OF THE CLASS AEACHNIDA. 



413 



several important particulars as regards their internal struc- 

 ture. In fact, all the arachnida have the head confounded 

 with the thorax, and have no antennae ; they have four pairs 

 of limbs, and never wings ; and they breathe in general by 

 means of pulmonary cavities, and have all a tolerably com- 

 plete circulatory apparatus. 



Fig. 409. Mygale, CUT. Aranea avic.ilaria, Lin. 



552. The tegumentary skeleton of these animals is in 

 general less solid than that of insects, and their body is com- 

 posed of two principal parts, almost always distinct ; the one 

 called cephalothorax, because it is formed by the head and 

 thorax confounded into a single segment ; the other, named 

 abdomen, and composed sometimes of a series of distinct 



