90 GASTKOPODA. 



number, and pattern of these teeth vary greatly, 

 though always constant in the same species. They 

 " are amber-coloured, glassy, and translucent ; and 

 being siliceous (they are insoluble in acid), they 

 can be used like a file for the abrasion of very 

 hard substances. With them the Limpet rasps the 

 stony nullipore, the Whelk bores holes in other 

 shells, and the Cuttlefish doubtless uses its tongue 

 in the same manner as the cat." * In each trans- 

 verse row, there is generally a variously-toothed 

 plate, pointing backwards, and overlapping the base 

 of its predecessor ; while on each side of this there 

 are several lateral teeth in the form of curved 

 spines, which arch inwards. The tongue of the 

 Limpet is longer, when extended, than the whole 

 animal ; that of the Whelk has a hundred rows of 

 teeth ; while the great Slug has one hundred and 

 sixty rows, with one hundred and eighty teeth in 

 each row.f 



The stomach is sometimes armed with horny 

 plates and teeth, as in the Sea-hare (Aplysia)^ 

 which feeds on leathery Fuci. The intestine usually 

 opens by an orifice on the right side of the animal, 

 not far from the head. In the Doris, the orifice is 

 in the middle of the back, and in the Chiton it is 

 at the hinder extremity of the body. 



A colourless blood, very deficient in the disks 

 which form in general so conspicuous a portion of 

 this fluid, circulates in the GASTKOPODA. A heart 

 is always present, divided into two chambers, a 

 very muscular ventricle, and a thin-walled auricle. 

 It is generally situated at the base of the breathing 

 organs, on the right side. The blood passes from 



* Woodward's. Manual of the Moll. 27. f Ibid. 28. 



