. The elasmobranch fishes . In one, forward movement is produced principally by the pectorals; in the other this function is performed chiefly by the caudal fin or tail. One of these types of locomotion we may therefore designate as pectoral, the other as caudal. Pectoral locomotion among the Elasmo- branclis is confined to the rays while the caudal type, so far as I know, is universal among the sharks. It is, however, by no means peculiar to them, since, as we have indicated, some transi- tional rays retain this method of swimming. To illustrate pectoral locomotion Urolophus may first be cons
. The elasmobranch fishes . In one, forward movement is produced principally by the pectorals; in the other this function is performed chiefly by the caudal fin or tail. One of these types of locomotion we may therefore designate as pectoral, the other as caudal. Pectoral locomotion among the Elasmo- branclis is confined to the rays while the caudal type, so far as I know, is universal among the sharks. It is, however, by no means peculiar to them, since, as we have indicated, some transi- tional rays retain this method of swimming. To illustrate pectoral locomotion Urolophus may first be considered. In Urolophus progres- sion is brought about by a synchronous wave movement in both pectorals. This wave begins at the anterior margin of the fin and passes backward, terminating at the posterior margin; thereupon another wave sets in and repeats this action. In the skate pectoral locomotion may be seen to excellent advantage. In it (fig. 23a), as in Urolophus, there is a wave motion which in- volves not only the margin, but also the greater width of the fin, throwing the pectoral into an inverted U with the sharper convexity directed forward. As this wave passes backward (1-9) it gains in size and evident momentum, serving as an effective pushing surface against the water. From this type of wave movement a beautiful gliding motion results. A further modification of the type is present in Cephal- optera, in which the stroke of right and left pectorals takes place alternately. Transitional rays are instructive in that they have not yet attained the method of pectoral locomotion although the pectorals are well de- veloped. In Rhinohatis these fins may be put to considerable use other than in directing the course, as may be seen upon grasping the tail and attempting to pull the fish out of the water. In the sharks, in which caudal locomotion is employed, the body itself is thrown into undulations which also provide concave surfaces with which to push against the water (fig.
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