Foot as Burrowing Organ
The Scaphopoda that usually have burrowing habit, possess foot suitably modified for burrowing purpose. The conical and protrusible foot of Dentalium is trilobed at its distal end. It can be thrust into the sand while burrowing occurs. A lobe encircling the cone can be erected to increase its anchorage. In between burrowing bivalves, in some like Pholas which bores into rock the blunt, short foot is employed for boring. They attach by the sucker-like surface of the foot; the muscles of the foot produce several movements producing the requisite cutting force; the anterior ends of the valves give the sharp cutting edge. The shipworm Teredo bores into wood using more or less similar mechanism. In Yoldia, the two sides of the foot can be folded to make a wedge shape for easy burrowing into the sand or mud. In some burrowing gastropods, for example Sigaretus , the anterior part of the foot, called protopodium, is as well specialized to become wedge like. The burrowing action in molluscs is typically accomplished through thrusting the blade of the foot into the sand or mud. The distal part of the foot is dilated along with the aid of protractor muscles and when this is accompanied by blood pressure it acts like an anchor. Contraction of the retractor muscles now pulls the animal downward.