The first Molluscs
Other kinds of annelids also developed in which the animal did not attach itself to the sea floor but continued to crawl and secreted a small conical tent under which it could escape from predators and probably represented the prototype for the Mollusc group, with a primitive representative being Neopilina. Today there are at least 60 000 different species of mollusc. Anatomically these animals usually possess a foot which may be used for locomotion, a shell, a mantle composed of thin sheets of body tissue that covers the internal organs, and an internal cavity that coats the central part of the body, in which most species have gills which extract oxygen from water.
ANATOMY OF THE MOLLUSC