The colouring matter of the little diatoms—swallowed by the million and digested—is taken up by the blood of the oyster from its stomach, and is excreted by certain corpuscles on the surface of the gills—as I showed some twenty-five years ago—just as red madder is deposited in the bones of a pig fed upon madder, and as the feathers of the canary take up the colour of cayenne pepper when it is mixed with the canary’s food. It used to be thought that the green colour of the green oyster is due to copper—and that opinion was supported by the curious fact that the blood of all oysters and other molluscs, and also of lobsters, scorpions, and king-crabs, does really contain a minute quantity of copper, just as our blood contains iron! It was also supported by the fact that occasionally a fraudulent fishmonger, when asked to supply green oysters, has been convicted of colouring the beards of ordinary oysters with green copper salt, so as to imitate the real article! The real history of the green-bearded oysters is now quite certain, and any one interested in the matter should look at the coloured pictures of the beautiful little blue-coloured _Navicula ostrearia_—the diatom on which this oyster feeds, published by me in the _Quarterly Journal of Microscopical Science_ in 1885. XV MATERNAL CARE AND MOLLUSCS The American and Portuguese species of oysters, which are called respectively _Ostrea virginiana_ and _Ostrea angulata_, as opposed to the common oyster, which is known as _Ostrea edulis_, are not hermaphrodite like the latter, but have distinct males and females. Moreover, the young are not fertilised within the parent’s body, nor do they pass their earliest stages of growth within the parent’s shell adhering to the “beard,” or gills, as in the common oyster. The eggs (Fig. 31) are, on the contrary, discharged by the females into the sea, and at the same time the males discharge a cloud of microscopic sperm filaments, or spermatozoa (Fig. 32), which dart about in the water and fertilise the eggs. That is a more prodigal and less certain process than that pursued by the common oyster. The American and Portuguese oyster have to pay for it. The female produces in one season not a million eggs, as does the common oyster, but nine millions. And out of every fifty million so produced (in some five or six years) only a single male and a single female individual, taking the whole oyster population of these species into consideration, survive to maturity.