206 THE MECHANICAL FUNCTIONS. 



greater than the shell itself; so that the opening 

 which it forms in the sand is large enough to re- 

 ceive the shell, when the latter is drawn down by 

 the contraction of the muscles which are attached 

 to the foot.* The foot of the Scyllcea is grooved, 

 for the purpose of enabling the animal to lay hold 

 of the stems and branches of marine plants, and 

 advance along them by a gliding motion. 



The head is generally furnished with tubular 

 tentacula, which the animal protrudes for the pur- 

 pose of feeling its way as it advances, and which 

 are quickly retracted, by the reversion of the tube, 

 when they are touched or irritated. This me- 

 chanism is matter of familiar observation in the 

 tentacula, or horns, of the snail and of the slug, 

 which are terrestrial moUusca belonging to this 

 order. The former of these has a turbinated shell 

 of the ordinary structure : the latter, though ex- 

 tremely similar in its internal structure to the snail, 

 is destitute of any external shell ; but is furnished, 

 instead of it, with a small internal plate of carti- 

 lage, giving support to some of the vital organs. 



§ 4. Structure and Formation of the Shells 

 of MoUusca. 



The structure and formation of the shells of mollus- 

 cous animals is a subject of much interest in com- 

 parative physiology, as presenting many beautiful 

 illustrations of the laws by which the inorganic 

 parts of the living system are increased in their 

 dimensions. 



* Osier, Phil. Trans, for 1826, p. 352. 



