The Structure and Classification of the Arachnida.

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STRUCTURE AND CLASSIFICATION OF THE ARACHNIDA. . 165
The Structure and Classification of the
Arachnida.
By
E. Ray Lankestcr, M.A., L.L.D., F.R.S.,
Director of the Natural History Departments of the Brit,ish Museum.
(Reprinted by kind permission of the proprietors from the tenth edition of
the 'Encyclopedia Britannica.')
ARACHNIDA is the name given in 1815 by Lamarck (Greek
apa\vr), a spider) to a class which he instituted for the recep-
tion of the spiders, scorpions, and mites previously classified
by Linnaeus in the order Aptera of his great group Insecta.
Lamarck at the same time founded the class Crustacea for
the lobsters, crabs, and water-fleas, also until then included
in the order Aptera of Linnams. Lamarck included the
Tliysanura and the Myriapoda in his class Arachnida. The
Insecta of Linnseus was a group exactly equivalent to the
Arthropoda founded a hundred years later by Siebold and
Stannius. It was thus reduced by Lamarck in area, and
made to comprise only the six-legged, wing-bearing " In-
secta." For these Lamarck proposed the name Hexapoda;
but that name has been little used, and they have retained
to this day the title of the much larger Linnsean group, viz.
Insecta. The position of the Arachnida in the great sub-
phylum Arthropoda, according to recent anatomical and
embryological researches, is explained in another article
(AKTHEOPODA). The Arachnida form a distinct class or line
of descent in the grade Euarthropoda, diverging (perhaps in
common at the start with the Crustacea) from primitive
Buarthropods, which gave rise also to the separate lines of
VOL. 48, PART 2. — NEW SERIES. 12
166 B. RAT LANKESTER.
descent known as the classes Diplopoda, Crustacea, Ohilo-
poda, and Hexapoda.
TIG. 1.—Entosternum, entostcrnite or plastron of Limulus
polyphemus, Linn. Dorsal surface. LAP, left anterior process;
RAP, right anterior process ; PAN, pharyngeal notch; ALR, anterior
lateral rod or tendon ; PLR, posterior lateral rod or tendon ; PLP,
posterior lateral process. Natural size. (From Lankester, ' Q. J.
Micr. Sci./ N.S., vol. xxiv, 1884..)
FLP.-
FIG. 2.—Ventral surface of the entosternum of Limulus poly-
pliemus, Linn. Letters as in Fig. 1 with the addition of NF,
neural fossa protecting the aggregated ganglia of the central
nervous system; PVP, left posterior ventral process; PMP, pos-
terior median process. Natural size. (From Lankester.)
Limulus an Arachnid.—Modern views as to the classifi-
cation and affinities of the Arachnida have been determined
STRUCTURE AND CLASSIFICATION OF THE ARACHNLDA. 167
by the demonstration that Limulus and the extinct Eury-
pterines (Pterygotus, etc.) are Arachnida; that is to say, are
identical in the structure and relation of so many important
parts with Scorpio, whilst differing in those respects from
other Arthropoda that it is impossible to suppose that the
identity is due to homoplasy or convergence, and the con-
FIG. 3. FIG. 4.
FIG. 3.—Entosternum of Scorpion (Palamnccus indus, De
Geer); dorsal surface, asp, paired anterior process of the sub-
neural arch; sap, sub-neural arch; ap, anterior lateral process (same
as RAP and LAP in Fig. 1); Imp, lateral median process (same as
ALB and PLR of Fig. 1); pp, posterior process (same as PLP in
Fig. 1); pf, posterior flap or diaphragm of Newport; w1 and m-,
perforations of tlie diaphragm for the passage of muscles; DR, the
paired dorsal ridges; GC, gastric canal or foramen; AC, arterial
canal or foramen. Magnified five times linear. (After Lankester,
loc. cit.)
FIG. 4.—Ventral surface of the same entosternum as that drawn
in Fig. 3. Letters as in Fig. 3 with the addition of NC, neural
canal or foramen. (After Lankester, loc. cit.)
elusion must be accepted that the resemblances arise from
close genetic relationship. The view that Limulus, the king-
crab, is an Arachnid was maintained as long ago as 1829 by
Straus-Durkheim (1), on the ground of its possession of an
internal cartilaginous sternum—also possessed by the Arach-
nida (see Figs. 1—6),—and of the similarity of the disposition
of the six leg-like appendages around the mouth in the two
168 E. RAY LANKESTER.
cases (see Figs. 45 and 63). The evidence of the exact
equivalence of the segmentation and appendages of Limulus
and Scorpio, and of a number of remarkable points of agree-
ment in their structure, was furnished by Lankester in an
article published in 1881 ("Limulus an Arachnid," 'Quart.
Journ. Micr. Sci./ vol. xxi, N.S.), and in a series of subse-
quent memoirs, in which the structure of the entosteruuin, of
the coxal glands, of the eyes, of the veno-pericardiac muscles,
FIG. 5. FIG. 6.
FIG. 5.—Entosternum of one of the mygalomorphous spiders;
ventral surface. Ph.N., pharyngeal notch. The three pairs of rod-
like tendons correspond to the two similar pairs in Limulus, and
the posterior median process with its repetition of triangular seg-
ments closely resembles the same process in Limulus. Magnified
five times linear. (From Lankester, loc. cit.)
FIG. 6.—Dorsal surface of the same entosternum as that drawn in
Fig. 5. Ph.N., pliaryngeal notch. (After Lankester, loc. cit.)
of the respiratory lamellee, and of other parts, was for the
first time described, and in which the new facts discovered
were shown uniformly to support the hypothesis that Lirnulus
is an Arachnid. A list of these memoirs is given at the close
of this article (2, 3, 4, 5, and 13). The Burypterines (Gigan-
tostraca) were included in the identification, although at
that time they were supposed to possess only five pairs of
anterior or prosomatic appendages. They have now been
shown to possess six pairs (Fig. 47), as do Limulus and
Scorpio.
The various comparisons previously made between the
r-4 W- ••
PA.
FIG. 7.—Diagram of the dorsal surface of Limulus poly-
piiemus. oc, lateral compound eyes; oc', central monomeniscous
eyes; PA, post-anal spine; I to VI, the six appendage-bearing
somites of the prosoma; VIT, probably to be considered as the
tcrgum of the genital somite; VII to XII, the six somites of the
mesosoma; XIII to XVIII, the six somites of the metasoma, of
which the first (marked XIII at the side and 7 on the tergum) is
provided with a lateral spine, and is separated by ridges from the
more completely fused five hinder somites lettered 8 to 12.
[This is a new figure replacing the Pig. 7 given in the ' Encyclo-
paedia. It is at present a matter for further investigation as to
whether the praagenital somite is merely represented by the piece
marked x at the hinder border of the prosoma, or whether the area
marked VII is the tergum of the praegenital somite, and that marked
VIII the tergum of the genital somite. The disposition of the
muscles and of the entopophyses should, when carefully studied, be
sufficient to settle this point.—E. R. L.j
170 E. BAY LANKESTER.
FIG. 8. structure of Limulus and the Eurypterines
on the one hand, and that of a typical
Arachnid, such as Scorpio, on the other,
had been vitiated by erroneous notions as
to the origin of the nerves supplying the
anterior appendages of Limulus (which
were finally removed by Alphonse Milne-
Edwards in his beautiful memoir [6] on
the structure of that animal), and secondly
by the erroneous identification of the double
FIG. 9.
•XV
xvn
FIG. S.—Diagram of the dorsal surface of a Scorpion to compare
with Fig. 7. Letters and Roman numerals as in Fig. 7, excepting
that VII is here certainly the terguni of the first somite of the
mesosoma—the genital somite—and is not a survival of the embry-
onic prsegeniul somite. (From Lankester, loc. cit.) The anus (not
seen) is on the sternal surface.
FIG. 9.—Ventral view of the posterior carapace or meso-mela-
somatic (opisiliosomatic) fusion of Limulus polypliemus. The
soft integument ami limbs of the mesosoma have been removed as
well as all the viscera aud muscles, so that the inner surface of the
terga of these somites with their entopophyses are seen.' The un-
sef*mented dense chitinous, sternal plate of the metasoma (XIII to
XVIII) is not removed. Letters as in Fig. 7. (After Lankester,
loc. cit.)
STEUCTUKJi AND CLASSIFICATION 01' THE AUACHNJDA. 171
sternal plates of Limulus, called " chilaria " by Owen, with
a pair of appendages (7). Once the identity of the chilaria
with the pentagonal sternal
FIG. 10. plate of the scorpion is
recognised — an identifica-
tion first insisted on by Lan-
kester—the whole series of
segments and appendages in
the two animals, Liniulus and
Scorpio, are seen to corres-
pond most closely, segment
for segment, with one an-
other (see Figs. 7 and 8).
FIG. 11.
]?IG. 10.—Ventral view of a Scorpion, Palarnnseus indus, De
Geer, to show the ariargemeut of the coxae of the limbs, the sternal
elements, genital plate and pcctens. M, mouth behiud the oval
median camerostome; I, the chelieerse; II, the chelse ; III to VI,
the four pairs of walking legs; Yllffo, the genital somite or first
somite of the mesosoma with the genital operculum (a fused
pair of limbs); Vlllp, the pectiniferous somite; IXs^ to XUstg,
the four pulmonary somites; met, the pentagonal metasternite of
the prosoma behind all the coxse; x, the sternum of the pectinifer-
ous somite; y, the broad first somite of the metasoma.
FIG. 11.—Third leg of Limulus polyphemus, showing the
division of the fourth segment of the leg by a groove S into two,
thus giving seven segments to the leg as in Scorpion. (From a
drawing by Mr. Pocock.)
The structure of the prosomatic appendages or legs is also
seen to present many significant points of agreement (see
172 E. BAY LANKESTER.
Figures), but a curious discrepancy existed in the six-jointed
structure of the limb in Limulus, which differed from the
seven-jointed limb of Scorpio by the defect of one joint.
Mr. R. I. Pocock, of the British Museum, has lately observed
that in Limulus a marking exists on the fourth joint, which
apparently indicates a previous division of this segment into
two, and thus establishes the agreement of Limulus and
Scorpio in this small feature of the number of segments in
the legs (see Fig. 11).
It is not desirable to occupy the limited sp"ace of this
article by a full description of the limbs and segments of
Limulus and Scorpio. The reader is referred to the complete
series of figures here given, with their explanatory legends
(Figs. 12—15). Certain matters, however, require comment
and explanation to render the comparison intelligible.1 The
tergites, or chitinised dorsal halves of the body rings are
fused to form a " prosomatic carapace," or carapace of the
prosoma, in both Limulus and Scorpio (see Figs. 7 and 8).
This region corresponds in both cases to six somites, as
indicated by the presence of six pairs of limbs. On the
surface of the carapace there are in both animals a pair of
central eyes with simple lens and a pair of lateral eye-tracts,
which in Limulus consist of closely aggregated simple eyes,
forming a " compound " eye, whilst in Scorpio they present
1 The discussion of the segmentation or metamerism of the Arachnida in
this article should be read after a perusal of the article AKTUEOPOBA bv the
same author (' Q,. Journ. Micr. Sci.,' vol. xlvii, n. s. p. 523).
FIG. 12.—The prosomatic appendages of Limulus polyphemus
(right) and Scorpio (left), Palamnaius indus compared. The
corresponding appendages are marked with the same Roman numeral.
The Arabic numerals indicate the segments of the legs , cox, coxa
or basal segment of the leg ; tie, the sterno-coxal process or jaw-
like upgrowth of the coxa; epc, the articulated movable outgrowth of
the coxa, called the epicoxite (present only in III of the Scorpion
and III, IV, and V of Limulus) ; ezx, the exopodite of the sixth
limb of Limulus; a, 6, c, d, movable processes on the same leg (see
for some suggestions on the morphology of this leg, Pocock in
' Quart. Journ. Micr. Sci.,' March, 1901; see also Fig, 50 on p. 235
and explanation). (From Lankester, loc. cit.)
STRUCTURE AND CLASSIFICATION OF THIS ARA0HN1DA. 173
PIG. 12.
174 E. EAT LANKESTER.
several separate small eyes. The microscopic structure of
the central and the lateral eyes has been shown by Lankester
and Bourne (5) to differ; but the lateral eyes of Scorpio were
shown by them to be similar in structure to the lateral eyes
of Limulus, and the central eyes of Scorpio to be identical in
structure with the central eyes of Limulus (see pp. 182, 183).
Following the prosoma is a region consisting of six seg-
ments (Figs. 14 and 15), each carrying a pair of plate-like
appendages in both Limulus and Scorpio. This region is
called the mesosoma. The tergites of this region and those
FIG. 13.—Diagrams of the metasternite si, with genital operculum
op, and the first lamelligerous pair of appendages ga, with uniting
sternal element si of Scorpio (left) and Limulus (right). (From
Lankester, loc. cit.)
of the following region, the metasoma, are fused to form a
second or posterior carapace in Limulusj whilst remaining
free in Scorpio. The first pair of foliaceous appendages in
each animal is the genital opercnlum; beneath it are found
the openings of the genital ducts. The second pair of meso-
somatic appendages in Scorpio are known as the " pectens."
Each consists of an axis, bearing numerous blunt tooth-like
processes arranged in a series. This is represented in
Limulus by the first gill-bearing appendage. The leaves
(some 150 in number) of the gill-book (see figure) correspond
to the tooth-like processes of the pectens of Scorpio. The