BIRTH, GROWTH AND REPRODUCTION OF OYSTERS. 1 09 



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, (h] 

 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, proto- 

 plasmic spheroidal body, about 2 ^th of an inch in diameter, 



(h] It must be remembered that the account here given holds good 

 only of the Ostrea edulis of England and Northern Europe. In the 

 Portuguese Oyster (O. angulata) and the American Oyster (O. virgini- 

 ana) the eggs are set free at once, and are not incubated in the mantle 

 cavity of the parents. 



