Oysters are the shell-fish most generally used, due to their palatability, digestibility, and high percentage of nutrients. China and Italy cultivated oysters 2000 years ago. There was a British oyster industry in 50 B.C.

It is heard that oysters are very similar in nutrients to milk. In quantity they are also in containing some of all food-constituents, even carbohydrates, so rare in animal food, (Its form in oysters is glycogen, the sugar in the liver.) But the proportion of the different food-constituents is widely different. (See p. 103.) Protein in oysters is nearly double that of milk. Compare other constituents.

Though oysters are more digestible raw, they are not wholly safe thus. When fattened in shallow water or kept in water, as they usually are while in market, they readily absorb any disease-germs the water may contain. These they then transmit. Such "floating" of oysters gives them a plumper appearance, but the smaller-appearing oysters may be safer. Oysters slightly cooked are digestible and safer. Overcooked oysters are toughened to indigestibility.

Shell-fish in general (clams, oysters, scallops, shrimps, lobsters) are not easily digested by all persons. Clams have a tough muscle; crabs and lobster, firm, compact fiber that requires thorough mastication. Shell-fish must always be fresh and from a near source of known security from disease.

In general, all-year fish have little fat, as also those available in winter; spring fish have more; summer, most. Range of cost varies similarly, though among those with fat, bluefish, mackerel, herring, eels, are cheap. Dried and smoked fish are nutritious, inexpensive, and safe. Canned fish must be taken from can when opened and used promptly. All these fish-foods build tissue and do not stimulate as do meats with extractives.