222 



THE SEA SHOEE 



ever, that their growth is slow on these artificial grounds, the full 

 size being attained in about seven years, while, in the natural beds, 

 they are full grown in a little more than half that time. 



Native oysters those that are reared on artificial beds are of 

 course removed as soon as they are ready for the market, but those 

 that live on natural banks are often left undisturbed till their shells 

 are thick with age. The latter, too, are often destroyed in large 



numbers by the boring sponge 

 (p. 124), which so completely 

 undermines the substance of 

 the shell that it finally breaks 

 to pieces. 



In the genus Anomia the 

 lower valve is concave, and per- 

 forated with a large oval hole 

 very near the hinge, while the 

 upper one is very convex, but the 

 shell is very variable in shape, 

 since the animal sometimes 

 clings permanently to an object, 

 and the shell, during its growth, 

 accommodates itself to the sur- 

 face of that object. The use of 

 the hole is to allow of the protru- 

 sion of a set of muscles which 

 proceed from the upper valve, 

 and give attachment to a plug 

 or button, more or less calcified, 

 by which the animal clings. 

 One species (A. ephippium), 



FIG. 153. 1. Anomia ephippium. 

 2. Pecten tigris. 3. Pecten, 



ANIMAL IN SHELL 



known as the Saddle Oyster, is 

 common on some parts of our 

 coast. It is seldom found on 

 the beach at low water, but the 

 empty shells are often washed up by the waves. 



The same family includes the Scallops, which constitute the 

 genus Pecten. In these the shell is nearly round, with ears on each 

 side of the umbones, those on the anterior side being generally 

 much more prominent than the others, and both valves are 

 ornamented by prominent radiating ribs. The shell is often very 

 prettily coloured, and the animal rests on the right valve, which 



