Page:The common shells of the sea-shore (IA commonshellsofse00wood 0).pdf/11



this work is entitled "The Common Shells of the Sea-Shore," it necessarily embraces several species of molluscs which do not secrete shells at all, and which are clothed simply by their membrane, or by a tough, leathery envelope. These animals must be introduced, because a knowledge of their structure is needed, in order that the shell-making molluscs may be properly understood; but they will only be mentioned for that purpose, and any description of them will be curtailed as far as possible. We will begin at the beginning.

There is a large group of molluscs which are appropriately called, because their bodies are wrapped in a tunic, or envelope, instead of being protected by a shell. This tunic has always an inlet and an outlet,