There are many other boring shells, most of which are related to the Pholas. Some of them burrow in rocks, others in wood, and some indifferently in either material. Of the wood-borers, the most remarkable is the Ship Worm (Teredo navalis), which penetrates deeply into floating or submerged timber, and lines the cavity of its burrow with a coating of shell. In this way the Teredo has often done much injury to piles and other woodwork exposed to the sea, and in 1731 and 1732 it excited so much alarm in Holland by attacking the piles of the great dikes, that even statesmen condescended to study its natural history. We must remember, however, that in the grand economy of nature even this destructive creature has its use; by penetrating in every direction through any floating mass of timber it promotes the breaking up of the latter, and prevents the surface of the sea from being encumbered with quantities of wreck.
1. THE MUSSEL. (Mytilus edulis.)
Like the oyster, the Mussel inhabits a bivalve shell, to which it adheres by a strong cartilaginous tie. The shells of several of the species are beautiful. The Mussel possesses the property of locomotion, which it performs with the member called its tongue, by which it gets hold of the rock, and is enabled to draw itself along; it has also the property of emitting a kind of thread, called the byssus, which, fixing the sides of the shell upon the ground, answers the purpose of a cable, to keep the body of the fish steady.