CHAPTER XV. 

 THE SNAIL. 



THE Common Snail (Helix aspersa) is an air-breathing terres- 

 trial mollusk. In its possession of an external shell into which 

 it can withdraw itself, in the complete absence of lateral appen- 

 dages to serve as organs of locomotion, and in the curious want 

 of bilateral symmetry, this organism differs from those which 

 we have hitherto considered. 



In the fully- extended snail (Fig. 85, A.) the head is seen to be 

 fairly differentiated, though it shades off into that part, the foot, 

 on which the organism creeps, which is in turn surmounted 

 by the shell, containing the visceral sac. The head bears two 

 pairs of tentacles, a shorter anterior pair (a. t.) and a longer 

 posterior pair (p. t.}, at the expanded ends of which are the pig- 

 mented eyes. The mouth (m.) is bounded by a circular lip and 

 lateral lips, and bears a brown horny jaw above. The thick- 

 ened ridge which is seen just within the lip of the shell is the 

 collar (co.). The shell itself is spiral, and consists of several 

 ivhorls gradually diminishing in size to the apex. The line of 

 junction between adjacent whorls is a suture. . The hollow 

 spindle around which the whorls are wound is the columella. 

 The lip of the shell is called the peritreme. Externally the 

 shell is covered by an organic cuticle. Beneath this the cal- 

 careous shell is composed of two layers. The first layer has a 

 thin superficial portion showing confused striation, and a thicker 

 portion (honeycomb layer) formed of vertical prisms. The second 

 or deeper layer (nacre or mother of pearl) is colourless, and con- 

 sists of several strata of prisms arranged horizontally, the 

 axes of the prisms in successive layers being set at right angles. 



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