CLASS OF THE GASTEROPODA. 517 



thus, the size of this last part of the shell and the form of the 

 opening are in relation with the size of the foot. In most 

 aquatic gasteropodous mollusca in which the shell is spiral, 

 there exists a horny or calcareous disc, named operculum 

 (Fig. 540, 0), which is fixed to the posterior part of the foot, 

 and which closes the entrance of the shell when the animal 

 retires within it. 



607. The heart is always aortic, and is composed of a 

 ventricle and an auricle; it is found near the back of the 

 animal, on the opposite side to that occupied by the repro- 

 ductive organs. The arterial system is in general well de- 

 veloped (Fig. 53) ; but the venous system is always more or 

 less incomplete, and sometimes is altogther wanting, so that 

 the blood returns from the different parts of the body towards 

 the respiratory organs only by traversing the lacunaB or 

 spaces existing between the organs. It is also to be observed, 

 that the abdominal cavity, in which are lodged all the viscera, 

 is always thus traversed by the venous blood. 



- 1 



p 



Fig. 541. The Pleurobranchus.* 



The organs of respiration are formed sometimes for an 

 aerian respiration, sometimes for an aquatic life. In the first 

 case, they consist in a cavity, on the walls of which the blood- 

 vessels form a complicated network, into the interior of 

 which the air penetrates from without by an opening under 

 the external edge of the mantle. This kind of lung (Fig. 167) 

 is situated on the back of the animal, and is lodged in the last 

 turn of the spire of the shell when the mollusc is provided 

 with such a covering. In the gasteropoda destined to breathe 

 in water, the disposition of the gill varies ; these organs are 

 often enclosed in a cavity analogous to that constituting the 

 lung in the preceding (Fig. 540) ; but at other times they are 



* m, the mantle raised up to show the gill (br) ; a, anus; J, mouth and 

 proboscis ; v, the veil; t, the tentacles ; p, the foot. 



