EXTEENAL CHAEACTEES 103 



The shell is placed obliquely on the back of the 

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

 feont, 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. 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. 



a. The head is the roimded 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 movable, and can be completely withdrawn by 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 com- 

 pletely withdrawn by invagination. 



b. The foot is a large flattened muscular expansion of 



the ventral wall of the body. It is of an elongated 

 oval shape, commenciag in front with a rounded 

 anterior border immediately below and behind 

 the head, and narrowing behind to a long pointed 

 extremity. A median longitudinal groove runs 

 along its ventral surface. 



