364 GENERAL CHARACTERS OF GASTEROPODA. 



carry in front, serve but for touch or smell. Their apparatus 

 for hearing, which has been only recently discovered, is very 

 simple ; consisting of a little sac on each side, which is almost 

 imbedded in the cephalic ganglion. The eyes, which are some- 

 times wanting, are very small, and of a very simple structure ; 

 sometimes they are situated on the head, and sometimes carried 

 at the base, the side, or the point of the tentacula. The Nervous 

 System is less developed than in the preceding Classes ; and is 

 chiefly composed of a cephalic ganglion, which is connected with 

 others, either placed immediately beneath the oesophagus, or 

 scattered in distant parts of the body, according to the position 

 of the organs they respectively supply, which varies considerably 

 in this group. (See ANIM. PHYSIOL. 438). 



906. Of these animals, some are terrestrial, some inhabit 

 fresh waters, but most live in the sea. In general they are 

 formed for crawling, as the Snail, the Whelk, the Limpet, &c. ; 

 but sometimes they are rather adapted for swimming, as is the 

 case with many of the naked Gasteropods. A few of this class 

 attach themselves to the surface of rocks, and pass a great part 

 of their lives with little variation in place. This is the case 

 with the Limpet for example ; which is frequently found par- 

 tially imbedded in a hollow exactly fitting to its shell, and 

 therefore evidently formed by its own action. But the attach- 

 ment of such is not a solid union like that of the Oyster and 

 some other Conchifera ; being only produced by the adhesion of 

 the muscular disc, or foot, which, acting like a sucker, can be 

 detached at any time by the will of the animal. 



907. As already remarked, the Shells of Gasteropoda, where 

 they exist, are usually formed in one piece, or are univalve. 

 There is no instance of a Gasteropod forming a bivalve shell, 

 unless we consider the large calcareous operculum of some of 

 these Mollusks in the light of a second valve, with which it 

 cannot be rightly compared. But there is a group, nearly allied 

 to the Limpets, which is distinguished by the possession of a 

 multivalve shell (Fig. 584); the valves being disposed like the 

 segments of Articulated animals, and being connected by a 

 complex muscular apparatus, which strongly reminds us of that 



