230 THE MECHANICAL FUNCTIONS. 



tion in the tentacula, or horns, of the snail and of 

 the slug, which are terrestrial mollusca belonging 

 to this order. The former of these has a turbin- 

 ated shell of the ordinary structure : the latter, 

 though extremely similar in its internal struc- 

 ture to the snail, is destitute of any external 

 shell ; but is furnished, instead of it, with a small 

 internal plate of cartilage, giving support to 

 some of the vital organs. 



§ 4. Structure and Formation of the Shells of 

 Mollusca. 



The structure and formation of the shells of mol- 

 luscous animals is a subject of much interest in 

 comparative physiology, as presenting many 

 beautiful illustrations of the laws by which the 

 inorganic parts of the living system are increased 

 in their dimensions. 



All shells are composed of two portions, the 

 one consisting of particles of carbonate of lime, 

 the other having the character of an animal sub- 

 stance, and corresponding in its chemical pro- 

 perties either to albumen or to gelatine. The 

 mode in which these two constituent parts are 

 united, as well as the nature of the animal por- 

 tion, differ much in different kinds of shell ; and 

 it is chiefly in reference to these circumstances 



