34 COMPARATIVE DENTAL ANATOMY 



or of chitin, and is derived from the superficial 

 layer of the derm. They are therefore ecderonic. 

 In insects and crustaceans the food apparatus is 

 modified from the chitinoid external covering. 



The forms of the food apparatus present great 

 and heterogeneous variety in this division. The 

 so-called teeth of Invertebrates are often but ser- 

 rated jaws placed about the oral opening. The 

 margin of the mouth may be raised into folds and 

 armed with cuticular plates. In the insects and 

 Crustacea the jaws and modified limbs are formed 

 from the exo-skeleton. In some — as the cuttle- 

 fish — there is a strong beak. In the Sea-urchin 

 there are five teeth set in true alveoli. In the 

 mollusks the teeth are supported by a movable 

 band, called the odontophore. 



Functions. Prehension is performed in this 

 sub-kingdom by cilia in many of the lower forms, 

 in the worms and gastropods by a suctorial form 

 of the mouth, in others by tentacles, and in insects 

 by their chitinoid jaws and modified limbs. Cut- 

 ting and dividing food is performed by jaws and 

 mastication by gizzards, when performed at all. 

 No true masticating teeth exist in the entire sub- 

 kingdom. Some have the food apparatus devel- 

 oped for the purposes of combat or sexual at- 

 traction, some for drilling through shells to get 

 the juices of the animal within, or even to drill 

 into rock. 



