CHAPTER XVI. 



OF THE CLASS OF GASTEROPODA. 



971. 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, repre- 

 sented by the snail, is extremely numerous ; and is chiefly com- 

 posed of animals living in a univalve shell, which is usually cone- 

 shaped and rolled into a spiral. Some species, on the contrary, are 

 perfectly naked, or destitute of an external 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 num- 

 ber from two to six ; the back is 

 enveloped in a mantle, which is 

 more or less prolonged backwards, 

 FIG. 651.-CASSIS. an( j w hich 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, occupying the superior part of the 

 buckler or cone formed by the shell, 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. 652, o), 

 which is attached to the posterior part of the foot, and which closes 

 the entrance of the shell when the animal withdraws itself. 



972. The heart is always systemic (AxiM. PHYSIOL. 281), 

 and is composed almost invariably of a ventricle and an auricle 

 (Fig. 639) ; it is placed near the back of the animal, opposite 

 that side occupied by the reproductive organs. The organs of 

 Respiration are formed in some instances for aerial, in others for 

 aquatic, respiration. In the first case they consist of a cavity, 

 on whose walls the blood-vessels form a complicated net-work ; 

 and into the interior of which the external air penetrates, through 

 an orifice in the outer border of the mantle. This pulmonic, or 

 lung-like cavity (Fig. 661), is situated on the back of the 



