shell, in zoology, hard outer covering secreted by an animal for protection. It is also called the test, crust, or carapace. The term usually refers to the calcareous shells of the many species of mollusk but is also applied to the exoskeleton of the crab and other crustaceans, to the bony covering of the turtle, and to the hard exterior of a bird's egg. People have made use of mollusk shells since early times as receptacles for food and water, as currency (see shell money), and for ornament. The scientific study of shells is called conchology.
See P. A. Morris, A Field Guide to the Shells (of the Atlantic coast, 1973; the Pacific, 1974); J. M. Eisenberg, A Collector's Guide to Seashells of the World (1980); The Audubon Society Field Guide to North American Shells (1981); S. D. Romashko, The Shell Book (1984); K. R. Wye, The Simon & Schuster Pocket Guide to Shells of the World (1989); M. G. Harasewych and F. Moretzsohn, The Book of Shells (2010).