186 



OBQANS OF DIGESTION. 



numerous pieces, and is much more complicated. Still, the 

 five fundamental pieces or jaws, each of them bearing a tooth 

 at its point, may be recognized, as in the Scutella; only, instead 

 of being placed horizontally, they form an inverted pyramid. 

 336. Among the mollusca, a few, like the cuttle-fishes, 



Fig. 192. 



Fig. 194. 



Fig. 193. 



Fig. 193. The dental organ of Fig. 194. The dental organ of a 



the Nerita Ascensionensis. Patella, from the Straits of 



Magellan. 



have solid jaws closely resembling the beak of a parrot 

 (fig. 192), which move up and down, as in birds. [But a much 

 larger number rasp their food by means of a tongue sometimes 

 coiled like a watch-spring, the surface of which is covered 

 with innumerable tooth-like points, as in the highly mag- 

 nified portions of the dental organ of Nerita (fig. 193) 

 and Patella (fig. 194). The teeth present a great variety 

 of patterns, which are constant in the different genera, and 

 even characterize the species. They consist of variously-co- 

 loured silicious bodies, generally of hook-like forms, ar- 

 ranged in triple rows upon a musculo-membranous band, 



