SNAILS AND SLUGS 



327 



two pairs of tentacles, which are hollow and can be drawn back 

 into the body. The front pair are short, while the others are 

 long, and each of them bears an eye at its tip. There is a well- 

 coiled visceral hump covered by a shell of corresponding shape, 

 into which the animal can be withdrawn, though there is no 

 operculum to guard the entrance. During the winter the snail 



TENTACLE 



BUCCAL GANGLIA 



LUNG VEIN 



AURICLE "J 

 VENTRICLE j 



PERICARDIUM 

 RTA 



Fig. 190. Structure of Garden Snail (Helix aspersa) 



A, General dissection, from upper side; roof of lung spread out to right. B, Buccal mass, right half removed. 



c, Nerve-ring, from back. 



remains in a torpid condition within its shell, or in other words 

 hibernates, under which condition the want of operculum is com- 

 pensated for by the formation of a limy partition across the 

 mouth of the shell, leaving, however, a small aperture for 

 breathing purposes. 



As would be expected in a form so twisted, the mantle-cavity 

 is in front, but no longer has a wide opening to the exterior, as 

 this would lead to its delicate lining being dried up. There is, 

 instead, a small aperture on the right-hand side, easily seen just 

 within the margin of the shell. When the mantle-cavity is opened 



