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shell
Dorsal view of a shell of Aporrhais serresianus. Photo: Jan Delsing.

shell:

1. ZOOLOGY. The hard or leathery outer covering of certain animals, such as clams and other mollusks, beetles and other insects, lobsters and other crustaceans, and some other animals such as turtles. Also, the hard outside covering of some eggs. 2. BOTANY. The hard outside covering of a nut or seed. 3. GEOLOGY. A hard, thin layer of rock such as the earth’s crust. 4. PHYSICS, CHEMISTRY. The patterns of electrons that surround the nucleus of an atom. Electrons are grouped in various “shells” around the nucleus. Each shell holds a certain maximum number of electrons.