, umbilical cord; c/i, chorion; c/,z, cotyledonary, discoidal, or zonary chorion ic villi ; dg, yolk stalk ; ds, yolk -, m , -i T sac ; r, space (extra-embryonic coelom), placentae, lo these Correspond between chorion and ammon;*, serosa. porti(mg rf ^ ^^ ];ning which are distinguished from the rest by becoming extremely vas- cular (uterine placenta). The cotyledonary placenta (fig. 659) consists of many small placentar patches, the cotyledons (most ruminants). In the zonary placenta the villous area takes the shape of a girdle or barrel (carnivores, Sirenia), while the discoidal (other mammals) is, as its name indicates, disc-like. Besides the placental structures the higher mammals are char- acterized by the disappearance of the cloaca, the unpaired vagina, and absence of marsupial bones and inflected angle of the jaw. The eh IV. VERTEBRATA: MAMMALIA, EDENTATA. 635 dentition, on the other hand, has undergone a progressive, diver- gent development, so that the distinctions are much more pro- nounced than in the marsupials, and hence of importance in differentiating the orders. Order I. Edentata. A few families, poor in species, are united under the name Edentata because teeth are absent or, as is more usually the case, are markedly degenerate. Persistent functional incisors are lack- FIG. 659.— Cotyledonary placenta and embryo of cow. (From Balfour, after Colin.) C1, cotyledons of uterine, C2, of foetal placenta; Ch, chorion ; U, uterus; V, vagina. ing, canines but rarely occur (Brady pus)', molars may be present, sometimes in great numbers (Priodongigas, the large armadillo, has about a hundred molars), but they are poorly rooted, prismatic, without enamel, and usually monophyodont. Since the aardvark (Orycteropus) and Tatusia have a heterodont milk dentition in embryonic life in which incisors occur, and fossil edentates (Entelops) with complete dentition are known, the absence of a replacement of the teeth is to be explained by degeneration, which may affect other parts, and is to a certain extent the reason for the low position accorded these forms. The great number of sacral vertebrae is striking, being as many as thirteen in some armadillos. The placenta is very variable, being diffuse, discoidal, or zonary in different species. The group is essentially tropical, but one species 636 CHORDATA. entering the United States. The oldest fossils occur in the Santa Cruz beds of Patagonia (eocene or oligocene). Sub Order I. NOMARTHRA. Old World edentates. FODIENTIA. Animals with strong digging claws, long tail, and long, vermiform, sticky tongue used in catching ants and other insects. Orycteropus capensis, aardvark, with long snout, sparse bristly hair, five small molars, and rudimentary milk dentition. SQUAMATA. Toothless, body covered with overlapping scales. Manis, pangolins of Asia and Africa (fig. 660). FIG. 660.— Manis longicaiidata, pangolin. (From Monteiro.) Sub Order II. XENARTHRA. Edentates of the New World. VER- MILINGrUIA, ant eaters. Resemble manids in toothless jaws, long ant- catching tongues, and strong digging claws, but are hairy and lack scales. Myrmecophaga. TARDIG-RADA, sloths. Hairy, head short, rudimentary tail, and few teeth, long strong claws by which they hang back down- wards from limbs of trees. Bradypus tridactylus, nine cervical verte- brae ; Cholcepus, six cervicals. Fossils allied are Megatherium, as large as an elephant, Mylodon, Megalonyx, these two extending north to Penn- sylvania. LORICATA, armadillos. Body with armor of bony plates, molars numerous ; insectivorous. In the extinct GLYPTODONTID^E of South America the plates fused to a continuous armor. One species twelve feet long. One species may have occurred in Europe. DASYPODID.E ; dermal armor in three or more movable transverse plates ; nocturnal. Genera based upon the number of bands: Dasypus, Xenurus ; Tacusia novem- cincta * enters United States. Order II. Insectivora. These primitive forms have a complete dentition, all the differ- ent kinds of teeth being present, although they vary in number. The roots are developed early and consequently the teeth are small. Since they end with sharp cusps, adapted for eating insects, they resemble the carnivores, from which they may be distinguished by IV. VERTEBRATA: MAMMALIA, CHIROPTERA. 637 the rudimentary condition or occasional absence of the canines (Talpa fjii. miny shrews ffff). There is great variability in the matter of replacement of teeth; in the shrews, for instance, the milk dentition is suppressed and the second only is functional, while in the hedgehog one incisor and one pre- molar in each jaw, a second premolar and the canine of the lower jaw func- tion in both dentitions. In many re- FIG. 66i.-skuii of Sorex. (From ,, . ,. Ludwig-Leunis.) spects the insectivores resemble the rodents : a clavicle is present ; there are usually five toes furnished with claws; there is a uterus bicornis, often divided its whole length, and discoidal placenta. Aside from the proboscis-like snout the insectivores resemble the rodents in appearance, forming parallel groups to those of that order. The ERINACID.E, or hedgehogs, of the Old World are spined like the porcu- pines ; the SORICID.E, or shrews (Sorex,* Blarina*), are mouse-like, as are the allied TALPID.E, or moles (Scalops,* Condylura,* star-nosed mole), which burrow in the earth and have the eyes more or less rudimentary. Some authors place here Galeopithecus of the East Indies, which has a similar membrane and similar sailing powers as the flying squirrels. It also presents resemblance to the bats and to the lemurs. The earliest known insectivores date from the eocene. FIG. 662.— Skeleton of bat. (After Brehm.) Order III. Chiroptera. The bats are the only mammals which actually fly, and this at once characterizes them. The flying membrane (patagium) is a thin fold of skin, richly supplied with nerves, which begins at the 638 CHORDATA. tail, includes the lower extremities to the foot, and extends thence to the fingers, leaving the thumb free. Fingers 2-5 are enormously elongated and support the membrane. Since flight requires strong muscles, the sternum develops a small keel, recalling that of birds, for the attachment of the large pectoral muscle. In con- nexion with the flying powers the clavicle is strong. The patagium is the seat of a very acute tactile sense, by means of which blinded bats can fly among all kinds of obstacles without disturbing them. The enormous ear conchs and a noticeable nose leaf, widely dis- tributed through the group, also have marked tactile powers. In the pectoral position of the mammary glands and in the discoidal placenta these animals resemble the primates. In temperate regions bats hibernate during the winter. The dentition is vari- able, often fllf . Fossils occur in the eocene. Sub Order I. MICROCHIROPTERA, with insectivorous dentition, only the thumb of the fore limbs clawed. VESPERTILIONID^, tail long, no nose leaf ; Vesperugo* Atalapha* PHYLLOSTOMID^E, with nose leaf, trop- ical America ; Desmodus, the blood-sucking or vampyre bat. Sub Order II. MACHROCHI ROPIER A (Frugivora), with smooth- crowned molars, claws on thumb and first two fingers. Includes the flying foxes, Pteropus, of the East Indies. Order IV. Rodentia. The rodents unite great similarity in appearance with a char- acteristic dentition. The canines are absent, and the molars are separated by a large gap (diastema) from the incisors (fig. 663). The latter are strong, chisel-like, have persistent pulps and grow at the lower end as they are worn away at the cutting edge. Since only the front surface has enamel, wear keeps them constantly sharp. Usually there is but a single in- cisor, and only in the Duplici- dentata is a second present in the upper jaw. The molars are cus- FIG. 663.— skull of porcupine. (From pidate or have enamel folds and Schmarda.) /, frontal; im, premaxil- * lary; k, temporal fossa continuous in frequently Continue to ffl'OW front with orbit; o, infraorbital fora- J . ° men, enormous on account of the por- throughout llie. Their number IS tion of the masseter muscle which „ , , , , . , _ passes through it. frequently reduced, the formulae varying between f-°-f | and -J-J-J-f. Many species have an inflected angle of the jaw like that of marsupials. The infraorbital canal is a striking feature in Muridae and Hystricidae (fig. 663, 0), a IV. VERTEBRATA: MAMMALIA, UNGULATA. 639 large opening in front of the orbit in which a part of the masseter muscle is attached. The rodents are distinguished from the ungulates, which, like them, are herbivorous, by the usually smaller size, the possession of claws, five toes (sometimes reduced to three), the occurrence usually of a clavicle, and a discoid placenta. The mammae are inguinal in position and, corresponding to the great fertility, are very numerous. The occurrence of glands with a strong-smelling secretion, which open near the anus, is common. About nine hundred living species are known, occurring in all regions except the Australian. The order appears in the eocene. Sub Order I. DUPLICIDENTATA (Lagomorpha), two upper incisors, includes the hares, Lepus,* and the picas, Lagomys.* Sub Order II. SCIUROMORPHA. The squirrels, SCIURID.E, are distin- guished by the soft fur and bushy tail. Sciurus,* squirrels ; Cynomys,* prairie dogs ; Sciuropterus,* flying squirrels. The CASTORID^E have soft fur and scaly tail. Castor fiber * beaver of Europe and America. Sub Order III. MYOMORPHA, rats and mice. Mus musculus,* common mouse; Mus rattus,* house rat, once abundant but now replaced by the gray rat, M. decumanus,* an immigrant from Asia. White rats are albinos of M. rattus. Fiber zibethicus* musk rat; Arvieola* field mice. Sub Order III. HYSTRICOMORPHA. The porcupines (HYSTRICID^E) have spines; the Old World forms, Hystrix, are terrestrial, ours (Erethyzon) arboreal. The CAVHD.E of South America have hoof-like claws. Cavia cobaya, guinea pig. Hydrochosrtis, capybara, the largest existing rodent. Order V. Ungulata. Under the heading of Ungulata, or hoofed animals, are here included two groups of living animals in which the body weight is supported on hoofs on the tips of the toes, and which are sharply marked off from other forms. If, however, the fossils are in- cluded, the limits of the group must be extended so that it includes the elephants and conies of the existing fauna as well as several extinct forms, for these so interlock and intergrade that sharp lines cannot be drawn. The ungulates, which arise from common ancestors, the Oon- dylarthra, the representatives of which occur in the eocene of America (Phenacodon), are preeminently herbivorous; the canines are rarely well developed, the molars numerous and adapted to grinding the food, more or less flattened and frequently with folded enamel. The mammae are inguinal, the uterus bicornuate, and the placenta either diffuse or (most ruminants) cotyledonary (fig. 659). The legs are exclusively locomotor structures and, to 640 CHORD ATA. permit freer motion, the clavicles are absent ; the feet touch but the tips of the toes, enclosed in hoofs, to the ground (unguligrade). Since the metacarpals and metatarsals are greatly elongate, the wrist and ankle are raised high from the ground so that they are frequently confounded with elbow and knee. With this exclu- sively supporting character of the limbs there is the same tendency to reduction and fusion of bones which was noticed in birds (p. 606). There is a constant increase in the development of radius and tibia to the chief supports of the body, the fibula becoming rudimentary, the ulna being developed sometimes throughout its whole extent, sometimes only in its upper part, which serves for the attachment of muscles (olecranon), and is more or less fused with the radius. The same tendency to simplification prevails in the feet, but is expressed differently in the odd-toed (perissodactyle) and even-toed (artiodactyle) forms. In the Perissodactyla the \ FIG. 664.— Fore feet of ungulates. (After Flower.) A-C, perissodactyle ; D-F, artio- dactyle. A, tapir; R, rhinoceros; C, horse; D, pig; E, deer; F, camel, c, trique- trum (ulnare); /, lunatum (intermedium); rn, capitatum; w2-w5, rudiments of metacarpals II and V; p, pisiforme; R, radius; ,s, scaphoid (radiale); td, trapezoid; tm, trapezium; U, ulna; w, hamatum; II- V, digits. axis of pressure passes through the middle toe (fig. 664, A—C, III), while the other toes disappear symmetrically around this. Since the first toe is early lost, toe V is next to disappear (J?), and then toes II and IV (C), so that at last there remain only the skeleton and hoof of the middle toe (horse), the rudiments of toes II and IV persisting as the small splint bones. In the Artiodactyla the axis of pressure falls between toes III and IV (fig. 664, /)), which both unite in supporting the body and are equally developed and frequently fuse, at least so far as the metacarpals are concerned (E, F}. The figures D-F show IV. VERTEBRATA: MAMMALIA, UNO UL AT A. 641 how the other digits disappear, digit I being lost still earlier. Since the weight of the body rests more upon the hind legs than upon the front ones, the former are the first to become modified. Since we are able, by using abundant paleontological material, to follow in detail the lines of descent of both artiodactyles and peris- sodactyles, the conclusion is certain that these form diverging series, distinct from the beginning. In each series most of the common characters enumerated above have been independently acquired so that the uniformity in appearance of the various groups of ungulates is in great part the result of convergence. The discussion of the fossils will be given under a separate head. Sub Order I. PERISSODACTYLA (Solidungula). The dentition is peculiar in having molars and premolars (with more or less pronounced enamel folds) of equal size; a second character is the predominant devel- opment of the middle toe, the others in the three existing families reduced to different degrees. TAPIRID^E, fore feet four-toed, hind feet three-toed; teeth i^||; nose elongate into a proboscis. Tapirus, tapirs, tropical Amer- ica and India. RHINOCEROTIDJE, three toes on all feet, teeth f££f; one or two horns on the nasal bones, these without skeleton; skin thick, hairless, hence these were formerly united with elephants as Pachydermata. Rhinoceros, a single horn, India; Ceratorhmus (Asia), Atelodus (Africa), have two horns. EQUID.E, a single functional toe, toes II and IV forming splint bones (fig. 664, c); teeth |{|| ; Equus cdballus* horse, a native of Asia; E. asinus, ass; E. zebra. Hybrids between jackass and mare are called mules; between stallion and she-ass, hinnies. Sub Order II. ARTIODACTYLA. Besides the features of the feet, these forms have the premolars, three or four in number, smaller than the molars. The species are much more numerous than the perissodactyles and may be divided into three sections. Section I, NON-RUMINANTIA (Bunodontia); omnivorous and have correspondingly a bunodont dentition, Jj|, the canines frequently developed into tusks; the stomach is usually simple, but is occasionally divided into three chambers (Dicotyles, Hippopotamus), although rumination does not occur. The leg skeleton is little modified (fig. 664, D), ulna and fibula not being reduced, and meta- carpals and metatarsals separate. HIPPOPOTAMUS ; all four toes reach the ground; skin thick (' pachyderm'), body heavy; living species all African. Hippopotamus. SUID^E; two functional toes, skin with bristles, snout proboscis-like. Sus scrofa, swine ; Dicotyles* peccaries of warmer America. Section II. RUMINANTIA (Pecora); teeth and stomach are adapted to the exclusively herbivorous diet. The stomach (fig. 665) is divided into two portions, each again subdivided. The first of these, the rumen, or paunch (ru), receives the food as it is eaten; then at a time of quiet it is regurgitated into the mouth and ground by the molars (' chewing the cud '). It then passes back, this time into the second division, the honeycomb, or 642 CHORD AT A. reticulum (re), thence to the many plies or omasum (o), and lastly to the abomasum, or true stomach (a). Usually not only the canines but the in- cisors of the upper jaw are degenerate, while the incisors of the lower jaw are strong and the canines have taken the form and position of incisors. The molars are seleriodont (have crescent-shaped cusps). With few excep- tions they are of large size and many bear horns on the frontal bones. These are larger in the males and may occur exclusively in that sex. In the sim- plest case (giraffes) these are cones of horn free from the frontals and cov- ered with skin. In others (Cavicornia) the horn cores fuse secondarily with FIG. 665.— Stomach of sheep. (After Cams and Otto.) a, abomasum (true stomach); o, omasum (manyplies) ; re, reticulum (honeycomb) ; rw, rumen (paunch). the frontals and are covered with a firm sheath of horn. Lastly, the horns are outgrowths of the frontal bone, in which usually the outer coats are lost and only the bone projects freely (antlers). These are shed yearly, the new antler which takes its place being larger and consisting of a larger number of branches or tines, thus constituting an index of age (Cervicornia). CAMELOPARDALID^E (Devexa), giraffes, long-legged forms (two genera) from Africa with persistent horns; teeth -$$ff, Giraffa. CERVID^E, deer, with deciduous horns in the male. Cervus,* common deer; Alces,* moose; Rangifer* reindeer; MOSCHID^E, horns lacking, males with enlarged upper canines and with a musk gland (the source of the familiar perfume) on the ventral surface; Moschus, central Asia. The TRAGULHWE, primitive Asiatic and African forms, includes the chevrotain, Tragulus javanicus, the small- est living ungulate. The CAVICORNIA include a large number of forms, some of great economic importance; teeth §fff. BOVID^E: Bos taurus, domestic cattle, probably descended from three distinct stocks (B. primi- genius, the aurochs, B. longifrons and B. frontosus); Bison,* including B. europeus, the bison proper, and B. americanus* our * buffalo,' so near extinction; Bubalus, the true buffalo of the Old World. OVHXE: Ovisaries, sheep; 0. montana* big horn; Capra hircus, goat; Ovibos moschatus,* musk ox. ANTILOPID^E: including a host of Old World forms (Antilope, Oazella, Rupicapra tragus, the chamois, etc.) and Antilocapra americana,* IV. VERTEBRATA: MAMMALIA, PROBOSCIDIA. 643 the prong horn, which sheds its horns, and Hoploceras montanus* the Kocky mountain sheep. Section III. TYLOPODA, stomach without manyplies, no frontal horns, diffuse placenta. Camelus, the camels of the Old World; C. drome- darius, one hump; C. bactrianus, two humps. Auchenia lama, A. alpaca of South America. Paleontology of the Ungulata. Extensive paleontological material, especially from the tertiary rocks of our western states, has cleared up many lines of ungulate descent and has rendered it probable that the CONDYLARTHRA of the eocene, with five-toed plantigrade feet, well-developed ulna and fibula, and an omnivorous dentition, formed the stock from which descended the artio- dactyles and perissodactyles, and possibly carnivores and primates as well, the ungulate line extending through the Amblypoda. From one group of these (the PHENACODONTID^E) the lines of rhinoceros and tapir have come, and in an almost complete series we know the ancestry of the horse. Hyracotherium (Eohippus) and Orohippus of the eocene had the fore feet four-toed (fig. 666. 1) ; Pafaotherium and Mesohippus (2} of the lower FIG. 666.— Evolution of fore foot of horse. (From Wiedersheim.) 1, Orohippus (eocene): 2, Mesohippus (lower miocene); 3, Miohippus (miocene); It, Protohippun (upper pliocene); 5, Pliohippus (pleistocene); 0, Equus. miocene and Miohippus of the later miocene were three-toed, while Mery- hippus and Hipparion (Pliohippus, 4) of the pliocene were near the horse in tooth structure. The single-toed horses appeared in the pleistocene with Pliohippus (5) and then Equus itself (6). It is a peculiar fact that the horse entirely died out in America, although the chief part of its his- tory was enacted here. The AMBLYPODA, mentioned above, were semi-plantigrade penta- dactyle forms, appearing in the lowest eocene, and reaching, in Uinta- therium (Dinocerus) an elephantine size. The TOXODONTIA of the South American tertiaries combined perissodactyle, rodent, hyracoid, and proboscidian features, while the TILLODONTIA of the eocene recall both carnivores and rodents. Order VI. Proboscidia. The elephants and their allies, with their hoofs and herbivorous •dentition, are closely related to the ungulates. They are charac- terized by their thick skin (« pachyderm '), the large, massive, five-toed legs, and especially by the nose drawn out into a 644 CHORD ATA. long proboscis with a finger-like process at the tip, lastly by the dentition. Canines are entirely lacking, but the incisors of the upper jaw have pulps and therefore continue to grow throughout life, forming the well-known tusks. In the living elephants there are but a single pair of tusks, but in some extinct Mastodons there were a second smaller pair in the lower jaw, while in Dinotherium only the lower in- cisors were developed, these pro- jecting downwards. The molars (in Mastodon and Dinotherium with normal replacement and cusps) consist of numerous plates of enamel and dentine united FiG.667.-inside~71e7t lower jaw of bJ cement, and undergo a lateral Sff owenri, fuLtfon^mXr;6!; displacement. Of the three large its successor. molars and premolars only one at a time is functional (fig. 667, ./); when worn out the next one behind (2) takes its place. Further features are a uterus bicornis, a zonary placenta, and two pectoral mammas. ELEPHANTINE : Elephas indicus, small ears ; E. africanus, large ears. E. primigenius, mammoth, in the pleistocene ; specimens found frozen in ice in Siberia have close woolly hair, in some places three feet long. Mastodon, with tuberculate teeth, range from miocene through the pliocene. DINOTHERIDJE, only lower incisors ; Dinotherium, Old World miocene. Order VII. Hyracoidea. The single genus Hyrax, including species from western Asia and Africa, with four-toed front feet, hind feet with three toes, the digits with nails, the placenta zonary, and the dentition |~§£f , forms this group, no fossils being known. Hyrax syriacus is sup- posed to be the ' coney ' of the Bible. Order VIII. Sirenia. This order consists of a few aquatic mammals which are whale- like in form, with the fore limbs fin-like, the hind legs lacking, and a horizontal caudal fin. They live in shallow seas or in the mouths of rivers, where they feed on the tang, which they chew with jaws covered with horny plates. The teeth (in the fossil Prorostomus f yf-f ) are reduced or entirely lacking. The fore legs are pentadactyle and often have rudimentary nails and always a flexible elbow. The two pectoral mammae have possibly caused these animals to furnish the germ of truth in the mermaid myth. Manatus americanus,* the manatee, six cervical vertebrae, eight to IV. VERTEBRATA: MAMMALIA, CETACEA. 645 ten molars; Halicore dugong, Indo-Pacific; Rhytina stelhri of the northern Pacific, exterminated in 1768. Order IX. Cetacea. In external form the whales resemble the sirenians, a result of an aquatic life, but the resemblance ends here. The whales are FIG. 668.— Restoration of skeleton of HaUtherium, an extinct sirenian. (After Miss Woodward.) so fish-like that they are commonly included by the laity in that group, and every one speaks of the whale fishery. Head and trunk are scarcely distinguished, the cervical vertebrae being very short and more or less completely fused. The hinder limbs are absent, and of the pelvic girdle only a small ilium remains, and no sacral vertebras are developed. The caudal fin is two-lobed and differs from that of a fish in being horizontally flattened ; the skin is thick and is sparsely haired or completely naked, in some hair being lacking even in the embryo. Most of the species inhabit the high seas, Inia botiviensis and Platanista gangetica occur in rivers. The fore limbs are modified into flippers, the bones of which are of nearly equal size and are jointed only at the shoulder. A dorsal fin ( ' fin backs ' ) occurs in some. The lack of hair is compensated by the thick layer of subcutaneous fat (blubber) which, like the fat penetrating the spongy bones, tends to lessen the specific gravity. In order that the ani- mals may breathe while feeding, the larynx is prolonged into a tube which extends up through the pharynx to the choanaB, from which the nostrils extend directly upwards to the single (Denticetes) or paired (Mys- ticetes) external opening. Since the air driven out with great force con- tains much moisture and this is condensed on contact with the cooler external air, the impression was natural that the animals in 'blowing' spouted water. Since the olfactory membrane is degenerate and the olfactory lobes are reduced, the nose is an organ of respiration only. The eyes are small, external ears are lacking, the mammas are close to the sexual opening. Tho teeth are either present in large numbers, similar and conical, and, since the second dentition is rudimentary, are mono- phyodont (Denticetas) or they are outlined early and then resorbed and replaced by plates of baleen (Mysticetaa). This is composed of large horny plates (whalebone) in large animals a dozen feet long (fig. 669, ?/;), of which several hundred are arranged in close succession extending inward to the tongue. They correspond to the transverse palatal folds which 646 CHORD AT A. occur in other mammals. As they are fringed on the inner edges they form a strainer which retains the small marine animals (plankton, Ceto- cliilus septentrionalis, a copepod, and Clione borealis, a pteropod) on which these whales feed. The oesophagus is too narrow for the passage of much larger animals. The origin of the whales is one of the unsolved problems. That they came from some terrestrial, quadrupedal forms is beyond question, and the FIG. 669.— Section through jaws of whalebone whale. (After Delage.) c, septum of nose; m, mouth cavity; ma;, maxillary bone; p, premaxillary (hinder end); t% vomer ; tt>, baleen. little evidence would seem to point to an ungulate or a carnivore ancestry. It is possible that the toothed and whalebone whales may have had differ- ent ancestries, and their resemblances may be the result of convergence. Sub Order I. ZEUGLODONTA. Extinct (eocene) forms with hetero dont dentition, the posterior teeth two-rooted. Sub Order II. DENTICETJE, toothed whales, carnivorous, some hav- ing but two teeth. Delphinus, dolphins ; GloMocephalus* black fish ; Monodon, narwal, with, in the male, a long maxillary tusk (possible origin of the 'unicorn'). Physeter macrocephalus, sperm whale, pursued for the spermaceti, an oily mass situated in the 'chair' between the cranium and the snout, as well as for ambergris, formed in the intestines. Sub Order III. MYSTACETI, whalebone whales, with baleen. Bala- noptera* rorquals and fin backs. B. sibbaldi,* the largest whale, reach- ing a length of eighty-five feet. Balcena, right whale. Order X. Carnivora. The carnivores live chiefly on the flesh and blood of other ver- tebrates, which they catch by craft, by coursing, or by pouncing upon them, overpowering their prey by their sharp claws and cutting teeth. With this mode of life correspond the high devel- opment of the brain (fig. 649, B) and sense organs, as well as IV. VERTEBEATA: MAMMALIA, CARNIVOEA. 647 structure of teeth and claws. Since this predaceous character in- creases within the order from the bears to the cats, and again tends to disappear in the aquatic species, there are few constant charac- ters, but a great variation in structure. In interest of greater mobility the clavicle is reduced or lost, ulna and radius well de- veloped. In the structure of the feet there is a gradual transition from the plantigrade bears, in which the whole sole of hand and foot rest upon the ground, to the digitigrade cats, which tread on the tips of the toes. In the latter the claws, which occur in all carnivores, are kept from injury, when not in use, by being re- tracted by an elastic ligament into pockets on the penult joint, from which they are extended by strong muscles. In dentition (fig. 650) the striking features are the almost constantly three incisors, and the great size of the canines; the molars, on the other hand, vary with the different families, the cusps assuming more of the shearing character (secodont teeth). The last premolar of the upper jaw and the first molar of the lower jaw become carnassial teeth (sectorial teeth), and acquire a dominating position in the jaw, while the others become smaller and tend to disappear at either end of the series. Further characters are the possession of a penis bone in the males, the abdominal position of the milk glands and the uterus bicornis in the females; the placenta is zonary. Anal glands, furnishing a strong, even offensive smelling secretion, are of wide occurrence. Sub Order I. FISSIPEDIA. These are the typical members of the order and are preeminently terrestrial animals with well-developed toes usually cleft to the base. The number of digits is frequently five on all feet, but is often reduced to four on the hind feet (Felidse, Canida?), rarely on the fore feet (Hyeenidse); but in these cases, as in the domestic dog, the reduced first toe may bear a claw. URSID^E, plantigrade; Ursus,* bears; Procyon lotor* raccoon. MUSTELINE; many species ofMustela * and Puto- rius* which include minks, martens, sable, ermines, and weasels, are valuable for their fur; Lutra,* otter ; Enhydris,* sea otter; Mephitis,* skunk; Taxidea* badger ; Grido* glutton ; anal glands common in this family. Fossils (Arctotheriwn, etc.) connect the bears and the CANID^E with five toes in front, four behind, claws not retractile ; which includes in the genus Canis* dogs, foxes, and wolves. The FELIDJE have toes as in the dogs, but with retractile claws. Felis domestica, our domestic cat. F. leo, lion; F. tigris, tiger; F. concolor,* puma or cougar. HT^BNIDJE, all feet four-toed; Hycena of Africa. Sub Order II. PINNIPEDIA. These are aquatic carnivores with the limbs flattened to broad flippers, the five toes long and webbed, the nails frequently rudimentary ; the dentition differs from that of the terrestrial forms in the similarity of molars and premolars (absence of carnassial) ; 648 CHORD AT A. the milk dentition degenerates early, without being functional. PHOCID.E. seals, without external ears ; Phoca vitulina,* harbor seal. OTARIID^E, with external ears ; Otaria,* sea lions ; Callorhinus ursinus, fur seal of Alaska. TRICHECHID^E ; incisors reduced, upper canines developed into large tusks ; Trichechus, walrus. The first carnivores appear in the eocene in the order CREODONTA, plantigrade forms with slightly differentiated dentition (no carnassial) ; FIG. 670.— Phoca vitulina, harbor seal. (After Elliott.) they present marked resemblances to marsupials, insectivores, as well as to the Condylarthra, the ancestral ungulates. True carnivores appear in the upper eocene and become abundant in the miocene. Order XI. Prosimiae. Linne united with the true apes a small group of animals known as lemurs (from India and the adjacent islands, and especially from Africa), because of similarity in body form and climbing habits, because they had grasping hands and feet (oppos- able thumb and great toe), and at least frequently nails on some of the toes. To-day many set them aside as a separate order on account of their lower organization. They have a less-developed cerebrum, uterus bicornis, and a diffuse placenta. Further peculiarities are the peculiar and variable dentition (Chiromys •}{)-if, Lemur f|||) and the presence of claws, which always occur on the second and frequently on the third finger of the hind feet, and in Chiromys replace the nails on all the digits of all the feet except the great toe. Their nocturnal habits have resulted in IV. VERTEBRATA: MAMMALIA, PRIMATES. 649 large eyes, which give these animals a most striking appearance. A distinction from the primates is the connexion of orbital and temporal cavities beneath the osseous postorbital ring. Usually there are a pair of pectoral mammae, to which are added in many Fio. 671.— Stenops gracilis, slender loris. (From Brehm.) species a pair in the abdominal or inguinal region, the latter alone occurring in Chiromys. CHIROMYID^E, digits long, all except the great toe with claws ; Chiromys madagascar ensis, aye-aye. TARSIID^E, second and third hind toes clawed. Tarsius spectrum of the East Indies differs from all Prosimite in having the orbits closed and a discoidal placenta like that of man. LEMURID^E, second hind toe alone clawed. Lemur; Stenops, loris. The old tertiary PACHYLEMURID.E and ANAPTOMORPHID^E are close to the most primitive mammals and to the creodonts and insectivores. The GALEO- PITHECID^E (p. 637) are often referred here. Order XII. Primates. The most highly organized mammals, the monkeys, apes, and man, are united in a single order because among them there is a great agreement in features of classificatory value. If we here, as elsewhere, ignore grades of intelligence and regard alone greater or lesser anatomical resemblances, we are forced to the conclusion that the anthropoid apes are much closer to man than to the lower monkeys. The primates have in common nails on all the fingers and toes (except the Hapalidae), orbits separated from the temporal fossae by a bony wall, and a cerebrum which covers the other parts of 650 CHORD AT A. the brain (fig. 649, c). They have a single pair of pectoral mammae, uterus simplex, and a discoidal placenta. The dentition is essentially the same throughout; in the Platyrrhinae f Jf |, in the Ilapalidse ffff , in the Catarrhinse and in man fff|. Yet there is a tendency to variation, since in the chimpanzee and in man the third molar (wisdom tooth) is in process of degeneration, while in the orang a fourth molar often occurs. In all the molars are bunodont. The skeleton of the hand and foot has played an important role in classification. As in the lemurs and opossums, the thumb and great toe can be opposed to the other digits, so that an ape can grasp objects with either hand or foot. In man this opposability of the thumb is increased, but that of the great toe, in consequence of the upright position, is only retained to a slight degree by chil- dren and primitive people. On this peculiarity rest the names often given of Bimana, for man, and Quadrumana, for the apes and monkeys. In contradiction of this it must be emphasized that the apes do not have a hand, but rather a grasping foot, on the hinder extremities. In the grasping foot (fig. 672) are the same bones, Fio. 672.— Hand and foot of gorilla, c, capitatum; ca, calcaneus; CM, cuboid; 7i, ha- matum ; i, lunatum ; me, metacarpals ; ??»t, metatarsals ; n, iiaviculare ; p, pisi- forrne; ph, phalanges ; s, scaphoid; t, triquetrum; 435 Astarte, 367 Astartidse, 368 Asterias, 337 Asterias, early development, 146, 148 Asteridae, 337 Asterinidae, 337 Asteriscus, 335, 337, 564 Asteroidea, 333 Astrsea, 261 Astrangia.- 260, 261 Astroides, 261 Astrophyton, 338 Asymmetrical form, 135 INDEX. 661 Asymmetron, 504 Atalapha, 638 Atax, 454 Ateles, 651 Atelodus, 641 Atheca, 595 Atlantidse, 380 Atlas, 581, 590 Atoke, 310 Atoll, 258 Atolla, 250 Atrium, in, 506, 508 Atrypa, 328 Attus, 453 Atypus, 453 Auchenia, 643 Auditory meatus, 545 Auditory nerve, 536 Auditory organs, 127 Auk, 615 Aulacantha, 196 Aulophorus, 315 Aulosphsera, 196 Aurelia, 245, 250 Auricle, ill, 548 Auricularia, 332 Aurochs, 642 Australian region, 176 Autoflagellata, 200 Autoinfection, 215 Autolytus, 313, 314 Aves, 603 Avicularia, 323 Aviculidae, 367 Aye-Aye, 649 Axial skeleton, 526 Axiothea, 313 Axis, 590 Axis cylinder, 96 Axolotl, 587 Axons, 94 Azoic era, 1 80 Azygobranchia, 379 Baboons, 651 von Baer, 17 Bsetisca, 479 Badger, 647 Balsena, 646 Balaenoptera, 646 Balancers, 491 Balaninus, 485 Balanoglossus, 513 Balantidium, 209, 210 Balanus, 423 Baleen, 645 Bandicoot, 633 Barbs 603 Barbules, 603 Barnacles, 423 Basalia, 330, 340, 527 Bascanion, 601 Basioccipital bone, 522 Basiopodite, 410 Basisphenoid bone, 522 Bass, black^ 577 Bassomatophora, 383 Bath sponges, 227 Bats, 637 Bdellodrilus, 315 Bdellostoma, 557 Bdelloura, 271 Bears, 647 Beaver, 639 Bedbug, 489 Bee, larva of, 105 Bees, 487 Beetles, 483 Bela? 380 Belemnites, 388 Bellovacensis, 9 Bell's law, 536 Belosepia, 388 Belostoma, 489 Belostomidse, 489 Beroe, 264 Beroidge. 264 Bestiarius, 9 Bicidium, 259 Bicoseca, 201 Bicuspid teeth, 626 Big horn, 642 Bilateral symmetry, 131 Bilharzia, 277 Bimana, 650 Biogenesis, fundamental law of, 34. 662 INDEX. Biology, 4, 57 Bipalium, 271 Bipinnaria, 332 Biradial symmetry, 136 Bird lice, 479 Birds, 603 Birds of paradise, 50, 616 Birgus, 432, 436 Bison, 642 Bittacus, 483 Bivium, 334 Black bass, 577 Black fish, 646 Black flies, 493 Black snake, 601 Bladder, urinary, 552 Bladder worm, 278, 284 Blarina, 637 Blastoderm, 153 Blastodermic vesicle, 155 Blastoidea, 342 Blastomeres, 151 Blastopore, 156 Blastostyle, 242 Blastula, 151, 155 Blatta, 480 Blattidse, 480 Blind fish, 576 Blissus, 489 Blister beetle, 484 Blood, 88, in Blood corpuscles, 88 Bloodvessels, no, ill Blow flies, 493 Blue birds, 6iC Boa, 60 1 Bobolink, 616 Body cavity, 109 Bojanus, organ of, 363 Bolina, 264 Bombycina, 495 Bombyx, 495 Bonasa, 614 Bone, 86 Bonellia, 317, 318 Book lice, 479 Bopyridse, 442 Bopyrus, 442 Bos, 642 Bosmina, 417 Botall's duct, 550 Bot flies, 493 Bothriocephalidae, 287 Bothriocephalus, 281, 283, 287 Bothrops, 60 1 Botryllus, 510 Bougainvillea, 144, 241 Bovidae, 642 Bow fin, 574 Box turtle, 596 Brachialia, 340 Brachiolaria, 332 Brachiopoda, 325 Brachycera, 493 Brachyura, 437 Braconidse, 486 Bradypus, 636 Brain coral, 261 Branchial arch, 524 Branchial chamber, 352, 506 Branchial clefts, 501 Branchial heart, 391 Branchial tree, 348 Branchiata, 408 Branchiomerism, 523 Branchiopoda, 416 Branchiostegal membrame, 56 Branchiostegal rays, 562 Branchiostegite, 431 Branchipidae, 416 Branchipus, 416 Branchiura, 422 Braula, 493 Breast bone, 518 Brevilinguia, 599 Brissus, 346 Bristles, 311 Bristles, tactile, 126 Bristle tails, 477 Brittle stars, 337 Bronchiole, 548 Bronchus. 547 Bryozoa, 321 Bubalus, 642 Bubo, 617 Buccal cavity, 106 INDEX. 663 Buccal ganglion, 390 Buccinidae, 380 Buccinum, 379 Bucerontidae, 616 Budding, 141 Budding and germ layers, 159 Buffalo, 642 Buffalo leaf hopper, 490 Buffon, 21 Bufo, 588 Bufonidae, 588 Bugs, 489 Bugula, 324 Bulbus arteriosus, 568 Bulbus olfactorius, 534 Bulimus, 383 Bulla, 381 Bulla ossea, 621 Bunodes, 259 Bunodontia, 641 Bunodont teeth, 626 Burbot, 578 Bursa, 331, 338 Bursa copulatrix, 471 Bustard, 615 Buteo, 617 Buthus, 448 Butrinus, 575 Butterflies, 494, 495 Butterflies, leaf, 47 Buzzard, 617 Byssus, 363 Cabbage worm, 496 Cacatua, 616 Cacospongia, 227 Caddis flies, 483 Csecidotea, 442 Caecilia, 587 Caecum, 106, 461, 627 Csenolestes, 633 Caenosarc, 231 Calamoichthys, 573 Calandra, 485 Calanidae, 421 Calappa, 437 Calcispongiae, 225 Caligidae, 422 Caligus, 422 Callianira, 263 Calliphora, 493 Callorhinus, 648 Calosoma, 484 Calycella, 242 Calyconectae, 244 Calycophorse, 244 Catycozoa, 250 Calyptoblastea, 242 Camarasaurus, 596 Cambarus, 435 Cambrian, 180 Camelopardalidae, 642 Camels, 643 Camelus, 643 Campanella, 242 Campanula Halleri. 564 Campanularia, 232, 233 Campanulariae, 239, 242 Camper's angle, 624 Campodea, 400, 477 Canal, radial, 331 Canal, ring, 331 Canal, semi-circular, 128 Canals of sponges, 223 Cancer, 437 Cancridae, 437 Candona, 423 Canidae, 647 Canine teeth, 625 Canis, 647 Canker worms, 494 Cannostomae, 250 Cantharidae, 484 Canthocamptus, 421 Capillaries, in Capillitium, 199 Capra, 642 Caprella, 440 Caprimulgidae, 616 Capsule, central, 193 Capybara, 639 Carabidae, 484 arapace, 410, 594 arboniferous, 180 archarinus, 571 archarodon, 571 G64 INDEX. Carchesium, 210, 211 Cardiidse. 367 Cardinal teeth, 359 Cardinal vein, 549 Cardium, 367 Cardoj 463 Caridea, 434 Carina, 424 Carina sterni, 605 Carinaria, 380 Carinariidae, 380 Carinella, 291 Carinatae, 613 Carnassial teeth, 626 Carnivora, 646 Carotid artery, 549 Carp, 576 Carpal bones, 529 Carpocapsa, 494 Cartilage, 86 Cartilage bone, 519 Cartilaginous cranium, 519, 520 Caryogamy, 184 Caryophyllaeus, 285, 286 Caryophyllaeidae, 285 Caryophyllia, 257, 260 Cassowary, 613 Castor, 639 Castoridae, 639 Casuarina, 613 Casuarius, 613 Cataclysm theory, 2O Catallacta, 220 Catarrhinae, 651 Caterpillars, 494, 495 Catfish, 576 Cathartes, 617 Cathartidae, 616 Catocala, 495 Catometopa, 437 Catostomidae, 576 Cats, 647 Cattle, 642 Caudal fin, 526, 562 Caudina, 347, 349 Causal foundation of theory of evolu- tion, 43 Cavia, 639 Caviare, 573 Cavicornia, 642 Caviidae, 639 Cavolinidae, 382 Cebidae, 651 Cebus, 651 Cecidomyia, 492 Cell, 58 Cell complexes, 71 Cell division, 68 Cell, nature of, 60 Cell organs, 183 Cell-reticulum. 61 Cell theory, 17, 58 Cells, adhesive, 264 Cells, blood, 88 Cells, contractile fibre, 92 Cells, division of, 68 Cell, egg, 80 Cells, ganglion, 94 Cells, gland, 76 Cells, goblet, 77 Cells, muscle, 92 . Cells, multiplication of, 68 Cells, nettle, 229 Cells, sexual. 143 Cells, somatic, 143 Cells, supporting, 83 Cells, thread, 229 Cells, yellow, 195 Cellular connective tissue, 84 Cellularia, 324 Cellulose, 172, 505 Cenozoic era, 181 Centipedes, 459, 461 Central capsule, 153 Central nervous system, 122 Centrifugal nerve tracts, 94 Centripetal nerve tracts, 94 Centrodorsal, 338 Centrolecithal eggs, 152, 153 Centrosome, 190 Centrum, 518 Centrurus, 448 Cephalaspis, 557 Cephalochordia, 502 Cephalodiscus, 514 Cephalopoda, 384 INDEX. 665 Cephalothorax, 399 Cephalothrix, 291 Cerambycidse, 485 Ceraospongias, 227 Ceratium, 203 Ceratodus, 579 Ceratorhinus, 641 Cercaria, 268, 276 Cercomonas, 202 Cercopidae, 489 Cercus, 477 Cere, 604 Cerebellar hemispheres, 535 Cerebellum, 535 Cerebral flexures, 624 Cerebral ganglion, 123 Cerebral hemispheres, 534 Cerebratulus, 292 Cerebrum, 534 Ceresa, 490 Cereus, 253 Cerianthus, 255 Cervicornia, 642 Cervidse, 642 Cervus, 642 Ceryle, 616 Cestidse, 264 Cestoda, 278 Cestum, 264 Cetacea, 645 Cetochilus, 421 Chaelura, 439 Chsetse, 311 Chsetiferi, 317 Chaetoderma, 358 Chsetognathi, 296 Chaetonotus, 295 Chaetopoda, 306 Chaetura, 616 Chain salps, 512 Chalcis, 486 Chalcididae, 486 Chalina, 227 Chameleon, 599 Chamois, 642 Charadriformes, 615 Charadrius. 615 Charybdea, 250 Chelicera, 445 Chelifer, 450 Chelone, 596 Chelonia, 594 Chelonidse, 596 Chelydra, 596 Chelydridae, 596 Chermes, 450 Chevron bones, 518 Chevrotain, 642 Chiastoneury, 373 Chigoe, 494 Chilognatha, 496 Chilomonas, 201 Chilomycterus, 578 Chilopoda, 459, 460 Chilostomata, 324 Chimaera, 572 Chimney swallow, 616 Chimpanzee, 651 Chinch bug, 489 Chiromys, 649 Chiroptera, 637 Chirotes, 599 Chitin, 398 Chiton, 357 Chitonidse, 356 Chlamydosaurus, 599 Chlamydoselachus, 570 Chloragogue cells, 116 Choanoflagellata, 202 Choloepus, 636 Chondrilla, 224 Chondrin, 86 Chondrioderma, 199 Chondrocranium, 520 Chondropterygii, 569 Chondrostei, 573 Chone. 313 Chorda dorsalis, 501 Chordata, 501 Chordodes, 304 Chordotonal sense organs, 406 Chorion, 148, 634 Choroid, 540 Choroidea, 13 I Choroid coat, 130, 131 Choroid gland. 564 666 INDEX. Chromatin, 65 Chromatophores, 387 Chromomonadina, 202 Chrysalis, 494 Chrysomelidae, 485 Chrysomitra, 244 Chrysopa, 483 Chyle, 550 Chyle vessels, 114 Cicada, 488, 489, 490 Cicadidse, 489 Cicindelidae, 484 Ciconia, 615 Ciconiformes, 615 Cidaridea, 345 Ciliata, 204 Ciliated epithelium, 75 Cilioflagellata, 203 Cimbex, 486. Cinclides, 253 Ciona, 507 Circulatory apparatus. 109 Circulatory organs of vertebrates, 548 Cirolana, 442 Cirri, 312 Cirripedia, 423 Cirrus, 120, 272, 308 Cirrus pouch, 272 Cistenides, 314 Cistudo, 596 Citigrada, 453 Cladocera, 417 Cladocora, 260, 261 Cladoselache, 572 Clamatores, 616 Clams, 368 Class, 10 Classification, difficulties in, 30 Clathrulina, 191, 192 Clava, 241 Clavellinidse, 510 Clavicle, 528 Claws, 618 Clear wings, 495 Cleavage cavity, 155 Cleavage planes, 151 Cleavage of eggs, 149, 151 Cleavage process, 151 Cleavage, types of, 153 Cleon, 479 Clepsidrina, 213, 215 Clepsinidse, 321 Clibanarius, 436 Clidastes, 600 Climbing birds, 616 Clione, 382 Clisiocampa, 495 Clitellio, 315 Clitellum, 315 Cloaca, 106, 223, 506, 546 Clothes moth, 494 Clupeidse, 576 Clymene, 313 Clypeaster, 343, 346 Clypeastroidea, 346 Clypeus, 462 Clytia, 242 Cnemidophorus, 599 Cnidse, 229 Cnidaria, 228 Cnidocil, 229 Cobra, 601 Coccidae, 490 Coccidiae, 215 Coccidium, 215, 216 Coccinellidse, 485 Coccus, 490 Coccygus, 616 Cochineal, 490 Cochlea, 128, 543 Cockatoos, 616 Cockroach, 480 Cod, 577, 578 Codlin moth, 494 Codosiga, 201, 202 Coelenterata. 228 Coelhelminthes, 295 Ccelom, 109, 158 Coelom of Vertebrates, 545 Ccelomic pouches, 158 Ccelenteron. 228 Coelodendron, 196 Coeloplana, 264 Coelopleurus, 343, 345 Coeloria, 260, 261 Coenurus, 285 INDEX. 667 Coiter, 13 Cold rigor, 63 Cold-blooded animals, 114 Coleoptera, 483 Collaterals, 94 Collembola, 477 Collozoum, 194 Coloborhombus, 49 Colon, 461 Colony, 164 Coloration, sympathetic, 46 Colossendeis, 456 Colossochelys, 596 Colubriformia. 601 Columba, 614 Columbidas, 614 Columella, 199, 256, 370, 525, 544, 598 Columns of cord, 533 Colymbidse, 615 Colymbus, 615 Comatrichia, 199 Comatulidse, 342 Commissures, 123 Complemental males, 424, 442 Compound eye, 403, 404 Conchiolin, 352 Conch of ear, 545 Condylarthra, 639, 643 Condylura, 637 Cones of eye, 540 Coney, 644 Conidae, 380 Conjugation, 184, 206 Connective tissues, 83 Conocephalus, 481 Conocladium, 200, 202 Conotrachelus, 485 Contractile fibre cells, 92 Contractile vacuole, 183 Conurus, 616 Conus, 380 Conus arteriosus, 567 Convergent development, 169 Cope, 24 Copelatae, 506 Copepoda, 417 Copperhead, 601 Copula, 524 Copulation, 147 Coraciformes, 616 Coracoid, 528 Coral, brain, 260, 261 Coral, deer's horn. 261 Coral, organ pipe, 259 Coral, precious, 259 Coral, red, 256 Coral reefs, 258 Coral snake, 601 Corallium, 256, 259 Cordyiophora, 239 Coregonus, 576 Corium, 514 Cornea, 130, 131, 541 Cornacuspongia, 227 Coronula, 423, 424 Corpora bigemina, 534 Corpora quadrigemina, 534 Corpus callosum, 623 Corpus striatum, 534 Corpuscles, Miescher's, 218 Corpuscles, Meissner's, 126 Corpuscles, Rainey's, 218 Corpuscles, Vater-Pacinian, 126 Correlation of parts, 14 Corrodentia, 478 Corti, organ of, 543 Corvus, 616 Corvidoe, 616 Corycaeidse, 421 Corydalis, 482 Corymorpha, 241 Costae, 256 Costal plates, 594 Cotingidae, 616 Cotton worm, 495 Cottus, 577 Cotyledonary placenta, 634 Coturnix, 614 Cougar, 647 Covering scale, 243 Coverts, 604 Cowries, 380 Coxa, 463 Coxal glands, 445 Crab louse, 491 Crabs, 437 668 INDEX. Crab stones, 433 Crangon, 434, 435 Crane flies, 492 Cranes, 615 Crania, 328 Cranial nerves,- 536 Craspedon, 235 Craspedota, 235 Crassatella, 359 Crassilinguia, 599 Crayfish, 435 Creodonta, 648 Crepidula, 379 Crepidula, cleavage of egg, 154 Cretaceous, 180 Cribillina, 324 Cribrellum, 452 Crickets, 481 Crinoidea, 338 Crisia, 324 Crista acustica, 127, 542 Crista sterni, 605 Crocodilia, 601 Crocodilus, 602 Crop, 106, 467 Crossbill, 616 Crosses, 27 Crossopterygii, 573 Crotalidae, 601 Crotalus, 60 1 Croton bug, 480 Crows, 616 Crura cerebri, 623 Crustacea, 408 Cryptobranchus. 587 Cryptocephala, 313 Cryptochiton, 357 Cryptodira, 596 Cryptoniscus, 442 Cryptopentamera, 484 Crystalline cone, 405 Crystalline style, 364 Ctenidia, 353 Cteniza, 453 Ctenobranchia, 379 Ctenodiscus, 337 Ctenoid scale, 558 Ctenolabrus, 576 Ctenophora, 261 Ctenoplana, 264 Ctenostomata, 324 Cubomedusae, 250 Cuckoos, 616 Cuculiformes, 616 Cuculus, 616 Cucumaria. 349 Culcita. 334, 337 Culicidae, 492 Cumacea, 437 Cunina, 242 Cunocantha, 239, 242 Curculio, 485 Currant worm, 494 Cursorial foot, 614 Cursoria, 480 Cuspidaria, 368 Cutaneous artery, 585 Cuticle, 75 Cutis, 514 Cuttle bone, 388, 395 Cuttle fish, 395 Cuvier, 14, 15 Cuvierian ducts, 548, 567 Cuvierian organs, 348 Cyamus, 440 Cyanea, 250 Cyanocitta, 616 Cycladidse, 368 Cyclas, 368 Cycloid scale, 558 Cyclometopa, 437 Cyclostomata, 324, 555 Cyclostomidge, 380 Cyclopidse, 421 Cyclops, 37, 421 Cydippidae, 264 Cygnus, 615 Cymbulidae, 382 Cymothoa, 440, 442 Cynipidse, 486 Cynocephalus, 651 Cynomorphee, 651 Cynomys, 639 Cynthia, 510 Cynthiidse, 510 Cyprseidse, 380 INDEX. 669 Cyprididae, 423 Cypridina, 423 Cypridinidae, 423 Cyprinidae, 576 Cypris, 423 Cypselidae, 616 Cypselomorphae, 616 Cyrtidse, 196 Cyrtophilus, 481 Cysticercoid. 285 Cysticercus, 278, 284 Cystid, 322 Cystidea, 342 Cystoflagellata, 203 Cystonectse, 244 Cytoblast, 58 Cytopharynx, 183 Cytopyge, 183 Cytosporidse, 213 Cytostome, 183 Dactylethra, 588 Daddy long-legs, 450 Daphnia, 417, 418 Daphnidoe, 417 Dart sac, 376 Darwin, 23 Darwinian theory, 25 Dasyatis, 572 Dasypodidse, 636 Dasypus, 636 Dasyuridse, 633 Dasyurus, 633 Datames, 450 Decapoda, 394, 429 Deer, 642 Degeneration, 167 Delamination, 157 Delphinus, 646 Demibranch, 566 Demodex, 454 Dendrites, 94 Dendrocoalum, 271 Dendrceca, 616 Dendronotus, 382 Dental formula, 626 Dentalium, 369 Dentary bone, 525, 582 Denticetae, 645, 646 Dentine, 515 Derma. 514 Dermal teeth, 5 15 Dermanyssus, 454 Dermatobia, 493 Dermatoptera, 480 Dermochelys, 595 Dero, 315 Derotrema, 587 Desmodont hinge, 359 Desmodus, 638 Desor's larva, 291 Deutocerebrum, 462, 468 Deutomerite, 214 Deutoplasm, 80 Devexa, 642 Devonian, 180 Diaphragm, 546 Diapophysis, 518 Diaptomus, 421 Diastema, 638 Diastictis, 494 Diapheromera, 480 Diastylis, 437 Dibranchia, 394 Dicotyle, 641 Dicyemida, 220 Didelphia, 632 Didelphys, 633 Didus, 614 Differentiation of tissues, 71 Difflugia, 198 Diffuse nervous system, 122 Diffuse placenta, 634 Digger wasps, 486 Digenea, 274 Digestive tract, 103 Digiti grade, 647 Dimorphodon, 602 Dimyaria, 367 Dinichthys, 579 Dinobryon, 200, 202 Diomedia, 615 Dinoflagellata, 203 Dinoceras, 643 Dinornithidae, 613 Dinosauria, 596 670 INDEX. Dinotheridae, 644 Dinotherium, 644 Dioecious, 118 Diopatra, 313 Diotocardia, 378 Diphasia, 242 * Diphycercal fin, 41, 562 Diphyes, 244, 245 Diphyodont, 625 Diploblastica, 230 Diplocardia, 316 Diplopoda, 459, 496 Diploria, 261 Diplospondyli, 570 Diplozoon, 273 Diplozoon, development of, 165 Dipneumones, 453 Dipneumonia, 579 Dipneusti, 579 Dipnoi, 579 Diporpa. 165, 274 Diprotodon, 634 Diprotodonta, 633 Diptera, 491 Dipurena, 240, 242 Dipylidium, 287, 289 Direct development, 160 Directive corpuscles, 140 Directive spindle, 146 Directives, 254 Discina, 328 Discodermia, 226 Discodrilidae, 315 Discoidal placenta, 634 Discomedusae, 250 Disconanthe, 245 Discophori, 318 Dispermy, 148 Distaplia, 510 Distichalia, 340 Distichopus, 315 Distomise, 274 Distomum, 116, 272, 275, 276 Distribution, 40 Disuse, 55 Division of labor, 72, 165 Dobsons, 482 Docoglossa, 378 Docophorus, 479 Dodo, 614 Dog-day harvest fly, 489 Dogfish, 569, 571 Dog, prairie, 639 Dog sharks, 571 Dogs, 647 Dolichonyx, 616 Doliolum, 512 Dolomedes, 453 Dolphins, 646 Dondersia, 358 Doridiidse, 382 Doris, 382 Doryphora, 485 Dorsal aorta, 548 Dorsal fin, 526, 562 Dorsal organ, 341 Draco, 599 Dragon flies, 479 Dranculus, 303 Dreissenia, 367 Drepanidotaenia, 289 Drills, 651 Dromseus, 613 Dromatherium, 632 Drum of ear, 544 Duck, 615 Duckbill, 632 Ducts, genital, 120 Ductus Botallii, 550 Ductus choledochus, 546 Ductus cochlearis, 543 Ductus ejaculatorius, 120 Dugong, 645 Duplicidentata, 639 Dynastes, 484 Dysmorphosa, 241 Dysodont hinge, 359 Dytiscidae, 484 Eagles, 617 Ear bones, 525 Ear of vertebrates, 542 Earth worms, 315 Ear wig, 480 Ecardines, 328 Ecdysis, 399 INDEX. 671 Echeneis, 577 Echidna, 631, 632 Echidnidae, 632 Echinarachnius. 345, 346 Echinobothrium, 286 Echinocardium, 34*? Echinococcus, 288 Echinoderidse, 295 Echinoderma, 329 Echinoidea, 343 Echinorhynchus, 304 Echinosphaerite3, 342 Echiuroidea, 317 Echiurus, 317 Eciton, 488 Ecology, 4 Ectethmoid bone, 522 Ectochondrostoses, 519 Ectocyst, 322 Ectoderm, 103, 156 Ectoparasites, 169 Ectopistes, 614 Ectoprocta, 322 Ectosarc, 189 Edentata, 635 Edriophthalmata, 438 Edrioasteroidea, 342 Edwardsia, 253, 255, 259 Edwardsiella, 259 Egg cell, 80 Egg of bird, 153 Egg, cleavage of, 149, 151 Egg, fertilization of, 147 Egg, maturation of, 146 Egg nucleus, 146, 149 Egg, segmentation of, 149, 151 Egg tooth, 593 Eichhorn, 13 Eiderduck, 615 Eimeria, 213 Elaps, 601 Elasipoda, 349 Elasmobranchii, 569 Elastic cartilage, 86 Elastic tissue, 85 Elastica externa, 5 16 Elastica interna, 516 Elastin, 85 Elater, 199 Electric catfish, 576 Electric eel, 576 Electric organs, 563 Elephantiasis, 304 Elephantidge, 644 Elephants, 643 Elephas, 644 Elytra, 312, 466 Embiotocidse, 577 Embryo, 1 60 Embryology, 3, 139, 160 Emu, 613 Enamel, 515 Enchylema, 62 Enchytraeidse, 315 Encyrtidium, 193, 196 Encystment, 184 Endite, 410 Endocyst, 322 Endolymph, 127, 543 Endolymphatic duct, 542 Endopodite, 410 Endostyle, 506 English sparrow, 616 Enhydris, 647 Enopla, 290 Ensatella, 368 Entalis, 369 Entelops, 635 Enteroccele, 109 Enteroponeusta, 512 Entoconcha, 349 Entochondrostoses, 519- Entoderm, 103, 156 Entomobrya, 477 Entomostraca, 414 Entoniscus, 441 Entoniscidae, 441, 442 Entoparasites, 169 Entophaga, 486 Entopldstron, 594 Entoprocta, 321 Entosarc, 189 Entovalva, 349 Environment, 54. Eocene, 181 Eohippus, 643 672 INDEX. Eozoon, 1 80, 198 Epaxial muscles, 518 Epeira, 451, 453 Ependyma, 124, 532 Ephemera, 479 Ephemerida, 479 Ephippium, 417 Ephydatia, 227 Ephyra, 246, 248, 249 Epiblast, 156 Epibdella, 274 Epididymis, 320, 552 Epigenesis, 16 Epiglottis, 547 Epimerite, 214 Epiotic bone, 522 Epipharynx, 463 Epiphragm, 372 Epiphysis, 535 Epiplastron, 595 Epipleural bones, 574 Epipodite, 410 Epipodium, 369 Epipterygoid bone, 598 Episternum, 528 Epistropheus, 590 Epistylis, 208, 211 Epitheca, 256 Epithelial tissues, 73 Epithelium, germinal, 118 Epitoke, 311 Epizoanthus, 170, 259 Equatorial furrow, 151 Equatorial plate, 69 Equidse, 641 Equilibration, organs of, 128 Equus, 641, 643 Erax, 493 Erethyzon, 639 Eretmochelys, 596 Erichthus, 429, 430 Erigone, 453 Erinacidse, 637 Eristalis, 493 Ermine, 647 Errantia, 313 Erythroblasts, 89 Erythroneura, 490 Eschara, 324 Esocidae, 576 Essence of pearl, 558 Estheria, 417 Estheriidae, 417 Ethiopian region, 176, 178 Ethmoidalia, 521 Ethmoid bone, 523, 620 Euchilota, 240 Eucopepoda, 421 Eucratea, 324 Eucrinoidea, 342 Eudendrium, 232, 242 Eudoxia, 166, 244 Euflagellata, 202 Euglena, 200, 202 Euglenidse, 202 Euglypha, 198 Euisopoda, 442 Eumeces, 599 Eunectes, 601 Eunice, 311 Eunicidae, 313 Eupagurus, 170, 435, 436 Euphausia, 429 Euplectella, 226 Euplexoptera, 480 Eupolia, 292 Euryalidse, 338 Eurypauropus, 497 Eurypterida, 444 Eurypterus, 444 Euselachii, 571 Euspongia, 221, 227 Eustachian tube, 544 Eustachius, 12 Eusuchia, 602 Eutainia, 601 Eutima, 240 Evadne, 417, 419 Everyx, 495 Evolution, 16 Evolution, Theory of, 19, 25 Evolution vs. Creation, 22 Excreta, 73, 103 Excretory organs, 115 Excretory organs of Vertebrates, 550 Exite, 410 INDEX. 673 Exoccipital bone, 522 Exocoetidse, 576 Exoccetus, 576 Exopodite, 410 Eugyra, 510 Extracapsulum, 193 Exumbrella, 235 Eyes, 129 Eyes of Vertebrates, 539 Eye spot, 183 Fabricius, 13 Face, bones of, 525 Faceted eye, 403, 404 Facial nerve, 536 Factors of evolution, 44 Fairy shrimp, 417 Falciform spores, 215 Falco, 617 Falcons, 617 Falconiformes, 616 Fasciolaria, 276 Fat body, 407, 468 Faunal provinces, 175 Fa via, 260, 261 Favositidse, 259 Feather tracts, 603 Feathers, 603 Feathers, molting of, 6li Fecundation, 147 Felidse, 647 Felis, 647 Femoral pores, 589 Femur, 463, 529 Fenestra oval is, 544 Fenestra rotunda, 544, 593 Fertility of hybrids, 29 Fertilization of eggs, 147, 148 Fertilization in Protozoa, 206 Fibre, 639 Fibrec, nerve, 94 Fibula, 529 Fibulare, 529 Fiddler crab, 437 Field mice, 639 Fierasfer, 349 Filaments, mesenterial, 252 Filar substance, 61 Filaria, 303 Filibranch, 362 Filibranchiata, 365 Fin backs, 646 Finches, 616 Fins, 526. 562 Fireflies, 484 Firmisternia, 588 Fish hawk, 617 Fishes, 557 Fishes, tails of, 41 Fishes, circulation in, 1 12 Fissilinguia, 599 Fissipedia, 647 Fissurella, 378 Fissurellidse, 379 Fissures of the cord, 532 Flabellum, 410 Flagellata, 200 Flagellate epithelium, 75 Flagellum, 376 Flame cell, 116, 280 Flamingo, 615 Flat worms, 267 Flea, snow, 477 Fleas, 493 Flesh flies, 493 Flies, 491 Flies, black, 493 Flies, blow, 493 Flies, bot, 493 Flies, caddis, 483 Flies, crane, 492 Flies, dragon, 479 Flies, fire, 484 Flies, flesh, 493 Flies, gall, 486 Flies, harvest, 489 Flies, Hessian, 492 Flies, horse, 493 Flies, house, 492, 493 Flies, May, 479 Plies, robber, 493 Flies, saw, 485, 486 Flies, Spanish, '484 Flies, stone, 479 Fluke, 276 Flustra, 324 674 INDEX. Flustrella, 324 Flying fish, 576 Flying foxes, 638 Flying squirrel, 639 Fodientia. 636 Fontanelles, 521 Food vacuole, 183 Food yolk, 80 Foot, 351 Foramen magnum, 522 Foramen Panizzse, 592 Foraminifera, 196 Fore brain, 533 Fore gut, 104 Forficula, 480 Formative yolk, 80 Formicarige, 487 Fossa rhomboidalis, 535 Fossores, 486 Fowl, 613 Fowl, digestive tract of, 105 Fowl, egg of, 153 Foxes, 647 Frenulum 494 Fringillidae, 616 Fritillaria, 506 Frogs, 588 Frons, 462 Frontal bone, 523 Frontal sinus, 624 Frontoparietal bone, 581 Frugivora, 638 Fulcra, 573 Function, change of, 100 Function, community of, 165 Fungia, 261 Fungiacea, 261 Funiculus, 322 Furca, 420 Furcula, 605 Fur seal, 648 Gadidae, 578 Gadus, 577, 578 Galea, 463 Galeidse, 571 Galen, 12 Galeodes, 450 Galeopithecidae, 649 Galeopithecus, 637, 649 Galeus, 571 Gall flies, 486 Gallinacea, 613 .:Galls, 486 Callus, 614 Gamasidse, 454 Gamasus, 4O°r 454 Gammarina, 439 Gammarus, 439 Ganglion, buccal, 390 Ganglion cells, 94 Ganglion, cerebral, 123 Ganglion, optic, 129 Ganglion, stellate, 390 Ganglion, supraoesophageal, 123 Ganglionic nervous system, 122 Ganoid scale. 572 Ganoidei, 558 Ganoin, 558 Gapes, 302 Garpike, 574 Garter snake, 601 Gasteropoda, 369 Gasterosteus, 577 Gastral tentacles, 246 Gastrochaena, 368 Gastropliilus, 493 Gastrotricha, 295 Gastrovascular space. 228 Gastrovascular system, 109 Gastrula, 156 Gastrulation, 156 Gavialis, 602 Gazella, 642 Gecarcinus, 437 Gecko, 598 Gegenbaur., 18 Gelasimus, 437 Gemmaria, 241 Gemmellaria, 324 Gemmulae, 227 Gemmularia, 241 Gena, 414, 462 Generation, asexual, 140 Generation by parents, 140 Generation, sexual, 142 IXDEX. 675 Generation, spontaneous, 139 Generations, alternation of, 144 Genital ducts, 120 Genital plates, 344 Genus, 10 Geocores, 489 Geodia, 227 Geographical distribution, 174 Geological distribution, 180 Geometrina, 494 Geonemertes, 291 Geophilidae, 461 Geophilus, 461 Gephyraea, 316 Gerardia. 259 Germinal disc, 152 Germinal epithelium, 118 Germinal vesicle, 81, 146 Germ layer theory, 17 Germ layers and budding, 159 Germ layers, formation of, 156 Geryonia, 242 Geryonid, delamination in, 157 Geryonid, germ layers, 157 Giant cells, 71 Gibbons, 651 Gigantostraca, 443 Gila monster, 599 Gill arch, 524 Gill arteries, 504, 548 Gill clefts, 501, 547 Gill leaves, 361 Gill slits, 501, 547 Gill, tracheal, 469 Gills, 108 Gills of fishes, 565 Gills of vertebrates, 547 Gipsy moth, 119, 495 Giraffa, 642 Girdles, 527 Gizzard, 106, 461 Glabelia, 414 Gland cells, 76 Glands, 77 Glands, castor, 618 Gland, choroid, 564 Glands, germinal, 118 Gland, Harder's, 542 Glands, hoof, 618 Gland, lachrymal, 542 Glands, lymph, 550 Gland, lymphoid, 331 Glands, mammary, 619 Glands, milk, 619 Glands, musk, 618 Gland, nidamental, 392 Gland, ovoid, 331 Gland, paraxon, 331 Gland, parotid, 584 Glands, sexual, 80, 117 Glands, sweat, 618 Gland, subneural, 509 Glands, suborbital, 618 Gland, thymus, 547 Gland, thyroid, 547 Glandular epithelium, 73, 76 Glaser's fissure, 621 Glass crab, 436 Glass snake, 599 von Gleichen, 13 Globiceps, 241 Globigerina, 197, 198 Globiocephalus, 646 Glochidium, 364 Glomeridae, 497 Glomerulus, 117, 552 Glossae, 464 Glossopharyngeal nerve, 536 Glottis, 547 Glugea, 218 Glutin, 85 Glutton, 647 Glyptodontidse, 636 Gnathobdellidse, 321 Gnathochilarium, 496 Goat, 642 Goblet cells, 77 Goblet organs, 307 Goethe, 14, 21 Goeze, 13 Gomphus, 479 Gonads, 117 Goniatites, 394 Goniodes, 479 Gonochorism, 118 Gonodactylus, 429 676 INDEX. Gonophore, 238 Gonotheca, 242 Gonys, 604 Goose barnacle, 425 Gopher turtle, 596 Gordiacea, 304 Gordius, 304 Gorgonidae, 259 Gorilla, 651 Gradientia, 587 Grallatores, 615 Grantia, 225 Grasshoppers, 48, 481 Gray matter, 124, 532 Grebes, 615 Green gland, 411 Green turtle, 596 Gregarina, 213, 215 Gressoria, 480 Gribble, 442 Gromia, 62, 198 Ground substance, 62 Grouse, 614 . Gruiformes, 615 Grus, 615 Gryllidse, 481 Gryllotalpa, 481 Gryllus, 481 Guanin, 558 Guard, 389 Guinea pig, 639 Guinea worm, 303 Gula, 462 Gulls. 615 Gulo, 647 Gunda, 271 Gymnoblastea, 241 Gymnodonti, 578 Gymnolsemata, 323 Gymnonoti, 576 Gymnophiona, 587 Gymnosomata, 382 Gynsecophoral canal, 119 Gynandromorphism, 277 Gyri, 535 Gyrodactylus, 273, 274 Habrocentrum, 453 Haddock, 578 Hadenoecus, 481 Haeckel, 18, 24 Haemadipsa, 321 Haemal arch, 516 Haemal ribs, 518 Haemal spine, 517 Haemapophysis, 517 Hsemoccele, 109, 113 Haemoglobin, 89 Haemosporida, 216 Haemuntaria, 321 Hagfishes, 555 Hair, 617 Hair necks, 302 Hair worm, 304 Hairs, auditory, 127 Hairs, tactile, 126 Halcampa, 259 Haleremita, 241 Haliaetus, 617 Halibut, 578 Halicore, 645 Halicryptus, 317 Haliomma, 135 Haliommidae, 196 Haliotidae, 379 Haliotes, 379 Halisarca, 226 Halitherium, 645 von Haller, 17 Halowises, 239 Halteres, 491 Halyclystus, 250 Hammerhead shark, 571 Hapale, 651 Hapalidae, 651 Harder's gland, 542 Hares, 639 Harpactidae, 421 Hatteria, 596 Haustellum, 465 Haversian canals, 87 Haversian lamellae, 87 Hawks, 617 Head kidney, 310, 550 Head, segments of. 536 Hearing, organs of, 127 INDEX. 677 Heart, in Heart shells, 367 Heat rigor, 63 Hectocotylus, 393 Hedgehogs, 637 Heliaster, 337 Helicidse, 383 Helioporse, 259 Heliozoa, 190 Helix, 383 Hell-bender, 587 Hellgrammite, 482 Helminthes, 169 Helminthophaga, 616 Heloderma, 599 Helodermatidae, 599 Hemelytra, 489 Hemerobiidse, 482 Hemibranchii, 575. 577 Hemichordia, 512 Hemimetabolous, 473 Hemiptera, 489 Hemitripterus, 577 Hen. 613 Hen clam, 368 Hepatopancreas, 106, 411 Hepatus, 437 Heptanchus, 570 Heredity, 67, 150 Hermaphroditism, 118 Hermit crabs, 436 Herons, 615 Herring, 576 Hesperornis, 612 Hessian fly, 492 Heterakis, 301 Heteraxial symmetry, 136 Heterocercal tail, 41, 562 Heteroconchiae, 367 Heterocotylea, 273 Heterodera, 300 Heterodont dentition, 625 Heterodont hinge, 359 Heterogony, 144, 145, 486 Heteromera, 484 Heteromyaria, 367 Heteronemertini, 292 Heteronereis, 3 1 1 j Heteronomy, 138 Heteropleuron, 504 Heteropoda, 380 Heteroptera, 489 Heterosyllis, 311 Heterotricha, 209 Hexacoralla, 259 Hexactinellidae, 226 Hexamita, 201 Hexanchus, 570 Hexapoda, 461 Hind brain, 533 Hind gut, 104 Hinge, 358 Hinny, 641 Hipparion, 643 Hippasterias, 337 Hippidse, 437 Hippocampus, 578 Hippocrates, 12 Hippocrene, 241 Hippoglossus, 578 Hippolyte, 434 Hippopotamidae, 641 Hippopotamus, 641 Hippospongia, 227 Hirudinei, 318 Hirudo, 321 Hirundinidse, 616 Hirundo, 616 Holoblastic cleavage, 153 Holoblastic eggs, 152, 153 Holocephali, 572 Holocystites, 342 Holometabolous, 473 Holostei, 573 Holostomate, 371 Holothuria, 349 Holothuria, gastrula of, 158 Holothuridea, 346 Holotricha, 209 Homarus, 435 Homaxial animals, 135 Homo, 651 Homocercal tail, 41, 563 Homoiothermous, 1 15 Homology, 14, 100 Homonomy, 138 678 INDEX. Homoptera, 489 Honey ant, 488 Honeycomb, 641 Hoofs, 618 Hooker, 24 Hoploceras, 643 Hoplorhynchus, 213 Hop worm, 495 Hormea, 324 Hormiphora, 262, 264 Horn bills, 616 Horns of cord, 533 Horned toad, 599 Horn tails, 486 Horse flies, 493 Horse mackerel, 577 Horses, 641 Horseshoe crab, 444 House fly, 492, 493 Human embryo, 35 Humerus, 529 Humming birds, 616 Huxley, 18, 24 Hyaena, 647 Hyaenidse, 647 Hyalea, 381 Hyaleidse, 382 Hyaline cartilage, 86 Hyalonema, 226 Hyalopus, 198 Hyalospongia, 226 Hyas, 437 Hyatt, 24 Hybrids, 28 Hydnophyton, 488 Hydra, 230, 240 Hydra, section of, 141 Hydrachna, 454 Hydrachnidse, 454 Hydractinia, 241 Hydranth, 231 Hydraria, 239, 240 Hydrichthys, 240, 242 Hydrobatidoe, 489 Hydrocaulus, 231 Hydrochoerus, 639 Hydrocorallina, 239, 241 Hydrocores, 489 Hydroides, 313 Hydromedusae, 230 Hydrophilidse, 484 Hydropolyp, 230 Hydropsyche, 483 Hydrorhiza, 231 Hydrosauria, 594 Hydrotheca, 233 Hydrozoa, 230 Hyla, 588 Hylesinus, 485 Hylidse, 588 Hylobates, 651 Hylodes, 586 Hymenolepis, 287, 288 Hymenoptera, 485 Hyocrinus, 340 Hyoid arch, 524 Hyoid bone, 524 Hyoid cartilage, 524 Hyomandibular, 524, 525 Hypsena, 495 Hypaxial muscles, 518 Hyperia, 439 Hyperina, 439 Hyperoartia, 557 Hyperotretia, 557 Hypoblast, 156 Hypobranchial groove, 503 Hypoderma, 493 Hypodermis, 398 Hypogeophis, 587 Hypoglossal nerve, 536 Hypopharynx, 463 Hypophysis, 535 Hypoplastron, 595 Hyporachis, 603 Hypotricha, 21 1 Hyracoidea, 644 Hyracotherium, 643 Hyrax, 644 Hystricidse, 639 Hystricomorpha, 639 Hystrix, 639 lapyx, 477 Ibis, 615 Ichneumonidse, 486 INDEX. 679 Ichthydium, 295 Ichthyobdella. 321 Ichthyodolurites, 570 Ichthyophis, 585, 587 Ichthyopsida, 555 Ichthyosauria, 594 Ichthyotomi, 572 Icteridse, 616 Icterus, 616 Idiothermous, 115 Idotea, 441, 442 Idoteidse, 442 Idyia, 264 Iguanidse, 599 Ilium, 528 Ilyanassa, 379 Imaginal discs, 476 Impennes, 615 Impregnation, 147 Inbreeding, 29 Incisor teeth, 625 Incus, 525, 544 Indirect cell division, 68 Indirect development, 160 Inermes, 317 Infrabasalia, 340 Infundibulum, 534 Ingluvies. 106, 467 Inia, 645 Inorganic bodies, 133 Inquilines, 486 Insecta, 458 Insectivora, 637 Insects, cleavage of egg, 155 Integripalliata, 367 Interambulacral plate, 335 Intercalaria, 516 Interfilar substance, 62 Interhyal bone, 561 Intermaxillary bone, 525 Intermedium, 529 Interorbital septum, 560 Interparietal bone, 619 Interradius, 246 Intervertebral ligament, 519 Intestine, 106 Invagination, 156 Inversion of retina, 541 Iris, 130, 131 Irritability, 62 Ischial callosities, 651 Ischium, 528 Isinglass, 573 Isis, 259 Isodont hinge, 359 Isopoda, 440 Isoptera, 478 Itch, 454 Iter, 534 lulidse, 497 lulus, 497 Ixodes, 454 Ixodidae, 454 Jacobson's organ, 539 Jassidae, 490 Jays, 616 Jigger, 494 Jugal arch, 526 Jugal bone, 526 Jugulares, 562 Jugular vein, 549 June bug, 484 Jurassic, 180 Kallima, 47 Kangaroos, 634 Karyokinesis, 68 Katydid. 481 Keyhole limpets, 379 Kidneys, 116, 550 Kielmeyer, 15 Kinetoskias, 324 King crab, 444 King fishers, 616 Kinosternon, 596 Kiwi, 613 Koenenia, 449 Kolliker, 18 Kowalewskia, 506 Kowalewsky, 18 Labial cartilage, 534 Labial palpi, 362 Labidura, 480 Labium, 463, 464 680 INDEX. Labor, division of, 165 Labridse, 576 Labrum, 463 Labyrinth, 128, 542 Labyrinthodonta. 586 Lac, 490 Lacerta, 599 Lacertilia, 598 Lacertilidse. 599 Lace wings, 482 Lachrymal bone, 590 Lachrymal gland, 542 Lacinia, 473 Lacteal dentition, 625 Lacuna, 379 Lacunar blood system, 1 13 Ladder nervous system, 124 Lady bird, 485 Lady crab, 436 Laemodipoda, 439 Lagena, 543 Lagomys 639 Lama, 643 Lamarck, 14, 22 Lamarckism, 53 Lamblia, 202 Lamellae, Haversian, 87 Lamellae bone, 87 Lamellibranchiata, 358 Lamellicornia, 484 Lamellirostres, 615 Lamna, 571 Lamnae, 618 Lamprey eels, 555, 557 Lampyridee, 484 Land crab, 437 Lanistes, 372 Lantern of Aristotle, 345 Larus, 615 Larva, 160 Larval organs, 161 Laryngeal cartilages, 524 Lateralia, 424 Lateral line, 537. 564 Lateral teeth, 359 Latrodectes, 451 Laurer's canal, 273 Leaf butterflies, 47 Leaf hoppers, 489 Leatherback tortoise, 595 Leather turtle, 596 Leda, 367 Leeches, 318 Leeuwenhoek, 13 Lemniscus, 304 Lemuridae, 649 Lemurs, 648, 649 Lens of eye, 130, 131, 541 Lepadidae, 425 Lepas, 172, 425 Lepidonotus, 313 Lepidoptera, 494 Lepidosauria, 594, 597 Lepidosiren, 579 Lepidosteidae, 574 Lepidosteus, 574 Lepidurus, 416 Lepisma, 477 Lepralia, 324 Leptalis, 48 Leptasterias, 337 Leptocardii, 502 Leptocephalus, 575 Leptochela, 441, 442 Leptoclinum, 510 Leptodiscus, 204 Leptodora, 417 Leptomedusae, 239, 242 Leptoplana, no, 271 Leptostraca, 427 Lepus, 639 Lernaea, 422 Lernaeidae, 422 Lernaeocera, 421, 422 Lernaeopodidae, 422 Leucania, 495 Leucetta, 225 Leuckart, 18 Leucocytes, 88 Leucon, 223 Leucones, 226 Leucortis, 226 Leucosoidea, 437 Leucosolenia, 225 Libel lula, 479 Libellulidae, 479 INDEX. 681 Libinia, 436, 437 Lice, 491 Lice, bird, 479 Lice, book, 479 Life, origin of, 140 Ligula, 286 Ligulidae. 286 Limacidaa, 383 Limacinidae, 382 Limax, 383 Limbs of vertebrates, 527 Limicola, 315 Limitaiis cxterna, 540 Limitans interna, '540 Limnadia, 417 Limnaea, 383 Limnaeidse, 383 Limnocnida, 239 Limnocodium, 239 Limnoria, 441, 442 Limnothrips, 479 Limpets, 379 Limulus, 444 Linckia, 334 Linear nervous system, 122 Linerges, 250 Lines of growth, 358 Lineus, 289, 292 Lingual ribbon, 355, 373 Linguatulida, 454 Lingula, 328 Linin, 65 Linnsean system, 10 Linnaeus, lo Liobunum, 451 Lion, 647 Liriope, 239, 242 Lithistidse, 226 Lithobiidse, 461 Lithobius, 461 Lithodidae, 437 Lithodomus, 367 Littorina, 379 Littorinidse, 380 Liver, 106 Liver fluke, 276 Lizards, 598 Lizzia, 240, 242 Lobatae, 264 Lobate foot, 614, 615 Lobi inferiores, 563 Lobosa, 189 Lobster, 435 Lobster, spiny, 436 Locomotion, 12 1 Locustidae, 481 Locusts, 481, 489 Loggerhead, 596 Loligo, 384, 395 Loligo, cleavage of, 155 Longipennes, 615 Loons, 615 Lophobranchii, 578 Lophodont teeth, 626 Lophogastridae. 429 Lophophore, 324 Lophopoda, 324 Lophopus, 324 Lophs of teeth, 626 Lorica, 200 Loricata, 435, 577, 601, 636 Loris, 649 Lota, 578 Love dart, 376 Loven's larva, 309 Loxia, 616 Loxosoma, 322 Lucernariae, 250 Luciae, 510 Lumbricus, 315, 316 Lumbricus, anatomy of, 118 Lunatia, 379, 380 Lung book, 443 Lung fishes, 579 Lungs, 109, 547 Lung sac, 443 Lung sacs of birds, 609 Lutra, 647 Lycosa, 453 Lyell, 23, 24 Lygseidae, 489 Lymph, 88, 90 Lymph corpuscles, 90 Lymph glands, 550 Lymph System, 550 Lymph vessels, 114 682 INDEX. Lymphoid gland, 331 Lyonet, 13 Lyre birds, 616 Lyriform organs, 445 Lytta, 484 Macacus, 651 Macaques, 651 Machilis, 458, 477 Mackerel, 577 Mackerel shark, 571 Macoma, 368 Macrsesthete, 357 Macrobdella, 321 Macrobiotus, 455 Macrochelys, 596 Macrochiroptera, 638 Macrodrila, 315 Macrogamete, 185 Macronucleus, 206 Macropodidse, 633 Macropus, 634 Mactra, 359 Mactridse, 368 Macrura, 434 Madrepora, 261 Madreporaria, 260 Madreporite, 330 Maioidea, 437 Malaclemmys, 596 Malacobdella, 291 Malacoderma, 259 Malacopoda, 456 Malacopteri, 574 Malacostraca, 426 Malagassy region, 178 Malapterurus, 576 Malar bone, 526, 620 Malaria, 217, 492 Maldanidse, 313 Malleus, 525, 544 Mallophaga, 479 Malpighi, 13 Malpighian body, 117 Malpighian tubes, 438, 445, 459, 461 Mammalia, 617 Mammals, 617 Mammoth, 644 Man, 651 Manatee, 644 Manatus, 644 Mandible, 401 Mandibles, 463, 464 Mandibular arch, 524 Mandibular cartilage, 524 Mandrils, 651 Manicina, 261 Manis, 636 Manna, 489 Manubrium, 235 Mantidse, 480 Mantis, 480 Mantis shrimp, 429 Mantle, 351, 505 Mantle cavity, 352 Manyplies, 642 Manyunkia, 313 Margarita, 379 Margelis, 144, 241 Marginal plates, 595 Marine faunae, 179 Marmosets, 651 Marsipobranchii, 555 Marsupialia, 632 Marsupial bones, 631, 632 Marsupium, 632 Marten, 647 Mastax, 294 Mastigamceba, 188, 201 Mastigophora, 200 Mastodon, 644 Maturation of egg, 146 Maturation and Fertilization, 147 Matuta, 437 Maxilla, 401, 463 Maxillary bone, 525 Maxillary sinus, 624 Maxillipeds, 401 May flies, 479 Measly meat, 284, 285 Measuring worms, 494 Meckel, 14 Meckelia, 292 Mecoptera, 483 Mediastinum, 546 Medulla oblongata, 534 INDEX. 683 Medullary plate, 501 Medullary sheath, 96 Medusae, 144, 230, 234 Megalops, 434 Megalonyx, 636 Megalosphaeres, 198 Megatherium, 636 Megascolex, 316 Megastoma, 202 Meissner's corpuscles, 126 Melanoplus, 481 Meleagrina, 367 Meleagris, 614 Melitta, 346 Meloidse, 484 Melolontha, 484 Melonites, 345 Melophagus, 493 Melopsittacus, 616 Membracidse, 490 Membrane bones, 515 Membranipora, 324 Membranellse, 209 Membranous cranium, 519 "Menopoma, 587 Mentum, 464 Menuridse, 619 Mephitis, 647 Meridional furrows, 151 Mermithidoe, 304 Meroblastic cleavage, 153 Meroblastic eggs, 152, 154 Meryhippus, 643 Mesectoderm, 222 Mesencephalon, 533 Mesenchyme, 157 Mesenterial filaments, 252 Mesenteries, 109, 545 Mesenteron, 105 Mesethmoid bone, 522 Mesites, 613 Mesoblast, 157 Mesobronchus, 609 Mesoderm, 104, 157 Mesoglcea, 230 Mesohippus, 643 Mesonemertini, 291 Mesonephros, 550 Mesonephric duct, 550 Mesopterygium, 529 Mesorchium, 546 Mesotroche, 309 Mesosternum, 462 Mesothelium, 158 Mesothorax, 462 Mesozoic era, 180 Mesovarium, 546 Metabolism, 172 Metacarpal bones, 529 Metagenesis, 144 Metameres, 137, 305 Metamerism, 137 Metamorphosis, 161 Metamorphosis of insects, 473 Metanemertini, 291 Metanephric duct, 550 Metanephros, 550 Metapodium, 369 Metapterygium, 529 Metastoma, 430 Metatarsal bones, 529 Metathorax, 462 Metazoa, 221 Metencephalon, 533 Methona, 48 Metridium, 259 Miastor, 492 Mice, 639 Micraesthete, 357 Microcentrum, 481 Microchiroptera, 638 Microconodon, 632 Microcotyle, 274 Microdrilaa, 315 Microgametes, 185 Microlepidoptera, 494 Microlestes, 632 Micronucleus, 206 Micropterus, 577 Micropylar apparatus, 148 Microsphaeres, 198 Microstomidae, 271 Microthelyphonida, 448 Micrura, 292 Midas, 651 Mid brain, 533 684 INDEX. Middle Ages, Zoology in, 9 Mid gut, 105 Miescher's corpuscles, 218 Migration of birds, 612 Migration theory, 52 Miliola, 197, 198 Milk teeth, 625 Millepora, 233, 241 Mimicry, 46 Mink, 647 Miocene 181 Miohippus, 643 Miracidium, 2"6 Mites, 453 Mitosis, 68 Mixipterygium, 570 Mnemiopsis, 264 Moccasin, 601 Modiola, 366. 367 Molar teeth. 625 Mole cricket, 481 Moles, 637 Molgula, 510 Molgulidae, 510 Mollusca, 351 Molpadia, 349 Monactinellidae, 227 Monadina, 202 Monascidise, 510 Monaxial symmetry, 135 Monera, 189 Moniezia. 287, 289 Monitor, 599 Monkeys, 651 Monocaulis, 240, 241 Monocystis, 215 Monodelphia, 634 Monodon, 646 Monogenea, 273 Monogony, 140 Monomyaria, 367 Monops, 271 Monophyodont, 625 Monopneumonia, 579 Monopylea, 196 Monorhina, 556 Monoscelis, 271 Monospermy, 148 Monostomum, 275 Monothalamia, 198 Monotocardia, 379 Monotremata, 631 Moose, 642 Morphology, 2 Morphology, development of, 12 Mosaic vision, 406 Mosasaurus, 600 Moschidae, 642 Moschus, 642 Mosquitos, 492 Mosquitos and malaria, 217 Moths, 494 Mouse, 639 Mud crab, 437 Mud puppy, 587 Mud turtle. 596 Muller, Fritz, 24 Mtiller, J., 18 Muller, O. F., 13 Miillerian duct, 551 Muller's fibres, 540 Mule. 641 Multicellular glands, 77 Multicellularity, 70 Multinuclearity, 70 Multituberculata, 632 Muricidse, 380 Mus, 639 Musca, 492, 493 Muscariae, 493 Muscidae, 493 Muscle cells, 92 Muscle fibres, 91 Muscular tissue, 91 Musculature, 12 1 Musk deer, 642 Musk ox, 642 Musk rat, 639 Mussels, 367 Mustela, 647 Mustelidse. 647 Mustelus, 571 Mya, 368 Mycetozoa, 198 Myctodera, 587 Myelin. 96 INDEX. 685 My gale, 453 Mygalidae, 453 Mygnimia, 49 Myidae, 368 Mylodon, 636 Myocommata, 531 Myomerism, 523 Myomorpha, 639 Myopsida, 395 Myosepta, 531 Myotomes, 531 Myrianida, 310, 313 Myriapotla, 408. 459, 496 Myriothelia, 241 Myriotrochus, 349 Mynnecocystus, 488 Myrmecophaga, 636 Myrmecophily, 169 Myrmeleo, 481, 483 Mysididse, 429 Mysis, 428, 429 Mysticetae, 645, 646 Mytilus, 363 Myxicolida, 313 Myxidium, 213, 217 Myxine, 557 Myxobolus, 217 Myxomycetes, 198 Myxospongiae, 225, 227 Myxosporida, 217 Myzobdella, 315 Myzontes, 557 Nacre, 361 Nageli. 24, 54, 55 Naiadse, 367 Naididse, 315 Nails, 618 Nais, 307 Naja. 601 Nandu, 613 Nanomia, 244. 245 Narcomedusae, 239, 242 Narwal, 646 Nasal bone, 523 Nassa, cleavage of, 154 Nassellaria, 196 Natatores, 614 Naticidae, 380 Natural selection, 44 Nauplius, 37, 413 Nauplius eye, 412 Nausithog, 134, 250 Nautilidae, 394 Nautilus, 387, 394 Nearctic region, 176. 178 Nebalia, 427 Nectonema, 304 Nectonemertes, 291 Necturus, 587 Nectocalyx, 243 Needham's sac, 392 Nematocysts, 229 Nematoda, 298 Nemathelminthes, 298 Nematophora, 228 Nematus, 486 Nemerteans, 289 Nemertini, 289 Nemocera, 492 Nemognatha, 483 Nemopsis, 242 Neocrinoidea, 342 Neogaea, 177 Neomenia, 358 Neotropical region, 176, 177 Nephilis, 321 Nephridia, 116, 308 Nephrostome, 116 Nepidae, 489 Neptunus, 437 Nereidae, 313 Nereis, 311, 312, 313 Nerve-end buds, 537 Nerve fibres, 94 Nerve hillock, 537 Nerve roots, 533 Nerves of vertebrates, 535 Nervous system, 122 Nervous tissue, 94 Nettle bodies, 205 Nettle cells, 229 Neural arch, 516 Neural plates, 594 Neural spine, 517 Neurapophysis, 517 686 INDEX. Neurenteric canal, 502, 532 Neurites, 94 Neuropodium, 308, 312 Neuropore, 503, 532 Neuroptera, 481 Never ita, 380 New Zealand, 177 Nictitating membrane, 541 Nidamental glands, 392 Night hawks, 616 Nipple, 619 Nirmus, 479 Noctiluca, 201, 203 Noctuina, 495 Nodes of Ranvier, 96 Nomarthra, 636 Nomenclature, binomial, 10 Non-Ruminantia, 641 Nosema, 218 Nothria, 313 Notochord, 501, 515 Notochordal sheath, 516 Notodelphys, 585 Notodelphidse, 422 Notogsea, 176 Notonectidae, 489 Notopodium, 308, 312 Nototrema, 585 Notum, 462 Nuclear plate, 595 Nuclear fragmentation, 70 Nuclear spindle, 68 Nuclear substance, 65 Nuclein, 65 Nucleolus, 66 Nucleus. 58, 64 Nucleus, cleavage, 149 Nucleus, egg, 146 Nucleus in fertilization, 149 Nucleus of Salpa, 511 Nucleus, significance of, 67 Nucleus, somatic, 208 Nucleus, sperm, 149 Nucleus, substance of, 65 Nucleus, structure of, 65 Nucula, 365, 367 Nuculidae, 367 Nuda, 264 Nudibranchia, 382 Nummulites, 198 Nurse, 144 Nutrition and reproduction, 64 Nyctotherus, 210 Nymphon, 456 Obelia, 240, 242 Obisium, 450 Obturator foramen, 622 Occipitalia, 521 Occipital bone, 521, 619 Occiput, 462 Ocellatae, 239 Ocellus, 129, 403 Ocneria, 119, 495 Octocoralla, 258 Octopoda, 395 Octopodidse, 395 Octopus, 384, 390, 394, 395 Ocular plate, 335 Oculina, 261 Oculomotor nerve, 536 Odonata, 479 Odontoholcae, 612 Odontophore, 373 Odontormse, 612 Odontornithes, 612 QScanthus, 481 CEcology, 457, 164 QEdipoda, 481 OZdogonium, 173 CEgopsida, 394 CEsophageal ring, 124 Oesophagus, 106, 546 OZstridae, 493 OZstrus, 193 Oikopleura, 506, 507 Oil bottle, 484 Olfactory organs, 126 Olfactory organs of vertebrates, 538 Olfactory lobe, 534 Olfactory nerve, 536 Oligocene, 181 Oligochaetae, 314 Oligosoma, 599 Oligotrochus, 349 Olividse, 380 INDEX. 687 Olynthus, 222 Omasum, 642 Omentum, 546 Ommastrephes, 388, 395 Ommatidium, 405 Oncosphaera, 283 Oniscidae, 442 Oniscus, 442 Ontogeny, 3, 160 Oospore, 185 Ootype, 272 Opalina, 209 Opercular bones, 562, 566 Opercularella, 242 Operculum, 210, 323, 371, 440, 566 Ophidia, 600 Ophidiaster, 334 Ophiocoma, 338 Ophiocnida, 338 Ophioglypha, 338 Ophiopholis, 338 Ophiothelia, 338 Ophisaurus, 599 Ophisthotic bone, 522 Ophiuroidea, 337 Opisthobranchia, 381 Opisthocoelous, 519 Opisthopatus, 458 Opossums, 633 Opossum shrimp, 428 Opoterodonta, 601 Optic ganglion, 129 Optic lobes, 534 Optic nerve, 536 Optic stalk, 541 Optic thalami. 534 Optic vesicle, 541 Oralia, 330, 340 Orang-utan, 651 Orange scale insect, 490 Orbitelarise, 453 Obitosphenoid bone, 522 Orchestia, 438, 439 Order, 10 Organic bodies, 133 Organisms, origin of, 139 Organ-pipe coral, 259 Organs, 99 Organs, animal, 101, 121 Organs, of assimilation, 102 Organs, auditory, 127 Organs, of Bojanus, 363 Organs, circulatory, 109 Organs, of Corti, 543 Organs, digestive, 103 Organ, dorsal, 341 Organs, electric, 563 Organs of equilibrium, 128 Organs, excretory, 115 Organs, excretory, of vertebrates, 550 Organ of Jacobson, 539 Organs of hearing, 127 Organs, lateral line, 537 Organs, olfactory, 126 Organs, pearl, 558 Organs, respiratory, 107 Organs, sensory, 125 Organs, sexual, 117 Organs, sexual, of vertebrates, 550 Organs of smell, 126 Organs, systems of, 100 Organs, tactile, 125 Organs, of taste, 126 Organs, of touch, 537 Organs, vegetative, 101, 102 Oriental region, 176, 178 Orioles, 616 Ornithodelphia, 631 Ornithorhynchidae, 632 Ornithorhynchus, 631, 632 Orohippus, 643 Oronasal groove, 538 Orthis, 328 Orthoceras, 394 Orthonectida, 220 Orthoneurous, 374 Orthopoda, 597 Orthoptera, 480 Orycteropus, 636 Oscarella, 227 Oscines, 616 Osculum, 222, 225 Os en ceinture, 581 Ossein, 86 Os transversum, 590 Os turbinale, 620 688 INDEX. Osmerus, 576 Osphradium, 354 Ossicle, auditory, 127 Ostariophysi, 575 Osteoblasts, 88 Ostracoda, 422 Ostracodermi, 557, 578 Ostracoteuthis, 388 Ostraeidae, 367 Ostium tubae, 552 Ostrich, 613 Otaria, 648 Otic ganglion, 537 Otica, 521 Otis. 615 Otocysts, 236 Otoliths, 127 Otter, 647 Ovibos, 642 Ovicells, 322 Ovidae, 642 Oviducts, 120 Oviparous, 161 Ovis, 642 Ovoid gland, 331 Ovoviviparous, 161 Owen, 18 Owlet moths, 495 Owls. 617 Ox warble. 493 Oxy haemoglobin, 89 Oxyrhyncha, 437 Oxystomata, 437 Oxyuris, 301 Oyster crab, 437 Oysters, 367 Pachydermata, 641 Pachydrilus, 315 Pachylemuridae, 649 Paddle fish, 573 Psedogenesis, 142, 472 Paguridea, 436 Palaearctic region, 176, 178 Palaemon, 400. 434 Palsemonetes, 434 Palaemonidae, 434 Palseocrinoidea, 342 Palaeotherium, 643 Palaeozoic era, 180 Palate, 539 Palatine bone, 525 Paleacrita, 494 Palechinoidea, 345 Paleontology, 4 Paleozoology, 4 Pali, 256 Palinuridae, 435 Palinurus, 436 Pallial line, 359 Pallial sinus, 360 Pallium, 351, 534 Palmate foot, 614, 615 Palm crab, 436 Palolo, 311 Palpi, labial, 362 Palpus, 430, 463 Paludicella 324 Paludinidse, 380 Pancreas. 1 06 Pandalus, 434, 435 Pandion, 617 Pandionidae, 617 Pangolin, 636 Panopeus, 437 Panorpa, 483 Panorpidae, 483 Pantopoda, 456 Paper nautilus, 395 Papilio, 496 Parachordals, 520 Paractinopoda, 349 Paradidymis. 552 Paradisea, 50 Paradiseidae, 616 Paradoxides, 415 Paraglossa, 464 Paragnath, 430 Paramaecium, 206, 207, 209 Paranuclein, 66 Paranucleus, 206 Parapodium, 312, 369 Parapophysis, 518 Parapterium, 604 Paraquadrate bone, 581 Parasita, 422 INDEX. 689 Parasitism, 167 Parasphenoid bone, 523 Parasuchia, 602 Paraxon gland, 331 Parietal bone, 523 Parietal foramen, 590 Parietal ganglia, 354 Parietal organ, 535 Parostoses, 519 Parotid gland, 584 Parrots, 616 Parthenogenesis, 142, 145, 472 Partial cleavage, 152, 153 Partridge, 614 Parypha, 241 Passer, 616 Passeres, 616 Patagium, 637 Patellidae, 378 Pathetic nerve, 536 Paunch, 641 Pauropida, 497 Pauropus, 497 Pearl organs, 558 Pearl oysters, 367 Pearls, 361 Pearls, artificial, 558 Pebrine, 218 Peccaries, 641 Pecora, 641 Pecten, 366 Pecten of eye, 611 Pectinatella, 324 Pectines, 447 Pectinibranchia, 379 Peclinidse, 367 Pectoral fin, 562 Pectoral girdle, 527 Pedal cords, 354 Pedal ganglia. 353 Pedata, 349 Pedes spurii, 475 Pedicellina, 322, 330 Pediculati, 575 Pediculus, 491 Pedipalpi, 448 Pedipalpus, 445 Pelagia, 246. 250 Pelecanus, 615 Pelecypoda, 358 Pelicans. 615 Pelmatozoa, 338 Pelobatidse, 588 Pelomyxa, 189 Peltogaster, 426 Pelvic fin, 526, 562 Pelvic girdle, 527 Pen, 389 Peneidse, 434 Penella, 422 Peneus, 434 Penguin, 615 Penis, 120 Pennaria, 241 Pennatula, 259 Pennatulidae, 259 Pentacrinus, 339, 342 Pentacta, 349 Pentadactyle appendage, 529 Pentamera, 484 Pentamerus, 328 Pentastomum, 169, 445 Pentatomidse, 489 Pentatoma, 489 Pentremites, 342 Perameles, 633 Peramelidse, 633 Perca, 574, 577 Perch, 577 Percidae, 577 Perdix, 614 Pereiopoda, 401 Perforata, 197, 198 Peribranchial chamber, 503, 505 Pericardial sinus, 470 Pericardium, in, 546 Perichaeta, 316 Perichondrium, 86 Pericolpa, 250 Peridinium, 203 Perilymph, 543 Periosteum, 87 Peripatidse, 456 Peripatopsis, 458 Peripatus, 456, 458 Peripharyngeal band, 506 690 INDEX. Peripheral nervous system, 122 Periphylla, 250 Periplaneta, 480 Periproct, 343 Peripylea, 195 Perisarc, 233 Perissodactyla, 640, 641 Peristome, 209, 343 Peritoneal cavity, 546 Peritoneum, 109, 546 Periwinkle, 380 Perla, 479 Perlidse, 479 Permian, 180 Perennibranchiata, 587 Peromedusse, 250 Perophora, 510 Peropoda, 601 Perradius, 246 Petaurus, 634 Petiole, 485 Petoscolex, 315 Petromyzon, 557 Petromyzon, cleavage of egg, 154 Petromyzontes, 557 Petrosal bone, 522 Phacellse, 246 Phoenicopterus, 615 Phoeodaria, 196 Phseodium, 196 Phaethon, 615 Phagocata, 271 Phalanges, 529 Phalangida, 450 Phalangistidse, 634 Phalangium, 451 Phallusia, 509 Pharyngeal bones, 560, 576 Pharyngognathi, 576 Pharynx, 106, 506, 546 Phascalosoma, 316, 317 Phascolomyidae, 633 Phascalomys, 633 Phascolion, 317 Phasianella, 379 Phasianidse, 614 Phasianus, 614 Phasmidae, 480 Phasmomantis, 480 Pheasants, 614 Phenacodon, 639 Phenacodontidse, 643 Phidippus, 453 Philichthys, 38 Philine, 381 Philonexidse, 395 Phlegethontias, 495 Phoca, 648 Phocidse, 648 Pholadidse, 368 Phoronidea, 325 Phoronis, 325 Phoxichilidium, 456 Phragmocone, 389 Phronima, 439 Phryganea. 483 Phrynicus, 448 Phrynoidea, 448 Phrynosoma, 599 Phrynus, 448 Phthirius, 491 Phylactolsemata, 324 Phyllium, 48 Phyllodactylus, 598 Phyllopoda, 415 Phyllosoma, 434, 436 Phyllostomidse, 638 Phylloxera, 491 Phylogeny, 4, 31 Physalia, 245 Physeter, 646 Physiological character of species. 27 Physiologus, 9 Physiology, 3 Physoclisti, 567 Physonectse, 244 Physophora, 244 Physophorse, 244 Physopoda, 479 Physostomi, 567, 575 Phytoflagellata, 202 Phytophaga, 633 Picariae, 616 Picas, 639 Pickerel, 576 Picus, 616 INDEX. 691 Pieris, 496 Pieris, cleavage of, 155 Pigeons, 26, 27, 614 Pigmented epithelium, 540 Pike, 576 Pilidium, 290, 291 Pill bug, 442 Pineal eye, 535 Pinealis, 535 Pinniped ia, 647 Pinnotheres, 437 Pinnulae, 340 Pin worm, 301 Pipa, 585, 588 Pipe fish, 578 Pisces, 557 Piscicola, 321 Pisidium, 368 Pituitary body, 535 Placenta, 634 Placentalia, 634 Placoid scale, 515, 558 Placophora, 356 Plagiaulax, 632 Plagiotremata, 597 Plagiostomi, 569 Planaria, 271 Planarians, 268 Planipennia, 482 Plankton, 179 Planorbis, 383 Plantigrade, 647 Plant lice, 490 Plants and animals, 171 Planula, 237 Plasma, blood, 88 Plasmic products, 64. 72 Plasmodium, 198, 21 6 Plastin, 66 Plastogamy, 184 Plastron, 594 Platanista, 645 Plathelminthes, 267 Platyonichus. 436, 437 Platyrrhinoe, 651 Plecoptera, 479 Plectognathi, 578 Pleistocene, 181 Pleopoda, 401, 402 Plesiosauria, 594 Plethodon, 587 Pleura, 414, 462, 546 Pleuracanthus, 572 Pleural cavity, 546 Pleural cords, 354 Pleural ribs, 518 Pleurobrachia, 262, 264 Pleurocercoid, 283 Pleurodira, 596 Pleurodont teeth, 599 Pleuronectidae, 578 Pleuroperitoneal cavity, 546 Plictolophus, 616 Pliocene, 181 Pliohippus, 643 Pliny, 8 Plover, 615 Plumularia, 242 Plumatella, 324 Pluteus, 332 Pneumatic duct, 567 Pneumaticity of bones, 608 Pneumatophore, 243 Pneumodermon, 382 Pneumogastric nerve, 536 Podocoryne, 241 Podophrya, 68, 212 Podophthalmia, 427 Podura, 477 Poikilothermous, 115 Polar bodies, 146 Pole field, 263 Poles of egg, 147, 151 Polian vesicles, 331 Polistotrema, 557 Polybostrichus, 311 Poly ch setae, 311 Polychoerus, 269, 271 Polycladidea, 269, 271 Polyclinum, 510 Polyclonia, 247, 251 Polycystidse, 214 Polydesmidoe, 497 Polyergus, 488 Polygordius, 309. 314 Polymorphism, 165 692 INDEX. Polynesia, 177 Polynoe, 313 Polynqidae, 313 Polyodon, 573 Polyodontidae, 573 Polyp, 230 Polyphemidas, 417 Polypid, 322 Polypodium, 239, 241 Polyprododonta, 633 Polypterus, 573 Polypterus tail, 41 Polyscelis, 271 Polyspermy, 148 Polystomeae, 273 Polystomella, 198 Polystomum. 273, 274 Polythalamia, 196, 198 Polytroche, 309 Polyzoa, 321 Poneridge, 487 Pons Varolii, 623 Pontobdella, 321 Pontodrilus, 308 Pontella, 421 Porcellanidae, 437 Porcellain crabs, 437 Porcellio, 442 Torcellio, nervous system of, 124 Porcupines, 639 Pore'lla, 324 Pori abdominales, 546 Porifera, 221 'Porites. 261 Porpita, 245 Portal vein, 548 Portuguese man-of-war, 245 Portunidae, 437 Porus branchialis, 503 Postabdomen. 401 Postfrontal bone, 526, 590 Postorbital bone, 590 Postpermanent dentition, 625 Potato beetle, 485 Powder down, 603 Praeclavia, 528. 622 Praecoces, 612 Prairie dogs, 639 Praya, 166, 244 Prefrontal bone. 526, 590 Prelacteal dentition, 625 Premaxillary "bone, 525 Premolar teeth, 626 Presphenoid bone, 522 Priapuloidea, 317 Priapulus, 317 Primaries, 604 Primary bone, 519 Primary yolk, 80 Primates, 649 Primnoa, 259 Primordial cranium, 521 Principal tissue, 99 Priodon, 635 Pristidae, 572 Pristis, 572 Proboscidia, 643 Proboscis, 373 Procoelous, 519 Procoracoid, 528 Proctodaeum, 104 Procyon, 647 Proechidna, 631, 632 Proglottids, 278 Profeet, 466 Progression, principle of, 55 Prolegs, 475 Promorphology, 133 Pronephric duct, 550 Pronephros, 550 Prong horn, 643 Pronotum, 462 Pronucleus, 149 Proofs of phylogeny, 32 Proostracum, 389 Prootic bone, 522 Propodium, 369 Propterygium, 529 Prorostomus, 644 Prosencephalon, 533 Prosimiae, 648 Prosobranchia, 378 Prosternum, 528 Prostoma, 156 Protamoeba, 189 Proteroglypha, 601 INDEX. 693 Proteroglyphic tooth, 600 Proteus, 587 Prothorax, 462 Protobranchiata, 365 Protista, 186 Protocaris, 416 Protocerebrum, 462, 468 Protoconchise, 365 Protodonta, 632 Protohydra, 239, 241 Protomerite, 214 Protonemertini, 291 Protonephridia, 115 Protoplasm, 61, 80 Protoplasm, discovery of, 59 Protoplasm, movement of, 62 Protopterus, 579 Prototheria, 631 Protovertebrse, 531 Protozoa, 183 Prortacheata, 408, 456 Protula, 313 Proventriculus, 467 Psammonyx, 198 Pseudelectric organs, 563 Pseudobranch, 570 Pseudocuticula, 597 Pseudolamellibranchiata, 365 Pseudonavicellae, 215 Pseudoneuroptera, 477 Pseudopodia, 187 Pseudoscorpii, 450 Pseudosuchia, 602 Psittaci, 616 Psittacus, 616 Psocidae, 479 Psolus, 349 Psorosperms, 217 Pteranodon, 602 Pteraspis, 557 Pterichthys, 557 Pterodactylia, 602 Pteronarcys, 479 Pteropoda, 382 Pteropod ooze, 382 Pteropus, 638 Pterosauria, 602 Pterotic bone, 522, 560 Pterotracheidae, 380 Pterygoid bone, 525 Pterygoid process, 622 Pterygoquadrate, 524 Pterygotus, 444 Pterylae. 603 Pubic bone, 528 Pugettia, 437 Pulex, 493, 494 Pulmonata, 383 Pulmonary artery, 549 Pulmonary circulation, 549 Pulmonary vein, 549 Pulp cavity, 515 Pulvilla, 493 Puma, 647 Pupa, 383 Pupae, 474 Pupipara, 493 Purpura, 379, 380 Putorius, 647 Pycnogonida, 456 Pygidium, 414 Pyloric caeca, 565 Pylorus, 546 Pyrosoma, 510 Pyrula, 373 Python, 601 Pythonaster, 337 Pythonomorpha, 600 Quadrula, 196, 198 Quadrumana, 650 Quadrate bone, 525 Quahog, 368 Quail, 614 Quaternary, 181 Raccoon, 647 Rachis, 603 Racemose glands, 77 Radial canals, 235, 331 Radial symmetry, 135 Radiale, 529 Radialia, 330, 340, 527 Radiata, 228, 329 Radiolaria, 192 Radius, 529 694 INDEX. Radula, 355, 373 Raia, 571, 572 Raiidae, 572 Rail, 615 Rainey's corpuscles, 218 Rallus, 615 Rana, 588 Ranatra, 489 Rangifer, 642 Ranvier, nodes of, 96 Raptores, 616 Raptorial foot, 614 Rasorial foot, 6*4 Rasor clam, 368 Rathke, 1 8 Ratitse, 612 Rats, 639 Rat-tail larva, 493 Rattlesnake, 601 Ray, 10, 20 Reamur, 13 Receptaculum seminis, 120, 471 Rectrices, 604 Rectum, 461 Red coral, 256 Redia, 276 Reduviidae, 489 Regulares, 345 Reindeer, 642 ;Remak, 18 Remiges, 604 Remora, 577 Renilla, 258, 259 Reproduction, asexual, 140, 143 Reproduction, sexual, 142 Reptilia, 588 Respiratory organs, 107 .Respiratory organs of vertebrates, 547 Reticularia, 196 Reticulum, 642 Retina, 129, 131 Retina of vertebrates, 540 Retinaculum, 494. Retinula, 405 Retitelarise, 453 Rhabdites, 270 Rhabditis, 300 Rhabdoccelida, 269, 271 Rhabdom, 129, 405 Rhabdonema, 145, 300 Rhabdopleura, 514 Rhachiglossa, 380 Rhachis, 414 Rhamphastos, 616 Rhea, 613 Rhegmatodes, 242 Rhinoceros, 641 Rhinocerotidse, 641 Rhinoderma. 585 Rhizocephala, 426 Rhizocrinus, 342 Rhizopoda, 187 Rhizostomeae, 250 Rhopalocephalus, 213 Rhopalocera, 495 Rhopalonema, 234 Rhynchobdellidae, 321 Rhynchobothrium, 286 Rhynchocephalia, 596 Rhynchonella, 325, 328 Rhynchophora, 485 Rhynchota, 489 Rhytina, 645 Rib, 517, 518 Right whale, 646 Ring canal, 331 Rocky Mountain sheep, 643 Rodentia, 638 Rods and cones, 129, 540 Root barnacles, 426 Rorqual, 646 R5sel von Rosenhofen, 13 Rossia, 395 Rostellum, 280 Rostrum, 389, 424, 465 Rotalia, 188, 198 Rotatoria, 293 Rotifera, 293 Round worms, 298 Rove beetles, 484 Rudistidse, 368 Rugosa, 258 Rumen, 641 Ruminantia, 641 Rupicapra, 642 INDEX. 695 Sabellidse, 313 Sabinea, 435 Sable, 647 Sacconereis, 311 Sacculina, 426 Sacculus, 128, 542 Saccus vasculosus, 563 Sacral ribs, 528 Sagartia, 259 Sagitta, 296 Sagitta (ear bone), 564 Sagitta. development of, 158 St. Hilaire, 14, 22 Salamandra, 585, 587 Salamindrina, 587 Salinella, 220 Salivary glands, 106 Saltatoria, 480 Saltigrada, 453 Salmo, 576 Salmon, 576 Salmonidae, 576 Salpa, 510, 512 Salpceformes, 510 Sand dollar, 345 Sand saucers, 380 San Jose scale insect, 490 Sapajous, 651 Sapphirina, 421 Sarcocystis, 213, 218 Sarcode, 60 Sarcolemma, 93 Sarcophaga, 493 Sarcophilus, 633 Sarcopsylla, 494 Sarcoptes, 454 Sarcosepta, 255 Sarcosporida, 218 Sarsia, 241 Saurii, 598 Sauropsida, 588 Saururae, 612 Savigny, 14 Savigny's law, 401 Sawfish, 572 Sawflies. 485, 486 Saxicava, 367 Saxicavidae, 368 Scala media, 543 Scala tympani, 543 Scala vestibuli, 543 Scale insects, 490 Scale, placoid, 515 Scales of fishes, 515, 558 Scales of reptiles, 597 Scallops, 637 Scalpellum, 424 Scansores, 616 Scansorial foot. 614 Scape, 603 Scaphander, 381 Scapharca, 367 Scaphiopus, 588 Scaphognathite, 431 Scaphopoda, 369 Scapula, 528 Scarabseidse, 484 Schafter, 13 Schizodont hinge, 359 Schizopoda, 428 Schizopodal appendages, 409 Schizosomi, 437 Sclerophylla, 261 Schleiden-Schwann theory, 58 Schwann, sheath of, 96 Scincidse, 599 Sciuridse, 639 Sciuromorpha, 639 Sciuropterus, 639 Sciurus, 639 Sclera. 131, 539 Scleral bones, 611 Sceleporus, 599 Scelrophyllia, 257 Sclerosepta, 255 Sclerotic, 539 Sclerotic bones, 593 Sclerotic coat, 130, 131 Sclerotomes, 531 Scolex, 278 Scollops, 367 Scolopax, 615 Scolopendra, 460, 461 Scolopendrelta, 497 Scolopendridse, 461 Scomber, 577 696 INDEX. Scombridse, 577 Scops, 617 Scorpionida, 447 Sculpin, 577 Scutellum, 489 Scutibranchia, 378 Scutigera, 461 Scutigeridse, 461 Scutum, 424 Scyphomedusse, 245 Scyphopolyp, 230 Scyphostoma, 245, 246 Scyphozoa, 245 Sea anemones, 251, 259 Sea cucumbers, 346 Sea fans, 259 Seahorse, 578 Sea lion, 648 Sea pens, 259 Sea otter, 647 Sea snakes, 601 Sea squirts, 505, 508 Sea urchin, fertilization of, 149 Sea urchins, 343 Sea whips, 259 Seals, 648 Secodont teeth, 626 Secondaries, 604 Secondary bones, 515 Secreta, 73 Sedentaria, 313, 453 Segmental organs, 116, 308 Segmentation cavity, 155 Segmentation of egg, 149, 151 Segments of head, 536 Selachii, 570 Selection, artificial, 43 Selection, natural, 44 Selection, sexual, 46 Selenodont teeth, 626 Semaeostomae, 250 Semicircular canals, 128, 542 Semilunar valves, 567 Semipalmate foot, 614 Semiplumes, 604 Sensations, 125 Sense organs of vertebrates, 537 Senses, 125 Sensory epithelium, 73, 82 Sensory organs, 125 Sepia, 386, 388, 395 Septibranchiata, 368 Septum, 306 Sericteria, 494 Serosa, 473 Serpulidae, 313 Serranidse, 577 Serripes, 368 Sertularia, 242 Serum, blood, 88 Sesiidae, 495 Seventeen-year locust, 489, 490 Sexual cells, 143 Sexual epithelium, 78 Sexual glands, 78, 80 Sexual organs, 80, 117 Sexual organs of vertebrates, 550 Sexual reproduction, 142, 145 Sexual selection, 49 Shad, 576 Shagreen, 569 Sharks, 571 Sheath of Schwann, 96 Sheep, 642 Sheep tick, 493 Shell gland, 411 Shell, layers of, 361 Ship worms, 368 Shore crab, 437 Shoulder blade, 528 Shoulder girdle, 527 Shrews, 637 Shrimp, mantis, 429 Shrimp, opossum, 428 Siala, 616 Sialidse, 482 Sialis, 482 Sicyonia, 434 Siderone, 47 von Siebold, 18 Silenia, 368 Silicispongise, 226 Siliqua, 367 Silkworms, 495 Silurian, 180 Siluridae, 576 IJSDEX. 691 Silverfish, 477 Simia, 651 Simiidse, 651 Simuliidge, 493 Sinupalliata, 368 Sinus frontalis. 539 Sinus, sphenoid, 539 Siphon, 345, 387, 360 Siphonaptera, 493 Siphonophora, 240, 243 Siphonophores, 166 Siphonostomate, 371 Siphonostomata, 422 Siphuncle, 388 Sipunculoida, 317 Sipunculus, 317 Siredon, 36 Siren, 586 Sirenia, 644 Sirex, 485 Siricidse, 486 Sixth sense, 125, 538 Skalis, 571 Skin, 76 Skull, 519 Skull of mammals, 619 Skunk, 647 Skylark, 616 Slime animals, 198 Slime eels, 557 Slime moulds, 198 Sloths, 636 Smell, organs of, 126 Smelt, 576 Snakes, 600 Snapping turtle, 596 Snout beetles, 485 Snow flea, 477 Social animals, 167 Soft- shell crab, 437 Soft-shelled turtle, 596 Solasteridse, 337 Sole, 578 Solemyidse, 367 Solen, 368 Solenoconchse, 369 Solenogastres, 358 Solenoglypha, 60 1 Solenoglyphic tooth, 600 Solenidse, 368 Solidungula, 641 Solifugae, 449 Solpuga, 450 Solpugida, 449 Somatic cells, 143 Somatic layer, 159 Somatopleure, 159 Somites, 305 Song birds, 616 Sorex, 637 Soricidse, 637 Sowbug, 442 Spadella, 298 Spadix, 238 Spanish flies, 484 Span worms, 494 Spatangoidea, 346 Species, 10 Species, nature of, 19, 25 Species, physiological characters of, 27 Spelerpes, 587 Speotyto, 617 Spermaceti, 646 Spermatophore, 392 Spermatozoa, 81 Spermatozoids, 202 Sperm nucleus, 149 Sperm whale, 646 Sphaeridia, 330 Sphasrogastrida, 451 Sphseroma, 442 Sphaeromidae, 442 Sphaerophrya, 212 Sphaerozoidae, 195 Sphargis, 595 Sphenethmoid bone, 581 Sphenodon, 596 Sphenoidalia, 521 Sphenoid bone, 523, 620 Sphenoid sinus, 539, 624 Sphenopalatine ganglion, 537 Sphenotic bone, 522 Spherical animals, 135 Sphingina, 495 Sphyranura, 274 Spicula, 300 698 INDEX. Spicules of sponges, 225 Spider crab, 436, 437 Spider monkeys, 651 Spiders, 451, 452 Spinal canal, 517 Spinal ganglion, 533 Spindle, directive, 146 Spindle fibres, 69 Spindle, nuclear, 68 Spinnerets, 452 Spinous process, 517 Spiny ant eaters, 632 Spiny lobster, 436 Spiracle, 459, 524, 544 Spiral valve, 565 Spirifer, 328 Spirorbis, 313 Spirobolus, 497 Spirula, 388, 394 Spirulidse, 394 Spittle bug, 489 Splanchnic layer, 159 Splanchnopleure, 159 Spleen, 550 Splenial bone, 582 Splint bones, 640 Spondyhdse, 367 Sponge, fresh-water, 133 Sponges, 221 Spongida, 221 Spongilla, 133, 221, 227 Spongillidae, 227 Spongioplasm, 61 Spontaneous generation, 31 Sporangia, 199 Spores, 213, 215 Sporoblasts, 185, 213, 215 Sporocyst, 276 Sporosacs, 238 Sporozoa, 213 Sporozoites, 185, 213, 215 Springtails, 477 Sprinkling-pot shell, 368 Spumellaria, 195 Squali, 571 Squalus, 571 Squamata, 597, 636 Squamosal bone, 526 Squid, 395 Squilla, 429 Squirrels, 639 Staggers, 493 Stapes, 525 Staphylinidse, 484 Starfish, 333 Statoblasts. 227, 323 Statoliths, 128 Stauromedusse, 250 Steganopodes, 615 Stegocephali, 586 Stegosaurs, 597 Stellate ganglion, 390 Stelmatopoda, 323 Stemma, 403 Stenops, 649 Stenson*s duct, 539 "Stentor, 209 Stephalia, 244 Stephanocyphus, 250 Sterna, 615 Sternaspis, 314 Stercoral pocket, 445 Sternum, 462, 518 Sticklebacks, 577 Stigmata, 459 Sting, 472, 486 Sting rays, 572 Stipes, 463 Stink bug, 489 Stolo prolifer, 512 Stomach, 106 Stomatopoda, 429 Stomodaeum, 104 Stomolophus, 251 Stone canal, 330 Stone flies, 479 Storks, 615 Stratified epithelium, 73, 74 Stratum corneum, 76, 514 Stratum Malpighi, 76, 514 Streaming of protoplasm, 62, 188 Strepsiptera, 483 Streptoneury, 373 Stridulating organs, 469 Striges, 617 Strix, 617 INDEX. 699 Strobila, 249, 278 Strongylidse, 301 Strongyloides, 300 Strongylocentrotus, 345 Strongylosoma, 497 Struggle for existence, 44 Struthio, 613 Struthiones, 613 Sturgeon, 573 Sturgeon, tail of, 41 Stylaster, 241 Style, crystalline, 364 Stylochus, 271 Stylohyoid ligament, 621 Styloid process, 621 Stylommatophora, 383 Stylonychia, 211, 212 Stylopidse, 483 Stylops, 483 Subcutaneous tissue, 514 Subintestinal ganglion, 374 Subintestinal vein, 548 Submentum, 464 Subneural gland, 509 Subumbrella, 234 Suckers, 576 Suck fish, 577 Suctoria, 212 Suidse, 641 Sulci, 535 Summer eggs, 416 Sun animalcules, 190 Supporting cells, 83 Supporting layer, 230 Supraintestinal ganglion, 374 Supraoccipital bone, 522 Supracesophageal ganglion, 123 Suprascapula, 528 Surf perch, 577 Sus, 641 Swallows, 616 Swallow tails, 496 Swammerdam, 13 Swans, 615 Swarm spores, 185, 195 Sweat glands, 618 Swell fish, 578 Swim bladder, 547 Swimming bell, 243 Swimming birds, 614 Swine. 641 Sword fish, 577 Sycandra, 222, 225 Sycon, 223, 225 Sy cones, 225 Syllidse, 313 Syllis, 311 Sylvicolidse, 616 Sylvius. 12 Symbiosis, 169 Symmetry, 134 Sympathetic coloration, 46 Sympathetic system, 537 Symplectic bone, 561 Symphyla. 497 Synapta, 349 Synapticula, 257 Synascidise, 510 Syncitia, 71 Synccelidium, 269, 271 Syncoryne, 241 Synentognathi, 575, 576 Syngamus. 301 Syngnathus, 578 Syringopora, 259 Syrinx, 608 Syrphidse, 493 Systems oi organs, 100 Systemic circulation, 549 Systemic heart, 391 Tabanidae, 493 Tabulae, 257 Tabulatse, 257 Tactile bristles, 126 Tactile corpuscle, 537 Tactile organs, 125 Tadpole, 586 Tadpoles of Rana temporaria, 35 Taenia. 169, 279, 282 Tseniadas, 287 Tseniolse, 246 Tsenioglossa, 380 Talpa, 637 Talpidse, 637 Tanais, 442 700 INDEX. Tanystoma, 492 Tapetum nigrum, 131, 540 Tape worms, 285 Tapiridae, 641 Tapirs, 641 Tapirus, 641 Tarantula, 453 Tardigrada, 455, 636 Tarsal bones, 529 Tarsus, 463 Tarsiidse, 649 Tarsius, 649 Tarso-metatarsus, 607 Tasmanian devil, '633 Taste, organs of, 126 Taste organs of vertebrates, 538 Tatusia, 636 Tautoga, 576 Taxidea, 647 Taxodont hinge, 359 Tectibranchia, 381 Tectrices. 604 Teeth, dermal, 515 Teeth of mammals, 624 Teeth of vertebrates, 547 Tejidse, 599 Tejus, 599 Telea, 495 Teleostei, 574 Teleostomi, 569 Tellina, 368 Tellinidse, 368 Telolecithal eggs, 152 Telson, 427 Telotroche, 309 Temporal bone, 620 Temperature of mammals, 630 Tendinous tissue, 85 Tenebrionidse, 484 Tentacles, gastral, 246 Tentaculata, 264 Tent caterpillars, 495 Tenthredinidse, 486 Terebella, 108, 313 Terebellidae, 313 Terebra, 486 Terebrantia, 486 Terebratulina, 328 Teredo, 368 Teredidae, 368 Tergum, 424 Termes, 478 Termitidse, 478 Terns, 615 Terrapin, 596 Terricola, 315 Tertiary, 181 Tessellata, 342 Tesseridoe, 250 Testicardines, 328 Testudo, 596 Testudinata, 594 Testudinidae, 596 Tethyoidea. 508 Tetrabothrium, 286 Tetrabranchia, 394 Tetracoralla. 258 Tetractinellidae, 227 Tetramera, 484 Tetraonidae, 614 Tetrapneumones, 453 Tetrapoda, 555 Tetrarhynchidse, 286 Tetrarhynchus, 281, 286 Tetrastemma, 290, 291 Tetrasticta, 453 Tetraxonia, 226 Tettix, 481 Thalamophora, 196 Thalamus, 534 Thalassicola, 192 Thalassicolidce, 195 Thalassima, 317 Thalassochelys, 596 Thaliacea, 510 Thamnocnida. 242 Thaumantia, 242 Theca, 255, 338 Thecasomata, 382 Thelepus, 313 Thelyphonida, 448 Thelyphonus, 448 Theridium, 453 Theriodonta, 594 Theromorpha, 594 Thoracici, 562 INDEX. 701 Thoracic fin, 526, 562 Thoracostraca, 427 Thread cells, 229 Thrips, 479 Thrushes, 616 Thylacinus, 633 Thymus gland, 547, 577 Thyone, 349 Thyroid gland, 547 Thysanoptera, 479 Thysanozoon, 271 Thysanura, 477 Tiara, 236 Tiaris. 242 Tibia, 463, 529 Tibiale, 529 Tibio-tarsus, 607 Ticks, 454 Tick, sheep, 493 Tiedemann's vesicles, 331 Tiger, 647 Tiger beetles, 484 Tillodontia, 643 Tim a, 242 Tinea, 494 Tineidse, 494 Tipulidae, 492 Tissues, 71 Tissues, accessory, 99 Tissues, classification of, 72 Tissues, connective, 83 Tissues, elastic, 85 Tissues, epithelial, 73 Tissues, muscular, 91 Tissues, nervous, 94 Tissues, principal, 99 Tissues, tendinous, 85 Toad, horned, 599 Toads, 588 Tobacco worm, 495 Tocogony, 140 Tomato worm, 495 Tongue bone, 524 Toothed birds, 612 Tooth shells, 369 Tornaria, 513 Torpedinidse, 572 Torpedo, 572 Tortoises, 594 Tortoise shell, 596 Tortricidse, 494 Total cleavage, 152 Totipalmate foot, 614, 615 Toucans, 616 Toxiglossa, 380 Toxodontia. 643 Toxopneustes, 345 Trabeculae, 520 Trachea. 109, 443, 458, 547 Tracheal gills, 469 Trachydermon, 357 Trachymedusse, 239, 242 Trachynema, 242 Tractus olfactorius, 534 Tragulidse, 642 Tragulus, 642 Transverse commissure, 123 Transverse process, 518 Trapdoor spider, 453 Tree cricket, 481 Tree hoppers, 490 Tree toads, 588 Trematoda, 271 Triarthus, 415 Triassic, 180 Triaxonia, 226 Trichechidae. 648 Trichechus, 648 Trichina, 302 Trichocephalus, 302 Trichocysts, 205 Trichodectes, 479 Trichomonas, 202 Trichoplax, 220 Trichoptera, 483 Trichotrachelidse, 302 Triclalidse, 269, 271 Triconodont teeth, 626 Tridacna, 367 Trigeminal nerve, 536 Trilobitse, 414 Trimera, 485 Trionychia, 596 Tristicta, 453 Tristoma, 274 Tritocerebrum, 462, 468 7.02 INDEX. Triton, sections of embryo, 39 Tritonidse, 382 Tritubercular teeth, 626 Tritylodon, 632 Trivium, 334 Trochal disc, 293 Troc banter, 463 Trochidse, 379 Trochilida;, 616 Trochilus, 616 Trochlear nerve, 536 Trochophore, 306 Trochosa, 453 Trochus, 379 Troctes, 479 Troglodytidse, 616 Troglodytes, 651 Trombididae, 454 Trophi, 294 Tropic birds, 615 Tropidonotus, 601 Trout, 576 Trunk fish, 578 Trutta, tail of, 41 Trygonidae, 572 Tubicola, 313 Tubificidae, 315 Tubinares, 615 Tubifex, 315 Tubiporidse, 259 Tubitelariae, 453 Tubular glands, 77 Tubular nervous system, 124 Tubulariae, 239, 241, 242 Tubulipora, 324 Tunic, 505 Tunicata, 505 Turbellaria, 268 Turbinated bone, 620 Turbinidse, 379 Turbo, 379 Turbot, 578 Turbidse, 616 Turd us, 616 Turkey, 614 Turkey buzzard, 617 Turritopsis. 240, 242 Turtles, 594 Twixt brain, 534 Tylenchus, 300 Tylopoda, 643 Tympanal organ, 128, 468 Tympanic annulus, 544 Tympanic bone, 526 Tympanic cavity, 621 Tympanic membrane, 544 Tympanum, 544 Type theory, 15 Typhline, 599 Typhlops, 60 1 Tyrian purple, 380 Tyrannidae, 616 Uca, 437 Uintatherium, 643 Ulmaris, 247 Ulna, 529 Ulnare, 529 Umbilicus, 370, 554 Umbo, 358 Umbrella, 234 Uncinate process, 605 Ungues, 618 Ungulae, 618 Ungulata, 639 Unguligrade, 640 Unicellular glands, 77 Unicorn, 646 Unio, 367 Unionidae, 367 Ureter, 550 Urinary bladder, 552 Urinator, 615 Urinatores, 615 Urnatella, 322 Uroceridse, 486 Urochorda, 505 Urodela, 587 Urogenital sinus, 553 Urogenital system, 120 Urogenital system of vertebrates, 550 Urosalpinx, 379, 380 Ursidse, 647 Ursus, 647" Use and Disuse, 55, 99 Uterus, 120, 629 INDEX. 703 ; Utriculo-saccular duct, 542 Utriculus, 128. 542 Varanus, 599 Vacuole, contractile, 183 Vacuole, food, 183 Vagabundas, 453 Vagina, 120, 629 Vagus nerve, 536 Valkeria, 324 Vampyre, 638 Vanessa, 496 Varan id oe, 599 Variation, 25 Vas deferens, 120 Vasa Malpighii, 461 Vascular arches, 504 Vater Pacinian corpuscles, 126 Vegetative organs, 101 Vegetative pole, 147, 151 Veins, 112 Velella, 245 Veliger, 355, 364 Velum, 235, 356 Veneridae, 368 Ventral aorta, 548 Ventral fin, 562 Ventral nerve cord, 123 Ventricles of brain, 534 Ventricle of heart, in, 548 Venous sinus, 567 Venus, 368 Venus' flower basket, 226 Venus' girdle, 264 Vermiform appendix, 627 Vermilinguia, 599, 636 Vermes, 535 Vertebra, 518 Vertebra (of ophiuroids), 337 Vertebral column, 516 Vertebrata, 514 Vertex, 462 Vesal, 12 Vesicle, auditory, 127 Vesicle, blastodermic, 155 Vesicle, germinal, 146 Vesicle, Polian, 331 Vesicle, Tiedemann's. 331 Vesicularia, 324 Vesicula seminalis, 120 Vespariae, 487 Vesperlilionidse, 638 Vesperugo, 638 Vibracularia, 323 Vibrissae, 618 Viperidce, 601 Visceral ganglia, 353 Visceral sac, 351 Visceral skeleton, 523 Vitellarium, 267 Vitreous body, 130 Vitrodentine, 558 Viviparous, 161 Vogt. 24 Volutidae, 380 Volvocina, 202 Volvox, 202 Vomer, 525 Vortex, anatomy of, I2O Vorticella. 211 Vorticellidae, 210 Wading birds, 615 Wagner, 24 Waldheimia, 326 Walking stick, 480 Wallace, 24 Wallace's line, 177 Walrus, 648 Warblers, 616 Warm-blooded animals, 114 Wasps, 487 Water bears. 455 Water beetles, 484 Water scorpion, 489 Water snake, 601 Water-vascular system, 115, 330 Weasel, 647 . Weevils, 485 Weismann, 24 Whalebone, 645 Whales, 645 Whelks, 380 White ants, 478 White fish, 576 White matter, 124, 532 UNIVERSITY OF CALIFORNIA MEDICAL SCHOOL LIBRARY THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW 1 8 is- I in- 11,' I 8 C1L48 Her twig, E57k A manual of poology, 1929 ^AY 3- 19g9 University of California Medical School Library UNIVERSITY OF CALIFORNIA MEDICAL SCHOOL LIBRARY THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW f. /W , . lm-1 1,'18 ertwig, A ^nanual of ^oology flY f - 19?9 MAY 3- 19g9 • University of California Medical School Library