xi MOLLUSCA 149 



concave, forming a basin in which lies the body of the oyster, 

 covered by the thinner, flat, upper valve, which can be lifted 

 up or closed tightly like a lid. The foot is rudimentary in 

 this sessile mollusc, which is entirely dependent for its food 

 on the nutriment brought by the currents of water which 

 wash over it. There are four gills. 



The fringed margin of the mantle protrudes slightly from 

 the shell, and on it are little pigmented masses which are 

 sensitive to light. Only one pair of large muscles for closing 

 the shell is present, situated nearly in the centre of the 

 body. 



The chief oyster- fisheries for edible oysters are to be 

 found off the coasts of Europe, wherever the ground and the 

 currents make a suitable habitat for them. 



Oysters reproduce in a wonderfully rapid way if they are 

 protected. One individual, after its third year of life, may 

 give rise to over a million embryo oysters. Nevertheless, 

 owing to the many dangers to which they are exposed in 

 their fixed lives, the death-rate amongst them is enormous. 

 There must also be a large mortality amongst the embryos 

 when first they escape, for they are then desirable morsels 

 for any hungry mouth, and, even when full grown, oysters 

 are attacked and devoured by whelks and starfish. 



They grow slowly ; at the end of the first year they are 

 about 1 inch in diameter, and this measurement increases 

 approximately by another inch each successive year, until 

 they are six or seven years old ; they may live ten years. 



The breeding time is in the summer, and then, from 

 the dealers' point of view, oysters are " out of season." 



The Pearl Oyster (Meleagrina margaritifera) is found chiefly 

 off the north and west of Australia. This oyster is valuable 

 not only for its pearls, but also for the very thick mother-o'- 

 pearl lining to the shell, used in making buttons, brooches, 

 and other ornaments. Some of the best pearls are obtained 

 from a smaller species of pearl oyster found in the oyster 

 banks off the coast of Ceylon, where they are collected by 

 native divers. 



Mytilus. The Sea-mussel (Mytilus edulis), though not 



The Sea- always wholesome, is also cultivated for food, 



mussel. es p ec i a iiy o ff t he coasts of France. It is this 



species which is found in countless numbers attached to piers 



