ARACHNIDES 791 



CLASS VII. — 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 anlennce. 



The animals of this class were arranged by Linnaeus in the last order of 

 his class Insecta, but were formed into a separate class by Lamarck, in 1800, 

 under Arachnides, from aqaxvTjg, 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- 

 times 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 

 for 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 

 (he 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 

 divisions ; 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 

 oack, with branches on each side. The respiratory organs, two in number, 

 and composed of minute lamina?, 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 papilla? 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 parasitical 



