Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

arachnides.
Thus, of those great divisions which appear to derive
their origin almost from the same point in the scale, the
first is formed of an enormous series of animated beings,
characterized by the strongly contrasted aspects of larva
and perfect insect; and the others are true Arachnides,
and consequently present no such marked distinction be¬
tween the conditions of the young and old. In the most
perfect of the aiachnidean class, such as spiders and scor¬
pions, Cuvier has demonstrated a muscular and dorsal
heart, subject to sensible systole and diastole; and on the
abdomen he discovered several stigmatic openings (from
two to eight), which lead to an equal number of particu¬
lar purse-shaped cavities, each of which contained a great
many small and very delicate laminae or plates. These
isolated cavities, and the plates with which they are fill¬
ed, are without doubt the respiratory organs of these
Arachnides. Cuvier regards them in the light of lungs;
while Lamarck himself is inclined to view them rather as*
branchial cavities analogous to those observable in the
leech and earth-worm—the property of branchia being,
in the first place, the power of becoming habituated to
the respiration of air (water being their usual medium),
whilst the true lung respires air alone ; and, in the second
place (and this character applies of course a fortiori to
lungs also), never to exist except in animals which are
possessed of a circulating system.1
Finally, from the dorsal heart already mentioned two
large vessels derive their origin, and are then ramified
over the membrane of the respiratory cavities. Cuvier
regards these vessels as pulmonary, the one acting as an
artery, the other as a vein—and other vessels spring
from the same dorsal trunk, and distribute themselves to
various parts of the body. In these animals there is even
a liver, composed of four pair of glandular clusters, which
discharge their fluid at four different points of the intes¬
tine.
It is among the Arachnides that we find the first es¬
tablishment of organs for the circulation of the animal
fluids; and in the same class we perceive, as it were, the
termination of tracheal respiration by ramified tracheae,
with a view to the substitution of the branchial system,
which, though in itself admitting of considerable varia¬
tion, is always characterized by its more local restriction.
Among the Arachnides also we find the commencement of
the principal of the conglomerated glands.
I he respiratory sacks, which Cuvier pointed out in the
spiders and scorpions, were also detected by M. Latreille
in the genus Phryna, in such a manner as to connect, by
that dominating feature in their structure, the Arachnides
pedipalpes and the Arachnides Jileuses of that author. Al¬
though among the Phalangides the respiratory sacks have
not yet become perceptible, yet the aeriferous tracheae
have changed their character, and no longer consist of
a double cord with a series of plexus, but are merely
branched or ramified. The same order prevails among
the Acarides, and results mainly from the reduced num¬
ber and altered position of the stigmata. Among the an-
tennated Arachnides, in which the stigmata are more nu¬
merous, and in general lateral, the tracheal cords, like
those of insects, have as many plexus as stigmata; and
such Arachnides, in fact, approach the most nearly to the
class of insects. Thus tracheal respiration changes by
degrees its nature a»nd mode of action, as the stigmata
undergo an alteration in regard to their number and posi¬
tion ; and, becoming more and more reduced and restrict¬
ed, it prepares the way, as it were, for branchial respira¬
tion, which only shows itself effectively along with the
establishment of a circulating system. It results from
the^e considerations that, in spite of the difference of or-
fharr-?bSerVable m dlfferent Emilies of Arachnides,
these families are nevertheless related and united by
would " es’ whlch 11 18 ™possible to mistake, and
would be improper to separate, and which appear to
place them at an almost equal distance from Insects and
Crustacea. In their aspect, however, they in general re¬
semble somewhat more nearly the latter class! For ex¬
ample, the cancendes or crabs represent in some respects
by their short bodies, and heads confounded with the
thorax, the usual form of spiders^ while the cray-fish and
1 halassmse {Cancer anomalus, Leach) recall the figure of
the scorpion. °
The greater proportion of Arachnides dwell on the land,
a few inhabit the waters, and a certain number are parasiti¬
cal on the bodies of other animals, of which they suck the
juices. In general they are carnivorous, and live on blood,
or at least on animal substances; a very limited number
existing on vegetable matter. Many are furnished with
mandibles, which perform the functions of a trunk or
sucker; and others are provided with an isolated or sepa¬
rate trunk, frequently accompanied both by mandibles
and palpi. They are for the most part solitary animals,
o. gloomy habits, and forbidding aspect; they court con¬
cealment, and avoid exposure to strong light. The bite
of many species is dangerous, in consequence of a poison¬
ous or irritating fluid frequently instilled into the wound.
The offensive organ in the scorpionides is placed at the
extremity of the abdomen.
The following is a tabular view of M. Lamarck’s system
of arrangement.
General Division of Arachnides.
Order I.—Antennated tracheal Arachnides. Head fur¬
nished with two antennae. Respiration effected by
means of double-corded plexiform tracheae.
Section I.—Crustaceans Arachnides. Two composite
eyes, of which the surface is granular or sub-granular.
They are sometimes called wandering Arachnides, to
distinguish them from such as are fixed or parasitical.
They are frequently of a scaly structure, and provided
with mandibles fitted for incision and division. This
section consists of two families, viz. Thysanoura and
Myriapoda. The following are the genera of Thysa¬
noura : Smynthurus, Podura, Machilis, Lepisma. Those
of Myriapoda are, Scutigera, Lithobius, Scolopendra,
Polyxenus, Julus, Glomeris.
Section II.—Acarideous Arachnides. Two or four smooth
eyes. These animals are parasitical, and provided
either with a retractile sucker or with two mandibles
hooked for the purpose of adhesion. Their bodies are
never scaly. The genera are Pediculus and Ricinus.
Order II.—Non-antennated tracheal Arachnides. No an¬
tennae. Tracheae for respiration branched, but not gan-
glionized. Two or four smooth eyes.
Section I.—Body either without apparent division, the
head, thorax, and abdomen, being united in one mass,
or composed of two divisions. This section is composed
of two families, viz. Acarides and Phalangides. The
former contains the following genera: Astoma, Lep-
tus, Caris, Ixodes, Argas, Uropoda, Smaris, Bdella,
Acarus, Cheyletus, Gamasus, Oribata, Erythraeus,
Trombidium, Hydrachna, Elais, Limnochares. Ihe
369
1 Analyse des Travaux des Sciences de I'lnstitut pendant l'annex 1810.
o A
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