PKOGRESSION ON AND IN THE WATER. 



69 



towards which the tail curves. In swimming, the body of 

 the fish describes a waved track, but this can only be done 

 when the head and tail travel in opposite directions, and on 

 opposite sides of a given line, as represented at fig. 32. 

 The anterior and posterior portions of the fish alternately 

 occupy the positions indicated at d c and tv v; the fish oscil- 

 lating on either side of a given line, and gliding along by a 

 sinuous or wave movement. 



I have represented the body of the fish as forced into two 

 curves when swimming, as there are never less than two. 

 These I designate the cephalic {d) and caudal (c) curves, from 

 their respective positions. In the long-bodied fishes, such as the 

 eels, the number of the curves is increased, but in every case 

 the curves occur in pairs, and are complementary. The cephalic 

 and caudal curves not only complement each other, but they act 

 as fulcra for each other, the cephalic curve, with the water seized 

 by it, forming the point d'appui for the caudal one, and vice versa. 

 The fish in swimming lashes its tail from side to side, precisely 

 as an oar is lashed from side to side in sculling. It therefore 

 describes a figure-of-8 track in the water (e fghijkl of 

 fig. 32). During each sweep or lateral movement the tail is 

 both extended and flexed. It is extended and its curve 

 reduced when it approaches the line 5 of fig. 32, and flexed, 

 and a new curve formed, when it reqedes from the line in 

 question. The tail is effective as a propeller both during 

 flexion and extension, so that, strictly speaking, the tail has 

 no back or non-effective stroke. The terms effective and 

 non-effective employed by authors are applicable only in a 

 comparative and restricted sense ; the tail always operating, 

 but being a less effective propeller, when in the act of being 

 flexed or curved, than when in the act of being extended or 

 straightened. By always directing the concavity of the tail 

 {s and t) towards the axis of motion {a h) during extension, 

 and its convexity (c and v) away from the axis of motion {a h) 

 during flexion, the fish exerts a maximum of propelling power 

 with a minimum of slip. In extension of the tail the caudal 

 curve {s) is reduced as the tail travels towards the line a h. 

 In flexion a new curve (v) is formed as the tail travels from 

 the line a k While the tail travels from s in the direction ty 



