THE TEETH OF INVEETEBEATES 



37 



Crustacea — we have an approacli to true jaws, 

 but they work laterally instead of vertically, as 

 in the Vertebrates. The mandible and maxillae 

 are very dense chitinous material, and the 

 "teeth" are merely serrations on the edges. In 

 the insects one pair of each — mandibles and 

 maxillae — make four jaws, which work trans- 



Fig. 3. — Head and Jaws of Nereis virens. 



versely in addition to the labia, which merely 

 cover the mouth. These organs are modified in 

 endless variety for various purposes, — from 

 strong jaws for cutting purposes to the long suc- 

 torial tubes of the butterflies. The so-called 

 dental plates lining the crops of insects and 

 Crustacea further comminute the food, and hairs 

 keep the larger particles back until finely crushed. 

 In the lobsters, crabs, etc., the "pinchers" (Fig. 



