In the fresh-water mussel the general results are much the same as in Mytilus. Movement of the animal is as a whole right-handed and slightly forward, though this is not invariably the case.

The movement appears to be brought about by the contraction and expansion of the foot, which is wedge-shaped, and larger in proportion than in the sea-mussel. When portions are detached, however, the same four parts, or pieces of them, exhibit decided movement as in the sea-mussel. The palps rotate, the gills and mantle-lobes also move, but remarkably slowly, while the ventral margin of the foot is pre-eminently active; it is par excellence the highly motile detached portion. Where not otherwise stated, the palps, gills, and mantle-lobes are always laid out with their inner surface uppermost, and for convenience of diagrammatic representation, as well as for clearness of explanation, the palps are always supposed to start from the half-round of the clock face. Thus right-handed rotation will have the successive positions of quarter to, hour, quarter past, ending with original position at half past.