CEES EEE " PEE EUEEEPEPEESEEEPEEER ELLE EA i | i| \ 4 jae 20 Rrra MA CORNELL UNIVERSITY. THE Roswell P. Flower Library THE GIFT OF ROSWELL P. FLOWER FOR THE USE OF THEN. Y. STATE VETERINARY COLLEGE 1897 Cornell University Library A MANUAL OF ZOOLOGY BY RICHARD HERTWIG Professor of Zoology in the University at Munich FROM THE FIFTH GERMAN EDITION TRANSLATED AND EDITED BY J. S. KINGSLEY Professor of Zoology in Tufts College Av P. £7 > Jf \ v rd LIBRARY. 3 s Py AS x NEW YORK HENRY HOLT AND COMPANY 1902 Copyright, 1902, BY HENRY HOLT & CO. ~~ ROBERT DRUMMOND, PRINTER, NEW YORK. PREFACE. 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 W. 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 Vermes 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 Verte- 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 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. Reference might here be made to those in the Anat- omies of Lang and Wiedersheim, the Embryologies of Balfour, Korschelt and Heider, Minot, and Hertwig, aud 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. ‘Turrs COLLEGE, Mass., Sept. 19, 1902. TABLE OF CONTENTS, GENERAT GANATOMY his cant tauaa s adglins VagNs. 2 Amataeetvea ae mice ede ous The Morphological Units of the Animal Body.................... The Tissues of the Animal Body........... 0.0.0. cece eee ee cues Fopitheltall Missiies: paeete on atennate we leusts cesreas Peel aune ate Gueciaeee eo CONNECTIVE TISSUES! Stee see deren eu eh eee aasthe lo eeencce austen ocelere Muscular Tissues in casas ts ee. cise oe See Ale rae digs eaun es anes Nervous: Tissues: sos padkcecs Saiaa- duane emnae Gen eens Duele ate cs UMMA Y sanded thles s Ha eas qa Tame U fait Munem. ara ace ates The Combination of Tissues into Organs..........0..0000 000 cca ee Vegetative Organs). ow cities casas pede das ee ee yee ee TA oe Organsiof Assimilationy, acsccieae deoudds ca dale ie aean ee ease’ Digestive: Tract. sy sa cowtusaki gran ac aera aan cee pene aie neste ag gneaees Respiratory OTgans. si sai 0a 6 ek sais Weal awceiece dale oath a Beles Circulatony-Apparatuss cb ce cued sane ciwedh wa naaisiay eee dons Excretory OffatiSine esc cans otede eal dad nv ewe one aes Sexital (Organs dio a ae Na en Wale ale au Mateh Maulana ia ntecany AnimalCOnansy.. ic.ucicn Vas Maageins qosagalaulied vase ae ey bee Organsor LOcomotlons weg c\aneenscdanasinewerelusa'e teases Gels 365 Order Il. Heteroconchige® 4. ose c.d4 ance oe bau loy vane da bles 367 Glass TIT) Seaphopodaw sao. cot be eg ac eh awe Sad Awe) 369 ClassLVy Gasteropoda ns ah 'ncyae mies Gtiow. De eens bees Oe cae tale 369 Order. L.. Prosobranchiata’ , 64 24 sawy sie cis eae HS RRO 378 Order II. Opisthobranchiata ....................0..0.... 381 Order ITD) sPulmiona tay sie 2 ike tad eaty actu tea eaten ace mana en wee 383 Class'V. Cephalopoda. soca. casas pn acuau icin ens ane wes alee ane 384 Order: Vetrabranchias is 544.444.n12acpanesn Gs oo aay rleasc scales 394 Order Il, Dibranthiain..c siete eee mana ae morn Ode oe des 394 NO UIMIMAIR Vio. 2ios ic Satu ene ANS e ai Sue IN NO Aon en BUS rec dive) akin ae lal ae 395 Phyl TX ARTHROPOD EB 32.345. ci: scaipdans- Sale hee RAMON AA eY Bm an eater alee 398 Class Tt Su St aCe a0 8 as ne ae ste can ores eather Aaah gate oe eee 408 Sub Classi, Urilobites co.) sodas Soc el awison ehh ee be ars 414 Sub Class II. Phyllopodas isc ccc ccc cede seen ned we bees 415 Orderl, -Braniehiopoda yy. pat ys tee cans 5 oceania nw hemes 436 Order Tl Cladocera i. cesses aye ate RaW A see eres ca ee Lala a 417 Sub:Class TIT “Copepoda. 0a. Sons og he pee ee ea Sande ae: 417 Order l: Hueco pe pod an. ese tie thie ha want hos Be Se Oe oe 421 Order ll. Siphonostomatay 2. fo suc Gana eis betas evew os 422 SubClass iV. Ostracodays, «h 284 on antag Geen puaeme es oe oe week 422 SubClass:Vi" Cirripediaic | dai. cg masts oakten oe ees eae he ee 423 Orders epadidae tA cere rd ce rn nace ae eae eet ee aioe 425 Order Thy Balamidaes tion. o.4 nen sie he Rach Rawiaee oeeereaeeas cored ee 425 OrderIIl.“Rhizoce pha aveeac3 ca sact ss.cep dee ee mean ve ees 426 Sub Class Vi; ‘Malacostraca sone sige coos, cee bad ow ad cnen 426 Tepionl, eptostiaca, sss ois sees. gate cstld hades et Re oon nee 427 Legion I. Thoracostraca.... 6.046 ccc ee Sane scenes cacsuavs 427 Orderly Schizopodai a, a2 Gic gs ud ontogueracaies oe wut hoen c 428 Order II; Stomatopoday.< ou cc. soe c4nh eh Cavan eee teicen 429 TABLE OF CONTENTS. PAGE Orderdil Decapoda: 2 satan, cae tie ds Manes woe eas as 429 OrderiIVi Cuma cla: eno: oe ote 22.8 aan eee aR aoe 437 Legion Ll. Arthrocostracds 23 44 ye ane tanec ned amines oho e 438 Order tT Am plnpodasi.s io 6 keen ei vac aloe eee ae 438 Ordér Te [sopoday oc 55 eo a yaar araecetneuaeanonn ane sheets et 440 Class TE ACerata vas sata mo un moe ceca aes eae ne eee aie 442 Stib: Class I. Gigantostrata ae ti.- Go adel ic de tke ee yak been vous 443 Order]. Xaphosura ss acdc. t esac s ates ee eu Oe Ee he 444 Ordetll. Hury pterdacc..« occstees ender ee ne Hea ae eas eed 444 SiibClassdl; ArachmMid ads «ac samckieiaba aadel scene ¢ Rew aided) ee 444 ecion |; Arthrogastridas scented G Galaaioe sanael waveeuaur: 447 Order. vSCOrpionsd ais cre ese ahece ie vaee « oeso. a eet Led Agee tenor 447 Orderdh. Phrynoideas.. 5. eat ee ies o beadh eieccioam se get 448 Order III. Microthelyphorida ......................... 448 Ordér LV... Solpigida. 54502 ccc4i4 erean gs ntyerus peewee ed 449 Order V2 Pseudoscorpil. whee as ds pane wea ndr esas enaet 450 Order Vi; Phalangiday. «2.007 <4 04 sek Bees eee y o8 450 Lesion Ll. -Spherorastriday, ocxccia sos Mea named SGaN ane 451 Order Amaneing: oi. cc sci oann aenvae dae ae aia eamen Am ames 451 Order Ti; Acarifia: 25.5.cc0hs 25 uk abe yes eeteae BH Ae tins 453 Order lil: Waingaatulida. <..:03 des yang eed ye hw bad See bees 454 Tardigrada:<. 24 anaseehnanach 4 menses Hodes Rance oe 455 PY ENOCOMG AL co Apmis a tee es RUE A hehe een eae 456 Class eELL Malacopoday ts. set stuvnpusni einen Aone ne eek a ayaa 456 Gla SoHU Ve CITI SEC tas coat ices fess Crean ences stats asa ease eR lah Site ma eA eee 458 SubClass Le Chilopodacc syed anenac eon weues Mateus hen enmtess 460 Siib Classi sHexapodar acc) asain omen watah tl oeite aeedae s 461 Order. Apteryeole: oun ick te she tee OR te eee sad 477 Order 11s. Axchipterars) - pat onde ns es eace ue vas ee Re ee 477 Order IIb, :Orthoptera., a5 ce sek ne Shea See odes ve ee ss 480 Order TV. sNeuropterdn wis comes cae acai, axe eects oe ee oe 481 Order Ve iStrepsipteray ion tise oie gaat aa athe a aautvea ww Raed 4g 483 Order Vis ‘Coleoptera: fn ciai.cus stanqeaians sve wae svn hos g-heen s 483 Order VIL) “Hymenoptera: ai. 00 tee a Milas oe eee eee 485 Order VILI2RDynChOla cs aah eA hee cis aoe cscs ha iceabhs asc avente lars 489 Order Tes Di pierre, c..a cena inds arecareas we. ceee am einen: awieainnlert 491 Order X, Aphaniptetan cccrsns acd cage ee daaed ea een ow hae 493 Order Xl. Lepidoptera... obec ot ag sone ee een tan Gaoe ae 494 Class'Ve MDiplopodar ys 2it. 54 cagiemeendt.sqeiws ay ae ea ee Goes 496 SUMMARY =..2cvies Sm ae nist WERE RANGE hdile HATS eb tenes sng RE aA 497 Phylum X CHORDATA. 9% Mani ia aa bese ulead ha Aetna HE a mes 501 Sub Phylum, Leptocandi.. 05s... 5 sarees cA ieee ete eee 502 Sub Phylum-iL, Dumeatas sc. o06acrsaceqeca deo aent Sas teneae eee aan OS Order tl. - Copelatan,) anc duducpeac menos be MANERA ROA ACNE S 506 Orderly ‘Tethyoideayn ss 26:8 ae avs aad ae Aes meee 508 Order TITS Pialiaceas ctracieseccconend, aathscee eo ene t Mn eh 510 Sub Phylum III. Enteropneusta ......... 0... eee eee eee 512 Sub: Phylum lV. Vertebrata scods-itied ere eae Agia us aaree ho stung BRO ao 514 Series Tenth y ops dans cea dase stars cea: sits secede: a ditebst aay bioas Waste Sopa AR 555 Class I. Cyclostomata TABLE OF CONTENTS. PAGE Sub: Class... Myzontesii3.2 oes gn-ce Mae Se oes ohare Ses 556 Sub: Class Il... Petromyzontes 3.4 sitio tw series Saree es dole 557 Class IT. Pisces jae gate satan seven nears Mt ie eee ee Mee oe Be ES 557 Sub Class I. Elasmobranchit 2. i. jaccg04 4. saceds va vate eas aas 569 Order I. Selachits.se .yesdwa hey Sock Hed aa Seer AY oe oe Ne 570 Order Ib. Holocephalit.+.27 os spice dbs ehh e Sh ato ooo de, oe 572 Sub Classll <:Ganoider winccretee eu et He bas ad Maeee Morne iS er 572 Order; Crossoptery ei sig jens renee aw eine sm ens wa alee 573 Order HT Chord rosten cic cah occas vine saide ne nesintssece eleacde es alesse) Bee 573 Order Tilo Holostomi: shoe eau iviesuuicaoscsun senses ale caer iier os 573 SubClass: LIL: “Leleostets 10 een acq-ahersianey won biguaea aati pouch bee ardes Bw 574 Order: (PhysSostOmiss ad sau gtepacm amas ee weak A oe Boe 575 Order Th: Pharyngopnathi . 222.5214 is tated eda eas te 576 Order III. Acanthopteri ........... 000.00. ee eee ee eee 577 Order IV; Anacanthinis. cao dse acces cease ahaa bo 577 Order Vi Lophobranehit .incccsdcanneg anaes ae eRe RaUNes 578 Order Vil; Plectognathion aco suins ay mere awe aa thse en ns 578 Sub: Class IV. DipnOk s4.sncuin ec oe che A LRA E EAR eRe Re as 579 Classi IL Amiphibiay occ kesdas samdosvoniae aaaloneaan nein se telaeee 580 Order. Stegocephalty oo scp paid wap sues ccausde Peuel bin nee ee 586 Order IL. Gymnophiona, cavenca cavicte ste ag aca cieuieag scons 587 Order LL. UrOd ela. ssscs soe. ee ane ek ae, Secadosn Boe sesohvaea enna Sore 587 Order TV." Amutatccaly cucte seo ag sabe aw Sane Sie aoeiaaee Skane ze 588 Series: Lie -AimmiO tars sees oe ns who eek ERS See Ee Se Se ERA ad 588 Class? In Reptilia: s.vsars ewiineceias ance nave n cide: ions oe Saw eens eset 588 Order], “Cheromorphay wicccwndeeas eee nak ak ne Het aaeares 594 Order li... Plesiosauriaos acc casrslew.cke Seowee we oak, wane wens 594 OrderIll, Tehthyosatitia: ...25 2 cass nak ne vols du ain en ae ome 594 Order IV. Cnelonia, space feng Gace ek ae ain as AIS cele eile ddepsako ee 594 Order V. Rhynchocephalia ..........0.0.00.0000 00000 cee ee 595 Order Vil2 DimGsatna: a'o 66:5 bacon naar ara ovation kaa ek 595 Order Vil Sa uiamatac ois 2s ena haat aude eed ces te oe 596 Order VATE CrOCOGMa x on). Scot cla dient Roe ae ae oe Gas ees 601 Order Ix: Pterodactyllayss.uc a uus bade v da og sawule oesekacese: 602 Classi A ViCStin eet nar ant eee Sid aie dacce yaa. et anenieeay gee ape neacs & 603, Orderly Saul nae ear aie ae iachmasecanlani lita cis ators lacs eilalh me 612 OrderTT. “Odontornithes: . ¢ en cece ds Sach ach oe ous 612 Order Le sRatitaes an seta nec aay ee ies Matte ows e cea cae 612 Orderly Carinae. aos saci tailon Sinnen tet Cee ee he tena 613 Glass Tis Matirmalliace eth natct Ae eee eens ete ne eh 617 Sub Class I. Monotremata, 224.005 564 05 een bensn decane. . 631 Sub; Class: Ti. “Marsipialtas sy <2. iced vag. dunes Re eetep eke bed 632 Order J. ‘Roly protodontar, . «ces Carak pause aeawas cay eee see 633 Order ILL Diprotodonta win.c.2-va es ou abies she ewan oclen each 633 Sub Class TIT; Placentalia 4:94. 64.04. sune duartinare utavaaie copie anes 634 Order ls Rdentata un ae scu cers tacks noeince cenan sa tees 635 Onderdls Tmsectivoral afc vate surges Gon tere css eed lee oe es 636 OrderlM.- Giirop tera es creeds restos hetietueen se eauiels ake ae 637 Order LV,“ Rodentilae sirens ch acstascunnlewcraien oan Meals ueetan 638 Order V_ Ungulata TABLE OF CONTENTS. X1 PAGE Order Vi, Proboscidia: ni cocci cacacsiouses MR ee 643 Order: VIE. Piyracoideas «sna tae et oe ad Ge ey cme as 644 Order VAULT, Sikenla oh Bas a4 wasted aun thea. ab a aes arenas allen 644 Orden IES, = Ceta Ged: mien aiataaen soonmiinia Saint ts a ee Bleue enn aendeh 645 Orden2k;= Car niviordscrn care wae SecA ey Memes eens eh eae 646 Order Del, PEOSIII TES ai dre isie.-giicus ss ss Soho hee Who dst aaa bse Weare aN 648 Order MIT: Primatess.c);o%e-acb sc aete seas a aeanneu aa eave ace: 649 GENERAL PRINCIPLES OF 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 different 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 developmental stages of man show definite regular agreements, not only with the structure of the adult human being, which in and of itself would be intelligible, but also with the 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 les 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 1s 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 aboye-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, ‘ Historia animalium,” ‘*De partibus,” and ‘De generatione,” works in which zoology is founded as a universal science, since anatomy and embryology, physiology and classification find equal consideration. Ilow 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 ve 8 GENERAL PRINCIPLES OF ZOOLOGY. 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 sy stematic cate- gories, with Eidos, species or kind, and yeévos or group. His eight yevn ueyzora 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: H Mammals (C@oroKobvta év avrois). 2. Birds (opriées). 3. Oviparous quadrupeds (retpamoda @OTOKOOYTA). 4, Fishes (iy@ves). 5. Molluses (uadakia). 6. Crustaceans (uadaxooTpaka). 7. Insects (Evropa). 8. Animals with shells (oorpaxodéppata). 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, evarpa (better, ental: with red blood), with the bloodless, avaiya (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. HISTORY 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 ‘ 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 religions or ethical teachings. In a similar way the religious element played an important réle in the many-volumed Natural History of the Dominican Albertus Magnus, and Vincentius 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 Linneus, has found its most brilliant exponents. Linnzus, 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 Rashult 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 Linneus.— Linneus’s most important work is his ‘‘ Systema Nature,” 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 Linneeus 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 ‘‘Systema Nature.” 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 Linneus 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 vulpes, 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. Linnus’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 Linneus 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. Linneus, on 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 Linnzus himself, the father of this tendency. For while in his ‘* Systema Nature” 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. Linneus divided the animal kingdom into six classes: Mammalia, Aves, Amphibia, Pisces, Insecta, Vermes. The first four classes correspond to Aristotle’s four groups of animals with blood. In the division of the invertebrated animals into Insecta and Vermes Linneus 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 Linneus himself, we see the damage wrought by the systematic method. The diagnoses of Linneus 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 physiologico-anatomical side. DEVELOPMENT OF MORPHOLOGY. Anatomists of Classic Antiquity.—Comparative anatomy—for this chiefly concerns us here—for a long time owed its development to the students of human anatomy; 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.p.), 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 Vesal’s Human Anatomy drawings of skeletons of vertebrates by the Nuremberg physician Coiter; 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, celenterates, 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 tracheex, 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, Réaumur in France, Lyonet in Belgium, Résel 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 O. F. Miller. 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, Gaze in Quedlinburg, Schiffer 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 Résel 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 amimals. 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 haye different functions, as the lungs of mammals and the swim-bladdey 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 Wirtemberg, 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’Héricy 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- luses, extended to the celenterates, arthropods, and vertebrates, living and fossil. Tle collected his extensive observations into his two chief works ‘‘ Le régne animal distribué d’aprés son organiza- tion’ and ‘* Lecons d’anatomie comparée.” Of quite epoch-making importance was his little pamphlet ‘Sur un rapprochement a ¢tablir entre les différentes classes des animaux,” in which he founded his celebrated type theory, and which in 1812 introduced a complete reform of classification. The Cuvicrian 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*); eithe the * This original meaning of ‘evolution’ is different from that prevailing at present. HISTORY 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 generationis” (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 Hpigenesis. 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 epigenesis. 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 Hihnchens, 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 GHNERAL 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. REFORM 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 Miller, 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 Kélliker; 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 celenterates 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 he termed them, the Zoophytes, to those animals with radially symmetrical structure (Echinoderms and the Plant-animals); separating all the others, he formed of the unicellular organisms the branch of * primitive animals’ or Protozoa; the higher organized animals he grouped together as worms or Vermes; at the same time he transferred a part of the Articulata, the annelids, to the worm group, and 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 Radiata 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 Cclentera (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: Protozoa, Celentera, 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 Articulata. Upon the ground of important anatomical and embryological characters the Brachiopoda, the Bryozoa, and the Tunicata have been separated from the Mollusca; they form the subject of diverse opinions. The 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 differences are such that they cannot be included in that group. The only way out of the difficulty is to unite vertebrates, tunicates, and some other forms in a larger division, Chordata. The Vermes, too, must be divided, as will appear in the second part of this volume. 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 oceupied 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 ZOOLOGY. 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 Linneus’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.’—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. Linneus said: ‘* Tot sunt species quot ab initio creavit infinitum Ens”; with this he built up a conception of species upon the tradition of the Mosaic history of creation, a procedure quite unjustified upon grounds of natural science, since it drew one of its fundamental ideas from transcendental conceptions, not from the experience of natural science. Linneus’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 semtnalis, 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 ‘* Philosophie 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 our globe; the terminal point of this series is man; the other animals are the descendants of those forms from which man has developed. Lamarck, in accordance with the then prevailing conceptions, regarded the animal kingdom as a single series grading from the lowest primitive animal upto man. Among the causes which may influence the change and perfecting of organisms, Lamarck emphasized particularly wse 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 (Geoffroy 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 ot 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 on 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 anda 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.—lIt 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 1800. 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 GHNEHRAL PRINCIPLES OF ZOOLOGY. 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. R. 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 Miller, 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 Hinglish naturalists are to be named particularly Huxley, Hooker, and Lyell. In America Gray, Cope, and IHyatt 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 beleved 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,’ ie., 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. 1c); 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. 1a, 1p); even the skeleton itself varticipates in this variation, as is shown by the fact Fig. 1A.—English carrier-pigeon. (After Darwin.) Fic. 1B.—English tumbler-pigeon. (After Darwin.) that the total number of vertebre 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- Fia. 1c.—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 prior/ 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. Rather 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- euishing characters ? iB. Lhe 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 Adramis brama and Blicca bjirkna, of Trutta salar (salmon) and Trutta fario (trout); among sea-urchins the spermatozoa of Strongylocentrotus lividus fertilize with great readiness the eggs of Lchinus microtuberculatus, but only rarely the eggs of Spherechinus 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 Hchinus microtuberculatus, but, conversely, the sperm of £. 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 Plewronectes platessa and Labrus rupestris by sperm of the cod (Gadus morrhua), frogs’ eggs (Rana arvalis) by sperm of two species of Triton, eggs of a starfish (Asferias 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 difti- 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. 20 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 (Dajfila acuta); 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 larve of slight vital force only in the case of Actias luna and A. tsabelle. 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 for 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 vulgaris; 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 difference 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 asa primary distinction between 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 Jong 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 GHNHRAL 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, theiz 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 medusew) 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. Fria. 2.—Archeopteryx lithographica. (After Zittel.) cl, clavicle; co, coracoid; h, humerus; r, radius; u, ulna; ec, 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 3 GENERAL PRINCIPLES OF ZOOLOGY. fortunately been found. Transitional forms connect the single- toed horse of the present with the four-toed Mohippos 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, ¢.g., between reptiles and birds, the remarkable toothed birds, und the Archeopteryx (fig. 2), a bird with a long, feathered, lizard-like tail. (3) Morphological Proofs. —When we employ comparative anatomy and embryology in support of evolution, we find that the two 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 peculharities which distinguish each type in structure and in the mode of development are not without intermediate phases. Hspecially from the branch of the worms there lead cff transitional forms to the other branches: Balanoglossus to both echinoderms and chordates, the annelids and Peripatus to the arthropods, the tunicates and Amphiowus 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 common ancestry. Hach animal during its development passes 30 HISTORY OF ZOOLOGY. through esseutially the stages which remain permanent in the case of lower or at least more primitive animals of the same branch, as 43 2 1 |__ Fia. 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; 1, nasal pit: h, cardiac region; ¢ [and e IJ, fore and hind extremities; m, mesodermal somites. Fic. 4.—Tadpoles of Rana temporaria. m, mouth; g, upper jaw; z, lower jaw; s, sucking-disc ; kh, external gills; ik, region of the internal gills; m, nose; a, eye; 0, auditory vesicle ; h, cardiac region ; d, operculum. the three following examples will show: (1) In the early stages of development the human embryo (fig. 3) possesses remarkable 36 GHNERAL PRINCIPLES OF ZOOLOGY. resemblances to the lowest vertebrates, the fishes. Like these it hag gill-shts, 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 Fia. 5.—Siredon pisciformis (arva of Amblystoma tigrinum). (After Duméril 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 Fra. 6.—Achtheres percarwumn. a, nauplius-, b, cyclops-stage ; ¢, adult female. (After Claus.) at all like their relatives. They are shapeless masses which were formerly regarded as parasitic worms. Their systematic position ras only determined by their embryology (fig. 6). Tlere it is HISTORY OF ZOOLOGY. BT eo shown that they pass through a nauplius-stage (fig. 6a), charac- teristic of most crustacea, and that they then assume the shape of small crustacea (fig. 6, 4), like Cyclops (fig. 7, A), so widely dis- Fia. 7.—Cyclops coronatus (A) and also the nauplius in lateral (B) and in ventral view (GC). T, head; T/-V, the five thoracic, and behind these the five abdominal seg- ments; F’, furca; 1, the first, 2, the second, antenne; 3, mandibles; 4, maxille; 5, maxillipeds ; (9, the first four pairs of biramous feet, while the rudimentary fifth pair are hidden; au, eye; 0, upper lip; e, egg-sacs; d, gut; m, muscle. tributed in fresh waters. Very often the males inake a halt in the cyclops-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 35 GENERAL PRINCIPLES OF ZOOLOGY. pass through the stage of organization of the lower, because they spring from ancestors which were Ss" 4. 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- 7 \ 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 Fundamenta)] Law of Biogenesis.’ ‘* The y-\ development history (ontogeny) of an A \ individual animal briefly recapitulates the history of the race (phylogeny); ee “ fe i.e., the most important stages of organi- (ater Bergeoel, 18: 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.—TZhe Nervous System.—This law applies as well to single organs as to entire animals. The central nervous system of the lower animals (echinoderms, celenterates, 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 of position by cross-sections through the dorsal region of embryos of different ages of any vertebrate (fig. ‘)). ; The Skeletal System.—The skeleton of vertebrates is a further example. In the lowest chordates, amphiorus and the cyclostomes, the vertebre 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 vertebre, 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 MIG. 9.—Cross-sections through the dorsal region of Triton embryos at different aoe (from O. Hertwig). In [the medullary plate (anlage of spinal cord) Me eels off from the skin (epidermis, ep) by the medullary folds (mf). In IT t he 1 et lary plate by inrolling of the medullary folds is converted into a groove. ae the groove has closed into a tube (7), the spinal cord, which has Separates rT mn the rest of the ectoderm (epidermis). C, body cavity (c@lom);_ ch, His . cp, cavity of primitive somite (myotome); dz, yolk-cells; ik, entoderm; (Ig, lume of gut; mk}, mk*, somatic and splanchnic layers of mesoderm. 40 GENERAL PRINCIPLES OF ZOOLOGY. completely ossified vertebral column; their embryos, on the other hand, have in the early stages only the notochord (amphioxus stage); later this notochord becomes constricted by the vertebra (fish-aiphibian stage) and finally entirely replaced; the vertebral column is in the beginning cartilaginous, only later becoming ossi- fied. Comparative anatomy and embryology thus give the same developmental stage of the axial skeleton: (1) notochord, (2) notochord and vertebral column, the latter at first formed of cartilage, then of bone. We have here spoken of a parallelism between the facts of comparative anatomy and those of embryology. But in reality we should expect a threefold parallelism: for according to the theory of evolution the systematic arrangement of animals is conditioned by a third factor—the historical development of the animal world, or phylogeny. The mile-stones of phylogenesis, the fossils, should give the same progressive series in the successive geological strata as the stages of forms found by comparative anatomy and embry- ology. We actually know instances of such threefold parallelisms. Comparative anatomy teaches that the lowest developed form of a fish’s tail is the diphycercal (fig. 10, A); that from this the heterocercal (/4), and from the heterocercal the homocercal form of tail-fin (C, 2) can be derived. Embryologically the most highly developed fishes are first diphycercal, later heterocercal, and finally become homocercal. Last of all, paleontologically the oldest fishes are diphycercal or heterocercal, and only later do homocercal forms appear. What has here been referred to is only a small fraction of the weighty proofs which morphology offers in favor of evolution; it can only serve to show how morphological observations can be employed. For the reflecting naturalist the facts of morphology are a single great inductive proof in favor of the theory of evolu- tion. Distribution of Animals.—From Animal Geography we learn that the present distribution of animals is the product of past hundreds and thousands of years. It will therefore be possible from this to figure out many of the earlier conditions of things, hy proceeding with the utmost caution and overcoming extreme difficulties. If we assume that from the beginning all animal species were constituted as they now are, they would then have been placed by the purposeful Creator in the regions best suited to their organiza- tion; their distribution would therefore have been determined by HISTORY OF ZOOLOGY. 41 favorable or unfavorable conditions of life prevailing in the various regions, as the climate, food-supply, ete. If, on the other hand, we assume that the animal species have arisen from one another through variation, then there must have been, as an influence determining the manner of distribution, besides the conditions of existence, still a second factor, which we will call the geo- Fia. 10.—Tail-fins of various fishes. (From Zittel.) A, Diphycercal fin of Polypterus bichir. (Vertebral column and notochord divide the tail into symmetrical dorsal and ventral portions.) B, Heterocercal tail of the sturgeon. (Asa result of an upward bending of the notochord and vertebral column the fin has become asymmetrical, the ventral portion much larger than the dorsal.) C, D, Homo- cercal fins, C, of Amia calva; D, of Truttu salar. (By a still greater upward bend- ing of the notochord and vertebral column the dorsal portion has almost en- tirely disappeared and the ventral portion almost alone forms the fin, externally apparently symmetrical, but in its internal structure very asymmetrical.) ch, chorda; a, b, ¢, cover-plates. logical. We know that the configuration of the earth’s surface has changed in many respects in the course of the enormous space of time of the geological periods; that land areas, which earlier were united, have become separated by the encroachments of the sea; that by the upheaval of mountains important barriers to the distribution of animals were also formed. From the fact that 42 GENERAL PRINCIPLES OF ZOOLOGY. these two changes—the changes in the earth’s surface and in the animal world established upon it—have gone on hand in hand there follows necessarily the consequence that greater differences in the faunal character of two lands must result the longer they have developed independently of one another, without interchange of their animal populations, and the Jonger the inhabitants have been separated by impassable barriers. For the various groups the character of the barriers is different; terrestrial animals, which cannot fly, are hindered in their distribution by arms of the sea; marine forms by land barriers; for terrestrial molluscs high moun- tain ranges, which are dry and barren, or constantly snow-capped, are effectual. Instances of Proofs.—Since attention has been called to these conditions, many geographical facts favorable to the theory of evolution have been ascertained: (1) Of the various continents Australia has faunally an independent character; when discovered it contained almost none of the higher (placental) mammals, except such as can fly (Chiroptera), or marine forms (Cetacea), or such as are easily transported by floating wood (small rodents), or such as could be introduced by man (dingo, the Australian dog); instead, it had remarkable birdlike animals (with beak and cloaca), and, the marsupials, which have become extinct in the Old World and the opossums excepted, in America as well. The phenomenon is explained by the geological fact that in the earth’s history Australia, with its surrounding islands, was certainly the earliest to lose its connexion with the other continents. While’ in the other four parts of the earth the higher vertebrates, which were developed from the marsupials and their lower contemporaries, came, by way of the lands connecting the various continents, to have a wide or even a cosmopolitan distribution, in isolated Australia this process of evolution did not go on, and its ancient faunal character was preserved. (2) As Wallace has shown, the Malay Archipelago is divided faunally into an eastern and a western half; within each group there are islands which, in spite of a different climate, have a very similar fauna. On the other hand, the faunal boundary (‘ Wallace’s line’) passes between the two islands Bali and Lombok, which have the same climate and geographically are very close together. But the depth of the strait in this region shows that here runs a boundary of extraordinarily long geological duration, and that in the earth’s history Bali has developed in connexion with the western, Lombok with the eastern chain of islands. More recent studies make it probable that there HISTORY OF ZOOLOGY. 45 is an island zone between the two in which a mixture of faunas occurs. Celebes especially belongs here. (3) A long time before Darwin, the renowned geologist Leopold von Buch, from the dis- tribution of plants on the Canary Islands, had come to the conclu- sion of a change of species into new species; viz., on islands peculiar species develop in secluded valleys, because high mountain- chains isolate plants more effectually than do wide areas of water. M. Wagner has collected many instances which prove that locali- ties inhabited by certain species of beetles and snails have been sharply divided by wide rivers or by mountain-chains, while in neighboring regions related so-called ‘ vicarious species’ are found. Causal Foundation of the Theory of Evolution.—The Dar- winian theory, so far as the above exposition shows, is fundamen- tally like the theories of descent advocated at the beginning of this century by Lamarck and other zoologists; it is distinguished from these only by its much more extensive foundation of facts, and further in that it abandoned the successional arrangement over- thrown by the type theory, and replaced it by the branched, tree- like mode of arrangement,—the genealogical tree. But still more important are those advances of Darwinism which relate to the causal foundation of the descent theory. The doctrine of causes which has brought about the change of species forms the nucleus of the Darwinian theory, by which it is especially distinguished from Lamarckism. In order to substantiate causally the change of species, Darwin proposed his highly. important principle of ‘Natural Selection by means of the Struggle for Existence.’ Artificial Selection.—_In the development of this principle Darwin started from the limited and hence easily comprehended subject of Domestication, the artificial breeding of our races of domesticated animals. Many of these undoubtedly sprang from a single wild living species; others arose from several species, but now have the appearance of a single species. Ilow have arisen such extraordinarily different races of pigeons—the fantail, the pouter, long- and short-billed pigeons, etc., the long- and short- horned cattle, the heavy, slow Percherons and the slenderly-built, fleet-footed Arabian horses? Undoubtedly through that same more or less conscious influence of man, which is still employed by the skilful animal-breeder. If he wish to obtain a particular form, he chooses from his stock suitable animals, which he pairs together if they in ever so slight a manner approach nearer than the others to the desired ideal. By repetition of this selection according to plan, the breeder attains a slow but sure approx- 44 GENERAL PRINCIPLES OF ZOOLOGY. imation to the goal, since he uses for breeding only the suitable individuals from each new generation. If he wish, for example, to breed fantail pigeons, he selects from his stock animals with the most numerous and strongest tail-feathers. In the course of generations, then, characteristics cumulate; the number of pigeons having an increased number of tail-feathers becomes greater, and thus material is obtained which is adapted to a further increase in the number of feathers. Factors of Evolution in Breeding.—The remarkable results of breeding which are well known to every observer of our domesti- cated animals depend mainly upon three factors: (1) Vartability; the descendants of one pair of parents have the capability of developing new characteristics, thereby differimg in appearance from their parents. (2) Hereditability of newly-acquired charac- ters. This consists in the tendency of the daughter-generation to transmit the newly-developed characteristic to the succeeding generation. (3) Artificial selection; man selects for breeding pur- poses suitable individuals, and prevents a new character which has arisen through variation from disappearing through crossing again with animals of the opposite variational tendencies. Factors of Evolution in Nature.—If we compare with the facts of domestication the conditions of animals living in the state of nature, we find again variability and heredity, as efficient forces, inherent in all organisms, though the former is not everywhere of the same intensity. There are many species which vary only shghtly or not at all, and therefore have remained unchanged for thousands of years. But contrasted with these conservative species are in every group progressive species, active species, which are in the process of rapid change, and these alone are of importance in causing the appearance of new species. Since heredity is present in all organisms, there is only lacking a factor correspond- ing to artificial selection, and this Darwin discovered in the so-called ‘natural selection.’ Natural Selection: Struggle for Existence.— Natural selection finds its basis in the enormous number of descendants which every animal produces, There are animals (e.g., most fishes) which produce many thousands of young in the course of their lives: not to mention parasites, whose eges are numbered by millions. For the development of this animal throng there is ‘no room on the earth; for even if we compute upon the basis of a slowly-multiply- ing animal, like the elephant, and assume that all the progeny live and reproduce normally, it would only be a few centuries before HISTORY OF ZOOLOGY. +5 the entire earth would be occupied by herds of elephants. In order to preserve the equilibrium in nature great numbers of unfertilized and fertilized eggs, as well as young animals and many that are mature but have not yet attained their physiological destiny, must perish. Many individuals will undoubtedly be blotted out by purely accidental causes; yet on the whole those individuals which are best protected will best withstand adverse conditions. Slight superiority in structure will be of importance in this struggle for existence, and the possessors of this will gain an advantage over their companions of the same species, just as in domestication each character which is or is fancied to be useful to mun insures advantage to the possessor. Among the numerous varieties that appear the fittest will survive, and in the course of many generations the fortunate variations will increase by sum- mation, while destruction overtakes the unsuitable varieties. Thus will arise new forms, which owe their existence to ‘natural selec- tion in the struggle for existence.’ The ‘Struggle for Existence.’—The expression ‘struggle for existence’ is figurative, for only in rare cases does an active con- scious struggle decide the question of an animal’s existence; for example, in the case of the beasts of prey, that one which by means of his bodily strength is best able to struggle with his com- petitors for his prey is best provided in times of limited food- supply. Much more common is the unconscious struggle: each man who attains a more favorable position by special intelligence and energy, limits to an equal degree the conditions of life for many of his fellow men, however much he may interest himself in humanity. The prey which by special craft or swiftness escapes the pursuer turns the enemy upon, the less favored of its com- panions. It is noticeable that in severe epidemics certain men do not fall victims to the disease, because their organization better withstands infection. Here the term ‘survival of the fittest,’ which Spencer has adopted in preference to ‘struggle for exist- ence,’ is better. Instances of the Struggle for Existence.—Although the fore- going general considerations suffice to show that the struggle for existence plays a very prominent réle in the organic world, yet on account of the importance of this feature it will be illustrated by a few concrete examples. The migratory rat (Mus decumanis), which swarmed out from Asia at the beginning of the eighteenth century, has since then almost completely exterminated the house- rat (Mus rattus) in Europe, and has made existence impossible for 46 GHNERAL PRINCIPLES OF ZOOLOGY. it in other parts of the world. Several European species of thistle have increased so enormously in the La Plata states that they have in places completely crowded out the native plants. Another European plant (Hypocheris radicata) has become a weed, over- running everything in New Zealand. Certain races of men, like the Dravidian and Indian, die off to the same degree that other races of men, like the Caucasian, Mongolian, and Negro, spread. The more one attempts to explain that endlessly complicated web of the relations of animals to one another, the relations of animals to plants and to climatic conditions, as Darwin has done, so much the more does he learn to appreciate the methods and results of the struggle for existence. He will become conversant with many interesting phenomena, formerly unintelligible, which immediately find an explanation through this doctrine. Islands lying in the midst of the ocean have a disproportionately large number of species of wingless insects, because the flying forms are easily carried out to sea. For example, on the Kerguelen Islands, remarkably exposed to storms, the insects are wingless; among them one species of butterfly, several flies, and numerous beetles. Sympathetic Coloration.—Very often, in regions which have a permanent or prevailing uniform color, the coat of the animals is distinguished by the same or at least by a similar hue; this phenomenon is called sympathetic coloration. Inhabitants of regions of snow are white, desert animals have the pale yellow color of the desert, animals which live at the surface of the sea are transparent; representatives of the most diverse animal branches show the same phenomenon. The advantages connected therewith scarcely need an explanation. Every animal may have occasion to conceal himself from his pursuers; or it may be his lot to approach his prey by stealth: he is much better adapted for this the closer he resembles his surroundings. Natural selection fixes every advantage in either of these directions, and in the course of many generations these advantages increase. Mimicry is referable to the same principle, except that the imitation is not here limited to the color, but also influences form and marking. Frequently parts of plants are imitated, sometimes leaves, sometimes stems. Certain butterflies with the upper sur- faces of the wings beautifully colored escape their pursuers by the rapidity of their flight; if they alight to rest, they are protected by their great similarity to the leaves of the plants around which they chiefly fly. When the wings are folded over the back, the dark coloring of the under sides comes into sight and the color on HISTORY OF ZOOLOGY. 47 the upper side is concealed. The parts are so arranged that the whole takes on a leaf-like form, and certain markings heighten the imitation of the neuration of the leat (fig. 11). Among the numerous species of leaf-butterflies there are different grades of completeness of mimicry; in many even the depredations of insects Fia. ce butterflies. .4, Kallima paralecta, flying; a, at.rest. (After Wallace.) B, Siderone strigosus, flying; b, at rest. (After C. Sterne.) is imitated; in others the form and marking are still incompletely leaf-like, the marking being the first to come into existence. Among the grasshoppers also there are imitations of leaves, like the ‘walking-leaf,’ Phylliwm siccifolium, P. scythe, while other nearly related forms more or less completely approach the appear- ance of dried, sometimes of thorny twigs (fig. 12, a and 4). Examples of Mimicry.—Very often insects are copied by other animals. Certain butterflies, Heliconia, fly in large swarms, 48 GENERAL PRINCIPLES OF ZOOLOGY. seythe 2. Fig. 18.—Methona psidii, a bad-tasting Heliconiid, copied by the Pierid, Leptatis orise. (After Wallace.) HISTORY OF ZOOLOGY. 49 clumsy and yet unmolested by birds, because they contain bad- tasting fat bodies. Another species of butterfly accompanies them (Pieridze), which does not taste bad, and yet are not eaten, because in flight, in cut, and marking of the wings they imitate the Heliconie so closely that even a systematist might easily be confused (fig. 13). In a similar way bees and wasps, feared on account of their sting, are imitated by other insects. In Borneo there is a large black wasp, whose wings have a broad white spot F1a. 14.--a, Mygnimia aviculus, a wasp imitated by a beetle; b, Coloborhombus fascia- tipennis. (After Wallace.) } nat. size. near the tip (Mygnimia aviculus). Its imitator is a heteromerous beetle (Coloborhombus fasciatipennis), which, contrary to the habit of beetles, keeps its hinder wings extended, showing the white spot at their tips, while the wing-covers have become small oval scales (fig. 14). Sexual Selection is a special phase of natural selection, chiefly observed in birds and hoofed animals. For the fulfilment of his sexual instincts the male seeks to drive his competitors from the field, either in battle or by impressing the female by his special 50 GENERAL PRINCIPLES OF ZOOLOGY. excellences. With strong wings and with spurs the cock main- tains possession of his flock, the stag by means of his antlers, the bull with his horns. The birds of paradise by means of beautiful coloring win the favor of the females, most singing-birds by means of song; many species of the fowl by peculiar love-dances. Since a Fic. 15a.—Paradisea apoda, male. (After Levaillant.) all these characters belong chiefly to the male, and since it is only exceptionally that they are inherited by the female (and even then are less pronounced), it is almost certain that in a great measure they have been acquired by the males through the struggle for the female. In the case of birds a second factor has un- doubtedly co-operated to impress distinctly the often enormous difference between the feathers of the male and of the female—as HISTORY OF ZOOLOGY. 51 is shown, for example, in the case of the birds of paradise (fig. 15); for the nesting female inconspicuous colors and a close- lying coat of feathers are necessary in order that, undisturbed by enemies, she may devote herself to incubation. On the Efficiency of Natural Selection.—In the course of the last decade there has been much controversy as to how far natural selection alone is a species-forming factor. A number of objectors dispute the possibility of fortuitous variations being utilized in the struggle for existence. It is not easy to see how many characters, = Fia. 158.—Paradisea apoda, female. (After Levaillant.) especially such as are used in classification, can be of use to their owners. It can only be said that they have developed in correla- tion, that is in necessary organic connexion, with other important characters. But useful characters must be considerable in order to be seized upon by natural selection. Fortuitous variations with which Darwinism deals are too inconsiderable to be utilized by the organism and so to be of value in the struggle for existence. In most cases, too, alteration in one organ alone is not enough to be of value; usually a whole series of accessory structures must be modified. In short, there must exist a harmonious co-operation of parts, which presupposes a progressive and well-regulated development extending through a long space of time during which the struggle for existence could have exerted no directing influ- ence. Thus, for example, the wing of a bird in order to be used for flight must have already reached a considerable size; the muscles for moving it, the supporting skeletal parts, the nerves running to it must have a definite formation and arrangement. Then there are difficulties in that most animals are bilaterally or 52 GENERAL PRINCIPLES OF ZOOLOGY. radially symmetrical, many in addition segmented. In all these cases the same organ is repeated two or more times. Organs which are repeated symmetrically and usually those which are segmental agree in general in structure. One must therefore admit that the alterations of chance must have occurred at at least two points simultaneously and in exactly the same way. A further objection is that the action of natural selection would under ordinary conditions be negatived by unhindered crossing of the varying forms. If, for example, we do not isolate fantails from other pigeons, they will cross with these, and their descendants will soon resume the character of common pigeons. Finally, it has been claimed that for the formation of new species a simple variation of forms is not sufficient; it must reach still farther: (1) a variation in different directions, a divergent development of the individual members of a species; (2) the disappearance of the transitional forms which unite the divergent forms. The objection that the struggle for existence cannot bring about the divergent development of individuals necessary for improvement is of least importance. It need only be added that of the many variations appearing at the same time in a species two or more may be equally useful; that then one set of individuals will seize upon one, another set upon the other advantage, and that in consequence of this both sets will develop in different directions. Consequently the intermediate forms which are not pronounced in the one or the other direction will be in an unfavorable position, and must carry on the struggle for existence with both groups of partially differentiated companions of their species, and, being less completely adapted, must fall. More important are the first two objections; they have led to theories which originally seemed destined to complete the Dar- winian theory, but in the course of discussion they have more and more raised the claim of entirely supplanting it. In the following paragraphs will be found an outline of these theories, but it is to be taken into consideration that, at the present time, we are still in the midst of the reform movement, and it cannot yet be said whether they will be able to stand beside the theory of the struggle for existence or will supplant it. Migration Theory.—T'o explain how characters newly formed by variation become fixed, and do not disappear again through crossing with differently modified individuals, M. Wagner has pro- posed the Theory of Geographical Isolation, or the Migration Theory. New species may arise if a part of the individuals of one HISTORY OF ZOOLOGY. 53 species should take to wandering, or should be transplanted, and thus come to a new place, in which crossing with the companions of their species who were left behind is not possible. The same might occur, if the region inhabited by a species should by geological changes be divided into two parts, between which inter- change of forms would be no longer possible. The animals remaining under the old conditions would retain the original characteristics; the wanderers, on the other hand, would change into a new species. Direct observations support this theory. A litter of rabbits placed at the beginning of the fifteenth century on the island of Porto Santo has in the mean time increased enormously and the descendants have taken on the characteristics of a new species. The animals have become smaller and fiercer, have acquired a uniformly reddish color, and no longer pair with the European rabbit. A further proof in favor of the theory of geographical isolation is the peculiar faunal character of regions separated from adjacent lands by impassable barriers, broad rivers or straits, or high mountains (comp. p. 42); especially instructive in this regard is the peculiar faunal character of almost every island. The fauna of an island resembles in general the fauna of the mainland from which the island has become separated by geological changes; it usually has not only these but also so-called ‘vicarious species,’ i.e., species which in certain characteristics closely resemble the species of the mainland. Such vicarious species have plainly arisen from the fact that isolated groups of individuals, scattered over the islands, have taken on a develop- ment divergent from the form from which they started. With all due recognition of the migration theory, it will never be possible by it alone to explain the multiformity of the organic world. In addition, it must be assumed that formerly the earth’s surface possessed an enormous capacity for change; but the more recent investigations make it probable that the distribution of land and water has not varied to the degree that was formerly believed. The experience of botanists, too, teaches that several varieties can arise in the same locality and become constant. Lamarckism.—While the migration theory agrees with Dar- winism in this, that the new characters appearing through varia- tion are to be regarded as the products of chance, yet it is just this part of the theory which has been subjected to searching criticism. Many zoologists have again adopted the causal founda- tion of the descent theory proposed by Lamarck and believe that the cause of species formation is to be found in part in the 54 GENERAL PRINCIPLES OF ZOOLOGY. immediate influence of ‘changing environment, in part in the varying use and disuse of organs, brought about by alterations in the conditions of life. Both principles, they say, are sufficient, even without the help of the struggle for existence, to explain the phylogenesis of organisms. Influence of Environment.—T'o what extent can the environ- ment bring about a permanent change in the structure of plants and animals? To decide this is no simple problem, on account of the complexity of the factors entering into the question. In cases where the food-supply is altered, organisms change in a very remarkable manner and within a short time; but these changes (Niigeli’s ‘ Modifications through Nutrition’) seem to have no permanence. Plants which, found in nature in poor soil, are transplanted into rich soil, or vice versa, soon acquire quite a different appearance, and preserve this through the following generations, so long as they remain in the rich soil; but the plant quickly returns to its former appearance when replaced in its previous surroundings. In general, a change seems to be the more permanent the more slowly it has developed. In researches upon the influence of environment, we can, therefore, rely soonest upon results if we experiment with slowly-working factors, such as light and heat, dry or moist air, different intensities of gravitation, of stimuli, etc., which can be excluded from the environment of the organism. Use and Disuse.—Regarding the efficiency of use and disuse, there is no doubt that the shape of an animal is influenced to a great extent by the manner in which the organs are used. The organs which are much used will become especially strong and vice versa those which are not used will become weak. The only ques- tion is whether these, in the strict sense of the word, newly-acquired characteristics are transmitted to the offspring, or whether the descendants, in order to attain to the same stage, must not repeat in the same way use and disuse. In the latter case the cumulation of characteristics, and with it the possibility that these may become permanent, is excluded. It is to be regretted that accurate results are still lacking on a point so well adapted for experimental treatment. At this time rudimentary organs strongly favor the Lamarckian principle; for we see that cave animals, which for many generations have lived in darkness, are blind, either having no eyes, or only vestiges of them, incapable of function. This seems to justify the view that this condition is attributable to lack of use, since it has brought about a functional and anatomical HISTORY OF ZOOLOGY. 5d incapacity, which has increased from generation to generation. Now we must believe that what is true for disuse must express itself in the reverse sense in the case of use. Nageli’s Principle of Progression.—In conclusion, there is still to be considered the change of species from internal causes, to which von Baer gave the poorly adapted because easily misleading term ‘‘ Zielstrebigkeit ” (the striving toward an ideal), and which Niigeli has termed the ‘ perfecting principle,’ or the ‘ principle of progression.’ It cannot, indeed, be denied that each species is compelled, by some peculiar internal cause, to develop into new forms, independently of the environment, and up to a certain degree, independently of the struggle for existence. In all animal branches we see the progress from lower to higher going on, very often ina quite similar way, in spite of the fact that the animals live under very different conditions of development. We see how the nervous system lying near the surface in the lower animals becomes in the higher animals concealed in the depths of the body; how the eye, at first a simple pigment-spot, becomes in worms, arthropods, molluses, and vertebrates, provided with accessory apparatus, as lens, vitreous body, iris, choroid, etc. Here we see an energy for perfection which, since it occurs everywhere, must be independent of the individual conditions of life, and must have its special explanation in the character of the living substance. It is by no means justifiable to call an assumption, as here expressed, teleological, and to reject it as unscientific; rather the organism seems to be just as mechanically conditioned as a billiard- ball, whose course is determined not only by contact with the cushions of the billiard-table, but also in a large measure by its indwelling force, imparted to it by the stroke of the eve. An organism, too, is a store of energy which must necessarily from itself develop more, but it is of more extraordinary complexity, and to an equal degree also is independent of the external world. A complete independence is naturally never present, and Nigeli has not so maintained. Along with it rather goes always an ‘action’ of the external world, a modifying influence which is carried on by the external conditions of existence, either directly or by the mediation of use and disuse. This exposition of evolution has been given in a rather detailed way, because in the history of zoology it is undoubtedly the most important feature. No other theory in the course of the develop- ment of zoological investigation has gained such a hold, none has propounded so many new problems and opened so many new fields 56 GENERAL PRINCIPLES OF ZOOLOGY. for research. There is no other zoological theory which compares with it in value as a working hypothesis. To the many objections which have been made that the theory is insufficiently grounded, it can only be replied that in the present state of our knowledge it is the only theory which agrees with our experiences and explains these in a simple way and on a scientific basis. In this sentence is given the merit of the theory, but at the same time also a limita- tion of its applicability. For on the one side the statement attrib- utes the merit in the applicability of the system to the necessity of the human mind for simple explanations of the facts of natural science, and on the other hand it makes the degree of correctness dependent upon the state, whatever it may be, of our knowledge. On both sides no constant quantities are involved. Many investi- gators see no necessity of reconciling paleontology and our knowl- edge of plants and animals. To such, therefore, the Darwinian theory proves just as little as any opposing theory. Meanwhile thoughtful naturalists will keep in mind that our knowledge of nature is making considerable advances, and is visibly becoming wider and deeper. It is possible, even probable, that these advances will lead to many modifications of the theory. For instance, the theory of the causes which condition the formation of new species will undergo numerous changes. On the other hand, we can affirm with great certainty that the principle of descent, which first obtained credence through Darwinism, will be a permanent landmark of zoological investigation. GENERAL MORPHOLOGY AND PHYSIOLOGY General Zoology: Animal Morphology.—lIn the vital phenom- ena of animals a certain degree of similarity can be followed through the entire animal kingdom; the way in which animals are nourished and reproduce their kind, how they move, and how they gain experience, is essentially the same in great groups, and even widely separated forms show many agreements. Corresponding to this, the apparatus which is concerned with the above-men- tioned functions, the organs of nutrition and reproduction, of motion and sensation in their grosser and finer structure, and in their ontogeny, must be similar to one another and show evidence of some fundamental characters which always or frequently recur. All this needs a general explanation before we can go into a description of the separate branches of animals. This explanation is the subject of general zoology, specially of general anatomy and embryology, or animal morphology. cology or Biology.—If by means of anatomy and embryology we have learned the general character of the animal organism, we must yet farther study its relations to the environment. For this study of the conditions of animal life, ecology or biology, we have to consider the geographical range of animals, their distribution over the surface of the earth and in the different depths of the sea; further, the reciprocal relations of animals and plants, and of beast to beast, as these find special expression in colony-build- ing, symbiosis, parasitism, etc. General Anatomy.—In the case of General Anatomy, with which we shall begin, the fundamental proposition will be, /fow is an organism formed from its constituent parts ? We shall thus in spirit follow the opposite course from that which anatomy actually takes, for this resolves the animal body into its elementary parts, its organs, tissues, and cells. Instead of analytical we will pursue synthetic anatomy. The synthesis of an organism, of which by general anatomy we can only gain an idea, actually takes place in nature during the ny o4 58 GENERAL PRINCIPLES OF ZOOLOGY. development of every animal. Embryologically every organism is at some time a simple element, a cell; this divides and gives rise to tissues; from the tissues are formed organs, and from the organs the regularly membered whole of the animal body is com- bined. If the general ontogeny proceeds synthetically, it then agrees in its manifestations with the processes which go on in nature and which are accessible to direct observation. GENERAL ANATOMY. The Morphological Units.—The expression ‘constituent parts of the animal body’ can be used in a double sense. We can speak of the chemical units, the chemical combinations, which form the tissues; these are the subject of animal chemistry, and may there- fore be passed over here. But we may also speak of the constituent units (morphological units) of the animal body; these are the cells. These and their transformation into tissues, organs, and entire animals are for us of vastly greater importance. I. THe MorpnonocicaL UNITS oF THE ANIMAL Bopy. The Cell.— The study of the morphological units of the organic body first found a firm foundation in the cell theory. Every scientific study of the anatomy of plants and animals must there- fore take the cell as its starting-point. History of the Cell Theory.—The conception of the cell of animals and plants has in the course of time undergone many changes, which must be known to some extent in order to understand completely the name and the conception. When, in the seventeenth century, Hooker, Marcello Malpighi, and Nehemia Grew introduced the term into vegetable anatomy, they meant small chambers surrounded by firm walls and filled with air or fluid contents. When, also, early in the nineteenth century, it was cor- rectly recognized that the cell is the anatomical and physiological vegetable unit from which all the other parts of the plant are formed, and when the English botanist Brown discovered in the interior of the cell that small body previously overlooked, the kernel or mzclews, the old conception remained, and as such was accepted by Schleiden in his cell theory. Schleiden added as new a completely erroneous view of the origin of cells: that in a sort of matrix (the ‘cytoblast’) first a granule, the nuclear body, was formed, then around this granule a membrane, the nuclear mem- brane, arose by precipitation, and around the thus completed nucleus a larger membrane (the cell membrane) was deposited. Hence for the formation of the cell the nucleus would be of most importance, The Schleiden-Schwann Cell Theory.—Since it is the nuclei which are most easily seen in the animal body, and even now are particularly useful GENERAL ANATOMY. 59 in deciding questions concerning the presence of cells, it is readily under- stood how Schleiden’s theory, which placed the nucleus so much in the foreground, should have led Schwann to apply the cell theory to the animal kingdom, and thus raise it toa principle of general application. We usually, therefore, speak of the Schleiden-Schwann cell theory. As a result of this theory the walls, the cell membrane, were regarded as most important for the function of the cell; through the cell mem- brane diffusion-currents must pass between the surrounding medium and the contents of the cell; the character of the membrane and of the cell-sap must determine the condition of the diffusion-currents, and hence the functional character of the cell; the different appearance of tissues depends chiefly upon the fact that the cells, spherical in the beginning, change their form; in the case of fibrillar connective tis- sue, for example, they increase enormously in length and become fine fibrillee. Since the life of an organism is nothing else than the co-operative work of all its cells, they flattered themselves that through the cell theory and the discovery, brought about by it, of the physical unity of the animal and vegetable body they had made an important advance in the great problem of the physical explanation of the phenomena of life. Cell gene- sis also seemed, according to the theory, to be just as satisfactorily explained on a mechanical basis as the formation of a crystal. In the ‘eytoblast’ the nuclear bodies, nuclear membrane, and cell membrane must be formed by deposition just as in the process of crystallization. Reform Movements,—Since that time our conception of the nature of cells has completely changed. The cell does not, after the manner of a erystal, arise as a new formation in a matrix, but it presupposes the existence of a living mother-cell, from which it arises by division or bud- ding. Just so also the cell is not a physical unit, but is itself an organism which shows to us all the enigmas of life, the physical basis of which our investigations must ever keep in view as a goal, though it be still indiscern- ibly distant. The membrane and cell-sap are of quite subordinate impor- tance for the existence of the cell; rather the most important thing in it is the previously disregarded substance, for which von Mohl introduced the name protoplasm. According to the newer conception the cell is practically asmall mass of protoplasm, usually, probably always, provided with one or more nuclei. This newer conception of the cell has developed so gradu- ally, and has so slowly supplanted the Schleiden-Schwann view, that the old name has been retained, although it no longer at all fits the new con- ception. We have indeed become so thoroughly accustomed to the name that we no longer notice the contradiction of terms when we call a solid lump without a membrane a ‘ cell.’ Discovery of Protoplasm.—The reformation of the cell theory was begun by discoveries which were made in very different regions and only lately have been brought to a focus. 1. At about the beginning of the nineteenth century, Bonaventura Corti and Treviranus had seen that the chlorophyl bodies, which cause the green color of plants, in many species stream around in a lively manner in the interior of the cell, but Moll was the first to find out that this motion 60 GENERAL PRINCIPLES OF ZOOLOGY. was not active, but rather that they are moved by a homogeneous sub- stance in which they are embedded. This substance, which Mohl, in order to bring it into prominence, named protoplasma, became by other studies still more important. In the reproduction of the simplest alge, it was found that the protoplasm, together with the chlorophyl bodies, col- lected itself into an oval mass, and that this body left the cell membrane aud swam freely in the water. Since the cell-wall no longer showed signs of life, while on the other hand the protoplasmic body came to rest and formed a new plant, it was shown beyond doubt that this was the most important constituent part of the cell (comp. fig. 115). 2. In the study of animal tissues the importance of the peculiar cell- substance, the protoplasm, was still more plainly brought out. Here, in spite of the long-prevailing preconceived idea, unbiassed observation led to the discovery that most animal cells had no cell-membrane. 3. Very important, finally, was the study of the lowest organisms, the Protozoa. Dujardin sought by extremely careful observations to prove that these animals had no organs, but consisted of a uniform granular sub- stance, the sarcode. The sarcode alone could produce all the vital phe- nomena, such as movement, sensation, assimilation, previously ascribed to many organs. Dujardin’s theory was stoutly contested by Ehrenberg and his school, but finally attained general acceptance through the epoch- making work of Max Schultze and Haeckel. Schultze’s Protoplasm Theory.—On the basis of these three series of observations, Max Schultze finally established the reformation of the cell theory briefly sketched above, when by accurate study of the appearance and the vital phenomena, and by means of numerous experiments, he proved that the cell-substance of animals, the sarcode of Protozoa, and the protoplasm of plants are identical, and that to this substance, for which he retained the name protoplasm, all the vital phenomena of animals and plants are referable in the ultimate analysis. The second important modification concerns the changes of cells into tissues. These follow not so much through changes of form and modification of the cells into the tissue elements, as Schwann thought, but rather by means of chemical changes. By means of its formative potentiality the protoplasm gives rise to non-protoplasmic structural parts, as, for example, connective-tissue fibrils, muscle fibrils, nerve fibres, etc. These give the various tissues their specific character and perform their functions. The tissues also retain as the source of life and formation the unemployed remnants of cells, the connective-tissue corpuscles, muscle corpuscles, ete. We will now trace out farther these two fundamental ideas in Max Schultze’s ‘protoplasm theory,’ and thereby briefly sketch the elements of the modern theory of tissues. Nature of the Cell.—The size of the animal cell varies to a considerable degree; the smallest elements are the male sexual cells, the spermatozoa, whose bodies, particularly in case of the mammals, are even less than 0.003 mm.; the largest, on the other hand, with the exception of the giant plasmodia of some GENERAL ANATOMY. 61 Mycetozoa, are the egg cells. The yolk of the bird’s egg, which alone forms the egg in the narrower sense, apart from its coverings, has for a time the morphological value of a cell, and in the case of the ostrich egg may reach a diameter of several inches. The form of the cell is hkewise variable. Free cells, whose form is not determined by the environment, are usually spherical or oval in the resting condition, as the egg cell shows; united into tissues, the cells, on the contrary, may be pressed together into polygonal or prismatic bodies, or may send out spindle- or star-shaped branching processes. Protoplasm.—So there is left to characterize the cell only the constitution of its substance: the cell is a mass of protoplasm with one or more nuclei. It is not known whether protoplasm is a definite chemical body, which from its constitution is capable of infinite variation, or whether it is a varying mixture of different chemical substances. So, also, we are by no means certain whether or not these substances (as one is inclined to believe) belong to those other enigmatical substances, the proteids. We can only say that the constitution of protoplasm must, with a certain degree of homogeneity, have a very extraordinary diversity. For if we see that from the egg of a dog there comes always and only a dog, and indeed an animal with all his individual peculiarities, that a sea-urchin’s egg, placed under the most diverse conditions, pro- duces always a sea-urchin, that a species of amceba always performs only the movements characteristic of that species, we must assume that the functioning constituent part of this cell, the protoplasm, has in each case its peculiarities. We are driven to the assumption of an almost unlimited diversity of protoplasm, even if we concede an impertant share in the prominent differences to the nucleus, of which we shall speak later. General Properties of Protoplasm.—The similarity of proto- plasm, still recognizable through all its variations, expresses itself in its appearance and in its vital phenomena. Under slight mag- nification, protoplasm appears as a faintly-gray substance, some- times colored yellowish, reddish, etc., by pigments taken up by imbibition, in which numerous strongly-refracting granules are embedded. The vital characteristics of this substance are move- ment, irritability, power of assimilation and of reproduction. By using higher powers a finer structure can be seen in the protoplasmic substance, the ‘homogeneous protoplasm ’ of earher writers. The nature of this is as yet in question: a fine-meshed framework (filar substance, spongioplasm, cell reticulum) the 62 GENERAL PRINCIPLES OF ZOOLOGY. interstices of which are filled with other material (interfilar sub- stance, enchylema, ground substance). The dispute les especially around the question whether this framework is formed of threads and trabeculae or whether the appearance is not formed by small chambers, bounded by fine partition-walls (foam structure of protoplasm). Movement of Protoplasm.—Movement expresses itself first in changes of form of the whole body—ameboid movement—and secondly in the change of position of the small granules in the interior of the protoplasm—streaming of granules. Examples of amceboid movement (fig. 16) are chiefly the movements of many Protozoa, and of the colorless blood-cells (leuco- cytes) of multicellular animals; here the protoplasmic body sends out coarser and finer processes, which may be again withdrawn, serving for locomotion and hence called pseudopodia or false feet. The streaming of granules can be observed in the interior of the cell- body, as well as in the pseudopodia Fic. 16.— Ameba proteus. (After extending from this. The pseudo- Bees cis eer ee podia may even be so fine as to be ice at the limits of visibility with our strongest magnifications (fig. 17), yet in them it can still be observed that the granules wander hither and thither like people on a promenade, simultaneously centripetally and centrifugally, some with greater, others with Jess speed. And yet the granules are only passively moved by the protoplasm, for if we feed the creature with some pigment granules, like tinely-pulverized car- mine, these granules show the same remarkable streaming. Indeed nothing better illustrates the great complexity in the structure of protoplasm than these extremely complicated phenomena of motion in such narrow limits as pseudopodia in general. Irritability of Protoplasm.—That ammboid movements and streaming of granules can be induced, brought to a standstill, and modified by mechanical, chemical, and thermal stimuli, is a sure proof of the irritability of protoplasm. Most important are the thermal stimuli; if the surrounding medium rise above the GENERAL ANATOMY. Fia. 17.—Gromia oviformis. (From Lang, after M. Schultze.) 64 GENERAL PRINCIPLES OF ZOOLOGY. ordinary temperature, the movements at first become more rapid up to a maximum: from that point begins a slowing, finally coming to a standstill.—eat-rigor. If the high temperature continue much longer, or if it rise still higher, death results. The fatal temperature is found for most animals between 40° and 50° C. (104°-122° F.); its influence explains a part of the injurious effects which high-fever temperatures have upon the human organism. Like the heat-rigor, there is also a cold-rigor, induced by a sharp sinking of the temperature below the normal. This is accompanied by a gradual diminution of mobility; it results in death by freezing, which is, however, not so easily produced as death by heat. It is a remarkable fact that many animals, conse- quently their cells, may be frozen; and in this condition can endure still severer cold without dying. (For example: goldfish, a temperature of — 8° to —15° C.; frogs, to — 28°; newts, to = 28°), Nutrition and Reproduction.—Irritability and power of motion are the prerequisites of assimilation, the change of food-substance into protoplasm. Most animal cells, for example almost all tissues cells, are not suitable for studying assimilation, because they live upon liquid nourishment. But certain cells of higher animals, the colorless blood-cells, and most unicellular animals can be fed also with solid substances; they take the food-particles into the midst of the protoplasm by flowing around them with the pseudo- podia. They extract all the assimilable and reject the indigestible portions (fig. 16). In the case of assimilation it is to be noted not only that the cells use the food which they have taken for their own growth and for replacing worn-out parts, but also that most of them have the power of producing substances other than protoplasm; for example, many Protozoa form organic shells or skeletons which are hardened with silica or lime. This formative power, the building of ‘plasmic products,’ is, as we shall shortly see, the starting-point for tissue-formation. Cell Nucleus.—The reproduction of protoplasmic bodies is synonymous with the division of the cell: but to understand this we must first consider the second important constituent, the nucleus. This is a body enclosed in the protoplasm, whose form, though definite for each kind of cell, shows in general wide varia- tions. Usually it is a spherical or oval vesicle; but it may be elongated or club-shaped, bent into a horseshoe, with constrictions like a rosary, or even be branched, treelike (fig. 18); in many cells GENERAL ANATOMY. 65 it is disproportionally large, so that the protoplasm surrounds it only with a thin layer, in others again it is so small that it can scarcely be found in the protoplasm among the other substances. Formerly, on this account, it was in very many cases overlooked, and even now it can often be demonstrated only by great care, Fig. 18.—Various forms of nuclei. a, horseshoe-shaped nucleus of an Acinete; I» branching nucleus from the Malpighian vessel of a Sphinyid larva; c, rosary- shaped nucleus of Stentor cwruleus. and by employment of a special technique based upon the micro- chemical reaction of the nuclear substance. The Nuclear Substance,—The nuclear substance is distin- guished from protoplasm, among other ways, by its greater coagulability in certain acids, e.g., acetic and chromic, which therefore are often used for demonstrating the nucleus. If ina living cell the nucleus be invisible on account of the similarity of its refraction to that of the protoplasm, the addition of 2% acetic acid will often bring it into sharp contour. Structure of the Nucleus.—In its minute structure the nucleus affords a wonderful variety of pictures varying according to the objects chosen, but which are not sufficiently understood to permit of a single description accepted by all. According to their reac- tions to stains two substances in particular are distinguished: chromatin or nuclein (fig. 19, ch), which is easily stained by certain staining-fluids (carmine, hematoxylon, saffranin), and the achroma- tin or linin, which stains not at all or only under special conditions. The achromatin forms a network or reticulum (according to another view a honeycomb structure) filled with a nuclear fluid, 66 GENERAL PRINCIPLES OF ZOOLOGY. bounded externally by a nuclear membrane, easily isolated in large nuclei. If little nuclear fluid be present, and the reticulum con- sequently be coarse-meshed, the nucleus seems compact. If the fluid be abundant, the nucleus appears vesicular. This is especially ab chp 6 4 Fic. 19.—Vesicular nuclei with achromatic reticulum and different arrangements of the chromatin and nucleolar substance. p, plastin (nucleolar substance); ch, chromatin; chp, chromatin plus plastin. land 2, nuclei of Actinospharium; 3, of Ceratium hirundella (after Lauterborn); 4, germinal vesicle of Unio (after Flemming); 5, nucleus with many chromatin nucleoli. the case when the lines of the framework are separated by con- siderable amounts of nuclear fluid (fig. 19, 4). The chromatin enters into close relations with a less stainable substance, the plastin or paranuclein (also sharply distinct from achromatin). In the nuclei of Protozoa plastin and chromatin are usually intimately united, the first forming a substratum in which the latter is embedded (chp). The united substances are most frequently closely and regularly distributed as fine gran- wes on the reticulum, so that the entire nucleus appears uni- formly chromatic (fig. 18). More rarely the mixture collects into one or more special bodies, the chromatic nucleoli (1, 2). The nucleolus is ordinarily a rounded body, more rarely branched (fig. 19, 7). In the nuclei of the Metazoa there may occur the same intimate mixture of plastin and chromatin (6). As a rule, however, the plastin (apparently not the whole, but a surplus) is separate from the chromatin. Thus there occur in the nuclei of many eggs GENERAL ANATOMY. 6T nuclei which contain, the one chromatin, the other exclusively plastin (4). In tissue cells only the plastin has the form of nucieoli (true or chromatin-free nucleoli, 5), while the chromatin is distributed on the nuclear reticulum (chromatin reticulum). Somewhat the same may occur in the Protozoa (fig. 19, 2). Significance of the Cell Nucleus.—For a long time the func- tional significance of the nucleus in the cell was shrouded in complete darkness, so that it began to be regarded, in comparison with the protoplasm, as a thing of little importance. The evidence that the nucleus plays the most prominent role in fertilization has altered this conception. Then arose the view that the nucleus determines the character of the cell; that the potentiality of the protoplasm is influenced by the nucleus. If from the egg a definite kind of animal develop, if a cell in the animal’s body assume a definite histological character, we are, at the present time, inclined to ascribe this to the nucleus. From this, then, it follows farther that the nucleus is also the bearer of heredity; for the transmission of the parental characteristics to the children (a fact shown to us by our daily experience) can only be accomplished through the sexual cells of the parents, the egg and sperm cells. Again, since the character of the sexual cells is determined by the nucleus, the transmission in its ultimate analysis is carried on by the nucleus. This idea has a further support in experiments on Protozoa. If one of these unicellular animals be cut into nucleate and anucleate halves, the latter sooner or later degenerates, the former persists and regenerates the lost parts. Within the nucleus it is probably the chromatin which controls the functions of the protoplasm and is accordingly (as observations on fertilization also seem to show) the bearer of heredity, while the achromatin is the seat of contrac- tility, and as such plays a part in cell multiplication. The Centrosome.—Besides the nucleus there frequently occurs a special body in the protoplasm, the centrosome, which on account of its small size and a behavior similar to achromatin with reference to staining-fluids was long overlooked, and even now its demonstration is difficult. It is apparently well distributed among the Metazoa, but is absent from most Protozoa. In many it appears only at certain times and then disappears. What is known of it makes it probable that it is a derivative of the nucleus, a part of the achromatin which has left the nucleus; in other cases possibly a second nucleus which by degeneration has lost the chromatin and retained only the active nuclear substance, the achromatin. In its function the centrosome is a specific organ of 68 GENERAL PRINCIPLES OF ZOOLOGY. cell division which controls both the division of the nucleus and that of the cell itself. Multiplication of Cells.—Increase in cells occurs exclusively by division or by budding (gemmaticn). Most common is binary division in which a circular furrow appears on the surface of the cell, deepens and cuts the cell into two equal parts. Multiple division is more rare and can only occur in multinucleate cells, Here the cell divides simultaneously into as many (sometimes hundreds) daughter-cells ag there were nuclei present. In all forms of division the simi- larity of the products is char- acteristic, while in budding the resulting parts are unequal. In budding one or more smaller daughter-cells, the buds, are constructed from a a large mother-cell (fig. 20). i Direct Cell Division. — Every cell division is accom- panied by nuclear division or Fic. 20.—Cell budding. _Podophrya gemmi- at least presumes that nuclear Bare een padeie) Waiga scuurstenod form division. has previously oé- 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. ete tremely rare in man. III, ANNELIDA. 305 Class III. Annelida. The segmentation of the body, which was shown ina slight de- gree in the Chetognathi, reaches its highest development in the Fic, 272.—Diagram of annelid somites (orig.). am, acicular muscles; c, ccelom: cm, circular muscles ; cv, circular blood-vessels; d, dorsal blood-vessel; i, intestine ; dm, longitudinal muscles; m, mesentery ; x, nerve cord; na, nephridium; ne, nv, neuro- and notopodia, forming parapodium; s, septum; so, somatopleure; sp, splanchnopleure; t, 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- WS |i (tn a ath Fig. 273.—Trochophore (Loven’s larva) of Polygordius. (From Hatschek.) 4, anus; dLM, dorsal muscles; £D, hind gut: J, stomach; J,, imtestine: Msfr, mesoderma band; 1, nerves; Neph, protonephridia; O, mouth; Oe, esophagus; oe LM, cesopha- 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 C@LUHELMINTHES. worms with colom 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 (W4r) 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 larve 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 Chetopoda and the closely related Archianellida. In other forms one or more features may be lacking—in the Gephyraa segmentation of the body; in the Hirudinei most of the celom 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. Chetopoda. 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 cclom is divided by the septa—delicate double membranes which extend ae pita in er end from the ectoderm to the alimen- and Jung.) 1, first segment with MATE Ae = ae ‘ r ahe J mouth and prostomium; 15, male tary canal—into as many cham sexual opening; 33-37, clitellum. bers as there are metameres, while a longitudinal mesentery, also double, separates the coelomic TI, ANNELIDA: CHA?TOPODA. 807 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 asupracsophageal ganglion (‘ brain ’) lying in the prostomium, from which the esophageal commissures pass around the esophagus to form the ventral chain, which con- sists of as many pairs of ganglia, united by longitudinal commis- Cal Ss / ani WI PHTT Fic. 275.—Anterior end of Nais elinguis. h, cerebrum, connected by commissure with ventral chain, n; dy, contractile dorsal, vg, ventral blood-vessel; m, muscular aA of skin; db, vb, dorsal and ventral chet; d, septa; k, prostomium; 0, 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 hemoglobin. 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 CQ@LHELMINTHES. body may function as hearts (fig. 276, c). Rarely, as in the Capitellidx, 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 conyo- ¢ oe dg ly a pr st ae Fic. 276.—Anterior end of Pontodrilus marionis. (After Perrier.) a, vascular arches; b, ventral nerve chain; ¢, ‘hearts’; co, esophageal commissure; dg, dorsal blood- vessel; ds, septa; gc, cerebrum; l, retractors of pharynx; lg, lateral blood-vessel: 0, Ovary; oe, @sophagus; Dp, receptacula seminis; ph, pharynx; pt, ciliated funnels of vas deferens; s, nephridia; vd, vas deferens, = ae sec sees / m up bm Fie. 277.—Schematic cross-section of an annelid. (After Lang.) ac, aciculum; ), cheetee ; bm, ventral nerve cord; dc, dorsal cirrus: dp, notopodium, ky, gill: Im, longitudinal muscles; md, digestive tract; mp, nephridium,; ov, ovary; rm, circu- lar muscles; tm, transverse muscles; tr, nephrostome; vc, ventral cirrus; vd, vv, dorsal and ventral blood-vessels; vp, neuropodium. ov luted tube, and the external opening (fig. 69). In many instances (Oligochetes, some Polychvtes) 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 III, ANNELIDA: CHA#TOPODA. 309 always arise from the ccelomic epithelium. In the Oligocheta (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 Oligochztes they may empty into the anterior or posterior ends of the digestive tract. In many (@lycera, Hesione, Nephthys, Gontada) 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 larvee occur. These, in spite of their many modifications, are comparable with the ‘ Loven’s larva,’ the trochophore already mes mes a Pra. 278.—4, larva of Polygordius; B, same changing to segmented worm. (After Hatschek.) a, anus; kn, 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 larva), 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 COLUELMINTHES. jointed portion the celom 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 hada metamorphosis. From the resemblance of the trochophore to the Rotifera the farther conclu- sion is drawn that the annelids have descended from Rotifer-lke ancestors, the body cavity, nephridia, blood-vessels, and veutral 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 polychwtes 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 WI. ANNELIDA: CHA#TOPODA. 311 somites, which contain the sexual organs, develop special bristles and para- podia (fig. 284, 4). Thus many species of Wereis pass into the ‘ Hetero- nereis’ stage. In other Polychzetes the sexual part (epitoke) separates from the sexless atoke portion and swims freely, while the atoke produces new epitokes. In the Samoan Islands Hunice 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 ave thus related. The Autolytide 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 ‘ Polybostrichus,’ the females as ‘ Sacconereis.’ 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 Chetopoda is the posses- sion of bristles or chet. 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 epithelhum 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 Oligochete. (After Vejdowski.) ¢, epithe- lium; rm, Un, circular and longitudinal muscles; m, muscle of the follicle; by,, cheta follicle, its cheta in function; b2, 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. Polychete. The Polychxtz owe their name to the fact that each group of bristles contains many chete; but more important is that the 312 CALUELMINTIES. 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 Fig. 281—A, head with protruded pharynx; 8, parapodium of Nereis versipedata. (After Ehlers.) ¢, cirri; k, jaws; l, head with eyes; p, palpi; t, 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- Fig, 282.—aAmphitrite ornata.* (From Verrill.) ing to position or function, as cirri, elytra, gills, ete.; onthe head, palpi and tentacles. The cirri are long processes on the parapodia, and like palpi are tactile (fig. 281). Elytra are thin lamelle which cover the back like shingles and thus protect the body. Ill, ANNELIDA: CHATOPODA. 3138 Nearly all Polycheetes are dicecious and undergo a more or less pro- nounced metamorphosis; with few exceptions (Manyunkia*-~from the Schuylkill, Weveis * 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 Errantia the head and trunk are not very different ; the anterior part of the alimentary tract can be protruded ag 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 Oligochetes 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£, mostly represented on our shores by small species, contains some species a yard in length. Diopatra,* Nothria.* The ALCIOPIDE are transparent pelagic forms with large, highly Fie. 283.—Head of Polynoe developed eyes. The SyLuIp# usually have © spinifera (After Ehlers.) three long tentacles ; Avtolytus,* Myrianida* Faek entirely covered win (p. 210). The PoLynoib& * (Lepidonotus,* Poly- projecting at the sides. noe* (fig. 284), Aphrodite aculeata,* the sea mouse, 6 inches long) are bottom forms with elytra covering the back. NEREIDE ; 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- D& the tube is membranous and there is a crown of gills ; Myxicola,* Chone,* Manyunkia.* In the SERPULID& the tube is calcified and is closed by an operculum on one of the gills. Hydroides;* Spirorbis,* forming coiled tubes on seaweed ; Protula.* The ARENICOLIDA,* which burrow in sand, have gills on the sides of the body. The MaALDANID#® (Clymene,* Azxiothea*) have extremely long segments and build tubes of sand. The TEREBELLID (Terebella,* Amphitrite (fig. 282), Thelepus *) have numerous filiform tentacles and branched gills on the anterior end. The ARCHIANELLID.E, which lack bristles and parapodia, must be placed near the Polychxte and are usually regarded as very 3814 C@ZLHELMINTHES. primitive forms which in structure and development (fig. 250) are of importance in the phvlogenesis of the Annelids. Polygordius.* Fia. 284.—New England Annelids. A, male Autolytus ; B, Sternaspis fossor ; C, Cise tenides gouldii; D, Clymene torquata. (From Emerton and Verrill.) Order II. Oligochete. The Oligochetes are almost as preeminently fresh-water and terrestrial forms as the Polychetes 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 chet may be regularly distributed around each somite (Pericheta) or gathered on the sides (Jfegas- cole) 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 ITI, ANNELIDA: CILHTOPODA. 315 presence of numerous glands, forming a clitellum (fig. 274), which secretes the egg cocoons. The clitellum also functions in copula- tion, secreting bands which 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 TUBIFICIDA, 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. Z'ubifea,* Petoscolex ; Clitellio inoratus * common on our seashores. The NaIpID# are transparent forms living on water plants which reproduce asexually throughout nearly the whole year. i ee Ot ow br dito a Fia. 286.—Sexual organs of Lumbricus herculeus (after Vogt et Jung); the seminal vesicles of the right side cut away. bm, ventral nerve cord; bl, bv, lateral and ven- tral rows of sete; di, septa; hy, he, testes, enclosed in sperm reservoir; 0, ovaries ; ov, oviducts; sbu, sperm reservoir ; sh,, 4 3, Sperm sacs (seminal vesicles) : sty seminal receptacles; t,, te, seminal *funne to, funnels of oviducts; vd, vas deferens. Dero* and Aulophorus* have gills around the anus. ENCHYTR#IDA; Distichopus, Pachydrilus. The DiscopRitips& (Bdellodrilus, Myzobdella) are parasitic. Sub Order II. TERRICOLA (Macrodrili). Here belong the terrestrial forms, the earthworms, our species of moderate size, in the tropics large s connected with the vasa deferentia ; 316 COLHELMINTHES. species (Megascolex australis four feet long). Our species belong to Lumbricus,* Allobophora*; Pericheta* 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. Gephyrea. The exclusively marine Gephyrea are distinguished at the first glance from the Chetopoda 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. Fie. 287.—Anatomy of Phascalosoma gouldi (orig.). a, anus; a, anterior retractors; d, digestive tract; g, gonads; m, mouth; 7, nephridia; nce, ventral nerve cord; pr, posterior retractors. Fie. 288.—Larva (trocophore) of Echiurus. (After Hatschek.) a, anus; d, intestine; hw, postoral cilia; kn, protonephridia; m, mouth; mes, mesoderm bands with indi- cation of segments; 7, ventral nerve cord; sc, esophageal commissure; sp, apical plate; vw, preoral ciliated band. III. ANNELIDA: GEPHYRAA. 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 Chetiferi 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 (ye@upa, bridge) Gephyrea. 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, esophageal collar, and ventral cord, the latter without division into ganglia. The relations of the Gephyrea to the Chetopoda are shown by the development. In some (Chetiferi) there is a trochophore (fig. 288) from which the worm arises, as in the Chetopoda, by growth at the hinder end; this at first has a segmented ceelom and nervous system, the metamerism being lost later. Order I. Chetiferi (Armata, Echiuroidea). With spatulate preoral lobe, often forked at the tip; at least a pair of ventral sete; a trochophore in development. Hchiurus pallasti* in our northern waters, Thalassema* farther south. In Bonellia there is a marked sexual dimorphism (fig. 389). 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 cesophagus of the female (fig. 289, B). Order II. Inermes (Achzta, Sipunculoidea). Distinguished by lack of chzete, 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. Priapulus, Halicryptus. 318 COG@LHELMINTHES. Sub Class III. Hirudinet (Discophort). Three points of external structure clearly distinguish the leeches from the chetopods. 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, Fia. 289.—Bonellia viridis. A, female (after Huxley); B, male (after Spengel). cc, cloaca; d, rudimentary intestine; g, excretory organ; 7, intestine; m, muscles sup- porting intestine; s, balls of spermatozoa in B, in A, proboscis (preoral lobe); u, single segmented 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 toasegment. The middle or one of the anterior rings is often distinguished by bearing strongly developed sense organs. Asinthe earthworms, certain of the somites at the time of reproduction may develop into a clitellum which secretes the egg cocoons. III, ANNELIDA: HIRUDINEI. 319 A third character is the marked flattening of the body in the dorsoventral direction (except in Ichthyobdellide and a few other forms), the animals thus recalling the flatworms. This may be the result of the very slight development of the celom. 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. Fie. 290. Fic. 290.—Transverse section of Hirudo medicinalis. (From Lang.) dm, lm, rm, dorso- ventral, longitudinal, and circular muscles; vl, vd, vv, lateral, dorsal, and ventral blood-vessels, the latter surrounding the ventral nerve cord, ni; h, testes; vd, vas deferens; md, midgut; np, nephridial tubule; enp, urinary bladder. Fig. 21.—Digestive tract of Hirudo medicinalis. (From Lang.) a, oesophagus; b, in- testine; d,, dy, 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- bdellidw, of four longitudinal trunks, a dorsal, two lateral, and a 320 C@LHELMINTHES. 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 Jat- ter containing frequently twenty-three ganglia (the first of five fused, the last of seven). Nerves from the brain go to the eyes. Right and left of the ventral cord are the hermaphroditic sexual organs. In Hirudo medicinalis (fig. 293) there are nine pairs of Fie. 292. Fic. 293. Fie. 292.—Hirudo medicinalis, medicinal leech. (After Leuckart.) a, anterior end with three jaws (x); 6b, a single jaw with its muscles. Fig. 293.—Nervous system, blood-vessels, sexual organs, and nephridia of a leech, ventral view. h, testes; hb, urinary bladder; /g, lateral blood-vessel; n, ventral nerve cord; nh, epididymis; 0v, ovary; p, penis; sc, nephridia; uw, uterus and vagina; vd, vas deferens; vg, ventral blood-vessel. testes (2), the ducts of which unite to form a vas deferens on either side (vd). These pass forward, form by coiling a so-called epididymis (xk) 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 (wz). 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 ccelom 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 acelom. In Clepsine there are the dorsal and ventral blood-vessels of the Cheetopoda and besides four longitudinal ccelomic 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 ewlomic, since the nephrostomes con- nect with it. In most Hirudinei a canal system filled with blood has arisen from the calom and blood-vessel, and in Nephelis is in part lacunar in character. Further relations are shown by Acanthobdella peledina, parasitic on fishes. This has both blood-vessels of the Oligocheetes, a IV. POLYZOA. 321 body cavity divided by septa and chete. On the other hand it is leech- like in other features; two suckers and sexual apparatus on the Hiru- dinean pattern. Order I, Gnathobdellide. The jawed leeches include Htrudo medicinalis, once extensively used for blood-letting but now little employed. Ha@madipsa includes land leeches, one of the terrors of travelers in the tropics. In Wephelis* the jaws are soft. Dacrobdella * includes our largest native species. Order II. Rhynchobdellide. Without jaws. The CLEpPsINIDH mostly feed on snails and fishes. Clepsine* in our waters. Hementaria officinalis of Mexico is used for blood-letting ; H. ghiliant of South America is poisonous. The IcHTHyo- BDELLIDA, cylindrical, occur in salt and fresh water, parasitic on fishes. Ichthyobdella,* Pontobdella,* marine ; Piscicola, fresh water, Class IV. Polyzoa (Bryozoa). In external appearance the Polyzoa closely resemble the hydroids, so that the inexperienced have difficulty in 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 Kcto- procta—differ so widely that one may doubt whether they belong together. The Entoprocta have no celom, and resemble in this respect the Rotifera; the Ectoprocta are true Celhelminthes 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 CQLHELMINTHES. open. The Bpaee between the horseshoe-shaped intestine and the body surface is completely filled by a pa- repchyma containing muscle cells, and correspondingly the excretory organs are protonephridia. In our fresh-water Urna- tilla gracilis * these organs are branched and begin with flame alls Pedicellina* and Loxosoma, marine. Sub Class II. Ectoprocta. In the Ectoprocta there is a spacious, often ciliated, celom between the alimen- tary canal and skin, so that these are separated and have a certain amount of independence (fig. 295). On this account Tee Sone Sigee ee has arisen a peculiar method (wholly years is pee ae indefensible morphologically) of treating J, intestine; 7, tentacles: Vs 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), asingle animal. a,anus; ek, ectocyst; en, entocyst; f/, funiculus; g, ganglion; hk, collar which permits complete retrac- tion; m, stomach, aiso dermal muscular sac; 0, esophagus. A, avicularium; B, vibracularium of Bugula. (After Clapardde.) The cystid is cup-shaped or saceular. 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. POLYZOA: ECTOPROCTA. 328 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 celom, 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 ceelom of adjacent cystids may be distinct or a wide communication may exist. The colonies grow by budding; in the Gymnolemata a part of a cystid becomes eut off as a daughter cystid in which the polypid—alimentary tract and tentacles—arises by new formation; or (Phylactolemata) 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 Phylactoleemata. Order I. Gymnolemata (Stelmatapoda). The tentacles in a ring around the mouth. The numerous species are almost exclusively marine and are abundant on every coast. In the 3824 CQ@LHELMINTHES. CHILOSTOMATA the cystids can be closed by an operculum: Gemmel- laria,* Cellularia,* Bugula,* Flustra* (fig. 295), Eschara.* The CYCLOSTOMATA have tubular cystids without an operculum. Crisia,* Fie. 296.—American gymnolematous Polyzoa. (After Busk, Hincks, Norman, and Verrill.) A, Uubulipora flabellaris, young; B, Flustrella hispida; C, Eucratea chelata ; D, Gemellaria loricata ; E, Kinetoskias smitti; F, Membranipora spini- Sera; G, Porella levis ; H, Lepralia americana: I, Cribillina puncturata. Tubulipora,* Hornera.* In the CTENOSTOMATA the cystid is more cal- careous and the opening is closed by a folded membrane. Alcyonidiwim,* Vesicularia, Valkeria,* Paludicella * (fresh-water). Order II. Phylactolemata (Lophopoda). Tentacles borne on a horseshoe-shaped lophophore extending on either Fie. 297.—Small colony of Lophopus crystallinus (after Kraepelin), with young and old, some extended, others more or less retracted. 0, statoblasts. side of the mouth, the tentacles on its margins. All are fresh-water species. Pectinatella,* Lophopus (fig. 227), Plumatella* V. PHORONIDEA. VI. BRACHIOPODA. 825 Class V. Phoronidea. The single genus Phoronis* occurs on our eastern shores. The animal was first placed among the Chetopoda 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 Fic. 298.—Anatomy of Rhynchonellu psittucea. (After Hancock.) a’, left, a%, right arm; a, opening into the cavity of the arm: d, intestine; ¢, blind end of the intes- tine; g, stomach with liver; m, adductors and divaricators of shell; 0, esophagus; p’, p®, dorsal and ventral mantle lobes; st, 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 (jp), one ventral, the 326 CQ@LUELMINTHES. 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 (h): 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 Fie. 300.—Waldheimia flavescens. (From Zittel.) .A, dorsal, B, ventral valve; a, b,c, impressions of muscular insertions; a, adductors; 0’’, adjustors (stalk muscles): c,c’, divaricators; s, hinge groove of upper valve in which the tooth (t) of the lower valve passes ; /, 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, BRACIIOPODA. BOT. 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 phylactolemate 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 ag 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 esophagus, 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 cwlomic pouch with a wide nephrostome, perforate the septum, and open to the exterior in the next somite. Since usually there are two septa, two pairs of nephridia may occur, hut one is usually degen- erate. The nervous system consists of an esophageal 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 Sugitta in thatin Argiope the colom 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 chetz 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@LHELMINTHES. Now there are but few species, some inhabitants of the greatest depths of the sea. Fie. 301.—Development of brachiopod. (After Kowalevsky.) A, gastrula with early enteroccelic pouches; B, closure of blastopore; C, celom separated, body annu- lated; D, cephalic disc and mantle developing, the latter with long seta; E, at- tached embryo, the mantle lobes folded over cephalic disc (seta omitted). ¢, ee disc; d, dorsal lobe of mantle; e, enteroccele; m, mantle; v, ventral man- e lobe. Order I. Ecardines. Hinge absent; valves similar when the stalk passes out between them (Lingula *), or unequal when the ventral is perfo- vated by the stalk (Disctna) or when the animal is directly attached by the shell (Crania). Order II. Testicardines. Hinge present, valves unequal, the ventral perforated by the stalk; anusdegenerate. Rhyn- chonella,* Terebratulina* in our colder waters. Fic. ie age alee ee tae oe Spirifer, Orthis, Pentamerus, Atryna, important tentrionculis,* fossil gener. Summary of Important Facts. (1) The CELHELMINTHES are characterized by a well-developed body cavity (ecelom). (2) The CH&TOGNATHI are hermaphroditic worms with three pairs of celemic pouches, with fins, and bristle-like jaws. (8) The NeEmaTopa are mostly di@cious, 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, Oxvyuris vermicularis in the large intestine, the blood-sucking AnAylostoma duodenalis, and the notorious Trichina spiralis, In hot climates occur Filaria sanguinis hominis and Dracun- culus 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&TOPOoD ANNELIDS have segmented bodies, the segmentation showing itself in ringing of the body wall and in the separation of the ceelcem into a series of pouches by transverse septa and the metameric arrangements of blood-vessels, ganglia, and excretory organs. (8) The CHa&TopopDA are distinguished from other annelids by the cheete (usually four bunches in a somite) arising in special follicles. The chvetee are few in the hermaphroditic Oligochetz, numerous and borne on special parapodia in the Polychete. (9) The GEPHYR#A are closely related to the Cheetopoda. 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 HrrupINEI are hermaphroditic Annelida which lack cheete 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 Potyzoa are like the Hydrozoa in being colonial and having a cireumoral crown of tentacles. They are distinguished by the complete alimentary canal, the large celom, and the ganglionic nervous system. (13) The PHoronIDEA are closely like the Polyzoa. (14) The Braculopopa 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 Celenterata, 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 ac@lom. In fact the radial symmetry of the echino- derms 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 Cclenterata is primitive, that of the Echinoderma, as development shows, is derived from the bilateral 330 ECIINODERMA type. In other words, the echinoderms have descended from bilateral, possibly worm-lke, 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 Pelagothuria), but even then shows itself as spicules and ‘wheels’ of lime. The spheridia and pedicellaria (fig. 303)—not always present— PIG. a close eg are characteristic appendages of the integument. open 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 larve, 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 culled the stone canal and leads Fa. 304. Fia. 305. Fig, 304.—Water-vascular system of starfish (orig.). a, ampulle ; ab, ambulacra; ¢, radial canal; m, madreporite; 7, radial nerve; p, Polian vesicle; 7, ring canal, beneath it the nerve ring; s, stone canal; t, racemose vesicle. Fic. 305.—Transverse section of stone canal of Astropecten aurantiacus. (After Ludwig.) ECHINODERMA. 351 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 ampulle, 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 dise 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 ampulle. 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 enterocelic 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 cceloin by special bands, while mesenteries support the alimentary tract and its derivatives. Respiratory organs are represented by very various structures: branchie, or thin-walled outpushings of the ccelom, either around the mouth, as in Echinoidea, or on the aboral surface, as in the Asteroidea, the burs of the Ophiuroidea, the branchial trees of the Holothuroidea and the various parts of the ambulacral system. 332 ECHINODERMA. 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. Fie. 306.—Echinoderm larve. (After J. Miller.) a, anus; m, mouth; the black line: the course of the ciliated bands. J, form common to all: JJ, IJ, developmental stages of auricularia (Holothurian); IV, V, stages of the Asteroid bipinnaria; VI, pluteus of a spatangoid; VJJ, larva (Brachiolaria) of Asterias (orig.). m, mouth; v, vent. Where there is no brood pouch the young escape from the egg as larvee which swim at the surface, and are distinguishable from the adults (fig. 306, 7) by their soft consistency, transparency, and bilateral symmetry. By the development of lobe-like processes and slender arms supported by calcareous rods the larve 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 larve are due to the drawing out of the ciliated band into lobes and arms, and also to its becuming broken into parts which unite themselves into complete rings (fig. 306, V). 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 celom and ambulacral system. This becomes divided, and one por- tion develops itself as a ring around the cesophagus, 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 alternaticn of generations, the larva being the asexual genera- F1a. 307. Fia. 308. F 1G. 307.—Formation of the celom in Echinus. (From Korschelt and Heider.) 4, first anlage of ccelom; B, later stage; C, complete constriction of c@lom (vaso- peritoneal vesicle) from archenteron Fra. 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 réle 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 ¥Fia. 309.—Comet form of Linckia multiflora. (From Korschelt-Heider.) One of the arms is producing a new animal by budding. LE RGS: ah a ell Fia. 310. Fig. 311. F1G. 310.—Ophidiaster ehrenbergi. (After Haeckel). Comet form: one of the original arms shown only in part. Fic. 311.—Culcita pentanguluris, 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- I. ASTHEROIDEA. 335 tion, and the animal can work its way through narrow openings. Of the skeletal pieces the ambulacral plates need special mention. B Fic. 312.—A, cross-section of starfish arm (orig.). a, adambulacral plates; am, ambu- lacra; ap, ambulacral plates; 6, branchi; c, ceelom: h, hepatic ceca; ¢, inter- ambulacral plates; », radial nerve; p, ampulla; 7», radial canal; v, radial blood- pone B, ambulacral plates, ventral view, showing the ambulacral pores etween. 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 ampullew. In each arm the pairs of ambulacral plates meet above the groove like the rafters of a Fic. 313.—Asteriscus verruculatus, aboral surface removed. (After Gegenbaur.) g gonads; h, hepatic ceca; /, 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 ECHINODERMA. The organs lie in part in the celom, in part in the ambulacral grooves. The alimentary tract is in the celom 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 Fic. 314.—Section through ray and opposite interradium of a starfish (orig.). B, branchie ; C, cardiac pouch of stomach; EF, eye spot; G, gonad; 4H, ‘liver’; M, mouth; N, radial nerve; P, pyloric part of stomach; RC, ring canal; RD, radial canal of water-vascular system; S, stone canal. constriction it is divided into a larger, lower cardiac portion and asmaller, 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 celom. The radial nerve, canal, and blood-vessel lie in the roof of the ambulacral groove between the ambulacra. The nerve ends at the F1a. 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 celom of the arm. The ambulacral system corresponds with the foregoing description II. OPHIUROIDEA. 8337 (p. 330), the ampulle as well as the five or more Polian and Tiedemann’s (racemose) vesicles projecting into the cwlom. 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#&, repre- sented on our shores by the five-finger Asterias* and Leptasterias,* and Heli- aster * with numerous arms. In the SOLASTERIDZ the ambulacra are two- rowed; arms sometimes numerous. Py- thonaster (fig. 316). In the ASTERINIDE& 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,* Ctenodiscus*) the body is more or less Fic. 316. — Pythonaster murrayi. : (After Sladen.) Aboral view pentangular, the margin being covered showing ambulacral grooves. 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 celom of the arms is greatly reduced, the hepatic ceca are lack- ing, and the alimentary canal, which lacks an anus, is confined to the disc. By the ingrowth of ven- Fic. 317.—Section of Ophiuroid arm tral plates the ambulacral grooves . a@,ambulacrum; b, blood ves- z ; ¢, celom; m, muscles of arm; 7, are converted into tubes, and the eye MT Ree eA cisaral Glses 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- SS SSN 338 ECHINODERMA. face. Also on the ventral surface are five slits which connect with as many burs, thin-walled respiratory sacs into which the sexual organs open. In many brittle stars (Ophioenida, Ophiothelia, 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), Ophioglypha,* Amphiura *), but in the EuRYALID&, or basket fish, the arms are branched (Astrophyton,* fig. 319), but not, as usually stated, dichotomously. Fig. 318. FG. 318.—Ophiopholis aculeata.* (From Morse.) Fig. 319.—Astrophyton arborescens, 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 Comatulide 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 dises and often bears five rows of outgrowths, the cirri. In the Comatulide (fig. 821) the adult is not thus attached, swimming about in the water with the arms or moving about on the tang. In these 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 S AAS a7 Sen) YS ; Oa ix : i D ‘ ‘i e P + B Ahead S zy Z ay Sy ow : oy. 4 = <5 = i’ bt ti z Bay gas BER He NES y) 4 7h ‘zl BY ~) ij Yard AN 7 ; ¥Fia. 320.—Pentacrinus macleayanus, (After Wyville Thompson.) Fia. 321. Fic, 321.—Adult of Antedon macronema, (After Carpenter.) eae Fic. 322.—Different Pentacrinus stages (a, ), ¢) of Antedon rosacea, 1, arms; 7, cirri; 3, stalk. FG, 322. 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 Fie. 323.—Hyocrinus bethleyanus. A, upper end of stalk with cup, and the bases of the arms; b, basalia; br, brachialia; r, radialia. B, oral surface of cup with mouth, five oralia, and the bases of the arms. 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. 823). 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 pinnule, 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 dise 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 wsopha- 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 pinnule. These are III, 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 Fia. 324. Fia. 325. Fre. 324.—Oral area of crinoid (Antedon), showing by dotted lines the course of the in- testine from the mouth (m) to the anus («); gy, 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; ¢, tentacles. here have no suckers nor ampulle 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 cwlom. Oppo- site their celomic 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 enterocele system, on the other hand, is well developed and forms the axial cord running through the brachialia and radialia to unite ina 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. 349 ECHINODERMA. Sub Class I. Fucrinoidea., The foregoing account applies entirely to the Encrinoidea, which may be divided into two groups : Order I. TESSELLATA (Paleocrinoidea), 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, theca 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# are fixed in the young, free in the adult. Antedon* (fig. 321). Sub Class II. Edrioasteroidea (Agelacrinotdea). Theca of irregular plates ; arms unbranched and lying on the theca. Possibly the ancestors of the noncrinoid echinoderms. Paleozoic. Ageda- 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, Echinospherites (fig. 326). Fie. 326. Fie. 327. Fia. 326.—Kchinospheerites aurantium. (From Zittel ) Fia. 327.—Pentremites florealis. (From Zittel). Lateral, oral, and aboral views. Sub Class IV. Blastotdea. Arms lacking ; the mouth surrounded by five petal-like ambulaeral areas. The group appears at end of Silurian and dies out with the earbon- iferous. Pentremites (fig. 327). IV. HCHINOIDEA. 348 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 spheridia and in the Echinoids five pairs of interambulacral gills. Be- tween peristome and periproct the ie see Ceara aanidanisetaaies body wall is composed of calcareous Agassiz.) Aboral ‘view, the spines : : ay tte removed to show the ambulacral (a) plates, which, except in the Echino- and () interambulacral areas, end- oe . fi ing respectively in the ocular and thuride, are immovably united. genital plates: in the centre the four Aside from the extinct Palechei- Ptes of the periproct. 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, Fia. 329. Fia. 330. Fig. 329.—Clypeaster subdepressus. (After Agassiz.) Aboral view, showing the peta- loid ends of the ambulacral areas. i ; Fria, 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 344 HCHINODERMA. distinguished from the interambulacral by the ambulacral pores by which the ambulacra on the surface are connected with the internal ampulle. 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 celom, to I 1] A st Fra. 331.—Sea urchin opened around the equator. 4, ambulacral area; J, interam- bulacral area; L, lantern; d, intestine; ed, anal end of intestine; g, gonads; nd, siphon; oe, esophagus; 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. 3845 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 hy five sharp-pointed calcareous plates, the teeth, ric. 332. — Aristotle’s which in the Echinoids are supported by a [anfern of imongine centratus lividus. (Af- complicated system of levers, fulcra, and mus- ee ee eee ne cles, the ‘lantern of Aristotle’ (fig. 332). cr 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 off two blood-vessels which run along the alimentary canal, while from the ring canal arise five ambulacral or radial canais which run on Fie. 333.—Oral (A) and aboral (B) surfaces of the sand dollar, Echinarachnius parma. a, anus; g, genital pores; 7, ambulacral areas ; m, madreporite ; 0, mouth. 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. Palwechinoidea. Paleozoic forms with five ambulacral areas, the interambulacral areas containing more than two rows of plates. J/elonites. 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,* Calopleurus* (fig. 328). B46 ECHINODERMA. Order III. Clypeastroidea. Trregular 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. 833), Ifellita,* 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. _) are upon the lower surface. Warmer seas. Spatangus,* Echinocardium, Brissus. Class V. Holothuroidea. The sea cucumbers are most re- Fic. 334. Young Spatangus pur- "4 : 0 ic ieee ee moved of any group from the typical removed, oral surface. In front, echinoderm appearance. At the first the slit-like mouth ; behind, the : anus. The bivium without tu- glance the skin appears naked and the ercies, . . 7 characteristic plates absent. Yet these are imbedded in the skin in the shape of plates, wheels, and anchors (fig. 835). The integment is tough, leathery, and muscular with POR SA2DOOS AVIIWNY Fig. 335.—Dermal plates of Holothurians. 4, Myriotrochus rinkii. (After Daniels- sen.) B, Thyone briareus; C, Synapta giraraii (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- V. HOLOTHUROIDEA. 347 Fia. 336.—Anatomy of Caudina arenata. (After Kingsley.) a, anastomoses of dorsal blood-vessel ; b, branchial tree; d, dorsal blood-vessel; f. mesenterial filaments; g, genital opening; /, alimentary canal; /, longitudinal muscles; m, mouth: 0, genital duct; p, pharyngeal ring; 7, gonads, cut away on right side; f, ampulle 348 ECHINODERMA. 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 foi ward in the left ventral interradins, and then back in the right dorsal interradius to the anus. It is held in position by mesenteries (fig. 337), and ed 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 outgrewths which are filled with water. The similarity of these to an = the excretory organs of some Gephy- “Int 7? yea (p. 317%) was one ground for re- Fic. 337.—Transverse section of garding those forms as intermediate Holothuria tubulosa. (After Lud- : wig.) d, digestive tract; db,dor- between worms and echinoderms. sal blood-vessel; g, gonad duct; ,,, E 2 h, skin; Im, longitudinal muscles: They are to be regarded as respiratory, betiegse See eitdiccralconmpiex since they are periodically filled with or ancium fambulacra} vessel) fresh water. In many species ‘ Cuvie- ee rian organs’ occur; these 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 esophagus 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 He 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 (Dendrochirotw) or bear frilled shield-shaped extremities (Aspidochirotw). A single Polian vesicle is usually present, and the stone canal (except in the Elasipoda) connects with the celom. Blood-vessels going from the vascular ring form rich anastomoses on the alimentary canal. Only a single gonad (or é pair of united gonads) occurs. This consists of numerous tubules which open usually interradially near the mouth. V. 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, Hntoconcha 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 Holothuride with peltate tentacles. Fia, 338.—Cucumaria frondosa, sea cucumber. (From Emerton.) Holothuria* in warmer waters, one species furnishing the trepang of Chinese markets. The CucuMArim. 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 Eva- sIPODA belong to the Pedata. The APODA are represented by Caudina * (fig. 836) and Molpadia.* Order II. Paractinopoda. No radial canals nor ambulacra. Tentacular canals arising from ring canal, Myriotrochus,* Synapta,* 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); (4) in that, as embryology shows, they have descended from bilateral forms. 2. Farther characters are the existence of a ccelom, the ambulacral system, and the mesodermal spiny skeleton, which has given the name to the phylum. 3. Theambulacral system 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 Fie 339.—Oligotrochus vitreus.* (After Danielssen and Koren.) ring canal and the radial canals to fill the ampulle 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 ceca extend. The ambu- lacral groove open. %. The Opnivrormea also have dise and arms, but the ambu- lacral groove is closed and the hepatic ceca absent. 8. The CRrINOIDEA have a cup-shaped body bearing arms, usually branching, with pinnule, 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. : MOLLUSCA. 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 Honornurormea 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 celom 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 calomate animals, and from forms in which, by encroach- ments of a connective tissue and muscular parenchyma, the eelom 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, 1) and the snails (fig. 340, 2), on the other hand, have an unpaired mantle which arises from about the central part of the back and either extends 352 MOLLUSCA. 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 Fic. 340.—Diagrams of three molluscan classes. 4.a cephalopod (Sepia); B, a gas- teropod (Helix); C, an acephal (Anodonta), a, anus; c, cerebral ganglion; fu, foot; m, mantle chamber; sch, 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 dran- chial chamber. Since most molluscs are aquatic, special vascular processes of the body, the gills or branchix, le 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 Jeft 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 ctenitdia, from their resem- blauce 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. 885). 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 molluse 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 esophagus and corresponds to the suprawesophageal ganglion of the worms; it is the brain (cerebrum) and supplies RNG pl. . _ & a LUE re * ; ae > Seemmeree ‘ \ par & pe. a re. at pe A B Cc ¥ 19. 341.—Nervous systems of Molluscs. A, most gasteropods; B, acephals; C, cepha- lopods and pulmonates. c, cerebral; pu, parietal, pe, pedal, p/, 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 gangha 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 Mollusea, and which from position and structure are regarded 354 MOLLUSCA. 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 wsophagus. Primitive Mollusca (Amphineura) have a simpler condition. The cerebral ganglia lie dorsal to the cesophagus and are united by a cord around the cesophagus (fig. 844). 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, 4). Thepleural 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. 841, 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 name 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 molluses (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 calom; for here, as in the annelids, the nephridia open by ciliated nephrostomes into the ccelom, and the sexual cells arise either from the celomic 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 celom (enterocawle) arises as diverticula from the archenteron. MOLLUSCA. 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 dicecious, but the gonads are always very large. Even more room in the visceral sac is demanded by the digestive tract in which wsophagus, 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). Reproduction 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 molluses (¢.9., Paludina vivipara) are viviparous. A metamor- Sp oS wS~ Ce , branchial siphon ; f, foot; k’, outer, 4’’, inner gill lamella; m, 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 feces, through the upper or cloacal open- ing. In many bivalves the free edges of the mantle grow together, Ee Fig. 350.—Section of shell of Anodonta. c, cuticula; p, prismatic layer; 7, 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, 2B). By further development the margins of these openings are drawn out It, ACEPHALA. 361 into two long conjoined tubes (fig. 349, 4), 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 calcie 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 lamelle 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. 2 K* Ks K' ml gf Fig. 351.—Anatomy of Anodonta, the mantle, gill, and liver of the right side removed, the pericardium opened. 1, ?, anterior and posterior adductors; I, II, IIT, cerebral, pedal, and visceral ganglia; a, anus; 1}, b?, upper and lower limbs of organs of Bojanus,; br, branchial siphon; «, intestine; ¢, nephridial opening ; fu, foot; g, gonad; hj, h®, ventricle and auricle of heart; k’, insertion of both lamelle of right gill; k%, k4, inner and outer lamelle of left gill; 1, left liver 2 I a its opening in m, stomach; ml, pallial line; 7, anterior, r?, 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. 851, 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 362 MOLLUSCA. 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 Nuculide—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 Fie. 352.—Projection of sections shown by the arrows in fig. 351. b1, b2, upper and lower limbs of nephridium (organ of Bojanus); @, intestine; e, nephridiopore; fu, foot; g, gonad; 4}, ventricle surrounding the intestine ; 22, auricle: k!, k2,inner and outer gill lamelle; /, hinge ligament ; m, mantle; n, cerebro-visceral com- missure; sp, hephrostome; 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 anda descending limb, it follows that each gill consists of an inner and an outer leaf (fig. 852), 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 IT, ACEPHALA. 38638 (Acephala). 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- de; 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 hy 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 byssus (fig. 853), 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; f, mouth; gy, labial palpi; h, 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, A’, 4°). The auricles receive the blood direct from the gills; the ventricle forces it out through anterior and posterior aorte (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 MOLLUSOA. 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 diwcious, the gonads being acinose glands. The digestive tract (fig. 351) begins with a short oesophagus, widens out to alarge 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 is 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. 355) are uncommonly wide apart. The two brain ganglia (cerebropleural ganglia) lie either side of the mouth at the base of the labial palpi and ventral to the anterior adductor. They are very small, since cephalic sense organs are lacking, and are united by a transverse supra- cesophageal commissure. The posterior ganglia, composed of the united parietal and pedal ganglia, he 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 seal- lops (Pectinide), arranged in a row like pearls on the margin of the mantle. Small tentacles with sensory powers may oceur both on the margin of the mantle and on the tip of the siphon. Veligers (fig. 842) are very common in development. When this stage is lacking the history may ecntain 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 IT, ACEPHALA: PROTOCHONCHLE. 365 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 themselves 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 Fic. Pee ae Resa: mantle, and adductor muscles have been (From Balfour.) ad, adductor ; by, used as basis of classification, the usual DySsHeG © Sense Mains ah Shell, divisions being founded on characters derived from only one of these organs. Order I. Protochonchie. The primitive character of these forms is shown by the struc- ture of the gills, which are either ctenidia (Protobranchiata) or ct Ob P Fic. 355.—Anatomy of Nucula. (After Drew.) aa, anterior adductor; b, byssal gland; c, cerebral ganglion; ct, ctenidium; f, foot; , heart; 1, labial palpus; 0, otocyst; p, pedal ganglion; pa, posterior adductor; s, stomach ; t, appendage of palpus; v, 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. ITlinge and ligament are symmetrical with regard to the umbo, or vary little from symmetry. The hinge may be Jacking, and the ligament is wholly or in part internal. The mantle edges are free, and rarely is there the first trace of fusion. 366 | MOLLUSCA. Fic. 356.—Yoldia limatula.* (From Binney-Gould.) Fie. 357.—A, Modiola plicutula*; RB. Pecten irradians*; C, Mytilus edulis.* (From Binney-Gould.) Il, ACEPHALA: HETEROCONCHLE. 367 Sub Order I. DIMYARIA. Two equally developed adductors. The taxodont NucuLID# have ctenidia, a broad foot, pleural and cerebral gan- glia separate, and gonads emptying through the nephridia, all points which show them extremely primitive. Mecwla,* Leda,* Yoldia.* The ARcID# are also taxadont, but filibranch. Scapharea,* Argina.* SOLEMYIDA, Sub Order I]. ANISOMYARIA. Anterior adductor rudimentary (Heteromyaria) or wanting (Monomyaria). With the exception of the isodont SPONDYLID#, all the families lack a hinge (dysodont). To the Heteromyaria belong the MYLILIDA, 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. JZthodomus* bores into stone. The AVICULIDA 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#ID& and the PECTINIDZ are monomyarian, The Ostreide, or oysters, usually become attached by the right valve. Our American Ostrea virginiana differs from the European species in having the sexes separate. The Pectinidee, or scallops, are free-swimming and are well known for their highly developed green eyes on the edge of the mantle. Order II. Heteroconchie. 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# (Naiade) 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 TripaentpZ, with small siphons, includes the Fic. 358.—A, Saricara arctica; B, Astarte suleata; 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 (CARDIIDE— 368 MOLLUSCA. Cardium*, Serripes*) and ASTARTIDA, marine, and the fresh-water CycLa- DIDE (Cyclas, Pisidium*), about the size of peas, belong here, as probably do the extinct RupIstTip& of the cretaceous. Sub Order II]. SINUPALLIATA. The VENERID# with swollen shells, represented by the quahog, Venus mercenaria on our east coast and by Fig. 359.— Te- redo neavatis, ship worm in its tube, the siphons (a, anal; b, bran- chial) drawn out of the tube (r); ky, shell. B,teeth of the shell enlarged. brightly colored species in the tropics; the MACTRIDZ or hen clams, and the flattened delicate TELLINIDE (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 MyIp#, represented in all northern seas by the long clam, Mya arenaria, and in the razor clams (SOLENIDE; Solen Ensatella*). The allied Sax1- cAVIDz 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- LADIDH, some of which are phosphorescent, burrow in wood, clay, or stone. The shell is well developed. In the ship worms (TERE- pIp#) 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 jn the sea, especially in the tropics. The GASTROCH/NID 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 me 3 eran. little-known Septibranchiata, in which the ee gills have the shape of a septum perforated Ludwig-Leu- by gill slits separating the branchial and clo- nis.) a, shell. ah ‘ % . acal chambers. Silenta, Cuspidaria. III, SCAPHOPODA, IV. GASTEROPODA., 369 Class III. Scaphopoda (Solenoconche),. The tooth shells are primitive forms which have some resem- blances to the Acephala in the paired liver and nephridia 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 diwcious 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 pig 361 —pentarium Seana secretes a shell shaped like the tusk of an — fim. tooth shell; left the elephant, from the larger end of which eae eee protrudes the long three-lobed foot used for boring in the sand. Dentalium (fig. 361), £ntalis *. 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 MOLLUSCA. 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 ina 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, 1), 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 (Vermetida, fig. 362, C). 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, #) 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, 2, 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. 871 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- FG. 362. Various forms of shells. (After Schmarda, Bronn, and Clessin.) A, Patella costata ; B, Haliotis tuberculata ; C, Vermetus dentiferus ; D, Lithoglyphus naticoides ; E, shell of Murex opened to show c, columella; 0, siphon. stome shells, in which the anterior margin is drawn out in a groove (fig. 362, £). 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 Fisswrella 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 metapodium. 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- binide. Land snails are usually without opercula, but at certain times, 872 MOLLUSCA, as in hibernation, they can close the shell by a calcareous plate, the epi- phragm. 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. 8363) 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, FIG. 363.—Sinistral shell “hich is opaque and contains the pigment. In rare of Lanistes carinatus. cases the mantle and consequently the shell are lack- (From Ludwig-Leu-. ‘ : a é nis.) 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, Fig. 3€4.—Three diagrams illustrating the torsion of the body and the twisting of the nervous system in gasteropods. (After Lang.) 4, bilateral, B, asymmetrical, C, streptoneurous condition. The reference letters are placed upon the organs ot the primitive left side. a,anus: c, cerebral ganglion: g, ctenidium; 7, auri- cle; m, mouth; 2, nephridial opening; o, osphradium; pa, parietal ganglion; pe, pedal ganglion; p/, 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 e ferred 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. 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 Fie, 365. Fie, 366. Fia. 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; ¢, ctenid- jium:em, columellar muscle; f, foot; h, heart in pericardium ; t, intestine ; 1, liver; m, mantle; mf, floor of mantle cavity; 1", nephridium, ns, Opening ot nephridium; 09, osphradium ; 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; oc, @sophagus ; 1’, radula; 7s, radula sac ; sp, salivary duct; 2, 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 MOLLUSCA. usually on the right side in front, in or beside the mantle chamber, tarely it empties in the middle line behind (figs. 365, 370, 371). (Esophagus, stomach, and intestine are slightly marked off from each other. The convolutions of the intestine are enveloped Fic. 367.—Row of teeth from the radula of Trochus cinerarius. (After Schmarda.) by the liver, which by its large size forms the chief part of the visceral suc.