Molluscs 135 



at the rate of not much more than half 

 an inch in diameter a year. Five years 

 or more may therefore elapse from the 

 date of its birth before it becomes an 

 oyster worthy of the attention of the 

 epicure. A French variety of the com- 

 mon oyster known as the " huitre de 

 Marenne " is remarkable for its bright 

 green colour, which is due, not to copper 

 as has been sometimes erroneously stated, 

 but to a greenish-blue diatom which 

 lives in the oyster ponds at Marenne. 

 Green oysters never occur in the open 

 sea, and the " Marennes " only acquire 

 their characteristic tint in the parks 

 in which they are placed for fatten- 

 ing. 



Our native oyster is peculiar in con- 

 tinually changing its sex, it having been 

 recently established at the Marine Bio- 

 logical Association's Station at Plymouth 

 that the creature may change from male to 

 female as many as four times in the year. 



