Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ARACHNIDES.
large vessel, which stretches along the back, and gives off
branches forwards and on either side.1 The feet are con¬
stantly eight in number. The head is always confounded
with the thorax, and presents two pincers (mandibulce of
authors, cheliceres or antenne-pinces of Latreille) terminat¬
ed by two finger-like projections, of which one is mov¬
able. or by a single hook or claw, always movable. The
mouth is composed of a lip (labium) ; of two palpi, which
sometimes assume the shape of arms or talons; of two or
four maxillae, formed, when there is only a pair, by the
radical article of the palpi, and when there are four, by
that same article and the corresponding portion of the first
pair of feet; and of a languette of one or two pieces. If
we were to assume as a basis the progressive diminution
in the number of the pulmonary sacks and stigmata, then
the scorpions which have eight, while the other Arach-
nides have only four or two, would form the leading ge¬
nus of the class, and the family of Pedipalpi would take
precedence of the Araneides or spinners; but these last-
named Arachnides are to a certain extent isolated, by rea¬
son of the sexual organs of the male, by the hook of their
frontal talons, by their pediculated abdomens, and the pe¬
culiarities of their spinning apparatus, as well as by their
natural habits; and the scorpions also appear to form a
more natural transition from the pulmonary Arachnides to
the family of the pseudo-scorpions, the first of the second
order.
Of all the Arachnides the Pulmonarice exhibit the great¬
est analogies to the crustaceous class, especially to the
genus Limulus and others of the paecilopodous order. The
pneumo-branchia and their stigmatiform openings may
frequently be detected externally by yellowish-white
markings, disposed in two longitudinal series. The first
two are placed immediately beneath the sexual organs, at
least in the females, at a small distance from the origin of
the abdomen, and on its second segment, when that part
is annular or divided into segments. Thus the second
segment in these Arachnides corresponds in its characters
to the first segment of the female Limulus. We may also
perceive in them the indications of conglomerated glands,
and even in certain species traces are observable of chili-
ferous vessels.
The claw of the mandibles in spiders, and the terminal
joint of the tail in scorpions, form a species of dart, perfo¬
rated by one or two openings, which give issue to a poi¬
sonous liquid secreted by special glands. This poison is
mortal to such small creatures as form the natural prey
of the Arachnides, and is even productive of dangerous
consequences to man and the larger animals. Its mode
of action on animals unprovided with a circulating sys¬
tem is not clearly understood, but the phenomena attend¬
ing it might reasonably be adduced in support of that
theory which advocates the agency of poison from the bite
of venomous reptiles as being carried on as much through
the medium of the nervous as the circulating fluid. The
almost instantaneous death of animals from the bite of
certain snakes has been regarded as a proof that the vas¬
cular system was not alone concerned; and the same sud¬
den effect produced upon those classes in which, as far
as we can perceive, no vascular system exists, demon¬
strates some other mode of action.
Notwithstanding the researches of Cuvier, Marcel de
Serres, Leon Dufour, Treviranus, and other observers,
om knowledge of the internal organization of several ge-
Arachnides is extremely deficient. Hence it is
difficult to trace the boundaries of the orders in a manner
at once natural and precise. At the same time it has
been remarked, that the accurate observation of the eyes
and other external organs furnish characters which coin¬
cide with the distinctions deduced from internal structure,
as far as the latter has been ascertained.
Family I—-Araneides.
This extensive family corresponds to the genus Aranea
of Linnaeus, and contains all those species commonly call¬
ed spiders—the Arachnides fileuses of Latreille. The cha¬
racters are, two or four branchial pouches; from six to
eight simple eyes; last article of the mandibles (cheliceres)
in the form of a corneous claw, perforated at the extre¬
mity for the emission of poison, and folded upon the pre-
ceding joint; abdomen usually soft, without divisions,
its extremity furnished with from four to six small teat¬
like appendages, pierced with numerous holes for the pas¬
sage of the silk or spinning materials ;2 feet palpi, without
pincers at the extremity, but terminated in the female by
small hooks, and in the male by the generative organs.
The maxillae are never more than two in number. The
languette consists of a single piece, always external, and
placed between the maxillae; its form more or less square,
sometimes triangular or semicircular. The thorax, usu¬
ally impressed with a form resembling the letter Y, indi¬
cating the space occupied by the head, is composed of a
single article. The legs, of which the forms are analo¬
gous, though the dimensions differ considerably, are com¬
posed of seven joints, of which the first two form the
haunch, the third the thigh, the fourth and fifth the leg,
and the remaining two the tarsi; the last is terminated
by a couple of hooks, which are usually toothed or pecti¬
nated, and in many species there is an additional tooth of
smaller size, but not pectinated. The intestinal canal is
straight: there is first a stomach composed of several
sacks; and towards the middle of the abdomen a second
dilatation occurs. According to Marcel de Serres the heart
is situated in the abdomen, and stretches throughout its
whole length; there is a considerable swelling towards its
superior extremity, after which it assumes and retains the
cylindrical form. It is very muscular, and its pulsations
are strong and frequent. The pulmonary pouches, usu¬
ally two in number, are always situated on the lower sur¬
face of the abdomen, near its origin, and are covered by a
coriaceous skin, generally of a red colour; the stigmati¬
form opening proper to each pouch is placed towards its
base, on the inner side. These pouches are formed of a
white membrane, strong but flexible, which presents on its
interior, transverse, projecting, parallel, nearly semicircu¬
lar folds or plates, which constitute the respiratory organ.
The liver is proper to the abdomen, of which it occupies
the chief portion. It is composed of an infinity of minute
glands fixed to the intestinal canal, and filled with a pe¬
culiar liquid, thick, and of a brown colour. The interior
of the abdomen also contains the silk vessels, four in num¬
ber, long, cylindrical, folded, yellow. They open into a
common canal, situated at the origin of the spinners.
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1 According to M. Marcel de Serres (Memoire sur le Vaisseau Dorsal des Inscctes), the blood in the araneides and scorpions pro¬
ceeds first to the respiratory organs, and from thence by special vessels to the various parts of the body. Latreille, however, seems
to think, from the relations which exist between these creatures and the crustaceous tribes, that such circulation may be effected in
the contrary direction. See the Regne Animal, tome iii. p. 212; and a memoir bv Treviranus On the Internal Organization of Arach¬
nides. 1 vol. 4to, Nuremberg, 1812.
2 Some naturalists are of opinion that the two small spinners placed in the centre of the four exterior ones yield no silk-