THE ELASMOBKANCH FISHES 



13 



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. 23b) . The wave here begins in the body just back 

 of the pectoral fins (1) and passes posteriorly and off at the tip of the tail (9), 

 and another undulation thereupon begins and repeats the course. At the same 

 time and with the greatest effect the dorsal and ventral lobes of the caudal fin 

 are directed by the strong muscles as a potent sculling organ. 



Fig. 23a. Diagram to illustrate 



the different phases (1-9) of 



pectoral locomotion in the skate. 



(From Marey.) 



Fig. 23b. Diagram to illustrate the different 

 phases (1-9) of caudal locomotion in the 

 shark, ScylUum. (From Marey.) 



