Almost invariably they are confined to salt water, and all the woods commonly used for piling are subject to their attacks.

There are two genera of mollusks, _Xylotrya_ and _Teredo_, and three of crustaceans, _Limnoria, Chelura_, and _Sphoeroma_, that do serious damage in many places along both the Atlantic and Pacific coasts.
These mollusks, which are popularly known as "shipworms," are much alike in structure and mode of life.

They attack the exposed surface of the wood and immediately begin to bore.

The tunnels, often as large as a lead pencil, extend usually in a longitudinal direction and follow a very irregular, tangled course.

Hard woods are apparently penetrated as readily as soft woods, though in the same timber the softer parts are preferred.
The food consists of infusoria and is not obtained from the wood substance.