MUSSELS. 137 



works by means of an acid solvent ; others that it bores 

 like an auger, by revolving, and rasps away the surface 

 of the rock with the rough points on its surface. The 

 question remains a knotty one, and my space forbids me 

 to discuss it here. The Mussels are another group of 

 bivalve shells, which inhabit rocky ground, but are in- 

 capable of burrowing into the rock. Nature has not, 

 however, left them unprovided with means for securing 

 their position. She has destined them to a sedentary 

 life on the naked surface of rocks, exposed to the great- 

 est violence of the waves. The common Mytilus rugosm, 

 or Rock Mussel, may be seen covering, by thousands, the 

 surface of rocks near low-water-mark, always choosing 

 the most open situations. But here it is as firmly 

 anchored as a ship in harbour. Its foot, which is so 

 small as to be useless for purposes of progression, is 

 employed in weaving silken threads of great strength, 

 which it affixes to the rock and to its neighbour mus- 

 sel; and thus mutually combined, and each grasping 

 the rock, the community of mussels live together in 

 security. 



But the great majority of the Mollusca which inhabit 

 rocky places belong to a very extensive group, called 

 Gasteropoda, the whole of the under side of whose body 

 consists of a strongly muscular, flattened foot, on which 

 they glide along with a slow but regular motion, leaving 

 generally a slimy track behind them. It is needless 

 to say that the Slug and the Snail are examples of a 

 naked and a shell-covered Mollusc of this kind. But 

 not merely these land Molluscs, but all the univalve 

 marine and fresh-water shells, and all the naked Sea 



