THE TEETH OF INVERTEBRATES 35 



Descriptive. All of the lowest forms, infusoria, 

 etc., are without any food-reducing apparatus, ex- 

 cept some which have an internal cylinder of par- 

 allel rods for the crushing of food. In the Roti- 

 fera, oral denticles are present in the shape of 

 denticulated plates placed transversely of the 

 mouth, which crush and comminute the food, 

 which is principally infusoria. 



In the Echinoderms, we meet with the Sea- 

 urchin, which has a highly developed and effective 



Fig. 2. — Dental Apparatus of Sea-Urchin (Aristotle's Lantern). 



dental apparatus of complicated mechanism. It 

 is very remarkable to find this highly organized 

 apparatus at this low stage of animal life, for it 

 is one of the most wonderful dental structures 

 in the entire animal kingdom (Fig. 2). It is pop- 

 ularly called "Aristotle's Lantern." It consists 

 of five rodent-like incisors of continuous growth, 

 arranged in the form of a pyramid, with their 

 points centering and finding exit in the middle of 

 the test. These teeth are hard and calcareous, 

 with the enamel thicker on one side than on the 



