STUDIIOS ON THE ARACHNID ENTOSTEBNITE. 225 Studies on the Arachnid Entosternite. K. I. Pocock. With Plates 13 and 14. THE investigations here recorded were set on foot in the first instance with the purpose of settling certain contra- dictions as to matters of fact in the extant descriptions and published ligures of the entosternites of various Arachnids, which a preliminary dip into the literature revealed. It was necessary to ascertain whether these discrepancies were attributable to a natural variability in the organ, to specific or generic differences between the species dissected, or to errors of observation on the part of the dissectors. In some cases, too, there was an entire lack of agreement on the part of observers in the interpretation of the facts recorded ; and the suggested homologies between the constituent parts of the entosternites of various species did not, on a priori grounds, appear to be in all cases satisfactory. I was anxious, moreover, to test the respective claims to recogni- tion of the two theories of the origin of the entosternite that have been put forward. I have made no examination of this organ in the Palpi- gradi, Pseudoscorpiones, Podogona, Opiliones, or Acari, and have nothing to add to what has already been said about the entosternite of these orders. The contents of this essay, which deals exclusively with the remaining existing orders, may be tabulated as follows : VOL. 4 6 , PART 2 . NEW SEKIES. P
Transcript
STUDIIOS ON THE ARACHNID ENTOSTEBNITE. 225
Studies on the Arachnid Entosternite.
K. I . P o c o c k .
With Plates 13 and 14.
THE investigations here recorded were set on foot in thefirst instance with the purpose of settling certain contra-dictions as to matters of fact in the extant descriptions andpublished ligures of the entosternites of various Arachnids,which a preliminary dip into the literature revealed. It wasnecessary to ascertain whether these discrepancies wereattributable to a natural variability in the organ, to specificor generic differences between the species dissected, or toerrors of observation on the part of the dissectors. In somecases, too, there was an entire lack of agreement on the partof observers in the interpretation of the facts recorded ; andthe suggested homologies between the constituent parts ofthe entosternites of various species did not, on a p r io r igrounds, appear to be in all cases satisfactory. I wasanxious, moreover, to test the respective claims to recogni-tion of the two theories of the origin of the entosternitethat have been put forward.
I have made no examination of this organ in the Palpi-gradi, Pseudoscorpiones, Podogona, Opiliones, or Acari,and have nothing to add to what has already been said aboutthe entosternite of these orders.
The contents of this essay, which deals exclusively withthe remaining existing orders, may be tabulated as follows :
VOL. 4 6 , PART 2 . NEW SEKIES. P
226 V. I. POCOCK.
I. The structure of the entosternite in the Xiphosuras,Scorpiones, Pedipalpi, Armiete, and SolifugaD.
1. The entosternite of the Xiphosurte, p. 226.2. ,, ,, „ Scorpiones, p. 227.3. „ „ „ Pedipalpi, p. 281.4. „ ,, „ Araneae, p. 233.5. The "so-called" entosternite of the Solifugse,
p. 237.II. The comparative morphology of the entosternite,
p. 239.III. Theories of the origin of the entosternite, p. 247.
I. STKUCXCKE OP THE ENTOSTERNITK IN THE XIPHOSURJE,
ScORPJONISS, PjEDIPALPI, AttANEiE, AND SoLlFDG^.
1. The Entostei-nite oE the Xiphosuras.
The form and structure of the entosternite in the AmericanLimulus is well known, thanks to the figures and descriptionsof it published by Ray Lankester (5, 6) and Benham (2).
It is a longitudinally oblong plate, with a pair of stoutanterior bars, or cornua, forming the pharyngeal notch, twopairs of long and slender apopliyses behind the anterior bars,diverging nearly at right angles from the main body of theplate, and a stout but short apophysis springing transverselyfrom its postero-lateral angle on each side. There is also anirregular-shaped posterior median process, as well as a pairof short apopliyses projecting subvertically beneath thelatter. In the Moluccan species T. gigas (= moluccanus),as was shown by Van der Hoeven, there is only a singlelong apophysis projecting from the lateral border in front.This represents the anterior of the two that are found in thisposition in X. polyphemus. This peculiarity obtains also,I find, in the other Asiatic species, Tachypleus triden-tatus (=longispina) and Carcinoscorpius rotundi-cauda, thus confirming the opinion I have already putforward ('Ann. Mag. Nat. Hist./ April, 1902) that these
STUDIES ON THE ARACHNID ICNTOSTERNIXE. 227
three forms belong to a group distinct from and morespecialised than that of polyphemus, as is clearly shownby the structure of the genital operculum, etc. Also thepair of posterior ventral apophyses fouud in polyphemusare missing in the two young specimens of trideutatnsand rotundicauda I liavo examined. In all species ofLimulus the upper side of the entosteruite is furnishedlaterally behind the middle with a short muscle-bearingexcrescence, suggesting a suppressed or undeveloped apo-physis.1
2. The Bntostemite of Scorpions.The variations that affect the entosternite of Scorpions
are principally correlated with the compression, antero-posterior or lateral as the case may be, of the exoskeletalmetasteruite. In Palainuteus thorelli the "body" ofthe entosternite consists of an irregularly transversely oblougplate. From its anterior angles rise the anterior cornua,which give off muscles to the appendages along their outeredge, and present a frayed or ragged appearance when clearedof these tissues (see fig. 20, PI. 14). On its underside theplate dips down on each side of the nerve-cord, and passingand fusing beneath it forms a complete and rigid ventralring through which the nerves pass backwards into themesosoma. The lower portion of this ring gives off in front
1 In liis paper on the anatomy of Li mulus po lyphemus ('Trans. Liiiu.Soc.,' xxviii, 1S73), Owen states (p. 4G9) that the entosternite of this speciesis furnished with a pair of " sclerous processes " which diverge from " nearthe fore-part of the dorsal surface," and reference is made to fig. 5 on pi.xxiviii, which is an acknowledged copy of Vau-der Hoeven's figure of theentosternite of L. moluccanus . Yet the original figures with which Owen'spaper is illustrated are all, apparently, taken from examples of the Americanform (L. polyphemus) . Hence it is difficult to account for his overlookingthe presence of the two pairs of processes in this species. It may also beremarked in passing, though the l apsus is of no great moment, that thestatement in the foot-note to p. 462 that " the species which he [Van derHoeven] dissected was the rapier-tailed Molucca crab (Limulus r o t u n d i -cauda, Latr.) " is an error.
228 B. I. POCOCK.
a median process which divides into a pair of divergingtendinous apophyses. From the sides of the neural ringexternally spring muscle-supporting processes. The musclesrising from the posterior of these processes are extendedlaterally and dorsally to become attached to the sides androof of the body-cavity, forming, with associated connectivetissue, a great muscular sheet or " diaphragm " which sepa-rates the cavity of the prosoma from that of the mesosoma.Interiorly this partition is completed by muscles which runfrom the posterior side of the lower edge of the neural canalto the floor of the body-cavity. In the middle line above the"body" of the entosternite the connective tissue of thismuscular sheet is perforated by two foramina; the inferiorgives passage to the alimentary canal, the superior to theaorta.
In addition to the muscles already mentioned, three pairsof dorso-veutral muscles arise from the eutosternite. Thoseof the posterior pair are attached to the underside of thetergite of the genital somite beh ind the diaphragm, and tothe posterior side of the entosternite in front of it. Hencein their passage from above downwards they pass throughthe diaphragm. The median pair extends from the aorticforamen in front of the diaphragm to the posterior borderof the upper surface of the "body" of the entosteruite.Just in front of their inferior points of attachment springthose of the anterior pair, which, rising vertically, meet inthe middle line above the aorta, before attaching themselvesto the underside of the carapace.
As might be expected, the entosternite of this speciesagrees in all essentials with that of Palamnasus indus( = B u t h u s cyaneus) as described and figured by Lankester(6) and Beck (1). In a general way the entosteruite of all{Scorpions is formed on this plan. In minor particulars,however, there is considerable structural variation. Inspecies with the metasternite antero-posteriorly compressed,the body of the " entosternite" becomes shorter as comparedwith its length, as shown in the figure of that of I u r u s
STUDIES ON THE ARACHNID KNTOSTEENVIE. 229
dufoure ius , one of theVejovidse. ID this species the lateralcrests which arise from the anterior cornua are betterdeveloped than in Pa lamnseas , and the homologue of thesolid lateral process of the latter is less solidified and rigid.Moreover the anterior process from the neural arch appearsto be undeveloped (PI. 14, fig. 21).
In H a d r u r o i d e s charcasus , a member of the samefamily as Iu rus , but with the sternum showing a markedlygreater degree of antero-posterior compression, tlio " body "of the entosternite is relatively much shorter than in the last-named genus, though in other respects the entosternitcs ofthe two are very similar.
The process of reduction in the size of the entosternite bylongitudinal compression is carried to an extreme in theBothriuridse (olim Telegonidte), where the sternum isreduced to a transversely linear sclerite wedged in beLweenthe genital operculum and the coxse of the appendages ofthe fourth pair ( = second walking leg). In B o t h r i u r u sb o n a r i e n s i s (see PI. 14, fig. 22) the portion of the body ofthe entosternite which forms the rooE of the neural canalis reduced to a narrow transverse bar. This modificationseems to have been accompanied by the disappearance ofthe anterior pair of dorso-ventral muscles; those of the secondpair pass up to the aortic foramen without fusing with thediaphragm. As in the genera of Vejovidte examined,the subneural process is apparently absent in the Both-riuridte. The structure of the entosternite in this familybears out the view I have elsewhere expressed that thesescorpions are a specialised offshoot of the Vejovidae.
In the ButhidiB, which are characterised by a triangularlycompressed sternum, the eutosternite shows unmistakablesigns of lateral compression, the " b o d y " being reduced toa longitudinal bar, from the posterior extremity of which,and rather between than behind those of the first pair, rise,in juxtaposition, the dorso-ventral muscles of the second pair.The lateral crests are well developed, as in the Vejovidas, andthe subneural arch is furnished with a median process
230 B. I. POCOCTC.
ending in two fan-shaped apophyses similar to those ofPalamnaeus, but stouter.
These characteristics are illustrated in the figure of theentosternite of O e n t r u r o i d e s m a r g a r i t a t u s (PI. 14,fig. 24), which may be taken as fairly typical of the ento-sternite of Buthidae in general.
Schimkewitsch (10) gives a figure of the entosternite ofA n d r o c t o n u s bicolor , which is quite unlike this plate inany member of the Buthidas I have examined. Presumablythe form he names A. b ico lor is the thick-tailed, dark-coloured species from Transcaspia which Olivier calledc ras s i cauda . In examples of this species the entosterniteclosely resembles that of C e n t r u r o i d e s m a r g a r i t a t u s(see PI. 14, fig. 24), having the same narrow median longi-tudinal ridge and large lateral crests and the same narrow,nay, even narrower bar, with broad, fan-shaped apophysesrunning forwards from the subneural arch. Yet Schimke-witsch represents the supra-neural arch as a transverselyoblong plate as wide in proportion to its length as in H a d r u -roides , fnrnisbed with lobate lateral projections, and a verybroad subneural process with unexpanding apophyses. Anentosternite of this description should belong to some specieswith a broad and short pentagonal entosternite.
Speaking of: the entosternite of the scorpions, Bernard(3) sa,ys that its points of attachment " to its parentcuticle correspond with the points of origin of the ento-sternite of Galeodes,"—that is to sa}r, to the integumentimmediately above the preaxial surface of the coxa of thefourth prosomatic appendage, or, as he elsewhere (4) ex-presses it, between the third and fourth segments. InPalamnaeus tho re l l i , the species examined by Bernard, Ifind that the anterior bar of the entosternite has a fibro-mnscular attachment to the in-projecting anterior rim of thecoxa of the fourth appendage (second leg). But I could notsatisfy myself that there was any union with the adjacentintegument,—certainly there was none such as to justify thespeaking of the integument as the "paren t" of this bar
STUDIES ON THK AHAOHNID TCNTOSTERNITE. 231
of tlie entosfcernite. Bernard also homologises the anteriorbars of the scorpion's entosfcernite witli tlie second pair ofventral apophyses which are affixed to the sternum oppositethe base of the third prosomatic appendage (first walkingleg) in the spiders. Since, however, these bars in thescorpion give attachment diagonally to great muscles whichsupply the second and tliird appendages (chela and first leg),it seems far more likely that they represent the anterior barsof the entosternite of the spiders, Thelj'phonns, etc., whichare similarly continuous with the muscles supplying theseappendages (PI. 14, fig. 20).
3. The Entos te rn i te of the Pedipalpi (Thelypho-n i d a3, P h r y n i d to).
In the Pedipalpi two types of eatosternifco are found, onecharacteristic of the Urofcricha (Thelyphonidas), the other ofthe Amblypygi (PhrynidEe). In the Thelyphonidre the mainportion of tlie plate is longitudinally oblong in shape. It isperforated mesially by two foramina, an anterior large andoval, and a posterior relatively small and circular. The twoare separated by a transverse bridge; a similar bridgeseparates the anterior foramen from the pharyngeal notch.Near the edges of the upper sin-face of the right and leftbars forming the external framework of the pharyngealnotch and of the foramina, rise five pairs of tendinous pro-cesses which are affixed by muscular fibres to the undersideof the carapace. The first rises at the extremity of theanterior cornu, the second just in front of the anteriorbridge. The latter apophysis is bifid and projects inwards,backwards, and upwards towards the central depression ofthe carapace. The others take a more lateral direction. Thetliird rises close to the second and a little behind theanterior bridge ; the fourth just behind the middle of thelarge foramen ; the fifth on a level with the smaller forameu.Below the latter may be seen a bifid tendinous crest runningdownwards and outwards. Behind this point the eutosternifce
232 R. I. POOOOK.
is laterally constricted, then expands into a subcircuhir softerplate, to the ragged edge of which are fastened manymuscles passing to the pregenital somite and to theappendages of the sixth pair. Sometimes at least there is, onthe upper side of this plate, a pair of short processes whichserially repeat apparently the longer tendons of the anteriorpart of the eutosternite. From the underside in front arisetwo pairs of processes, the first passing from the anterior ex-tremity of tho cornua to the coxte of the chelte, the secondto the prosternum from a point on a level with the anteriorbridge (PI. 13, figs. 2, 9).
The figure and description of the entosternite of Thely-phorms published by Laurie (8) do not agree with thisorgan in the species examined by myself. The anterior threepairs of dorsal processes and the two pairs of ventral pro-cesses, as well as the lateral crest, are omitted from the figureand unmentioned in the text; and I find no process pro-jecting from the sides of the posterior lobe such as herepresents and describes. Bo, too, is the figure twicepublished by Schimkewitsch (9, 10) from a sketch byTarnani and copied by Bernard (3) unlike, in certain particu-lars, the entosternites of the Thelyphonidte I have dissected,although resembling them in general form and in thenumber of the processes. For example, the bifid processnumbered Sa.' in my drawing is represented as rising fromthe side of the anterior bridge slightly behind the levelwhence the processes numbered 2tg. diverge; and the pro-cesses numbered 3tg. spring further back in line with theposterior bridge, not just behind the middle of the largerforamen, as shown in my drawing. The lateral crest, too,was apparently unnoticed. Considering the uniformityin the structure of the prosoma throughout the family Thely-phonidte, it seems hardly probable that these discrepanciesare due to specific differences between the specimens examined.I find a practically complete uniformity of structure in theentosternite in species of the genera The lyphonus ,Hypoc tonus , and Mas t igoproc tus .
STUDIES ON THR ARACHNID ENTOSTERNITE. 233
The entosternite of the Amblypygi is very different fromthat of the Urotricha. The pharyngeal notch is semi-circular and the anterior cornua large. Each bears a pairof dorsally directed apophyses near tlie apex, also one on theunderside, which dips down beneath the pharynx, and oneabove, at the base, which projects upwards and inwards.Tlie body of the plate itself is wide, narrowed posteriorly, andsolid, i.e. without foramina. Near its lateral border on eachside arise four apophyses which extend upwards and outwardsto be inserted by means of muscular bundles to the under sur-face of the carapace. The first is very slendei-; the secondand third are approximated at the base ; the fourth is thestoutest. These four spring from a common ridge beneathwhich the edge of the entosternite runs ont externally into ashort angular crest to contribute support to the greatappendicuhir muscles (PI. 13, fig. 3).
I t would be unfair to criticise the figure of the entosterniteof " Phrynus" given by Bernard (3), because " the prepara-tion was accidentally destroyed before the drawing wascompleted." Four pairs of dorsal apophyses are represented,but I cannot satisfactorily homologise them with the sixpairs shown in the figure here published (PI. 13, fig. 3). It isstated, moreover (op. cit., p. 20), that this plate has " onlyone attachment to the ventral surface, and that is to theinfcersegmental membrane between the second and third pairsof limbs corresponding with the first pair of npodeinesforming the entosternite in Myga le . " It is true that thereis only one pair of ventral processes, and that they representthe similarly situated processes in Mygale . They are notattached, however, in the position Bernard states, but to thecoxa of the second appendage, the point of their insertionappearing as a horny subcircnlar patch on the soft membranebelow the mouth, when these appendages are pulled apartand examined from the front.
4. The Entosternite in the Aranete.
In typical members of the Araneee the eutosteruite closely
234 R. T. POCOOTC.
resembles that of the Ainblypygi in general form and in manystructural details. In E p h e b o p n s m u r i n u s and othermygalomorphous spiders of the family Aviculariidaj it is alongitudinally oval imperforated plate, with large anteriorcornua bounding the pharyngeal notch. The upper side isfurnished with four pairs of dorsally directed tendinousprocesses, arising, as in P h r y n u s , from a common ridge.Below this the edge of the plate runs outexternally into angularprocesses which afford attachment to some muscles of thelegs. From the underside four pairs of processes pass down-wards to meet the sternum; the first pair arising from theanterior cornua and running to the bases of tbe appendagesof the second pair, immediately behind the prosternnm ; thesecond, third, and fourth radiating from a common medianexcrescence, behind the pharyngeal notch, and reaching thesternum opposite the third, fourth, and fifth appendages.Remnants of a similar apophysis corresponding to the sixthnppendage, but failing to I'each the sternum, are traceablenear the posterior end of the entosternite. The chiefdifference between this entosternite and that of the Ambly-pygi lies in the preseuce of the ventral apophyses corre-sponding to the four posterior pairs of prosomatic append-ages.
The figure of the entosternite of a M y g a l e given byBernard (3, figs. 3 and 5), and taken from a specimen inthe College of Surgeons' Museum, is diagrammatic. It is tobe noticed, however, that the ventral processes of the firstpair are correctly represented as fused in the middle line.The entosternites of the species examined by Lankester (5, 6)1
and Wasmann (11) agree closely with that of E p h e b o p u sm u r i n us.
In the great majority of the Mygalomorphae the ento-sternite is in the main like that of E p h e b o p u s , retainingthe four dorsal and the four ventral apophyses, the pointsof attachment of the latter being visible on the external
' In the figure published in the second of the two works enumerated abovethe dorsal side is by an oversight represented as the ventral, and vice versa.
STUDIES ON THI'! ARACHNID ENTOSTEBNITIi. 235
surface of the sternum as the so-called sigilhi. In manygenera there is a tendency for the posterior pair to increase insize and shift their point of insertion from a snbmnrginal to asubcentral position. Tliis is particularly noticeable in theso-called "trap-door" spiders, where the muscles and append-ages of the prosoma are specialised for fossorial work.
In a few genera, e. g. Atypus , Ei'iodoiij and Act inopns,all the four pairs of ventral apophyses have moved from themargin of the sternum towards its centre, the convergencereaching an extreme in Act inopus , where their points oEattachment meet in the middle line, forming the well-knownrosette or star-shaped sternal impression characteristic ofthis genus. The union of these four apophyses on eachside with one another and with their fellows of the oppositeside resnlts in the formation of a solid plate beneath thenerve mass, which is thus enclosed, as it were, in a basket,the lateral nerves to the limbs passing out through thespaces between the upright portion of the apophyses.From the middle of the anterior border of this ventralplate a short median process runs forward, fanning themedian unpaired lobe of the rosette-like impression on theouter side of the sternum.
In A t y p u s the four apophyses retain their primitive dis-tinctness, and are arranged on the underside of theentosternite in the form of a circle, following the curvatureof the pharyngeal notch. A fibrous strand rnns forwardfrom the anterior apophysis to the prosternum.
A reduction in the number of ventral apophyses takes placein the typical genera of the Ctenizinas, e. g. Pachy-lomerus, S tas imopus , and of the Idiopinse, e. g.Acanthodon and H e l i g m o m e r u s . In Pachy lomerusthe first and fourth apophyses persist, the second and thirddisappear. S ta s imopus resembles P a c h y l o m e r u s in thisparticular, but differs in that the apophyses of the anteriorpair fuse across the middle line to form a complete collarround the nerve mass. In Acan thodon the first apo-physis is retained as in all the Mygaloniorphte, and the second
236 E. I. POOOCK.
and third also as slender pillars with a marginal attachmentto the sternum, but the fourth pair has vanished. In someof the genera of this group, e. g. S tas imopus and Pachy-lomerus, an additional apophysis is found on the dorsal sidearising from the crest just behind the second apophysisfrom the anterior end, and directed inwards. Indications ofa similar tendon are also observable behind the next suc-ceeding apophysis, and in A c a n t h o d o n similar supple-mentary tendons are observable behind the posterior twopah-s of apophyses.
In an immature specimen of L i p h i s t i u s I find the fournormal dorsal apophyses of exceptional thickness, and repre-sentatives of the two supernumerary apophyses that occurin Stas imopus, well developed. The entosternite in thisspecimen, however, perhaps on account of its immaturity, hasno ventral apophyses extending to the sternum, although themuscular scars are visible at the sides of this plate. Thisabsence of ventral apophyses is full of interest, on account ofits repetition in the Arachnomorpbas, with which Li phis t inshas other features in common (PI. 13, fig. 7).
On the structure of the entosternite in the Arachnomor-phous spiders (olim Dipneumones) my observations have notbeen far extended. A few examples of genera belonging towidely separated families have been examined, however,without the discovery of any very marked differences in thestructure of this plate. In all there are four pairs of dorsalapophyses corresponding exactly to those of the Mygalo-morphfe and L iph i s t ius , and in all, except F i l i s ta ta , anadditional pair arising, one on each side, between the normalsecond and third pairs, and directed obliquely inwards andbackwards. This represents, no doubt, the muscle, sometimeswith a tendinous base, which arises in the same position insome of the Mygalomorphfe, e. g. Pachylomerus . In manystrong-legged species, such as Lycosa, Otenus, aud Eresus ,it is noticeable that the dorsal tendons are broad and divideddistally into two branches. The extension of this split to theroot of the tendon would give rise in each instance to two
STUDIES ON THE A1UCHNID ENTOSTEBNITK. 237
complete and distinct apophyses. This cleavage appears tohave taken place, probably once, possibly twice, in the caseof the eutosternite of P h r y n u s .
In none of the ArachnomorphsB have I found ventralapophyses extending to the sternum, such as are found inall the Mygalomorplite. The underside of the entosterniteof Lycosa i n g e n s , however, is furnished in its anteriorhalf with a high median crest, from which five short andslender tendons arise on each side. These tendons appearto be homologous with the five inferior tendons seen inE p h e b o p u s (fig. 13) and other genera of Avicnlariidas. Iuthe latter, however, only three pairs spring from a commoncentre, the first lying far forward; the fifth, often obsolete,far backwards.
The diagrammatic transverse section of the entosternite,with its associated muscles, of the Araneae, figured bySchimkewitsch (10), shows on each side two dorso-ventralmuscles, a lateral muscle, and two that pass to the legs, anexternal or elevator of the trochariter (second segment), andan internal (the depressor of the coxa), which passes ventrallyto an entapopliysis between the sternum aud tlie base of theleg,—all rising from distinct apophyses. The last-namedmuscle is, I believe, the ventral portion of the tergo-sternalmuscle, and tlie pair of dorsal muscles on each side repre-sent the tergal portion cleft to the base.
5. The " S o - c a l l e d " E n t o s t e r n i t e of the Solifugaa.
The "so-called" entosteruite of the Solifugas, botli in itsstructure and attachment, is qnite unlike the true entoster-nite of other Arachnids. It consists of a pair of stout, rigidchitinous pillars united, with or without articulation, to thenarrow prosternal plate, wedged in between the coxas of thethird pair of appendages. From the prosternum it extendstransversely along the narrow strip of integument joining thejuxtaposed coxas of the third and fourth pairs of appendages.Internally the two pillars, runniug obliquely or subvertically
238 R. i. POCOCJI.
backwards from their base, converge and meet, without actualfusion, in the middle line, expanding beneath the alimentarycanal to form a somewhat saddle-shaped enlargement formuscular attachment. This structure is supported beneathby a pair of slender chitinous rods which rise, one on eachside, from a poiut on- the ventral integument of the fourthsomite close to the inner extremity of the tracbeal stigma.In front of this entosternite there is a pair of fibrousnodules, each of which forms a centre for the attachment offive tendons, one passing backwards to be attached to aforwardly directed process from the expanded portion of theentosternite, a second passing vertically downwards towardsthe point of attachment of the entosteruite, a third passingforwards, a fourth obliquely downwards and outwards, andthe fifth downwards and inwards to the base of the rostrum.Bernard (3 and 4) says the entosternite of Graleodes "risesas a pair of iufoldiugs of the cuticle between the third andfourth segments," and his drawings represent the two pillarsus attached some distauce from the middle line to the externalportion of the membrane between the eoxte of third andfourth appendages, no connection with the prosterual platebeing shown or mentioned. The only addition to be made tohis description relates to the attachment of the entosterniteto the prosternal plate as mentioned above.
This attachment is of two kinds. In the case of a specimeno f S o l p u g a s a g i t t a r i a the continuity of the entosternifce withthe prosternum and the intercoxal integument is plainlyindicated after clarification in caustic potash aud immersioniu glycerine. I can find neither articulation nor sutural line,to attest its obliteration, between the two. On the contrary,the strengthening strands of thick chitin which traverse theentosternite pass without interruption into those of the pro-sternum, the two forming a rigid and continuous whole. Asimilar state oE things is shown in the figiu'e of the skeletalelements of the prosoma of Graleodes given by Schimkewitsch(10).
The treatment mentioned above applied to the entosternite
STUDIES 0NT THE ARACHNID KNTOSTHRNITE. 239
of Galeodes a r abs revealed, however, a joint between theposterior extremities of the bars of the V-shaped prosternumand the diverging pillars of the entosternite.
The question as to which of these two arrangements is themore primitive must remain unanswered until the origin ofthe Galeodean entosteruite is settled. If, as Bernard main-tains, it is nothing but an exoskeletal entapophysis, the con-dition of unbroken continuity with the exoskeleton, asmanifested in So] puga, must be regarded as the original, andtlie jointed condition seen in G-aleodes the derivative. If,on the other hand, the entosternite in this order proves tobe an entoskeletal element like that of other Arachnids, itsfusion with the exoskeleton must be a secondarily acquiredcharacteristic, and its separation therefrom a primitivefeature (PI. 14, figs. 26, 27).
II . THE COMPARATIVE MORPHOLOGY OF THE ENTOSTE UNITE.
The evidence favouring the hypothesis that the prosorna ofthe jjrimitive Arachnid was furnished with a broad segmentedsternal area separating the post-oral appeudages of the rightside from those of the left needs no recapitulation. It may beclaimed that the possession of a wide sternal area by theAinblypygous Pedipalpi and all the typical Araneaa is anarchaic feature; and that, in this particular at least, theseorders are less specialised than the Scorpioues, Solifugte,Pseudo-scorpiones, and Opiliones, where the encroachment ofthe coxas of the appendages, aided in the case of the firstand last-named orders by antero-posterior compression accom-panying the forward movement of the generative aperture,has more or less obliterated the sternal sclerites in the middleline. Although modified to a very appreciable extent inthe direction of sternal suppression, the prosoma oE theUropygous Pedipalpi is more primitive than that of thefour orders last named, more primitive even than that ofthe Amblypygi and Araneaa iu the lesser constriction of itsposterior somite to share iu the formation of the waist.
240 B. I. PO0OCK.
What is true of the Pedipalpi and Araneas is also true ofthe Xiphosurse. In Limulus there is a relatively wide sternalarea extending from the mouth to the posterior extremity ofthe prosoma, and strengthened by a pair of strong ineta-sternal sclerites behind and a weakly chifcinised promeso-sternal plate in front.
Correlated with this primitive development of the sternalarea we should expect to find eutosternites of a more archaictype in the Pedipalpi, Aranese, and Xiphosuraa than in theother orders of Arachnids. This expectation is justified bythe unmistakably inetameric arrangement of the constituentelements of the entosternite exhibited in these three orders.On almost any theory of the origin of this plate, segmentalrepetition of its parts must be postulated as a primitivefeature. It is obvious that this characteristic is manifestedin a far greater degree in the entosternite of the three ordersnamed than in that of the Scorpiones, Psendo-scorpiones, orOpiliones.
A satisfactory settlement of the homologies of the severalparts of the various types of eutosternites already described isa matter of no little difficulty on account of the variation innumber of the apophyses that arise from them. In theAraneaa the dorsal apophyses range in number from four tosix. In the Phrynidse there are six; in the Thelyphonida?five, with indications of an additional pair on the posteriorlobe of the entosternite.
In the Araaea3 the apophyses in question are referable totwo categories, those that are directed obliquely inwardstowards the central apodeme of the carapace, and those thatarise subvertically to be inserted serially along the areabetween its middle line and lateral border. The latterare invariably present, and invariably four in number on eachside ; the former are either absent or represented by one ortwo pairs. In the two types of entosternite presented by thePedipalpi there is a single pair of apophyses directedinwards and backwards, arising in each case close to thebase of the anterior cornua.
STOD]ES ON THE ARACHNID JCNTOSTKRNITE. 241
As explained below (p. 249), there are good reasons for sup-posing that the four apophyses of constant occurrence in theAraneae represent the tergal elements of the tergo-sternalmuscles of the second, third, fourth, and fifth somites of tlieprosoma, those of the sixth somite being undeveloped as anaccompaniment oE the compression this somite has suffered.
Seeing how nearly related in many particulars the Araneaaare to the Pedipalpi, it can hardly be doubted that the fourapophyses in question in the Aranese are homologous to thefour that project laterally in the Thelyphonidas, the fifthpair which is suppressed in the Spiders being retained in arudimentary state by the Pedipalpi. In the case of P l i rynusthe question is complicated by the presence of an additionalapopliysis on each side, making a total of five. The posteriorof these might be held to represent the apopliysis which ismissing iii the Aranese and rudimentary in T h e l y p h o n u s ;but since the lastprosomatic somitoin Pl i rynus is compressedin its sternal portion almost to the same extent as in theAranese, and since even in Thelyphouus, where this somiteretains its more primitive condition, the apophysis is scarcelydeveloped, it seems more probable that this apophysis is alsoundeveloped in P h r y n u s , and that the fifth apophysis inthis genus actually corresponds to the fourth in the Spidersand The lyphonus . The likelihood of the truth of this viewis enhanced by the basal juxtaposition of the third and fourthapophyses in Phrynus , which forcibly suggests their commonorigin from a tendon representing the third apophysis of theSpiders and The lyphonus secondarily subdivided into two.The possibility of the subdivision of these apophyses is clearlyshown in the case of many Araneas, such as Otenus andLycosa where they are deeply cleft, in Acanthodon wherethose of the third and fourth pairs are double down to the root,and in T h e l y p h o n u s where the apophysis rising from theupper side of the anterior coruu gives off a secondary branchtowards the middle line.
Cleavage of primary single tendons may account for thepresence of the one or two pairs of supplementary tendons
VOL. 4 6 , PART 2.—NEW SE1UISS. Q
242 R. i. POCOCK.
which run obliquely inwards and backwards towards thecentre of the carapace. The constancy in position of thesetendons in the vSpiders suggests their horaology throughoutthe order, and their origin from the second or the second andthird of the larger normnl apophyses. Only one suchapophysis is developed in The lyphonus , and this arises alittle in front of the second marginal tendon, not behind itas in the Spiders. Interesting, therefore, it is to observe thatin Phrynus—a genus in many respects intermediate betweenThe lyphonus and the Spiders^the apparent homologue ofthis tendon lies a little farther back than in The lyphonus ,alittle farther forwards than in the spiders. It is also noticeableas a peculiarity in P h r y n u s that in the third tendon, which,for reasons already given, may be regarded as reduplicated,the extra branch takes the same direction as its twin. Oneother small structural feature bears out the homologies heresuggested. This is the presence in many spiders of a trans-verse thickening of the entosternite just in front of the fourthmarginal apophysis. The posterior bridge of the entosternitein The lyphonus has exactly the same relations. Similarlythe posterior margin of the pharyngeal notch in the Spidersis generally thickened, so as to suggest its correspondence withthe anterior bridge in The lyphonus . As for the ventralapophyses, there cannot be much doubt that the pair passingfrom the anterior cornua to the basal segments of the secondappendages in Ph rynus and The lyphonus are the homo-lognes of each other and of the anterior pair, which have thesame origin and are affixed to the sternum close to the baseof these appendages in the Mygalomorphse. So, too, mustthe second apophysis attached to the sides of the pro-sternnm in Thelyphonus represent the second apophysisattached to the sternum opposite the base of the thirdappendages (first leg) in the Mygalomorphte. Thus itis possible to bring into complete accord the apophysesdeveloped on the dorsal and ventral sides of the entosternitesof the three orders here considered.
Scarcity of material for comparison seems to have prevented
STUDIES ON THE AJfACHNID ENTOSTERNITK 243
Bernard's recognition of the homologies existing between thepnrts of this plate in different orders, homologies which are,at all events, fairly obvious in the case of the Pedipalpi andAraneae. To quote his own words (4), " I n the Spiders . . .the ontostei:nite consists of four pairs of apt)demes whichmeet in the centre, the second pair of which correspond withthe entosternite of Galeodes and Scorpio, whilst the firstpair is perhaps represented in Galeodes by the fibrous plateabove described. In P h r y n u s the entosternite is difficult tounravel; it may perhaps represent only the first pair ofapodemes of the spiders with secondary attachment of dorso-ventral muscles. In TIi el yphonus we have a longfenestrated entosternite which may correspond with that ofthe Spiders ; the component apodemes not, however, meetingin a point."
I venture to think that the new facts and theories concerningthe entosternite of the Pedipalpi and Aranese put forward inthis essay will show that the homologies are by no meansso vague and difficult to unravel as the passage quoted wouldlead one to suppose.
Schinikewitsch (10) terms the apophysis that rises fromthe upper side of the anterior bar in The lyphonus a dorso-ventral outgrowth, and those numbered ltg., itg., 3tg., and Mg.in fig. 2, PI. 13, as lateral outgrowths, homologising the latterapparently with the lateral crest, and the former with one of thedorsal apophyses in the Aranete. I cannot think this interpre-tation correct in view of what obtains in the entosternite ofPhrynus . The Pedipalpi and the Aranete are so very closelyrelated that the conclusion as to the homology between theapophyses of the entosternite appears to me inescapable.
The entosternite of Limulus forcibly recalls that ofThe lyphonus . The anterior bars correspond in the two.Following these in L. polyp hem us come the two long,slender apophyses running out towards the bases of the thirdand fourth appendages, and representing, I believe, the dorsalportions of the tergo-sternal muscles of the correspondingsomites. A. comparison between these and the second aud
244 ]{. I. POCOCJv.
third pairs of lateral processes seen in The lyphonus andSpiders is obvious, and is fortified by the evidence favouringthe view that in the spiders at least these processes corre-spond with the second and third pairs of post-oral appendages.It is only necessary to horaologise the muscle-bearing stumpin Limulus with the fourth lateral process in The lyphonusand the Spiders, and the strong postero-lateral apophyses inthe cntosternite of Limulus with the vestigial processes on theposterior lobe of the entosternite in The lyphonus to com-plete the parallel. On the underside similarity between theentosternite of Limulus and the Arachnomorphous Spiders isto be found in the absence of ventral apophyses, with theexception that in L. po lyphemus a single abbreviated pnir ispresent at the posterior end of the plate exactly as in someof the Mygalomorphous Spiders, e.g. Ephebopus .
The entosternite of Scorpions has been so affected by thecompression of the prosoma that it is not easy to bring it intoexact line with those of the orders hitherto considered wherea more primitive condition persists. That the anterior barsframing the pharyngeiil notch are comparable to those ofLimulus, the Pedipnlpi, and Aranese hardly admits of a doubt(Lankester, 5, 6, and 7). Similarly the lateral tendinouscrest supporting the Ipg-muscles, aud so well developed in theButhidse and Vejovidas, forcibly recalls that of the Aranese.But the dorsal apophyses which form so conspicuous a featurein the entosternite of the Pedipalpi and the Araneas remainiindeveloped. They are represented by the two pairs ofdorso-ventral muscles which lie in front of the diaphragm,those of the third pair which perforate this partition beingusually regarded as the tergo-sternal muscles of the genitalor first somite of the mesosoma.
To which three of the four or five pairs of dorsal apophysespresent in the Araneas and Pedipalpi do these two pairs in thescorpion correspond ? Probably, I think, to the fourth and(iftli pairs,—that is to say, to those that belong to the fifth andsixth segments of the prosoma. This homology is suggestedby their positiou at the posterior extremity of the prosoma,
STUDIES ON TH15 ARACHNID ENTOSTEliNlTIi:. 245
and by the fact that the somites in question have retainedtheir sternal elements, and are therefore, in that particular atleast, less modified than the somites in front, in which thesterna have disappeared. If, then, we suppose that thepharyngeal notch in the scorpions has been prolonged back-wards almost as far as the position of the posterior transversebridge in Thelyphonus, or as the corresponding thickenedridge in the Araneae; that the anterior three pairs ofapophyses have been suppressed upon the two cornua; that thelateral tendinous crests represent the similar crests in theAraneas and Pedipalpi, those of Pa l amnasus in particularrecalling those of Thelyphonus ,—it is evident that the ento-sturnites in the orders now under discussion are more alike inreality than appears at first sight on the surface. Furthermore,if we suppose that representatives of the muscles which radiatefrom the margin of the posterior lobe of the entosternite inT h e l y p h o n u s extended dorsally and. laterally to meetthe walls and roof of the prosomatic space, leaving a channeliti the middle line for the transmission of the aorta and thealimentary canal, it is possible to bring even the diaphragminfo harmony with parts already existing in the T h e l y -phouus . In short, strip away the apophyses lettered \ty.,2tg., Sty., and sa.' iu the figure of the entosternite ofThelyphonus, remove the anterior bridge (a. b.), and fill up theposterior foramen, and the homology o£ the remainder oftlie plate with the supra-neural portion of the entosternite iuthe Scorpions becomes obvious. The annexation by theentosternite of the tergo-sternal muscles of the genital somiteprobably took place before the upgrowth of the posterior flapshut oft' the cavity of the prosoina from, that of the mesosoma;and this consideration points to the formation of the dia-phragm after the suppression of the pregenital somite andafter the forward movement of the ventral area of the genitalsomite, which brought its tergo-sternal muscle into contactwith the entosternite.
The origin of the dorsal portion of the diaphragm iu thiswiiy from a pair of upgrowing muscular ilaps embracing tlie
246 R. I. POOOCK.
alimentary canal and aorta is attested by the persistence ofthe divisional line between its right and left halves. Theyare merely united by a strip of connective tissue, perforatedabove and below by the aortic and alimentary foramina,which must be regarded as the sole remnants of the openspace which originally separated the right and left portionsof the flap from one another.
The neural ring in the Scorpions has its counterpart inActinopus, even to the development of an anterior medianprocess. It may have arisen in the same way by the fibroussolidification and subsequent subneural fusion of the ventralmoiety of a pair of tergo-sternal muscles. If so, the viewthat only one such pair of muscles is involved in its con-struction, and that that pair belonged to the sixth somite ofthe prosorna, is suggested by the absence of lateral perfora-tions iu the sides of the neural arch for the exit of nerves tothe appendages, and by the situation of the ring behind thepoint whence the nerves to the appendages of the sixth pairdiverge. Equally well, however, may the sides of the canalhave arisen from the downward growth of the lateral portionof the underside of the posterior portion of the entosternite.
Lankester (6) suggests that the lateral process marked or.iu the figure of the entosternite of Palamnssus (PI. 14,fig. 20) corresponds to the antero-lateral processes of theentosternite in Limulus. More likely, I think, is it that thisprocess is the thickened and solidified representative of theposterior part of the crest developed (PI. 14, iigs. 21, 24) inC e n t r u r o i d e s aud Iu rus , and finds its homologue iu thesimilar crests in The lyphonus , and not in the dorsalapophyses, to which. I believe the two processes in Limulusare comparable. Nor can I agree with the opinion ofSchimkewitsch (10), that the lateral processes he findson the entosternite of the Scorpion named Androc tonusbicolor (see p. 231) are the hotnologues of the processes Ihave numbered ltg., 2tg., aud 3tg. in The lyphonus .
STUDIES ON THE ARACHNID TiN't'OSTIBKNITB. 247
I I I . THEORIES or THE OmGiN OF THE ENTOSTERNITE.
As long ago as 1881 Lankesfcer (5), when describing theeutosternite of Limulus, said it may be regarded as an en-largemenfc and interlacing of the respective tendons of themuscles which are attached to it. By implication a similarorigin was predicated of the entosternites of Scorpions andSpiders. This opinion was accepted by Schimkewitsch (9,10) iu the case of the entosternite of Scorpions, Spiders, andother air-breathing Arachnids, and for that of Limulus byBernard (3), who, however, regarded it solely as a derivativeoE the ventral longitudinal muscle-bands. Bernard's views asto the origin of the entosternite iu the terrestrial Arachnids,which he considers to be in no way related to Limulus andits extinct allies, will be referred to later on.
It appears to me that the evidence in favour of Lankester'sview of the mode of production o£ this plate in both groupsof Arachnids is overwhelming; a comparative study of theentosternites in this class precludes, to my mind, any otherhypothesis as to their source.
What muscles, then, have taken the largest share in theirformation ?
There is reason to believe that the prosoma was originallysupplied with five pairs o£ tergo-sterual (dorso-ventral)muscles serially repeating those of the opisthosoma, andpassiug vertically from the under surface of the carapace tobe inserted ventrally on the sternum close to the points ofarticulation of the post-oral appendages. There were also adorsal and a ventral pair of longitudinal muscles traversingthe prosoma from end to end (see Laukester, 7).1 With the
1 " The simple musculature in the ancestor consisted of—(1) a pair of dorsallongitudinal muscles passing from tergite to tergile of each successive seg-ment; (2) a similar series of paired longitudinal ventral muscles; (3) a pairof dorso-ventral muscles passing from tergite to sternite in each segment; (4)a set of muscles moving the coxa of each limb in its socket. The. confluenceof the prostomium and the six anterior tergites to form a prosomatic carapace,as well as the specialisation of the six pairs of appendages of the prosoma,was common to the ancestors of both Limulus and Scorpio. This modi-
248 K. I. POCOCK.
welding together of the external skeletal elements to form acompact iuexpansible whole, the function of these muscles asdilators and contractors of the prosoma would cease, leavingthem available for other purposes if required.
The fusion of tergites to form a carapace, accompanied nodoubt by the partial or complete cessation of function of thelongitudinal and vertical muscles, took place, as may be seen, inthe Trilobites, before the five pairs of post-oral appendages ofthe prosoma were set apart as the exclusive organs of locomo-tion and prehension. This specialisation, demanding an in-crease in the size and strength of the limbs in question, wouldbe advantageously accompanied by an increase in the area forthe attachment of the eularged and subdivided muscles thatcontrol them. This area might be supplied, in the first in-stance, by the fibrous solidification of the central portion ofthe tergo-sternal muscles, aided perhaps by that of theadjacent portion of a longitudinal muscle on each side pass-ing from the anterior to the posterior extremity of theprosoma above the nerves radiating to the appendages.
Does the structure of the most primitive types of ento-sternite known to us furnish justification for the opinion thatthey have originated in the manner here suggested ? Agood deal may be said, i thiuk, in favour of an affirmativeanswer to this question.
The points of attachment oE the tergo-sternal muscles ofthe opisthosoina are generally apparent euougli externally,both on its dorsal and ventral walls. On the prosoma theyare usually much less apparent. In the Mygaloniorphousspiders, however, the sternum is typically marked with fourpairs of " scars," the so-called " sigilla," one on each sideclose to the proximal end of the coxa of the second, third,fourth, and fifth appendages (i. e. the palpi and first threepairs of legs). The position and nature of these scars at
Dcation of form anil specialisation of body regions entailed a correspondingmodification of the muscular system. The dorsal and ventral longitudinalmuscles of the prosoma were suppressed. Tlie muscles of the prosomaticlimbs acquired large size and became subdivided."
STUDIES ON THI5 AliACHN-JD I'lNTOSTERNlTE. 249
once suggest their correspondence with the similarly placedscars upon the sterna of the opisthosoraa in, e.g., P h r y n u s ,which admittedly indicate the ventral attachments of thetergo-sternal muscles. Dissection, however, shows that thescars on the sternum are the points of insertion, not ofmuscles, but of the tendinous processes which project down-wards from the lower surface of the cntosternite. Thesetendinous processes are, I believe, the solidifiedventral moieties of the primitive prosornatic tergo-sternal muscles. Apart from other considerations, theirmuscular origin is attested by their representation in theArachnomorphcB (e.g. Ctetius) by muscles passiug fromthe lower surface of the eutosternite and affixed by a fibrousstrand to corresponding points on the sternum.
The dorsal moieties of these same muscles are represented,I think, by the paired tendinous apophyses springing fromthe upper side of the eutosternite and passing into muscularfibres which fasten them to the lower side of the carapace.There is never a sternal scar near the base of the sixth append-age in the Mygalotnorphas, and no apophyses, either dorsalor ventral, of any appreciable length, corresponding tothis limb, on the entosternite. Short chitiuous ridges areobservable, however, on this plate in the appropriatepositions. These considerations suggest the suppression ofthe fifth pair of apophyses as a concomitant, no doubt, of theconstriction as the last somite of the prosoma.
The acceptance of this view of the nature of the dorsal andventral processes of the entosternite carries the conclusionthat a large part of this plate in the Araneas results from thetendinous solidification of five pairs of tergo-sternal muscles.
Evidence that a share in the formation of the plate hasbeen contributed by at least one pair of longitudinal musclesis supplied by the following facts. Apart from the appen-dicular muscles, which originally took a transverse direction,both the anterior and posterior extremities of the ento-sternite afford support to muscles; those from the anteriorbars passing forwards to the front wall of the prosoma,
250 n. i. POCOOK.
those from the posterior extremity running backwardsinto the opisthosoma iu continuity with the longitudinalmuscles of this region. This is well shown in Limulus(Benham, 2); in Epeira by Schimkewitsch, and in thespecimen of A t y p u s figured iu PI. 14, fig. 17. Again, in theThelyphonidae, the anterior two thirds of the entosternite,apart from the dorso-lateral apophyses, consist of a pairof stout parallel longitudinal bars, united by an anterior anda posterior transverse bridge, the posterior lobe alone con-sisting of an undivided subcircular plate.
The solidification of these muscles was no doubt broughtabout to afford a firm support for the muscles of the proso-matic appendages. To resist the action of these muscles,which would tend to draw the bars asunder, tendinous bridgeswere developed across the middle line serving to hold theentire structure in place. As a later development in theArauete and the Amplypygi, the intervals between the bridgeswere filled in aud the projecting marginal angular crests socharacteristic of the entosternite of the AraneEe were formed toincrease the attachment-area of the leg-muscles. Latentpotentiality for transverse fusion between the originallyseparated right and left halves of the entosternite may beinferred from the fusion of this nature that has actuallytaken place between the ventral apophyses in Act inopus .
It is possible that the longitudinal muscles have notplayed so important a part in the formation of the ento-sternite as here suggested. The entosternite-may be almostequally well derived on theoretical grounds from dorso-vcntral and crural muscles alone, the anterior bars formingthe pharyngeal notch resulting from the fusion of the tendonsof the dorso-veiitral muscle of the second somite with those ofthe appendages of the second and third somites, aud the longi-tudinal direction of the bars being assignable to the forwai'dmovement of the second appendage pulling the tendons in asemicircle round the pharynx and brain, the notch thusformed becoming deeper aud deeper to accommodate itselfto the concomitant backward movement of these organs.
STUDIES ON THti ABACHNID ENTOSTKUN1TB. 251
The opinion put forward by Lankester in 1881 (5) that theentosternite was formed by the enlargement aud interlacingof muscular tendons was modified in 1885 (7) by the furthersuggestion that the prosomatic and smaller posterior (mesoso-inntic) entosteniites are merely the original subepidermicconnective tissue of the sternal surface of the segments inwhich they occur, which has become thickened and carti-laginoid, and, in the case of the prosoma, has been at thesame time floated off, as it were, from the sternal surface,taking up a position deeper, that is to say nearer theaxis of the animal than that which it originally occupied." Aud, again, in both Limulus and Scorpio the proso-matic entosternite or plastron represents the mid-sternalarea of several segments fused, probably in both cases ofall the prosomatic somites, though possibly in Scorpio thefirst segment is not included." The position o£ the euto-sternite above the nerve-cord is esplaiued on the hypothesisthat the detachment of the mass of connective tissue frumthe sternal surface occurred at a period when the nerve cordswere still quite lateral in position, their union takiDg placeafter the flotation.
This hypothesis assigns an immense antiquity to theentosternite, an antiquity dating back probably to theTrilobitic stage of Arachnid phylogeny, possibly earlier still.But supposing that the entosternite owes its origin to thedetachment of subneural fibrous thickenings of connectivetissue, a later phylogenetic stage can be ascribed to it byassuming its derivation from paired thickenings which floatedoff on each side of the uuited nerve-cords, and subsequentlyfused with one another both transversely and longitudinallyto form a gate-like framework beneath the digestive tract.Maybe the fenestration of the entosternite of The ly pho imsis a survival of this early stage. But whether the entoster-nite had its origin in subneural thickenings of this nature,and, if so, the manner and purpose of their assumption oftheir present position, or whether it was derived from thefibrous solidification of muscular and connective tissue in the
252 R. I. POCOCK.
mid-region of the prosoina, as I am inclined to believe, arequestions which must for the present be left unsettled. Theevidence we possess that at least the dorsal and ventralprocesses of the entosternite in the spiders are modifiedmuscular tendons seems to make it unnecessary to look else-where for the source of the formation of the entire plate.
This conception of the origin of the entosternite frommuscular and connective tissue differs entirely from that heldby Bernard (3, 4), who would derive this plate in allterrestrial Arachnids from integumental apodemes andsegtneutal constrictions. It may be inferred from what hesays about the entosteL-nite of Myga le that he regards itsfour pairs of dorsal and ventral processes as the remainsof integumental infoldings marking the line of union of.the origiually separated tergal and sternal elements of theprosoina.1 He adds, " The shape of the whole fused masshas been, no doubt, much altered by the action of muscles,but its essential nature as a fusion of metamerically recurrentapodemes cannot be mistaken" (3, p. 20). In his paper on themorphology of the Galeodidas (4, p. 327) he says that in thePedipalpi and Araneas "four pairs of dorso-ventral muscleshave been retained, more or less modified, as the dorsalattachments of the entosternite, and are now largely fibrous;they suspend the entosternite and separate the alimentarydiverticula in the typical manner." Hence may be inferredthe admission that part at all events of the dorsal processesof the entosternite have been derived from dorso-ventralmuscles. If part, why not the whole of the dorsal process?Aud if the dorsal process represents the part of the muscleabove the eutosternite, what reason is there for refusing to
1 Bernard slates that tlie original distinctness of tlie terga of the carapacein the Arancseis sliowu by the furrows on its dorsal surface. These furrowsarc in reality the external indications of the radial arrangement of the greatdorsal appendicular muscles, and mark the lines of attachment of tlie musclesrising dorsally between them from the entosternite. If the grooves indicatedtlie union of tergal plates, such plates should be more clearly defined in theembryo, but so far as I am aware the carapace of spiders at no period of itsdevelopment shows division into separated tergal plates.
STUDIES ON THE ARACHNID ENTOSTERNITE. 253
regard the ventral processes as the representatives of theventral moieties of these same muscles ?
Bernard's hypothesis involves the assumption o£ a degreeof dislocation and rearrangement of muscles and intcgn-meutal apophyses for which it is difficult to find justification.I can discover no evidence that the sternal scars of theMygalomorphaa and the ventral processes of the entosteriiitewhich rise from them are the remains of integumentnl dis-sepiments. If this were the case we should expect to find anintimate unseverable union between the sternum and the pro-cesses in question. No such union exists. The expanded ex-tremities of the pi'ocesses inay be readily detached, the extentof their union with the sternum being quite compatible withthe theory of thoir muscular origin, but hardly reconcil-able with that of their derivation from ectodermic ingrowths.
The basis of Bernard's hypothesis is to be found, firstly,in the structure and relations of the so-called entosternite ofthe Solifugse, which is shown by its histology and unionwith the integument to be an ectodermic entapophysis;secondly, in the assumption that this skeletal piece is themorphological equivalent of the entosternites of otherArachnids; thirdly, in the conception that the Solifngaeretain a more archaic type of prosouia than that of the otherorders of this class.
Assuming the truth of the propositions here stated orimplied, the conclusion Bernard draws as to the ectodermicorigin of the entosternite in the Spiders, etc., necessarilyfollows. Bat from what is known of the structure anddevelopment of the entosternite in the two orders there islittle doubt that the fii-st proposition is true, and that thesecond is untrue. As for the third, it is a matter of opiniondepending upon the standpoint from which the morphologyof the Arachnida is regarded.
The available evidence is, in my opinion, decidedly infavour of the view that the " entosteruite' ' of the Solifuga?must be regarded as a post-oral entosclerite comparable tothe crescentic pre-oral entosclerite of the Scorpions. But
254 ii. I. POCOCE.
if there are any who see in it the homologue of theScorpion's entosternite, they will remember that chitin hasbeen shown (Laukester, p. 6) to be present in the ento-sternites of Scorpio, Mygale, and Limulus, and will realisethe possibility of the formation of the rigid and hornyGaleodean entosternite by increased development of itschitin, followed or accompanied by fusion with the exo-skeleton of the second post-oral somite.
Briefly, then, of the three suppositions that may be enter-tained with regard to the "entosternite" of the Solil'ugse,each points to its being a specialised, not a primitivestructure. (1) If it is an en tosc le r i te, as Bernard andSchimkewitscli maintain, it is not the bomologue of: theentosternifce of other Arachnids, which is shown by itsmorphology and development to be an entochondrite, pro-duced by the condensation of connective tissue and the fusionof muscular fibres and tendons. In this case it lias func-tional!}' replaced the true entosternite, and is a recentspecialisation, not a primitive structure. (2) If it is anendochondrite and the homologue of the entosternite of otherArachnids, its structural similarity to, and fusion with, theexoskeleton also attest high specialisation. (3) If it hasresulted from the union of the true entosternite with a pairof exoskeletal ingrowths—if, say, the expanded portionsupporting the alimentary canal corresponds with the trueentosternite, and the pillars diverging therefrom to the exo-skeletal elements,—the absence of all trace of union betweenthe two, the complete continuity of their tissues, againindicate great specialisation.
The evidence in favour of the truth of the first suppositionis almost strong enough to enforce its unquestioned accept-ance. But whichever of the three prove consonant with factand be ultimately adopted, the Solifugas must be regardedas the most specialised type of Arachnid known, so far as theorgan under discussion is concerned—a conclusion which isperfectly in accord with many, nay most, of the structuralfeatures of this order.
STUDIES ON THE ARACHNID ENTOSTERN1TK. 255
The one hypothesis of all others which, in my opinion,has least in its favour is that in the Solifugre we find aprimitive type of prosoma and a primitive type of ontoster-nite clearly attesting the exoskeletal origin oE this platein all orders of Arachnida; and the conclusions deducedfrom these disputable premisses that the true entosterniteshave been derived from chitinous integumental apodemes iscontradicted by their structure in the adult and their meso-blastic origin in the embryo.
Lastly, according to Bernard (3), the "diaphragm" ofScorpio, "like that of G-aleodes, is the homologue of thegreat constriction between the sixth and seventh segmentsforming the 'waist' of other Arachnids, . . . . a diaphragm orwaist being typical of Arachnids." It is not at all clear howa partition like the Scorpion's diaphragm, composed of mus-cular and connective tissue and without exoskeletal elements,can be the homologue of an exoskeletal constriction. Aua-logons structures, structures with the same phj'siologicalsignificance, they no doubt are; but homologous, surelynot.
According to Bernard's theory of waist formation, I pre-sume the condition initiated in Thelyphonns and cul-minating in the Spiders, a condition which results from theconstriction and reduction of the pregenital somite, precededthe condition now met with in the Scorpions and Solifngte.In that case the " diaphragm" in these two orders mustrepresent the pregenibal somite, insuuk and overgrown, plusthe dorsal and lateral arthrodial membranes which connectedthis somite with the prosoma in front and the mesosomabehind.This double partition then became united into one, the dorsalarea of the pregenital somite disappeared, setting free theaorta and the alimentary tract, which were previously confinedwith the nerve-cord in a narrow canal, and enabling them to riseand take up respectively their original positions in the dorsaland central regions of the body-cavity, cleaving the partitionas they rose. Only by entertaining some such conception asthis is it possible to hold that the " waist " of the Spiders and
256 U. I. POCOCK.
Pedipalpi is the homologue of the "diaphragm" in theScorpiones and Solifugte.
This theory of the formation of the diaphragm seems to mescarcely more plausible if the pregenital somite, which wasnot recognised as such when Bernard wrote, is left out ofconsideration ;' and the following quotation shows that I havegiven no exaggerated rendering of liis hypothesis. He says (4),"Between the sixth and seventh segments . . . there is in theGaleodidae . . . a strong intersegmental constriction. Inter-nally this constriction has given rise to a very strikingdiaphrngm. It forms a very complete wall between theinterior of the cephnlothorax and that of the abdomen, andis pierced by the dorsal vessel, the alimentary canal, thenerve-cords, and the tracheae. Close examination shows thatthe diaphragm is due to a strong indrawing of the interseg-meutal membrane between the above-mentioned segments, sothat it is composed partly of a chitinous infolding and partlyof muscle-bauds. It is clear that if the opposite two internalfaces of such a deep segmental constriction fuse together, theyform a diaphrngm; if they remain unfnsed they form a waist.In the G-aleodidaa we seem to have an unspecialised arrange-ment, the interseginental infolding being fused only in itsdeeper parts, forming the diaphragm, while the outer partsof the fold remain open, making an approach to a waist."
Such an infolding, in its unspecialised state, must havedisturbed the position of the dorsal blood-vessel, forcing' itdown towards the central axis of the body on to the alimen-tary canal; but, as a matter of fact, the blood-vessel andalimentary canal show no trace of any disturbance of theirprimitive positions, the former perforating the diaphragmhigh up beneath the dorsal integument, the latter traversingits centre, while below, in line, is seen the canal for the nerve-
1 Bernard has pointed out to me I bat his figure of the seclion of tlie"waist" of a Spider published on pi. xxxiii, fig. 6, of liis paper " On theMorphology of the Galeodidse," shows the presence of a pair of dorso-ventral muscles, and thus confirms the view that the waist is a genuinesomite.
STUDIES ON THE ARACHNID KNTOSTKBNITE. 257
cord, to say nothing of the large tracheal apertnres on eachside. Hence if the diaphragm originated from an integu-mental infolding ib has secondarily encircled the three medianorgans above mentioned, find is a highly specialised sfcrncturc.But, as a matter of fact, in the "diaphragm" of Ga leodesI can find no evidence of such a derivation. I t appears tobe formed of muscular and connective tissue like that of theScorpions, and to have had an internal origin quite apartfrom the integument. The infolding of the integumentBernard speaks of appears to be quite superficial, and tooccur only at the periphery of the diaphragm.
Whether this diaphragm has been developed independentlyof the diaphragm of Scorpions, to which it is similar in itsstructure and position, or whether the two are to be regardedas a heritage from a common ancestor, are matters of quiteanother kind. The absence of such an organ in the Pedi-palpi, Araneae, Pseudo-scorpioues, Opiliones, and Acari,coupled with the wide structural divergences between theScorpions and Solifuga?, points to the independence of theorigin of the diaphragm in these two orders in response tosimilar physiological needs.
NOTE.—I have elsewhere suggested (see ' Ann. Mag. Nat.Hist./ 1893) that the value of the structural characters of theorders of terrestrial Arachnida may be expressed by groupingthem into four divisions of snperordiual rank : the first tocontain the Scorpions; the second the Pedipalpi and theAraneas; the third the Solifugffi; the fourth the Pseudo-scorpioues, Opiliones, and Acari. A study of the entosternitesconfirms this classification iu a remarkable and uuexpecteddegree, especially as regards the isolation of the Scorpionesand Solifngte, and the association of the Aranete with thePedipalpi.
BIBLIOGRAPHY.
1. BECK, E. J.—" Description of the Muscular and Endoskeletal Systems ofScorpio," ' Trans. Zool. Soc. London,' xi, 1885.
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258 E. I. POCOCK.
2. BENIIAM, W. B.—"Description of the Muscular and Endoskeletal Systemsof Limulus," ' Trans. Zool. Soc. London,' xi, 1885.
3. BERNARD, H. M.—"The Eudosternite of Scorpio, etc.," 'Ann. Mag.Nat. Hist.,' xiii, p. 18, etc., 1894.
4. BERNARD, H. M.—"The Comparative Morphology of the Galeodidce,"'Trans. Linn. Soc. Zool.,' vi, 1S96.
5. LANKESTER, E. R.—" Limulus iu Arachnid," ' Quart. Journ. Micro. Soc.,'xxi, 1881.
6. LANKESTEK, E. R.—"On the Skeletotropliic Tissues, etc., of Limulus,Scorpio, and Mygale," ' Quart. Journ. Micro. Snc.,' xxiv, 1884.
7. LANKESTEK, E. 11.—" Comparison of the Muscular and EndoskelelalSystems of Limulus and Scorpio," 'Trans. Zool. Soc. London,' xi,1885.
8. LAURIE, M.—" On the Morphology of the Pedipalpi," ' Journ. Linn.Soc. Zool.,' xxiv, 1894.
9. SCHIMKEWITSCTT, W.—" Sur la Structure . . . . de l'Endosternite, etc.,"' Zool. Anz.,' 1893.
10. SCHIMKEWITSCH, W.—" Ueber Bau und Entwicklung des Entost.ernitesder Aracliniden," ' Zool. Jahrb. Anat.,' viii, 1894.
11. WASMANN, —.—"Anntoniie der Spinnen," ' Abh. Ver. Hamburg,' 1846.
EXPLANATION OF PLATES 13 & 14,
Illustrating Mr. Pocock's paper, " Studies on the ArachnidEntosternite."
With the exception of Fig. 1 the figures have heen drawn by the authorfrom entosternites dissected and preserved with others, not here figured, inthe Natural History Museum, where they are available for examination. Inthe figures representing the dorsal and ventral views of the entosternite theanterior extremity is uppermost; in those showing the lateral surface theanterior extremity lies to the right. In the case of the so-called entosterniteof the Solifugse, however, the distal or anterior extremity is directed down-wards.
LETTERING COMMON TO MOST OF THE FIGUBES, AND INDICATING
SUGGESTED HOMOLOGIES.
A. P. Anterior process or cornu of right side forming part of the frameworkof the pharyngeal notch (PA. N.). ~\tg.,'i,tg., Ztff., 4tg., $lg. Dorsal processes
STUDIES ON THE ARACHNID ENTOST.EttNl'J'E. 259
or muscles representing the dorsal moieties of the tergo-slernai muscles of thesomites bearing the first, second, third, fourth, and fifth post-oral appendngcs.1st., 2st., 'Asi., 4st. Ventral processes representing the ventral moieties of thesame muscles. Or. Lateral crest developed mainly to support some of themuscles of the appendages.
PLATE 13.
FIG. 1.—Dorsal view of enloslernil.e of the American Limulus (X. poly-phemus), showing the two pairs of long slender ant.ero-lateral processes(ilg. and &tg.), the stunted muscle-bearing process ('hlff-), find the large pos-terolateral process (5lg.). (After Lankesler.)
FIG. 2.—Dorsal view of the entosternite of a Thelyphonid (Mastigo-proctus giganteus) showing the dorsal processes (ltff. to 5/g.), which areconsidered to represent the dorsal moieties of the tergo-sternal muscles ofthe second to the sixth somites of the prosoma; sa.1, supernumerary or addi-tional apophysis, which has perhaps arisen from the fission of the apophysisnumbered 2lff.; a. b., anterior bridge, and p. b., posterior bridge, boundingthe large foramen in front and behind; the smaller foramen is shown behindthe posterior bridge; 1.1/., lateral bar, showing perhaps the origin of I hisportion of the entosternite from a great longitudinal muscle, or from pairedsubepidermal ventral entochondrites ; P.F., posterior plate, with frayed edgeindicating the attachment of radiating muscles, the suggested homologue ofthe dorsal portion of the "diaphragm " in the Scorpions (see PI. 14, figs. 21,22); or., lateral crest.
FIG. 3.—Dorsal view of the entosternile of a Phrynid (Damon John-ston i), showing the duplication of the apophyses numbered \tg.and 'i/g.; cr.,lateral crest, to which leg-muscles are attached; 1st., anterior ventralapophysis, the suggested representative of the sternal moiety of the tergo-sternal muscle of the second segment of the prosoma.
PIG. i-.— Dorsal view of entosternite of one of the Mygalomorphre (Ephe-bopus murinus), with same lettering as in the last figure, showing theabsence of supernumerary apophyses and I lie presence of a thickened ridge(p.b.), the suggested homologue of the posterior bridge in the enl.osternite ofThelyphonus.
]?IG. 5.—Dorsal view of the entosternite of Stasimopus Schonlandi, aMygalomorphous spider of the family Clenizidse, showing the presence of twosupernumerary apophyses (sa.1 and sa.1) and the fusion of the anterior ventralapophyses (1st.) to form a neural collar.
FIG. 6.—Dorsal view of entoslernile of Actinopus Wallacei, a Mygalo-morphous spider of the family Aetinoporiidec, showing the fusion of the fourventral apophyses to form a subnciirul arch with perforated walls for the exit
260 R . i . POCOCK.
of the appendicular nerves; 1st., anterior ventral apophysis ; m., median pro-cess from the floor of the subneural arch.
FIG. 7.—Dorsal view of entosternite of a young Liphist ius, sp. ?, show-ing the thick dorsal apophyses (1/^. to Ug.), the two pairs of supernumeraryapophyses (««.' and saF), and the absence of anterior ventral apophyses.
PIG. 8.—Dorsal view of the entosteriiite of Nephila femoralis, anAraclinomorplious spider of the family Argiopidse, with the single pair ofsupernumerary apophyses (sa.1), and without anterior ventral apophyses, as inLiphist ius.
PIG. 9.—Lateral view of entosternite of a Thelyphonid (Mastigoproctusgiganteus) 1st. and 2sl., first and second ventral apophyses ; other letteringas in Fig. 2.
FIG. 10.—Ventral view of entosternite of Pachylomerus nidulans, aMygalomorphous spicier of the family Ctenizirlse, showing the persistence ofthe first (1st.) and fourth (4*/.) ventral apophyses, and the disappearance ofthe second and third.
FIG. 11.—Lateral view of the same, showing the four dorsal apophyses{\tg. to &lg.) ; p. m., posterior median crest.
FIG. 12.—Lateral view of entosternite of Actinopus Wallacei (seeFig. 6), showing the four dorsal (ltg. to 4lg.) and four ventral (lsl. to4s/.) npophyses, the latter meeting in the middle line beneath the nervousmass, leaving lateral spaces for the exit of nerves; m., median process fromsubneural arch.
FIG. 13.—Ventral view of entosternite of Ephebopus murinus (seeFig. 4), showing the persistence of the four veniral apophyses (1st. to 4s/.),wilh indications of I he fifth (5s/.).
PLATE 14.
FIG. 14.—Lateral view of the entosternite of Ephebopus murinus,showing the four dorsal (llg. to Hff.) and the four sternal (lsl. to 4s/.) apo-physes. This figure clearly indicates the correspondence between the dorsaland ventral apophyses, which suggests their origin from tergo-sternal muscles(see PI. 13, figs. 4 and 13).
FIG. 15.—Ventral view of entosternite of Acanthodon opifex, aMygalomorphous spider of the sub-family Idiopinffi, showing the persistenceof the first, second, and third pairs of apophyses (lsl. to 3s/.), those of thefourth pair (4s/.) being rudimentary.
FIG. 16.—Lateral view of the same, showing the duplication of the thirdand fourth dorsal apophyses, and the absence of the fourth ventral apophysis.
FIG. 17.—Mygalomorphous spider of the genus Atypus, dissected from the
STUDIES ON THJL ARACHNID JENTOSTERN1TE. 261
dorsal side with entosternite in situ to show the muscles radiating to thepost-oral appendages II to VI, tlie anterior longitudinal muscles which passfrom an entosclerite above the rostrum to the extremities of the anteriorcornua, and the longitudinal muscles which pass backward into the pregenitalsomite.
FIG. 18.—Lateral view of entosternite of the same, with the four dorsal (Ug.to Ug.) and four ventral (1J/. to isl.) apopliyses, and the tendon runningforwards from the first ventral apopliysis to the prosternum.
FIG. 19.—Ventral view of the same, showing the arrangement of the fourventral apophyses in a circle round the pharyngeal notch.
FlG. 20.—Entosternite of PalamnaeusThorelli , with the posterior flap ordiaphragm removed, showing the fibro-muscular attachment of the anteriorcornu to Ihe coxa (ex.) of the fourth appendage ; cr., lateral processes repre-senting the muscle-bearing crest seen in Thelyphonus; 4clg., 5tg., anteriorand posterior pair of dorso-ventral muscles, the suggested homologues of theapophyses numbered Ug. and 5(g. in the entosternite of Thely phonus(PI. 13, fig. 2), and representing in all probability the tergo-sternal muscles ofthe fifth and sixth somites of the prosoma; V.P., median process ofsubneuralarch dividing into a pair of apophyses; Da., median dorsal portion of " body "of eutosternite forming the roof of tlie neural canal.
FIG. 21.—Entosternite of one of tlie Vejovidse ( lurus dufoureius), withposterior flap (P. F.) or diaphragm attached, to show its corretpondence inorigin with the lateral crest (Cr.), and its median perforations for the gut (Al.C.)and aorta (Aoc), between which lies the channel for the lodgment of UieaorUi,formed by the dorso-ventral muscle of the second pair (5ig.) and a strip ofconnective tissue, which binds the right and left portions of the diaphragmtogether.
FIG. 22.—Entosternite of Bothr iu rus bonariensis, showing the re-duction of the median dorsal portion of the "body," forming the roof of theneural canal, to a narrow transverse bar (Da.).
FIG. 23.—Anterior view of entosternite of the same, showing the neuralcanal (N. C), dorsal arch (Da.), and subneural arch (Sa.).
FIG. 24.—Entosternite of Centruroides margaritatus, with most of thediaphragm removed, showing the lateral compression of the " body " or dorsalarch (Da.) of the neural canal, the juxtaposition of the second pair of dorso-ventral muscles (&(g.), and the tips of the apophyses of the median processof the subneural arch (P.P.).
FIG. 25.—EntosteruiLe of the same, wiLh its dorsal portion removed to showthe cut ends of the lateral walls (La.) of the neural canal, the floor (Sa.) ofthe latter and the median process terminating in two expanded fan-shapedapophyses (V.P.).
FIG. 26.—The so-called eutosternite of Solpuga sagit taria, cleaued
262 B. T. POCOC.K.
with caustic potash, to show the continuity of its supporting chitinous strandswith those of the prosternal plate (pis.), which is wedged in between thecoxa? of the appendages of the third pair and its attachment to the sternalmembrane (;«.), between the third and fourth somites of the prosoma; Ls., leftbar of the entostcriiite, which expands at the free extremity to form with itsfellow of the opposite side a supporting channel (dl.) for the alimentary canal.
1'IG. 27.—Lower extremity of the two pillars of the entosternile ofGaleodes urabs, to show this articulation (•«.) with the prosternal plate(prs.), its union with the sternal membrane (»/.); ear., coxa of appendage ofthird pair.
