CHAP. IV.] THE COMMON SNAIL. 273 



semblages ; hence it is no uncommon thing to find, during 

 the warm season, individuals, to the exterior of whose shells 

 there adhere one or more (often a great number) of these 

 hybernacula, cast off by their fellows on emerging from the 

 dormant state. 



The body of the snail is soft and unsegmented, and, 

 unlike that of any other animal dealt with in this work, 

 asymmetrical — inasmuch as the anal, respiratory, excretory 

 and genital orifices all open to the right side, the latter 

 being situated far forwards near the mouth. The ventral 

 surface of the body is thick and fleshy, giving rise to a 

 locomotor foot^ by the wave-like contractions of which the 

 sluggish movements of the animal are performed. So deli- 

 cately adjusted are these, that the creature can crawl with 

 ease and comfort over a knife-edged surface. The anterior 

 end of the body is differentiated into a well-marked head 

 segmetit bearing two pairs of tentacles — a shorter labial pair 

 adjacent to the mouth and a longer ocular pair situated 

 above and behind these. The integument covering the apex 

 of each tentacle is especially modified in connection with a 

 nerve supply derived from a large underlying ganglion, 

 whence it follows that the eye-bearing tentacle performs a 

 double function. It may be that the labial one is either 

 tactile or olfactory, but the exact functions of these sensi- 

 ferous areas have yet to be fully elucidated. 



Between the head-segment and the free anterior end of 

 the pedal-disc there is a cleft, at the base of which opens a 

 large mucus secreting pedal- gland which extends far back 

 to the hind end of the body. 



The dorsal surface of the body is produced into a spirally 

 roiled hump, within which the whole digestive gland and 

 portions of the alimentary and reproductive viscera are 

 odged — it is hence termed the visceral sac. This sac, to- 



M. 18 



