Besides the varieties of fish that have already been considered, the general term fish also includes SHELL FISH. Fish of this kind are different in structure from bony fish, for they are acquatic animals that are entirely or partly encased in shells. They include mollusks, or bivalves, such as oysters, clams, and scallops, and crustaceans, such as lobsters, crabs, and shrimp.

The popularity of the edible varieties of mollusks and crustaceans mentioned depends largely on whether they can be easily obtained and whether they are pleasing to the local or individual taste. As they are found in salt rivers, bays, and other shallow salt-water sources, their greatest use is among people living near the seashore, but they are much favored where they can be procured in edible condition. They are not so cheap as many other fish foods; that is, a certain amount of money will not purchase so great a quantity of shell fish, lobster for instance, as some of the well-known varieties of fish proper, such as halibut or whitefish. Lobsters and crabs are usually more expensive than oysters and clams; consequently, they are used more often to provide a delicacy or to supply something more or less uncommon for a special meal.

Several precautions should be observed in purchasing shell fish. For instance, crabs and lobsters should be purchased alive. They are usually shipped on ice so that they will remain in this condition for some time, and they are displayed on ice in the markets for the same reason. Such shell fish should be kept alive until they are plunged into boiling water to cook. Oysters and clams bought in the shell must also be alive when purchased. A tightly closed shell indicates that they are alive, whereas a slightly open shell proves that they are dead. If these two varieties are bought out of the shells, the fish themselves should not be accompanied by a great quantity of liquid. Considerable liquid is an indication that the oysters or clams have been adulterated by the addition of water. Formerly it was the custom to keep oysters in fresh water, as the water they absorb bloats or fattens them. This practice, however, has fallen into disfavor.

Shell fish lend themselves admirably to a large variety of dishes, including soups, entrees, salads, and substitutes for meat dishes. They possess a great deal of distinctive flavor, their food value is comparatively high, and, provided they are in good condition and are properly prepared, they are healthful and easily digested. It can therefore be seen that shell fish have much to recommend their use. There is considerable danger, however, in using any varieties that are not perfectly fresh or freshly cooked. In the case of mollusks, or bivalves, much harm has resulted from the use of those which have been grown or bred in unsanitary surroundings. Because of these facts, it is of the utmost importance that great care be exercised in selecting and preparing shell fish.