ARACHN1DES. 305 



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 monodactyle chelae, improperly 

 compared to the mandibles of Insects, and so denominated, moving in a 

 contrary direction to the former, or from above downwards, still how- 

 ever 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 segment of two small legs or palpi, or by au 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 tri- 

 angular labrum, and of an inferior longitudinal carina, usually very 

 hairy, 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 

 rather cephalo-thorax, which, except in a small number, is only formed 

 of a single segment, and is frequently intimately united to the ab- 

 domen. This latter part of the body is soft, or but slightly defended, 

 in most of them. 



Most of the Arachnides feed on Insects which they either seize alive, 

 or to which they adhere, abstracting their fluids by suction. Others 

 are parasitical, and live on vertebrated animals. Some of them how- 

 ever are only found in flour, on cheese, and even on various vegetables. 

 Those which live on other animals frequently multiply there to a great 

 extent. Two of the legs, in some species, are only developed by a 

 change of the tegument. 



Division of the Arachnides into Orders. 



Some have pulmonary sacs, a heart with very distinct vessels, and 

 six or eight simple eyes. They compose our first order, or that of the 



PuLMONARIvE. 



The others respire "by tracheae, and have no organs of circulation, or 

 if they have, the circulation is not complete. The tracheae are divided 

 near their origin into various branches, and do not, as in Insects, form 

 two trunks which run parallel to each other throughout the whole 

 length of the body and receive air from various points by means of 

 numerous stigmata. Here, but two, at most, are distinctly visible, 



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