Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.7. Locomotion
The foot is the chief locomotory organ and its size and shape are always changing in the living animal. The wedge-shape foot is adapted for progression in the mud or sand at the bottom of the river of lake where it lives. In a buried clam, the shell valves slighty agape ventrally and through this opening the fleshy foot protrudes and burrows through the mud like ploughshare. As the mussel wants to move, the pointed foot is extended forward, as far as possible into the mud by the contraction of the protractor muscles. An influx of blood now takes place into the cavity of the foot so that its tip swells up, becomes td and acts as anchor. Sphincter muscles round veins prevent the return of blood from foot. Next, contraction of the retractor muscles pulls the body of the mussel forward through or deeper into mud. The blood is forced out of the foot which 1'ns down again and can be extended forward in the mud. The repetition of these movements of the foot results in a slow progression of the animal, while a narrow wedge-shaped path is left behind.