THE SHELL 107 



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 sheU. The ianer layer 

 is formed by the integument covering the vis- 

 ceral mass, 

 i. The periostracum or outer layer is homy 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 ghstening, 

 pearly appearance, and a very smooth surface. 

 It is formed by the superposition of a number 

 of delicate, densely calcified lamiase. 



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 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 seen through it. 



For convenience of description the basal or largest 



turn of the spiral, commencing with the coUar, will 



be spoken of as the first turn, the next as the second, 



and so on. 



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 animaVs right. 



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

 of a yellowish- white colour and granular appear- 

 ance, lying about half-way round the first turn 

 of the spiral, opposite to the collar. 



