Studies on the Arachnid Entosternite.

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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. 46, 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 American
Limulus is well known, thanks to the figures and descriptions
of it published by Ray Lankester (5, 6) and Benham (2).
It is a longitudinally oblong plate, with a pair of stout
anterior bars, or cornua, forming the pharyngeal notch, two
pairs of long and slender apopliyses behind the anterior bars,
diverging nearly at right angles from the main body of the
plate, and a stout but short apophysis springing transversely
from its postero-lateral angle on each side. There is also an
irregular-shaped posterior median process, as well as a pair
of short apopliyses projecting subvertically beneath the
latter. In the Moluccan species T. gigas (= moluccanus),
as was shown by Van der Hoeven, there is only a single
long apophysis projecting from the lateral border in front.
This represents the anterior of the two that are found in this
position 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 put
forward ('Ann. Mag. Nat. Hist./ April, 1902) that these
STUDIES ON THE ARACHNID ICNTOSTERNIXE. 227
three forms belong to a group distinct from and more
specialised than that of polyphemus, as is clearly shown
by the structure of the genital operculum, etc. Also the
pair of posterior ventral apophyses fouud in polyphemus
are missing in the two young specimens of trideutatns
and rotundicauda I liavo examined. In all species of
Limulus the upper side of the entosteruite is furnished
laterally behind the middle with a short muscle-bearing
excrescence, 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 exoskeletal
metasteruite. In Palainuteus thorelli the "body" of
the entosternite consists of an irregularly transversely obloug
plate. From its anterior angles rise the anterior cornua,
which give off muscles to the appendages along their outer
edge, and present a frayed or ragged appearance when cleared
of these tissues (see fig. 20, PI. 14). On its underside the
plate dips down on each side of the nerve-cord, and passing
and fusing beneath it forms a complete and rigid ventral
ring through which the nerves pass backwards into the
mesosoma. The lower portion of this ring gives off in front
1 In liis paper on the anatomy of Li mulus polyphemus ('Trans. Liiiu.
Soc.,' xxviii, 1S73), Owen states (p. 4G9) that the entosternite of this species
is furnished with a pair of " sclerous processes " which diverge from " near
the 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 the
entosternite of L. moluccanus. Yet the original figures with which Owen's
paper is illustrated are all, apparently, taken from examples of the American
form (L. polyphemus). Hence it is difficult to account for his overlooking
the presence of the two pairs of processes in this species. It may also be
remarked in passing, though the lapsus is of no great moment, that the
statement in the foot-note to p. 462 that " the species which he [Van der
Hoeven] dissected was the rapier-tailed Molucca crab (Limulus rotundi-
cauda, Latr.) " is an error.
228 B. I. POCOCK.
a median process which divides into a pair of diverging
tendinous apophyses. From the sides of the neural ring
externally spring muscle-supporting processes. The muscles
rising from the posterior of these processes are extended
laterally and dorsally to become attached to the sides and
roof of the body-cavity, forming, with associated connective
tissue, 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 run
from the posterior side of the lower edge of the neural canal
to the floor of the body-cavity. In the middle line above the
"body" of the entosternite the connective tissue of this
muscular sheet is perforated by two foramina; the inferior
gives passage to the alimentary canal, the superior to the
aorta.
In addition to the muscles already mentioned, three pairs
of dorso-veutral muscles arise from the eutosternite. Those
of the posterior pair are attached to the underside of the
tergite of the genital somite behind the diaphragm, and to
the posterior side of the entosternite in front of it. Hence
in their passage from above downwards they pass through
the diaphragm. The median pair extends from the aortic
foramen in front of the diaphragm to the posterior border
of the upper surface of the "body" of the entosteruite.
Just in front of their inferior points of attachment spring
those of the anterior pair, which, rising vertically, meet in
the middle line above the aorta, before attaching themselves
to the underside of the carapace.
As might be expected, the entosternite of this species
agrees in all essentials with that of Palamnasus indus
(=Buthus 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. In
species with the metasternite antero-posteriorly compressed,
the body of the " entosternite" becomes shorter as compared
with its length, as shown in the figure of that of Iurus
STUDIES ON THE ARACHNID KNTOSTEENVIE. 229
dufoureius, one of theVejovidse. ID this species the lateral
crests which arise from the anterior cornua are better
developed than in Palamnseas, and the homologue of the
solid lateral process of the latter is less solidified and rigid.
Moreover the anterior process from the neural arch appears
to be undeveloped (PI. 14, fig. 21).
In Hadruroides charcasus, a member of the same
family as Iurus, but with the sternum showing a markedly
greater 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 of
the two are very similar.
The process of reduction in the size of the entosternite by
longitudinal compression is carried to an extreme in the
Bothriuridse (olim Telegonidte), where the sternum is
reduced to a transversely linear sclerite wedged in beLween
the genital operculum and the coxse of the appendages of
the fourth pair (= second walking leg). In Bothriurus
bonariensis (see PI. 14, fig. 22) the portion of the body of
the entosternite which forms the rooE of the neural canal
is reduced to a narrow transverse bar. This modification
seems to have been accompanied by the disappearance of
the anterior pair of dorso-ventral muscles; those of the second
pair pass up to the aortic foramen without fusing with the
diaphragm. As in the genera of Vejovidte examined,
the subneural process is apparently absent in the Both-
riuridte. The structure of the entosternite in this family
bears out the view I have elsewhere expressed that these
scorpions are a specialised offshoot of the Vejovidae.
In the ButhidiB, which are characterised by a triangularly
compressed sternum, the eutosternite shows unmistakable
signs of lateral compression, the "body" being reduced to
a 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, and
the subneural arch is furnished with a median process
230 B. I. POCOCTC.
ending in two fan-shaped apophyses similar to those of
Palamnaeus, but stouter.
These characteristics are illustrated in the figure of the
entosternite of Oentruroides margaritatus (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 of
Androctonus bicolor, which is quite unlike this plate in
any member of the Buthidas I have examined. Presumably
the form he names A. bicolor is the thick-tailed, dark-
coloured species from Transcaspia which Olivier called
crassicauda. In examples of this species the entosternite
closely resembles that of Centruroides margaritatus
(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 apophyses
running forwards from the subneural arch. Yet Schimke-
witsch represents the supra-neural arch as a transversely
oblong plate as wide in proportion to its length as in Hadru-
roides, fnrnisbed with lobate lateral projections, and a very
broad subneural process with unexpanding apophyses. An
entosternite of this description should belong to some species
with a broad and short pentagonal entosternite.
Speaking of: the entosternite of the scorpions, Bernard
(3) sa,ys that its points of attachment " to its parent
cuticle correspond with the points of origin of the ento-
sternite of Galeodes,"—that is to sa}r, to the integument
immediately above the preaxial surface of the coxa of the
fourth prosomatic appendage, or, as he elsewhere (4) ex-
presses it, between the third and fourth segments. In
Palamnaeus thorelli, the species examined by Bernard, I
find that the anterior bar of the entosternite has a fibro-
mnscular attachment to the in-projecting anterior rim of the
coxa of the fourth appendage (second leg). But I could not
satisfy myself that there was any union with the adjacent
integument,—certainly there was none such as to justify the
speaking of the integument as the "parent" of this bar
STUDIES ON THK AHAOHNID TCNTOSTERNITE. 231
of tlie entosfcernite. Bernard also homologises the anterior
bars of the scorpion's entosfcernite witli tlie second pair of
ventral apophyses which are affixed to the sternum opposite
the base of the third prosomatic appendage (first walking
leg) in the spiders. Since, however, these bars in the
scorpion give attachment diagonally to great muscles which
supply the second and tliird appendages (chela and first leg),
it seems far more likely that they represent the anterior bars
of the entosternite of the spiders, Thelj'phonns, etc., which
are similarly continuous with the muscles supplying these
appendages (PI. 14, fig. 20).
3. The Entosternite 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, one
characteristic of the Urofcricha (Thelyphonidas), the other of
the Amblypygi (PhrynidEe). In the Thelyphonidre the main
portion of tlie plate is longitudinally oblong in shape. It is
perforated mesially by two foramina, an anterior large and
oval, and a posterior relatively small and circular. The two
are separated by a transverse bridge; a similar bridge
separates the anterior foramen from the pharyngeal notch.
Near the edges of the upper sin-face of the right and left
bars forming the external framework of the pharyngeal
notch and of the foramina, rise five pairs of tendinous pro-
cesses which are affixed by muscular fibres to the underside
of the carapace. The first rises at the extremity of the
anterior cornu, the second just in front of the anterior
bridge. The latter apophysis is bifid and projects inwards,
backwards, and upwards towards the central depression of
the carapace. The others take a more lateral direction. The
tliird rises close to the second and a little behind the
anterior bridge ; the fourth just behind the middle of the
large foramen ; the fifth on a level with the smaller forameu.
Below the latter may be seen a bifid tendinous crest running
downwards and outwards. Behind this point the eutosternifce
232 R. I. POOOOK.
is laterally constricted, then expands into a subcircuhir softer
plate, to the ragged edge of which are fastened many
muscles passing to the pregenital somite and to the
appendages of the sixth pair. Sometimes at least there is, on
the upper side of this plate, a pair of short processes which
serially repeat apparently the longer tendons of the anterior
part of the eutosternite. From the underside in front arise
two pairs of processes, the first passing from the anterior ex-
tremity of tho cornua to the coxte of the chelte, the second
to the prosternum from a point on a level with the anterior
bridge (PI. 13, figs. 2, 9).
The figure and description of the entosternite of Thely-
phorms published by Laurie (8) do not agree with this
organ in the species examined by myself. The anterior three
pairs of dorsal processes and the two pairs of ventral pro-
cesses, as well as the lateral crest, are omitted from the figure
and unmentioned in the text; and I find no process pro-
jecting from the sides of the posterior lobe such as he
represents and describes. Bo, too, is the figure twice
published by Schimkewitsch (9, 10) from a sketch by
Tarnani 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 the
number of the processes. For example, the bifid process
numbered Sa.' in my drawing is represented as rising from
the side of the anterior bridge slightly behind the level
whence the processes numbered 2tg. diverge; and the pro-
cesses numbered 3tg. spring further back in line with the
posterior bridge, not just behind the middle of the larger
foramen, as shown in my drawing. The lateral crest, too,
was apparently unnoticed. Considering the uniformity
in the structure of the prosoma throughout the family Thely-
phonidte, it seems hardly probable that these discrepancies
are due to specific differences between the specimens examined.
I find a practically complete uniformity of structure in the
entosternite in species of the genera Thelyphonus,
Hypoctonus, and Mastigoproctus.
STUDIES ON THR ARACHNID ENTOSTERNITE. 233
The entosternite of the Amblypygi is very different from
that of the Urotricha. The pharyngeal notch is semi-
circular and the anterior cornua large. Each bears a pair
of dorsally directed apophyses near tlie apex, also one on the
underside, which dips down beneath the pharynx, and one
above, at the base, which projects upwards and inwards.
Tlie body of the plate itself is wide, narrowed posteriorly, and
solid, i.e. without foramina. Near its lateral border on each
side arise four apophyses which extend upwards and outwards
to be inserted by means of muscular bundles to the under sur-
face of the carapace. The first is very slendei-; the second
and third are approximated at the base ; the fourth is the
stoutest. These four spring from a common ridge beneath
which the edge of the entosternite runs ont externally into a
short angular crest to contribute support to the great
appendicuhir muscles (PI. 13, fig. 3).
It would be unfair to criticise the figure of the entosternite
of " Phrynus" given by Bernard (3), because " the prepara-
tion was accidentally destroyed before the drawing was
completed." Four pairs of dorsal apophyses are represented,
but I cannot satisfactorily homologise them with the six
pairs shown in the figure here published (PI. 13, fig. 3). It is
stated, moreover (op. cit., p. 20), that this plate has " only
one attachment to the ventral surface, and that is to the
infcersegmental membrane between the second and third pairs
of limbs corresponding with the first pair of npodeines
forming the entosternite in Mygale." It is true that there
is only one pair of ventral processes, and that they represent
the similarly situated processes in Mygale. They are not
attached, however, in the position Bernard states, but to the
coxa of the second appendage, the point of their insertion
appearing as a horny subcircnlar patch on the soft membrane
below the mouth, when these appendages are pulled apart
and 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 many
structural details. In Ephebopns murinus and other
mygalomorphous spiders of the family Aviculariidaj it is a
longitudinally oval imperforated plate, with large anterior
cornua bounding the pharyngeal notch. The upper side is
furnished with four pairs of dorsally directed tendinous
processes, arising, as in Phrynus, from a common ridge.
Below this the edge of the plate runs outexternally into angular
processes which afford attachment to some muscles of the
legs. From the underside four pairs of processes pass down-
wards to meet the sternum; the first pair arising from the
anterior cornua and running to the bases of tbe appendages
of the second pair, immediately behind the prosternnm ; the
second, third, and fourth radiating from a common median
excrescence, behind the pharyngeal notch, and reaching the
sternum opposite the third, fourth, and fifth appendages.
Remnants of a similar apophysis corresponding to the sixth
nppendage, but failing to I'each the sternum, are traceable
near the posterior end of the entosternite. The chief
difference 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 Mygale given by
Bernard (3, figs. 3 and 5), and taken from a specimen in
the College of Surgeons' Museum, is diagrammatic. It is to
be noticed, however, that the ventral processes of the first
pair 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 Ephebopus
murin us.
In the great majority of the Mygalomorphae the ento-
sternite is in the main like that of Ephebopus, retaining
the four dorsal and the four ventral apophyses, the points
of attachment of the latter being visible on the external
' In the figure published in the second of the two works enumerated above
the dorsal side is by an oversight represented as the ventral, and vice versa.