TEETH. 39 



(mussel); 'teeth' around margin of certain univalve molluscs, 

 as in Nerita pdoronta (bleeding-tooth shell). 



PROTOZOA. For the most part without teeth; when ali- 

 mentary canal is not denned, form of jaw simple, as in Dys- 

 teria. 



RADIATA. Coelenterata without teeth. With some Echino- 

 dermata, as Toxopneustes (sea-urchin), the dental apparatus 

 (Aristotle's lantern) is very complex. Five prismatic seg- 

 ments or jaws are arranged to one another around the mouth 

 (which presents inferiorly), like the segments of an orange. A 

 slender tooth extends through the longitudinal axis of each 

 jaw, its basal or formative portion being above, and its free 

 and trenchant portion exposed at mouth. The jaws are 

 moved upon one another by abductor, adductor, and con- 

 strictor muscles. 



MOLLUSCA. No teeth below Gasteropoda. Here teeth hav- 

 ing a siliceous and chitinous basis are placed in numerous 

 rows transversely to a narrow cartilage-like band (lingual 

 ribbon), running along the floor of gullet. Each row is com- 

 posed of denticles having narrow bases, but free, scalloped 

 edges. The apparatus is moved to and fro by muscular action 

 upon a simple curved plate (upper jaw) in the roof of gullet. 



Scissor-like teeth in Tritonia. Calcareous plates upon walls 

 of stomach (gastric teeth), in Build and Aplysia. 



Many Pteropoda possess two horny, dentigerous jaws, op- 

 posing laterally ; and a tongue armed with booklets directed 

 backwards. Simpler form in Hyalea. 



Cephalopoda. Dibranchiata have two horny beak-like 

 processes at mouth. 



ARTICULATA. Teeth with few exceptions move laterally. 

 Entozoa. Commonly none. Gizzard-like teeth in Oxyuris 

 (thread-worm). 



Rotifera. Chitinous cesophageal apparatus; composed of 

 a central piece with lateral jaws; constantly motile, and pro- 

 trusile. 



Annelida. With many the pharynx has a horny masti- 

 catory apparatus, sometimes of a very complicated structure, 

 and which, when the pharynx is protruded, often extends 



