66 



Class GASTEROPODA. 



Head distinct, furnished with eyes and tentacles. Body- 

 usually protected by a conical or spiral shell. Lower part 

 of body formed into a thickened, expanded, creeping disk 

 or foot. 



The Gasteropods, so termed from the circumstance of 

 their crawling on their belly, may be known from the other 

 Molluscous classes by the head being always present and 

 provided with eyes and tentacles, the former being either 

 sessile, or on the ends of ommatophora or peduncles, which 

 are either free or connate. They constitute by far the most 

 numerous section of Mollusks, and comprise not only all 

 those that live upon the land, but likewise a very large pro- 

 portion of those that breathe by gills and inhabit the water. 



With the exception of the air-breathing families, the 

 Gasteropods are differently shaped when very young, the 

 fry having ciliated wings on the sides of the head by which 

 they swim freely about, and the body contained in a little 

 clear spiral shell with the aperture closed by an operculum. 

 As they grow, however, the head-wings are absorbed, 

 the foot or creeping disk becomes developed, and the 

 shells assume the forms peculiar to the different genera. 

 In the Nudibranchs the shell falls off and disappears when 

 the locomotive foot appears, but sometimes it remains ru- 

 dimental within the folds of the mantle. In their adult 



