218 Lamarck's Genera of Shells. 



jaws, (the cephalopoda,) shaped like a parrot's bill, also live 

 on animal food. The limaces, helices, bulimi, and all that 

 have cartilaginous jaws, furnished with very minute teeth, 

 almost invisible, but sensible to the touch, live on herbs or 

 fruits. 



The foot consists of a fleshy, muscular, and glutinous disk ; 

 it serves the animal to crawl with, and is placed at the lower 

 surface of the body, either on the fore part, or extending 

 through its whole length. The crawling foot is peculiar to the 

 gasteropoda and the trachelipoda. The moUusca, which have 

 non-operculated shells, have but one muscle of attachment, 

 situated near the middle of the back ; those with opercula have 

 two, one which connects the animal with the shell, the other 

 belonging to the operculum. The operculum is usually round, 

 sohd, horny or calcareous, and serves to close the mouth 

 of the shells when the animal is in a state of repose ; when it 

 comes out of the shell, it carries the operculum with it, and on 

 retiring it re-adjusts this natural door to the entrance of its 

 dwelling*. Some mollusca are naked, that is, have no exter- 

 nal shell, and are quite soft in all their parts — others, though 

 naked without, are provided with one or more solid bodies 

 internally, which sometimes are simply cartilaginous or horny, 

 sometimes cretaceous and lamellar, constituting a true internal 

 shell. This shell is usually spiral, and its cavity simple or 

 undivided, as in the bulleese, bullae, sigareti, ^c, but in many 

 of the cephalopoda, it is multilocular, its cavity being divided 

 into several regular chambers, by transverse partitions. 



Other mollusca have shells, which are wholly external. 



The mollusca are in general aquatic animals. Most of 



* The helix pomatia has a very solid, calcareous operculum; with 

 which it firmly closes the mouth of its shell at the approach of winter. 

 The wonderful rapidity with which the animal secretes the matter to form 

 this external defence, is strikingly exhibited in the following experiment, 

 communicated by Mr. Henry Stutchbury. Tliis gentleman and his brother, 

 took a helix pomatia on a warm summer's day, wheji it was quite desti- 

 tute of any operculum, (for it casts it off at that sea-son,) and placed it in a 

 vessel, surrounded by a freezing mixture. In the sliort space of twelve 

 hours it formed a complete solid operculum, in every resjiect similar to the 

 natural oue, except that it was not quite so thick. — Tr. 



