CLASS OF THE GASTEROPODA. 473 



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. 44) ; but the venous system is always more or 

 less incomplete, and sometimes is altogether wanting, so that 

 the blood returns from the different parts of the body towards 

 the respiratory organs only by traversing the lacunae 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. 



The organs of respiration are formed sometimes for an 

 ae'rian respiration, sometimes for an aquatic life. In the first 



P 

 Fig. 455. The Pleurobranchus.* 



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

 vessels form a complicated network, into the interior of 

 wfyich the air penetrates from without by an opening under 

 the external edge of the mantle. This kind of lung (Fig. 144) 

 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. 454) ; but at other times they are 

 lodged between the mantle and the foot, or even on the back 

 of the animal, so as to float freely in the surrounding liquid : 

 as an example of the pulmonary gasteropodes, we may men- 

 tion the slug and snail, which live on the land ; the lymnaeus 

 or helix (Fig. 143), the planorbis (helix vortex), and the 



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

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



