For the cook's purposes, shellfish can be divided conveniently into three categories: crustaceans, mollusks, and cephalopods. For the cook, they share at least one characteristic in common: freshness is so important that they are often sold alive.
Technically, crustaceans are animals with external skeletons; their bony outer shells, jointed for movement, are shed periodically as their owners grow larger. Lobster is most prized of all, closely followed by crab, various kinds of shrimp and prawns, and freshwater crayfish.
Mollusks are univalved or bivalved, meaning that they have one or two shells (or valves) that expand as the animal grows. They can be hard to extract from their shells, leading to expressions such as “winkle out” and “clam up”. Apart from the snail, univalves such as abalone, conch and whelk tend to be neglected in the kitchen. Bivalves are another story, for they include the more popular oysters, mussels, scallops and clams.
Cephalopods—namely, squid, cuttlefish and octopus—are classed as mollusks. They have a reduced internal shell (a “pen” or, in the case of cuttlefish, a cuttlebone), and share many of the cooking characteristics of shellfish. So do an ill-assorted band of exotic sea and land creatures, such as turtles and frogs, all of which are covered in this chapter.

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