Shellfish

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.