PBIMAEY DIVISIONS AND CLASSES. 



205 



ganglions are but little developed, and present a remarkable 

 uniformity in structure and functions. We may thus sub- 

 divide this primary division into two secondary groups 

 namely, the articulated animals, properly so called, and the 

 vermes or worms ; but this classification is not sufficient to 

 represent all the great differences in the nature of these 

 beings. 



Fig. 147. lulus. 



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

 we find insects (Figs. 148 and 149) which receive the air into 

 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. 148. Agrion. 



Fig. 149. Bethylus. 



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

 myriapoda (Fig. 147), which resemble insects by their mode 

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

 not the trunk divided into thorax and abdomen ; which have 

 from twenty-four to sixty pairs of feet, aud even more, but 

 which never have wings : the spiders (Fig. 151), which have 

 not the head distinct from the thorax, which have always 

 only four pairs of limbs or feet, and which respire the air like 

 all the preceding, but which have no tracheae, and receive the 



