80 STUDIES IN ANIMAL LIFE. 



issue from them are irregularly disposed in the 

 mollusks, and symmetrically in the insect. 



But the recognition of a mollusk will be easier 

 when you have learned to distinguish it from one 

 of the AETICULATA, forming the third great divi- 

 sion the third animal type. Of these, our vases 

 present numerous representatives prawns, beetles, 

 water-spiders, insect-larvae, entomostraca, and worms. 

 There is a very obvious character by which these 

 may be recognized : they have all bodies composed 

 of numerous segments, and their limbs are jointed, 

 and they have mostly an external skeleton from 

 which their limbs are developed. Sometimes the 

 segments of their bodies are numerous, as in the 

 centipede, lobster, etc. ; sometimes several segments 

 are fused together, as in the crab ; and sometimes, 

 as in worms, they are indicated by slight markings 

 or depressions of the skin, which give the appear- 

 ance of little rings, and hence the worms have been 

 named Annelida, or Annulata, or Annulosa. In these 

 last-named cases the segmental nature of the type is 

 detected in the fact that the worms grow, segment 

 by segment ; and also by the fact that- in most of 

 them each segment has its own nerves, heart, stom- 

 ach, etc. each segment is, in fact, a zooid.* 



Just as we recognize a vertebrate by the presence 

 of a backbone and internal skeleton, we recognize 

 an articulate by its jointed body and external skele- 

 ton. In both, the nervous system forms the axis 



* The term zooid was explained in our last chapter. 



