106 THE SNAIL 



g. The structure of the shell. The shell, like that of 



the mussel, consists of three layers, of which the 



outer and middle ones are formed by the collar, 



and consequently can only increase by additions 



round the mouth of the shell. The inner layer 



is formed by the integument covering the whole 



■visceral mass. 



i. The periostracum or outer layer is homy and un- 



ealcified. To it the colour of the shell is due. 



It is usually rubbed off the uppermost turns 



of the shell, leaving the middle layer exposed. 



ii. The middle layer is much the thickest of the 



three. It is densely calcified, and has an 



opaque porceUanous appearance. 



iii. The iimer or nacreous layer has a ghstening, 



pearly appearance, and a very smooth surface. 



It is formed by the superposition of a number 



of delicate and densely calcified laminae. 



S. The visceral hump is the part of the snail which is per- 

 manently lodged within the shell. It is the elongated 

 and spirally coiled dorsal part of the animal, and 

 contains the greater part of the viscera. It is 

 covered by a very thin layer of integument which 

 secretes the nacreous layer of the shell, and which is 

 sufficiently transparent to allow the several organs to 

 be readily seen through it. 



Place the snail in a dissecting dish under water, and 

 determine the positions of the organs mentioned below. Note 

 that the outer side of the spiral corresponds to the animal's left 

 side, and the inner side to the animal's right. 



For convenience of description the basal or largest turn 

 of the spiral, commencing with the collar, will be spoken of 

 as the first turn, the next as the second, and so on, 



a. The kidney is a large, somewhat triangular body, 

 of a yellowish-white colour and granular appear- 

 ance, placed about half-way round the first turn 

 of the spiral, opposite to the collar. 



