ARACHNIDES. 321 



CLASS II. 

 ARACHNIDES. 



The Arachnides, which compose the second class of articulated 

 animals provided with movable feet, are, as well as the Crustacea, 

 deprived of wings, are not subject (o changes of form, or do not ex- 

 perience any metamorphosis, simply casting their skin; but they 

 differ from them as well as from Insects in several particulars. Like 

 the latter, the surface of their body presents apertures or transverse 

 fissures called stigmata^ for the introduction of air, but they are few 

 in number — eight at most, and usually but two — and confined to 

 the inferior portion of the abdomen. Respiration is also effected 

 either by means of air-branchiae, fulfilling the functions of lungs that 

 are contained in sacs, of which these stigmata are the apertures, or by 

 radiated tracheae. The visual organs merely consist of ocelli, or 

 simple eyes, which, when numerous, are variously grouped. The 

 head, usually confounded with the thorax, in place of the antennae, 

 has two articulated pieces in the form of small didactyle or mono- 

 dactyle chelae, improperly compared to the mandibles of Insects, 

 and so denominated, moving in a contrary direction to the former, or 

 from above downwards, still however co-operating' in the business 

 of manducation, and replaced in the Arachnides, where the mouth 

 has the form of a siphon or sucker, by two pointed blades which act 

 as lancets. A kind of lip, or rather ligula, produced by a pectoral 

 prolongation; two jaws formed by the radical joint of the first seg- 

 ment of two small legs or palpi, or by an appendage or lobe of that 

 same joint; a part concealed under the mandibles, composed of a 

 projection in the form of a rostrum, produced by the union of a 

 very small clypeus terminated by an extremely small trianL'ular la- 

 brum, and of an inferior longitudinal carina, u^aally very liairy, are 

 the parts which, with the pieces termed mandibles, constitute with 

 some modifications the mouth of most of the Arachnides. The legs, 

 like those of Insects, are commonly terminated by two hooks, and 

 even sometimes by one more, and are all annexed to the thorax, or 

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