PRIMARY DIVISIONS AND CLASSES. 



219 



and the vermes or worms ; but this classification is not suf- 

 ficient to represent all the great differences in the nature of 

 these beings. 



In fact, amongst the articulated animals, properly so called, 

 we find insects (Figs. 171 and 172) which receive the air into 



Fig. 180. Social Ascidiae.* 



all parts of the economy by means of tracheae, which have the 

 body subdivided into three dissimilar parts the head, the 

 thorax, and the abdomen, which have always three pairs of 



Fig. 181. Cypraea ; Cowry. 



Fig. 182. Shell 

 of the Palu- 

 dina ; Marsh 

 Snail. 



feet, and which are almost always provided with wings : the 

 myriapoda (Fig. 170), which resemble insects by their mode 

 of respiration, and which have also a distinct head, but have 



* Ascidise of the genus Porophora : &, mouth ; &, stomach ; i, intestine ; 

 <7, anus ; t , common stalk. The arrows indicate the direction of the current 

 of water serving for respiration. 



