These are invertebrate aquatic animals with an external shell or carapace that are used in cookery. There are two main classes of shellfish: crustaceans and molluscs.

Crustaceans  - These are arthropods and most of them are marine. The marine crustaceans include lobsters, langoustes (spiny lobsters), crabs, and shrimps; the only freshwater crustacean used in cookery is the crayfish. Crustaceans must be bought very fresh and are best bought live if one is to prepare them oneself. If buying ready-cooked, go to a good fish­ monger. Crustaceans are also available frozen and canned. The heaviest crabs and lobsters are the best; they should still have their claws on. Crustaceans can be prepared and cooked “ a la americame”, as bisque soup, fried, poached in court-bouillon, or grilled (broiled), depending on the spe­ cies. They are also served in cold hors d'oeuvres: the large ones are shelled (legs, claws, and trunks); the small ones are left whole or served in the form of tails by themselves, shelled.  

Molluscs - These are either bivalves (clams, cockles, mussels, oysters, scal­ lops, etc.) or gastropods (winkles, whelks, ormers, limpets, etc.). Unless they are bought where they are caught, only shellfish which come from reputable areas and are subject to some sort of quali­ ty control should be used. Only molluscs with closed shells or which close on touching should be re­ tained. Molluscs which are cooked in their shells and which remain closed after cook­ ing should not be eaten. Similarly, only live gastropods, with a healthy smell of the sea, should be consumed. Cephalopods are also a subgroup of Molluscs and include squid, octopus and cuttlefish. They have a soft body with tentacles that grow from around the head and a sac of dark brown or black ink inside the body cavity which they can squirt as a protective screen.