42 OSTREID^E. 



case. In the Report of the British Association for 

 1856 Mr. Eyton has given some farther information as 

 to the appearance and habits of the oyster-fry. He 

 says, " The animal was semitransparent, with two 

 reddish elongated dots placed on each side behind the 

 cilia, which were in constant and rapid motion. They 

 were exceedingly tenacious of life, the cilia moving until 

 the water was dried up upon the glass. Some that I 

 placed in a little salt and water were alive the next 

 day." After a short enjoyment of freedom they attach 

 themselves to a stone or some other object ; the mantle 

 soon afterwards begins its work of secretion, and converts 

 the case into a shell ; the latter becomes agglutinated 

 to some extraneous body ; the cilia and eye-like spots 

 disappear, and the permanent organs are developed. 

 This metamorphosis has its parallel in the Cirripedia 

 and other classes of invertebrate animals. The parent 

 oyster is slow in recovering from its long-continued par- 

 turition and it is not fit to eat until about the middle 

 of August. Indeed, it is not considered to be in full 

 flavour until September. The period of its longevity is 

 not known. It is said to be in prime condition from 

 the fourth to the seventh year, and rarely to live beyond 

 its fifteenth year. If the numerous laminae or plates of 

 which the shell is composed denote the marks of annual 

 growth, some individuals must attain a very venerable 

 age; but these plates are formed inwardly, instead of 

 outwardly as is the case with the trunks of coniferous 

 trees, and the analogy therefore fails. A severe winter 

 causes great mortality among those which are laid in 

 park sor shore-beds, in consequence of the valves being 

 closed by ice during the recess of the tide. In all pro- 

 bability the stock of sea-water, which had been taken 

 in before the oyster was laid bare, requires occasional 



