Shell

[AS.] The outer part of an egg or nut, the pod of peas, and the hard covering of some habiting shallow places near the shore and exposed to the beating of the waves have stronger shells than those living in deep water. Fresh-water mol¬ luscs generally have delicate shells. The thin layers of the oyster-shell are deposits of shelly matter showing the lines of growth. All living shells have an outer layer of animal matter called epi¬ dermis, and they have no lustre till this is taken off. Mu el-shells show beautiful blue tints when the epidermis is removed. The bodies of all sliell-fishes are enclosed by a delicate membrane called a mantle, which secretes the shell. Among other animals the tortoise has a shell,whose upper part is made of the flattened spines of the verte¬ brae, and of the ribs, the shelly plates being merely portions of the skin hardened into shell. (See Tortoise.) There are many other animals that form shells, some of them being microscopic. Chalk is largely made up of the shells of these.