CHAPTER XVII. 



OF THE CLASS OF GASTEROPODA. 



902. GASTEROPODS are those Mollusca, which are provided 

 with a head, and which move from place to place by means of a 

 fleshy disc, or foot, placed under the abdomen. This Class, 

 represented by the Snail, is extremely numerous ; and is chiefly 

 composed of animals living in a univalve shell, which is usually 

 cone-shaped and rolled into a spiral. Some species, on the con- 

 trary, are perfectly naked, or destitute of shell : the Slug, for 



example. The body is elongated, 

 and terminates in front by a head 

 more or less developed, bearing 

 the mouth, which is provided 

 with fleshy tentacula varying in 

 number from two to six ; the 

 back is enveloped in a mantle, 

 which is more or less prolonged 

 FIG. 554. CASSIS. backwards, and which secretes 



the shell ; and the belly is covered 



on its under side by the fleshy mass of the foot. The viscera, 

 lodged on the back, occupy the superior part of the buckler or 

 cone formed by the shell, and always remain inclosed there ; but 

 the head and foot project beyond it, when the animal unfolds 

 itself for the purpose of walking ; and re-enter the last turn of the 

 spiral, when it again contracts. Hence the size of this last part 

 of the shell, and the form of the opening, are in keeping with 

 the size of the foot. In most aquatic Gasteropods whose shell is 

 spiral, there is a horny or calcareous disc, called the operculum 

 (Fig. 555, o), which is attached to the posterior part of the foot, 

 and which closes the entrance of the shell when the animal with- 

 draws itself. 



