212 THE MECHANICAL FUNCTIONS. 



colours, and presenting altogether a close resem- 

 blance to porcelain, that have procured them the 

 name they bear. 



When the transparency and brittleness of these 

 shells are very great, they have been considered as 

 forming another class, and they have been termed 

 Vitreous shells, from their making a nearer approach 

 to glass. Some shells present intermediate textures 

 between the membranous and the porcellaneous. 



All those surfaces of the shell on its outer side 

 which are not in contact with any part of the animal, 

 are originally covered with an epidermis:* which, 

 however, is frequently rubbed off by friction. 



The process employed by nature for the formation 

 and enlargement of the shells of the mollusca was 

 very imperfectly understood prior to the investiga- 

 tions of Reaumur, who may be considered as having 

 laid the first solid foundations of the theory of this 

 branch of comparative physiology, t His experi- 

 mental inquiries have fully established the two 

 following general facts ; first, that the growth of a 

 shell is simply the result of successive additions 

 made to its surface ; and secondly, that the ma- 

 terials constituting each layer, so added, are fur- 

 nished by the organized fleshy substance, which he 

 termed the skin of the animal, but which is now 

 known by the name of the mantle, and not by any 

 vessels or other kind of organization belonging to 

 the shell itself. 



If a portion of the shell of a living snail, for in- 

 stance, be removed, which can be done without 

 injury to the animal, since it adheres to the flesh 



* This membrane has been termed the Periostraciim. 



t Mem. de TAcad. des Sc. 1709, p. 367, and 1716, p. 303. 



