ARACHNIDES 791 
CLASS VIIL.—ARACHNIDES. 
Oviparous animals with articulated members, and not undergoing a metamor- 
phosis ; respiration tracheal or branchial, the openings for the admission of 
the air stigmatiform ; no antenne. 
Tue animals of this class were arranged by Linneus in the last order of 
his class Insecta, but were formed into a separate class by Lamarck, in 1800, 
under Arachnides, from agaxvys, a spider, as denoting animals which can- 
not properly be included either among the Crustacea or insects. They differ 
from the Crustacea in having their respiratory organs always in the interior 
of the body ; and from the insects, in not undergoing a metamorphosis. 
The head in the Arachnides is not distinct from the trunk; the eyes are sim- 
ple, and vary in number from two to eight. Some have two jointed mandi- 
bles or forceps at the posterior extremity of the trunk, such as the scorpions; in 
others, these parts take the form of the sucker. The Arachnides are also 
destitute of a labium, or under lip, as in the insects; the part designed under 
this name being a dilatation of the space between the fore feet, which some- 
fumes forms part of the sucker. The mouth is generally accompanied by 
two palpi. The number of feet in the animals of this class is eight; al- 
though some have six, and the females of others have two additional ones . 
ior the purpose of carrying their ova. These feet are arranged around the 
sides of the breast, and are composed of seven joints; the first two forming 
the haunch, the third corresponding to the thigh; the two next to the leg, 
and the last two to the tarsi. The feet are terminated by two hooks, gene- 
rally dentated or pectinated below; and a smaller simple one in the middle. 
The trunk of the body, except in one family, is soft, and without apparent 
livisions ; the envelope being a kind of bag or sack, including the organs of 
circulation, respiration, and intestines, and the secreting vessels of the mat- 
ters which forms their web. The heart is a large vessel running along the 
pack, with tranches on each side. The respiratory organs, two in number, 
and composed of minute laminz, are contained in the interior walls of two 
sacks, situated at the lower part of the belly, one on each side, and covered 
by a membranous operculum. A transverse cleft affords a passage for the 
external air, and two yellowish or whitish spots generally indicate the place 
of these organs. The intestinal canal is short, with two dilatations, the last 
surrounded by the liver. The vessels containing the matter of the web, 
generally six in number, extend on each side internally, are of a tortuous 
form, narrowed abruptly towards their extremity, and terminate in a straight 
filament ending at the membranous papille from which the threads are 
exuded. The generative organs of both sexes are placed at the base of the 
belly, and are double in all the pulmonary Arachnides. Some of the Arach- 
_nides live on land; others in the water; and a third group are parasitica] 
