I08 GREEN OYSTERS chap. 



centres of the industry are Arcachon, Auray, Cancale, and la 

 Teste. 



It is at Marennes, in Normandy, that the production of the 

 celebrated ' green oyster ' is carried out, that especial luxury of 

 the French epicure. Green oysters are a peculiarly French 

 taste, and, though they sometimes occur on the Essex marshes, 

 there is no market for them in England. The preference for 

 them, on the continent, may be traced back as early as 1713, 

 when we find a record of their having been served up at a 

 supper given by an ambassador at tlie Hague. Green oysters 

 are not always green, it is only after they are placed in the 

 ' claires,' or fattening ponds, that they acquire the hue ; they 

 never occur in the open sea. The green colour does not extend 

 over the whole animal, but is found only in the branchiae and 

 labial tentacles, which are of a deep blue-green. Various theories 

 have been started to explain the ' greening ' of the mollusc ; the 

 presence of copper in the tanks, the chlorophyll of marine algae, 

 an overgrowth of some parasite, a disease akin to liver complaint, 

 have all found their advocates. Prof. Lankester seems to have 

 established ^ the fact, — which indeed had been observed 70 years 

 before b}" a M. Gaillon, — that the greening is due to the growth 

 of a certain diatom {^Navicula ostrearia') in the water of the tanks. 

 This diatom, which is of a deep blue-green colour, appears from 

 April to June, and in September. The oyster swallows quantities 

 of the Navicula ; the pigment enters the blood in a condition of 

 chemical modification, which makes it colourless in all the other 

 parts of the body, but when the blood reaches the gills the 

 action of the secretion cells causes the blue tint to be restored. 

 The fact that the colour is rather green than blue in the gills, 

 which are yellowish brown, is due to certain optical conditions. 



Not till the young white 03^ster has been steeped for several 

 years in the muddy waters of the 'claires' does it acquire the 

 proper tint to qualify it for the Parisian restaurant. The 

 ' claires ' are each about 100 feet square, surrounded by low 

 broad banks of earth, about 3 feet high and 6 feet thick at the 

 base. Before the oysters are laid down, the gates which admit 

 the tide are carefully opened and shut a great many times, in 

 order to collect a sufficient amount of the Navieida. When this 

 is done, the beds are formed, and are not again overflowed by 

 1 Quart. Journ. Micr. Sc. xxvi. p. 71. 



