THE SHELL. 113 



wall of the several turns, each of which overlaps 

 rather more than half of the preceding one. 



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 integiiment covering the vis- 

 ceral mass. 



i. The periostraeum or outer layer is horny and un- 

 calcified. 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 porcellanous" appearance. 



iii. The inner or nacreous layer has a glistening, 

 pearly appearance, and a very smooth surface. 

 It is formed by the superposition of a number 

 of delicate densely calcified laminte. 



2. 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 greate)' 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 seen through it. 



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. 



Place the snail in a dissecting dish under luater, 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. 



I 



