The tellinid Macoma balthica moves extensively about the surface of the sand, its activities not being restricted to vertical movements as hitherto supposed.

These horizontal migrations comprise a movement towards the sun followed by a movement away from it. If there is heavy cloud movements still occur, in summer at least, but are at random.

Other intertidal species indulge in similar movements, possibly indicating a widespread habit which confers the advantage of bringing the animal into contact with surrounding areas while maintaining its station on the shore.