SWIMMING OF FISHES 



i33 



rays, the ceratotrichia. The radials of the paired fins form a series 

 attached to larger rods at the base. These more basal rods are attached 

 to a 'girdle' of cartilage embedded in the body wall. The pectoral 



Fig. 90. Successive positions of a swimming dogfish at intervals of o-i sec. The 

 lines are 3 in. apart. The passage of a wave is marked by dots. (After Gray.) 



Fig. 91. Successive positions of a swimming eel at intervals of 005 sec. Scale 3 in. 

 The wave-crests are marked. (After Gray.) 



girdle is a hoop extending some way round the body, but the pelvic 

 girdle is simply a transverse rod in the abdominal wall. The origin of 

 these girdles and of the fins will be discussed later (p. 136). 



2. The swimming of fishes 



The propulsive forces that move a fish through the water are usually 

 produced by the longitudinal muscle-fibres of the myotomes, but in 

 some forms the propulsion is produced by movement of the fins, 

 whose function is usually rather to give the fish its stability, enabling it 

 to keep on a constant course, and also to change its course. 



