MOLLUSCA. 263 



moutli towards us on the glass, we see his black-striped 

 snout beyond the circle of his foot. In the centre of this 

 we see a pair of lips open, and then a glassy organ rolling 

 out between. This is the tongue, which, giving a kind of 

 sweep, rolls back into the mouth, and the lips close over 

 it again. So the scavenger goes on, taking one sweep after 

 another, and even leaving a series of curved marks behind 

 him, like those left by the scythe of a mower. On taking 

 a dead specimen and dissecting the head, the tongue may 

 be found. At the mouth-end it is formed like a narrow 

 spoon turning back upon the throat, to which it is fixed; 

 the other end is a thread-like coil lodged in the stomach. 

 Along the spoon and the whole length of the coiled thread 

 are three rou^s of curved teeth. What we have seen, in 

 witnessing the Mollusc feeding, is the convex surface of the 

 spoon, rasping up the food and coiling backwards as k 

 rasps. The food is passed along the triple row of teeth, and 

 becomes fully masticated in the coils of spiral thread in the 

 stomach. 



Most persons are acquainted with the horny operculum 

 with whicli the Periwinkle shuts himself up in his liole. It 

 is fixed on the back of his foot ; so that, in retiring, his 

 head enters first, then his neck and the forepart of his 



