YET even the ranks of the Mollusca can boast members of no mean athletic prowess. Scallops, for example, are vigorous swimmers, travelling backwards through the water, when alarmed, in convulsive jerks, propelled by the expulsion of water as the two valves of the shell are alternately opened and brought forcibly together again. Cockles, too, are expert at high jumping and, when left stranded by the receding tide, are capable of leaping four or five feet into the air in a series of bounding movements as they make their way down-shore to the edge of the water. For the most part, however, the Gastropoda and the Lamellibranchiata are content to travel very little or even not to travel at all. Oysters, for example, swim vigorously in the larval, or young, state, but once they have settled down they become almost literally rooted to the spot.
To understand the movements of molluscs, whether the slow creeping movements of a snail or the leaping of a cockle, it is necessary to make clear what is meant by the foot, as all locomotion is effected by means of this organ. This is not
easy, however, since the foot varies in shape, size and disposition as we pass from one group of molluscs to another. In snails it is the broad, muscular expansion on which the animal creeps; in oysters it is the fleshy part at the centre of the animal; in cockles it occupies the same position as in the oyster but is larger and longer and is capable of being protruded outside the shell; in some of the free-swimming molluscs of the open seas, the foot has become much expanded and can be spread like a sail, enabling the animal to be carried along by the currents or can, by undulating movements enable its possessor to swim; finally, in the cuttlefishes the foot has become completely altered in shape and forms the arms and tentacles.