Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

eight legs, is, strictly speaking, of a Hexapod type ; for the
anterior pair, ordinarily regarded as legs, and performing
their function, are really the analogues of the maxillary
palpi of perfect insects. This will be evident to you if
ou examine any species of Galeodes. These animals, if we
ook at them cursorily, we should regard as decapods; but
when we trace the two anterior pairs of apparent legs to
their insertion, we find that both proceed from the head,
which in that genus is distinct from the trunk; while the
three last pairs, which alone are furnished with claws, are
planted, as legs usually are, in the latter part. The first
A R A C H N I D E S.
Phrynidea.
371
l
Dee. Mandibles unguiculate. Anterior palpi chelate
01 unguiculate, very robust. Posterior palpi pediform,
very long and slender. Abdomen divided into seg¬
ments. Spiracles two pairs. Anus terminating in a
mucro, and sometimes in a filiform jointed tail, with¬
out a sting at the end.4
In the Pecjne Animal (2d edition, 1829), Latreille in¬
cludes in the class Arachnides only those species which
... correspond to the Arachnides palpistes of M. Lamarck,
pair represent the ordinary palpi of Arachnida, are analo- By this arrangement he is of opinion they maybe classed
gous to the labial ones of hexapods, and, as likewise in and defined in a more simple and rigorous manner. Accord-
Phrynus and Thelyphonus, are more robust than what are ing to his most matured views, then, the Arachnides, like
usually taken for the first pair of legs ; but they differ in the Crustacea, are destitute of wings, and are not subject
being considerably longer, and, instead of terminating in to metamorphoses, but only to simple changes. Their
a chela, are furnished with a retractile sucker.1 The sexual organs are removed from the posterior extremity
second pair are more slender and shorter than the first, of their bodies, and situated, with the exception of cer-
They correspond precisely with what are deemed the first tain males, at the base of the abdomen. They bear a re¬
pair of legs of Octopods and Arachnida, and are clearly semblance to insects in as far as the surface of their bo-
analogous to the maxillary palpi of insects. Whether the dies present openings or transverse clefts named stigmata,
base of the first pair of these palpi is in any respect ana- for the entrance of air, but in smaller number than in in-
logous to the labium of insects (as that of the second sects (eight at most, generally two), and placed only at
seems to be to their maxillae), I am not prepared to assert; the inferior part of the abdomen. Their respiration is
it will therefore be most advisable to name these palpi an- farther carried on either by aerial branchia, performing
terior wad posterior ; but as they evidently proceed from the office of lungs, inclosed in the cells of which the open-
the head in Galeodes, and in that genus are clearly analo- ings just mentioned are the entrance, or by means of ra-
gous to those of the Phrynidea (which in their turn as *—1 rr'1 n • • • ” •
clearly represent those of the Aranidea), it follows, that
in all they are organs of the part representing the head,
and therefore not in a primary sense legs, although in
a secondary, as M. Savigny2 has proved, they may be so
called.”3
diated tracheae. The organs of vision consist of small sim¬
ple eyes, variously grouped when numerous. The head,
usually distinct from the thorax, presents, as analogous to
the antenna- of insects, two articulated appendages, in the
form of small talons of two or more pieces, which have
been improperly compared to the mandibles of insects,
The following are the secondary groups of the class though moving in a different direction. They, however, co-
Arachnides, according to Mr Kirby’s exposition, in the
work last quoted.
1. Aranidea.—M‘Leay. (Aranea, L., Araneidce, Lat.)
The Aranidea or spiders seem resolvable into two
sub-orders—the Sedentaries and the Wanderers;
thus forming, perhaps, what Mr M‘Leay would de¬
nominate the normal groups of a circle oi Arachnida.
Dee.—Mandibles armed with a perforated claw. Head
and trunk coalite. Palpi pediform, anterior pair
without claws. Abdomen without segments or elon¬
gated tail. Spiracles two. Anus furnished with an
apparatus for spinning.
2. Scorpionidea.—M‘Leay. (Scorpio, L. Latr.)
Dee.—Mandibles chelate. Head and trunk coalite. An¬
terior palpi chelate. Posterior palpi pediform. Pec-
tens two. Abdomen divided into segments, and ter¬
minating in a jointed tail, armed at the end with a
sting. Spiracles four pair.
3. Galeodea.
Dee.—Head distinct. Eyes two. Mandibles chelate,
with dentated chelae. Palpi pediform, the anterior
pair thickest, with a retractile sucker. Trunk con¬
sisting of two principal segments, with a minute sup¬
plementary posterior one. Spiracles two, placed in
the trunk. Pseudo-pectens two. Abdomen divided
into segments. Anus unarmed, and without a spin¬
ning apparatus.
operate in the action of the jaws, and are represented in
those Arachnides of which the mouth is formed like a sy¬
phon or sucker by two pointed plates or lancets.5 A sort
of lip (labium, Fab.), or rather tongue, formed by a pecto¬
ral prolongation; two maxillae, formed by the radical part
of the first article of two small feet or palpi,G or by a lobe
or appendage of that article; a beak-shaped projection,
named sternal tongue (langue sternale) by M. Savigny,
produced by the re-union of a very small chaperon, ter¬
minated by a minute triangular lip, and of a longitudinal
inferior keel, usually hairy:—these, and the parts called
mandibles, compose, under certain modifications, the gene¬
ral structure of the mouth in the class Arachnides. The
pharynx is placed in advance of a sternal projection, which
has been regarded as a lip; but when we consider its po¬
sition behind the pharynx, and the absence of palpi, it is
probably rather analogous to a tongue (languette). The
feet, like those of insects, are generally terminated by two
crotchets, sometimes by three, and are all attached to the
thorax, which is almost always formed of a single articu¬
lation, for the most part intimately united to the abdo¬
men. The abdomen in the greater proportion of species
is soft, or but slightly protected.
When considered in relation to their nervous system,
the Arachnides are obviously distinguished both from the
Crustacea and Insects; for, with the exception of the scor¬
pions, the ganglia or swellings of the nervous cords never
exceed three in number.
The greater proportion of Arachnides feed on living in¬
sects, the bodies of which they seize or adhere to, and
1 Leon Dufour, Six Nottv. Arach. S[C. Ann. Gcn.dcs Sciences Physiq. iv. iii. 17, t. Ixix. f. 7- b-
2 Mem. sur les Anim.sans Veriebres. 3 Introduc. to Entom. vol. iv. p. 3CG. (1826.) 4 Tnd. p. .188.
4 Chcliceres or Antenne-pinces of the French writers.
* According to Latreille, these do not differ from feet properly so called, except in the tarsi, composed of only a single joint, and
usually terminated by a small crotchet; they almost entirely resemble the ordinary feet of the Crustacea.