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Las diagrammas suivants illustrant la mAthoda. irrata to pelure, n d 1 2 3 32X 5 6 INVERl J. I Pi h A TEXT-BOOK or INVERTEBRATE MORPHOLOGY BY J. PLAYFAir'McMURRICH, M.A., Ph.D. Professor of Anatomy in the University of Michigan SECOND EDITION, REVISED \ Q. U 0^S NEW YORK HENRY HOLT AND COMPANY 1896 Copyright, 1894, BY Henry Holt & Co, KinRRT DRUMMONF,, Et-ECTROTVPER AND PR'NTER, NEW YORK. The Morp either from tl the zoologica to recommen( method has p the subject i method empl present work has been fol peculiarities c iug the brick uuite them to{ opportunity h to the compar I affinities of th( A word is ] jadopted, which I usually empio;; liimple of Claus Ithan twelve of iber has resulte iuized, namely, 1 former it has h collection, and tt is true that )hyl() iNv; In tl plieiionK peculiar quickly i animal, ^ orgauiza kuown c kuowled this mat and it b fully un Life, ma Mud rate kno this subs it as a SI number of which things it substauci since our mining ai act of su relationsl INVERTEBRATE MORPHOLOGY. CHAPTER I. PROTOPLASM AND THE CELL. In the exainiuatiou of organisms presenting the series of phenomena which we term life, the invariable presence of a peculiar semi-fluid transparent or hyaline substance becomes quickly apparent. Whether the organism be a plant or an animal, whether it be of the simplest or of the most complex organization, it is still composed of this substance, which is known as protoplasm, and it may be said that so far as our knowledge extends life never exists except in association with this material. Protoplasm is " the physical basis of Life," and it becomes of great importance that its nature should be fully understood, in order that the results of its activities. Life, may become more intelligible. Much has yet to be accomplished, however, before an accu- rate knowledge of the structural and chemical characters of this substance is obtained, and indeed it is incorrect to regard it as a substance, since it is rather the aggregate of a large number of exceedingly complex chemical compounds, none of which are sufficiently known. From the very nature of things it is impossible at present to get a correct idea of these substances and the relations which they bear to one another, since our present analytical methods are not capable of deter- mining and isolating them in living protoplasm and the mere act of subjecting ])rotoplasm to analysis destroys those very relationships which are the cause of the vital manifestations. IN VERTEBRA TE MORPIIOL OGY. Dead protoplasm is sometliiug very dili'ereut from living pro- toplasm, and our present knowledge only imperfectly extends to this much-altered material. Furthermore even in the dead material the chemist has to deal not only with the complex substances which constitute protoplasm proper, but also wicli numerous secondary prod- nets either in the process of being built up into protoplasmic molecules or else resulting from the destruction of these I molecules- For both these processes are contimally going I on, the living organism continually uniting simple chemical compounds to form ne-v complex molecules, a process known as anabolism, and resulting in growth ; and just as continually it is resolving into simpler comijounds the complex mole- [ cules already formed, a process known as catabolism, and resulting in the manifestation of energy in its various forms, ; such as heat, motion, electricity, and even light. Growth and I the manifestation of energy are then two most important phenomena exhibited by living organisms, standing in oppo- sition to one another and determining the general condition of the organism. If anabolic changes are the more active, then the animal or plant grows, as we express it, adds new protoplasm and increases in size ; if the anabolic and catabolic changes are practically equal in amount, stability results ; Avliile the preponderance of catabolism leai^s to a lessening of misterial, and finally to what we term death. These changes constitute a cycle occurring in the life-history of probably every organism and causing the periods which we ■ denota as youth, matarity, and old age. Dead protoplasm th-^u, together with the anabolic and catabolic constituents which are inextricably associated with it, wil b«> f( rnd on analysis to consist to a largo extent of tln^ chemical elements Carbon, Hydroy;en, 0.:ygen, and Nitrogen, together with Sulphur and Phospl orus, as wall as a number of substances ])resent in varyiiig amounts, such as Chlorine, •Potassini.i, Sodium, Iron, Calcium, and Maghesium. Exactly l)ow these various elements are united together it is dilVicult to detei'miii(», but es))eciu] importance has beon assigned to t' the (\ ii, (), N, Jllid S i'<)iii|i()irn(ls whirii tHCiil' Mill wliich foriii a group jf cheuiicul compounds known as ]*roteids. Of sucli compou Nuclein, probabl the pre Plastiu importai Plastiu trated alk; and eonsis teristic of miclcus, o closely, be t cliomical e f tlio niicleii 1^ seems pi'ol I from wliici ; compounds I With ous salts, Potassiui other sue to estima dead has As rej: moderate and more peripliera than the cially dis eiuloplasi usually vi prot()[)lasi and knowi sin)plor oi toplasm, )i tnre, the i occupyiiig .Jetm later := protoplasrj protoplusi] PROTOPLASM AND THE CELL. 3 compouuds several, sncli as Albumin, Globulin, Fibrin, Plastin Nueleiu, have been isolated from protoplasm, som'e being probably secondary products resulting from the alteration of the protoplasmic molecules proper, but others, such as Plastin and Nueleiu, are especially constant, and seem to be important constituents of the protoplasmic complex. Plastin ronns wlien isolated a sticky fibrous mass, i-soluble in concen- trated alkaline solutions atui unaffected bytlie peptic and tryptic ferments and consists of C. H, O, N, S, and P. Nuclein is more especially charac- teristic of a speca. portion or modification of protoplasm termed the imcleus of which more will be said hereafter, and resembles plastin very c osely, beniK, however, less insoluble than it, and consists of the same chemical elements. Analyses of these substances, however, differ Kre-itlv the nuclein from spermatozoa, for instance, containing no sulphur • ami ii seems probable not only that they differ materially according to the source irom whicn they are obtained, but also that they are not reaMy chemical compounds, but a mixture of several highly complex substances. With these proteids, then, there exist in protoplasm vari ous salts, such as Potassiuiu, Sodium, and Calcium phosphate Potassnim and Sodium chloride, Magnesium sulphate, and other such salts, the exact significance of which it is difficult to estimate. Fow living protoplasm differs chemically from dead has not up to the present been accurately determined As regards its general structure protoplasm appears as a moderately consistent jelly-like substance, usually coh.rless and more or less granular in appearance. As a rule the peripneral portion of a mass of protoplasm is less granular than the central, appearing therefore clearer, and is espe cndly distinguished as the ectoplasm from the more opaque endoplasm. Imbedded in the eudoplasm are to be found usually various bodies, the products of the activities of the pintoplasm, such as largo, clear spaces occupied by fluid nnd known as vacuoles, food-particles „f various kiml.s in the simpler organisms, starch granules and crystals in idant-,),... toplasm, and depositions ,,f pigment. One particular struc- ture, the nucleus Jiowever, seems to be invariablv present l-HH-npying the central portion of the mass, and, as will bj .^een later, playing u very important lole in the life of the ^ pro n|> asm. it is ind-ed a specially nio.liaed portion of the piotoplasm and cannot, therefore, be placed in the same 1 i /iV^ VEUTEBltA TE MORPIIOLOG Y. category as the vacuoles autl other accidental or secoudaiv constitueuts which have been mentioned, and every mass of protoplasm may be considered as consisting of tv/o essential parts, the protoplasm proper or cytoplasm and the special modification of it, the nucleus, Avhich for convenience is termed the caryoplasm. Such a combination of cytoplasm aud caryoplasm forms what is technically known as a cell, and ail living organisms are composed of one or more such struc- tures, which are to be regarded therefore as morphological units. If the more intimate structure of the cytoplasm of such ii unit or cell (Fig. 1) be examii^ed, disregarding the various urn -, nl Pig. 1.— Diagram bhowtno the Structurk op an Animal Cell. c = cc'iitrosoiuo. m — iniciosoine. cl ~ cylolyiiiph. nl = nucleolus. cr — cbroumtia nm = niKleiir meinbiane. r = reticulum. secondary constituents it may enclose, it will be found to consist of a network of oxceodiiigly fine fibvils, along Avhicli, and more especially at the points where two or more of them meet, are to be found minute granules which stain deeply with the ordinary niicrosco})ical staining reagents. The fibrils constitute the reticuhun (Fig. 1, r) of the cytoplasm, and tln' granules are termed the micrnsonics (iii). The reticuluni seenis to be formed princii)ally of the ^.loteid substance already mentioned as ])lastin, and its meshes are occupied by a more fluid substance >vhich has been termed the cyin- hjmph {cl). Several o] [plasm, ill ad I compared to *cytolymph. I fibrils of varj Itlio retiouluii ■According to •in the cytopl; -less matrix ii ^sometimes se It he upholder Ircga'-ding tin lits .structure seems probabl products of tl ordinate valu more nearly lust be admit true reticulufr In some cases Au imitatii |;ure of thickt bhioride of so globules each tefraetive indi he cytolympli bo collect at t pgethcr, and tinder certain ( |olumnar form possible that eticular theon le a sjKxigy .sti ivided into •,\ llastin. At ])i bost acc!n'atel; Ssioiially acccj The cary( bont the mi iUihles it, t usually ro fongated, h( liirs, .iiiii is i iin'uibrune PROTOPLASM AND TUE CELL. Several opinions have been given in regard to the structure of the cyto- [plasm, in addition to tliat here presented, according to whicli it may l>e jconipared to a sponge the meshes of wliose networlc are occupied by the Icytoiympli. According to anotlier view it is composed of a number of jtibriis of varying lengtiis imbedded in a matrix, thetibrils corresponding to Ithe reticulum of tlie reticular tlieory and tlie matrix to tlie cytolympli [According to still another theory which rests on tlie appearance produced jin the cytoplasm by a special method of treatment, there is present a color- Jess matrix in which are imbedded numerous exceedingly small granules sometimes scattered and somelimes united together into chains Indeed tiie upholder of this granular theory has carried his view to the extent of m-eganiing the granules as structural units of which the cell is composed |Kts structure being comparable to that of a zoogloia of micrococci It «^eems probable, however, that the granules are to a large extent secondary products of the activities of the cytoplasm and have therefore but a sub M-d-.nate value in its composition. The reticular theory seems to stand hiiore nearly in harmony with the majority of observations, thou-rh it ynust be admitted that some observers do not seem to have perceived the true reticulum, confining their attention to the coarser network produced pi some cases by extensive vacuolization of the cell. An imitation of tiie cytoplasm ha. been recently obtained by the mix- ture of tiiickened olive-oil with a solution of potassium carbonate or of chIor.de of .sodHim, the watery solution taking the form of polyhedral globules each surrounded by a thin layer of oil which from its lii-dle befrac ,ve index gives the appearance of the plastin reticulum surrounding the cy ..lymph, hdid particles finely .livided and mixed with tlie nil teud 0 collect at the points where the oil-films of three of the glolmles come logoth-r, and resemble the microsomes, whil,- it is further noticeable tC bnder certain con.lit.ons the superficial globuh-s of th. ,.,„,dsion take on a [olumnar form and may be compared with the , .pla.sm of the cell It imss.hle that the cytoplasm may have this structure, in which case the itu ula, theory would n>q,iire to be moditied, since there would no longer ha spongy , structure, but rather an eu.ulsion in which the cvtolymplfi Jvi ed imo a number of globules eac-h surrounded by a thin ^"^r .l f^^a. in. At ,,r,.s,.n , howev.r, the reticular theory seems to correspond Thcy^aryoplastii or nuolons, as already stated, lies usually I.OP the nmldle of the cytoplasm an.l to a certain extent re- ^nMenit, thou^rJi it presents certain peculiar feaUires It i usually round or oval, though occasionally it n.ay assume Jongated. horseshoe-shaped, moniliforni, or even f.ranchimr ^nns, uui iH as H nde clearly nnirke.i off from the cvtoi.lasm l.v 1 membrane (I ,g. 1. ,nn>\ whi.h. lM>wever. at certain periods .if 6 INVERTEBRATE MORPHOLOGY. nuclear activity seems to disappear, a new one subsequently forming. Traversing the space enclosed by the membrane, so as to form a network, are fibres which do not stain very deeply with the usual staining fluids and which are composed of a substance termed linin, which does not, however, appear to differ essentially from the plastin of the cytoplasm. In- deed it is not improbable that the linin network is con- tinuous through the nuclear membrane with the plastin reticulum and that both are identical, as is also the caryo- lymph contained in the meshes of the linin with the cyto- lympli. A more characteristic substance is the chromatin (Fig. 1, cr), so called from the strong aflinities it shows for many staining fluids, such as carmine, hsematoxylin solutions, and certain aniline stains. It seems to consist of the substance nuclein, already alluded to, and in the resting nucleus forms a reticulum intimately associated with the linin network, which it usually to a considerable exteut obscures. Where the various strands of the network meet, thickenings of the chromatin sometimes occur, producing densely staining bodies (r?i) to which the term nucleoli is given, though it is probable that bodies of a somewhat difl'erent composition are also included under this name ; for there are usually to be found in the nucleus, imbedded in the substance of the network, one or more spherical bodies whose chemical re- actions differ noticeably from those of the chromatin nucleoli, the substance of which they are composed being termed piranndein or py renin. There are then in the coll the following structural con- stituents : I. Cytoplasm [ membrane (cell-wall), ■I reticulum (plastin), ( cytolymph. membrane, reticulum (linin), II. Caryoplasm : -\ caryolymph. i (:iiri;iii;i,tiii Iji'-tvvork fuucloiu), (^nucleoli (nuclein and parauucloiu). In additioi be mentioned undergoing m various restiuj kinds of endo structure kno\ an exceedingly stain with the deuce, but has such as safran centrosome is p even more mny the neigh b(,rli( ■slight concavity ing the centroi especially in d ■sphere, and froi directions, whe ^onibination of i niic rays. (See The significance Domena of cell-divi '"'liiiml into at tl 'iiattor, according t( ""clous and at a cei In favor of this vie^ "iicloiis are poiiitc( 'louiiccd during col ti'dsonie appearing tc 'livision into two pai "'woino is a cons! it ii ''" \vitli (ho luicloiis. i'lca has been suggesl '"■ '<••"'« than ati aggn iii'c cytoplasmic fil)fo '"'L'ur along their coi "lioro the fibres ineol " ""iiy l>e pointed out '"''I liieir powers of n lirrejrated jnU-rnHomc:, tliiit (ho theory cxpi 'vpieal aster during th PROTOPLASM AND TUB CELL. .tructm-e k„„w. a., the c.„tro.orn! m„lT- n '' " "f r =r: ~ ":r; s a^^^ the iieij.liborhoocl of fl.a , . ^^^"'^^ed m the cjtoplasm in .li.*rfio„s, whence IheW ^ , ' '''''■''' P""*' °»' "' "■' -.„biua.i,„ ooeulo! "•*'•/'■':'"'' i-^ ■■'PPlie.l to tl,e ...ie ra,.. («ee Kg atlTr""' "' ' "" "'"' "" "^■'"''''^^- The significance of tlie oentiwom*. „;ii i "-"ena of cell-division are nl^ ^I. ,^ ^^^rbu^'T "'"" '" ^^"«- ""l'"re(I into at this place. Two v^^ s .^ ' •. . ' "'"'^'" '"''^^ ''« '"atter, according to one of which tlT T ^* '" '"'^''^'■'^ *» f''^ l-Ieusandat /oertai;:,, ri: ^\^^e nf ::;r ^'^^ '^^ ^^^'^'" «» t'- I" favor of this view the intim-. P -Z /• '"'!'"''''' '« extrndcd from it. -■-s are pointel r^" 1 Z T- l''/'" ^-'^^«-"'e and tha •—1 during ceii-divLi T^l nvS ""T, "'^^'^'""^ '"- '-some appearing to penetrat;,,:,'^/;^ rTtf.^'"' "" ^^"- 'I'vision into two parts. AccordinL' tr"... i ^'^ '"■'"^' ''^'^""^ *'* '— is a constituent of t^ ^ ^I ^,1; 'j;^ r'''''^: ""--•• ^'- -n- i'loa has been suggests o t .o off t n , T '"^ "'"pliHcation of this. "•• lf.s tlKina,.4.vJi n of evol ^^""•-«ome is nothing „,oro: h-^'tn- ah,ng their course an ^---ve ',ti , f ' /'"p ""'" ""^'••"-"nes ■ I «''">'o the fibres meet. This i,' .i •"'" '''"''^''^ '"'"^''^ ^e found j " '"ay be pointcl out that the absn.c n ™'''^ '" ''^''^'l ''f-''.., but '•-' tlu-ir powers of repro.l, Mo ?'. 1 . '''''''"''''''" "' ''^^' ^^'"^•'' ""ve L'r..L.n...d U-rn.-, "''''''^ oxphcaWe on thi. id.., ,!- -, - p>i-' -tor during !h:d;^:;;:;i;;::^^^^^^^ 8 IN VERTEBRA TE MORPHOL 00 Y. Sucli a combination of cytoplasm and caryoplasm consti- tutes a morphological element capable of carrying on all the functions of life. It is not only a morphological but also a physiological element. It is capable of assimilating the necessary substances and building up protoplasm ; metabol- ism and the consequent evolution of energy goes on in it ; it excretes waste products ; it is contractile and may therefore be capable of motion ; it responds to stimuli of various kinds, or in other words it is irritable ; and, finally, it is capable of reproduction. The question naturally arises, however, whether this combination of the two substances mentioned is essential— whether, that is to say, organisms without nuclei do not exist and manifest all the phenomena of life. At one time the existence of unicellular organisms destitute of a nucleus was recognized, the term cytode being applied to them to distinguish them from nucleated cells. Within re- •cent years, however, a growing skepticism has come into existence as to the non-nucleate character of these organ- isms, the recent improvements of the microscope and the application of modern staining reagents having revealed the existence of nuclei in many of the forms at one tnne regarded as typical cytodes. It would perhaps be going too far to state that cytodes do not exist, but the evidence at hand indi- , ijates that their existence is highly problematical. This conclusion is strengthened by the results which have "been obtained irom the observations of artificially produced cytodes. Some of the larger unicellular organisms have been cut into fragments some of which can be definitely shown to be destitute of nuclear or caryoplasmatic substance. In such cases it is found that the nucleated fragments if placed under proper conditions Avill regenerate and carry on Aheir existence as before, while the cytode fragments, thou^di manifesting signs of life for a considerable length of tinie,^ will not regenerate and do not possess the power of repro-' duction. The nucleus seems to possess a marked rogulatiiif;; or coofdinatiug action upon the cytoplasm, coordinating the: 4inabolic and catabolic activities upon which the coutiuuanci' of life depends. , It would be beyond the scope of the present work to enter! into a discussion of the various forms of physiological activity of the eel tion, must remarkabi the dispro epigrammj watchword it finds it nucleus e m assumed t' cell, and th of the anc previously is accompa under cert occur Avith multinuclea of tlie cytc rated witho might be e the nucleus is usually p This lati begin as a s: deeper and ( ion of the c that each of the original is rather ra branes of th ion, to distin method whi( jiiienomena t w applied. Starting v nieutioued oa K'lrded as afl |g<"tlierwithtl '"'itiii. Thocf jKi'iuliially Sep h'(> fit oppositt ch have rocluced | tis liavei efiiiitel) bstauce. nents if 3any oil , thouj^'li of tinio,^ »f re pro- gulatiiif;^ iting the' tiuuauci' to euterl I activitv PROTOPLASM AND THF CELL. c of the cell, but one of its physiological functions, reproduc tion, must receive special attention in conuectioL w th the j^markable sti.c ural changes which accompany it Si^ce the disproval of the doctrine of spontaneous generation «! epigrammatic statement Ononis eelMa e cellulhr^Z lie vatchword of modern histology and embryology, and t .d Y i.imens e nuoleo. hvery cell at iji-esent iu existeucp mi,- 1,= assume.! to have desceuded from some pre WK exSiu! cell aud the nucleus it contaius to be a porlio. of the uuetus of the aucestral cell. New cells arise by the divisteu o' previously ex.stmg cells, aud each division of the c y onhsm IS accompanied by a division of the nucleus. NotCt that under certaaa conditions a division of the uuc en ",' occur without a correspoudini; division of tl,. .,? > -^ multinucleated cells th^s arisni«, auHoli" /idSo™' of the cytoplasm may possibly in certain cases be W„ rated «thout entailing a division of the carvoplLm . bi' ^^J might be expected from the relation which ext , \T' the Bucleus and the cytoplasm, the dM^on of he r" is ..sua ly preceded by a division of the caryo llai '^ '""''^ Ihis latter process may take place in two wavs Tf ,„„ begin as a simple constriction of t le nucleus which b.J ^ deeper and deeper, finally separates off a po t on o 'i^ 7'"'' louof the cytoph,s.„ iu asLilarmannerCn oc«n.t:: that each of the new cp1I« fl.,1^ <-.,. i . • "^^^"^""n. s>o the original nucleu:" tl^^'^X:^:^!^^ 1 |s rather rare, occurs for instance in e,nb . '"v " '..anes of the Scorpion and is termed ^^.-'c;. ' ,v • I"'- H.,_towhichtheU..i4m^^.;ir7;:^ Hiartiug with a typical cell, consisting of the varion^ ,.nvf haentioned .bove, the karyolduetic pheno... n. ' .Tbe K'u-ded as affecting two constituenf« i n +i ' '"''• «"M.er with the su.;„,.,,ding'ttTlt ll ^r." '■-r-, '^ j''"ini.. Tlio centrosome first dividpL ,.'" -'J- ^^Ucicur ehro- «-iuai,y separating fr,,,n^Ur.t;i:.;t'-i"T;:f,::;;:r^ I'"' «t opp„„te poles of the nucleus, ..s„„„;-..:. ^ :;, l! ;;;;: 10 INVERTEBRATE MORPHOLOGY. tion ninety degrees distant from the point at which the origin nal ceutrosome lay. During this process the radiating fila- ments which surround the centrosome become especially distinct and may be divided into two portions, those which come in contact with the nucleus and which from their appearance in later stages are termed the spindle-Jibres, and Pig. 2.— Diagram showing the Phenomena of Cell-division. A, sL'paratioii of tho ('L-iitiosomt's niid aichoplasm; chromatin in skeiu-stage. B, fully formed spiudle ; chromatin loops formed. V, longitudinal division of tlie clironuitiu loops. 2>, separation of chronuitin loops and coninieucenient of the division of the cytoplasm. those which radiate outwards and are lost in the cytoplasmic network and form the aster. In the meantime, however, im- portant changes have been taking place within the nucleus. The chromatin substance, which originally was scattered in a reticulum, begins to arrange itself in a band (Fig. 2, A) which with many turns traverses the nuclear substance, the | nucleoli which were present at the same time gradually van- ishing. This stage of the process is termed the skein stage. T^i.r. D>->i»|{llp_fll^i'Qt! (]f f.]>o ppjjf |'{"jR(^n!fi tlip.n 'ipiif^sir tn '•"SPiif^fi'Mf!^ through the nuclear membrane, whicli sooner or later disap- pears, and by their growth push the chromatin skein toward-i I the equator c iug into a ui number of tl cells of any g able variatior served cases i 16, 32, or to t in different fo dumbbell-sha; less definite r: this stage, wh appearance pi (i) of the nuc plasm and b; extending toAv s )nies (cr) lyin complex the te Fig. 2 bis.—Km-y Pulgin- carica. Ill the next g I as a tyj)ical shf j V towards the u jtliat there Avere ( Jl'l.ite, as the re.s l^'Uiged in 2)airs '•' Jiiove towards f'> ''le otlier, sc ;n>u]. uf six chro [I'av be seen strel pi ''ippearance wii I'ltOTOl'LASM AND THE CELL. jj the equator of the nucleus, the skein at the same time break .ng j„to a number of fragments, termed chroMOso,,es 4t number of these chromosomes is practically constant for t I cells of any species of animal, and though there is consider able variation an different species, yet in the majori y of ol: served cases the number belongs either to the i-ies 2 4 8 16, 32, or to that of 6 12, 24. They vary considerably in s ze i^ different forms, being in some cases V-shaped or i,( others lumbbell-shaped, and arrange themselves flnilly in a mo,t or e s definite ring sui-ronnding the equator of the nucler I {") of the nilcleiis is a centosoine « "" .^^ ""'' ''"'^ plasm and by the astrll ,! i^ «"'™"''tled by archo- e^teudiug to^/ardt^rim ; t^^X, '^T'''' -mes (cr) lying at the equatoi^of tl e rnicleutl t, 7" ™mple. the term a,npMa.t.r is sometlJeTipUed? """"" as a t ;L;;rlte'\'l';i' r^''"'"" -""—■"-. t- take this fli.-.t there were mil ,11- >»"Rlt"'ll"«llv. Assu.ning ^Pl-te, as fl e res" It r' "r 'r''.''™'-"'"''' ■" «■« equatorial o ...ove towards one of 'n T T ""'' '""'' """' P™eeeds ■> ''- <.tliei so W e rlr "' '™ """'""^^ """' "'" ""'«- •»W»**ilfl«»* 12 IN VERTEBRA TE MORPIIOLOQ Y. cases at the equator of tlie egg there is to be seen on these fibres a number of darkly staining dots which may be termed the intermediate bodies (Fig. 2, D). At about this stage the cytophism begins to divide, the plane of its division passing through the equator of the nucleus, aud there are thus formed two cells, each containing a nucleus composed of six chromo- somes and a centrosome. The chromosomes now begin to become irregular in shape, they gradually fuse and are finally scattered in the form of a chromatic reticulum through the substance of the nucleus, which thus passes again into the resting stage, developing a new nuclear membrane. Our knowledge of many of the details of karyokinesis is yet imperfect, and especially is this the ease with regard to the mode in which the cen- trosome exerts its influence. It has been regarded as a simple centre of attraction, similar to the pole of a magnet, but the spindle-fibres seem to be more than passive in the phenomena. A comparison of the centrosome with an aggregation of microsomes has already been referred to, and if this idea be extended some light may be thrown upon the spindle-fibres. Tliey would then naturally be regarded as reticular fibres, i.e. fibres of pliistin to which a certain amount of contractility ar'. extensibility may be ascribed. During the earlier stages of karyokinesis their extensibility is more manifest, and extending into the nucleus they compress its chromatic substance, the contractility manifesting itself later and determining the migration of the chromatin loops or chromosomes towards the poles of the nucleus. Furthermore, since the linin reticulum of the nucleus is probably continuous with the plastin reticulum of the cytoplasm, it is conceivable that the activities of the centrosomes may call out in it changes of contrac- tion or extension which may suffice to bring r.bout the characteristic skein formation of the chromatin and the subsequent fragmentation of the skein into the chromosomes, as well as the formation of the connective fibres, the intermediate bodies upon these being regarded as microsome. LITERATURE. 0. Hertwig. Die Zelle und die Oewcbe. Jena, 1893. W. Flemminpf. Zellsubstam, Kern iiiid ZcUtlieilinig. Leipzig, 1882. 0. Btitschli. Untersiu'hiiugcii iibcr mikroskapische Schdume und das Proto- pldsma. Leipzig, 1893. C. Rabl. Ueber Zelltheilung. Morpliolog. Jahrbuch, x. 1884. G. Platner. Beitriige zur Kcnntims dcr Zelle und ihrer Theilung. Archiv fl'ir mikrosk. Anatomie, xxxiii, 1889. M. Heidenhain. tlber Kern und Protoplasma. Leipzig, 1893. E. E. Ziegler. Die Uologische Bedeuluuy der amiloliiichen KerntJieilung im Tkierrcich. Biolog. Ceutralbl., ix. 1891. A 8IMI power of queutly tl Such org number oj iug to a grouped 1 termed P: show but majority o upon then this physi moditicatic ous forms permanent and even absorption horny, cah ])rotective i high degrei lular organ various poi For the mo they associ the colonie functions o labor amon exists is in «neli as Vol entiation ; n identical pi SUDKINGDOM PROTOZOA. la CHAPTER II. SUBKINGDOM PROTOZOA. A SIMPLE cell, as has already been stated, possesses the- power of performing all the fuDctions of life, and conse- quently the existence of unicellular organisms is possible. Such organisms, together with those which consist of a. number of cells grouped together, each cell, however, retain- ing to a greater or less extent its own individuality, ara grouped together in a subkingdom and are collectively termed Pkotozoa. In its simplest form a Protozoon may show but littlo ditferentiation of its protoplasm, but in the majority of cases various portions of the cell-substance take upon themselves special functions, and in accordance with this physiological differentiation undergo various structural modifications. Locomotor and prehensile structures of vari- ous forms may be developed, excretory pulsating vacuoles, a permanent mouth and pharynx, special contractile bands, and even pigment spots presumably connected with light absorption may occur, and in addition the power of secreting horny, calcareous, or siliceous skeletons, serving either as protective or supportive structures, is frequently present. A high degree of complexity may therefore occur in a unicel- lular organism, a complexity produced by a differentiation of' various portions of the protoplasm composing the individual, j For the most part the organisms are simple, but occasionally they associate together to form colonies. The individuals of the colonies are as a rule all alike, each carrying on all the functions of existence for itself, and there is no division of labor among the various individuals. The complexity which exists is individual and not colonial. A few forms, however, such as Volvox, do present a certain amount of colonial differ- entiation ; all the cells composing the colony are not perfectly Identical physiologically, some becoming, ''for instance, spe- w 14 INVERTEBRATE MORPHOLOGY. cialized for reproductive purposes, while the rest take but little part in this process. Such a colony presents indica- tions of a passage towards a higher grade of individuality, some of the various cell-individuals merging to a certain ex- tent their individualities in that of the entire colony, and becoming somewhat dependent for existence on the coopera- tion of their fellows. This dependence, however, never reaches a high degree of development in the Protozoa and is for the most part entirely absent. It is in this respect that colonial Protozoa differ from the higher organisms, but th,^ difference is one of degree, not of kind. Four well-marked classes may be distinguished nniong the Protozoa : I. CI. Bhizopoda. II. CI. Sporozoa. III. CI. riagellata. IV. CI. Infusoria. I. Class Bhizopoda. The simplest Ehizopods present an approach to the least complicated condition under which protoplasm is known to us. They are simply small masses of protoplasm, more or less granular towards the centre, clearer towards the periph- cv Fig. %.—Anmha proteus (after Gruber). w = co.'ilractile vacuole. n = nucleus. ps = pseudopodium. try, and continually alter their shape by pushing out lobe- or thread-like processes known as paeiuiopvdia (Fig, 3, ps). By throT were, loc genus of gulfed by in coutac food-subs iutracellu part of th the geuer pie proces It is 1 this; even of the pre absent in j ferentiatio: and the c( noticed in the cell, a even in th covered, j which has and that ii been undii I exists is qu iu its fund solution ace protoplasm jcal contraci jexpelled fro Various jthe Rhizopc [degree of di; Ireproductioi Iteristics ma-i The For ^lass. In tL the simple c BUBKISODOM PltOTOZOA. 25 By throwing out such a proocr.s aud iiowiug after it as it wero, locomoliou is performed, wliicl. fro.,, a wel i,„ gem,, of the c1u.b is termed am^ioid. F^d i, sI,ll,T gulfed by the protoplasm flowing around it, at it .^r io, fe" n contact w.th a pseudopodium, and the digest m of food.substance takes place within the protopla™ ?" .. intracellular. Undigestible material' i^cCcardeda^I part of the body ; respiration and excretion a ecttdlnW he general surface; and reproduction is limited toihe "im pie process of division. ®^™" i:: raTrhetfrn-s -LI z T'Z ^^rB Wiations most usually oec„rrinra:e%he'i:c:;^^^^^^^ the cell, and it'is quesfion bt w t firL"Zn "'T '^ been undifferentiated, but whether sue fa r" '^ ^"^ exists is questionable The 0™^,-!^^! , '"?''"■'"" '"'»' ia its function, fluid contl ^'^tlXT meTab"r"'°'^' solution accumulating at one or mo,-! r« •? ""^ '*''°''™' '" 1. Ordei' Foramlnifera. »Ias^'^!T'"'"""'^"™'''"'^ "'^ «»"Pl'='- "embers of the Liass. In the genus Amrphn m\,^ q\ • ^^ "^ i-ne tlio «,-aiple c]i.r~-f 1 ^ ?• ^""'^ organisms presenting .-mple chaiucters above alluded to, being simple naked T 16 JN VERTEBRA TE MORPHOLOG T. XJ^K Fig. 4. — Arcella mitrata (after Leiuy) masses of protoplasm coutaiuiDg a nucleuf? and a contractile vesicle and presenting a slight differentiation into a peripheral more transparent ectoplasm and a central more granular eiidoplasm in which the nucleus is imbedded. The pseudo- podia are as a rule blunt loV)ose processes, though in some species they are more or less fila- mentous and may even be some- what permanent. The majority of forms, however, secrete a protective shell of varyiiig composition and com]^lexity. In Arcella (Fig. 4) it is chitinous and smooth, and len- ticular in shape, completely sur- rounding the protoplasm, the pseu- dopodia projecting from the cir- cular opening on the flat surface ; in Eughjpha it is similar in composition, but sculptured on the convex surface ; in Dijffugia the shell is flask-shaped and composed of particles of sand and similar foreign bodies cemented together, while in a large number of forms, es- pecially those which are marine in habitat, the shell is calcareous in composition. It is in these forms with calcareous shells that the great- est complexity of structure occurs. In some, such as Gromia, the shell is simple and flask shaped, the protoi'lasm pro- truding from the mouth of the shell and covering its entire surface as a delicate layer, from whi'*h ilie long, slender, and fre([uently anastomosing pseud()))odia take their origin. Al- though tlie ]iseudopodia are practically permanent in form their protoplasm is continually changing, carreuts streaming from the body towards the tii)S of the pseudopodia and re- turning again to tlu^ central mass, a constant circulation beiii}^ thus maintained, and food-] (articles caught by the delicato pseudopodia conveyed to the central mass, there to bo di- gested. A simple shell is, however, comparatively raro among these calcareous fornis , more frecpiently it consists of several chambers, as in Miliola, the chand)ers varying in size, the first- formed one being the smallest, and, in addition, in very many forms the shell is perforated by minute pores SI througli which the p chambers are arrang( as in Nodosaria, som axis as in Textularm sometimes as a heli more or less irreguLn ■'.■ <■'.■■', /It Fio. 5.—Ro(alii Notwithstanding < protoplasm retains tl and thougii in the nn may be replaced by more marked differe genera. In the second on slender as in the ct\]c. and somewhat rigid, being differentiated i nials are usually glob radiating (uit from t thus produced which "sun-animah'uln " wh such as Avtiiiopliri/.s SVBKINQDOM PROTOZOA. 17 througli which the pseudopodia are emitted. The successive chambers are arranged in various ways, sometimes end to end as in Nodosaria, sometimes alternately on opposite sides of an axis as in Textularia, sometimes as a spiral as in Globigerina sometimes as a helix as in liotalia (Fig. 5), and sometimes more or less irregularly as in Acervidaria. V; /. ■ Fk>. 5.— liotalia venatn Mftei-M. Schi-ltzk from Hatschek). Notwithstanding the complexity of the shell, however, the protoplasm retains throughout tin' order its simple structures and though in the more complicatec'l forms the single nucleus may be replaced by several, yet beyond this they present no more marked difl'erontiation than is found in the simpler genera. C. Order Heliozoa. In the second order, the Heliozoa, the pseudopodia are slender as in the ciilcart ous Foraminifera and are permanent and somewhat rigid, the central i)r(>t()plasm of each one b<'ing diti'erentiated into an elastic axial su])port. The ani- mals are usually globular in siiape, the slender i)seud()p()dia radiating (mt from the ccMitral mass, an appearance being thus produ(^ed which is sullicient oiuse for the ]»npular term "sun-animah'uln" which is applied to several cf the genera, such as AdiHophnjs and AvtiiiosftjKvrhnn (Fig. e travel is seen in i^^ SUBKINGDOM PROTOZOA. 1» the body is occupied by a spherical mass surrounded by a firm chitiuous covering and forming the central capsule. This contains usually many nuclei as well as vacuoles, oil-globules, and in some cases crystals and pigment-granules. The wall of the capsule is probably comparable to the shell of the Foraminifera, being perforated as in those forms by minute pores through which the intracapsular protoplasm becomea \ \ * '■ f I • i J. ^ 7 w# vfw*!*-' ft - - .,^.:."- • ^.M-^ Fig. l. — T/tiibisHi-oUapdagica (afUr Haeckki, from Hatschkk). continuous with the extracapsular. This latter portion on this supposition, notwithstanding its greater relative thick- ness, is etpuvalent to that jjortion of the protoplasm of the Foraminifera which is outside the shell and from which the ])seutlopodia arise. It is usually richly vacuolated and i)ig- niented, but contains no nuclei : the axial supjwrts of the pseudopodia traverse it and take their origin from the inner layers which in) mediately surround the central capsule and are more homogeneous than the outer portions. The shell is very various in form in the diflfereut genera, reaching a high degree of ditterentiation in some forms, such as IMiosphwm (Fig. 8), where it consists of a fenestrated ^lol.e traversed by )'adiating spines. Its greatest simplicity is seen in Splio'wzuum, in whii-h it is represented by scattered *20 INVEHTEBRA TE MORPIIOLOG 7. spicules, while in Thalassicolla, already alluded to (Fig. 7), it is eutirely absent. As stated, it is usually siliceous in char- acter, though in Acanfhomelra it is composed of a peculiar horny material termed acanthin. Scattered through the protoplasm of the Eadiolariaus there are usually to be seen numbers of small yellowish bodies long known as the "yellow cells." They are not con- stant, however, individuals of any species frequently beiu^' destitute of them, a peculiarity due to the " yellow cells " not being really constituent parts of the Eadiolarian, but / Fig. %.—HeUospli(vra actinota (after Hakokkl from Hatschkk). foreign bodies, in fact unicellular plants, for which the term Zooxanthella' has been proposed. They cannot be consid- ered parasites, since they do not appear to exist at the ex- pense of the host, but, on the contrary, their presence seems actually to be beneficial. Mutual benefits are conferred bv the plant and the Radiolarian, the coexistence constituting an example of the ])henomen()n known as Symlnosis. I{eprod,urtion in the lihizopods. — Throughout all the grou)»s the simpk'st form of reproduction, fiHsioiij is prnbably ])reY:i- lent (Fig. 9), though it is not yet definitely known to occur the Fig. 7), it in char- peculiar iolariaus yellowish uot con- ly beiu^' V cells" dan, but ). the term cousid- t the ex- e seeijis 3rred by stitutiiii.' i groups ;o occur aVBKINODOM PROTOZOA. ' 21 among the Eadiolaria. lu the fresh-water Foraminifera and Hehozoa it is, however, the usual method in genera both with and without shells, and it also occurs among the marine Fora minifera. Where the shell is thin it may be divided durin- the process but where it is thicker the protoplasm divides within it one of the individuals retaining the old shell, while the other wanders forth and constructs a new house for itself This is the case, for instance, in Arcella, in which the wanderin- iudi Fig. 9.— Division of Amoehn (n ter Schclzk) vidual protrudes from the mouth of the parent shell until it Ws^its new shell, only separating when thisT 1^1 Colonies produced by repeated divisions and the imper- onne'l'rtT '' ^'^ '"'"^ ^^ ^^"^^"^^^'' ^'^ occasionally med, but they are simply aggregations of similar individ- -ulH. no difterentiation or individualization of the colony as a !,._ oceuiiiiig. Among the fresh-water lihizopods this io the case with Micro,ro,ua, a shelled form, numerl Ldivid ^ I 22 INVERTEBRATE MORPHOLOGY. uals of which may remain iu connection with one another by means of their profusely-branching pseudopodia. Colonies n^ Adinophrys are also formed in a similar manner, and among the Radiolaria the forms with rudimentary shells — such as Sphcerozoum, produce, apparently by the division of the cen- tral capsule, numerous individuals which remain in contact. A moditication of fission known as budding or gemmation also occurs in some forms. It differs from fission only in that the products of the division difi'er in size, so that it is possible to regi.rd the larger individual as the parent and the one or more smaller ones formed from it by budding as the progeny. The process is, however, fundamentally the same as fission and is a derivative of that process. In Arcella bud-like processes arise from the periphery of the parent protoplasm, separate, and assume amoeboid movement, leav- ing the shell in an Amoeba-like condition, and some marine Foraminifera and probably certain Heliozoa reproduce in a similar manner. Spore-formation also occurs, the parent protoplasm break- ing up more or less completely into a number of small por- tions termed spores, which later increase in size and assume the characters of the parent. This process is sometimes pre- ceded by encysfment, a phenomenon not, however, in its origin connected with reproduction. It is more prevalent among fresh-water than among marine forms, and seems to have been originally developed as a protection from injurious ex- ternal conditions, such as the drying up of the pools in which the organisms live. When about to encyst, an Amoeba, for instance, withdraws its pseudopodia and assumes a spherical shape, and then secretes a more or less dense chitinous case or cj'st which completely encloses it. In virtue of the resist- eut and non-conductive nature of the cyst the organism may, while in this state, suffer uninjured prolonged exposure to conditions which would quickly entail the death of the non- encysted individual, and on the return of favorable condi- tions niay leave the cyst and reassume its active life. Occa- sionally, too, encystment may occur as the result of good nutrition, an individual which iui« engulfed a number of diatoms, for instance, secreting a cyst around itself witliiu which it rema digested, whei diatom shells Plentiful n under this te related to a ci the two proces be associated 10, A) feeds in on a stalked i contents of tl Fig. A. Vampyi B. Vampyi them aside and pied by them. spores (Fig. IC becomes a new Among the most usual met among them in some cases the sular protoplas others some of others small (j spores may be vidual mayproc it is easy to dc spores or isospo 8UBKINGD0M PROTOZOA. 33 which it remains until the food-matter has been thoroughly digested, when the cyst is thrown off together with the empty diatom shells and the animal again becomes active. Plentiful nutrition and reproduction by division (including under this term the various modifications of fission) are related to a certain extent, and it is easy to understand why the two processes of eucystment and spore-formation should be associated together. The Heliozoan Vampyrella (Fig. 10, A) feeds in its active condition on diatoms, and especially on a stalked form, Gomphonema. After having digested the contents of the diatom frustules which it engulfs, it pushes A B. Fig. 10.— Vampyrella (from Haeckel after Butschli). Vampyrella feeding upon the stalked diatom Gomphonema. Vampyrella encysted upon the stalli of the diatom. them aside and encysts itself upon the stalk previously occu- pied by them. Within the cyst the animal divides into four spores (Fig. 10, B), each of wliich escaping from the cyst becomes a new Vampyrella. Among the Eadiolaria spore-formation seems to be the most usual method of reproduction, and a complication occurs among them in that spores of two kinds may be formed. In Horae cases the spores, which are formed from the intracap- sular protoplasm, are all equal in size {isospores), while in others some of the spores may be large {macrospor&'i) and others small {microspores). Both macrospores and micro- spores may be formed in the same individual, or each indi- vidual may produce only one of the two forms. In such cases It is easy to determine whether one has to do with macro- spores or isospores, which closely resemble each other in size, MMMttMii^ 24 INVERTEBRATE MORPHOLOGY. from the fact that the isospores are spherical in shape and each possesses a peculiar whetstone-like crystal, wanting in the macrospores. All the spores are provided with siuf>Ie whip-like processes, flagella, by which they are propelled through the water when set free from the parent. The various processes so far mentioned concern a single individual ouly aud are therefore non-sexual, Sexual repro- tluctiou, consisting of a fusion of two individuals (covjugation), seems to be a rather rare occurrence among the lihizopods, though it has been observed preceding spore-formation in several instances, the fusion being probably the predisposing cause of the spore-formation. In the Foraminiferan Gromia the production of spores has been observed, and subsequent conjugation of some of the spores occurs, and it seems exceed- ingly probable that the macrospores and microspores of the Eadiolaria are sexual cells, their further development de- pending on the conjugation of a micro- with a macrospore, but the fate of these spores has not as yet been ascertained, and their conjugation can only be imagined from analogy w-ith other forms. II. Class Sfobozoa. The Sporozoa, which constitute the second class of Proto- zoa, are all parasitic, living in the cavities, cells, or tissues of other animals and deriving their nutrition from their )i'>«x.s. At present much is lacking to an adequate knowledge ^i various members of the group, but at least three orders m to be recognized. 1. Order Gregarinida. The Gregarinida include some of the largest Sporozoa, ?ind are parasitic either in the body-cavity, intestine, or organs of various luvertebrata (especially in Annelids and Tracheata), or in the cells especially of Vertebrated Animals, these intracellular parasites being .usually known as the Coccidia in contradistinction to the former, the Gregarinid.i proper. The members of both groups show a marked ditfer- entiation of their protoplasm into ectoplasm aud endoplasm, I a relatively known to pi many Greg covers the sometimes d tuberculatec garinida she what has b( nida which i present the of two cells, of the body of ectoplasm addition to t cases is fun finger-like pr the animal to it lives. Eve a single nuc] lar. Reproduc in some case or gemmatioi division beinf formation, pr sumes a sphe ,/eater portic number of jver, remaini: mature the s have received however, dev( plasmic conte spores (Fig. li the formation these crescent cases {Po/'ospi seems to becoi later elongates 8UBKINOD0M PROTOZOA. 25 a relatively large nucleus lying in the latter, and none are known to possess pseudopodia. Indeed in many Gregarinida a well-marked cuticle covers the exterior of the body (Fig. 11), sometimes distinctly striated or occasionally tuberculated. The Coccidia and many Gre- garinida show little differentiation beyond what has been mentioned, but the Gregari- nida which inhabit Tracheate hosts usu"ally present the appearance of being composed of two cells, owing to the anterior portion of the body being separated by a partition of ectoplasm from the posterior part, and in addition to this the anterior moiety in some cases is furnished with hooks, bristles, or ^ finger-like processes (Fig. 11) of use in fixing ^^«- ^^•-nopl(yrhyn- the animal to the walls of the cavity in which "''"* oiigncanthus it lives. Even in these cases, however, but ^^"•^'- '™"'""^- a single nucleus is present and the organism is unicellu- lar. Reproduction is carried on by spore-formation, preceded in some cases by conjugation (Fig. 12), but simple division or gemmation is not known to occur, apparent instances of division being more probably cases of conjugation. In spore- formation, preceded or not by conjugation, the animal as- sumes a spherical shape and forms a cyst about itself, the ..reater portion of the protoplasm splitting up into usually number of nucleated spores, a small portion of it, how- ever, remaining undivided {residual body) (Fig. 12) When mature the spores are usually spindle- or boat-shap-^d and Jiave received the name oi pseudonavicellce. They do not however, develop directly into Gregarines, but their proto ' plasmic contents break up into 2, 8, or more crescentic spores (Fig. 12), a residual body being again formed as in the formation of pseudonavicella3. The further history of these crescentic spores is not thoroughly known, but in some cases {Po,ospora from the intestine of the lobster) each seems to become converted into an amoeboid structure which later elongates to an actively moving thread-like organism % 26 INVERTEBRATE MORPHOLOGY. the fseudofilaria, and this, gradually losing its motility, de- velops into the adult form. _pm---l^ PiQ. 12.— Reprodcction of Gregarine (from Hertwio). 1. Clepaidrina blattarum in coiijugiition; ck = ectosarc, en = eudosarc.cjt = cuticula, pm = anterior portion, dm = posterior portion, n = micleus. 8. Cysts in transformation into pseiulonaviceliae; pn = pseudonavicellae: rk = residual protoplasm. 3. A, a pseudonavicella strongly magnified; B, the same divided into spores, sk; n ■■= nucleus, rk = residual protoplasm. One of the Coceidia, C, oviforme, is very common in the liver of rabbits and has also occasionally occurred in Man. It is found encysted in the interior of nodules in the liver, and the development goes no further than encystation in the body of the host. On escaping from the body, however, each Coccidium breaks up into four spores, and the contents of each sporo later divide into two comet-like bodies (sporozoites) which, when swallowed by ix rabbit, make their way to the liver, where they encyst. 2. Order Myxosporidia. The Myxosporidia are found almost exclusively parasitic in Fishes, af in the intern bladder. Th toplasm, som falling consid enclosed in c; when not so amoeboid mo^ entiated from tion a large ni Keproduct formation bei Myxosporidiui Pike the prot masses each ( all of these m six nuclei, foi two portions trinucleated b pseudonavicell sisting as the at the extremit structure cout? ment which is ure and proba body of a hos thoroughly knt escape as amoE sporidia. In many respe^ but the possibilitj plant nature must lations have been f view which certain The Sarcosp i» the muselo- of Mammalia, 1 SUBKINODOM PROTOZOA. 27 iu Fishes affecting principally the skin, but also occurring in t le internal organs, such as kidneys, spleen, an ' urinary bladder. Ihey consist of irregularly-shaped masses of pro- oplasm, sometimes reaching a length 0.1 mm., but usually falling considerably short of this size. Frequently they are enclosed in cysts developed from the tissues of the host but when not so enclosed seem to possess the power of 'slow amcBboid movement. The endoplasm is usually well diffeiT entiated from the ectoplasm and contains in the adult condi- tion a large number of minute nuclei Keproduction by division is not known to occur, spore- formation being the only method as yet observed. In the Myxosporidium occurring in the urinary bladder of the Pike the protoplasm breaks up into a number of spherical masses each containing a number of nuclei. The fate of all of these masses is not known, but some, containing only SIX nuclei, form a wall about themselves and divide into wo portions each of which contains three nuclei. These tnnucleated bodies elongate, develop a wall, and become ixseudonavicella-like spores, one of the thre; nucIeT per! siHting as the spore-nucleus, while the other two, situLd a the extremities of the spore, seem to give rise to a sac-like structure containing within its interior a spirally rolled fila inent which is emitted when the spore is Jubje Jed to pre 1 re and probably serves for the fixation of the spore to the hody of a host. The further history of the spores is not thoroughly known, but it seems probable that the contents Toridia^^ ^"^^boid masses which develop into adult Myxo- In many respects the Myxosporidia resemble closely the Grecarinirl. but the possibility of their being in reality not of an anMbut of ; ant nature must not be overlooked. By some authors theTr nearest re ^U.ons have been found in the Myxomycetous and Chytridiaceous fu. li t view which certainly has not a little to recommend it. ^'"'''^''°"' ^""S'' '' 3. Order Sarcosporidia. i.. '!it' l""f-T^^' T' "^"^ "^r^^^ ^^^^P««-- parasites If M ^"'^f.^'^f^^^"^ «f warm-blooded animals, especially of Mammalia, being found in the interior of the primitive 28 INVERTEBRATE MORPHOLOGY. fibrils of the striated muscles, whose contents they more or less destroy. They form somewhat elongated sacs 1-2 mm. in length, the wall of the sac being formed of a distinct membrane which has the appearance of being covered with fine bristles. The contents of the sac consist of a protoplasmic ground- substance in which a large number of nuclei are imbedded, sometimes aggregated into masses each of which is sur- rounded by a delicate membrane. It seems probable that these masses represent a process of spore-formation, but as yet nothing is known regarding the further development of the spores. 4. Order Heemosporidia. The HsBmosporidia are a group of forms which occur in the blood of various Vertebrates, one of the most interesting of them being the cause of intermittent fever in Man. This form occurs as an amoeboid body in the interior of the blood- corpuscles, causing these to enlarge. The growth of the amoeboid stage is completed in forty-eight hours, and the or- ganisms then divide into fifteen or twenty spherical " spores " which pass out of the corpuscles into the blood-plasma, enter- ing later new corpuscles. This is the history of the form which produces tertian ague ; the quotidian and quartan varieties of the disease are produced by forms requiring re- spectively twenty-four and seventy-two hours for the com- pletion of their life-cycles. In association with these Ilcemamceba forms there also occur sickle-shaped bodies termed Laverania whose exact sig- nificance is not yet understood, and, furthermore, there is sometimes found free in \,he blood-plasma a flagellate struc- ture which is probably a degeneration stage of the Ham- amceha. III. Class Flagellata. The Flagellates are characterized by the possession of one or more long filamentous processes of protoplasm, knovvn as JlageUa, which, by whip-like movements, propel the organ- riatfellum usus iletinite cutich 8UBKINGD0M PROTOZOA. 29 isms through the water in which they live, and at the same time by the production of currents in the water bring food- particles within their reach. Some forms possess pseudo- podia in addition to the flagella, which are indeed simply at- tenuated and mobile pseudopodia, but the majority have a mcxe or less permanent body-form. This in many species is accompanied by the formation at the exterior of the body of a skin or cuticle which in some cases, as in the Dino- flagellata, may assume a sufficient density and thickness to entitle it to be termed a shell. 1. Order Autoflagellata. In the Autoflagellata the body is usually more or less oval, and while in many forms it is naked and capable of changing form (Fig. 13, /I), yet in others special cuticular in- vestments may be present, taking the form in some cases of a simple cuticular covering, as in Evglena (Fig. 13, B), in others forming a stalk by which the organism is attached to a foreign body ; in some forms, as in Codosiga (Fig. 13, C\ a cuticular collar surrounding the base of the flagellum is pres- W\W A ®"*' ^^^"^® ^" others, such as Dinohryon, a cup ^i^oaos^ga flagellum, the food - particles drawn to the organism by the currents established bv the Fig. 13. -I, Oikomonas (after Butschli). (after Butschli). Hatfellum usually im]ii iletinite cuticl aud iiigmg at this point ; where, ho shell is developed a definit vvever, a mouth occurs. in s.>me cases, as Euglena (Fig. 13, B), this leads int o a 30 INVKIiTEBRA TE MOKPIIOL 00 T. distiuct tubular pharynx projecting some distance into the in- terior. No hollow digestive tract is, iiowever, present, but the food-particles, after traversing the gullet, are received directly into the proti)i)lasm of the body, and are digested there as in Amoeba. A localized egestive region, situated usually towards the posterior end of the body, has been as- certained to occur in some species, but in no instance is it a permanent oritice, as is the case with the mouth. In addition to the nucleus, contractile vacuole, and food-particles, other definitely organized particles, such as starch-like granules and pigment-granules, may by imbedded in the protoplasm. In Eiujhna the pigment is green and resembles plant-chloro- l)hyll, probably too possessing a similar function. A red pigment-spot (stigma) is also present in this and other genera at the base of the fiagellum and is supposed to be concerned in ]ight-])('rception. The typical Flagellate is a free-swimming single organism, but many forms are fixed, developing a stalk by which they are fastened to foreign bodies ; the stalk may be very much branched, each terminal branch supporting an individual, the -whole thus forming a colony, without, however, any differ- entiation among the individuals. Free-swimming colonies also exist, such for example as VoJvox, in which a large number of individuals are grouped together to form a spheri- cal hollow cohmy. Each individual contains chlorophyll- granules and a red stigma, and is i)rovided with two fla- gella by the action of which the entire colony is propelled through the water with a rotai'y motion. 2. Order Dinoflagellata. The Dinoflagellata are distinguished from the members of the preceding order by the almost general occurrence of a rather dense shell comi)osed of i)lates of a substance resem- bling closely vegetable cellulose. Some of the forms, such an Cerniium (Fig. 14), ])resent a rather bizzaro shape on account of the shell Ix'ing jtrolongtnl into horns, and in the majority the shell-{)lat('S are delicately sculi)tured, while arround the equtttor of the shell runs a furrow, and from an opening in the lijie oj ])()ssesses t acter, whil row and in of a delicat r(>phyll-]ik( ably preseij Peculiar c} protoplasm of a hollow Avitliin it a vx'casion ni and no doii tion, resem structure tl The ordi NodiJuca 1 structure n is almost gl the flagella the mouth-( sessing an ( like outer v strands of ] the central nucleus and nt is almost iiivari- ablj present, as is also the red sti^nii;i. Teculiar cysts are also i)resent in tin; protoplasm of nnxuy forms, consistinj,' of a hollow capsule havinj^ rolled up within it a hollow thread, which on occasion may be rapidly eva^'inated and no doubt has a protective func- tion, resembling,' very closely in its structure the uematocysts of the Cadenterates. 3. Order Cystoflagellata. The order of the Cystoflagellata includes only two genera Nixiihica nd LepMwcm. The latter is a somewhat •— ^'oc<*V«m miliaru •vhitively to the Hagellum, thick pro! , . '*',""• "^"•^•'">- c-ass of the internal protophis.n. ' = ^^■"'"^•'^' "^-'uclcus. t.i , u,ii^-j,5e.,eijicii ill jAinmUscm^ which OlLef- wise closely resemldes NtKiilKca. I 3 82 INVERTEBRATE MORPUOLOGT. Noctiluca is of considerable physiological interest, since it is one of the forms to wliich the phosphorescence of the ocean is due. The cause of the liglit and its character are, however, as yet unknown. Reproduction in the Flagellata. — The most frequent method of reproduction in all the orders of the Flagellates is simple division, either transverse or longitudinal. Encystment, fol- lowed or not as the case may be by spore-formation, is also common, and when accompanied by spore-formation may be preceded by the conjugation and fusion of two individuals. In Cercomonas the spores are exceedingly abundant and small, presenting the appearance of minute granules even under the highest powers of the microscope, but in other forms, as Chlamydomonas, the spores are larger and much fewer in num- ber, being only 4 or 8 in this particular case. An interesting modification occurs in closely-related species (Fig. 16), some individuals of which divide into a number of small spores {microspores), while others undergo a more restricted division and give rise to a few large spores [niacrospores). The latter develop directly into the adult forms, but the microspores show a tendency to conjugate in pairs before undergoing further development. This differentiation of two kinds of spores is carried still farther in other forms where neither Fia. 16.— 1. Phacotus lenticttlaria ; 3, Macuosi'ouks and Mickosi'okes op TIIK HAMIfi Sl-EOIES (after Ui^TsiHi.i). «cA = shell. n = nucleus. macro- nor microsi)orea develop directly but further develop, ment is contingent upon the conjugation of a micro- with a mucroHpoio. In this ] certain celh spliere, enla when they 1 cells Avhich (laughter co' to this a se vidu.'ils grad surface of tL to enlarge, : frecpiently, f tliigella ; tlies ova develop tion with spt colony do nc tegrate and < it is clear tL eutiation of of the consti iudividualit}" In NoctilHc< occurs which p ceded by tlie C( plasuis coming lii'peiited divisi t likes place ace tliat (he surfac( liki' structures. on tiu! chanict adult Noctihim The Infu Protozoa, HJK tained by otl izod by the \ of liumorous cilia by mea procured. I SUBKINGDOM PROTOZOA. 33 lu tills respect considerable interest attaches to Volvox ; certaiu cells, usually those situated in the posterior heiiii- spliere, enlarge and project into the interior cavity, dividing when they have reached their full growth into a number of cells which arrange themselves in a hollow sphere forming daughter colonies in the interior of the parent. In addition to this a sexual process occurs ushered in by certain indi- viduals gradually enlarging, and leaving their position at the surface of the colony. In the interior some of them continue to enlarge, forming ova (macrospores), while others divide fie(piently, forming packets of elongated cells furnished with tiagella ; these may be termed spermatozoa (microspores). The ova develop into colonies similar to the parent after conjuga- tion with spermatozoa. Since many of the cells of the parent colony do not participate in this reproductive act, but disin- tegrate and die on the development of the daughter colonies, it is clear that we have in this form a rather marked differ- entiation of the individuals of the colony, the individualities of the constituent cells being to a slight extent merged in the individuality of the colony. In Noctiluca in addition to simple division a process of roi)fodui;tioii occurs which partakes of the character of budding. It is apparently pre- coded l)y the conjugation of two individuals, the combined central proto- plasms coming to the surface of the cyst where they form a protuberance. Repeated division of the nucleus into 2, 4, 8, etc., up to 250 or juore now tiikes place accompanied by only u partial division of the protoplasm, so tliat (he surface of the protuberance is covered tiy a large number of bud- like structures. Eventually these separate, develop a Magt!llum, and take on the character of motile spores. Their further developmeut into the adult Noctihim Las, however, not yet been followed. IV. Class Infusoria. The Infusoria are the nii.st highly specialized of all the Protozoa, showing a ditterentiutioii of the protophism uiiat- tained by other meiiibers of the group. They are character- ized by the possession during the whole or part of their lives of numorous delicate short motile haii-iike processes termed vU'nt by moans of Avhlch locomotion is performed and food procured. In one of the orders into which the class may bo 34 IN VERTEBRA TE MORPHOLOQ Y. Jivided, tlie Ciliata, these structures are present during the iidult life of the orgauisms, while iu the other, the Suctoria, though preseut iu the jouug stages they are replaced later by immovable processes of the body, which extract the nourish- ment from the food-particles which come "nto contact with them. 1. Order Ciliata. The Ciliata are for the most part free-swimming organ- FiG. 17.— A, Paramecium; B, Stentor ; C, Vwticella ; D, Euplotea. ev = coiitnictlle viicuole. n = nucleus. m = inoutli. n' = uiicrouuclous. my = iuy()i)!mno. tr = trioliooyst. isms, tlioiujfh some, o.ff. Vortwella ("PxiJ. 17. /7\ ndlmra fr» fz-krai"" b()di«>s by means of a stalk, similar to that found in Flagel- lates, and c In these sti case, of whi site the stal by cilia wh tected by tl free-swimmi distributed, forms, Fig, some of the bristles up< Fig. 17, D). A detini situated fre and leading cilia, thoug] {(Mlodon). food-particL the body-pr( is a localize definite ana ellates by th iug minute c cilia surrouu The bod. tral part, an( tion. Pigme sist of Chloi contractile vi ally single, t genus Opalin are numerou; cleus may I horseshoe-shi ^fanfor (Fig. ; nucleus there found in its ■ (nictiun and i iliU'tuentiatioi 8UBKINOD0M PROTOZOA. 36 I ]ate«, and colonial stalked forms also occur as in that class, lu these stalked forms the body is enveloped in a chitinouJ case, of which the stalk is a prolongation, the surface oppo- site the stalk being, however, left naked and being surrounded by cilia which are absent on the portions of the body pro- tected by the chitiu {Peritrichous forms, Fig. 17, C). In the free-swimming forms, however, the cilia are more universally distributed, covering either the entire surface {Holotrichous forms, Fig. 17, A) or else oje surface of the flattened body, some of them in this case being modified into stout movable bristles upon which the animal creeps (HypotrichoK^ forms Fig. 17, U). A definitely localized mouth-opening is always present, situated frequently at the extremity of a peristomial groove and leading into a gullet of variable extent, usually lined by ciha, though sometimes furnished with a chitinous support (C/nlodon). There is, however, no special digestive tract the food-particles after traversing the gullet being received into the body-protoplasm, where they are digested. Usually there IS a localized cgestive region, and in a few cases there is a definite anal opening. The food is procured as in the Flag- ellates by th. ^urrents set up in the water by the cilia carry- ing minute organisms to the neighborhood of the mouth, the cilia surrounding this opening directing them to the gullet. The body-protoplasm is usually very granular in its cen- tral part, and filled with food-vacuoles and products of diges- tion. Pigment-granules are sometimes present and may con- sist of Chlorophyll, as in Stentor, and one or more excretory contractile vacuoles are always present. The nucleus is usu- ally single, though occasionally two are present, and in the genus Opalina, which occurs in the intestine of the Frog, they are numerous in the adult condition. When single the nu- cleus may bo very large and either spherical, elongated, iiorseshoe-shaped as in Vorficelhi (Fig. 17, r), moniliform as in 6tcntor (Fig. 17, B), or otherwise shaped. In addition to the iHicleus there are one or two minute structures usually to be found in its vicinity which play an important part in repro- (mctii)n and are known as micronndci {Fiir. 17, y/, n'). Other ilillerentiations of the protoplasm are also found in certain UtiSiilif'imtSKn 36 INVERTEBRATE MORPHOLOGY. forms, as, for instance, special bands differentiated so as to be specially contractile and therefore corresponding in function to the muscles of the higher animals, and hence termed onyo- phanes. In Vorticella a more striking differentiation of spe- cially contractile protoplasm occurs (Fig. 17, C, my) ; running in an open spiral through the centre of the supporting stalk of this organism is a strong ^yopha7ie terminating above in the protoplasm of the c ^ • ' When the latter is stimulated the myophane contract: , .ng the stalk into a close spiral and withdrawing the animui from the source of irritation. In some of the Holotricha, such as Paramoecium, numerous mi- nute rod-like structures occur imbedded in the protoplasm near the surface of the body (Fig. 17, A, tr). They are appar- ently defensive in function, since when stimulated they sud- denly, as if by an explosive action, become transformed into long threads or needle-like structures projecting beyond the cilia. These trichocy.^ts also occur in some Flagellates. 2. Order Suctoria. The Suctoria lack the active movements of the Ciliata, being destitute in the adult stage of cilia, and many of the forms, o.g. Acineta (Fig. 18), are attached to foreign bodies by a stalk. They do not possess any mouth, but a number of simple or branched {Demlrocometes) saff pro- cesses project from the body which serve for the prehension of the organisms, principally AuKebio, upon Avhich they feed. A contractile vacuole and nucleus are always present, the nucleus having sometimes a very complicated shajje. It seems pretty clear that they have been derived from the Ciliata, since in their young stages they are free-swimming ciliated sti'uctures; the tentacular processes have been compared to the pseudopodia of the Rhizo- fio. \%.'—AcinetM pods, but good reasons for such an homology gramiis (after do not exist, and. it is more probable that thev saville kknt). are structures ])eculiar to the group. The JicproducfioH of the Jnfmoria. — In the Infusoria tho re])roductive cation than ( cesses of fis former occui mode of repi e rations. L many cases unless the pi Encystme under variou has been m( not be follow a resting cys resistent so , insufficient a< ciated with ei tiou. The di closed, and ■« or four parts. within whicL gradually grc it contracts t( Olid cyst wit] animal from fractive bodii bnrsting of th in which they develop. Los amoeboid; th which elougat liliizopod to t withdrawn, ci the adult form has been obs' taL'hes to it as ! Infusoria. Conjugatio] 'here it suems SVBKINGDOM PROTOZOA. 37 reproductive processes reach a much higher grade of compli. outiou than occurs iu other Protozoa, though the simple pro- cesses of fission and spore-formation likewise occur The former occurs in the majority of forms, and may be tlie only mode of reproduction occurring throughout a number of gen- eiations. Long-continued fission seems, however, to lead in many cases to structural and physiological derangements nuless the process of conjugation be interposed. Encystment is also of frequent occurrence and may occur under various conditions. In Colpoda, in which the process has been most thoroughly studied, encystment may or may not be followed by reproduction. In the latter case' the cyst a restmg cyst, is perfectly closed, and the walls are thick and resistent so as to withstand unfavorable conditions, such as insufficient aeration or dryness. When reproduction' is asso- ciated with encystment it may be either fission or spore-forma tiou. The division cyst is thin-walled and is not completely closed, and within it the animal undergoes division into two or our parts. In spore-formation a thin cyst is first formed within which the animal slowly rotates, at the same time gradually growing smaller by the expulsion of fluid. Finally it contracts to a round mass and surrounds itself with a sec ond cyst within the first. At the surface of the encysted amnial from eight to thirty minute spherical and highly re- ructive bodies appear which are the spores, an.l by' the burHtm of the cyst they, with the remains of the protoplasm in which they arose, escape to the exterior and soon begin to deve ojx Losing its spherical shape each spore becomes .uncBboid ; then, drawing in all the pseudopodia Init one V nch elongates and becomes a flagellum, it passes from the K .opod to the Flagellate stage ; and finally \he flagellum L hdrawn cilia appear and the animal gradually assumes lie adult form. Hpore-development somewhat similar to this ms been ob,^ rved also in Vorticella, and special interest at- mZX ""^ "^'''^'-''^^y ^"^^i^'ating the line of descent of the ^ CJoujugation is a frequent process among the Infusoria, lieie it seems lo have u rejuvenating rather than a strictly 88 IN VEHTEBHA TE MOIiPJIOLOG Y. reproductive function. If prevented, and fission goes ou through a number of generations, marked degeneration en- sues ; while if it be aUowed, the same number of generations may be produced without any signs of degeneration. Tlio process consists of a renewal of the nuclei and micronuclei of the conjugating forms, and the process as it occurs in Colpid- ium colpoda may be described thus. Two individuals com<^ into contact by tiie anterior portions of their body, actual fusion of the two protoplasms taking place at the point of contact. The microniicleus in each individual then enlarges and divides, the two thus formed subsequently dividing again, so that each of the conjugating individuals contains four micronuclei and oue nucleus. One of the four micronuclei ill each individual now divides, and one of the two thiis formed (the male pronucleus) crosses over to the other indi- vidual and unites with the other product of the division, the female pronucleus, there being thus a mutual interchange of aiicronuclei. The individuals now separate and resume their independent existences, and a rearrungement of the nuclear structures accompanied by fission takes place. The three micronuclei which did not take part in the formation of the pronuclei of conjugation degenerate, as does also the original Fig 19.-DIAQUAM to Illusthatk the Beiiaviou of the Nuclei and| Micronuclei duuino Conjugation in Infubouia (after Maupas). nucleus. The conjugation micronucleus, formed by the fusioul of tho male and female pronuclei, divides twice, forming foiuj micronuclei, and this is' followed by a fission of the entirej Infusorian, each of the daughter forms so produced possessniJ two micronuc iiiu'Ieus, while ('()iiil)Ii(';ited I accdihpanying In the mil [)rocess, the tv pioimclei. In J>y j-epeated 1< into a number land swim abo ('OHIO into con i])orni;uient fusi I. Class Rnizop( 1. Order Fo when p (a) ^ (b) S (0) S (d) g (e)S! 2. Order ^cif present (a) S {b) SI 3. Order lia( usually present. (a) SI (6) SI (e) Sh II. Class Spouozo, 1. Order (Tfre;, Invertel 2. Order Jfiji internal •i. Order y repeated longitudinal fission a Vorficella becomes divided I into a number of small individuals which leave their stalks and swim about freely in the water. Should one of them (■oiiie into contact with a large individual a complete and h)ornianent fusion of the small with the largo one occurs. SUBKINGDOM PKOTOZOA. I. Class RmzopoDA. -Protozoa willi lohe-likoor filamentous psoudopodia, 1. Onlor i'T>r«w'e,jari,nf1,,,jcllnta.-Wnhunt shell, protoplasm not especially vacuolated. ^ III 40 IN VEllTEBllA TE MOEPUOLOG 7. (a) Without coWav.—Monas, Cercomonas, Chlamydomonus, Eiif/Ieiia, Volvox. (6) Witli collar. — Codosiga, Dinohrijon. 3. Order Dinu/lof/ellaia.— With shell composed of cellulose. Cera- tium. 3. Order Cysto/lageUata.— Without shell, protoplasm highly vacuo- lated, marine. Noctilnca, Leptodiscits. IV. Class Infusoria.— Provided with cilia or immovable processes. 1. Order CZ/m^a. —Provided with cilia in adult stage. (a) Cilia of nearly uniform length all over the body {Holo- tricha). Paminoecium, Colpoda, Colpidium, Chilodon, Opalina. (6) Cilia around anterior end of body longer tlian tne rest {Heterotricha). Stentor. (c) Cilia limited to anterior end of body (PeritricJia). Vorti- cella. (d) Cilia or setae only on ventral surface of the body {Hypo- tricha). Stylonyehia, 3. 0-der Suctoria.— With cilia only ir. the young stages, in the adult with immovable processes. Fodopliryn, Acineta. LITERATURE. 0. Biitschli. Protozoa. Bronn'sKlassen u. ORimmgendesThierreichs. Leip- zig u. iI'jiQylberg, 1888-87. W.S.Kent. A Manual of the lnjumiia. Lomloii 1880-83. J. Leidy. Fresh-water lihizopods of North America. U. S. Geological Sur- vey of the Territories, xii. 1879. B. Hertwig. Bemerkungen zar Organisation und systematischen Stellung dtr Foraminiferen. Jeuaische Zaitschr., x. 1876. E. Haeckel. Die Radlolarien. Eine Mon-jgraphie. Berliu, 1862-88. K. Brandt. Pie Kolonie-bildenden Radiolavien {Sphmrozoen) des Oolfes von Neapd. Fauua u. Flora des (iolfes vou Neapel. Mono^rapLie, xni. 188'). A. Labbe. Rrxherches zoologiques et hiologiques .mr les parasites endrglobidaires da sang des Vertebris. Arch, de Zool. Expcr., Ser. 3™", ii. 1894. F. Stein. Per Organisnius der Infusionsthiere. Leipzig, 1859-79. E. Maupa. La rajeunissement karyogamique chez les Cities. Archives de Zo(j1. experimeutale, 2'"" t^er. vii. 1889. L. Rhumbler. Pie verschiedenen Cystenbildungen und die Entteicklungsqr- schichte der holotrichen Lifmoriengatlung Colpoda. Zeitschr. fur wis- sensch. Zoologie, XLVi. 1888. B. Hertwig. Ueber Podophrya gemmipara, etc. Morpholog. .Jahrbuch • 1876. be ; aggregates miBKlNGDOM METAZOA. 41 CHAPTER III. SUBKlNGDOx¥ METAZOA. The Metazoa are equivalent to colonies of Protozoa, the mdividual cells of which have differentiated in various direc tious, some being more especially contractile, others nutritive' othei-s irritable, others reproductive, etc., instead of each one for Itself performing equally all the functions necessary for existence. A physiological division of labor of a more or less perfect kma is introduced among the individuals composing the colony, and the welfare of each individual becomes de pendent upon the proper performance by its colleagues of their special functions; in short, the individualities of 'the component cells are merged in the higher individuality of the whole organism. ^ Physiologically a Metazoon is equivalent to a Protozoon but morphologically it is the equivalent of a large number of them. Each IS physiologically an individual, but morpholog- ically the Metazoon is a colony of Protozoan individuals To harmonize the physiological and morphological conceptions 0 an individua it is necessary to recognize several grades of morphological individuality of which the cell mav be -is sumed to be the lowest. In the Metazoa the physiologilal diffei-entiatK^ns of the cell-individuals are accompanied by ti-uctural differentiations, so that it is possible, as a rule to determine from its structure what the function of a cell mav be ; aggregates of similar cells are termed tissues or tissne-indi- Ms, and as the simplest Metazoa are complexes of various tissues, such a complex forms the third grade of individu- ality and may be termed an Organ-individual. A complex of organ-individuals united to form a physiological unit consti- utes an individual of the third grade, the 3Ietamere.indi,idual, Awhile the fourth grade, the Cormus, is formed bv a similar umon of a number of metameres, as, for instance, in the l.arthworm, each joint or segment of which is a metameie ^S^^^lP 42 INVEHTEBRA TE MOHPHOLOO Y. It has been pointed out that the FhigelUite Volvox presents a tendency towards a higher individuality, being somewhat higher than a mere colony of cell-individuals and yet not quite reaching the dignity of an organ-individual ; similarly intermediate conditions between the other grades may occur. In certain worms, for instance, considerable independence of the I nstituent metameres exists, any one of them, when de- tached, being capable of carrying on an independent exist- ence, and of developing into an organism similar to that of which it was originally a part. In the Earthworm the depend- ence of the various segments or metameres upon one another is greater than this, but in it, too, a certain amount of inde- pendence is shown by the power it possesses of regenerating lost metameres. In other cormi, as, for instance, in the Lobstei", the interdependence of the component metameres proceeds still farther, and a differentiation of the various meta- meres occurs, a process carried to its greatest extent in the higher Vertebrates. A physiological division of labor amouj^f the metameres develops, some of them losing, for instance, their excretory organs, while in others these organs lose their excretory functions and serve as ducts by which the repro- ductive elements may pass to the exterior. The subordina- tion of the metameres proceeds most rapidly and is most complete at the anterior extremity of the organism, leadiug to the formation of a head bearing highly developed sense- organs and containing a complex nervous system, which rep- resents originally distinct metamere nervous systems, now fused and destitute of all independence. Sexual Iieproducfion in the Metazoa. — In cell-individuals it has been seen that fission is the most frequent and simplest mode of reproduction ; in the Metazoa this method and its modification, budding, also occurs, but, as a rule, only iu forms of a low grade of individuality or in a transition stage between a lower and a higher grade. In organ-individuals it is of frequent occurrence, the imj^erfect separation of tlie individuals so produced leading, in many cases, to the forma- tion of colonies, and in cormi in which tlie integration of the constituent metameres is but slight it also occurs. In the Protozoa cell-division naturally entails reproduc- SUBKINGDOM METAZOA. 43 tiou, but iu organ-individuals reproduction of the constitu- ent cell-individuals is not necessarily connected Avith the reproduction of the entire individual, but may simply increase the number of lower-grade individuals of which it is com- posed. Similarly multiplication of the organ-individuals of a luetamere, or of the metamere-individuals of a cormus may occur without producing reproduction of the whole • it is snuply growth. From growth to reproduction l,y budding the path is short, and various intermediate stages connecting the two processes can be found. Hence reproduction has been .iptly dehned as "discontinuous growth," though perhaps it would be even more apt to define growth as reprodiccHon icith- out discontinuity, growth in a Metazoon depending on the I reproduction of the lower-grade individuals of which it is composed. It is possible to carry this idoa still farther back and refer the growth of a ceil to the reproduction of the constituent elements, plasomes, of which It .nay be miagmed, it is composed. In the simplest cells the various onus of plasomes are distributed throughout the cell, but in the hLWier 1 rotozoa, for n.stance, an aggregation of similar plasomes occurs, givin<^ nse to such structures as the myophanes. In a similar manner h. th^ hmv Meta.oa, although a division of labor and structural differentiation has taken place among the constituent cells, yet the cells possessing similar functions, as, for mstance, the nerve-cells, are more or less ir^gular y scattered throughout the body, only becoming aggregated in the higher fornis mto d.stmct tissues, and giving rise to the most perfect type of an 0 Kan-nuhvulual. Likewise in a metamere-individual a multipHcation o le organs leads to a transition form with discretely arranged parts, the 1 hnite aggregation of which produces a cormus, composed in the sin pier fonns of d,s Hict metameres, which become more and more integrated and :::;;:!;f:;:;;Udm;r ^-^^^^^^^^^^^ -' ''- --- - ^^^'- ^^- - ^'^- According to this view the segmentation or metamerism of the higher M.'tazoais the result of the multiplication and subsequent integration of i>' organ-.nd.vKluals of an ancestral metamere-individual, and explains •IH. occurrence of imperfect metamerism in certai.i forms o that So , n u, ,,in,y (2W,VW/«nV;). Some authors have considered metan.fnl o I Ji^e a men by he reproduction by budding of an ancestral metamero an ea which fails io explai..« satisfactorily the condition Just referred io 1 If view presented here considers metamerism to be the result of c^rowth' "!;"iuj;'"'r" '7 *"V^^^^'^^-''- -^ ^'- '-^amere, but by tlna of its o'^aiiN just as a typical organ-individual has arisen by the reproduction and integration of its constituent cell-individuals. louuction i c i 2 44 IN VEliTEBRA TE MORPIIOLOQ Y. As a mode of reproduction iu tlie Metazoa division plays but a secondary part, the sexual process being the character- istic method. Attention has already been called to the par- tial specialization in Volvox of reproductive cells Avhich servo to perpetuate the species, the remaining cells of the colony perishing. This condition is a premonition of the more per- fect specialization found in the Metazoa of reproductive or gervt cells and non-reproductive or somatic cells, the hitter serving for the nutrition and protection of the germ-cells, t(; whicli the perpetuation of the species is entrusted. Comparatively early in the development of an individual certain cells differ- entiate from the others, not undergoing like them a physi- ological and structural specialization, but retaining a general- ized character. Tliese are the germ-cells usually grouped together to form the reproductive orgaus. In describing the methods of reproduction occurring in the riagellata, the manner of the development of sexual repro- duction was indicated. It appears to have been originally a more or less accidental fusion of two similar cells or spores, and from being accidental this fusion gradually became the rule on account of the greater vitality which the conjugate in- dividual possessed over cells which did not conjugate. The next step was the differentiation of microspores and macro- sjiores, which reaches a high development in J^tJi'Ojr, where it is associated also with a differentiation into somatic and germ cells. In the Metazoa both these differentiations are carried to a higher degree, the macrospores being known as ovd and the microspores as spermis both elements. This ct)ndition of livrttiaphi'oditism, which oo- SUBKINGDOM METAZOA. 45 (Ills iu many pcarasitie forms and in certain sponges, Flat- Avorms, Mollusks, and Crustacea, seems to have been second- iivily acquired. It is probable that the ancestral Metazoa were unisexual, possessing reproductive elements of only one kind, a supposition borne out by the frequent association of liermaphroditism with a parasitic or sessile mode of life, such conditions being what may be termed abnormal, and usually accompanied by marked structural characters which are to 1).' regarded as secondary modifications. On the other hand, it is noticeable that the lowest free Metazoa (such as the free' swimming Cnidaria) are unisexual. An ovum is a single cell, and iu its typical form ccmsists of a mass of protoplasm containing a nucleus, and may or may not be surrounded by a membrane. Seldom, however, does such a simple ovum occur; usually more or less yolk, consisting ^ « of fatty and albuminous globules, is distrib- uted throughout the protoplasm, and fre- (jiiently the amount of yolk far overbalances the amount of protoplasm. Other structures, such as albumen and one or more enveloping ineinbranes, may be added, the ova of different sj.ecicis differing greatly iu this respect. Among the lower forms th«^ ova are usually extruded freely from the body of the parent, hut in many of the higher Metazoa they are eiu;U)sed Avithiu protective cas'^s (cocoons), as in the Earthworm, or imbedded in jelly-like masses, as in the common Pond-snails. In the ovary of a young individual all the Pru 2().-Ovahial germ-cells are alike, and all are pottuitially 'I'l'i'i-: ok a Mkk- reproductive colls; very fre(iuently, however, many of the p'iniitive genii-c(Uls relimiuish their reproductive function and serve as pur- veyors of nutrition to certain of their com- liidos which enlarge and bcconu^ mature ova. .'/ = yolkcdlH. This is well seen in insects, in which each ovary '^ " f"'"it'l<'fiH8. (iMg. 20) consists of a number of tubes tapering to a point at one end, while at the other they open into u common duct, i)xfT. I.ni Ti,|.; (ufi.T IIOCK). g — ^(Miiiiiml re 0 — OVIl, Ijk'ioM. « ^^miiluu' ovmii. t 46 INVERTEBltA TE MOltPIIOLOG Y. the oviduct, leading to the exterior. At the tip of each tube the primitive germ-cells (Fig. 20, g) are located, and lower •down ova (o) in various stages of development towards matu- rity are to be found, each surrounded by a number of small undeveloped germ-cells, known as follicle-cells (/), whose func- tion it is to transfer food-yolk (y) to the growing ovum. As the latter approaches maturity the follicle-cells secrete around it a thick, sometimes highly sculptured shell and finally degenerate. As a rule, conjugation with a spermatozoon, i.e. fertiliza- tion, is necessary as an antecedent to further development. Before this takes place, however, certain modifications of the ovum are necessary, the phenomena which accompany them being known as the mat unit ion of the ovum. In this process PS- A B FlO. vl.— DlAORAMR ILLUSTUATING TIIK MaTUUATION OF THF: OVUM. A = foriimtiou of the liisl poliir globule (pff). li - foimiitioii of the second polar globule iind eiilianct' of the spenn-iiuclcus (Fig. 21, A) the nucleus ai^proaches the surface of the ovun) and there undergoes u karyokiuetic division which is pecnljjir in that iii the e(iuat()rial-i)liit(^ stage twice as many chronio- souios are foruied as are typical for the species. Thes<> do not undergo longitudinal division, and by tlie karyokinesis th(>ir number is reduced to the typical number, a small cell, the polar (jlooule ( pg), being separated from the ovum with half the chromosoiues, whih^ the others are retained within the ovum. The nucleus of the t)vum, instead of now returning t.. tin, resting stage, divides again iFig. 21, //), a second polar globule being formed and receiving half (he chroniosimies SUBKINGDOM METAZOA. 47 whicli remain, so that the nucleus of the ovum now possesses oiilj half the number of chromosomes which are character- istic for the species. At the time of the formation of the second polar globule the hrst frequently divides without its uuclsus passing into a resting stage, so that as the result of this maturation process four cells have been formed, three of which are small, while the third is relatively very large and will alone undergo further development. AVhen these divi- sious have been completed and the chromosomes have been reduced to one-half their proper number the nucleus of the ovum passes into the resting stage, migrates back towanls the centre of the ovum, and is ready for conjugation with the nucleus of a spern)atozoon. The spermatozoa are always much smaller than the ova, and are, as a rule, capable of active motion, though in certain Crustacea, for instance, they lack this power. The ova and spermatozoa have specialized in opposite directions in this respect. The ova of the Metazoa are specialized as the nutritive cells of conjugation, possessing abundant protoplasm and usually a considerable amount of yolk for the nutrition of the young embryo. They consequently have lost their motility, and in order that conjugation may be made prob- able the spermatozoa lack all unnecessary material which would interfere with their motility, no yolk beiug stored up and the i)rot<)plasm even being reduced to the smallest amount ^consistent with the development of a locomotor organ. The nuclei, as will be seen later, are essential ele- ments in conjugation, and the si)ern)atozoa are to all intents locomotor nuclei, the ova supplying the })rotoplasmic nidus ne(!essary for the growth and division of the nucleus formed l)_v conjugation. In their typical form spermatozoa are composed of a globuhir or pyriform head consisting of a nuchuis surrounded by a small amount of protophism, and a long lilamentous (nil continuous with the protoplasm and fre(|uently provided with u delicate fringe-like niembrane (Fig. 22, A'). By the rai -' wliipj)ing movements of the tail the organism is prnpnUo,! tlirough the water, or other tinid in which it may iind itself, ttiid so may come into contact with an ovum. § c i 48 IN VERTEBRA TE MORPUOLOG Y. The trausforniation of the germ-cells present iu an em bryo into spermatozoa is usually a somewhat complicated process. In the liouud-worm Asians, iu which it retains somewhat primitive characters, the process closely resem- bles what takes place during the maturation of the ovum» Fig. 23.— Diaohams to ii.i,u8tratk the .Matuhation of the Spkum-cell. A — liivisioii of tlie spermogoue. B — (livisiou of the two speruiocytes. C = the four spermatids. D, E — conversion of a spcrniaiid into a spcrniiitozoon. F — fully (levt'loped speiinalozoon. The embryonic germ-cells {spermatogones, Fig. 22, A) undergo karyokiuetic division, the number of chromosomes being, as in the ovum in the division which results iu the formation of the tirst polar globule, twice that which is characteristic for tl.>e species. Tliey do not undergo longitudinal division, and one half of them passes into one of the daughter cells {spcr- iiKiforyft's) and tlie other half into the otlier, so that these two cells possess the numl)er of chromosomes characteristic for tlie species. A division of these daughter cells (Fig. 22, />') immedial'ilv takes place without a return to the resting stage, nnd uuacc( uipanied by a longitudinal division of the chromo- Hcunes, so that four cells {sfM'i'nudids, Fig. 22, C) are formed, each of which contains only half the ty]ucal number of cliro- jjj(-ya(-)jj5P,ij •j.jjd p.jich one of these cells becomes a s')erm;il'i- zoou. This process is comparable step by step with the 8UBKINQL0M METAZOA. 4^ maturation of the ovum and seems to indicate tliat the polar globules are to be regarded as abortive ova. The conversion of the spermatids into spermatozoa ia simply a diflerentiatiou of structures already present In the air-breathing Moliusca, for instance, the spermatids consist of a mass of cytoplasm containing a nucleus, in close proxim- ity to which may be found the centrosome, .vhile an irregular mass of filaments represents the remains of the spindle-tila- ments. In the differentiation which follows (Fig. 22, 1) E iind F) the nucleus elongates and its chromatin-filame'ntJ liise to form a homogeneous mass ; the cytoplasm likewise elongates, and m it appears an axial filament which later will form the tail-tilament. The origin of this filament is doubt- ful, some authors maintaining that it is a difterentiation of the cytoplasm, while others believe it to be a prolongation of the nuclear substance ; but, however that may be, the spiral Irmge which surrounds the axial filament is certainly the remaiiis of the cytoplasm of the spermatid. The remains of the spindle-fiiaments disappear, while the centrosome prob- ably persists as a structure lying behind the head and termed the "Mittelstiick." Ill some cases as the insect P,,nh.eoyl, and the crnsfacean mrptomm, I..' duubhnjj of the chromosomes previous to division into spermatocytes .I0..S not take place. In />,n-/.>c../,vtwenty-four ^^.::::^T^. .ally present and twelve of tln-se pass into each of the spermatocvtes u.d .1. the d.v.s.on of these to forn. the spern.atids each of tl.e twelve eh'r'omo .Jom^d.VKles so that each spermatid possesses half the typical nnn.her. . JhupUunn. the sann,> resnlt is brought about somewhat differentiv he sperniatoKoncs possess eij,d>t ehro.nosomes which assume a dund.bni sLape anddnule transversely, so that each spermatocyte has tl.e t pi a ■unnber of chron.oson.es ; ,he spern.atocytes divislms been completed, the nucleus of the ovum ..ugntteH towards the centre of tlie protoplasm ami is th„ tnnule^nuden. (Fig. 23, ./>,) of conjugation. The penetia- t">n o the spermatozoon may occur at any p,)rtion of th.« sur- I'-e of the ovum and may take place before, during (Fi« 21 A. sp\ or after the formation of the polar gh.bules, a single U&t'liii m W VERTEBRA TE MORPHOLOG Y. spermatozoon, as a rule, in healthy ova, penetrating and tak- ing part in the conjugation, though apparently in some cases polyspermy, or the penetration of several spermatozoa, may occur. The head of the spermatozoon comes into contact with the protoplasm of the ovum, which in some cases rises up to meet it, and is rapidly engulfed. The tail likewise of C D Fm. 23.— Diagrams to Illustkate the Piiknomena of Fertilizatiox. (From flf?ures by E. B. Wilson.) A, the approximation of the male and female pronuclei. B, division of tiie arclioplasm. V, separation of the arclioplasm spheres. D, fusion of the pronuclei, and formation of the segmentation spindle. //) = female pronucleus. a = arclioplasm. mp = male pronucleus. sn = segmentation nucleus. the spermatozoon is taken into tlie ovum and seems to ho completely absorbed, the head alone being visible in later stages ; it constitutes the viale pronuclem (Fig. 28, mp), and moves towards the centre of the egg until it comes into'con- tactwith the female pronucleus, fusing with it to form the sey- menfation nudeus. A spindle now makes its appearance, and the Bogmentatiou nucleus passes through the various karyokinetic stages, forming an equatorial plate with the typical number SUBKINGDOM METAZOA. 51 of chromosomes, which divide lougitudiually in the usual in.'umer, cue half of them passing towards each of the archo- plasin spheres. The ovum then divides into two cells, the nuclei of which each possess chromatin elements from both the male and the female pronuclei. It will be seen from this that the conjugation or fertiliza- tion process consists of the union of two distinct nuclei, whose complete fusion however does not necessarily occur until after the first division or segmentation of the ovum. The archoplasm of the segmentation nucleus is derived as a rule, from the sperm-cell alone (Fig. 24), that which ex- isted in the ovum during its maturation disappearing before fortdization takes place. It has been claimed, however, that ill some cases the ovum archoplasm persists, and unites with tiiat from the sperm when the pronuclei unite, and that more rarely the ovum archoplasm alone persists. These ob- servations, however, need confirmation. Furthermore, in some cases at least, it is possible to dis- tinguish the nuclear elements derived from the male and female pronuclei respectively in stages later than the first segmentation, owing to a slightly different behavior to certain staining reagents which chara,cterizes them. The pronuclei undergo a morphological fusion during the first cleavage of the ovum, but a physiological difierentiation persists. Segmentation and EarJy Ih'relopinent of the Ovum.— The development of the ovum into the embryo consists in its divi- sion into a number of cells, which gradually undergo a pliy- sioiogical and morphological difierentiation resulting in the formation of tissues, organs, etc. These divisions constitute the segmentation of the ovum. The first division has already been described ; it bears a definite relation to the formation of the polar globules, the l)lane of the division ])assing through the p.)int at which they Mere separated from the ovum. Considering this point to represent one pole of the ovum, the first division is meridio- iial, and the second divisi(ui likewise, though its plane is at right angles to that of tlie first division (Fig. 2^ A). The third division is, tm tho otlier iiand, equatorial, its plane cutting the planes of [)revious divisions at right angles (Fig. 24, Hl i I i i ^mammmmmmm m INVERTEBRATE MORPHOLOGY. Eight segineutatiou-cella are tlius furiued which remaiu in ^outaet with each other aud enclose a small cavity, the seg- meiitatiou-cavity or blastocoel. The further divisiou of the cells (Fig. 24, C) results in the formation of an oval or spheri- cal organism (Fig. 24, JJ) which may be compared to Volvox, consisting of a single layer of cells enclosing a more or less voluminous blastocuel. This embryonic stage is known as the hlostula. In its simplest form it shows no special differ- entiation into tissues, its cells being uniformly ciliated, and Fig. 34 —Diagrams illusthating the Segmentation op the Ovum. A, four-celled stage. B, ei,i;lit-celleil stujje of a telolecithal ovum. f, sixteeu-ccUed stage. 1), blualula. Tlio arrows ludicate the mode of divisiou. the organism free-swimming, moving through the water with a rotatory movement about a dehuite axis, one and the same end of which is always anterior. In many blastulas, however, especially in those which for one reason or another are not free-swimming, an early differentiation of the cells takes place, especially at the extremity which is posterior in the free- swimming forms or which corresponds to that pole in the non-motile embryos. These posterior cells are usually sonie- xirlinf Ini'dfor flimi flirisft n.^. flip nufftvinr r)ole- n.ud if lYinnli foo - jolk is present in the embryo it is especially concentrated lu 8UBKINOD0M METAZOA. 03 these cells, which in the later developmeut will assume the vegetative fuuctious of the orgauism. In many ova the processes just described are modified to a greater or 1..SS extent, but from tlie frequency of tlieir occurrence they must be re garded as fundamental and the modifications as secondary Ova which contain but little yolk usually follow more or less closelv the typical processes, but wher^. the yolk is abundant, being an inert sub stimce, It acts as a drag upon the protoplasmic activity and produces modi lioation of the segmentation-processes. Two methods of arrangement of tl.e yolk may be recognized : (a) it may be aggregated more or^less com- pletely at, one pole of the ovum, such ova being termed telolecithal or (h) It may be distributed in the meshes of a protoplasmic network a small (luantity of yolkless protoplasm being concentrated around the nucleus of tlie ovum, while another portion of it forms a thin peripheral layer sur rounding the yolk, this arrangement being termed centrolecithal In telolecithal ova the third segmentation-division results in the forma tion of tour cells containing very little yolk at one pole of tlie ovum while nearly all the yolk is concentrated in the four cells at the other pole (Fi- 24, B). This arrangement, which occurs in many Mollusca, constitutes what IS termed a total irregular segmentation, in which, owing to tlie larn), while that l/ng imn eZ Lv below the ectoderm is the somatic layer (m) and iTl f ^ cavitj is the ccelom (C) or body-cavitv T, enclosed protoplasm destined to give r^e to tl 1 n.. 7 ' ""' '^" iato a small number of ce Is or sonw '"• ''^''^^^''^ i ^^...P tLo 1 ''^ ^^ considered the blastula stage. Ihese cells, known as mesoMasts, give rise by rene.f. division in one direction nn.l i.„ +i i ^ lejaeated ' the dauditer o.ul 7 } ^ ^'^ subsequent division of iu,)j, and later crrowino- rlm-u-illTr ..^ i , ^ ^" tive tropf Tl. , "^ ^'^'^'^^^y «o as to enclose the di«es. £-::,. ;t,r '2 ':;:: ,xxtA:i - j: same appearance as in the former case Presents the Jbeing minute scattered cavities in n mnL ^f ^ 'lerm, it would seem that Zim Z ^ "''"" ''^'^ ""^"" "^ *''« '»««o- 'f ti>o nature of a scliiJl 'f.1 f ^ S";'"'" '""* '^ ^""^"'"'"^^ "'^'^ -norodifferentiatedfrr^eend d ™^^^ '^^""^ '"-'^ -'^ ^s a separate germ-layer .t an e-^J ? P'-oper and either tendeesoblasts or wa del- v'd i n N T'"^ ^'^^'^^opment in the fonn of >f ••"■ PHmitivo di..e3 i 0 hnct 7. ^^^^'l^P'"'^"^ ""^il after the fofmatio. --'ouches. ird::;r:oth- rth^t^^^^^^^^ 111) I I 58 INVERTEBRATE MORPHOLOGY. derivative of the endoderm, and the endoderm of the diploblastic organ- isms is equivalent to the endoderm plus mesoderm of tiie triploblasiic forms. The apparent derivation of the mesoderm from the ectoderm in .some of the latter (e.g. Annelida) is to be regarded ao resulting from tins l)recocious segregation of the mesoderm at an early period of development and is not to be regarded as indicating its original derivation. Ko7i-sexual licproduciion in the Metazoa. — Reproduction hx division and by budding, though playing by no means so im- portant a part as in tlie Protozoa, is neverthe- less of frequent occurrence in the Metazoa, \m especially in certain groups. In certain Tiir- bellariau worms (3Iicro.sto)na) division is the '" usual mode of reproduction, replacing almost ,, completely the sexual method, and, the indi- viduals so produced remaining in connection fin with one another, longitudinal chains aie ]>roduced, ct)nsistiug of individuals in various m» degrees of separation (Fig. 28). In certain Annelids also ) division frequently |ni"< takes place, occasional!}' each metamere beinif capable of developing into a new animal, as in Ctenodrilths. >' Budding, however, is a rather more fre- (juent method and is characteristic of certain groups, such as the Hydroids, Anthozoa, and Fid. 28. — - D I A- Jjfyoijoa. In some cases, as in Hydra and ouAM smnviNo ^^^[^^^^ medusie, the buds se])arate "from the NON-8KXUAI, ItK- i i n • i i ruoDicTKiN OK P"^'"^"'' •'^''*' ^6ad an independent existence; A Tchuki.i.ahian but frcnpuMitly the se])aration is not com])leto, WouM MiiTOMto- resulting in the foiinution of colonies tlie '"" "'^'*''' """ individual components of which are in or- ganic connection with each other. In sucli colonies a physiological division of labor aniong the con- stituent individuals may talu^ ])lace, as in the Hydroid //ydrnefiiiiii (seehind and do not take part 1" i\y^ ditterentiation, and later give rise to a new bu.l ; and sundarly ,n the Annelid worms the tissues of a regeneratin.^ I'Mi-l Hhow an appearance and nn.de of ditierentiati,.n similar ywhat hey present in the development from the ovum. ...iversely the greater the degree of dim-rentiation anirthenogenetieallj, giving rise to a second generation ditlering in form from the' Hrst and reproducing by the sexual method. 'J'vpical exam- )'I<"s of this process are to be found among the Trematode worms ((,.v.), where the sexual worm gives rise to a sporocyst n. the interior of which ova, developing parthenoger.'ticallv give rise to a larva which later on transforms to the adult worm. In a less perfect form heterogony occurs in many lower Crustacea {Daphma), which throughout the warmer portion of the year produce "summer eu'gs " which dfivelop IMrthenogeneticaily, male animals a])pearing onlv f..r ,■. slinrf l»i rind in the autumn, as a I ule, when the lemules produce 62 IN VEHTEBliA TE MORPllOLOG Y. " winter oggs " which develop after fertilization. Here no difference of form exists between the two generations, but such cases, as well as those in which two sexual generations unlike in form and habitat alternate Avith each other, are usually associated with the more typical examples as in- stances of heterogony. TltWUOPLA LITERATURE. 0, Hertwig. Beitrage zur Kenntniss der IHldung, Befi uchtung tind Theilung dfs thierisrhen Eics. Morpholog. Jabrbucb, r., iii. and iv. 1875-78. E. van Beneden and A. Neyt. Nouvellcs rccherches sur la tWondntion et la dmsion mitodque rhez I'Asmride megalocephale. Bulletin de la Societ(' royale Belgiqne, xiv. 1887. Th. Boveri. ZeUenstudien. Jenaisclie Zeitsclirift, xxii. 1888 and xxiv. 1890. E. B. Wilson. Archopltism, Centrosome, areutly parthenogenetic and no male Dicifcnia is as vet known. In young individuals iii;: f^orm-cclls sngincnt in the interior of tlie coitral cell and give lise to "vermiform" ^ '-C mi{ TRICHOPLAX, THE DIGYEMID^ AND ORTHONECTID^. 65 embryos (Fig. 31, V) similar to and developing directly into the adult form. Auotlier form of embryo is, however, produced by older individuals, its formation being iiecompanied by a peculiar behavior of the germ-cells. The nucleus of each one first divides into two unequal parts, the smaller l)art separating as a paranucleus and under- going no further development. The germ- cell now segments, and an embryo (Fig. 31, e) consisting of a single large cell partially surrounded by smaller cells re- sults. The smaller cells are now thrown off and separate somewhat from each other, and the larger cell repeats the segmentation- l)iocess, the smaller cells being again thrown off; and this may happen three or four times, the result being the production of tliree or four concentric layers of small cells surrounding a single larger one, all lying in the central cell of the parent. The large cell undergoes no further develojo- ment, but the smaller ones, except those of the last generation, develop into " infu- " soriform" embryos of a peculiar and com- l)licated structure. The cells of the last generation develop into "vermiform" em- 1^, o, „ ""^ l..:.yos si,„ilar to those f„„.,.l i„ yo^/'^^'Z^^:')! l>lcyemids. several fiKures by The fate of the '« infusoriform " embryos c -"0X1 cell 1ms not been deternnue;l. Since they are «- embryos, eihated it seems not imj>robable that they .v - germ -eel Is. serve for the dissemination of the species '' = "'"^•'-'usofceutial -ulits transference from one CephalopcKl v = ::^,orm e.n- l'"^t to another. It has, iiowover, been bryo. suggested that they may develop into males. The OjtTHONECTIDA. The Orthonectids are i)arasitic on Echinod.M-n,^ .,p.1 nuortean worms and resemble in structure the JJicvemids «P--h- g .\ 66 INVEHTEBHA TE MOliPIIOLOG Y. TmCIIOPLi the ectoderm cousistiiig of a number of ciliated cells arranged in a single layer and enclosing a mass of germ-cells which correspond to the central cell of JJicyeina. Between the germ- colls and the ectoderm fine nucleated fibres occur which are presumably muscular. Three forms of individual are known to occur in the genus Ithopalura, one being a male, and the other two females. Fig. d2.—R7iopnhira Oiardu (nftM- Ji-un). A, mule ; D, round fomule ; C, flat I'euifile. The male (Fig. 32, .1) is about half the size of the females, which measure about 0.'^' mm. in length, and presents a met- americ arrangement of the ectoderm which does not extend to the internal cells. The cells of the anterior segment havo their cilia directed anteriorly-, and are succeeded by a segment consisting of several rows of small non-ciiiated cells eacli containing a refractive body, and behind this there follow throo or four segments formed of cells provided with cilia directed backwards. One of the female forms (Fig. 82, B) is eloii gated, and is segmented like the male oxce])t that the segments are more numerous and the second non-ciliated seynifint. (>in\- sists of a single row of cells destitute of refractive bodies. TlilCHOPLAX, THE DIOYEMID^ AND ORTllONECTIDJE. 67 The other female (Fig. 32, G) is, on the contrary, ovoid, flat- toned, and unsegmented, beiug ciliated all over ; it differs furthermore from the elongated female in possessing on one side near the anterior extremity a granular niass containing a large nucleus whose significance is entirely problematical. Associated with the difference of form of the two females there is a difference of function. In the elongated form when the ova are mature the anterior two segments si)lit off as a cap and allow the ova to escape, and, on fertilization, these ^ive rise to males. In the ovoid form, however, the ova are imbedded in a gelatinous mass, and are liberated by the breakiug up of the parent into a number of fragments; from the ova females of both forms develop. The systematic position and affinities of the Dicyemidfe and Orthonee- iidif IS a mutter of uncertainty. Tliey i.ave been held by some authors to IM.SS..8S atH.mies witii tiie Greg.-vri.iida and by otliers to be degenerate Ihit worms, vvliile others iiave sought to trace resemblances to the Rotifers llie granular mass with the large nucleus which occurs in the ovoid RhopaUira has been supposed to represent a rudiment of a digestive tract while the superficial metamerism of the male and elongate female of the Orthouectida may possibly point to a derivation from more highly organ- ized aace:^trai forms. There can be but little doubt that the Dieyemidiel.nd Ortiioiiectida are closely related, but at present sufflcienc evidence is want- ing to warrant any detinite conclusions as to their relationships to other torms. LITERATURE. TRKIIOPLAX ADn.KRENS. F. E- Schulze Uher Tnehnph,v adhanrm. Abhandl, Akad. Wiss. Berlin. 1»'J1 (see also /oolog. Anzeiger, vi. 1883). DKYEMID^. C 0. Whitman A Contnhutiou to the Emhr},olorn,, Life-history, and aa.xi- J>«,ttoa oj the Bwyemtds. Mitth. a. ,1. Zool. Station zu Neapel. iv. 1882. OKTIIONECTinyE. C. JuUn^ Contribution a I'histoire des Mesozoairea. Arcliives de Biologie. iii. ^IStjl «8 IN VEHTEBltA TE MOliPHOLOG Y. CHAPTER V TYPE CaaENTERA. The Coeleiitera iwcliKlo the (li])h)Wastic Metnzoji, ouly two gerin-hiyei'.s, the ectoderm and eudodenn, being re})re- seuted in tlieir organization (Fig. 83). Between these two hiyers, Iiowever, a third (Fig. 38, mg) is invariably present, which in its primitive condition is not cellular, but consists of a gehitiuous or fibrous sub- stance secreted by one of the two cellular layers. Usually, however, cells from the endoderm or ectoderm wander into it, and sometimes are so numerous as to give it the appearance of a cellular layer. Even in such cases, however, the gelatinous matrix is the fundamental substance of the Tig. 33.— Diagham op Hy- layer, which it seems preferable to lira TO snow t.ik Gen- term the 7/ie6rP««nf i • formed of spicules consisting of carbonil'oTte Th: group contains forms of various comnlpvi-fv^^ / \ , U. simple cylindrical /. J:!^ tl^tn ^ l;;::;.^^ Ascoatype through Sycon forms such as OmntiauZ sentutives of the third and fourth types. Imleed ft i o ' "" this group that the Ascon and Svcon tvnesof .h 7 ^ found AH flm v.. • ^•>^"" t.^ pes ot structure are sLViudeptL '■" "''"'" "" '""""^ "■"> '-■« "' only 2. Order Cornacuspongise. { The skeleton of the Cornacuspougiro consisf., «,-fl <• ; sihceous,needle.liIf shallow M-ater and are n,' T ^'"•'' '''^ 1-HlH upon the entire absence J H^^' ^^^^^^^^ ^^'■ ^vi'ioh is found in the shallo 4t s I ?'""^'"' '"^^ of the MeditGrran«n.. ,-.. t^ p "f " ^ ""'*"'" I"*'^'"" ^Bi ^^^" 74 m VERTEBRA TE MORPHOLOQ 7. 3. Order Spiculispongise. The skeleton in the SiDiculispougiae is occasionally entirely wanting, as in the genus JIalisarca, but usually consists of siliceous spicules usually tetraxial or rod- or club-shaped, sometimes interlocking with one another so as to form a firm Fig. 37.— a small Spongilla with oNiiV a single Osculum (from Huxlkv). a — iiilialenl pore. c = ciliuted clianiber seen through d = oscuUini, tlie tissues. skeleton- One of the members of the group is the ** boviu*,' sponge," CUona, which excavates channels in and assists iu the disintegration of oyster-shells, frequently attacking tho idiells of living animals and contributing to their destruction. 4. Order HyalospongisB. The Hyalospongiw are essentially deep-sea forms, and are characterized by the possession of six-rayed siliceous spicules as skeletal elements. The spicules inuy becomt> fused to- gether to form a firm siliceous network having the appear- ance of spun glass, as in the genus Eupleciella, commonly known as Venus' Flower-basket. Reproduction of the Porifern. — Soxiia] reproducfcion occur* ])robab]y tliroughout the entire group of tho S])ouges, the nv TTPE CaSLEXTEHA. n« rrodnctive elements, ova and spermatozoa, differentiatm^ i.«m mesosteal cells Many Sponges a.e hemaph" dte t »,,«-,atozoa developing usually somewhat in advluee of'the «v.. but some forms seem to have separate sexes The o™ an. lertihzed while still within the tissues of the parent and ...HLngo a portion ot their d6velopn,eat there, laterXelun^ t n,ush into a canal and so passing to the exterior a a c h .te,l free-swimming structure. The seixmentiti,,,, f . OV-.UI iu typical eases results in the fo™S ' '"abla t. 1' "iuch becomes converted into a solid ciliated steiruTalv "...ulgnition After swimndug about for a tim t e terruH loses Its ciha and settles down a c.,v,f>. ., "."'«. s-'ei ' ula tenor, which later, in forms:!, tlt ^o^^^, 'Sl^f j: ''^ '"" gives rise to these structures as a .i .^ f "''ambers, tio. with which canals :-e ■^"';:i^Zt^ '"'T' u j^astrulu results ustejul of a st^- rnL, Ti . , In addition to the sexual method most Sponges also po, <-. abilities for regenerathui. A detached portion of ,. •<, ;n under favorable conditions, regeneia!: i ' L^'' :r I>i"'lnctiim of the ommerc a Spouues In -n,,,,. f,.. •■■•■I"-;; '.. this a process of interl.al t ddi ^ ursl ';:,:" ;;;;■ ■; '-'-"KI-:'! ™"s aggregating together r,dev: 2 "■'" '■ ' oval, ciliated, sterrnladike structure which leaving •'"""■••• •■* - ' "' ■i'>..... .1.1.1. II,. ,„,:;;;;ii I I 76 INVEHTEBHATE MORPUOLOG 7. dividual cannot withstand. Thev are spherical bodies con- sisting of a mass of cells richly laden with food-matter, and enclosed in a double chitinous wall, with an opening at one point, a number of siliceous spicules, in the allied genus iLphydatia of a very characteristic form and known as amp/ii- discs, being arranged between the two layers of the wall. Ou the approach of cold weather the Sponge dies down and the gemmules thus fall to the bottom of the ponds or streams, where they remain unchanged until the approach of warmer weather, when the internal cellular mass flows out through the pore (which is closed only by a thin membrane) and de- velops into a new Spomjilla. The relationslni)s of the Sponges have long been a matter of discussion. For a long time they were regarded as plants and later as colonies of Pro- tozoa, but the discovery of sexual rei)foduction in them and of their mode of development demonstrated that they were to be considered Metazoa. At present the question as to whether they are to be associated with tlie Cnidaria among the Ccelenterates or regarded as a distinct type is still open, though the weigiit of evidence and authority is in favor of their Cuilenterate character. 8uch simple forms as Leucusolenia certainly point in that direction, and, if tlie occurrence of a sterrula formed by immigra- tion prove the typical mode of development, the embryology of the Sponges presents stages up to the formation of the ciliated chambers which are step by step comparable to what occurs in the Cnidaria. II. Subtype Cnidaria. The Cnidaria, like the S})onges, liave in their simplest forms tlip general form of a hollow cylinder open at one end and consisting of but two cellular layers, the ectoderm and endoderm, between which is interposed a tibrous or gelatinous mesoghva which may or may not contain cells. Differences from the S|)onges are found in the occurrence, exce])t in one or two forms, of a number of elongated, contractile i)r()coss('s or tentacles around the mouth of the cylinder (see Fig. H^it, and in the absence of inhalent pores upon its surface. Sucli simple forms are known as polyps, and they are usually attached organisms with little or no [)<)wer of locomotion. V large number of C^uiduria present u very different form, how- ever, Ix'ing disk- or bell-shaped, a process comparable to tlio cla})per of a beil hatiging (hnvn fnuu tin? centre and liav- TYPE CWLENTERA. 77 ii)g at its extremity the mouth-opening. This leads into u central cavity, the coelenteron, lying in tlie substance of he bell and Irom this pouches or iine canals radiate out towards the rim, where, in some cases, they are united by a circular canal which runs completely round the bell at this region. To the margin of the bell tentacles are usually attached, and sense-organs, presenting frequently consider able complexity of structure, are found in the intervals be- tween the tentacles or at their bases. These forms, known as medusae, are, as a rule, free-swimming, propelling them- selves through the water by vigorous contractions of the bell All Cuidaria, whether of the polyp or medusa form, pos- sess in their tissues peculiar elements altogether i.urem-e- .- cl t - seated in the Sponges. These are the nematocysts or so-called thread-cells. Each consists of an ow'. or spherical cyst (Fig. 38, c) with a membranous wall and Hiiid contents, the wall being pro- longed at one end into a long, exceed- ingly delicate, hollow, thread-like lila- iiK'iit sometimes furnished with sjiines at its base, and, in an undisturbed cyst, is invagiuated, and coiled up in the interior (Fig. 38, t). These nematocysts are pro- duced by and enclosed within special Fig cells known as cnidohhists {en) lying . ^ „_, principally in tlie ectoderm, and in their " ~ "enmtocyst. most highly developed form difr»n-en- 't - """'t','!'- tiuting below into a supporting stalk (.) Z Z r^'il^.^ation wliu'li rests upon the outer surface of « = Mi|,,„„ti„g process, the luosogloea. From the outer extreini- ' "^ ""■*^'''"'- ty there projects l^evon,! the surface of the ectoderm a short i.nr-likeproce^ ,.:. cnidocU(cl),^n^, iu addition, an excoe.ling- y hno process of Kn.e length («) is given oti" at the juncti,;n of tlio stalk with the cyst-co)>taiuing portion uf the cell The.se two processes a/e supposed to be sensitive, the longer"ono per- mj.s bringing the cni,lob!ast into connection with nerve-cells lying elsewiiore 'u the ectndHrtp Tm -i-m ^ eily uuderHt<.od, a stimulation, such us a touch by some foreign 38. — Nematooyst Cell of Phyaalia. »«.x. 78 IN VERTEBRA IE M0RP110L06 Y. body, produces au evagiuation of the coiled thread from the cj'st, ill the interior of wliich it has been bathed hy the fluid contents. The evagination is of suflicient force apparently to puncture the skin of many animals and so inoculate the contents of the cyst, which are of a poisonous nature, and produce inflam- iiiutory disturbances, and in minuter organisms paralysis or death. To the presence of these structures jelly-iishes owe their stinging powers, and they form eflicient weapons both for obtaining food and for warding ofl' enemies. The Cnidaria may conveniently be divided into three classes : I. Class Hydromedusce. II. " Scyphoine^ 'uslienomenoii is in some cases carried to a much greater ex- tent than in that group. In some forms special gonopolyps ■•uo i)resent from which all the medusie arise, but more fre .;uently any of the hydrauths may produce these structures, llie gonopolyps when t'ley occur are never enclosed within u gonotbeca.and hence the term (jumnoblastic, frequently applied to the polyps of this order. In the genus Hydractinia (Fig 44) a complicated division uf labor occurs. The various li.xirauths comj)osing the colony arise from a flat expansion rms which others lack ; it will be convenient, therefore, to consider an ideal form iu which the various modifications are present. Each colony (Fig. 47) consists of an axis or stolon on which the various individuals are seated and which places them in connection with each other ; it is usually long and slender, but iu some cases, as in Porpita, may he reduced to a disk. At the ex- : treniity of the stolon may be found ' a lioat or pneumatophore (Fig. 47, p\ a double-walled sac containing air in the interior, and which is to be re- yarded as a modified medusa form. Xext to it come usually several medusa fonas lacking manubrium, tentacles, imd sense-organs, and possessing a locomotor function; these are the swnii-bells or nedocalyces (n). At in- tervals along the rest of the stolon ^ -^ x are situated groups of individuals, Fig. 47.-DrAGKAM of . Si- each group covered over by one or " more scale-like structures (cs), which are again highly modified medusae, and lu each group is to be found a fropho- M'//> (;'r), a vase-like polyp form with a wide trumpet-shaped "mouth, and iiavmg near its base a single tentacle 10 which bears along one side a row of numerous secondary PHONOPHORE Colony. cs = covering scale. n = uectociilyx. P = pneuiiiatocyst. r ~ reproductive pol^'p. s = sensory polyp. ir = nutritive polyp. t = tentacle. 92 INVERTEBRATE MORPHOLOGY. branches, each richly provided with nematocysts. Associated with this nutritive individual is usually a reproductive form, which in some cases may take the form of an Anthomedusa, separating from the colony and leading a free life, as iu Velella, or may be medusoid, presenting a medusa form, but lacking a mouth and tentacles and never separating from the colony, or finally a gonopolyp (?•) may occur which bea.'s numerous much-degenerated medusoid buds. In some forms there is still another form of individual {s), resembling a tro. phopolyp, but being destitute of a mouth and having a sim- ple tentacle without the secondary branches. From its great sensibility to stimuli this is supposed to be a sensory polyp. In some forms, such as Diphyes, no pneumatophore occurs, but nectocalyces are present ; in others, as Agalma, both occur and the colonies resemble somewhat the diagrammatic form described ; while iu a third group, including the Portuguese man-of-war Caravdla, the jDueumatophore becomes largely developed and nectocalyces are wanting, the stolon at the same time being contracted to a disk lying on the lower sur- face of the pneumatophore. In Velella and Porpita the stolon is reduced to a disk, but the pneumatophore is wanting. Alternation of generations of a typical form, complicated, however, by the polymorphism, occurs in such forms as Velella, which possess a free-swimming medusa ; in the ma- jority, however, it is obscured, as in many Tiibularian hy- droids, by the greater or less degeneration of the medusa An alternation of another kind, however, occurs in some forms, the bunches of individuals separating from the stolon and leading for a time an independent existence, during which their medusoid reproductive individuals become mature. The complicated polymorphism of the Siphonopliore colonies leads to a merging of the individualities of the component individuals in that of the entire colony, a process which reaches its highest pitch in such forms iis Velella. The various polyp and medusa forms of the colony may be cnii- sidered as organ-individuals, and by their integration an individuality of a higher grade — a metamere-iudividual— is produced. Development of the Hijdromedusa^. — It has been mentioned as one of the characteristic features of the Hydromedusa? tliat the reproductive elements arise in the ectoderm. They reach TYPE C(ELENTEIiA. 9» tlieir maturity in the medusse or medusoid buds except in the Hydrarioe, in which this stage is entirely wanting, and in cer- tain Hydrocoralliuie, in which it has disappeared, and may likewise first become differentiated in the medusa. In many forms, however, in which an alternation of generations occurs they arise in the polyp, sometimes at a point far distant from ^vhere the medusa-buds will arise, and reach these structure* only after, it may be, a rather extensive series of wanderings. Thus, to take an extreme case, in Endendrium the ova arFse iu the ectoderm of the main stem of the pinuately branching colon:, a short distance below the terminal hydrauth ; as new branches are formed in this same region the young ova niigrate into them, passing through the supporting layer and Miuideriug among the endoderm cells. Later on* when the gouopolyps arise on the lateral branches the ova wander into them, still in the endoderm, and finally when the medusa- buds develop on the gonopolyps the ova continue their eudo- dermic course into them and eventually, again passing throu INVERTEBRATE MORPHOLOG Y. primitive Hydromc(lusr>,n tlio medusa form lias been regarded as a socotkI- arily specialized reproductive organ, winch in the Narcom<>dusa^ nii'l Trachymedusa3 has become so imuortant that the ancestral polyp kivm js practically suppressed in the life-history. On this view it must be sup posed that organisms so similar as the medusa; of tiie Tubulariau and Campanularian polyps hiive been developed entirely independently of uim another, a view which carries with it man; ditiicullios, aiid that tlic medu- soiu buds represent stages in their evolution. It seems more probable, however, that, leaving the Hydrariit; out of tiiu question, in all the other groups the medusa was the parent forta. Tlii,, i:, borne out by the fact that hi tlie Narcomedusa;, which, with their broad pouch-liko ext<;nsious of the gastric cavity, are the most primitive of all the craspedute medusa>, there is no lixtd polyp form, i', lias been shown, Ihav- e\ei, that the Nareomedusee andXraeliy- medusie in their development i)ass tl\rough a stage wliich may be considi'icd 'o represent the polyji form, and if, wliilu in this form, non-sexual reproduction should have taken place, the buds re- senil)llng the immature form which gave rise to them, a polyp colony would rcsull, some of the buds of which might con tinuo their development and becoiiiu medusa'. B> this view the dilliciillits presented by the similarity of the medusa throughout all the groups where ihcy occur are overcome and the medusoul bnds are regarded as imperiectiy Uivei- oped or degenerate medusa'. Furili"!- more this view is rendered more than prol.able by the development of Cuim- f< bud (Fig. 4!)i, tl.i' buds resembling the original ei bry(» which gave rise to (hem, and all tlio bud^, the parent embryo included, later develop into medusa". In t'liiiiim. liowever, tiie parent embryo whiel gives rise to buds undeif; )es no fiinliii developuiont, only the buds continuing on their course of gr>wth l(» nutlii .sii'. In this case a true and typical alternation of g •), rations occurs aii'i ]Kiiiits out a simple expla'iM.ion of the nlternalion which is found in \.v' Anlhonudiisa' and I.epiouie liisie. In these the jiolyp colonies are tlicn- «i!lt.^ of n;in-.sexual reprodiictio'i of a larval medii.sa. and some only of Ihi' j individuals s > formed continue their development ie medii.sa'. Tiie relittionships (if the Uydrocoialiina' t(» the other groups av nel vet I quite demonstrated. It woidd seem, however, from tiie uie(hisa-bud wiiiilij Fio. 49.— Brpnrxo Laiiv\ of Cnnoct.inti.il octoiuiv'a caftt-r McOrkadv. from Hudoks). a — egg larva. ub = buddeil laivte. TYPE CCELENTEUA. 97 occurs in one species of Millepora having its reproductive organs in tlie walls of tiio manubrium that the amnities of the group are with the Antho- incdiistB, and that an exceptional amount of degeneration of the medusa liad occurred in correspondence with tiie development of the calcareous conillura. The Siphonophores are evidently allied to the Anthomedusa?, judging from Mie characters of their medu3ie. The colony, however, contains both iiHMlii?a and polyp individuals, the former not being in all cases reproduc- tive as in the Anthonieduste. The emin-yology of those members of the or(i(>r which have been studie oovoring the st'iLso-orgiiii. « = sfiisn-orgaii. m - inoiiili. ig = suligciiilal cavity. vif = lucseiiteiial filaments. t = toiitacle. bell a communication between adjacent ])()U('hos is present comparable to the craspedote circular canal. This condition is, however, oidy retained in the simpler forms ; in the higlit r Hcyphomedusu* the lines or points of oblitenition nuiy 1h' omitted, and by secondary oblit(M-ations taking ])]ace ov((f')iifiil rhaniherfi (V\\: 60, s(f), above which lie the hor.seshoe-shaped reprodm-tiw organs, r, devsloppd in the c«e|ej!toric endixlerm, nrm lim!) -M «ach horseshoe lying in each of the adjacent radial pouches. TYPE COiJLENTEUA. 9^ Fi..;tlly, ill the line of each interradius there project into the n.lontonc cavity a number of coarse thread-like tilaments. i^^ mesenterua JiUuuaU. ^,./), which are unrepresented in the llydromedusat. Such is the |,.eneral structure of the Scyphon>edus,x) ; the. n...d.hcat,on.s will be better described in connection with the various ordrs <,f both eyes and otocysts. The> are usually short l.-Kor-hke stalks, lym^r i^ a notch of the rim of the bel a.ul covered over by folds (corerhu^ ploU., Fi. 51, cp) ansin« trn.u the .djucent substance of the bell on either side of ha cp- Fu.. 51.-Mai.«,nat, SKNSK-oaoAN „F Rhopalonema(nn.v lunrmo) cc = cHcnlcri,- .-nvily. ^n . eudoduu.. cp = (ovcrii.u pliilo. ^ ^ ot = otocyst. •iH-ho ami frequently uniting so that the st.lks lie in pouch- '■'- <'.''v.t,es. The ectoderm of the lln^er-like stalks contains •"•n.erouH sensory and ^.tn^dion cells, and at <.ne or more ■■.•K.O..S pigment-cells are associate.l with these to form the ;'.v;;(" . whu-h nnt^. be further perfected by the a.ldition of a >' u-nhu. lens The stalks are hollo, v. containing a prolon,u- o h. , len .r.c n.vity (cc) lined by cn.lodern, and at Uie tip nf the sta k the en.loderm-cells are lilled with crvstals of oaibonato of l.me, the whole mass of crystals formin^/a rath... i--^. ulnc^vst (o/,. The covering plates, furthermore, above tlie •SH.sory stalks are usually grooved, tin- bottom of the groove (*'• 100 INVERTEBIiATE MORPHOLOGY. being lined by sensory cells to which an olfactory function is attributed. The marginal sense-organs of the ScyphomedusiB are thus much more complicated in structure and in additiou have a different mode of origin than those of the Hydrome- dusse. 1. Order Stauromedusse. In this order some of the forms are fixed throughout their adult life, e.g. Zucernaria (Fig. 52); while others are free- swimming when mature, as Tessera. The deep bell has at the margin eight tentacles — the primary tentacles, none of which become modified into sense-organs, these structures being wanting in the group. In some species of Zucernaria Fig. 52— JJalidystua auricula divided i-ongitudinally (after H. Jamks-Cl^uk / = funnel. inf = mesenterial liliiuieuts. po = lepiodiictivo oijfans. in = iutenailial adhesion. rp = radial pouch. t = uioditied priuiury tentacle. these primary tentacles {t) are somewhat alteretl, and in all the margin of the bell is produced into eight knob-tipjuHl lobes, to each of which a bunch of secondary tentacles may be attached. The CtX'lonturoii ei'teads out io the niar^nu of the bell, and is interrupted along tlie interradii by a poiut TYPE CCELENTERA. 101 {L aa)o. I ue Z^,-n«na) of adhesion. The depression, ol the subumbrellar surface, the funnels (/), are verj deep in Lucermxria, extending almost to the summit of the bell. 2. Order Peromedusae, In this order the adult medusu3 are always free-swimmin^. and are characterized by the bell being pointed in shape and about Its middle marked with a distinct constriction The CO. lenteron IS obliterated at only four points, as in Tessera, Hud they difler from the Stauromedusje bj possessing four sense-organs, the four interradial tentacles of the i^rimury .series of eight becoming modified to form these structures, while the radial €ue;s retain their original character. 3. Order Cubomedusse. This order, of which CJiaryhdea (Fig. o3) is a typical example, is characterized by the bell being of a cubical shape. The interradial obliterations of the Cd'leuteron are linear, and, as in the pre- ceding order, four of the primary teu- taclos, these being the only ones which tlevelop, are modified to form seuse- or^'uns. In this order, however, it i^Y^o.^^.-CkaryMeamar^ tlie lour radial tentacles which form tiie •■"/;»•'«//.' u,f.erci^u8,. seuso-organs {so), the Tour interradial per- "^ = scuse-orguu. sistiiig as tentacles {t). ' = tfiiiacle. 4. Order DiscomedusBB. I., this order, which includes the maiority of the known S(Tphomedusa^ all the eight priinary ten'.., '.s are converted int.. sense-organs, a number of secondary tentacles usuallv h,U couditiou iPA,gia), without ever "lum^S t^ My.mM there i.roiect in^i ' H ? ' '"'• ''■°'" *''« mli (.ueseutede „h e L k 'Lr f "■"" ■ ^"■ "'"^"^ ll» mouth to the posterior e„,rVM f "''e''l>«l'oo'l of «ive rise to the .L^ ^uVu^"''""'- ^"" ""^^ «.p.»a,«.>proa„eeS^^^^ The Scyphostoma may develon dirn^fi,, • . ., . «»--.iuK ,„e.lu.., but in 'a uuIZofS it' 1 '^ "■ series o( transverse divisions (Fig 55 /ft T,!t ' "'"''"8°'*'' » p..osto ja j:rc„urj;.~ rs '1' ) rr '"^- ■«lo the adult Biscomedusau by theiutrv,') ."" "P""*-' «Kl.t lobes, whieh earr, the J.^^^.^'t^T h" 'levelopraeiit of tentacles, and by the irrowH, f " , ' ^ "'° .U,pica> alternation of KeneraLt^TZ^tS^atr- ;«- Of d„so„„t arose. b„.„ ..uSt , „ w7:f ' ^'■™ ""» '»- .'- "'^tJ i^iscoraeuusjt! resiiltinr- in iha. „,.i • .. "'fcauas; icspec- 104 IN VEIi TEDHA TE MOIWIIOLOG Y. Epliyra seems to represent an ancestral stage, since some matnre medusae resemble this stage very closely and it occurs in the life-history of all, ami earlier than this is the Scyphostonia representing tlie Lnccntarfa stage of evolution. The iScyphobtoma has superficial resemblance to a hydroid Fio. 55.—^, Scyphostom of Aurclia ; B, Strohila of Aurelin ; C, Ephyra of Pekujia (aU after Aqasbiz). polyp, which resemblance is almost an identity in the earlier stages of the 81'yphostoma before the development of the mesenteries and funnels. This suggests a relationsliip of the Scyphomedusae to the Hydromedusa; only through the polyp, the separation of the two classes having occurred be- fore the appearance of the medusa? on the scene. III. Class Anthozoa. The Anthozoa never assume the medusa form, but are ses- -sile, usually colony-produciug polyps of the Scyphostoina type. Typically they are cylindrical structures (Fig. 56) at- tached at one extremity, the base, and bearing at the other extremity the mouth in the centre of a flat surface, the disH, around the margins of which are a number of hollow tenta- cles. The coelenterou is imperfectly divided into a number of chambers by !ongitudi?!nl j^-urtitiring arising from the b^xlV' wall, the mesenteries (Fig. o7, me), the various iutermesenterial TYPE Caer.ENTEllA. JQ5 cliauibers commniiicatiDs fredv ivitl, „ , . i .....nth does ..ot opeu dhecti! n. , ,"■"' 'P'"'^- ^Le 1..V.I.OJ.1 poi„«. L il: tr;, el,::'?';™' "» - «- by ectoderm aud cou^muuica uTf,: , """«"'" <">> ''"«' o-leuteric space. CerZ" .L ^ ^' ! " *'"' ""^ ''''""■^' p..rtK."» «e attachedtTreUodZr "r '"."'"'■ '■"'^"'• '■"l-".n, but below it. lower ™d an J ""' "' ""* '''"""'• -^ ^- ed«e tbere r„., ,. ™::i;:L'':r re-t^it:? Fig. 56. - Metrfdinm margina- tum, Le8. Fig. 57.-DiAGRAjfMATic Traks- VEHSE SECT10i< THKOUCiU AVmrd- «« IN ItEoxoN OP StoMATOD^UM. '"ie = iiieseulery. rm = rcliuclor iiiubcle. «' = sipbonoglypiie. St = sloiiiuioiiteiiiji. I-IV = uasenteiies iu iLc order of tlitir developuifcui svslen, of coils aud iwkT T. i,^ " " •'"'"V^'^'^i'^d «U:eut sr erirrrlTidrd" T ■"^-■''-.V» "buucb of Hue l^ei.« l»otr„deVfZ' bet urh":";!?'^ T" """"^'■' "^ l'«lywall. Tbe upper part rH»^°l x"*''' P"''"" '" "'« fe'«'t structure l,T' ' "'efilameut is usually of dif- h'i«tea e Is "L'e ful"" T'T'^ " ■"""• "' «'™8«'«I l«- fl»ij. iu ibe cr,eu.:,t " '° "'"'""^ " "■•'•''■■"^«™ "f '"' -P'oductive cells develop iu the . ,deru, of tba 106 INVERTEBRA TE MOBPHOLOQ T. mesenteries, whence they are shed into the intermesenterial chamber and make their exit by the mouth. The nervous system is well developed especially in the ectoderm of the disk and tentacles, though it also occurs in the endoderm. It possesses the general character of the Coelenterate nervous tissue consisting of sensory cells, nerve-fibres, and gangliou- cells. The muscular system is very well developed both iu the ectoderm and endoderm, the muscle-fibres being generally longitudinal in the former layer, and in the latter circular ia their direction. At certain regions of the body the muscle- fibres are especially abundantly developed, the mesogloea being thrown into complicated folds for their support, so that it is possible to distinguish certain definite muscles. One of these is developed upon one face of each mesentery, and, its fibres being directed longitudinally, it forms a strong retractor (Fig. 57, rm) for the disk and tentacles ; a second is developed in the endoderm of the body-wall a short distance below its junction with the disk, and its fibres may, by the growth of the mesoglcea around them, become imbedded in that layer ; it forms a more or less powerful sphirwte?', serving to cover iu the disk and tentacles when these have been retracted by the mesenterial retractors. The Anthozoa are constructed upon a radial symmetry, as are the other Coelentera, this symmetry appearing in the arrangement of the mesenteries and tentacles and in the cylin- drical form of tho body. Nevertheless it is always possible to divide the Anthozoan by a single plane into two simihar halves, that is, a bilateral symmetry is also present which is produced by the arrangement of the retractor muscles on only one face of each mesentery and by the flattening of the sto- matodffium. This latter feature is furthermore usually made more pronounced by the occurrence, at one or both ends of the longer transverse axis of the stomatodseum, of a distinct groove lined by high columnar cells with long cilia, these grooves forming the siphonoglyphes (Fig. 57, si), and by the mesenteries which are attached to the stomatodteum in the neighborhood of the siphonoglyphes usually having their retractor muscles on different faces from those on whicli they occur iu the other mesenteries. Fig. 58.— DiAGii. TTPB CWLENTMBA. jqj Frequeatlj the ectoderm of the autl.o;,oau polyns secr.t„ ». sle etal substauce wl.ich may either be carbo.ateof me o el,e au orgauic substauce of a horuy eo.dstency iTib' corals the secrohou takes the fonu of earbo.ate rf lime ' d io.n,s a cup (F.g. 58. ap) in ,vhich the polyp is sealenj;:' Fro. Sa-DiAGUAM OP THE Stbuctcke ok a Coiul (after voK KOCH, from u.a, «;? = exotlieca. ;,„ fP = basal plau,. t I ^"J""^- Calcareous skeleton, wLi.e; ectoderm, sbaded, mcsog,„„, black; e.doderm dotted. ' (sepfa H over which the soft tissues of the uuimal are Moulded, projecting np fro„> the botton, of the c, p Tl e septa may be united by delicate tauKential bars, sJ,ptic2 a« from the botton. of the cup a somewhat cyl nS o S: * may project, other upright rods. the^„?t, inter en mb t»eeu the free edges of the septa and the columella The „ ; ' / <=f l'^"«o'>« enp and n,ay produce ridges, co.I.e 0" ts outer surface corresponding in position with Vhe sen a ami „,as,„„ch as the cup is continually increasing in eptl lo V^Z he'T ■'■" '""■""'' "'^ >'"'-n' -'-voccupis theC 1.0 ton. he lower part m, ay from ti„,e to time be separated oa liy a transverse partition or dissepiment Ii. other forms, such as the Alcyouarians, however the skeleton .s secreted only by the basal ectoderm and thTcolon; 108 iiV VKRTEliHA TE MOlWUOWa Y. becomes moukletl over it (Fig. 69), so that it forms a ceutml lioruy or more or less calcitied axial support, aud iu udilition the mesoglcjoal cells secrete scatten d particles of carbonate of lime, liaviiig a more or less detiuite form for each spe- cies, but uot uuitiug together to form a firm skeleton. The development of the Autho^ioa is always direct, aud the diploblastic Fig. 59. — DiAGitAM op condition is produced by delamina- Young Gohoonian (af- tion of the cells of the blastula. Muuv ter VON Kocu). i li. r ii adult lorms possess the power oi division, either transverse or longitudinal, the latter giving rise to complex colonies in many cases. In other forms the primary polyp may develop a stolon from which other imli- viduals may bud, producing a diffuse colony, or the intervals between the individuals may be filled up by a growth of mesogloea traversed by a network of canals, forming a tissue, the coenenchyme (Fig. 60), in which the various individuals are imbedded. The class Anthozoa may be divided into a number of orders, whose existence depends mainly on the arrangement of the mesenteries. 1. Order Alcyonarise. The majority of the Alcyonarians produce colonies by bud- ding. In some the individuals are scattered on stolons, iu others imbedded in a coenenchyme {Alcijonium, Fig. 60), or in others united to form flesh}' colonies of a feather or renifi^rii' shape {Renilla), the whole being imbedded in the sand by ,. fleshy stalk. In some of the groups a horny or calcareous skeleton is present in addition to the calcareous spicules im- bedded iu the mesoglcea and may form a central axis enclosed by the coenenchyme (Fig. 59) and of a horny consistency, as in Gorgonia ari(» Lep^ogorgia, or more or less calcareous, as in Isis aud Cv..''ta?\(:>?, tlie skeleton of the latter constituting the red coral of commerce. In the Organ-pipe Coral, Tuhipam, each individual lives in a calcareous tube, the various tulies TYPE CCRLENTBRA. 109 .oiug united by transverse plates, and in Helio^nyra the skeleton l^ocomes very massive, resemblir -^ tlmt c.f the ..rdinarv corals even to the occurrence of septa projecting into the interior of tlie cups which contain the individual polyps xNotvvithstanding these manifold variations of the skeleton H.ul of the colony form, the individual polyps present through- out a great similarity of structure. They possess only eight pinnate tentacles and eight mesenteries whose retractor mus d.s are arranged in the manner shown in the annexed dia- grammatic cross-section of a polyp (Fig. 61). lu li^^Ma and rm- Pia 6O.-D1AGKAM <,K Y<,t;NG F,o.61.-DuorammaticTkan8vehbe v.?N K^L**^ ^^cyo»*MWJ (after Section of an Alcyonauian. ^"« = retractor muscle. »i = siphouoglyphe. I-IV = mesenteries. allied forms, such as Pmnatula, a slight polymorphism occurs certain polyps possessing no tentacles and functioning as n^/./m^ «oozc^. through which currents of water pass into the cclenteric cavities of the colony through which they circulate. 2. Order Edwardsise. The Edwardsitie never produce nnlr.r.iVc j i, a skeleton, tl.ongh frequently 1 ee"^Wo7«,:'^7'^"^'' c-nsted by foreign particles. Thev I ve ",„'.' i , T/.™- M'l, the base beinf> rouofled ^rl Zt T ' ^^^^^"'^ » eight (sometimes si^teeTor tbi tv r\ • 'T'' """^ ""^^^'^ oi.Tl,tm„-,. I • ' tliirtr-twol simple tentaole= „„j o.„ht mcontenes, a,fleri«g from those of the AIoyonaria.s i^ 110 INVERTEBRATE MORPHOLOGY. the ari>.ugement of tho retractor muscles as shown in Figuro 67. 3. Order Ceriantheae. The Ceriauthoro are, like the Edwardsi.T, solitary forms destitute ci v. skeletou, uiul live imbedded iu saiu! or mud. Tho basal region is roundod and not adhesive, having at the centn? a pore which conimuniea 3s with tho eoeleuteron. Jji Ceriivnthun a tibrous investment surrounds the body ay a tabc, secreted by the ectoderm, this layer of the body being further characterized by an enormous development of muscle-tibres arranged longitudinaiij and supported upou slender processes of the mesoglwa of the body- wall. The tentacles are simple and very numerous, being arranged in two setS; one surround- ing th{» margin of the funnel- shaped disk and the other im- mediately surrounding the mouth. The mesenteries are also very numerous and are distinguished by the absence iu the adult of '-e tractor muscles, the ectodermal muscles playing the part of the retractors, and the tdiaracteristic Antln)Z()iin sphincter is also absent. The arrangement of the meyanteries (Fig. 02) is peculiar to the group, new ones continuing to form during the entire life of the animal and making their appearance one on each side VEU^K Skition ok a YotiNO Ceri- anthus (according to Caiu-ohkn). Hi = slphonofflyplic. I-IV = tlie Edwftrdsian inosciiteries. 1-4 = si'coiidiiry incscntcrios. of the sagittal plane between the two which immediately preceded them. The older mes- ontories are thus cowdod to one surface of the body, tlie dors.d surface, at which the single sii)honogiy})he {si) occurs in the sfomatodaMim, and the four on either side of flie mid-dorsal line (/-/F'> are the homologues of the ei^lit mosonteries of the Edwardsiir, tiie rest being secondary structures not represented in that group. TYPE V(ELENTERA. Ill 4. Order Aniipathariae. The members of this order are jill colonial aiul secrete a hnuichiiig axial skeletou of u black horuy material. The lH.lji)s possess usually but six simple tentacles, and as a rule only six mesenteries are ])resent, of which only the two lyin^ ill the transverse axis bear reproductive organs and mesen'^ t.^rial filaments ; in some forms four or six additional imper- t(!ctly-developed mesenteries are present, but six seems to be the number tyjjical for the group. 5. Order BrotactinisB. This order includes a group of forms, all sim^ le and with smiple tentacles, but showing ccmsiderable vari tion in the iminber of the tentacles. They all agree in this particular, however, that there are twelve mesenteries arranged in pairs (Fig. 03, I-VI\ the two pairs attached to the si- l)honoglyi)he region of the stoma- todioum having their retractor muscles on the faces turned away I'lom each (jther, while in the other four pairs they are on adjacent laces. The two former pairs are termed the dirertive imsc7ih;rie^ p,„ n^ y. ^ ^ 11' ig. 63, D and I)'), their constit- vkhsk Skctk.n <,k aonactinia. uent mesenteries lying one on each A // = diieciiv.- moseiiifri.-s. >iide of the sagittal ])lano, and to- ^'^^ = *''*^^ Kdwunlsiiin meseu- ^vther with one mesenterv (//and v vr ^*"'''"'" : , 1) iinm each of the other pairs imi.s wi,i. //nnd / represent Mie eight Edwardsian 7 = .secoiidaiy i.uir ..f mcs- mesenteries. To these six primary <'nieik',s. pairs a varying number is added in the difierent forms- it jnay be, on each si.le, one between one of the j.airs of dir.-c. ives an.l the a.ljacent lateral pair {Sr,jtn,>hnrm), or a pair in tl.. Han.e locality (r/omW/„m, Fig. (53, 7), or two pairs l.ne of whirl, corrosiKinds to the pair of (ionardma, the second pair lying between the two lateral primary pairs {Ormii.s). 112 INVERTEBRATE MORPHOLOGY. In all these forms there is a strictly bilateral arrangement of the mesenteries, and a tendencj' for them to arrange them- selves in pairs. 6. Order ZoanthesB. The Zoantliea3 form very frequently colonial aggregates either of a diffuse stoloniferous character (Zoanthus) or of a more compact form, the individuals being imbedded in a ca'- nenchyme (Palyfhoa). No skeleton is present, though many forms have a dense crust on the outside of the body formed of particles of sand, sponge-spicules, radiolarian and fora- miuiferan shells, etc., imbedded in the outer portion of the meso- ghiea. They possess a varying number of simple tentacles, and 1 there is only a single siphonn. glyphe which marks the ventral surface of the body. The mes- enteries are arranged in pairs, six of which (Fig. G4, /- F, //- V/, ///and IV) correspond with the six i)rimarv pairs of tlie Protac- tinifo ; of thesn the dorsal direc- Fiu. 04.— Di\«;uAMMATic TuANR- tives (/>) are never united to the vKusK Skction of Zoanihus. .stomatodfPum and the dorsal J), I) - directive nic'senterios. ij i ■ / rr jr rs • . e T ,,, . ,, . lateral itair (//, I I) consists of 1-4 = i)iiir,s of secundiiry nicseu- ""® perfect and one imperfect teries. mesentery, th«i latter being ventral to the former. The ventral lat- eral primary pair may consist of two perfect mesenteries or may have the same arrangement as tlie dorsal lateral |)air. To these six ])airs a varying number of s on. Iu a few forms, owing to the precocious development <>t one or two of tlus secondary pairs on each side, the symmetry becomes converted from an h«"xamerous one to an octamerous (Aiptasia annulata) or a decamerous one (Tealia). — u..^ .!..|! m tonutritiwu witxi ine spaces Ix'twoen the mesenteries, they are arranged iu cycles cone- -Sf*')ll8S«*4P'#t*aHBIBB 114 IN VEHTEBUA TE MORPUOLO G Y. spondiug to the mesenteries. Usually but a single tentacle communicates with each space, but in some forms a series may arise on the roof of each sjjace so that the tentacles have a radiating arrangement {Discosoma) or may appear to be irregularly scattered, as in some corals (Fwigia). They are usually simple in form, though they may be in some cases pinnate {Phymanthus, Thalassianthus) or even branched. The order is usually divided into two suborders : 1. Suborder Malacodermata. This includes the Sea-anemones or Actinians, all simple forms, not producing colonies, and usually attached by au adhesive base. They never form a skeleton of any kind, though they may develo^i an enveloping cuticle, usually very thin and in some cases encrusted with foreign matter ; this is more especially the case with deep-water forms, the shallow- water forms, such as 3Ietridiinti, Bunodcs, etc., lacking a cuticle. Many forms possess the power of division, the in- dividuals so produced separating completely and not formiiij^ colonies ; furthermore some forms reproduce nou-sexually by separating oil' portions of the tissue at the margin of the base, each portion eventually developing into an adult Actinian. 2. Suborder Sdvrodennnta. This suborder includes the ordinary corals, which secrete a calcareous skeleton of the character already described (p. 107). A few forms are sinii)le, but the majority produce complex colonies by longitudinal division and by buddiu^s while iu others the division is only carried to the extent of the formation of an individual with a number of mouths, as in Fujigia and Manicina. In most of the forms the corallum is tolerably dense and may be either brandling, as in (h'ul!7ia^/,a,,-„,._CoIonial forms with axial horny support ■ 6. Order Protactinic,.-^un^\^ forms with twelve primary mosenter- los arranged in pairs, and in addition one unpaired'me^^t; Omc^f! ' '' '' ""' '' '^" ^'""''^- '^^•^^^^^/'-•"•^•. Go„actinil '6. Oi^er Zoanmec..-H\nn.h or cobnial forms with twelve primary niosent.nes arranged in pairs, and in addition a varying n, „- ber ot secondary pairs developed on each side of the primary pair occupying the n.id-ventral line. Z,>antU„s, Palytkoa 7. Older Hem,tunw.-^■nx^^A,, or colonial forms with twelve nri- Zirof'"''""/'"'"""''' '" ^"""' "^'^ '" '''^"'''-^ - variable umbel of secondary pairs arranged in cycles, the newer pairs d veloping in the intervals between the pairs already pre et An external calcareous skeleton present in many forms (a) Without calcareous skeleton {Malacodennnta) Hal- /M xvT'^'": ^'^'^"•"■«' ^^«^'«' Metridium, Bunodes. (6) W ith a calcanM,us skeleton merodennata). Madrepora, J^unyia, Manicina, Mwandrina, Astraugia 118 INVEKTEBHATE MOIWHOLOQT. Leip- LITERATURE. A. PORIFEUA. Vosmaer. Porifera. Bronn's Klassen und Ordnungen des Thierreichs, zig u. Heidelberg, 1887. E. Haeckel. Die Kalkschwamme. Berlin, 1872. F. E. Schuize, UntersuchuiKjen iiber Ban und Entwicklimg der Spongien. Zeitscbr. fl\r vviasenscb. Zoologie, xxv-xxxv. 1876-81. A. Dendy. Obnerxations on the titracture and Classification of the Calcarea heteroca'ld. Quarterly Journal of Microsc. Science, xxxv. 1893. B. von Lendenfeld. A Monograph of the Horny Sponges. London, 1889. r. E. Schuize. lieport on the llexactinellidce. Reports on tbe Scientific Results of tbe Voyage of H.M.S. Cballenger. Zoology, xxr. 1887. G. von Koch. iMonogr. x\ A. von Kdlliker, VII and VIII 6. Brook. Itcpo tbe Voyage A Andres. Le . IV. 1883. 0. & R. Hertwig. isebe Zeitscli J, P. McMurrich. 1891. B. Cnidakia. GENERAL. L. Agassiz Ccntribvtions to the Natural History of the United States. Vols. Ill and IV. Boston, 1860-62. A. Agassiz. North American Acaleplm. Illustr. Catalogue of tbe Museum of Comp. Zoology, ii. Cambridge, Mass., 1865. E. MetschnikofiF. Embryologische Studien an Medusen. Wien, 1886. 0. & R. Hertwig. Das Nervensystem und die Sinnesorgane der Medusen Leipzig, 1878. Ray Society. HYDUOMEDUS/E. G. J. AUman. A Monograph of the Oymndblastie Hydrcids. London, 1871-72. E. Haeckel. System der Medusen. I. Graspedoten. Jena, 1879. H. N. Moseley. Report on certain Hydroid, Alcyonarian, and Madreporarian Corals. Reports of tbe Scientific Results of tbe Voyage of H.M.S. Chal- lenger. Zoology, II. 1881. W. K. Brooks. The Life-history of the Hydromedusm. Memoirs Boston Soc. Nat. Hist., III. 1886. A. Weismann. Die Entsteliiuig der Sexualzellen bei den Hydromedusen. Jena 1883. H. V. Wilson. ral Stages. IV. 1886. E. Haeckel. lieport on the Siphonophorw. Reports of tbe Scientific Results of tbe Voyage of H.M.S. Cballenger. Zoology, xxviii. 1889. A. Agassiz. The Porpitidw and VdcllidiB of tlie Gulf Stream. Memoirs Mu- seum of Coinp. Zoology, viii. 1883. The Structure of Cunoctantha octonaria in the Adult and Lar- Studies from tbe Biol. Laboratory Johns Hopkins University, SCYPnOMEDUS^. S. Haeckel. System der Medusen. II. Acraspeda. Jena, 1880. TYPE C(ELENTEliA. 111> ANTnOZOA. G. von Koch. Lie Oorgoniden. Fauna u Flor,. a « ,/ iMonr.gr. xv. 1887. ^'*''^* <^«s Golfes von Neapel A. von KOlliker. Die PennatuUden. AbLandl '^Pn.i. , VII and viir. 1872. ^'^iiandl. feenckenbiirg. Nat. QesellscL., 6. Brook 7?.^;c;;-<,,,, ^/^c ^«<*>«%-^ .«.«•..., Jena- .P.^M^MurriC. .'/. ,V.,.,.., .,,,, ^,,_. Journal of Morpbolog,. ., lao iJV VEliTKBUA TK MOIWUOLOU Y. CHAPTER VI. TllK crrKNOlMlOKA. Thk group of forms known ns ilio Ctonopliora, to wliicli the sjHtoniatic vnluo of ji rliiss iniiy In; }j;iv(Ui, j)r('8('nt no littlo gonoral restMuhliuuH^ to tlui (-(MltMitiM-a., but ut tlio siinu) tiiiio ili'part so wiilely in struoturul juul liistologieul chjiructcrs fnun tlui (hiicliiria and Porifora that it hcumiis a(lviHaI»lt>, until further cvidouco is forthconiinj^, to consider them as a grou}) a})art. Ail the Ctenophoros are pela}j;ic and are of groat trans]i;i- renoy and delicacy, due to the nature of the mesoglaial tis.siu!. mo Fig. QCy.—Iioltna hyihxtina (after Cih'nV cp =■ ciliatod pliUi's. tc = fuiinel-ciuml. g = stomach or t'uunel. I = lobe. These plate; arranged one al arising from ect ional rows of th each row a d(di towards the abo "»»* as tliey a] reduced to four il 'Inme-shaped to^^!thor at the c wicldsed is Hom transverse axis ai riK cTKNimmiu. 121 ../™.), <.tl,„rH I„„„K ,il,l,„„-]il<.. as «.,,/„,„, t|,„ V„„„.s- .-i,.,)!,. «v,u„„,t,.y ,.,.0 s,.,.„ in tho ,.i«|,t l.,„Kit,„l,„.l I | f . ' H|'to« c,, wh„.l, »„„„ ,„ |„„,„„„t„, „ I ' ; - " ■> t... l..u-kK-n.„l by tho „„„■„ ,„",„„„„,.,„1 , t :" I i.o »t,„„.Kl„„„,, ,„ „,u„„„,, i„ ,„„^ M, „ s ■<■' '■■■»it.v (I/) n, a pla,,., at n'Kht anf.|„s l„ this tiv„ t .„ I ;■'; ""'^'' -,'" "[ " .V. iyi..« ;:,»„ h, thir;"t :',:::," l"> HC,».nl,„(,b- t" .•.■«w,i.„ a sa„ittai , la,„. Ih t „ .T '"". "■■■- •■";!' ■' '™-e..s„ ,,h.„„:that „!■ t «a . v " Tlu! nu.uth lie.s .-it t],o oxtr«>initv of the vfufi....! • i • , r ;'"'7";" '"";'"-'•« ■•- '"-■",.,• li ,, : :• ;i,: : ' '';"'""' »"".'•>''-"■". «hh.|, is Halt,., ,,a.alh.l t„ th, v it l" ;' "■ «''»"■- -"'.V (,/, ...■ s„.,.ali,„l "f„ „, .1 " , ,, : i, us„any tw., „,„.„i„„s ; two oth..,. pass iw.nval pa r i »;n, the l„.„a,l surface of th,. sto no,h,.,„„ („ . . " l'l""ll.>-; whih, th„ „th,„- t,vo (Fi,, ,;7 „. ' ; . ■"' '""' f t lo I r '■'"'"■"■'T """ '■' "■" 'y -'• "'"• •■" 'I'-' i« 1 vi ,^ ti ;■ ' , ■' ''"'■""" ''"'■■"■" "-■'■ ton„i„ati,„ "'""*',•',' ''™'"l"-«. ""< .ach si,l„. These hrujohes »">' 'I'vule a,„l Kive nse each to t,vo ca„als wh cl , u ;i::t;,::;';^:::X:"' -"'" - "-.i-n,::;,; hZ These i-lates, whi,.h coastitnte the loc,„„„tor or.-an, are ...n,n.e,l „uo above the other and are con.pose.l of fn^se , C' a "ns.nf! fro„, ecto,h,r,„al thicke (,». There are ei, U -1. row a ,lel,cate groove line.l with ciiiate,! ce'l e2 ' "••"■(Is the aboral pole, each groove nuMin^ with au a,H ace "«■> lis tlioy approach the pole so tl,.,t tl, "'>>«'' leduceato four (Fi., C7 c«1 T, ' """''""' '" 111 l,ii„. ij(, est), lliese pass m upon the floor of I me-shaped cavity euelose.l by fuse.l ei ia which ^r ludsccl IS Homuwhiit brojuler in the Hauii^-A fl,..n .• xi ".-verse axis and coutaius the aboral seultl^ltl 'The'al':: 122 INVKRVKBllA TE MOUrilOLOG Y. of the cavity is foinietl of liigh ciliated cells probably uervous in fuuctiou, and above tlieru is a mass of otoliths suppoiicd on fonr incurved rods of fused cilia, one of which forms tlie termination of each of the four meridional grooves. In addition to this sense-organ, \vhi!en(erates and the Turbellarians, and h -nneeUon w.th tins idea two aberrant fonns n.ay be brieHv described ^n.Uemi^,„a, .s a flattened forn., on the n.i.ldle of whose chu'sal s..rfaee ..s he ot,,l.ta sac, and at a s ,.rt distance from this are eight short rows of ' ilM-pI.i es each u, a slight d..p,,>ssion. Two tentacles lie in the transvers,» ;ix:s, and the month .s sitnateu.,phores. AuuHig these nuty be n.entione.l .' structure and p..s,ti..u of the sense-organ, the structure an.l position ..f - mes..gl,eal musele-libn.s, the structure of (he t.M.tacles, the ptvsence of tlH' a.lh.'s.ve cells which cannot possibly bo homologized with nemafM-vst- Hls. an.l finally the early .litr..n.ntiati..n in flu, on.bryo of cel|«, rese.nbiing 0 uu.s.,.lerm-eells of (riploblasti,.animals. which giv.- rise to the muscles of III!' tentacles and ixTJuips to some .if (h.^ !!iivi,'tt.-{,y=,| -.■ ; , t_ It seoms not improbal.l,. that fhe HUlnitieH orihJ'cfon.mhores woul.l b.. 'Mniv accurately indi,.a(ed i.i the cla.ssiflcafi..n iffhoy wore onf irely removed 126 IN VERTEBRA TE MORPIIOLOO T. from the Coeleiitera and associated with the TurbeUaria, being regarded as highly modified forms, adapted for pehigic life, descended from Tur- bellariau ancestors. The evidence which lias been brought forward in favor of a relationship of the TurbeUaria to the Ccelentera througii iht; Ctenophores would support this view as well as that it was intended Ut support, and to this may be added the fact that while the peculiar adhesive <;ells of the Ctenophores cannot be homologized with any of the histological •elements of the Cnidaria, they may readily have been evolved from (ho adhesive cells which occur in the ectoderm of many Turbellarians. SUBKINGDOM METAZOA. Class Ctenophora.— Pelagic organisms provided with eight meridional rows of plates formed by the fusion of cilia. 1. Order Ten.tamlata.—V,\{t\m\)\w\-A provided with tentacles. (a) Without lobes ; more or less oval in shape. Pleurohrachia, Mertensia. {b) Lateral lobes occurring at oral pole. Bolina, Mnemiopsis. (c) Kibbon-like form. Cestum. 2. Order EKrystomiiv.—yiWXumi tentacles ; stomodajum wide aud bell-like, lie roe, Myia. LITERATURE. K. Hertwig. Ueher den linu der Cteiiiphoreu. Jenaisolie Zeitschr., xiv. 1880. C. Chun. Die Cteiiophoren des (io/fes von Neapcl. Fauua uud Flora des Golfes vou Neapel. Monogr. i. 1H80. TYFE PLATYUELMINTHE8. 127 CHAPTER VII. TYPE PLATYHELMINTHES. The Platjlielmiuths constitute a group wliicli, though preseuting a much liigher grade of orgauizatiou thau the (.Jd'leiitera, nevertheless show certain general structural similarities to the representatives of that type. Thus upon the exterior of the body there is a thin ectoderm (Fig. 09, ^ b'm vd n Fid. OO.-DiAOHAMMATic TuANHVKtiRR Srctionh Tnnomn vARiors TimnEi • i-AiMA A, m Aci-lan ; li, a„ AllcioccHan ; C, a Hl.abdocu'lun ; D, a 1 riclud. bm - hasonu'iit inembianc o = nvarv. rf = iulL'Sli.lC. od = OVidilil. ec = (.ct.ult.rm. ^ ^ pairnchvine m = niusclclayer. t = tentis " = '"^^••^^' V = vitdlarimu. vd = vns defert'iis. />. re), below which is a fxrscneuf-mi'mhranr (hni) sometimes till.., strnctureiesH, and destitute of cells, sometimes thicker H...1 enclosing branduMl cells, and strictlv comim.able in iha uu'so^hvn ,>f the (;..>lenterates. AVitl.in the basement-men- hiMi.o there is a compact mass .»f tissue surrounding, i„ the 128 IN VERTEBRA TE MORPllOLOG Y. majority of furius, a cavity, the eiiteroii {d), the cells liiiiiij. the walls of this beiug dirterentiated into a cli<,a^stive epithe- liuni ov eiulodenii. The space between the euteron and tluj basenient-inenibrane is occupied by the mesodenn, consistiiifr l)eii])lieially of compact layers of circular and loiigitudi-^ I muscle-libres {m), whih> below these it forms a mass of nu- cleated cells, usually vacuolated so as to resemble a network of libres enclosinj-- spaces and constituting,^ the pumichi/nia (p). It is traversed by dorso-ventral muscle-libres and iuis im- bedded in it various or<,'ans most of which are further dif- ferentiations of this middle germ-layer. These two layers, the emloderm and mesoderm, are together comi)arable\vith' the inner layer of the Cielenterates, the mes-endodern), and when the enteron exists it communicates with the exterior, as in that f^roup, by a sin«,de oj)(>nino-, the mouth, the Nemer- teans only, the most hi<>hly or«>ani/ed class of the Platvhel- minths, possessin<,f a second opening, the (nin.H. These homologies are, however, associated with a com- plexity of organization unrepresented in the Co'lenterates. The Platyhelminths all present a tyj.ical bilaterality of form, and show furthermore a well-marked antero-posterior as \\v\l as, in most cases, a dorso-ventral dittenrntiation. The body is usually riatteued and more or l(>ss vermiform, whence the mune of the group, and is adai)ted to a creeping habit, certain j)ara- Hitic forms, and some Nemerteans which live buried in sand, being the only forms not ])resenting such a mode of life. The greatest contrast to what occurs in the CrHte» however, is presented by the development of compact organs. The nervous system is no longer an altogether diffuse tissue, scattered in a thin layer throughout the body, but a large uumber of ganglion-cells are aggivg.-ited into a "compact mass, the brain, embedded in the mesochMin j)arencliyma near tlie anterior end of the body, and fr.)m this there pass backwartis two or more longitudinal cords of nerve-libres which give iA\ branches extending to all parts of the body and forming a network below the basement-membrane from which the \iv.- rii)hera,l muscles deiiv(* their nerve-supply. In some cases nerves hav(> been observ(>d to pass fn.in this network throu'Mj the basementmembraue to come into conuectiou apparently TYPE PLATYUELMINTUES. 13» with nerve-cells \y\\i<^ between the inner ends of the ectoderm- cells us well as with sensory cells reseniblinfr in general form those ulreudy described us occurrinj,' in the Cniduria. It nuist not be understood, however, that the gan<,'lion-cells are limited ill their ls there is a ciliated depression on each side of the head richly^ supplied with nerves forming what has been considered an olfactory or-aii 1. Older Accela. The Accxila form a group of lowly-organized Turbellaria exclusively marine in habitat and leading an active and free existence. They all possess a mouth (Fig. 71, m) situated (,n the ventral surface and leading into a short pharynx, though in some forms this may be absent; but beyond this there ""is no trace of a digestive tract, the food passing from tlu> pharynx into the parenchyma [p), where it is digested. Ow- ing to the lack of a digestive tract these forms are strictly tNvo-layered (Fig. m,A), only the ectoderm and mes-endoderni being represented, and consequently are exceedingly interest- ing as indicating the manner in which the ditlerentiation Of the triploblastic condition has been derived from the dipio- blastic. The nervous system has been described in Convohda as consisting of a bilobed ganglion surrounding the otocyst, ami in front of this and united to it by commissures is a second pair of ganglia. From the anterior ganglia there arise by a common stem two nerves on each side which pass backwards one along the edge of the btxly and the other a little internal to It, while the posterior ganglionic mass gives rise to two nerves which pass backwards, one on each side of the median line. All six nerves send ott" numerous transverse branches which unite to form with the nerve-cords a square-meshed network. In addition to the single otocvst (Fig. 71, of) two pigment-spots lying in the ectoderm and' representing li.rht- percipient organs (e) are present, as well as a ])ecnliar refrac- tive highly-movable organ, lying in the median line on the T YPE PLA TYlIELMINTima. 133 anterior margin of the body, which is supposed to be tactile lU luuctioii. No excretory apparatus has as yet beeu described for the Acuila, but a reproductive system with some iuterestiug peculiarities occurs. The male apjiaratus consists of numerous spherical testes (/) whose ducts unite to two vasa deferentia, dilating bek)\v to form the seminal vesicles (y.v) and uniting in the mus- cuhir intromittent organ. The female organ is, however, relatively simple, consisting of two club-shaped ovaries (oy) whose short oviducts open almost directly to the exterior near the pos- terior end of the body by a pore (^ 9 ) common to both male and female ap- paratus. There is no vitellarium, no shell-gland, no seminal receptacle, and no special uterus, a state of affairs indicating great simplicity of structure compared witli what is found in the other orders. 2. Order AUoi.ocoela. The members of this order are marine with the single exception of Plmjiostoma lemani, which is found in the deep waters of the Swiss lakes. They present a distinct advance upon the Acoela m that a well-deiined diges- tive' tract is present (Fig. 09, n)^i\i& interval between it and the peripheral ^ ^ = >ei>i<)diKiivo oiitice. ".usculature being completely filled up by the usual paren- chyma and the organs in.bedded in it. These forms are then tnploblastic, possessing well-defined ectoderm, mesoderm, and eiidoderm, a condition found m all tl.ie higher orders The mouth varies somewhat in position, Iving either near Fig. 71.— Diaoham of an Ac(KI,(»»:^^ Tl'KHELLAKTAN (uftf r VON Gbakk). e = i.'ye. til = mouth. Ot =. OtOCViit. ov = ovaiy. P = parenchyma. t = testis. V8 = vcsicul.i seminalis. I § c I 5 134 INVERTEBRATE MORPHOLOGY. the anterior or the posterior eud of the body, and opens into a pharyngeal pouch, whose walls are thickened by muscle-fibres in such a way as to form a somewhat bulbous mass sharply marked off from the parenchyma which surrounds it. In Monotus, however, the pharynx is more developed, projecting as a strong circular fold into the pharyngeal pouch and form*"- ing what is termed a plicated pharynx. This at its imier extremity communicates with the sac-like intestine, usually quite simple but occasionally somewhat pouched, and ter- minating, as in all the Turbellaria, blindly. The nervous system consists of a bilobed ganglionic brain- mass from which pass backwards two nerve-cords Avhich may {Monotus) or may not present transverse anastomosiu^^^ branches, and in addition a number of smaller branches pass forward to be distributed to the anterior end of the body. Eyes, consisting of pigment-spots seated upon the brain, are frequently present, and in 3Ionotus an otocyst is fouad, while l^'teral ciliated depressions on each side of the head occur in Pla(jL''*oma. The excretory system is present, but presents no notable departures from the typical arrangement. As regards the reproductive organs, the testes resemble those of the Acoelji, but the ovaries are comparatively small and the separate vitellaria are large and sometimes branched, opening into a cavity, the genital atrium, common to them, the oviducts and the intromittent organ, and communicating with the ex- terior by a single median pore situated near the posterior end of the body. In a few forms the vitellaria are not differen- tiated from the ovaries, presenting a condition similar to that found in the Accela. 3. Order Rhabdocoela. The Rhabdocoela are found both in fresh and salt water and are usually small. They possess a distinct tubular digestive tract (Fig. 69, C, d) without lateral pouches or branches, but the principal characteristic lies in the presence in the paren- chyma of large spaces resembling the coelomic cavities of higher typos, a feature not repeated in any other Turbellariiu TYPE PLATYHELMTNTHE8. 135 Tlio moutli is situated at various regions of the body iu diftereut forni.s, being anterior in Microstorm, while in Me^o- stoma (lig. 72) it is situated at the middle of the ventral sur- lace. The walls of the pharyn- — geal pouch {ph) n,ay be quite simple, as in the Acrela which possess a pharynx, or may pre- sent a muscular thickening form- iiJg a bulbous pharynx, but no further complexity occurs, aJ- tliough in certain forms, such as Prorhyiiohus, the pharynx is capa- ble of being protruded from the mouth, acting probably as j, delicate tactile organ. The nervous system (w) is essentially similar to that of the Alloiocoela; two or four eyes {oc) frequently occur, though otocysts Jiie wanting, while the ciliated depressions on the side of the head supposed co be olfactory in fuucHon occur in Microstoma and riorhynchus and allied forms. The excretory system consists ^^ occasionally of a single nephri ^"*-'''^~^ ^«abdocngitudiiiHl iiephridia open on tlio iloraal sur- face of the btxly by numerous pores, which iu Gimda corre- spond in number with tlie iutestinni diverticuhi and nerv(!- eommissures. The re})roduc'tive apparatus consists of nu- merous testes (Fig. 7;5, /), as iu the Aco'la (arranged uK^tameii- cally in (iinxfa), whose ibiets unit(> to vasa defereutia (rjit'- like expansions. Compared with the members of the other orders they may be said to be as a rule large, though few reach the length which has been mentioned for sonu' IuihI Trichuls. The mouth varies greatly in position, as in the Khabdoc(els, and opens into a spacious pharyngeal poueli containing a plicated ])harynx (Fig. T.'l, p/i). The intestine consists of a ctMitral cavity, into which the pharynx ojiens ;it its iuutu- end and from which numiM-ous braiu-hes (hence tlio name of the order) pass olV into the compact parenchvni.i, where they branch and may anastomose with one another to frri'ii) tentacles !iris<» from thf^ ihiry,'!} t(urfact- = r elue from the margin near the anterior eud of the bodv. As TYPE PLATYIIELMINTUES. 189 Vd in the Triclads otocyHts are wuutiug, uor have ciliated lateral dei)roHHious been described as occurring in the order. Little is kuowu couceruiug the excretory system. The reproduc- tive system diflers from that of tlie other orders iu that the male nud the female ap])aratu.s each possess a separate opening ( i* imd ? ), there being no genital atrium common to both, lioth apertures he behind the mouth-o])ening, near tiie })osterit)r end of the body, the male ap])aratus opening an- teriorly to the female. The former is simihir in structure to what has been described for the Triclads. The female ai){)aratus possesses no vitellarium, and the ovaries {ov) are very numerous, lying in the ^^ lateral parts of the body, their various ducts uniting to form wide canals which serve as uteri {nt). These open into a single tube, the ■rtu/ina, which receives the secretion of the numerous glands (sg) which form the shell-gland. In some forms there is situated ^^^ «. . ,, about the muldle of the ventral .11.,*^, 7 , , , ■ ■■ surface of the body a muscular ofier lanu). sucker which serves as an organ of "^ "" nccfssoiy frlund julhiisiou. Hince the presence or absence of this organ is in eitiior case associated with the occurrence of other important structural pecu- liariti(>H, the order has been divided into two sul)orders — the (oltflnt, provided with a suclier ( /'////.yrOH*. ($ . 2 = "mlt'iiml fi'iimlcoiiiices. zoo,,, Eimjhpta), and the Amfylm, m which it is absent {Planocera, Leptoplomt). ce = cciehrul gaiigHou. t>» = oviiry. l>h = plmiyiix. sg = slu'llgluud. te ~ testis. itt = titerus. vd -- VHH (leferons. vs - vt'siculii .siMiiiniilis. 140 INVERTEBRATE MORPHOLOOT. Reproduction of the Turhellaria.~^on.mxnii\ reproductiou is not characteristic of the x'urbellaria, though it occurs iu cer- tain Bhabdocoels. Iu Jlicrostoma a transverse partition, con- sisting of two closely-applied laniellrt% forms, extending from the outer wall of the body to the wall of the digestive tract, Aviiich it constricts slightly without dividing. Later a constriction of the outer surface of the body appears, the two lamella} of the partition separate slightly, and the indi- vidual lying behind the partition develops a new mouth and pharyux and a new brain, so that it resembles exactly the anterior individual with which it is directly connected by the uninterrupted digestive tract. Before these processes are complete, however, they are repeated in each of the two indi- viduals, so that a chain of four imperfectly separated individ- uals results, and by further repetitions of the process chains of 8, 16, or 32 individuals may arise, each provided with mouth, pharynx, and brain, the anterior individual possessing the original structures, and all connected by the digef,tive canal which runs uninterruptedly through the entire chain (see Fig. 28). Eventually the various individuals separate from one anotlier and become sexually mature. The sexual method, however, plays a much more important part in the life-histories of the Turbellaria. The development of the three lower groups has not as yet been as thorouglily investigated as is desi.-able, but the i)henomena which occur in the Triclads, find especially in the Polychids, have been fol lowed. The Triclads deposit their ova iu chitinous cocoons, which c mtain, besides the ova pro])er, large numbers of am(id)oid cells, originating in the vitellarium-pouches of t\u\ parent, and serving as food for the young embryo. In asso- ciation with this condition of affairs many peculiarities of segmentation and growth occur in the Triclad embryos, all of which must bo considered as secondary adai)tations. In the Polydads, however, a more primitive state of alia i is occurs, the food-yolk being incor])orated with tlie protophism oi the ovum, a more or loss distinct irregular segmentatidii resulting from its telolecithal arrangement (p. 63). 'J'Ik^ dii>l:>})l;iHtic eouditiou arises by an invagination either of tlie embolic or epibolic type, but at an ejirly period of the Hegnien TYPE PLATYHELMINTHEa. 141 tfition the cells which are to form the mesoderm are separated off from those from which the ectoderm and eiidoderm are to be derived, so that even before the iuvagination all the three lajei-s are represented. This, however, is to be regarded as a juecocious segregation of the germ-layers, and even within the limits of the few forms whose embryology is known consider- iible variations in the time and manner of the diflferentiation of the mesoderm occur. The result of the invagination is in some cases a solid, bilaterally symmetrical, ciliated embryo consisting of a layer of ectoderm enclosing a central mass of eudoderm and mesoderm, in tiie interior of which a cavity ap- pears surrounded by the eudoderm. A depression appears on the ventral surface, which, deepening, finally unites with the cuteron and forms the pharyngeal pouch, and gradually the characters of the adult are assumed. In some forms whose ova are jirovided with comparatively little yolk the embryo leads from an early period a free-swim- iiiiiig existence, and in accordance with this a specialized form has been acquired and a slight metamorphosis is neces- sary for the conversion of this Inroa into the adult ccmdition. In Sfi/hchiift the embryo develops into what is known as docftes larva, a bilateral ciliated structure with an anterior and posterior tuft of strong se».sory hairs, while fronj the ven- tral surface on either side of the mouth tlicre hang down two ciliated ear-like lobes or lappets. In another form {Thymmzmw) these ]ai)pets are much more developed, ])assing round to the dorsal surface of the body, and their edges are drawn out into four or eight lobes, one of which lies in frimt of the nnrnth and another on the dorsal surface, the other two or six lying at the sides of the body and being arranged symmetrically on either side. It seems p,o. 7r,.-LAuvA op probable that this larva, known as MilVcr's ni/sanoJoi,, Mm- hirra (Fig. 75), may be traced back to a con- '•"■'» iMyvn («fter dition such as that described in (loette's '"""'^' larva, the two lapjiets of that form having united in front of the mouth, while their lines of attachment have become more 142 INVERTEBRATE MORPHOLOGY. i ' and more oblique until wLat were ori^riually the posterior edges of the lappets meet on the dorsal surface. The edj^es of the lobes of the lappets are fringed with long cilia, aLd consequently a lobed prseoral baud of cilia is produced. These larvee pass into the adult form by gradually becoming more and more flattened dorsoventrall/, the ciliated lappets or lobes at the same time growing smaller and smaller until they finally disappear. Relatiomhips of the Turbellaria.—A relationship of the Turbeliaria especially of the Polyclads, with (he Ctenophores has l)een advocated within recent years, and through this relationship genetic affinities with the Cnidaria have been sought. The question of the affinities of the Cteno- phores has already been discussed, and it has been pointed out that it is probable that, instead of being a connecting link between the Cnidaria and the Turbeliaria, they are rather highly modified Turbeliaria adapted to a pelagic life. In this sense the idea of a genetic affinity between the Turbel- iaria and Ctenophores may be correct, though it seems i)robable that the Polyclad affinity should be given up and the relationship sought for among AUoiocoelan forms. The Ctenophore-Polyclad theory necessarily viewed tiie Polyclads as tlie most primitive Turbeliaria, and came into contact in this way with the more simple organization of the Acoela, Alloiomla, and Rhabdocvela a difficulty which was avoided by assuming that these were degenerate groups derived from Polycladan ancestors. No good grounds for such an assumption exist however, nothing in the mode of life suggesting a cause for degeneration ; and until embryological evidence of degeneration is obtained, it is preferable to consider their simplicity primitive. This latter view is strengtiiened if it In rmonizes rtith a probable phy- logeny. It has already been pointed out that the solid embryo or stermla is to be recognized as an ancestral form of the Cnidaria. With .such an ancestral form the .\ca>la show affinitii-s in the absence of a differentiation of the central mass into well-(l(«finod endoderm and mesoderm. The local- ization of a definite region for the ingestion of nutrition would lead to the formation of a mouth in the Sterrula, ju.st as it has done in the FlmielUita. The differentiation of mu.scle-fibres from the mesendodermal cells would naturally follow the assumption of a creeping hal)it, so that it is only tlic l)ossession of a definite nervou^ system imbedded in the mcsoghea (in which ti.s3ue, however, Cnidarian characteristics are yet discernible, as already pointed out) and the occurrence of a com])lieated reproductive apparatin that render a eljse comparison with the Sterrida difficult ; but (jven tlir «xplamition of the presence of these .structures makes fewer demands upon our ideas of developmental possibilities than does the a.s8umption that the Ai-ii'la owe their peculiarities to deg(Mieration. Upon this view of the phylogeny the AvwUx are united with the Co'lcii TYPE PLATYHELMINTHE8. 143 era only through the Sterrula ancestor common to both, or more probably hrough an ancestor in which the mouth had developed, as well as a slight differentiation of muscle-flbres, but in which no hollowing out of an entemn had yet occurred. This appearing in a primitive acoelan form gave rise to xh^ Allmoccela from which two divergent lines of descent arose, one leadin-^ to tiie Rhabdocoels and the other to the Triclads and Polyclads If this be the true phylogeny of the class, some evidence of it ouglit to be found in the embryological history of some of the liigher members of the group in accordance with what is termed the Biogenetic law, which is to the effect that an individual in its development recapitulates more or less accurately its phylogenetic development, or, to put it more briefly the ontogeny is a recapitulation of the j^hylogeny. Secondary modifications especially in the form of the abbreviation or omission of certain sta-es' may intervene in the individual development, f.)rming what are termed cenogenetic modifications, but notwithstanding exceptions produced in this way the law is of general application. In 8tylochas the young larva is a solid body without any enteron and represents, therefore, an Acoelan stage of development ; later the central mass becomes hollowed out to form an enteron whose walls are not at first clearly marked off from the surrounding parenchyma, and a representa- tion of the Alloiocffilan condition results, from which the Polyclad condi tion gradually develops. Consequently in mylochus the ontogeny indi- cates a primitive nature for the Actrla, and agrees with the phylogenv which has been outlined above. It must be recognized, however that ail reconstructions of the phylogeny of the Turbellaiia and all views as to tlieir afiinities to the Cnidaria must Ijo accepted with much reservation until the muclMiecded facts as to the developmental history of the Acala and Alloiocoda are available. 11. Class Tkematoda. The Trematodes or Fluke-woniis are throughout parasitic Snckers are preseut in all, consisting of cup-like depressions whose walls are richly supplied with niuscle-eells, by the i'ontraction of which a vacuum is formed, and in many forms, in udditiou to these, chitiuous hooks occur. The suckers vary 111 number from one {MomsUymnm) or two {DiHtomnm, Fig. 76) to several {Pohjstomnm\ and at the bottom of one situated at the anterior extremity of the body is the mouth-opening. This l»"ii(ls into a tubular o>sophagns whose walls are thickened ix'ar Its anterior end to form a muscular phnn/iHf,'„l hnJh which functions HH .., pump for the ingestion of the nutritive fluids of the host. At its posterior extremity the u-sophagus branches 144 INVERTEBRA TE MORPHOLOO T. into two limbs wbicli are continued backwards, in some cases giving off secondary branches, to near the posterior end of the body, where they either end blindly or unite together in the middle line (Polystomum) to form a loop. The body is covered by a distinct cuticle secreted by the ectodermal celJK, which in the adult may undergo a con- siderable amount of degeneration, or probably in some cases the cuticle is formed in part by the transformation into chitin of the ectoderm. Spiny ele- vations of the cuticle are present in many forms, and the large chitinous hooks which occur in many ectopara- sitic forms are but further develop- ments of these structures. Below the ectoderm lies the usually thin base- ment - membrane, below which again lie the circular and longitudinal pe- Fw. 7Q.-i)i.tom,nn cyg. ripheral muscle-sheets, and between HOe«^« (from a (IrawitiL' l)y ,1 • , ,. 1 ,, , . the intestine and these muscles is the parenchyma traversed by dorsoventral muscle bundles and ha\ iiig imbedded in it the re])r()ductive and other organs of the body. The nervous system (Fig. 77) con- sists of a transversely elongated gan- glion lying dorsal to the (esopluigus— usually between the bottom of tln' ant(>rior sucker and the i)haryng»'iil bulb. The ganglion is somewhat swollen at each extremity, indicating its origin by the approximation of two ganglionic masses, and from these thickenings nerves arise which \y,\M both forward and backward. The anterior nerves are shoit and slender, and supply the musculature of the anterior sucker and the sides of the anterior end of the body, whilf the })osterior nerves are much sti'onger and longer and varv from two to six in number ; in the latter case four run aloni^ C Lan(!Knhkcki. as = iiiitcrioi' sucker il = iiiU'stine. ov = ovary p = reproductive orilico. ph = iiliiiryngoal bulb. pe — cirrus. fe = testis. u( = uterus. vi = yolk-giand. va = veutral sucker TYPE PLATYHELMINTUES. 145 the ventral surface of the body, two on each side of the mid- dle line, the other two having a more dorsal position, while when only two are present they correspond to the two more median ventral nerves of this arrangement. Sense-organs are but feebly developed as a rule, especially among the endopar- asitic forms, but in some ecto- parasites eyes are present con- sisting usually of four spots of })igment seated upon the brain- ganglion and sometimes provid- ed with a lens-like structure. Fid. 77.— Nehvous System of Tke- MATOUE, Triitomum molui (after Lanu). Fig. 78.— Excketouv System OF TiJEMATouE, Distomum divergens (after Fraipont). A, euiiie system; B, terminal fuuuels. / = fuuuel. Jl — flume of ciliii. n ~ muiii iruuk. ph = pburyngeal bulb. s = anterior sucker. tc = termiuiil cell. vs = ventral sucker. vt — coutractile vesicle. The excretory apparatus (Fig. 78) consists, as is usual in the Platyhelminths, t)f two longitudinal, more or less irregu- larly twisted tubes {n) from which arise the funnel-bearing l)runches (/). A peculiarity of the Trematodes is, however, the union of the two longitudiual tubes in a terminal vesicle [vt) which opens to the exterior at the hinder end of the body by a single pore. The reproductive system is exceedingly complicated.though essentially similar to that of the higher Turbellaria. It opens 146 INVERTEBRATE MORPHOLOGY. to the exterior by two pores lying close together on the ventral surface rather nearer the anterior than the posterior end. The male apparatus consists in the Polystomecv of numerous closely-aggregated testes, or else, as in the Distomece (Fig. 76), of only two situated in the posterior half of the body ; the ducts from the testes pass forwards towards the genital pore, near which they unite to form a sac-like seminal vesicle,' from whose anterior end the single vas deferens is continued on towards the pore, passing in the latter part of its course through a muscular protrusible intromittent organ, the cirrus. The ovary is single, and its duct shortly after leaving it receives the ducts coming from two yolk-glands situated one on either side of the body, and is surrounded at about the same region oy a shell-gland, consisting of a number of uni- cellular glands arranged in a radiating manner around the oviduct. Beyond its union with these ducts the oviduct either runs almost directly to the genital atrium, opening into it in close proximity to the cirrus, or else pursues a winding contorted course through the parenchyma and serves as a litems or ooti/p, within which the ova undergo a portion of their development. From the oviduct in the region where the duets from the vitellaria and shell-glaud open into it one or more canals may arise whose significance is to a certain extent problematical. In the Distomcas one such canal occurs, and when a seminal receptacle is present it stands in more or less close relations to this canal, known as Laurer's canal, which, after a short course, opens to the exterior on the dorsal surface of the body. In some Pblystomea two canals arise from the yolk-ducts and pass forwards parallel to the uterus to open by a number of pores situated on the margin of tiie body. Those canals liave been termed the vagina, and in some forms are represented l.y a single canal. In addition to the vagina, however, another canal is pres- ent which has been shown m PoJystnminn awl Sphymnurato o\^(in into the digestive tract, and has been homologized with Laurer's canal of the Distomeae. It seems pretty certain tiiat the vagina of the Polystomese functions in copulation, the genital orifice of one Polystomuvi having been observed to come into contact with the vaginal openings of the other during that act. But the Laurer canals do not seem to have any such function, and it hiis been suggested that they may serve for the removal of surplus yolk- material produced in aceordancu' with, the favorable nonditions for nutri- tion offered by the parasitic mode of existence of the Trematodc^s. TYPE PLATYUELMINTHE8. 147 Two orders may be recognized as occurring in the Tre- matoda. 1. Order PolystomesB. The PolystomesB are for the most part ectoparasites and present fewer signs of degeneration than do the endopara- sitic members of the class. The apparatus for adhering to their hosts is usually strongly developed, several suckers usually being present, as, for instance, three in Tristomum and seven in Polystomum integerrimum (the latter parasitic in the urinary bladder of the Frog), and in addition a number of chitinous hooks may occur, as in Gyrodadylus and Sphyranura (the latter parasitic on the skin of 3Ienobranchus). In accord- ance, too, with their mode of life, sense-organs in the form of eyes and probably of tactile papill* on the skin occur, and furthermore the processes of development are much simpler than in the endoparasites, as will be seen later. Some peculiar anomalies occur in the life-histories of some of the Poly- stomeae, as, for instance, in the Gyrodactyhis, which lives upon the gill? of the Carp. It is a viviparous form, and the young while still within the body of the parent may already have become mature and contain young likewise, which again may contain ova in course of development, four gen- erations being thus enclosed one within the other. Di^lozoon, which livts likewise on the gills of Cyprinoid fisiies, is peculiar in that at tiie time of sexual maturity two individuals become fused with one another in the form of an X, the fertilized ova giving rise to a single form formerly known as Diporpa. 2. Order DistomesB. This order includes endoparasites which show a more marked degeneration than do the members of the preceding order. Eyes may be present in the young but are absenl in the adult, and furthermore a very complicated metamor- phosis is passed through in the development. The suckers for adhesion to the host are either one {Monostomnm) or two {Distomum), and as a rule no chitinous hooks are pres- ent. A...oiig the more interesting members of this order are Distomum liepatimm, a large form measuring 2-3 cm. iu length and inhabiting tho 148 iNVKitTRimATK MouriroLoa r. biliMhioIrt of Shoop, in wlii(!l» it pnMluoos vviiuL is toniiod V.w " Uof," wliicli, in tht> low-iyiiiK pasf.ims of KuKluntl and tiio (lontinont, is I'nM.iiontly tlu) cans." of llKHloslniclion of larjjjo nnml»ors of slioop. in oxct*pl'ional casos il lias boon k-nown to owur in man. In K^ypl. liowcvcr, tlir l, tlm so.xos jiff scpMnilod ii'i tcrs and blaii iiillanunation. accompanied by suppuration, of these organs. Di'vclofwicvf of f/iv Tmttn(s.~-'n)o ova of Tronuitodii consist of two ilistinrt j jirts, a j>vnii-coIl, tlu^ i>rotluct of tlio ovary, surroumltMi Uy a inaH.s oi food-niaiiMial, the socrotion of tho vitellaria, tlio wliolo boiii^; oucIosimI in a sluUI fonnod by tlie slioll-n;lniu]. In tlio roljstoinoni tho (lovtih)j)nioiit, as a ndo, is initiivlv carriotl on ontsido tho body of tho ])anint, tho stalkod ova boing attached to tlio body of tho host, tii(>u<,'h (jifrH^driifhis is vivi|)arous, in tho Distoiuoas Iiowover, th(* rovoiso is tho iido, tho ova iuuh>i{roiiio; u cortain part of thi>ir dovolo|)iiioiit ill tlit^ utorns *)f tlu^ i)aront, and loavin^' tli.^ o'u^ shortly after its oxtrnsion as a larva, sonietinios ciliated, soiiiotinios provided in the plaoo of tho cilia with a strncturo- leaii cutu'lo, and furthornioro in those ondoparasites tl>er<> occurs a reniarkablo alternation of ^'onorations of the kind already referred to as liotoroji;ony (see p. (51). The lu'torogony may be of various (h^nibryo (Fi^^ 7<), J\ which may bo a free-swim mini>- ciliated organism i)rovido(i with a short pouch-shaped intestine and with a mouth, and froipioutl possessin«;t also a nervous system and })i}rnuMit .\ve-spot ad well as excretory tubes ; in other cases, however, as stated, the embryo is destitute of cilia, usually in this ca,j;e beino; pro\ided with one or more spines at the mouth-end of the body, and all gradati()ns of dej^eii oration of the eve- spot and nervous system, as well as of the excretory tub(>s and dicrestive system, may bo observed. In all, however, tho space between the more or less developed di^^estivo tract and the body-wall is occupied by uumerous uuspecialized cells TYPE I'LA TYlIKLMINTriKS. 149 (;/<;), whicl are 'n reality gorm-c,3llH or ovu cai)a],le of under- «oinK' a i,artlien(,g,„iotic dovolopmeiit. EvxMitnally tliis larva u it. way int., thn interior of an anin.al of „ome kind usually a Mollu.sk, and there undergoes a further develon' ...ent, either retaining ,t,s .ligostive apparatus and elongating somewhat to loriu a JMia (Fig. 79, JJ), or beeondng an oval Fm. 79.-^, CiliHtod larv,v, and B, Redla of Distomum hepnticum (after l^cck- AIIT). d = intcslino. ,„ ^. „Hiiitli. ffc = gorm-,oils. ,•■ = second gen.Tntion of R,,,li,c. sac without mouth or digestive tract, the Sporocyst. The Kedia iH a niueh more liighly organized form than the Sporo- cystand is fnupiently capable of motion, two blunt proiec t.oHs near the hinder end of the body serving as supports in a somewhat similar manner to the sucker-like feet of cater pilhirs It adheres to the wall of a cavity of its host, from winch by energetic action of its muscular pharynx it is able to absorb nutrition. From this stage onwards the development varies in com- plexity m various forms. It is simplest in Momstommn muta. /"/^S whose ciliated embryo, while still free-swimming, contains within 1 a small sexually immature Monusfomum, and after it 1ms made its way into the interior of its Molluscan host the :., r). During the summer, however, the Kedic-e produce Cer' caria3 which, leaving their host, swim about for a short time I lusT^^ir^'"' ^'^"^^^^^-' ^^' necessarily in a second 31ollusk, but on grass or any other object with which thev may come in contact, the tail at the same time being lost If, now, these encysted forms are swallowed by a sheep the Simple Mode. i - I Embryo. Water. Sporocy.st or Redia I. Host (Mollusk) a 0 a 0 1 1 V Eiicapsuled Distoine I. Host n. Host 1 a Mature Distonie V hH Usual Mode. Kiiibryo. ! Water. Spoi-ocyst j I. Host or Redia l(Mollnsk) Cercari8B. Encapsuled Distome Water. Complicated Mode. 0) _ £.2 Mature Distome II. Host. III. Host Embryo. Water. Sporocyst I. Host (Mollusk) a a O Red i 88 CercariBB. Water. Encapsuled Distome Matui'e Distome H. Host or on {fiass, etc. III. Host fjenus Holo^tomum, who.se e.nbrv ' aft n ml V " '"'^'"'^^ "^ *''« ;;.e «.t host, see.s to ^:t^izz:^:^z::':^' '^'y ^^^ D'stome, without any alternatio.) of generations immature A very peculiar life-liistorv is found in n,\fn„>„,» ^^ parasitic in insect-eating birds. tLIo.^^^^^^^^ ^^'^'^'I' is peculiar in that it asstunes a br hinrfo' he h \" ''""'' ^"' network among the tissues of the ho t "uhlends o .^ ? ''''''''''' '' work young Distomes develop without th n er enion ofTc "'■ ""' '"^- '"Hl by their development and its own l'i nvf. tlT ?''""''' '*^^'^' 152 INVEliTEBHATE MOliPlIOLOG Y. m hi it 4 tlie snail, wliich tluis become enormously distended. Tlie club-shaped structures so formed are abundantly supplied with muscle-fibres, and bv visrorous movements linally burst the distended wall of the tentacle, and separating from the Sporoeyst fall to the ground. There tiiey ni()v,> about, resembling an insect larva in gen(>ral ai)pear:ince, a resemblan-e iucreased by banded markings of green and white, which render them very coM.spicuous, and they are apt finally to be snapped up l)y some bird, iu whose digestive tract tlie young Distomes are set free and become matui'e. There can be little question but tiiat tiie .simple metamorpliosis of ihc Pbli/stome(z represents the original method of development of the Trematoda, tlie heterogony characteristic of most DistomecB being a secondary accpusi- lioM developed in accordance with their ondoparasitism. An ich a of llic mode in which this alternation of generations has been l)rought about is furnished by such forms as Qyrodactylus, in which the development of tlio ovum takes place witliin the body of the parent, the young in (heir turn developing embryos before being borti (see p. 147). Tills acceleration of sexual maturity, accompanicjd by parthenogenesis, has l)r()ught about (lie condition seen in the Sporocyst or Redia, which are embryos provided witii ova capable of parthenogenetie development. Thus fundam(>ntally the heterogony is a piedogene^pis (see )). 60), and may b(; compared, in a gen- eral way, with the formation of a hydroid colony by the budding of a medusa larva. III. Class Cestoda. Like the Treniatoila the members of this class are para- sites, but are throughout eii(h)i)arasit('s, and i)i'eseiit a much j,'reater degeneration of structure than is found in the Disto- niPflB, accon)panied bj peculiarities of development dil}erin,^' somewhat from wliat occurs iu these forms. The Cestodes or Tai)e worms lack all trace of a di<,'estive tract and of a mouth, living in their mature state attached to the wall of tlie digestive tract of their host, and immersed in the nutriti\e fluids contained in the intestii.'c. In some forms, such as Cin-j/ophj/Utrns (Fig. 81, ^I), para- >4itic in the intestine of Cyprinoid fishes, the similarity stive tract. The worm consists ot a somewhat dilated head, succeeil'nl by auarrower portion which may be termed a nock and gradually enlarges to the ratlier cylindrical body, which contains a single set of reproduclive organs. In Li)/nl,i, which is found in the intestine of aip.iatic birds, there is likewise an .ihsenco of suckers, but the re]>ro. TYPE PLATYHELMINTIIES. 153 (luctive organs are present in sever-.] «pf« other, without any extpr.nl i TV succeeding one an- ■ '^i^,« 1. i '''^'\<^^ei, extevnallj by indis- ictions ol the bodj, an indication of a tendency . L. iiiilLiioi eutl of 7. «(fr/<««pnH,nt nui;J^,V /;'?'' i-rtion. th« .^.r^F^-'si ;-f^' "^"f \^'*''^'^.^«* "^ -' -"terior tlie form of Huckers. accompanied or not by chitinous 154 INVERTEBRATE MORPHOLOGY. hooks and followed by a varying number of segments or pro- glottides, each possessing a set of reproductive organs and cap- able of separating from its fellows, maintaining for a time an independent life. The proglottides towards the hinder end of the chain or strohila are the most advanced in development, and one after another drop off and pass to the exterior of the host's body with the fieees ; more anteriorly the proglottides are sexually immature, and still nearer the scolex they are to be found in various stages of formation. In fact the hinder end of the scolex may be regarded as a zone of growth, new proglottides being successively formed at this region. The process of proglottid formation resembles not a little what has been described as the non-sexual reproduction of the Dis- comedusai, the scolex corresponding to the jiarent Scyphos- toma and the proglottides to the Epliyra?, the entire aggre- gation in both cases being termed the Strobila. The exterior of the body of a Cestode is formed by a cuti- cle without any trace of cellular structure, and is perhaps to be regarded as a basement-membrane, the ectoderm, originally present, having disappeared. The cuticle varies mucii in thickness, and is throughout traversed by fine pores which allow of the absorption into the body substance of the nutri- tive fluids in which the Tai)eworm lives, either directly or by permitting the passage to the exterior of fine protoplasmic processes from the subjacent tissue. Special developments of the cuticle in the form of chitinous hooks are frequently present, arranged in some Ta-nias, for example, in a double circle upon a prominence, the rostellum, at the apex of the scolex, and forming a very eflicient means of attaching the worm to the wall of the intestine of its host. Beneath the cuticula there is to be found a very thin muscular layer, tlic peripheral musculature, but the main bulk of the musculM- ture consists of those fibres which traverse the parencliynia. These, especially the longitudinal and transverse ones, are massed into strong bands, the former lying usually exterior to the latter, and both eucl<»sing a central mass which is trav- ersed by weaker bundles of dorso-ventral muscles, and con- tains tiie reproductive a]i})arntus. In connection with tiie muscular system mny be mni- TYPE PLATYHELMINTIIES. 166 tioned the suckers which frequently occur upon the scolex and serve with the hooks, when these are present, to attach tlie j.arasite to its host. In Timia these suckers are four in muaber, and have the form of circukr depressions whose walls are nchly supplied with muscle-fibres, while iu Bothrio cephalus they have the form of elongated grooves, situated on the edges of the somewhat flattened scolex. As might be expected from the great development of the muscles, a well-defined nervous system is present. It consists of a brain lying imbedded in the tissues of the anterior por- tion of the scolex, evidently composed by the union of two ganglionic masses and giving rise to two main nerve-cords which pass backwards through the entire lenr^th of the strobila without interruption (Fig. 82, n). So, too, the excre- tory system (Fig. 82, ne) extends through the entire strobila uninterruptedly. It consists of two nephridial tubes, which m the anterior part of the scolex may be united by a cross branch, as they are at the posterior edge of each proglottid and open to the exterior by a pore situated at the centre of he posterior edge of the last proglottid. After the loss of he original terminal proglottid two pores, corresponding to the extremities of the longitudinal tubes, place the system HI communication with the exterior. The reproductive system (Fig. 82) possesses a complexity similar to what has been described for the Tremaioda. and lernrnphroditism prevails throughout the class. In the stio lulnr Cestodes each proglottid contains a complete set of organs, both male and female ; the testes (Fig. 82 te) are n^sually very numerous, consisting of small spherical' masses scattered through the parenchynut. each being provided witii a sn.all duct, which after a short course unites with sinular peus to the exterior after passing through a muscular organ, the cirrus-sac, by the con- tractum of which its terminal portion, often provided on its mnor surface with barbed hooks, i,s protruded to the exterior as an mtromittent organ or cirrus (r,. The female aunarnt.w vanes somewhat in its arrangeu.ent. In the majority of fornis the ovary is a bilobed organ (o.). lying near the' posterior 156 INVEliTEBKA TE MOltPlIOLOG Y. end of the proglottid. The oviduct soon after leaviug the ovary unites with the yolk-duct {vid) coming from the albuminous vitellarium {vi), which consists of a number of glands scattered through the parenchyma similarly to the testes. At the point of union with the yolk-duct the oviduct Fm. 82.— PROOLOTTIO o.' T , ruDH almost dii^ctly f'i ; when it is swallowed by the -secondary host the head evaginates, and the cyst remains for some time attached to the hinder end of the scolex, but later disappears, and the formation of the proglottides occurs. Further modifications arise by the formation in the wall of tlic cyst of not only one but several invaginated heads, forming the Ccenurv.s (Fig. 83, O) ; or even secondary c.ysts may ariso from the inner wall of the original vesicle, and each of them m».j develop several heads, forming what is known a,s the Echinococcus (Fig. 83, D). Rovoral of tho Cestoda are especially interesting from a madJral stan.l- poiiit, inusmuch as tliey are parasitic in man eitiier during the adult or tlic TYPE PLAT YHELMIN TEES. 159 larval stage. Among these may be mentioned Bnthriocephalus latus, which occurs in the iiuman intestine, where it may reach a length of as much as 13 metres, in such cases consisting of many thousand proglottids. These may readily be recognized by tlie convoluted uterus, and by the open- ings of the reproductive organs on the median line of one oi the flat sur- faces, while the head is characterized by being flattened, and provided on the margin with two elongated suckers. The ova give rise to a ciliated larva which becomes transformed into the six-hooked embryo, this latter making its way into the tissues of certain fish, which serve as tlie first host. Man becomes infected with the worm by eating improperly cooked or salted fish, the Pike being the more usual primary host, though this part may also be played by other forms. Tlie genus Ttenia furnishes two Iiuman parasites. The genus is char- acterized by the head being providfrivation of the Cestode from the Trematodes, and from Trematodes in whicii the Cercaria-stage liad already been established. 162 IN VERTEBRA TE MORPHOLOO 7. IV. Class Xemertina. The three precediug classes show marked evidences of genetic affinity, the char;ieteri:stic differences of structure in the Trematodes and Ce«todf^s being due to the parasitic habits of these forms. The Nemerteans, on the othv . hand though apparently tracing descent from a Turbellarian-like ancestor, show a marked advance in structure, and must be regarded as organisms of a considerably b7^]ier grade than the other Platyhelmiuths. They are for the most part marine, though a few forms ^ inhabit fresh water or even damp earth, and are usually elongated ribbon-like forms, reaching a length, in some cases, of several centimetres. The body is ex- ternally unsegmented, though a more or less marked metamerism of the internal organs, due to their repetition at definite intervals, may be present. The ectoderm of the body resembles that of the Tur- bellaria in being throughout ciliated, and rests upon a basement membrane, which in some cases contains cells. Within the membrane are a varying number of muscle layers, differentiations of the outermost portions of the mesodermal tissue, which in the form of a parenchyma occupies the interval between them and Fig. 84.-STirHosTEMMA the digestive tract. This (Fi- 84 rf, is Ell-HAUDI (after Montgo- i - , • , , ^ ^" * '' '^ MERv). ^" almost straight tube, except in the h = blood-vessels. genus J/alacoMelh, and is pushed out ou c. = cerebral ganglion, each side into sac-like pouches, which cfi' = ciliated funnel. „,.a ov,.o.> ,. i • . ^ = intestine. ^'^ anauged m some cases with a oc = eyes. regularity of succession almost meta- ov = ovary. meric. It opens to the exterior at the F- = proboscis. anterior end of the body bv the mouth rm = retractor muscle. ^ short non-muscular oesophagus inter! veiling between the intestine proper and that oi)ening ■ and at the other end of the b.nly is a second communication' with TYPE P'LATYIIELMINTIIES. 163 the exterior, the anus, an opening unrepresented in other Platyhelmiuths. The digestive tract is no longer a blind sac, but has the form of a tube, as in all the higher types. lu the anterior end of the body, above the digestive tract, is a structure, the proboscis (Fig. 84, pr), essentially peculiar to the Nemerteans, although indications of such an organ are to be found in the Rhahdocxla. It consists of a closed tube, the proboscis sheath, with muscular walls, imbedded in the body parenchyma and extending backwards in some cases almost to the end of the body, and within it lies the proboscis, also a tube, united to the wall of the sheath near its anterior end aud in fact closiug it at that region. From this lino of attachment the proboscis stretches back in the cavity of the sheath, the space between it and the walls of the sheath being filled with fluid. It is a simple invagination into the cavity of the sheath of the external body-wall, whose musculature as well as ectoderm are coutiuuous with that of the proboscis. From the tip of the invagination a band of muscle fibres, forming the retractor muscle {rm) of the proboscis, passes to the wall ot the body. By the contraction of the muscular Avails of the sheath the fluid contained in its cavitv forces the proboscis to be evaginated sometimes with sufficient force to tear itself loose from its line of attachment ; but should this accident not happen, the proboscis can be reinvaginated by the contraction of its retractor muscle. The function of this organ is doubtful. In some cases it is undoubtedly a weapon of oifence and defence ; but it seems not improbable, from its rich nerve-supply and from the probable function of its prototype in the Rhabdocoela, that in some cases at least it may be a tactile organ. A well-developed ervous system is always present, though it may show in some cases, as Carinellc, the primitive character of being still imbedded in the ectoderm or else lying immedi ately beneath it. In other cases, however, as Cerebratulns, it is enclosed in the muscles of the body-wall or may even be completely within them, imbedded in the parenchyma. It consists in its most usual form of two ganglionic masses (Fig. 84, ce) from which short nerves pass forwards and which iire united by two iransverse commissures, one of which passes 164 INVERTEBRATE MORPHOLOGY over or m front of the (Bsophagus, wJule tlis otlier arches from oue gau-houic nuiss to the otlier over the proboscis sheath. Each gaugliou is bilobed, the smaller posterior lobe being iu some cases united to the larger one bj a rela- tively thin baud of uerve-tissue so that it appears to be almost mdepeudeut. From the larger lobe of each side a nerve-cord passes towards the posterior end of the body whera the two may unite to form an arch passing over the' posterior part of the intestine. Iu addition to these a third nerve originating from the comniissure passing over the pro- boscis sheath and running backwards in the median dorsal line IS frequently present as well as, in some forms, another nerve running along the dorsal wall of the proboscis sheath, to which it sends branches. In many forms, such as Cerehrai ulus, a fine plexus of nerve-tissuo, lying between the muscle layers of the body-wall, unites the three main nerve-cords some of the strands of the plexus being sometimes larger than the others and forming circular commissures between the nerve-cords ; iu Tetmstemma and AmpJuporus, for example, these circular commissures may be strongly developed and have an almost metameric arrangement, the general plexus being in such cases wanting, ^ Eyes (Fig. 84, oc) are present iu some forms occasionally in considerable numbers, but are frequently wanting, and uto- cysts occur but seldom. The lateral ciliated grooves which occur on the sides of the head of some Rhabdocoela reach in the Nemerteaus a high development (eg), in some forms, e.g. Cerehratulus and Tetrastemma, becoming ciliated funnels of some length, whose inner ends are imbedded iu the substance of the posterior lobe of the brain. An olfactory function has been assigned to these organs, though some authors have consid- ered them mainly respiratory. The excretory system consists of a longitudinal canal on each Side in the anterior portion of the body, sometimes re- placed by a network of canals, which opens to the exterior by one or more ducts leading to pores situated on the margin of the body. In some cases these lateral ducts and the pores may be quite numerous and, like the intestinal pouches and the circular nerve-commissures, may have a somewhat metameric moveman TYPE PLA TYIIELMIXTIIES. 165 arraugement. The various termiual branches of the nei^hridial tubes are chib-shaped aud closed, a flame of cilia projectiuj? from the closed end into the lumen of the tube. The cajials aud tubes are liued with ciliated cells, aud are therefore inter- cellular and not intracellular, i.e., do not perforate cells, differ- iuj,' in this respect from the nephridia of other Platyhel- iiiinths. The blood-vascular system is peculiar to the Nemertea among Platyhelminths, and consists in the simijle forms, such as Carinella, of two lateral vessels which anteriorly open into lacunar spaces without definite walls. In the more highly organized forms, however, three longitudinal trunks, two lateral and one dorso-mediau, are present Avith definite and st)metimes muscular walls, aud unite in a J_-shaped manner at the posterior end of the body, while in front they may (Uthor oi)Pn into a system of lacuna), or they may, as in Sficho- stemma, unite with each other, a perfectly closed system thus resulting (Fig. 85, h). Transverse connecting branches be- tween the dorsal and lateral vessels occur in regular succes- sion, a metamerism being again suggested. The blood-vessels and lacume contain a fluid in which float round or elliptical corpuscles, which in some of the higher forms have a red color, due to the presence of haemoglobin. No heart or special contractile organ is present, the blood being driven through the vessels, without any definite direction, by the movements of the body. The occurrence of a blood vascuiar system in the Nemerteans and its eliaracter in the lowest members of the group suggests a mode of origin for the system which agrees well with what may))e deduced from embryological observations on other forms. It may be supposed tiiat in the pi-imhive Nemerteans a system of spaces filled with fluid existed, in which cells derived Irom the parenchyma floated. Tliese spaces would represent a simple ctelom, and were lacunar in character, lacking definite walls, the cir- culation of the fluid they contained being very irregular. In time tlie .spaces along the sides of the body might arrange themselves iu a linear manner, and might acquire definite walls, the rest of the spaces remaining lacuiuir, when a condition resembling that in Carinella would ensue, tlie arrangement found in higher forms resulting from the couversiou of the remaining lacunar spaces into vessels with definite walls. According to this view the blood-vascular system is to be regarded as 166 INVEliTEBHA TE MOHPIIOLOU r. in reality ji portion of tlie coeloni separated off for a special purpose, and otiier instances bearing the same significance will Ijo noticed later. The reproiluctive Kjsteni dirters from tluit of the other Phityhelminths iu its much ^'renter simplicity, no vitelhuia or shell-ji;hiud being present, unci furthermore tlie Nemerteiiiis are almost without exception of separate sexes. The ovaries (Fig. 8-1, ov) or testes are present in considei-able numlxM's, cue lying in each interval between two lateral diverticula of the intestine, so that they partake in their arrangement of the more or less pronounced metamerism of that organ. Between the intestine and the genital masses there is in some forms a distinct cavity, or ccelomic si)ace, and at tlie time of maturity a separate communication with the exterior forms for each ovary or testis. The class Nemertiua niay be divided into four orders, whose chief characteristics may b(^ briefly stated, having been for the most part already described. 1. Order Paleeonemertini. To this order belong the genus (JarineUa and allied forms, all characterized by structural peculiarities which are to be regarded as i.rimitive. The lateral ciliated organs are, as in the Khabdocd'la, mere grooves, not being continued inwards to the brain ii' the form of a funn(d ; and furthermore the nervous system is either imbedded in tin? ectoderm or lies immediately below it. To these chnractiMs may be added the more or less lacunar nature of the blood vascular system, and the communication, in some cases, of the nephridia with it. 2. Order Schizonemertini. In the Schizonemertini the ciliated funnels are well devel- oj)ed, and the nervous system is imbedded iu the muscular layers of the body-wall ; and though the nerve-cords are still, as in the ])receding order, united by a plexus, nevertheless there are indications of a dcn-elopment of commissural con- necting nerves. The blood vascuhir system is still lacumir anteriorly, though ])osteriorly three well-deHned ve.SHels are present. The genus CVHn-dtnlnH Ixdongs here. This ^ Amphipoi ciliated fi complete] the nerv< ])lexus be series of t most stri structure (that is, ^ spines or of being t poisonous tion of th into the w ])robo8cis of the pre; This or fouml iu tl: the connno ///(/ iu ma] (M'gans, anc is a con vol 1 <3nd of the ' Devvlojnm nf cm Ilia and j •■yliiidrical eil Willi bundles find graduall} 'orni. The ni '•'It shaped di f*trii('(ure oidy iiK't.unorphosif i's (Irst deacrili -' Usv niidy tht TYPE PLATYIIELMINTIIEa. 167 3. Order Hoplonemertini. This order, whicli iudu.les the genera Tetrastemma and Amphporus nienti.med above, has, like the preceding order diluted funuelH as lateral organs, and the nervous system lies completely within the muscular layer of the body-wall and the nerve-cords are united by transverse commissures, the j.loxus being wantiug. The blood vascular system is a closed series of tubes, not communicating with lacunar spaces The most striking characteristic of the order is, however, the structure of the proboscis, whic-h is armed near its posterior (that IS, while invaginated) end by one or more dagger-like spines or stylets The most posterior portion is not capable ot l-eing evaginated, and its walls are glandular, secreting a poisonous fluid which is poure.l into the more anterior por- tion of the tube, l,athing the stylets and thus being carried into the wound which n.ay be made by the forcibly evaginated IH-oboscis with tlie stylets coming into contact with the body ot tiie prey or enemy, ^ 4. Order Malacobdellina. This order contains a single genus, Mahcohdelln ,v],ich is found in the mantle-cavity of marine Lamellibranchs, such as he common Mussel and Clam. It resembles the Iloplnncnrr. ton in many particulars, but is destitute of lateral ciliated organs, and its proboscis possessos no stvlets. The intestine IS a convoluted tube without lateral dive'rticula, and the hind <3nd of the body is provided with a sucker. Devehpmeutof the Nemntiva.~\n .so.no Nomortoans. snch as Tetro .•y m, .,cu c.hau.,1 larva, usually provided at the extremHies " 2 . Lunches o U.u^rv eilia, whi.-h n.ay p,..ssii,Iy be sensory in en nd gr^u^ually changes without any .narked nn.amorphosis nfo TZl "n... The month <,pons upon the ventral surface of'the body o .TO o,dy appearing nnn.h later. In ,nauy forn.s. however, a pecu "• ..no phosis occurs during (he transformation of the larva k ,m , " n; nrst desc.^,er an ^W. lann, into the adult. (.„ „.e ven,;;:; "l ':;! - -•■• "'H.y there appear four invaginationsof the octoderm. two situate,! in 168 INVERTBBUA TE MOltPIIOLCG 7. front of the month and two behind it, wliicli gradually separate from the ectoderm to form four single-layered plates lying immediately beneath it. By a subsequent growth and fusion of these plates a new ectodermal cover- ing is formed enclosing the internal organs, and on its completion the orig- inal larval ectoderm is thrown off. In some species a somewhat more com- plicated process occurs. The larva, kuovni as the Pilidium (Fig. 85), has the shape of a helmet from whose rim two ear-like lappets hang down, be- FlG. 85. — IHUdiuni Lauva (after Sai.knsky). aj) = apical plate. m = mouth. h = digestive sac. tvveen which lic^s the mouth-opening (/«), while at the ape.\ of the helmet there is an ectodermal thickening («p), nervous in character, from whicli projects a bunch of strong sensory cilia. As in the Desor larva four invagi- nations of the ectoderm of the ventral surface occur, which, however, sqia- rato fi-om the larval ectoderm as four hollow sacs which unite together, their inner walls thickening to form the ectoderm of the young Nemertcaii. while tiie outer walls iM-conie thin and form wliat is termed tlie ainnioii sur- rounding a cavity within which lies the young worm. During the process of fusion of the four saes i he entenm ( S) and a portion of the mesoderiu of the PHhUum are enciosed and give rise to the digestive tract and mcM)- denn of the young worm, which later breaks through the amnion ami I'ilidium wall to become free. Tho signifk'unce of this metamorphosis is decidedly oKscnre. Souh' authors regard it as more jtriniilive than the direct method of developniciii, on the ground that the I'ifiifiinii with its lappets presents general sinn- larities to thf Miiilerian larva of the I'olyclades and is derived phylogeiic: cally from sucli a I'orin. iteing therefore more ancestral in its characNis tiian the simpler larva', ll must be recogni/.tMl, however, that there is lU) indication of mt'laiiiorpliii.^i> in llw I'ulyclad larviv, and furthermore tliit TYPE PLATYJIELMINTHE8. 169 the Nemerteans perhaps show greater similarilies to AUoioccelan Turbel- ana than to Polyclads. Perhaps an exphiuation of the proceL i to be found jn the sloughing of the ectoderm and the formation If ne" oil ated col s winch IS seen in the larva of a Pal.Tonen.ertean, C>.Ma/o^/.V, he motamorphos,s o Desor's larva and of the P///.//.,. being a greater a.ni more complicated ecdysis derived from the simpler one Some niteresling evidence as to the morphological significance of tho anus ,.s to be derived from a study of its devLopm^nt in X Nemer i' I ,. un open.ng which has been considered by some to have arisen bvi; closure m the nuddle of an elongated slit-like blastopore-the two en L hot ever, remammg open to form respectively the mouth and anus- and it h.s been thought that the direct transforn.ation of the blastopore ntotlpt maneu, mouth in some case., and in others into the permanent am receives on this theory an explanation. The phenomenon of the osu of tl 0 blastopore ,n the middle does actually occur in the Annelid-like TrI 0 eate Per,pat,,s and in many forn.s both mouth and anus stand in cbe outogenetic relationship to the blastopore. In th., Nemerteans are reme sented the most lowly organised animals which possess ..oth i.;::i tl la ml" anus and accord.ngly it might be expected that in them the original rela no-'''l« wdl be n.ost clearly seen. The young Nen.ertean pots e no . "US. t resembles, so tar as its digestive tract is concern,.,!, an Alloioco^l-.r 1 .s only relatively very late in its life-history that the anus app, " mi HM. in a region of the body which has no relation whatever to he riinn bia.s.opore. This fact .shouhl carry considerable weignt w t lisp en v asm the „u,,>rity of forn.s the anus is, in co.npait.n with , .^ m u^ of cativey late formation. It seems not improbable that primitive; UK'} ai,' entn-ely secondiiry. The in.lieation ,>f m,.tau>erism .seen in the Nemerteans n.n.ls no f„rtl„.r discussion aff.r what has been said on p. 43 with reference to similar 'ci lanties m the Turbellarians. ' I Sli|iKlN«D()M MiyPAZOA. TYPE PLATYllHl..\riXriIES. I. Cla.ss TrunKiXAi«A.-Eet,)d,.rm eiliat,.,!; no anal openiuK 2" tw ';;;■':' -^'•""^'I-eM.nt, but n.. .iigestiv,- tra,.t; Co>n.n.ta^ 2. (>.de.l/A.o,,Wa.-I),geslive tract pre.s..nt; space betu.Hw, it and s Jf"'-^-;;,"' "^^''I'H-.ll.y parenchyma. M.„nt„s, Pl,n,i„s,n,n„ 8. Or,l,.r /^.,/./o,.,W,,._I)igestive tract straight rod- .,r .sae-Mke- simce between it an,l bo-ly^wall n.>t till,-,! with parencl.vma' Mu'.n>Htoma, Misostama, Prorhy„rh„.s, Vorte, 4. Or,ler '/VA7«,/m.-l)iges,iv.> tract branched, three principal limb« mm nso to 8econ,h.ry branclu-s: mnio and femJe repin hot ^ 170 INVERTEBRATE MORPHOLOGY. organs with common opening. Gunda, Playiaria, Phagocata, Dendrocoslum, Bdelloura, Bipalium, Syncoelidium. 5. Order i^/ycZadeo.— Digestive tract branched, tlie primary branches being numerous; male and female organs having separate open- ings. (a) With terminal sucker (Co^yZea). Thysamzoon, Eurylepta, (b) Without sucker (Acotylea). Planocera, Leptoplana, i^ty- lochus. IL Class Trematoda.— Ecto- or endoparasites; ectoderm not ciliated; with digestive tract and suckers. 1. Order Poly stomea.— Suckers more than two; development direct; usually ectoparasites. Polystomum, Sphyramira, Tristomum] Gyrodactylus. 2, Order i>j*i!omecE.— Suckers one or two; development indirect; usu- ally endoparasitic. Distomum, Monostomnm. III. Class Cestoda.— Endoparasites; ectoderm without cilia; no digestive tract or mouth; usually strobilated. Tit7iia, Bothriocephaliis, Caryophyllmis, Liyula, Trianophonis, Archigetes. IV. Class Nemeutina.— Ectoderm ciliated; not parasitic; anus present; with protrusible proboscis. 1. Order Palmmemertini.—l^iGv&l ciliated funnels shallow; nervous system imbedded in ectoderm; proboscis without stylets. Cari- nella. 2. Order ScJuzonemertfni.—Lnteriil ciliated funnels deep; nervous system imbedded in muscle-layer; proboscis without stylets. Cerebratulus. Order Hoptonemertini.— Lateral ciliated funnels deep; nervous system within muscle-layer; proboscis with stylets. Tetra- stemma, Ainphiporus, Order MafacobdeUiua.—'No lateral ciliated funnels; proboscis without stylets. Mnlocohdella. 8. B leuokan Heidel A. Lang. Nerven tion zu C. Clang. ., wurmkt W c. Helm London 0. Bttrger. Monogr LITERATURE. TrilBELLAniA. I vonOraflf. Monographie der Turhellarien : \. Rhnhdoewliden. Leipzig, 188:' L. von Graff. Die Orgatnmtioii der Tiirhellaria tirrdn. Leipzig, 1891. L. BJJhmig. Uiitersitchutigen iihrr r/inbdocwkn Turbellarien, H. Zeitscbr. fur wissciisrli. Zoologie, i,F, 1890. W. M. Woodworth. Confvihiifions to the Morphology of the Turbelluria, I. Bui- Ictin of tbo Muspiim of Comp. Zool., xxi, 1891. A. Lang. Die Po/yrl,id,n, Fauna w. Florn des (ilolfe.s von iSeapel. MonoLT XI, 1HH4. W. M. Wheeler. Synrolidiim pellneidum, a new marine Tridad. Jour, of Morphology, IX, 1894. TYPE PLATYHELMINTHE8. THEMATODA AND CE8TODA. 171 B. leuokart. Die Parasiten dea Memchen. 2te Aufl Bd T ^^^^ • Heidelberg, 1879-1889. -^ipzig u. A. Lang. f^«««,.««cAM«^,„ g«^ vergleichenden Anatomie nnd Hiatoloai. d.. Ner^ensystems der Turbellarien. II u. III. Mittheilunrn a d ZcTsfr tion zu Neapel, ii 1881 """ocn a. a. IacoI. Sta^ NEMERTINA. § 5 4m I 172 mVEHTEBRATE MOHFEOLOOT. CHAPTER VIII. TYPE NEMATHELMINTHES. The Nemathelmiuths are, like the members of the preced- ing type, characterized by the form of the body, which is cylindrical and usually elongated or even thread-like, Avhence the popular terms Round-worms or Thread-worms which are frequently applied to the hi. The ectoderm is covered by a thick layer of cnticle which it secretes, and in counectiou with which spines, bristles, or hooks may be developed at various parts of the body. There is no trace of segmentation or reduplication of orgru^;, with the exception that in some forms the circular nerve-commissures uniting the longitudinal cords may succeed each other with tolerable regularity ; the cuticle, it is true, especially when taick, is ringed by numer- ous grooves succeeding one another at short intervals, l)ut this cannot be interpreted as an indication of metamerism, but is more probably a provision to counteract the rigidity of the cuticle and to give a considerable amount of mobility to the body. The Nemathelmiuths accordingly have the same grade of individuality as a simple Platyhelminth, such us un Alloiocoelan, and are to be regarded as metaiuere iiuiividuals. One important ditierence of structure which these worms show from the Platyhelininths is the presence of a capacious coelom, the interval between the digestive tract and the mus- culature of the body- wall not being tilled up by parenchyma- tous mesoderm, but being a sim})le undivided cavity in which lie the reproductive orgnus. These latter are simple, the animals being as a rule bisexual, and there is no separation of the female organ into ovary and vitellarium. Structures of an excretory nature occur in one of the two classes into which the type is divisible, but a blood vascular system is entirely wanting. The lial)it of life varies greatly in the various members ..1' TYPE NEMATIIELMINTHE8. 173 the group. In the class Nematoda many forms live freely in the sea, fresh water, or damp earth, while others are parasitic during a part of their lives, and others again are parasites practically throughout their whole existence. The Acantho- cephala aro without exception parasitic. I. Class Nematoda. The Nematodes are distinguished from the members of the second class by the presence in nearly all cases of a dis- tinct digestive tract, usually with mouth and anus, and by the absence of a retractile proboscis furnished with hooks at the anterior end of the body. The arrangement of the muscles of the body-wall are also peculiar inasmuch as longi- tudinal muscles only are present (Fig. 86, m), which iustead"of forming a closed sheath are interrupted along four longitudi- Fio. 8G.— TuANRVEKSE SECTION OP Amir>\i Inmhricoides at the Lkvkl of PlIAUYNX (from HEHTWKi). c = cuticle. m = loiigitudiuiil muscles. d = dorsal line. s = latenil line. ?i = hypodeimis. v = vcutriil Hue. to = utpbridiuni. nal lines {d, v and . tlie coelomic cavities of the Rhabdocoela or the blood-sinuses of the Nemerteaus. The digestive tract is a straight tube traversing the bodv from one extremity to the other, opening posteriorly in thi sp i = intestine. p?i = pharynx sp = spicules. te = testis. female commor anterior rectly c( while pc Jayer of cular tia The appears lining hj in the tli terior p( which o] behind t This the ante] ganglion one of w other, w] median ' nerve-rin the dorss backwar( nect the body not not sue^g( commissi are as a i The r. male the^ in its up] dilating 1 short ejac of this la small sac ble of bei with the 1 the other which dih TYPE NEMATHELMINTHES. 176 female directly to the exterior, in the males into a cloaxia common to it and to the male organ of reproduction. Its anterior part is a muscular oesophagus lined with cuticle di- rectly continuous with that covering the surface of the body, while posteriorly it is a delicate tube composed of a single layer of cells, not being surrounded by any mesodermal mus- cular tissue. The excretory system is not as yet fully understood. It appears to consist of a pair of tubes, for which no cellular lining has as yet been made out, which lie, one on each side, in the thickened hypodermis of the lateral lines. In the an- terior portion of the body they unite to form a single tube which opens to the exterior in the median ventral line not far behind the brain (Fig. 88, B). This latter ^jonsists of a ring or nerve-collar surrounding the anterior part of the oesophagus on which lateral masses of ganglion-cells occur and which gives rise to two main nerves, one of which runs back in the median dorsal line, while the other, which in some forms appears to be double, lies in the median ventral line. Other nerves pass forwards from the nerve-ring to the anterior part of the body, and in addition to the dorsal and ventral nerve-cords two lateral nerves pass backwards a short distance, while circular commissures con- Dect the two main nerve-cords, those of the two sides of the body not, however, being opposite each other, so that they do not sue^gest a pseudo-metamerism so strongly as the similar commissures of the Hoplonemertini. Special sense-organs are as a rule absent, though a few forms possess eyes. The reproductive organs are exceedingly simple. In the male they are represented by a single convoluted tube, lined m its upper part by the mother-cells of the spermatozoa and dilating below into a seminal vesicle, to which succeeds a short ejaculatory duct which opens into the cloaca. The walls of this latter cavity are frequently invaginated to form two small sacs in each of which lies a chitinous spicule capa- ble of being protruded from the cloacal opening and serving, with the bursa, as copulatory organs. The female organs, on the other hand, consist of a pair of convoluted tubes, each of which dilates into a uterus and unites with its fellow to form a i I 176 INVEHTEBliATE MOIiPUOLOG Y. single tube, tbe vagina, sometimes with muscular walls, which opens to the exterior in tlie ventral mid-line some distance in front of the anus. As a rule the sexes are separate, her- maphroditism occurring only in a few isolated cases. Many Nematodes are free throughout their entire exist- ence, living in the sea, fresh water, or damp earth, and fre- quently possessing eyes. Others are found in some domestic products, such as the viuegar-eel {Anguillula), found in vine- gar and sour paste ; Avhile others, again, are parasitic on plants, such as Tylenclms, which lives upon the young grains of wheat and hi some cases produces very rerious damage to crops, and Heterodera, which is quite as injurious to root- crops. More interesting, however, are a number of forms occurring as parasites in animals, many affecting man, in some cases producing serious results. Life-histories of the Eunematoda.—Tha f uving forms show no peculiarities of development, the immature a. I'liai developing directly from the egg. Among the parasitic forms, howev interesting variations from direct development, due to a change of host, occur, a wel'.-markod lieterogony occasionally being found. An example of this is seen in lihah- ditis iii;/i-ovenosa, which at one stage of its existence lives in damp earth, the females being viviparous and producing young which make their way iuto the lungs of frogs, where they assume a form which led them to be assigned to the genus Ascan's, and where tliey become mature. At this stage tiiey differ from the Rhabditis forms in being hermaphrodites, and from the eggs deposited by them the Rhabditis generation again results. From a medical standpoint one of the most important forms is Trieli/iia spiralis, which occurs enc^psuled in the muscles of various warm-blooded animals, such as man, the pig, rat, mou.se, and occasionally in the fox, cat, and ral)bit. Tiie capsules are oval and about 0.6 mm. in length, and occa- sionally have a white color, due to the deposition of calcareous matter in the wall. In tlie interior of the capsule lies coiled up an iminatu)'(! Trichina, which may retam its vitality in tliis condition apparently during the lifetime of its host. Should, for instance, improperly cooked or sahcd pork which contains such capsules Ijc eaten l)y man. tlio capsule becomes dis.solved by the digestive juices and the young Trichina is .set free in the small intestine and in tlie course of a few days becomes sexually matuiv. The males die shortly after having reached maturity, but the femali's l)()re into tlio intestinal walls, some i)assing through into the mesentery, where tliey may contimie to exist for several weeks. They are viviparous and <'acii individual may produce as many as 10.000 young, whieli make their way to the voluntary nniscles. especially to those of the neck and to the diaphi active mi; about 0.01 tiiey prod reached tl cysts mea they prodi capsuled : though th Other 1 worm (Fij length anc an earth-M inhabits tl (Fig. 88), anterior h the in test slender p tion hang intestine. forms ma of tlie til their ova, fer very g developme there is ik are tjikc-n I'er.sonal drinking-\ In add been ment more espe mates. Z form about teeth or bl which fast ■small intes of its host develop in and probat t'ected. It known as ( tunnels, ha tunnel, whe workers in is a very sh TTPE NEMATEELMINTHES. 177 tlie diai»hragm, partly by the lymphatic and blood vessels, and partly by active migration in the connective tissue. The young Trichince measure about 0.01 mm., and duiinjr their i)assage into the substance of the mu.scles tiicy prodi more or less .severe inrtaniinatory disturbances. Having reached the muscle tissue they become encysted therein (Fig. 89, A), the cysts measuring about 0.4 mm. in length, and if the disturbances which they produce have not proved fatal to the host the danger is past, the en- capsuled Trichina undergoing no further development within this host, tliough they may retain (heir vitality for many years. Other forms which o. ir in man are Ascaris lumhricoides, the round- worm (Fig. 88, B), a large form, of which the female measures 40 cm. in Icnglh and the male 25 cm., and which bears some resemblance in shape to an earth-worm, Oxyurisvermiculans, asmaller form, 1 cm. in length, which inhabits the rectumespecially of young children, and Trichocephaiusdispar (Fig. 88), which measures 3-5 cm. in length and is characterized by the anterior half of the body being exceedingly slender, the worm boring into the intestinal wall, especially in the neighboihood of the caecum, by this slender portion, the hinder thicker por- tion hanging freely in the wall of the intestine. The presence of these three forms may be recognized, independently of the finding of the actual worm, by tlieir ova, whose respective characters dif- fer very greatly. So far as is known the development of these forms is direct and there is no intermediate host, but the ova are tjikc-n into the body with the food. I'cr.sonal uneleanliiiess and imperfect drinking-water may be causes of infection. In addition to the forms which have t)een mentioned there arc a few which are more especially frc(iuent in tropical cli- mates. ^ocAm/,.sw/«o. 1.4 - 6" 1.6 ^ P}^ ^ # §!■ ''^> Photographic Sciences Corpordlion /'.%^^ ^/q V ^ 4^ \ \\. rS W1«'T MAIN STRfIT WHSTIV.N.Y 1 4310 (714) •7i.4503 g%>A^ 1^ \\v «> #,^ „„, 178 INVERTEBRATE MORPHOLOGY. tive tissue beneath the skin, producing ulcers, at the bottom of which tha worm lies coiled up. The ova develop in water and the embryos pass probably into small Crustacea, which are swallowed with drinking-water. Mlaria sanguinis hominis, also solely tropical in its distribution, receives its name from the fact that it lays its ova in the blood of mm, which may thus swarm with countless numbers of small worms. These make their way to the exterior of the body by the kidneys, producing hemorrhages or minute abscesses in that organ and, as' the result of these, milky or bloody urine. 2. Order Oordiacea. This order includes the families of the Gordiidce and Mer. mithidcE, long slender thread-like worms, which differ from the Eunematoda in several important respects. They occur in their mature state iu fresh water ; in their immature stages, however, they are parasitic in insects. In the adult Gordhis the mouth is usually closed by an overgrowth of the cuticle, and the anus is lacking iu Mermis. The musculature of the body- wall consists only of longitudinal fibres (Fig. 89, m), which 11 Fio. 89.— Tkansverse Section or Oordius (after Vwdovbkt). eu = cuticle. d = intestine. Ay = iiypotlennis. ff» = longitudinal muscles. OP = oviduct. me = mesentery. n = nerve-cord. p = peritoneum. ut = uterus. differ in their I'rraugement from those of the Eunematoda in being interrupted only in the mid-ventral line. The c(»lom is lined by a peritonea) epithelium (p) lying beneath the TYPE NEMATHELMINTHES. 179 iP'^ muscle-cells, and is divided into two lateral chambers by a mesentery (me) running the entire length of the body and con- sisting of two layers surrounding the intestine (d), and inserted into the body-wall dorsally and ventrally, their outer surfaces being lined by a continuation upon them of the peritoneal epithelium. No excretory system has been as yet discovered. The nervous system consists of a ganglionic ring surrounding the oesophagus, from which a number of nerves pass forward, while a single nerve-cord (w) passes backwards in the mid- ventral line, dilatiiig at the posterior end of the body into a ganglionic mass. The reproductive organs consist in the female of a series of ovaries {ov) attached one behind the other to each mesen- tery above the intestine. In the mesenteries two tubes (ut) pass backwards which receive some of the ova and function as uteri, near the hind end of the body bending ventrally to open into the cloaca, whose wall is invaginated to form a single seminal receptacle. The testes have not yet been found, but two seminal vesicles, corresponding to the uteri of the female, occur and open likewise into the cloaca, which in the male is evertible and serves as a copulatory organ. The Affinities of the Nematodes. — Tlie relaiionshipsof the Nematodes are exceedingly obscure. Their unsegmented character and the character of the nervous system seem to ally them more closely with the Platyhelminths than with higher forms, but the relationships to any of the known Platy- helminths must be exceedingly remote. The parasitism which occurs so frequently in the group is to be considered as secondary, since so many forms lead a free life and peculiarities of structure can hardly be attributed to degeneration. The Gordiacea stand on a Higher plane than the Eu- nematoda, as shown by the possession of a mesentery and the arrange- ment of the reproductive organs and nervous system, which Iwjar some sim- iliirities to those of the Annelids, but their Nematode eharacteristies uro must pronounced. Perhaps the ancestors of the Nematodes are to bo fountl in the yet unknown intermediate forms between the Platyhelminths and Annelids, a view which would account for their similarities in cortaiu respects to both these groups. II. Class Acanthocefhala. This class contains a number of })arHBitic forms which occur more especially in thu digestive tract of tishes, though to 180 IN VERTEBRA TE MORPHOLOQ Y. lit rm ^ <« also found in Mammalia and in exceptional cases in man. A great uniformity of structure exists thvoughout all the species, so that they are all referable to a single genus, Echimrhynclms. The body (Fig. 90) is cylindrical and as a rule not very long, and a marked distinction froni the -Nematodes is found in the retractile proboscis {pr) occurring at the anterior end of the body. It is a cylindrical prolongation of the anterior portion of the body and is provided M'ith a number of chitinous hooks by means of which it adheres to the intestinal wall of its host. The proboscis may be invagi- nated into a double- walled muscular proboscis-sheath by whose contraction it may again be protruded, a strong retractor muscle, extending from the tip of the proboscis to the base of the sheatli, serving for the invagination ; and from the base of the sheath re- tractor muscles {rm) pass to the body- walls and serve to hold the sheath in position. No traces of a digestive tract occur. The body is covered upon the out- side by a thick cuticle secreted by the subjacent hypodermis, which is a rather Fig. 00. -Male Echinarhyn- thick layer consisting of a protoplasmic ' niatiix in which nuclei are scattered but in which no cell-outlines are to be distinguished. Beneath the cuticle the matrix has a fibrillar character, and near its inner surface it is hollowed out into a network of anastomosing canals of which mention will be made later. Beneath the hypodermis lies a basement-membrane within which are Uvo layers of muscle-cells, having the same ei.i- thoho-muscular character as those of the Nematodes, the fibres of the external layer having a circular direction, while g - glands I = leiuiiisciis p = pt'uis. pg = proboscis ganglion pr ■= proboscis. nn — retractor muscle. t = testis. TYPE NEMATUELMINTHE8. 181 those of the iuuer layer have a lougitudiual course. The body-wall encloses a well-marked coelom, not liued by a special peritoneal epithelium, but which contains the repro- ductive organs and is traversed by the retractor muscles of the proboscis-sheath. The nervous system consists of a ganglionic mass {pg) lying within the proboscis-sheath which sends forward nerves for the supply of the walls of the sheath and -^f the retractor muscle of the proboscis. Posteriorly two lateral nerve-cords extend backwards along the sides of the body, and in male individuals are connected near the posterior extremity with a ganglion lying beneath the reproductive ducts and from which nerves pass to the genital apparatus. The system of lacunar canals which form a network in the lower layers of the hypodermis is probably excretory in function. The canals are found throughout the entire hvpo- dermis, both in the proboscis and f, the body-wall, in' the latter there being indications of two larger lateral trunks. From the point of junction of the proboscis with the body- wall two muscular sacs, the lemnisci (l) , hang down into the ccelom. The cavity which they contain commun.cates with a circular lacuna which surrounds the base of the proboscis and with which the lacuntc of the proboscis-hypodermis like- wise communicate, this system of the proboscis-lacuna) and the lemnisci being shut off from the system of the body-wall by a partition extending from the basement-membrane to the cuticle. The lemnisci have been regarded as possible repre- sentatives of a digestive tract, but it seems more probable that they are reservoirs for the reception of the fluid con- tained in the lacunro of the proboscis when it is driven from them during invagination. The reproductive organs are much more complicated than those of the Nematodes. The sexes are separate, the male individuals being usually smaller than the females. Tlie ovaries are paired bodies enclosed within a muscular Ugnment attached anteriorly to the base of the proboscis-sheath and posteriorly to the reproductive duct. At an early stage of their development, however, the ovaries split up into masses which float about in the coelom together with large numbers 182 IN VERTEBRA TE MORPHOLOO Y. of separated ova. They pass to the exterior by a complicated system of ducts, the most anterior portion of which is a wid& funnel-shaped structure, the hell, to whose wall the ligament i» attached and which, by a rhythmical expatjsion and contrac- tion, engulfs the ova and ova-masses floating about in the cob'om. From the lower end of the bell they escape, the ova- 3e8 to be returned to the coelom, while the fertilized sepa- rate ova pass into a short tube, the ovUmt, which opens below into a muscular uterus, which finally communicates with the exterior at the posterior end of the body. The male apparatus consists of usually two testes (Fig 90, t) enclosed within the ligament, which is attached below to the wall of the evertible bursa. From each testis a duct passes backwards, the two soon uniting to the single vas de- ferens winch, after receiving the ducts of some unicellular glands {gl), opens into the bursa at the tip of a muscular penis (p). The bursa when everted is a somewhat funnel-shaped structure at the bottom of which is the penis, the edge being furnished in some forms with hooks by means of which it serves as a copulatory organ. The life-history of the Acanthocephala includes a change of host The arv8B are found in the body-cavity of Crustacea or insects, and reach ma- turity only when the intermediate hosts are swallowed by the proper final host. The largest species of Echinorhymhus is the E. gigas, which occurs in the intestine of the pig ; the intermediate host of this form is the June bug {Melolontha, Lachnosterna). Nothing can as yet be stated with any certainty concerning the relation- ships of the Acanthocephala. They are usually associated with the Nema- todes, to which they certainly present similarities, but no intermediate ionns bridging the gap between the two classes are yet known, and the embryological history throws little light upon the question SUBKINGDOM METAZOA. TYPE NEMA TIIELMINTHES. I. Class NEMATODA—With digestive tract; without proboscis furnished with chitinous hooks 1. Order A'^«i^//w/rWa. -Musculature of body-wall interrupted along the lateral line ; no mesentery ; no peritoneal epithelium An- (fuil/ula, Tylenchus, Heterodera, Trichina, Asvaris, Oxyuris Tnc/iocep/ntlus; Dochmins, Filaria. TYPE NEMATHELMINTHE8. 183 2. Order t?oyv/weea.-Musnilature of body-wall not interrupted along the lateral line ; wuh mesentery and peritoneal epithelium. Qordms, Mermis. II. Class ACANTHOCEPHALA. -Without digestive tract; with proboscis armed wuh recurved chitinous hooks; parasitic throughout. EcJitnorhymhus. I LITERATURE. R. Leuckart.-2?^e Parasiten des Menschen. 2te Aufl. (In course of publica- A. Sohnnier.—MonogmpMe der Nemntoden. Berlin, 1866. L. Orloy.—Monographie der AvgtdUuliden. Buda-Pesth, 1880 J. G de V.^n.-DieeAnheimuche,,, frei in der reinen Erd'e und in sussen Wasser lehendenmmatod,n. Tijscbr. cK Nederland. Dierkund. Vereen v 1880 T.Vejdov8ky-* 190 INVERTEBRATE MORPilOLOOY. !=i;i M having iu such cases the form of a sucker {Philodhw), or ter- miuatiug m two movable lamellae, Brachionus (Fig, 93), or else being entirely wanting (Asplanchna). The body, with the exception of the anterior portion or trochal disk, which bears the bands of cilia, is enclosed in a chitiuous cuticle, occasiojiallv comparatively thick and firm, forming a case, the lorica, into which the softer parts may be withdrawn, and frequejitly pre- senting a delicate sculpturing GJ or prolongations into spines. A few forius (Floscularia) se- - JV Crete a gelatinous case within Qy wi'.ich they live, foreign parti- cles being sometimes added to the secretion ; a species of Melicerta, for instance, building a case for itself of pellets man- ufactured from foreign bodies and arranged in obiiijue or spiral rows and cemented to- gether i)y the gelatinous secre- tion. ,p,., no D ; • . . '^'^^^ frochnl dink wiiich oc- *xo. vA.—BniciiioHus nrceolaria (after • ,i , . , „ cupies the anterior end of the body is but rarely circular in outline ; more usually it is ECKSTKINI, Br — ueive-ganj^lioii. cv = coMimctile vesicle. (-H = (linostivegliviid. M = miiscie. Ma = miistax. N = nepliiidicl caual. 0 = oye. Oc = ocellus. (h = oviuy. 8p = calcnr. lobed at its margins and mav even be se))arated into two parts. The margin o.i the disk is surrounded by one or two bands of i'ilia which follow the lobations, when two bands are present one being entirely prnooral and the other postoral in its position, so that the mouth lies between the two on the ventral side of the disk. Various differences of arrangement oi the bands are found iu dillerent species, one of them, the i)raH)ral, being som*- CLASS liOTIFEHA. 191 times discontinuous, as in BracMomis, or retluced to a few isolated patches, as in Asplanchna. It is a question whether the forms with a double band of cilia or those ill wliich it is single represent the more primitive arrangement. It may be supposed that originally there was but a single band which later became double, but it seems more probable that the double condition is tlie more primitive, from the fact of its frequent occurrence and also P'nce, when a single band is present, it seems to represent in some cases the pra>- oral baud and in others {Floscuhiria) the postoral one. Such a condition of affairs can be most plausi^'y explained on the assumption that originally two bands were present, and that in some forms the prttoral one gradually gained i)re-eminence in its development, the postoral one disappearing pari passu, while in the Flosculariidoe the reverse was the case. Beneath the cuticle lies the ectoderm, consisting of a layer of cells whose outlines cannot be distinguished, and within this comes the musculature of the body, which does not, how- ever, form a more or less continuous layer beneath the skin, but consists of aggregations of muscle-fibres into bundles which traverse the body-cavity in various directions, some running longitudinally and forming retractors of the foot and of the trochal disk, while others have a circular direction. The coelom, in which the muscle-biiudles and the various organs lie, is not lined by a special peritoneal layer of cells, but may be traversed by a greater or less number of delicate fibrils arising from amceboid cells and representing uudiflerentiated mesoderm. The mouth lies near the ventral border of the trochal disk, the ciliated bands serving to produce currents which con- verge toward the month-opening, and so carry to it food-parti- cles, which are then carried through the ciliated «i!Sophagus to the i-harynx, whose walls contain a somewhat compliciated comminuting apparatus, the ma-sfax (Fig. U3, ma), consisting of two calcareous bodies, the laaUei, of varying shajx', and sometimes also of a median body, the iitrm. By tl i action of muscles attachec] to the mallei, these parts of the appiiratus can be brought into contact with each other, and with the iucub when this is ])r('seut, the food-particles btung thus comminuted. From the pharynx the food passes through a shorter or longer tube lined with chitin, which is to Ik» regarded as a continuation of the pharynx, to the stomach, 192 iiV VERTEBRA TE MORPHOLOO Y, 3 usuiilly a globular cavity, whose wall i« formed by a layer of ciliated cells coutainiug fat-globules aud various other par- ticles, probably absorbed food-particles, these cells being covered externally by a layer of couuective tissue. Into the stomach there opens from either side the duct of a gland (gl), whose secretion is probably digestive iu function and which may be termed a digestive gland from its resemblance to similarly located glands in other invertebrates. The stomach opens below into the shorter o)- longer intestine, whose walls are lined by ciliated cells ; aud this in turn communicates with the terminal cloaca, which receives iu some cases the termi- nations of the excretory tubules and m..y be contractile. The cloaca opens to the exterior, usually on the dorsal sur- face, near the base of the foot, though iu some forms which live within a case the intestine bends forward upou itself, so that the cloacal opening lies further forward. The nervous system consists of a relatively large ganglionic mass {lir) lying on the dorsal side of the ajsophagus, from which nerves pass anteriorly to the trochal disk, and posteriorly to sui)ply a dorsal sensory papilla, the calair (Sp). In addi- tiou to this, two pairs of posterior nerves have beeu described, one of which i)asses to a sense-organ situated on each side of the body in its posterior third, while the other pair runs back- wards on each side of the middle line to near the posterior end of the body, giving off branches to the musculature as it goes. Among the sense-organs eyes (O) are very geuerally present, varying in number from one to several, and situated in the region of the supraiesophageal ganglion, with which thev are connected. Tiiey consist of patches of red, brown, or black pigment with which sensory or retijial cells are asso- ciated, ami which are in some cases covered by a refracting lens formed as a special cuticular thickening. Other sense organs to which a tactile function has been ascribed con sist in their simplest form of one or several cells beariii ■ stiff cilia. A jjaii of such organs is usually present, one on each side immediately above the ganglion of the iateiul nerves, and anteriorly in the mid-dorsal line just behind Uw. trochal disk a third occurs, the cnlcar (Sp), which freoueutlv is situated upon the extremity of a tubular extensible ])iocess CLASS ROTIFEHA. 193: of tlie body-wall, supplied with muscles for its retraction, aud to which uerve-fibres pass from the supraoesophageal gauj^lion. Ill a few forms, such as Melicerta, the calcar is double. No blood vascular system exists, b'lt a well-developed excretory apparatus {N), resembling that of ilie Turbellaria, is present. It consists of two longitudinal tubes, one on each side of the body, from which arise a varying number of tiner lateral branches, each of which terminates in a funnel closed by a iianie-cell, as in the Turbellaria. Anteriorly the two Uibes may be united by a transverse connecting tube, and posteriorly they may unite together to form a contractile bladder which opens into the cloaca, or in some cases may Dpen directly to the exterior. The female reproductive apparatus consists of a relatively largo ovary {Ov) which in some cases at least consists of a vitellarium portion and an ovary proper, the whole being surrounded by a thin membrane a backward prolongation of which forms an oviduct opening into the cloaca. The preceding descrii)tion of the structure of a llotifer is that of such a form as is most frequently met with. It was for a long time believed that these were hermaphrodite, but no trace of a testis could be found. It was later found, how- ever, that they were all females, and the males of several species have been discovered, differing decidedly in size and structure from the females, and besides being usually rather rare in their occurrence. They are considei ny smaller than the female, and possess like it eyes, nerve-ganglion, muscles, and excretory system ; but the ciliated band of the trochal disk is single, and the digestive tract, with the exce])tiou of the cloaca, is reduced to a solid band of tissue. The single testis occupies the greater portion <>f the body-cavity, and the short vas deferens opens into the cloaca, passing through an evertible iutromittent organ. This marked difference of ft)rm of the male and female indiv'duals of the same si)ecies constitutes a phenomenon known as sexual (iliuorphism. An expbunitiou of the usual numerical preponderance of the femah^s over tiie males is to be found in tin* fact that under favorable condi- tions the females produce '>vh capable of developing }>arthe. uogenetically, and giving rise in all cases to females. A series 194 IN VBHTEBliA TE MOliPIIOLOG Y. inn of geiieratious reproducing by these so-called "summer ova " may thus succeed each other without the interveution of a male. Under certaiu coiulitious, however, certain females pro- duce " summer ova " of a smaller si;«e than usual, which, devel- oping partheuogeuetically, give rise to the males. In addition to these two forms of " summer ova," some species produce a third kind of egg, the so-called " winter ovum," which differs from the summer ova in containing more yolk and in being enclosed within a stout resistant shell. It seems probable that these ova develop only after fertilization. Thoro.-uv two liotifers wliicli do.servo a spocialdescriiXion on account of thoir having served as a basis for pliyio^c^nctic spocnlation. One of tiiose, TrovhosplHrm ( Fig. 94), is spherical in sliap«' ; a l)aiid of cilia runs rouisd the equator of the sphere, not encirc- ling it completely, however, but leav- ing an unciliated region on the dorsal In surface. Anteriorly this baiul passes |M above the mouth-opening, which is ^^^** bounded below by a very small post - oral band and opens into a pharynx provided with a inastax (Mi), from whicii the stomach, with digestive glands, passes towards the centre of the body and liiere bends at rigiit Fm. QL—Trochosph,rra (tquatovialis angles to open liirough tlu- intestine ' i'l'o ii cloaca (C/) whicii receives the cxcrelorytubuIes{/i» and the oviducts nnd opens to the exterior at tiie lower pole of the sphere {A). The brain (.V) lies above the pharynx and sends nerves to the two eyes siliialed, one on each side, below the e(iuatnri,il band of cilia, and also to a small sensory papilla (.SV)), proi»abIy the calcar. lying on the dorsal surface. this nerve (//) encireling the anterior half of the sphere, and running in a plane at right angles to that in which the eili;ii(>d l»and lies. The other form belongs to the genus mrarffiiv and differs from other Rotifers principally in the oeciirrenee of six hollow processes of the ImmIv arising from the ventral surface and arranged in pairs diminishing in si/e trom before Iwickwards. Kaeh is terminated by a bunch of .stiff i)ri.stles „>■ seta', and all are supplied with mu-scles whereby they can be rapidly swept (iiftor Sk.mi'kh). A = anus Cl = eloHCM. E.r - excretory tube. M - iiiouih. Ma = iiitistax. mil - muscle. N = nervegaiigllou. n = nerve. 0 = ovary. 8o - seiiseorgaii. OIWER GASTROTlilCUA. 195 backwards in the manner of a paddle and so serve as locomotor organs, i)ro- ducing a quick jerky movement quite different from the steady progression ojuised by the cilia of the trochal disk. In another nearly related form, I'edalhn, six processes are also present, but are ari-anged somewhat differ- ^■ntly from those of Hexarthnt, the largest one arising from the ventral and another from the dorsal surface, while the other four are lateral iu position, two occurring on each side. The Affinilies of the. Jiot/feru.—Saveval views have been advanced as to the affinities of the Rotifers, especially as regards their relationships to higlier forms ; these opinions will not, however, be fully considered here, l)ut merely indicated, attention being directed first to the relationships in which the Kotifcrs stand » > organisms lower in the scale. In this connec- tion the excretory .system becomes of no little importance on account of its resemblance to that of the Turbellaria, a resemblance which is further emphasized by the nervous system,— consisting of the simple brain, from wliieh posteriorly-directed nerve-cords arise,— by the combined ovary and vitellarium, and by the ab.sence of a blood vascular system. Here, how- ever, the resemblance ceases, and the presence of an anal opening to the digestive tube marks the Rotifers as standing on a higher level than the Turbellaria. It .seems probabh;, however, that the similarities do indicate the ancestry, and that the Rotifera have been derived from the Turbellarian type. Another possibility which has been suggested is to the effect that they are derived from the form represented by the Trofihophore larva of the Annelida (see p. 2i;{). The principal argument for this view is found in the arrangement of the trochal cilia, which, in the occurrence in many cases of both ])rieoral and postoral bands, certainly resembles not a little that of tlie Troehophore larva. It must be remembered, however, that the similar- ity ii: the arrangement of the cilia is not quite perfect, and that it may be without phylogenetic significance, having been accpiired indejtendently in the Rotifers and in (he Troehophore larva ; and furthermore it is noticeable that in one important eiiaracter at least a marked difference is found, the nervous ganglion lying in the Rotifers beliind instead of before the pru'oral band of cilia. The most that can be said at present is that the Rotifers show closer struetural aflinities to the Turbellaria than to any oth(!r group, and that it is jirobable that they represent the culmination of aline i>\' cpht posteriorly to end usually in oue or two cercnl jirocesses, uud anteriorly show n dilutatioii succeeded b- u more or less well-niiirked narrow region, the two giving rise to a head and neck. The body is covered upon the outside by a ciiticle, which may be smooth as in the genus Jchtluidinin, ov take the for-n of overlapping scales as in Cha'tonoim\Yh^. 95), sometimes bearing spine-like prolongations. Along the ven- tral surface two bands of cilia run from the posterior part of the head region almost to the hind end of the bodv, and in addition to these patches i)f cilia are found upon the ventral surface and on the sides of the head, some of which are undoubtedly sensory in func- tion. Beneath the cuticular covering lies the ectoderm in the form of a laver of i)rotoplaHm in which no cell outlines can be perceived, but which contains numerous scattered nuclei. A pair of longitudinal muscle-bands lie be- neath the ectoderm on the dorsal sur- face, and other bands traverse the coelom in anantero-posterior direction. Transverse and circular muscles are, however, absent. A distinct cadom is present, the greater portion of which is occupied, however, by the internal organs ; it is not lined with a i)eri- toneal epithelium, nor are any mesen- teries j)resent. The mouth is situated at the an- terior extremity of the body and opens into a muscular (esophagus {w), which opens in turn into the cylindrical stomach (/"). To this succeeds a short intestine opening to the exterior at the posterior extremity of the bodv. No blood vascular system is present, but the excretory sys- tem consists of a single i)air of much-convoluted tubes (nephv) -which terminute at one ep.-l i.. « closed ciliated "funnel" Fio. 9r). — C/i,ftoiiotus maxi- III NH lafier Zklinka). Cff = IHTVt' ifJl'lglioil. Ur - gliiiuis. i = iutt'siiiie. vi = liiiigiiiKiinal inusclo.s. iie/)/ir = nepluidiii. w = u^isoplmgiis. or = ovary. OlWEIi OASTItOTIilCflA. 197 while ut the other the_^ open on the ventral side of the both' to the exterior. The reproductive system (ov) ccnisists of two groups of geriu-cells lying in tlie posterior part of the body, one on each side of the digcistive tract, but no oviduct has been (IcHnitely made out to exist. With regard to the testes some uncertainty exists, an oval body lying in the same region of the body as the ovaries, but beneath the intestine, having been (Itiscribed as such an organ, though the idtMititication is o])en to (piestion. If, however, the body in question be the testes, the animals are hermaphrodite. As in the case of the female organ no ducts have been observed leading from the testes, and nothing is known as to the method by which the sexual products are extruded. The nervous system {n) consists of a large ganglionic mass which lies above the (esophagus in the head region, and from the posterior border of which two processes, one on each side of the middle line, are directed posteriorly and dorsally, p(U-haps representing the origin of a pair of nerves, Avhile the l)ostero-external angles of the ganglionic mass are continued l»ackwards to near the posterior extremity of the body to form the hiteral nerves. Certain of the elongated cilia found on the head no doubt function as sense-organs, coming into intimate connection at their bases with the cells of the supra- (esophageal ganglion ; in additicm to these sense-organs eyes have also been described as occurring in some species, either in the form of sim})le patches of pigment lying in the integu- ment above the brain, or else of such i)atches provided with lens-like structures. TIk! jiffinitics of tlio Gastrotrkha seem almost certainly to be with tiie Uotifora, iiiiuiy of lii« stnictiiral f.'atiu-cs being oxfoodiiigly similar in the two groups. The principal ditrereiices arc to be found in the arraiigeuHitit of the cilia and in th.^ structure of the nephridia. With regard ''to the former it seems not improbable that in the arrangement seen in the Gastvo- trirhi a relic of a more primitive uniform ciliation is presented, and that in this particular as well as in the greater simplicity of the digestive tract, ■■•lid in the general form of the Ixxly and life-habits, the Omtrotrhha api)roach more n.'arly an ancestral Turbellarian form than do tlui liolifera. Tlie nephridia depart much more widelv, however, from the Turbellarian condition tlmn do those of the liolifera -a fact whic^h argues against fh( more primitive character of the Gastrotrich of ducts for llio teproductiv n. as does likewise the ai.'scn* e organs. Whether, therefore, the Gaslrotrk'ha 108 INVERTEBRATE MORPIIOLOOT. i ! m '■ \ are to bo considenui as representing the ancestral form from which both they and the Kotifera liave descended more nearly than the latter group, or wliether they are moditieations of the Kotifer type of structure and havo had tor their ancestors forms whicii were Kotifer-like in structure, it is difficult to say ; thougli the balance of evidence seems to tij) in favor of the former view. Attention should be called, however, to a possible affiliation of the Oas- trotrutlia with the Echinodera. If, as has been suggested (p. 186), the seg- mentation of the latter has no phylogenetic signiticance, it is not difficult to trace similarities of structure in the two groui)s, the principal differ- ences being connected vvitli external parts. It is by no means improbable tliat the Gastrotriclia, Kotifera, and Echinodera form a series, eacli of the groups being of equivalent rank, and related to each other somewhat as are the Turbellaria, Trenuitoda, and Cestoda. Genus Dinophilus. The geuus Dinophilus includes some small marine organ- isms all of which are • ferable to a small number of species. The body (Fig. 96) is cylindrical and consists of a head segment followed by from 5-7 trunk segments (tlie number varying according to the species), each of which bears a ring of cilia, inter- rupted ventrally by a uniform ciliation which covers the entire ventral surface. The head is likewise provided with a ring of cilia which is usually double, one of the constituent bands passing in front of the mouth and the other behind it, the area in- tervening between these two bands being, in one species at least, occupied by smaller cilia. The musculature of the body-wall is but weakly developed, though both uu external layer of circular fibres and an in- ternal one of longitudinal fibres may be found, both layers being absent iu o\w species in the dorsal region. The coeloin is traversed by a network of branching cells, there being no special peritoneal layer, and no musculature in the walls of the intestine. The mouth is situated on the ventral surface at the juno- 1 of the head and first trunk segments, and leads into a Fig. QQ.— Dinophilus fU/rociliatus (after ."^UVKRt. «(' = nepliridium. or = oviuy. sg = salivary gland. OENUS DJNOPIIlLVa. 109 wide ciliated oesophagus, beneath which lies a mu^.ular pro- boscis contained in a special sheath and protrusible through the mouth-opening. Behind the oesophagus is a proventricvL, a sma 1 thick-walled ciliated cavity, into which, at its junction with the CBsophagus, a pair of salivary glands {sg) pour their secretion. Behind, the proventriculus communicates with a cylindrical stomach, upon which follows the short straight in- testine, terminating in the anus at the posterior end of the body. There is no blood vascular system. An excretory system is present, consisting in D. gyrociUatus and D. tceniatus of five pairs of nephridia (ne) which open externally on the sides of the body and terminate in the coelom-spaces in a funnel con- taining a flame-like bunch of cilia. Whether a direct commu- nication between the lumen of the nephridial tubes and the CO. om exists in all cases has not been definitely ascertained, but a similarity of structure to the Platyhelminth tvpe of iiephndium is shown by the flame-like bunch of cilia ^nd by each nephndium being composed of a series of perforated cells Jhe reproductive organs are separated in diflferent individu- a s ; and in one species, D. gyrodUatm, a marked sexual dimor- plnsm similar to that occurring in the Rotifera exists, the luale being much smaller than the female and possessing only- he ciliated ring of the head and the ventral ciliation ; and furthermore the digestive tract and the principal sense- organs are entirely wanting. The reproductive elements (ov) are shed into the coelom-spaces and find their way to the ex- tenor in some species at least by means of the most posterior pair o nephridia, which in the male of D. ta^niatm are trans- tormed into seminal vesicles and are connected with an intro- ■nittent organ situated in the posterior segment The nervous system consists of a brain or supraoesophageal .aughonic mass which occupies the greater portion V^the dsTr iT l'-'^'^ .^'^'^'^ *- nerve-cords pass back- AN aids m the lateral region of the body, and in D tmiiatm possess ganglionic enlargements equal in^'number to t .e: t nephridia and the trunk segments and are connected by tiausverse commissures. In otb^r «r.n.ieo i.^^.^^- ., ^ -uctures have .ot been o^.^,. ^ E^lt'oetTLbeS 200 INVERTEBRATE MORPUOLOOY^ iu the substance of the supraoesophageal ganglion, and tactile hairs occur at various regions of the body. Affinities of Dinophilus. — The descriptions given of the various known species of Dinophilus indicate a considerable variation in the structure of certain parts, more especially of the nervous system, which in D. tivniatus partakes of the metamerism shown by the uepliridia and the bands of cilia, while in other forms it is apparently non-metameric. Tliis would indicate either that the metamerism has been acquired within the limits of tliu genus, or else that those forms lacking it are degraded in this respect and have descended from metameric ancestors. There is little justification to be found, however, for the calling in of degradation to explain obscure re- lationships unless there is sufficient collateral evidence to support such an appeal ; in the present case this seems to be absent, and the marked simi- larity of the non-metameric nervous system to that of the Turbellaria sug- gests an origin from these forms and favors the first hypothesis as to the origin of the metamerism. The nephridia also and the character of the coelom strengthen the probability of a Turbellarian ancestry. A close relationship to the Rotifera has also been suggested and is not debarred by the supposition of a descent from Turbellarian forms ; but it seems doubtful if such a relationship can be other than a very distant one. Tlie position of the supraoesophageal ganglion relatively to the cephalic cilia ov prototroch, and the paired arrangement of the nephridio as well as the occurrence of circular fibres in the subepidermal musculature, stand in opposition to the view, and the most that can be said is that both Dinophilus and the Rotifera are to be referred back to closely-similar ancestors. The affinities of Dinophilus and the Rotifers to the Annelida will bo discussed in connection with the latter group (p. 217). SUBKINGDOM METAZOA. Order Eehinoder a.— Body cyUndrical, with 11 rings ; no cilia ; with pro- boscis ; minute forms ; marine. Echiiioderes. Class Oh^tognatha. — Marine; body divided into three segments; witli lateral and tail fins ; mouth with chitinous jaws composed of series of strong bristles. Sagitla, Spadella. Class Rotifera. — Anterior end provided with a retractile crown of cilia ; minute forms both aquatic and marine. Floscularia, Melicerta, Lacimilaria, Philodina, Brachionus, Asplanchna, Trocho- sphcei'a, Pedalion, Hexarthra. Order Gastrotricha.—Mmnte forms both marine and aquatic ; ventral sur- face of body ciliated ; no anterior crown of cilia. Ichthydiuin, Chittonotus. Gqhws, Dinophilus.— ^maW marine forms; body with 5-7 segments, each with a ring of cilia. GENUS DlJSOPHlLUa. 201 LITERATURE. ECHINODERA. W.Keinhard. Kinorhyncha (EcMuoderes), ihr anatomischer Ban und ihre btdlung im System. Zeitschr. fUr wissensch. Zoologie, XLV, 1687. CHiETOGNATHA. O.Hertwig. Die Chmtognathen. Eine Monographie. JeBaische Zeitschr. . xiv. ROTIFERA. C. T. Hud.on and P. H. Gosse. The PMifera or Wheel-animalcuUs. London, ^ ^^T' -.Sf '^^' ''''' ■^«^«^5'««cA^A«e der Rotatorien. Jenaisclie Zeitschr. XIX, looO. ' C.Zelinka. Studien ilher mderthiere. Zeitschr. fttr wissensch. Zoologie XLiv 1886 ; XLVir, 1888 ; liii, 1891. ^ GA8TR0TRICHA. C. Zelinka. Die OastrotricJien. Zeitschr. f ttr wissensch. Zoologie, xlix, 1889. DINOPHILUS. 1. Korachelt. UeberBau und Entwicklung des Dinophilvs apatris. Zeitschr tur wissensch. Zoologie, xxxvii, 1883. ^xx;n;788r°' ^*''^'^^"^"*^^«*- ' Q"^"«^Jy J«^r'^- of Microscop. Science, iii-iiiiMiiif 202 INVERTEBRATE MORPHOLOGY. CHAPTER X. TYPE ANNELIDA The type Annelida includes a series of forms amoiif:^ which metamerism reaches a high grade of development. In what may be considered a typical Annelid (Fig. 97) a number of segments or meta- meres succeed one another from the head to the tail, each one resembling its predecessor and its successor in all its parts ; the nephridia, reproductive organs, nerve-ganglia {n), and appendages, when present, are repeated in each successive segment, and each metamere is marked off from its fellows, externally by a groove surrounding the body and internally by a partition or dissepiment extending transversely across the ctelom from the body-wall to the digestive tube. This latter structure and the blood vascular tubes cannot well from the nature of things be divided metamerically, but are continuous from one end of the body to the other, showing, however, in the meta- K^a meric pouches and intenne tumeric con- ■n. nry rkr.^« ,, ^™ strlctlous of tlie digest'* ve f .ict and in Fig. 97. — Diagram of . . General Plan of an the metamerically arranged lateral vessels of the blood vascular system which encir- cle the digestive tube, indications of the di- vision which has affected the other organs. Two segments, however, the heiul (pr) and the tail, usually present differ- ences from the rest in their structure ; the head or anterior metamere boars senso-orgaus when ANiNELID. a = :uius. ce = cerebral ganglion. m ™ uuiu It n — "'n •• ' nerva co»"d. pr = j>ro9'< laiiiui. TYPE ANNELIDA. 208 tliese are developed, is destitute of uephridia in tlie adult, and contains priuiaril" the sui)i-aoesopliageal ganglion of the nervous system (Fig. Ml, ce), the ganglia of the trunk meta- nieres (n) lying ventrally to the digestive tube; while the tail segment bears the anal opening and usually presents other characteristics which distinguish it from the preceding seg- ments. It is rare, however, leaving aside this antero-pos- terior differentiation, that a perfect metameric condition is found in any Annelid. Secondary changes may interfere with the similarity of all the metameres ; a suppression of parts usually present in some of the segments may occur, as, for instance, where the reproductive organs are confined to one or two metameres, or again there may occur a differentia- tion of the anterior appendages for a special function where- by a marked dissimilarity between the anterior and posterior metameres is produced. Finally, owing to peculiar habits of hfe, the metamerism may be almost or completely lost, being indicated only, perhaps, by one set of organs, such as the nerve-ganglia, or else only evident in the larval stages. Para- sitism or a fixed or tubicolous habit of life are among the principal causes of this degeneration, examples of which will be seen later. In consequence of this degeneration some Annelids pre- sent a metamerism of a lower grade than that found in such forms as the Nemerteans. Other peculiarities of structure occur, however, which serve, together with the indications of metamerism, to mark out the Annelid type. One of these peculiarities is the occurrence in nearly all forms of a series of nerve-ganglia along the ventral nerve-cords ; this feature i^- of course a part of the metamerism, but it is not usually marked in the metamerism of the nervous system seen in lower forms. In these scattered ganglion-cells occur all along the nerve-cords, which extend backwards from the brain, while in the Annelids these scattered cells are associated together to form metameric ganglia. Another peculiarity is fonud in the structure of the nephridia. These are no longer m all cases rows of perforated cells closed at the inner end by a flame-cell, but may consist of more or less convoluted tnhp.n lined by ciliated epithelium and open as a rule into i 5 204 IN \ Kli TKnitA TK MOUPUOLOU Y. the cMoloin by a wiilt^ l'miiu>l-lik(> dxirornify. Provision;*! ki(lll(^vs of tlio Tiirbollarijiu ty[)o oinuir in tlie liirvir of injui\ Annolids, but tho iio[)hri'lipod in the i)oritc>iioal lining of the Cddom ami are not usually (except in the Hirudinoa) provivlod with siiecial dm'ts. When mature the ova or Hpermatt)zoa are simpiy shed into the C(elomic cavity and u>ake their way to the ext«M'ior through the ordinary nei)hridia, or through nephridi-ii MptH'ially nu)ditied for Hie pur})ose. Fii.iilly it may be mem- tionetl that a blo(«d vascular system is usually preseut. I. Class riiii>to|MMla. The Olnetopoda are Anuelids in which the external t^eg- mentation of the boily corresponds with the internal seg- mentation of the organs, and which bear along tho sides of the body two rows of pouches, the seta-sacs, the cells lining which Hocrete chitinous spicules or seta' of various shapes, which serve for the purpose of locomotion or in some cases consti- tute a d(>fensive armanuMit. The class is ctinvenientlv divisible into two aubclasaes. Subclass I. POLYCHJETA. The forms iududeil in this subclass are exclusively ma- rine, and are characterized by the presence ou tho sides of a gr»>ater or less number of the i;i(>tameres of a })air of hollow processes of tlu^ body-wall ui>on which the seta-sacs occur and which are known as pto'ttpoitia. In a few forms {iSerpiiln) the i^ara podia, and indeed the seta* as wi>ll [I'oIi/i/ontiNs), may be al»s(Mit, anil in others, such as (Ij^nH'm'lla, they may be very nnu'h reduceil in size, but as a rule they possess a higii ile- gree of development. In its typi«^al form a parapodinni (^Fig. 08^ consists of a iU)rsal and a ventral lobe (>ach of which bears s»»ta-sacs ami setoB {s). Towanls the base of each lolit> there may fretjuently be found a slender hollow process, the dorsal and ventral cii'via, {dc and ve), and j)late-like or more or less dendritic appendages, the hrancliia' (ftr), either modili- TYPK ANNELIDA. 205 cutions of tho cirri or bruiiclies uriHing from tlioin, and respiratory in function, also occur. Muscles puss from the l)()(ly-vvjill to tin; piirai)o(lia, which thus l)()(H)nio important organs of locomo- tion and in somo of the actively swim- ming species assume a more or less flattened ])late-like form. Tli(! hiiad segment is generally well d'fierentiatcul from those which succeed it, l)(>,ing destitute of parapodia and setie, and as a rule carrying a iiund)er of appendages sensory in fun(;tiou, and being likewise usually provided with eyes. The cephalic appendages nun ^''"- ^«— T'AKAi'onnrM <»p 1 1 , 1 ,, L L f • " Nereis nrciiti. i)e short and rather stout, formmi; what are termed •/)(»/;>/ (lig. 100, p), ,/« ^ ,io,sal dmus. or somewhat longer and more slender, s = sciu'. forming the ('//t/ (r), or eveii still moit^ "" = ventral cirrus, slcmler, being then known as tentacles {t). Tlui body is enclosed in a chitinous covering secreusd by the sul)jac(!nt ectoderm, here known as the hifpodermw {Vifr, '.>;), ////). 'V\\o. musculature of the body-wall which lies bidow the hypodermis is separated from this by a basement-mem- lirane and consists of an external layer of cinndar fibres (rm) aiiil a subjaci^nt layer of longitudinal fibres {hn) which is, as .1 ruh), interrupted iu the iidd-dorsal and ventral lines and also in the region of the two lobes of the parapodia so as to form four bundles. Special muscles extend from the bod}-- wall to the base of the seta-sacs, and fiirtheimorea ]>Mir of muscle-bands cross the cavity of each nnitamere, in typical cases passing from the lateral regions of the dorsal surface downwards and inwards to be inserted into the v of the median line. The inner surface of the longitudinal musch^-layers is liniul with .^ layer of fx-ri- foiinil rclls which ct)mpletely enclose the codom {soph(i(jc<(l ganglionic mass situated in the head segment, fre(|uently presenting a division into s(n- oral lobes. From it various nerves arise passing to the an- terior segment, and in addition a strong cord passes from it ventridly on eith(>r sid(> of the (x>sophagus to unite with a gaiiglion lying below the (i'soi)hagus in the second metamore, fi)rming the circiiiiucsop/Kiijnd c()mini.s.snr('. To the suha-Nop/Kt. (ji'dl tjaiKjIlou of tlu^ second nu^tamere there succeeds a pjiir of ganglia in each nn^tamere, each pair being united with the prec(>iling and succetMling pairs by two longitudinal cords of nerve-hbres, the ran iicrf Ives, the whole constituting the ventral m*rve-chain, and furthermore the ganglia of each pair are united by a transverse commissure. The ventral uorve-chain has therefire a distinctly ladder-like arrange- ment, freipiently somewhat obscured, however, by the api)roxi. mation of the ganglia of each pair and a consecpient shorten- ing of tlu> traiisv(>rs(> commissures. From the v.irioiis ganglia nerves arista which pass to the musculature of the metam(>res and to the hyptxlermis and its stMisi^-organs. In the major- ity of forms the nervous system Ii»>s freely in the co'lom surrouml ■ I bv a special sheath, but occasionally in various forms widely separattMl g(Mietically from one another, such as /'olj/iftti'i/im and the Opheliaceio, it presents a })rimitiv(^ char- acU-r in being completely imbedded in the hvpod(>rmis, rei'.dling the condition in ciMtaiu Nemerteans and in the Cniilaria. Special nerves arising from the sui)ra(est))thageMl ganglion are supplied to the walls of the digestiv(> trai't, form- ing the so-called sfonittfoifttNtrii' nerves. Sense-organs of various kinds are of fr(H]uent occurrence at ditVerent ])ortions of the body of tlu^ Polyclneta. In addi- tion to the cephalic and caudal cirri which are richly supplied with nerves and are presumably tactile in function, eves are of very general occurrence. Thev are nsiiiilly sit!i:ited on the heatl, sometimes iu connection with the hypoilermis and TYPE ANNELIDA. 2()» soinetiniea imbedded in the dorsal surface of the brain. For tho most part they consist of a cu}) of pigment-cells, in which numerous sensory cells are ])resent — a lens being in soiimj instances developed above each eye. Occasionally, however^ as in the pelagic genus Alciope, the eyes reach a high grade of development. In some forms they are not confined to tin* region of the head, as for instance in the genus Pohjophthdhum — so named from the fact that pairs of eyes are found on the sides of a number of the trunk mctameres ; in the majority of tubicoious Annelids eyes are found in considerable num- b.;rs upon the branchial lobes of the head sej^mrnt, <"lie genus Vermilui possessing somewhere in the neighborhood of 11,000 separate ocelli in this region. These eyes are simply differ- entiations of the ectoderm, and in many cases an; still situated in the hypodermis ; they consist of a number of cells which ar(! prolongtul at their inner ends into a nerve filament, while })eripherally tht^ir ])rotoplasm is converted into a refractive substance, each of these cells being separated from its neigh- bors by pigment deposited in its peripheral layers, as well as by a number of smaller pigment-cells. On account of this ])igment-sheath it is presunnible that each of these optic elements or omniatidin functions more or less independently of the rest, and the eyes are to be considered as compound, composed of a number of iudei)eudent parts each of which is ]>hysiologically an eye. Auditory organs or otocysts also occur in certain forms, but cannot be considered as typical of the Polyclneta. In Arenicohi they consist of two sacs lying in close proximity to the circunuesophageal comnnssures and connected with the i^xterior by a narrow canal, indicative ot their origin as invag- inations of tho hypodermis. The walls of tiie sack are formed by columnar cells terminating below in a plexus of n«>rve- tihrils and covered on the surface turned towards the cavity of the otocyst with a firm homogeneous cuticle, and not pos- sessing any terminal hairs. In the cavity a varying number of spherical particles of carbonate of lime, the otoliths, are found. In some forms a number of such otocysts are present, as iu Aricia, where four or five pairs have been found in adult 210 IN I ■KliTKIiliA TK MOUrilOlAHl Y. I iii(Hvi»limlH; Iml. in tlio majority of wpocioH thoy do not seem to l>« dovtilopotl. Ciliatotl (lopn^HHionH wliieh liavo beou Hn[)i)OH(Hl to ho olfiu'tory ' Hvt> h.uMi (IohciIIumI jih occuniiif,' in tlio untorior rt>j'it»n of tli«^ Itotly in vjiriouM HpooioH, roiiching n high «|«u'ol- opni.'ntm l,ho (\ipil,olIi,h,., wjioro thoy f,)rni cIub-.shiip.Kl hiicIch ^«Hplll.K^ of luMng oviiKinjitod. In .uldition th.uo mo to bo f»)uiul HCHttt>ivd on tho Huifiu'o of tho body niinuto hnlcn'-shapcd doprossions, at th.> b.)lioni of which uio colls boaring long hairs and piosuniably sensory in function ; and fnithonuoro iu a fow forms, such as tho Ca|)itolIida> and roliiophtlndrnm, a Horios of sensory hillocks occur along tho sidos of tho body— a pair in oach motanu>ro, f»)rming tho sniscoiyaiiN of ffic la(eral litiv. In tlu' Capitollidio those organs aro in the nntorior mot- anuM-os containod in doprossions, hut uioro posteriorly thoy {>r.>joct slightly from tho surface. Tho central part oi oacli projection is retractile and is formed of a number of hair-colls, oach oi which is in coiin.>ctiou at its inner end with a norvo' fibril. No little interest attacJies io those organs, which forcibly recall, both in thoir structure aud distribution, tho lateral liiu> organs i>f tho lower Vortobratos. The nophridia (Fig. [)[), m% iu typical adult fi)rms, occur as a single pair in each motamoro »»xcept tho two terminal ouoa. Kach consists of n usually conti)rtetl or coiled tube lined with colls opening by a funnel-shaped mouth int.) the oa>Iom oi the motamoro, perforating tho dissepiment between it and tho next metamero in which tho greater portion of it lies and in which it opens to tho exterior by a sniall pore sit- uatoil on tho ventral surface oi tho body at tho base of tho parap..dium. It is rare, however, that "any such motamerio regularity of arrangement *)ccurs. and very fretpiontlv they become retluced to a small number, or even to two pairs ; in tho tubicolous forms a fow pairs are frocpientlv found in tho anterior portion of the body much larger than any of the rest. In aildition to their original excretory function thev may also serve as outlets for the reproductive elements, and income oases become specially moditied for this puri)ose aud lose their original function. TYPK ANNELIDA. 211 Til CapUt'Ua only one pair, thiit of tho oiglith motamorc, bocomcs con- voHdd into a k.U8 teeth. To this order l)Ginng the genera Nereis (Fig. 100), usually found lurking beneath GO 212 INVERTEBRA TE MORPHOLOQ T. •Jil stoues during the day-time, but becoming, in some species at least, free-swimming at night ; Lepidonottts, characterized by the possession of elytra arranged in overlapping series on the dorsal surface ; Diopatra, which forms tubes for itself by glu- ing together particles of foreign matter; and Autolytus and Syllis, which are peculiarly pelagic in habit, as is also Alciope, characterized by its large highly-organized eyes. 3. Order Sedentaria. This ordor includes a number of forms which manufacture for themselves tubes of various substances— some being merely composed of particles of sand glued together by an adhesive secretion, while others consist of a chitinous sub- stance, to which foreign bodies may be added, or even of car- FiG. 100,— Anterior End op Nereis virena, c = cirrus. p = piirapodium. t Z tentacle ^^^' ^^^•~^'"P''"'<»''^ ornaia (after Vehrill). bonate of lime. Within these tubes the animals permanently reside, and in conformity with this mode of life numerous adaptations of structure are found. The head is usually pro- vided with a number of loner nirri nnA fViQ l^voT^plj."— „-.„ J_„ xu^ most part confined to the head region. In some forms, such TYPE ANNELIDA. 213 as Serpula and Sdbella, plume-like branchiae supported by an axial cartilage-like skeleton occur upon the sides of the head, and numerous eyes may be found in the hypodermis of these structures. Parapodia are as a rule but slightly developed, sometimes being entirely wanting though the setje persist, those of the lower parapodial lobe being usually hook-like. The protrusible pharynx with chitinous teeth does not for the most part occur. Amphitrite (Fig. 101) lives in tubes in sand, while Terebella composes tubes by gluing together particles of sand. In Sahella the tubes are membranous in character, Avhile Serjnda manufactures more or less contorted tubes of carbonate of lime. Development of the Poli/chceta. — An important feature in the development of the Polychseta is the occurrence of the Trochophore larva. A typical example of this larva is to be found in the development of Polygordius ; it is a transparent organism, haying the form of two Ioav cones united by their bases (Fig. 102). Just below the junction of the two cones is the mouth (31), leading by a short stomodmcm or oesophagus into a retort-shaped stomach, the intestine opening at the apex of the lower cone. Above the mouth, along the line where the two cones are united, lies a band of strong cilia arranged in two rows and forming an almost complete girdle for the body, being wanting, however, in the mid-dorsal region. This is the prototroch (pro) or praeoral band of cilia, and par- 1 to it is a second weaker band which passes behind the . th — the paratroch (po) or postoral baud. The slight , between the two bands is lined b}- fine cilia, the adoral Cilia, and in some Trochophores a band of fine cilia extends backwards along the ventral surface of the body towardo the apex of the lower cone. At the apex of the upper cone is a strong thickening of the ectoderm, the apical plate {ap), which is nervous in function and bears a number of strong cilia and may also have imbedded in it pigment-spots which function as light-percipient organs. From the apical thickening four nerve-cords (w) extend back- wards, one being dorsal, the other ventral, and the remaining two, stronger than the others, lateral. A series of fine nerve- rings arranged concentrically about the apical thickening unite 214 INVERTEBRA TE MORPUOLOG Y. these cords at regular intervals, the lower riug being con- nected with the cells which bear the protrochal cilia and forming the prototroch nerve. At the apex of the lower cone and ventral to the intestine lie two cells, or two masses of small cells, which constitute the mesoblasts and give rise to two longitudinal mesoblast bands {mb). A few scattered cells are also found between the ecto- FiG. 102— Tkochophoke of Polygohdius (after Hatschek). ■4 = fiuus. mb = inesoblast-baiid. ap = apical plate. ne = uepliridiiim. M - luouth. pro = prteoral band of cilia. m, m', m" = muscles. po = postoral band of cilia. derm and the digestive tract, some of which elongate and be- come muscle-fibres (w), and which have been thrown off from the mesoblast bands. In some forms a band of muscle-fibres underlies the prototroch cells. In the neighborhood also of the mesoblast bauds in the posterior cone there occurs on either side of the digestive tract a small, sometimes branched, tubular body, the head-kidney (ne). Each kidney consists of a row of perforated cells, terminating in a funnel-shaped structure closed at its mouth by a cell, the whole structure thus agreeing closely with the uephridia of the Platyhelminths. From such a larva the adult condition is derived bj' the gradual elongation of the posterior part of the body an elongation wHh Avhich the mesoblast bands kepn nanp i^lie mesoblasts retaining their position at the posterior extremity^ TYPE ANNELIDA. 215 and continually adding to the bands by the formation of new cells. The bands as growth proceeds break up into a n mber of masses, the mesoblastic somites, in the interior of which cavities appear, and adjacent pairs of masses growing dorsally and ventrally finally come into contact above and below the digestive tract, the dorsal and ventral mesenteries of the intestine being thus formed, and later a metamerization of the body-wall, corresponding with that of the mesoderm, also takes place. The anterior cone of the larva, which at first sur- ])assed in size the posterior one, gradually becomes smaller, aud the prototrochal cilia, and in some cases the cells also, are thrown ofi". The apical plate takes i)art in the formation of the supracje.sophageal ganglion of the adult, aud the lateral nerve-cords arising from it form the circumoesophageal com- missure, becoming connected with thickenings of the ventral hypodermis arranged metamerically and representing the ven- tral chain of ganglia. The head-kidneys gradually disappear, being merely provisional larval structures, and now nei)hridia of the Annelid type develop from the mesoderm of the trunk- metameres. Altliough the Trocliophore larva occurs in the hfe-history of many of the Annelids, as well as in other groups as will be seen later, yet never- theless it is not invariably present. In some forms a single l)and of Iar<'e cilia runs around the middle of the body, whicli is elsewhere uniformly ciliated, while in others the cylindrical larva is surrounded by several I'iiiids of cilia succeeding one another at definite intervals. In certain siK'cies the larva is provided with very long sette which are thrown off (iiiriiig larval life, and are interesting on account of similar setis having ))('en found in fossil forms, though absent in recent adult species. It is also worthy of note that in some forms the Trocliophore larva is succeeded by a well-marked stage in which, in addition to the head seg- ment, three trunk segments are developed. It is possible that this may represent an ancestral form from which certain other groups have taken tiieir origin. In what may be considered exceptional cases a non-sexual rci)ro- duction by budding also occurs. In the genus Protnla a zone of growth occurs in the sixteenth segment, and at this point later separation takes place, a new head developing for the po.sterior individual from the ori.rjual seventeenth segment. In one species of Syllis the new individuals arise not only in a linear series but al brandling colony is produced raniifviiig tl IIyalos{ ion so as lateral buds, .sn t!;at irough the canal system of iIk* 00 ge in which the form lives. The buds eventually separat e as 216 INVEHTEBHATE MOliPlIOLOG Y. sexually mature male and female individuals wliich, since they differ frta had been de- veloped by a process of linear budding, each metamere of the Polyclia-t body being equivalent to the original Trochophore and the adult organism being a co-ordinated succession of Trochophore individuals. Other authors, however, who do not see in metamerism the result of a budding of tlie individual, but rather the multiplication of its subordinate parts, are in- clined to refer the Annelids to a Nemertean-like ancestor and to consider the Trochophore larva a purely secondary adaptation. Between these two views it is difficult to decide, and it is possible that in their plain statement neither is quite correct, though each may contain certain elements of truth. It seems exceedingly probable that a larval form which is met with in the life-history of the Annelids and Mollusca, as well as in a modified form in other groups, has some ancestral significance. It is difficult on any other hypothesis to explain its occurrence in widely different groups, since it seems hardly probable that it coidd have arisen independently in scver.il instances. Convergent evolution could hardly be carried to such an ex- tent as to produce in the Mollusca, quite independently of any genetic relationships, a larva resembling in all its structure that of an Annelid. If the Trochophore occurred only in the Annelids, it might be quite po.ssible that it had made its appearance in the life-history of some primitive Annelid as a secondary modi lication of a more priniitiv(> larva, and had reappeared subsequently in the life-history of all forms descended from thi.s Annelid ancestor, but this would not explain its occurrence also in the Mollusca, unless it be supposed tliat the members of this group have been derived from the primitive segmented Annelid, a view that has little to recommend it. Tlie working of the biogenetic law (see p. 143) is interfered with in innumerable instances, and the distinguishing between examples of its action and secondary modifications is the most difficult task of the TYPE ANNELIDA. 217 cmljryologist. The evidence at present available seems, liowever, to point, in the case of the Trochophore, to its being an example of the law, and to tliis extent the first of the two views stated above is probably correct. But, on the other hand, this may not be the case with the second part of the theory. If the view as to tlie origin of metauieri.sui which is advo- cated in this work be correct, then the Annelid cannot be regarded as having ' risen directly by a prucess of reproduction by budding of the Trochophore. It is not a colony uf Trochophore individuals, but a single elongated Trochophore whose organs have undei'gone repetition, producing a high grade of metamerism. To this extent the second of the views may bo correct, but this does not necessarily imply that the Annelids are to be derived from a Nemertean-like form in which the metamerism is not quite so perfect. Metamerism, as here explained, is simply the following out into the higher individualities of the phenomenon of discontinuous growth or reproduction by division which characterizes the cell, and it is quite ])ossil)lo that there may be no more genetic connection between the meta- merism of the Nemertean and that of the Annelid tiian there is between tiiat of the Cestode and that of the Nemertean. It may have arisen cpute independently in the two forms, and in fact when the details of metamerism are examined in the two groups considerable differences are to be seen. The view here advocated in regard to the origin of the Polychaita may be briefly expressed as follows : The Polych«ta— and with them the Annelida in general— have had for their ancestor a non-metamenc form of ivhieh the Trochophore is the hirval represe)itntim, and this in the course of its decelopment elongated, the elongation being accompanied by tht^ repetition by a budding process of certain organs, a high grade of iiu'tanierization being thus produced. The relationships of the Trochophore seem to be with the Turbellaria. The nervous system, consisting of the apical thickening and lateral nerve- ords, is very similar to that of Turbellaria, and also it is interesting to notice the similarit^y of structure of the head-kidney with the Turbellarian nephridium. The exceedingly small development of the parenchyma is (tfobably a secondary condition, and the presence of an anus is an im- poitant advance upon the Turbellaria. An undoubted similarity in many respects exists between the Rotifera and the Trochophore, and the former have been regarded as persistent Trochophores or else as forms descended from the Trochophore. This latter view in one of its phases has already l-een considered (p. 195), and an important difference in the relation of the MipracKsophageal ganglion to the prototroch mentioned. Reasons have also been given for the belief that the Rotifera are descended from Turbellarian ancestors, and it seems probable that the line of descent of the Rotifers was identical for a time with that followed by the Trochophore the former group branching off from it shortly before the Trochophore ancestor Biade Its appearance on tiie scene. In this respect the Rotifers and the ")| 218 INVERTEBRATE MORPIIOLOG Y. gl Troclophore are related to each other, but hardly with so close an afflniiy as would be implied by a statement that they are persistent Trochophoros. II. Subclass Oligoch^eta. The OHgoclipeta are with few exceptious fresh-water or ter- restrial ChtBtopods, aud present a much simpler body form thau do the Polycha^ts. The first segmeut or prchstomiuiu (Fig. 10;j, pj') is devoid of tentacles or cirri, aud ouly iu a few forms are eyes present upon it. The body is divided into well-marked segments, but parapodia are lacking, though in the majority of forms setfe. arranged iu a more dorsal (s') and a more ventral is) group, occur on the sides of each metamere ; in a few forms, however, a single series of groui)s only is present, while iu PerichtvUt the setio are arranged in a rim: around each metamere, and in Amt- chvta they are Avanting, their place being indicated only by the sacks in which in other forms they are Fio. 103.-ANTEmonENuoF ^evehuied and which project' into Liiiiibrkiis ,, , , , , , the c(elom as large hypodermal glands. As a rule, too, no branchim are pr^sent, the blood being aerated through the walls of the body, mi- nute brancdies of the blood-vt'ssels » - iaictal MctiP. i)enetrating into the hypod terrestrial forms ; a few aberriint forms, however, possess either dnrs;d or ventral {(]ha'tnhrneai cells which along the dorsal and ventral lines is reflected towards the digestive tract, which it surrounds, form- ing a dorsal and ventral mesentery. The former of these fre- > ion, performing perhaps t xteut the part of the liver-cells of the Vertebr <) a cer- ata. 220 IN VEIi 1 'EBRA TE MOIiPlIO L 0 G Y. A circulatory system is always preseut, aud consists of a dorsal lougitudiual vessel lying on the dorsal surface of the digestive tract and a veutral oue lying below it, the two being united in more or fewer of the metameres by one or two lateral vessels ou each side. The dorsal vessel is con- tractile, the blood in it flowing towards the anterior extremity, and the lateral vessels are also usually contractile. Branches are giveu off to the muscles and to the various organs in reg- ular metameric succession, and additional longitudinal vessels are also found accompanying the ventral nerve-cord, two lateral and one ventral. In the terrestrial forms tine branches l)enetrate into the hypodermis, the aeration of the blood being thus effected in the absence of special branchire. 'ilia blood in the majority of forms is red from the })resence of hjenio- globiu dissolved in the plasma and contains colorless cor- puscles. As in the P(^lychieta the c(elom contains a hfcnio- lymph in which corpuscles tloat. The digestive tract forms a straight tube, extending from the mouth, situated on the ventral surface at the junction of the prostomium and tirst trunk metamere, to the terminnl anus. The mouth opens into a short mouth-cavity, and this into a more or less muscular pharynx, which in most cases can be protruded from the mouth and is slung to the body- wall by numerous radiating muscular bands. To it succeeds a smaller (esoi)hagus, which communicates posteriorly in ter- restrial forms, after in some cases dilating to form a sack-like thin-walled crop, with a muscular ijizzord. The intestine which succeeds this is usually somewhat pouched, being con- stricted in the regiou of the dissepiments and bulging out into the intervening cadomic cavities. In the terrestrial Oli- gt)cluets its absorptive surface is increased by the projection into it along tlu^ dorsal surface of a longitudinal fold, ilie tiiphlosnU; the chloragogue cells lying in th(* furrow |)roducTd by the fold. Various glands open into the digestive tract iit dill'ereut regions, as, for instance, salivary glands which optn into the anterior part of the (I'sophagus in some forms, and calciferouH glands (Morren's glands) which contain jjartich s of cinbouate of lime and arc found opening into the (Ksoplia- gu8 in terrestrial forms. TYPE ANNELIDA. 221 The nervous system consists of a supi-aoesoplia«,'e!il ^^an- gliou (Fig. lO-A, ce) of a somewhat complicated structure lying in the anterior portion of the body. In ^^o/osow(t it is "con. nected with the hypodermis (i.e., the ectoderm), but in most forms it lies upon the anterior part of the digestive tract, quite separate from the hypodermis. It differs in position from the corresponding ganglion of the Polyclupta in that it is not usually situated iu the anterior metamere or prostomium, but has passed farther back and may lie in the second, third or fourth metamere or even more posteriorly. It sends off nerves to the sensitive i)rostomium and gives rise to two connnis- sures which pass backwards and downwards on either side of the pharynx to unite with the suboesoi)hageal ganglion (.so), which, like the brain, is formed of two more or less fused lat- eral masses, each of which in many forms shows indications of being compound and formed by the fusion of two or more ganglia {Lumbricns). To this there succeeds in each meta- mere a pair of ganglia each of which is united to its i)rede- cessor and successor by a pnir of connecting cords, the whole ventral cord (??) so produced having a characteiistic ladder- like arrangement. Usually the connecting cords are closely ujiproximated, and the same may be the case with the gan- glion pairs, the whole being ensheathed iu connective tissue so that the cord seems to be single. From each pair of gan- glia in LuinhrirxH three nerves pass out on aach side, the two posterior ones being closely related so as to ajjpear to be one. Nerves especially connected with the digestive ti-act, the sloiiiafo(/n.sfrw nerves, seem to be present, but their distribu- tion and connections have not yet been thoroughly studiiMl. in some aquatic forms a hferal nerve imbedded in the hvpo- dermis and united anteriorly with the supradisophageal ganglion runs along the lateral line between the two rows of setie, recalling the lateral-line nerve of the Capifeflhhv anion"'- the l*olychu'ta. Sejise-organs of various kinds are present. T( ntucles are absent throuj^^hout the group, and in only a few fc.rins {Nais) <1() eyes, consisting of pigment-spots imbedded in the hypo- tlermis, occur. Ciliated dp]tiT5Ssi«»iis at the side of tini pro- Ktomial segment occur in J'Jithmvna and a few other genera, 00 5 222 ly VERTEBIiA TE MORPIIOLOG Y. while tactile set* or impilht- are scattered over the body. lu those forms which possess a lateral nerve seuso-orgaus re- sembliug those of CapiteUa occur metamerically along it, and iu the geuus Slavina are increased in number so as to form a circle of from fifteen to twenty papilla surrounding each metamere and innervated by a branch from the k.eral nerve. Cup-shaped organs, supposed to be gustatory, occur especially abundantly on the prostomial metamere. The excretory system has usually a typically metameric arrangement— a single pair of coiled tubules lying in each metamere. Each tube opens by a ciliated funnel into one cuilomic compartment and then passes backwards, perforatiug the dissepiment, into the next succeeding compartment in which the coiled portion lies, aud opens to the exterior in this metamere between the dorsal and veutral rows of seta'. The lumen of the coiled portion of the tubule is intracellular, the tubule cousisting in this region of a series of perforated cells recalling the condition found in the Platyhelminths. In a certain number of anterior metameres the nephridia may be wanting in the adult condition, though in younger stages provisional nephridia are to be found in these metameres later disappearing. Considorublo variation is to bo found in tlio noplwidial systi-ni of (Ik; Oiigocliu'ta, sonic of Uio variations siiKj^'cstin,-? important tlicon'tical fon- sidcrations. A /aw/-/.vV///r// similar to that dcscnhed as occnrrint,' in tliu Troehoplioro larva persists in ilin adult staj,'(> in somo Olijroclia-tarand in Ctenodtilas appears to bo tho only nophridiuni which oxists. In othoi' fonns, suoh as C/i(rfoi/iiiiiil lit till < i-.i n.!!', .Kill. 1.1 i>>l,. ,,!>.. it, ...I.:..!, 4i._ _„,- i • • . ., , - ' '"'' '!!!<• Oil*, iii iVimii liii; canal is nitorceiiiiiai'. The nephridia of ('hatnijuder are uiupiestionably homologous with tlioso TYPE ANNELIDA. 223 of Lumbrieus, for instance, whicli possess a terminal funnel, and are like- wise similar in structure to and may be regarded as repetitions of the head-kidney, thus establishing the homology between the two forms of nephridium. In carrying the homology of the Annelid nephridium back to that of the Platyhclminth the question arises whether it is equivalent to the whole branching system of the Turbellarian or only to a part of it. Whichever view of metamerism be taken the various nei)hridia of tlie Annelids arc to be regarded as bud-products, and each, therefore, equivalent to a branching Turbellarian nephridium. It has been suggested that the Annelid nejjhrid''- ial system has been produced bj the fragmentation of an originally con- linuous system, but for this there is no ombryological evidence. Each nephridium being a bud from an undifferentiated nephridial blastema is just as much an organ-individual as is the branched nephridium of a Tur- bcllarian -^ust as much an individual, though of a lower grade, as is the bud of a Polyzoon developed from an undifferentiated blastema. It might be supposed, then, that the Annelid nephridium migiit show a branched structure in certain primitive forms, and indeed a branched liead-kidney occurs in Polychaet Trochophores. A branched condition is however, rare in the trunk nephridia, though it does occur in ccrt-.in ter- restrial Ohgochajta in which, however, it must be regarded as a purely .secomlary phenomenon without any phylog,>netic signiticance, since in the development of such nephridia a single tube 's first formed which 'ater on bec.mes solid and then gives off the branches, the various nephridial branches of successive segments becoming sometimes united This branched condit. passes into one in which the various branches separate aiHl acquire independent openings ; several pairs of nephridia, four in the anterior segments of a .spedes of Perfc/uHa and a greater number in other lorms. occurring in a single segment. This branching ami multiplication <>t nephridia is confined to terrestrial forms which in their conditions of existence are farthest removed from the primitive state, and it is not improbable that the multiplication bears .some relation to the assumption 01 a terrestrial m.uJe of life. In the genus Lnmhrica.s, in which the nophridia are simple coiled tubes, a duplication of the nephridia in some seoments is to be found. The reproduc.ive ducts are probably modified n.'plindia, and ... all aquatic fo.-ms other nephridia are ab.sent .n the nu-ta,ne,.es in which t hey occu.-. I„ /.o>,Ma,,^ however, in. for ins.ance, ..■ metamere which contains the oviducts, two pairs are p.-esent. one o} ^^1"; ' nianis its original excretory function, while the other has been "I'Mlilied ... t„nn m duct for the reproductive elements. The leproclnctive orKmii.s Imve a very .lifferent arrui.oeineut iiom what iH fouii.l i„ the Polydi.otn, boiuj. li...it.,l to a <'n...p,ir,itiv«ly feu- .u...t..i,npro« ; aiKl fnrthorinoiv ii,,. ()]i. K..ehM>ta are thr(,„^.h<,„t horn.aphroditio, tho n.ah- a.ul fe.nale GO 2 224 INVERTEBRATE MORPHOLOGY. i organs both Ijiug in the auterior portion of the body, usually between the ninth and fourteenth metameres, or sometimes even farther forward. There are either one or two pairs of testes (Fig. 104, t), which arise like the ovaries from the peritoneal epithe- lium and early break down to m grf-rt:»'' ..■/ !o«^' extent, their cells p? •— , 'uto seminal vesicles (t'.sj, whb they undergo further de- velopment into spermatozoa. The vasii de/erentia {vd) are moditied nepliridia, and are either two oi' four in number according as there are one or two pairs of testt?s. "When four are present they nmy open separately, or may unite in pairs on either side in a common atrium, through which the}- open to the exterior, or finally those of Fig. 104 —Nervous Systkm ., ., ., , , AND liKPRouucTivE Okoanb ^^]^ ^''^J^© Side may unite a short distance below the funnels, f()rmiii<^' OF IjUiubricus. n — ventnil iiervecord. od — oviduct. ov = ovary. rs = leceptaciiliim soiiiiuis. «o = suboesopliageul ganglion. t = testis. vd = vas deferens. vs = vesieula .seniinalis. ee - siipracBsophageal ganglion, for the greater part f)f their course a single tube. Tliere is only a single pair of ovaries (ov), to which in some forms ovarian receptacles similar to the seminal vesicles are addfHJ ; and in all but some of the lower forms oviducts {od), which are modified nephridia, are ])res- ent. In front of the metameres which bear the testes one, two, or occasionally three pairs of invaginations of tlie body- wall occur, producing pouches projecting into the body-cavity —the seminal receptacles (rs) — which receive the seminal fluid during the mutual interchange of it which takes place on copulation. A satisfactory subdivision of tlio Oligochajta into orders has :iot yet bct'ti possible ; indeed the various fiuniiics are ao rclutcu to one .•uiotiiet' that such a subdivision .seems unnecessary. Formerly it was tlu' custom TYPE ANNELIDA. 225 to recogniie two orders, Limicokv and Tenicolce, aquatic forms being referred to t!ie former, and terrestrial ones to tiie latter— a division, how- ever, which is decidedly artificial. Less so, but still unsatisfactory, is a division into Naidotnurplia, reproducing non-sexually, and Lumbrico- muipha, reproducing by tlie sexual method only. It seems on the whole better to omit a subdivision into larger groups, and recoguize one into families only. Development of the Oligochceta. —lu the development of the Oligochaita there is practically no larval stage, but a sufficient amouut of uutiition is supplied to the enibvjo, either in the form of yolk in the egj^- itself or as an albuminous substance stored up in the interior of a cocoon in which the ova are contained, to enable it to pass through all its early stages while still within the egg-shell or cocoon, and to assume a free life only when it has reached the form of the adult. The Trf)chophore larva under such conditions is useless, and is suppressed in the ontogeny, the development becoming thus direct or of the foetal type. This mode of development has been acquired as an adaptation to the aquatic or terrestrial life, in which, for obvious reasons, the occurrence of a free- swimming larva would be an iucouvenieiice rather than an advantage. In the Polychrpta it was stated that usually at a very early stage of development one cell, Liter dividing into two, differ- entiates from the rest as the primary mesoblast, and gives rise to all the mesodermal tissues of the adult"- rm. This is an example of a precocious segrcfjation of the nn^sodtMinal material into a single cell. It is to be presumed that in more l)rimitive forms the mesoderm separated off from the eudo- derni only at a relatively late period of development; the tender.cy, however, for the a))pearance of an im;,ortaut struc- ture to be thrown farther and farther back in the individual development, to api)ear at successively earlier stages in the developmi "f, has asserted itself to such au extent that the mesoderm in the Polychu'ta makes its appearance while the embryo is still composed of but a few ceils, becoming there- fore segregated in a single cell. Such a process has further- more the ndvuntage of permitting a rapid growth, the original embryonic mesoblasts retaining their position at the posterior 00 2 226 INVERTEBRATE MOItPHOLOOY. t I : xe X - eud of the body aud giving rise by division in a transverse plane to rows of cells, the mesoblast bands. Such a pre- cocious segregation of the mesoderm also occurs in the Oli- gochaeta. In a Lumhricus embryo there may be seen near the posterior extremity of the body the two mesoblasts (Fig. 105, m), lying one on each side of the middle line, with the meso- blast-bauds {nib) extending for- wards from them. A little in front of them and on either side ma}' be seen another cell {nh), giving rise to a band extending anteriorly, which later on will become differentiated into the ventral nerve-cord, the cells Avhich give rise to it being neuroblasts ; Tr,« Ac\K «... ^ \'? r. w^^^^® ^ ^i^*^6 behind and exter- liQ. 105.— hunFACE View OF Pos- n , ,, TEHioK ExiKEMiry OF Embkyo "''^^'>' *° th&iiQ, on either side, two OP Ltimbriciis (after E. B. Wilson), other cells (?ieaud x) occur, giving rise likewise to germ-bands, whose further fate is undecided, though it seems probable that the inner of the two bauds gives rise to the nephridia, the cells being nephro- bJasts. Thus from a small number of cells the entire nervous system, with the exception of the supraoesophageal ganglion (which arises as a local thickening of the ectoderm, comparable to the apical thickeniiigof theTro- chophore), the nephridia and all the other mesodermal tissues arise, the precocious segregation of these organs being carried to an extent only equalled in the Hirudiiiea. Indications of it, however, are found in the Polycha^ta, not only in the meso- blasts but also in a layer of cells occupying the ventral surface of the embryo, and forming the so-called ventral plate, from which the ventral nerve-cord, the nephridia, and some of the musculature seem to arise. A reduction of the number of cells constituting this ventral plate to the smallest number consistent with a bilateral symmetrv. that is in two. for each ec — ectoderm. m = inesobliist. mb = inesoblast-baud. nb = ueuiobliist. 7ie = iiephrohlast. X = lateral teloblast. TYPE ANNELIDA. 227 ig- sei o structures formed from it, would give rise to a condition sucli as is found in the Oligochseta. A number of the simpler Oligochaets, in addition to repro- ducing in a sexual manner, also reproduce by division, and in some forms it plays a much more important part than the sexual method, which in jEolosoma is not yet known to occur. In the simplest form of this method of reproduction the animal simply divides at the middle, each portion after sepa- rating regenerating the parts which are wanting. In one spe- cies of Cfenodrilus each metamere except the anterior one may separate and become a new individual ; a phenomenon which might be regarded as illustrating the bud theory of metamerism, but which seems more properly to be a case in which the gradual integration of the multiplied organs has reached its highest development— the case standing as the culmination of the process of metamerization rather than as au example of its mode of origin. In Nais a division of the new individuals may begin before they have separated, and chains may thus be produced composed of individuals vary- ing in the stage of regeneration which they have reached, but which eventually separate and may later become sexually mature. As might be expected from the occurrence of this mode of reproduction, the power of regeneration of lost parts is pos- sessed in a high degree by the Oligoch^ta ; and not only in those forms which habitually reproduce by division, but also iu forms like Lumhricus, in which under normal conditions this method of reproduction is unknown. Affinities of the OUf/ochata.—Tlniva is Utile reason to doubt that the OlitrochaQtii liave ))eon derived from the Polychieta, and represent members of that subclass wliich have become specially adapted to aquatic or terrestrial modes of life. A few OligocluTts. such as Ifulix/nlns, are marine, livinj? below stones between tides; but tiiey are undoubtedly derived from aquatic forms, and cannot be regarded as having any ancestral significance. As re- gards the more definite aflinities of the group little can at present be stated with certainty. They have been referred to forms like thv. Capitellidse, in some of which the parapodia are very much reduced, as is likewise the distinctness of the head, while, as in the Naids. lateral-line sense-organs are present. A more remote relationship through the Archiannelida has also been suggested, but at present no detiuite evidence is forthcoming as to which view is to be preferred. 59 "228 INVERTEBRATE MORPHOLOOT. It ■ IP i^^ V II. Class Uirndinea. The Hirudiuea differ from the CluDtopotla iu their external form, beiug destitute either of parapodia or seta?, aud possess- lug at the anterior end of the body a muscuhir sucker at the bottom of which the mouth is situated, while a second larger sucker used for attachment occurs at the posterior extremity of the body. The outer surface of the body is distinctly ringed, but a comparison of the rings with the internal organs shows that they have not a raetameric value, but that a num- ber of them, varying in different forms, are included in each true segment of the body. In Branchellion and Clepsine three such rings correspond to a metamere, iu Ichthyohdella and Pontobdella six, in Piscicola twelve, aud in all the group of the GnathobdellidfB five. Towards the anterior and posterior ends of the b(.dy a reduction of the uum jer of rings corre- sponding to a metamere is found, as for instance in the genus Macrohdella (Fig. 106), which has in the middle region five rings to a segment. The first two metameres consist of but one ring each, the third of two rings, the fourth, fifth, aud sixth each of three rings. At the posterior end of the body the twenty- third metamere consists of four rings, the twenty-fourth, twenty-fifth, and twenty sixth of two rings each ; while prob- ably no less than seven metameres whose rings are not reailily distinguishable are represented in the posterior sucker. The entire animal consists, therefore, of tliirty -three metameres, ■and this number is characteristic for all the Hirudin ea — a definiteuess of number which contrasts strongly with the wile variations found in the Chictopoda. This number does not include a sm.,'1 lobe in front of tlie mf)st anterior metamere, which may be equivalent to the prostomial lobe of the Oligo- chfeta, and may represent another metamere. As iu the Oligochrota the gland-cells of the hypoderrais at about the time of reproduction become enlarged and more abundant iu a definite region of the body, forming a clitellum which is usually in the neighborhood of the tenth, eleventh, aud twelfth metameres. As a rule no brancliitc occur, though an exception is found iu the marine genus Branchellion, iu TYPE ANNELIDA, 229 which each ring of the middle region of the body bears an appendage which functions as a gill. In another marine form, PontobdeUa, large warts occur on certain rings, which prob- Fig. 106. — Antkkiok and Pos- terior EXTHKMITIES OP Ma- crobdellu senteriia (afiei- Whitman). an — alius. fo = opeuiug of oviduct. gc ~ copulutory glands. mo = opeuiug of vasii deferentia oc - eyes. p - uepUiidial pores. sp = seuse- papilla. 1 100 = anutili. i-xxv = meluiueres. ov-.n vB ^ Fig. 107. — Diagrams to show Ar- KANGy.MENT OF Bl.OOU-SXNU.SKS OF (^4) Hirado, (B) Clepsine, and (C) Nephelis (after Bolhne). al = digestive tract. c = ccelom. ds - dorsal sinus. Is and Iv = lateral sinus or vessel. n and mc = veuiral neive-cord. 7ie = nopliridiuui. ov = ovaiy. te = testis. m = veutral sinus. ably are mainly respiratory in function, being richly supplied with blood-vessels. ^ The exterior of the body is covered by a cuticle, beneath which lies the hypodermis. The muscular tissue which nn- derlies the hypodermis consists, as in other Annelida, of laj-ers CO 230 IJSf VERTEBRA TE MORPIIOLOG Y. of longitudinal and circnlar fibres ; and in addition, between these, a layer composed of fibres which cross one another obliquely is usually present. A marked distinction from what occurs in the Cluetopoda is found in the coelom, which in the Hirudinea is traversed by a parenchyma, recalling that of the Platyhelminths, so that the actual cavity is to a great extent obliterate! and the dissepiments only to be distinguished with difliciilty. Those portions of the coelom which persist (Fig. 107, Cc) are occupied by a red or colorless fluid containing corpuscles and identical and continuous with that found in the blood-vessels. The coelom is in fact represented by a number of blood-sinuses, which in some forms are lined by an epithe- lium, while in others such a lining is wanting. On account of the manner in which the blood-vessels anastomose with the sinuses it is exceedingly difficult to distinguish Avhich spaces should be considered as belonging to the circulatory system proper and which to the coelom — if, indeed, the two are to be considered fundamentally distinct. As a rule four main lon- gitudinal vessels or sinuses are to be found — viz., one dorsal (Fig. 107, (Is), which may be wanting {Nephelis, Fig. 107, C) and which probably corresponds to the dorsal vessel of the Chiic- topoda ; one ventral (vs), sinus-like in character and frequent- ly destitute of an epithelial lining, which surrounds the ventral nerve-cord ; and two lateral vessels (Iv and Is) unrepresented in the Chfetopods, and perhaps also to be regarded as rem- nants of the coelomic cavity. Communications between these longitudinal vessels occur through the medium of smaller vessels ; and in some forms, such as Nephelis, the connection between the lateral and ventral vessels takes place through ampullse, globular vesicles arranged in two pairs on each side of a number of metameres and receiving blood-vessels from the ventral sinus, while other vessels passing to the main lateral vessels arise from them. In many forms, espe- cially among the Gnathobdellidae, a rich plexus of capillary vessels penetrates the hypodermis. The union of the blood vascular system with sinuses which most prob- ably represent portions of the ccelomic cavity suggests an intimate relation, so far as its origin is concerned, of the vascular system with tlie odRloni : and this view is borne out by what has already been seen to occur in the TYPE ANNELIDA. 231 ^'^■ Nomerteans p. 165), the lowest forms that possess a distinct blood vascular system In th>s group the co^lom, so far as it exists, consists of «„.all space withou any definite walls scattered through the parencliyma. In some Is : n ""?'"■ ''''''" —- -ates with these spaces thr ugh «hich the blood circulates, it being only in the most highly differentiated th 1. that the blood-vessels were simply coelomic spaces which had acquired dehn.te walls; and it seems probable that such has been their on^-n Jn the Annelida a somewhat different state of affairs occurs. Here, as a rule there IS a definite coelom lined with peritoneum and completely separated from the cavity of the blood-vessels, which seem to represent rather the rema is of an original cavity, the so-called blastocoil (see p. 52) which Ins be.,', almost obliterated by the growth of the mesodermal Lgment!^ the holbw- mg out of which the coelomic cavities have been formed (see p 56) It seems certain that the ccelomic spaces of the Nemerteans are likewise the remains of a primitive blastoecBl, so that to this extent the homology of the blood-vessels holds in the two groups. In the llirudinea, however, the blood-sinuses, if they are coelomic cor- respond wiHi the ecelom of the Polych.ta ; and furthennore, in '"01 ^o- luvts and Polych^ts, as well as in the Gephyrea, as will be seen later, the iuemolymph contained in the ea^lom is very nearly if not quite identic.;! in composition w,th the blood contained in the blood-vessels. These facts would .seem to indicate a close relationship between the Annelid coelom and .)e more primitive blastoccBl; or, in other words, would lead us to suppose at theco^Iom of the Annelids lined with peritoneum is not some ling apai and distinct from the blastocoel cavity, as has usually been supposed The Mew which maintains the distinctness of the two forms of coelom has Its origin in the fact that in some forms, such as Sagitta, a ca^lom lined with pentoneum is formed as an outgrowth from the primitive digestive tract- and It ^^•i^s supposed that all coelomic cavities with definite walls were nri- marily of a similar origin, and hence were termed enteroccels in contradis- tinction to the scMzocceh or simple spaces in the mesoderm without defl- »ite walls, which are in reality remnants of the blastocoel. The significance of true enteroccels will be discussed later. In the mean time 1^ maybe pointed out that there is no embryological evidence in favor of the Annelid t,w 7'"! '?''?' '" '''''' '^ pouch-like outgrowths from the primi- ' ne chges tive tract. It is rather to be regarded as a schizoco^l whose char- . ter has been a tered by metamerization, and by the manner of its forma- lon from mesoblasts. On this view the union of the cavity of the blood- essels with .he c(^lom in the leeches, and the similarity of the h^molymph to the blood in other forms, cease to be morphological puzzles. The mouth lies at the bottom of the anterior sucker pnd opens mto a muscular pharynx, which in some forms (e a Clen sine) IS folded similarly to that of some Turbeilaria (see p lU) so as to form a protrusible tube, while in others (e.g. Hirndo S3 s 232 INVERTEBRATE MORPHOLOGY, i Fig. 108.— Diaqkam of THK EXCKKTOUY Uk- I'HOUIICTIVK AND NeU- VtJUS iSVHTKMS OK Iliiudo (iftcr UoUHNK). ■ce = ctMeldiil giiiijfliou. ep = ('|)i(ii(lyiuu9. f/l = oviiluciil gliiud. Ic = liiteral blood-vessel. n = iK'pliridiii. i)V = oviiry. pe = i)euis. te = testis. Macrobdella) it is thrown iuto three longitudinal muscular ridges whose edges may become converted iuto chitin, thus forming teeth. Salivary glands open into the pharynx in some forms. The large stomach into wliicii the pharynx opens behind gives off a number of lateral pouches (eleven pairs in Uirudo, seven in Clepsine), sometimes branched and increasing in size from before backwards, the most posterior pair being usually quite long and directed backwards parallel to the straight narrow intestine which opens to the exterior on the dorsal surface of the body, just anterior to the posterior sucker. Occasionally onl}' the poste- rior pair of pouches is present, and in a few forms they are entirely wanting. The nervous system (Fig. 108) is constructed on the ty])ical Annelid plan. It consists of a circunupsopha- geal ring and a ventral nerve-cord composed of fibres which have their origin in ganglion-cells grouped to- gether at definite intervals into gan- glionic masses. Several of these gan- glionic masses correspond to siuglo .segments, but at the anterior and posterior extremities a cousiderabh^ amount of fusion of the metameiic groups of ganglia has occurred. Tn Clepsine phina the jiortiou of the ner- vous system which lies above the ce80})hagus consists of a transverse band of fibres passing hiterally into the (iicunursophageal commissures aiit 1 'ce, and two marginal. In the anterior and posterior segments whose width is reduced the marginal pa- pillsB may be wanting, but throughout the rest of the body the number of rows is constant. In structure these papilla? are somewhat complicated, consisting of an axial bunch of elongated sensory cells bearing fine cilia at their outer ends, and lying in the connective tissue in their immediate vicinity is a varying number of large cells, each containing a large watery vacuole in the interior, the nucleus, in consequence, being pushed to one side. A strong nerve runs to each pa- pilla and is supplied to the large vacuolated cells as well as to the axial sensory cells. Slight differences are to be found in various forms in the structure of these organs. In Clepsine there is an axial bunch of hair-bearing cells to which the terminal fibres of the nerve run, and posteriorly and Vjelow the nerve are found the largo vacuolated cells. In Hirudo and NephtUs no hair-bearing cells occur, the nerve occupying the axis of the organ and the vacuolated cells being arranged symmetrically around it^ It is probable, in view of the two kinds of constituent elements in Clep- sine, that in this and similar genera a double function is possessed by the sensory papillao, the hair-bearing cells having perhaps a tactile function, while the vacuolated cells are visual. It seems probable also that primarily the papilliB were similar in structure and function to the orgaii.s of tln;^ lateral line of certain Polychif ta, such as the (!apitellida% or perhaps it would be better to compare tiicMu with the tactile pai)illa3 of certain aquatic Oligocha^ta, which in the genus 8lavina have uti arrangtmient on each meta- mere recalling that found in the llirudinea. Towards the anterior extremity of most of the Hirudiuea a varying number of eyes are found. In some s])ecies of Clepsine but t\vo such organs occur, while in others there are six, aud iu llirmlo, MacrohleHa (Fig. 10(5, oc), and allied foiius tlicre are always ten. In the latter forms the eyt^s are always arranged in a definite manner: one pair is situated on the anterior ring (when more than one ring occurs) of each of the five metameres immediately following the prostomial lobe, aud if their position be determined it will be found that they occup3' the place of one of the dorsal sense-pupilhe, tlie eyes boing serially homologous with the Hense-papillir of one of the dorsal rows. This conclusion is verified bv their struc- TYPE ANNELIDA. 235 aH ture, since they differ from the sensory papillae only in the ^'reater number of the large vacuolated cells and in the pres- ence of a quantity of black pigment in the surrounding tissues. Other sense-organs somewhat beaker-shaped in character 411 e found upon the prostomium and have been regarded as gustatory in function. JSephridia occur in a number of the metameres of the middle portion of the body, there being in Hirvdo (Fig. 108, n) and its allies seventeen pairs. Each nephridium has a ter- minal funnel, which in Glepsine has the typical Annelidan structure, but in Hirudo has been modified so that the inner extremity of each nephridium is constituted by a lobed spongy ciliated mass without any definite central lumen. The funnel lies in a blood-sinus, either the ventral one as in Glepsine (Fig. 107, B) or the dorsal as in Pontohddla, or in a sinus which surrounds the testes as in Hirudo (Fig. 107, A), or in a special sinus which is to be regarded as a coelomic space as in Nephelis (Fig. 107, G). The canal which traverses each nephridium is intracellular as in the Oligochseta, and in some forms minute canals traverse the substance of each coll, open- mg into the central lumen. As a rule the various nephridia are quite separate and distinct from each otlier, but in Pontob. deUa and one or two other genera they unite to form a net- work of intracellular canals traversing several metameres. Immediately before their exit to the exterior the canals enlarge in some forms to bladder-like vesicles, from which a short tube leads to the exterior, the opening being situated either upon the anterior {Glepsine) or the posterior {Hirudo} ring of the metamere to which the nephridia belong. The rei)roductive organs differ from those of the Chieto- poda in possessing ducts which do not seem to be modified nephridia and which are continuous with the walls of the ovaries or testes. All the Hirudinea are hermaphroditic. The ovaries constitute in Glepsine two elongated organs which lie in the middle region of the body, expending through several metameres, but in Hirudo (Fig. 108, or) they are small oval or spherical bodies ; th«ir ducts dilate to form a uterus auu finally unite to open on the mid-ventral line usually in the eleventh metamere (Fig. lOG, /o). The testes (Fig. 108, tt) 236 INVERTEBHA TE MORPHOLOO T. consist of a number of pairs, varying from twelve or more to six {Clepsine), of spherical bodies lying in the same region of the body as the ovaries. Each testis has its own duct, which opens into a longitudinal vas deferens common to all the testes of the same side of the body. Anteriorly the two vasa deferentia unite to open in the mid-ventral line of usually the tenth metamere (Fig. 106, mo), frequently through a strong muscular penis (Fig. 108, pe). In many forms special glan- dular thickenings, supposed to be useful in copulation, occur on the ventral surface of one of the metameres behind that bearing the opening of the oviduct (Fig. 106, gc). The Hirudinea are at present usually divided into two orders, though it seems probable that further division of one of them will be necessary later. 1. Order GnathobdellidsB. In this order are included the leeches which are provided with chitiuous jaws in the walls of the muscular pharynx. In addition to this all the members of the order are charac- terized by possessing five rings to each fully developed meta- mere. To this order belong the Hirudinidee, characterized by possessing ten eyes arranged in pairs on the five anterior metameres behind the prostomium, and including Hirido, the medicinal leech, a native of Europe, instead of which Macro, bdella is sometimes used in America. The Nephelidifi, with the genus Nephelis, differ in possessing fewer eyes (four pairs), and in having distinct segmental sense-organs either wanting or occurring on all the rings of each segment. 3. Order Rhynchobdellidse. The Rhynchobdellidro are characterized by possessing a protrusible pharynx, as well as by possessing three, six, or twelve rings to a metamere. In the Ichtliyobdellidjo, or fish- leeches, the larger numbers are found, the number six being characteristic of Pontoklella, while twelve occurs in Piscicola. In the ClepsiuidjG but three rings are found to each meta- mere, and the eyes are either two or six in number. To this family belongs the genus Clepsine, a conimuu fresh-water form, as well as the tropical land-leech, Hmnvnterin. TYPE ANNELIDA. 237 Development of the Himdinea.—The Gnatliobdellid* deposit their eggs in chitinous cocoous, tm do tlje Oligocliaeta, and the development is of the foetal type, in coutradistiiictiou to the larval, the ova containing as a rule a considerable amouui of yolk. Ttie mode of oviposition of the majority of the Rhynchobdellid* is unknown ; but in the genus Clepsine the eggs are fastened to the veutral surface of the body of the parent, where they un- dergo development. This resembles closely the development of Lumbricns, allowing for the greater amount of yolk which is usually present. The same precocious segregation uf mesoderm, nervous system, and nephridia in special budding cells, the mesoblasts, neuroblasts, and nephroblasts, i.s likewise found, and lu later siages the mesoblast is distinctly segmented and coelomic cavities are present, which later become to a great extent obliterated. The Affinities of the Hirudinea.—lX is exceedingly probable that the ancestors of the Hirudinea were to be found in tha Oligochffita, the two groups having not a few structural features in common. The embryologi- cal peculiarities found in the two groups are strikingly similar ; and fur- thermore the aquatic or terrestrial habits are not a little suggestive, for although some leeches are marine, nevertheless the majority are aquatic and a few terrestrial. The complete disappearance of parapodia may be considered a further development of the tendency towards their oblitera- tion in the Oligoclueta, where only the setae are present, these even having disappeared in the Hirudinea in consequence of the development of the suckers and a new mode of locomotion. The suggestive arrangement of tlie sense-papilljB of the Oligochaete Slavina has already been mentioned. It must not be forgotten, however, that the differences between the two groups are many and important. Such are, for instance, the disappearance of the original coelomic spaces, the communication of the blood vascular system with sinuses, and the occurrence of special ducts for the reproduc- tive organs. These differences have, however, equal or even greater im- portance when (he attempt is made to trace the Hirudinea directly to the Polycha'ta, and it seems more satisfactory at present to refer them back to the Oligoclueta. III. Class (jephyrea. The Gepbyreans coustitute a ji^roup of mariue worms wliii'li differ from the CLujtopoda principally in the more or less coin])lete absence of metamerizatiou. All trace of it is ab- sent upon the outside of the body; for although ^he thick cuticle may be marked by distinct riugs, these bear no relation to the internal parts and are, as in the Nematoda, due simply to the thickness of the cuticle. Ail traces of parapodia are lacking in many forms, while in others they are representinl 238 INVERTEBRATE MORPHOLOGT. 0 only by a pair of setae situated on the ventral surface of the body, nearer the anterior than the posterior end. The body- wall presents a close similarity in its structure to that of the Chsetopods — differing, however, in the occurrence of a more or less pronounced layer of fibres having an oblique direction. The coelom is lined by a layer of fiat peritoneal cells, but shows no division into more or less distinct compartments, no trace of metamerism, but, as in the Chaetopods, the peritoneal lining is refiected upon the walls of the digestive tract, form- ing mesenteries suspending the intestine. As a rule the dor- sal mesentery disappears, and in some cases the ventral one is almost wanting, the intestine being slung only by a number of irregular strands of connective tissue extending from it to the body-wall. In some forms {Sipunculm) the sur- face of the peritoneum, especially that covering the intestine, is dotted with numerous irregularly scattered minute depres- sions, whose openings are guarded each by a peculiar ciliated cell, and which contain cells comparable in function to the chloragogue cells of the ChaBtopoda. The coelomic cavity is occupied by a haemolymph, which in some cases is colored, and contains numerous cell-elements, some of which may be circular in outline and colored by haemoglobin, while others are amoeboid and colorless. A blood vascular system, principally developed in the an- terior portion of the body, is present and appears to be com- pletely closed, though connections with the coelom are said to exist in some forms. In iSipuncithis, for instance, the system consists of a collar surrounding the a3sophagus, sendiug braucLes into the tentacles which surround the mouth, aud tlorsally dilating into a wide sinus lying just below the brain ; and from this sinus a dorsal vessel (Fig. 109, Bs) passes backwards along the digestive tract for a short distance, end- ing blindly where the oesophagus joins the stomach. In Echiu- rns a ventral vessel runs the entire length of the body just above the nerve-cord, and it is united with the dorsal vessel by lateral vessels at its anterior and posterior extremities. The digestive tract may be either straight {Priapulus) or considerably convoluted {Echiurm and Siptinculus, Tig. 109, Jilt), and the anus is in some forms terminal {Echiuriui), while TYPE ANNELIDA. 239 in others the intestine bends upon itself and passes forward to open on the dorsal surface near the anterior end of the body (Fig. 109, A). Throughout the greater extent of the intestine there runs along its ventral surface a ciliated groove which is no doubt homologous with the accessory in- testine of certain Polychseta (see p. 207). The nervous system partakes of the absence of distinct metamerism which characterizes the other parts. It consists of a brain lying in the anterior portion of the body above the oesophagus and sending a com- missure downwards and backwards on each side to form the circum- cesophageal collar. These two com- missures unite to form a single nerve-cord (Fig, 109, n) extending the entire length of the body in the ventral median line, differing from the ventral cord of the Chaetopoda in the absence of ganglia. Nerve- cells are scattered along the entire length of the cord and are not aggregated into special ganglia, though slight indications of such an aggregation are found in Priapu- lus. Nerves are given off at more or less regular intervals on either .side, a somewhat metameric ap- pearance being thus produced, but ne = nepliridinm. the corresponding nerves of op- ^* = a'soplmgus. posite sides do not invariably arise *^,! " '^'"^7\ . f ..^ , , 1 -i . , , -^ , vli = ventml retractor muscle. Horn the cord opposite each other. One, two or three pairs of nephridia (Fig. 109, ne) are as a rule present and form conspicuous brown tubes, which com- municate by a funnel with the body-cavity at one extremity and with the exterior of the body at the other. They are Fig. 109.— SxnucTunE op 8i- punculus Qouldii (after An- drews). A = anus. Bs = blood vessel. dR = dorsiil retractormusclo. Lit = iiitcstiiie. N = nerve-coid. too s 240 INVERTEBRATE MORPHOLOG T. undoubtedly homologous with the nephridia of the Chffitop. oda, possessing the same relations. In a few forms {BonelUa, Phascolion) a single nephridium only is present. In addition to these in Echiurus, Thalassema, and allied genera there is a usually much-branched organ on either side lying in the body- cavity and opening into the terminal portion of the intestine. Numerous ciliated funnels occur upon the branches placing the organ in communication with the body-cavity. This so- called " respiratory tree " (so named from a supposed homol- ogy with the similarly named organs of the Holothuria (q. v.) are probably nephridia, though whether or not they per- form excretory functions is not quite clear. In Priapulus these organs are represented by branched tubes, the branches of which terminate blindly in flame-cells, resembling thus the excretory organs of the Platyhelminths, and in Sipunculus rudiments of these organs have been described as short tubes. The Gephyrea are bisexual, the reproductive organs {oc) forming small digitate, elongate, or ovoid processes arising from the peritoneal lining of the body-cavity; but in some forms {Sipunculus) their products early escape into the ca> lomic cavity, in which they float. The exact manner in which the ova and spermatozoa escape to the exterior has not been definitely ascertained for the majority of forms, but it seems probable that the nephridia serve as the generative ducts. In Priapulus the " respiratory trees " are said to give rise to the reproductive organs, and also to serve as the reproductive ducts — a behavior which would render exceedingly probable the supposition that they are modified uepliridia. Two orders are recognizable in the Gephyrea. 1. Order EchiuresB. The Echiureae, sometimes known as the Gephyrea armatu, are characterized by the presence on the ventral surface ui the body, in front of the openings of the nephridia, of a pair of setffi— the genus Echiurm possessing, in addition to these, two circles of setse at the posterior extremity of the body. The anus is terminal in all the known species, and the ter- branched respiratory trees. The anterior end of the body is TYPE ANNELIDA. 241 prolonged into a prostomium of considerable size overlying the mouth ; it may be short and broad as in ^chiunts, more elongated and slender as in Thalassema, or deeply bifurcated at the extremity as in JBorteUia. A Fig. IIQ.—Bonellia viridis A, Adult Female opened so as to show thb PRINCIPAL OuGANa ; B, male much enlarged in proportion to the female (trom Hebtwiq). c = cloaca m = muscles. d - rudimentary intestine. s = proboscis. g = lespiiatory trees. a (in Fig. B) = spermatozoa. i = iutesliue. vd = vas deferens. u = single uephridium which serves also us the oviduct. The last-named genus is interesting as affording an exam- ple of sexual dimorphism, the males being small Turbellarian- like organisms which live parasitically in tlie anterior portion of the digestive tract of the female, only coming to the exterior lor the purpose of copulation. 2. Order Sipunculacea. The Sipunculacea, to which the term Gephyrea inermes is also applied, is an order including forms which lack all traces 342 mVERI'EBRATE MOBPHOLOGT. Mi r of setae. In Priapidus the iutestiue is almost straight and the iinus terminal ; but in Sipunculus and the allied genera, such as FJiascolosoma and PhascoUon, the digestive tract is convoluted and bent back upon itself, so that the anus lies on the dorsal surface near the anterior extremity of the body. A " respira- tory tree " is absent or rudimentary as a rule except in Pria- pulus and allied genera, and the large prostomial lobe char- acteristic of the EchiureoB is lacking. The anterior por- tion of the body, however, is capable of being invaginated by means of strong retractor muscles (Fig. 109, dR and vR) into the fore part of the digestive tract, forming the so-called in- trovert. The extremity of this is provided with a circle of finger-like or branched tentacles in the centre of which lies the mouth, and which are supposed to have a respiratory function, being riohly supplied with blood. In Priapulus these are absent, but at the posterior end of the body there is a prolongation which bears papilla-like processes which probably fur.ction as respiratory organs. Development and Affinities of the Gephyrea.~T]:e early development •of the Gephyrea resembles closely that of the Polyeh-jeta, more especially in the Echiureae. In this ordera Troehophore larva is formed resembling very closely the typical Polygordius troehophore, the similarity extending even to a segmentation of the primitis^e mesoderm bands. In later stages this metamerism of the mesoderm disappears, no trace -uf it being found in the adult forms. In the Sipunmlacea the larva differs from tiio Troehophore in lacking the typical praeoral band of cilia, thougli this may t ^ weakly ■developed in some forms, such as Phaseolosoma. The postoral cilia are, on the other hand, strong. A further dilTerence is found in the absence of metamerization of the mesoderm, which at a very early st.ige of develop- ment forms a layer lining the interior surface of the body- wall, and also covering the digestive tract and enclosing a cojlomic cavity continuous through the entire body. Notwithstanding these important differences thore seems little room for •doubt but that the Sipimculus larva has arisen as an aciptation of the typical Annelidan Troehophore still represented in the development of the Echiurese. By these forms a clone relationship is shown to the Polychrota ; and the Gephyrea are to be regarded as Polychaeta which have secondarily lost a metamerization originally present in the adult ancestors and still represented in the Echiurus larva, but lost even in the larval stages (»f the Sipunculacea. Since the discovery of the larval forms of certain Echiurid and Sipuu- <5Ulid forms there has been a tendency to regard these two orders as being TYPE ANNELIDA. 24^S laucb less closely related than thoy are here supposed to be. The Echiure® are stiU held to have Anuolidan affinities, while the Sipunculacea are as- signed to the next type to be described. This tendency has its origin in the attachment of too great importance to the metamerism which i& indi- cated in the Echiurid trochophore but lacking m the Sipunculid larva Ihere seems no good ground for suppohxng that; its absence in the latter group may not be sufficiently explained by the assumption that it repre- sents the final stage of the reduction of metamerism ,f which the transient segmentation of the Echiurid is a stage. In their anatomical character- istics the adult forms of the two groups are tea much alike to be assigned to different types and the similarities of deiail too numerous to warrant the belief that they have been independently acquired. It seems much more probable that both orders liave descendod from segmented aucestors—the degeneration, if degeneration it can be called, having been carried to a greater extent in the Sipunculacea than in the Echiureee, and having in consequence been thrown back upon the laival stages and so obscuring the developmental evidences of the phylogeny. A connecting link between the Echiureqe and the Polychseta has been traced by some authors in the genus Sternaspis, atone time associated with the Gephyrea but now universally assigned to the Polychsta. In this genus the metamerization, though to a certain extent reduced, is still pro- nounced, S. arcuata consisting of from twenty to twenty-two metameres. of which the anterior seven, together with the head-lobes, may be invagi'- nated— the introvert of the Sipunculacea being thus recalled. On the ven- tral surface near the posterior extremity of the body are two shield -like plates armed with setae, and at the posterior exlremity, as in PnapiiUts, are a number of filamentous appendages which are regarded as brancliite. Setae are present on all the metameres except the fifth, sixth, and seventh • those of the eighth to the sixteenth metameres being, however, concealed beneath tne hypodermis. The digestive tract is somewhat convoluted, but opens terminally ; the ventral nerve-cord shows traces of ganglionic swell- ings, and at the posterior end of the body possesses a marked enlargement ; and only two nephridia are present. The musculature and the vascular system resemble those of the Polychaeta rather than those of the Gephyrea, while the reproductive organs are peculiar in possessing special ducts, which, it has been held, show no indications of being modified nephridia. In many respects, accordingly, Sternaspis does hold a position interme- diate between the Echiure* and the PolychaBta, and it seems not improb- able that it may represent an offshoot from near the base of the line along which the Gephyrea have been differentiated. Whether this be the case or not, it is exceedingly probable that the Gephyrea have been derived from tiie Polychaeta, the Echiurea* preserving more numerous traces of their an- cestry than do the Sipunculacoae. 244 INVERTEBRATE MORPHOLOGY. IV. Class Myzostomeae. The MyzostomesB constitute a group of Annelids which pre- sent but few traces of a typical metameric form, being much modified by their parasitic habit. All the known forms are parasitic upon Crinoids, some producing malformations of the pinnules of their host in the form of cysts in the interior of which they live. The body of Myzostomum (Fig. Ill) is flattened and oval, a number of finger-like processes or cirri (c) projecting around the margin. There is no trace of external segmentation, although five pairs of parapodia ( /)), each with an axial supporting chitinous rod and a single hooked seta, occur on the ventral surface. On the same surface too, near the margin, are to be found in most species three or four sucker-like depressions {su) on each side, which have been supposed to represent highly-modified nephridia. The body is covered by a thick cuticle beneath which lie the hypodermis and the musculature of the body-wall, which has the characteristic Auuelidan arrangement. A body-cavity can hardly be said to exist (unless it be indicated by the space occupied by the ova), the interior of the body being completely filled up by the internal organs and by numerous muscle- bands passing both dorso-ventrally and from side to side, these latter in some forms being arranged in such a way as to represent incomplete dissepiments. There is no blood vascular system. The mouth is situated near the anterior end of the body on the ventral surface and opens into the proboscis-sheath, within which lies the proboscis {ph), constructed upon the same plan as that of the llhynchobdellid Hirudinea. Around the extremity of the proboscis are arranged a number of short tentacles, and its walls are very muscular; behind it opens through a short oesophagus into the wide intestine (s) from which three (or two) branched pouches project on either side towards the margin of the body. The short and relatively narrow rectum (r) opens near the posterior end of the body, uniting shortly before its termination with the oviduct. The nervous system consists of a circumoesophageal com- missural ring upon which lie numerous scattered ganglion-cells TYPE ANNELIDA. 246 likewise surrounding the oesophagus and apparently represent- ing the supraoesophageal ganglion. Numerous longitudinal nerves pass forward from the ring to unite with another ring around the base of the proboscis from which nerves pass to the tentacles. Below the intestine lies a large ganglionic mass with which the circumoesophageal commissures unite and which gives off a number of peripheral nerves. This mass is mo Fig. 111. — Myzostomum (after von Graff) c = cirrus. do = cloacal opening. fo = opening of uterus into cloaca. 0 = opening of male reproductive organs p = parapodium. ph = proboscis. r = rectum. s = stomach. 8U = sucker. t = testes. u = uterus. composed of several (probably 6) united ganglia and represents the ventral nerve-cord of other Annelids. Nerves pass pre- sumably from the supracesophageal ganglion-cells along the dorsal wall of the intestine and seem to constitute a sympa- thetic system. The only structures which can be considered sense-organs are the marginal cirri and the tentacles of the proboscis, which probably have a tactile function. No traces of eyes have yet been observed. Nephridia, unless they be represented by the snclcer Hire depressions and the oviducts, are wanting. The Myzostomese 1 24« INVERTEBRATE MORPIIOLOOY. are as a rule lierraaphrodite. It seems doubtful if the ovaries have actually been made out, the large masses of ova {ov) lyiug between the branches of the intestinal pouches, which have been considered ovaries, being more probably original ccelomic spaces which have become filled with ova set free from^ the ovaries; while the so-called uterus (?<), lying im- mediately above the intestine, and which in mature animals is closely packed with ova, is probably of the same nature. Three oviducts, one dorsal and two lateral, pass from the uterus to open ifo) into the rectum near its termination, though the dorsal one in some forms may open directly to the exterior near the anus. If the uterus is correctly ident'fied as a ccelomic space, then it seems not improbable that the oviducts may represent modified nephridia. Their opening into the rectum is a secondary condition and does not necessarily stand in opposition to their nephridial character, since practi- cally the same conditions obtain in some Rotifera. The testes {t) are branched organs lying for the most part between the intestine and the nervous system, though isolated masses occur in some forms near the margin of the body. On each side two vasa deferentia, one anterior and one posterior, convey the spermatozoa to a muscular sperm-vesicle opening to the exterior at the margin nearly opposite the centre of the body (mo). In some species, notwithstanding their hermaphroditism, " comple- montal males," small individuals which possess ripe spermato/oa while lacking ova, have been described as occurring. Fiiithcr observations have not, however, tended to conlirni tliis idea in its original sense, since these small individuals have been found to be, like tiie larger ones, hermai)hr()- dites, being secondary adaptations from the prevailing hermaphroditic condition, and not Iniviug, therefore, the same significance as the "com- plcnienlal males" of the Cirrhipedia U{. v). There can be little room for doubt but that the MyzostoraoiB are Ainielida degenerated by parasitism, and that they are most closely related to the Polycl.inta. It is interesting to note in this connection the effect their parasitic and sessile mode of life under eijuable oxteruul conditions has had in producing indications of u radial symmetry. TYPE ANNELIDA. 247 APPENDIX TO THE TYPE ANNELIDA. Class PHOEONiDa;. The class PlioronidaB iuchules a siugle genus, Phoronis, of wliich but a few species are kuoAvu. Tliey are all marine forms of comparatively small size, reaching in some cases a. length of 50 mm. Each iudivitlual is contained within a. chitinous tube to which particles of sand are in some cases agglutinated, and is worm-like and cylindrical in form, the anterior extremity of the body being provided with a horse- shoe-shaped fold, termed the lophophore (Fig. 112, a), bearing a number of tentacles arranged around its margins. Between the two circles of tentacles is sitiu.ted the mouth (/>), over which hangs a fold known as the epistome, representing the prostomium or pnt'oral lobe of the larva. Outside the area enclosed by the tentacles is the anus, on either side of which a i)ore, the opening of a nephridium, is found. The ectoderm of the body-wall is separated by a distinct basement-membrane from a layer of circular muscles, within which is a second layer of longitudinal muscles (?)— an arrauge- mcut resembling that found in the body-wall of the Annelids, internally the longitudinal muscle-layer is lined by a layer of peritoneal cells enclosing a spacious ccelom. Near the- anterior end of the body there is a transverse septum sepa- rating off, more or less perfectly, an anterior chamber, with which the cavity of the epistome and of the lophophore com- nninicates, from a larger posterior chamber in which lie the intestine and reproductive organs, and which is divided lon- gitudinally by three mesenteries extending from the intestine to the body-wall. One of these mesenteries accompanies the. intestine throughout its entire extent, while the other two. lateral mesenteries are in connection only with the sides of the descending limb of the intestine. The tentacles are processes of the body-wall, with a ciliatotl ectoderm, and contain a chitinous axial supporting tissue, A completely closed blood vascular system is present, coiiHisting below the transverse septum oif two longitudinal 246 INVERTEBRATE MORPHOLOGY. vessels {li aud /). One of these (/) divides {g) near the an- terior extremity of the body, the two branches passing into a circular vessel lying at the bases of the tentacles and sending branches up into them. The vessels Avhich return the blood from the tentacles open into a second ring external to the first, aud from it two vessels pass ^-ick wards and unite to form the second longitudinal trunk from which numerous cjccal pouches arise. All the vessels have contractile walls, aud the blood which they contain possesses nucleated red cor- puscles. The digestive tract is bent Fig. 113.— Lateral view of An- upon itself (t^ and e), tli.' TEHiou Keoion of r/iorouis (after mouth and anus, as already described, lying in close prox- imity at the anterior extrem- ity of the bod}'. Several regions, such as a;80])hagus, first stomach, second stoni- acli, aud intestine, are to Ik; distinguished, aud along one Bol'RNK). a = lophophore. b — inoiitli, surrounded by epistome e = lophophoral disk. d = OBHOpluigUS. f = intestine. / = ventral bloodvessel. g = brimcb of/. h = dorsiil blood-vessel. i = Ioiif?itu(iiniil inuscli! of body-wnll. surface of the oesophagus and k = interlentiicular membrane. fl^.^^ stomach ruus in I\ architecfa a ridge, becoming a groove in the stomach region, of ciliated gland-cells, which recalls the accessory iutestiue of the Gephyrea. There are no special digestive glands. The nervous system is com])letely imbedded in the ecto- derm. It consists of a nerve-ring, following the outline of the lophophore at the bases of the tentacles and surround- iug, therefore, the mouth but not the anus. From it a nor\t' ruus backward asymmetiicidly upon one side of the body to near the posterior extremity. It contains a largo clear rod-like structure which seems to be a colossal nerve fibif. TYPE ANNELIDA. 249 The only sense-orgaus which have beeu described are a pair of ciliated depressious lyiug one on each side in the concavity of the lophophore ; no definite statement can be made as to their function. A single pair of nephridia is present, opening into the posterior chamber of the coelom by fuuuel-like mouths, and to the exterior on each side of the anus. They serve not only for excretion, but also as ducts for the reproductive elements. The various species of Phoronis, \yith the possible exception of P. arcMteda, are hermaphrodite, the ova and spermatozoa developing from cells of the peritoneum lyiug in the vicinity of the pouched longitudinal blood-vessel. They are shed from their place of formation into the coelomic cavity and thence pass to the exterior through the nephridia. Frd. 118.— Metamoki'Iiosih of Actinotrochn (after MitTsrHNiKOFF from BALrot'R). in = invMiiiiiMfion. Development of the Phownida-.—Iu their deve'opmeut the various species of Phoronis ho far as kuowu undergo a very r.'inarlaible metamorphosis. Tlie larva which develops from the ovum is knowji as Actinotrochn (Fig. 113, ^)aud is a some, what eloijgated Htructure possessiug at the anterior end a large hood which overhangs the mouth, its edge bearing 250 IN VERTEBRA TE MOEPHOLOG T. strong cilia. Beliiud the mouth are a number of ciliated tentacle-like processes arranged in a horseshoe-shaped curve, their cilia, together with those of the edge of the prostomial hood, f(5rmiug a baud encircling the mouth. The digestive tract opens to the exterior at the posterior ex- tremity of the body, and the axis of the body is the axi» passing through the anus and the centre of the prostomial lobe. A little later {Ji) an invagination (in) of the body-wall into the coelora of the larva develops on the ventral surface behind the band of ciliated processes and becomes of a con- siderable size. At the time of the metamorphosis this in- vagination is suddenly everted (Fig. 113, C and D), the intes- tine being carried with it as a loop, and entirely new axial relations are thus brought about. The long axis of the body is now (D) almost at right angles to what it was in the Acthiotrocha, and since the invagination originally formed on the ventral surface of the larva, the body of the adult l-horonis must be regarded as formed by an excessive de- velopment of the veutral surface, the dorsal surface beiug represented only by the short interval between the mouth, or ratlier the epistome, and the anus. The epistome rei)resents the prostomial lobe of the larva, and the ciliated processes represent the lophophoric region, though they themselves are afterwards replaced by the permanent tentacles. There can of course be no question but tliat this remarkable metamor- phosis is a .secondary plienomonon, and it .seems probable that its acquisi- tion stands in relation to the tubicolous habits of the adult which nocos- .sitato the change of the principal axis of the body. The metamorphosis is the means of avoiding a slow and tedious change necessitated by tlie 'lifler- ent habits of the larva and the aduU, just as the occurrence of the ehry- snlis stage in the development of the butterfly is required on account of the great differences between the mouth-parts of the larval caterpillar and the aduli biittcrliy. The affinities of Phoronis cannot bo considered to be finally settled as yet, though there has been a tendency of late years to associate them with the Polyzoa. They also seem to show affinities to the Gephj-roa, and hy Korae authors are considered more correctly referable to that group. The r.ndency to develop the ventral surface of the body at the expense of ilic dorsal and so to form a new body-axis is seen in Sipnmmlus and carritd t;; its eriijri:rir;iiuii in Phurnnis, and furtiicr ,>iinilaiilit's iwiweeu tiio two forms lire t(» he found in the character of the nephridia and in the occur- TYPE ANNELIDA. 251 rence of a closed blood vascular system. The lophopliore of Phoronis, and tlie epistome, are on the other hand characteristic Polyzoan features, and it seems not at all improbable that Phoronis occupies an intermediate posi- tion between the Gephyrea and the Polyzoa. There is this at all events to be noted concerning the Prosopygia (see following chapter), and that is that they are certainly closely related to the Annelida. If the supposition advanced on p. 343 to the effect that the Sipunculacea are to be regarded as Annelida which have secondarily lost their metamerism be correct, and li Phoronis really indicates a derivation of the Prosopygia from Gephyrean- liko ancestors, then the Prosopygia too must be regarded as Annelida in which all traces of metamerism have been lost. This view seems preferable to that which would refer the Polyzoa, for instance, back to unsegmented ancestors— back, that is to say, to the non-segmented ancestors of the Annelida. SUBKINGDOM METAZOA. TYPE ANNELIDA. I. Class Ch^topoda. — Metamerism usually well marked ; with dorsal and ventral rows of set* along the sides of the Ixuly. I. Subclass PoLYCHJiTA.— Marine forms ; with the setaj usually borne upon lateral lobes of the body (parapodia). 1. Order ^rt'//)fUhalmus, Arenicola, Aricia. 3. Order ^edentaria. —VsunWy tubicolous ; anterior metanieres more or less different from the rest. Amphitrite, USerptila, Sahel'a, Terebella. II. Subcla.ss OLKiocH.CTA.— Aquatic or terrestrial forms; with setas but without parapodia. 1. Order Naidomorpha. —¥ov the most part aquatic; fre- quently reproflucing nousexually ; nephridia servo as re- productive ducts. Nais, Dero, C/i(etobrunchii.s; uEolc- sonia, Ch(vt(uinster, Cfenodrifiis, Tubi/ex. 2. Order Lnmbriiomorpha.~Y(n' tlie most part terrestrial; not i'oproducing non sexually ; special reproductive dtiets. LuiiihriruSy Perich(tta, Aikh 'fa. II. Class IIiiiUDlNEA.— Metinnerism \x(>]\ marked ; without setip ; with anteiior and posterior suckers. 1. Order 6' m///*(*/*f/('//«(/(r.— Mouth with three more or less well developed teetii ; pharyn.x not protrusible. Ilirudo, Ma- crobdtlla, Nep/ieli.s. 252 IN VERTEBRA TE MOIiPHOLOG T. 2. Order RhynelwhdeUidcB.—W ifhont teeth and with protnislble pharynx. Clepsine, Povtobdella, Piscicola, Branchellion. III. Class Gephyrea.— Metamerism indistinct ; without parapodia. 1. Order Ec1dure>iiy cf -'iiinculiis Gotildii, Pourtolts. Studies from the Hiolog. l-iilior. Johns Hopkins Univ., IV, 1890. TYPE ANNELIDA. ^m MVZOSTOMIDA. L. von Graff. Das Genus Myzostoma. Leipzig, 1877. Report on tlie Myzostomidm. Scient. Results of the Voyage of H.M.S. Challenger. Zool, x, 1884. J. Beard. On the 'Life-history and Development of the Genus Myzostoma. Mittb. a. d. zool. Station Neapel, v, 1884. W. M. Wheeler. Tlie Sexual Phases of Myzostoma, Mittb. aus der zoolog. Station zu Neapel, xii. 1896. PHORONIDA. E. B. ■Wilson. The Origin and Significance of the Metamorphosis of Actinotroeha. Quarterly Journ. Microscop. Science, XXi, 1881. C. J. Cori. Untersuchungen iiber die Anatomic und Histologie der Oattung Phorohis. Zeitscbr. fUr wissenscb. Zoologie, Li, 1891. s 254 INVEliTEBEATE MOliPHOLOO Y. s/ CHAPTER XI. TYPE PROSOPYGIA. The members of the type Prosopygia are compact, soli- tary, or coloiiial orj^auisms destitute of a true metamerism iiud liaviug the digestive tract usually bent upon itself, so that the auus lies iu more or less close proximity to the mouth and therefore uear the anterior end of the body. A <3hitinous or more or less calcareous investment is formed •about the exterior of the body, and in some cases assumes the form of a calcareous bivalve shell, similar to that of the Pelecypoda in its general appearance, although in the rela- tions of the valves to the body-surfaces and in other particu- lars (see p. 327) there are very decided differences, the simi- larity being simply an analogy. A more characteristic feature, however, is the presence at •the anterior end of the body of a circular or horseshoe-shaped fold, or else of two armlike lateral processes, forming what is termed the lophophore, upon which are borne a number of tentacles which play important roles not only iu obtaining food, but also iu the process of resj^iration, no branchial or other special respiratory organs being present. A more or less spacious coelom is usually present, trav- ersed by muscle-fibres and some specially developed muscle- bands, though the must lar system is on the Avliole poorly developed. The coelom contains a Incmolymph, but a sepa- rate blood vascular system and heart is entirely wanting. The nervous system, in accordance witli the absence of met- amerism, is exceedingly simple, consisting either of a single ganglion, lying between the mouth and anus and sending off nerves to the various regions of the body, or else of a nerve- ring surrounding the oesophagus, with more or less distinct supra- and sub-cjesophageal ganglionic enlargements. Special sense-organs are wanting. TYPE PliOSOPTGJA. 255 A pair of simply-constructed nephridia are present in some forms, but in many a special excretory organ seems to be entirely wanting. Bisexuality is the usual arrangement, although in the Polyzoa hermaphroditism is of not unfre- quent occurrence. The great majority of the Prosopygia are marine in habi- tat, though a number of Polyzoa are inhabitants of fresh water. The type may be divided into two well-marked classes, the Polyzoa and the Brachiopoda. cinl I. Class Polyzoa. The Polyzoa, a group usually spoken of by German zool- ogists as the Bryozoa, are almost without exception colonial organisms, forming encrusting, massive, or more or less den- dritic masses composed of a large number of small individuals or polypides, each of which is enclosed within a chitinous or in some cases partially calcified investment, the zooecium, from the mouth of which the anterior portion of the body bearing the lophophore may be protruded. This outer investment or edoci/st (Figs. 115 and 116, ec) is lined upon its interior sur- face by a layer of ectoderm-cells, within which is a delicate peritoneal lining, these two layers together constituting the true body-wall or endocyst (Fig. 115, en) practically destitute of muscle-tissue, though a sphincter is usually present at the mouth of the cup, which may thus be closed over the re- tracted polypide. A more or less spacious coelom (Fig. 115, co) is present in the majority of forms, containing a lucmolymph and tra- versed by a number of muscle-strauds (m) which may be aggregated into special retractor bands ; but in one order, the Eiidoprocta (Fig. 114), these are wanting and indeed the ccelom is reduced to a very small space between the body, wall and the digestive tract. This latter structure has tlie characteristic U- or Y-shaped form and presents but little difterentiation into special parts, though an oesophagus, stomach, lined with glandular so-called liver-cells, and rectum may be distingni.slied. An anus (Fit^s. 115 and 116 a) is iilways present and may be situated either within or without 256 IN VERTEBRA TE MORPIIOLOG T. :f the area enclosed by the lophophore. The nervous system is exceedingly simple, cousistiug of a siugle gangliouii mass (Figs. lU and 115, ce) lying between the mouth and auus» nerves lamit'ying from it to the various parts of the body. The only sen.se-organs that have been detected are situated upon the free portions of the bod; more especially on the lophophoral tentacles {t), and are represented by scattered ectodermal cells each of which bears a strong cilium and is in connection with a nerve-fibre ; the}' have been assigned a tactile function, thoi gh ic seems probable that they react to stimuli of various kinds and have a much more generalized function. The arrangement of the excretory and reproductive organs varies considerably in different forms and may be more con- veniently described in connection with the various orders. 1. Order Endoprocta. This order contains but a small number of forms, which, with one exception, Urnatella, are marine in habitat. They all possess the power of reproducing by budding, colonies being thus formed, as in PediceUina, Ascopodaria, and other genera with the exception of LoxosomUy in which the buds separate completely from the parent at a relatively early stage of their development. Each in- dividual (Fig. 114) is a cup-shajjed structure, prolonged posteriorly into a stalk (6'^) and upon the rim of tlie cup, which represents the lophophoral fold, or slightly below it on the inner Fig. 114.— Single Indi- *^"ii'ace, the tentacles (t) are arranged viDUAL OF PediceUina in a circle surrounding a depression, the vestibule, into which open both the mouth and the anus, the situation of tha latter opening within the circle of tentacles having suggested the name given to the order. The tentacles can be coiled in a circinate manner, so as to lie completely t .- (after Nitsche). ce = ganglion. I = so-c'iiUed liver. s = stomach. St = stalk. t = tentacles. r^ TYPE PEOSOPYGIA. 26T within the vestibule, and the rim of the cup can be closed over them, owinj^ to the presence in it of a circular band of muscle-fibres. The entire body is covered by a delicate cuticular ectocyst similar to the cuticle of the Annelids, below which is the ectoiierm containinj^ numerous gland-cells, as well as scat- tered sensory hair-bearing cells which, however, have not been found to exist in some genera {Ascopodaria). Scattered mus- cular fibres occur in the body-wall, but they do not as a rule reach an extensive development. The coelom is of very slight extent and in Lo.rosoma is re- placed by a gelatinous matrix enclosing branching cells and muscle-fibres and recalling the parenchyma of the Turbel- laria. Imbedded in this parenchyma is the U-shaped diges- tive tract, beginning with the mouth situated in the vestibule and overhung by a well-marked epistome. The mouth leads into a narrow oesophagus lined by ciliated columnar cells, and opening below into a saclike stomach (Fig. 114, s) which forms the lower transverse portion of the U. The cells of its anterior (vestibular) wall (?) are large and destitute of cilia, and contain numerous granules, on which account they have been termed " liver-cells." The intestine forms the ascending limb of the U, and like the oesophagus is cili- ated, opening into the vestibule at the siimmit of a well- marked papilla. The nerve-ganglion lies below the floor of the vestibule between the epistome and the anal papilla and is a single tluuib-bell-shaped structure from which from one to three nerves arise on each side, branching to be distributed to the tentacles and muscles of the body. A single pair of nephridia occurs, opening into the vesti- bule, and each is composed of a number of perforated cells, the lumen being ciliated. It is doubtful whether a flame-cell occurs at the inner extremity as in the Annelid head-kidneys, Avhich otherwise they resemble. Most of the Endoprocta seem to be bisexual, though Pedicelliva is perhaps hermaphrodite. The reproductive organs arise from the mesoderm of the body- wall, forming masses projecting into the parenchym.a, ,i.nd are provided with special ducts which either remain independent 3 S ^ 258 INVERTEBRATE MORPHOLOGY. of each other (Pedicellina), or unite together to form a single tube and open into the vestibule, between the epistome and the ganglion. 2. Order Ectoprocta. The order Ectoprocta includes the great majority of forms Avhich are referable to the class Polyzoa. They are without exception colonial forms of small size in which the tentacles are arranged either in a circle or in the form of a horseshoe surrounding the mouth, the anus being invariably situated, contrary to the arrangement in the Endoprocta, outside the limits of the lophophore. The tentacles, too, when retracted are not flexed or coiled as in the Emloprocta, but are simply approximated to form a bunch, each tentacle being straight And parallel to its fellows. The most characteristic peculiarity of the EctojDrocta, hoAvever, is the power which they possess of withdrawing or retracting the anterior portion of the body with its crown of tentacles within the posterior part (Fig. 115). This latter portion is enclosed in the ectocyst {ec) to which the body- wall is closely adherent and which forms a chitinous or, in some <;ases, more or less calcareous cell, termed a zoaecium. At the mouth of the cell the cuticle becomes suddenly exceedingly thin, so that the anterior portion of the body is quite mobile, and by means of special retractor muscles (w) may be with- drawn within the zooecium. The retraction is a process of in- vagination, similar to what occurs in the withdrawal of the pharynx of the Annelida, the most anterior lophophoric part of the retracted portion not, however, sharing in the invagina- tion : the whole process indeed is similar to what may be obtained when one finger of a glove is caught from within somewhat less than half way from the tip and drawn down towards the palm ; half of the lower portion will thus be in- vaginated within the other half, while the tip of the finger remains uninvaginated. The portion of the body-wall enclosed by the ectocyst is thin, its longitudinal muscles being for the most part sepa- rated in the form of bands traversing the coelom and funetiou- FlG, TYPE PROSOPYOIA. 25£> iug as retractors, while the circuhir muscles are specially de- veloped as a rule ouly arouud the mouth of the cup, which may by their action be closed over the retracted tentacles. A relatively spacious ccelom, contaiuiug a colorless corpuscu- lated ha3molyraph is preseut, aud is liued by flatteued perito- ov Fig. 115.— Diagkam op the Stkuctuke of Alcyoridium alMdum (after PH(r(]('r Phiilactiihvmata. The members of this suborder are exclusively inhabitants of fresh water and are characterized by the tentacles being arranged in a horseshoe-shaped manner (excejit in the genus Fmlvn'rella, wliere they form a circle), and by the occurrence of a well-develoi)ed h)be or cplstomc overlapping the mouth. The colonies assume various shai)es in difl'ennit genera, being sometimes dendritic and incrusting stones or other TYPE PliOSOPYGIA. 261 bodies, as iu Fredericello, or fonniug compact masses, as in Ahyonella and Lophopus, or even being ca])able of motion, as in (Jnstatella. In some forms, e.g. Lophoptis, the ectocjst pos- sesses a gel-itinous consistency, tliough usually it is cliitinous, and the various zooecia are in free communication with one another, nt)t being separated by transverse partitions. In addition to multiplying by the usual processes of bud- ding and by ova, the Phylactohcmata develop upon the funiculus special internal buds, termed stafohla.sts, which are (mclosed Avithin dense chitinous capsules. These are set free by the dying and disintegration of the parent and, being pro- tected by the i-esisti'nt capsule, retain their vitality under conditions, such as cold and dryness, that destroy the adult individual. They are evidently a special provision for tJie l)erpetuation of the species developed in accordance with the fresh-water liabitat, iu v.hich the organisms are exposed to various conditions not apt to be met witii in the ocean ; it is interesting to note in this connection the occnrrence of gem- mules in the fresh-water sponges which are stri^itly compara- ble to the statoblasts and have a similar siguiiicauce. 2. Suhordor fii/mnolmnafn. The Gymnolivnuita are distinguished from the PliylactolK'- mata by being, with the single e.vce))tiou of the genus Pah(di- cclhi, marine in habitat, by the tentacles being arranged in th(; form of a circle, and by the invariable absence of an epi- stome. As in the Phyhictoln'mata, the colonies vary greatly in form, being in some cases encrusting, Menthmnipora, Llns- h-(i, in others branching, Scrnpocellaritf, or in others again massive, Alci/oiiuiium ; and furthermore the ectocyst ])re- sents varying d(«grees of consistency, being frequently chitin- ous, but occasionally somewhat g«datinous or calcified to a greater or less extent. The zod-cia are not in free communi- cation with each other, but each is closed below or posteri- orly by a transverse cliitinous ])late in which })erforations are said to be present, though doubt has recently been thrown upcm tlieir existence as perforations. In shape, too, the 2<}u?ei;i like the colouies vary greatly, especiuliy so fur as their to 262 IN VERTEBRA TE MORPHOLOO Y. av mouths are concerned, and it is possible to divide the Gymno- Isemata into three groups or tribes, based upon these differ- ences. In the tribe Oydostomata the zooeeia are usually cylindrical, and the mouth is circular and destitute of any appendages ; in the Ctenodomata the mouth is closed during retraction by a series of bristles which surround it {Alcyonid- ium); Avhile in the ChUostomata, in which the ectocyst is. usually lirm and frequently calcified, the mouth is closed by a lid, the ojoc/'cm/mw, furnished with spe- cial muscles {Hugula, J/emhranipora). In this last-named tribe a poly- morphism of the individuals conipos- .. avi^n ^ colony is frequently found. Scattered among the ordinarj' indi- rm viduals others, the Avicularia (Fig. 116, av), may be found having fhe appearance of a bird's head, the lower beak being fastened to the head by a hinge and having inserted into it strong muscles ; bunches of sensory hairs are also ])resent, and when these are stimulated the lower beak is ra})idly snapped against the upper and the stimulating organism thus caught. There can be little doubt but that these Avicularia are specially modified individuals whose head and ujjper beak represent tiie ordinary iiidividnai, while the lower beak may ])i>ssibly be the equivalent of the operculum ; physiologically they liavc beeu usually regarded as specializ;<>d for the purj)()He of catching food for the ordinary individuals, but it is not improbable that their services may rather be of a cleansing nature, re- moving from the colony particles of dirt Hiid the extirota, which l)v accumulating might interferr' with the proper function of the tentacles. Another polymor. 00 _bb Fin. 118. — PouTioN OF Colony of Hugula. a = uinis. av = ftviculiida. bb = brown body. ee = ectofysl. / -- funiculus. oe = ovici'll. rf» = reUa(!lor muscle. t = tentacles. TYPE PROSOPYQIA. 263 phic form is known as the Vihracula, and consists of a slender filament movably articulated to a rounded structure corre- spondiug to the head of the Avicularia ; the filameutr wave continually to and fro and are probably tactile in fuiiction. In many forms, too, in the neighborhood of the mouths of the ordinary individuals sac-like pouches occur, in which the ova undergo their development. These structures, known as ovicelh (Eig. 116, oc), or ooecia, have also been considered modi- lied individuals, but seem rather to be organs of the ordinary individuals, arising as a pouching of their walls. Finally, not iufrequently certain individuals relinquish their nutritive Fio. 117.—^, Lauva op PedkelUna fafter Hatschek) ; B, CyphonauUs (after Pkouho). ad = adhesive orgau. « = stomiicli. cal = calotte. sh = sliell. cor = coroMu. pyr = pyrifonn organ.' function and serve as root-like anchors or stem-like supports for the entire colony. As regards the internal structure of the Gymnolwmata it IS unnecessary to add anything to what has already been stated in describing the general characteristics of the order Ectoprocta. Development of the PoJyzoa.—The larva of PcdkrUimt (Fig. 117, A ), which may be taken as a type of the Endoprncta, is a somewhat donie-shapiul organism, the summit of the dome IxMug occupied by an apical thickening {cat) bearing a tuft of ciliji, while at the marmn tlif»ro iu n uf,.i •iliary band, ti iO I 264 INVERTEBRATE MORPHOLOGY. corona {cor). The cavity of the dome is occupied by the U- shaped digestive tract (s), the mouth aud anus both opening Avithin the circle formed by the corona, a deep depression, the vestibule, lying between the two. In the coelom above the floor of the vestibular depression are a number oi meso- derm-cells, and also a cili.^ted canal composed of a single row of perforated cells and probably excretory in function. Upon one surface of the larva between the marginal ciliated band and the apical thickening is a peculiar glandular organ termed the cement-gland (pyr), around the mouth of which are situ- ated a number of strong cilia. The development of thir larva into the adult form is accompanied by a number of remarkable changes, which in their details and significance are not yet tlioroughly understood. Tlie larva settles upon the ventral or oral face and shortly thereafter one wall of the vestibule becomes pushed over towards the other and eventually unites with it, the original vestibular cavity becoming divided into two portions, one of which remains in con- nection with the surface of fixation and later degenerates, wliile the other has opening into it the mouth and anus, tliough tlie former ()i)ening at about this period becomes closed. Later a remarkable rotation througli 180° of the digestive tract, together with tlie portion of the vestibule in coniu'otion with it, occurs, the portion of the body immediately above the point of fixation elongating to form the stalk of tiie adult, becoming at the sami time filled with mesodermal tissue. The mouth opens again into the vestibular cavity, the tentacles arise from the wall of the cavity which later opens to the exterior, tlie adult form being thus assumed. The fate of the apical tiiickening and of the cement-gland is uncertain ; they have been described as degenerating without taking any part in the formaticm of the adult organs, tliough it has been suggested tiiat the apical thickening may give rise to the nervous system of the adult. The form of the larvie in the Eeto[)r()cta is subject to con- siderable moditication. In A/einhnaiiporii and some other genera the larvti is known as the ('i/pli())H(>tf<',s (Vi^. 117, /I), having been so designated before its life-history was eluci- dated. It has a somewha* triangular outline and is character- ized by being enclosed in a bivalved chitinous shell (fih). At the apex of the triangle is the apical thickening {col), with its elongated cilia, while around tl.ll-develope(! digestive tract is ])re8ent. both the mouth and anus upcning upon the basid surface of the tri auLde. aud therefore within the area enclosed bv the con)nn. TYPE PROSOPTGIA. 265 This vestibule is a deep depression of the oral surface of the larva, diflfenng from that of the Fedicellina larva ouly in hav- ing an arch-like thickening of its walls (only one side of the arch IS represented in the figure) which imperfectly separates an oral portion of the vestibule from a posterior or anal portion, a glandular depression situated in the roof of this latter portion constituting the adhesive organ (ad). In front ot the oral vestibule is situated a ciliated depression from which projects a tuft of long cilia and which appears to correspond to the cement-gland of the Pedicellina larva and () a glandular structure in the moru modified Ectoprocta larvae known as the piriform organ {pyr), by which name it may be known here. The similarity of this larva to that of Pedicelhna is clear, the details of organization ox the two forma ugreeing part for part ; in other Ectoprocta, however, great aifte.-ences are to 1 . found. In the genus IJugnla, for exam- ple,the arva (Iig. .18) is a barrel-shaped organism at the one extremity of which is a thickening, the calotte (cal), which appears to correspond, in part at any rate, to the apical thickening or dorsal organ, as it is sometimes termed, of Pedicel- linn and Cyphonantes. The sides of the barrel are formed by a circle of elongated cells forming the corona and equivalent to the marginal corona of the other larva? ; it does not, however, form a simple band in Bugnia, but its cells are much sliorter on one of the faces of the eiubryo than elsewhere, producing a Avell-marked groove at the apex o^f which lies the pyriform organ (pyr^ whose homologies in Cyphonaute, have already l)eeu pr.nt,;,| out. A peculiarity of this larva is the entire al)sence of -i uigestive tract, the lower end of the barrel being orm-layers It is possible, however, to reganl the tissue from which buds arise as undiffer- entiated embryonic tissue passed on from polypide to polvpide and trace- able back to the embryonic ti,s8 le of the ovum. In the foVmation of each polypide a certain amount of the t; ,.ae becomes differHntiated but some still retains its embryonic character, .i continuation of the buddin- process bfiiiig thus possible. Affinities of the Ihlysoa. -There seems to be little room for doubt but that the Endoprocta represent more nearly the original Polyzoa than do t he Ectoprocta. Their colony formation is of a more simple form than mat of the other group, they possess nephridia which are wanting in the "lajority of the Ectoprocta, and their development is much simpler the i;;lily modified larva of the marine EctojH-octa having undoubtedly been .lorived from one approximating in structure that of Pedicelliua, Cmho- nantes representing a stage in the evolution. Similarities have been traced between the Pedirdlina larva and the Anne Hi Irochophore, and it is not improbable that this may have been the tme derivation of the group, m which case the Polyzoa are' to be re-ard^d as forms which have never possessed any traces of metamerism. but"8tan.i m al.ni.t. the snme relationship to the Annelida us do the Kotifera 268 INVERTEBRATE MORPHOLOGY. Another view, however, Avhich has had ardent supporters is that which recognizes a relationship between the Polyzoa and Phoronis. There is a lophophore in both, likewise a U-like bending of the digestive tract, and the nephridia of Phoronis may be considered comparable to those of the Endoprocta. But here the similarity ceases. The anus in Phoronis is outside the limits of the lophophore and is comparable in position with that of the Ectoprocta, a point which tells against tlii,, phylogeny since these forms have been shown to be less primitive thiin tl . Endoprocta. If, however, this phylogeny should prove to be correct, ii will show a descent for the Polyzoa from metameric Annelids, through tlic Gephyrea, since it is to this group that Phoronis seeras to be most nearly related. II. Class Brachiopoda. The Brachiopoda constitute a very well-defined group whose present poverty in species is in striking contrast to itn great development during Palaeozoic times. Like the Poly- zoa they possess a tentaculute lophophore (Fig. 120, Ip) Avhich usually takes the form of two exceedingly elongated, some- times spirally-coiled, arm-like processes projecting, one on either side, from the anterior portion of the body, and fur- nished upon their outer or posterior border with tentacles. The body is usually somewhat short and stout, and prolonged posteriorly into a peduncle (pe) or stalk Avhich is in some cases at least provided with adhesive papilla) and serves us an anchor. The most characteristic feature of the Brachiopoda is, however, the presence of a bivalved shell (Fig. 119) similar to that of a bivalve Mollusk, with which forms the Brachio- pods were until comparatively recently associated. From near the base of the peduncle, upon the dorsal and ventrul surfaces of the body, a fold of the body-wall is found, Avhieli contains a cavity in communication with and indeed in rriulits a portion of the euilojnic cavity. These two folds are of sulii cieut size to enwrap or enclose the body and the lophoph( re and are termed the mantle-lobes (Fig. 120, m), the space between them and the body being known as the mantle-cavity. Thev subserve largely if not entirely the function of respiration, the portion of the ccelom which they contain being more or less divided up into a system of latMintp through which tlm TYPE PR080PYQIA. 269 hyemolymph circulates. Upon the outer surface of each mantle-lobe, and formed by it, is a valve of the shell, composed of a certain amount of organic matter, but largely of carbonate of lime, yiuce the mantle-lobes are dorsal and ventral in position, so too are the valves of the shell, and consequently their hinge-line is posterior and their mouth anterior. In a number of forms, which may be grouped together as the suborder Testicardines, the shells along the hinge-line are provided with interlocking teeth, a true hinge being present, the peduncle in these cases perforating a backward prolonga- tion or beak of the lower valve. In a few genera, however, forming the suborder Acardines, no such hinge is present, Fig. 119.— Dorsal Valve op »'?;«n/e;', showing Arm Skeleton (after Leunjs). the peduncle passing out between the two valves of the shell. Special muscles are present extending from one valve of the shell to the other and are necessary both for the opening and tlie closing of the shell, and furthermore it should be noted that except for a slight difference in concavity both valves of the shell are similar and symmetrical. It will be seen by comparing the various facts mentioned liere witii what is said on p. 827 regarding the shell of the bivalve Mollusks that the structures in the two groui)s are very different. Tiiis difference is emphasized by the presence, ill the majority of the Testicardines, of a calcareous support for the coiled lophophore attached to the inner surface of the dorsal valve (Fig. 119). It consists of a pair of calcareous rods which project downwards and forwards, uniting to form a transverse arch, and may give rise on each side to a spirally- wound ])r()cess upon which the coils of the lophophore rest. The body-wall is ( omposed of an outer laver of ectoderm 2 ^ 270 JN VEIiTEBHA TE MOBPIIOLOO F. from which numerous papilho or in some cases branching processes arise, projecting into corresponding cavities or tubes in the substance of the sholl-valves. Bek)w tlie ecto- derm is a more or less homogeneous connective tissue con- taining cells and recalling the mesogkua tissue of the Coelen- terates. Scattered mascle-libres, arranged transversely and longitudinally, occur in the mantle-lobes and in the body-wall, but there are no definite muscular layers such as are foui)d in the Annelida, though the longitudinal muscles of the peduncle are well developed. Special muscles, which cannot be con- sidered differentiations of the musculature of the body-wall, traverse the ctelom from one valve of the shell to the other, one pair, the divaricators, being inserted in such a way as to cause by their contraction a separation of tiie two valves, while another pair, the adductors (Fig 120, ant), approximate them. Other muscles also occur, si: 'h as the adjustores, which produce lateral ements of the shell-valves, and pro- tractors and retractors ^. 120, rm) of the peduncle. The coelom is lined cy a peritoneal epithelium and con- tains a corpusculated hsemolymph which is driven about through the coelomic spaces, and the lacuna) in the mantle- folds and the lophophore which communicate with them, by the contractions of the body-wall and the musculature, there being no distinct heart or blood-vessels. A dorso-ventral mesentery which slings the intestine is present and divides the body-ccfilom more or less completely into two lateral chambers, and furthermore two transverse partitions or dis- ssepiments occur in several forms and divide the coelom into iintevior, middle, and posterior compartments, an arrangement recalling the metamerism of such a form as Sagitla (p. 1 37). The mouth opens at the anterior end of the body between the two lophophoric arms and leads into a short, somewhat muscular oesophagus, which posteriorly communicates with a stomach-like dilatation (Fig. 120, i) into which open the •ducts of one or more pairs of branching tubular glands — the so-called liver or digestive glands {J). Behind the stomach lies the intestine, which, in most of the Ecardines, such ;is Lingula, bends upon itself and opens into the mantle-cavity in the mid dorsal line near the anterior end of the body. In Crania, TYPE PROSOPYGIA. 271 however, it opens posteriorly, while iu TerehratuUm, Argiope, ami WaUheimia, iu fact iu all the Testicardines, it euds bliuclly, the auus boiug wautiufj. The nervous system consists of au cjBso[)hageal riuf,' lying iu the connective tissue substance, the lower portion being in connection with the ectoderm and slightly swollen, represent- ing probably the subcesophageal ganglion of the Annelida ; <) Fig. 120.— Stuucture op TerebratuUna septentrionalis. am = adductor muscle. ne = ncphridium. i = intestine. ov — ovary. I = liver-lobes. pe = peduncle. Ip = lophophore. rm = retractor muscle. m = mantle. g = shell. a similar swelling in the dorsal portiou of the ring reju-esents the supraoesophageal ganglion, and iu addition there are usu- ally two further lateral ganglion-like swellings. Nerves pass off from the upper ganglion to the lophophore and other regions, and from the lower one to the mantle, muscles, etc., l)oth sets terminating in the superficial layers of the lopho! l)hore-tentacles or of the mantle in a network of ganglion cells and fibres. No trace of a ventral nerve-cord in addition to the subcesophageal ganglion is present. Sense-organs are but poorly developed, neither eyes nor auditory organs occurring. The tentacles on the lophophoric arms are in all probability sensory, as indicated by their rich nerve-supply, and the papillae of th« mantle-ectoderm whioli oei IMAGE EVALUATION TEST TARGET (MT-3) 4. 1.0 1.1 1.25 UilM 1^ 1^ 2.5 2.2 1.8 U ill 1.6 vm (^ W. ^".^ '\ % op. Pbotogrdphic Sciences Corporation 23 WfST MAIN STREIT WI»'HII,N.Y 14910 (7<6) 173-4503 C5> 272 INVERTEBRA TE MQRPHOLOG Y. M i I I, b !#■ ?S' project into the canals of the shell have been stated to be sensory, containing an axial nerve-fibre terminating in a sen- sory cell. The nephridia (Fig. 120, we) are represented by two or four {Bhyrwhonella) funnel-shaped short tubes which open by a fimbriated mouth at one extremity into the coeloraic cavity and, rapidly narrowing towards the outer end, open by a small pore into the mantle-oavity. In addition to their probable excretory function, th'^se structures, as in some of the Anne- lida, serve also as ducts for the passage to the exterior of the reproductive elements. These are derived from the coelomic peritoneum and form branching masses (Fig. 120, ov) lying in some cases in the coelomic spaces of the mantle, or in addition extending into the body, as in most Ecardiues, or, as in Tere- braMina, confined to this region. Most of the Brachiopods are bisexual apparently, though it is possible thai Lingida and perhaps some other forms may be hermaphroditic, the male and female elements maturing at different times. Development and Affinities of the Brachiopods. — The Testi- cardines are characterized by the occurrence of a free larval stage destitute of a shell. In Argiope (Fig. 121) it is appar- ently divided into four segments, the most anterior of which bears two eye-spots and assumes an umbrella-like form, long cilia projecting from the margin. The third segment develops two folds which enclose the posterior segment and bear on their margin bunches of setjc inserted in seta- sacs and recalling the sette of certain Annelid larvae. After swimming about for a time the larva settles down and fastens itself by the posterior segment and the mantle-lobes turn forward to enclose the anterior seg- ments. The posterior segment becomes the peduncle of the adult, and the shell de- velops on the surface of the mantle-lobes, whose bunches of setio are thrown oflf. The mouth makes its appearance only after fixation just ventral to the eye-spota, and around it there develojm a ring of tentacles jdaced somewhat obliquely, and Fio. 131.— LxnvA OK Argiope (after KoWALBWMKI). TYPE PBOSOFTOIA. 373 later elongating laterally to form the coiled lophophore with its numerous tentacles. The early stages of the development of the Ecardines is not known, but in Lingula the larva is free-swimming long after the shell has formed, the peduncle being late in develop- ing. In this form also the lophophore arises as a circle of tentacles surrounding the mouth and subsequently elongates laterally. The affinities of the BracLiopods have long been an open question- They were by early writers regarded as Mollusca, later as Annelida or closely related to thai group, but are now usually considered to be more nearly related to the Polyzoa than to any other forms and to be most properly associated with them, the general likeness of a young Lingula, for instance, to a Polyzoan being very striking. The presence of the mantle- lobes and the shell seem to mark the Brachiopoda as something far removed from the other members of tiie type Prosopygia, but it must be remembered that in the larval Ectoproctous Polyzoa the corona behaves in a manner closely similar to the Brachiopod mantle and it is not impossible that the two structures may have something in common. Another distinguishing feature of tha Brachiopods is the indication of a segmentation. The presence of two dissepiments and in Rhynchonella of two pairs of nephridia certainly suggests metamerism, hut objection has been raised to the dissepiments having any metameric significance, on the ground that they do not bear the proi)er r«lationsiiips to the body axis to be regarded as comparable to the dissepiments of the Annelida. It ha* been stated by some authors as a characteristic of the Pro.sopygia that tlieir body axis is bent upon itself so that the two ends are approximated and one surface, the dorsal, is almost obliterated, while the other, the ventral, is very much enlarged, as seems to be actually the case in Phoronis. It must be remembered, however, that the terms dorsal and ventral are not to be defined by reference to the digestive tract alone, but otner structures have also to be taken into consideration. Thus it is quite l)ossiblo that in the Polyzoa the approximation of the mouth and anus indi- cates simply a bending of tin digestive tract and a migration forwards of the anus and not necessarily a bending of the body axis; and the varying position of the anus in the Ecardinate Brachiopods tends to support this idea, the Ijody axis in Crania with .i terminal anus certainly besiug similar to that of Lingula, in which the anus lies far forwards. In this connec- tion, too, the arrangement in Sipmicu/us is of interest, the nerve-cord show- ing the usual relations to the body axis, while the digestive tract is bent upon itself and the anus opens fai' in front of the posterior extremity, in the Sipunculacea there can be no question of a difforonco of the body axis in the various forms, and it seems probable that the supposed bending of the body axis in the Prosopygia has not really occurred, but that there ,n &i ■TP 274 INVERTEBRATE MORPHOLOOY. has been simply a bending of the digestive tract and a migration forwards of the anus. If this be the correct way of regarding the matter, then there is no reason for disputing the homology of the dissepiments of the Brachiopoda with those of tiie Annelida, and the idea that they represent a metamerism is borne out by the arrangement of the two pairs of nephridia of RJiyn- ■chonella. The question is then. Does the metamerism of the Brachiopods indicate a descent of the Prosopygia from metameric ancestors, i.e., from Annelids through Gephyrean-like forms, or is it a structural feature inde- pendently acquired by the Brachiopods ? The evidence at our disposal is not sufficient for the solution of this problem, and all that can be main- tained is that a very close relationship exists between the Polyzoa and the Brachiopoda. 'Ii te« SUBKINGDOM METAZOA. TYPE PROSOPYGIA. 1. Class Polyzoa.— Small, usually colonial forms ; lophopbore circular or horseshoe-shaped ; no bivalve shell ; no mantle-lobes. 1. Order Endoprocta.—Houih and anus both within the area enclosed by the lophophore. Loxosoma, Pedicellinu, Ascopodaria, Urnatella. 2. Order Ebto^rocto.— Anus outside the area enclosed by the lopho- phore. 1. Suborder P^yZactotowato. —Fresh- water forms ; lophophore usually horseshoe shaped ; epistome present. Ftederi- cella, Alcyonella, Lophopus, Cristatella. 2. Suborder %»iwotema^a.— Usually marine; lophophore cir- cular ; no epistome. Mouth of zooecium without bristles or operculum (Cydo- stomata). Crista. Mouth of zooecium usually surrounded by bristles which close over it (Ctenostomata). Paludicella, Alcyonidium. Mouth of zooecium provided with an operculum {Chilo- stomata). Membranipora, Bugula. Flustra, Scrupocel- laria. II. Class Brachiopoda.— Non-colonial and of moderate size ; lophophore usually arm-like and coiled into a spiral ; mantle-lobes and bivalve shell present. 1. Order Ecardines. — Shell-valves not hinged ; peduncle protrudes between the valves ; anus present. Lingula, Crania. 2. Order Testimrdines.—^hnW-ynXsa^ hinged ; peduncle when present protruding tiirough perforation in the ventral shell ; anus wanting. Terebratula, Waldheimia, Aryiupe, liliyncho- nella. TYFE FHOSOPYGIA. 275 LITERATURE. 4 POLYZOA. G J. AUman. A Monograph of the Fresh-icater Polyzoa. London, 1856. T. Hincks. A Biatoi'y of the British Marine Polyzoa. London, 1880. H. Nitsche. Beitrdge zur Kenntnisa der Bryozoen. Zeitschr. fUr wissensch. Zoologie, XX, 1869 ; xxi, 1871 ; xxv, Suppl., 1875, A. Hyatt. Observations on Polyzoa. Suborder Phylactolamata. ProceedingR Essex Institute, iv and v, 1866-68. S. F. Harmer. On t/ie Structure and Development of Loxosoma. Quarterly Jouni. Microscop. Scieace, xxv, 1885. C. B. Davenport. Papers in Bulletin of the Museum of Comp. Zoology, xx» 1890 ; XXII, 1891 ; and xxiv, 1893. BRACHIOPOnA. E. 8. Korse. On the Systematic Position of the Braehiopoda. Proceed. Boston Soc. Nat, History, xv, 1893. W, K. Brooks, The Development of Lingula. The Chesapeake Zoolog, Labo- ratory. Scient. Results of the Session of 1878. J, F. van Bemmelen. Unters,. .angen iiber dew anatomischen und histologischen Ban der Braehiopoda testicardinia. Jenaische Zeitschr., xvi, 1883. M. A. Sohulgin. Argiope Kowalewskii. Ein Beitrag zur Kenntniss der Braehio- poden. Zeitschr. fllr wissensch. Zoologie, XLi, 1884. H. 0. Beyer A Study of the Structure of Lingula (Glottidia) pyramidata. Studies from the Biolog. Labor. Johns Hopkins University, iii, 1886. F. Blochmann. Untersuchungen iiber den Ban der Brachiopoden. Jena, 1892. *i76 INVERTEBUATB MORPHOLOGY, CHAPTER XIL TYPE MOLLUSCA. While the Annelida are characterized by an elongated form of body, the Mollusca present the opposite condition, being compact, nou-metameric organisms, though at the same time primitively bilateral in the arrangement of their organs. Upon the external surface of the body a cuticular secretion is formed in which usually particles of carbonate of lime are deposited, a calcareous shell being thus developed, which encloses more or less perfectly the soft body, assuming, however, very different forms in the various groups. It is essentially a dorsal structure developed in the majority of forms from a depression on the dorsal surface of the body — the shell-gland (Fig. 122, /) — and in some forms may be entirely confined to this area. Usually, however, a circular or bilateral fold of the body, the mantle (c), arises peripheral to the margins of the shell-gland and extends downwards towards the ventral surface, and the growth of the shell may accompany that of the mantle-fold, so that the entire body is enclosed by or may be retracted within the greatly-developed shell. Even in cases, however, in which the shell is but slightly developed *^^he mantle-folds retain their development, forming a marked structural feature of the Mollnsca, and en- closing a more or less spacious cavity, the mantle-cavity, in which lie the respiratory organs and into which the intestine and nephridia and reproductive ducts open. The body-wall is formed of an external layer of eetodern), below which a more or !ess thick layer of muacle-tissne is found whose fibres sometimes show the arrangement in cir- cular and longitudinal layers characteristic of the Annelida, but usually the simplicity of this arrangement is interfered with by a development of connective tissue in which irregu- larly-arranged muscle-bundles lie. Upon the ventral surface li TYPE MOLLUSC A. 277 of the bodj there is a special thickening of the muscle-tissue to form a "foot" (Fig. 122, p\ which assumes a great variety of forms, and special muscles are developed for its protrac Fig. 122. ope P ^i Jc 'I -Diagrams showing the Ahrangement op the Organs in an Ideal Mollusk (after Lankestek). a z= tentacle. b = head. c = margin of mantle d = margin of shell. e = edge of body. /= edge of shell depression. g = shell. gc = cerebral ganglion. gpe = pedal ganglion. gpl 7= pleural ganglion. A = osphradiiim. i = ctenidiiini. k = reproductive pore, I = uephridial pore. m = anus, n and p — foot. r = coelom. « = pericardium. t = testis. u = nephridiuni. « = ventricle of heart. el = liver. tiou or retraction when this is necessary, as well as for the closure of the shell in those forms (Pelecypoda) in which it is II bilateral structure. The coelom is in some forms a relatively spacious cavity, traversed, however, even in these cases by thin bands of con' 278 INVERTEBRA TE MORPUOLOG T nective tissue, but more usually it is reduced to a system of lacunar spaces (a so-called schizocoel) by the development of muscle-bundles traversing it in various directions. A special portion of it (the so-called enterocoel) is, however, always en- closed in definite walls lined by a peritoneal layer of cells,, forming a cavity, the pericardium (Fig. 122, s), which lies nor- mally near the dorsal surface of the body, containing the heart and having the inner ends of the nephridia (w) opening into it. The blood vascular system consists of a primitively three-chambered heart (Fig. 122, r) enclosed within the pericardium and composed of a tubular muscular ventricle and two wing-like auricles which open into the ventricle, their openiugg being guarded by valves which prevent regur- gitation. From the anterior and posterior extremities of the ventricle aortas arise, which, however, as a rule soon lose themselves in the coelomic lacunae. There is thus no distinc- tion between the blood and pseud-hsemal fluids in the Mol- lusca, since the blood vascular system is not closed. The blood is a colorless fluid in which numerous amceboid cells float and which holds in solution a substance, hsemocyanin, which subserves a respiratory function in a manner similar to the hsemoglobiu of the Vertebrata. The heart is a systemic heart, as is usual in the Invertebrata, and con- tains only aerated blood, which it propels through the lacunae of the body. Returning from these, the blood passes either directly to the respiratory organs or branchiae, or else a greater or less portion of it traverses first the walls of the nephridia and then passes to the branchiae. From these, in which it is aerated, it is received into the auricles, and on their contractioh is forced into the ventricle. In some Mollusca respiration is carried on by tbe general surface of the body, but such an arrangement must be re- garded as the exception. As a rule special respiratory or- gans are present in the form of one or more pairs of plume- like processes {ctenidia) of the body-wall (Fig. 122, i) lying free in the mantle-cavity. They have various forms in the different groups, but consist essentially of a central axis con- taining an afferent and efferent canal for the blood and bear- ing a single or double series of filaments whose walls are thin and whose ectoderm is ciliated, an interchange of the gases TYPE MOLLUSC A. 279 of the blood with those of the water contained in the mantle- cavity being thus readily effected, a renewal of the water con- stantly taking place in consequence of the action of the ecto- dermal cilia. The thin-walled mantle-fold is, however, a very efficient adjunct to the branchiae in respiration, the spaces within the fold being portions of the lacunar coelom and con- sequently containing blood ; indeed in some cases the mantle assumes completely the respiratory function, the ctenidia becoming rudimentary. The digestive tract is a usually more or less coiled or con- voluted tube in which various regions may be distinguished. In a few forms, characterized either by the slight develop^ ment of the mantle or its development as two lateral folds the anus is terminal in position, but when an extensive cir- cular mantle-fold is developed the intestine bends upon itself and opens upon the side of the body, more or less anteriorly into the mantle-cavity. Immediately behind the mouth chi' tiuous teeth (Fig. 123, hj) are usually developed in the wall of Fig. 123. -Buccal Mass and Radula of Helix (after Howes). ce = cerebral ganglion. re = nidular cartilage. hj — horny jaw. tm = intrinsic muscles. pgl = pedal gland. rd = radula. I'd' = radular sac. St = opening of salivary gland. the pharynx, and behind these a large muscular thickening generally occurs, the buccal mass, in connection with which is developed a characteristic Molluscan structure, the lingual ribbon or radula (rd). The sides and floor of the pharynx in tins region are largely thickened by the development in them of muscular tissue (wi). The thickening of the floor is usually so extensive as to project into the pharyngeal cavity, forming I IB Jis m i liltt 280 INVERTEBRA TE MORPHOLOG Y. the so-called tongue, and in addition to the muscular tissue two or mox-e pieces of cartilage, the radular cartilages {re), are frequently found in it. Oovering the tongue is a stout chi- tinous membrane, the basal membrane, which bears upon its surface a usually enormous number of chitinous teeth arranged in transverse rows, so that the basal membrane and the teeth, together constituting the radula {rd), recall somewhat the appearance of a flat tile. Behind the tongue the floor of the pharynx is produced downwards and back- wards into a pouch, the radula-sac {rd'), sometimes of con- siderable length, into which the radula is continued on the ventral wall, the cells {odontoblasts) which form the teeth as a cuticular secretion, lying at the bottom of the sac. The tongue, with its radula, can be protruded to a greater or less extent from the mouth b}' special protractor muscles, and its intrinsic muscles serve to give it a slow licking movement, whereby the radula acts as in the manner of a flle or rasp upon the object with which it is in contact. Owing to this action the radula is continually being worn away at its anterior end, but is also continually being pushed forward upon the tongue by the addition of new teeth to its posterior portion at the base of the radula-sac. In connection with the digestive tract various glands are usually present, of which the most constant are the salivary glands and the " digestive glands." The former open into the pharynx and in some cases reach extensive devolopment ; their function for the most part is but little understood, but in some predaceous Ga^teropods their secretion has been found to contain a considerable amount of free sulphuric acid which probably serves to soften the calcareous shell of Echinoderms and other MoUusca which serve these forms as food. The " digestive glands " open into a dilated portion of the intestine, usually termed tlie stomach, and are usually paired, voluminous, much-branched tubular glands whose function is indicated by the name applied to them. They seem to be the physiological representatives of the pancreas of the Vertebrata, and to secrete digestive ferments which are brought into contact with the food in the stomach. The nervous system of tne Mollusca (Fig. 124), in accord- TYPE MOLLUSC A. 281 ance with the absence of metamerism, lacks the ladder-like arrangement which characterizes the Annelida. Nevertheless there are two ganglionic masses, each in typical cases com- posed of two ganglia which may be homologized with the su- pra^scphageal and the most anterior suboesophageal ganglia of the metameric forms, and are known respectively as the <.6r.W (Fig. 124, ce) and pedal (pe) ganglia. The foimer lies above the oesophagus behind the buccal mass and is connected by nerve-cords termed connectives, surrounding the oeso- phagus, with the pedal ganglion. The cerebral ganglion gives off nerves which pass to the eyes and otocysts {ot) and to the tentacular structures of the head, while the pedal ganglion receives its name from the fact that it sends nerves to the muscular mass forming the foot. In addition to this system of nerves and ganglia there is another system highly developed in the Mollusca which would seem to correspond to the visceral system found in some other forms. It consists bu = buccal gauglia. typically of a pair of pleural ganglia (pi), ^^ = cerebral guuglion. one of which lies upon either side of the "' " *'^''^^''*- pharynx, being united by connectives with both the cerebral and pedal gauglia. From each pleural ganglion a nerve-cord passes backwards to unite with one or more visceral ganglia {vi), situated below the intestine near its posterior termination, and on each of these visceral cords a ganglion occurs, the /janeto? ganglion {pa\ from which nerves imss to the gills, or rather to the sense-organ which is in con- nection with them. The pleural ganglia innervate especially the mantle and the body-wall behind the head, the visceral ganglia send branches principally to the various viscera, while the parietal gauglia, iu addition to the branches which go to the gills and their sense-organs, also assist in the innervation of the mantle. Besides these principal ganglia, however, others connected Fig. 124.— Diagram of Nervous System of -MOLLUSK. pa = parietal gangliou. pe = pedal ganglion. pi = pleural gauglion. VI = visceral ganglion. 'W UM 282 INVEHTEBRATE MOliPHOLOOY. with either the cerebro-pedal or pleuro-visceral system raaj be developed, the most constant of which are the buccal gan- glia {bv) which lie at the sides of or more usually below the buccal mass which they innervate and are united by commis- sures with the cerebral ganglia. Two nerve-rings in such cases surround the oesophagus, i.o., that formed by the cere- bro-pedal and that of the cerebro-buccal connectives. This description has reference oidy to what may be con- sidered a typical condition, and it must be remembered that frequent modifications of it may occur. In the Gasteropods, for example, in which, in accordance with the development of a circular mantle-fold, the anus comes to lie on the anterior portion of the body-wall, a peculiar crossing of the pleuro- visceral commissures occurs in some cases, and as a result what was originally the right parietal ganglion comes to lie upon the left side of the body and the original left ganglion upon the right side. Further consideration of this arrange- ment may, however, be postponed until the Gasteropods are under discussion. Mention should, however, be made here of another not unfrequent modification of the typical arrange- ment of the nervous system, which consists in the concentra- tion of the ganglia and the shortening of the various connec- tives. This may affect only the cerebral, pedal, and pleural ganglia, bringing them into close approximation, or, as in some Cephalopods, the visceral ganglia may also be carried forward so that all the principal ganglia are united into a single lobed mass closely surrounding the oesophagus behind the pharynx. This condition constitutes of course the culmination of the concentration process, but various gradations of it are to be found in the different groups. Sense-organs are as a rule well developed in the Mollusca, and descriptions of many of them may be more conveniently given in connection with the detailed account of the various groups. The general ectoderm of the mantle and body-wall has scattered in it numerous sensory cells v/hicli may become specially aggregated at certain points to form definite sense- organs. Thus tentacles are frequently borne upon the head which are tactile or in some cases olfactory in nature, and at the bases of the gills special aggregations of sensory cells are m TYPE MOLLUSC A. 283 185.- Otocyst op Ptero- trachea lafter Glaus). to be found formin- the osphradia, also supposed to have an olfactory function. Otocysts (Fig. 125) are present in nearly all the groups, consisting of a vesicle with a membranous wall, the interior of which is lined by sensory cells bearing bunches of hairs projecting into the vesicle which contains one or more calca- ^/^f reous otoliths. An auditory func- tion has usually been attributed to these organs, but it seems probable that, as in the lower forms (see p. 82), they are rather to be regarded as organs of an equilibrium -sense, and in fact that they subserve such a function in part at least has been experimentall}' determined in the Cephalopods. Eyes are very frequently present and in the Cephalopods reach an exceedingly high development. They occur usually upon the head, but may also be found, as in the Pelecypoda, upon the edge of the mantle, or even on the dorsal surface of the body, as in the Pulmonate Onchidium. They vary, however, so much in structure in different groups that an account of the various modifications which they present may be postponed. Excretory organs in the form of a pair of uephridia are present, each nephridium consisting of a tube which opens at one extremity into the mantle-cavity, while at the other it communicates^ with the cavity of the pericardium, which, as has been seen, is a portion of the coelom. The relationships of these structures are therefore the same as those of the nephridia of the Annelids, and, as in those forms, they receive a rich supply of blood, most of the venous blood returning from the tissues passing through their walls on its way to the branchiae. The reproductive organs are unpaired in the majority of forms and in some cases come into relation with the nephridia, which serve as reproductive ducts. More usually, however, they open directly to the exterior, a con- dition which is probably a secondary one. The majority of the Mollusca are bisexual, but hermaphroditism is by no 284 IN VERTEBRA TE MORPHOLOG 7. I-, f.% means uncommon, the single reprodu«itive gland producing both ova and spermatozoa and being therefore an ovo-testis.. Accessory structures are frequently added to the essential parts of the reproductive apparatus, such, for instance, as al- buminiparous glands, introraittent organs, spermatophore- sacs, etc., so that a relatively complicated arrangement may occur. I. Class AMPHlNEFilA. The Amphineura are Mollusca iu which the primitive bi- lateral symmetry is fully retailed and which seem to approach most nearly to what may be considered the primitive Moliuscau condition. All the known members of the jjroup are marine in habitat and are more cr less elongated forms in which the elongat: )n of the ventral surface or foot is accompanied by a corresponding elongation of the visceral complex, which ac- cordingly is not elevated at right angles to the long axis of the foot to form a visceral dome. In a general way, therefore, in the form of their body the Amphineura may be compared to the Platyhelminths, especially to such forms, sometimes flattened, sometimes more or less cyliudrical and elongated, as are found among the Folyclad Turballaria. The mouth and anus are situ- ated at the extremities of the body, and to either side of the anus are situated the one or more pairs of plumelike brauchiie and the openings of the single pair of nephridia. The shell may con- sist either of a number of scattered calcareous spicules im- bedded in or projecting from a thick cuticle, or else may take the form of a number of plates arranged in a ) )ugitudinul series upon the dorsal surface of the body, and as a rule the miMitlo-f^ld is but slightly developed and ma}' be in son^e forms almost rudimentary. The foot, too, wJiich is so characteristic for the ISiollusca, may iu soDie forms be practically un- developed, but in other forms is a broad flat mu8cuh\r sunace, showing no differentiation into special regions such as are found in the higher Mollusca. Little need be said iiere as to tlie internal organs except to emphasize the fact that both the heart and the nephridia have a perfectly bilateral arrangement. The nervous system is characterized by the absence of a deiiuite agrregatiou of the TYPE M0LLU8CA. 285 nerve-cells into concrete ganglia ; they are scattered along the lougitucliual nerve-cords, of which there are two pairs, i.e., the pleuro-viscer,",! cords, which run along the lateral portions of the bod}', and the pedal cords, which are situated jQore ventrally and whi.'ih, as well as the pleuro-visceral, are fre- quently united by cross-commissures which suggest an imper- fect metamerism. In front tiiese cor<^3 unite together to form the civeumoesophageal ring in which the ganglion-cells are someyd^at more numerous than elsewhere, without, however, forming distinct ganglia. Sense-organs are but slightly de- veloped throughout the group, which is divisible into two well- marked orders. Fig. 126. — Neomenia ca- rinata (after Nansen). ct — ctcnidium. m ~ mouth. eg ~ v'jiitriil groove. 1. Order Solenogastres. The members of this order are for the most part elongated worm-like animals, though some forms are short (JTig. 126) and more nearly approach the typical Molluscan lorm. The exterior of the body is covered by an exceptionally thick cuticle traversed by bauds of cells extending into it from the ecto- dermal layer of the body and termina- ting in cui)-shaped groups of cells whicli secrete the calcareous spicules which are scattered through the cuti- cle {Proneomenia) or may project upon its surface {Chtvioderma), and which are the sole repre- sentatives of the shell of the higher MoHusca. Upon the ventral surface of the body is a Inngitudiual furrow (Fig. 12(5, vg) at the bottom of which lies tiie but sliglitly developed foot, represented by a small ciliated l(>ngitudinal ridge, which in Chvtoderma \\m\\ be (piite undeveloped, tlie furrow being in tliis form also barely indicated or entirely absent. The lips of the furrow which enclose the foot probably represent the mantle- folds of higher forms, here very nnu^h reduced, though more extensively developed at the posterior end of the body, where they projoet to frirsn a fnnutl'liko structiiro (Fig. 127) whose cavity — the cloaca — receives the openings of the digestivo 286 INVEliTEBltATE MOIiPJIOLOOY. Fig. 127. — DiAOKAM of Au- KANOEMENT OF ()H(JANS AT lIiNU End of Glurtcth'rma (after Hubhboht from I^ankebtkb). ct — ctenidiuin. n — nepluidium. 0 = ovary. p = periciirdiuni. r = rectum. tract (r) aud the uephridia ^n) and contains the branchiso {ct). These hist are either a single pair of structures each consisting of a central axis with pinnately- arrauged lateral appendages or in some cases are represented by bunches of ciliated filaments. The ectoderm rests upon a layer of muscular tissue in which both circular and longitudinal layers can be distinguished, and numerous bands of transverse libi'es, in some cases arranged to form septa placed at regular intervals, traverse the body- cavity. A fairly-capacious peri- cardium is present, lying dorsally to the i«)sterior porticm of the digestive tract and into its upper portion the heart i)roject8 slightly, not being, however, completely enclosed by the pericardium. No auricles seem to be developed, nor are any definite blood-vessels present, the circulation being throughout lacunar. Tliis condition of the heart in relation to the pericardium is interesting as sliowing its original independence of that portion of the body-cavity. Its enclosure in the pericardium in the higher Moilusca is a secondary con- dition, the heart and its cavity belonging to the schizoccelic structures rather than to {\w, so-called enteroca>lic i)ericardium. This agrees perfectly with tiio relationships of the blood vascular system of the Nemerteans and Annelids. (See pp. 105 and 381.) The mouth is a longitudinal slit upon the ventral surface «'f the body aud opens into u pharynx i)rovidod usually wHh a iMchila and with salivary ghinds, though both these structures ure absent in Neomeuw. Tlie intestine pursues a straight course towards the anal opening, being, however, in some forms i)ouched, owing to its constriction at more or less regu- lar and close intervals by muscular transverse se'pta. The walls of the pouches thus formed are glandular and represent the digestive gland of other Moilusca, though in Olia'ttHfcrmn there i« a simple ontgrowth of t!io digestive tract wiiich rep- resents it more perfectly. TYPE MOLLUSC A. 287 The nervous system varies iu the details of its arrange- uient iu the different species, but is characterized 'u general by a tendency to form ganglia, although nerve-cells are scat- tered along the nerve-coru.' throughout their entire length. In rroneomenia there is present a well-developed and closely- approximated pair of cerebral ganglia from which arise the pleuro-visceral cords which extend backward along the sides of the body and possess a number of ganglionic swellings near their posterior extremity. Two nerve-rings surround the oesophagus: (1) the cerebro-pedal connectives, which end below in the pedal ganglia, from which two pedal cords extend backward along the foot, in some forms {Dondersia) connected at regular intervals by transverse commissures in an almost nietameric manner, ganglionic enlargements of the cords being developed iu connection with the commissures ; and (2) the <;erebro-buccal connectives, which pass to two buccal ganglia lying below the pharynx. Special sense-organs have not yet been discovered in the Solenogastres. The nephridia consist of a pair of tubes which communi- cate internally with the pericardial cavity and, bending around FiQ. 188.— DiAOHAMMATic LONGITUDINAL SKCTIO^ OF Chiton (after Hallkb) e = perivisceral ca'lom. n = iiervf. h = heart. m — iiiotitli. 1-8 p = |)eric('.r(lliiin. ro — reproductive orgiin. sUell-|)lates. the posterior ])art of the digestive tract, unite to o])en into the cloaca ventral to the anus by a common n-itice. The walls of the tubes are glandular and jnobabjy, therefore, excretory iu function, but the nephridia also serve as the ducts for the " i '--.i.- !iir-.i IT nil luc cvs^trptiv/ii !>i r^rirrtuficrriKl the tJoleuogastre ; are hermaphrodite, the single reproductive 288 INVERTEBRATE MORPHOLOGY. l,^ > , !H gland producing both ova and testes. This hermaphrodite gland is a hollow sac divided into two principal compart- ments by a longitudinal partition and lies above the digestive tract. It is a hollow structure (Fig. 128, ro), the reproductive elements developing from the cells lining its walls and pass- ing from its cavity into that of the pericardium (p), with which the reproductive sacs communicate. They are in fact simply prolongations of the pericardial body-cavity, and the epithelium lining them is continuous with that of the pericar- dium. From the pericardial cavity the ova and spermatozoa pass to the exterior by the nephridia. The Solenogastres are especially interesting on account of the many structural peculiarities of a primitive character which they present and in consequence of which they have been regarded as representatives of ances- tral Molluscan forms. By others, however, this important position- is denied them on the ground that many of their peculiarities are due to degeneration produced in accordance with their life in the mud at the bot- tom of the ocean. The absence of a shell, the reduction of the mantle- lobfp, foot, and radula may with plausibility be accounted for in this manner, but there are other peculiarities that are certainly primitive which are not thus explicable. The relation of the heart to the pericardium is one of these, and others are the communication of the hermaphrodite gland with the pericardium, and the functioning of the nophridia as ducts for the reproductive organs. The Solenogastres are unquestionably primi- tive MoUusca; the only question which is yet to be settled is to what extent, if any, degeneration is responsible for their external peculiarities, such as the absence of a shell, the reduction of the mantle-lobes and of the foot. It must be noted in this connection that one form belonging to tlie genus Pondevfiia has been described as passing through in its development a stage in which indicatiojis of a shell consisting of several plates and simi- lar to that of the Polyplacophora was present, a condition wliicli would seem to indicate the derivation of the members of this group from forms provided with a distinct shell. 2. Order Polyplacophora. Tlie Polyplacophora, like the preceding order, contains only marine forms. For the most part they are somewhat fl.it- toned animals with a rather broad foot occupying the ventral surface, while from the sides of the body a sliglit fold, the mantle-fold, uroiects. In one crenus, ChitoneRus. tlie form of the body is more cylindrical and tlie foot is rather narrow TYPE MOLLUSC A. 28& Fig. 129.— C/«Bto- pleura apiculata. and situated, as in the Solenogastres, at the bottom of a median ventral furrow, the lips of which correspond to the more dorsally situated mantle-folds of such forms as Chiton, Trachydermon (Fig. 129), etc. In all cases, in the groove be- tween the mantle-folds and the foot a number of gills, pinnate processes of the body-wall, are to be found, in some cases occurring at definite intervals along the entire side of the body, in others {Chitonellus) limited to the posterior part only. One of the most characteristic features of the Polyplacophora is, however, the shell, which consists of eight calcareous plates arranged in a longitudiral series along the dorsal surface of the body so that the posterior border of one overlaps the anterior border of the other. The series covers only the median portion of the surface, the more peripheral portions and the outer surface of the mantle-lobes possess- ing a large number of scattered spicules, plates or granules imbedded in their wall. The body-wall has not so definite an arrangement of the muscle-fibres lying below the ectoderm as is the case in the Solenogastres, but, on the other hand, the body-cavity is well developed. Indeed the schizocoelic lacunae play a rather sub- ordinate part in the Chitonidre, as the order is sometimes termed, the enterocoelic cavity (Fig. 128) being very large and divisible into three usually separated parts united by bands, which indicate the original continuity. One of the parts (c) surrounds the intestine and the digestive gland ; another, lying iiitlier towards the anterior end of the dorsal portion of the body, contains the reproductive cells (ro); while the third part ip), lying dorsally and posteriorly, is the so called pericar- 'f»l! il>o ]»1 1 +- il 1 "" • - ''• >Ti.jLii liit Ejjwwvi piiaaus U) tiie iacnnar Hl)ace8 of the schizoccjel. Two vessels w distinct walls r un 290 mVERTEBRA TE MORPHOLOO T. 4 'a longitudiually in the foot, aud presumably receive the blood which they contain more or less directly from the aorta aud distribute it to the lacunar spaces of the foot. The mouth lies on the ventral surface, in front of the anterior end of the foot, and leads into a pharynx provided with a well-developed radula characterized by a somewhat complex arrangement of the teeth. Into the oesophagus a pair of glands opens in Chiton whose secretion contains an amylolytic ferment, and in addition a pair of small glands open into the mouth-cavity. The oesophagus communicates with a sac-like stomach, into which open the ducts of the paired digestive gland, and the intestine, being considerably longer than the body, is thrown into numerous coils, and terminates by a short rectum which opens at the posterior extremity of the body. The nervous system is characterized by the diffuse arrangement of the nerve-cells, no well-defined ganglia oc- curring on the principal nerve-cords. These consist of a strong circumoesophageal ring (Fig. 130), the upper part of which gives off numerous nerves and evidently corresponds to the cerebral ganglia of other Mollusca, while the lower part, corresponding to the pedal ganglia, gives rise to two nerve-cords (pc), the pedal nerves, which pursue a parallel course throughout the foot, giving off a number of nerves laterally and being connected by a number of somewhat irreg- ularly arranged transverse commissures, which almost suggest a metameric arrangement. From the sides of the circum- oesophageal ring two other strong nerves, the pleuro-visceral nerve-cords, arise and pass backwards along the sides of the body, uniting with each other posteriorly above the terminal portion of the digestive tract. These cords {ji), like the cir- cumuusophageal ring, present no distinct ganglionic enlarge- ments, but contain the elements of the pleural, visceral, and parietal ganglia, sending off numerous nerves to the brauchias tlie mantle, aud probably also to the heart and nephridia. In addition to these principal nerve-cords others of smaller size also arise from the circumtwsophageal ring. One pair of J - beneath the buccal mass and send nerves to the oesophagus le blood orta and t of the provided omewliat upliagus itaius au 11 glands mnicates s of the dderably oils, and posterior I diffuse nglia oc- sist of a c part of responds he lower B to two parallel f nerves lat irreg- t suggest circum- )- visceral 3S of the terminal B the cir- enlarge- eral, and »ranehias lidia. f smaller e pair of ia. Ivi!!!/ (ophagus TYPE MOLLUSCA. 29 L while another pair pass to a pair of ganglia lying below the radula and in intimate connection with a peculiar subradular organ, probably sensory, lying in this region. Fig. 130.— Diagram of Neuvous and Excretohy Systems of Chiton sieulus (uoiiibiiiation of two figures by Haller). an — iimis. no = uephridial orifice. Br = ctenidia. oe = ccsophagus. go = genital orifice. pc = jiedal nerve cord, ft = uepliridiiiin. pi = pleural nerve-cord. As regards sense-organs, in addition to this subradular organ whose function is entirely problematical, ridges of sensory epithelium exist along the sides of the body in the lu.intle-cavity. One such ridge runs along the inner wall ot tiie mantle-fold, while the other is found at the bottom of the mantle-cavity passing over the bases of the branchial l)hinie8and sending a short prolongation outwards upon each IJ— •" -^ -^- •-• vf •*• i'*s • • ■■ ' ....... ^ ^...- '- * -t I ■_ !:75j'^„'iii.t TTiLii tiitr osphradia of other Mollusca. mm 11 I 292 INVERTEBRA TE MORPHOLOG 7. A much more peculiar series of organs, found, however, in their perfect form only in certain species, is developed in connection with the shell of the Chitonidae. They consist of club-shaped structures contained in pores which traverse the shell-plates and possess a definite arrangement, being ar- ranged in groups of larger and smaller organs {megalcesthetes and micrcesthetes). Each group is in connection with a num- ber of large glandlike cells, which terminate in the megal- iesthete, covered externally by a cup-shaped layer of chitiu, and from this cell-mass more or less numerous branches arise, the micrsBsthetes, which terminate in club-shaped swellings likewise covered by a chitinous layer. Below the group of cells is in connection with fibrils which unite to form a nerve probably passing to the pleuro-visoeral nerve- cords, and it thus seems tolerably certain that these struc- tures are sensory and perhaps tactile in function. In some species the megahesthetes become modified into eyes consist- ing of an external convex chitinous cap, the cornea, below which is a lens and below this a layer of retina-cells con- nected with nerve-fibrils and surrounded by a cup of pig. ment-cells. No eyes other than these occur in the Polypla- cophora, nor are tactile tentacles or otocysts, of such fre- quent occurrence in other Moliusca, found. The nephridia (Fig. 130, n) are paired, one lying on each side of the body and consisting of a long tube giving rise to numerous dendritic branches. Posteriorly the tube branches, one of the branches opening into the mantle-cavity in its posterior part, while the other communicates with the peri- cardial portion of the enterocoel. In function these organs of the Chitonidie difiers from the corresponding ones of the Solenogastres in being excretory only and in not serving as ducts for the reproductive elements. These are developed in a portion of the enteroccjel which lies anteriorly to the pericardium and make their way to the mantle-cavity and so to the exterior by special ducts arising one on each side frotn near the posterior part of the reproductive enterocoel and ending (go) on the sides of the body slightly in front of tho openings of the nephridia (no). The Polyplacophora are with- out exception bisexual. TYPE MOLLUSCA. 293 The structural peculiarities of the Polyplacopliora point strongly to their i)rimitive chantcter, though in many respects they are less primitive than the Soleiiogastres. Thus they possess special reproductive ducts, in all probability a secondary acquisition, and furthermore the reproductive and pericardial moieties of the enterocoel no longer communicate freely. If tlie ISolenogastres have been derived from forms with Chiton-like shells (see p. 288), then it must be supposed that the two groups r;;present di- verging lines of development from a common ancestor whose character- istics have been partly retained in the one group and partly in the other. II. Class Gasteropoda. The Gasteropoda form a very complex group, the various members differing so much in the details of their organiza- tion that it is difficult to give a general description which will apply to all the forms. Certain features may, how- ever, be considered typical of the class, and these may be mentioned here, reserving notice of the more important varia- tions until the various subdivisions are being considered. One of the most characteristic features is the occurrence of what may be termed the '• visceral hump " whose presence is responsible for many of the peculiarities of Gasteropod struc- ture. It consists of an elevation into a dome-like structure of the dorsal region of the body, the digestive tract and gland being contained within the elevation. The mantle arises as a circular fold surrounding the hump, but usually is more liighly developed, and therefore encloses a deeper cavity, upon the right side or anterior surface of the hump, and in the cavity so arranged lie the structures which usually are associated with the mantle-cavity, namely, the braucliisB and the openings of the digestive tube and of the ne2)hridia. There is thus a very decided asymmetry in most Gastero])ods, usually emphasized by the visceral humj) being coiled into a spiral, a coiling which is shared by the shell, usually present and consisting of a single tubular structure surrounding the visceral hump, but usually sufficiently ample to permit of the retraction within it of the rest of the body. In a number of forms the visceral hump may be very i)^^^/>]} vfi|(|i|f>orl and with this reduction there is feuerallv con- comitant a redu.^^ion of the shell, but such conditions are ^f '11 i 294 INVERTEBRATE MORPHOLOGY. pliiinly secondary iuasmucli as the primitive asymmetry is iudicated in certain of the organs in all cases. In order to understand the exact nature of this asymmetry it will be nec- essary to consider what may have been the original form of the Gasteropoda. Judging from what is known of the Amphi- neura, it may be supposed that in the primitive Gasteropod (Fig. 131, A) the anus (a) was terminal and opened into a mantle-cavity, the mantle being, except posteriorly, only a slight fold. In this mantle-cavity there was present also a single pair of branchial plumes (c/), and into it the two nephridia opened («), passing frftm the posteriorly-situated pericardium which contained the heart provided with two auricles. It may be imagined now that in such a form the visceral hump enclosed by a dome-like shell became elevated to such an extent that it could no longer be retained in an erect posi- tion, but fell over to one side — it may be supposed the left side. The result of this would be an interference with the development of the mantle-cavity towards the left side, and a prevention of the pf^rfect growth of the left branchia and of the proper functioning of the left nephridium. There would be a tendency then for the mantle-cavity, and with it the anus and indeed the entire posterior region of the body with the heart and nephridia, to be pushed over towards the right side (Fig. 131, B), and this process might in some cases be con- tinued until the mantle-cavity and the organs associated with it had been pushed round through 180° (Fig. 131, C\ D) and had come to lie apparently in front of the visceral hump (Z>). The anus in sucli a case would open into the mantle-cavity in the mid line, dorsal to the mouth, and what was originally the right branchia would lie upon the left side of the body ; the digestive tube, which may originally have been practically a straight tube, would now be bent upon itself, and furthermore the original right parietal nerve-ganglion would have passed over to the left side of the body and the original left ganglion to the right side, a crossing of the pleuro-parietal connectives {vc) being thus brought about. The original pressure of the shell upon the left half of the mantle-cavity would, however, as pointed out, have TYPE MOLLUSCA. 295 tended to produce a retardation in the growth or even the complete abortion of the organs lying in that region. Accord- ingly the original left uephridiuni is in many Gasteropods com- pletely suppressed as well as the original left branchia, and in accordance with the disappearance of this latter structure Fig. 131.— Diagrams to illustrate the Rotation of the Mantle-c wity AND ITS Organs in a GASTEitorou (after flguifs by Butschli and Lakg fiouL KoRscHELT and Heider). « — Hims. m = mouth. ao = aortu. n = uephiidial pore. ct - ctenidhmi. peg = pedal ganglion. eg = cerebral ganglion. p/g = pleural ganglion. vc = visceral connective. there is a disappearance also of the left auricle of the heart which receives blood from it. The visceral hump does not, however, retain its original conical form, but, owing perhaps to unequal pressure, grows more rapidly upon one surface, the anterior, and so becomes coiled into a right-handed spiral, the shell covering the hump naturally assuming a similar form. In the majority of 296 IN VERTEDUATE MOltPnOLOO Y. 'f % Gasteropods consequeutly a, shell coiled in a right-baiuled spiral occurs, but this rule has uot a few exceptions. Where the shell forms a left-handed spiral it is to be explained by supposing that in such cases the visceral hump tended towards the right side of the body rather than the left, and this is confirmed by the fact that in most left-handed forms it ia the left branchia and nephridium that have persisted. It must be pointed out, however, that the extent to which the rotation of the mantle-cavity, the abortion of the orrjans of either the left or right side of the body, and the crossing of the pleuro- parietal nerve-cords has been carried varies m different forms. In some the rotation has been curried so far that the original right branchia, etc., has passed the median line in front so as to lie on the left side of the bodj', and in such cases the crossing of the nerve-cords (chiastoneurism) is completed. Many forms, however, stop short of this, and numerous gradations are to be found. The rotation, however, is present in all forms to some extent and forms a character- istic feature of Gasteropod morphology. The anterior portion of the body (Fig. 132) is usually well marked off" by a more or less distinct constriction or neck, and consequently it is possible in the Gasteropods to speak of a head in contradistinction to the trunk rb„ion of the bod}' ; in- deed so prevalent is this character that the term Cephalo- phora has been applied to the group. Tentacles, either one or two pairs, are borne by the head, and furthermore eyes are usually present upon it either at the bases of one of the pairs of tentacles or else borne at the tips of these structures. The foot is generally well dp f lo^ed and usually has a flat creeping sole. It undergoes muDv i>vodificatjou'', however, sometimes becoming more kee' In^f^ or bocoming differentiated into three regions differing in form, the propodium, mesopo- dium, and metapodium, the last-named portion frequently secreting a chitinous plate, the operculum (Fig. 132, op), which serves to close the mouth of the shell when the animal is withdrawn within it. In addition to these portions an epipo- dium is frequently highly developed, consisting in its primi- tive form of a fold arising from the sides of the foot where it passes into the body-wall. In many cases, however, it loses TYPE M0LLU8CA. 297 this simple form, its margin becomiug fringed or tentaculate, or elst it may be reduced to one or more separate lobes or tentacular ])roce88es on either side of the body. Openiu<^ upon the surface of the foot is frequently to be found a so" called "foot-gland " which secretes a sticky raucous fluid and is comparable to the byssus-gland of the Pelecypoda {q. v.). Fi(i. 132. — Buecihum undatum. op = opcicuUiiii. 8i = siplio. The respiratory organs (Fig. 133, d) consist in typical cases of a single pair of pinnate branchial plumes lying in the mantle-cavity, but, as has already been mentioned in conuec- tion with the rotation of that cavity, one of these structures is very frequently aborted. Other changes, however, also occur, such, for example, as the fusion of the central axis of the branchial plume throughout its entire length to the inner surface of the mantle {Haliotis), or the disappearance of the pinntB from one side of the plume in connection with such a fusion {Sycotyptis, Fig. 133). In some forms accessory bran- chijG may be produced as folds of the mantle, richly supplied with blood, and their development may be carried to such an extent that they may entirely supplant the branchiae proper (Patella). From such a condition as this a passage is not dif- Hcult to such a condition as is found in the air-breathing Gas- teropods (Pulmonata) in which the entire inner surface of the mantle serves a respiratory purpose, an interchange of gases tidiing place between the air contained in the mantle-cavity and the blood which is richly supplied to the mantle. The musculature of the body-wall does not as a rule pre- sent the Annelidan arrangement in layers, as in some Amphi- neura, but usually are irregularly arranged as dorso-ventral and oblique bands traversing the schizocoel. Special muscles, t4 298 INVfiliTEBltA TE MOliPHOLOGT. however, are developed in many forms, the most important beiu^ those connected with the foot and servinj^ for h)comotiou, r«>tractor muscles in connection with the head, proboscis, and tentacles, and the spind'e-niuscle, which has a general vertical direction running along the right side of the visceral hump from its insertion into the shell to the -foot in whose wall its fibres spread out, intolacing as it were with the horizontal and trans erse muscles there developed ; it serves to retract the entire animal within the .ihell, and its development is naturally in proportion t> that of the shell, those forms in wh'ch the shell is rudimentary or absent frequently lacking it. The enterocady pointed out, the asymmetry produced by the develop- ment of the visceral hump iiih>cts the heart, resulting in tht^ sui)pression of one of the auricles, that of the left lor right) siile (Fig. i;)3). In such cases the j)ersisting auricle nav secondarily assume a terminal position with regard to the ventricle, and the latter, instead of being continued into an ar- tery at either extremity, gives oil' a single artery at the end op- ])osite to that at which the blood enters from the auricle, this artery dividing into two nniin trunks which distribute the bh)od to the various regions of the body. Those arteries nun- be coutiuued as distinct tubes with definite walls for some TYPE MOLLUSC A. 299 nportaiit oiuotioii, seis, jiiul [ vertical il liiinip I wall its jrizuiital ;) retract )iueiit is onus in ,ckiii, tliis )ute the riea nuiy or some distance from the heart, but sooner or later the blood passes into the system of lacunar spaces constituting' the schijjoccel, whence it is again returned to the auricle through a series of veins. The i)osition of the single auricle with reference to the hod.y axis differs in different orders of Gasteropods, in accord- Fio. 183.-STHTICTDUK OF Si/cott/pi>.9 ra„nlk„lni,is. The miintlc Is divided in tlie iniddltj liuc aud uuiicd aside, exposing liu; nuuitleciivitv. an = anus. cl = ctenidium. d(/ — digestive gland. t = intestine. n = nepiiiidiiini. no = nepliridial opening. 0 = eye. op = opereiilinn. OK - osplinidinMi. p = pericardial cavity. )>e = perus. pr = proboseLs. ««■ = siplio. / - lenlacle. te = testis. V = ventricle. I'd = vas deferens. Tlie arrows show tlic op.inings of ncpIiiidiiini to llie nianlle-olmmher and lo the periciirdiuni. ance with the varying jjosition of the branchia. In those t'Tnjs in which the l)ninchia lies in front of the liwirt the fiiiricle lies at the anterior end of tlie ventricle, while when till! branchia is posteriorly situated the auricle lies behind t\iv veutricle. rii.^ mouth lies in all Gasteropods at the anterior ex- tremity of the body, towards tl le ventral surface of the head 300 IN VEIi TEBRA TE MOIiPIIOLO G T. I 1-^ and opens into a mouth-cavity frequently provided with two or more chitiuous teeth. The pharynx usually receives the ducts of a pair of salivary glands, contains a well-developed radular organ in practically all cases, and communicates posteriorly with a tubular (esophagus. In many cases the anterior portion of the digestive tract is capable of being protruded as a proboscis (Fig. lo3, pr), which lies when re- tracted within a proboscis-sheath, formed by a circular infold- ing of the body-wall around the mouth. The intestine (t) is usually more or less coiled, extending into the visceral hump, and presents a stomach-like enlargement Avhich receives the ducts of the digestive gland {dg), a structure usually well developed and forming the greater portion of the visceral hump. The intestine terminates in a straight i)ortion, the rectum (r), which passes forward to the anus {an), Avhich, as has already been indicated, lies in the mantle-cavity, slightly to the right, but occasionally' to the left, of the middle line, its position depending upon the amount of rotation which the mantle-cavity and the associated organs have undergone. It should bo mentioned that in one suborder of Gasteropods the pericardium and ventricle have wrapjjod themselves around the rectum in Buch a way that the diges- tive tube seems to have pejietrated the ventricle, a features which will later be seen to be characteristic of one of the other groups of Mollusca. The nervous system has the arrangement which has been di scriluid as characteristic of tlie Mollusca (Fig. 124), the peculiar feature being the crossing of the p hi uro- parietal con- nectives which is found in many forms. Numerous modifica- tions of the typical condition are to be found, consisting ])rincipally in (1) th(^ concentration of the ganglia, more especially the cerebral, pleural, and ])edal, or the ])edal, phniral, parietal, and visceral (Fig. 139), to form a singli' mass; (2) in the su])])res8ion in Home cases of one of the parietal ganglia ; and (3) in the occurrence of several visceral ganglia. In accordance with the flat elongated form of the foot in many species, the nerve-cords jiassing backward from the pedal ganglia may be of considerable size, and further- more nniy be connoctod by regularly-arranged transverse TYPE MOLLUSCA. 301 commissures, recalling the condition seen in the Chitons, as well as the ladder-like arrangement of the ventral nerve-cords of the Annelida, though there cannot in the Gasteropods be any question of metamerism in this connection. Special sense-organs are very generally well developed in the Gasteropods, The tentacles so usually found upon the head have probably a tactile function as well as the tentacular or winglike processes sometimes found in connection with the anterior extremity of the foot, and the epipodial ten- tacles Avhich occur in some forms {llaliotis). On the ventral side of the bases of the epipodial tentacles of some forms s})ocial sensory thickenings have been found which havt^ suggested a compariscm with the sense-organs of the lateral line of the Annelida, a comparison which, however, at present seems rather strained ; it seems probable, notwithstanding their innervation from the pedal ganglia, that these sensory })atches are to be placed in the same category as the osphra- dia and the sensory ridges of the mantle-cavity of the Chi- tons. Tlie osphradia (Fig. 188, os) in all Gasteropods which are provided with branchiie are associated with these "igans' and even where one or both -mi^J^ branchiio have been suppressed the osphradia may still persist. Eyes (Fig. 134) are very generally j)resent in the Gasteropods, being situated at the base of tlie ten- tacles, or at tlieir summit in some forms. Tiiey present a veiy uni- form structure throughout the group and arise as a depression of tin; integument, the lips of the cavity fusing and giving rise to a glol)uhir sac lying beneath th(i ejiidermis, which I'emains thin and transparcmt, forming an outer i-ornea U'<>\ I'he cells of the outer wall of tlie sa lemaindtu" of the wall of the sac they are sensor) in function, ]>igniented cells being scattertMl anh.ng them, tl Fid, 184. — KvK <»K lliiliotiH u\iU'r Pattkn). CO — coiiita. I — lens. rt — reliiia. iO two together forming the retiini (>W). The nerve-fibres pass- i m 302 INVERTEBRATE MORPHOLOGY. iug to the eye from the cerebral ganglia pass through an optic ganglion lying beneath the optic sac and are distributed to the sensory cells, and the centre of the sac is filled up by a cuticular mass which serves as a refractive lens (/). In some forms {Patella, etc.) the development of the eye ceases while it is still in the cup form, there being then no formation of corneal layers and no central lens, though the retina is usu- ally covered by a thin cuticular layer. In some species of a peculiar genus of the air-breathing Gasteropods, Onchidium, eyes are developed upon the dorsal surface of the body, the shell being lacking and the visceral hump undeveloped. In structure these eyes differ very materially from those usually occurring in the Gasteropoda and will be described later tin- position of tiio iifi.liridiii n^i iliey ate supposed to litive becu niniuged in llic primillvo aymmelricul Oiistciopod. TYPE MOLLUSC A. 303 duct {vd), having apparently no relation to the nephridia and opening into the mantle-cavity to the right side of the anus. In the more primitive Gasteropods, however, such as Haliofis, Fissurella, and Patella, the nephridia, as in the 8olenogastres,' serve as reproductive ducts ; and it has been suggested that the special reproductive duct of the remaining Gasteropods may represent the left nephridium, which is usually described as having disappeared. The reproductive duct, especially in hermaphrodite forms, has developed in connection with it accessory glandular structures a;i well ^^s external copulatory organs, the whole reproductive system becoming highly com- plicated. An account of the more important arrangements will be more satisfactorily given in connection with the various orders. 1. Order Prosobranchia. The Prosobranchia are, with very few exceptions, marine Gasteropods, provided with well-developed shells, which are usually spirally coiled, the height of the spiral varying, however, in different forms. In some, such as Patella and Fvisun'lla, the shell has a simple conical form, without any indication of a spiral ; and since these forms in many respects show primitive charac- ters, it might be supposed that this type of shell was also primitive. These very forms, however, show also that asymmetryof parts, which is character. F.a.l35.-SHKu-s OF Pmoso- istic for the Gasterojjods, and which "hancii (Jastkuoi-odh. accompanies the rotation of the mantle- ^' Acimm ti'studlnnliH ciivity, and furtluM-moro, in Fi.s.surella "*"**'' ""'''"*= Z^- '/"''""^ 11 J f .• ..,. . 'lifter Lkunimi; C. Ttirvltt'lln. Ht least, a distinct indicaticn of a (after leun.h) si)iral coiling, is present in the shells of young animals. It seems more prol)able, accordingly, that these conical shells are to be regarded as secondary niodid. cations of an originally s])irally-coiled shell. The mantle-cavity is situated in front of the well-devel- oped visceral hump, and is usually somewhat cajjacious, com. r« 304 IN VERTEBitA TE MORPHOL 00 T. nuinicating with tlie exterior freelj-. lu some forms the mantle is slit from itb margin upwards and backwards, a corresponding slit occurring in the shell {Ithiarginula) . In HaUotis and Pleurotomaria the alit in the sliell becomes closed at regular intervals, producing a row of round perfora- tions, beneath which lies the niantle-slit, and through which water finds a read}- exit from the mantle-cavity, and, in Fis- surelhi, in which at an early stage the margin of the shell possesses a slit, by the subsequent growth and obliteration of the spiral coiling the slit becomes converted into an ai)erture which lies almost at the apex of the conical shell and leads into the mantle-cavity, functioning as a means of exit of the water and excrementa from that cavity. In the greater num- ber of forms, however, such slits or apertures do not exist ; but one finds frequently the margin of the mantle produced at one point on the left side into a projecting narrow lobe whose edges may be brought into opposition, thus producing a tube or siphon through which water may pass into the mantle-cavity. Where this siphon is Avell developed a dis- tinct notch is found in the margin of the shell, through which it may be protruded, or else the lips of the notch are pro- longed so as to form a grooved process, the siphonal canal, in which the siphon lies, being by these arrangements able to function even Avhen the moutli of the shell is closed by the operculum. In many forms the mantle-folds are sufficiently large to allow of their being reflected over the outer surface of the shell when tlie body is fully protruded. The foot is as a rule adapted for creejjing, but in many cases is differentiated into pro-, meso- and metaj)odium, tlui last usually bearing a chitiuous or more or less calcified operculum. In certain forms belonging to a group of pelagic forms, however, which Avere formerly associated together as a distinct order, the IMeropoda (Fig. 138), the pro- and meso- podium are modified into a keol-like structure and bear a peculiar sucker. The ei)ip()dium is frequently developed in the Prosobranchia, especially in the more primitive sjjccies - most frequently, however, being reduced to tentacle- or lolu!- 3esses arising from the sides of the foot. lik< proc< In the majority (Fig. 133) of forms there is but a singl< TYPE MOLLUSC A. 305 irms the wards, a ula). In becomes . perfora- ;li which I, in Fis- bhe shell iratiou of a})erture iiid leads xit of the iter nura- ot exist ; produced •row lobe »rodueii)g into the ed a dis- gh which are pro- . canal, in s able to id hy the itliciently r surface in many diuui, the calcified i)f pelaj^ic ather as a lud meso- id bear a eloped iu sjjecies- - B- or Idbe- t a singltf branchia Avhich lies iu front of the heart, whence the name of the order, but iu a few genera the original left gill also per- sists. In many forms a gland is developed in the floor of the mantle-cavity close to the rectum— hence called the adrectal gland — which in some forms, e.g. Murex secretes a purple pigment. The rotation of the mantle-cavity and the associated organs has called forth a crossiug of the pleuro-parietal nerve- cords, a feature which is lacking in the other orders and therefore forms a characteristic of the Prosobranchs. In all but a few cases the members of the order are bisex- ual, the unpaired reproductive gland lying in the visceral hump. The oviduct hii^s iu connection with it one or more receptacula semiuis and dilates into a glandular uterus in whicli the eggs are su])p]ied with the albumen in which they are usually imbedded and also surrounded by a shell. In the males, except in the more primitive forms, there is present a well-developed intromittent organ or penis (Fig. 133, jot^), situated upon the right side of the head or neck and there- fore removed at some distance from the opening of the vas deferens into the mantle cavity. A groove or tube extends, however, from the reproductive orifice to the grooved or tubu- lar penis, and along this groove or tube, by the ciliary action of the cells liuing it, the seminal fluid is carried. 1. Suborder Diotocardia. This suborder includes the more primitive Prosobranchs, iu which, although a considerable rotation has occurred, yet nevertheless the abortion of the organs of the original left side of the body has not been carried very far. Thus, except in Patella and some allied forms, there are two auricles to the heart, although iu Turho, 7'r'' - , i. ii • . C7 = buccal can <;]ioii. features of their anatomy con- n ^ .as.ro-a.so,,i,a.real ganglion, siderable variations are to be e = pedal conmii.ssure. found. Thus in some forms a ^' = visceral conunissme. well-developed spirally-coiled visceral hump is developed, while in others it loses its spiral arrangement, and in others again is elongated in the direction of the foot and can hardly be said to exist, S(j, too, with the occurrence of the shell, mantle, and branchije ; all are well developed in some forms, but entirely absent in others. These peculiarities will be more conveniently referred to in connection with the various groups, and it is only necessary here to refer to another feature in addition to those already given, which is common to all the members of the order— i.e., the hermaphroditic character of the reproductive gland. This forms part of the vi^-eral mass and is usually com- posed of numerous lobes, these again being divided into secondary lobes or acini, the lining epithelium of which give rise to both ova and spermatozoa. In some forms, such as Bulla and Aplyda, both elements are formed in ail the acini ; but in others, such as Doris, Janus, Pteropoda, etc., the epithelium of the terminal acini gives rise to ova only ; the epithelium of the lobes, i.e., the central portions of the gland, producing spermatozoa. Whether or not, however, there be such a separation of the epithelium into male and female a i mi ii4 .8..: t 312 INYERTEBRA TE MORPHOLOG Y. areas, the reproductive elemeuts make their way into a com- mon hermaphrodite duct, which presents variations of structure in diti'erent forms and receives the secretion of certain acces- sory ghmds. In its simplest form, as seen for instance in Aplysia, the duct runs forward, pursuing a somewhat tortuous course and becomes surrounded by an (dbummiparom ghaut, from which it receives a viscid secretion, Avithin which the ova become imbedded just in front of the point where tlie ghmd opens into the duct. The Litter lias attached to it a pouch-like structure, the vesicida seminalls, and is continued on as a somewhat wider tube to open to the exterior at the genital pore situated on the right side of the body, shortly before reaching the pore, however, receiving a duct from a globular sac, the spermatheca. From the anterior edge of the pore a groove, the seminal groove, extends along the right side of the body to the neck region, where it e?ids in a mus- cular evertible penis, situated near the anterior right tentacle. It seems probable that the spermatozoa mature before the ova, and passing to the vesicula are stored up there. During copul!.,tion the seminal fluid is transferred through the penis to the sjiermatheca of another individual (])erliaps the trans- ference is a mutual interchange), and when later the ova pass along the duct they are im])regnated by the spermatozoa so stored &yfSi,y, a cross-fertilization being thus brought about. This arrangement of the reproductive duct is found in tlie more primitive Opisthobranchs, i.e., in those in winch the mantle-lobe still persists, and in the group Pteropoda] in the more highly-modified forms, such us Doris, A'Jolis, etc., and, among tlie more siinph^ forms, in PlenroJyninvhn'a tlic heriiiaplirodite duct divides into an oviduct and a vas deferens. 'J'he former after receiving the spermathecal duct o])ens into n genital atrium, with which communicate also the albuniiiii|)ii- rous gland and a nidnmentoJ pore by which the atrium communicates ' Si TYPE MOLLUSC A. 313 o a cora- structure liii .acces- stance in ; tortuous mfi gland, liich the here the 3ll to it !l soutiuueil or at the y, shortly it from a Ige of the the right 11 a m US- tentacle, efore the During the penis the trans- ova pass rniatozoa brought nd in the hich the >p()da ; in hJis, etc., u'luvn the i ile/eycvu. i)pnH inti) >uniiiii|);i- tnrcH the 'ons, after ir Hadiki' riuni; tin s capalili inmicalcs ™ilh tl,e exterior. This coudition seems to be a seeoiidarv mochhca .on of oue in which the oviduct aud JultZ opeu nulepemlently at widely .separated ,,oint.s-a couditZ which ,8 represented by a few Opisthobrauchs. 1. Suborder TecUiranclUa. The Tectibraiichiates are those Opisthobrauchs which present the smal *st amount of modification from wha 1, ' been considered the typical Gasteropod structure. A n e or less developed mantle-fold is usually present, someti.ne! sufcciently voluminous to cover in the s ngle branchiTw i h pei..sts (/«.). but frequently represented only' a li , , ,nf\ ; ™'"^' «■"""■"">■ V^o^oat, sometbnes well de- veloped (BMa), but in other cases reduced to a plate-1 ke structure enclosed within the mantle which has been reflected overit and the lips of the reflected portion meeting .and?: n1 (.Vy.v«,, /•fc«™/«.,„,c;,.,). The visceral hump, h.nvever is "^ a n, e ow and elongate,! in the direction „f the loug axis o the body mstea,! of at right angles to it, as in the matritv o IVosobranchs In many n.en.bers of the group the Wt poB s sses a broad creepn.g s„rfa..c b„t its n.argins are prolonged mto broad thm wmgs, the pern;,,*',,, which may be bent m, wards, as „, Aply,;,,, ,„ ,, „,„„,,» j,, „„^.|„^^ ^^^^ "P Ihe lectibranchiates are divisible into two groups accor,! ".« to he,r habits, in accord.ance with which tl.e fo, of7 J loot an.l especally of the parapo.lia is n,o,Iitied. Those for s w noh ,«,ssess a broa.l flat sole to the foot have a c^e! i:; liabit, but there are n.any hums wliich are pelagic in Imbit U .H ,m,d„ of l,le, and were conse,p,ently classified a one tin.e as a d.stn.ct or,ler, the P™,„n,„u, an,l conse.inentiv cal r ^|ec,..d menfon. ()m, of the n.ost characteristic features . «n,up ,s the foot, which is limited to the anterior po rti, „ "I the body and insists of a small .n-.li,,,. „„,,-o, -,1, i.Ueral wi Hl,„ ,: .■ . 7^ • r"!t.oi. .md tw, ileral wing-like parapodia tiled ■an portU)u, au teropods in that they are, with the exception of a single genus^ Onchidium, either terrestrial or aquatic ; and in adaptation to this assumed habit certain well-defined changes have occurred. In some genera, more especially the aquatic forms, such as Limncea, Physa, and Planorbis, the visceral hump has its typi- cal Gasteropod development, and is spirally coiled; but in many terrestrial forms, such as Umax (Fig. 142, A), Arion, and Vaginula, it is low and elon- gated parallel to the long axis of the foot with which it is fused. The mantle is in all forms well developed, but pre- sents the peculiarity that it is fused by its edges to tlie body- wall except at one point upon the right side, where an open- ing is left by which the (other- wise com[)letely-cIoHed mantle- cavity communicates with the exterior and through which air may be taken into the cavity. The position of the mantle- cavity, when not interfered with by secondary changes, is ui)on the right side of the body and somewhat in front of the visceral hum]i when this is present. A spirally-coiled shell is present in all forms in which the visceral hump is well developed, as in Limnim, J'hysc, /Mix (Fig. 142, //), and PlamrHs, but in the elongated terrestrial forms a rudimentatif)n of the shell accom])aiiies the diminution of the visceral hump. 'J'hus in Dundebardia, in which only a slight truce of the hump j)ersists, the shell has become Fio. 142. — A, Liinax VKirimus ; B, Helix (after Howes). TYPE MOLLUSC A. 317 quite small, though still showing plainly a spiral form ; but in Limax it is represented only by a partially calcified plate, im- bedded in the roof of the mantle-cavity by the closure over it of a fold of the mantle. In Avion only a few isolated parti- cles of carbonate of lime persist, while in Vaginula and Onchi- dium all trace of it has disappeared. A marked characteristic of the Pulmonata is found in the character of their respiratory organ. A ctenidium is entirely wanting, the culy trace of its existence being the occurrence in some of the aquatic forms {Limncea, Physa, etc.), of an os- phradium near the mantle-pore. Its place is taken by the roof of the mantle-cavity, which receives a rich vascular net- work and functions as a lung, the mantle-cavity containing air which can be renewed through the mantle-pore. The heart is situated far back in the mantle-cavity, its auricle lying in front of the ventricle and receiving the blood from the more anteriorly-situated lung, so that the relation of the respiratory organ to the heart is the same as obtains in the Piosobranchs. In tJrf), receives the duct of a uidamental gland and dilates into a TYPE MOLLUSC A. 319 large pyriform structure, which tapers somewhat to form a vagina opening to the exterior and receives a duct from the receptaculum seminis. The vas deferens {vd) shortly after Its separation from the hermaphrodite duct dilates into a glandular structure, the prostate gland, from which the nar- row duct passes onward to terminate in an enlarged penis- sheath (^e) which contains the muscular protrusible penis and opens to the exterior quite independent of the opening of the vagina. In the majority of the Stylommatophora (Fig. 143), however, the two ducts open iuto a common atrium so that only one genital orifice occurs, as in some of the Opistho- branchs (see p. 312). Other- wise the arrangement is simi- lar to what has been de- scribed for the Basommato- phora, except that in some forms, as Helix, one or two additional accessory struc- tures are added. Thus the atrium has communicating with it a sac which contains a sharp calcareous rod, the "dart," which serves as a stimulus during copulation, being plunged into the body of the other party to the act ; and again just at the point where the vas deferens opens nto the penis it has communicating with it an elongated tubular structure, the "flagellum," which perhaps furnishes the material of which the capsule of the spermatophores is composed. Development and Affinities of the Gmteropods.-— The devel- opmeut of the Gasteropods is made interesting on account of Fig. 143.— Reproductive Organs of Limax maximus (after Simroth). al = alljumluipuiousglaud. hfi = hemiapLrodite duct. hg = hermaphrodite gland. U =■ ligament. od = oviduct. pe = penis-sheath. ?•« = receptaculum seminis. ut ~ uterus. vd = vas deferens. V8 = vesicula semiualis. 3 „™«„.>»_ II 11 320 INVERTEBRATE MORPHOLOGY. the occurrence in the majority of forms of a larva knowu as the Veliger (Fig. 144) which presents many interesting affin- ities to the Annelid Trochophore. In the early stages of de- velopment the embryo is strictly bilateral, with the mouth and anus at the extremities of the longitudinal axis. Upon the dor- sal surface posteriorly is a de- pression lined with columnar cells which secrete the larval shell (Sh), and in front of this is an area enclosed by two rows of cells bearing stout cilia and forming the velum (F). This band of cilia is prseoral (Pro) in posxfciou, and in addition to it a second band of smaller cilia is to be found which passes ventrally to the mouth and constitutes a post- oral band (Poo), the groove be- tween it and the prseoral baud being occupied by the adoral cilia. On the ventral surface is found a prominence which represents the foot. In later stages the lateral edges of the velum are drawn out so as to form a broad lobe, sometimes divided into two arms, projecting on each side of the head ; the prseoral and postoral bands of cilia extending round the margin of the fold, not, however, completely enclosing the velar area, but re- maining oi)en on the dorsal surface. The shell area increases markedly in size, the shell becoming spirally coiled, the vis- ceral hump which develops in the shell area likewise assuni- iug the coiled form. At the margins of the shell area a fold appears, the rudiment of the mantle, which gradually increases in size as the shell area extends, and at the same time the anus becomes rotated forwards from its original terminal posi- tion along the right side of the body to a greater or less ex- tent. As these changes i)rogress, the embryo gradually ap- proaches more and more to tlio auuit form, uinering from it mainly in the existence of the velum, by means of which it Fw. 144.— Vkligek Lauva. F= foot. M = moutb. Oc = eye. Poo = postoral band of cilia. Pro - prsooral band of cilia. 8h = shell. T = tentacle. V = velum. TYPE MOLLUSCA. 321 leads a free-swimming pelagic existence, assuming the adult habit only after a further growth which is accompanied by a reduction of the velum. Such a Veliger larva occurs in the life-history of the majority of the Gasteropoda, though, as might be expected, it undergoes certain modifica- tions more especially in terrestrial forms, though even in these there are ample indications of its existence. Indeed the Veliger is so frequent in its. occurrence that the conclusion is almost unavoidable that it has an ances- tral significance and represents in a more or less modified condition a primitive form from which the Mollusca have descended. A comparison of the Veliger with the Annelid Trochophore brings out, as already men- tioned, numerous similarities. These are especially noticeable in the ar- rangement of the ciliary bands, which resemble those of the Trochophore part for part, even to the dorsal break in their continuity. It is difficult to believe that such marked similarities should have been acquired inde- pendently in the larva? of two different g.oups of animals and Lave become so characteristic, a difficulty rendered all the greater by the occurrence of other points of simihirity, such as the development of the mesoderm, in some forms at least, from a pair of mesoblasts situated at the posterior extremity of the blastocoel ; the existence of a thickening of the ectoderm in the centre of the velar area in some forms, corresponding to the apical plate of the Trocophore ; and the occurrence of a larval excretory organ or uephridium in some Veligers which may be compared to the larval ne- phridium or head-kidney of the Trochophore. The probable significance of tills larval form will be more suitably discussed at the conclusion of this chapter ; it remains to be said here regarding it that the occurrence among the Pteropods of la.vae with several bands of cilia surrounding the visceral hump is probably to be explained as a secondary adaptation, just as the niesotrochal Annelid larvae are probably secondary modifications of a Tro- chopliore. As regards the relationships of the various groups of Gasteropoda among themselves, there is little doubt but that the Diotocardiate Proso- hraiiehs are, on the whole, the most primitive of all the groups and stand nearest to tlie Amphineura, and from them the Monotocardia iiave devel- oped. The Opisthobranchs and Pulraonates are apparently closely re- lated, tlie latter group having been derived from Tectibranchiate ancestors sdinewhat more generalized probably than any Opisthobranch now living. The orthoneurous character of the nervous system and the structure of tlt is pro- traded, and two posterior, the branchial and anal openings. The mantle around these latter frequently becomes prolonged so that two tubes, or siphons as they are termed, are formed, sometimes in contact with one anotlier (Pholas), sometimes quite separate (Fe/iws), sometimes capable of retraction within the shell, sometimes so large as to be incapable of retraction (Mya). Fig. 147.—^, Mya nrenaria with the siphons slightly expanded; B, inner sur- face of the riglit valve of the shell of Mi/mporarily fastened to nx'ks, etc. Occasionally acMilionid cal- careous plates are added to the usual shell, as in the horiiiR mollusk 7'/(f>/«.v. in which three accessory calcareous plates are devel()i>ed on tlH> dorsjd sur- fi'CO of the hody. In the Siiip-worm, or Tendi), wiiich hores extensively into timber and is in some eases exeecdiiiKly destructive, the true sla^ll- v-vlv'.'P. are Terv sniall and situated at the anterior end of i\w body, aJid I he mantle projects backwards far beyond them and secretes a thin calcareoiiv lube which lines the interior of the passages excavated by the animal. A TYPE MOLLUSCA. 329 t being ited by jonding )getlier. 'essious alue in argin of pallial t of the ell. In lie shell genera IS it is Other on into iniport- 1), large contrac- ,ud clos- wo snc'h portion :)i other dduetor- UHUally nmsfU's lly similar y is foil 11(1 mill rortts, is smaller wliicli hr- lioiial cal- isk Pltiiliin, iloi'sal sur 'xteiisivclv true slicli- ly, and iIh' cjilcart'dii- iiiiniul. A similar peculiarity is found in the Aspergillum. Here, too, the true shell- valves are exceedingly small and are united together by and imbedded in a calcareous tube secreted by the mantle, which projects far beyond the shell proper and is fused throughout the greater portion of its extent The calcareous tube is open behind for the passage of the two siphons' but anteriorly is closed by a perforated plate, the margins of the perfora- tions being sometimes prolonged into tubes which may branch dichoto- mously. The animal lives imbedded in the sand, the posterior ex- tremity of the shell being directed upwards, and seems to have been derived from forms originally possessing a boring habit, such as is seen in Teredo. The foot of the Pelecypoda is as a rule very simple. lu the most primitive members of the group, such as Nucula (Fig. 151), it is a flat disk-like structure, recalling somewhat the foot of the Gasteropoda, but more usually it is a keel- shaped structure (Eig. 149, p). The modifications in shape which it undergoes are, however, numerous and it may even in some cases be almost absent, as in the Oyster {Ostrea), but special developments, such as epipodia, are never found in connection with it. A " byssus-glaud " is a characteristic development of the Pelecypod foot, consisting of a cavity witli usually greatly folded walls lying in the tissues of the foot and connected with the exterior by a canal opening on the sole of the foot. By the cells lining the cavity threads of a horny consistency are secreted by means of which the animal is enabled to fasten itself to stones, etc., or even in some cases, as Mytilm, to move about in the absence of a uell-developed foot, throwing out byssus filaments, attaching them, and tiieu drawing itself forward towards them. The re8])iratory organs (Fig. 149, hr) of the Pelecypoda consist of a pair of i)latelike structures situated on each side of the body, and being attached along tlu'ir doisal margins hang (lown between the mantle and the body-wall. Notwithstand- ing their plat(>like forn. they are moilificaticms of the i)lnniose .'tenidium of the Gasteropods. If the typical bipinnate eteijidium be imagined tu be directed i)arall(>l to tin* long axis of the body and the median axis to have fused with the body-wall, so that the two rows of piunie are bent down so as to lie parallel to one another, the simi)lest form of the Pelecypod ctenidium, such as occurs in Nucula (Fig. 151), will 1 830 INVEUTEBRATE MOliPlIOLOGT. bt) obtained. lu the majority of forms, however, the arrange- ment is much more complicated than this. Thus in Mytilus it Avill be found that the various piunfo composing each plate arc held together by a series of patches of strong cilia> ^^^^ F/G. 148.—^, (liftgramnifttic section through Pecten, and B, through Anodon; C, section through gill-luuicUti of Pecten, iiud D, of Anoilon. au = a\uicle. "' = outer lamella of outer gill. / = foot. P*^ = pericardial cavity. gf = gill-filament. i' = P<>'«- iil = inner lamella of inner gill. « = blood-sinus. il = intcrlan\ellar juucliou, »br = suprabranchial chamber. ne = nephridium. «'* = shell. which interlock forming the "ciliated junctions," and further- more the pinnro are at their free ends bent abruptly \\\w\\ themselves, those of the outer row outwards and those of tlip inner row inwards, so that each gill-plato is composed of two lamellm (Fig. 148, A). This ctmdition may be regarded as tlio next step in the modification, which is continued even further by the permanent union of the outer and inner linihs TYPE MOLLUSCA. 331 of the pinnsB, or gill-fikmeuts as they may be called, by hollow processes, the " iuterlamelhir juuctious " (Fig. 148, C, il) A still greater departure from the primitive condition is found, however, iu the greater uumber of existing Pelecy- pods, consisting of a fusion of all the filaments of each lamella into a plate (Fig. 148, />;, small openings {p) only beiug left here and there between adjacent filaments; furthermore the inter- lamellar junctions become very well developed, so that the two lamelhe of each gill become firmly united together to form a plate, containing in the interior a cavity, the interlamellar si)ace. In addition to these various modifications which lead to tlie formation of a true lamellate gill, the edge of the external lamella of the outer plate fuses with the inner surface of the mantle, and the internal lamella of the inner plate fuses sim ilarly with the side of the foot (Fig. 148, H), and the mantle- cavity thus becomes divided into two chambcus. luto the ven- tral chamber the inhalent siphon opens, and the water which enters by it passes through the openings left between the filaments and so reaches the interlamellar spaces which com- municate above with the dorsal or suprabranchial chand)er {.shn, whence ifc passes to the exterior through the exhaleut siphon. In the region of the foot the su]>rabvanclrfal cham- ber IS of course divided into two portions, one of which lies on each side of the base of the foot, and eacli of these is a-ain divide;! longitudinally into an innar and an outer portion by the hue attachment of the gills to what may be considered the roof of thd mantlo-cavity. Behind the foot the inner cavities "f the two sides unite and in some forms open ventrallv into the mantl,>-cavity i.rop.-r ; in others, however, the inner lamellro nt the inner gdl-phites fuse with one anotlua- ahu.gthe middle hue so that a distinct partition, formed by the gills, sepa- rates the suprabranchial chamber from the ventral mantle- <'l'and,or throughout its cmtire length In a few forms, such as r//,v;„,/,rns and separatirg the two cham- I'J'i's, practically all indication of the original ctenidium chitrncterH liuving disaj)])Ciii(Ml. The muscular system of the Pelecypoda reaches a some- ^ 332 INVERTEBRATE MORPHOLOGY. what extensive development in connection with the presence of the bivalved shell. The mantle-folds are as a rule some- what richly provided with muscle-fibres especially near the margin ; and where siphons are developed some of the fibres are specialized into retractors for these organs. For the closure of the shell- valves, however, more extensive muscular bands are present which seem, like the siphonal retractors, to be special- ized portions of the mantle musculature. Of these shell- adductors there may be one, as in Ostrea and Pecten, or two, as in Anodon (Fig. 149, aa and pa), which pass transversely across the body from one shell-valve to the other, in the form of stout compact muscular bands. In connection with the foot special bands are also developed which function as protrac- tors {pp), retractors {rp), and elevators arranged in pairs and extending from the inner surfaces of the shell-valves to spread out below in the foot. These various bundles seem to cor- respond to the spindle-muscle of the Gasteropods. The coelom presents an arrangement similar cO that of other Mollusca, both schizocoelic and enterocoelic portions being distinguishable. To the former portion belong the numerous lacunar spaces which traverse the body and mantle- folds, and to the latter the pericardial cavity (Fig. 149,^9) and the cavity of the reproductive glands. The blood-vascular system consists of a ]'eart provided with two lateral auricles and lying in the pericfi,rdium. In the majority of forms the ven- tricle {v) seems to be traversed by the terminal portion of the digestive tract, a condition produced by its having folded itself longitudinally around the rectum, and which recalls what occurs in certain Diotocardiate Gasteropods (see p. 305). This arrangement does not, however, obtain in all forms, some of the more primitive {Nwcula, Area) having the ventricle entirely dorsal to the intestine, as it is in the Amphineura, for example, while in a few others (Ostrea) it has assumed a secondary position ventral to the intestine. From both the anterior (ao) and posterior extremities of the ventricle arteries arise which, after branching a number of times, pour the blood into the schizocoelic lacunar system. Traversing this the venous blood is roturucd to a iongitudiiial sinus lying in tuo middle line of the body just below the pericardium (Fig. TYPE MOLLUSC A. 333 Dresence ie some- lear the be fibres } closure ands are ! special- 36 sliell- wo, as in y across form of the foot protrac- airs aud 0 spread L to cor- that of portions ong the 1 mautle- 9, jj) and vascular auricles 1 the ven- )n of the led itself lis what 5). This some of ventricle eura, for mmed a both the I arteries he blood this the ig in the iin (Fig. 148, B, s), whence the greater portion passes into the compli- cated network of the nephridia aud thence to a blood-vessel, the branchial artery, running along the base of the gill of each side. After traversing the gill-filaraeuts it becomes arterial and is returned to the branchial veins which run parallel to the branchial arteries and thence is returned to the auricles of the heart. The digestive tract has a much simpler structure than in the majority of the Mollusca, lacking all trace of a radula and muscular pharynx. On each side of the mouth are two usually triangular plates, the so-called labial palps, the upper- most of which meet above the mouth forming a sort of upper lip, while the lower ones similarly form a lower lip. At the bottom of the space separating the two palps of each side is a groove which, starting at the sides of the mouth, runs back- wards along the sides of the body to the gills. This groove serves for the conduction to the mouth of the particles of food brought into the mantle-cavity by the action of the cilia of the gills, the food of the Pelecypods consisting of diatoms and other minute organisms capable of being captured in this manner. The oesophagus opens into a stomach (Fig. 149, 5) which receives by numerous openings the secretion of the usually voluminous digestive gland (?), the so-called liver, and passes posteriorly into the intestine (i), which, usually in sev- eral convolutions, lies imbedded in the tissues of the base of the foot. In the wall of the anterior portion of the intestine is a groove, frequently converted into a canal, which may open into the stomach by an independent opening ; the epi- thelium of this groove or canal secretes a substance which forms a transparent glass-like rod lying in the canal and pro. jecting into the lumen of the intestine. The function of this crystalline style, as it is termed, has been the subject of much speculation, the most plausible theory being that the secre- tion serves to surround sharp-edged particles of sand or simi- hir substances, taken into the intestine with food, with a jelly- like coating which will prevent them from injuring the delicate walls of the intestine. Towards its posterior end the intes- tine bends upAvards, i.e. dorsally, to a point in front of the heart aud then passes directly backwards to terminate in the anus 9 I'-'-'- ■■- ■ ■-- 334 INVERTEBRATE MORPHOLOGY. (a) which opens iuto the suprabranchial chamber {sbr) iu the vicinity of the exhalent siphon. The relations of this rectum to the heart have ah-eady been noted (p. 332). The nervous system of the Pelecypoda differs somewhat apparently from that of the Gasteropods, a smaller number P y ne np' a = nnus. aa — iuiterior adductor. ao = aoi'ta. hr = gill. cff = cerobriil ganglion. eo = exhalent oiilice of siphon. / = foot. go — genital orifice. i = intestine. to = inhaleut orifice of siphon. I = liver. m = mantle. ne = uepluidinin. Fig. 149. — Structcke of Anodon. np^ = nephridial opening into supra- branchial chamber. p z= pericardial cavit3^ pa = posterior adductor. pfj — pedal ganglion. pp — i)rotraetor pedis. y =: reprodnctivi; organ. rp = reUaetor pedis. s = stomach. sbr - supiabranchiul chamber. s/i = shell. V = venliiolc. vi ~ visceral ganglion. np^— nephridial opening into pericar- dial cavity. of ganglia being discernible. Above the fKSophagus a short distance behind the mouth is on either side a well-marked ganglion (Fig. 149, eg) connected with its fellow of the ()})po- site side by a transverse commissure. In the more primitive forms {NhchJo) two ganglia are found on either side, of which one evidently corres])()nds to the cerebral and the other to the jiloural g.iiiglioii of the {liistcrojiods. Where, thei* ore, as in the majoritj' of the Pelecypods, but a single ganglion occurs on TYPE MOLLUSC A, 335 each side, it is to be regarded as a cerebro-pleural ganglion. From eacli of these a pedal connective passes downwards into the foot to terminate in a paired pedal ganglion (p^), and a second strong connective passes backwards on each side of the base of the foot to terminate in a large ganglion ivi\ sit- uated below the rectal portion of the intestine and frequently in close proximity to the posterior adductor muscle, and which from its relations is evidently to be regarded as representing both the parietal and the visceral ganglia of the Gasteropods and hence may be termed the viscero-parietal ganglion. The sense-organs are of essentially the same nature as in the Gasteropods. Tactile cells exist scattered over the sur- face of the body, and are especially numerous in certain lo- calities, as upon the siphons Avhen these are present. A pair of osphradia are also present situated above the viscero-parie- tal ganglion close to the insertion of the bases of the gill- plates into the side of the body ; and imbedded in tlie tissues of the foot, usually in close proximity to the i)edal ganglia, though innervated by the cerebro-])]eura], are a pair of^to' cysts haviug the usual structure (see p. 283). In a number of forms paired elevations, evidently of a sensory nature, have been found in the neighborhood of the inner ends of the siphons, or on the sides of the body a little in front of the anus ; the function of these is doubtful, though it has been suggested that they are olfactory. Eyes are present in a number of forms and present various degrees of comi)lexity. In some cases a perception of sudden variations in the intensity of light is present, as in the siphons of some forms, without any distinct optic sense-organs being developed. 8eus(ny and pigment cells are present, howevHi*^ and may be regarded as forming a diffuse oi)tic organ. No eyes occur upon the head, nor are tentacles developed in any of the Pelecypods, but large numbers of eyes are developed upon the edge of the mantle of many forms, such as Pevten and Spombjlm. These eyes may be simple depressions of the niautlo-margin, the bottom of the depression being lined with piguiented and sensory cells, a cuticle of varying thickness cov- »»»«i 4-/M>li<>vt>a niviMl OI witii iibuiMiaiii pignu3iii.-cuiir. trr tisioi.irtvj i-- - which is provided with a muscular arraugemeut by which its TYPE M0LLU8CA. 343 t lies in form of t'eu fused lI iu the al being of their 3 termed im of the ects iuto )ody-Avall homolo- apodiuin, isteropod Aium and :nlium by e funnel, the outer ing eleva- becomes L of water g the vis- lally only is a wide )ody-wall. e nephri- being ex- le funnel, the pres- •action of h1 in size, ig around lie cavity, firmly iip- ,. In the the oult'r providod each of which its size may be rapidly diminished, remarkable flushes of color passing over the surface of the living animal. Iu the Nautilus (Fig. 159) a chambered calcareous shell is present having a rather complicated structure which will be described later, and iu one or two other living forms, such as Argonauta and Spirida, an external shell also exists, but in the majority of forms the edges of the mantle close over the shell, which thus becomes internal and takes the form of a plate lying along the anterior surface of the body, being some- times calcareous as in the common Cuttlefish bone of com- merce obtained from the Sepia (Fig. 152, sh), or else chitiuous as iu the common Squid, Loligo. In connection with the mantle there are also frequently developed fiulike expansions with a cartilaginous support and provided with muscles, sometimes running along the sides of the visceral hump or iu other cases situated near its dorsal extremity. The respiratory organs or ctenidia (Fig. 154, ct) are present as either one or two (Nautilus) pairs of pinnate structures lying iu the mantle-cavity. Each consists of a central axis attached throughout its entire length to the body-wall, forming a rather high ridge upon it and containing near its outer edge two blood-vessels running throughout its entire length. The vessel nearer the summit of the ridge is the branchial vein carrying the aerated blood back to the body,, aiul between it and the branchial artery is a cavity or canal which communicates with the mantle-cavity between each pair of branchial ])innie. These structures arise from near the free edge of the axial ridge, but each is IxMind to the ridgo by a thin triangular meuibraue so that they possess the form (»f lamelhe rather than of pinnie. Near the line of attachment of the axial ridge to the body- wall is a cord of cellular tissue richly supplied with blood coming from the branchial artery, forming what is termed a blood-gland, from which the blood is collected into two hjngitudiual canals which conduct it back to the heart. The co'loni of tho Ce[)halopods is characterized by the great development of the pericardial cavity (Fig. 153, pr), which Vf-callH ila^ couilitioii fonn<] iu the Arnphinmra, aud may perhaps bo bettor termed the viscero-pericardial cavity. If: lit^ ■ 344 INVEliTEBHATE MORPHOL007. lu the majority of forms it is a large sac occupying a con- siderable portion of the apex of the visceral hump and ex- F»a. 168.— Diagram of Body cavity of /S8pi« (after Qrobden). bh = brauobial heiiit. od = ovitiuct. / = funnel. '>v - ovary. (fo = leproductive opening into coiloui. p = pancreas. JI = heart. P** = partition partittlly dividing ca*- • = intestine. lorn. lb = ink-bag. pc = coplom. l = liver. « = stonnicb. id = liver- duct. Bh = shell. inc - mantle cavity. « = external opening of ncphrfdium. N= nepbridium. V^ = opening of nephridinin into cn'. ioni. teutliiig veutraily a considerable distance, the more ventnil ■pwrtion being incompletely separated from the more capacionn ^ a cou- aiid ex- TYPE MOLLUSCA. 345 lEM). vidiug ca>- rphridiiini. Ill iiili) ergeut behind, and in which the various ganglia are but indistinctly indicated, the condition which occurs in Chiton in this rei-:pect beiug recalled. That portion of tin liiig uiiich lies in front of the oesophagus rep- resents the cerebral ganglia ; the lateral portions of the more ventral of the two rings found on the pos- terior surface are the pedal ganglia, giving rise to the nerves to the pedal lobes and the infundibuluni ; while the more dorsal posterior ring rep- resents the combined visceral, pa- rietal, and pleural ganglia. In other forms the ganglia become more perfectly marked oil' and at the same time more concentrated. A cerebral ganglion (Fig. 355, c) is always dis- tinguishable, and with it are con- nected pleuro-parieto-visceral {pi and v) and i)edal {p find p') ganglia ; the latter, however, are usually divi- ded into two portions — a more ven- tral mass {p') which sends branches to the armlike prolongations of the pro- and mesopodium and which is hence termed the brachial ganglion, and a more dorsal one (p) which supplies the infundibulum and is known as the pedal ganglion proper. A study of a number of different forms shows clearly that the brachial ganglion is merely a separated portion of the pedal, and that the arms are to be cou- sidered portions of the foot and are not cephalic appendages. At the sides of tlie cerebral ganglia there are to be fi>und a pair of large ganglia {op) which stand in relation to the eye and are termed tlie optic gai'.njlia ; they are undoubtedly spe- cializations of the cerebral ganglia, owing their separate exist- ^^ PP' B Fig 155. — Nervous Ganglia OF {A) Loligo and (i?) Octopus (after Pklseneeh). b — buccal giingliou. c = cerebml ganglion. 'p = putlul giinglion. p' = bracl.iiil gangliou. op = o()ti<: ganglion. v = pltiuro - parielo - visceral gauglica. TYPE MOLLUSC A. 351 ence to the remarkable development and differentiation of the eye which is found in the majority of the Cephalopods. A sympathetic system of nerves is well developed and con- sists of one or two pairs of buccal ganglia (&) innervating the large pharyngeal mass and united to the cerebral ganglia by connectives and giving rise to a strong nerve which runs dorsally along the oesophagus to end in a large gastric ganglion from which nerves pass to the viscera. Mention should also be made of two other ganglia, the ganglia stellata, which belong to the central system and are situated in the lateral portions of the mantle, being united with the pleuro- visceral ganglia by strong nerves ; they corres])ond probably with the parietal ganglia of the Gasteropods, sending branches to the tissues of the mantle. The special sense-organs are exceedingly well developed, nnd especially is this the case with the eyes. In NantUus, Jiowever, the eye (Fig. 156, A) stands on a much lower grade Fig. 156.—^, Eye of Nautilus (modified from Henskn): and B, of Loligo. c = cartilage. I = lens. CO - corueu. n - nerve-layer. (J = layer of ganglion-cells. op = optic nerve and retinal ganglion. ir = iris. pg = pignient-hiyer. r = layer of rods. of organization than that of the other Cephalopods, con- sisting of a cup lined by a retina composed of several layers and richly supplied with nerves. The outermost layer consists of rodlike bodies (r) below which is a layer of pigment (pg), below which again lies a layer of :anglion-celh \h- 1- No refractive struetur cs ure. lowever. jV^-^-W 1 present, the cavity of the cup communicating freel}^ with the 352 m VERTEBRA TE MORPHOLOQ Y. external water through a small circular opening in the front flattened wall of the cup. The eye is a camera constructed on the " pin-hole " type, the image being defined by the exclusion of all the more divergent rays of light which pass in from the object towards the eye. In the remaining forms the eyes (Fig. 156, />) are large globes imbedded in an orbit formed by the lateral portions of the cephalic cartilage and its processes. The retinal por- tion of the eye closely resembles that of Nautilus, consisting of an external layer of rods (r) bounded beneath by a pigment- layer ipg) beneath which is a nerve-layer (w) enclosed within a connective tissue-sheath in which cartilage (c) is developed. The optic nerve dilates into a retinal ganglion before being distributed to the retina, the rods of which, it will be notetl» are turned towards the source of light. The eye-cup differs^ however, from that of Nautilus in being comjdetely closed, and the cells which form the outer and inner layers of the outer wall of the cup secrete chitinous material Avhich acts as a lens (?), forming a powerful biconvex condenser. In ad- dition to this the eye is further complicated by the develop- ment of a series of folds from the skin in its neighborhood. One such fold is developed from the front Avail of the optic sac, surrounding the region occupied by the lens and form- ing an iris (n-), the circular opening in its centre correspond- ing to the pupil of the Vertebrate eye. A second likewise forms nearer the base of the optic sac and, growing forward, may enclose a space bounded behind by the iris and lens, resembling the anterior chamber of the Vertebrate eye, the portion of the fold immediately in front of the lens becoming transparent and forming a cornea {co). The iinterior chamber is not, however, closed in all forms, but remains in communi- cation wdth the exterior by an aperture produced by a failure of the edges of the fold to unite completely. Finally, in sonu^ forms other folds, which from analogy have been termed eye- lids, develop. The reseaibliince of such an eye to that found in the Vertebrates is ex- ceedingly striking, but a detailed study of tlie structure and mode of .origiiv of the various parts demonstrates conchisively tliat tlie similarities are ana- logical only and not homological. Uue of the most important of the ilitfer TYPE MOLLUSCA. 353 he front icted on xclusioii rom the L-e large portions 11 al por- >iisi8tiii)}n's. I'oth dui'ts u!'o ')rt>seiit. In otlipr toruis but a single duct persists, which, contrary to what occurs in TYPE MOLLUSCA. 365 Nautilus, is that of the left side. The oviduct opens into tJie inautle-cavity at the extremity of a well-iuavked papilla, its terminal portion being richly supplied Avith glands, and iu addition in some forms two small pear-shaped glands are attached to it in this region. In connection with the female ducts there should be mentioned a pair of glands which take \nitt in the formation of the investments of the ova, but Avhich open quite separate from the oviduct into the mantle-cavity. These are the nidamental ghinds which are present in the majority of forms, excluding the Octopoda, and consist of a pair of large pyriform structures lying on the posterior sur- face of the visceral mass ; iu connection with them in some forms are developed accessory nidamental glands consisting of a central and two lateral portions whose ducts open into the mantle-cavity in chjse proximity to those of the nidamen- tal glands proper. As stated, the gelatinous mass within which the ova are imbedded is probably manufactured by these glands. The testis iu its general relations resembles the ovary, being single and eucloseil in a capsule whicli is a portion of the visoero-pericardial cavity. The organ is .ittached to the wall of the capsule by a thin band of tissue and is iu most cases uluiost completely surrounded by the capsule, into the cavity oi! which the spermatozoa are slied when niatuie. From the wall of the capsule the vas deferens arises and is usually a single tube opening upon the left side of the body into the mantle-cavity. In NuntiJuH there are, as iu the female, two tlucts, tho right, however, being functionless, l)ut in other forms a paired arrangement is very rare. The proxinjal por- tion of tlui duct is a coiled vas detVrcns, which ()[)ens into a tliick-walled glandular seminal vesicle which on its part by means of a narrow duct ])asses into a saclike structure kjiown as Xeedham's pouch which finally passes into the muscular penis. In most forms the duct connecting the seminal vesicle with Needham's pouch receives tho secretion of a special gland known as the ])rostate. The majority of the accessory structines connected with the nude ducts are concerned in tlu* formation o*' cases or spermatoi)hores iu whicli a number of sjjcrmatozou are eu- 366 INVERTEBRATE MORPHOLOGY. closed. Such cases are cyliudrical structures with a double wall, and are provided at one extremity with a somewhat complicated apparatus for the ejection of the spermatozoa. The exact method of their formation is not understood, but apparently the seminal vesicles and the prostate plaj' an im- portant part in the process, the Needham's pouches being a reservoir in which they may be stored up until required for fertilization. Since the genital capsule is a portion of the viscero-pericardial cavity, and the reproductive ducts are continuations of its walls, these structures must also be regarded as prolongations of the enterocoel; and indeed second- ary communications may exist between them and the viscero-pericardial cavity proper. The genital <.'apsule is not completely separated off from tiie rest of the enteroca^l, so that it might be possible for the reproductive elements to pass from its cavity into the viscero-pericardial cavity proper, and so to the exterior through the nephridia, though this method of exit does not seem to l)e made use of. A I'emarkable moditication of one of the armlike processes of the foot occurs in the males of certain species in connection with reproduction. The arm — in Trcmocfopus and l^hilonexis the third arm of the right side of the body counting from the anleric)r mid line, in Aryonuuta (Fig. lo8) A B Fio. 158.— Male of Arqnnnuin wxtm IIe( ■iocotymzed Arm (ttfler H. iMi'i.LKH from IlA'rmiiEKi. A - ami slill enclosed williln ii membranous sac. B = ui m freed from the sue. the third of the left side— is at first enclosed within a Sftc, by the bursting of which it l)ecomes free, the walls of the sac l>eing reHectoil back so as to form a pouch which in some unexplained manner receives a ™i TYPE MOLL use A. 357 sperraatophore. The terminal portion of the arm, which is traversed throughout its entire length by a canal, is developed into a long terminal filament through which the spermatozoa may pass During copulation the arm is probably thrown off and passes into the luantle-cavity of the female, the manner in which the spermatozoa reacli the ova being, however not yet understood. When first discovered in the mantle-cavity of a female tlie arm was regarded as a parasitic worm, and the name Hevtocotijlms was applied to it-a term which is still retained on account of its convenience In other genera of Cephalopods one arm is generally peculiarly modified in the male— in the Decapoda usually the fourth of tlie left side and in the Octopoda usually the third of the right side, though frecjuent exceptions are found. This arm is termed the liectocotylized arm, though it is doubtful •whether it takes any part in copulation. As will be seeu from the above deseriptiou the genus Nautilus differs iu many important particulars from the re- maiuiug genera of Cephalopods, and the class is therefore divided into two orders. " 1. Order Tetrabranchia. This order, of which the genus Nnuiilus (Fig. 159^ is the sole living representative, was in former periods of the earth's history the dominant group of the Cephalopods— the Ortho- cerites of the Pala'ozoic and the Ammonites of the Mesozoic heiug extinct members of it. It is characterized bv its mem- bers possessing four cteuidia, four auricles to the heart, and lour iiephridia; and in addition there maybe mentioned, as further peculiarities, the presence of paired reproductive ducts, of wl.u'h the right one alone is functional, and also of direct ♦•omi.iunication of the viscero-pericardial cavity with the ex- terior by two pores, and by the occurrence of a single pair of osplmidia. For a more detailed account of the peculiarities of NitniUus the preceding general description may be con- sulted. It remains to discuss here the shell and the structure of the foot-lobes- structures which, with the other characters mentioned, serve to distinguish Nautilus from all its living congeners. The shell is voluminous, coiled, and calcareous, its cavity H'lug divided l)y a seri(>s of transverse partitions into a nunl- her of chambers, in the last— that is to say, the youngest— of which tlie aiiihml lives, while the remaining ones are flih-d with ,¥••»•«« "•JCI n^ 1 ? ■ 1 i ! 1 i i 1 ■ ' I i* 368 INVERTEBUATE MORFHOLOGY. gas. The centre of each partitiou is perforated, aud through the opening there extends to the tip of the shell a prolongation of the body of the animal, termed the sipuncle. The foot of Nautilus, or at least that portion of it which fuses with the head, has already been described as forming a number of tentiiculiferous lobes. These lobes are arranged in the female in two series — one ventral, consisting of threa Fig. \n9.— Nautilus pompilius,—FEii\i.ii, with the Shell sectioned Longi- TUDINAM-Y TO SHOW ITS InTEUNAL StkUCTUUE (after Lkunis from Hkrtwio). 1 = nmiitle. 7 = nidiimeulal glaud. 2 = dorsal lobe of niantle. 8 = sliell-uiuscle. 3 - tenlades. 9 = terminal chamber of shell. 4 = lu'ad cap. 10 = partiiions between the various 5 z= oje. ciiambers. 6 = funnel. U = sipuncle. lobes which immediately abut upon the mouth, aud a inoro dorsal ringlike lobe the anterior portion of which is de- veloped into a hood (4) which arches over and protects the re- tracted tentacles. Around the margins of both the veutial and dorsal lobes are arranged the tentacles, each of which is filiform and capable of being withdrawn into the basal jxu- tion, which thu:i serves as a sheath. In addition to tlic.se tentacles two other tentacles are f(nind in close ]n-oximity to the eye, one being on its ventral side and the otiior on its dorsal. In the male the arrangement is very similar, except that the median lobe of the ventral series is transformed into a lannllated structure aud does not bear tentacles, wiiih^ a- portion of each of the lateral lobes of the inner series is sepii- TYPE MOLLUSC A. 359 through aii'-atiuu it vvliicii »rmmg a irrauged of three ;ed Lonoi- 1 Hkrtwio). lell. ie various a nioro h i.s iIb- ts the re- B veutral )f whith asal |)()r- to these siniity to u' on its ,r, ext'('|it iiied iiili* , Avhih> ii 9 is sepii- rated from the rest of the lobe—that of the left side becoming modified into a conical structure, lamellated at the extremity and destitute of tentacles, forming what is termed the spadix, probably homologous with the hectocotylized arm of the male Octopods and Decapods. 2. Order Dibranchia. Tlie members of this order, which includes the majority of living Cephalopods, possess but a single pair of ctenidia, uephridia, and auricles, and lack tJie direct communication of the viscero-pericardial cavity with the exterior as well as the osphradia which occur in Nautilus. The portior. of the foot which is fused with the head is diawn out "uto a number of arms provided with suckers, which seem to represent the tentacles and their sheaths found in Nautilus. The suckers are very numerous and may be arranged in from one to four rows on the ventral surface of the arms, the margin of each sucker being in some forms strengthened by a horny ring, which may be toothed. The number of the arms varies, being either eight or ten ; and, since, other struc- tural differences are associated with this difference, the order "uiy l,e divided into two suborders Pro. m.-F^Ugo pallida, D<,nsAr, -the Octopoda with eight arms, Vikw (after Emkbton from Vkkuiu,). mcluding tiio genera Octopus, Tremodopm, and Argonauta (iMg. 08), and the Docapoda with ten arms, the genera Spt- ruh, OmmaMrephes, Sepia, and Loliyo (Fig. lOO) belonging to this gri)up. " / fob In the Deeapoda tho ten arina are not of equal size, one i4ip 360 INVERTEBRATE MORPHOLOOY. on each side of the head, the fourth, counting from the ante- rior mid-line, being k)nger than the rest, usually destitute of suckers except towards the tip, and in most species kept retracted within a groove on each side of the head except when required for prehension. They are all good swimmers, and the body is elongated and provided with lateral tins of greater or less extent. A shell is present in the Decapod a, but shows a great reduction in size and complexity from that of the Tetra- branchiates ; and in order to understand its homologies in the different genera it will be necessary to obtain an idea of its form in the fossil members of the group which occur in the Mesozoic rocks forming the fam^y Belemnitida?. In^ the genus Iklemnites (Fig. IGl, JJ), for instance, the shell consisted of a terminal conical solid portion, termed the rostrum (?'), the base of which was liollow and contained a chambered shell, the phragmacone (/j//), corresponding to the Nautilus shell, the anterior portion of the last chamber of this being elongated into a broad Hat process termed the proostracou (pr). By various nu)diticati(nas of this structure the shells of the different living Dec-apods have been developed. In Spiruhi the shell is coiled into a spiral and is partly enclosed by tlie mantle, the rostral and proiistracal portions having disap- peared. In all other forms the shell has a more or less flat- tened for]ii and becomes completely enclosed within the mantle, folds of which .^row up around it. In Sepia (Fig. 101, A) the proostracou becomes almost obliterated and tlic rostrum (/•) is exceedingly small, the. phragmacone (pi) form- ing the })rincipal bulk of the siiell. This, however, has become very much modified— that iiortion of it which lies ])osteriortu the sipuncle {.s) ceasing to develop, or rather becoming exceed- ingly compact by the various partitions lying in close con- tact with one am)tlier witlumt any intervening air-chambers. These chambers are, however, developed in the portion ant' - rior to the sipuncle, but ar(^ compaiatively Hat and travers. d by calcareous spicules, so that the shell has a somewlml spongy appearance and is exceedingly light. In other forms, however, the ].roostracou is the portion that i)ersists, the rostrum ami i)iinignia (•inn liotli (]isa]5]iC;iring (Fig. K>1, C TYPE M0LLU8CA. 3C1 lie ante- dtute of es kept I except imuiers, I lius of a great e Tetra- es ill the 3a of its r in the In the ionsisted ;rnm (;•), ambered Nmdilus lis being lostracou shells of II Spirilla d by the g disaji- less Hat- thill the pia (Fij^'. and tlic pi) form- s become steriorlc g exceed- lose c'oii- hainbois. tiou aiiti'- travel's* (I .oniewli.'ii er foriu-^, sists, tlif 01, 0), -- that fiually nothing is left but a plate imbedded in the man- tie, formed entirely of chitinous material and destitute of any calcareous substance. This forms the so-called " pen " of such forms as Loligo and Ommastrephes {Fifr. IGl, D), a slight FiG. 161,-DrAORAMs r.v SiiEM. ov A, Sepia: R BelemviUs (fossil); C Os- tr„cou;iti.i. (lassi!); and D, Ommadrn'hes u.u.v v^m',) ph = i.hm-mocone. ;. ^ ,,„„,,„„ pr - l.roOstracon. , ^ ,i,,„„^.|^. tj'iekening of the dorsal end of it in some forms representing tlie remains of the phragniacone. In the Octopo.la the eight arms are practically equal in i""g h, and the body is more massive than in the ])ecaj)ods and less suited for active swimming. The visceral hum,) is a more or less globular mass, destitute as a rule of lateral lins. ;"ul m all forms a shell comparable to that of the J)ecapods ■^ wanting. Tn the females of Jnmanta, however, a non- '• 'a.nlHn-ed calcareous shell is present, to which the body of |l'<' ammal does not closely adhere, but whicl. is iield in I'osi- "uu by the broad plate-like anterior arms which embrace it 362 IN VERTEBRA TE MORPHOLOO Y. \ \ It seems to be a secretion of the ectoderm of the mantle and visceral hump, the anterior arms contributing only a thin external layer. Development and Affinities of the Cephalopods. — The ova of those Cephalopods which have been studied are richly provided with yolk, in consequence of which the development becomes considerably modified, definite traces of the Veliger condition being entirely lost. It seems clear, however, from the marked development of the head, the presence of a radula, and the general arrangement of the viscera, that the ancestry of the group is to be sought for among the primitive Gasteropods, but in forms more primitive than any existing forms. The symmetrical shape of the body and the character of the viscero-pericardial cavity suggests forms intermediate in development between the Amphineuraandthe Diotocardiate Prosobranchs. So far as the various groups are concerned there can be little doubt but that the Tetrabranchs are the more primitive forms, showing as they do less specialization of the foot, what must be considered a more primitive shell, and a more general tendency towards a paired arrangement of the organs than is found in the Dibranchs. The duplication of the ctenidia and nephridia mu§t, however, be considered a secondary acquisition. The Decapods, again, seem to be on the whole more primitive than the Octo- pods, the character of the crelom and the presence of a shell in the former being points to which attention may be called in this connection. The Affinities of the i/oZZwsca.— Attention has already been called to the similarity of the typical Gasteropod and Pelecypod larvas to the An- nelid Trochophore, and the evident conclusion has been pointed out that the Annelids and Mollusca are to be traced back to a common ancestor represented by the Trochopliore. It is difficult otherwise to understand the remarkable similarity which exists between the two larva?— similarities, including not only the general arrangement of the locomotor cilia, but ex- tending as well to internal organs, such as the nephridia. In two respects, however, the Molluscan Veliger differs from the Annelid Trochophore ; it possesses a shell and a foot. These features are, however, readily ex- plicable as a throwing back in the ontogeny of important structures origi- nally developing at a much later period in the life of the animal— a phe- nomenon of by no means unfrequent occurrence. It must bo admitted, however, that frequent modifications of the Trochophore arrangement arc to be found, as has been indicated in the descriptions of the Amphineura, and these become especially interesting from the fact that in the former a primitive arrangement of the parts oi the body must be recognized. If the Trochophore represents an ancestor, then it might be expected that it would be found more perfectly represented in the Amphineura than in tiie highly specialized Gasteropods, or even than in the Pelecypods. It is important, then, that the possibility of some of the similar struct- ures of the Trochoi)hoi'e and Veliger having been iiidependently acquired TYPE MOLLUSC A. 363 Qtle and r a thin of those [i yolk, in modified, ems clear, ence of a stry of the t in forms ipe of the 3sts forms tocardiate doubt but is they do primitive ent of the le ctenidia tion. The the Octo- the former 1 called to to the An- d out that n ancestor inderstand imilarities, ia, but ex- o respects, ophore ; it eadily ex- ures origi- lal — a plic- admittod, icemen t are nphineura. le former a ^nized. If ited that it, than in the lilar struct- !y acquired should be kept in mind, especially in view of another idea as to the gene- alogy of the Mollusca which has been advocated by certain authors Ac- cording to this theory they have sprung from Turbellarian-like ancestors the creeping surface of the worm having become more muscular, and so having given rise to the foot of the MoUusk, the dorsal region of the body elevat ing into the visceral hump. The nervous system of the Amphineura, with Its ladder- Ike arrangement, miglit readily be deduced from the arransre- inen found in the Platyhelminths, and thus many points on which the Tro- chophore theory throws no light become intelligible. This tlieory concerns itself mainly with tlie adult forms, yet it is not unpossible that a reconciliation between it and theTrochophore theory mar be possible. It has already been pointed out that the Trochonhore mav possibly be the representative of a Turbellarian larva, and the same idea may be applied to the Veliger. In other words, it is possible that the Mollusca may have been derived from the Turbellaria, and that the ances tral worms possessed in tlieir life-history a larva whicli, independently of the adults, underwent a series of modifications leading to the Veliger The Vehger would then be the descendant of a Turbellarian la'-va while the adult xMollusk would be directly descended from the Turbellarian This view may be contrasted with that which regards the Trochophore (includ nig the Vehger under this term for convenience) as the ancestor by means or the following scheme: TUEBELLAEIAN THEOEY. Turbellarian larva==Turbellarian (ancestor) \ "Troeliophore TEOCHOPHOEE THEOEY. ''Turbellarian Annelid MoUusk Turbellarian larva (ancestor) Trochophore Annelid Mollusk SUBKINGDOM METAZOA. TTPE MOLLUSCA. I. Class AMPHINEURA—Visceral hump not developed; bilaterally sym- metrical; shell represented by scattered spicules or by a series of ealuareous plates; anus tormiual. aBIJS** I 364 INVERTEBRA TE MORPHOLOO T. 1. Order Solenogastres. — Shell represented by scattered culcareous spicules. JVeomenia, Proneomenia, Choetoderma, Dondci'sia. 3. Order Polyplacophora. — Shell formed by eight plates on dorsal surface of body. Chiton, Trachydennon, ChitoneUns. II. Class Gasteropoda. — Visceral hump usually well developed body asymmetrical; shell univalved and usually spirally coiled, some- times absent; anus not terminal. 1. Order Prosobranchin. — Clenidia present, situated in front of the heart; auricle in front of ventricle; mantle edL j not fused with body. Heart with two auricles; two nephridia (Diotocardia). Haliotis, Turbo, Trochus, JYeritina, Fleurotoinaria, Fissurella, Patella, Acintea. Heart with a single auricle and a single nephridium {Monotocar- dia). Dentition tienioglossate. With creeping habit. Cijprad, Paludina, Natica, Ampul- laria, Littorina, Ci/clostoma, Calyptraa, Strombus. Wi h pelagic habit {Hetei-ojioda). Atalanta, Carinaria, ] terotrac/iea. Dfc.-' :ion ptenoglossate. lanthina, Saalaria, Solarium. Dentition rachigiossate. Fusus, Buccinum, Nassa, MureXy Purmra, Oliva, Marginella. Dentition toxiglossate. Terebra, Conus, Pleurotoma. 3. Order Opisthobranchia. — Clenidia frequently absent, when pres- ent behind the heart; auricle behind ventricle; mantle vdien present not fused by its edges to body-wall; shell frequently absent. Mantle present {Tectibranclda) . Foot with broad Hat sole; with creeping habit. Bulla, Jamis, Ai)lysia, Pleurobrandius. Foot with winglike parapodia, jielagic (Pteropoda). With shell (jf7teco.w«t«to). Limadna, Styliola, Cymbuliopnis, Without shell (Gymiinsoinata). Pnenmodi'rma, Ciione. Mantle not developed {Nndibrauchia). Phurophyllidia, PhyV liiholK Limapontia, Doris, ^oli.s', Facellina. 3. Order Pnlmonata. — Ctenidia wanting; mantle fused by its edges to body- wall; terrestrial or aquatic. Eyes at base of tentacles {BosoinmatopJiora). Limncca, P/iysa, Planorbis. Eyes at tip of tentacles (Sti/loni inafophora). Helix, Liinax, Arion, Vaginula, Daudebardia, Ondiidium. in. Class ScAPHOPODA. — Visceral hump developed; bilaterally symmetrical; shell cylindrical, open at both ends. Dentaliuin, Hipliodenta- Hum, Cadulus. I calcareous ndcrsia. oil dorsal y. ped body iled, some- ant of tlie fused with Haliotis, I, Patella, Monotocar- a, Ampul- nbus. '^armaria. irium. ia, 3Iure,v, la. vlieii prcs- ntle when fi-equeutly 'la, Janvs, nbuiiojxsis^ 'lone, ilia, PhyJ^ y its edges xa, P/ii/sa, a,r, ArioiL minctrical: iphodeii/ii- TYPE MOLL use A. 365 IV. Class Pelecypod A— Visceral hump not developed; bilaterally sym- metrical; mantle forms two lateral folds; shell bivalved; anus terminal. 1. Order P/7>to6/-rtMc/i p ■l ^ J 1 ' 1 1 :l % t 11 et 14 366 INVERTEBRATE MORPHOLOGY. Quarterly Journ. Microscop. Science, Ar- on the Structure of these Organs. XXV. 1885. H. J. Pruvot. L'orgaimation de quelquea Neomeniens dea Cotes de France. chives de Zool. exper., 2me Ser., ix, 1891. S. Wiren. Studie7i uber die Solemgastres. I. Monographie des Cheetodei-ma nitidulum. Svensk Vetenskap, Akad. Handl., xxiv, 1892. GASTEHOPODA. Alder and Hancock. A Monograph of the British Nudibranchiate Mollusca. London, 1850-51. J. W. Spengel. Die Oertichsorgane und das Nervensystem der Moilusken. Zeitscbr. fUr wissenscb. Zoologie, xxxv, 1881. H. L. Osborn. On the Gill in some Forms of Proaobranchiate Mollusca. Studies from the Biolog. Labor. Jobns Hopkins Univ., in, 1884. B. Haller. Beitriige eur Kenntnias der Niere der Proaobranchier. Morpholoff Jahrbucb, xi, 1885. H. Simroth. Versueh einer Naturgeschichte der deutschen Nacktschnecken und threr europaiachen Vericandten. Zeitscbr. fUr wissenscb. Zoologie, xlii, 1885. W. 0. Binney. A Manual of American Land-ahella. Bulletin U. S National Museum, No. 28, 1885. BelaHaller. Unterauc/mngeniiber marinenRhipidoglosseii. Morpbolog Jabr bucb, IX, 1883 ; xi, 1886. E. L. Bouvier. Syatime nerveux, morphologic genernle, et rlaaaification dea Gas- terojmlea proaobrancliea. Annales Sciences Nat., Zool., Ser. 7me, in, 1887. B. Hallar. Die Morphologie der Proabranchier gcmmmelt aiif einer Er'dumae- gelung durrh die konigl. ital. Corvette " Vettor Piaani." Morpbolog Jabr- bucb, XIV, 1888; xvi, 189() ; xviii, 1892. P. Polseneer. Report on the IHeropoda. Scient. Results of tbe Voyage of H. M. S. Cballenger. Zoology, i.viii. 1887 ; Lix, 1888 ; i.xv, 1888. J. I. Peck. On the Anatomy and Iliatology of Cyvibuliopsia calceola. Studies from tbe Biol. Labor. Jobns Hopkins Univ., iv, 1889. A. Lang. Verauch einer Erklarung der Aaymmetrie der Gasteropoda. Viertel- jabresscbr. Natnrf. Oesellscb. Zl\ricb, xxxvi, 1891. E. von Erianger. Ziir Entwickelung vonPaliidinavinpara. Morpboloir Jabr bucb, XVII, 1891. **■ H. von Ihering. Morphologie und Syatematik dra Genitalapparatea von Helix Zeitscbr. fUr wissenscb. Zoologie, i,iv, 1892. SCAIMIOI'ODA. H. de Lacaie-Dnthieri. Iliatoire de V organ iaaf ion et dv developpement dii Dentak Annnles des Sci. Nat., Zool., 4mH Ser., vi, 1856 ; vii, 1857 ; viii 1858 L. H. Plate. Ueher den Ban und die Veru>,ridfaehaftsbeziehung'en der Sole no- conehcn. Zoolog. .labrbUcbor, Anat. Abtb., v, 1892. rEI.KCVI'ODA. R. H. Peck. The Strurture ■)/ the Lame.llibranch Gill. Quarterly Journ. Wl- croscop. Science, xxvi, 1876. C. w. B. P. J. B. J.: €. I P.] TYPE MOLLUSCA. 367 )p. Science, ranee. Ar- Vtcetodeiina i Mollusca, Moiluaken. 1. Studies tforpliolog. lecken und ogie, XLii, 5. National olog. Jabr- n des Gas- , HI, 1887. Erd}imse- ilog. Jabr- ^'^oyage of 8. . Studies ;. Viertel- )log. Jabr- von Jlelix. C. Orobben. Ueber die Pericardialdrme der Lamellibranchiaten. Ein Beitrag zur Kenntnias der Anatomie dieser Mollmkenklasse. Arbeiten a. d. Zool. Institut Wien, vii, 1888. W. M. Bankin. Ueber das Bqjanus'schen Organ der Teichmuschel (Anodonta Cygnea, Lam.). Jenaiscbe Zeitscbr., xxiv, 1890. B. Bawitz. Der Mantelrand der Aceplialen. Jenaiscbe Zeitscbr., xxii, 1888 ; XXIV, 1890 ; XXVII, 1892. P. Pelseneer. Oonti'ibutio?i8 a I'etude des Lamellibranchea. Archives de Bio- logie, XI, 1891. J. Tbiele. Beitrdge zur vergleichenden Anatomie der Amj)?iineui'en, Zeitscbrift ftlr wissenscb. Zoologie, lviii. 1894. CEPHALOPODA. E. Q^on. Article " Cephalopoda." Todd's Cyclopaedia of Anatomy and Pby- siology, I. London, 1836. J. Brock. Zur Anatomis und Systematik der Cephalopoden. Zeitscbr. fUr wis- senscb. Zoologie, xxxvi, 1882. C, Grobben. Mjrphologische Studien iiber den Harn- und OeacMechtaapparat soicie die Leibeahohle der Cephalopoden. Arbeiten e. d. Zoolog. Instiiut Wien V. 1884. P. Pelseneer. Sur la valeur morphologique des bras et la composition du systems iierveux central des Cephalopodes. Arcbives de Biologie, viii, 1888. uDentale. I, 1858. er Soleiio- ourn. Mi- 868 INVERTEBRATE MORPHOLOOT. CHAPTEB XIII. TYPE CRUSTACEA. The group Crustacea includes a very large number of forms, most of which are mariue, though many are found iu fresh water and a few are even terrestrial. A great diversity of form is found m the various members of the group, but at the same time the general structure, except in. forms degen- erated by parasitism, shows comparatively close similarity throughout. The body is enclosed in a thick chitinous cuticle which not infrequently becomes hardened by the deposition of calcareous matter in it, producing what may almost be con- sidered a shell and giving origin tv) the name aj)plied to the group. This covering serves not only for protection, but also as a point iVappui for the insertion and origin of muscles. Where it reaches a considerable thickness it becomes more or less regularly divided into segments, separated by intervals in which the cuticle remains thin, so that movement of thu various segments upon each other are possible. As a rule there is attached to the sides of each of these segments an api)endage, also inclosed iu a more or less thick cuticle and jointed, this jointed character having suggestt'd the reference of the Crustacea together with the Arachnida and Tracheata to a single group termed the Arthropoda. An examination of the internal parts, especially of the nervous system, shows that these various body-segments are in realitv metameres, and that the Crustacean is, like the Annelid. ;i motameric organism. A characteristic of tlie Crustacea, how- ever, is a tendency towards a greater » a vai'ving number of the nu^tanieres fuse more or less perfectly together to form a distinct head, bearing the ~3M ijttifa TYPE CRUSTACEA. 369 aumber of B fouud iu t diversity >up, but at nis (le^eu- similurity icie which ositioii of ii be cou- ied to the 11, but also f muscles. es more or itervals in ut of thu 1 of these less thick sugtj^ested A.rachui(lii locla. All e nervous ! iu realitv A.nueli(l. ;i acea, how- aiul coii- uuelidii, a ion of till) lO more or iariug the principal seuse-orgaiis aud the orgaus of mastication (Fig. 162), there beiug behind this head, more or less perfectly dis- ti'iguishable, a thorax and an abdomen. Judging from, the number of pairs of appendages arising from this head region it seems that the typical number of metameres consolidated to form it is live, but to these there must be added an anterior segment which does not bear appendages but upon which the eyes are developed. To these six somites there are added, especially in the more highly-differentiated forms, a number of additional metameres which properly beloug to the thorax, the apparent extent of the head region beiug thus increased. Fig. 162.— :A Dedapod ChustaCKan, Cambarua. There is indeed throughout the Crustacea a tendency towards what has been called " cephalizatiou," i.e., a condensation of the anterior metameres, and as a rule tlie higher the form the greater is this condensation aud the greater the apparent ex- tent of the head region. The number of segments composing tlie thorax aud abdomen is exceedingly variable in the lower forms, but in the higher there are constantly eight thoracic iuid seven abdominal segments, the posterior one, termed the telson, being alone destitute of appendages. Frequently, especially in the higher forms (Fig. 162), the thoracic seg- ments consolidate to a greater or less extent, the segmentation (»t' this region of the body being indicated iu some forms only by the apjiendages and the nerve-ganglia, and furthermore? lateral folds of the body-wall may project backwards from the sides and dorsum of the head or anterior thoracic regions, enclosing the thorax or even the entire body in a firm cara- pace or else in a bivalved shell, someiimos provided with adductor muscles. s***-. 370 IN VERTEBRA TE MORPHOLOG T. The study of the embryology of some of the highor Crustacea has brought out the fact that in these there are indications of a segment desti- tute of appendages but represented by a pair of nerve-ganglia, immedi- ately succeeding the eye-bearing anterior segment. In tliese cases, then, the head really consists altogether of seven segments. AVhether this seg- ment represents the first appendage-bearing segment of the lower forms Pig. 163. --Crustacean Appendages. A, antennule of Craytish, Camharus; B, auteuuulu of Copepod, Oitliona (after UIK8BHKCHT); C, auteuuii of Camharus; D, antenna of Phyllopod, Eulimnadia (after Packard); as, sensory hairs. or whether in these also it exists in a degenerate condition has not yet been determined; for convenience at present the six fully-developed head- segments may bo considered homologous throughout the group. The appendages vary much iu form in diflfereut parts of the body and iu different forms. Those of the head region TYPE CRUSTACEA. 371 Listacea has jment desti- lia, immedi- cases, then, ler this seg- ower forms 'thona (after Vulimnadia ot yet been ped bead- parts of d region are modified to serve as seuaory organs and organs of masti- cation. The first pair, termed the first antennce or antennules, are usually sensory in function, though occasionally also loco- motor (Ostracoda and some Copepoda), and are frequently supplied with peculiar setpe supposed to be olfactory in addi- tion to others probably tactile in function (Fig. 163, B). They consist in their typical form of a basal portion composed of three or four joints, the terminal one bearing one (Fig. 163, B) or two (Fig. 163, J) many-jointed flagella. The second pair, the second antennce termed also simply antennce CFig, 163, C and D), are also principally sensory and consist typically of a two-joint- ed basal portion, bearing two many-jointed branches. One of these, that upon the outer side, frequently becomes reduced to a scalelike rudiment (Fig. 163, C), the inner branch persisting as the fiagellum. The third pair, the mandibles, serve as mas- ticatory organs and are generally much modified in correspon- dence with this function. Typically (Fig. 164, A) they consist of a two-jointed basal portion bearing two branches. The proximal joint of the basal portion, however, becomes much indurated by the thickening of the chitinous cuticle and also toothed, forming the mandible proper, while the remaining joint and the two branches undergo reduction even to disappear- ance, being krown when present as the mandibular palps (Fig. 164, C, mp). Tho fourth pair are the first maxillm (Fig. 104, B and D) and serve like the mandibles for mastication, un- dergoing a somewhat similar modification. They do not, how- ever, become so indurated, though one or both of the basal joints may be provided with stift' setee and serve as a jaw, and the two branches more frequently persist than in the mandi- bles. The fifth pair, the second maxilla', are also masticatory and resemble the first in the modifications which they undergo. The thoracic and abdominal appendages in all but the lowest forms can be reduced to a typical appendage consisting of a two-jointed basal portion tipped by two branches also jointed. In appendages employed for swimming both branches persist (Fig. 165, A), and may possess a broad ])latelike form, but when modified for walking the outer branch disappears. From limbs modified in this latter it 372 INVERTEBRATE MORPHOLOGY. Fi0. 164.— Ckdstacean Appenuages. A, mandible of Copepod, Notodelphys (from Bronn); B, first maxilla of Noto- delphys (from Bhonn); C, nmndible of Cambarus; D. first maxilla of Cam- barus. en = eudopodite. ex = exopodite. mp = mandibular palp. ex Fig. 165. — Cuustacean Appendages. A, second thoracic appendage of Mt/sis (after Sahs); B, second tlioracic appen- dage of an Amphipod. Hi TYPE CRUSrACEA. 373 ilia of Noto- 11a of Cavi- ir palp. mauuer the grasping claws (Fig. 165, B) or cLeli» are devel- oped by tl'.e liexioii of the terminal joiut on the subterminal or by the elougatiou of the angles of the latter into a more or less strong process against which the terminal joint may be approximated. The description given above of the various appendages is of course general, the modifications found in the various forms being almost endless. Indeed in parasitic forms the ap- peudages, except those concerned in mastication, may entirely disappear, all gradations between fully-developed append- ages and the merest rudiments being found in various forms. icic appeu- Fig. 166.— Sixth {A) and Second (B) TnoijAcxc Appendages op Bkanchio- POD, Apus (after Zaddach from Bronn). hr = bract. fl = Uabelluni. l~6 - iuuer lobes. From what has been said, however, it may be seen that typi- oally the Crustacean appendage may be considered a biramous structure, consisting of a two-Jointed basal portion termed the protop(,dite and two jointed branches termed the exopodite and emfopodite (Fig. 165, ex, en) accordinj,' to their relation to the median axis of the body. Additional rami are frequently developed upon the i)r()t()podite— such, for example, as that termed the cpipudife (Fig. 167, ep) and the branchia (hr). How- ever, although such a limb may be considered typical, it is not necessarily also the most primitive. Indeed wheii the simplest forms, such as the Phyllopoda, are examined it will be found that the more posterior appendages have a very clilierent composition. Thus in the genus Apm the sixth tlioracic appendage (Fig. 166, A) consists of a central two- a'— - M:;: 374 INVERTEBRATE MORPHOLOGY. jointed axis ending in a rounded lobe and bearing upon its inner edge six lobes (1-6), some of which are united to the axis by a joint. On the outer side are two large lobes, the distal ono being termed the fiabellum (/), while the proximal one is the bract (br) and serves respiratory purposes. The entire appendage has thus a leaflike form. In one of the more anterior appendages, however, an interesting modifica- tion of this will be found. Thus in the second thoracic ap- pendage (Fig. 166, B) the axis will be found to be more dis- tinctly divided into two joints, each bearing two of the internal lobes somewhat reduced in size, while the terminal one in addition carries two other lobes, the fifth and sixth, which have become somewhat elongated. The fiabellum and bract remain nearly the same as in the first post - genital appendage. If now such an appendage be compared with the second tho- racic appendage of the Shrimp Palcemonetes (Fig. 167), a direct homology of the parts may be discovered. The axis of the Phyllopod limb is represented by the protopodite while the exopodite (ex) and endopodite {en) represent the two terminal inner lobes, the others having disappeared; the fiabel- lum is represented by the epipodite (ep) and the bract per- haps by the branchia (br), attached to the epipodite in this par- ticular limb, but free on the more posterior ones. It would appear probable from these facts that the bira- mous limb is really a derivative from the more complicated foliate appendage possessed by the Phyllopods ; the foliate condition, however, has given place to such a great extent to Fig. 167.— The Second Maxilli FED OF Palmmonetes. br = braucbia. en = endopodite. ep = tpipodite. ex = exopodite. TYPE CRUSTACEA. 375 the biramous that it is most conveuieiit to regard tlie latter as the typical coutlitiou in the Crustacea. Respiratory organs are not always present, but wVen they are they take the form of thin-walled outgrowths of the body, wall. lu some forms in which the surface of the body-wall is increased by the development of a bivalved shell or carapace the hning-surface of the fold serves for respiration, and may be thrown into a number of folds so as to increase the extent of surface, as in the Gasteropod Patella. In the majority of cases, however, more or less branched hollow processes are seated upon the sides of the body or on a greater or less number of the appendages, their cavities communicating with the lacunar spaces of the body, so that the blood can circulate through them and receive aeration through thea- thin walls In the Isopoda a certain number of the appendages are de- voted to the respiratory function, both the exopodite and endopodite being lamellar and thin-walled, or else the endo- l)odite alone may have this function, the exopodite serving as a covering-plate for the protection of the inner respiratory ramus. As already stated, the body is covered by a chitinous or more or less calcareous cuticle. This is secreted by the cells of the hypodermis, as it is termed, which correspond to the ectoderm of other forms and rest below on a more or less well-developed layer of connective tissue. A dermal muscu- lar system is entirely unrepresented in the Crustacea, owing no doubt to the development of the thick cuticula ; but never- theless muscles are well-developed. These take the form in the body of four longitudinal bands, two situated dorsally and two ventrally, giving off slips to be inserted into the cuticle of each metamere, flexion and extension of the various meta- lueres upon one another being thus permitted. In addition muscles extend from the body-wall to the various appendages and between the various joints of these structures, being in all cases, it is needless to say, situated within the body and the appendages. In some cases, more especially in those forms in which the appendages are adapted for walking, special chitinous plates or processes project into the body- cavity from the ventral surface forming the endophragnml sys- ^31 11 ll# 376 INVEHTEDIiA TK MOlil'lIOLOG Y. tern aucl serving for the attaclimeut of the muscles passing to the appendages. In forms furnished with a bivalved shell special adductor muscles for its closure are frequenth' devel- oped ; and in the higher Crustacea, in which the so-called stomach is usually provided with a series of chitiuous teetli, special muscles are developed for their movement. The ccelom of the Crustacea consists for the most part of a series of cavities, without definite walls, between the viscera and the muscle-bundles and extending out into the ajjpend- ages and the braucliiio. One of these occupies the mid-line below the dorsal surface of the body, and contains the heart, whence it is known as the pericardial sinus. It is bounded below by a distinct partitif)n, the pericardial septum, but seems to be a schizoccelic space, since it contains blood, and is therefore not comparable to the pericardial cavity of the Mollusca. A true enterocoel does exist, however, in some of the higher forms (e.g., Palcemonetes), consisting of a sac Ij'ing in the anterior thoracic region. It surrounds the anterior aorta as a narrow cavity and behind expands so as to cover the anterior portion of the reproductive organs, and then })asses ventrally into the schizoccelic cavity which surrounds the intestine. It is a perfectly closed sac, having no com- munication with the pericardial sinus beneath which it lies, and contains a coagulable fluid in which no corpuscles have been observed. The sticlike cavity into which the anteiuiary gUmd, to be described later, opevis is also to be regarded as a true enterocoil; but attention miisl ;i»gain be called to the iuadvisability of maintaining a wide distinction be- tween schizocoelic and enterocoelic spaces. (See p. 231.) The circulatory system is comparatively simple. In many forms a heart and distinct blood-vessels are entirely wanting, the blood circulating through the lacunar ccelom by tlio niovemeuts of the appendages. In the majority of forms, hoAvever, a pulsatile heart (Fig. 168, /*) is present, lying near the dorsal surface of the body in the pericardial sinus, ex- tending in some forms throughout the entire thoracic and abdominal regions of the body. More usually, however, the heart is limited to the thoracic region, or occasionally is almost eutirel}' confined to the abdomen, its anterior ex- !■'» TYPE CRUSTACEA. 877 tremity encroaching but slightly upon the thorax (Isopoda) It IS provitied with a varying number of openings alon/^ its sides, through which the blood gains entrance to its cavity from the pericardial sinus— the«e openings, termed ostia being guarded by valves opening inwards and preventing regurgitation of the blood during systole. From either end of the heart arteries arise which, after a longer or shorter course and many or few branchings, open widely into the lacunar spaces. From these the blood passes in some forms into a venous sinus situated on the ventral surface of the an Fig. 168. -Diagram of Structure op Crust ackan {Cambariuls an = anus ca = carupiice. ee = cerebral guugliou. h = heart. i = intestine. I = digestive gland. m = mouth. mp = opening of vas deferens. ne = nephridium. s = stomach. sa = sternal artey. te = testis. tl = telsou. vd = vas deferens, vn = ventral nerve. 1-6 = abdominal segments. body, and thence is distributed to the branchi*, passing from them back to the pericardial sinus, and so to the heart a.^ain The blood IS usually colorless, tliough occasionally greenish, m which case it contains a respiratory copper-containing pig! nient termed hsemocyanin, or reddish, in which case the pig- went is haemoglobin. It consists of a plasma in which float aiiKeboid nucleated corpuscles. The digestive system consists of an almost straight tube extending from mouth (Fig. 168, m) to anus (an) and divisible luto three regions. The mouth is bounded in front by an overhanging lip, and behind by a lower lip which arises as two separate parts, which by some writers have been regarded i '^ 378 IN VERTEBRA TE MORPHOLOG Y. [4 as appendages, though the absence of a corresponding nerve- ganglion tells very stron[,'ly against such an idea. The ante- rior portion of the digestive tract arises in the embryo as an ectodermal invagination, and is fre^iuently lined throughout by a chitinous cuticle. In the higiier forms (Malacostraca) the posterior portion of this foregut is enlarged to form a so- called stomach (Fig. 108, s), in which the chitinous lining thickens to form a co)nplicated arrangement of teeth, Avhicli, moved by special muscles extending frcu the stomach to the walls of the body, serve for the comminution of the fooil. No salivary glainls occur. The midgut is frequently of very small extent, and has usually connected with it a digestive gland (Fig. 108, I) consisting either of from one to four pairs of simple or but slightly branched cciecal tubes, or else of ;i mucJibranched compact gland opening into the intestine 1).\ two or more ducts. The hindgut (i), like the foregut, arises as an ectodermal invagination, and is usually lined with chitin and unprovided with special glands. The nervous system preseniis a typically metameric condi- tion throughout the greater portion of the body, n pair of ganglia occurring in each segment, united by paired connec- tives with the gangUa of the preceding and succeeding metu- nieres (Fig. 108, vn). In the anterior portion of the body, however, as well as posteriorly, a certain amount of conccn- tration and fusion of the various ganglia occu-s. An ideal condition in wldch no fusion has taken place would show a pair of cerebral ganglia (Fig. 101), cr) with which more or less complicaied optic ganglia are connected. From the cevel)r;d ganglia couiu ctives pass backward and unite with a pair of '^auglia {(/), clearly indicated in the embryos of many of tli»; higher forms, but not yet detinitely known in the Entoni.^ straca, though ii seems probable tliat they occur in tli.^' also. The inetaiaere and api)endageo wlu(-h shouhl proiiei U bt^ associated with them seem to have disappeared ; that is to say, they are the sole representatives of a metanie'e mh r- veniiig between the cerebral and antennulary segujents. Thrso ^^an'_^lia are united by !i pair of connectives with a third piiir Mending nerves to the antennuU'fl (if), and these again with a fourtli^pair belonging to the antennary metamere {y*), and so liug uerve- Tlie aute- bryo as au ;lirouglioiit .laoostraca) form a so- lous lining' etli, Avliioli, iiaeli to tlie t' tlie food, itlv of very a digestive ) four pairs >r else of a utestine bv egut, arises witli cliitin Qeric coiuli- -, a pair of I'fcd coiiiicc- ediug luetu- F the body, ; of c'oiu'cii- . Au ideal mid show ;i i»)re or less tho cevebri'd th a pair of inaiiv of tliti he Eut»^iiiO' lur in these lid properly red ; tliat is iiiiufvo iiitcr- ents. Thesis a third pair igaiu with a ; {(/), and so TYPE CRUSTACEA. 379 on, -i. paa- of ganglia occurring in each metamere throughout the body. Such a condition as this is found only in em- bryonic stages, and even there not always perfectly. The ganglia, representing the preanteunulary metamere fuse with the cerebral, as do also the antennularv, and in higher forms the antennary ganglia, there bein^- thus formed a complex a'.._.,p FlO. 169.— DfAOUAM OP Neuvous System ok Ckihi acean. ee = cerehrul giinglion. P'' = second " g' = antemiiiliiry " i/'' - anteiiimry " vin = iimiiilil)uliir " mx\ vix'-' - iiiiixilliiry ganglia. (e — (Bsopliagiis. (>!•■- "jifiivi -n- -» " u • t...ii,.l, ..1 i__ 1 » ri much shortened, if: fuse. Thus in the C or a greater or less nund)er of thoni may rayiisii the ganglia of the three posterior : "■. Iw'W ' J.^ 380 INVERTEBKATE MOItPJIor.OG Y. liead-metameres unite with those of the two iiiiterior thoracic segmeuts to form a single gangliouic mass lying behind the ojsophagus autl sending nerves to the appendages of the somites represented in the fusion. Similarly, in the posterior region of the body of the Isopoda all the ganglia of the abdominal region may fuse to a more or less simple mass (Fig. 170, A, ah), and an extreme condition of fusion is to be found in some Crabs (Fig. 170, B), in which all the ganglia behind the antennary segment fuse to a single mass {i(d)), lying in the thorax — a condition standing in relation to the reduction of the abdomen and the extensive concentration of the head and thoracic regions which are characteristic of these forms. A sympathetic nervous system seems to be generally present, consisting in its most complete condition of an un- })aired nerve arising from the syncerebrum and passing buck- wards to be distributed to the stomach, and of a medi.ni nerve (Fig. 170, A, m) extending from one pair of post(es(»])li- ageal ganglia to the other, lying between the two connectives. Sense-organs reach a high degree of develo})ment in the grou]). Hairs occur in abundance on the appendages nnd body, the majority no doubt having nu^rely a mechanical function ; but among them will be found some beneath wliicli lie one or more ganglion-cells, giving rise to a nerve wliicli passes into the hair. These hairs ai'e supposed to be tactile in function. On the autennules of many forms and nioit; rarely upon the antenme, hairs of sjiecial forms occur, usually in bunches or in rows. They may be club-shaped or cylin- drical, iind each has a nerve-libre extending into it without dilating into a ganglion-cell bmeiith its base. To these liairs an olfactt)ry function has been assigned, and it is noticoahlf that they are usually more abundant u})on the antennuli's >>( the males than on those of the females — an arrangeiiiiiit v/hicl» suggests a probable service as guides in finding llif latter. Eyea are very generally ])resent in the Crustacea, ami reach usually a hi^h degree of etliciency. Two forms of cm' are known a metlian unpaired t)ne, frecpiently spoken of ;ih the simi)lo eye, anil the lateral or comjiound eyes. The iia- TYPE CRUSTACEA. 381 lYAiveA eye is present in the larval stages of probably all Crustacea, and persists iu a more or less perfect form in the adults of most Entoniostraca,— a group wJiich contains the more primiti.-^ forms,— and has even been detected in those of some of the higher forms (e.g. Cmngon). It consists when well developed of three patches of pigment, forming cups iu each of which lies a group of clear cells from whi di nerve- libres arise passing to the optic nerve. The lateral eyes are composed of a number of units each of which possesses all thii parts of a visual organ and is termed an ommatidium, and consequent- ly these eyes have been regarded as an aggregation of a number of individual eyes, whence the term compound usually applied to them. Each ommatidium is a complicated structure consisting of several parts (Fig. 17i). The outermost layer of each is a transparent cornea which is continuous with the general cuticle of the body, and in some forms is only distinguished horn this by its trjuisparency. More frequently, how- ever, this cuticle becomes more or less porfectl}' divided into a series of corneas of an hexagonal or tetragonal shape, one correspt)nding .o each ommatidium, 1.1 i« e ,^ It . . CltrsTACKAN the surtace ot the eye thus acquiring j^^j^^ a faceted appearance. Below the cuti- c = cone-cell, ch' come (he hypodernial cells (^7/) ^'//= corneal liypodcimis. which secrete it, arranged irregular) v ^'''''= <'iy^'"lli"<' <<»iif. without reference to the ommatidia in ^J'J^ ''i^'"' •'■';>•"'"• the simpler non-faceted eyes, but m Uh ^ iiiabdom. the faceted eyes with two hypodernial cell.--, lying beneatli each cornea and constituting the corneal hvpodermis. IJelow these come the cone cells (T), two to four in niunber as a rule ; these are elongated cells a portion of wliose pro- toplasm becomes converted into a refractive translucent bodv. Fig. 171. DrAOlJAM OF O.MM ATI- le CI ystall ine cone {('(n, comoosec I of as maiiv seijments a> !KK: 53 i f52 there are cone-colls taking part iu its formation, and sur- 1 382 INVEUTKBliATE MOliPIIOLOG 7. rounded upou the outside by a delicate layer of protoplasm placing the part of each cell above the cone in continuity with the part lying below it. In the higher forms there occur, partially surrounding the cone-cells, two pigmented cells which seem to be sensory in function and are termed the distal retiuular cells {DR). They are, however, unrepresented in the lower forms, in which the sensory portion or retinula is represented by a single circle of usually 5 cells {PR) lying l)roximally to the cone-cells and surrounding a chitinous rod which is manufactured as a secretion from their approximated surfaces, and is termed the rhabdom {Uh). These cells are also pigmented and are prolonged below into uerve-libres, which, piercing the basement-membrane u])on which the om- matidia rest, pass to the optic ganglia. In the higher Crus- tacea, in which a distal retinula is present, the rhabdom is formed by a circle of eight cells (one of which is almost aborted, so that there appear to be only seven). These con- stitute the proximal retinula, and appear to correspond to the single retinula of the simpler forms. Finally, a number of accessory cells, usually pigmented, may surround each oni- matidium, separating it from its neighbors, but not appearing to be essential constituents of the e^'e. The view according to wliich those lateral eyes are regarded as an ag- gregation of a nurnbor of indepemlent eyes has already been referred to. It seems qiiestional)le, Ijowever, if tliis be tiie correct interpretation of tiiem in view of tiie occnrrencc of so-called compound eyes in tiie Mollusca (Area) and tlie Polycha'tous Ainielida. It seems more probal)le that, as in these forms, the Ci'iistacean eye is to be regarded as a separation into a number of more or less isolated parts of an originally continuous retina, a corresponding division of tin* originally simple refra(;tive apparatus also taking place. This view seems to harmonize «iost satisfactorily with the facts of development. Occasional de})artures from the usual arrangement of the eyes are to be found — as for instance in Phronhna, one of tlio Amphi})oda, in which two pairs of compound eyes occur on the head. Mention may also be made hero of the peculiav eyelike structures occurring in /'jiplnnisia, one of the Schizo ])oda. They occur on the basal joints of the aj)})eudages of certain of the thoracic metameres, as well as upon the ventral TYPE CRUSTACEA. 383 otoplasm uity with :e occur, ted cells •raed the )reseuted iliuula is 'A>) lying uous rod :3ximated cells are ^'e-fibres, I the oni- ler Crus- ibdom is s almost liese cou- ud to the amber of 3ach om- ppeariug [ as an ag- •eferrod to. retatioii of le Molliisca )1(' that, ii.s tioii into 11 loiis retina, appanitiis itorily Willi iiit of the ne of the occur on I peculiar e Schizo- jdagrs of le vi'iitral surface of the abdomen, and appear to be rather phospho- rescent than optic organs. Otocysts occur throughout the group Decapoda, to which the Crayfish, Lobster, and Crab belong, and consist of sacs lined by sensory seta3 and containing otoliths. They aro situated on the basal joint of each of the antennules and in some forms are completely closed, though usually their cavity communicates with the exterior, being guarded by a number of closely-approximated bristles. In the Schizopoda similar otocysts occur in the endopodite of the last pair of abdominal api)endages, and iu the Amphipod Oxycephalm two lie above the syncerebrum. These structures, which are usually spoken of as auditory organs, seem to be rather sense-organs of equi- librium. In the larva) of many forms and in the adults of some Eutomostraca one or two papilla-like processes project from the anterior surface of the head and are supposed to be sen- sory in function, though what purpose they may subserve is unkuovv^n. Strong nerves pass to these frontal sense-organs which appear to be of considerable importance. The excretory system consists of two pairs of nephridia, one or other of which may be absent in many forms. One of these develops in connection with the antennary segment and opens to the exterior on the basal joint of the antenna) (Fig. 108, ne), whence it is known as the antennary gland, some- times, however, receiving the name of the green gland. It reaches its highest devel()])ment among the Malacosiraca, occurring iu many Eutomostraca only iu larval stages, later on degenerating. In its simplest condition it consists of a coiled tube whose lunien a])pears in souie cases to be intra- cellular, though iu others it is undoubtedly intercellular, and which terminates internally in a saclike dilatation whose wall is richly supplied with blood-lacuna-. In the higher xornis (Fig. 172, A) a great complexity is brought about by *; ■ development of lateral branches from the tubular portion, and the terminal sac {s) may enlarge and fuse with that of the oj)po8ite side to form a cavity of considerable size lying in the anterior portion of the thorax and termed the nophro- l)eritoiieal Hac. 384 INTERTEBliATE MORPHOLOGY. fir n The second uephridium (Fig. 172, B) develops in couuec tion with the secoud maxillary segment, and opens usually upon the appendage of that segment. It is especially devel- oped in the Entoniostraca, in which it may lie in the folds of the body-wall which form the shell, and hence is usually known as the shell-gland. It occurs also in the larval stages of many Malacostraca, and may possibly persist in a degen- erated condition in the adults of some forms. In structure it sa4 — til Fig. 172.—^, Diagham of Nephuidium (Gueen-gland) op Aatacus (after Marchal); B, SiiELL-GiiANO «)P EuUmnndia. a = teimiiml sac. sa = saccule. resembles closely the antennary gland, but does not present tlie complexity frequently found in that gland. The majority of the Crustacea are bisexual, hermaphro- ditism occurring only in forms wliich have a ])arasitic habit and in some which are sessile in adult life (Cirrhipedia). The ovaries or testes (Figs. 173, A and B) are paired orgaus lying alongside of the intestine or slightly dorsal to that orguii^ transverse connecting bars in some cases passing from tlic organ f)f one side to that of the oth(>r. Each organ mav h.' regarded as a tube, sometimes simple, sometimes branchtMJ, and lined on its interior by an epitludium which gives ris(> to the germ-colls. Speciivl germ-producing regions are fr(>- quently developed, as, for instance, at the extremities of the tubes or ahmg one side (Isopoda), the cells in otlior regions ceasing to give rise to ova or S})ormat()zoa. The reproductiv(> elements pass to tlie exterior hy s])ecial ducts, oviducts {od) [n couuec- is usually illy devel- e folds of s usually i^al stages 1 a clegeii- ructure it TYPE CRUSTACEA. 385 itacus (after t preseut rniapliro- itic habit ia). The aus lying at oi'gaii, from the 1 luay 1)1' ) ranch I'd, ;ives rise are fre- es of the •r regions rod active lucts {(kI) or vasa deferentia {vd), connected with each organ, and open- ing usually upon the ventral surface cf the body at or near the junction of the thoracic and abdomiural regions. The origin of these ducts has not yet been discovered, but it has been suggested that they may represent a third pair of ne- phridia. Accessory structures, such as receptacula semiuis and cement-glands, for the attachment of the ova in the fe- males, and spermatophore-sacs, in which the spermatozoa are encapsuled in spermatophores in the males, are frequently de- ov Fm. 173.— 4, OvAHY, AND B, Testis ok Mysis (after sars). oil — oviduct. tb = transverse bar of ovary. ov = ovary. te = testis. vd = vas deferens. veloped in connection Avith the ducts, and in the Malacostraca certain of the appendages in the neighborhood of the genital openings are, especially in the males, moditied so as to serve as copulatory organs. Owing to the great variety of form and structure met Avith in the various species of Crustacea the grouj) is sfc])ara- ble into a large number of subdivisions. Two principal classes are, however, readily discernible, of which the tirst is I. Class Eiitoiiiostraea. In this class the number of segments (»f which the body is composed varies greatly in the various groups and even in closely-related genera. The abdominal region is in some forms very mucli abreviatetl and is destitute of appendages, a nile which, however, finds exception in certain Plvllopods iu s .■"::3i3 386 INVERTEBRATE MORPHOLOGY. ma ¥• ^i which some of the segments behind the genital openings, which may be taken as indicating the line of separation be- tween the two regions, are provided with appendages. Folds arising from the head region and forming either a carapace or a bivalved shell are frequently present and the animals are for the most part small, the largest reaching a length of about €ight centimetres, while the majority measure less than a milli- metre. The unpaired eye usually persists in the adult, as does also the shell-gland, the antennary gland, on the other handj being usually rudimentary or absent. A masticatory stomach is never present, and a further characteristic is found in the fact that the larva which hatches from the egg is almost invariably a Nauplius (see p. 417). 1. Order Phyllopoda. The Phyllopoda are principally confined to fresh water, the genus Artemia, however, being found in salt lakes, while a few Cladocera are marine. They seem to be the most primitive of all the Crustacea and present the greatest variation in the number of metameres composing the body, some spe- cies possessing over forty pairs of appendages, while in others again the number is reduced to nine. All the thoracic ap- pendages, however, as a rule bear branchial lobes, and in some oases (Apus) present the mauy-lobed and imperfectly-jointed condition which has been considered the most primitive form of the Crustacean limb (see p. 373). The antennules are usually small and abundantly provided with olfactory hairs, while the antenna (except in Apus, in which they entirely disappear) are long and serve as locomotor organs. The mandibles are reduced to simple masticatory plates without palps, and the maxilliB undergo likewise considerable reduction. A heart is always present, but no blood-vessels exist, the blood passing from the heart into lacunar spaces. 1. Suborder Bra?ichiopoda. The Branchiopoda have all a plainly-segmented body con- sisting of many segments, and, with the exception of Branehi. pus and Artemia, are provided with a fold of the body-wall TYPE CJiUSTACEA. 387 •penings, Ltion be- . Folds carapace mals are of about u a inilli- adult, as be otber sticatory is found is almost li water, is, wbile le most ariatiou me spe- i otbers acic ap- iu some -joiuted form of usually s, wbile appear) bles are and tbe heart is passiug dy con- iy-wall wbicli may form a dorsal carapace, as iu Apus, or a bivalved sbell, as in Limnadia, Limnetis, and Estheria (Fig. 174), an adductor muscle being developed for tbe closure of tbe sbell witbiu wliicb tbe entire body may be witbdrawn. Tbe anten- nules are as a rule small and are provided witb olfactory bairs; tbe antenna?, on tbe otber band, are well developed ex- cept in Apus, in wliicb tbey are in some species quite small and in otbers entirely wanting. In tbe sbelled forms tbey are biramous, consisting of a several-jointed protopodite termi- nated bj' two many-jointed fiagella, and serve as oarlike loco- motor organs, but in Branchipus tbey are sbort strong struc- FiG. 174. — Estheria com'ph.rimanus (after Packard). a<' = auteuiiule. at- — iUilLiiiiii. m = slioll-jniiscle. tures witbout any locomotor function, serving in tbe males as cbasping organs of use in copulation. Tbe mandibles are reduced to tootbed plates, lacking a palp, and tbe first max- illiP sliow an almost similar reduction, wbile tbe second are entirely wanting in some genera, sucli as Limnetis. The suc- ceeding appendages are not limited to the thoracic region of the body, taking tbe genital opening as tbe limit between the two regions. Tbus in xipus cancri/orniis there are eleven thoracic append.ages, while bebind tbe genital ring tliere are no less tban over bfty locomotor limbs, and in sucb forms as Limnetis and Estheria (Fig. IT-l) it is difficult to distinguisb between tbe tborax and the anterior abdominal segments. Tbe beart of the Branchiopoda is a more ox less elongated organ witb several ostia and is usuully limited to the anterior portion of tbe thoracic cavity, thougb in Branchipus it extends 8S: 2'l 12 :« 5 * 388 INVERTEBRATE MORPHOLOGY. into the anterior abdominal region. Lateral eyes are present in addition to the unpaired median eye. In Branchipm they are situated upon the sides of the head upon well-detined stalks, but in Apiis they are closely approximated on the dorsal surface of the cephalo-thoracic carapace, while in the shelled forms they are united together to form a single eye whose double nature is revealed only by a study of the details in its arrangement. A peculiar feature in the life-history of the members of this group is the €ompai'utive iufrequency of males, their proportion to females being so small that for some time they were not known to exist. The females are able to reproduce parthenogenetically— males appearing only under certain conditions which are not as yet satisfactorily understood. The eggs de- velop generally in brood-pouches situated upon certain of the thoracic ap- pendages {Apus, Limnadia) or else are affixed to fiiamentar processes of these appendages (Estheria). 2. Suborder Cladocera. The Cladocera are distinguished from the Brauchiopoda by the segmei;tati(jn of the body being much less clearly defined and by the small and more definite number of appenda'-^es, there being only from four to six pairs of thoracic limbs. A bivalved shell arising from the maxillary segments and pro- vided with an adductor muscle is always present ; it does not enclose the head, but the rest of the body may be completely withdrawn within it except in some genera, such as Ji'vadne and Polyphemus, in which it is transformed into a brood- chamber, leaving the body almost unprotected. The antennules are always small unjointed structures pro- vided with a bunch of olfactory hairs usually terminal in po- sition, and the antenuje are strong biramous locomotor organs. The mandibles are simple toothed plates without palps, and the second maxillae are usually entirely wanting in the adults. The tlioracic limbs are six in number in the genus Sida and are all lamellate and abundantly supplied with marginal setas but in Daphiia (Fig. 175), Moina, and allied forms the number is reduced to five, and the more anterior ones are more or less modified tovr'ards simple cylindrical jointed appendages, a condition found in all the four thoracic appendages of Evadne TYPE CRUSTACEA. 389 J present pv^ they l-deliiied on the le iu the Dgle eye e details roup is the s being so imales are ler certain le eggs de- loracic ap- ocesses of ipoda by ' defiued 3iiduit. er forms, a undergo space in- ace of the itated, the dapted to >cera, es- show an nerations pearance. ttle yolk, aud this lie coudi- sfactory ; * summer le coudi- hable by ind their r the de- ;es wliicli id at tln^ larger in siderable )parently } a thick to render the ma- ternal shell is sloughed and serves as a protecting case ^ut more usually, as in Daphnia, Moimt, aud others, a sac"' c- shaped thickening, the eplnpnimn, appears on the dorsal wall .if the brood-pouch at the time of the passage of the winter egg into it, and this thickening is thrown off with the egg and forms a protective covering for it. 3. Order Ostracoda. The Ostracoda resemble the Cladocera in the segmentation of the body being but slightly marked aud in possessii..^ a bivalved shell provided with an adductor muscle. The shell however, encloses the head as well as the thoracic and ub-' domiual regions, and furthermore but two thoracic limba exist. The auteuuules and autennro are both uniramous append- ages and serve for creeping, though the former are also pro- vided with olfactory hairs. The maudible consists of a tooth- bearing plate and a strong jointed palp which in some forms also functions as a creeping limb, and behind it are two well- developed maxilhB. The first of these is distinguished bv the development of the jaw portion and the reduction of the palii and in Cypris and Cijthere bears a large plate with nume' ,)us' marginal setre which is usually termed a branchial lobe. The second maxilla, on the other hand, shows considerable modi- ficatiou in different genera. In Cypridma (Fig. 176, .l/c') it is- jawlike and bears a large branchial lobe (wanting on the first pair), and in Cypris is adapted for the same function, but bears m addition to the rudimentary branchial lobe a short two- jointed palp, which in Haloc,/pris becomes enlarged to form a three- or four-jointed limb, while finally in Cythere the append- age is practically a walking limb, its jaw function not being, developed. The tirst thoracic appendage is an elongated mauy-jointed limb except in Cypridina (Fig. 170, t'), where it possesses a jaw function, and tlie second is also limblike. In I/alocypris this latter appendage is, however, rudimentary, aud in Cypris and (),pridma (Fig. 17(), T') it is dorsally directed and serves for cleansing the inner surface of the shell from J-ieigu bodies, in the latter genus arising some distance ui> Sit SIfS;: ■^ 'i 892 INVEIiTEBltA TE MOliPIIOLOO Y. upou the sides of the body aud forming a long cylindrical uiijointed appendage. Eespiration is usually effected by the general surface of the body and the inner walls of the shell duplicature, thou<,di in certain Cypridinidre a double row of respiratory processes are situated upon the dorsal surface of the body near the second thoracic appendage. The so-called branchial lobes on the inaxilhe probably sr.bserve the respiratory function only by renewing the water in contact with the body surface. A oOc H Fig. nQ.—Ci/priiUna mediterranea, Female (after Claub). At^ = auteiiiiule. A(^ = iiiiteuuu. h = heart Mnp = niiiutlibiilar pulp. Mx\ Mx' = tiist (lud suc'outl maxilla. 0 = simple eye, Oc — roinpomid eye. Pr — fioiitiil orgai). 8m = sliLll-rnu.scle. 0, T* = first and second thoracic appendages. single mediun eye alone is ])resent in Cypru and Cytherc, l)iit in addition a pair of lateral compound eyes occurs in ('i/pr!- dina. The frontal sense-organ is a single strong process, in certain forms lying slightly al)()ve and betwec the antennults. A heart is ])res(Mit in Cifpridimi and Ihilorypris as a sacliko organ with two lat«^ral ostia and is not j)r()longed into arterios. In (hjprifi and i'ythere it is entirely wanting. The Ostracoda occur both in fresh water and in the ocefiii. The genus CypriH and itn allies are for the most part atjuati*-, wliih^ the other genera mentioned are exclusively marine. cylindrical surface of ire, tliouf,di y processes ly near the al lobes on iction only lurface. A TYPE CRUSTACEA. 393 lU8). ^e. !oik1 tliomclc 'ijtherc, ])iit 1 iu Cypri- jrocess, ill mteunult's. H a sacliko to arteries. the ocBiiii. rt aqiiatio, lariue. 3. Order Copepoda. The members of the order Copepoda present great varia- tions in form, due to the fact that there are a number of para- sitic forms belonging to it some of which show so much de- generation that their relationships to the non-parasitic forms only become apparent by a study of their development. Typically, however, the body is generally elongated (Fig. 177) and consists of ten segments in addition to those of the head, the five anterior ones usually bearing appendages and con- stituting the thorax, while the live posterior lack appendages and form the abdomen, the terminal segment of which bears a pair of cau lal inrcm provided with setfe. la female indi- viduals the tv T anterior abdominal segments fuse together to form a genital doable segment, and in all cases the head seg- ments fuse together, while the anterior thoracic segment usually fuses with this consolidated mass. Xo shell-duplica- ture occurs. In the parasitic iovmn there h a tendency for the various segments to beco)>\e indistinct and all trace t)f them may vanish, the abdomen in some cases becoming also extremely reduced in size. Add to this that lobes and pro- cesses are frequently developed upon the body and it vill be understood lunv far these degenerate forms depart from the typical arrangement. The antennules (Fig. 177, af) in all free-swimming Cope- poda form long nniny-jointed swimming-organs used in an oarlike manner. They consist of a certain nund)er of stout basal joints, terminated by a single hmg multiarticulate Hagellum, no trace of a biramous condition being apjiarent. In addition to their locomotor function they also, as in otlier forms, serve as sense-organs, olfactory hairs being scattered along the flagellum, and in male individuals they are si)eciallv iiioditied to form c]as,)ing organs for use in copulation. The uiiteunie {nf) are much smaller and are frecimmtly biramotis, and the mandible {mn) has usually a palp, wiiile the first niaxilh© (wx'), bearing strong masticatory bristles on their biiSiU joints, also show more or loss iiidicatiou of a biraiuous condition. The second raaxillie (mx*), so^uetimes termed the mi',xillipeds, have generally been regarded as representing a ■lk« Slls 394 INVERTEBRA TE MORPIIOLOO T. mx' - mx' single imir of appendages whose exopodites and endopodites liaA'e separated from each ^Jj ., other. It is now known, how- ever, that they really repre- jj^a sent two pairs of appendages which have lost their exopo- ™° dites and have been approxi- mated. The anterior pair are small plates provided with numerous masticatory setro ou their inner edges, and are the true second maxilhc, while the posterior pair, elongated and limblike, are really thorii- cic limbs and so may l)o termed maxillipeds. Tli(> other thoracic appendages (<'-<') are typically biramous and serve for swimming. This description refers to the free-swimming forms ; in parasitic species much modi- fication of the a[»pen(higt's ensu(>s. The antennuhis lose their long oarlike character and may even be degenerated to strong hooks wliich serve to fasten the animal to its host, a degeneration whieh the antennje may also under- go. The mouth-parts beconu' Ml.—Calanua hyperboreus (after adapted to a piercing func- tion, and the mandibles mk^ represented by sharp stylet- like structures, sometimes eii- closed in a tube formed by the lips, a sucking-organ being thus produced. The tirst nuixilho undergo consideralde Fio (JlKHDHKCHT). (ui, = iinus. aC — aiiU liiiiilti. af = iintennii. mn - intuidililu. mx\ nix' = first mid sec-oiid nmxilla' <'-<' = ihomcic uppoiidiigcs. TYPE CRUSTACEA. 395 Qdopodites rom each lowu, how- illy repre- ppeudages leir exopo- n approxi- or pair are ided with )ry setro on ud arc the Ihp, while e hi 11 gated sally thoru- > may l)e Is. T]i(> [)peiidages biraiuous luing. 1 refers to forms ; in inch modi- ;»l)eiula}j;es imiles lose churact«n' egeiierntcd liich serve lual to its ioii whith lIho uudei- rts become reliig fiiiic- idibles jiiti arp stylet- iietimes eii- med bv the •J •gaii beiiij^ iiiHiderahle reduction, while the second pair is frequently adapted to form organs for adhering to the host, and finally the thoracic appendages may undergo various stages of degeneration, iu some forms entirely disapjjearing. Branchial organs are entirely wanting throughout the order, respiration taking place over the entire body surface. A heart is present in a few forms (Calani(he) consisting of a saclike organ with but a single pair of ostia, but in the majority of cases it is wanting. A single median eye is gen- erally present, and in a few forms, Pontella, Corycceus, and ArgidiiH, lateral eyes are also present, though absent as a rule throughout the group. Each lateral eye in Corycanis consists of a single onimatidium, but in Argnliis is compound and similar to the lateral eyes of the Brauchiopoda. The Copepoda are throughout bisexual even in the cases of the parasitic forms. The vasa defereutia are provided with an enlargement in which the spermatozoa are included within a capsule, forming a spermatophore which during copulation is dei)()sited in the neighborhood of the female genital open- ing. The spermatozoa being discharged from the spermato- ])liore-capsule, by a special discharging apparntus with which it is provided, make their way into a receptaculum seminis which communicates with each oviduct, the ova being fertilized iluring their passage to the exterior. These are usually carried iu one or two masses attached to the first abdominal .segment of the female, though in some forms, such as NofotieJ. /)////.s', they undergo their development in a brood-chamber formed by the duplication of the integument of the dorsal surfaces of the fourtii and fifth thoracic segments. A peculiar dimorphism of the sexes occurs in some of the most highly iiiodihed parasites, such as Chondvacanthun, Achtheros, and others, the male being very much smaller than the female Jind showing much less degradation, frecpieiitly presenting Avrll-developed eyes and more or less perfectlv-developed .■ili|)endagef;, «o that it is able to lead for a time a free exist. I'lii-e, It is to be regarded as a larval stage sexually mature, sincir it resembles clo.sely the female when in the sbige immedi- ately before fixation to its host, tiie greater ])art of the (leirpn. e after that has been acco ■lir sib:; ■51 1' Two suborders are recognizable. ipliubt m 396 INVERTEBRATE MORPHOLOGY. 1. Suborder Eucopepoda. This suborder iucluJes the majority of the Copepoda, and its members are characterized by having only tlie first thoracic segment fused with the head and by possessing usually a well-developed abdomen. Many are free-swimming, some in- habiting fresh water, as Cyclops and Canthocamptus, while others are more especially marine, such as Ilarpacticus, Valamis nix« Fig. 178.—^, Philirhthyfi xiphin seen from the nonsAt, Surface (nfi.T C'LAfs); li, Acfilfieres percarum (fiom Bkonn). (Fig. 177), find Cetochihis, the latter sometimes occurring in enormous schools, and forming an important food-supply for fish and the baleen whales. Some, on the other hand, lead a commensalistic life, occurring in the branchial chamber of Tunicates, e.g. Noiixh'Jphijs, while a large number of forms are ])arasitic. The degree of parasitism varies greatly in dirtercnt forms ; thus many are ca))ablo of free existence, beconiini^ parasitic only occasionally, such naCori/avm and tlie brilliantlv- c(jlored Sttpphin'uu, while others, such as h)y(i.sihis, ])arasitic on the gills of fislies, and Cnh'yus and Pduddru/i, though essou- epoda, and rst thoracic 5 usually a ig, some iu- ptus, while 'ius, Calamis TYPE CHUSTACEA. 397 lUnFACE (nfi.T .^curriug in -supply for aud, lead a jhamlxM' of f forms arc in dirteroiit , becoiniiii,' brilHaiitly- iH, ])arasitic rnigh essen- tially parasitic, still retain more or less perfectly the segmen- tation and general appearance of free-swimming forms, the modifications which they have undergone affecting principally the antennae, which are modified for purposes of adhesion to the host, the mandibles, which are piercing organs, and in some cases the maxillre, which may, like the antennjB, become hook- like. Frequently, however, the body assumes aberrant forms, as in Philichthys (Fig. 178, A), aud the segmentation may en- tirely disappear, as in Penella, Lerncea, Chondr acanthus, Achtheres (Fig. 178, /?), and Anchorella, these last two forms presenting a peculiar modification of ...le second maxilhe in the females, the two appendages fusing at their tips to form a chitinous adhesive disk which serves as an organ of adhesion. In the majority of these forms, as already noted, the thoracic appendages may become more or less rudimentary; indeed even in the less modified forms, such as Erqamlu.s, the appendages of the fifth thora- ic segment may be wanting. nix 3. Suborder Branclnura. In the Branchiura the cephalic and thoracic seg- ments are fused together to form a shield-shaped ce})halo- tliorax, while the abdomen is small and divided into two platelike halves which have a rich blood-suppl}', appar- ^^^- '^"^^■—Arpnlim foliaceua (after ontly serving resi)iratory ])ur-. „<, ^ amrnllles. poses, and in the males l = di>resiive glaud. contain the testes. wa; = second muxilla. The basal joint of the an- '"'= '*>'''• tennules (Fig. i7(>, at') is devel- ' " ^''"''• oped into a strong hooked process, aud the mandibles and 12 -SI tirst maxillo), which are stylet-like, are enclosed lU a tube } I 398 INYERTEBRATE MORPHOLOGY. iormed by the fusion of the upper aud lower lips. The second maxillse {mx) develop at their bases large suckers, while the first thoracic ai)pendages, here termed maxilli- peds, are limblike aud have also hooked processes upon the basal joints. These are succeeded by four pairs of biramous swimming appendages. A well-developed heart is present, giving rise to arteries extending throughout the length of the body. A pair of lateral compound eyes {oc) are also present, and a further difterence from the majority of the Eucopepoda lies in the fact that the eggs are not carried by the female, but are de- posited on foreign bodies. All the forms are parasitic, in some cases, as Argulus, upon fresh-water fishes, but they also possess the power of swimming actively. 4. Order Girrhipedia. The Girrhipedia or Barnacles are without exception ma- rine forms, and in the adult condition either adhere to foreign P. Fig. 180.— Cypuis Larva of Lepas (a.ner Cumb). Ah — abdoiiR'n. Oc = eye. AV = iiutennuk's. Ov = ovary. Cg = duct, of cement-gland. p = penis. 0 = opeuinjr of oviduct. 2''' = third tborncic foot, bodies, leading a ])erfectly sessile life, or olse bore in the slioll^^ of certain Mollusca, or finally are parasitic. It will be con- venient to describe first of all the organization of the HcsHilc and boring forms, later considtu-ing briefiy the parasitic forms which show many peculiarities due to degeneration, course of de^ [•ing 'P ipe( P through a larval stage similar in general appearance to .in TYPE CHUSTACEA. 399 ips. The ;e suckers, d maxilli- 3 upon the t biramous to arteries A pair of a further ies iu the )ut are cle- s Argulm, 5 power of sption ma- to foreign foot. the she]l>< 11 be con- he HCS.Hilc iitic forms ia all pass lice to iiii OstracoJe and hence termed the Cypris-stage (Fig. 180). The body is enclosed in extensive folds of the body-wall termed the mantle, and the antenuules {at') are characterized by being directed forwards and terminating in an adhesive *, cir) which contains the terminal })ortions of the vasa defereiitia. The mantle-folds which occur in the Cypris-larva persist in the adult, and calcification of their walls takes place, giving lise to a calcareous shell, composed of several pieces, which encloses the animal. In the genus Lepus, the goose-barnacle, this shell consists of five ])ieces. On the dorsal side there is a single unpaired piece which receives the name of the carhia (Fig. 181, ^1, C(i)') ; at the sides and resting below cm the uppar- ::» "'"1 f" '-•Si ' .If— •- .■..3iCJ '^'"^ -'35 400 INVEHTEBRATE MORPHOLOGY. most part of the stalk are the two acuta (sc), while above these are the terga {te), also paired, the opening into the in- terior lying between the terga and the scuta of opposite sides. In Scalpellmn between the two scuta a sixth, unpaired, piece, the rostrum, is inserted, and in the same genus between the scuta, terga, and carina and the summit of the stalk small accessory pieces occur ; and if one imagines a disappearance Fig. \?>1.—Lepas fascicular is. A, exterior; " "•'icture. ag = antemiary gland. Car = cariim. Cir = cirrus M — shell -muscle. Od = oviduct. Ov — ovary pe = peduncle. Sc = scutum. t = testis. Te — tergum. Vd = vus deferens. of the stalk of such a form, an enlargement of these accessory pieces, usually six in number, and their articulation to form a wall-like circle around the body of the animal, the scuta and terga closing it in and forming as it were a roof, an idea of the arrangement of the shell of Balanus, the acorn-barnacle, will be obtained. No special respiratory organs exist, the entire surface of the body probably performing this function, nor does a heart seem to occur in any member of the group. The nervous TYPE CRUSTACEA, 401 die aboA'e bo the iu- site sides, ed, piece, tweeu tlie alk small )pearauce accessory a to form the scuta f, au idea •baruaele, urface of 3S a heart } uervouii system consists in Lepas of a syncerebrum and five or six ventral ganglia, — of which the last is probably composed of at least two fused ganglia, and a certain amount of fusion has also probably occurred in the first. In Balamis the fusion has reached its greatest extent, the entire ventral chain of ganglia having fused to a ringle mass. The median unpaired eye is usually represented, and in some forms rudimentary lateral eyes are present, showing, however, a marked degen- eration from the large compound eyes which occur in the Cypris-like larva. As a rule the Cirrhipedia are hermaphrodite in accord- ance with their sessile or parasitic life. The testes (Fig. 181, B, t) lie one on each side of the digestive tract, and the vasa deferentia {vd) after dilating into seminal vesicles pass to the long cirrus (cir) borne by the abdomen, at the tip of which they open by a short common duct. The ovaries lie in Lepas (Fig. 181, B, ov) in the stalk, and in stalkless forms, such as BaUinus, in the basal fold which corresponds to the stalk, and the oviducts {od) passing upwards and then back- wards open on the basal joints of the anterior thoracic ap- pendages. Although hermaphroditism is the rule throughout the order, yet in some cases small males have been found which have received the name of " complemental " males. These occur in the genus Ihla and in some species of Sccdpel- liim and live like parasites in folds of the mantle of the her- maphrodite forms. In form they do not advance greatly beyond the Cypris stage, and possess in addition to the anten- uules only four pair of small thoracic limbs, the mandibles and maxilLe as well as the mouth being entirely wanting, while the digestive tract is rudimentitry. In other species of Seal- pellupi, and in the genera Alcippe. and Cri/pfopJiialus, these pigmy males are also present, but the forms in which they live are no longer hermaphrodites but females, so that bi- sexuality with sexual dimorphism occurs in these forms, It might be supposed from the u;ener.al occurrence of Ijisexuality ainoiig^ the Crustacea that these last cases represented the first stage in the dis- appearance of the males, leading finally to hermaphroditisiu. Since, how- ever, Alcippe and Cri/pti)phialus are tlie most degenerate of the Cirrhi- peds so far discussed, it would seem that this is not the case, but rather 403 INVERTEBRATE MORPHOLOGY. 1 4 that, on the assumption of a sessile life berraaphroditism became character- istic of the order, the bisexualism of these boring forms l^eing secondarily acquired. The fact that the pigmy males present larval characters sug- gests the idea that their occurrence may be an extreme case of proterandry. If in the hermaphrodite forms it is a rule that the spermatozoa mature earlier than the ova, thus preventing self-fertilization, it is conceivable that this early maturation of the testes might be carried back almost to the Cypris stage and pigmy males be thus developed. Not uufrequeiitlj barnacles choose the bodies of other animals upon which to fasten, as for instance upon the cara- pace of Limulus, or on the skin of whales, and the genus Anelasma fastens itself upon the surface of the body of a Shark, its stalk penetrating into the tissues and developing rootlike processes and so enabling it to lead a parasitic life. As a result of this the calcareous plates cease to develop, the niantle having merely a leathery consistency and the mandibles and maxillcB remain rudimentary. This degenera- tion is carried still further in Proteolepas (Fig. 182), which lives as a jJarasite in the mantle-cavity of other Oir- rhipeds and has a maggotlike appearance, the body being distinctly divided into eleven segments and lacking all traces of a mantle. The mouth-parts are modified so as to be suctorial, and the thoracic feet are entirely wanting, while the digestive tract becomes rudimentary. Finally, a group of forms, known as the liJdzocepJiala, fasten themselves to the abdomen of crabs and become transformed into cylindrical or saclike structures entirely destitute of digestive tract and appendages, rootlike processes arising from the anterior end of the body and traversing the body of the host, by whose juices the parasite is nourished. The genus SaccuUna consists of an an- terior short cylindrical portion from the extremity of Avhich the rootlike processes arise and which perforates the integu- ment of the host. From the base of this a circular fold arises Fig. 183. — Proteolepas (from Bronn). in = muscle. ov = ovar}'. vs = vesicula seminalis. le character- secondarily racters siig- )roterandry. •zoa mature eivable that nost to the of other the caia- he gemis oclj of a evelopiug isitic life, develop, and the legeuera- 2), which ! ill the her Oir- aggotlike :ly being to eleveii king all e. The dified so and the entirely digestive uieutaiT. if forms, abdomen r saclike •eudages, the body lices the of an au- of which J integu- Id arises TYPE CRUSTACEA. 403 which encloses between its walls and the wall of the body a cavity which serves as a broo'l-pouch and communicates with the exterior by a terminal opening capable of being closed by a sphincter. The body proper contains only the nervous system, reduced to a single ganglion, and the ovaries and the paired testes, as well as a pair of cement-glands connected with the female genital openings. The development of Saccidiiia presents some extraordinary features. It resembles in its early stages the develupment of the other Cirrhipeds and reaches a typical Cypris stage during which it fastens itself by the anteiinuk's to the body of a crab. The tissues of the larva then retract lliemselves from the cuticle, and a remarkable degeneration of the body together with an amputation of the entire thoracic and abdominal regions then ensues, leaving an oval mass of tissue, richly pigmented, attached to the body of the crab by the empty cuticle of the antennules. At the anterior end of this mass a hollow dartlike process arises which is l)uslied forward through the hollow cuticle of the antennules and pierces the body-wall of the host, the parasite apparently flowing then through the dart and so becoming an endoparasite. Within the body of the crab the development of the Saecidina takes place from the apparently undiffer- entiated mass of tissue by which it is represented, and growing rapidly produces an absorption of the ventral integument of the host, which allows the saclike body to protrude to the exterior. It is to be noted that para- sitic Cirrhipeds (Laura) have been found in the stem of a Gorgonian and also in the body-cavity of Echinoderms {Dei id rog aster). These forms show many peculiarities of structure and have been grouped together iu the suborder Ascothoracida. II. Class Malacostraca. The Malacostraca are distinguished from the Entomo- straca by the definiteness throughout the entire class of the number of metameres entering into the composition of the body. The head consists of five segments which are invari- ably fused, and the thorax is composed of eight, of which the anterior one, or indeed all, may unite with the head to form a perfect or imperfect cephalothorax. The abdomen is the onl}' region in Avhich variation of number takes place, and this variation is confined to a single group of forms (Lepto- straca). In these the abdcnnen is composed of eight segments, while in all other forms it possesses only seven, counting iu both these cases the terminal segment which bears the anus •mi 8S» wtuS J:* IBS;; It*-'— 3 r 404 IN VERTEBRA TE MORPHOLOG Y. and is known as the telson. All these segments with the exception of the telson, and in the Leptostraca of the seg- ment immediately in front of it, bear appendages. Folds of the integument forming a eephalothoracic carapace are fre- quently present, but it is rare that a bivalved shell occurs. The stomach is always provided with chitinous tteth and forms an efficient masticatory organ, and lateral eyes are present except in some Cumacea and in some forms belong- ing to other groups which inhabit caves or the depths of the ocean, under which conditions the eyes become rudimentary. The openings of the female reproductive organs are always situated on the basal joints of the appendages of the sixth thoracic segment, and the male openings on the appendages of the eighth segment. The antennary gland is usually well developed, while the shell-gland is either rudimentary or wanting in the adult. Although numerous rather small forms belong to this class, yet on the whole they much surpass in size the Ento- mostraca, some forms even reaching a length of over 50 cm. A few forms, such as Euphamia and Fenams, leave the egg as a Nauplius, but in the majority this stage is passed before hatching, the embryo first leading a free existence at a later Btage in the larval form known as the Zijea, though in some cases hatching may be retarded until later stages, in fact sometimes until the adult form is acquired. I. Subclass Leptostraca. The Leptostraca are exceedingly interesting forms, present- ing similarities to the Entomostraca on the one hand and to the Malacostraca on the other, thus connecting the two classes. They are exclusively marine in habitat and possess a, thin bivalved shell- duplicature which is provided with an adductor muscle and is prolonged in front into an unpaired plate which covers the dorsal surface of the head. The antennules (Fig. 183, at') consist of a three-jointed basal portion bearing in addition to the multiarticulate fiagel- lum a scalelike exopodite, a structure wauling in the anteuutu (af), which otherwise have a similar form. The mandil'os TYPE CRUSTACEA. 405 witli the f the seg- Folds of B are fre- Dccnrs. tteth aud eyes are is beloug- ths of the imeutary. re always the sixth peiidages lally well Biitary or ; to this lie Euto- sr 50 cm. J egg as a d before it a later in Some ;, in fact , pre sen t- d aud to the two 1 possess i with an unpaired e-jointed ,te flagel- antenn.u landil'os bear a palp, as do also the first maxilhe, it being in these latter appendages prolonged into a long slender limblike {mx) structure which is directed dorsally and serves for cleansing the inner surface of the shell. The second maxillfo are biraraous foliate structures, as are also the eight thoracic appendages {t), each of which bears upon its basal joints a platelike epipodite which is respiratory in function. The four anterior abdominal appendages {ah*) are strong biramous swimming-legs, while the two posterior are small aud unira- mous. Behind the last appendage-bearing segment are two sm nix Fig. 1%^.—Nehalia Oeoffroyi, Male (after Clacs). ab* = abdominal appendage. Ji = heart. adr = antennary gland mx = process of first miixilla. ae = antennule. sm = shell-muscle. a(^ = antenna. t = thoracic appendage. te = testis. others without appendages, the terminal one being the telson, the Leptostraca possessing one more metaraere than the rest of the Malacostraca. The heart is an elongated organ extending from the maxillary region as far back as the fourth abdominal seg- meut ; it possesses several ostia, and is prolonged anteriorly iiiid posteriorly into aortre. The antennary gland is present aud a rudimentary shell-gland also persists. The lateral eyes are borne upon short stalks. The group contains but few species, the majority belong- ing to the genus Nehidia (Fig. 183). Mil,. •llir ■»•. 1% ■I? •iS;: ■s. 406 INVERTEBRATE MORPHOLOGY. II. Subclass Thoracostraca. The Thoracostraca are characterized by the occurreuce throughout the group of a well-developed duplicature of the body-wall, arising from the posterior head-segmeuts aud covering iua greater or less uumber of the thoracic segments, constituting what is termed a cara])ace. On the dorsal sur- face it fuses Avith the body-wall, but, at the sides encloses a respiratory chamber in which the branchiic, when present, lie. According as the carapace extends over all or only over the anterior thoracic segments a mf)re or less perfect ce})hal()- thoi-ax is formed, a fusion of the covered thoracic segments with each other and with the head-segments occurring, the abdominal segments remaining in all cases distinct. Branchia>, consisting of bunches of hollow thin-walled processes whose cavities communicate with the lacunar spaces of the body, are borne by certain of the appendages except in the Mysidea'. The lateral eyes except in the Cumacea are stalked and the autennary gland is usually well developed. 1. Order Schizopoda. The carapace in the Schizopoda covers in the entire thorax, but a certain number of the posterior thoracic seg- ments remain ununited with it. The antenuules are biru- mous, as are also the antenuse (Fig. 184), tiie exopodite iu the latter case being represented by a scalelike structure. Tho Liioracic appendages are all similar and are biramous, the endojjodites being limblike structures tip])ed by claws, while the exo])odites are multiarticuhite tiagelhi. In tlie genus A)(phan.si(i the two last ))airs are quitu rudimentary, their brauchia' remaining, however, well develoj^ed. The two anterior pairs iu the genus 7l///.s/.s' have their basal joints en- larged to form jaws and cousecpiently are distinguished as maxilliiKHls, but in Euphnmut. this distinction does not occur. The abdominal a[)[)endages in the females are generally small with tlio exception of the sixth j)air, and in the genus il///.s/v are quite rudimentary. In the mah»s of all genera tliey arc, liowever, well-developed bii-amous swimming-f('(»t, and tln' sixth pair iu both sexes forms witli the telscjn a tail-iiu. TYPE CRUSTACEA. 407 Braucliise are present in 3Iysis only in the form of small epipodiul elevations of the thoracic appendages, and in Siriella as coiled tubular structures on the protopodites of the abdom- inal appendages of the males. In Euphamia, however, they form large ramified bunches attached to the protopodites of the thoracic limbs and are present even on the rudiments of the seventh and eighth pairs ; they are not, however, enclosed Fig. 184. — Mysis relicta (after Sars). hp — biood-poucb. ot = otocyst. within a chamber formed by the lateral portions of the cara- pace, but project freely to the exterior. Otocysts occur in the inner lamelLe of the sixth abdominal appendages (Fig. 184:, ot), and in Kuphansla a number of eye- like phos})horesceut organs occur on the basal joints of the second and seventh thoracic appendages as well as upon the ventral surface of the four anterior abdominal segments. They are spherical in shape and each consists of a cup of cells containing red pigment covered in by a lens. The Schizopoda are essentially marine, though some species of the genus Mysis (Fig. IBl) occur in fresh and brackish water. «!5 tSS;: '5:30 ■?. 2. Order Gumacea. In tliis order the carapace covers only the anterior three or four thoracic segments, five or four of them remaining dis- tinct. The antennules are short and in the male biramous, while the antennie, though in the female almost rndimentarv, may be as long as the entire body. The two anterior thoracic ■ippendages form nnixillipeds, their basal joints serving for niasiicntion while the succeed ling V )like, all hilt the last or three last possessing snuill exopodites. The 408 IN VKHTEBItA TE MORPIIOLOG Y. sixth abdomiual segment bears a pair of biramous ap- pendages with a long single- jointed protopodite, the re- maining segments being in the female destitute of ap- pendages, but in the male the anterior 2 {DiastyUs), 3, or 5 {Campylaspis) segments may bear biramous swimrning-feet. The lateral eyes are never stalked and may be closely approximated or even fused on the dorsal surface of the cephalotliorax. They uro generally composed of but few omniatidia and in some species are entirely wanting. The Cumacea are exclu- sively marine and are more especially characteristic of the colder seas. 3. Order Stomatopoda. As in tln) Cumacea tlir carapace covers oidy some of the anterior thoracic seg- ments, the last three or four remaining distinct, but tlif abdomen, instead of boiiiLr slender, is even stouter tliaii th(i tliorax and ends in a ter- minal tail-tin. The anterior . -, portion of the head, bearint,' Pio. \Hry. —Dtasii/hn sfygtn, Mai,k (oftor ' 1,1/ HARsfroniLANo). the eyes and the two ])itirs «, = (uitciiimlc. «'« = eii(l()|)o(lito. of autenuie, is separated from M.=:llllt(!l)Da. t'JJriL'XODoditL'. , , , .. , .. aft=iil)(lominRlnp ?) = iil)(r()miiml up- «"hI movabie iipoii Ihf ThmI ul cl/i= p.-n(luj?oH. ,„ „,,l'«''"l'ws- the cephulo-thorax, and onlv = cumimcc. Il-l III = thovnilv. ., ' ..,,•" st'giiicnis, the more anterior thoracis' TYPE CliVSTACEA. 409 segment amous ap- >ug siugle- 3, the re- beiug iu ite of ap- e male the is), 3, or ") leuts may imiug-feet. \ are never be closely sven fuseendageH being also biramous and somewhat lamellar swimming-feet. The last pair are especially enlarged and di- rected backwards, forming with the telson the strong tail-fin Bunches of branchial filaments occur upon the outer lamel- 1m> ol the abdominal appendages with the exception of the last ])air. The heart is much elongated, extending from the ante- rior thoracic region as far back as the fifth abdominal seg. i.ient and i)ossessing numerous pairs of rstia. It is prolon-tnl .•u.toriorly and posteriorly into aortro and gives off laterally lu each segment a ])air of avtia-ies. The Stomatopods are all marine and pass through a com- l>lioated tl Kvnm oi ni«iam»»rpii().s«!s during development. S ( I mux principal genera are SquUla (Fig. IHii), Ly.iosqnilla f defy Iks, ome and imp Mix "HA I 410 IN VERTEBRA TE MORPIIOLOG Y. 4. Order Decapoda. lu the Decapods the carapace is well developed, covering in the thorax completely (Fig. 1G2), the segmeutsof that region of the body fusing with ii dorsally, so that a perfect cephaio- thorax is present. The autennules generally possess two terminal multiarticulate flagella, and the antenuie frequently lack the scalelike exopodite which occurs in other groups (e.g., Schizopoda). In the second maxilliTe the exopodite is transformed into a platelike structure which, swinging to and fro, serves to renew the water in the branchial chamber lying between the lateral portions of the carapace and the body- walls. On account of this action this appendage is usually spoken of as the scaphognathite. The three anterior thoracic appendages are maxillipeds, the third one frequently becom- ing almost limblike, a characteristic which distinguishes the live posterior pairs of appendages which are adapted for walk- ing and are hence termed the pereiopods. Tliey lack all traces of exopodites, though usually bearing epipodites and branchiae, and a certain number of the anterior ones are fre- quently chelate, thus serving for the prehension of food. The number of the pereiopods has suggested the name given to the order. The abdominal appendages are sometimes want- ing or very rudimentary, but when present are biramous swim- ming-feet and are hence termed pleopods — a term equally applicable in some other groups. The branchia' lie entirely within the branchial chamber and are developed in connection with the thoracic append- ages. They may be seated U})on the basal joints of tlie ap- pendages (podobranchia), or upon the joint between the ap- pendage and the body-wall (arthrobranchia), or finally upon the body-wall itself (pleurobranchia). All three kinds may occur on the same segment, so that the entire number of gills may be much grej'ier than that of the appendages, amount- ing in the Lobster to no less than twenty in each branchial chamber. The heart is a short saclike organ lying in the thorax and possessing as a rule three ]iairs of ostia, one pair being situ- ated on the dorsal surface, one upon the sides, and the third t)n TYPE CRUSTACEA. 411 the veutral surface. Arteries pass off from both ends of the heart. Otocjsts are always developed iu the basal joiuts of the autenuules. 3. Suborder J/«(?n/ra. Iu the Macrura the abdomeu ts well developed aud usu- ally as loug as the cephalothorax, aud is provided with its full complenieut of appendages, the sixth pair formiug with the telsou a tail-fiu. Exceptious to these arraugeuieuts oc cur ; lu the Heriuit-crabs, Eupagurus, which inhabit the empty .shells of Gasteropod Mol- lusks, the abdomen is gener- ally soft aud uusymmetrical, since it is coiled around the columella of the shell, but terminates in a movable tail- fin which serves, together with the remaining pleopods and the last (and sometimes also the penultimate) pereiopod, wliich is bent dorsally, to re- tain the animal in the shell. The chehB of the anterior pereiopods are generally uu- 0(]ual in size, serving to oc- clude the mouth of the shell, and occasionally the abdomi- Jial a])pendiiges of only one side are develo])ed. In the genus llippa too the abdomeu, though Avith a well-develoi)ed Mild calcified cuticle, it, short, the terminal half being bent M|. under the thorax, the F.o. 187.-J. a vorxo /.^r^yir (n.i«p,e.i condition characteristic of '''"•" Hhooks); n, Eupayurus biru the Crabs being thus n,,. '""'/"""fn'r i-ki-nks). pioachod. In some fo ap- t'.ef.mrth andfiftl as Sorqpstt's and T.uolfi er. aent, but more usually all th 1 peren)pods may be rudimentarv or even ab- •on.. i^^S'" 5 eso appendages are well dovel- 412 IN VERTEBRA TE MORPHOL 0 G Y. oped, the anterior ones becoming chelate. Id the Crayfish, Carnbarus, and the Lobster, Homarus, the first pereiopod is an exceedingly strong chela, and the same arrangement is found in Alpheus, while in the Shrimp, Pulcpmonetes, the second pe- reiopod is somewhat longer than the first. The branchiie are usually numerous and are for the most part bunches of cylindrical processes, but in Palo'inonetes and the jal-.' "•^is 'xiid prawns in general, which form the family Ca- rididc n the Hermit-crabs they are lamellate. In Lucifer branclu;.. are entir3ly wanting. The Macrura are essentially marine, a few forms, such as Camharus and some species of Palwrnon, occurring in fresh water. The genus Birgus, one of the Hermit-crabs, commonly known as the robber-crab, is almost entirely terrestrial, living in holes in the ground and climbing cocoa-nut palms for the sake of the nuts, on which it lives. In harmony with its terrestrial life the inner surhice of the branchial chamber is thrown into folds richly supplied with blood-lacunae, a lunglike structure, recalling the lungs of the Pulmonate Gasteropods, being thus developed. 3. Suborder Bracliyura. In the Brachyura the body is exceedingly compact, the abdomen being very much reduced in size and usually desti- tute of a tail-fin, and in addition is bent up so as to lie in a groove upon the ventral surface of the cephalothorax. In some casts the cephalothorax is almost glo- Ituhii', though prolonged anterior- ly into a strong rostral spine, as in Ijhhi'm, the s})ider-crab ; -^yfliilo in other cases it is more fiatteiu'il and triangular in shape and lacks a distinct rostrum, as in lli' edible crab, CaUinectes, the hulv- crab, Plaiyovychns, and the com mon crab, Cancer, ami in others again is more or les- (juadninguhir and thicker, as in /*i)inot/ieres. the ovHtpr-criih Ocypuda, the sand-crab, and Gdanimiui, the tiddler-crab. Tlii Fio. 188. — Panopmis deprcssus (afiiT Embrton from Vkrral). TYPE CRUSTACEA. 413 } CrayfisL, opod is an t is foimd second pe- r the most lonetes aud family Ca- in Lucifer essentially species of jus, one of jr-cral), is ound aud , on wliicli ler surface f supplied e luijgs of mpact, the :ally desti- 1 addition ii a groove .ce of tht» >me cases liuost gl<»- 1 anterior- . spine, as •ab ; •^vhilo 3 tiatteiu'il aud lacks ,s in tin the ladv- thc com e or less kster-criili. )rab. Til. antenuules are small aud they and the eyes can be partially concealed in a groove on the anterior edge of the carapace. The abdominal appendages, with the exception of the anterior cue or two pairs Avhich are adapted for copulation, are ab- sent in the males, while the females generally possess four pairs, to which the ova are attached. The gills are generally few in number, except in Porcel- hina and some allied forms, and are usually lamellate in form. While essentially marine in habit, the Brachyura are fre- (piently more or less terrestrial, the sand-crabs, Ocypoda, and the fiddler-crabs, Gelasimus, living in holes in the sand just above high-tide mark, while the land-crabs, Gecarcinus, of the tropics may live some distance from the sea, migrating to it in armies during the breeding-season. A few forms, such as the genus Telphusa, are aquatic. III. Subclass Arthrostraca. The Arthrostraca, with the exception of the small group of the Anisopoda, are destitute of a carapace, and the tho- racic appendages, with the exception of the first pair, are jointed walkiug-limbs lacking an exopodite. The anterior, or in some cases the anterior two thoracic segments fuse with the head, the appendages of these segments differing from those of the free segments, being modified to assist in the process of mastication, whence they are termed maxilli- peds. The abdomen is , .nposed of six segments provided with ai)pendages, and of a terminal telson ; occasionally the va- rious segments fuse together, and in some forms the abdomen is reduced to a small unsegmented structure. Platelike ap- pendages attached to the basal joints of some of the thoracic limbs form by their meetii.g and overlapping a brood-pouch in which the ova undergo their development. The lateral compound eyes are not, except in Tanais, sup- ported on stalks, a characteristic which has suggested the tinin EdriophfJmhnata sometimes applied to the group. 1. Order Anisopoda. Tlie Anisoiiodn., are evduuivfily marine forms in which the two anterior thoracic segments are fused with the head and is;: .Jyi».,J 414 IN VERTEBRA TE MORPIIOLOO Y. covered in at the sides by duplicatures of the body-wall, M'hich euclose a small respiratory cavity. The anteimules and autemiio are iiuiramous except in ApFCMies in which the auteuimles carry two terminal 'Hagella. The palps of the anterior maxilhe project into the respiratory chamber aud serve for cleansing it, and the first thoracic limbs, the maxillipeds, bear each an epipodial branchial ap- })eudage lying in the respiratory chamber. This limb aud the succeeding one are chelate, the inner angle of the penul- timate joint being prolonged into a process against which the terminal joint may be apposed. The abdominal appendages are biramous swimming-feet in Tanais aud Apseudes, the last pair being in Anthura especially enlarged to form with the telsou a terminal finlike structure. 2. Order Isopoda. The majority of the Isopoda are marine, the genus Asellns (Fig. 189), however, occurring in fresh water, while Oniscm, ForceUio, and ArmadilUdium are terrestrial, being commonly known as Wood-lice or Sow-bugs. The body in all forms is more or less flattened dorso-veutrally aud only the anterior thoracic segment is fused with the head, the remaining sevei' remaining perfectly distinct. There is no trace in the aduJt of a carapace, and the abdominal segments are usually small and may be fused more or less completely. The maxilhe are destitute of palps and the maxillipeds {mxp) usually fuse together to form a sort of lower lip. The remaining thoracic appendages are limblike and do not bear any respiratory appendages, though lamelhe are attached to the basal joints of several of them in female individuals, serving to form a brood-pouch. The five posterior abdominal appendages are biramous aud lamellar {ah), serving both for swimming and for respiration, the anterior pair (op) usually becoming hard and forming an operculum which covers in the posterior more delicate appendages and in the terrestrial forms may have branching sjjaces containing air (trachea) ramifying through them. The heart (Id), in confornuty to tlif position of the respira- tory organs, is situated print'i[)ally in the abdomen, exteudiui; bodj-wall, except in lal ^agella. .'espiratory it thoracic iriichial ap- limb am] the peuul- which the ppeuda^es is, the hist a with the TTFE CRUSTACEA. 415 le Oniscu.s, comnioL»ly 11 forms is e auterior liiig sevei' I the atluJt lally small laxillipetls lip. The o Lot bear ttached to uliviiluals, ibdomiiial ^ both for yp) usually covers in terrestrial (trachea' ) ie res])ira- exteudiuu forwards only a short distance into the thorax segment. It possesses one or two pairs of ostia and is closed behind, giving off in front anti at the sides numerous aortse. A shell- gland has been observed in some Isopoda, but the antennary gland is wanting. Although the majority of the marine forms, such as Idotea and Sphmroma, lead a free existence, nevertheless there are certain parasitic forms. Thus the genera Cymothoa and jEga are parasitic on the skin or in the mouth of fishes, but also retain the power of swimming and consequently are not much at' e vn _ 1 ] hf Fia. \^9.—Asellu8 communis, Diagram op STnucruiiE. ab = abdominal appendages. l = liver-ciBcn. (10 = aoila. tnnp = maudibular palp. «<• = antennule. mxp = maxilliped. at- = auteiina. r = rectum. ce = cerebral ganglion. « = stomucli. ch = chelate limb. t = tiioracic appendage. ht = heart. mi = veutial uerve-coid. modified. The genus Bopyrus, which lives in the branchial cavity of shrimps, becomes in the female some\\ hat distorted in shape and asymmetrical, and the mouth-parts become transformed into a suctorial proboscis and the eyes disap- pear. The male, however, which is mucli smaller than the female, retains the eyes and does not depart from the usual symmetrical body form. The degeneration of the female proceeds much farther in the genus Entoniscus, which lives either partly or wholly included within the body-cavity of other Crustacea and assumes a saclike unsymmetrical form, recalling to a certain extent that of some of the ])arasitic Copepoda. At tlse time of pairing both sexes are alike fully •segmented and with an almost full comi)lement of appendages. i— tail 416 INVERTEBRATE MORPHOLOGT. After copulation, liowever, the female assumes the degener- ated form, while the male dies. Sill ^^ c t f! ill 3. Order Amphipoda. Like the Isopoda these are essentially marine forms, though the genus Gammarus is aquatic and Orchestia (Fig. 190) partly terrestrial, living among the v.-rack on sea- beaches just beyond the reach of the waves. The body in ht Fig. 190.— Diagram of Stuuctuke op Orchestia cavimana (after Nebeski). aO ~ nntennule. m = uioutb. at^ = auieuiia. mt - Malpighian tubule. br — brail cilia. oc = eye. ce = cerebral ganglion. r = rectum, ch = chf^laif! limb. rd = reprcductive duct. 7tt = heart. ro = reproductive organ. I = liver-cajca. vn = ventral nerve-cord. the Amphipoda is laterally flattenud and presents therefore a very different appearance from that of the Isopoda, though, as in that group, lacking all traces of a carapace. The first thoracic segment is fused with the head, and in Caprella and Ci/nmus the second segment likewise. The appendages of the head and the maxillipeds resemble those of the Isopoda, and the remaining thoracic appendages are limblike, a certain number of the anterior ones frequently possessing a terminal joint capable of flexion upon the succeeding one, or eveu being chelate. The five posterior limbs or the third and fourth only beai epipodial lobes which serve as branchiae, and le degener- ine forms, Orchestia ,ck on sea- he body iu TYPE CRUSTACEA. 417 ter Nebeski). iibule. duct. organ. !-cord. ;lierefore a ia, though, The first iprella and iges of the )poda, auJ a certain a terminal 3, or eveij third aud iichifc, and a number of the limbs also iu females bear lamella) which may enclose a brood-pouch. The three anterior abdominal limbs are biramous and serve for swimming, while the three posterior ones, also biramous, are frequently directed back- wards and serve as springing organs, the springing powers of Orchestia having gained for it the mime of the Beach-flea. In Caprella, which crawls about over colonies of Hydroids and Polyzoa, and Cymmis, which is parasitic upon the skin of whales, the abdomen becomes almost rudimentary and is des- titute of appendages. The heart {U) lies in the thoracic region in the anterior five or six segments and possesses from one {Corophiuni) to three ostia. It is prolonged into an aorta at either end. In connection with the mid-gut portion of the digestive tract, iu addition to the four so-called liver-caeca (l) is a pair of gluud- ular cfBca which seem to be excretory in function and have been termed Malpigluan tubules {mt). An antennary gland occurs, but the shell-gland is apparently unrepresented in adults. Development of the Crustacea.— The majority of the Crus- tacea pass through a more or less complicated series of metamorphoses, the larval forms being highly suggestive when studied from the phylogenetic standpoint. A few forms, especially those inhabiting fresh water, abbreviate their de- velopment considerably, so that the young animal when it leaves the egg practically may diff'er from the parent only in size {Cambarus), and among the higher forms the development is generally abbreviated to the extent that a greater or less number of the larval stages, characteristic of lower forms, are passed through while the young animal is still within the egg- membrane, only the final stages being free-swimming. Throughout the Entomostraca the first larval form Avhicli li:.tclies from the egg is termed the NaupUus (Fig. 191) and diliers markedly fr(,m the adult, chiefiy, however, in the small number of appeiidagos it possesses. The body in typical iorms shows no trace of segmentation and possesses a single median eye geuf^rally \-shaped. But three pairs of limbs Hie present, which become transformed later into the anten- imles, antennie, and the mandibles of the adult. The NaupHar !!S s Si* ■—•IS go «... 418 INVEliTEBliATE MOltPIIOLOOT. i'( antennules are uuiramous aud, like tlie other limbs, but indis- tinctly jointed, the antennso and mandibles bein<,', however, biramous and possessing strong setiB at their bases which function as jaws, though both j^airs of appendages are essen- tially locomotor. Judging from the appendages, therefore, the Nauplius may be regarded as consisting of live segments, one corresponding to the prostoinial lobe of Annelids and contain- ing the primitive cerebral ganglion (archicerebrum), one cor- FiG. 191.— Nauplius op Cetochihis septentrionalis (utter Qhobbeh). responding to each pair of appendages and one to the region of the body behind the mandibles. A Nauplius of this simple form may be regarded as typical and is that which is found in the majority of the Copepoda aud in the Cirrhipedia as well as in some Branchiopoda {Edherhi, Limnodia). In the Ostracoda the arrangement of the limbs and segments is the same, but the bivalved shell characteristic of the adult is already developed, giving the Nauplius an ap- pearance very different from that of the Copepoda. Not uu- frequently, however, as for instance in Apus among the Branchiopoda, and Leptodorn among the Cladocera (the re- maining Cladocera, so far as is known, leave the egg with the adult form), the Nauplius, though jiossessing only the three . pairs of appendages, yet shows indications in the post-maudih- ular region of a varying number of additional segments, and to this form it is convenient to apply the name BletanaupUm. As a rule in the Entomostraca further development con- sists of a series of moults (ecdyses), an increase in the number of segments and appendages and modifications of the hitttu' taking place at each ecdysis, until the adult form is attained. No special larval forms beyond the Nauplius are common to TYPE CRUSTACEA. 419 bs, but iudis- 11 J,', however, bases which es are esseii- herefore, the jgmeuts, one autl coutain- im), cue cor- ■ Qrobbkn). io the re Fjg. 193.—^, ProtozoMsa op Lucifer (after Brooks); B, Zoea of Palmmoneies (after Faxon). At^ = autenuule. AC = antenna. 7nx\ c = cerebral ganglion. mp^, E = compound eye. h = heart. 8 = stomnch. m = mandible. 7nx^ = maxillae. mp'^ = maxillipeds. oe = simple eye. r = rostrum. majority of Malacostracaus, though occurring as the first larval stage of some Stomatopods. It is characterized by the development of two maxilla) aud the two or three anterior i kS"* •■III, se 420 IN VERTEBRA TE MORPHOL 00 Y. Ill thoracic appendages in addition to those already present in the Nauplius, and furthermore by the distinct separation of the body into an anterior cephalo-thoracic portion covered by B. carapace and a posterior abdomen which is usually but imperfectly segmented. This stage is succeeded sometimes after two or more ecdyses by the Zota (Fig. 192, B), a stage in which the majority of Decapoda leave the egg. It is distin- guished from the Protozoea principally by the perfect segmen- tation of the abdominal region, tli ugh it still possesses no appendages, unless it be rudiments of the sixth pair, and it is furthermore characterized by the compound eyes beiug stalked, a feature but slightly indicated in the Protozoea, in which stage they make their appearance. The Zoea stage in the Brachyura is generally characterized by the development of spines, sometimes of enormous length {ForceUana), ui)on the dorsum and sides of the carapace. In such a form as Euphausia the next stage is the adult, but in the Decapods other larval stages intervene before tlio adult condition is reached. The first of these is characterized in the majority of the Macrura by the appearance of the re- maining thoracic appendages which were unrepresented in the Zoea, in the form of biramous structures closely resem- bling the thoracic appendages of the Schizopoda, whence tlie stage ia generally termed the Mysis stage (Pig. 193). Tlu; Via. 198.-Mvfiis8TAOE ov LonsTEH, ITomarua americanua (afters. I. SMiin abdominal appendages also develop dining this stm^e. Among the Hermit-crabs (Paguridie) and the Brachyura ihv, development is to a certain extent abbreviated, the pereiop.Mls never being represented by biraniouH nppHMdjvt'es but biiun from the first uniramous, and in these forms therefore a true Mysis stage never occurs. To the correspoiidiu^r Htagr, present iu paratiou of covered by usually but sometimes I, a stage iu [t is distiu- !ct segmeu- >ssesses no pair, aud it 3jes being otozoea, iu ea stage iu ivelopmeut ), upou the the adult, before tlie iracterized of the re- 5seuted iu ely reseui- .'heuce the [93). The TYPE CltUSTACEA. ir 8. I. SMirn). lis stage, lyura the ereiopodrt [)ut bciug lereforo a \i\,:, stage, 421 or rather to one in which the pereiopods are indicated but not fully developed, the term Metazoea is applied. Further- more in certain Ma crura, such as Scyllarus and Palinurus, the Mysis stage is represented by peculiarly-shaped transparent larvse which have been termed Phyllosoma, or glass-crabs. The carapace is divided into two portions, of which the an^ terior or larger covers iu the head region and the posterior the thorax, the body being throughout flat aud the ab- domen very small The pereiopods, of which in the earliest stages there are but three, are biramous, and the flrst maxil. Fia. 104.-MEGALOPA-8TAOE OF Cancev irroraius (after Emkrton from Vkrrill). hpeds are either entirely wanting or very rudimentary. Dur- ing successive ecdyses tlu> missing ai)peiulage8 are gradually developed, though the actual trausformntiou of the /^hijUoHoma into the youngest Sci/llnrm w Palinnrm stage (whic'ii is de- ndedly smaller than the oldest Phylhmma) has\iotyet been observed. The oni the MyRi.« stagp (n tJio adult ;., ., n. a.s a rule chancre ) S| among the Macrura. In the IJrachyura, however, gradual, and no special!} definite larval forms are to bo found I IP I lift ;.3H3 "SI 422 INVEltTEBliA TE MORPUOLOG Y. i the Metazoea becomes trausformed iuto a well-marked form, the Megalopci (Fig. 194), so called from the usually largo size' of the cephalothorax. It resembles closely a Macruiau, diflfering only iu the abdomeu being relatively small, and becomes converted iuto the adiilt form by the doubling of the abdomeu beneath the thorax. A Megalopa stage occurs also in the Hermit-crabs, but is not so well marked off from the young fully-formed auimals as iu the Brachyura. Affinities of the Crustacea.— Tho relationships of the higher groups of the Miilacostraca to one another are clearly siiown by their larval forms, the Megalopa showing the origin of the Brachyura from Macruran forms.' and the Mysis stage that of tlie latter from Schi/.opod ancestors. When attempts are nuMle to go still further difficulties stand iu the way. As regards the Stomatopoda it is to be noted that they pass through a stage, the Erichthns, in which the thoracic appendages which are present are biramous, and it seems probable that both they and the Cumacea are re- ferable back to Schizopod ancestors. The Arthrostraca, on the other liaiid, are probably traceable to Cumacealike ancestors, while the Leptostraca represent more nearly the Entomostracan ancestors than any other group, though widely diiferentiated from them in certain particulars. It is even still more difficult to trace out relationships of the various Entomostracan orders, but it seems fairly clear that IMiyllopodan forms such as Aptis are to be considered as representing more nearly than any others the primitive Crustacea. As regards the affinities with other groups verj' interesting questions arise, two possil)ilitics seeming to be open. According to one the Crustacea have been derived directly from segmented Amu'lids, through forms repre- sented in a modified condition to-day by Aptis. The lol)ed iii)pen(lage ol ApHs is a modified parapodium, and thesegnuMiiation of the body has been inherited. What then as to the Nauplius? According to this view it has praeti('ally no ancestral signiflcatice. or at best can be considered only as representing a Trocliophoro larva highly modified and with many aiiiili characters thrown back upon it. This latter idea does not seem, however, to agree with tlu> facts, since tlie Trochopliore is an unsegmented structure and can be eomparal)le only to the prostomial and first aj)pendage-l)earin- segments of the Nauplius. In other words, the Nauplius is comparalih. if comparai)l(^ at all, to a Trochopliore p/ns certain additional segnu'Uls. It has recently been suggested that po.ssibiy the Nauplius may represent nut till' Trochopliore but the larval Annelid with three parapodia, which, as iiulieated (p. 215), is a well-marked stage in the development of maiiv Polyehnita. Th(* number of segmenta is jipparently similar in the Iwii forms, and the idea is plausible. If, however, iu all Cni.stacoa a gauiflio.K representing a segment, intervene.-, b.>t ween the archicerebral ganglia ami the antennulary (see p. iSTH), then the Nauplius has potentially one 80g me th( pr( wil a s is t the cat thii 11. TYPE CRUSTACEA. 423 rked form, '^ larger size Macruiau, small, and oubling of age occurs narked olt' shyura. ler groups of larval forms, riiran forms, itors. WluMi he way. As lugh a stage, present are aacea are r(!- ) other liaiul, Leptostraca other group, I. It is ovt'ii itoinostracaii as Apus arc the primitive ng questions he Crustacea forms ropre- l)pen(iage of ody has l)eeii J view it lias ored only its many adult m, however, ied siruetiiie lage-lM'ariiii,' comparalile. id segments, ay represent in, which, m nt of many in the two I H ifanifliuiii ganglia and lly one su^: ment more than the Annelid larva and the comparison will not hold If the direct Annelid origin is to be accepted, it seems most satisfactory at present to regard the Nauplius as a secondarily acquired larval stage without any ancestral significance. Another suggestion has, however, been made which gives the Nauplius a significance and traces the Crustacea back to unsegmented ancestors It IS to the effect that the Nauplius can be referred to Rotiferliko ancestors the remarkable Hexarthra with its six processes being supposed to indil eate the line of descent. It is exceedingly doubtful, however, whether this similarity can be regarded as anything more than a superficial one TYPE CRUSTACEA. I. Class ENTOMOSTUACA.-Number of segments varies ; abdomen without appendages ; larva a Nauplius. 1. Order Phyllopoda. -Nambev of segments variable ; appendages with branchiae. 1. Suborder Bmnchiopoda. -Body plainly segmented and seg- ments of thorax more numerous than six. Apm, Branchipus, Esthena, Limnadia, Limnetis. 2. Suborder Cladomra.— Body indistinctly segmented ; with bi- valved shell ; four to six thoracic appendages. Daphnia Moma, Sida, Emdne, Polypheimis. ' a. Order Ostracoda. -W\t\i bivalved shell ; body indistinctly seg- mented ; two thoracic appendages. Cypris, Cythere, Cypri- dina, Halocypris. 8. Order Vopepoda.-WMhont shell ; five pairs of thoracic limbs ; many forms parasitic and degenerate. 1. Suborder EHcopepoda.-Tw^i thoracic segment only fused with head ; abdomen cylindrical and segmented except in higlily degenerated forms. Cyclops, Cai,ff,omii,ptu,s, HarparticMs Cafanm, Cetochilns (free-swimming) ; Notodelphi/s (commen- sal istie) ; Corycuiui, Happ/ifrina, ErnasiluH, Calfym, Panda- rus (piirtly parasitic) ; Philichthys, Penella, I^rima, Chon- dracaiithiis, Achtheres, AiichnreUa (parasitic). 3. Suborder liranrhiara.—AW tlioracic segmcnits fused with head • abdomen small and lamellar, partly parasitic. Arynlus. 4. Order ^CV;T///>f.r/,V,._s,.ssiIo or parasitic; segmentation indis- tinct; SIX pairs of thoracio appendages; pass through Cypri.s stage. Lepa.s, Scalpellxm, J Ua, Jia/anm (m^mile) ; Ak'ippe, Cryptophialus (boring) ; Proteolepas, HaocuUna, Ixiura, Ihndroyaster (i»arasitic). 11. Class MALAC()8TUACA.-Numl,er of se^ ...it« constant ; thoracio seg- monts eight, abtiominai seven or eigiu. 1. Subclass hptostranu With l)ivalv(>d shell ; abdomen with eight segments. NebuUa. I irl Sii I? !!! 111 424 INTEliTEBliATE MOliPUOLOOT. 2. Subclass Thoracostmca.—\S\t\\ carapace covering the whole or a part of the thorax ; abdominal segments seven. 1. Order /St7a>o/»orf«.— Thorax completely covered ; thoracic append ages biramous. Mysis, Eaphatinia, SirieUa. 2. Order Cwmacea.— Last four or five tiioracic segments not covered by the carapace ; eyes sessile or rudimentary. Diastylis, Campylaspis. 8. Order Stomatopoda.— Last three or four thoracic segments not covered by the carapace; eyes stalked; five maxillipeds. Squilla, Lysiosquilla, Gonodactylus. 4. Order Decapoda. — Thorax completely covered ; five posterior appendages uuiramous and three maxillipeds ; otocysts in antennules. 1. Suborder ifocrara.— Abdomen usually well developed. 8er- gestes, Lucifer, Fenoetis, PaJdmonetes, Alplieus, Cambarus, Homarus, Eupayurus, Binjiis, Hippa. 3. Suborder ifmc/i^ttm.— Abdomen small and concealed beneatli cephalothorax more or less i)erfectly. I'brcellana, Libinia, CalHitectes, Platyonychus, Cancer, Pinnotheres, Oci/poda, Gelasi)nus, Gtcarcinns. 8. Subclass ArtJirostraca.—No shell or carapace as a rule ; with seven (or six) walking-limbs; eyes sessile. 1. Order ^ /J /.so;w(/a,— Carapace slightly developed : first two tho- racic segments fused with head ; branchii« on anterior maxillu". A seiiJes, Tanais, Anthnra. 2. Order Tivo^wf^a.— No carapace ; first thoracic segment fused wilh head ; body flattened dorso-vcJitrally ; braneliitu on abdomi- nal appejidages. A.selliis, Onisen.s\ Porcellio, Armndillid- ium, Idotea, Sphmoma (free) ; Cymothoa, JEga, Bopyriis, EntnniscuH (parasitic). 8. Order Amphipoda.—^o carapace, first thoracic segment fused with head ; body flattened laterally ; branchim on thoracic appendages. Gammarus, Vrr/ie.sfia, Corophium, Cyainns, Caprella. LITERATURE. GENKHAIi, H. Milne-Edwards. IlMoire Natiirelle des Crustacea. Paris, 1884-1840. r. Mttller. Fiir Darwin. Leipzig, 1804. H, Oerataeoker. Arihropodn. nronn'-H Klnsson und Ordnuiigeu des Tliici- reichs, Btl. v. Ahtii. 1. 18(i(!- (not \ot completed). C. Claui. UnterHii chilli gen zur Krforschung der genealogisehen Orundlage ikx Crmtnceen-St/nteviH. Wien, 187ft. 0. Orobben. 7)w Aiitennendri'me der Vrusfaceen. Arboiten ji. d. Zoolog. Inst. Wieu, 111, 1881). ie whole or a Dracic append ts not covered y. DiastyUs, segments not ) maxillipeds. five posterior ; otocysts in .'doped. Ser- !s, Cambarm, oaled beneatli tna, Libinia, 'es, Ocypoda, 0 ; with seven first two tlio- erior maxilla". !nt fused with 3 on abdonii- Armndillid- 'ga, Bopi/riis, Bgment fus(>(l ) on thoracic sm, Cyamits, TYPE CRUSTACEA. 423 J4-1840. m dus Tliici- Orundliige dfs Zoolog, Inst J. E. y. Boas Studusn iiber die Verwandtschaftsbeziehungen der Malakostraken Morpbolog. Jahrbucb, viir, 1883. J. Cwriere m Selm-gane dsr Thieve, mrgleichend-anatomisc?i dargestellt Munich and Leipzig, 1885. C. ClauB. Ntue Beitrdge zur Morphologie der Grustaceen. Arbeiten a d Zoolog. Inst. Wien, VI, 1886. " ' S. Watase. On the Morphology of lU Compound Eyes of Arthropoda. Studies from the Biolog. Laboratory. Johns Hopkins Univ., iv. 1890 G. H Parker. The Compound Eyes in Crustaceans. Bulletin Museum of Coiup. Zoology, XXI, 1891. C. Grobben. Zur Kenntniss des Stambaumes und des Systems d^r Grustaceen Sitzuugsber. Akad. wissenscb. VVieii, ci, 1893. PHYLLOPODA. A. S_ Packard. A Monograph of North American Phyllopod Crustacea. Twelfth Annual Report U. S. Geolog. Survey. Washington. 1883 C. L. Hernok. A Final lieport on the Crustacea of Minnesota. Twelfth An- uual Report of the Geolog. and Natural History Survey of Minnesota Minneapolis, 1884. C. Claus. Zur Kenntniss des Bams und der Entwieklung von Branchipus staa nalts und Apus cancriformis. Abhandl. k. Akad. wissenscb Gottiutren xviir, 1878. v^uiuufeen, A. Weismann. Ueber Ban und Lebenserseheinungen von Leptodora hvalina Zeitscbr. fUr wissenscb. Zoologie, xxrv, 1874. Beitrnge zur Naturgeschiehte der B^iphnoiden. Zeitscbr. fUr wissenscb Zoologie, xxvii-xxxiii. 1876-1879. E. Ray Lankester. Observations and Reflections on the Appendages and on tU Nervous System of Apus cancriformis. Quarterly Jouru. Microscon Science, xxr, 1881. '^' C. Claus. Untersuchungen iiber die Organisation und Enttcicklunq von Branchipus und Artemia. Arbeiten a. d. Zoolog. Inst. Wieu vi 1886 P. Pelseneer. Observations on the Nervous System of Apus. Quarter! v Jouin of Microscop. Science, xxv, 1885. P. Samasia. Untersuchungen iiber das centrale Nervensystem der Grustaceen Archiv fllr mikr. Aiiat., xxxviir, 1891. G. M. Bernard. The Apodidm, a Morphological Study. London, 1892 C. Grobben. Uie Entwicklungsgeschichte der Moina rectirostris,' etc Arbeiten a. d. Zool. Inst. Wien, ii, 1879. OSTHACODA. G. E Brady. A Monograph of the Reeem British Ostraeoda. Transactions Linnaean Soc. London, yttt. C. Clang. Ueber die Organisaiion der ^^pridinen. Zeitscbr. far wissenscb Zoologie, TV, 1865 Beitrdge zur Kenntniss ier siUauxmet Ostracoden. Arbeiten a d zoolog. Inst. Wieo. xii, 18«8. G W. Muiler, ihtracoden. Fauna und Flora des Golfes von Neapfl. Monoirr XXI. 1H»4. % >: 111 MM.. — .,» """"'^llBij- 426 IN VERTEBRA TE MORPHOLOO Y. Sii % lie »■« COPEPODA. C. L. Herriok. A Final Report on the Crustacea of Minnesota, Twelfth An- nual Report of the Geol. and Nat. Hist. Survey of Minnesota. Min- neapolis, 1884. W. Giesbrecht. Pelagische Copepoden. Fauna und Flora des Golfes von Neapel. Monogr., xix, 1893. C. Glaus. Ueber die Entwicklung, Organisation und systematische Stellung der Arguliden. Zeitschr. fllr wisseusch. Zoologie, xxv, 1875. C. Holder. Die Qattung Lernanthropus. Arbeiten a. d. zoolog. Inst. Wieu, II, 1879. M. Hartog. 2he Morphology of Cyclops and the Relations of tJie Copepoda. Trans. Linnseau Soc. London, 2d Series, v, 1888. F. Leydig. Ueber Argulus foliaceus. Archiv. fUr mikr. Anat., xxxiii, 1889. C. Grobben. Die Entwicklungsgescfiichte von CetocMlus septentrionalis, Qoodsir. Arbeiten a. d. zoolog. Inst. Wien, in, 1881. CIUUHIPEDIA. C. Darwin. A Monograph of the Subclass Cirrhipedia. London, 1851-1854. H. de Luoaze-Duthiers, Ilistoire de In Laura Oerardim. Archives de Zool. exper. et. gen., viii, 1880. P. P. C. Hoek. Report ok the Cirrhipedia. Scientific Results of Voyage of H.M.S. Challenger. Zool., viii, 1883; x, 1884. Yves DelBge. Evolution de la Sacculine. Archives de Zool. exper, et. geu., 2ine ser., 11, 1884. LEPTOSTKACA. C. Claus. Ueber den Organismus der Nebaliiden, und die systeriatischen Stellung der Leptostraken, Arbeiten a. d. Zool. Inst. Wieu, viii, 1888. BCIIIZOPODA. G. 0. SaiB. Report on the Schieopoda. Scientific Results of the Voyage of H.M.S. Challenger, xiii. 1885. CUMACEA. A. Dohrn. Ueber Ban und Entwicklung der Cumaeeen, Jenaische Zeitschr. filr Naturwiss , v, 1870. G. 0. Sara. Report on the Cuniacea. Scientific Results of the Voyage of H.M.S. Challenger, xix, 1887. 8Tt)MATOPODA. C. Claus. Die Kreislaufsorgane und Blutbewegung der Stomatopoden. Arbeiton a. d. Zoolog. Inst. Wieu, v, 1883. W. K. Brooks. Report on the Stomatopoda, Scienliflc Results of the Voyage of H.M.S Challenger, xvi, 1886. DECAPOUA. S. I. Smith. Vnrioas Papers in Trnns. Connecticut Academy and in Reports of the U S. Commissioner of Fish and Fisheries. Twelfth An- uesota. Min- s Golfes von 5 Btellung der J. Inst. Wleii, tJie Copepoda. XXXIII, 1889. nalis, Qoodsir. 1851-1854. lives de Zoo). )f Voyage of ;per. et. geu., iysteriatisclien VIII, 1888. lie Voyage of 3che Zeitsclir. le Voyage of len. Arbeitoi) )f the Voyage id in Reports TYPE CRUSTACEA. 427 V. Hensen. Studien uber das OehOrorgan der Decapoden. Zeitscbr ftlr wissenscb. Zool., xiii, 1863. C. Grobben. Beitrdge zur Kenrdnm der mdnnlichen OmJdechUorgane der JJecapoden. Arbeiten a. d. Zoolog. Inst. Wieu, i, 1878. T. H. Huxley. TJie Crayfish. Loudon and New York, 1881 W. K. Brooks. Lucifer: a Study in Morphology. Philosoph. Trans. Royal boo. London, clxxiii, 1882. H. Keichenbach. Biudien, zur Entwicklungsgeschichte des Flusskreoses. Ab- bandl. Senckeuburg. Gesellsob. Frankfurt, xiv., 1886. W. F E. Weldon. Ccelom and Nephruiia of Palcemon serratus. Journal Marine Biolog. Assoc, i, 1889. G. H^ Parker The Hutology and Development of tJie Eye in the Lobster Bulletin Museum Comp. Zoolog., xx, 1890. P. Marchal. Recherches anaiomiques et physiologiques sur Vappareil excreteur des Crustaces decapodes. Archives de Zool. exper. et gen., 2me ser., x, W. Z Brooks and F. H. Herrick. The Emh-yology and Metamorphosis of the Macroura. Proc. U. S. National Acad., v, 1892. E. J. Allen. Nephridia and Body-cavity of some Decapod Crustacea. Quarterly Journ. Microscop. Science, xxxiv, 1893. * ARTBR08TRACA. 0. Harger. Report on the Marine Isopoda of New England and Adjacent WaUrs. Report of the U. S. Commisslouer of Fish and Fisheries for 18<8. A. Delle Valle. Oammarini del Oolfo diNapoli. Fauna und Flora des Golfes von Neapel. Monogr., xx, 1893. P. Mayer. Caprelliden. Fauna und Flora des Golfes von Neapel Monoffr VI and XVI, 1882, 1890. ^ '' 0. Nebeski. Beitrdge zur Kenntniss der Amphipoden der Adria. Arbeiten a d. Zoolog. Inst. Wien, iii, 1890. E. Kossmann. Studien uber Bopyriden. Zeitscbr. fUr wissenscb. Zoologie XXXV, 1881 ; Mitth. a. d. Zoolog. Station zu Neapel, iir 1882 ' Yves Delage Contribution d felude de I'appareil circulatoire des Crustaces ednophthalmes marins. Archives de zool. exper. et gen., ix, 1881. APPENDIX TO THE TYPE CRUSTACEA. Order Xiphosura. The Xiphosura is a group which possesses many Crus- tPceau peculiarities, and also many foreign to that group and more especially characteristic of the Arachnida ; consequently It IS advisable to consider it as an order by itself, intermediate between the two types. A single genu?,, Lhmdns (Fig. 195), with few species consti- tutes the order, the members of which are popularly known =^: I 428 INVERTEBRATE MORPHOLOGY. as King-crabs or Horseshoe-crabs. They are large forms measuring a foot or so in diameter, and the body is composed of three portions. The anterior is a broad semicircular ceph- PiG. 19fi.—Limulu8 Polyphemus, Female, from the Ventral Surface. ab = abdomen cp = cepbalothorax, an = uiiiis. ol = olfactory organ. ch = chelicera. op = operculum. chi = chilnrium. ap = spine. alothorax (cp), prolonged backwards into sharp points at its posterior angles and bearing upon its dorsal surface a pair oi compuuud eyes towards the sides and x ear the median line two simple eyes. The middle region is the abdomen (ah). TYPE CRUSTACEA. 429 arge forms s composed cular cepli- L Surface. ft an. oiuts at its 36 a pair ol aediau lino lomen {ali). showing but faint indication of segmentation, and bearing on its terminal segment the anus, behind which is a long mova- ble spine {sp), the post-abdomen, forming the third region and to be regarded probably as a movable prolongation of the dorsum of the last abdominal segment. The cephalothorax bears seven pairs of appendages. The first pair, the chelicerce (Fig. 195, ch), which lie in front of the mouth, are small and, liiio the following four pairs, are chelate. These together with the sixth are much longer and surround the mouth, their basal joints being provided with strong bris- tles and serving as jaws. The sixth pair of appendages differ from their predecessors in not being chelate and in possessing upon their basal joints a peculiar process which has been termed the flabellum and by some is regarded as representing an exopodite. The seventh pair of appendages is very differ- ent from the others, forming a broad fiat plate, the two appendages of the opposite sides meeting in the middle line. This plate covers in the abdominal appendages to a certain extent and hence is termed the operculum {op). The abdom- inal appendages, o* which there are five pairs, resemble the operculum in form, and like it allow an external larger exopo- dite and an inner smaller endopodite to be distinguished. They carry upon their posterior surfaces series of large leaf- like, thin-walled folds which function as brauchif©. The heart (Fig. 196, ht) is an elongated tubular organ lying in the posterior part of the cephalothorax and the anterior part of the abdomen, and possesses eight ostia. Arteries arise from it which carry the blood to various parts of the body, eventually, however, opening into the general lacunar system. The blood has a distinct bluish color which deepens on exposure to the air and is due to a copper-containing respiratory pigment, hiumocyanin. The body is enclosed in a hard chitinous cuticle, and in addition a peculiar fibro-cartilaginous plate, the endosternite, is found in the cephak)thorax between the intestine and the nervous system. It is formed by the fusion of a number of tendons and may be regarded as an endoskeleton. The mouth is an elongated opening lying between the bases of the anterior cephalothoracic appendages and is ■^ 430 INVERTEBRATE MORPHOLOGY. §i bounded behind by a pair of processes which rejiresent a lower lip and are known as the chUaria (Fig. 195, chi). Tlie oesophagus passes upwards and forwards and dilates into a large proventriculus (Fig. 196, pr) in the front part of the cephalothoracic rhield, and this, bending upon itself and constricting again, opens into the stomach (s), from which the intestine {%) passes straight back to open on the ventral sui*- face of the body at the base of the terminal spine. The inner wall of the hind-gut, oesophagus, and proventriculus is lined by chitin, which in the last-named structure is thrown into ^•^ ht vn Fig. 196.— Longitudinal Section through a young Limulus polyphemus, Diagrammatic (after Packard). ce = cerebral gnuglion. I = liver. es = eiidosternite. pr = proventriculus. ht = Lean, s = stomach. i = iutestiue. vn = ventral uerv<;-oord. folds and recalls the masticatory apparatus in the stomach of the Decapodous Crustacea. Into the stomach there open the ducts of two pairs of voluminous digestive glands (I) which occupy the greater portion of tlie cephalothorax and are much branched greenish structures. The nervous system consists of a syncerebrum (ce) com- ])osed apparently of three pairs of ganglia. It lies in front of the oesophagus, sending branches to the compound and simi)Ie eyes. Behind the oesopliagus and united with the syncere- brum by circumoesophageal connectives comes a series of seven pairs of ganglia closely approximated, the first pair innervating the chelicerre and the remaining six the other thoracic limbs in succession. A chain of six pairs of ganglia lying in the abdomen is connected with the cephalothoracic series and innervate the abdominal appendages. enresent a chi). Tlie ates iuto a art of the itself and which the entral sui*- The inner IS is lined irowu into TYPE CRUSTACEA. 431 i polyphemus, IS. i-oord. 6 stomach there ojjeu glands (/) horax and I (ce) com- iu front of -nd simple e syncere- series of first pair the other of ganglia lothoracic As already noticed, a pair of simple eyes are borne upon the dorsal surface of the carapace, one on each side of the median line, while a pair of larger compound eyes are situated lat- erally. The structure of these compound eyes is peculiar (Fig. 197). Over their surface the cuticle is considerably thickened and shows upon the outer surface no indication of corneal facets, but its inner surface is prolonged into a num- ber of papillae (]) each one of which projects iuto a depression of the ectoderm. At the bottom of each depression is a bulb- FiG. 197.— Compound Eye op Limulm polyphemus, Two Ommatidia (after Watase). c = central cell. ms = mesoderm. i = leas. opn = optic uerve. rt = retliiula, like structure composed of a number of cells arranged in a circle and constituting a retinula (rt), the lower ends of the cells being continued inwards to form part of the optic nerves {opn). Upon the face which is turned towards its fellow eacli retinular cell secretes a layer of chitiu, and these various chit- inous rods being in contact there is formed a structure com parable to the rhabdom of the Crustacean eye. In the centre of the retinular cells and below the rhabdom is a .single clear cell [c) whose lower end is also prolonged into a nerve-fibre. Each depression with its retinula and the chitinous pjtpilla wniCti tit." lUtO lu and represents its cornea is an winmatidiiim and the development shows that the ommatidia arise as I 432 INTERTEBRATE MORPHOLOGY. number of separate invaginntions of the ectoderm, the sides of the retinular cells which secrete the rhabdom being iu reality those sides which before invagination were at tbe surface of the body, and the xhabdom may therefore be regarded as composed of portions of the general cuticle which have been separated by the invagination. On the under surface of the carapace in the median line in front of the chelicerre is a small tubercle (Fig. 195, ol) which contains an organ supposed to be olfactory in function, and probably some of the setse upon the basal joints of the limbs may also possess a similar function. Nephridia are represented by a single pair of large reddish bodies lying at the sides of the cephalothorax. Each gland communicates with the exterior at the apex of a papilla situated on the interarticular membrane of the basal joint of the fifth thoracic appendage, and is at first a tubular organ and nephridiumlike, later becoming a lobate and complex structure. Chemical analysis indicates an excretory function for these glands, which have been termed the " brick-red " glands and also the coxal glands, the latter term indicating the situation of their openings on the basal joints (coxae) of one of the pairs of limbs. The Xiphosura are bisexual, the genital ducts opening on both males and females on the posterior surface of the oper- culum near its base. The ovaries are much branched paired structures, the various branches frequently anastomosing even across the median line. The testes are numerous spherical bodies scattered through the body and situated on branching and anastomosing vasa deferentia. Development and Affinities of the Xip/ioswra.— When tlio young Limulus leaves the egg it presents a remarkable resem- blance to a Trilobite and suggests a possible affinity with these forms which are known to occur only in the Palaeozoic rocks. In these same rocks there occur also the remains of forms known as the Eurypteridce which seem to have been even more nearly related to Limidus than were the Trilobit(;s. In them the cephalothorax bore apparently only six pairs of appendages which resembled more or less closely those of Zimvlus, except that the sixth pair was broad and oarlike. TYPE CRUSTACEA. 433 I, the sides n being iu ere at tbe erefore be tide which lediau Hue ig. 195, ol) u function, ints of the ' of large rax. Each )f a papilla sal joint of )ular organ d complex ry function brick-red " indicating ;s (coxae) of apening on f the opev- hed paired astoraosiug numerous situated ou -When the able reseni- ,ffinity Avitli 3 Palaeozoic remains of have beou 3 Trilobites. six pairs of ly those of md oarlike, probably serving for swimming. The abdomen was com- posed of twelve segments, the anterior six of which were much more massive than the others and bore five pairs of platelike appendages on whose posterior surface were the branchiae. The terminal segment bore a spine or fiulike structure. Such a form as this, represented by the genus rterygotus (Fig. 198), presents strong similarities to Zimulus and also to the Scorpions, bearing out the numerous similarities of structure occurring between Zimidus and those forms. This side of the affinity may be postponed, however, until the next chapter, and the comparison of Limidus with the Crustacea discussed here. Its chitinous cuticle, its jointed and biramous appendages, and its branchial respiration show similari- ties to the Crustacea, as do also the form of the heart and the compound eyes. Whether or not the coxal gland is comparable to the shell- gland is at present uncertain, but the other similarities are sufficient to justify the recognition of a Crusta- cean origin for Limulus. It iormi^ YmAm.—Pterygotus angliens indeed a connecting link between the ^^'■°"* Nicholson). Crustacea and the Arachnida, presenting probably on the whole more affinities with this latter group than with the former. Since, however, a Crustacean ancestry is probable, a com- parision of the appendages of Limulus with those of a repre- sentative of the ancestral group ought to be possible. It has already been noticed that the brain of Limulus is a syncere- brum composed of three segments ; it represents, therefore, two segments of which the appendages and other parts have disappeared. Furthermore, recalling that, in the higher Crustacea at least, a ganglion occurring between the cerebral autennary ganglia iu the embryo indicates a lost pair of 'll, 32"** 434 INVERTEBRA TE MORPHOLOQ T. Sip 2- »: lie appendages in these forms, the following table may represent the homologies of the appendages of the two groups. Crustacean. Limulus. 1 segment no appendage no appendage 2 • * • • « (( 3 antennules tt (( 4 .... antennae chelicerae 6 mandibles 1st pair of legs 6 ' 1st maxillsB 2d " " " 7 ....2d " 3d " " " 8 .... 1st thoracic appendages 4th " " " 9 .... 2d (( 5th " " '« .0 .... 3d « operculum LITERATURE. A. Oerstaecker. Crmtaeeen. Bronn's Klassen und Ordnungcn des Thierreichs Bd. V. 1. Abtb., 1866-79. A. S. Packard. The Anatomy, Histology, and Embryology of Limulus poly- phemua. Memoirs Boston Soc. Nat. History, 1880. E. K. Lankester. Limulus an Arachnid. Quarterly Journ. Microscopical Science, xxi, 1881. 8. Watase. On the Morphology of the Compound Eyes of Arthropods. Studies from the Biol. Lab. Johns Hopkins Univ., iv, 1890. W. Patten. On Vie Origin of Vertebrates from Arachnids. Quarterly Journ. Microscop. Science, xxxi, 1890. J. 8. Kingsley. Tlie Embryology of Limulus. Journ. of Morphology, vii 1892 ; viri, 1893. W. Patten, On the Morphology and Physiology of the Brain and Sense-organs of Limulus. Quarterly Journ. Microscop. Science, xxxv, 1898. TYPE ARACHNID A. 435 y represent ps. Limulus. appendage (( (( liceraB pair of legs (( « « « <( « <( « (( « (( i( irculum 5s Thierreichs, Litnulua poly- Microscopical pod*. Studies arterly Journ. •phology, VII, I Seme-organs 893. CHAPTER XIV. TYPE ARACHNIDA. The Araclmida are essentially terrestrial forms, for though a few species lead an aquatic or marine life, they are evi- dently descendants of forms which led a terrestrial existence and have only secondarily acquired the power of living under water. In all members of the group the body is covered by a more or less ^^ ick chitinous cuticle and the appendages nre as a rule jointed. A characteristic feature of the group is the fusion of the head and thorax to form an unsegmented cephalothorax bear- ing usually six pairs of limbs. The first pair of these are the chelicerce (Fig. 201, ch), composed of one to three joints aud terminated either by a claw or a chela ; they lie in front of the mouth, which is bounded at the sides by the basal joints of the second pair of appendages, the pedipalps (pe), which may be long and lind)like, or chelate, or in some cases* cluwlike, their basal joints serving in all cases as mandibles. Behind these follow four pairs of legs composed of six or seven joints, the basal joint being termed the coxa, the next, usually short, the trochanter, the third the femur, the next two to- getlier form the tibia, then follows in some forms a metatarHus, while the terminal one, provided with two claws, termed ungues, and in some mites also with a suctorial disk, consti- tutes the tarsus. Variations from this structure of course occur, the chelicerir, for example, in some mites being re- duced to short stylets, and in others the two posterior pairs of legs maybe quite rudimentary {/'ht/foptus). The most imjmr- timt variation is, however, that found in the members of the order Holifugu', in which a head is distinctly marked off from a thorax composed of three segments. The abdomen iu some loinis is segmented, in others all trace of the segmentation is lost, and, tinully, in the Mites it 111 436 INVERTEBRATE MORPHOLOGY. 5- '1! JjS. may be united with the cephalothorax. In the Scorpions it is divisible into an anterior portion, the prseabdomeu, much broader and stouter than the posterior postabdomen, an ar- rangement also indicated in certain other forms. In the adults the abdomen is usually destitute of appendages, though they may be present in the embryos ; the Scorpions, however, possess two highly-modified pairs, and it seems probable that the four or six papilliD upon which the ducts of the spinning- glands open in the Spiders represent also modified append- ages. A special respiratory system is entirely wanting in a few forms. In the majority there occur on the sides of the body from one to four pairs of pores termed stigmata (Fig. 201, st^'^). In the Scorpions and some other forms each stigma opens into a cavity lined with cliitiu continuous with that which covers ^ the general surface of the body, f and into this cavity there project * a number of lamellae arranged ^ like the leaves of a book (Fig. ^ 199), whence the term lung-books frequently applied to them. Each lamella is hollow, trabecular ex- tending across the cavity troiii one wall to the other, and the cavities communicate with tlie c, so that blood can readih' flow into them and so cliange its gases through the thin TiiRonim TiiK Li'No-BooK OF Avails of the lamellro. In otln'V Sim DKK (lifter McLkod). cases there occurs in connection ch = rliitino,j,'(Mious tiHsue. ^^itj^ the lung-book apparatus, or ff = ImmIv Willi. , J- 1 1 • -i 1 , ,■ , „ else entirely rei) acing it, a tia- «( = stigma. choal system consisting of a niini- t = lust coinpnrtnipnt of Iupg:- bor of tubes ramif^'ing thron^li book, tiiid.ualikc In clmr- tlio l,ody. Ill some cases a stron.^ tube or trachea arises at each ntigiiiti jxuii iTaverses tne ixjily, giving s>ii ijriiiiclieH to all p.'ti'is as it goes ; iu others there is iu couueutiou with each stigma Fio. 199.— TiiANHVEUSK Section TYPE AliACIINIDA. 437 icorpious it )meu, much neu, au ar- is. In the ges, though IS, however, obable that le spiuniug. ed appeud- Dg in a few )f the body g. 201, st'-% igina opens k'ith chitiu hich covers I the body, ip.re project B arranged book (Fig. lung-books lem. Each ^beculfB ex- !avity troiii r, and the with till) t bh)od ciiii 111 aud so gh the tliiii In other connection )paratns, or g it, a tr;i- jj of a nniii- ig thr 5e ^vhose nuclei are situated towards tl eir inner ends, while near 440 INVERTEBRA TE MORPUOLOG Y. the outer ends are situated a uumber of rodlike bodies (rliab- doms, h), whence these eyes have beeu termed prebncilhir ; the nerve-fibres are continuous with the inner ends of the cells. In the posterior dorsal and lateral eyes (/>*) an inversion of the retina (?•) has taken place, so that the rods (J)) are situated at the apparently inner ends of the cells and the nuclei at their outer ends, whence the term postbacillar applied to these eyes. The optic nerve-fibres enter at the sides of tlic eye and are distributed to the nuclear ends of the retinal cells, recalling the arrangement occurring in Feden among the Mollusca. The innermost layer of the eye upon which the ends of the rods rest is cellular, numerous minute crystals being deposited in the cells, whence it has the function of a reflector and is termed the tapetuni it). It is quite wanting in the prebacillar eyes. The signiflcanee of the structure of tlio Araclinid eye may be under- stood by siijtposing it to have been derived from u compoiiiid eye similar tn that of LiiiiulNs (sec p. 4;U), tlie iiidividiialities of the various ommatidia being more or less sul)ordinated. The cuticular cornea in Liiniiliis i.> smooth upon its outer surface, the inner surface 'oeing produced intd papilke, one of which corresponds to eacli ommatidium. In the Arachnids even these papillie are wanting, (he cornea showing no evidence of tlic j)rescnce of ommatidia. The lateral eyi's of the Scorpions aiti)roach moio nearly in their general structures the eyes of Liinuliis, though the conden- sation of the ommatidia has been carried further than in tlie median eyes of that form, or in the posterior dorsal and lateral eyes of (he Spiders. But in these eyes the condensation is associated wi(h an invagination of the en- tire eye, a process which, it may be remarked, is indicated in the niedi;m eyes of Limidnn. Tills invagination has been regarded as a pushing in, under and parallel to (he liypcxhu-mis, of a pouch of tliat layer, a prucc,>> which gives in cross-sec(i()n the appearance of an S-shaped fold. The outermost layer of the fold forms the vitreous cells or corneal liypoderniis, the middle layer (he retina, the inversion of whicli is plainly seen in (lie ])os(erior dorsal and laleral eyes of (he Spiders, while I lie innermost layer forms the postretinal layer in (he Scorpions and (he tapetuni of the jSpiders. The ommatidial re(inuh« are more or less retained in these eyes, as is shown by the structure of (he rhabdom, which in (he Scorpions i> composed of five ])ar'-, ui the Spiders of two, and in the Harvest -spid'V-^ of three. The auteei )r dorsal eyos of the Spiders do not seem to li,i\e undergone an invi^ination, hence the absence of a tnpetum and (he pra'ba- cillar structure of the retina; a corneal hypodcirmis is, however, preseni. and would seem t" wdicate an invagination, but its mode of orighi seems at present but imj>ori 'etiy understood. If ageneraliza(iou is to be iiiaile, it TYPE ARACUNIDA. 441 will be to the effect tha. the eyes of the Arachnids have been derived from compound eyes simihir to those of Limulus, and tiiat in the median eyes of tlie Scorpions, and the posterior dorsal and lateral eyes of the Spiders the entire optic area has been invaginated, making them comparable to the median eyes of Llmulus, while the lateral eyes of the Scorpions and the anterior dorsal eyes of the Spiders liave not undergone invagination and hence are comparable to the lateral eyes of Limnlus. Whether the com- parability indicates also the homology from a phylogenetic standpoint of eye to eye must remain at present uncertain (see p. 457). In cidditioii to the Malpishian tubules already meutioued as excretory organs occurring in connection with the digestive tract^ of the Spiders, there exist in many forms additional glands which probably are also excretory in function or sig- niticauce. These are the coxal glands, so called on account of their openings when present being on the basal joints (coxro) of one of the pairs of legs. In the Scorpions and Spiders the ducf;S of the glands open on the third pair of legs (i.e., the fifth pan- of appendages) in the embryo, but are usually wanting in the adults. In the Solifugic and Harvest-spiders coxal glands also occur in connection with the fourth pair of legs, and similar glands have also been observed in several genera of Mites, opening, however, at varying points. Glands are also of frequent occurrence in connection with the pedipalps, having apparently varying functions in differ- ent genera. They do not, however, seem to belong to the same category as the coxal glands and are in no case excretory. The Arachnida are bisexual throughout. The ovaries not infrequently fuse to form a single mass or a circular band, and in connection with the oviducts, which are in direct com- munication with them, there is usually developed a receptac- nlum seminis, and in the Harvest-spiders, an elongated ovi- ]H)sitor. The testes are also frequently fused, and the vasa deferentia are provided with vesicuho seminales and usually terminate in a copulatory organ. The majority of forms are oviparous, excepticms to the rule beiug found, however, as in tlie genus Phrynus and in the Scorpions, which are viviparous. 1. Order Scorpionida. In the Scorpions (Fig. 201) the body is composed of an uusegmeuted cephalothorax and an elongated segmented Ml «ll :1 1^' ,-itai» 442 INVERTEPRA TE MORPIIOLOO Y. abdomen. The seven anterior segments (the prfeabdomen) of the abdomen are broader and thicker than the remaining five segments (the postabdomeu), the last one of which ter- minates in a curved stout spine which bears at its extremitj' the openings of two ducts leading from a pair of glands lying in the twelfth abdominal segment and secreting a poisonous liuid. The chelicerre (c/i) are small chelate appendages situated in front of the mouth, while the pedipalps {pe) are long and provided with strong chelae, their basal joints and those of the two succeeding appendages surround- ing the mouth and serving as jaws. The four pairs of appendages be- hind the pedipalps are all similar in form, being six-jointed walking- limbs. Upon the abdomen modi- tied appendages are also found, the second abdominal segment bearing a pair, each member of Avhich consists of a single joint whose posterior edge is beset Avith a number of processes which give it the appearance of a comb, whence the name pectines [pt) applied to these appendages. In front of the pectines lies the geni- tal opening, protected by a small genital operculum {op) which may possibly represent another pair ol appendages belonging to the first abdominal segment. Upon the ventral surfaces of third, fourth, fifth, and sixth abdominal segments elongated pores are to be found which are stigmata {sV'*) leading into the respiratory cavities containing the lung-books, of which there are in all four pairs in this group. No tracheae occur. The intestine is quite straight in the Scorpions and laclvs cffical outgrowths excepting the two Malpighian tubules sit- FlG. 201.— SCOUPION (after Owen). c>.. = clielicerae. op = genital operculum. pe = pedipalp. pt = pecten. «('-■* = stiffuiata. TYPE AnACHNIDA. 443 uated at the posterior eucl of the mid-gut. The digestive gland 18 a large live-lobed structure which empties through several ducts into the mid-gut. ^ The nervous system consists of a syncerebrum lying aoove the cesophagus and giving rise to nerves for the eyes and for the chelicerae. It is connected with a suboesophageal mass from which the pedipalps and the three anterior legs are innervated, the fourth pair of legs receiving its nerves from a pair of distinct ganglia separated only by a short distance from the suboesophageal mass. Behind this in the abdomen IS a chain consisting of seven pairs of ganglia united by long connectives. The eyes vary in number from two to six pairs one pair being situated on or near the median line, while the' others are lateral. Coxal glands occur in connection with the third pair of legs, and the heart is an elongated structure lying in the an tenor portion of the abdomen and possessing eight pairs of OStlJX, The Scorpions are viviparous. The ovaries are situated m the anterior abdominal region and are elongated, that of one side of the body being united with the other by several transverse connections. The oviducts, which are short, serve as uteri, and open to the exterior by a single median opening situated on the ventral surface of the first abdominal segment Ihe testes consist of four tubes, those of the same side bein- connected by transverse anastomoses, and unite together to open into a protrusible penis, accessory glands, vesicuhe sem- males, occurring in coniic-ction with each vas deferens The single genital orifice occupies the same position as in the temale. The Scorpions are confined to the warmer regions of the globe, but few genera being known. Of these the genera Eu- ■scorpius and Buthm are perhaps the commonest tHttk '■IM IRliifei ';* ;;i53 2. Order Pseudoscorpionida. This order includes a number of small forms which are found under the bark of trees or among dead leaves or moss one genus, Chelifer (Fig. 202), occurring occasionally between 444 IN VEUTEBHATE MORPIIOLOO Y. the pages of books, aud lieuce being known popularly as the Book-scorpion. The cephalothorax is unsegmented, and is followed by a broad flattened abdomen composed of eleven segments. A prseabdomen and a postabdomen, such as can be distinguished ';i the Scorpionida, does not occur, nor is there a terminal poison-spine nor a poison-gland. The chelicene and pedipalps resemble those of the Scor- pions, beiuf.' chelate, and the four succeeding appendages are walking-legs, while the abdomen pos- ^tL sesses no appendages in the adult. Both Si the secon(l and third abdominal seg- ments bear upon their ventral surfaces a pair of stigmata which are the openings of tubular trachetxi which extend througli the body sending otf branches, except in Cherncs, in which bunches of un- brauched tracheae arise from each stig- ma. A heart is present, but consists of Fiu. 202.— Chelifer card- a siniple tube with either a single pair H(?/(/es(fio.n(:iiviKR). Qf (^ytia near its posterior extremity (Olnshun) or with four ostia {Chernes). The endodermal portion of the digestive tract gives rise to a pair of lateral ctecal diverticula branched at the apex and to one unpaired ventral one. Two eyes are present in Chelifer and four in Ohisium, Avhile they are entirely wanting iu Chernes. The reproductive organs open upon the ventral sur- face of the second abdominal segment, and the opening is surrounded with glands which secreve a fluid which quickly hardens to silky lilaments and serves to fasten the eggs to the abdomen of the parent. These glands are hypodermnl iu origin and correspond to the spinning-glands of the Spiders. 3. Order Solifugse. The members of this order are characterized by the head- region being separated from a thorax consisting of three se.u- ments and bearing the three posterior pairs of legs. TIih abdomen is also segmented, its ton sognicnts showing no dit- ferentiation into pr«3abdomen and postabdomen, nor does it TYPE ARACIINWA. 445 i openings possess any sting or poison-gland. The chelicenc are chelate but the pedipalps are long and leglike and possess glands which in Galeodes have been supposed to be poisonous. The anterior pair of legs lacks the terminal ungues found on tho others, and functions as a second pedipalp rather than a walkiiig-leg. No appendages occur on the abdo- men. Three pairs of stigmata occur on the ventral surface of the body, the most anterior ])air being situ- ated on the lirst thoracic segment, while the other two are on the second and third abdominal seg- ments. The anterior position of the first pair is probably to be regarded as secondary, and produced by a forward migration of the pair which should occur upon the first abdomi- nal segment. The stigmata lead ^'^' ^^^-—Gnleodes spinipalpus into tubular trachese which branch ^f'o'" clviku,. extensively. A comparatively simple heart is situated in the abdomen. The mid-gut possesses numerous branched diverticula as well as Malpighian tubules. The nervous system consists of a syncerebrum connected with a subu-sophageal mass which represents all the thoracic and abdominal ganglia fused to- gether. Two eyes are present, situated on a common eleva- tiou at the front edge of the head. The reproductive organs resemble those of the Scorpions except that transverse anastomoses do not occur, and the oen- ital opening is situated upon the ventral surface of the Srst abdominal segment. The Solifugno is a small order living more especially in warm sandy regions. They are usually, on rather insufficient grounds, supposed to be capable of inflicting a poisoned wound. Only two genera, Solpuga and (kdeodes, belon- to the order. I 5 •5!: nor does it 446 IN VEIiTEDliA TE MOliPUOLOQ Y. 4. Order Pedipalpl. The order Pedipal[)i includes two genera, Phrynua and Thelyphonm, both of which are inhabitants of the warmer regions of the earth. The cephah)thorax is unseginented ; the abdomen in Phrynus is eh)ngated and oval, and conn)osed of eleven segments showing little differentiation of form, while in Thelyphoniis there are twelve segments, the last three of which are much smaller than the others and bear a long, many- jointed terminal filament. The cheli- cera3 are not chelate, but the terminal joint may be Hexed upon the basal one and contains the duct of a poison- gland which opens at its extremity. The pedipalps in Phrynus are long and leglike, though richly provided with spines, and terminate with un- gues, but in Thelyphonus they are rel- atively short and stout with a flexible terminal joint as in the cheliceru' ; in both genera the basal joints of the , , , two pedipalps are fused. The iirst datus (from CuviER). • ■ i n t leg IS long and slender and termi- nates in a filament-like structure, the other three pairs being typical walking-legs. Four stigmata occur, one pair situated in the second and another in the third abdominal segment, and they open into cavities containing lung-books. The digestive tract is com- paratively simple, but the nervous system shows a concentra- tion of the postcjesophageal ganglia similar to that described for the Solifugffi, except that a single pair of ganglia occurs in the abominal region united by long connectives with tlm cephalothoracic mass. Eight eyes are present, two of whicli are larger than the others and situated at the anterior edj^v of the dorsal surface of the cephalothorax, while the other three pairs are situated laterally^ The reproductive organs are paired and open by a median TYPE ARACHNID A. 447 orifice situated on the ventral surface of the first abtlomiual segment. J*hrynus is vivipairous. r>. Onlur Phalangida. Tlie Phalangida (Fig. 205), popularly known as the Harvest .s|)iders, possess an unsegiuented cephalothorax (ct) and have from six to nine segments composing the abdomen {ah). The ii- larly applied. Other forms, such as (umiilcptiis, with spinoso jHMlipalps, are tropical in habitat, whih^ CijphophfjKfJmm aiid (iibhiK'cUum have a limited distribution, and on account of tho many diflnrences of structure which they present when com. jmred with other forms are sonu^times grouped together to form a s(>parate order. It is to be noted especially that these two forms possess upon the second abdominal segment a ]i;iir of wartlike elevations at tho summit of which the ducts df numerous spinning-glands o])en. G. Order Araneae. The order Araneie includes a large number of forms possessing very tleJinite characteristics. The ceplialothoi.ix TYPE ARACHNID A. 449 is uusep;menteil, as is also the abcloinen, which is an oval, si)herieal, or sometimes irreguhirly-shaped region wliich narrows suddenly anteriorly so as to be much narrower than the cephalothorax. The chelicerfe project somewhat in front of the cephalothorax and each consists of a broad basal joint iiiid a terminal strong claw which may be Hexed upon the basal joint, and has opening at its tip the duct of a poison- ghmd (Fig. 200, p(j) which lies in the cephalothorax. The ht, dK spg ''■* tr FlO. 30G. — DlAGUAM OF STnUCTUIlE OF A SlMPKIi 'Hftcr LkitkAHT). iio = aorta. ce — cerebral ganglion. ch = c'lielicera. (Iff = digestive ginnd. gp = genilal pore. ht = heart. l/j = liing-l)ook. int = M;iii)igliian tul)nle. oc = eye. ov = ovary. J)e = |)eili|ijil|). Pff ■- poison gland. rb = rectal liladder. rs = receptucuhmi seminis. « = stomach. sd = stomaeii = spinneret. »pg = spinning glands. tff - tlionieie ganglion. ir = trachea. p((lipalp;-i of the fen)ales are leglike structures usually with a terminal unguis, but in the male are more or less swollen to serve as ticcessory organs in copulation. The four pairs of seven-jointed legs are all simihir in structure and serve for walking, dilToring in relative length v.\ ditierent genera. In thti emlu-yc the abdomen is distinctly segmented and bears five or six pairs of rudimenl.iry ap])endages, the more ante- rior of wiiicii later disappear, while the two or three postcn'ior pairs persist as the spinnerets («/)), so called from the occur- rence on tlieni of the openings of the ducts of the spinning- elands (HpcfX These are very numerous and oj)en at the apices of the spinnerets, eiuh gland i)rodiiciug a fluid .secretion which ouiekly l.-.nh-- i^^^ exposure to the air io f,,rm a silken I 450 INVFAITEBRA TE MOIiPUOLOG T. thread. The thickuess of the thread may be modified by uuitiug together the secretions of a greater or less number of the glauds, which, moreover, differ amoug themselves, some producing, for instance, a sticky secretion with which certuiu of the threads may be covered. In some forms there is situ- ated upon the abdomen just in front of the spinnerets ,t chitinous plate, the crihellum, which is perforated, like the spiunerets, by the ducts of numerous spinning-glands. Its ])reseuce is associated with that of a calamistriim, a peculiar modification of the metatarsus of the last pair of legs, it beiu"' furnished with a double row of bristles which are rapitllv waved over the cribellum and draw from its glands their secretion. Tlie threads are used for several purposes, as, for example, to fasten the ova to the body of the parent or to form a cocoon for them, or else to form a snare by which insects may be caught to serve as food. These snares iu some cases are composed of an irregular network of threads arrauged without any definite pattern, as in Theridium, ])iit some other forms show a certain amount of architectnnil skill, weaving a i)latform of felted threads which tenninatcs in a tubelike place of concealment for the spider (e.g., ^b/r. letia, Tegenaria) or webs composed of threads radiating fioiu a central point and united by other threads arranged in a spiral or in concentric circles (e.g., hpeim, the common gardm- spider), or else using the threads to form a hinged trap-door covering in a burrow in the earth which serves as a doniicilf as in the Tra})-(K)or Spider. The digestive tract exjjands in the thoracic region into a saclike structure (.v) from each side of which thive or jiioic usually five ca'cal diverticula (.sd) arise, the anterior pair sometimes anastomosing so as to form a ring, while in souk; cases {Epaiva) secondary diverticula (>xt(>nd from tlu! iiioiv postiu'ior ones into tiie coxal joints of the legs. In the ahdo- mon the intestine is more cylindrical, giving rise to niurli- branched lateral div) which a short TTPE ARACHNID A. 451 modified by ss number of iselves, some vliich certuiu there is sitii- spinnerets ;t ted, like the -glauds. Its m, a peculiiir legs, it being are rapidly glands their poses, as, for parent or to re by which 3e snares iu k of threads wridium, l>iit irehiteotnral li termiuatfs er (e.g., Jr/r- diating fioiu •ranged in a mon garden- ed tra])-door iS a domicile Bgiou into a ive or jnoi't' iiterior pair lile in soino a the molt' [n the abdo- ^o to nitifli- ;he Ho-ciillt'd it joins (he , TllO (Mld- liich H short rectum connects with the anus situated at the posterior ex- tremity of the body. In the genus iMygale and allied f..rms two pairs of stig- mata are found near the anterior portion of the abdomen, l)oth of which lead into cavities containing lung-book.s! In the majority of forms, however, bat one pair of luirg-books (U)) occurs, the second pair of stigmata opening into a tracheal tube {tr) extending into tlie cei)halothorax and terminating in a bunch of unbraached trachere, a similar bunch arising near its base and extending backwards into the abdomen {Seges- tria).^ In some iovivn the second or tracheal stigmata may be situated far back upon the abdomen, and may be united to a single median transversely-elongated cleft, from which a l)unch of unbraached {Aftus) or branched trachese aiises. The heart {ht), which lies in the abdomen, is enclosed Mithiu a so-called pericardium and possesses three pairs of ostia. It is continued anteriorly and posteriorly into aorta', and gives off also lateral arteries, all of which open after rel- atively short courses into the lacunar spaces. T]ie blood is returned u) the pericardial cavity, whence it passes into the lieart, the greater portion on its way to the pericardium pass- ill,!;- through tJie lung-books. The nervous system consists of a syncerebrum (ee) and a Jarge cephalothoracic ganglionic laass [tg]. In addition to the nerves to tlie appendages, a posterior nerve arises from this mass and })asses backwards towards the abdomen, in Mugnh' dilating at the junction of that region with the cephalo- thorax into a pair of small ganglia. A sympathetic or visceral bvstcm, ccmsisting of a nerve arising l)y paired trunks from the brain, is distributed to the anterior ])ortion of tlie dige.s- tive tract. The eyes arc; usually nunuMous, threci or four jjairs orourring on tlie anterior ])orti()n of tlie cephalothorax, their arifuigement varying in different genera. C'oxal glands have been found in several forms in connec- tion with the first i)air of legs, and in Mjigale tliey occur in connection with the third pair. The reproductive organs «'|"'ii in botli sexes by a singl" opening situated near tlie ■interior end of tlie abdomen between the .-interior stigmata. 'Iho ovaries {ov) are paired, or may unite to form a rinLr, and t on I 462 INVERTEBHA TE MORPHO LOG Y. tlie two short oviducts unite to form a vagina with which may be associated receptacuhi semiuis {rs), though more usu- ally these structures open independently in front of the genital orifice and may be single, or paired, or in some ca.si ,s even three in number. The testes are cy- lindrical structures whose long, slender, uiJd frequently-contorted vasa deferentia unite just before opening to the exterior. A remarkable copulatory organ is formed liv the terminal joint of the i)edipalp of the male (Fig. 207), which bears upon its inner surfjjce a process cojitaining a spirally- coiled tul)e. This tube opens at the ex- Fnj. 207.~PEDiPArp ^^"^["^^^J »/ t^^*^ process, and is filled by tlie OF Mai,i.; SnuEu ^pi^^er Avith spernnitozoa, and during cojui- (after ukktkau). hitiou is inserted into the receptacula semi- uis of the female. The males are usually smaller than the females, and tluir approaches are frequently resisted by the latter, who en- deavor to capture and destroy the persistent swains. In the Attidte a process of courtsiiip has been observed to occur, the male posturing before the female and displaying to their best advantage the highly-colored hairs with which the body is covered. The ova are in nniny forms (Lycosa) attached to the under surface of the abdomen, while in others tiiev are enclosed in a silken cocoon which may either be carried about by the female or suspended in the webs or deposited in protected situations. Two suborders are recognized, accoi-ding as there are two pairs of lung-books or only one. The TcfrapiicMmoui's in- clude the forms with two i)airs of lung-books, among whieh are the Trap-door Sj)iders, Cfenizft, already mentioned, and the Tarantula, Jhnjifh', tlie largest of all the spiders and reputr,^ to attack oven small birds. The Dipneumone.s have bin ,t single pair of lung.l)ooks, the majority of living spiders he longing to the subonhir. Some, such as /i/>r/\vf, Jf/ehna, Tt'ijc- vnri'ii, T/ict'idinni, and Seijesfria, spin wol)s, while others c.Arh thoir j)rey by their rapid movenjenta (Lycihsti) or by sudd(>iilv springing upon it (Attufi). TYPE ARACHNID A. 453 with wLich ;h more usii- iout of tli(> 1 some ea^s(^s istes are cy- wleiider, uud fentiu unite ixterior. A s formed by palp of tlie )oii its inner a spirally- at the ex- died by the uriu^ cojm- tacula seiai- !s, and their ir, \vh(j eii- ius. lu the o ofcur, the <) their best the body is attaclietl to srs they are be carried lepositod in ere are two VKmoin's iu- iioij^ which led, and tiio Lud repiitr ' liave bin ,t spiders i'v /fluiK, Tr(Ji'- 'tilers Catcii •}' suddeiily 7. Order Acarina. The Acarina are for the most part small forms, many ])(5iug almost microscopic, while the largest, the Ticks {Ixodes) do not when at tlieir greatest size exceed a centimeter iu inngth, the males being much smaller. Some forms, such as <}ribates and Nothrus, live among moss and in similar situa- tions, while others, such as Hydrachna and Afax, are aquatic Many forms are, however, parasitic either ujxin plants (^^ fmnychm and Phytoptus) or on auiunils, the genus Sarcoptes l)eiug the cause of the disease termed the Itch in man the symptoms being produced by the Mites Imrrowing beneath the skin. Other forms affect various animals and birds the genera Dermakichm, AnaUjes, etc., feeding upon the feathers Fi.i. 208.-^, Sarcoptes scabiei ; /?. Demodev phyllokles (after CsoK.m fron, Whkiht). <'f various birds, while others, such as Deumlex (Fig. 208, B), liv.3 1.1 the hair-follicles or sebaceous glands of the skin, pro- (Fig. 201), Md) are I INMM 454 IN VEltTEBliA TE MOltPHOL OGY. 5 It ;! frequently chelate, but in parasitic forms are reduced to stylets enclosed by the fused basal joints of the pedipalps, a proboscis being thus produced which can pierce the integu- ment and thus render the juices of the host available as food. The pedipalps (Alxp) undergo various modifications, being sometimes long and limblike, sometimes chelate, while their basal joints may or may not be fused. The four pairs of legs are generally adapted for walking, and terminate in ungues or bunches of hairs or, in some parasitic forms, in suctorial disks, while in the Water-mites they are provided with usually long bristles along the sides, serviceable swimming-orgaus being thus produced. In the genus Demodex the four legs are reduced to short unjointed structures each provided with four ungues, while in the Leaf-mites, Phytoptus, Avhicli pro- duce galls on the leaves of various plants, the two pairs of posterior limbs are reduced to wartlike elevations l)earing bristles, the two anterior pairs being on the other hand fiviv jointed. The chitinous covering of the body is usually thick and delicately wrinkled. It usually bears numerous setji* and occasionally also plates or lateral prolongations, as in Ori- bates and its allies. Dermal glands also frequently occur, ]>roducijg oily tluids and sometimes odoriferous secretions. Spinning glands opening on the pedi})alps occur in Tetniinj- chns, frequently ])arasitic on the leaves of the Rose, but as u rule they are not developed. A pair of stigmata (Fig. 209, fit) occurs in many forms, situated usually near the coxse of the last pair of legs, l)iit not unfrequently they are much further forward, lying near tiie basal joints of the pedipal})s or even of the chelicerji'. They open into trachea^ which branch once, bunches of lateral trachew being situated at intervals upon the two branches. Fre({uently, however, especially in parasitic and aquatic forms, both tracheiP and stigmata are wanting, as is usually also tlu^ heart. When j)resent ((riimd.sius, /xodcti) this latter structuiv is Hunill, with but a single pair of ostia, and is prolonged an- teriorly into a slender aorta. The digestive tract is frecjuently provided with glanabdomen and a postabdomcn (see Fig, 198). In the internal structure quite as striking similarities are to be found in the presence of an endosternite in both groups and of coxal glands in connection with the fifth pair of appendages, in the tendt .cv towards the eencentration of the postoral ganglia, and in the invaginate'origin of the median eyes, to mention but a few points. The Arachnida are accordingly to be traced back to Limulus or Euryp. temsAxkQ ancestors, and through thes.,' finally to the Entomostraca, perliaps a C rustacean ancestry being clearly indicated. As to the relationslni)s of tiio .anous orders little that is definite can l)e said, differentiations liaving taken place along different lines in the various orders, so that while the LEY). 1 = indifferent .stage. // = Limulus stage. A = Aia(;lini(ian staffc. 3 458 INVEliTEDliA TE MOIWIIOLOO Y. Pedipalpi arc more pi'itnitive as regards the number of abdominal seg- ments and their distinctness than tiie Aranea", yet the hitter and especially the Tetrapneunioiies show a much more primitive condition of tlie respira- tory oi'gans. With regard to tliese organs it may be at; d that the con- dition in which they are represented by buuciies of un iunched tracheae is more primitive than that in which they are branching tubes, the bunched condition being probably derived by a modification of original lung- books. TYPE ARACHNIDA. 1. Order /Scorp/omrfa. — Abdomen segmented and differentiated into pra;- abdoraenand postabdomen ; postabdomen terminating in poison- spine ; pedipalps chelate ; two pairs of abdominal api)endages ; four of stigmata and lung-books. Euscorpius, Bathus. 3. Order Pseiidoscorpioithla. — Abdomen segmented but not differentiated ; no terminal spine ; pedipalps chelate ; no abdominal appen- dages; two pairs of stigmata opening into trachea;: first pair of legs adapted for locomotion. CheUfei\ Obisiuvi, Cheriiea. 8. vU'dei Solifugw. — Head separated from tliorax with thret; segments; abdomen segmented but undifferentiated ; no terminal spiiu; ; pedipalps palplike ; three pairs of stigmata leading into trachea\ Oaleodes, Solpuga. 4. Order Pedipalpi. — No distinction of head and thorax ; abdomen seg- mented, and either undifferentiated or with three small segments terminated by a multiarticulate tlagellum ; pedipalps leglike or subchelate ; two pairs of stigmata and lung-books ; first pair of legs elongated and palplike. Phryntts, ThehjpUonus. 5. Order Pltalamjlda. — Abdomen segmented but undifferentiated and without appendages or terminal spine ; pedipalps leglike ; one pair of stigmata leading into trachea; ; no spinning-glands. Leiohunum, P/ialanyium, Opilio, Qonyleptus, Cyphophthalmus, GibbocelluiH. 6. Order Araiieir. — Abdomen nnsegmonted and with two or three pairs of rudimentary papillalike appendages bearing the openings of ducts of numerous spinning-glands ; abdomen not fused with eephalothorax ; pedipalps long and palplike or leglike. 1. Suborder Tetrapneumonen. — With four stigmata opening into sacs containing lung-books. Mygale, Cteniza. 2. Suborder Dipnenmones. — With four or three stigmata, the anterior pair opening into sacs with lung-books, the posterior one or two with trachea?. Epeira, Agelena, Tegenaria, Tlierkliniit, Segestria, Attus, Lycosa. 7. Order Acarina. — Abdomen unsegmented, without appendages, ami fused with the eephalothorax ; pedipalps sometimes long uml leglike, sometimes chelate ; stigmata wanting or present as a TYPE AUACUNIDA. ilorainal seg- nd especially [ the respira- thiit the con- ;he(l tracheie ; tubes, the I of original 469 single pair leading into tracheue ; many forms parasitic • fre- quently with complicated metamorplioses. ' Nonparasitic, or ])arasitic only in larval stage ; terrestrial Ori- Oates, JVothntfi, TrombkUum. A(iuatic. HydracluM, A tax. Living on organic matter. Ti/rof/li/p/nis. Parasitic on animals. Deinodex, Sarcoptes, Dermaleiclnts Anal- (jes, Oamasus, Ixodes. Parasitic on plants. Tetranyehus, Phytoptus. cd into praa- ingin poison- api)endages ; thus. tTerentiated ; iiinal appon- aj : first pair <, Chernes. e segments ; minal spine ; into trachea\ bdomcn sog- iiall segments ps leglike or ; first pair of us. Bntiated and leglike ; one ining-glands. lophthalmus, ;hree pairs of openings of Dt fused with ike. opening into , the anterior terior one or , Theridiniii, 3ndages, anil 3CS long uml present as a LITERATURE. GENEIIAL. H. Grenacher. Unterauchunge7i iiber das Sehorgan der Arthropoden. GC.ttingen, ^' ^uT^T.xi'"'''^''" ""^ ^'■"'^' ''^- ^'"^'■^^'•'^ J"""-"- Microscop. Sci- vIKC, A-XI, loo J.. J. Macleod liecherckes sur la structure el la signification de I'appareil respira. toire des Amchnides. Archives cle Biologic v 1884 ^' 'wTc^rix^tso?''"''^'"*''' '^''' ^'•"'''"^"^^«' '^'''"^'^^ a- ^- Zoolog. Inst. J. S. Kingsley. The Embryology of Limulus. Part II. Journ. of Mor- phology, VIII, 1893. ^^^*^* BCOIJPIONIDA. 1. Dufour. Ilistoire cmatomique et physiologique des Scorpions. Memoirs Aciid. Sciences. Paris, xiv, 1856. ^tuiuiis E. K^I;anke8ter. On tkeCoxal Glands of Scorvio, etc., and the Brick-red ^^l^ndsoJUmulus. Proceedings of the Royal Society, xxxi v. 1884. G. H Parker. Ike Eyes m Scorpions. B.llctiu of the Museum of Compar- ative Zoology, XIII, 1879. v^umpar- W, Patten. The Origin of Vertebrates from Arachnoids. Q.mrterly Journal of Microscop. Science, XXXI, 1890. J -Jouinai M. Laurie. The Embryology of a Scorpion {Euscorpius itaiicus). Q.mrterlv Jouiu. Microscop. Science, xxxi, 1890. V£<"iutriy rSEUDOSCOKPIONIDA. A. Croneberg. Beitragzur Kennt7riss des Banes der Pseudoscorpione. Bulletin hoc. Imp. Nuturalistes Moscou, ii, 1888. -tsu'ittm SOLIFUG/E. 1. Dufour. Anatomic, physiologic, et histoire naturelle des Galeodes. Me- moires Acad. Sciences, Paris, xvii, 1858. niALANGIDA. ^ wisf ■ 1 T7' '"' t"''"""'' '^"' ^^^^^^^^'mlen- Zeitschrift fur \\issensch. Zool., XXX VI, 188:2. i 'SUM.. %* ^ w IMAGE EVALUATION TEST TARGET (MT-3) / O 1.0 I.I 1.25 ■ 45 110 ^ 1^ 12.2 IT 1^4 1^ IIIIIM 18 U_ IIIIII.6 6' %^ n\ ■li^ ^ 6^ ^Lfc^'^ C/j ^ mm 460 INVERTEBRATE MORPHOLOGY. C. M. Weed. A Descriptive Catalogue of the Harvest- spiders (Phalangiida) of Ohio. Proceedings United States National Museum, xvi, 1893. 2;-« ARANEiE. E. Zeyserling. Die Spinnen Amerikas. Nttruberg, 1880-91. G. W. and E. 0. Peckham. North American Spiders 'of the Family Attidm. Transactions of the Wisconsin Acad. Sciences, 1888. H. C. McCook. American Spiders and their Spinning Work. Philadelpliia 1889-90. ' J. H. Emerton. Papers in tbn Transactions of the Connecticut Academy vii 1889, and viii, 1891. 0. Marx. Papers in tlie Proceedings of the Entomological Society of Wasli- iugtou, 1891. and iu the Proceedings of the U. S. National Museum xii 1890. ' ' W. Sohimkewitsoh. Etude sur Vanatomie de I'Epeire. Annales des Science* Naturelles, 6meser., xvii, 1884. Ph. Bertkau. Ueber den Verdauungsapparat bei Spinnen. Archiv. fUr mi- krosliop. Anatomie, xxiv, 1885. Bcitrage zur Kenntniss der Sinnesorgane bei Spinnen. I. Die Augai. Archiv fUr miliroskop. Anatomie, xxvii, 1886. £. L. Mark. Simple Eyes in Arthropods. Bulletin Museum of Comparative Zoology, XIII, 1887. A. T. Brace. Observations on the Embryology of Insects and Arachnids Baltimore, 1887. ACARINA. 0. Haller. Zur Kenntniss der Tyroglyphen und Verwandten. Zeitscbr. fiir wissensch. Zoologie, xxxiv, 1880. H. Henkingr. Beitrdge zur Anatomie, Entwicklungngeschichte and Biologic roii Troinbidium fuligiiiohum. Zeitschr. fUr wissensch. Zoologie xxxvii 1882. " ' ■ A. D. Michael. British Oribatidm. Lond(;n, 1984. A. Nalepa. Anatomie der Phytopten. Sitzungsber. Akad. wissoiisch Witii xcv, 1887. W. Winkler. Das Ilerz der Acarinen nebst vergleichenden Bemerkungen uber das Ilerz der Phalangiden und Chernetiden. Arbeiten a. d. zool Ins Wien, VII. 1888. Anatomie der Oamasiden. Arbeiten a. d. zoolog. Inst. Wien vir 1888. APPENDIX TO THE \UACIINIDA. There are three orders which show a certain amount of affinity to tlie Ar-ichuida, but which are not so ch).sely reiatid as to warrant the actual association of them with the orders which have heen assigned to that tvpe. They will be de- lih TYPE ARACHNID A. 461 Zeltscbr. fiir L'nsch. Willi, i scribed here, and are the orders of the Fentastomidce, the Pycnogonida, and the Tardigrada, Order PentastouidsB. The PentastomidsB are all parasitic, living in the adult stage in the lungs or nasal cavities of various animals, one species, PentoMomum tcenioides, occurring in the nasal cavities or sinuses of dogs and Avolves, VAhile several species have been found in the lungs of different sptcies of snakes (Fig. 211). They are all elongated wormlike forms, some- times slightly flattened and usually distinctly aunulated, the annuli, however, not repre- senting a metamerism. The anterior end of the body is rounded and bears on the ventral surface the mouth, upon each side of Avhicli is situated a pair of strongly-recurved hooks {h) supplied with special muscles and serving for the attachment of the animal to the tissues of the host. With the exception of these hooks no appendages are present. The body is covered by a cuticle secreter" by the ectodermal cells (hypodormis), be- neath which lies a layer of circular muscle- libres, and beneath these again a lnyex of longitudinal muscles. The coelom is ample and is traversed by dorso-ventral muscle- bands, which divide it into a central com- partment containing the various organs, sus- ). ended by mesenteries, and two lateral ones. ;„„,^ Femalr .after lliere is no heart or circulatory apparatus, spkncbr). and trachefP or othor respiratory organs are '* "^ Jiooks. also wanting. ^" = *^«"'^''' "••^^'■«- The digestive tract is a straight tube extending through tlie body from the mouth to the terminal anus, giving off uo lateral diverticula throughout its course. The nervous sys- tem (Fig. 212, wo) consists of a ganglionic mass lying below tlib (imophagus, a c<)m})aratively small commissural ring passuig round that portion of tlie digestive tract, without, Fm. %n. — Penta- Htomu7,i teretiusru- ISWu. 462 INVERTEBRATE MORPHOLOGY. however, possessing any ganglionic enlargement which can be termed a cerebrum. Various nerves are given off from the mass, two of which extend backwards throughout nearly the entire length of the body. The only sense-organs pres- ent are a number of small papillee on the anterior portion of the body, which are probably tactile in function. Glandular organs are highly developed. Scattered over the surface of the body are numerous flask-sliaped glands apparently ectodermal in origin, while lying in the coelom on each side of the mid-gut and extending back almost to the posterior ena of the body are two long c^cal tubes, a glan- dular structure being also connected with them anteriorly These glands open in the vicinity of the hooks and have hence been termed the hook-glands (Fig. 211, hg), and it has been suggested that they secrete a fluid which serves to keep the blood which the parasite ingests from coagulating, being thus similar to the glands in the pharynx of the Leeches Fig. 212.— Diagham op Structuhe op Femat,e Pentnstomum (after Spenkcb). go = genital orifice. od = oviduct. hff = hoolc-glaiid. ov = ovury. I = intesliue. j-g = seminal receptacle. ng = nerve-ganglion. ut = uterus. which serve the same purpose. Unless the eciodermal glands are excretory, no special organs for the carrying on of that function occur. The Pentastoniidre are bisexual, the mule being smaller than the female, aud recognizable l)y the situation of the geni- tal orifice (Fig. 211, 70), which is near the anterior end of the body, wljilo in t\w female it is near the posterior end. The ovary and testis are both uii])aired organs situated beneath the dorsal surface of the body and extending almost its eutiro length. Anteriorly a pair of oviducts (Fig, 212, <)\Mmng Pentastomum to be related to the Arachnids is the occurrence of the four-legged larva, which resembles so tar as Its external form is concerned, a Mite. The internal structure is \ my different, however, although corfain Arachni.l features are indicated ■ I'ut It IS evident that the.se forms must have undergcu.e an enormous de- l-amire from the ancestral form during which the remaikal)le life-historv an.l peculiar structure have been acquired. The parasitic habits of many liu-s. and the general similarity of tiie body form of Demmlex to that of I'Htastom.im, suggest the Mites as the ancestors of the latter, a theory wlacii IS as plausible as any other which can at present be suggested. Order Pycnogonida. The Pycnogonida are exclusively marine in habitat, and vary considerably in size, the smaller forms, such as Tamjsty. It Ml' «... 'X i 464 INVERTEBRATE MORPHOLOGY. lum, being only about a millimetre in breadth, while the purple Fhoxichilidium measures over three millimetres from tip to tip of the legs, and the deep-sea form CoUossendeis has a span of over sixty centimetres. The body proper is compar- atively small, the four pairs of long legs which arise from the thorax being exceedingly conspicuous, a feature which has Fig. 213. — Phoxichilidivm mnxillare (after Morgan). suggested the term Fanfopoda sometimes applied to the group. An'oriorly there is a ivell-marked proboscis carrying:; the mouth at its anterior end, and at the base of this there arise the cheliceriB, which are rather short chelate limbs. The next segment of the body succeeding that which bears the cheliceru} bears upon its dorsal surface the eyes, and may be regarded as a fusion of three segments since it bears three pairs of appendages. The most anterior of these are slender ' TYPE ARACUNIDA. 465 jointed palps; the econd pair, wautiug in the females of some species, but always present in the males, arise from the ventral surface of the segment, and are curved jointed struc- tures serving to carry the ova ; while the third pair are ex ceeding long jointed walking-legs. The next three segments nlso bear long walking-legs, the last one having attached to it the usually unsegmented rudimentary abdomen. The body and the appendages are encased iii a well-de- veloped chitinous cuticle, and there are no indications of special respiratory organs. The heart lies immediately be- neath the dorsal integument and is a simple tubular organ with from two to three pairs of ostia. The portion of the digestive tract which lies within the proboscis is lined with chitin and opens behind into an elongated mid-gut, from which long diverticula extend out into the chehceraa and the proboscis and into the walking-legs sometimes reaching even into the terminal joints of the latter' A short hind-gut leads to the anus at the tip of the abdomen The nervous system consists of a supracesophageal gan- glionic mass, from which arise the optic nerves and those for the chehcerae, as well as certain nerves passing to the pro- boscis. Connected with this brain by circumoesophageal com- missures IS a ventral chain consisting of five pairs of ganglia the first pair of which is really formed by the fusion of two pairs, distinct in the embryo, and innervates the palps and the oyigei-ous legs, while the four pairs of walking-legs are sup- phed by the remaining four pairs. Finally one or two small guugha also occur, innervating the abdomen. The eyes are imir in number, situated at equal intervals upon a small (lomelike elevation on the dorsum of the first thoracic seg- ment, which, it is to be remembered, is compound. Each eye IS covered by cuticle, sometimes thickened so as to form a lens, below which is a layer of cells forming the corneal or cuticular hypodermis. Below this comes a thick layer com- posed ot retinal elements with nuclei in their outer portions and rodhke bodies towards the inner ends where they rest iil)on a layer of pigment. , Tlim' eyes recall the poslbacillar eyes of tl.o Arachni.ls by their struct- me, but show one renmrkable peculiarity, i.e., a distinctly bilateral ar- ^mmmm 466 IN VERTEBRA TE MORPHOLOO T. P? ,««• nil rangement both of the corneal hypodermis and of the retinal elements, a distinct raphe being observable upon the inner surface of the eye, the reti- nal elements being arranged on either side of it. Such a condition as this cannot readily be explained by a simple unilateral invereion such as was described as probably occurring in Arachnidan eyes; it suggests rather an invereion of two sides of a primitive optic cup, the posterior wall at tlio same time forming the pigmented layer of the eye. "Whether the Arachnid eye is not also traceable to such an arrangement, all traces of the original raphe being lost, is a question, though at present it seems more probable that it has been produced by a suppression of the inversion of one side of the cup. Glands, occurriag in the palps and ovigerous legs, have been regarded as excretory in function, but no Malpighiuu tubules or coxal glands seem to exist, though an homologj- of the excretory glands just mentioned and of glands occurring iu the walkiug-legs of the males with the latter is not impossible. The Pycnogonids are bisexual, the reproductive organs lying in the thorax and sending out branches into the walking-legs, on the fourth joints of one or more of which they open. As already stated, the male carries the eggs upon his ovigerous legs, fastening them as they are extruded by the female by means of the excretion of the gJauds occurring upon the walking-legs. Development and Affinities of the Pycnogonida. — The young Pyciio- gonid leaves the egg as a six-limbed embryo, which recalls, in a general way. the nauplius of the Crustacea, and indeed has suggested a derivation of the Pycnogonids from that group. The resemblance is, however, but superficial, important differences being found in the structure of the eyes and in tiie absence of an anus, to say nothing concerning the details of the early development. On the other hand these last, as well as the structure of the eyes, recall the Arachnids, and it seems most probable that the Pycnogonids are to be regarded as having descended from ancestors wIiIlU might have been included in the type Arachnida. Order Tardigrada. The Tardigrada are small forms not exceeding a milli- metre in length, with an uusegmented body provided with four pairs of short conical appeudages tipped with claws, tiie last pair being situated at the posterior extremitj- of the body. The body is covered by a cuticle secreted bj' the subjacent hypodermis, below which and traversing the coelom is a well- TYPE ARACHNID A. 467 developed system of muscle-bands. There are no special organs either for respiration or circulation. The mouth (Fig. 214, m), surrounded by papilla, lies at the anterior extremity of the body, and leads into a tubular mouth-cavity containing, imbedded in its walls, a pair of chitinous or partly calcareous teeth, and receiving the ducts of two glands {sg) which have been regarded as salivary or perhaps poisonous in function. Behind, this cavity opens into a muscular pharynx which is connected by a short a3so- phagus with the mid-gut (s). At the junction of this with the rectum or hind-gut is a pair of cfiecal diverticula {mg\ possibly Malpighian tubules, and into the hind-gut there also open the ducts of the reproductive organs, the hind-gut thus serving as a cloaca. It opens on the ventral surface of the body a short distance from the pos- terior extremity and therefore in front of the last pair of appendages. The nervous system consists of a supracfisophageal ganglion (ce) united with a chain of four pairs of ventral ganglia. No special sense-organs occur except two eyes situated at the sides of the head. The sexes are distinct, the reproductive organ being un- paired and opening into the cloaca, into which opens also in both sexes au unpaired accessory gland. The Tardigrada occur in water usually, especially in such locations as the gutters on the roofs of houses, though some- txines found also among moss. The group contains but a small number of genera, of which Macrohiotus is perhaps the most common. Fig. 214. — Diagram op Stkucture op Macrobiotua megalonyx (after von Erlan- oek). an — iiuus. ce = cerebral gauglion. e^— e* =: appendages. h = bindgut. m = moutb. mg = mid-gut diverticula. oc = eyv. a = midgut. iff = salivary (?) glands. so = suboesopbageal gan- glia. 468 INVERTEBRATE MORPITOLOOT. «3 lis 2 "5 Affinities of tlie Tardigrada.—ThQ presence of four pairs of limbs has usually been regarded as pvMnting to a relationship of these forms with tho Acarina, but the absence of all mouth-appendages, the structure of the legs, and the position of the last pair with regard to the anal opening, not to mention the peculiarities of the internal organization, are opposed to any close relationship with the Arachnida. The Tardigrada must be con- sidered as holding an independent position, without distinct indications of relationship with any of the types, until further information as to their primitive relationships has been secured. In addition the embryological history, which has recently been worked out, furnishes no evidence in favor of Arachnidan afiflnities, and it seems quite probable that the Tardigrades should be removed entirely from among the Arachnida and possibly associated with such forms as the Echinodera. LITERATURE. PKNTASTOMIDA. B. Leuckart. Bau und Entwicklungsgeschichte der Pentastomen. Leipzig uud Heidelberg, 1860. C. W. Stiles. Bau und EntwicklungsgescMchte von Pentastomum pi'ohosddeum und P. subcylindrictim. Zeitschr. flir wissensch. Zoologie, Lii, 1891. A. B. Spencer. 17ie Anatomy of Pentastomum ieretiusculum {Baird). Quar- terly Journal of Microscop. Science, xxxiv, 1892. PYCNOGONIDA. I. B. Wilson. The Pycnogonida of Neto England and Adjacent Waters. Re- port of the U. S. Commissioner of Fish and Fisheries for 1878. Wash- ington, 1880. A. Dohrn. Die Pantopoden des Oolfes ton Ncapel. Fauna und Flora des Golfes von Neapel. Monographic, in, 1881. P. P. C. Hoek. Report on the Pycnogonida. Scientific Results of the Voyage of H.M.S. Challenger. Zoology, iii, 1881. T. H. Morgan. A Contribution to the Embryology and Phytogeny of the Pycno gonids. Studies for the Biol. Laboratory, Johns Hopkins University, v, J891. TAIIDIGKADA. X. Plate. Beitrdge zur Naturgeschichte der Tardigraden. Zoolog. JahrbUcher, Anatom. Abtheilung, iii, 1888. E. von Erlanger. Beitrdge zur MorpJiologie der Tardigraden. Morpholog. Juhrbuch, xxii. 1895. TYPE TB ACHE AT A, 469 CHAPTER XV. TYPE TRACHEATA. The Tracheata are, like the Araclmida, essentially terres- trial forms, for, though a few Insects have adapted themselves to an aquatic mode of life, they are nevertheless air-breathers, living either at the surface of the water or coming to the sur- face from time to time to renew the air contained in the tracheae which ramify through the body and serve as respira- tory organs. However, a few Insect-larvse have acquired the power of extracting oxygen from the water by branchia-like processes of the body, but, even in these cases, trachero form the organs by which the respiration is carried on, the branchi^ being richly supplied with them. The body is distinctly segmented (except in Peripahis,) and is covered by a chitinous cuticle secreted by the ecto- dermal cells, which constitute the so-called hypodermis. The appendages are usually uniramous, and with few exceptions {Peripatus) are jointed. The anterior pair in all cases are more or less elongated multiarticulate structures provided with sense-hairs, and are situated prajorallv, while of the remaining pairs, varying in number in different groups, the most anterior pair is specialized to serve as mandibles, while the succeeding one or two pairs usually form maxillte. Numer- ous glands of varying function are developed in the hypo- dermis, the most interesting of which are the crural glands, well developed in Peripatus, and represented more or less perfectly in certain other forms. In addition to these, glands which secrete an acrid or offensive fluid {repugnatoricd glo.nds) are frequently present, as well as others which secrete waxy substances, or even in some cases silk. The coelom except in Peripatus is lacunar throughout, pos- sessing no definite walls, and is traversed in various directions by muscles, serving to flex or extend the body and to move i» » IN»' i^«p«i«1 470 INVERTEBRA TE MORPHOLOQ Y. «£ llM I '} the appendages. A marked difference between the Tracheata. and the Arachnida is the universal absence of an endo- sternite, a structure of considerable phylogenetic significanco in the latter group. A heai't is invariably present, lying above the intestine, and situated in a pericardial sinus incom- pletely partitioned off. In the major.ty of forms the parti- tion is composed of a varying number of triangular muscles, the alar muscles, which are attached by their bases to the walls of the heart, and by their apices to the body-wall. While at rest they are somewhat vaulted, the convexity being dorsal, and on contraction flatten down, thus enlarging the sinus and causing a flow of blood into it. The heart (Fig. 227, h) is elongated and imperfectly divided into a series of chambers, separated by pairs of valves which allow the blood to flow from behind forwards but not in the reverse direction, the heart being closed behind. Ostia are present in the lateral walls to allow of the entrance of blood into the heart- chambers, whence it is propelled through very short arteries which open widely into the lacunar spaces of the ccelom. In many forms a central sinus surrounds the ventral ganglionic nerve-chain, the blood flowing in it from before backwards, but with this exception definite vessels are wanting. This is compensated for by the rich branching of the tracheae, which, as stated, serve as respiratory organs and convey air to all parts of the body ; the air is in fact brought directly to the tissues, instead of being carried to them by the blood from limited portions of the surface of the body. The bbod is usually colorless, but in some cases is of a bright yellow or green color, owing to pigment contained in the plasma, and it contains in all cases colorless amceboid corpuscles. The tracheae (Fig. 215, tr) communicate with the exterior along the sides of the body by a varying number of pairs of stigmata {st), and may either consist of bunches of unbranched tubes connected with each stigma, or of a number of richly- branching tubes, each one arising from a separate stigma and anastomosing in some cases through some of its branches with the tubes from other stigmata. Each stigma is usually provided with an apparatus by which it may be closed, ami in the Insects the air is expired from the tracheae by the ecu- isS' TYPE TRACHEATA. 471 traction of certaiu dorso-ventral muscles of the abdomen which cause a compression of the organs in that region of the body inspiratioL following on their relaxation and the conso- quent re-expansion of the abdomen. In structure the trachea are simply to be regarded us invaginations of the body-wall aud consist of a single layer of cells continuous with the hypo! dermis of the body, lined within ^^ —that is to say, on the surface with which the air is in contact —with chitin, which is thick- ened in such a way as to form a spiral band extending along the tube and serving as a spring to keep its walls apart. The digestive tract is in most groups a straight tube, but in Insects (Fig. 227) it may be coiled in a more or less com- plex manner and differentiated into several parts. Glands of various kinds are usually asso- ciated with it, salivary glands (Fig. 227, sg) opening into the anterior portion and Mulpighian tubules {inv\ in connection with the posterior portion, being the most constant in occur- rence. It is to be noted that the fore-gut and hind-gut are ectodermal in origin, and that the Malpighian bodies arising as outgrowths from the hind- gut are also ectodermal, differing thus in origin from the similarly- named organs of the Arachnida, which are apparently of eudodermal origin, arising from the mid-gut. In function both orgaus are similar, the Malpighian bodies of Tracheates being excretory. The nervous system in the less-differentiated members of Fig. 315. — Figure showing the D18TKIBUT10N OP Trachea in Aylm pelargonii (after Witlaczil). At = antennse. g = glaud-duct. St = stigma. tr = trachea. 1, 3, 3 = thoracic appendages. h !' I \ 472 INVERTEBRATE MORPHOLOGY. the type consists of a siipraoesopliageal ganglionic mass, con- ncs.ted by circumcBsophageal commissures with a chain of ventral ganglia, a pair of ganglia corresponding typically with each segment. In the Insecta (Fig, 228) more especially, however, considerable concentration occurs, a number of the })ostoral ganglia, or, in some cases, all of them, fusing to a single mass. A well-developed stomatogastric or sympathetic nervous system occurs in all forms, arising from the supra- uisophageal ganglionic mass by two trunks, which unite to form a single nerve, passing to the digestive tract, and in some cases provided with ganglionic enlargements both paired and unpaired. Sense-organs of various kinds are well developed in the Tracheata, with the exception of Peripatus, in which the only definite orpaus of special sense are the eyes. In other forms the antenna and other portions of the bodv are pro- vided with hairs connected with nerves and serving as tactile organs, and seta) situated upon the mouth-})arts and associated with peculiar nerve-endings have been supposed to represent organs of taste, and others again, on the antenuje, olfactory organs. Eyes .are very generally present. In Peripidns ami most Myriapoda simple ayes or ocelli are alone present ; in l*evipatus they resemble closely in structure the eycK of the Annelids or Mollusca (e.g. Ilalioiis, see Fig. 134), but in the Myriapods and Insects they are usually more complicattnl. Tims in a young larva of AciUm (Fig. 216, A\ a water- beetle, the chitin is thickened to form a cornea (J) whicli lies over a depression of the liy^^oderMiis, the cells at the bottom of which are modified to form a retina, each being contiuuoiiH at its inner end with the optio nerve (7t), while at its outer end it bej-rs a layer of chitin (/). The cells of the lip of tlio depression have ';onverged together h".' as to meet beneatli the cornea, which is indeed formed by those cells, and a cavity is tlius enclosed into which theio i)r(.trude from among tli« retinal cells largo c^lls {rmje) with chitin dejiosited on tiicir adjacent siirfaces. In a hitev sti'.ge (Fig. 21(5, II) the lij)s of the dopreK;:ion have unitevl, a continuous corneal liypoderiiiis [vh) being thus jjrodiu'ed ; pigment lias been deposited in the lateral cells, and the ictiinil cells, i)ig!heuted near their outer TYPE TRACHEATA. 473 eud8 and in continuation witii the optic nerve, have developed distinct rods (r) at their outer ends. In the Insecta and cccasionally in the Myriapoda (Scuti- gera) there are m addition to these simple ocelli compound eyes situated at the sides of the head and similar in structure to the compound eyes of the Crustacea. Each of the .m- luatidia of which the eye is composed, and tlere may be F,G. 216.-8ECTIONS THKOaon AN OCELLUR OF A LaUVA OF ^.wJ'l^N {A) A Veuy loUNG AND IN (B) AN OLDEIi iSPECIMKN (after Pxtten) ir = inverted rods. ,, = ^erve ' = '"";!^'''- Pff = pigment. mgc = niedjiiu gliint cells. r = rods. vb = vitreous body. 8.v( >al thousand of them in each eye, conftists of an external cornea (Fig. 217, ro), usually more or less hexagonal in outline giving the eye a faceted appearance. Beneath the covuoa iUB two cells which secrete it and form the corneal hypo,l,n- "lis, and below these again come four cells, the crystallinp- ou.u, cells, which may (emonom eyes) or may not (acmn.s oyes) manufacture a crystalline cone (c\ and finally 1 eneath tliose IS a circiG of seven retinular cells (four in Zepmna) t'ucj one of which is pigmented and manufactures a portion jf the chitinlike rhabdoni (rh) whicli th cell,"' are i)robably contiisuous at their inner ttiFwujiu j blltise ends with the optic nerve. Additional pigment-cells {pg) separate the •i i lil V 474 INVEBTEBRA TE MORPHOLOO 7. various ommatidia. CO pg' Rh Pg*" T Fig. 217. — Ommatidium OF Eve of Munca (after HiCKBUN). c = crystiiUiue coue. CO = c'onieii. Pff = piijint'iit cells. r = retiimlii-ceUs. Bh = rlmlHloia. T = tiacheii. tv = tmchctil dllutatioD. Other sense-organs occur in the various groups, but may more satisfactorily be considered in the special descriptions of these groups. True nephridia similar to those of the Annelids occur in Peripatus, but iu the Myriapods and Insects they are en- tirely wanting, their place as excretory organs being taken by the Malpighiau tubules. The Tracheata are bisexual, the reproductive organs being typically paired and opjning to the exterior by ducts, which may unite before reachiiij^ the genital orifice. Accessory struc- tures, such as a bursa copulatrix for the reception of the penis and a recep- taculum seminis occur iu the female, ami vesiculiB semiuales and accessory glands in the male. The region of the body- wall in the viciuit}' of the reproductive orifice is in the Insects frequently iu- vagiuated, adding a still greater com- plication, and furthermore the terniiual portion of the duct in the male is frequently capable of being evaginated and thus serving as a penis, while integumentary elevations or processes of the last abdominal segment iovm ovipositors in the females. I. Clash Protraeheata. This interesting group contains but a single genus, Pcripntm, which has, lii>u- ever, a wide distribution, species Ixiiij,' found in tne West Indies and South Anu'rica, at the Capt' of Good Hope and in New Zealand, thus indicating an origimd wide distribution of the genus which has become extinct except iu these few widely separated regions. Feripiitm is an elongated cylindrical form, measuring iu TYPE TR ACHE AT A. 475 jraeut form the Cape species (Fig. 218) from about five to six and a half centimetres in length, and is found beneath stones or bark o- amongst decaying wood. The body -wall is finely annulatedi the annuli not, however, corresponding to segments, and the cuticle is thin, small papillae being scattered all over the surface of the body, each terminating in a short bristle The head is but poorly marked off from the rest of the body and bears a pair of many-jointed anteunje, at the base of each of which towards the sides of the head is situated an eye. The mouth lies in the middle of the ventral side of the head and IS surrounded by t umerous papilla?, and within its cavity is situated a pair of jaws furnisljed with strong chitinous sickle- shaped teeth. These jaws really represent the second pair of Pig. 'i\9—Penpatm capensis (after Moseley from Balfour). appendages, the third pair being represented by two short papillsB lying at the sides of the head and having at their tips the openings of a pair of glands which extend far back into the body-cavity and from which, when tlie animal is irritated there is violently emitted a sticky fluid, whence the glands have been termed the slime-glands (Fig. 219, s(i). There is IK) division of the trunk into thoracic an ..bdomiual regions and It bears a number, varying according to the species from' seven to twenty-one pairs, of ambulatory appendages, each of which consists of a proximal stouter and somewhat conical portion bearing rings of papillae and a more slender short distal portion which bears at its tip a pair of claws (ungues). Ihese limbs are unsegmented, differing in this respect from those of the Myriapods and Insects, and are also soft owing to the thinness of tlie cuticle, a feature wliich has suggested thfl name Mnlampixla formerly api.lied to the tyne, as the presence of the terminal ungues has suggested 'the term (hnfvhophom. The anus is situated at the posterior extremity Ox the boaiMs (F^K- ^20)» and is lined throii^'h- out by a peritoneal epithelium which covers the various or- gans. From it are, however, separated certain cavities with detinite walls, which staud in relation to the nephridia and the reproductive organs ami will be spoken of in connection with these organs. A heart (Fig. 220, h) of a tubular form extends throughout almost the entire length of the body,lviiig oc) incompletely separated fenestrate (from Hkbtwio). a = anus. ai = nnteuniB. ftwt = ventral nerve-cord. d = inlostiiie. go = opening of reproductive organ. 0 = ovary. og = brain. p = ))liarynx. sd = slinic-glands. 80 = nepliridia. sp = salivary glands. tr = trachea). M = uterus. in a pericardial space (Fig. 220 from the rest of the ccjelom by rausverse par- Ik 13- !■ TYPE TRACHEATA. 477 tition. A pair of ostia are situated on the dorsal surface of the heart in each metamere, aud pass the blood into the heart from the pericardial space. Respiration is performed by trachesB (Fig. 219, tr) consisting of slender unbranched tubes which arise in bunches from stigmata, either scattered ir- regularly over the surface of the body in considerable numbers or else arranged, as in P. capemis, somewhat imper- fectly in two rows upon the dorsal and two on the ventral surface of the body. The mouth opens into the mouth-cavity containing the mandibles, and this communicates posteriorly with a muscu- lar pharynx, and has opening into it the ducts of two long tabular salivary glands {sp) which extend through more than half the length of the body. The pharynx (p) communicates by a short oesophagus with the stomach, which extends as a straight tube almost to the extremity of the body, where a short rectum places it in connection with the anus. The pharynx and oesophagus and the rectum are lined with chitin and represent the fore-gut and hind-gut of other Tracheates, the stomach being the mid-gut. No Malpighian tubules or other diverticula of the intestine occur. The nervous system shows several highly-interesting features. There is, as is usual in metameric animals, a supra- (jesophageal ganglion-mass (Fig. 219, og) composed of at least two and probably three pairs of ganglia, of which the first supplies the antenuie aud the second the mandibles, while a third pair lies at the sides in close contact with the second pair and sends nerves to the oral papilho. These latter are, however, postoral and ventral in position, and from them tluu-e extend back two ventral cords {hm) which in each iiietauiere dilate into a gaugliouic swelliug. The two ventral <'<»r(ls are, however, widely separated, lying in the lateral chauibers of the cojlom (Fig. 220), and are connected by a liirge number of cross-commissures — a condition which recallH the arraugemeut in i •, Amphiueurous Mollusca (see Fig. 124), the similarity being further increased by the facts that the two cords unite behind and above the rectum, as in the Soleuogastres, aud that the ganglion-cells are not confined to the enlargements but are scattered all along the cords. Tho It 478 IN VERTEBRA TE MORPHOLOO Y. eyes are the only special organs of sense ; their structure has been already indicated (p. 472). One of the most interesting features of Peripatus is the occurrence in it of typical nephridia (Fig. 219, so). Upon the under surface of the proximal portion of each limb, with the exception of the penultimate or last pair, there is a slitlike opening which leads into a more or less coiled tube lying in the coelomic compartment, which extends into the limb and Fig. 220.— Transverse Section op a Peripatus (after Sbdowick) c = central compartmeut of ccelom. mg = slime-glands. g = reproductive organ. h = heart. / = intestine. / = lateral compartment of coilom. ■ju = muscles. JV = ventral nerve-cords. ne = nephridia. p = compartment of ccelom which extends into the limb. pc = pericardial compartment of the ccelom. terminates in a thin-walled vesicle. These tubes are ne- phridia, the terminal vesicles (Fig. 220, ne) representing por- tions of the coelom into which the nephridia open — a fact indicated by their erabryological history. The nephridia are thus exactly comparable in every respect with the nephridia of Annelids, communicating at one extremity with the exterior, and at the other with th.« coelomic cavity. It i^ interesting to note that the development of the salivary glands shows that TYPE TRACHEATA. 479 iructure has shows tliiit they are the modified nephridia of the third segment of the body, that which bears the oral papillse, and furthermore it is to be noted that in the last or next to last (according to the species) limb-bearing segment, in which nephridia are wanting, are found the ducts of the reproductive organs— a fact whicih suggests that these are also modified nephridia. This idea is confirmed by the development of the genital ducts, and car- ries with it the corollary that the cavities of the reproductive organs (Fig. 220, g) are portions of the coelom, just as they were shown to be in the Mollusca (see p. 288). In addition to the nephridia there are associated with cer- tain of the appendages glands which open on the under sur- face of their basal moiety and are termed the crural glands. In P. capemis they are present in all the appendages except the more anterior one, and the slime-glands are simply the highly-modified crural glands of the oral papillae, those of the last pair of appendages in the males of this species being similarly elongated though possessing a difi'erent function. In P. Edwardsii, however, crural glands occur only in the males, and in these only in a few segments immediately in front of that bearing the reproductive opening. The Protracheata are bisexual, the female usually being somewhat larger than the male. The ovaries are paired, thvmgh included within a common capsule, and lie in the pos^ teiior part of the coelom. They are continuous with two uteri, which immediately at their origin are united by a trans- verse tube, and each bears a receptaculum ovorum and a receptaculum seminis. Beyond this each continues its course along the side of the body, passing backwards to finally unite at the common orifice, lying a short distance in front of the anus on the ventral surface of the body. The testes are slender paired structures whicli are continuous with a slender yas deferens. This dilates a short distance from the testis into a vesicula seminalis and then unites with its fellow of the opposite side to form a slender somewhat coiled tube, the ductus ejaculatoriuH, in the terminal portion of which the spermatozoa are united together into a spermatophore. The 1 rotraciieata arc viviparous. I Wlk' 480 INVERTEBRATE MORPUOLOOY. r<\ i Affinities of the Protracheata. — Peripatus\& a highly-suggestive form on account of possessing both Annelidan and Tracheate characteristics, so that it has been generally regarded as indicating a descent of the Tracheate forms from the Annelids. Its Annelidan features are, first, the presence of a distinct dermal muscular system; second, the occurrence of crural glands wh'ch seem to be homologues of the glands which secrete the set® in the Annelida; third, the possession of nephridia corresponding closely to those of the Annelids; and fourth, the structure of the eyes. On the other hand, its Tracheate aflBnities are shown by the claw-tipped feet, by the adaptation of the feet (mandibles) for masticatory purposes, by the ten- dency towards a concentration of the anterior segments to form a head, and by the occurrence of tracheae. Both these sets of features are highly important, and, taken with the wide distribution of Peripatus, point strongly to its being the representative of a connecting link between Tra- cheates and Annelida, a phylogeny which may be considered more in de- tail at the close of this chapter. II. Class Myriapoda. The Myriapoda possess a distinct head composed of a num- ber of fused segments and followed by a distinctly-segmented body formed of a varying number of segments, all of which are more or less similar, there being no differentiation of a thorax and abdomen. A single pair of appendages as a rule is borne by each segment, with the exception, in some cases, of the last. The most anterior pair are usually long multi- artic'ilate antennae, the second pair mandibles, and the third and fourth, or the third alone, are modified to form maxilhi ; the succeeding pairs, with one or two exceptions, are ambula- tory, and are jointed and tipped by a claw. The chitiuous cuticle is generally thick, and consequently no definite system of dermal muscles is developed, a number of separate muscles occurring in each segment for moving the appendages and the various segments u])on one another. Glands of various kinds opening upon the surface of the btxly occur, the most important being glands or protrusible gland- ular sacs situated upon the basal joints of a number of the appendages and apparently homologous with the crural glands of Peripatus. The heart is in all forms verv loner, extendiner tlirnncfli tlie entire length of the body behind the head, and possessing I TYPE TRACHEATA. 481 just as many chambers and pairs of alar muscles as there are trunk-segments. The number of stigmata vary, in some forms only a single pair occurring, while in others there is a pair on each segment of the trunk; and the form of the tracheae varies also, as they are ciometimes branched and sometimes arranged in bunches composed of a number of un- branched tubes. The digestive tract is almost always a straight tube, ex- tending through the body to the terminal anus. The mouth is guarded in front by a well-developed upper lip or labrum, while the fusion of the maxillse behind it in many forms pro- duces a lower lip. It leads into an ectodermal fore-gut, and this into an endodermal mid-gut, which is usually provided with a number of unbranched diverticula termed hepatic creca. One or two pairs o{ Malpighian tubules open into the ante- rior end of the ectodermal rectum, and serve as excretory organs. The nervous system except in the head region shows but little trace of concentration, there being as a rule in each segment of the trunk a pair of ganglia. The antennal ganglia are fused with the supraoesophageal ganglionic mass which sends off branches to the ocelli ; these may be quite numer- ous, though compound eyes do not as a rule occur. A sympa- thetic system is present as in other Tracheates. There are no nephridia so far as known in the group, the excretion being performed by the Malpighian tubules. The reproductive organs are paired, and open to the exterior in some cases by paired orifices, but more usually by a single opening, which may be situated either far forwards, or else in other cases near the posterior extremity of the body. » It irouch flie 1. Order Pauropoda. The order Pauropoda contains a few small forms in which the trunk possesses twelve metameres and but ten pairs of appendages, which, with the exception of the rudimentary first pair, are six-jointed and terminate in a claw. When viewed from the dorsal surface, the segments appear to be less nu- merous than the appendages, a condition which results from 48d INVERTEBRA TE MORPEOLOO Y. Fig. 221. — Pauropus tlie fusiou of certain metameres iu pairs, so that two pairs of appeudages appear to belong to some of the segments, the double nature of which is further shown by the occurrence iu them of two pairs of nerve-ganglia. The antenna) (Fig. 221) are remarkable in form, consisting of a four- jointed basal portion which bifurcates at the tip, one of the bifurcations bearing two long liagella and a peculiar spherical stalked body, while the other one bears a single flagellum. Mandibles are present, and there is also a single pair of but poorly- developed maxillae. Tracheae or other respiratory organs are 'suxkyHtrom'hzvyilh ^ot yet known to exist, nor have any in- dications of a circulatory system been dis- covered up to the present. The reproductive opening is situated upon the second trunk-segment. None of the members of the order exceed a little over a millimeter in length. The genus Pauropus i^ characterized by the body being elongated and cylindrical, and by its active movements, while in Eurypauropus the body is flattened and relatively broad, and the movements are sluggish. 3. Order Diplopoda. The Diplopoda, sometimes termed the Chilognatha, are popularly known as the Millipedes on account of the commoner forms possessing an unusually large number of appendages. The body is usually cylindrical and provided with a hard cuticle, and many forms are in the habit of rolling themselves when disturbed into a ball or a helixlike coil, thus protect- ing the more delicate ventral surface of the body. The an- tennae (Fig. 222, at) are generally seven-jointed and are never very long, and the mandibles are strong jaws without pulps. In front of the mouth is a well-developed upper lip (ul), while behind it is a lower lip formed by a fusion of the maxillte (nix). According to some authors this lower lip represents two pairs of appendages, but its innervation and embryological history seem to be opposed to this view. The segments behind the TYPE TRACES AT A. 483 head vary in number in different genera from eleven {Glo- meris) to over one hundred, and the number of appendages is much greater still, since the majority of the segments bear two pairs of limbs and in reality represent each two meta- raeres. The four or five anterior trunk-segments are, however, single, bearing but a single pair of limbs (Fig. 222), and one of them — in some cases the first, in others the second, but more usually the third (lulus) — is entirely destitute of appen- dages. The last few segments also carry but a single pair of appendages, as does also the seventh segment in the males, the appendages of which are usually modified to serve as copulatory organs. Stigmata occur on each of the trunk-segments, the double segments bearing two pairs, situated on the ventral surface near the coxal joints of the limbs. Each one has in connec- tion with it a bunch of un- branched tracheae, a condition recalling somewhat that of Peri- patus, although the location of the stigmata is much more regular and definite. Upon the dorsal surface of the body there is in most species a row of pores which have been mistaken for stigmata, but are really the openings of glands (gl repugnatoria) secreting an oily evil-smelling fluid which serves as a means of defeuca. In the genus Polydesmus the secretion contains hydrocyanic acid. Crural glands do not as a rule occur, but protrusible warts occurring on the coxal joints of a number of legs in some genera {Lysiopetalum) have been regarded as homolo- gous structures. Tlie nervous system has the cliaraftteristic Myriapodan arrangement, each of the double segments possessing two Fig. 232.— Anterior Portion op a DiPIjOPOD. at = antenna. cop = copulatory appendages. mx = maxilla. ul = upper lip. [segments. 1, 2, 3, 4 = the four anterior trunk- \t 484 INVERTEBRATE MORPHOLOGY. iSi *• 11 pairs of ganglia. Eyes are usually present and are always simple, varying in number from two to as many as eighty. The Diplopoda are bisexual, and the ovaries or testes form a single mass from which two ducts, or one which later divides into two, arise and pass forward to open on the ven- tral surface of the body between the second and third trunk- segments. The embryos when first hatched out possess but three pairs of legs, situated upon the first, third, and fourtli segments in Strotigylosoma, and on the first, second, and fourth in Iidus, one or more segments without appendages lying behind the fourth pair. By successive moults new segments and appendages are added and the form of the adult gradu- ally acquired. The Diplopoda live for the most part under stones, etc., or among dead leaves, and find their food in decaying vege- table matter, though some forms will attack living vegetation and may prove thereby injurious to gardens. The commonest form, Iidus, may readily be obtained under stones or boards all through the summer. 3. Order Chilopoda. The Chilopoda, or Centipedes, are very different in their habits from the Millipedes, being carnivorous and provided with poison-glands which render the larger forms of iScolo- pendra dangerous even to man. The body is as a rule some- what flattened and less hard than that of the Diplopoda. The antennae (Fig. 223, at) are usually long, with at least twelve joints, and may be as long as the body, while the mouth-parts are much more complicat * than in the Dip- lopoda. The mandibles and upper lip ve.f n^vie the c '''r,;- spouding parts in that group, but the mavillsB {mx) are jaw- like, are not fused together, and in some forms (Geophihis) bear a palp. Behind the maxillfe comes a pair of second maxillse (mx^), which, however, do not serve as jaws but are TtaCvUved to a pair of palplike structures, and behind these again iz u. pirir of maxillipeds (mxp), the appendages of the first trunk-segment, with their basal joints fused to form a lower lip supporting a four-jointed palp, the last joint of which is Hi TYPE TRACHEATA. 485 Fig. 323.— Anteriou Pou- TION OP A CiuLoroD. at = antennae. mx^ = umxilla. mx'^ = second maxillu. mxp = luaxilliped. clawlike and is perforated by the duct of a poison-glaud. Each trunk-segment, of which there may be over a hundred] bears but a single pair of appendages, there being no compound segments as in the Diplopoda. Each leg is as a rule seven-jointed, the coxal joints of those of the same segment being widely separated, and there is no modifica- tion of the seventh pair to serve as copulatory organs., though the pair of tlie penultimate segment are much reduced in size and lie at the sides of the reproductive orifice. Stigmata are usually wanting in the first three trunk-segments, but occur in a certain number of the others, lying usually laterally between the segments except in Scutigera, in which they have a dorsal position. They open into branched tracheal trunks which usually an- astomose with one another, though in Scutigera they open into sacs from which a large number of simple uubranched tracheal tubes arise arranged in a bunch as in the Diplopoda. Crural glands occur on the coxal joints of several of the posterior appendages. The nervous system is arranged as in other Myriapods, and simple eyes are usually present, in Scutigera only being closely aggregated together to form a faceted eye. Tliis, however, is not a compound eye exactly similar to that of the Insects, but is to be regarded simply as a close aggregation of simple eyes. The reproductive organs are usually paired, and the sexes separate. The ducts unite before opening to the exterior, so that there is but a single opening situated on the antepenulti- mate segment of the trunk, the appendages of which are j,neatly reduced in size. The embryos of Scolopendra and GeopJiilm leave the egg with almost the same number of ap- pendages as the adult, while those of Scutigera and Lithohius possess but seven pairs of legs (in addition to the maxillipeds) HUil gradually acquire others by successive moults. 486 INVERTEBRATE MORPHOLOGY. In Scutigera, a form wLicli frequents the warmer parts of the world, the dorsal surface of the body is covered in by eight shieldlike fok^s which conceal a certain number of the segments, which are about fifteen in number. Lithobius has the same number of segments a.id is common under stones, etc., as is also Geophilvs and Scolopendra, both elongated forms, the former usually without eyes, while the latter usually possesses them but has only some nine or ten pairs of stig- mata. Some of the species of Scolopendra, especially those living in warm countries, grow to a considerable size and are capable of inflicting a dangerous wound. 4. Order Symphyla. The order Symphyla contains a number of small forms referable to one or two genera, of which the best known is the genus ScolopendreUa (Fig. 224). Unfortunately the details of ti'e structure of the members of the group are by no means well known, a circumstance all the more to be regretted siuce /ScolopendreUa seems to possess certain Insect-like features. The body is elongated, and on the dorsal surface possesses a number of plates which overlap slightly, but which do not correspond in number with the ap- pendages. Tlie head bears a pair of h)iig many-jointed antennre, and behind these, in the region of the mouth, is a pair of mandibles and a single pair of maxilhe, both these last-named aj)- pendages l)eing deeply imbedded as it were in the tissues of the head, their Fio. 224.- ScolopendreUa ^'P« ""^.V projecting. The first pair of im7naculata (fiow lkunib). truukappeudages is not transformed into maxillipeds as in the Chilopoda, but is ambulatory in function, and most, but not all, of the succeed- ing segments, of which there are apparently fourteen, bear a pair of tive-joiuted legs terminated by two claws. Attached to the coxal joints oi most of tiiese appendages is a peculiar Hpurlike process, internal to whicli is situated a protrusible TYPE Tli ACHE ATA. 487 glandular sac which is probably to be regarded as a crural glaud. The last pair of appendages may be uiijoiuted, each bearing a tactile seta, and attached to the last segment is a pair of conical processes each of which has opening at its tip the duct of a spinning-gland. Two stigmata, situated at the base of the antennre, are the ..uly ones which occur, their position being very remarkable Tliey open into bunches of branched trachea3 which extend throughout the greater portion of the body, leaving only the appendages and the posterior part of the trunk destitute of uu air-supply. The Malpighian tubules attached to the limd-gut are very long, and salivary glands opening upon the lauxilliu are present. Eyes do not occur. Both t) a male and female repro- ductive organs are paired. There is but a single genital pore iu either sex, situated in the mid-ventral line towards the front of the fourth truuk-segmen', and the oviducts or vasa (loferentia Uxiite before communicating with it. Very little is kucwu as yet concerning the development of SaP ■^pendrella. It Jias been considered by some to show marked affinities with the lusecta, and the name given to the order implies the l)f)ssibility of its being a connecting link between that group iiud the Myriapoda. The spurlike processes have been re- garded as limb-rudimeuts, in which case it seems possible tliat each segment may really be a double segment and that the Sym])hyla have close affinities with the Diplopoda. III. Class Insecta. The class Insecta is far riciior in species than any other clasHof animals, some two hundred thousand s})ocies belonging to it being known to exist, and of these about eighty thousand are beetles. A very large number are i)rovided with organs of flight and may be termed aiirial ; others are terrestrial, living either upon the surface of the earth or excavating bur- rows beneath its surface; while some have adai)ted them- -■" -"' • .....,.- ..,..!5. .-I iii-., aaa t^ihcrs aiu even murme, members of the genus IhthhaU'K l)eing found on the surface of the ocean many miles from land. Many species, living as 488 IN VERTEBRA TE MORPUOLOO Y. they do upon vej,'e table food either in the adult or larval stages, aud occurriug occasionally iu enormous numbers, form powerful enemies to the horticulturist and agriculturist, the Eocky Mountain Locust, for example, devastating at times the crops, while fruit and forest trees are injured by the at- tacks of other forms. The Insecta differ from other Tracheata iu having the body divided into three well-marked regions. The most an- terior of these is the unsegmented head, bearing the autenuio and the masticatory appendages, and immediately following it is the thorax, composed of three segments, the prothorax, mesothorax, and metaihorax, each of the last two bearing usually a pair of wings upon its dorsal surface, while pos- teriorly is the segmented abdomen composed typically of ten segments, sometimes as broad as the thorax at the junction with that region, sometimes contracted to a narrow stalk. In many cases, however, the apparent number of segments falls below ten owing to the fusion of certain of the posterior seg- ments or the union of the anterior segment with the thorax, and in the Butterflies and two-winged Flies the thoracic seg- ments seem to be reduced to two owing to the close associii- tion of the metathorax with the first abdominal segment. Four pairs of appendages are borne by the head. The autenme, and indeed all the appendages, vary greatly in shape in the various groups, but are usually long slender raultiarticulate structures i)r()vided with sensory hairs. The masticatvry appendages are a pair of mandibles aud two pairs of maxilla", which are variously specialized for bitinj,', l)iercing, or sucking. The most typical condition is that iii which the entire apj)aratuH is adapted for biting and that niav be described here, leaving special modifications to bo con- sidered in connection with tlie orders in which they occnr, The mandibles (Fig. 225, C) are strong unjointed toothed plates which meet together in the middle line and are ))!()- vided with strong muscles. The first maxilhe, or, as they are usually termed, the nnixilhe (Fig. 225, li), on the other hand are distinctly i(jinte(l, luuX cniiHiHt of u !)as•'/'> tVf Ui t r t ; t ' • •, •oars on its lates in one or two unsegmented plates of which the innermost is usually toothed. The second maxill* (Fig. 225, A) are also jointed and are fused together to form a lower lip, or laUum. The fused basal joints form the sKbmentum {sm), the second joints the mentum (m), which bears, as does the stipes of the maxillcTO, a jointed pal}) (p) and terminates frequently in two uujointed plate-like processes. The three pairs of appendages of the thorax are typically ambulatory, but are niodilied for clasping, swimming, digging, etc., according to the habits of the insect. They typically con- ^. sist of a basal joint, the coxa ^^"" 225.— Moutii-paiits op a Bkr- •succeeded by one or two small ''"^f ^''7/'"""«««- ... ,, "^ , A = labium, joints, the trochanter, upon B = miuilla. which follow a femur, a tibia, C = miiudible. and a tarsus, the last consisting of five (occasionally four) short joints, the terminal one bear- ii:g two claws or ungues. Abdominal appendages are ropre- sented only by cerci, which are usually many-jointed structures occurring near the posterior end of the body in many groups of lusecta (Fig. 23G), and by the pairs of spurlike processes which occur on a number of the al)dominal segments of the Thysanura and which recall the spurs on the trunk-segments of Scolopendrella. In the embryos of probably all forms rudimentary appendages are found on some of the abdominal segments, but they later disapjiear, showing, however, a de- scent of the Insecta from forms in which abdominal a])pen- ss- ing poison -glands in ccninection with a complicated stinginj,'- apjmratus, which is a modified ovipositor. The respiratory stigmata vary greatly in number in dif- ferent groups of Insects. In the wingless Thysanura and Collembola there are usually ten stigmata on each side of the groups which Pulex, Meh. □aites, Neuter , when their ictures, pro- blood-lacuu;e ter, however, ity being ob- i which they at plates so ) wiug, which ;aut arrauge- )ecome more 3f atklitioual . 239) which wings, which 3 two-winged iich reduced, id structures I metathonix. e, usually of hairlike i)ro- jrlands opeii- inection with )le glanduhir t, on the jtb- bola close to those forms, liniilar struc- >ly represent o be wautiii;,' ■)ssess glands ■glands occur also possess- ,ted stingiiifj;- miber in dif- ysanura and )h side of the TYPE TRACHEATA. 49^ body, two being situated on the sides of the thorax and eight ou the abdomen, but in Cmnpodea the number is reduced to hree pan-s. which occur upon the thorax. In the winged orms the number also varies somewhat, but there are agdn typically ten pairs, arranged as in the Thysanura. They Tea into short trunks, which, in a.mpo.te, ramify through the body without anastomosing, but more usually they are uni ed on each side of the body by a longitudinal 'tube, to^;! et pass off numerous branches penetrating to all parts of the body, and transverse connecting tubes passing between t le systems of the two sides (see Fig. 215). In certain fonns dn t cW "f ' '"' *'' longitudinal tubes are frequenS; dilated to form air-sacs, as in the Bees, or numerous air-sacs may occur which may be more or less emptied or expanded according to the will of the iijsect, the specific gravitv of the body being thus altered. 'lu the jupiatic larvfe Avhich occur in some forms, such as the May-fiies (Epheme- iKhe, Fig. 220), adaptations occur for the breathing of air dissolved in the water, the sides of the body in the abdominal region being prolonged into ;i number of pairs of platelike process- es, into which branches of the trachero project, an interchange of the gases contained in the trachea? for those dissolved in the water taking i)lace through the walls of the plates, simi- liuly to what occurs in the brauchia) ^'"- 226.-Lauva op an of the Crustacea, though in these fornw ^''"'^^'*'"^«""'""" ''«'msj. the exchange is directly with the gl": " = '''''''' '''''''^^ of the blood These structures are consequently termed -heal branchuo, and while they are functional, the'stign.ata '•' closed only opening when the adult stage is reached and a torrestrud life adopted. As a rule the tracheal bn.,..].! te i"own off at the m.)nlt by which the adult L, nit m a few forms they persist throughout life orni is reached. 1'' i I ! 492 INVERTEBRATE MORPHOLOGY. m \ I A dermal muscular system does not exist, but complicated and well-developed muscles are present for the movement of the various parts of the body, those occurring in the thorax bein^ especially well developed and serving for the movement of the limbs and wings. As in other Tracheates the coeloin is lacunar, and the heart lies in a pericardial sinus below the dorsal surface of the bodj', alar muscles extending from it to the walls of the body and partly dividing the sinus into a dor- sal and a ventral chamber. In the Thysanura the heart extends from the posterior thoracic region throughout tho greater part of the abdomen, and consists of nine chambers separated by valves and each provided with a pair of ostia ami a pair of alar muscles. In the majority of forms (Fig. 227, //), however, the heart is entirely confined to the abdominal region, and the number of chambers, though frequently as high as eight, may be greatly reduced. An aorta extends forwards from the anterior chamber into the head, in the Butterflies (Fig. 227) dilating in the thorax to form a second- ary heart {ciK), and sends off branches which quickly empty into the lacunar spaces. The greater portion of the abdomen is occupied by a peculiar tissue, termed the fat-body, in which the various organs are more or less imbedded, and which receives its name from the fact that its cells contain globules of fatty matter, and in the adult insect usually also concretions of uric acid. It is not necessarily confined to the abdomen, but may extend into the thorax or even into the head. In certain Beetles— the Fireflies (LampyridjB) and Pyrophorus of the West Indies — certain regions of the body, especially the abdomen, and, in Pyrophorus, two spots upon the thorax, give out under certain conditions, apparently under control of the animal, a very bright light, usually spoken of as a phosphoies- cence. The tissue which produces the light is tlie fat-body, or special portions of it abundantly sui)plied by trachea', ami the process seems to be one of oxidation of phosphorus, containing substances. The exact nature of the phenoniciiou is but poi)rly understood at present, and it is not possible hy any means at our disposal to produce in the laboratory a TYPE Tit ACHE AT A. 493 light equal in iuteusity to that of the Firefly with the espenili- tare of as little euerg3% The digestive tract is as a rule much more complicated than ill other classes of Tracheates aud is generally more or less twisted or contorted in the abdominal region, so that usually it is longer than the body. The mouth is bounded in front by a usually large upper lip or labrum, generally described with the mouth-parts, but distinguished from them in not representing a pair of appendages. The anterior portion of the intestine, the fore-gut, is ectodermal in origin ah as Fig. 227.— Structure op Butterfly, Danais archippus (after Buroess). a ■-= ttiiteuim ag = accessory gland. ah = accessory heart. an = abdominal ganglion. be = bursa copulalrix. e — crop. cc = canal uniting be aud oviduct. ce = cerebral ganglion. h = heart. I = thoracic limbs. mv = Malpighian tubules. t)cl = oviduct. ov = ovary. ph =z pharynx. pi = palp. s = stomach. sg = salivary gland. tg = thoracic ganglion. I-TIT = thoracic segments. 1-9 = abdominal segments. as in other Tracheates and consists of a mouth-cavity into which, or in its neighborhood, the ducts of one or more generally well-developed salivary glands (Fig. 227, .vr/) open. The secretion of these glands varies considerably in different forms, one of the pairs present in the larva) of "the Biitterflies and certain Moths being transformed into silk- spinning glands, the silk of the Silkworm being a product of their activity. When digestive the secretion seems to have a peptonizing effect as well as the power of transforming starch into suLmr, jiiid is consequently of fongidorable diges- tive importance. The mouth-cavity opens behind into an 494 IN VERTEBRA TE MORPHOL OQT. oesophagus, whose posterior region is frequently dilated iuto a crop (Fig. 227, c) which in some Beetles is lined with chitinous teeth or bars and whose walls are muscular, the apparatus probably serving for a further mastication of the food. The mid-gut which succeeds the crop is usually dilated into a stomach (s), lined in some cases by glandular cells, or, in others, having opening iuto it numerous glaudukr diver- ticula, the so-called liver-pouches. The hind-gut, like the fore-gut of ectodermal origin, has opening into its anterior extremity the Malpighiau tubules {mv), which vary consider- ably in number, amounting to nearly one hundred in some Hymenopterans, though more usually limited to from four to eight. They are excretory in function, and are apparently the Tig. 228.— Different Akiiangements op the Nertous System in Insect j (from Qeoenbaur). A, Termes ; B, Dytiscus ; C, a fly. only excretory organs which occur. The anus is situated at the extremity of the body, and in close proximity to it odor- iferous glands frequently open into the hiud-gut, serving as organs of defence. In some cases they secrete an acrid fluid which, as in the Bombardier beetle {Brachimis), can be ex- pelled with almost explosive force. The nervous system in forms where it shows the least amount of modification (Fig. 228, A) consists (1) of a supia- oesophageal mass composed apparently of three pairs i)f ganglia and supplying the eyes and the antennre ; (2) of a sul)- CGSophageal mass composed also of three pairs of ganglia supplying the segments indicated by the mandibles, the TYPE TRACHEA TA. 495 ilated iuto a th cLitinous e apparatus I food. The Llilated iuto ir cells, or, dulcr diver- ut, like the its auterior vy cousider- 'ed iu some rom four to pureiitlj the EM IN Inbect: Y- situated at to it odor- serving ilS acrid fluid can be ex- s the least )f a supiii- e jjairs of 2) of a sul)- of ganglia libles, the laaxillee, and the labium ; (3) of three pairs of ganglia in the corresponding thoracic segments ; and (4) of a chain of ab- dominal ganglia, a pair occurring in each segment except usually the last two or three, in which a compound ganglion occurs. Frequently, however, this typical condition is modi- fied by a greater or less concentration of the various ganglia, the thoracic ganglia fusing to a single mass, as may also, more or less perfectly, the ganglia of the abdominal chain (Fig. 228, 7>), and the latter may even unite with the thoracic ganglia to form a single mass situated in the thorax, as in certain two- winged flies (Fig. 228, C). A visceral system is usually pres- ent arising from the supraoesophageal (cerebral) mass and being distributed to the walls of the digestive tract. The antennffi of insects seem to act as sense-organs, and serve apparently to control the flight, since when removed the insect is not able to fly with its accustomed ability. So too it seems probable that in the Ants and Roaches these appen- dages are the seat of the olfactory sense, and iu the Mosquito it seems that certain hairs upon them may be auditory in function. Compound eyes, frequently consisting of several thousand omraatidia, are usually present, as well as a small number of simple eyes (ocelli) situated upon the dorsal sur- face of the head. Special organs, which have usually been considered auditory, also occur in many forms, varying con- siderably in complexity. In its simple form such an organ vnsists of a single nerve-fibre which dilates into a ganglion- prolonged into a terminal hair which is enclosed within . sheath fastened at one end to the wall of the body. This whole apparatus is termed a chordotonal organ, and T.iere is usually attached to the sheath just where the hair arises from the ganglion a ligament, which is also inserted iuto the body- wall. In the majority of cases a number of ganglion-cells and liairs are associated to form a chordotonal organ (Fig. 229) the various hairs sometimes being grouped within a single slieath,— sometimes, however, spreading out in a fanlike man- lier, each possessing its own sheath. These organs occur in vari ous parts of the body, on the antennsp or on the lim])s In tlie grasshoppers (Acridiidre) the first abdominal segment hears on either side a thin tense membrane, a thinned portion 496 INVERTEBRATE MORPHOLOGY. of the cuticle, recalliug the tjmpauic membraue of the human ear, beneath aud iu couuection with which is a chordotoual apparatus, further improved by the occurrence in close prox- imity to it of a saclike enlargement of a trachea which serves as a resonator. Similar organs occur in pairs on opposite sides of the tibiro of the first pair of legs in the Crickets (Gryllidse), and seem from their structure to be auditory Fig. 229.— Subgenual Chordotonal Organ of the Tibia op the Second Thoracic Appendage of Isopteryx (after Graber from Lang). bk = blood-corpuscles. gz = ucrvo-cells. c = intcgmeut. tr = trachea. es = teiralnal ligaiueut. se = teniiiual balrs and sheaths. organs, whence the conclusion that the more simple chordo- tonal organs also subserve this function. It is interesting to note that the males of the forms provided -vitli a tympanal organ possess the power of making a harsh or sharp chirpiiiir noise, produced in the Grasshopper by rubbing the femora of the hind legs which are furnisiied on their inner surfaces with a row of line teeth, ovor the strong marginal veins of the anterior pair of wings ; and in the Crickets and Locustida? by rubbing together the two anterior wings, a row of toetii upon a vein of one wing working upon a projecting smooth vein of the other. The male Cicadas also make a similar noise, the stridulatiiig appa- ratus resembling that of the Grasshoppers, and in all cases it seems to be a sexuj'l characteristic serving to attract the females. The sexes are separate, and a more or less distinct sexual dimorphism occurs, the males being usually smaller and more slender than the females. In some cases, as in the TnKS(i('k- moth (Orgyia), the female lacks wings and has a very different TYPE TRACHEATA. 497 appearance from the males, and iu many Beetles the male is adorned with spines and tubercles upon the head which are but rudimentary or absent in the female. Differences in the shape of the antenna and the presence or absence of stridu- lating organs also serve to distinguish the two sexes iu some of the groups. In a few forms a polymorphism is produced by the failure of certain individuals to reach sexual maturity and by the assumption by them of certain special structural characteristics. Examples of such cases are afforded by the Bees, Ants, and Termites, the workers of the first two groups bemg immature females, while in the Termites (Fig. 237) the so-called neuters may be either males or females, always, however, immature. The ovaries (Fig. 227, ov) are paired and consist of a varying but usually rather large number of tubes, which start from a common basis. At the extremity of each tube is the germ-producing region, the rest of the tube beiug divided into a series of chambers each of which contains an ovum sur- rounded by a lajer of follicle-cells. Not unfrequently the chambers are arranged more or less distinctly in pairs, the lower one of each pair containing an ovum, while the upper contains a number of small cells similar iu appearance to the primary germ-cells, but which serve as nutrition for the ovum which gradually absorbs them (see Fig. 20). From each ovary an oviduct arises, the two, however, soon uniting, and receiving, usually not far from the unpaired orifice, the ducts of various glands {ag) whose secretion serves to cause an adhesion of the ova to the structures on which they are deposited. A receptaculum semiuis is usually present, and there is frequently a large pouch, partially separated from the oviducts, which receives the male organ during copulation and is termed the bursa copulatru- (be). The genital orifice is situ- ated on the ventral surface of the ninth abdominal segment and is usually surrounded by a number of papilhe, or some- times by long processes, which serve as ovipositors and are to be regarded simply as processes of the segments from which they arise and not as modified limbs. The testes are also paired, each being composed of a. number of separate spherical or tubular portions. The ducts. tss 498 INVERTEBRA TE MORPHOLOO Y. from these various portions on each side unite to form a vas deferens which may dilate into a vesicula seminalis and then, uniting with its fellow of the opposite side, forms the ductus eJHCulatorius. Occasionally the vesicula is unpaired arising from the point of union of the two vasa deferentia, and very frequently accessory glands occur. The ductus ejaculatorius opens usually on the ventral surface of the tenth abdominal segment, and projections of the body-wall in the vicinity of the orifice form a groove or tube through which the sperma- tozoa, usually united into spermatophores, are introduced into the bursa copulatrix of the female. Parthenogenesis occurs as a normal process in certain Insects, though always associated with true sexual reproduc- FiG. 2.2Q.— Aphis mail. Winged and Wingless Forms (from Packard). iiou. Examples of it are found in certain Coccidee {Aspidio- tits) and in some of the Gall-wasps (Cynipidse), the fertilized ova producing both males and females, while in the Bees, for example, in which both fertilized and unfertilized ova are de- posited, the latter give rise to drones or males alone, while workers or queens, i.e. the females, develop from the fertilized ova. Occasionally heterogony occurs, as in the Plant-lice (Aphidaj). These forms under favorable conditions of tem- perature and food produce viviparously usually wingless indi- viduals, not, however, from true ova, but by a process which may rather be compared to internal budding, as in the Rediiv of certain Trematoda. Generation after generation of such individuals may be produced during the summer, but on the TYPH TU ACHE AT A. 49^ 5m Packard). approach of cold weather or ou the exhaustion of the food- supply males and fen. ales appear by which true fertilized ova are produced, and fr nn these, surviving the winter, viviparous lieterogonous females develop. In the genus Phylloxera, whicli has played such havoc ou grape-vine^ ill France, a greater complication of generations occurs. A winter egg, which has survived beneath the bark of the vine, gives rise to wingless forms which migrate to the roots, and there produce numerous genera- tions. After a time winged forms appear which ascend from the roots and, reproducing parthenogenetically, increase rapidly in number and serve to distribute the species over wider areas. Certain of these jjroduce small ova from which males develop, and others larger ones which give rise to females, both sexes being destitute of both wings and digestive tract, and by these forms the fertilized winter eggs are produced. In certain flies {3Iiastor, Cecidomyia) psedogenesis occurs» the female reproductive organs becoming mature while the iusect is still in the larval stage, and the ova, developing par- thenogenetically within the body, give rise to another gen- eration of larvsB. This process may be repeated several times, the last generation of larva) developing into the adult form (see Fig. 29). The more primitive Insects, the Thysanura and Collem- bola, leave the egg in a form resembling the adult, differing from it only in size and in the immaturity of the reproduc- tive organs, and i)ass through no marked metamorphosis during their post-embryonic development. Such forms are termed ametaholic. A similar absence of metamorphosis i^ found in certain forms degenerated by parasitism and lacking Avings, but these have evidently descended from winged forms which passed through a certain amcmt of metamorphism» so that the ametabolism is secondary and should be distin- guished from the primitive ametabolism of the Thysanura. In the majority of winged forms, however, a more or less pronounced metamorphosis occurs. In the simpler cases the young are distinguishable from the adults by the absence or but slight development of the wings, which become larger after successive moults, the adult form being thus gradually acquired. In tliese cases of gradual metamorphosis the habit ' of the adult and larva are similar, but where they S3 500 INVERTEBRATE MORPHOLOGY. differ greater cliauges result, leading to hemimetahoUsm. This; occurs, for iustauce, iu the Fish-flies (Ephenieridse) aud Dragou-llies {Lihelhda), in which the larva) are adapted for au aquatic life and possess tracheal brauchiie (Ephemeridse) aud other features which are lost, either gradually by successive moults or suddenly at the last moult, the adult winged Dragon- fly, for instance, issuing from a peculiar aquatic larva with the merest rudiments ol wings. Finally, a large number of forms are hohmetaholic. In such cases the habits of the larva3 are different from those of the adults; for instance, the larvje of the Butterflies, Wu^ caterpillars (Fig. 231), are wornilike creatures with power- mpmmmmm Fio. 231.— Lauva, ri:i>A, and l.MAfio uv I'lcris oleracea (ivom Riley), ful jaws feeding ou plant-tissues, while in the adults the mouth- parts are adapted for sucking. The transformation from the larva to the adult is accomplished by the intervention of a resting stage or pupa, dur-ng which no uutiition is taken, ami when the transformation takes place the fully.deveh)}i('(l insect or imago issues from the ru])tured skin of the ))ii)ia. The pupa varies considerably in form in different grou]).s, in some being enclosed in a silken case manufactured by the larva before the last moult and termed a cocoon. In some cases the adult apjiendages project from the bpdy of the pupa (pupa JiberaX but iu other cases they are united with the surface of the body and but indistinctly visible {pupa ohtecta), au arrangement usually found in the Buttor- flies, whose pnpjo, owing to their frequent brilliant coIoim- tion, are termed chrysaluh. a term which lias been somewhat incorrectly extended to the mummy-like puptr of other forms. TYPE TRACHEA TA. 501 liually, m some of the two-winged ilies the pupa is euclosed Mithiu the Lxst larval skin, possessing then a cvliudiical form without any indication of the adult Umbs (pupa cocwctata). A metamorphosis in ^vhich a distinct pupa-stage occurs is said to be "complete" in contradistinction to the hemimetabolic form frequently spoken of as "incomi)lete." Mention sliould bo made here of the dimorpliisu, or polymorphism whidi o.:ct.rs m certai.. adult Insects. It has already received' passing n.e.itiun M>. 497), but in addition to the frequently-occurriuf,' sexual dimorphism there occurs in forms which live together in colonies a i)olvmorphism asso- ciated with a division of labor on the parts of the members of the colony liius m the Bees there are found the drones or males with heavy bodies Ih.'queen or female, as large as the drones but with a much more fjlonder body, and the workers, which are sterile females (listingiiish,.d bv their smaller size and by other features, such as a peculiar moditication'of the lilMas of the last pair of legs which a.lapt them for the collection of pollen Ironi the (lowers which they visit. Among the Ants a similar trim..rpl,ism orcurs, males, females, and neuters or workers constituting the colony and iM some f epical forms the workers are of two kinds, namely, ordinary workers wh 1 small heads and mandibles, and soldiers with large heads anil strong prominent mandibles, whose functions are indicated by the popular name applied to them, though guards would perhaps be nrnre" appropriate luially, among the Termites, popularly known as the White Ants four lorins. I.e., males, females, workers, and .soldiers, al.so occur. In certain Buttertiies a peculiar form of dimorphism ..r trimorphi.sm termed "seasonal dimorphism" occurs, an excellent exami)le of it bein- <'thT..d by the American Popff'o AJax, of .vhieh there have been described (luve distinct varieties, diflVriiig markedly in e.,l..rati..n both in the males aiul (ho females, and distinguished as the yarielies W„fs/u,\ Tvhmouhhs iiiu\ Marce/lus'. From ehry.salids which have pas,sed the winter then. i,ateh out in the early days of spring forms belonging to the variety ]Va/sf,;i mu\ somewhat lat('r, from tlx.se whose d.-n-lopment lias been reiarded' the Ti/amonide,s f(nms. Drring the early i art of summer th,. WaMuf f,",rms (ii.'out and a little later the Telamnnhhs also disappear, both forms pre- vioiisly, however, .lepositing ova, ino.st of which develop into larvie and Wuvsahdsand hatch out in the later moi.lhs (.f summer as the J/r,,w//?/.» fonu. whose ova, again developing int., ciiry.salids. pass the wiui,-r in (hat M;ii>'. and give rise in the following spring successively to the Wal.shii and hhnnonides forms. The three varieties are evidently prcMluced by influ- <'n'( .s a..(iug upon (he ehry.salis and differing nceording to the sea.son. por- liiips iieeording to temperature, whence the distiniruisl mis lorm of dimorplii.er, being usually ten, thougli in Canipodta they are reduced to three, and the trachew in this TYPE TR ACHE ATA. 503 same form are interesting in being destitute of longitiuliual ami transverse anastomoses. Lepismai. frequently found in houses, in attics and similar places, feeding upon woollen, linen, and other fabrics, but also on meal or sugar. Campodea (Fig. 232). on the other hand, is o be found under stones or dried leaves and is a small white torm, by no means uncommon. 2. Order Collembola. _ The Collembola are distinguished from the Thysanura by the abdomen consisting usually of but six segments, and in some cases the number is even smaller. The body in Podura IS covered with scales, and the terminal segment of the body IS usually provided with two processes which may be bent up underneath the abdomen and then suddenly extended, pro^ pelling the lusect to a considerable distance. These structures are absent in the adult Anurida, but occur in young spec" n.ens and their occurrence and function have suggested the ]H>lHilar name of Spring-tails applied to the order Neither Hbdomiual appendages nor coxal glands occur, but the first segment bears a peculiar organ, having in Anurida the form ol a saccular protrusion, which is probably adhesive in func .on The antenna are usually short, and bear in some fonns a.i antennal sense-organ similar to that of Carnpnt compound eyes never occur. A peculiar oilman! ying 1 "^ In.ul the bases of the antenna, and hence termed tL pl 4. Order Odonata. The members of this order, the Dragon-flies, are elongated forms with two pairs of nearly equal, abundantly-veined winj,'s of usually large size, all the forms being excellent fliers and seeking their prey in the air. The head is united to the thorax by a narrow stalk which permits extensive rotation of the head, and the abdomen, terminating in two unsegmeuted platelike cerci, is long, and in the large Dragon-flies, jEsvhim and Diplax (Fig. 235), and in the brightly-colored Agrio7i very slender, though somewhat stouter in the genus LihelMa. Tlie anteuntB are very small and the mouth-parts adapted for bitiiip, while the legs are slender, the anterior pair being directed somewhat forwards so as to serve for grasping the prey. The lateral compound eyes are very large, meeting on the dorsum of the head, and in front of them are situ- ated a pair of small ocelli. The larv8B are aquiitic and are characterized by the remarkable develoj)- ment of the labium, wliicli is very much enlarged, terminating iu two power- Fia. i'in.— Diplax elisa (from Packahd). £jj| j^^yj^ j^^j provided witli a hiugo, 30 tliat it can be flexed so as to lie beneath the In:! ! or suddenly thrust out to capture the unwary prey. This TYPE TB A CHE ATA. 507 apparatus is termed the "mask." Respiration is carried on by tracheal gills, consisting in Agrion of three leaflike pro cesses situated at the posterior end of the body, and also by the terminal portion of the intestine, into .vhich Avater is taken and which is abundantly supplied with tracheae The water can be forcibly expelled from the intestine, serving to l.ropel the insect through the water if it so desires. The metamorphosis is incomplete. 5. Order Plecoptera. The Plecoptera, or Stone-flies (Fig. 236), are found in the viciniiy of water and have a somewhat elongated body fre- quently terminating in two long cerci {Perla). The antennse are long and filiform and the mouth-parts adapted for biting, while the legs are strong aud used for walking. Two pairs of wings occur almost e.iual in size, but lacking the com- .ucated venation found in the Odo- liatu, aud when at rest lie fiat upon Fig. 236. -Stone fly, Perla. tiie abdomen, completely concealing it. The larva) are a(iiiatic, and are usually to be found in considerable numbers u.i.ler stones in swiftly-runiiing streams. Tliey recall the ili.ysanura in their appearance, and possess tracheal branchhB ou the under surface of the thorax, which in some forms are retained m the adult. The metamorphosis is gradual or in- oomplete according as these structures are or are not retained m tlie imago. G. Order Corrodentia. The mmnbers of this group possess biting mouth-parts and are su.netimes destitute of wings. The Termites, or A\ lute Ants, live in colonies aud show a ,,olym„rphisin. TJie males aud feuiales, termed kings and queens (Fj.. 237 I ]}) a.v at first provided with largo wings resembliug those of the i !o<^optera, but after the maniage flight settle to the .n-ouinl and become wingless. The workers select from the many pairs I 608 INVERTEBRATE MORPHOLOGY. one for each nest, the remaining uuselected ones soon dyin.). The Aphid» also posses a a rule upon the antepenultimate abdom/nal segment pair of tubular elevations or papilte from which a sweet secreUon «..nes the so-called "Honey-aew," which covers he Wes ami stems of the plants npon which the Insects live ZZ -gerly sought for by various Insects, more especL™ by The larv* of the Ehynchota as a rule resemble the aduU, even to the .tructure of the ..outh-parts, anrthe metam^r P .OS. .s consequently gradual. 'bI Ckada forms, I o leve " mf..ce of the ground and living „,,„„ the roots of tree, I becomes ransformed into a pupa, which, however con t»-es to lead an active existence, becoming quieslen ' lort time before tlie moult which only results in the formation of 512 INVERTEBRATE MORPHOLOGY. I! : M the imago, very different iu appearance from the pupa. The metamorphosis here approaches the complete type. 9. Order Coleoptera. The order Coleoptera includes the Beetles and is richer in s]iecies than any other order of animals. The members of the oup are characterized by the anterior wings bein^ coii- vertetl into hard chitiuous plates, the elytra, which cover iu and protect the posterior membranous wings and the abdo- den, being short only in a few forms, such as the Buryiujr. beetles {Necrophorus), in which the tip of the abdomen remaiiTs exposed, and the Staphylinidae, or liove-beetles, and /deloc, in which they cover only the more anterior portions of the ab- domen, the posterior wings in the last-named form beiu^r wanting, as they may also be iu some of the Weevils. Occa" sionally, as in the Fireflies {Lampt/rls), the elytra are but slightly thickened, and in some forms they may be completely fused together. The autenn.ne vary greatly iu shape, being usually filiform and sometimes very long, as in the Boring-beetles {Monohum. Fig. %'i^. — Cotalpa lanigera and its Lauva {a) (from Packard). mils, Olytus, Saperda, etc.), though occasionally, as iu tlie Lamellicorn beetles {JIelolonfha~the June Bugs and CotaJpa, Fig. 239), the termiual joints are flattened aud folded together like the leaves of a book. The mouth-parts (Fig. 225) are in all cases adapted for biting, and the legs for locomoticm. Iu the Lady-bugs {Coccinella) the tarsus consists of but lour joints, one of which is rudimentary, while in the Weevils (Curculiouida3), in wdiich the anterior part of the head is pro- TYPE TIUCHEATA. S18 lougecl into a cyl.mliical s„o„tIike process at the extremity otwnchis l.e fflo„th,iuthe Bori„g.beetle«, and iu tl.e Po Uto-beet e (Doryphra) it is forme.l of five joints, one of ^,^t » exceed.n,l3. .s.uall. In otl.er forms, such as Mdoc and the -hster-beetles (Z,«„), the tarsi of the two anterior pairs of leKs are five.j„,nted and those of tl,e last pair fonr-L ted and n others again, snch as tl,e Fireflies, the Cliclc-beefe (tlaternla,) the Lamellicornes, the Bnrjing-beetles ml' he Ca,ab d,e (U.dosoma, Cnmhus. l[arp_du., /ijhinns, etc and the I,ger.beetles (CkhuMa). all the tarsi are Hve-io n ted and all the j.nnts approximately equally developed. Iu the'l TJT '""'' r"'^' '" '""" '" "'^ "^'ff"-™' genera. lu the L„d. bugs audson.e other forms they are Thysanuri- ..■u, the three anterior trunk-segu.ents (corresponding to he hie the abdomen terminates in a pair of cerci. In some ,,ter-beetles ((?,„„„) tracheal gills are present, and Z la V e ot the Lamelhcorns (Fig. 23U. „) are soft-bodied eyele ; wh.te h,rms, characterized by a saclike dilatation of tlie lasl abdommal se,.nent, and live beneath the surface of t e K oum feeding up.n the roots of grasses. In the Click-bet t e., (E laternte the body of the larva is elongated and slen- .k- and very hard, these forms being kno™ as the wire- >1 plants In the Bormg.beetles, the larvre, which excavate "..rows beneath the bark or in the wood ^f various ee, ave he l.mbs almost or quite rudimentary, while maggot.' like larvffi are characteristic of the Weevils The larva, whose life nnay be prolonged through several ■ears, passes finally into a resting p„ pa stage of*lhe'Z onn,resembhngin the body form and%he mouth-part the .HI. l.o,.oy, .he parasite shp, „,«>„ L eg«, wlnl'Mrcoll."": 1!^ 614 INVERTEBRATE MORPHOLOOT. transforms into a maggotlike second larva which lives upon the honey on the surface of which it floats, and after a time passes into a resting pseudo- chrysalis stage, from which a larva similar to tlie second one emerges, and this finally transforms into a pupa which gives rise to the adult. m 10. Order Neuroptera. The Neurojjtera are characterized by the abundant and rich venation of their wings, in which numerous cross-veins extend between the longitudinal ones. The mouth-parl.i are adapted for biting, the mandibles being in some forms {Cory, dalis) very large. The lace-winged flies {Chrysopa) also be- long to this group, as does also the Ant-lion {Ifyrmeleon, Fi". 240), whose larva excavates a funnel in loose sand, and buries itself at the bottom with only the head and powerful Fig. 2i0. —Myn)>eleon obsoletus (from Packard). mandibles projecting, ready to snap up any insect which s]ii)s down the yielding sides of the trap. The larvM> are usnullv Thysanuriform, those of Chri/sopa attacking Aphides, whence they are frequently termed Apliis-lions, while tliose oi Ci>nj. dalis are acpiatic and possess tracheal branchiae upon tlu' abdomen. This larva is familiar to anglers as the Holl- gramite. The metamorphosis is complete. 11. Order Panorpata. This order contains a small number of forms, the majority of which possess membranous wings reseml)ling tlios(> ol' tlic Neuroptera, except that the cross-veins are not so numoruiis. The anterior part of the head is produced into a downwaidlv projecting snout, at the extremity of whicli are the smail biting mouth-parts, the arrangement recalling tliat foum! in the Curculioniihe among the Coleoptera. In the genus l' the corre- ,' insects, ox- ler teriniuiil re united to I the higher i), however, m) are quite um is nnu'li ts pulps (//)) id well (Ic- ube is coiii- !ed into two nd by their I, the hirvii' irs. Thi'ir TYPE TR ACHE ATA. 5^7 of the Cabbage; many other similar examples might be given A few of the Microlepidoptera possess aquatic'lal^ ).utthey orm exceptions. In the typical cateiillar the^ are, in addition to the three pairs of thoracic legs, Le pa s of •short stout prop-legs situated upon the third, fourt lifi th s.xth, and tenth abdominal segments, and the body may be covered with hairs of various lengths, as in the larvafof nnn' -oths (e.g. the Woolly Bear, Spilosonu,), or may possZ spiny processes, as in the larvae of the Mourning-cloak Butter % {Vanessa) which feeds on the Willow, ''or v.dou l" shaped tubercles, as in the American silkworm iTelea).^ e ^o.op.a arva. In one group of moths, the Geome tril bu two or three pairs of prop-legs occur, situated on tlfe oe postei-ior segments, and in progression these for ai uv these legs up close to the thoracic limbs, throwing tl e intervening portion of the body into a loop, whlnce the ten s measuring-worms" or "loopers" often applied to 1.17 rare cases, as in a few Microlepidopteri, the lai s iviHiout feet ami luaKKotlike. ""v.iis The i,„p,. „,. ohi-ysulis is of the ohMa variety, au.l i, f™ n bythe larva whose salivary glau.ls are couverte iu to >! mm„K-«lau,ls. A ceoou is „„,re Renerallv present i/tl Moths than ,n the B„ttorllies, whose chrysali.fs Lre snspen.ed .V a patch of silk to which the hin.l en,l of the p„ a is I tadiecl or may be in acljition slnn.- in ., sille,, 1 „ ■ ro..n,l the body near the nml.lle (Kg. Lii) ""'"'' 14. Order Kymenoptera. The Hymenoptera possess fonr membranous wings with "mparatn-ely few veins and not covered with scah-s ^r ,m . pos enor. The al d.m.en is sometimes l,roa,Ily attached lo thorax as „, the Saw-flies (Tenthredinida,),bnt r" - ally the anterior one (Bees) or two (Ants) abdo ninal s el . N..;..t» are very narrow, so tl„>t the al„lon,en Meen.s to CL -..".'" by a stalk. The fenndes possess ovipositors which -,■ be retractile and provide.l with a poison^iand, fo™,- '! 518 INVERTEBRATE MORPHOLOGY. If 1 mjcp efficient organs of offence and defence, as in tlie Ants, Bees, and Wasps, or else long and slender and but partially retrac- tile and destitute of a poison-gland, as in the Saw-flies, Gull- flies, and IchueumonidsB. The mouth-parts are adapted partly for biting and partly for licking. The mandibles (Fig. 242, mn) are well developed and fitted for biting in all forms, and in the Teu- thredinidse the maxilloe are also like those of biting insects, while the inner of the two terminal plates of the labium are united to form a tube, the outer plates remaining separate. In the Bees and Wasps the maxilhe {inx) become elongated and are uo longer adapted for biting, and the inner terminal plates of the labium are fused together to form a luiij; touguelike structure, the glossa [I), the outer plates forming what are termed the paraglossae ( pg). The entire apparatus is adapted for biting aud also for licking up the huiiej contained in the nectaries of flowers. The great majority of forms are solitary, but a few Bees {^pis, Bom- hits) and Was])S {Vespa, Sphex) and the Ants [Formioi, Camponohis) form social aggregations with more or less pronounced polymor})hism, to which reference has already been made. The Gall-flies {Cynips) lay their eggs upon the leaves or stems of plants, at the same time injecting a jxiisou which causes a proliferation of the plant-tissues, forming' ;i gall in the interior of which is the larva of the insect ; while many forms, such as the Ichneumon-flies, Proctotrupes, Ptero- malm, Microgtister, etc., are parasi<^ic in their larval stago, the eggs being deposited in or xipon the bodies of the larNicnf other insects, a very decided cluv'k being exerted U])oii tin' larvio of injuriouH insects, such as the Cabbage-butterfly, h.v these forms. Pig. 243. — MouTH-rAUTB of Bee, Anthophora (after Newport rroin Gbggnbaur). I — glossii. Ip = labial palp. mn = mandible. mx = maxilla. mxp = maxillary palp. pg = paruglossa. TYPE TRACHEATA. 619 lie Ants, Bees, irtially retrac- ^aw-flies, Gull- The lar™ of the Tenthredinida,, for example that of the Poar.sl„g (&fa»J«,), „hi„h feeds upon the leaves of the pear ue resemble the caterpillars iu possessing prop-wf o forms, however, owing to parasitise, or to being in contec Pig. 343.-, which ar„ those of the mesothorar, the 620 INVERTEBRATE MORPHOLOGY. metathoracic pair being usually represented by a pair of club- shaped bodies on the sides of the segment, termed hcdteres or balancers. The wings are always trans- parent and the veins by no means abundant. In a few forms, such as the Sheep-tick (Melophagus) and the Flejis {Ihdex), the wings are entirely wanting,' in harmony with the parasitic habits which these forms possess, but they form exceptions to the general rule. The mouth-parts are adapted for sucking and also for piercing; the labruxn (Fig. 245, Ir) and labium {la) are prolonged into grooved processes, forming together a tube within which lie, in the female Mosquitoes (Oulex) and Gadflies (Tabanus), two pairs of elongated needlelike rods which repre- sent the mandibles (md) and maxillio (mx), to which a lifth unpaired stylet may be added which arises as a growth from the lower wall of the pharynx {hy). In other forms the maxillae only OF A Gnat, Culex, the ^^^.ve the ucicular form, the mandibles Labrum turned to fusing with the labrum, and in all cases ONE SIDE (fro,u h.ht- the maxillary palps are present, whil,> the labial palps are undeveloped, lu the ordinary House-fly (Mtisca) the ex- tremity of the sucking-tube is expanded into a disklike structure, and in all forms the salivary glands open near the extremity of the tube, usually niaggotlike (Fig. 244), entirely destitute of feet, and in some forms the head even is indis- tinguishable. The metamorphosis is complete, the pupa boiiig in the Mosquitoes active, swimming about in water, thou^^h more usually it is incapable of motion, and enclosed witliiu the last larval skin, thus belonging to the coarctata variety. Fig. 245.— Mouth-parts WIG) hy = hypopharyux, a process of labium. I'i = liibiuin. Ir = labium. md — mandible. 7IU --= maxilla). p — maxilliiry palp. The larva) are TYPE TR ACHE AT A. 031 ^^^^iopment and Affinities of the Imeofn Th. i . dovelopment cannot be discussed 11. ^^^y stages of Insect books of Embryology, but ^2' ^Id Se 7n '"'"l^ *^ *^^^- uoinenou which occurs duriue the IZf 1 ^ *" remarkable phe- nnaginal conditions in tl^Xm '^7'" """^ '''' ^""^^^ '^ ^he describing the developme t o ^ A^^^^^^^^^^^ " ""''''" '" the transition from one sta^eto hVn / . ^""'"^"'^ ''"* ^^^'-^^ ^"^''^g e-tion of certain parL ^^^^21:^^2 7\ ^"'"^""^ '■^^'^"- tl.o same process occurs durinr he Z ^ ^" ^^'f ^'o'^metabolic Insects t'.o majority of the muscle ad' t e S^difl "\'""^ '^^P^^^'™'^' «...,, so that ti,e idem t'v of tl. v '''' '°' ''^'°"•^^- scrved throughout the process. The im 1 fal d ^'^"°'^\«''g^n« ^^ Pro- portions of the original anla-^en o? Zl '''' '^ ^' '^^^^^ed as rained during larvanifeir.!:r:b;onL"Sn"^^^^^ ""^' '''"^ '^■ be N^t :tt :r;^r :rrc:^^.:^^^^ r-r- ^^ '-- eates a descent from Apterygote ance or !nd ^^"^""5 "^'"^ '''"'* ■"^"- larva. of a wormlike ot maLtUke form r T ''"^''' ''^'''^' P''^^*^-^' highly specialized. It is Tn thes.^^^^ I" ''" probability the most occurs, and it is selJl::drnt t r^- d .?:n:r^'''^^^ ?^^^""^^^ i;l.o.ses are more primitive than the nf, i^ '^^ uITT'I "^''^'"«^- •icponds upon the differences in habit and stn ctu tf the b '"''''^''^'^^'^^ and becomes more and more conn.tpt. o„ '.''^"'^ "^ ^he larva and imago, •Icpart more and u^oreZL^r^l^^^^^^ V '''''' "'^^ ''"'^«'"«« Consequently it may be cone udnh^tnorf""'™ ''^' '' ^^^^^^ure. which retain most perfectly Zh 1^1. "I' "'' '^'' "^°«^ ^''^^'^^^ elnu-acters. These L ^l^" ^^XiX^"^'' ^"^ "'^'"'^""''''^ both in the adult and larval stages' t. Cor d^iiarT''^ not become modified by parasitism) a 1 tlw n h / ^' '" ^^'^ ^'^'^ •'^^f,-,d it is intorestL^otri;^st.^ cZZu?" ''T'' '- •-r g^^;';;j-~- - - -r r ^-- - ...o^rr :;S:rtJ-rtS --;;;:- r-s in the .rv. f"">'naptera, is that of the Thysa.m terfwh i '"'" ''^"'" ^^^ *''« '""i"Hte affinities with the R Stf t L 7 ' ' "°^ °^«»th-parts '^ --d group t^ceable back tr^ ^im^ P^^^: '''''''' ^-"^"« ;je resemblances l^i.^ll^Za':^:::;:^^-:'^ T""''''^ ^^- character of the wings and by the mouth^arts."";:: .l!: I^;^' ;^ 622 INVERTEBRATE MORPHOLOGY. ^\ lated are the Neuroptera with Thysanuriform larvae, probably to be re- garded as a group which has undergone a development parallel to that of the Ephemeridae and Odonata, the relationship bei.ig traceable back to an ancestor common to it and the Ephemeridae. To this group may also bo referred the Plecoptera. A fourth group includes those forms in which the larvfB are provided with prop-logs, secondary forms in which all indications of the Thysanurid ancestors have disappeared. Of such forms the Panorpata show relation- ships on the one hand with the Ephemerid group, and somewhat closely related are the Trichoptera, whose entire organization points to a close affinity with the Microlepidoptera. From the primitive Microlepidoijter-i two lines of descent are probably to be traced, one leading to the Maero'- lepidoptera and the other to the primitive Hyraenoptera, the resemblance between the larvae and the mouth-parts of the Tenthredinid*, and these of the Microlepidoptera being very striking. The two remaining orders, the Coleoptera and Diptera, are very hi-rhiy specialized, both being holometabolic, and the temptation is to look" for their ancestors in forms with a similar metamorphosis. This tempfuiou may be justified in the case of the Diptera, whose larva are the most modified of all, and it is not impossible that they hnve descended from primitive Hymenopteran ancestors, their nearest existing relatives beiii- the Tenthredinidae, whose sluglike larvae, suggest not a little the k-i.s't modihed Dipteran maggots. With the Coleoptera, however, the case'is different, and it seems more probable that their holometabolisin has l.,.,-n acquired quite independently of that of the other holometabolic orders The larvae of some beetles, notably those of the Coocinellid*, are markclly Thysanuriform, and prop-legs do not occur in the order. To which of the groups they are to be referred it is very difficult to say, though the mouth parts and the arrangement and structure of the wings in the adults point to an affinity witli the Orthoptera. Granting a descent of the Pterygota from wingless ancestors, it boco.nos an interesting problem to discover the origin of the wings. Attempts have been made to show that they are modified tracheal branchias a theory which necessitates tiie derivation of the Pterygota from aquatic ancestors bucli a derivation, however, is unsupported by any evidence at pivsnK af our disposal, it being much more probable that the immediate ancestors of the Pterygota wore terrestrial, just as Campodea is to-day. The wiix's arise in the emt^ryo as dorsal outpouchings of the meso- and metathora'x, traeliea3 later pushing out into them, and transient indicaiions of o„t- pouchings of the prothorax also occur in some embryos. It has been s,,,.- gcsted that primarily the wings were platelike outgrowths of the thoracic seginents which served to break the fall and increased the distance irav- ers.'d by jumping Insects, and in support of this view tiie fact n.av he mentioned that many Apterygota are saltatorial. The limitation ..f' ihc wings to the meso- and metuthorax may stand in some rnlutinn („ the centre of gravity of the body. TYPE THACIIEATA. ^^3 The Phylogeny of the Trachentn if t. u together the Crustacea, Araor^r:^^ ^X ^ •''' ^"^^''^ *« "-*« Arthropoaa^ characterized by iX^^.^^'^T ^l^ ''''^'' ^^'«"P' ^^e -ourrence of jointed limbs, and by Te Ts ,1 '"""' '""''•^' ^^ "'« i"nb,s; and furthermore it ms been cuVZ "'^ ^'^^"' ^*^'"S °'«d'fied -"1 the Tracheata as closely related '"fr^^^/^ ««"«ider the Arachnida ..oup. of trachea. The eiriy ^oc ^ oT'^^^^^^ groups also show many points of sinXTtvH 'fP™'"* in the three «i.ows decided differences in the deTa Is h1^^-^ ^ ''''''' examinaUoa '"ay have operated to produce the Z?.i.- ''' ^""^^'•gent evolution -'i it can be settled only by a onsZit '';',;'' P^*^'^^"^ *° ^« ^^^tled" winch indicate the phyloger.; of t' e v '" "'" '"^^« ^^ «"^ <^i«Posa wouM prove entirei;be'yofd[h:'limL^^^^^r^ ^^^^ to ^^St r r ^ - "tirfP^^ - - Crustacea is descended trom ^-..P^^ll Ice a^^^^^^^^^^^^ '^-e in all probability ;n the,r affinities. Are the Tracheair then « T'-^^^^t^'^'y Crustacean aeea and from forms which poLesl tel^V O^ ''' '"" ''' ^-- of the group negatives any such sunnnJn t "^ P'"^^^'^* knowledge Tracheata should, like the^p.^er^rv de^^^^^ ancestors ; it must rather be conceded that h • f'"^ ^'-^i'^-^.-like and the Arachnida are due to conterln?" *'^^^''°•'^^'ties between them ant.es to the acquisition of comraSv I^ "' *'" ^'"^'•^'^"'« «•'«'• he ova, distributed in a similar mannri'T """T' ^' '"^^'y^'^ ^« to the exigencies of a terrestrial TiJe 11^0 '""''^""^^ of the adult groups seems at first an importan noinlof T"'' ^^ ''''^''^ *" both only by a community of descent butl ""^^""'^ *^ '^« «««o»»ted for terrestrial Isopoda trache^ So ocL 'i^ u" br " r?''^"'^ ^^^' - ^^e their occurrence in these forms ranireW. ,""' "P'^^*^"'^' ^"^ that anyphylogenetic significance, i ievidenf hTf."^ "'''^P^""°"' ^'thout catjons of affinity is much reduc d It 'I J T "^^P^-^^nce as indi- Malpighiantubulesof theArachnidaandcrul ' P''"*'^^ '^"^ ^''^^ ^he -e X.eheata they are eetoder^rS:^— ^1:;:^— - Ariidl;^"^^^ . closely related to «o that the conclusion seems inevitable '1?"^^'''^'' f ''''' P'-^no^nced, nvo,l from Annelid-forms and hav. h f Tracheata have been de {-dent of that of the J^^^^ ^^^ ^^ P"^'«^-y P-tically indt Id ancestors of Peripatus and those of If.; n V'"''^' '^''' ^^^ ^"ne- «losely related, and that certain of 1 p''»«tacea were more or less ^-'Ps are thus to be acco'^^/fL t^S" ^'T'*"'"^« ^^ ^» '^^ *h- P'-e^-'nt ,„ a position to judge One'r o n T''^* '^*^"* ^'« ^^^ "ot at pound eyes of similar stn.cf. re ^l^l^r '' "'^ "««"--ce of com- tion, sine« it --^ « ■ " groups, seems w^-thv -f -- -^ 624 INVERTEBRA TE MOBPHOLOQ T. \\t markable instance of convergent evolution. It is to be noticed that tlm most primitive Insects, the forms througn which affinities +o the Crustiicoa if they exist must be traced, are as a rule provided only with simple eyes, a condition repeated in the eyes of Insect larvae — a fact which indicates tluit the compound eyes are structures which were not characteristic of tlie primitive Insects, but have developed within the limits of the group aiid can therefore have no phyletic connection with the compound eyes of the Crustacea. Adding to this fact the independently-developed tendency to form compound eyes seen in certain Annelida and Pelecypod Mollusks, it seems probable that notwithstanding their remarkable structural siinilaii- ties the compound eyes of Crustacea and Insects have been independently acquired. Instead, therefore, of uniting the three groups together as a typo Arthroprda equivalent to the other types, it seems preferable to separate them as distinct, just as is done with the Annelida and Mollusca, and the Annelida and Prosopygia. Starting, then, with the supposition that PeripaUis has descended from Annelid ancestors and represents the ancestors of the Myriapoda, the rela- tionships of the various orders of this class and of the Insects remains to be traced. Unfortunately a large gap exists between Peripatus and any recent Myriapods, and it is possible that this class is a heterogeneous group ; indeed oy some recent authors it has been sugj^ested that it should be done away with as a class, the Chilopods being united with the Insecta to form one class, while the Diplopods (perhaps witl; the Pauropoda associated with them) should form a second. There is no doubt but that Peripatus possesses many tracheate peculiarities, but its affinities to the remaining Tracheates are much more remote than those which exist between the vari- ous groups of Myriapoda, or between any of the.w groups and the Insecta. The character of the various appendages considered in relation with the nervous system seems to afford an admirable means of indica' "ng the rela- tionships of the various groups. The brain of Peripatus seems to be formed by the fusion of three pairs of ganglia ; the most anterior and dor- sal of these gives rise to the antennal nerve and the most posterior inner- vates the mandibles, while upon the middle one, which is closely related to the mandibular ganglion, the eye seems to be placed. It may be assumed that the ganglia with which the e}-es are associated represent the Annelid supraffisophageal or cerebral ganglia and may therefore be termed the jiro- tocerebrura, while the antennary ganglia form the deu.,ocerebrum, and tlie mandibular the tritocerebrum. In the Myriapods and Insects the brain is also composed of three parts to which the same names are applied, tlio antennae being innervated from the deutocerebrum, while the tritocerebrum lacks a corresponding appendage, though in certain Insects transient indi- cations of a tritocerebral appendage have been seen. Bearing these facts in mind, the ganglia and appendages of the various groups may thus be tabulated, and to make the comparison complete the Crustacea are also in- cluded. f TYPE TRACHEATA. 526 Ganglion. Crustacea. Peripatus. Diplopoda. Chilopoda. Inseet/i. Deutocare- hral. Tritiicere- bral. Ist postoral •Jd " 3.1 nh " .5th " 6th " Antennules Antennae Mandibles Ist maxillae ad Ist thoracic limb ad " Antennae ? Mandibles Oral papillae 1st legs 2d '• 3d " 4th " 6th " Antennae Antennae Antennae Mandibles Maxillae Ist legs ad " 3d " 4th " Mandibles Is maxillae 2d Maxillipedes 1st legs 3d " Mandibles 1st maxillae ad " 1st legs 2d " 8d " ;ea are also in- It will be seen from this that in the Diplopoda the arrangement is intermediate between that found in Peripatus md that of ihe Chilopoda while these latter approach closely the Insecta, and this seems to be the ac- tual relationship, Scolopendrella forming an intermediate link between the Chilopods and the Insecta, approaching the Thysanura closely in the ar- rangement of the mouth-parts and in the number of segments of which the body is composed. The Diplopoda, it is true, pass through a larval stage in which but SIX legs are present, and it might at first sight be supposed that this indicates an affinity with the Insecta, but these legs do not belong to the .same segments as do those of the Insects, and furthermore the occur- rence of rudimentary abdominal appendages in some Thysanura, as well as 111 the embryonic stages of probably all Pterygota, indicates that the In- secta have been derived immediately from forms with many pairs of appen- dages, and these forms seem to be represented most accurately by the exist- ing Scolopendrella. SUBKINGDOM METAZOA. TYPE TRACHEATA. I. Class Protracheata. -Annelid-like forms ; trunk not differentiated into thorax and abdomen ; w ith nephridia. Peripatus II. Class Myriapoda. -Elongated forms; trunk not differentiated into thorjix and abdomen ; posterior trunk-segments with appen- dages in the adult. 1. Order Pauropoda. -^m^W forms ; with only one pair of maxillje • antenn* ending in three flagella ; reproductive orifices at basi,s of third pair of trunk-appemlag.'s. Pauropus, Eurypauropns 2. Order Diplopoda.-Wnh only one pair of maxill* : antennae simple ; reproductive orifice on .second or ),etweeu second and third truiik-segmonts ; mo.st of the trunk-.segments with two pairs of legs. luhm, Lysiopetalum, Polydesmus, Strow/vlo- soma, Glomeris. 3. Order Chil»poda.-W ith two pairsof maxilla; and with maxillipeds- antenn* simple; reproductive orifice on the antepenultimate segment: each tr!,!,k-sec.n3ent with a single pair of legs. Qeqphilus, Scolopendra, LitUobiun, JScutigera. 11 526 IN VEKTEBUA TE MORPHOLOQ T. m 4. Order Symphyla. —Vfith only one pair of maxillEo and no maxilli- peds ; antenme simple ; most of the trunk-segments with u single pair of legs. Scolopendrella. III. Class Insecta. — Trunk differentiated into thorax coniposea of throe rings and an abdomen with typically ten segments. 1. SubcUiss Apterygota.—T\\ov&.x without wings; abdominal segments sometimes with rudimentary limbs in the adult. 1. Order Thysanura. — Abdomen with ten segments, bearing two or three cerci ; abdominal appendages frequently present. Le- pisma, Campodea. 2. Order Collembola. — Abdomen with six segments terminating in two springing-orgaus ; abdominal appendages wanting. Fodura, Anurida. 2. Subclass Pterygota. — "With usually two pairs of wings situated on the meso- and nietathoracic segments ; abdominal appendages wanting in adults. 1. Order Dermaptera.—KhAomQn. with forceplike cerci ; anterior wings small and chitinous, posterior folded like a fan and also transversely; mouth-parts biting; metamorphosis gradual. Forjicula, Labia. 2. Order Orthoptera. — Abdomen usually with cerci ; anterior wings chitinous, covering the posterior, which fold fanlike and some- times also transversely ; mouth-parts biting ; metamorphosis gradual. Caloptenus, Gryllus, Qryllotalpa, Periplaneta, Dia- pheromera. 3. Order Ephemeridce. — Abdomen with two long cerci ; wings mem- branous and richly veined, the anterior larger ; not folded when at rest ; mouth-parts biting, but reduced ; metamorphosis in- complete. Ephemera. 4. Order Odonata. — Abdomen with two platelike cerci ; wings mem- branous and richly veined, not folded when at rest ; month- parts biting; metamorphosis incomplete, sometimes approaching completeness. Libellula, jEschna, Agrion, Diplax. 5. Order Pteo/>^era.— Abdomen usually with cerci; wings membra- nous, moderately veined with few cross-veins ; the anterior cov- ering the posterior when at rest ; mouth-parts biting ; meta- morphosis incomplete. Perla. 6. Order Corrodentia. —Abdomen without cerci ; wings sometimes wanting (parasites and neuters), membranous, the anterior cov- ering the posterior when at rest ; mouth-parts biting ; meta- morphosis incomplete or wanting. Termes (with polymorphism), Atropos, Liotheum, Trichndectes. 7. Order Thysanoptera. — Abdomen without cerci ; wings sometimes wanting, narrow, poorly veined, fringed with hairs ; the anterior pair covering the posterior when at rest ; mouth-parts piercing and sucking; metamorphosis incomplete. Thrips, Phl(Xothrips. linal segments present. Le- «. TYPE TliACHEATA. 537 Order Rhpnchota.-Ahdom.n without cerci ; basal portion of anterior wings chitinous. posterior wings and tips of anterior membranous, or else both membranous, the anterior the larger or both wanting ; mouth-parts piercing and sucking ; meta.nor- phosis incomplete. Anterior wings chitinous at base (Henuptera). Anasa, Notonecta, Belostoma, Ranatra, Hydrometra, Halobates, Cimex, PediouUs (wings wanting in the last two) "^"Jrr'n "f ; ^"''' "^^^^--^^^^^ iHomoptera). Cicada, Aspidi. in^letsf S:r '^^'"''"'"■' ^"'"' ^^'"^^ "^'^ '^ ^^"""^ Order 6>.^.o^^.m. -Abdomen without cerci ; anterior wings chi- tinous, covering in the posterior when at rest ; mouth-parts biting ; metamorphosis complete. "''" paus (a) Tarsi of four joints, one of them ver> small (Cryptote. tramera). Coccinella. (b) Tarsi of five joints, one of which is very small (Cn/ptopen- tainera). Ourculionidae, Clytm, Saperda, Monohammus, Boryphora. ' (c) Tarsi of posterior legs four-jointed, of two anterior pairs tive-jomted {Heteromera). Meloe, Lytta. (^Tar.si all five-jointed and all the joints of equal size {Pentamera). Lampyris, Elateridfe, Melolontha, Necro- phorus, Staphylinidae, Hydrophilus, Qyrinus, Bra- ofmms, Harpalus, Carahus, Calosoma, Ckindela. OvA^Neuroptera.-AMom^n without cerci ; wings membranous richly veined with numerous cross-veins ; mouth-parts bidng • 11. Order Panorpato.-Abdomen sometimes with cerci ; wings mem- branous with few cross-veins; mouth-parts biting, af end of cylindrical rostrum ; metamorphosis complete. Panorpa. Order r..c/.op<.,.a._Abdomen without cerci ; wings covered with hairs or scales, posterior ones larger and folded fanlike wlieu at rest ; mouth-parts sucking ; metamorphosis comi)lete Phnj. ganea, Anaholia. " Order Lepidoptera.-Ahdomen without cerci ; wings covered with seaK not folded when at rest, though they may overlap; mol h- parts usually sucking ; metamorphosis complet^ Small forms (Microlepidoptera). Tinea, Carpocapsa, Pyralid* Oigyia, Telea, Pierts, Vanessa, Papilio nous, without scales, not folded ; mouth-parts biting and lap- ping ; metamorphosis complete. Ovipositor rfitrnnti!f> with n'^i"'iri f-! of North America. Proc. and Bulletin U. S. Nat. Museum A. S. Packard^ Monograph of the Geomeirid Moths. Report U. S. Geolodcal Survey of the Territories. X, 1876. ^. o «eoiogicai E. T. Cre..on. Synopsi.. of the Families and Genera of the Hymenoptera of A^nencanorih of Mexico together mth a Catalogue of DescnLl SpZa^d hibhography. Trans American Entomolo^rical S(»ciety. 1887 H Loew and Baron Osten Sacken. Monograph of the Diptera of North America bnulh,sonian Institution. Miscellaneous Collection, v,, 18C3 • vi 1864 viii, 1869; XI, 1873. «'"« , vi. 1004 , 8. H. Scudder. The Fossil Insects of North America. New York 1890 J. H^ aiul A. B. Com.tock. A Manual for the Study of Insects, iihaca! N. Y. See also the publications of the U. 8. Entomological Commission and the o> o. V. KUey, A. 8. Packard, and others, STKrcTl'HAL. H. Orenaolier Untersuch-mgen iiher das Sehorgan der Arthropoden. Omiin- E Burge.. Contributions to the Anatomy of the MUkweid liutterfl., (Danai, "'-^y. Anniversary Memoirs B<..ton Soc. Nat. History, 1880. 18m ^'^'"'^ ««'* 0/ tf>e Garden of the Gods, etc. Philndelphla, V. Qrab«r. We chonhUmal Sinnesorgane und das GehOrder Insekten. Archiv [ti'" niikroHk. Anat., xx. 18H'.J. ■'»-<»-utv ^ "i^'^SSa. ^"^ ^"""^^ '^'' ^^''^"- Arbeiteu a. d. zool. Inst. Wien, jJO Sir John Lubbock. INVERTEBRA TE MORPHOLOO T. The Oi'igin and Metamorphosis of Insects. London, 1883. Ants, Bees, and Wasps. London and New York, 1883. J. A. Palmen. Zur vergleicJienden Anatomie der Ausfurhrungsgiinge der Sex- ualorgane beiden Insekten. Morpholog. Jahrbuch, ix, 1883. J.Carriere. Die Sehorgane der Thiere vergleicJiendanatomisch dargestelU. MUnchen, 1885. B. Orassi. Anatomia comparata dei Tiaanuri e considerazioni generali tulV organizzazione degli Inaeiti. Atti della R. Accad. Lincei, Berie lY. it, 1887. X. Heider. Die embryonale EiUwicklung von Hydrophiln piceus L. Jena, 1889. J. Van Rees. Beitrdge zur Kenntniss der inneren Metamorplioae von Mu&ca vomitoria. Zoolog. JabrbUcher, iii, 1888. V. Oraber. Vergleicliende Studien am Keimstreif der Insekten. Denkaclir. Acad. Wisseusch. Wien, lviii, 1890. H. T. Fernald. The Relationships of Arthropods. Studies from the Biolog, Labor. Johns Hopkins Univ., iv, 1890. B. T. lowno. Anatomy, Physiology, Morpliology, and Development of the Blow- fly. London, 1890-91. W. M. Wheeler. A Co7itribution to Insect Embryology. Journ. of Morphology, VIII, 1893. T. H. Huxley. Anatomy of Cockroach. Text-book of Anatomy of Invertebrate Animals. New York, 1878. W. K. Brooke. Anatomy of Oratahopper. Hand-book of Invertebrate Zoology. Boston, 1890. 'X. TYPE ECHINODERMA. 631 sA dargeatelU. CHAPTER XVI. TYPE ECHINODERMA. The Echinoderms are exclusively marine organisms and vary considerably in shape, some forms being elongated and vermiform, others stellate, and others again almost spherical. Whatever may be the shape, however, a well-marked radial symmetry can be distinguished, which suggested to the older zoologists the association of the members of this group with the Ccelentera in a type Radiata. The radii in the Echino- darma are, however, almost invariably five, instead of four or six or some multiple of these numbers as in the Ccelentera ; and, furthermore, while in the Ccelentera the radial symmetry represents a primitive condition and any departure from it towards bilaterality, as in the Anthozoa, is secondary, the reverse is the case witii the Echinoderma. The larval forms of this group are strictly bilateral, and even in the adults certain organs or parts of organs interfere with the regularity of the pentamerous arrangement and bring about a more or less pronounced bilaterality. This may be clearly seen if one of the stellate forms, such for instance as the ordinary five-rayed Starfish (Fig. 240^, be examined. This animal consists of a central disk, at the centre of one surface of which, the oral surface, the mouth is found, while the anus occupies a somewhat excentric position on the other surface, which may be termed the aboral or ai)ical surface. From the edge of the disk the five arms or rays project outwards, and ahmg the median line of the oral surface of each arm there extend outwards from rings around tlio mouth a nerve-cord and a hydroc(el canal, this latter forming a part of a peculiar system of vessels characteristic "• =•-.• t.i,-. ixi ^.„^-^.j^jj^,jjj.g .J. j^^-.j_ ruaiation of these structures out along the arms, and the arrangement of 532 INVERTEBRATE MORPHOLOGY. the other organs for the most part in conformity with the radiation, the arms may be regarded as representing the radial axes of the body, the interradial axes lying in the in- tervals between them. If now the aboral surface of the disk be examined, there will be found upon it, in one of the inter- radii, a peculiar tubercle, known as the madreporiform tuber- FiG. 'iiQ.—Asterias arenicola (after Aoassiz from Vebrill). cle, which serves to place the hydrocoel system of canals in comnuinicati< u with the exterior water. There is but ono such tubercle, and but one canal leading down from it to tlio hydrocrel ring which surrounds the mouth, and consequently there can be but one plane in wliich the anini;il can be di- vided into two siniihir i)artH. Therefore the Starfish, thou;^di superficially appearing to j)()ssess a radial symmetry, is funda- mentally bilateral— a statement which applies etpially well to any member of the Echiuoderm type. It does not necessarily follow, however, that the ])lHiie which passes through the madreporiform tubercle is tin* luediau plane of the body. The larvo) of the Echinodcniis are strictly bilateral organisms, no sign of radiality beinj,' found in thftm in an early stage of development, and it would seem more satisfactory to take as tlie median j)lane of the TYPE BCUINODBBMA. 533 adult animal one which ccresponds as closely as possible -th the larval median plane. The madreporiform tubercle ...•rather the pore which corresponds to it ancrthet, winch leads from it to the rudiment of the iyd oLl '''t can readily be made out in the larva, of most forms and can be seen that it lies to the left of the median p aTe "r he l>«ly. Indeed in the larva, of some Starfishes two no e "ccur at an early stage of development, one to the left and the other to the nght of the median plane, the latter subse . nently disappearing. The madreporifor,; tnbercle mi .ht" therefore be regarded as lying to the left „f the median ,Ze However it is impossible to tell how much modiHctio., has taken place during the transition from the bZ. ra to l.e radial condition and it is not impossible that the g eat r portion of the adult represents one of the halves tf uL embryo, the other half remaining more or less „n evelper Furtherniore a secondary bilaterality supervenes in ceiLt S tl f .^'"""f," .™'' H"l"".-"'oidea which does no a ^ e with that indicated in the precedinc imra.rra,,!, ., , . "'™ indeed the same in the two groups. n'::r;;i .;;;:"; able to assume a perfectly arbitrary method of in, Lit uj he radii of the body, calling that radius whicl, lie, on t e madreporiform tubercle A, that which lies to th ' l',^ .s when the animal is held with the oral surface p war K ''wlir' Z^' f- ''•/'.'"'"™'*'' "-"'-tion of the h 1? .. atch. The interradii may be in.licated by combininir the ■Iters of adjacent radii, the interradiu, beLeen i n.l « l'"MJg denoted by vl «. "veeu .1 ami H The bo,Iy-wall in the Echinoderma is covered on the out ..le by a usually delicate, and in .some cases cille e" enn which ,„ay, however, be indistinguishable fr, n ^ , ■■ nt inesodennal tissue, in certain parts of t le b, v" l-l.-w this ectoderm, when present, comes a layer oi me . maal connective tissue consisting of relatively few Tel "iil edded m a more or less Hbrillar matrix, and in tlds co , "ective tissue there are imb„d,l„,l „„.„„,...,... 1., '"", ".""- ". ^..me forms, such as the Ho.oThuri'an;; ^L^'XT t^^^^ 634 INVERTEBRA TE MORPHOLOO T. separated from one another so that the body-wall has a more or less leathery consistency, but more frequently placed almost or quite in contact with each other, and uniting in most of the Echinoids or Sea-urchins to form a firm test en- closing the principal vegetative organs, a small area or peri- stome around the mouth alone remaining but partially calci- fied and retaining a leathery consistenc}'. Spinous elevations are frequently developed upon these dermal plates (whence the name of the type) and may assume various forms, being in some cases quite loug, movably articulated with the plates, and supplied with muscles so that they may aid in locomo- tion. The arrangement of the calcareous plates diflfers greatly in the different classes which com- pose the type, but certain of them, distinguishable by their position and relative arrangement, reappear in the majority of the classes. These plates are situated at the oral and aboral surfaces of the body. The oral plates are not so constant nor so numerous as the aboral or apical, and show a tendency, even in those groups in which they are most highly developed, to undergo a greater or less amount of resorption during development, being frequent- ly more pronounced in larval than in adult life. Typically the oral system consists of a central oral plate, the orocentral, unknown in recent forms, but occurring in cer- tain fossil genera, and this is snr- rounded by a riog of five plates, which may bo termed the oral plates, and which nave an interradial [)osi- tion. Tin ujiical system has as a central pli* ■ the so-called ceiifro- dorsfd (Fig. 247, CD), which in some i-tmg is re])laced by a Fig. 247.— Disk and Akm of Zoroaxter, s-iiowino the Apical System of Platks (after Si.adkn). an = umis. cd = ceutio-ilorsal. 7Tit = madieporite. T = tertliinal plate. 2 = u.'ider-basals. 8 = basal':. 4 = radials. n^-Jf^^^^ TYPE ECIIINODEIiMA. 535 unmber of small plates between which the anal openiug of the digestive tract is to be found. Formin- a ring around this are frequently live plates possessing a radial position which are termed the nnder-basah (Fig. 2^7, 2) ancl are un- represented in certain forms ; next to these comes a secoud circle of five plates, the bamls (Fig. 247, 3), which are inter- radial and correspond to the oral plates, while next to these again is a third cycle, also of five plates, the rndials (Fig 247 4), whose name denotes their position. Numerous ''other plates may intervene in the various groups between the radials and the orals, but their number and arrangement is not suiH- ciently constant to permit of homologies ; the oral and apical systems are, however, represented more or less perfectly in all but one of the classes, and consequently deserve special mention. A well-developed dermal muscular system occurs in the Holothurians in which the calcareous plates are scattered aa.l the body-wall consequently capable of considerable con- traction and expansion, but in other forms it is very much reduced. In those forms in which the calcareous plates are simply in apposition strands of muscular tissue pass from plate to plate, a considerable amount of movement being pos- sible, but in the Sea-urchins, for example, the dermal muscu- lature IS almost wanting, being reduced to bands passing to the bases of the movable sp,ines and to the complicated mas- ticatory apparatus. The C(Klom is somewhat complicated in its relations, which vary considerably in the ditierent groui)s. In all enterocctaic and schizoc(Blic portions are distinguishable, the former in the embryo arising as pouchlike diverticula from the primi- tive intestine or enteron, and later becoming completely con- stricted off from it. Mucii variation occurs in the later his- tory of the pouches in the various groups, but in general it •nay be stated that one of them, the left, has a portion con- stricted off from it, which forms the adult water va.<^cuhir s-ys- >''i>i or hfilrocopi, a structure characteristic of the Echinoderms • iiu.l furthermore this same left enterocoel communicates with' 'li(> exterior by a dorsal jiore, situated in the interradius CD ni.l represented in the adult by one or many pores opening 536 INVERTEBRA TE MORPIIOLOO Y. upou a sievelike calcareous plate known as the mmhrporifonii tubercle or madreporite. The hydroccel in the adult commniii- cates with the left enterocoel by a tube, termed the stone- canal from the deposition of calcareous matter which occa- sionally takes place in its walls, and so indirectly opens to the exterior through the madreporiforni t\ibercle (see l\. 265). The various departures from this arrangement which occur will be more conveniently considered in connection with the special descriptions of the various groups ; the con- dition just mentioned may be provisionally acce])tecl as rep- resenting the typical arrangement. After the separation of the hydroccel from the left entero- coel, the latter and the enterocoel of the right side increase in size and finally apply themselves closely to the inner surface of the bodj^-wall and to the outer surface of the digestive tract, forming the peritoneal lining of these structures. Wlieie the two ccelomic sacs meet there are formed, of course, two partitions extending from the body-Avall to the intestine, ami suspending that structure between them. These partitions are the mesenteries, but before the embryo reaches the adult stage one of these mesenteries disappears, the oilier persisting in a more or less perfect form. The coiling of the intestine, which occurs frequently in the adult forms, brinjiis about complications of the course of the >aesentery, coui))!!- cations further increased in most cases by the formation of other partitions which may traverse a greater oi less portion of the coelom either longitudinally or transversely. One of the transverse partitio" ■', most frequently present, separates off more or less completely from the rest of the coelom, a por- tion of it surrounding the pharyngeal region of the digestive tract and hence termed the peripharyngeal cavity, while iu gome cases a perianal cavity may similarly bo formed. The hydrocoel, whose origin has been described, develojw into a tubular ring (Fig. 248, cc) surrounding the oesophagus quite close to the mouth. Upon this ring iu the interradii one or several saclike diverticula, termed Polian vesicks (/)), occur, and in one interradius a canal, the stoue-canal (sr), passes aborally to open into a thin- walled sac termeh In connection with the stone-canal a peculiar body is developed r\ most forms. It? function is a matter of ques- tion, it hnviug been at one time taken for the heart and at another for a gland. It is generally termed the ovoid gland (Fig. 265, og) and consists of a mass oi cells, derived from the peritoneal lining of the enterocoel, grouped together to form a more or less solid mass. The oral end ci the gh^nd is pro- longed into a cordlils biructure which seems to enter into close relationships with the oral lacunar ring (see below), while at the other it is continued out to enter into close relationships with the reproductive organs in a manner that will be de- scribed when treating of those organs. Surrounding the gland is a sinus — the axial sinus (Fig. 265, as) — sejjarated off from the enteroccel and, in some forms, in communication with the ampulla of the stone-canal, and the portion of the gland which passes off towards the reproductive gland is also surrounded by a sinus, or rather lies in the wall of a sinus which may or may not communicate with the axial sinus but has, like it, origin from the general enterocoel. "What has been termed a blood system is usually present, consisting of a tubular ring surrounding the cesophagus, and lying between the hydrocoel-ring and the nerve-ring. Five branches may extend oft' from it along the radii, preserving the same relatio]is to adjacent structures as does the ring. These spaces seem to be schizocoelic in their character, and may be termed the schizocoelir ring and radial schizocoelic sinuses in order to avoid confusion with another system of vessels which sometimes lie within the sinuses and have also been termed blood-vessels. This latter system may be termed the lacunar system, and is composed of a network of vessels lying in the walls of the intestine, and collecting usually into a perioesophageal ring or plexus (Fig. 265, Ir), with which also the ovoid gland comes into connection. In the Echinoids, as has just been indicated, prolongations of this perioesophageal ring or ])lexus extend out in the radial schizocoelic sinuses. The fluids contained in the sinuses, lacuuio, hydrocrel, and enterocoel are all very similar, consisting of a plasma contain- ing amoeboid cells sometimes deeply pigmented. In a few forms hsemoglobin is present ; in the Ophiuran Ophiactis it is II TYPE ECIIINODEBMA. 539 contamed iu flat nou-uucleatecl disks, resemWiug Mammaliau red blood-corpuscles, floatiug iu the plasma "f 11""™*"" ™seula.- system ; m the Holothuriaus, Thyo,^ aud CucJ^T, >l .s howevev coutaiued iu au^boid corpuscles, ^hicTa e ..ost ab„u,laut m the ccelomic fluid, though occur iug dso L tlie water vascular tubes. ^ The digestive tract is generally more or less twiste,! iuto a :i!ht T IT' "' '" r'"^ Holothuriaus, it appea to b .straight, ,t m to be regarded as a much-drawu-oit spiral .nee he mesentery still retains a spiral arrangement. I ' Ho othunaus, Eclnuoids, and Starfishes it opens on the abo d ™ ace of the body, but in the Crinoids it is bent upcn itse hat he anus ,s on the oral surface. In some Itarfls .e tr ,ctu,'.e ^"'"rr "° ™"' '•" l^'-''^»'- ^'""O"'' accessory t.uctu,es masticatory apparatus, ccecal pouches, etc are Sjuitnirtr ''""'""'' "-'" "-*'- -/^« The nervous system may be regarded as being composed of meut as the water vascular tubes, consisting of a circumoral or a pencesophageal ring from which fivA ,.^,i;,l "™"'°"" offiFiiT 9fi^ «,. 1 . """'"""""' nveiiidial nerves pass >« (lig. 26o. vr and ,■»). In the Starfishes and Crinoids the »tne system is imbedded in the ectoderm, but in other for ' .inks within the body-cavity. From it branches pa s n ulsat the mouth to supply the walls of the oesophagus ami ft. branches form a network covering the surface " te W>, supplying the sense-organs which m.ay occur thereo , Uu. radial nerves, in addition to sending branches to joi. tl e p derma plexus, supply the ambulacral system. Th » po of th, nervous system may be termed the epideral 1". .on, and the second, inasmuch as it supplies the m i,, ritv :.;: "'ti-':;"' f --3-. ■"■^3- be ter^iirthe^r::;; wiuoi is, but when present accompanies in gener.al the eni :^'s:r;:tf::t'''^fz.rr:t--:^;;"'^' - '.<• entirely wanting in the Holothurians, b!it w,:eV:!r:: '> if- 640 TN VERTEBRA TE MOliPUOLOG Y. cousists of a xm^ situated at the aboral surface of the boilj', sending off branches to tlie reproductive organs as well as, in some cases at least, forming anastomoses with the epidermal 83'stem. Sense-organs of various kinds are developed. Tactile tentacles occur at the extremities of the radii of some forms and round the mouth in others, while in the softer-skinncnl Holothurians tactile papilhe may occur. Eyes occur at the extremities of the radial nerves of the Starfishes, and have also been described as occurring in some Echinoids, while otocysts occur in some Holothurians, sometimes in considera- ble niimbers. No special excretory organs occur in the Echinodermuta, the am(i'boid cells of the cadomic fluids perhaps serving iu some cases to remove the Avaste substances. They have beeu observed to pass through the bodj'-wall, in regions where it is thin, to the exterior and there degenerate. For the nujst part, however, the waste products are deposited in the tissues, or else pass to the exterior by osmosis. In the Holothuriaus special branched appendages of the terminal jjortion of the intestine apj^ear to take some part in excretion, but suck organs do not occur in other groups. The Echinoderms are almost invariably bisexual, and the reproductive organs are usually situated in the interradii. They are enclosed in a special coelomic sinus, the genital .sinus, in whose wall may be found the branches of the aboral nerves. From each organ or mass of reproductive cells a cellular ccud, the genital racMs, surrounded by the sinus may be traced, except in the Holothurians, to the ovoid gland, and it a))pears probable that iu some cases at least the reproductive cells originate in a part of the ovoid gland and migrate to the reproductive organ along the rachis, becoming mature in tlieir final position. The openings by Avhich the reproductive ele- ments pass to the exterior vary both in number and position in the different groups, but are usually situated on the aboral surface of the body. TYPE ECIIINODEltMA. fi41 I. Class Crinoidea. The Crinouls, or Sea-lilies (Fi<,^ 249), coustitute a group of forms whicli lii the earlier geological i)erioas reached a hi-h orade of developmeut, but to-day the class is represented by eumparatively few forms, for the most part couHued to deep Fig. 24!d.—Pentiicvinus madenranus (after Wyville Thompson from Hertwig) water. One of the most characteristic features of the group IS the presence of n more or less elongated cylindrical stalk one end of which is attached to stones or other objects which snrve as supports for the animal, while at the other end is the body proper, which has a more or less cuplike form. In the peculiar genus Ilolopus the stalk is thick and short, and may be described rather as the prolonged apex of the body 642 mVEUTEDUATE MORPHOLOGY. U 4 than as a ilistiuct stalk, while in other forms, such as Antedon and Actinometm, the stalk, though present in young forms, is entirely wanting in adult life, during which the animal is free- swimming, though having the power of anchoring its.'lf temporarily to solid objects by means of a number of slender processes termed cirri which project from the apex of the cud (Fig. 251, c). ^ The lower portion of the cup, or calyx, is formed by a num. ber of series of calcareous plates united to each other l)v sutures, while its mouth is covered in by a flat or dome- sliaped disk in which calcareous plates may or may not lie present. In the centre of the disk is the mouth of tlie animal, while to one side is tlie anus, lying in the int^rradius ClK From the mouth Ave grooves, known as the anibulucral (jrooves, extend outwards towards the margin of the cup, and, near the margin, brjinch, being tiien continued outwards on the f)ial surfaces of ten arms which arise from the junction of the disk and calyx, frequently branching in tlioir course, and bearii]v he second: and furthermore in the same forms the b sals have also been pushed, as it were, within the calvx lave fused to iorm a single plate, the so-called .o.sW/. MI g. 251, 7?o,v). winch rests upon the centrod<.rsal. par- uly dosing a cavity in that plate. The terminal ra.lials sually present two articuL-tting facets in their distal sur- t.U'es and are generally know., as the r,.r;7AnvV.v (Fi. 2r,0 a) -cc he arms articulate with them. In the ^.n^TljJ: ... betw^^n each pair of first radials an interradial plate -n s. a condition frecpiently found in fossil genera ii), but sua^y wanting in recent forms. These various plates which <;;•. t.tute the apical system are united by sutures, the edce« ^. .lie various senes of plates coming into contact, s.. that a *H'"Hupportisationlod for the arms BnoNN), a - Hxillary plates. * ~ ))!isa! plates. i = iiilenadial plates. r = radial plates. 544 INVERTEBRATE MORPHOLOGY. These are iu reality continuations of the radial series of plates ; iu fact, iu some forms certain of the radials appear to enter into the formation of the arm. Iu most forms, however, a series of arm-plates arises from each facet of the live axillar}- plates, so that the arms are ten iu number — a condition which finds an exception in the remarkable genus Thaiimatocrinns^ which possesses but five. In some forms these ten arms branch dichotomously ; the plates intervening between tlio axillaries and the first branchiug are termed brachials, those between the first and second branchings distichals, and those between the second aud third branchings palmars — terms which are useful in systematic de ^criptious. These various plates are united together b}' ligaments and muscles, or else by ligaments alone (this last form of union being known as a syzj/(jy), the movements of the arms noted above being thus rendered possible. The pinnules repeat the arm iu tlieir structure, though usually on a much-reduced scale. They are situ ited on the joints separatiug consecutive plates of tlie arms, aud are j)laced alternately on the right and left sides of the arm which bears then). They appear at first to have been produced by lateral budding from the joints, but closer examination indicates that in reality they represent a braiidi- iug, one of the brauches remaining small, while the other in- creases in size and ])laces itself in the direction of the axis of the arm. The whole arrange nnuit is comparable to that form of inriorescence termed by botanists a scorpioid cynu^ tlie pinnules re]>resenting the riower-])edicels. Owing to the ])inuules being iu reality one of the branches of a dich(;tt)my, it is evident why, in those forms iu which the arms braucli, there is uo pinnule at the joiut where the brauching ot-curs; iu addition, however, pinnules are also lacking on sy/ygi.il joints, so that their r^igular succession )uay be somewhat dis- turbed. As regards the (U-al systmn of plates an nro-ccnfral is found in souu' fossil forms, but is unrepresented in recent gouerii. A circle of five interradial oral plates is found iu Jlolnpns, /thiziates are developed, and from which strands, frequentlv with cal careous spicules imbedded in them, usually travers; the body-" cavity- Ihe ligaments which unite the plates of the arms and stern are formed of this connective tissue, and contractile hbres of a peculiar character are sparingly developed in it stretching across the non-syzygial joints of the arms, pinnules' and cirri, and probably also reaching a slight developmen; 111 the stem. ^ The internal structure of the Crinoids is known i^rinci- pally from observations on Antedon, and the following account represents what occurs in that form. The ccelom, as alreadv stated, is t.-aversed by numerous strands of connective tissue ami primarily consists of two cavities separated from each other by a mesentery, each cavity being continued out into he arms, forming the oral and alu.ral canals of these struc ures, at the extremities of which they unite. The mesentery does not however, long persist in its entiretv, but the two cavities fuse, new n.embranes, h.nvever, arising and ,livi,lin- then, m some species of Antod,n,. One of these mend.ranes (Ug.2;>l, y.s.) Mil-rounds the intestine and forms the visceral sac Its presence rendcM'ing the evisceration of Aiitrdon an I'asiy.accomplished proc-ess and one which is n.ade use of •y the annual in unfavorable conditions, a new visceral mass -.ng later regenerated. The portion of the ccelom which li.'H peripherally to this sac is termed the circumvisceral l""-tion (r.), and that within it the intervisceral He), the latter containing an axial cavity (J..) enclosed by a membrane sim. p. VI HC- rul Hut= and continuous with the oral co>i I cavities {oc) of the arms, tiie aboral omic cavities ((/(•) coiumuuicat- 046 INVERTEBRATE MORPHOLOGY. iug with the circumvisceral coelora. A portiou of one of the coelomic cavities at au earl_y stage becomes cut off from the rest of the coelom aud divided iuto five chambers whose walls are formed of a deuse fibrous membraue. This constitutes •\s Fig. 251.— Veuticai. Section rnRoroii Antedon (combination of flgures liy l,ri)w:i; ami MAi'snALi.t. cc — ahoml caiml nl' ann. an — aboral iutvc. Aj! — nxial sinus. Br = hr.Mi'hiiil jjliitcs. C = cirri. cc = circuiiiviscoml ciivity. CD = ccntiodorsal plute. CO — central oryaii. Do — ilori-al tirgmi. gr — geiiiliil nichls. /= intcHliuc. id =: intcrvlsceral cavity. M — uioulii. oc = oral cavity of arm. on = oral nervf-riuj.^. R — radial piatts. rh — radial lacunar vessel. rn — radial epithelial nerve. VOB — rosette plate. r?o = radial hydrocojl-caual. «c = stone-caual. T — oral tentacle. vs — visceral septum, wp — vvaler-pore. the chambered organ (ro). whicli in Antalun. lies in a cnviiy in tlie centrodorsal pltite and is roofed over by the rosette- phite, l)ut in other forms simply rests upon the centrodorsal; communioating with it is the lower end of a somewhat citih- suaped structun* termed the ihprmil 'Wfjun [Do), wliicli \no- jects orally i)arallel to the axial co'lomic ctivity. The e|>itlieliuni of the aborid codouiic cavities of the atiiis is ditferentijited hero tind there into peculiar organs the ciii- TYPE ECIIINODERMA. 547 1 of flKiires by uted cups, consistiu^^ of slight tlepressic.ns lined by columnar cells eacli of wiii.h bears a long ciUunu These cups are especially abundant in the pinnules, and serve to create a circulation of the crelomic Huid, which, as in other Echino- (lerms, contains numerous amueboid cells lioatiug freely in it The water vascular system, or hydroccjel, consists of a riuL^ surrounding the mouth, and sendi.ig outwards five radial canals {rw) which lie below the ambulacral grooves and are continued along the arms and pinnules. Occasionallv sub- ambulacral calcareous plates are developed in the connective tissue bel(,w the radial canals, and in some fossil forms these plates assume a regular arrangement in two rows. At regu lai- lutervals along the arms are situated the ambulacral ten- tacles, which are fingerlike outpouchings of the radial canals . estitu e of terminal > -ckers and are arranged in groups of three, the canals being somewhat enlarged in .he region where they occur, an indication perhaps of the ampulho found in other groups; in some forms the cavities of the tentacles seem t<. be united with those of the canals only by exceed iiigly small orifices, which may be closed, since the tentacles 111 their greatest contraction always remain filled with fluid 111 the neighborhood of the mouth ar,> a number of oral ten- tacles (Fig. 251, 7') arising directly fron, the oral rin.^ and .lilleriug from the anibulacral tentacles in not being armn.>ed in grcnips of three. From the oral ring there also arise" in Anteaon a uumber of ciliated tubes (.r) which open into the cjelomic cavity, each one corresponding to a stone-canal of t h; othei Echinoderms. In AntaJon there are as many as thirty of these canals in each interradius. and in Prntavrinm an even greater number occurs ; but in other forms they may he fewer, Mizucrinus, for exrunple, possessing onlv ftve in all one being sitna' h1 in each interradius. In the larva of Jnfri 'l<»> there is .,.t tvi early stage only one, communicating with a I'-rtion of on- >! the primary cielomic cavities, which on its jMit o])en8 to the exterior by a pore, an arrangement Nvhich may be regarded as typical for the Echinoderms. Later iowever, this portion of the c.^lo.aic cavitv degenerates, and the canal ihi ,i opens directly \nU. tiie ireueval cu-lom, and t[ ns communicates with the exterior by th y the pore. In subse- 648 INVERTEBltA TE MORPIIOLOG Y. n queut stages additioual stoue-cauals develop from the oml riug, and at the same time additioual pores develop iu the walls of the body, forming what are called the calyx-pores (icp). These may reach a considerable number, it being esti- mated that iu Antedon there are uo less thau fifteeu hundred of them scattered over the disk; iu rjiizocrinus, llyocrinnH, and Holopus, iiowever, there are only five pores, one pierciiig each of the oral plates present in these forms. The schizoccelic system consists of live radial sinuses (Fi- 251, rh) lying between the radial hydroco^l vessels and tl?e more superficial radial nerve, together with, according to some authors, a circular sinus surrounding the mouth iuto whicli til" radial sinuses open. A plexus of lacuna occurs in the Avails of the intestine, and another surrounds the a'soplui- gus, ,;his latter in part aggregating itself into a structure re- sembling r, lymphatic gland and known as the spongy body; the dorsal organ likewise contains a dense network of tnl)e,s lined with epithelium. Ahmg the sides of the hydrocol- canals, in the disk, arms, and pinnules, alternating in the two last with the triads of tentacles, in the walls of the intestine, and occasionally elsewhere, there are imbedded in the con- nective tissue yellowish spheiical bodies known as the hocvh};. The interior of ench sacculus is lined with cells, and contains a number of pyriform masses formed of small highly-refrac- tive spherules, apparently of an albuminoid substance. Tlie function of tliese bodies is very obscure ; they have been le- garded as organs for secreting carbonate of lime, as excretory organs, as parasites, as mucous glands, and lately as organs of reserve iu which proteid matter may be stored up for future use. At present, however, the question is an ojn^ii one, and a function cannot with certainty be assigned to tlicni. The mouth (Fig. 251, m) is usually situated at the centre of the oral disk, and opens into a simple tubular intestine which coils once round the cadomic cavity in the direction of the hands of a watch and then, ])endiug upon itself, turns orally to open in the interradius CD upon tlie disk. In .\<- iinometra, a genus closely related to Antedon, the intestine lies iu four coils, but there is as a rule little variation from tlic TYPE ECIIINODEUMA. 649 i" the lower" i^Tr''" """"«'^«"* *J-»- -^tire course i^jeis ol the ectoderm. The aboral system is Fm. 253.-DiAonAM of the Arkang^micnt op the Abor.i at « = arms. B;- = bmchiul plates. c^= centrn.iorsal. ->« tl.e otJ,er ),au,I, ,„„ch more str„u«ly .levelm,e,l .,,,,1 .t > "1 mtimate aasociatiou with the ch^imhe, " • ""''' «"m,,lex-,a, is show,, i„ Fi/252 , i , ■!'/""'«whut ;;'">^l; five str„„K cor,',s rllial o, ,f ^.l , ^ 1^;? ^f™' ""■ plates of >vl,iol, these are compose B tj', '^ "'"""^ K;u.f-l.o«-oells enter into the comn, Litio,, of f *"'' ;;.«. ami a oo,„plioate,l »,stem o I ™,,:L ^s^ V'"' '!« central portion branches are also sen to ^hf • ,' K.. abh. in s.,.,^e„ forms . brand trverles t ,:'™;;: '"■' "* "" ""'"'' ""--.'a-yi^K prolonsations of the ^.dtLi 550 INVERTEBRATE MORPHOLOGY. of the chambered organ. The terminal branches of the radiul aboral nerves pass to the integument of the oral surfaces of the arms and to the muscles which unite the various plates, so that the system governs and coordinates the movements of the arms and pinnules as well as of the cirri. The epithelial system, on the other hand, controls the movements of the aiii- bulacral and oral tentacles, stimulation of it causing move- ment of these structures in the immediate viciuit}' of the region to which the stimulus is applied. Another system of nerve-fibres, consisting of a perioesoph- ageal ring which sends off two branches to each arm, one lying on each side of each of the ambulacral grooves, and which is connected with uerve-libres passing from the dorsal organ, has been described as occurring, but its signiticauoo has not yet been satisfactorily determined. No special sense- organs occur in the Crinoids. The reproductive organs are developed for the most part in the pinnules, occasionallj^ a slight development of tiieiii appearing in the arms or even in the body proper ; in llolopnn ahme they are confined to the arms. They consist of tubes lined with germinal epithelium on their inner surfaces and enclosed within a prolongation of the coelom. They lie be- tween the two ccelomic prolongations of the arms already mentioned, and though the reproductive organs are develoj)ed only in the pinnules as a rule, nevertheless each genital tube or raehis (Fig. 251, gr) can be traced through the arm to the body, where it terminates in connection with the dorsal organ. In their develoi)ni(nit indeed they grow out from this oi^an, and it seems prcjbable that the (jva and spermatozoa mother- cells migrate out from it along the rachides to reach matuiity in the pinnules. Comparing this with the condition in other Echinoderma, it seems clear that the so-called dorsal oi^an of the Crinoids is homologous with the ovoid gland of tlie other forms. The reproductive elements i)ass to the exterior by one or tw(^ ducts connected with each reproductive mass- the origin of these ducts is unknown. Tlie Crinoids seem to liave been closely related to two groups of lonib known only as fossils. These were the Ci/stohl,i, which apiJeur in the Lkwci Silurian rocks and die out in the Carboniferous, and the Jilantoids, wiiicii TYPE ECIIINOBEliMA. 651 nppear in the Upper Silurian and also disannear in fT,. r i •. For a description of these fonns roforcnceSl'vuf^to Jh^^^^^^^^^ works on Pa]a3ontology. On account of their sim l.dt v f^r> f l r 1"^''^ have been associated with them in the cl ,s '^ ^ ^ *' ' ^"""^^'^ ^^'^^ .l.e groups fonnod an order "^1 i',;^::"^^;' ^'' ^^'-'^ -h of eoncerned only witli recent forms if .' . ^ ' ' "" ^"'""""^ "'"'"'^ ^^ .•egard the Crinoid.s ."a c,L^ ' " '"" ""^'^'"^ "^"^ —"-"t to Tlie class Crinoidea has been divided into two nnl,.,.^ ti r> , mda^ chiefly Pala-ozoic forms, eharacteriz hn^inei.v ?' H '^'''''"■■ "uder-basals and of a series of nl.,f »'»^'l>all} by the presence of Pletely, to which u,ay^ d ed t 'u IZ^f "^ f' '''^ '^''"^^ ^- ...ater wldn. of one of the inte .: , r^^^;; ./'ir"'''^ '''''''' (-0 Fig. .50). TbeiV.0...,,, on tl. olhe ,1 ' ,^, ,11:,^"-^ lonns. te group making its first appearance in tli" M "^"^^ , '^ "r acterized by the disk bcin.r n.,i,r ;.„. ,. ., 'Hl.sozoic, and is ciiar- nnder-basa^ and inti^hlds ^ g^X rl^r^::; '^ ^'^^If' '' ''^ width. Transition forms between t it l " o s^^^ ™^ ''' ^^"^' '" genera H>,ocnnas and Ca/a^.ocnV..., f<,r cCn 1 , ' T'""''' *''" i'aheoerlnld peculiarities combined with N. ei Id^s 'r^"' '"'"^ satisfactory to divide the class into families only leavh;.: / T' question. ^ ' '^'^^ '"» ""^^''"^ "ut of the Development of the Crinoids.—Antedon is tl.P nni n • • , , -l-olopment has been studied. The In.n^o lead^ ' "^"""^ "''^^^ tor a time a free-swimming existence, and possesses a somewhat ovoidal form (Fig. 253) with a tuft of 01 la at the smaller anterior end and five rings of ciiia surrounding the body. Not far from tl.e ante- '•i<.r end ,s a slight groove, and lower down uiwu the .side IS a much larger one. This larva settles down upon tiie anterior end, the slight depression near >liis end serving as an organ of fixatl.,,,, and then a rather remarkable rotation occurs, the large groove •sii-fting round together with the interior organs until It comes to lie at the free end of the organism, and at 1 10 same time its lips unite so as to enclose a cavltv tl)o vestibule. Calcareous plates have ere this de- veloped in the connective tissue of the embryo and ^^^^^^ outhne a stalked Crlnoid into which the larva is^io. SSS.-Lauva of gradually transformed, the larval skin shrinking as ^^'■''- >t were, so as to closely surround the stalk and calyx Wilde the vestibule opens to the exterior by the ,^nidual thinning and final disappearance of its' roof lis floor forming the ectoderm of the disk A ft,.,- nr.lr • ■ »».ue time ., „ stalked Cri„„id, t ; , t, aIi^TZI '""T"; "" .!.< stalk, a„d thereafter leads a free Listenec" ^ '""'"''■' '™ Antedon (combination of figures by Thompson AND (ioETHK after Koii- SCHELf AND HeiDERJ. 552 INVEHTEBRA TE MOEPUOLOO Y. II. Class Astekoidea. The Asteroidea, or Starfishes, are all flattened forms, at no period of their lives attached by a stalk, but creepinj^r about freely upon the oral surface. In some forms the Ixxlv is a flattened disk pentagonal in outline {Asterina), but Uh m usually (Fig. 246) the Ave radii are prolonged out into live stout unbranched arms, and in some forms, such as Brisimid, the arms may be long and slender and more than five iii number. The mouth is situated in the centre of the oial surface, and the anus slightly excentrically upon the aboral surface, while the hydiocoil system of tubes is confined, as in the Criuoids, to the oral surface of the body, except tliat the madreporiform tubercle by which the system comniuiii- cates with the exterio? ipou the aboral surface in the iu- terradius CD. The ectoderm is throughout ciliated, and contains usualh numerous mucous glands, while in its lower laj'ers ganglion- cells and nerve-fibrils form a plexus extending over the entire surface of the body. Calcareous matter is deposited in the connective tissue, but in the majority of forms the primitive apical plates are not recognizable iu the adult; more usually the aboral ! n v" e. though upon he marginal and adan.bulacral plate t" H.e very frequently longer, united to the plates byT rudf ...en ta.7 articular surface and supplied with muscle fibres b! « . oh they can be n.oved. In addition to these append- „es ..1 the dern,a skeleton, others are to be found in he 8^., W.es, such, tor example, as the ciliated spines found i " le.- forms, such as Z«fc, upon the margin d plates These ^.mes are small and delicate, and gro,';ped togelher ^e I....e.pal pecuhanty being that thev are covered by an ; e^^ium ol h,gh columnar cells which bear strong ,ia' l" .o.-t bta.hsh also peculiar structures termed pedlcelhru, are .•veloped in connection with the skeleton, but'^theb de^,-; on may be deterred until the Echinoids are under di'cu ......a winch group they reach a high ,rade of d t , ,. . u.t. lecu nir to certain genera of Starfishes, e.g. /.«,/ tl.e body. 1 hey consist of small columns of carbonate of I \^- IMAGE EVALUATION TEST TARGET (MT-3) 4 sr J^ MP., K 1.0 I.I 1.25 i^g^ 12.5 •S m ut u 11^ 2.0 1.8 U nil 1.6 ^' '"^ W- ^ # -% ^>^ w ^/W/ m Oy- Photographic Sciences Corporation 33 WIST MAIN STRICT WIMTII.N Y. I4SI0 (>i») ^73-4103 4ss ^ 654 IN VERTEBRA TE MORPIIOL 00 Y. lime imbedded iu the coune(3tive tissue, aud bear upon tlieir free extremity a number of radiating spines, which vary in the amount of movement of which they are capable iu ditrcr- eut species. Tlie paxilla are frequently found iu grou})s around the dermal branchijB, over which the spines may bo bent so as to serve for protection. These dermal branchije (Fig. 2o4, h) are pouchiike evagi- uations of the coelomic cavity with thin walls composed of ectoderm and a layer of ciliated cells continuous^ with the peritoneal lining of the cfeloni. between these two layers there bein^ but a slight developuient of connective tissue "and Fig. beei' asserted that the amceboid cells of the coelomic Lain - TYPE ECIITNODBUMA. 555 iiodifled from ^'^ :'r::- r^r -: ™tt;; : - r-'- "-« ''■■"•eve... n det.n«, .lescipti,;, "si , ft s" ""/, """"■''■ ra.lial ciocmu „f the (li.rpsliv,. f.. , , " •^''■>' ""»» each »i.ieh, wi,l^tl,eL^ ,"';«''"'•'''■■'' "--'enes (Fi,,. ,,i, sti-oiiK wall, ,vl,ioh i.s i„ e„n„„n„ . "' '"•""y '''"■'< l'"rti"u of ti.e ocBlom wlrcl, i. 1 r •"""•"■ ^''"» " "■ ;;■■ .; ".e e,„.,,o :,:^;:lx'i:r:r r-' "■« ■-'• ' .«!. .•...lain s|,,„.,.;X,',''^r ','''"'' '" ""• ^»'e'-'"v means of the muscles of the wall of the ampulhe water can be forced into the tube-feet, which may be thus extended, a circular valve occurring in the branch which passes from the radial canal to the ampulla preventing the water from })assing back into the canal Contrary to what occurs in the Crinoids, there are several ai)pendages to the oral ring, iu addition to the stone-canal. This leaves the ring iu the interradius r/> and, passing abovally, communicates with the axial .sinus wliieli, as already stated, opens to the exterior by the madrei)orite. This isacom])licatod calcar(>ous sieve.plate of some thickness, and the union of the canal and the sinus takes place wilhiii its substance, so that in r(>ality the canal seems to open to the exterior. The embryonic history, and the fact that injee- tions forced through the tubercle pass into both the sinus nnd the canal, shf)w that what has been described is the true TyPM KClllNoDKttMA. 657 H-'la .o„8l.,i,. I„ ti,e w.lls „f tho st„„o-c,„,al culearooas the surface „.■ the ciliate.l „,,it,,„,iu,„ ,,„i„,. th„„ iu^-ea.".! ' Ihe „,,,,o„,laHe.s „t the oral riu„ are of t«-o kinds, both he,„« situated m the iuto>>-a,lii, that coutaiuiu;. the stoueia," howevo,, usually lacki,,., any other appe^da^e. T, s t e l-n,s hollow saclike struc:ures opeu 'iiuo the ri.,« by ..."•nnv uock. and are termed the Polian vesicles; thc^Ir walls -msist of conuective tissue iu which are situated n.uscle hlnes, and theu- interior is line.l by an epithelium wh „I appears „ separate and fjivo rise to tho an„eb„id cells of the |."l.-oc,„l 1 ,„d. The other kind of appendages occur generally throu^.hout he group and are know,i as Tiedemau's vesicles c"..s.st,n(; „ n,asses of hollow tubes arranged iu pairs iu oae ;;;■ -- <>' '!»> ".ter..adii. Tho epithe.iunHining 'tl.e wall o icse s rm..tures als„ . en,s to .-ive rise to the a,„,eboi,l cells, I ..tl, k.mis ol organs l,e,„g tuerefore comparable to Iv.nphntic «lands, though the l-oliau vesicles have also been regard ! reservoirs lor the hydrociel Hi. id. " The „n,»th is situated at the centre of the oral surface of he disk and opens into a short .esophagus which, i„ some Tonus, has connected with it ten glanduhir pouches. The ...sophagus opon.s iuto a usually capacious car.liac stomach "Inch is re,,ucntl.y lobo.l (l-ig 2.55, c), is ovorsible and pro. vMlcl with specnil muscles for its retraction. Above his "unes the pyloric stoma.d, which gives rise to Hve radial P-uches, which son,, branch into a pair of sacculate,! pouches «e."."g out into the arm,,, an.l being termed th'e radial la ). From the p.vloric stomaoh a short rectum passes .'l>.Mally, mterra,l,al ««„a being .somotimes found close to its w'l ll ?''■■ "™ "'"'":■''•'""• "!>«.« upon the dorsal ^ "face In a few forms, such as /.Mi,,, .-(.,/,,,,,..,„„, and their •Nhes, the anus ,s wanting, but more usually it is present iu till! r(><,M()ii indic'uted. The epitliolial norvous .svHteni consists of a i,lexii8 of K.iu^' lon-ce Is a.ul fthros i.abod.lecl in the ectodern. and cover mg the siirfaco of tlio l,o,Iy. and of an oral rin« and Hve ra.'ial nerves (Fi,. 254, S) which, as in the Cnnoid^are siti;a;;;:; • ! 658 IN VKHTMBIiA TM MOUPIIOLOG T. the lower liijers of the ectoaerm. Upou the aboral siirfaco of the oral rill-,' and the radial nerves sections show distinct bands of fibres separated from the rin-like form and do not d(n-elop suckers a( the extremity. Their walls are richly supplied with nerves, .md TYPE EVIIlNODEIiMA. ggQ they ure surrouude,! mid nmy be c(,voiecl in by the movable spiues „t the iulHinbuhicnil a.ul i„ai-inul plates. That thev have a sensory function se.mis ch'ar, Init what the evaJt nature of the function nnty be is as yet uncertain. At the base of the terminal tentacle of each arm is situated an eye <'<>UHist.n^ of a laroe number of conical depressions, lined bJ an ep.thehum contaiuiuK a red pi^Mnent, covered on tlie out- side by a cutich. and ri cutich^ and these eyes can only c..nvey to the aninnil im- pressions of chanoes in the intensity of the lij^bt fallin.^ upon them ; they cannot form ima-es of external ,d,jects Th.^ repro.luctive o.-^^ans are ten in nund.er, two beiii.^ situated in each arm (Fio.. 2.15, .,). Eaeh consists of a mass ot roproductive cells, and is enclosed in a genital sinus (Fi... -.;.4, I), winch, as already stated, communicates rith the axi^l sinus. Ihe proximal end of each gland is connected with a <-;dhke structure, the genital eorning later. In later stages two additional arms are developed at the sides of the apical lobe, which becomes like the new arms destitute of ciii i and tipi)ed wiLh a group of wartlike elevations. This form of the larvi 'i ■ known as the limchiolaria. ' ' '^ A peculiar process, amounting almost to a metamorphosis, occurs durin.r the translormation of the larva into the Starfish. Calcareous plates of the aboral system make their api)earance on the dorsal surface of the stomich Fig. 256.— Btpinnatiia ow Asleracanthion (after Aoassiz) an = anus. ],,, = i.ydn.ca'l. m = mouth. near the posterior end of the body, and oral plates on the ventral surfa. .> o. the same organ. The.se two systems, at first rather widely senan.l'ni gradually approach each other, and at the same time the iutonvA or....K assume the adult form. Finally the two series of plates unite, encl.rsin.^ between (hem the hydroccel, a portion of the digestive tract an.l of tlH> n.^. ""; ,J . T'^?''^ "'""^'' •'""^ '"■•"' •'"« obliterated, and indeed the anl.r- lor half of the larva takes no part in the formation of the adult animal. but IS gradually absorbed. A highly-developed faculty for regeneration occurs in the Asteroid,;,. the disk being able to regenerate lost arms ; and indee.l an arm, with wi,i,li u small fragment of the disk is in connection, has the pow.>r of regeii.w.il- ing all the missing parts. Specimens of the eommeii Starfish Ast.na.s ar," in consequence frcpiently found with one or more of the arms bifid a( il- tip, or even with an abnormal number of arms, tW^'--'- TYPE ECIIINODEIiMA. 661 III. Class Ophiuecidea. .•.oI';^"';;::;:?!?,::;,7,,;^f ':-'•"■«. -emi-e »,« starfishes i" -ill cases slH,„Ier a,„l HsH,,: ^' "'■"'"' ''"»"'^'»-' »'«> y 10. 257. -Op/iioffh/pha aculeatu from tiw AT.on*T «r. I^KKs..^K^. A.C.. z^:::\^:::^^ '" ^"^^ ^"•^ (Th... arms are cut oil close to the disk) 1 = centioilorsal plate. .. 3 = under basals. = ''"'"*'«• 4 = ladials. face, there are no visible nmlmlnnvni -liicl. a,e move,,,. Ins 'i,™ ■ f"™',"" "'^ """«• t»iu e,.oal processes ,>f tl e t . i .i; T" T'll'" "'"' '"■"■ each side of each of the fiv» ? , !, ' ^'"'•""'"""■•e, o., tlic oral surface , t ,,1 k 'mii " " """" "'" '■''«" "' ;» the, a,.e ^^U'z a:'z:::^\:'z!':::::jr- liey seen, to have a respirat,,,.,. f„„„tio„ .„T """"/"'■"«• ''"■ «it of the ,.e„.„a„ctile el^^ent::,',' ;:;,:e"fors:et l^X 662 INVERTEBRATE MORPHOLOGY. phiura squamata, even serving as brood-pouelies in which the young develop. The ectoderm is indistinguishable over the greater portion of the body in the adults, becoming, as in the Crinoids, con- founded with the mesoderm. Calcareous plates are largclv developed iu this tissue (except in Ophiomyxa and its allies), giving to the disk and arms a brittleness which has suggested the popular name for the group. The extent to which the apical system of plates is distinguishable in the adults varies considerably even in members of the same group, and while in some forms (Fig. 257) all the plates represented iu the {Starfish Zuroader can be distinguished, in others only the radials or the basals or both are visible. At the tip of each arm is a plate comparable to the terminal of the Aste- roidea, and iu addition there are frequently present series of interradials or interbrachials, the most aboral plates of which separate the radials from each other and extend round to the oral surface, abutting on five large plates known as the buccal shields and corresponding to the orals of other forms. On the aboral surface of the disk above the origin of eacli aim there is a pair of plates termed the radial shields, whicli must not, however, be confused with the radial plates extend- ing ak)ng the aboral surfaces of the arms. These latter form a complete series extending from the disk to the terminal plates, and form the aboral wall of the arms, their lateral walls being formed by another series of plates, the adamhulacmls (Fig. 258, Ad), while still another series, the superawhulacrals, form their oral walls. Between each adambulacral plate and its successor is a pore (usually bounded by a number of small plates) through which tlie tube-feet are protruded, the radial water-vascular canals being situated in the interior of the arm. The cavity of the arms is occupied almost entirely by a linear series of calcareous masses termed the vertebral or amhulacral ossicles (Figs. 258 and 2G0, A\ each of which consists of two halves, usu.illv firmly united by suture. The ossicles are united by well-dii- veloped articular surfaces, and have attached to them muscles, whereby a considerable amount of motion is possible for the arms as a whole, the motion being almost entirely in a hori- TYPE ECIITNODEHMA. 11 wliicli the 563 /ontal plane, except in Astrophyton and its allies, in which the arms may be coiled up over the oral surface, in a manner smnlar to what is found in the Crinoids. These ambulacrax ossicles seem to correspond with the similarly-named plates of the Asteroidea. In the neighborhood of the mouth certain modifications m the arrangement of some of these plates occur. The two halves of each iirst ambulacral ossicle (Fig. 258, A ) are widely separated, and come into close relation with the simMarlv- separated ossicles of adjacent radii, forming a buccal shield lie pla e so formed rests upon the aboral surface of the first adambulacrals {Ad,), which unite in pairs in a similar manner Ad 4 Ad, ^y^\ Ad, A = nmb„l„c™l plates. „ = p.|„ , ,„^ Ad = adambulacrals. y _ to,.„g / = iutenadial. ^ ^ „,^, ^^^^^^^^^ wr = radial hydroccel-vessel. forming a triangular plate, termed an oral angle-piece, lying in an in erracius, and partly covered on its oral surface by a buccal shield At the sides of the buccal shield are the so- called lateral buccal shields {AcQ, which are in reality the second adambulacrals of adjacent arms, and cover in the second ambulacrals (A,), which serve as supports for the oral angle-piece. Along the margins of the oral surface of tins are a series of spines, the hnc<^al papilla, while, at the apex of the triangle, are the dental papillce. The vertical edge of the piece is furnished with a number of stout projections the 504 mveitTESBATE iroitmoLoor. palm angulares (Fig. 260, p), whose bases severally fuse to form a supportiug plate, the ton^ anyulm-u (T) Spmes dejeh.ped iu coiiuection with the dermal skeleton, ea™,g out of ccsKleration the oral a.gle-pieees, ma,- b ™' tirely wanting, b,,t lu mauy forms they are bor^; iu ™,ti ' 1 rows „p„„ the adambulacrals, aud are usually movable lu JZZZ7'°"''' ""?" '■"'""""'K ■-'^y bottom ,,e- cliai hooked spmes are situated ou the oral surface of l,e a ms towards their extremities, and seem to serve au ad . ::: aS;:,.,:'^''"""'^ -« ""-■"' -^' '- -^-"^t; The coelo,,, (Fig. 2«0, e) is of comparatively slight e.te.t he cavity of the ,lisk beiug largely occupied byt^e ige ^v e t act, and th.at„f the arms by the ambulacra! ossicles, it disk the cavity IS traversed by numerous bauds which tend from the body-wall to the wall of the digcstiv-e st and from the wall of the esophagus a membr nt exteu ^ outwards and orally to be attached to the peribuccal p ate forming a septum (Fig 2C0, ,), enclosing a cavity surroum ' n.g the esophagus, the peripharyngeal %ac, (^ whiT , completely separated from the rest of the c»lo,f \n X,', thn. and some other forms a second septum occurs pa^ TsZZ''Z"'T''1\T "'*' "'^ I'eripharyngea',:*! is double. T e coelom of the arms consists of two portion, one lying on the aboral and the other on the oral side of,' series of ambulacral ossicles. The aboral cavitv is expand laterally so as to partially surround the ossicles, but t iT a lamella, and is thus separated into a series of chambers Jhicb open into the undivided aboral portion, term d th aboral or dorsal caual. Au axial sinus, standing in close r atlonship to the ovoid gland, exists, but presents som.tfel. tures uot found in the Asteroidea. It consists in A,„p,. Z. ara ed from one another; one of these is the so-called aL • pulla (am) of the stone-canal ; tlie second (.) lies iu clos w u ^hu 'r ?™' •''"""• ^*'"'''' ''"-'"p^'' - '" -i wit 'tlT "l '^ '", """'P'"-''"™'.^ -mall, and is associated With the genital cords (;,,•), and the mass of cells in the ovuid TYPE ECUINODSIiMA. 66ff f^'laud from which these arise. These two last cavities are .u d to be portious of the general coelom which become sepa rated oil dunug development, and are not simple exTensions" ol that portion of the ccelom into which the stoie-canal opens m the embryo, and which persists as the ampulla ^ am pc Fm. 259.-DIAGRAM showing the Relationships of the Stone can*t Axial Sinus, etc.. in Amphiura s,uunu,ta (arte.- jucBrz^k, ' am = ampii la of stone-canal. ^ ^ ri„g-nerv, U = geuital bursa. «_.„*, ,'\ mp = madreporite. ^' = Peripharyngeal space. mu = muscle. * = ^'""*^- sc = stoiie-canal. »»». the ..p„„a „, the s.one-eanafo, «,e oThT^ a'^d 1:1^' Lying on the aboral surface of each radial nerve-cord is a adial sclmocoehc sinus (Fig. 260, br), which communicate' .th an oral s.nns surrounding the mouth. The reTatbns o^ .s system are similar to those of the schizoeceli tem „ . ■ 1 « "° r' "'"' "-'r'""' »■— ictions beLe" it am the ccelomic cavities occur. It contains, however a sjstem of canals, which correspond to the lacu^se occurring 566 INVERTEBRATE MORPJIOLOG Y. f i iu the walls of the oesophagus iu the Crinoids. They huve been termed blood-vessels in the Ophinroideii, the siuuses ivhich surround them beiug termed the perihiemal canals; they follow the course of these latter, a process of the ovoid glaud coming into connection Avith the oral lacunar ring. This glaud (Fig. 259, o) is, as iu other grou;)s, im^tly asso! dated with the lacunar system and partly with the genital ap[)araius. It lies in the wall of the axial sinus and projects into it so as almost to till it. At ou(. extremity, as stated, it comes iuto connectior with the oral lacunar ring, and at one poiut in it!-' wall it contains a nuiss of cells from which the genital cords pass cut to the reproductive organs, accom- panied by strands of the lacunar tissue. The hydrocoel has the usual arrangement, and is confined to the oral surface of the disk and arms. The radial canals (1^'ig. 260, wr) lie on the oral surface of the ambulacral ossi- cles, extending to the terminal plate, and ending, at least iu those forms which have simple arms, in a terminal tentacle. At regular intervals, corresponding iu number to the ambu- lacral ossicles, the radial canals give of! transverse branches, which pass outwards in the substance of the ossicles (Fig. 2')H), and make their exit through the ambulacral pores betweeu successive adambulacral plates to terminate as tube-feet. No ampulhe occur ou these transverse branches, though a circu- lar valve oc(!nrs just where each branch becomes continuous Avith thu tube-foot. The feet are simple conical structures destitute of a terminal sucker, and do not therefore serve for locomotion. Their walls are richly sup])lied with nerves, uud iu some forujs are i)royided witli numerous papdlii' ap- parently sensory in function. Surrounding the mouth are ';en buccal tentacles (Fig. 2()0, hi), which correspond to tiie first two pairs of tube-feet of each radius of the Astor.tids, but arise by fine braaches, which later divide, and are directly connected witli the oral ring-canal (Fig. 258, i). These seem to be undoubtedly sensory and perhaps olfactory iu function. Th(> oral ring-canal usually has attached to it iu each interradius, excejjt that in v.hich the stone-c.tnal lies, a single Poliau vesicle (Fig. '2(50, /*."), though iu Oplnarlh two, three, or even four vesicles may occur iu eac'a iutcira TYPE EClIINODEhMA. 5^37 oe Hchizoco'Iip 'Vi,^ ".ai r,n« o the nervous system is not enclosed n„„ Ij .1... c..n.,i, but icmuins m connection with the ectodeL .t the lower e.trenuty of the ccso„ha«i,s, being .Lued ll.,: 1 1« 1 1 568 INVEliTEBRATE MORPUOLOGY. aborally by the development of the or«al angle-piecea. The radial nerves are, however, contained in the wall of the sinus, coming to the surface of the body at the tips of the arms, where they terminate by fusing with the general ectoderm. The muscular nervous system is, as in the Asteroidea, closely associated with the oral ring and radial nerves, lying on tlioir aboral surface and separated from them only by a thin layer of connective tissue. The aboral system consists of a riii.<' situated beneath the aboral surface of the body, from which branches pass oft' towards the reproductive organs. ludee'l the entire system is intimately associated with the genital 260.— Section tiiuough an Oimuuhan showing Sthuctuhe (after LUDWIG), A = auibuliicial ossicles. 0 = moutli. br = scliizoca'lic sinus. p =. pulu angularis. bt - buccal teutacles. p« = periplmryngenl space. G = cuiloui. PV= Polian vesicle. Jf = nuisile. s = peripharyngeal septum nr = radial nerve. T = torus angularis. wr = hydroco'l- vessel. cords, and its course can be understood from a descii))tii)ii of these structures. No special sense-organs other than the terminal and buccal tentacles and the tube-feet, already de- scribed, occur in the Ophiuroidea. As already stated, the genital cord arises from a groni> of cells in the wal' of the o\oid gland (Fig. 259, gr) and pusses in an interradius towards the aboral surface of the bodv, carrying with it a jmrtion of the axial siuus. Arrived at tliis point the sinus and cord form riugs, the aboral nerve-ring lying in the wall of the sinus, while the genital cord lies in its interior, ftttnplifid in ifa wnll 1)v n lo»p«11.. ,^t -»-^t.. *i,-„ +i „- From the genital-cord ring ten short branches are given oft" {UCTUHE (after TYPE ECUmODERMA. ggg P-longatif.. of t Lul" St' 1""'; ''^ "-'-" iu the reproductive poucbe" Ci »' *"""'' ""'' """"« lobes of the reDrodrtf *^ *''° spawuiug-time the rushing befr:c'rL:T:L,7;h:'e^ ''''°, '"^ •""■»•»• .eproductive elements wheurtlb !*"'."■""'' "'" cavities of the bursm wl,o„„ *i 'lirongh mto the exterior, or else as In' J ^^ »ake their way to the From what has been said it may be seen tl.,t n Astro^Kyton and its a.iies differ ilr^l^Z^tTZ FiQ. 261.— Pluteub Larva of if.T^ , ■ « = (rs„plmgus ^'•'"««r«M«o« p«;.^„ (after Fewmb) «= rudiment of adult. '« = momh. , * ~ t^alciireous skeleton. ™!;:b,e?/1;:i*'^,:;:™^ »™s s„.neti„.es branei,ed and Peonliarities. CV,„se,,ut„tirtt b l',;' ■'" "'■?""" ""'«'' regarded as consisting of .1 . ,"l''""''""i«" "my Iw ijK ^i»'^.My.o„, ^^nLr ::„':•«'«: T^r '"''""- '*"ne, ch of w h,oh « represented by one of the pair. ....ii^ir^LZ"''^::, '"'■•"V'''' "^ '"^ «■="""• '"-'- « l»"tiu „ther.s the limits of fL fnl , "^'^^^^^ '-^^f^'- -^^^ H iinutH ot the tubercle may exteucl so an to 672 INVERTEBRATE MORPIIOLOOY. include all the plates of tlie apical system, and at the same time the aual opening may leave its position near the centre of the apical system and become situated in the interradius A /i, either at the margin of the flattened disklike test, or even on its oral surface. A marked bilaterality of form is thus de- veloped, which may become still more pronounced by a mi- gration of the mouth away from the centre of t^e oral surface along the line of the radius D, which at the same time be- comes more or less altered in size and form, and consequently dissimilar to the other radii (Fig. 263). In these cases it is possible to recognize in addi- tion to oral and aboral surfaces anterior and posterior poles and a right and left side, the median line of the body pass- % ing in front through the radius '* D and posteriorly through the interradius AB. Three of the radii, C, D, and E, thus lie in the anterior half of the body, and for descriptive })urposes these have been termed the trivium, while the two posterior ones, A and JJ, constitute the hivium. The mouth, which is usual- ly situated in the centre of tlie aboral surface, is surrounded by an area, the peristome, which has imbedded in it only a few scattered calcareous plates and consequently possesses a somewhat leathery consisteucj. An oral system of plates cannot be distinguished in adult Echinoids. Fig. 363— a PETALosTrcHous Eciir- NOID, Britisopm lyrifera, FiiOM THE AlJOHAL SUHPACK WITH THE Spines hemoved (after a. agassiz). D = modified ainbulucnini. / = fusciole. The marked bilateral symmetry referred to above aa occurring in cer- tain Ecliiuoids ia undouhtciHy a secondary condition, those forms in wiiii'h tlie Miouth is central and tlio anus approximately so, and wiiose bilati'i'.iliiy is indicated only by tiie madreporiforin tubercle, being, there is cvi ry reason to believe, the most primitive. The l)ilaterality cannot be regarded as a reversion to the more primitive symmetry of the hvrva, since in the fc> TYPE EVIIINODERMA. 573 uj vv nch .t is „,ost pronounced are the most Inghly SIC tiatek t^^^^^^^ Projectiuf; iuwarck from the i.mer surface of tbe te»t i,,' H.e neighborhood of the peristome Me frenuentlv to L f , ^b.', au), which may either be coiifiue.l to the inter-imb,,! „ i' Pla es or occur also ou the ambuh.crals, unit u "il it t l! h lougli w „eh ,,,„ radial hydrocel-caiials ami nerve- ords V^. In tlie flattened dislclihe forms, sncli as Echi,uZZus the e ,,1 hxrs are much more numerous, extending fr m the M ace of the test are nnmerons spines, each of which is l".ll"wed out at Its base, the hollow fitting over the coi ve.i ' "1 a tubercle npou the test. This b-ill •„, 1 „ i , f """"'y allows of a fre'e »n,veinent of tl^i^'irtv dif: ■''"'' ...nement which is effected by mns'ces «L ,g'";i: N base Tiir "1; "'"'"'' '""' '"'■""■"« " «'-"' a i.,™^ s base. The spines thus serve as eflicient organs of locorao t . n, usurping this tuuction entirely iu some forms w Tn others they are ahled by tli,i tube-feet Thev 1 ■ '...■",„ serve as defensive ■:truct„rcs, allt ^^,^1^:^: a e l,mg and slender and readily penetrate th, U "f less p.-otected animals, or in .«/,»„„.„„.„, ;„ ,,,;„„ \ ^ j-^ 674 IN VEBTEBIi A TE MORPHOLOQ Y. spines are soinewliai; eulargeil towards the tip, the enlarge- ineut contaiiiug a poison-gland whose secretion is injected into the wound produced by the spine. Pedicellarise, which have already been noted as occurring in the Asteroidea and the Euryalid Ophiuroidea, are richly developed in the Echi- noids, more especially in the neighborhood of the mouth aiul anus. They assume varying forms, in the typical one (Fi p-C Owing to the presence of the firm test the muscular system \Ul Hi' Fig. 265.— Diagram showing the Al = Aristotle's lantern. g«"l ".'S the stone-ctnal W passes aborally to open into the axial sinus close to the ^adreporiform tubercle, and in addition in the lad al Ectf noids the ring has attaehe,l to it in each interradius a pongy rncture which is usually termed a Polian vesicle Zf ongh t ^se structures in other groups are saclike The abe-feet (y) which perforate the ambulacral plates are in tl» uajori(,y of forms, and especially in the radial ones v ry eUensible and provided at the tip with a sucking-disk, and o assist he spines in locomotion. Two pores as a me exist or each oot ; through one of these the branch issuing ZL 0..ck tiom the foot mto the interior of the body to terminate m a saclike ampulla. The feet, however, near the a K,r 1 .urlaoe are frequently branched and lack a sucker, .^eivbg a c piratory function rather than a locomotor, and in the only on the aboral surface of the test, nearlv all the feet may assume a tentacle like or pinnate form and become respirator/ The digestive tract in all those forms in which the mouih -cupies the centre of the oral surface is provided with a 578 INVERTEBRATE MORPHOLOGY. li '4 pharynx surrouucled by a complicated calcareous masticatory apparatus usually termed Aristotle's lantern (Fig. 265, Al, aiul Fig. 266). Wlieii most highly developed it has the form of a pentagoual pyramid, whose apex is directed towards the mouth aud consists of five similar portions united together. Each portion contains an elongated ribbonlike tooth (Fi(^. 266, t) lying in an interradius and projecting slightly beyond the lips of the nouth, though for the greater portion of its length imbedded in ;i calcareous socket or alveolus (a) composed of a right and a left h;i]f united above by epiphyses (e). Between each pair of alveoli, at their basal ends is another calcareous piece termed a radius, and below each of these, i.e. on its oral surfaces, lies another piece, the radula (r). Muscles pass to this complicated apparatus from the auriculse and from one piece to the other, producing approximation and divarication of the projecting tips of ihe teeth. The presence of this apparatus brings it about that the circumoral hydrocoel and lacunar rings are forced back some distance from the moutli, surrounding the oesophagus just where it leaves the lauteru. It seems well accordingly to speak of these rings as beiug pericesophageal rather than circumoral. On leaving the lantern the dieostive tract, starting in the interradius 1)E, passes around th'' coelomic .'avity in the direc- tion of the hands of a AvaLcL, until it reaches the interradius CD, when it bends abruptly on itself and, on another pluue, nearer the aboral surface, retraces its course almost to its point of starting, whence it passes to the anus. The portion of the intestine immediately succeeding the pharynx is termed the oesophagus aud is succeeded by a slightly wider intestine, the junction of the two parts being in some forms furtlier indicated by the occurrence at that point of a large caecum. As a rule, however, appendages to the digestive tract are rare, Fia. 268. — Aristotle's Lan TERN FKOM Avbacia. a = nlveolus. e = epiphysis. r = nulula. t = tooth. ii .if TYPE BCamODEKMA. 67» th. oiily one occurriuB with any ,„arked ,lw-,.„e „f coiiHtam.v l.en,K the mphon (Fig. 265, «•), a tube which aiiseVf™' !l ! -sophas„s an»«u .uto .t at the extremity of ti,e oral coil. Th fuL ion J t t if """"rro-". to be respi..atory, but it is to b ^ that It 13 wanting m all the members of one of the or,le,-«^H ay^astroir '; : ""■""■^^^■»'«'» '--^ 'l'" ---l arrangen,ent consisting of a perioesophagea ring (Fie w.r, ...i .,f , „ radial ooras M. As m the^phiui-ijl^l^.t; t e '2™ ith , H r ' '"™ "'^"-'''^«» *■'■"» «« ectoderm and s uuk «lthi. the body-eavity, and accordingly there is to l,e fn ,^ 1 an epineural sinus lying below the i"ve-c X. Be ol he nerve-ring, however, no sinus is to be found ■ nd ./ cttr":^" r T" ;r i"' '"^ ^^^^^^ :;:':: liie extiemity of eacli radial cord fuses with the ectodenu in SI t:t^' Tn ^" '^^ "^"^^^ ^^^*^' -dt t:;," uted to the walls of the terminal tentacle. A muscular uerrous system is present, consisting of five mM.t« 1 the aboral surface of the radial njJJll t ^1 rtW s\ th^^r'^^-ti ""''^r:^ ''-''' - ciiLct^rec^i^z utli each othei , they send fibres to the muscles of the mas w^Wh^ pass to the walls of the ducts It the T^tduX: Sense organs of various kinds have already been referred o, such as the terminal tentacles of the hydroacl canals he fascioles, and the splneridia. In addition to thei „t me spots occurring on the ocular plates have be n r^VaS as" eyes, and somewhat complicated structures of a br ght blue color which occur abundantly over the surface of t l.l a .P~f .>,We„. have also been regarded l^eti;" --e-ul/"-! l'"' '7T- "" '^"•■°^'™'"'' -y^'em consists of the ge-ital «,rds and the reproductive organs. The former hnve their origin from a single cord, which is a hollow t„be ltd mo INVEHTEBRATE MOHPHOLOQT. iuternally by immature germ-cells aud i.s connected at its on,! extremity with the ovoid gland. It passes thence to Uie aboral surface of the body, where it forms a ring (Fig. 26G, r/n from which in each intorradius a branch passe:-; outwards to expand into a highly raconiose sac, the reproductive org.ui {G). In some forms the number r,f the organs may be^v. duced to lour or even to two, though live is to be regarded ;is the ypical number. Each organ oi)ens to the exterior by ;i special duct {Gd), usually opening on a genital plate, but sojuetimes in au int^ -radius outside the genital plates. As already noted, there is considerable variety in the rela- tive positions occui)ied by the mouth and anus, and many diiierences of structure are associated with these variations. It IS possible, in fact, to divide the Echinoidea into tlir, ,. orders, which are marked out by the positions of the opeuin-.s of the digestive tract. " 1. Order Desmosticha. In these forms the mouth occupies the centre of the oval surface, and the anus approximately that of the aboral sur- face, the radial symmetry usual among Echinoderms beini; well marked. The body is usually more or less spherical in form, though occasionally soraew)iat flattened ; all the anihu- hicral ])lates are perforated hn- the emission of tube-feot, and all live ambulacra! areas aie equally deveh)ped (Fig. 202). In the members of this order, conseijuently, the bilateralitv is marked externally only by the position of the madreporifi.nn tubercle. The prinniry ambulacral |)lates frequent] v fuse to form secondary plates each of whicli i.s i)crfoiated by several ])air,s of pores, as nniny as six occurring on some phites i., AV/v^m/y. locenfrnfits. The spines are sometimes exceedingly long.'/is in Diiuhnw, and are usually well developc^d. being hi Arh„'m equal in length to about half the diameter of the bod v. The auricuhn are the only representatives of the calcareous i)lafos or bars which extend from the oral to the aboral surface, and au Aristotle's hmtern is always w(>ll developed, its ahv..|i being much longer than broud. In this order external brauchio) TYPE ECIIimDEUMA. -0% tl.erefore, to clivide the orT ! "'^^'"'- ^* ^« «»«t<>m- ENTO.K.XCHU.., i,,, „'^.:C^,:;^'^ ^- sul,.,.oups: the ATA, iuclnding all other fon s fudT V^'^ "^' J^ctobiunchi. 2. Order Clypeastroidea. •■".'"» ■■« situate,! iu t! . in e' !Z :« .T '"■"' l"""' '"""«' "f tlie flattened test •, ■ i, A' ; ', ' """"" "' "« ■"".■.-(« '"•" siuface, as iu JM,V,, i,, '• **• ''"'>' "'■ "u Us «4.7«a*.tl,eb„.lyis|,„t»ii,,|,t, ('■•'tteued, but iu ti.e t,v„ otl,er Jjeuem nh-eadv n,eu,i„„ecl the fl.-.tteu,ug is eaiTied to such „„ "■^'7" tl,,.t the test has „ ,uo.e " ess .hsklike shape, wl.euce "";'""" ^'"'''-'"""•"•s a,,,,lie,l to '•ortniu foruis. 1.1 aocordauce witi, '.he shift. -^.- 'V-Pf^Si^ 7 "f '1- «..»» fa.u, the centre ^\*^^^*^ l,,,'" "l^;""' ».v»te". cert,.iu'":-"--'*'".vp..:.v»r „,:,„,. 'l''-"Ke.s take plaoe i„ it, t,.e .Z f"'"""'""""". ,».,„", ■;p ma,ke,I heiu« au e.teusio, «;:.,. r.,,t,;r',;;„ «---^ i-- -.oiu« We?": sr ortr;' '"■ "'""■^- "- --■" :""'-l-eral plates, au., iu si™,'"'" "" ""f ,"' "- i"te,- """■■•■'"lius .//,au,i the eonesn ,< , I'"™,""'"' l««t«i.,r » ■'"«"« (/:W»-„,„w/,„,-,„) The ' .'■"'"■'"'"'■'"•" "W". is "'■" "» " .-"h, eouHue,! to the .il'" '."'"'"''"'•'■"' I'l'""" »'"f-e,thep,a,e,ueaMho 1 '■''''■''"" "' ""' »I'"™I "'" "■•"> ™rfa..e heiu, tp.^^^ L"' "," "'»' ",■"' "-" " ■ «i" of course v,„.y uar,;>v a , ;.„, l";" ••!':"'"««' i''"l-H f(''H(Iiially onj.t! «'viug thus the appear o'" fi'^ tiu3 »UNM •aiice npical end <,f th ;7PH«MtowardHtheod^eof th. 'I live liower-petals, aud h e oerioH, -mhI !i(^ tesf, oiice 682 INVERTEBRATE MORPHOLOGY. the ambulacra or areas occupied by perforated plates are termed petaloid. lu Echinanichnius, for instance, the })lutes after having reached their greatest widtli retain it to tlieir abrupt termination (Fig. 267), the petals being then termed open, but in other forms, e.g. Jlellita, they contract again l)eripherall3', in which case the ambulacra are said to bu closed. The pores belonging to each pair are generally united by a groove, and are termed yoked pores, and iu Mellita, for example, in addition to the pair of yoked pores on each ])lato there is a third one situated near the middle line of the am- bulacrum. The tube-feet which project from the yoked jujres are frequently pinnate in form, while those emitted through the single pores are simple and tentaclelike. The spines are generall}' very small, though those of the oral surface serve for locomotion. In Mellita, towards the periphery of the test, the imper- forate ambulacral plates of the radii A, B, 6', and E ([.o not meet, leaving elongated holes passing through the test, and the same thing also occurs with the plates in the interradius AU, so that altogether live such holes exist. In other forms, instead of holes, notches occur at the margin of the test, ami other interambulacra than that in which the hole occurs in MelUUt nniy be atlected. Calcareous columns extend from the oral to the aboral surfaces of the test, being especially almn- dant towards the peri|)hery, and calcareous plates uniting the two surfaces occur on either side of each and)ulacrum. Au Aristotle's l.mtein is present, but the alveoli are usually broader than long. 3. Order Pettlosticha. In this order, as its name indicates, the ambulacra are usually petaloid, ami the bilaterality indicated in the Cl\ |i( as- troids is more pronounced, since neither the mouth nor tin' anus rical surface. The anus lies in the posterior inttuiailins yl />, while the mouth has moved forwards to a greater oilissl extent along the radius D. The test is oval or, freijui iillv, TTPn BcmmmHMA. 533 somewhat heai-e-shaped, owintr tn fl,„ . • ■"o>-e or Jess depressed ol to ,,.™ ' ""^^' ^ ^^^ The madreporiform tnl», ? ? " *'™°™ (^W 263). -"'.e of the ap ea Vstrttr r'' "^""^ '"'-«'' «^« ""■« 'iividf,.K the'apiea?:^;;,, "'"*'' 'T"°' i-i<^^r.ai.., « obliterated in ail members of .,^''%P"^'''™>- gemtal pore live glaad ™.respo„di2 to .t dt/ ' """' **"> '■«P'<'^uc- >«pi-oductive orRans aid ,1 <'"'''PI""'r3. so that but four -«e forms, ho^te , « e redToLT ,T'"' <*'"'"''-)• I" »'"'. pores is carried tilf furth 7v t '!'^';''P^°d»"«ve organs |i«ht anterior iuterradius^XftlT' TT"™"^ '" "'« ...terradins Z^^' may also disan^r /"'^ " '"" ■""«"<»■ ■"« pan of the n>ad^eporif' .^rbtrersoTh'r'/'''^ ''^^°"'- ■■.■productive organs and nlacru,u All There is L^ a w''^"^" Posterior inter- l^etalosticha. '" "° Aristotle's lantern in the l>eoelotment of the Echimidea -Th„ » , ' "•""••OS very .si,„ii,„. t„ that of ho rw '.'?^"'"n'^^"t is i„ if., general Ml. prroneoU j;:.:;:^^^^ eases be .is:;: „ J:! "" I <>.st.M-i,„. ,,orti„„ „f eho body whip . . ^"^'"'''''".'/'^v) processes „no„ «"' ''"•" occur „,,ou th, ,i,,,., ,,; '/ ■ ^'^ t«-ooarliko lolu-.s fri,,,^., - "own as Ciliated epauiett:.";: ^ T'^ ''"^'r" "^ ^"« '>"" - PoM.Tior portion of fl,,, body of the ..r"^ •'""■"'"" '^^'^'^^ i > the '" ;-olH.ion.s,nps of the vario, ,' , ^v' '7'« '<''-'-"'".V 'Vsorbed. I *'i'' ':eod not b(, ji^Hiii -li • ■ *»"*"!'« "Jive already l.e".. -' - ■ , T i 684 INVERT EBlt A TE MOliPUOLOG Y. primitive, while the Clypeastroidea and Petalosticha are secondarilydo- rivi'd forms. The bihitcrality of these latter forms is not to be regarded, therefore, as having any phylogenetic significance. Class V. Holothuroidea. The Holothurians (Fig. 268) are characterized so far as tlieir form is concerned by being elongated in the oral-aboral axis, having thus a somewhat wormlike form, the mouth be- ing at or near one extremity and the anus at the otlier, exee])t in the genus lihopalodina, in which the two oj)enings are ap- proximated. As a rule the l)ody is cylindrical, but in some forms, such as Psohm and the /iUmpodd, there is a well-marked flattened ventral surface. Three of the radial hydroco'l- canals lie upon this ventral surface, the other two being dorsal, and it is usual to a])ply the term trivium to the ventral radii and bivium to the dorsal. It must l)e recognized, however, that this use of the terms does not imply a homology with the radii similarly named in the Ecliin- oidea, since in the latter the radii (', 1), and A' constitute the trivium, whereas iu the Holothurians it is the radii J, U, and K The mouth is surrounded by a circle of tentacles varying in number from ten to thirty. There are at first five primary (Zoflrt. A IloLoTHiiuiAN. t''"t''^^*l^'^' iJiterradial in position, wliioh are formed in connection with five ca'cal outgrowths of the h3'droco>l-ring, and the tentacles subso- t^uently formed receive branches from the five primary ca'ca. In shape the tentacles vary considerably, being cylindiii'iil iu some forms, arl)orescent or pinnate in others (Fig. 'iCKSi, and in others peltate, and iu some forms they are retractile. The exterior of the bod}- is usuall}' covered by an epithe- lium over which a cuticle may be developed, but iu some forms the ectodermal cells siid; into and become fused with the subjacent connective tissue. The calcareous skeleton if* Fro. 2QS.—Pentacta fron 585 TYPE ECIIINODEllMA 1 , » , , ■ woo skeleton is represented by llJZZ' TTZ' "" "'"— - I'orforated, tuobbed, so„Jti " , If t "/ ™""- ^'-1-S ".■ associated with au auchorii e ^''*"^. "' '" <^'''"-odo,a, a.e uot suffieieutly r,.„„ro„ ' 'r^ spicules iu Synaptu, aud "»"t, which i« 6V„„w" Z *^ ^ ■ "«"'"^ '° "«' i-tef-u- Tl-e isBoi-KlicatlrofarCr. ': '"'" '""• *--'--*. WfenV. a circle of fi™ phte ^' "^ "^ "'*'*' """-fi'' '" 0-1 syste,,, is represented t T"""^' "'" """• >"" "'« Piates which may be cWd 1 '',""■'""' "'' ^'~'"« ^y five I" other parts of the hlZl'tt'T'' """ '-'-'-• ■"•■'tter is also frequently iL u" , ""*'.«r<^'" calcareous 'ive tissue of the wall of ft ^1"'*?' •''■ '" "'" ""'"'-<'- «-lial ossicles, gro ,ved o .W TT'"'' '^'^''"""y •>< «ve :-n.ydroca.l.catal»;rud yCi!: r ,"" '""'"' """"^ "'« with them, thouJh iu tlK„!7 """"•"''f ' "■''««1«» alternat. '^..t-.es is greater'th,: Ttl ™rb:r''f h'" """"•- <" «s»icles may be increased X, ^ ^ "'" ""^'ra-Iial "-seutery, au,l i„ son efn,,^ Its'" "'' t" '""■"' "' «'« •bvelops in the wall TZ T ' '^ T "'"'''' "''^''"'^ I'Wseut, though the plates of ^7 •""'■. ''"""' "" '•■"■e'y otlier forms bear tlim td ,f "'r"'""' ^"'"'''"' ™'> " '«* The ca,l„n, is trav ; ed b^ eT ,"'" "'" ''""'"'^ "'-«"«■ 'lisostive tract to th ".> 'ti M •""r"'"'*- uniting tl!; -ailed dorsal niesen:^-'^^ ";,;";: 7.»'-'. ^-ing the ■"' ■no.- portion of the ini radius (V^tI'"'' 'T '" "'« «'l<>m which surrounds the o-o, , , • '""""" "*' "'^ '«t, as in the Echinoid 1 „,;''«'" ''< »"';"■"'-• '■-" the '"•' -.ilariy i" son,e forms Zt 'J^l^'r^"^-'^' »P"ce, hpaco surrounds the lern.i, al , , ''"'""""■'")" Perianal '■' ^map>a and its alii " , ' IZ T f ,",'" '"«''-''"™ "«'. 586 INVERTEBRATE MORPHOLOGY. \ of the Crinoids iu maintaiuiug a circulation of the coelomic fluid. So far as is known, the portion of the coelom which in the embryo opens to the exterior by the water-pore and with which the stone-canal communicates in the Asteroids and Echiuoids does not persist iu the adult Holothurian, and consequeutly there is no axial sinus, and it is doubtful if a structure com- parable to the ovoid gland of other forms exists. Schizoca-lic sinuses corresponding to the perihaeraal canals of the Echiuoids occur in their usual position betweeD the nervous system find the hydrocffil-canals, and consist of a ring accompanying the nerve-ring and five radial canals which abut against the ring at their oral ends but seem to be completely separated from it by septa. A lacunar system is well developed, consisting of a plexus in the walls of the intestine, the various branches uniting to form a dorsal and a ventral intestinal vessel, which, passing forwards, unite with a lacunar ring surrounding the oesophagus at about the level of the hydrocoel-riug. From this ring five radial lacunte extend backwards, lying in the connective tissue between the radial perihsemal sinus and tlie hydrocoel-canala, and giving branches to the tentacles and the tube-feet. A lacuna also extends from the pericesophu^^Md lacunar ring to the reproductive organs arising from a thick- ened portion of the ring, and this thickening has been re- garded as the rudiment of the ovoid gland. The hydroccel has the usual arrangement, consisting of a ring (Fig. 261), o) surrounding the oesophagus behind tho ring of peripharyngeal ossicles, and having arising from it a stone- canal which in tho majority of forms hangs freely in the coelomic cavity, where it terminates iu a madreporiforni ]tlate. Iu the embryo it as usual opens upon the surface of the body, and this condition is retained in many Ehisipoda, in wliich tho canal opens upon the dorsal surface of the body, probuhly indirectly through the intervention of an ampulla, as in other forms. In the majority of forms, however, the connection with the exterior becomes lost, the ampulhi which is present in the embryo disappearing, and occasionally a number of secoii '*» f^om the tnviumoftheEladpoda; wLu Z." ! T''""' ""■" "' "'« "l™g the lines of tlTe ra. iaTotn ? ''"^ ""^ ''« "^''ged 208) or naay be .eat er: 1 ^Xit ""T*"' ''''^'^' ^'«- body (Tk,one). In form tW a —'"■ "'^r'''"'« "' "'« e.tljer simple fingerlike processes o7 T'"!^''"''''^' "'^'"g sucker. Frequeutly the tnbe Iff '^ ''I'P'"^ ^""' a ".e Elasipodl the/talTe tfe form "oTsr' '"^'^f'- """ '" corneal processes arranged in pairs ^^ «-ell-developod Owing to the absence of a firm lest in n tr , tl.ere is a much more extensive del ! ' ^"'"""•rians 8.vstom than in other Echlnodermr tI'™"" "' "'" '""^^'a^ body-wall is formed by a layer rfci J l" """"■ ""*■'"' »' "'« 0" each side of each radi,tf Wdro " "' "'•"•'"'"'""•«^' ""d muscle-bnndlo (Fig. 269, „) rom L.t^ " '' " '""si'-'taal '»'"<"«« pass to the p;r1p „;"!elf "'-T' '""'"" "l'""'*' ■■ee^ctors of the tentacles aSh-dir"'^^ """ ^""^ ^ As staled the mouth is usualN f '""'y at the centre of a dk™ .•,;,";", ™'™'"'- '^'"' »' "- '" tl.e Elasipoda it has a so ,"« 1 f " '""'"'o^' '™' •gestive tract is a simple tube '"hi , ' ""■'*"''""- T''« '"•tlj straight course fr ,„, mZhl "'T"'"'"^ '"'« a per- " '»bent twice „p„„ it. 1 ",:;";': ''""""■■« fequeitly -"Ji"K limb (Pig. 2fi9,/>,'.„ a t tr^^^ "" «"'-ior de. 'lexeendmg one (/,). The term!,, J n*' ^''}' ,""'' " Posterior , --"■■« li."b is dilated, t i ; / *^^ ? f" P"^'"''"' •'«- .l»H,cle-bands(^)rad,^aet,.tl,.!r.,';^'?"''l')f™'« whose wall 'Hiythmically contractile, and lar^ltrtj^^^^^^^^ 588 INVERTEBRATE MORPHOLOGT. the SynaptidsB two much-branched structures termed the re- spiratory trees {k). As their name indicates, these structures are supposed to have a respiratory function, but it is possi- ble that they may also aid in excretion, the waste products of metabolism collecting in the cells lining the interior of the Fig. 269.— Diagram representing the Internal A«Ai'oAi; THUKIAN (after Ludwiq from Leunis). ^'^0- i' = cloucal opening. k — respiratory trees. I = Cuvierian organ. m = dorsal mesentery. n = duct of reproductive orgau. 0 — reproductive organ. p — lougitudinal muscles. a = tentacles. b = calcareous pharyngeal ring. c = hydrocoelring. d = stoiie-cauals. e = Polian vesicle. /, g, h — intestine. i = cloaca. 9 = radiating muscles of the cloaca. tubular branches and being carried to the exterior In ii- desquamation of the cells. In addition in a small number of forms {Holothuria) there occur upon one side of the cloucii a large number of slender tubes (/), which, at the will of the animal, can be evaginated so as to project through the aiial opoiitjg. These constitutG the orgau of Cuvier, the fuiictiou of which is not as yet satisfactorily explained. TTPB BOHlSODEttUA. 589 The epidermal nervoas system consists of a perioesoni,, geal nug aud five radial nerves as iu other forms !udTu addT tan hve ,nterradial nerves pass from the ring to be tent^l ' extremities passing tUrongl, the tissues of ILIo,^:,-^,":^;;;:: ».th the ectoderm. In accordance with this arran Jmen tan epiueural smus accompanies each radial nerve thou^r„\! \ ^"m the pericesophageal ring. The mnsculaV ne^U .^ .ervous system has bee'n dLoverot in'Ihe H^lt ir^-' uthal lieive close to its origin from the rinrr T?, i . ..nteins . number of otolith! and I t:t^^^X ^^^.uch greater, amounting to ajuf t^CsuL" r;:. ^Tf The 8.ynaptids and Molpadids are hermaphrodite bnt nil other Holothuriaus are bisexn-.l Tl.. «*F"iou te, but all (Firr 9fiq ^\^ . 7 '^^^^*^^"'^^- Awe reproductive ortraus 590 INVERTEBRATE MORPHOLOGY. - r. : (lifFerences are associated with the absence or reduction of the ovoid ghmd and of an aboral nervous system. The number of the organs is very much reduced, and no genital cords have as yet been discovered. It is interest- ing to note, however, tlie existence of a genital lacuna mentioned above, in association with which the reproductive organs seem to develop, and it may be, as stated, that the lacunar thickening from which it arises 'is to be regarded as representing the ovoid gland, which, as has been seen, is intimately connected with the lacunar system in other forms. It seems probable that in harmony with the shortening of the stone-canal and its separation from the body-wall, and with the abortion of the axial entero- ccBl, there has been a shortening of the genital cords so that the aboral ring no longer exists, and the reproductive organs, reduced in number, develop directly upon the wall of the genital lacuna. It must be remarked that in some forms there is no distinct genital lacuna, but the reproductive organs are associated with the intestinal lacunae, a condition which may be secondary. Development of the Holothuroidea.— The typical larva of the Holotlm- rians is known as the Auricularia (Fig. 270), and is distinguished from that of the Asteroids, Ophiuroids, and Echinoids by being destitute of armliko processes. In later stages the ciliated bands fuse in such a manner as to form St a series of circular bands surrounding the barrel-shaped larva and recalling tlie <'P larva of the Crinoids. By the gradual elongation of this larva and the disap- pearance of the ciliated bands the adult form is acquired, there being no absorp- tion of any extensive portion of the larval body as in the Brachiolaria and Plntous. The Phylogeny of me Echinoderma. —The Echinoderms form a well-defined group siiowing little indication of aflini- PiG. 270— Auricularia Larva OP ties with other forms, and the establish- Synapta (after Semon). dp = ciorsal pore. -fir = hydroccel. pt = primary tentacles. at = secondary tentacles. ment of a plausible phylogeny is an unusually difficult task. One tliiii;,^ however, seems certain from their de- velopmental history, and that is that tliey have been derived from primitive bilat- eral forms, and that the radiality charac- teristic of the adults has been secondarily acquired. The larv* are strictly bilateral, there being indications that originally two water-pores, situaled symmetrically upon the dorsal surface, existed. The first question to he decided then is the cause of the radial symmetry seen in the adult. Bilaterality in the animal kingdom is usually associated with an antoro- popterior differentiation, and this with a definite axis of progression, 'i'lnis TYPE ECHINODERMA. 691 become specially provided with sense oZTf' m"^ ^^"^^^'^^"tly it has new conditions whether favorable ouJro''' ''''''^^''" ^^ ^h««« affecting the dorsal and ventral su^facf^^^^^^^^^^^^ '"!"''*'''^ "^« conditions dorso-ventral differentiation exists „.. ?*"''"' ^"^ consequently a affecting the two sides ofXb 'y ^^l;" o ? " T^ '"^' "" ^''"'^^«- the differentiations which occur on each side If h' "''''' ""' c««««q»ently con^:^::;^-: ^~ ^^" ^~tr ^^ -^^^ .-sion or w^atrod^ ^i^rar fh'e ^^"^ """^ ^ ^f pro- radiahty of the Echinoderms may b Ihe re u ^ ^ f^^'''' '''''' ^^^^ ".e larva The majority of recent /ch noderrarp > "^f""'' '' "^^ ^^^^'^''^l f?rr '^•^ '"^"^ ««^i'«' but it wm be fouTd I 'I '"'' '''' ^«™«' t'^e foms are the latest to appear, and tC the Pp,. * geologically the free actenstic of the Paloeozoic ro ks. Ths would T'" "'"' ^^P^^'^"^^ •-'''ar- 0 be regarded as the most perfect representltir'' "^ ''"' '''' ^"^^^^^^ ^^e he ancestral types, an idea Ihich is borne on/h """"""^ '"^^^"^ ^^r^n^ of derm development. Thus the calyx of the p"^ '''. ^'^"'^ '" ^^^^^^o- posterior portion of the oval larv^ fh *''^^"'^o^d is developed in the |hest,a,k; intheBrachiolarTain/SuteLtr^.r"^" '^'"^ ^-P-^ t 1" the posterior portion of the larl 'ti/ '•"''"^' '^'^'^^'^ ^^^Wd ' ser., v, 1887. 0. W. Field. Th, rarva of Asteriaa vulgaris. Quarterly Jourual of Micro- scop. Science, xxxiv, 1892. ornn7noiDB.\. Th. Lyman. OpJiiurida and Astrophytidiv. lllustr. Catalogue Museum Coiup. Zool.. 1865. B. Ludwig. Beitrdge zur Anatomie der Ophiuren Zeitschr. fUr wisscuscb, Zool.. xsxi. 1878. H Ludwig. Das Mundskelett der Asterien und Ophiuren. Zeitschr. fUr wis- senscb. Zool., XXXII, 1879. H. Ludwig. Neue Beitrdge zur Anatomie d$r Ophiuren. Zoiischr fQr wis- senscb. Zool., XXXIV. 1880. TTl>ll KCmmDKHMA /i95 xxm, 1889. ^mmen u,ul Crmoiikn. Jcuaiscbe Zeitsclir., E. W. MaoBride. 7%e Development of tJie (Umtni n and Aboral Sinuses in Amphium 1 ^'"f"'^^7«"*. Ovoid Oland, Axial euce. XXXIV. 1892. ' '• Q'^'"t«''.V Jour.,. Alicrosco,,. Sci- ECHINOIDEA. ;■ "M^t- ,£ rrrMlsi^r,^' * * — - .» ,„. ■ S...f!"t^.'"' "• *"•■'•*•• K»"8>. 8vo„a.. Vetc^k. A.„„ UOLOTnnuoiDKA 0. Hamann. Die Ilolothurien. Jena, 'ibn/' H. Ludwii Die ^.„n,, ' , *-'^'''-"- '^<«j'"«:ie, Lvn. 1894. 596 INVERTEBRATE MORPHOLOGY, CHAPTER XVII. TYPE PliOTOCHORDATA. The type Protochordata coutaiiis a u umber of forms which present certain features of similarity to the Chordata (Verte- brata), oue member of the type, AmpMoxus beiug frequeutly cousidered as belougiug to that group, which is to be regarded as the most highly ditfereutiated of all the types composing the Animal Kingdom. The various groups of the Protochordata differ greatly in general appearance, but certain structural features of great morphological importance are common to all of them. These may be briefly stated as (1) a notochord, consisting of a more or less well-developed rod, arising from the mid-dorsal line of the digestive tract and either extending the entire length of the body, or else limited to its anterior or its posterior^part, or even present only during larval life, as in the majority of the Tunicata ; (2) hramhial slits which place the cavity of the pharynx in communication with the exterior and serve us respiratory organs ; (3) a central nervous system, situated in the mid-dorsal line of the body, and arising in some forms as an ectodermal invagination. Metamerism is but feebly indicated in the majority of cases, some forms possessing only three mesodermal somites, while others, such as some of the Tunicata, show traces of it only in the posterior region of the body, AmpMoxus being the only form in which it is at all well marked. Limbs do not occur in any members of the grouj), nor are there auv special jaws or organs of mastication. All tlie members '" ''^^ ^l'-'"- tion with the eeto^lerm and Z, '^ "f '■'""""" '" """"ec m the collar region '' P""""""' P"""" i» situated '• Order Pterobranchia. (^pMCl'lClttl oocJZ.TT"' ^''^r^'^^ra and "f Norway, while the latte^wL obLl A' "^ ^^'""' °"' ""^ '"""* Expedition in the Straits ofTaget: ' ""^ " ^'"'"'^"«-- " '""*^""^"^'l)--,oniaH„rn.. consisting or a stolonhko systoni „f tubes ru„„-f • »'"ues, etc., and Kiving ff ^0,^ Y? 'T "'" "'"•'»-"> "f »lich c„„t„i„„ .'^. ; t'. . . "'""'"'• 'atorul tubes e„,.l, „,,_ J -l..tmlike material and fonn 'a " hou 'r-7 ,'"" ~"'1"'»«' "f " '"' "'f <">l"i'y. and 598 m VERTEBRA TE MORPHOLOG Y. are traversed, except towards the extremities of the latorul tubes where the iudividuals occur, by a chitinous rod which results from the chitinization of what was ouce the stem of the various polyps. Each of these J- stalked (Fig. 272), the stalk (C) becoming continuous below with the chitinous rod, and Fio. 273.— iNDlvlDtTAl, OF Rhnhdopleura (slightly modlfled after Lankkster). (N.B.— Tlie teiitiicles of oue side of oue arm oiil^'^ are represented.) B = iiiial piipilla. G = tentacle. C = stalk. Oa = arm. Z> = epistomc. / = intestine. E= trunk region. K = sensory papilla. F = collar region. JV = notoeliord. each consists of three well-marked regions. What may ho termed the anterior portion of the body is formed by a hii>,'e disklikc iipistome (/>), beneath which on the ventral surface is TYPE PROTOCUORDATA. ("rr,; ^^^^-r r-' '-^ -"- -- processes (ft,), each carry L,r^,"''^'""' '"» '""S •■'™'ike ( O) arranged pinuately 'T^t "d ' "" °' "I""'"' '-'"«>- (^;), from the posterior and vL .1 T" '" "'" "''"""■"' ^"<' arises, while dorsally Ind alrt i'";""" °' """'' "» ^''"'^ (li). at the extremity of which , '""™' " "°'" P"?'"'' 'I'l.o ,i; 1- . "I'lcu tile anus opens Jl'e digestive tract consists of , <,t. ■ T . "■■"."g the collar, and liavkr,o, r*' "'«'" «sopLagns trav. -"•face a short b i„d , re"s ^l 1 '""'■ " "" "'« 'l''''™' with that of the ceso, 1 Zs T "''.'"'!, "''" '3' communicates cl'ord. The «sopha ~ns fnto f /'^ """'"-taiy ii„to. fromthelowerendof wliiXh.,? r^'^" ""''"«' "'"'"ach, i-K "Pou itself, runs frrwlrd, '" "' ^'} ""'''• "■"'• ''-<'• , The nervous syst m co'dst T: tr ' " ™"' '^»P'"- 'lerm on the dor.al surface of ,t M "'"='^'""■'8 "' «'« ecto. found a small ciliated eWtiLlrT" "'"^T' ''''"'' '" ■•''- -> other special se.ise-oi™':" 'cc u^'T" 'r "^r^'^''' collar a pore occurs which hv » ,"'""='' ^"^o "I the p.- torates the wall of the bodv , [, '"'""^'' """' '"'"d' i" commiinication wi h tie e tSri " "'"'r "' ""' <=""- .•epresentingaiie«retory„r,4n Z' 1 ""^ '"' ''"«"'''='' ■■« slits have yet been observed ^° 'I"«""™-P°'-e or branchial ".e house is gelutin uT'ntut "^""'V" t>eing colonial, but »l'ort stalk do not eni'ain ' ' '":■'■" '^'"''' '"'"'"' ''■""' "'e early separate from Z pa"r"E 1'" "i"' ^"'' ""-■• ''"' xists of three regions-al ^1^ ''"'•''' <^'«- 273) con. a large epist,,,ne^ a idX CO 1 ,7™'"''"\P"'''"" '^''i"'' f^^ -c ; the body-cavity b in," i ; ,!*^'™l'''': " P"^'""™ "■"ceral -«iouB. Two epistle ;o^er:;te ", 7"-'>"-'-« tl.e c»lom to them passii g tl ro ,g 't ^T:?"' ''"'""« '■""' awvons system („). The dnr»! '"""'' I""'' "' tli« i» thickened to f ,rm t e ceZ „" "" "' "" """'"• ••««'"" »i'le of this is a cluster of kLr'T'%"'''''"' "'"' "" <«"^1' l'"'Wike dilatation a, ,1 Larii' , " "'' 'f' '""""« '" - •'"■angediii two rows. At m" ^^."""^""M'"--'" 1'""".Ich At tlie aides the coll -'<- an ,1 pairoi luternl folds whi ar iM iv.i.f.' •luuOU '•"itorior portiou of the li slightly overlap the VLsceral sac and for.n the operJ^Z. 600 IXVERTEBRATE MORPllOLOG Y. A I i w \ I upon the inner surface of wliicli there is on each side a colhir- pore. The mouth (m) opens beneath the epistome into an esoph- agus, which iu the collar region bears a dorsal cliverticuluin, the notochord {x), projecting forwards into the epistome, In this same region !^'ere is on each side a branchial slit {sp), structures v !.;.!■ 're apparently wanting in lihahdopleura. Behind the a „u,gus opens into a saclike stomach from Fio. 373.— DiAGUAMMATic LoNoiTruiNAT- Srctton TiniouGH Cephalodiscus (after Ehi.eks from Kokschelt mikI Heideh). a = anus. n = nervous system. ex = excretory organ. sp = branoliial slit. g = ovnr3\ t = tentacles. m = mouth. X — notochord. which the intestine, bending upon itself, passes forwards to open (a) upon the dorsal surface of the visceral sac, a shuit distance behind the collar. The collar-))ores probably serve as excretory organs, ami it has been stated that the epistome-pores open into well- developed tubes (ex) terminating in the epistome cavity in a dilatation ; they also have been regarded as excretory. The reproductive organs {(/) are paired sacs, which c^peu on the dorsal surface just in front of the anus. No circulatory system ide a collar- 601 TYPE PltOTOCUoUDATA. -"tailed i. the order. '"^^^^^^'-^^ -^ -tLer of the l^ : Cepliolodhi'M ^, ^- ^"'^^" Enteropneusta. xlie o)'der T^nf '" »tn,c-ture over iL Pterob.r i ! " "'«<''''<«' ^'Iva-u-e '>•-« M ami the au„s t«-„,i„af Til "'f ■'"''"« l'"'clical]y '".a.'c,,ra ,■ «1 surface of wl.i,-!, is a „ 1 , ""'■'■""■ '"''^' '>l".u H,e ,]„; «- p.'"bo...i..„,„.„„ ,-: '„ :;X';;-".v t.o) „,,el, ,„a ', »'".'« "l'»" it« ventral .surfa e i^ ""' !""' *''« -'''«io,, ■■-«;<;.., i.s the „,o„tl,. "' '""' "^'^'^ " J'-i-^s the second J^ije second remo,, ic, „ '■""-■(«), it. post;;;:. „,z'br "■'."'""■' ='■"' f-"- «- ■■«o..t distance in the fo T^Zl"''^'' ''-"-«!« i".' ;"" "'« «i-l- "t the «nterio ,;:,:"■''?. ""^''- ""-■ -alls «>;Iy a space ,vhich connn ,„':'"' ,''f .";'■'' I«"«™ "f the I le„oran,Iislnown„s the ',!■ T. I""' "-'.V «ith the «.on of the bo.Ij. is „„.eh hthan ■/;""'"'"'■ "■■ """k : «""••"■ ■■"Kions, and contains H """"' "'» l'"'l'o»<-is '''««st.ve tract and the re t , r *'"'""""' l"""'"" "f the ;:""■ '»' fattened and e::;'::"«r\ ^■""'"'b' it i "Ise, on either side of ,vh ..hll tn , ''" ''"'•''''' ^'-'face a ;: '!;!'v-ve tra:!:!:i;-:;;:r^^^^^ iiicr lienor ■ea.se ones I1I "'' ^'"'" »' "'« bIIt?^' cavity eo.a,,,utCL„ w;;*: tT.'r" " "■"- » I^O'sal to the uot,«h„rd lies »"'""' ™«l'''»Kus. ;' fo>r' "'• '" «o,„e oases many " ' t? ' ," '"" ^'"^ e„„taimus l«««B.vesie]e, as it has bee fe™ J"' "'"'""""• '^'''" l»o- >"'tion of the prob,,scis-c„lor V ;vT'''''''m ''''^''■"''■■--■'t a "o separate cavities, oril ' , ^ ^'" '' "■""''' '!«'" consist of «comes very iar«e aid i' fe'f "'"' '^"- ""e of which «-««. while the other rem-^Z ^ . ' ''"'■""" "^ "'e I^'o- I'e^e structures are a mZ ;<,?'"!/""'"• S»"ouu<{iug ajersof the proboscis-cJ™ ("y"."'' "^ ""^ BUauch.it b'^e m the folds, while tl^Tel Is i' ■'" "^ •''""''■vessels "'"tarn yellow ,„.,„„,^^ , l"\!r""« "'"» frequently Sianules indicate a Klaudul!, f . '^"Pl'osed that these *'<"ently the entire'la'^i'fi™:^ ""' "" '"'''''■ "" " ''"scis-ghmd. '°''''' ''"^ I'eeu teriued the pro- The coilomio cavities of *l, i, "■""''• ••»« umch siu rr al '''"'"' ^^'«- ^^S, 7/ cop) and "«'" and left sides bS^ se L!"ed'fr"''' "f """'^^ °^ -J ventral mesenteries." iC tt ' " 7°'' ""'-• ''J-'Io-sal "■•» end of the trunlc-cceh ,„ „" ' '?"' '^"'■«'"' "' the an. •»'•<' '"to the collar. ]yin,, '" '"V "'""«""°™ extend for- ---1 a„,I ,„„„i„^, t « petil™,, d "■ ' "' ""^ '•™-'" '''ool. ;;'■■<;'' are longitudinal ,,„ Ifih """t^" '" ,"■« '"'-ior of "";■'' I'^'l^Kation of the tr„uk„lll , T •"''"■ ^'""'-- '''■- --"-'-..ophagus, fort-^r ;5;:-^^^^^^^^ '""ar and trunk of a dorsa ''"'"'' <=°"«i»tHig in the !■"■ '>i«tin.t n,u.eui:r ™ , ' H ., T'!"'"' . '™Si'-Ii"al ves P'<"" the dorsal vessel bm ch'e ^"'^' '" "'" ""■"enteries ;'•»« of the branchial s i s T, d 1!,""' ',"'' '™«"'^'"'- I'«- "vard into the proho,,,:' "t" ^r ","" '" ™""''-J '----^ - t^e "otochor^rir ^^-n,-,J- 604 IN VERTEBRA TE MORPHOLOG Y. t The bloocl-spiices iu the proLoscis-ghmd commuuicate with the heart, aud the dorsal aud ventral vessels of the collar uinl truuk are imited by a double set of tine lacunar capillaries, one set being situated in the body-wall, and the other in the cop — colliii-cceloin. ' (j — tongue- bill- of skeleton. ]i = lieurt. Jal = bnuicliial valve. kh - bnuichii.l portion of oesophagus. ks — bnaichial septum. kp = branchial porv. Im — loiigiiudiuul umst'les. nc = uotoeliord. nd = dorsal nerve. no = ventral nerve. 0 = a-sojiiiagus, pc = pioboscis-coeloui. pfl — proboscis-gland. ps = proboscis-vesicle. wall of the intestine. The blood is a colorless coagulable fluid, apparently destitute of corpuscles. In the posterior portioJi of the proboscis is found a plate of chitiuliko material produced into two horns posteriorly, and frequently somewhat hollowed out in front. It is evi- dently supportive in luiietioii, and forniK the proboscis-skele- ton. In connection with tlie branchial slits a similar chitiiioiis skeleton is formed (Fig. 276) consisting of a series of tiiii verse bars placed cn'er each septum between adjacent slits From the middle of each bar a rod (really double) passi down each septum (.sA), ixJuI from the extremities a bar (// passes into each of the adjacent tonguelike calves, each valv. thus possessing a ))ar from the arch lying in front of it ;;ii' another from that lying behind it. The septal bars ami ti is-l TYPE PliOTOClIORDATA. tlie bruiichin! slits. ^^^"'^ ^^'^"'^^ tl^^ valves of ;'"'-'":.-ate«t,„e (Fi,, 275, /• «) auJ eventually opening t„ ■■'»rfy i" soa,e species the wall A^ 'i""';.^'^^''- ^^f'"''' I'"«'«- ';;*o »ac„„lati„a, .Lie,, have , e ' f',"" '^ ''"""'""' "»' ""»'"« m the ,„id.lle of the liv' '''?•'•""'''»""'' «•«".•», '"ee of the i„te.,ti„e i„ the for. f /",'"," *''""' ""^ ''"'■'"'1 »"■■ .'■""■'3'. "Pe,« a«ai„ i„t , t ie « , ," '"^' "'-". '"-- p.«te. -i.testi„e of eertai,, A„„eli, f v ' '""''"" "'" "ccessory '"■' i» P«,„har ei„, „i .r''''-'?;^'"- ""'' Echiuo.Ier„,s «'«. «ter.„r have hee„ fo,„„ ,,•""'"""'"'" ""1' «"™.» at present „.li„v„ "' ""'■*'"■•«• "'«■• «i«uifl. '"' '■l<'"8atVd''errriv!Crrt!,e''n,n' i' '"?"" '" "'« f<""' "f l-.^ion. with the eel..,,;,,: o , ' h iT"' "™ "' "'^ »"-"' ' " "* '" oouuectiou at ■'' ^ syiiiipiicula. tb = toiio-uf Ijjir. t\ 606 INVERTEBRATE MORPHOLOGY. either end, though free throughout the greater portion of its length. It contains in young forms a central lumen, which may be represented in adults by a series of separated cavities and which results from its formation as an invagination of the ectoderm. From this dorsal cord a plexus of nerve-fibres extends all over the surface of the body, lying in the lower layers of the ectoderm and being at certain regions specially developed so as to form nervelike thickenings. One of these surrounds the dorsal and lateral surfaces of the base of the proboscis, being perforated by the proboscis-pore ; another occurs at the posterior edge of the collar; M'hile two others occur in the trunk region, one in the dorsal (Fig. 275, B, ml) and the other (wv) in the ventral mid-line, extending the entire length of the trunk. No sj^ecial optic, olfactory, or auditory organs seem to be developed. The short canal opening by the proboscis-pore has been regarded as excretory, but the assignment of such a function to it seems questionable. A similar function has been as- signed to two short tubes with folded ciliated walls Avhich communicate internally with the ccelom of the collar, and open to the exterior by the collar-pores, situated, one on each side, on the edges of the atrial folds. More definite informa- tion is required concerning these organs before they ciiu finally be accepted as excretory ; they evidently correspond to the collar-pores of the Pterobrauchia. All the known species of Ualanoglossus are bisexual, the reproductive organs, ovaries or testes, consisting of simple or branched pouches situated in the trunk, beginning in the braucial region and extending some distance backwartls. Each pouch opens to the exterior by a special duct, upon the dorso-lateral portions of the body. Development of the Enter opnezista. — Some species of Bahnio- glossus (B. Koivaleii'sldi) develop directly without the iiitti- vention of a larval stage in the life-historj', but the majority possess a characteristic free-swimming larva known as the Tornai'ia (Fig. 277). It is a barrel-shaped organisnj, buli^eil out slightly at either pole, and possessing a locomotor iijipa- ratus in the form of somewhat complicated bands of ciiii. One of these surrounds the posterior portion of the bu(l\ ;i^ TYPE PltOTOCUOItDATA. q^^ <'4^ex of the body in an ectocler- '' ""Z''''' "* "^« ;;;f f/f^^^i^^^ the apical plate («)> which bears two ejes Tlie moutJi (J/) opens by a s ^ort oesophagus into a capacious , «ton.ach iS)^ separated by a per! '' forated partition from the short rectum (i?), which opens by the terminal anus. In the anterior portion o the body is a saclike siucture(;,c), united to the apical Piate by a muscular band and opening to the exterior by a pore --^-^ O') situated a little to the left of ^^«- /^^-Tokkakia Lakv. o. tiie mid-dorsal line. Thivj «n • -^«^«"»^^o.wm«. « = apical pliiie. cc = coilar-cffilom. ^= month. P = dorsal pore. Pc ~ piobobcis-coelom. li = reciiim. ^ = slomncli. ic = triiiik-ca^Jo^ t ie mid-dorsal line. This sac is the proboscis-ccelom, and the pore the proboscis-pore, and in connec- t on w,th th ,,^^ .^ ^ ^^^^^^^ the so-called heart, which becomes the proboscis-vesicle of the adult At a ater stage of development - = .n..,k.ea^,o„ t^vo other pairs of ccelomic sacs (cc and tc^ i T' pearance at the sides of fl.n . ^ f ^ "'''^^'^ their an- coHar and trunk c^:' '^Z ""IZlf '"' ''"' ''^^ '^ ^ '^ ^-adual transformation of this h v. T '\ '''^"""'^^ '^^ ^^'^ ^netamorphosis. ''''^'^' *'^^^'^ ^^ing no sudden T^>^ Affinities of the Hemichordatn ri '>"t that tho Pterobnu.chi. ::, te - 7 '''^^ -;nanysi„nhu,,„a at the same til ptu ::"::?: "" f''^^^^^ ^•'^'^^^-^' - h grcps. Thus, to mention or.ly some i; tl "'"' '""' ^"""^^ "^ '>•> " Krou.^s possess a notochorcl of "mil' "'"''' ''''^''^ '^^''^^'^^^ eollar-pores and branchial slits and r -. ? ■"''''' '^''^'^^^^-i^-pores ;'"■- .stn-ctlv.con.parable re^^ion Th « I "•',*';"' "^^'""' ^''-"-^ i"t(^ 608 IN VEIl TEBRA TE MOliPHOLOG Y. be postponed for the present, and attention called to the sugj^estivc ehar- acterof the I'urnaria. Its first describer took it for an Ecliinoderni larva, and tiie majority of succeeding antiiors have been inclined to regard it as indicating aiiinities with that group. Tlie arrangement of tlie pneoral and postoral ciliated bands, and the occurrence of the proboscis-pore, suggest the Ecliinoderni larva witliont doubt, but it must still })e regarded as a decidedly open (picstion whellier or not these fcatur'-s indicate an alliiiiiy. Furliier inforauition is recjuii'ed both in regard to the ancestry of ilic Echinoderms and as to tlie life-histories of the I'lcrolinmchia, before llu^ question can be settled. Anollier line of ancestry niu.st also be mentioned, namely, one wliicli leads back to ancestors common to tiie Ilemichordates and the Prosopygia. The similarities of the Fterobrancliia to the I'olyzoa are striking, ilici'C! being the same bending of the intestine, similar lophophorelike tentacular structures, ami, what is of considerable importance, a dor.sally situated nervous sy.stem arising as an invagination of the ectoderm. A furihn' point perhaps of some imiKirtaiice may also be mentioned, i.e., (he occur- rence of three :>ectioiis in the body-cavity of the Brachiopoda. In following out the line of difscent suggested by these similarities, we are, howevci', quickly brouglit to a halt by the uncertainty connected witii the origin nf the Prosopygia. and we ai'c left standing Ix^twecn two lines, one leading back to till! Prosoiiygia and the other to tins Echinoderms. Whether or not the.so two lines conv(n'ged to common ancestors in pre-Cambrian timo can- not be ascertained, and the solution of the problem must be left to fuliue eml)ryi)logical investigations. 11. Cl.vss Cephalochorda. The class Cephaloclioi'dii oontaiiis a single body on the; left side id' tin' cauihil tin is situated the anal optnung (Fig. 278, hi- 1 lial»it;it, suiid, flic 0. 11(1 SdllH'- tlie iiiid- oilv w.ill, [.'till i'»l.^ . Vn^U: niil.il I'm till' \(ii- itll tiM'-^<' - U' .il til- TYPE PItOTOCUOlWATA. gQ^ (4J. The oltVwa ). f tli^': ".r""^' '^'■"'"' "'« ""■"- .Mes of the boa;'t .1 "" e ''e", ■^^.r fl, '"'t • l"' "'^ crea.„,K i. «,e, f„«e t„«ether beW ete ':; t \ '"^ l».-e thus e.ch,«i,.g the utrial c.vit/a, .'^w ,,,;;, n"-'' hue,l thr.,„(.ho„t by ectoderm aud sur ■om.d; 'l '? veutiHl surface of the auterior two thirds '■" '^ "'"' liorly the cavity is ch,spd l,v .1 V , "" ^'"''^'- '^"'«- rt« r: eye "* = "•"'*'c.|,lni,Iiun.. ch = n,.t„d.or,|. '^ "^ ""•""'• i' = rc-producdvc. organ. i> - "inal pore. / = liver. ''"'■'•''' '"''vc'-conl. «i' - biniicliial cltft. torn of which Ii.» th. oriKii.al n.onth, th a.-ins „f th„ l,n , i'niso.Iint,,nuMUM-onss.n.so,vpa,;ilIa>. ' "^'■^"^** T iie cctodenu is vcrv simnio in ;♦ , i ««;;.;^ i..r of c„n.;c:;:,i^;;;,:r; ::•::;;— ^^^^^ ;:":r::^b;::;:';;:r:;:;:r""'''r^^'"-^ -'■■"^''"M.Had.vc;,i,c.i::t:::t:;;:i.:^::;;::;r-;'-;:--;: 610 INVERTEBRA TE M0RPH0L06 T. time these folds are gradually constricted off from the intes- tine, and at the same time are divided transversely into a number of sacs lying one behind the other, their number in- creasing as the folds are separated from before backwards from the intestine, until in A. lanceolatm there may be as many as sixty-one. TLese sacs are the primitive mesodermic somites, and the cavities they contain are the primitive ca»- lomic cavities. At first entirely dorsal in position, the various sacs later on extend ventrally, those of opposite sides meeting below the intestine ; and still later the cavities of these ven- tral extensions fuse to form a continuous cuelom extending the entire length of the body on the ventral surface, and forming what is termetl the splanchuoccel. This becomes eventually separated by a layer of connective tissue from the more dor- sal portions of the somites, which remain distinct from each other throughout life and are termed the myocoels. The future history of the two portions of the mesoderm thus formed is very different. The walls of the si)lanchnocc)el remain thin, and the cavity well marked (Fig. 279, co), but in tlit* myoccels the cells forming the median walls become converted into longitudinal muscle-tibres {m) which traverse the entire length of each myocoel, filling it almost completely, and are inserted into plates of connective tissue which develop be- tween the various mjoceels and separate them from one an- other. At the same time each mjocad becomes bent, so that its dorsal portion is directed downwards ami forwards ami its ventral portion downwards and backwards, each muscle- plate having in a longitudinal s(!ction of tlie body a <-shiipL'd appearance and fitting into the one in front of it. AViien the epipleural fohls develop, both the s})lanchnoc(t'l ami the muscle-plates are continued into them, the muscle-])lat{'s lying to the outer side of the sphinchnociel, and their tihrcs here having for the most part a transverse direction, in- stead of a longitudinal one, as in their upper portions. Owing to the myociels being practically obliterated h\ tlif miiscie-])lates, the (-(elom of the adult is j)rincii)ally formcil of the splanchnoc(t'l, but other spaces also occur which an; ]n'()ljably schizocd'lic in origin and form various lacnuM' throuL'hout the body. a b c CO : e : TYPE PIWTOCHOBDATA, 611 ia wLich floM numerous coTn.T ""'"'""'I'S » ""'"'ess blood Byseem eo.»i.ts o r 11, rH""''? ""P"-'-- The .-. po..o„ o. u. oo„:::Lnjs,a r £r - l^IG, 279.— TnAN8VEH8E SppTrnw m„^ •lesccndhiff aoi-hi ^' '"" "«"twio). « = (lesct'tiding aorta. * = atrial clmiiiher. c = riotochord. oo n co'lorn. « = liyp<)l)n»Khfnl groove beneath wliich is the n.scci.ding aorta. 9 — reproductive orgim kb = branchial arches ^ ~ pharynx. I = liver, w = Miiisrlca. n = nepliridiiim, r = nerve-cord. *» = nerves. li.e bmudnul resiou „f tl,e intestine It ,.,,,1. „ff i va.-.o„s reKious „f the h„,ly whid, ,*,!!'""'''"''" f^'I.ilhu-ies, unite ,.„„in to fi,! ,l '"■""'<'"K up into V"i". which, „u»si„; „ :.,I '«""•""-.'-. ■■ vonfal «."uehi,;r.--:/r^!e'i^r:L":s^:r:::!: 612 INVERTEBRATE MORPHOLOGY. W sally to unite witli the right aortic vessel. The blootl which passes from the dorsal aorta to the iutestiue is not, howevei-, returued directly to the ventral vein, but the intestinal capil- laries unite to form a vena porta which passes to the liver and there breaks up into a second set of capillaries, these finally emptying through the hepatic vein into the ventral vessel. An hepatic portal system, resembling that found in the Yertebrata, thus occurs in Amphioxm. While passing be- neath the branchial region of the intestine the ventral vein gives off paired vessels, the branchial arteries, opposite each branchial septum, and these passing dorsalwards in the sep- tum open into the dorsal aortic trunks. There is no definite heart, but certain of the vessels, notably the vena porta and branchial arteries, seem to be contractile. The notochord (Figs. 278, ch, and 279, c) has a much more extensive development than in the Hemichorda, since it traverses the entire length of the body. It arises from the dorsal surface of the digestive tract, but early loses all con- nection with the intestine ; and though in early stages it con- tains traces of a lumen, this cpiickly disappears, the cells becoming richly vacuolated, so that the notochordal tissue assumes a characteristic appearance. At either end it is pointed, and throughout its entire length it is surrounded by a sheath of dense connective tissue, which is continuous below with the partitions separating the splanchnoctel from the muscle-plates and these from one another. From each side of the dorsal surface of the sheath a longitudinal lamella extends dorsally, the two lamelhe enclosing the central nervous system and being continued above it as a strong neural ridge (Fig. 27',)). As has been already stated, the adult month is formed by the margins of the oral hood, the original larval mouth lyini; at the bottom of the f)ral cavity enclosed by the hood and beiu'f surrounded by a (drcular fold of tissue termed tho velum. A short tube leads from the mouth to the branchial or pharyr.geal region of the digestive tract, whose walls me here perforated by numerous slits (Fig. 278. up) ])lacing its <«avitv in conuiiuiucation with the atrium (see Fig, 270). In the adult the slits are elongated ami are i)lac»nl (iblicjuely to TYPE PliOTOCIIOHDATA. 613 the brauchial re^.ion of IITZT ' "^"^^'^eplates of tious between each pair of slits ,;^"^^^^"'^^^' ^^^ ^Jie parti- tl^e same side of tlxe body skeletal |■ «tu,n..s of d..v,.|o,HM,. :. ; i" ""■; '" ""• "■"-'•-Pl.-.t.s. In ■••'-I>"'Hl .m.ta,,u.rin.ll \ ^ , " "''^''^■^ '"•*' ''"-'"i-'i "Lich do ^--•<' H^ <'.o ...no , s. J *;;;;; "" "''""' '"'"^ -••■ •-'-! ,„ , "K'ifimon.! coiT(.spoii,i,.,,e,« is lost. Fin. 280.-nrAo„AM ok tiir BiiANdriAi, Wkki.kton of ^»ip^'ioxilS (after Si-KvaEL). * -«Vniipliculii. «* - sf|t;,il bar. <6 = tongue bur. ive ciliate tlirou i»Joo(J-vesselH occm- l.,>fi. ; n into ti,.; ■■;■,"*""''■>■ "'•"■"">'iM««»e8 614 IN VERTEBRA TE MOliPUOLOQ Y. exterior by the atrial pore. Along the dorsal and ventral mid-lines of the branchial region is a distinct ciliated groove, the ventral one having projecting from its floor a longitudinal ridge, while ventral to it is a chitinous skeletal plate composed of paired moieties having a metameric arrangement. This ventral or hypopharyngeal groove (Fig. 279. e) is termed the endostyle, and from its anterior end a band of ciliated cells passes dorsally on each lateral wall of the pharynx to unite dorsally with the epithelium of the dorsal or hyperpharyngeal groove. From the digestive tract behind the branchial region a diverticulum, termed the liver (Fig. 278, I) arises, and pro- jects forwards, covered of course by the body-wall, into the atrial cavity (Fig. 279, I), and behind this the intestine passes straight back to open at the anus (Fig. 278, a), situated, as already indicated, upon the left side of the body, some dis- tance from the posterior end. The nervous system consists of a thick-walled tube (Figs. 278, and 279, r) which lies immediately above the notochord and is enclosed by the connective tissue lamellre which arise from the notochord-sheath. It extends throughout the entire length of the body, tapering rather suddenly at either ex- tremity. Throughout the greater part of its course the lumeu Fig. 281.— Diagram of the Antekior Portion of the Nervous System OK Amphioxiia (after Hatschkk). cA = uotochord. JV = hypophysis. 1, 3, 3 = are placed over the three ventricles. is very small, forming the central canal from which a well- marked cleft, the dorsal fissure, extends to the dorsal surface. At the anterior end of the tube, however, the lumen ealar-os to form an anterior ventricle (Fig. 281, 1) which has been compared with the anterior of the three primary vesicles nf the Vertebrate brain, and behind this the lumen couUucts rYPS PBOrOCHOHDATA. VOD9 Stbtesi 615 formiug the aqueduct of Svlvins nf n,= • , i • belaud this again au expaudon !f fh 1 ^ '"''"" ^^^^ "''"« do«ul fissure form, a /C T / .'?1 .'''"■"''' I«"«°" of the the Vertebrate hiuTbrfr the , n" ^'^ '"""" *° "'»' "' Vertebrate braiu I^TCI :;t':rir,*° "'" ^"'^-•^•"-<' which is increased bv tl,„ ^ umrked— a resemblance (^) of the :S ten r e^r^'r ?/ " '""'""-"'''' -'--on l^od,, where it ZT^^Z^^'^ ''«-' ^f- of the Jerm. This is tiie rem J,„ nf JeP'-essiou of the ecto- -itl. t.,e oxter""; rh e,ftsrtT"'"f °'' °' "-^^-'Wele -verare";:rffrc28i) trrr ''-- "^'^- °' come from the dors- 1 nm-t; „ , '""' ""°'"' of wldch 1-i.- on each sll " ,, X ™ " '^™"' ^''""^ ""^ "-"J tl.e dorsal surfa e of e b,'.! n"'?"'' '! " '•"<" ■■"'"■''« '-"' v-tral side, a condi i :,', J " iT;' r.lfr ' " m""« '"'"• "- ...etamerically-arranged nerv T, ,"' ,'" ' " ^-'eJing ■■ever unite to form a comm™, ... """' ™''"'''' "-oots '-0 the .^.-inerve:r. :, ;::trb;;t r^tfr*'"'''''^ are sensory iu fauctiou, while he 1 IV^ •"=^'='-"»Iess they -.W;binK only the mus^lature t^ tCbol""" "" "'°'"^' I" addition to^hese a;\>: .l'\:''';''''-^'" '•-«»'• ai. exceedingly simple evl Z ^ ' ""'' '""^ »-ei>rosent 'l.e brain, am/in "^ ' .S^'.f r„,::'r: t -"""""^ ""' "' ;>■■«-, -..-rounding a depr ss „ irS." ''f ^■'' "' -"^ «a„ng l„„g refractive hiirs. Tl, 1 j:''"""''""- ™"» 0 a sense-organ of some kind, but its exlct f !• """'"' '" ku(nvn. ^ ^^^c* function is ujj. Differeut stiiicture.s Iuivp fmm r *lered excretory organ I„ 2 «"! '? """' ''«°" «™- '•-«".. l.a» been assi'I^^ J'l "^^^ .•"7. - ---tory of the oral cavity on 'tl.e^ fcft's d '':,' !'" ""^' '" "'" """ siiK, and conimumcating by u 616 INVERTEBliA TE MOliPIIOLOG Y. fuunel with the coelom just behind the level of the velum. {Secondly, although iu all probability they are not uephridia, the "browu cauals" may be here meutioued. These lie in the sphiuchnocuel at about the level of the twenty-seveutli muscle-plate in A. lanceolatus, and open by wide funnels into the atrium, though it is uncertain whether the inner end lying in the coelom is perforated. Thirdly, iu the pharyngeal I n s r 11 Fig. 283.— Excuetoky Organ of Amphioxus (after Bovkri). nc = uephrldium. tip = uephridiiil pore. nd = nephridial fuiuu'ls. « = synapticiilum. / = braucbial septum. //= bniuohial tongue. region a number of nephridial canals have lately been de- scribed. They are situated above the upper ends of the branchial slits (Fig. 282, nc), each opening into the atriuiu ()pi)OHite a tongue-valve {np), and from the short tube whuli passes inward from this opening an anterior and a posterior l)ranch arises, each of which opens into the coelomic cavity by a terminal funnel. Between these two funnels three or four others may occur (nd), and around the mouth of eiu:h funnel are a number of threadlike jn-ocesse i which end iu round strongly-refractive cells. That these structures are UPDhridia seems indicated by their relations to the cwhm and furthermore by the fact that iu the neighborhood of eacii TTn PBOTOCHOBDATA. of tLem the brancliial blood-vessels fr,,™ „ , may be regarded as a glomeru ussuH " "" ' P'"''™ ""«'' t."-vith the uriuary ttbulerol" .rVertlrrtru, " """"^"^ Ampho^,^ is bisexnal. The nmZZt ^^• or testes (Figs. 278 ami 279, M occH iu the ' °T" '' "''""'' "UJ are arranged metau.o i.,!! / epipleural folds, Tl.ey lie at the level oi the ^f T ""'■':>'->'i^"' metamere. =-1 the ventral trins Lt , C! e" ,"' '"'"•"' '""^itudinal « cceiomic cavity wh • h L lit' ""f 7' """'"'"'d ''"''in of the segment! o ' • i h tiL'"' 1 "" °"«'''^' "^o^^'^ Tl^ey lie on the inJ^^'t^^T^'^-^Zr ''^ covered ou their iuuer surf-.PP« 1 li !, ^°^'^''' ^^^ a^'e -i-1^ the rern^oan C ;^^^^^^^^^^ ;^to the atriun. and thence 1 iullo I^ " T'""' '^"^"^' the atrial pore Thpvp ..v *'^^^'"^^^ *« the exterior through poie. iLeie are no reproductive ducts. considered the mos;,,n„-uX^^^^ ^'-- -»all> been •" =^ 'nore or less modified condi on ,p\7 7^^^«"^'"g. ^" other words, i-e deseended,-and there a em?' "^ :'"/':«'" ^^'^'^'^ the Vertebrata o~^'ns and the larval L'Mnm-J.rZUTl^'T^^^^^^^ -I'Ported. The character If ::r:f t',! 'f ?"'' ^^ ^'^•^^' ^^^ '- «V«tem, the arrangement of the ne X nP . , ""'"'^^"'•'^ '"^"^ "^^'vous If ^tes, the occurrence of an h pit ,o ,. i ', 'T'^ *'" '''''^' "^"^de. f''« early stages of devel,>pme t 'rf ^ i '''7^'^^'"" '-^"^1 the character of -Plioable only on the snpp u^. o'a'son "v. . ' "'"""''^^ ^^'^"^'^ «««'" Ou the otiier hand ni i.,.. . '^o^'e^'Iiat close affinity. , '- -r.ed. It trlr '™ ^ ; e Enteropneusta ar^ hut slightly the arrangement of the ^^r..^T^J^J^^'^r ^^""'"•'^'- ^^ <'^'Pen..,,. is nndonhtediv much more 1 . ' ''^ ''^"''"">.'^^»*v"*-- , !''au yy./„.o,//..v.,, ,,Kl both are 1 ; Iv ' "'"' '''^ ^^"'^^'''^■"^•^ ""« ;-- ^'- '"^--'-'''Positions, Extending through 0 etelt a™ ""'"'""' "*' """^■ 'vhich communicate .^1 Ihe Won7"''"7 '^™-'-« tubes They arise a. outgrowths of the bT r''"'" °^ ""^ """'y- l.od.y.wall before fhem beii L^ T''"™?^"'^ ""'' P»«>' "ie ectoderm, beneath wl ch is Tl. .'■' '"""' «"'™''Hy by each is separated int^ J: cirr^ eT^r""^^ "^^"^' ""^ partition which does not L ' '' ^>' " '""si'u.liual enlargement with .tier; iThZ', "''"J' '"*" "'« "'»'""- :--c_.ati:^n^rb\t\- oppoiiiirbiariiiXHSTatr "'"T -'■- -- out into two '"^uiar pr„;est:rthe'"c.';i:i'rd':: •:: II 620 INVERTEBRATE MOliPHOLOOT. siphons, in whose walls circular muscles are developed to serve as sphincters of the opeuiiigs. T}ie branchial siphon opens posteriorly into the branchial region of the digestive Fig. 283.— Figure of a Tunicate, ILterotrema, removed prom the Test (after Fiedler). A =• atrial pore. pc = periphiuyugeal ciliated band. an = auiis. « = stomach. CG - cerebral ganglion. m = subneural gland. en — endostyle. »i = branchial stigma. ex — excretory organs. t = testis. / = intestine. iid = vas deferens, tract, the opening being known as the mouth, and usually being surrounded by a number of tentacles (Fig. 288) wliich arch over it. The atrial siphon, on the other hand, does not open into the body proper but into a cavity, lined probaiily TYPE PROTOCnORDATA. Qg^ anterior and posterior em TL ' . "•"'' f ' "'" ''""■^■"'^ Ws walls are tenued tl e taufle iT- '^' '" "'" ""■""■' ""-1 atrial o..ityoiA,npMoJXT^:Jll:°7r'"' '" "« sive developueataml arises ia tie Kvv ■""'■" '*'"'"■ ".en> at t,i sa,„e t^L'dl't,; r ,! t":;; t^l" 'r '"''"''^" become coiitiuuous. ^' ^'''^ ^'^'^^ cavities The external surface of botJi flm ,» n proper is covered witl. elde™ tu ret ,"",'' ""^ ''°''^- layer of mesodermal connective hsuei'T f"'" "'""' * iibres. The cu,l„,„ic cavity 1,^1 of °7''""*' """"='''- ■spaces which l.ave,e»pecii„;r,t ais'of «'■" '""'""■ '■es-on, a more or less cleiinitei.ra.l;e,, „ ^^^^ . '« """"r'""' vessels. In a somewhat ,H«t;,„.t *' "' "'"' '^"^ as blood- t'.e bod, is situated at hi 'tri^ir""' "T, '''"' "'"' "' of a single layer of cells. Z t^l^" 7"'^. ?'" '""'""' verted iuto muscle-Hbres Tl,„ . ' ''''"•"'' "'•« «'u- wavelike, starting f™,„^een^a J '■"'!™' "' '"" ''^•■"■' -- rather .nieUl, to;ards ^21:^^^:^!^'^' 'f""*-"' « that after a certain number of beafs,'/'™'"""^ contraction-wave be..iu, «„,„„,!' ""''' "^ ^'•''''='' ""> andf„rasimihu.n„i:fratnh;:i™r:j This change takes place with a certJn f ? "'' '''"'• and at each change The course o^ til 7 > ;',°""' "' '■''^"""' at least of the l,ody is reve led 1 7 "■""«'' '■• l""""" ;..'" a large lacnua, one I ' ^ Lh nn f ir'n'"' ''^'"■' ""-» i"e of the branchial sac, while tl't,'^ '«:;:;'?'' ""''■ branches to the intestine niid t^^t , ^ ^ "" lacunar line of the same r giorsmalle. C'"' " °"" """ '''"■^''' ■■"''- ■ cl.;al bars, nuitiug ^^the" Zf^^r^^^Z^^" ''-"■ colorless and contains amoeboid oornnscL vK V '"""' '" usually coloriess, though a few coWed nn ' "'■" '•''"" pl«mentedeeUsofthe^st,are7:°;::!tr;:;;r"''""« '"^ li-e mouth opens into a capacious pharyngealor branchial 622 INVEliTEBRATE MORPHOLOOT. \ l\ 't sac whose walls are, perforated by numerous slits or pores termed stigmata (Fig. 283, st), arrauged iu transverse or spiral rows. The bars separating the stigmata enclose lacuna) which place the ventral and dorsal branchial lacunre iu com- munication so that the walls of the sac are richly supplied with blood, opportunities for its aeration being provided by currents of water drawn by the cilia which border each stigma through the mouth and out into the atrial cavity, whence it escapes by the atrial aperture. The transverse bars which separate the rows of stigmata are generally stouter than the longitudinal ones, and iu most species there is a second series of longitudinal bars lying on the inner surface of the sac, less numerous than the bars which separate adjacent stigmata, united with each transverse bar by a short connecting branch, and bearing opposite each junction a hollow papilla which projects into the cavity of the branchial sac. Running al()nrauchial cavity ; these are termed the dorsal languets. The remaining portions of the digestive tract is in i\w «imi)le Ascidians generally situated iu the mantle on the left .side of tlie body, owing to the enormous development of the branchial sac;, but iu other forms it constitutes a part of a viscv^iid mass lying immediately below the posterior end of the b"-c. The tt3S(jphagus, l)egiuning at tlie lower end of thii sac, forms a short tube wliich opens into a fusiform stoma. Ii, from the further end of which the inte.stiue (/) arises. This is generally bent twice upon itself, forming thus two loons, and ends in a straight piece, the rectum, which opens by tho TYPE PROTOCHORDATA. 623 eutue length of the muer surface of tlie iutestiual wall form HR the typhlosole, aud a number of brauche.l tubule ' open" «ud aud giving oir nerves both anteriorly rnjl^f^T'' tbe anterior part of the brancSre ' "tt Lt X" Jet ,^arkod papilla which n=av possibly be .JoILL:^^ rhe gland, from .ts relation to the nervous system and tT^ £:rr"r .:;„i:rr,' rtSif r-ss orsal languets, and the papilla at the opening of the tl ^ the subneural j, serving as fins. Tjinh the anteri"umcate .«-t .e.t.kable ^'^^ ^tJ^ ZIZJTT " ''^ :-:rtL: zt:^s^z :!,;t;r ^-"r J-Ces of ,vMeh it «e.„« „n .en-l^la . ::.' ^ tube throughont nearly it, entire ieugth, lie, a uotochoil 1 wlwch servex a, a skeletal support to the tail, au o" ii" «.le of ,t « a plate of lougituib-ual .m.sde-fibres iiy uioauH of energetic lateral movements of the tail this arva s,v,,ns about for s,„„e tin,e, but when about to tra' one';,;;; >;■'" '"'"" " '''"'"■" '- -•- -""io 41 "; me ol the a,lhes,ve papilla,. The tail with its nervous svT t"n, n,„scles, a„ tail and i!h1 tjjut tlie ndiiltH, except in tl the fi !lH ■eo-Hwiimning Iwihit 626 INVERTEBRATE MORPHOLOGY. Several are persistent, are to be regarded as degenerate, orders of Tuuicates may be recogoized. 1, Order Larvacea. To this order belongs the genus Appendictdaria which has already been several times mentioned. It is throughout life free-swimming and retains the larval tail, greatly resembling in general appearance a tadpole larva. It secretes an exten- sive test which is gelatinous in consistency and is but loosely attached to the body, being frequently thrown off shortly after its formation. The body (Fig. 285) is comparatively f .Fig. 285.— An Appendicularian, Oikopleura cophocerca (after Fol from Hrrtwio). a " unus. /= ciliiited groove. c = uotocbord. (f = brain with auditory vesicle. d' = pharyux. g' = first guiigliou of tall, d" = stoiiwich. /i = testis. en — eiidoslyle. ov = ovary. « — brauchial cleft. «mall, the tail being attached to its ventral surface, while its posterior extremity is somewhat enlarged and contains th«^ reproductive organs {ov and h). The branchial sac has but a single pair of stigmata («) which open to the exterior b) a pair of funnel-like tubes situated behind the anus. This ar- rangement represents exactly a condition present in the larvio of other Tunicates, two stigmata lirst forming and the atrial sac arising as two separate invaginations of the body-wall J — -A- — — l.r-.l. Al-a. •»*.».*»..»•»» <.iw».v. <^i.« ^^*\r\*% 4-l»/^ 1 tiiraryi 1) < f loTlu rvtilv Several rhicli has ;»liout life isembling au exteu- it loosely tf shortly jaratively ter Fol from esicle. , while its itaius tlu^ has but !i erior b) a This ar- the larvio the atriiil body-wall -tions nulv TYPE PROTOCUORBATA. 627 later fusing to form the extensive atrium. The eudostvle ie.\ (cWipts ht.lo« *i 1 , h'^^h^i'i IJ/ ;• llie uotochord 2. Order Ascidiaceae. iiiey ditter from the Larvaceje cliipflv ,■„ +i i J°"-^«". tail iu the acIuU aud iu the at^e d ve/„, l^t oft," ','" "V't sac aud the m„ue.-„„« stif-matt '"'"'"P""^"' "^ "'« '"''■"^""al Ow,n„ to the complexities prclueed by the methods of InuUms .t ,s cstoma.., to divide the o..de/i„to ..boSate 1. Suboriler Aaeiilm simplkes. Tl,e simple Aseidians agree with the description civeu a, typ.oal and ,„ not re.p.ire any further notice llr, eSe.t Z moufou the fact t u.t there are included within t!,.: suIk^'i innpima. 1 ^ formation of new individ.mls takes „l,„-e i„ these cases from stnlonlike oiitirrowths of the nnl. , uud each bnd remains seated upo'n tl " ttln „ 4 1 d',',"; ts own test. The stolon (Fi,,. 28r.) arises fron, the lower 'f ->n of u.e body of the parent and pushes before' it a por i i 628 INVERTEBRATE MORPHOLOGY. of the test ; the cavity it contains is continuous with the body- coelom and is therefore lined b}' mesoderm, and is divided into two compartments by a longitudinal partition which may be traced back to its origin from the posterior wall of the branchial sac of the original individual. Since ectodermal tissue lies between the mesoderm and the inner surface of the test, the stolon contains portions of all three germ-layers, and a portion of each jjasses into each bud (b) as it arises. The first indication of a bud is a slight wartlike elovatio'i of the wall of the stolon which increases in size, its cavity being a diverticulum of the stolon -coelom. Fig. 286.-PonTioNoPAiuto the elevation a process (en) of ihe Stolon of ^^'^^p;;-"- gndodermal stolon-partition extends, and, (after Kowalewsky from _ ^ _ ' ' KoRscHELT and heidbr). forming a hollow saclike body, gives rise to the digestive tract of the bud. The various layers give rise to their respective organs with one exception, and that is that the atrial walls, the man- tle, arise from the endodermal branchial sac as diverticula which unite together, the atrial cavity being thus lined throughout with endoderm. Such anomalies are not infre- quent in the Urochorda, and indicate a necessity for further study of the nature of the germ-laj^ers in these forms. The simple non-budding forms are quite numerous. Com- mon genera are J/olgnla, Cynthia in which the test has a leathery consistency owing to the fibrillar character of the matrix, and Boltenia, a stalked form. b = bud. ir — braiicli of stolon. ec — ectiiderin. en = eudodeiiu. 2. Suborder Ascidim composite. All the members of thi?- order re])roduce by budding iu .some form or other, and «liffer from such forms as ClanelUna in that all the individu.-' ii remain imbedded in a common tost whether or not they rem.!i5) in organic connection with (uie another. The gmup seemn 'a lie a ^omewlsat composite one, ^V the body- is divided ugitudiiial il back to all of the udividual. tweeu the ice of the ins of all n of each it arises. $ a slight 11 of the its cavity on-coelom. en) of the ends, and, dy, gives the bud. 1 to their exception, , the muii- liverticula iius liued not iufre- jr further IS. us. Corn- iest has Ji iter of the uddiiig iu Clanellhia lumoii tost 1 with one 30site one, TYPE PUOTOCliORDATA. ggg aud it is probablp Hiaf ;* ; possessing a short »tol„a eLet ,„.! ^''Y"^' *■"' *'^''"'Pl«. former geuus, from which b X ^ ^ "'."^'f '" """ "' ti"* separate fro. the ^^^ ::^^^-!^~ ^^ ^-.^ox geuus, trom which bnrla « • , • 7 **" °^ '^e «efro. the -oJL''u t.T iSj^H^ ^ OK THK TEST (a..e. Kow...wskv .C Kohs hI.'? r'""""' ""^ «"«^^^^ « = parent iudividu«il h ,** . , '*'"' «•=">«•»). * = *^"'on a„d so the oo.on;'.:r„drr ieTtrrs;:j'';:;;,£rr'''*' connect on with thp nrimnai „i«„„ u "'«»wuuais tailing to form a colon,, stni, horvLrSdedrih^Zlotr '" " "" "'^"""« 3. Suborder PyrosomidcB. the central cavity or cloaca into whid the! it, f ?" open, the br.ancltial apertures opening on ILelwT" cylinder. Each individual 'resembles in ,!;.^ , "' ""* Ascidian, the principal differ cebehrthatt'llr"'^''' ture, as in Botryllu., is at the posteri™ end of t L bo "''"'; tl.at each individual has the poler of repto ,:i gl^ddW parent .o.ms uot, uowever, degenerating after giving rise to. 682 INVERTEBRATE MORPUOLOOT. buds, as iu Botryllus. The buds arise from the branchial saos behind the eudostyle, and, on separating from the parents, piyce tJiemselves oetween them and the opening of the com- moii cloaca, so that the okiest members of the colony lie at the closed end of the cyli. ler. On each side of the branchial sac of each individual near the anterior end, or more precisely near the peripharyngeal ciliated bands, is a mass of cells which are brv.^l'fly phosiihorescent, the entire colony, which may reach a length of over a metre, emitting a brilliant light when stimulated. The development of Pi/rosoma is exceedingly interesting inasmuch as it presents an alternation of generations. From A B Fig. 289.— Lakval Budding of Pyrosoma. A, embryo divided into the cyatl'ozooid aud four iiscidiozooids; B, Inter stage shovviug the asciuiozo- olds twisting to form the circle of four primary individuals (after Kowa- LEWSKY). cl = cloaca. en = endostyle. el = elseoblast. /* = heart. n — nerve ganglion of ascidiozooid. the embryo which develops from the egg at a very early stage a stolon develops (Fig. 2d9), containing a prolongation of what corresponds to the embryonic branchial sac and aiso •of the embryonic mesoderm. The embryo itself nevei' reaches a full development and is termed the Cyathozooid, serving to supply the individuals developed from the stolon with nourishment until they have reached a certain stage of development. This it is able to do on account of the ovniii being plentifully supplied with yolk, which the Cyathozooid gradually absorbs. The stolon at an early staf^e divides iuU^ TYPE PROTOCHORDATA. 633 in a commou test ami ,1. f ^' ^^ ^-^'o^ed with it end of the cvliiulnVnl ..r^i^, a ''"*-" occupy the closed .ooia cie«e:^!;t": a°s,/: "2t r r' ""'"^"'""- cloacal cavity of the coin,, 7,2 ."°™P''' ^'^ 'lisappears, tlie p.-es.io„ of t'he tet :t;~re;;;af d 7""' ''' ' "«- imlividuals arise by budding ^ ""^ '''''''^'' "' ""^*- the first generation of Botrmus Sids wl ^^.^"'"^''^'d' ^'l"«h represents and that it alone degenerates the P^^^^^^^^^^^^^^ '" -^ embryonie eondition, listing and forming LrtsTfthffnir , «»««««d,ng generations per- zooid there oeeurs bd nd ^e br h: 7^ ^^''^ ''^'"^- '" "^^^'^ ^^^i^io- cells termed the 2^ Fil' 'rrlHr"'';' ^""''^^ '"''^^^^^ ^' tl^ough it has been ^J^^^ii^^Z^^^ '^ ----''^i". larval tail. •' •' represent the rudimentary 3. Order Thaliacea. The Thaliacea are with a siugle excenfmn r.oi • isms, and present a lifo-histo../„o2 e'aW C X °'"'"'- rence of an alternation of Keneratiors T„ , ^ °r'"'" (Fig. 290) a well-developed t"t is pi.enfa'dr''" ^"t" ture of the mantle is arranged in bands wl.t.. "f™ '" surround the body and fuHher^l'thoVltndtn S together on the dorsal surface of tlm 1.^ V '^^^'e^cj to unite posterior end of the bodv %!« . ^ , ''*"^*'^ ''^^ *^^ - grouped toJIher^oL^^tr^n^ (-) lying behind the branchial sac anrveZll T""f " some forms the intestine is more elonrted nnd ^ ' T^ '^ Avhat from the nuclen. Tl.. ^^°"8ated and projects some- ■rm and positzon ; U i,as m connection with it three pigmented 634 INVERTEBRATE MORPHOLOGY. A \- spots probably represeiitiug eyes, and the subneural gland is present as usual. Each species of Salpa, however, presents two distinct forms {A and J5), differing in shape and in the number of the muscle-bands which are found in the mantle and having like- wise a different origin. In the sexual form (A) reproductive organs are developed, the ovary usually containing but a em ma Fig. 290.—^, Salpa mucronata, the sexttal Potim, and B, Salpa demoera- tica, THE Non-Sexual Form of Salpa democratica-mitcronata (after Claus). c, cl = cloaca. ni = brauchial pore (mouth). cp — ciliated pit. ma = test. em = embryo. n = nerve-gauglion. en = eudostyle. nu — uucleiis, h — heart. si = stolon. single ovum. This when fertilized (em) is passed into the atrial cavity, the follicle-cells with which it is surrounded forming an adhesion to the wall of the cavity, and later modifying to form a structure recalling the placenta of the Mammalian Vertebrates by which nourishment is conveyed from the parent to the embryo. As the result of the devel()]i- ment of this ovum the non-sexual form {B) is produced, which is characterized not only by its general form, but also by the possession of a stolon {st) arising from the branchial sac iust irPE PBOTOcaOBDATA. ^35 behind the posterior end of the endostvie Thi, «(^i eventual^, divides into a large number of pX each f ^^ after undergoing certain somewhat complioa ed >;l,if;,r i position on the stolon, develops into Z e fal t^Lf 2 r"fn^tz:',:trr a'^^inT'^^^^^ iriJJrsii^i^tvs^^^^^^^^^^^ into the non.sexual form (Kg 291 Jri'l,. T'' T^*^^ possession of nine circular m« J! .^ ] diaractenzed by the .eea some distance ^^:i:^:::'7t^^:, tlie boay, a ventral stolon (st^ anrl l a 7 ^ directed process (.,, From^t t olol aribr^tT rior:nX-lt't::d^° t'^ ^'^«- — e\:?£ .i.emselvesZtgrattX~{rd1r^r' means of amceboid cells, prob.blv n^^f. P *■' . ''^"^' ''^ »ttach themselves in pair; £ t ba'se of eahtd?/'"" to convey it to the dorsal process uLn t, ■ ""^ ""'"^ Uds arrange themselves inSiree ,^„,, t,ie i -r'T"', "'" lateral rows developing into form, « ^ li" '^'"'^r'^ °^ '''^ resulting from the development of the buds ottT T "'°'^ The lateral buds when fre^Iv ,L i T "'*' """'"'" '■°»-- the possession of a "i t,nS°'' T "''r'"'^"^**" "^^ - i_i„e oiauchial aperture, which occupies r^T-^fi^ 636 INVERTEBRATE MORPHOLOOY. almost the entire length of one side of the body and leads into a branchial sac whose stigmata ojjeu directly to the exterior, the atrial cavity disappearing during the course of develop- ment. The intestine is well developed, but the muscles are but slightly indicated, while the reproductive organs, rudi- ot en li St P'iG. 291.-^1, Tin: Non-sexual, and B, the Skxual, Foum of DolioUim (after Ulianin). cl = cloiutt. i - iult-'StiiK". (//) = dorsal process. n = iifrve-giiiigliou. en = eiitloslylc. ot = otocyst. g = reproduotlvu orgau. M = plinryu.x. h = heurt. «< -• stolon. ments of which were present in the young buds, complt'tcly atrophy during tlie process of dfMelopment. Thusc ImuIs are incapable of lending a free existence, serving only us nutritive and res])iratory individuals for the median buds, as well as for the parent, whose digestive tract deg(uierates, its muscle-bands and nervous system at the same time umlor- OF Doliolum rrPB PUOrocilOHDATA. J37 goiiig enlai-semeut, so tlmt it serves ev^nh„?i iiiJiviaaal for the eutire ■,.,„,T , '''^"'•"'"y •is a locomotor With recarilU H "*'«"'*'"'"'' °' ""'"iJnals. occur. ctTiu Xt;":;" '"r" ""'"■'""- °' oP'-ou 1'ei.g set free, develop ill" '"'■*-' '" ™^ "-"-rit.v all, ■nechau process upou which bura'fotV Tf '' ™''""' to the orij-in of these buds that tL r^ '' '" '"W""' exists. According to oue v ew ' T" "' ""'"'»'' ventral process which is"3e. uf aT!"'" "?'' '"" "'^ Uiird generation, while acm,! f ^, " "■"' ''epreseDt a certaiS member of he ma 'I '" » """*'• "^>^ "--^ -« tl.e forms bearing thttl ^ ' 't" vt X h ' ''^r"''""""; serving us nurses for them TOi . ' "'*'«''* ""'1 l^owever. the buds eventn.'.ll :"' """^ '"' "'"'■ °"«i"^ '-«vid„;i., (f!^"'L 1 ;^^Ui:i,'rar'/"'" ■^^^""' .nenced. The two views as ioiktoXlnDju "' "","'- schematically represented thus : "" '""^' ^« Ovum =. non-sexual form^Nurses Nutritive individuals -Nutritive individuals f^oxual forms— Ova Ovum --. non-sexual form! 'Nutritive individuals 'Nurses - S(;xual forms Qyji Nutritive individuals '-'«-! into oi,h, iaper 1 ui^'s tI' '"f ' ''^ '""•«'"« '^^'"^ 1''- "i«k and lead, nito a ^^:^ClZ:iJt'"T "" "" '"" ""''■"- •^' "- l<"ms and the inte.stine and vis ,••».; I ';""""" '' ^■'""l-'.ratively '•- form of a nucleus. Noth . ' 'V't '" """'"'^ '""--1 ^"«<-t''«-'r iu this form. " ' ''' ^'^ '^""^^" "« to Hie life-histor.v of Affl»it>(s of the Vriirhiivih, ti "■"' M,„ ,■,,,.:,„„,„, ,:; ;; :, ■™^'i,:;;;.;;:,:.'"'" ■■•"™ '"■■ ■> la,. »'■"! I.... ...any „„„„„„„ siruolun,! f,.. I ,, "" '"'''""l" 1">- a I". TI, i.,M,., ,/.'.,,;:"■;'''■■■',;•"'''-■ tl«„. ,„ ,„, chordat "iitin line of cvolut I .'iiid ieadiny (<> tjw. v„r(,.|jj.jjj., -j lie e '•""■("|)res(.,it.'(| hv (iirl-r,,!, ■'•Hy stages of devch.i.ment INVERTEBRATE MORPHOLOGY. 638 of the simple Tunicates (see text-books of embryology) are so very siunlar to those oiAmpkioxus that it must be concluded that he evolution ot 1 Uvochor.iaandCephalochorda proceeded for some distance along sim.Ui lines, and the general affinities of the Protoehordata may possibly be mdi- cated by a scheme thus : Vcrtebrata Cephalochorda llemichorda Ancestral Urochorda 'rotochordata T-ikinu tlie larval Tunicates as a basis for comparison, we And as fettrnv 'on n o ito them and Amt>kio.us a dorsal nervous syste.n ans- ;« ' an magiiution of the ectod.rm and extending the entire length « the dv in the anterior porthm.l.e lumen of the nerve-cord expands :: ;!;:::;t;in .^..^. m ..../<>,.. opens in eany ^ages t. t lie ^t...or ■i.ul in the Tunicates into the anterior portion of the bianchul^ac .t, Z ec odermal portion, the canal of communication in lie latter to.n . osin Tnl tcf stages its connection with the brain and fon.ung the sub- ^ Ida An'urial cavity occurs in both, which, th.ugh ans.ng in a ^ u. iKit diiferent manner in the two groups, nevertheless sc.nis qu e h oU Otis, an.l homologies have also been pointe.l out between the • r^^^^^ The increased lu.n.ber of stigmata and their arrange- ,^ U,e Tunicates is a secondary character resulting probably from he : :n;:x:s;ence ; and the development of the test and the H"'-.- o ^; „„(„,,,ovdtothetail are also probably see.mdary characto.s 11".'^^' bhinces are important one., ami when taken into eous.deratio„ with the embryonic development point very stmuglv to a close "A"" >; Xs re-mrds the relati<.nships of the various groups of I .o<'ho.d.v to on., .noi er n tsLlerabie .lillV.vnce of opinion exists. The Appeudu-ulanans " ^ .1 . Sim., se..m to be the most primitive of all the orders pres..,. rtain remarkable pecul.arities, such as the separate openings o the at. 1 TviU ^ and the amis, and some authors are inclined to reganl thnn no a ■ u tive forms, but as sexualiv-ma.ure larva- of sessile lonns m w ue ■ e«t had alreadv developed an.l de.^eneratum tar a.lvaneed. As.egaids ^erl ai i"«f L ron en teveral groups originating independently, all the members n..t " ; l:^,M le/froln a,i ancestral simple forn.. bn. some fnmi one Ir and others fro.u uuother. uuU .o on. The ThaUu..n, tlnuUy. ha.e very siiuilar ntion of the long similar bly be indi- TYPE PROTOCUORDATA. , wo fliid as .system uris- ontirc length cord expands 1 the exterior 'hial sae, i.e., latter forms ning the sub- li arising in ii a seems ([uite between tlu; heir arrange- )al)ly from th(^ litiition of the . The resem- itiou with the [•horda to onc^ (Midieiilarians, irders, present ijs of tlie atrial rd them not as •ms in wiiich a I. As regards nost j»rijnitive, iisilioi\ of noli oehi really I" ' nuMubers not Dtne from one !i lliudlv. have 639 probably been derived fiv»« „ affinities in its b^n '^ S^t/f^'o^^^'"" ^"^^'^"^^ ^-^n solitary not necessarily indicating a\,rimitTv. ? '"'"' ^"^'^''^"als being posite forms no organic union ext sCe „ t^"' '' """^ '" ^"« «"'"- colony when they have readied m ituHfv .. ' ''^"^"^ individuals of the rather aggregations than c I'^t f^," ^'^' *'" ^*'"^"*^^ ^-''--^^ -« «oc,althan colonial or oven composite ™' '"''**^ Properly termed SUBKINGDOMMETAZOA. TYPE PROTOCmiil)AT\ I. Class HEMrcHouDA.-I3ody divided into three distinet • c'liord a .small flngerlike div. H . '■'^"'"'' ^ "^to- from anterior po.-t on of d ' ? '^^"J^^^"'^' ^V.rwards retains connection ^""''''' '"''"'^ ^^'^'^ ^vinch it 1. Order I^e,v6ranc/na. - .sessile colonial f ;'''on.se- intestine bent u^;^^^^ '^^'^-^'"^' a lopliophorelike, tentacle-beirim , 1' " '''^'"' ^^■'"' cus, Rhahaopleura ° P'ocesses. Ceph^Uodis. 2. Order E,itiroj,neusta.~Fvoo form« ,. f . -retingatest; i;::3su"l"^./""'"r "''"'^^ ' "^^ lophopi,orelike , roc esse Z ' , '"" '■'^''•^" "'^ ''«"* n. Class CE.MiAL(,cHo,M)A.-Frec form .; , ^"f "'"^'"-^''^-v- bodynotdivi:^tt:;r;;:^;;«;;;--tsecretingatest; ous metameres ; .iot<,clH , ^ L r"' "" I'"""- dige.stive tract and traver.i,. / ^ separated fn,,,, body, .l.,;./.-,;:;;: ''""•^'"^' ^'"^^ -^^ire length of the nJ. Class UROCHOHlu.—Sc'ssili. r>. f,. , . -1 »i".wu.„ ,.„ i,„n..,. '„'!1.' '''''"' ■■'*""" 1. Suborder .l.vcvV/m. .v/W, , 1 ' "••'"""^"" a.iult. b-.g. Sinllou ZL Neapol, ix, 1889. E R. Lankester and A. Willey, The Derelopment of the Atrial Chamber of Ainphioxtix. Quaiteily Journ. Microscop. Sciciioc, xxxi. 1890. F. E. Weill. Excretory Tubules tn Amphioxus lanceolatus. Quarterly Joiuii. Microscop. Sciouce, xxxi, 1890. i a|)er by Uiifv.. fj T^PE PROTOCBORDATA. nation of A. Wllley. The Late T — '^^^^. g^j V of the Vertebrates. Ne^ York, 1894. C. Kupffer. i?,-, ^^„,„,„'^' ^""^ «^''- x, 1866. ^««(/«««. 3Ienioires I, 1872. ^'' -^^''^^^^iV*. Arciiivea do 7o.i ^ H-'ieLaca.e-Buthiers Te. A r " '^ "^^^^ -'--t geuerale. A- Kowalewsky Ced , ' Archives ,Ie ^^ toM.ie, X, 1874. ' ''"^'"''^"^"ff^ierAm-dien. Archiv fn- •, *• van Benaden et C Juiin A • ^"0"ogi., C M«url«. A , '"■'■''"■<■» ■'« Biol., VI ^"Pt'i- bv .>[ M Ar . ,;'^'' " Moaoqranh Win. '"'^''' °- ''""*'' '"'"IS ilc.pkiua Grorip roman tj ACANTl Acant/h ACAKIJN Acervul Achther Acicvla Acineta, Aenum, ACCELA, ACOTYL ACRASP) Aclinian Actinovh Actinovii Aetinoph ActinoHpi ACUJ,EA1 ^^ga, 41 i u-EoUs, ai ^'Eolosomi ^Hquorea, ^■Eschna, Affalina, I Agilena, ■ Aglaophei, Afirion, 5( Aiptmia, Alc.iope, 2 Alcippe, 31 Alcyonai Alcyonclla AlcyonnUu AUyo Ilium Ali,oiocu<:i Alpheus, 4] • / INDEX OF PROPER NAMES. roman type. capitals, generic names in italics, and popular names in ACANTHOCEPIIALA, 179, 183 Acanthometra, 20, 39 AcAUiNA. 45y, 458 Acertularia. 17, 38 Achtheres, 397, 423 Acicula, 322 Acineta, 36, 39 Acnuta, 364 ActELA, 132, 1G9 ACOTYLEA, 139, 169 ACUASPEDA, 97 Acliniun, 114 Actinometra, 542, 592 Actinomma, 39 Actinophrys, 17, 39 Actinosphcerimn, 20, 89 ACUJ,EATA, ,'^27 -*;9rt, 415, 424 .iJolis, 315, 304 ^-Eolosoma, 219. 251 ^■hJquorea, 80, 116 ^■Eschna, noo, 526 Af/alma, 92, 116 Agdena, 450, 458 Afilmphenia, 87, 116 Av'Vo//, 506. 526 AipUuia, 113. 117 Ali'iopc, 212, 251 Alrippe, 399, 423 Ai.cYONAiu/i';, 108, 117 Alcyondla, 261, 274 AlcyonuUum, 261, 274 Alcyouium, 108, 117 ALi,oi<»ca:LA, 133, 169 Alp/ieus, 412, 424 Amaroecium, 629, 689 Amceba, 15, 38 Amphineuka, 284, 363 Amphioxus, 608, 639 Amphipoda, 416, 424 Amphiporus, 167, 170 Amphitrite, 213, 251 AvqMura, 564, 592 Atnpullaria, 308, 364 Anubolia, 515, 527 Aiiachcftu, 218, 251 Anulgen, 453, 459 Anasii, 510, ,527 Anchorella, 397, 423 A)iel/», 518, 527 Aplynia, 313, 364 Apod A, 593 Api^ixliculiiria, 636, 089 ApHindiH, 414, 424 APTEt; luuiA, .wi, 526 -4j!JM«, 387, 423 648 €44 INDEX OF PROPER NAMES. ii AUACIINIDA, 435 AuANii.«, 448, 453 Arbacia, 580, 593 Area, 339, 365 Aveella, 16, 38 AUCIUANNKUD^, 211, 251 Archigetes, 161, 170 Aricia, 309, 251 ArenicoUi, 209, 251 Argiope, 271, 274 Argonauta, 359, 365 Argulus, 397, 423 Avion, 316, 364 ArmadiUidium, 414, 424 Artemia, 386 AuTHKoroDA, 368, 523 AUTUHOSTKACA, 413, 424 Ascavis, 48, 177, 182 AsciDiACE.K, 627, 639 Ast'ioi/K coMi'osiT^:, 627, 639 Asciui^: siMrucES, 627, 639 Asc'iiUiins, 617 Ascopodaria, 256, 274 ASCOTHOKACIDA, 403 Asellus, 414, 424 Asperg ilium, 329 Aspidiotits, 510, 527 Asplanchna. 189, 200 Asterias, 550, 592 Asterina, 552, 592 Asi'EKOiDEA, 552, 592 Asthenosoma, 570, 593 Asirangia, 114, 117 Astropectcn, 556, 592 Aslrophi/toii, 561, 593 Atalanta, 309, 364 JKaiC, 453, 459 AtropoK, 509, 526 ^«««, 451, 458 Aitrelia, 101, 117 AUTOFl-AGKLI-ATA, 28, 39 AiUolytus, 212, 251 Balanoglossus, 601, 639 Balamis, 400, 423 Baruiicles, 398 UA8OMMATt)PU0UA, 317, 364 lidellara, 136, 170 Beacliflea, 416 Bees, 518 Beetles, 512 Belemnites, 360 Iklostoma, 510, 527 i?,'?w, 121, 126 Bipalium, 136, 170 Z?M-5r«s, 412, 424 Blastoids, 550 Bolina, 124, 126 Bolteuia, 628, 639 Bombus, 518, 527 Bonellia, 241, 253 Jiook-seorpion, 444 Bopyrus, 415, 424 Bothriocephalus, 153, 170 Botvyllus, 631, 639 BntchiiiHS, 513, 527 Brachiontts, 189, 200 Bkachiopoua, 268, 274 Bkacuyuka, 412, 424 Branchellion, 228, 253 Buanciiiopoda, 885, 423 Bvanchiostoma, 608 Branchipus, 386, 123 Bkanciiiuka, 397, 423 Brisinga, 553, 593 Bnstle-tails. 501 Hrillle-stiirs, 561 BiiYozoA, 355 Biigi, 510 Bugula, ,262, 274 jB«;/«, 313, 364 i?«rt«W, 510, S ^'inndela, 513 QidariH, 530, j IJSDEX (^amponotus, 518, n^l Campylaspia, 408, 434 Cancer, 413, 424 G^mthocamptm, 806, 433 Capitella, 206, 251 Capretla, 416, 434 (Jm-abus, 518, 527 Garavdla, 92, 116 Cariiuiria^ 309, 364 Garinella, 106, 170 C'arpocapm, 516, 527 (iNATnA, 186, 200 ^iKHoHotus, 196, 200 C|h^:t()poda, 304, 351 ('/Kirybdea, 101, II7 ('fielifer, 443, 458 Oheriies, 444, 4,158 Vhllodon, 34, 39 OlIILofjNATHA, 483 C'lULopoDA, 484, 525 <'iiju)sToMATA, 262, 274 C'nrodota, .585, 593 ^''''ito/i, 389, 864 (^"litimelhts, 388, 364 fl'lumydomonas, 31, 39 ^''ondracnnthm. 397, 423 <-''iryaopa, 514, 527 ^^wififffi, 510, 537 <^<<-indeia, 513, 537 ft'/'*m, 530, 593 CiLIATA, 33, 39 C'lUiuiiPKDiA, 398, 433 C'l-ADOCKUA, 388, 423 (-''urn, 339 Clat/irulina, 18, 39 tVaoa. 87, 116 ClavelUna, 637, 639 Clej)sine, 336, 253 6Vitf/<«, 74, 1J5 CVi.Me, 814, 304 Clmocainpu, 5i6, 537 ('iymeneUu, 304 ' OlypeoMer, 581, 593 Cmi.kastuoioka, 581, 593 f-<'A««, 513, 537 ^'NIUAHfA, 76, 116 COCCID/K, 510 (^'OCCIDIA, 34 Cocci iiella, 513, 527 t'ockroiicli, 504 Codosifja, 28, 89 Cuil.KNTKKA, 68, 115 Cmloplmm, 135 Coi.Ecn'TKUA, 512, 527 COLLKMHOLA, 503, 526 CoUosHendeift, 464 Colpidiiiw, 37, 39 Volpoda, 36. 39 Coiivolutii, 182, 169 CoiMci'ODA. 393, 423 C'oial, 114 Coraltium, 108, 117 t'ouNAcusroNoiA, 73, 115 Cwophiiim, 417, 424 CoHUODKNTfA. 507, 526 GorycivuH, 396, 433 ' Corydali8, 514, 527 ^w^/ic, 87, 116 CoTYMiA, 139, 170 Cml)s. 412 Cmnia, 271, 274 Oniylislj, 413 Crickels, 504 CJiiNoiDKA, 541, 593 Cvisia, 274 CriHtateUa, 260, 274 CnusTACKA, 368 Chyptopkntameha, 527 645 646 INDEX OF PliOFER NAMES. Cryptophialus, 399, 423 C'jiYPTOTETUAMEHA, 527 Cteniza, 452, 458 CteiiodHlus, 58, 222 Ctenopiiora, 120, 126 Ctenoplana, 125 Ctenostomata, 262, 274 TUBOMEDUS^. 101, 117 Cucumaria, 585, 593 Vulex, 520, 528 CuMACEA. 407, 424 Cunina, 96, 116 Cunqctantha, 84, 116 CURCULIONID.E, 512, 527 Cuspidaria, ;i89, 365 Cyamua, 416, 424 Cyanea, 101, 116 Cyclnti, 3:39, L Cyclops, 396, -. ,!3 Cyclostoma, 30t ' 4 Cyclostomata, 262, 274 CymhuUopm, 314, 364 Cymothoa, 415, 424 Cytiips, 518, 528 Cynthia, 628. 639 Cyphopliihulmus, 447, 458 (Vp^vpa, 307, 364 Cypridina, 391, 423 C:y;»'is, 391, 423 C'ystoflagellata, 30, 39 Cystolds, 550 Cy there, 391, 423 Dactylopius, 510, 527 Daphnia, 388, 423 Daudebardia, 316, 364 Decapoda (Cepbiilopoda), 359, 365 Decapoda (Crustacea), 410, 424 i)emrf, 585, 593 Demodex, 453, 458 BendrociBlum, 136, 170 Dendrognster. 403, 423 Dentalinm, 323, 364 L'-rmnlekhus, 453, 458 Deumapteua, 504, 526 /fero, 218, 251 Desmosticha, 580, 593 Dindema, 580, 593 Diopheromera, 505, 526 DiantyliH, 408, 424 Dibkanciiia, 359, 365 Dicyema, 64 DiCYEMiu.*;, 64 Dideinnum, 630, 639 Difflugia, 16, 38 Dinobryon, 28, 39 Dinoflaoellata, 30, 39 DiiiophUuH, 198, :iOO JJiopaira, 212, 251 DioivcAUDiA, 305, 364 Diphyes, 92, 116 Z>«>?(!«a;, 506, 526 DiPLopoDA, 482, 525 Diplozoon, 147 DiPNEUMONEs, 452, 458 Diporpa, 147 DiPTEUA, 519, 528 DiscoMEDus^:, 101, 117 IJincosoma, 114, 117 Distaplia, 629, 639 DiSTOME^, 147, 170 Distomum, 147, 170 Dochmius, 177, 182 LoUolum, 635, 640 Uuiidermi, 287, 364 Z>o?'M, 315, 364 Boryphora, 513, 527 Dragou-flies, 506 Earwigs, 504 EcAuuiNEs, 269, 274 Echinarachnius, r81, 593 Echinocucuntis, 585 ECIIINODEKA, 184, 200 Echinoderes, 184, 200 EciIINODEUMA, 531 EcHiNoiuEA, 570, 592 Echinorhynchus, 180, 18S EciiiuuE,*;, 240, 252 Echinrns, 240, 252 EcTOPuocTA, 257, 274 Edriophthalmata, 413 Edwardsid, 117 EowAKDsi^:, 109, 117 Elasipoda, 585, 593 Er.ATEKin.K, 513, 537 Elpidi Endo] Ensate Enteh Entom Entoni Epeira, Epherru Epheme Ephyda ErgasUi Eruani Esperell, Estheria Eucope, EUCOPEI Eudendi\ Euglena, Euglypha EULAMEI EnNEMAl Eupagun Euphausu Euplectelli EuRYALrn Eurylepta, JSurypaui't EURYPTER EuTypterus EURYSTOM] Eiiscorpius, Empongia, Evadne, 38fe EacelUna, 3 Eilaria, 177, FiLIBRANCH Eiona, 311 Fissurella, 3( Elagellata I'^Jies, 519 Eloscularia^ ^''lustra, 261, EoHAMr]snFEp ^orficula, 50J Formica, 518, Fredericella, 2 ^nngia, Hi, 1 IN3BX OF PROPER NAMES. Elpidia, 585, 593 Endoprocta, 256. 274 Enmtella, 339, 365 Enteropneusta, 601, 639 Entomostraca, 385, 423 Entoniacus, 415, 424 Epeira, 450, 458 Ephemera, 504, 526 Ephemeridce, 505, 526 Ephydatia, 73, 115 Evgasilua, 396, 423 Erkantia, 211, 251 Esi)ereUa, 75, 115 Estheria, 387, 423 Eucope, 86, 116 Eucopepoda, 396, 423 Eudendriutn, 93, 116 Euglena, 29, 40 Euglypha, 16, 39 EULAMELLIBRANCHrA, 339, 365 EUNEMATODA, 174, 182 Eupagurus, 411, 424 Euphauaia, 406, 424 Euplectella, 74, us EuRYALiDA. 569, 592 Eurylepta, 139, no Eurypauropus, 482, 525 E UR YPTERID^, 433 Eurypterus, 433 EuRYSToME^j, 125, 126 Euscorpius, 443, 458 Euspongia, 73, 115 ^i'ttcJwe, 388, 433 FacelUna, 315, 364 Eilaria, m, iS2 PlUBRANCHIA, 339, 365 Eiona, 311 Eissurella, 305, 364 Elagellata, 28, 39 Elies, 519 Elosculana, 189, 200 Eliistra, 261, 274 EoRAMlytFERA, 15, 39 Eorficula, 504, 526* Formica, 518, 527 Eredericella, 260, 274 Eungia, 114, 117 647 Galeodes, 445, 458 Oamasus, 453, 459 (^arnmarua, 416, 424 Gasteropoda, 293, 364 (iaateiopteron, 313 Oecarcinua, 412, 424 Oelaaimua, 412, 424 Geometrid^, 517, 527 Geophilua, 484. 525 Gepiiyrea, 237, 253 (^eryonia, 85, il6 Oibbocellum, 447, 458 Olobigerin", 17, 39 Olomeria, 483, 525 Gnathobdellxd^, 236. 251 l^onactinia, 111, 117 Oonodactyhta, 409, 424 Oonoleptua, 448, 458 GoRDiACEA, 178, 183 Gordiua, 178, 183 Gorgonia, 108, I17 Grantia, 73, 115 Giasslioppeis, 504 Gregarinida, 24, 39 Gromia, 16, 39 Gryllotalpa, 504, 526 Gryllua, 504, 526 G'j^jfte, 136, 170 Gyainol/emata, 261, 274 Gymnosomata, 314, 364 Gyrinna, 513, 527 Gyrodactylua, 14 7, 179 Il'vmenteria, 236 Ilalcampa, 113, II7 Halecium, 87, 116 HalioUa, 305, 364 lialiaarca, 74, 115 Ilnlohates, 510, 527 Halorypris, 391, 403 Halodrilm, 227 Harpacticus, 396, 423 ■'^'rpahia, 513, 527 Harvest-men, 448 Harvest-mite, 453 Harvest-spider, 447 JIdiopora, 109 Uelioaphmra, I9, gg ^, ^> # .v. %^, i^vi <6. ^ IMAGE EVALUATION TEST TARGET (MT-3) 1.0 I.I 11.25 - 6' Photographic Sciences Corporation 1^ 1^ 1 2.2 If 1^ 12.0 118 1.4 ill 1.6 4^^ ^^U as WIST MAIN STRUT WSBSTM.NY UJiO (?-)6) •7a-4&03 •P 4. toVW x$> ^ ^ 648 INDEX OF PROPER NAMES. Heliozoa, 17, 39 Helix, 316. 364 Hemichokda, 596, 639 Hemiptkka 510, 527 Hermit Crab, 411 Heaione, 207, 251 Heterodera, 176, 183 Heteuomeua, 527 Iletei-onereis, 216 Heteropoda, 309, 364 Heterotuicha, 40 Hexactim^, 113, 117 Hexarthra, 194, 200 Eippa, 411, 424 Hirudinea, 228, 251 Hirudo, 236, 251 Uolopus, 541, 592 Holothuria, 585, 593 HOI.OTHUROIDEA, 584, 593 HOLOTRICHA, 40 Homariis, 412, 424 HOMOPTERA, 510, 527 HOPLONEMERTINI, 167, 170 Horseshoe Crab, 428 HVALOSPOl.GI^. 74, 115 Ilydra, 58, 83, 116 Hydrnchna, 453, 459 Uydractinia, 58 87, 116 Hydrari.*:, 83, 116 Hv'DUOCORALMN^., 89, 116 Hydromedus/E, 78, 116 Hydrometra, 510, 527 UydropJiilus, 518, 527 Hymenoptera, 517, 527 Ilyocrinua, 542, 592 Hypotriciia, 40 lanthina, 807, 864 ma, 401, 428 Ichneumon, 528 Ii'hthydium, 196, 200 IcIMyolHleUa, 228, 252 Tdotea,iA\ 424 Idyia, 125, 126 Infusoria, 38, 40 Inbkcta. 487, 52C /«>. 108, 117 IsopoDA, 414, 424 lulus, 483, 525 Ixodes, 453, 459 /awM«, 311, 364 King-crab, 428 Kolga, 589 Labia, 526 Lacmularia, 189, 200 Lam::li.ibranchia, 326 Lampyris, 513, 527 Larvacea, 626, 639 Laura, 403, 423 Leiobunum, 448, 458 Lepas, 399, 423 Lepidonotus, 212, 251 LEPiDOPTEr,A, 515, 527 Lepisma, 502, 526 Lepiodiscus, 3C 39 Leptodora, 418 Leptogorgia, 108, 117 Leptomedus^, 85, 116 Leptoplana, 139, 170 Leptosthaca, 404, 423 Lernom. 397, 423 Leucosolenia, 73, 115 Libellula, 506, 526 Libinia, 412, 424 Ligula, 152, 170 Limacina, 814, 364 Limapomia, 315, 864 Limax, 316, 864 Limnadia, 387, 423 Limuaa, 316, 364 Liinnetis, 887, 428 Limulus, 427 IJnguln, 271. 274 LioVieum, 509. 526 Liriope, 85, 116 Lithobius, 485, 525 Lobster. 412 /x>%o, 359, 865 Lopfiopus, 261, 274 Lo^rosorna, 256, 274 l.ucei'naria, 100, 116 Zuci^fr, 411, 434 iT.T/iV^ui, 568, 692 LUM Lum Lycoi Lysic Lysio Lyttu Macrc Macro Macr< J*Iacim Mad I e^ Malucc Malac Malac AfALAC( Malac< Malloj Margeli, May-fly, Melicertt M-llita, Meloe, 5] Melolonti Melophai Membran Mernm, ] Mertensia Mesostovu Mesozoa, Metazoa, Metridium Mierogaxle Microgrom MrcHoLEp Microstoim Miliola, 16, Millcjxira, i ^'illipodes, Elites. 453 Mnemiopttia^ Modiolaria, yfoina, 388, ^foira. 588, '^(olguld, 62f Molpi.X OF PROPER NAME8. 649 Monostomum, 147, 170 MoNo'iOcARDIA, 306, 364 MoHoius, 134. 169 Aimeria. 585, 593 'l/Msca, 020. 5^i8 Musse), 339 ^y<'ypodn, 412. 434 ONATA, 506, 536 650 INDEX OF PROPER NAMES. Oligoch^ta, 218, 251 OinmastrepJm, 359, 365 Onchidium, 316, 364 Oniacus, 414, 424 Onychophora, 475 Opalina, 36, 39 Ophiacti8, 566, 592 Ophioderma, 561, 592 Oplmlepia, 567, 592 Ophiomyxa, 561, 562 Ophiothrix, 564, 592 Ophiura, 592 Ophiubida, 569, 592 Ophiuroidba. 561, 592 Opilio, 448, 458 OPISTHOBRANCHIA, 310, 364 Oractis, 111, 117 Orgyia, 516. 527 Oribatea, 453, 459 Orthonectida, 65 Orthoptera, 504, 526 OSTRACODA, 391, 428 Ostrea, 339, 365 Oxyuris, 177, 183 Oyster, 339 Palmmon, 413 Palmmonetes, 412, 484 Pal^kocrinida, 551 Pal^onemertini, 166, 170 Palinurua, 421 Paludicella, 261, 274 Paludina, 807, 864 Palythoa, 112. 117 Pandartta, 396, 428 Punorpa, 514, 527 Panorpata, 514, 537 Papilio, 527 Paranmcium, 85, 89 Patella, 305. 864 Pauuopoda, 481, 535 Pauroptia, 482, 525 PiiUa, 409, 424 Staphylinid^e, 512, 537 Starfish, 553 STAUROMEDUSiE, 100, 116 Stentor, 35, 40 Sternaspis, 343 Stomatopoda. 408, 424 Sloinclophus, 101, 117 Slouu-aies, 507 Stroml/ua, 307, 364 Strongylocentroiua, r)80, 593 Strongylosoma, 184, 520 Stylaster, 90, 116 %;eWrt, 314, 364 Siylochus, 141, 170 STYLOMMATOPnOUA, 318, 364 Stylonychia, 40 SucToiUA, 30, 40 Sim-animnlcule, 17 Syllia, 313, 351 Symphyla, 48C, 536 Synapta, 633, 040 Synccelidium, 186, 170 Tabanus, 520, 528 Tmnia, 153, 170 Tanaia, 414, 424 Tanysiylum, 463 Tardigrada, 466 rc«Jja, 113, 117 TECTIBRANCHfA, 313, 364 Tegenaria, 45(t, 458 7e/«i, 517, 537 Tentaculata. 134, 126 TfTPhella, 213, 351 Terebkantia, 537 Tevebratulina, 371, 274 Teredo, 839. 365 Tfe/'me.'i, 508, 526 Termites, 507 Teaa&ra, 100, 116 tk8ticahdine8, 269, 274 Tetrabranchia, 857, 365 Tetranychua, 454, 459 Tetrapnettmones, 452, 458 Tetraatemma. 167, 170 Textularia, 17, 89 Tualaaaema, 241, 253 niamsaianthua, 114, 117 INDEX TliaUmicolla, 18, 39 Thaliacea, 633, 640 Thaumaiocrinus, 543, 593 Thecasomata, 314, 364 Thelyphonus, 446, 458 Theridium, 450, 458 Thohacostraca, 406, 434 Thripa, 510, 536 Thyone, 587. 593 Thtsanopteka, 509, 536 Thysanozoon, 139, 170 Thtsanura, 501.' 636 Ticks, 453 Tinea, 516, 537 Tracheata, 469 Trachydermon, 289, 364 Trachymedus^, 85, 116 Trematoda, 143, 170 Tremoctopua, 859, 365 TrianopJm-ua, 153, 170 TricJiaater, 569^ 593 Trichina, 176, 183 TrichoeepJialua, 177, 183 Trichodectea, 509, 526 TricJioplax, 63 Trichoptera, 515, 527 Tricladea, 136, 169 Triatomum, 147, 170 Trodmphcera, 194, 300 Troc/iua, 305, 864 Trombidium, 453, 459 'Jubipora, 108, 117 Tubulana, 89, 116 TUBULARI^. 87, 116 TUNICATA, 617 OF -mopeh names. \ TURBELLARIA, 130, 169 Turbo, 305, 364 Tyroglyphua, 453, 459 Unio, 339, 365 Urnatelia, 356, 374 Urochorda, 617, 639 Vaginula, 316, 364 Vampyrella, 23. 39 Vanessa, 517, 537 Velella, 93, 116 Vemis, 339, 335 Venus' ginJle, I3i Vermilia, 309 Veapa, 518 Fb^ioa;, 30, 33, 40 Vortex, 135, 169 Vortieella, 34, 40 Waldheimia, 271, 374 Walking Stick, 504 Wasps, 518 WLeel-auimalcule, 189 WLite ants. 507 Wood louse, 414 Xiphosura, 437 Toldia, 339, 365 ZOANTHE^, 118, 117 Zoanthus, II3, 117 ZOOXANTHELL^, 80 Zoroaster, 553, 593 668 »." 605; Ecbinoldea, 579; Qep^yZ', ~a9; Polycbaeta, 207 flconous eyes, 473 Actinotrocha, 249 ^iil rectal gland, 305 adliesive cells, 133, 131 alar muscles, 470 albumi-jiparous gland, 312 tocla, 176; Scyphoniedusffi, 103- Trenmloda, 148; Urochorda 633 amitosis, 9 ' ^°° ametabolic insect?, 499 amoeboid motion, 15 ampbiaster, 11 ampbidiscs, 76 ampulla, 537 an ten naiy gland, 383 apical plate, 213, 607 arclientcrou, 54 archiceiebrum, 379 Aiistotle's lantern, 578 artbrobraucbia, 410 ascidiozooid, 632 Ascon, 70 aster, 7 ^""•"•". 596; Oepbnlocborda. 612; En leropneusta tiOi- v>, i. r.09 600. IT , •, ^'^'•"branchia, ■>•'», 000; Urocborda, 622 atrnim (genital), I34 aiiiiniias, 573 Auricularia, 590 aviciilaii-i^ ggg axial sinus, 538 INDEX OF SUBJECTS. Basement-membrane, 127 biogenetic Jaw. 143 Bipinnaria, 559 Wviuin, 572, 584 blasfoccBl, 62 blastopore, 54 bbislula, 52 blo^-vascular system. Arachnida 437. Cepbalocborda, 611; Cepbalopl' 346; Cbaelopoda. 206. 220- C^!' cea, 376; Enteropueusta eo^ « '? ' yrea. 238; Hirudinea 2^ M i?'^**" 278; Nemertina 165 P, "^'"''"• 247- T,^ ., ' Pboron dte. 621 xTr "'"• ''"' Urocborda O'^i , Aipbosuia, 429 Bojanus, organ of, 837 tfrachiolaria, 559 brancbial heart, 347 bmncbial skeleton, Cepbalocborda. oirf, Enteropneusta, 609 brown body, 267 brown canal, 616 bursa copulatrix. Nematoda. 174 182- Tracheata, 497- T^^^^.^^^ • ' 138 Juibellana, 136, byssus gland, 329 Calamistrum, 449 f^alcar, 192 calciferous glands, 220 calyptoblastic, 86 caryolympb, 6 caryoplasm, 5 cell, 4, cell-division, 9 cellulose, 30, 619 cenogenetic, 143 central cai>sule, 19 ceutrolecitbal, 63 centrosome. 7, 51 cepbalization, Sflo 655 656 INDEX OF SUBJECTS. ceratn, 815 Cercana, 150 cerci, 489 chambered organ, 546 chela. 373 cheliceroB, 439, 435 chiiisloueuiisin, 296 chilaria, 480 chloragogue-cells, 219, 238 chlorophyll, Flagellatu, 30, 31; Hy- drariae, 83; lufusoria, 35; Porifern, 78 chordotonal organ, 495 chromatin, 6 chromosome, 11 chrysalis, 500 cilia, 33 cilia-plates, 121 cirrus, 146, 155 cirri, Cephalochorda, 609; Crinoidea, 542; Myzoslomeae, 244; Polychoeta, 204, 205 clavuloe, 574 clitellum. 219. 228 cloaca, Nemutoda, 175, 179; Rotifera. 192; Urochorda, 631 cnidocil, 77 cnidobiast, 77 coelenteron, 77 poelom. 57 coeneuchyme, 108 coeiiosarc, 79 Ccenurus, 158 colony-formation, 5, 8; Anihozoa, 108, 111, 112. 114; Flagellar, 30; Hydro- medusoe, 78, 85, 87, 91; Polyzoa, 255; Rhizopoda, 21; Urochorda. 628 columella, 90, 107 compiemental males, 401 conjugation, 24. 25, 32, 37 contractile vacuole, 15 corallum, 89 cormus, 41 costse, 107 coxal glands, Arachnida, 441; Xi- pbosura, 432 crlbellum, 449 ciural glands, Insecta, 502; Myriop- oda, 485, 487; Protracheuta, 4:9. crystalline style, 333 ctenidium, 278 Cuvierian oigiins, 588 cynthozooid, 632 Vyphonautes, 264 C'ysticercoid, 158 CysHcercus, 158 cytode, 8 cytoli mph, 4 cytoplasm, 4 Daclylozoid, 90 delaniinution, 55 Desor's larva, 167 deutovnin. 456 development, Acaniliocephala, 182; Acariiia, 456; Asleroidea, 559; Brachiopoda, 272; CVplialopoda, 36->; Cestoda, 157; Criuoideii, 551; Crustacea, 417; Echinoidea, 583; Enleropneusta, 605; Gasteropoda, 819; Gephyreu, 242; Hirudinea, 237; Hoiothuroidea, 590; Hydro- medwste, 92; lusecta, 521; Nema- toda, 176; Nemertina, 167; Oli- gocliteta, 225; Ophiuroidea, 570; Pelecypoda, 339; Pentastomidoe, 463; Phoronidfle, 249; Polychteta, 213; Polyzoa, 263; Porifera, 74; Pycnogonida, 466; Scaphopoda, 324; Scyp} omedusaj, 103; Trenia- toda, 148; Turbeilaria, 140; Uro- chorda, &2\\ Xiphosura, 432 digestive gland, Arachnida, 437; Brachiopoda, 271; Crustacea, 378; Mollusca, 880; Rotifera, 192; Xiphosura, 430 digestive system, Amphineura, 28(1; Arachnida, 437; Brachiopoda, 270; Cephalocorda, 612; Cephaloiioda. 348; Choetognatha. 187; Chujtopo- da, 206, 220; Crustacea. 377; Di- nophiliis, 198; Ecliiuodera, 185; Echinoderma, 539; Enleropneusta, 605; Gasteropod -, 300; Qastrotricha. 196, Gepliyrea, 238; Hirudinea, m INDEX OF SUBJECTS. 231; Mollusca, 279; Myzostome*. ?Ao' i^f™'''^^'"' 174; Nemertina. lOv; Pelecypoda. 333; Peuiastomi- doB. 461; Phorouidee, -47; Polyz,oa. 205; Pycuogoiiida. 465; Kotifera. 191; ScapLopoda. 323; Turdigruda. ?!!' J""'"'''^'"' ^'^= Trematoda. 144: Tuibellaria. 133, 135. 136. 138; Urochorda. 62-J; Xiphosura. 430 dimorphism, sexual, 193, 199 241 395.496 ' ' dimorphism, seasonal, 501. dissepiment, 107. 187, 202. 270 dissogoiiy, 123 divisio..oflabor,85, 87, 91 (see also polymorphism) docoglossate dentition, 306 dorsal organ, 546 dorsal pore, 219 Echinoeoecxis, 158 ectocyst, 255 ectoderm, 54 ectoplasm, 3 elseoblast, 633 elytra, 490 embole, 54 eiicystmeiit. 22, 37 eiidocyst. 255 endoderm, 54 endophragmal system, 375 endoplasm, 3 endopodlte, 373 eudosteruite, 429, 457 endosfyle, Cephalocorda, 614 Uro. chorda. 622 enterocoel, 57 epliippium, 391 Ephyra, 103 epibole, 54 epipleural folds, 609 epipodite. 373 epipodium, 291 epistome, 247. 260. 597 epithelio-mnscid.ir cells, 80 Erklithus, 422 euconous eyes. 473 excretory system, Acanthocepbala. 657 !*i' ^™P^'«eurtt,287.292; Arach- nida,437;Brachiopoda.273;Cepha. locorda. 615; Cephalopo.ia. 853; Cestoda. 155; Chujtopoda, 210, 222- Crustacea. 383; Dinophilus, m- Ijchmodera, 185; Enteropneusta,' 605. Gasteropoda, 302; Gasterotri- cha,196;Gephyroa.239;Hirudinea, £ Mollusca. 283; Myzoston.e*. 245; Nematoda. 175; Nemertina. a!^' ?.f "'^P«''«' 337; Phoronid*. 249; Platyhelminthes. 129; Polv- fioo ^^'•J''^'' ^^^•■«^'-««ehia. 599. 600; Rot.feru. 193; Scaphopoda,324; Iracheata. 474; Trematoda, 145' Turbelhiria, 135, 138; Urochorda.' o«3; Xiphosura. 432 exopodite. 373 eyes-Amphineura. 292 ; Arachnida. 438; Asteroidea, 559; Cephalochor- aa. 615; Cephalopoda. 351; Chteto- g'latha, 188; Chaetopoda, 208; Crus- tacea. 380; Gasteropoda, 301, 318- Hirudinea. 234; Hydromedusffi. 82- Nemertina. 164; Pelecypoda, 335- Pycnogonidn. 465; Rolifera. 192' ScyphomedusiB, 99; Tracheata, 472- Tui-bellaria, 131; Urochorda, 625- Xiphosura, 431 Fascioles, 574 fat-body, 492 flagellum. 28 flame-cell. 129 follicle-cells. 46 fossa rhomboiilalis. 615 funiculus, 259 Gaslerozooid. 89 gastrula, 53 gemmation, 22 gem mules, 75 genital bursae, 561 germ-cell, 44 germ-layer. 54 OlocJiidium, 339 Goette's larva. 141 gonopolyp, 85 668 INDEX OF SUBJECTS. gouotheca, 86 greeu-gliuid, 888 gymuoblastic, 87 Htemocyauiu, 278, 377, 429 haemoglobin, 877, 588 hsemolyiupb, 206 halteres, 520 head-kidney, 214, 222 heart— Arapbiueura, 289; Arachnida, 487; Cepbulopoda, 346; Crustacea, 376; Gasteropoda, 298; Pelecypoiia, 332 ; Pycnogouida, 465; Tracbeatii, 470; Urochorda, 621; Xiphosura, 429 Hectocotylua, 856 hemimetabolic insects, 500 herinapbroditism, 44 heterogony, 60, 148, 498 histolysis, 456 holometabolic insects, 500 hook-gland, 462 hydrant h, 79 hydrocaulus, 79 bydrocoel, 535 hydrorhiza, 79 bydrotheca, 79 bypermetamorphosis, 513 hypodermis, 174 hypophysis cerebri, 615, 623 hypostome, 79 Imago, 500 immigration, 55 individuality, 41 ink-bag, 349 interteutacular organ, 260 invagination, 54 Karyokinesis, 9 Keber's organ, 337 Lacunar system, 538 languets, 622 lateral-line organs, 210, 222 Laurer's canal, 146 lemnisci, 181 Lencoti, 71 linin, 6 liver, 614 lopltopbore, 247, 254 lung- books, 436, 457 Madreporiform tubercle, 586 madreporite, 586 malpigbian tubules — Ampbipoda, 417; Arachnida, 487; Tracbeata, 474 mantle, 268, 276, 621 manubrium, 81 mustax, 191 Medusa, 77, 80 ; Craspedote, 81 ; Gymuophthalmatous, 82 ; Ocel- late, 82, 89; Vesiculate, 82 megaloestbetes, 292 Megdlopa, 422 mesendoderra, 68, 132 mesenterial filaments, 99, 105 mesentery, 57, 104, 179, 187, 206, 270 mesoblasts, 57, 214, 225, 237 mesoderm, 56 mesoglcea, 68 mesopodium, 296 mesothorax, 488 metagenesis, 60 metamore, 41 metamerism, 43 metamorphosis of insects, 499 Metanaxiplius, 418 metapodium, 296 metathoiax, 488 Metazoea, 421 microestbetes, 292 micronucleus, 35, 38 microsomes, 4 mitosis, 9 Morren's gland, 220 Mllller's larva, 141 muscular system — Acantbocepbala, 180; Amphineura, 286, 289; Anllio- zoa, 166; Brachiopoda, 270; Ceplm- locborda, 610 ; Cepbalopoda, 348; Cestoda, 154 ; Cbajtognatba, 187 ; Chaetopoda, 205, 219; Crustacea, 875; Dinophilus, 198; Ecbiiiodera, 185; Ecbinoderma, 585; Gasleropo- INDEX OF SUBJECTS. da, 298; Qastiotiiclin. 196; Hirudi- ^^ 329; Insecla. 492; Nenmtoda, ', • ^^°'^ Ptilecypoda. 882; Tia- clieata, 469 myotOBi, 610 myopliaues, 36 659 Natipliui, 417 neciocalyx, 9l Needham's pouch, 855 nematocyst, 77 nephiidia (sec Excretory System) uephroblasts, 326, 287 nervous system _ Acautl.ocephala, i81: Amphmeura, 287. 290; Aiach- DJda. 487 ; BiacliiopodM. 271 • Ce- Phalochorda. 614; Cephalopoda. 850:Cestoda. 155; Cha^tognatha. 187; Choetopoda. 208, 221 ; Crusta- cea, 378; Ctenophom, 124; Binophi- ins, 199; Echinodera, 185 ; Echiiio- derma. 589 ; Enteropnensia, 605 • Gasteropoda. 800; Gastrotricha, 197- Gephyrea. 239; Hirudiuea. 232- Hy* dromedus8B, 80; Molhjsca, 281 • My. zostomeaB.244; Nematoda. 175. 179- Nemertina. 163 ; Pelecypoda, 834;' Pentastomida,. 461; Phoroi.id(B,247- PIatyhelminthes.l'>8; Polyz<,a.257;' Ponfera. 73 ; Pterobrauchia. 599. 600 ; Pycnogonida. 465 ; liotifera. 192; Scaphopoda. 823 ; Tracheata. JJ|'J''«'"a'«da, 144;TurbeIlaria. lol, 133, 137 ; Urochorda, 623- Xi- phosura, 430 neuroblasts, 226, 237 nidamental gland. 812, 355 notochord — Cephalochorda. 612 • Entero'.-neusta. 603; Pterobranchia' 599, 600; Urochorda, 625 nuclein. 3 nucleolus, 6 nucleus, 5 nymph, 456 Scj^phomedusoB, lOO ; Tracheata. 47^; Xiphosura, 482 omaiatidium - Ch^topoda. 309- Crustacea. 881; lusecta. 472- Pele- cypoda. 387; Xiphosura. 431 ooBcia. 263 ootyp, 146 operculum-Gasteropoda, 296- Polv- zoa, 262; Scorpiouida, 442; Xipho- sura, 429 organ, 41 °'"fjr f ^^^J'^""^' 337 ; of Cuvier, Oo8; of Stewart, 576 orthoueurism, 310 osculum, 69 osphradium, 283 otocysts-Chaetopoda, 209; Crusta- cea, 888; Cteiiophora. 122; Holo- thuroidea,589;Hydro.ne,lu8ffi,82. »4, 85. 86; Mollusca. 283 ; Scypho- medusae. 99; Turbellaria, J31, 132, 184; Urochorda, 625 ovary, 44 ovicell, 268 ovum, 44 ; Fertilization of, 49 • Ma turation of. 46; Segmentation of 51 Odontoblasts. 280 olfectory organ-Cephalopoda. 853. Chaetognatha, 188 ; Mollusk. 282 • PflBdogenesis, 60, 499 pali. 107 palpi. 205 parapodia. 204. 313 paratroch. 213 parenchyma, 128 Parenchymelia, 55 parthenogenesis. 60. 498 paxillsB. 553 pectines, 442 pedicellariae. 574 pedipalps. 435 Pentactma, 591 pereiopod. 410 pericardial glands, 298, 337. 345 pericardium, 278, 437 ptirisarc, 79 peritoneal cells. 205 phaosphere, 489 phosphorescence. - Crustacea, 382; 660 INDEX OF SUBJECTS. Cystoflagellatii, 32; lusecta, 492; Urochordu, 682 phruffiiiocoue, 360 Phyllosoma, 421 PHidium, 168 piunules, 542 plasome, 48 plastiu, 3 pleopod, 410 pleiirobrancLia, 410 Fluteus, 570 pueuumtophore, 91 podobniuchia, 410 polii'- bodies, 46, 49 Polian vesicle, 536 pol^p, 76, 78 " polypiue, 255 polymorphism, — Ahyouariae, 109; lusecta, 497, 500, Polyzoa, 262 (see -ilso divisiou of laboiy polyspermy, 50 porlril systeai, 612 proboscis, — Acautbocepliala, 180; Gasteropoda, 300; Myzostomea-, 244; Nemertina, 163 proglottid, 154 proOstracou, 360 propodiiim, 296 profjopyle, 71 proslonuum, 218 prothorax, 488 protopla&m, 2 protcpodit; 573 prototroch, 213 Protizotd, 419 p8et:;Iot^Iiiria, 33 pseudouaviceila, 25 i.aeudopodlum, 14 pteiioglossjite dentition, 308 pupa, 500 Uachi^dossate dentition, 307 radula, 273 Jfedin. 1 19 regeneration, R9 reproduction —Flagellata. 32; Inftiso- ria, :}«; Mciazoa, 42; Myxosporidia, 87; Ubizopoda, 20; yporozon. 25 leproduciiou,— by budding, 22, 08, 71, 83. m, 114, 215. 256, 266, 627*; by conjugation. 24, 25, 31. 37; by divisiou, 21, 37, 58, 114, 227: by spore - formation, 32, 25. 82, 37; «exunl, 44 reproductive system.— Acantbocepb- ula, 181; Ampbiiieura, 287; An- tbozoa, 105; Aracliniibi, 44) ; Rrach- iopoda, 272; CcphalochorUa, 617; Cephalopoda, L54; Oestoda. 155; Cliieutgnalha. 188; Chuetopoda, 211, 223; Crusli.cea, 384; CliMio- pbora, 123; DinophiluH, 199; Ec;lii- uodera, 185; Echiuoderina. 540; Gasleropoda, 302, 305, 311. 318; Gasuotricha, 197; Gepbyrea, 240; Hirudinea, 235; Hydromedusa;, 83, 85, 86; Myzostomea;, ::;46; Iseuiii- toda, 175, 179; Ne.nerlina, ](jG; Pe'.ecypoda, 337; Penlasloniida', 462; Platybelminthes, 129; Poly, zoa, 257, 260; Pterobriincbiii, 6U0; PycnoL-onida, 466; Rotifcra, 193; Scypbomedufiw, 98; Traoheata, 474; Trematoda, 146; Turbellaria. 133, 134, 135, 187, !39; Urocliorda, 623; Xipbosura, 48£ rt'piiguatorial glands, 488 respiratory system.— A.nneliila, S-^M; Aracbnida, 436; Asteroidca, .^4; CephrJochorda. 612; Cephalopoda, 343; Cnstacea 875; Ecliinoidea, 576; Enteropneusta, 601; GastiT- opoda. 297, 217; Mollusca, 278; Pelecypoda, 329; Plerobrancbia, 599, 600: Tracbiala, 470; Uro- chorda, 622; Xiplion-ia, 429 re8|)iratory trees, 240, 5f(8 Rhabditia, 131 rhipidoglossate dentition, 806 ./Mtollum, 164 Saoculi, 548 salivary glands, 280, 498 acapboguathite, 410 schizocd'l, 87 foolvx, 158 INDEX OF SUBJPJCm 661 Scyplmtoma, 103 seasonal diiiiojpliism, 501 Semites, 574 septa, 107 setae, 204, 218 seUi-sucs, 304 sexupl dimorphism, )93. 199 241 395, 4f>6 ' ' sbell.-Amphiiieura, 289; Bracbiop. oda, 269; Cephalopoda, 343, oo7, 360; Gasteropoda, 303, 316; IVle- cypodrt, 327; Scaphopoilu, 3^2 shell-glaud, 383 siphou, 304. 827, 493, 579 siphouoglyplie, 106 sipuucle, 358 skeleton,-Ceph(.]ochorda. 613; Enle ropiieusta, 609 somatic cells, 44 somatlo mesoderm, 206 sperinalid, 48 spermatocyte, 48 spermatogenesis, 48 spermatogone, 48 spermatophore, 355 epermatozoa, 44, 47 splia'ridia, 574 spimiju-.gjands, 449 splaiichnic mesoderm, 206 splauchnocoel, 610 spongiolin, 72 sporocyst, 140 statohlasi, 261 Slerrula, 6G Stewart, organs of, 576 stigma, 80 8tigmata,-AracImi(i„. m- Tra cbejifa. 470; U.ochorda, 682 fitomalod.i'um, 105 etoniodoiim, 218 stone- caual, 536 strobila, 104, 154 subiieurai gland, 623 Sycon, 71 symbiosis, 20, 83 syucerebrum, 379 Tffiuioglossate dentition, 307 tapetum lucidum, 836, 440 telolecithal, 53 teJson, 369 testis, 44 thorax, 488 Tiedemaun's vesicles, 557 tissue. 41 Tornaria, 606 toxiglossate dentition, 308 414; rracheata, 470 tracheal brauchia', 491 trichocyst, 35 trivium, 572, 584 Trochophore, 213 trophopolyp, 85, 91 tube-feet, 587 tympanal organ, 498 iyphlosole, 220, 623 Veliger, 320 vehjm, 81, 820, 612 ventral plate, 226 vibnicula. 263 vitellarium, 180, 155, 193 Waler-vascular system, 685 wax-glands, 490 wings, 489, 623 Zo9a, 420 zorecium. 255 zooxanthellte, 20