346 Professor Ruxley [May 11, 



smallest of the young fixed oysters. Perhaps the most eouvenient 

 course will he to use " fry " for the eggs or embryos which are 

 contained within the mantle cavity of the parent ; '* larvsB " for the 

 locomotive stage ; and " spat " for the final condition. 



In order to become spat, the larva appears invariably to fix itself 

 by one side (almost always the left) ; and, if the surface is favourable, 

 the extent of the surface of adhesion becomes very considerable, and 

 the oyster is fixed throughout life. But, if the surface of adhesion is 

 small, the oyster, as it increases in size, readily becomes detached and 

 lies free, though motionless, on the bottom. 



The young oysters grow very rapidly. In five or six months, they 

 attain the size of a threepenny piece ; and, by the time they are a 

 twelvemonth old, they may reach an inch or more in diameter. The 

 rate of growth varies with the breed of oyster, and with the conditions 

 to which it is exposed ; but it is a roughly accurate and convenient 

 way of putting the matter to say, that, at two years, the oyster 

 measures two inches across, and at three years, three inches. After 

 this, which may be regarded as the adult age, the growth is much 

 slower, and the shell increases in thickness, much more than in 

 circumference. 



The natural term of the oyster's life is not known, but there is 

 reason to believe that it may extend to twenty years or more. An 

 excellent authority, Professor Mobius, is of opinion that most of the 

 adult Schleswig oysters are from seven to ten years old, and that, 

 though oysters over twenty years of age are rare, he has met with 

 occasional specimens which had attained between twenty-five and 

 thirty years. 



Oysters breed long before they are full grown, very probably in 

 the first year of their age, certainly in the second. Their productivity 

 appears to reach its maximum at five or six years, and afterwards to 

 decline ; but much further observation is needed before any definite 

 rules can be laid down on this subject. 



These are the most important obvious phenomena presented by 

 the reproductive processes of the oyster.* We must now consider 

 them a little more in detail, and under those aspects which are hidden 

 from ordinary observation. 



The oyster, like other animals, takes its origin in an egg, or ovum, 

 a minute, relatively structureless, protoplasmic spheroidal body, about 

 2^i-Qth of an inch in diameter, by a long series of developmental 

 changes which take place in that ovum after it has united with 

 another living particle of extremely minute size, the spermatozoon, and 

 in consequence of the fertilisation etiected by that union, just as the 

 ovule of a plant develops in consequence of the influence of the pollen 



* It must be remembered that the account here given holds good only of the 

 Ostrea cdulis of England and Northern Europe. In the Portuguese oyster 

 (0. atujulata) and the American oyster (0. virginkmd) the eggs are set free at 

 once, and are not incubated in the mantle cavity of the parents. 



