Studies in Animal Locomotion Viii. the Kinetics of Locomotion of Nereis Diversicolor

(With One Plate and Eight Text-figures) THE locomotory movements of a typical polychaete worm, such as Nereis diversicolor, are of interest in that they are effected by two distinct mechanisms, (i) a series of parapodia which act as levers comparable to the appendages of terrestrial animals, (ii) the longitudinal muscles of the body. When Nereis is moving slowly over a solid surface, only the parapodia are active; during more rapid motion, or when the animal is swimming through water, the movements of the parapodia are coordinated with those of the longitudinal muscles and the two mechanisms combine to give a highly coordinated locomotory mechanism. Under all circumstances the movements of one side of a segment alternate with those of the other, and during normal forward progression the unilateral activity of any one segment begins slightly after that of the segment situated immediately posterior to itself. During slow forward movement, waves of activity appear to pass alternately over the parapodia of each side of the body from a point situated at the posterior end of the animal. Such a picture is, however, confusing, for no such centre of locomotory activity is in conformity with the fact that relatively short fragments from any region of the body may, if suitably stimulated, exhibit a well-defined and normal locomotory rhythm. A more useful picture of the facts is derived from the observation of a Nereis, originally at rest, starting to move in a forward direction. Cinematograph records taken under such conditions show that the first parapodia to be active are always situated near the anterior end of the animal, and that in a very short period of time a pattern of parapodial activity spreads posteriorly over the body by the activation of the parapodia of every fourth to eighth segment (see Text-fig. 1). As soon as one of these active parapodia begins its effective stroke, a cycle of activity begins in the parapodium of the segment lying immediately anteriorly to it, and this in turn is followed by a similar movement of the next anterior neighbour. The original pattern of active parapodia thereby moves anteriorly and gives rise to a series of waves moving, at a relatively low velocity, from the tail of the animal towards the head. The acquisition of the fully active ambulatory state clearly involves two distinct phenomena, (i) the rapid establish