The question is raised how pelagic larvae of marine bottom animals reach a type of substratum suitable for adult life. Attention is drawn, to a few forms in which it is not entirely due to chance, since their larvae have been shown to exercise some degree of choice of the soil on which they will settle, and to be capable of preserving the ability to metamorphose for several days at least. These powers, if found to be fairly general in marine bottom animals, would have important bearings on distribution problems. There is supporting evidence from the investigations of Notomastus latericeus Sars here described. The adult is found in mud or muddy sand. Experiments showed that larvae of this species if provided with mud or sand generally metamorphose several days before those kept in clean glass vessels or with clean shell gravel. Mud perhaps induces slightly more rapid settling than sand. After metamorphosis the growth and health of the young worms is generally much better in mud or sand than in gravel or clean glass vessels.