322 ARACHNIDES. 



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

 formed of a single segment, and is frequently intimately united to 

 the abdomen. 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 adiiere, abstracting their fluids by suction. 

 Others are parasitical, and live on vertebrated animals. Some of 

 them however are only found in flour, on cheese, and even on vari- 

 ous 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 Pulmoxaeoi;. 



The others respire by tracheas, 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 through- 

 out the whole length of the body and receive air from various points 

 by means of numerous stigmata. Here, but two, at most, are dis- 

 tinctly visible, and they are situated near the base of the abdomen. 

 The number of ocelli is at most but four. They constitute our se- 

 cond and last order, or that of the Tkacheaki^. 



ORDER I. 



PULMONARIiE. 



We here find a well marked circulating system and pulmonary sacs, 

 always placed under the abdomen, announced externally by trans- 

 verse openings or fissures (stigmata), of which there are sometimes 

 eight, four on each side, and at others four, or even two. The 

 number of ocelli is from six to eight, while in the following order it 

 never exceeds four, and is most generally but two; sometimes they 



