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 the 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[221] the fact,--which indeed had been observed 70 years before by 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 oyster 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 _Navicula_. When this is done, the beds are formed, and are not again overflowed by the sea, except at very high tides. The oysters are shifted from one ‘claire’ to another, in order to perfect the ‘greening’ process. About fifty million of these ‘huitres de Marennes’ are produced annually, yielding a revenue of 2,500,000 francs.