118 THE SNAIL. 



preceding turn. In the fully-formed shell there are 

 about four and a half turns in the spiral. 



The shell is placed obliquely on the back of the 

 animal with its apex to the right : its mouth is in 

 front, and faces downwards and to the right. Like 

 the shell of the mussel, it is a cuticular structure, 

 i.e. it is outside the epidermis and is non-cellular. 

 It has therefore no power of interstitial growth, but 

 can only increase in size by the addition of new 

 shell-matter round its free edge or mouth. 



The shell will be more fully described after its 

 removal from the body. 



2. The part of the animal outside the shell. This consists 

 chiefly of the head and the foot, and will vary 

 greatly according to the degree of protrusion of the 

 animal from the shell. It has a mammillated 

 surface, capable of holding in the wrinkles a con- 

 siderable amount of water. By increasing or 

 diminishing this amount the activity of the snail is 

 heightened or diminished. 



a. The head is the rounded anterior extremity of the 

 animal. It bears the two pairs of tentacles and 

 the mouth. 



i. The posterior or dorsal tentacles are a pair of 

 long fleshy cylindrical processes of the dorsal 

 surface of the head. They are very freely 

 movable and can be completely withdrawn by 

 invagination when the animal is disturbed. 

 Each bears an eye at its extremity, which 

 is only visible when the tentacle is fully 

 protruded. 



ii. The anterior or ventral tentacles are a pair of 

 much smaller processes arising from the sides 

 of the head, below and in front of the larger 

 tentacles. Like these latter, they can be 

 completely withdrawn by invagination. 



