OF THE CLASS ARACHNIDA. 455 



changes are not analogous to those we have seen take place 

 in insects properly so called, and consist only in the forma- 

 tion of new rings, and in a corresponding augmentation in the 

 number of the limbs. 



.550. Two natural groups, easily distinguished by the 

 form of the antennae, compose this latter class namely, the 

 chilognathus or iulus, and the chilopodes or scolopendra. 



Fig. 494. Poly desmus, Lat. lulus complanatus, Fab. 

 Flattened millipede. 



The chilognathi have a cylindrical body, and feed on organic 

 matters more or less decomposed ; their pace is slow, and they 

 often roll themselves into a spiral or ball. They are known 

 by the names of iulus (Fig. 493), polydesmus (Fig. 494), and 

 of glomeris. 



The chilopoda have the body flattened and more membra- 

 nous than the preceding ; they are carnivorous, and run very 

 fast. Three principal genera compose this group : the scolo- 

 pendra (Fig. 164), the lithobius, and the scutigera. 



OF THE CLASS ARACHNIDA. 



551. The class of the arachnida is composed of articu- 

 lated animals, having a strong analogy with insects, and, like 

 them, organized to live in air, but which maybe distinguished 

 at once by the general form of the body, and by the number 

 of their limbs ; they differ also from those animals in several 

 important particulars as regards their internal structure. 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 complete circu- 

 latory apparatus. 



552. The tegumentary skeleton of these animals is in 



