130 INTRODUCTION TO ZOOLOGY chap. 



Living on the ground, the snail has often to 

 The^Trall "'^^^ its way over rough surfaces ; to obviate the 

 hindrance this might be to its slow gliding motion, 

 the snail discharges a mucous substance from a special 

 " pedal " gland, which opens at the upper front end of the 

 foot, just below the mouth. The mucus forms a smooth 

 bed over which the sole of the foot can easily glide. It is 

 this that causes the slimy trail left behind by a snail. 



These trails enable us to learn something of the snail's 

 wanderings at night in search of food. It will go relatively 

 long distances and climb walls to reach some favourite food, 

 and yet — as the trail tells us — it will return to its original 

 home after it has fed. This " homing instinct " seems 

 curiously well- marked in snails. 



The sense of smell is well developed, and 

 of^me^l^ the concentration of special sensory cells in the 

 longer tentacles seems to suggest that these 

 organs have a special olfactory function as well as that of 

 bearing the eyes ; probably the shorter tentacles also possess 

 this sense, and possibly even to some extent the whole soft 

 skin. 



In all univalves the sense of taste is probably 



of Taste* located in the upper lip, which is always very 



mobile and sensitive, and often curiously lobed. 



This lobing is well seen in the Black Slug (Arion ater), and 



in many others, but is not so marked in H. aspersa. 



Hibernation usually begins in October, when 

 ' snails are in specially good condition. The snail 

 may merely hide itself in some crevice in a wall, usually 

 behind ivy, or it may bury itself in loose earth, always with 

 the mouth of the shell uppermost, and over this it forms 

 the epiphragm. If the winter is a very cold one, the 

 body contracts further and further into the shell, and fresh 

 epiphragms are formed one within the other. As many as 

 six have been found in Helix aspersa.^ It is stated that in 

 some districts with calcareous rocks, H. aspersa excavates 

 tunnels in the rock, each tunnel being about one inch in 

 diameter and three or more inches long. It is thought 

 that the wearing away of the rock is chiefly due to the 

 friction of the foot, but the rubbing action is doubtless 



^ J. W. Taylor, op. cit. 



