248 AN OYSTER'S LONGEVITY. 



shelly bottom is best, the pad disappears. But it is a 

 mistake to suppose that the adult individual is incapable 

 of progress. Diquemarc says that it can transport itself 

 from place to place by the simple process of absorbing 

 sea- water and ejecting it violently from between its 

 valves. And thus it defends itself against its enemies 

 among the minor Crustacea ; particularly the smaller 

 crabs, which endeavour to enter its shell when the 

 valves are partly open. There is reason to suppose, 

 moreover, that our favourite mollusc is not altogether so 

 dull and inert as the earlier naturalists represented it. 

 Oysters exposed, says Diquemarc, to the daily ebb and 

 now of the tide, seem aware that they are likely to be 

 exposed to dryness at certain recurring periods, and con- 

 sequently retain a supply of water in their shells to serve 

 their need when the tide is out. This is true, however, 

 of those oysters only which are found close in-shore. 



The longevity of the oyster is another point on which 

 our information is not very exact ; but many authorities 

 allow it a " span " of twelve years. The young oyster, at 

 the end of three days, measures three lines in diameter ; 

 at the end of a month it is as big as a pea ; in six months 

 it is as large as a half-crown. In three years the best 

 oysters are large enough for sale. Such, at least, is M. 

 Coste's calculation ; but the rate of growth varies in 

 different localities. The oyster on the Yellette bank does 

 not acquire its full proportions in a shorter period than 

 five years ; in the bay of Cancale it comes to maturity in 

 eighteen months. 



There are several oyster-farms in the Thames all con- 

 ducted on much the same principles. That at Queens- 



