70 



ANIMAL LOCOMOTION. 



the head travels from d in the direction w. There is there- 

 fore a period, momentary it must be, when both the cephalic 

 and caudal curves are reduced, and the body of the fish is 

 straight, and free to advance without impediment. The 

 different degrees of resistance experienced by the tail in de- 

 scribing its figure-of-8 movements, are represented by the 

 different-sized curves ef,gh, ij, and ^ / of fig. 32, p. 68. The 

 curves e f indicate the resistance experienced by the tail 

 during flexion, when it is being carried away from and to the 

 right of the line a b. The curves g h indicate the resistance 

 experienced by the tail when it is extended and carried towards 

 the line a b. This constitutes a half vibration or oscillation of 

 the tail. The curves i j indicate the resistance experienced 

 by the tail when it is a second time flexed and carried away 

 from and to the left of the line a b. The curves k I indicate 

 the resistance experienced by the tail when it is a second 

 time extended and carried towards the line a b. This consti- 

 tutes a complete vibration. These movements are repeated 

 in rapid succession so long as the fish continues to swim 

 forwards. They are only varied when the fish wishes to turn 

 round, in which case the tail gives single strokes either to 

 the right or left, according as it wishes to go to the right or 

 left side respectively. The resistance experienced by the tail 

 when in the positions indicated hj ef and ij is diminished 

 by the tail being slightly compressed, by its being moved 

 more slowly, and by the fish rotating on its long axis so as 

 to present the tail obliquely to the water. The resistance 

 experienced by the tail when in the positions indicated by 

 g h, k /, is increased by the tail being divaricated, by its being 

 moved with increased energy, and by the fish re-rotating on 

 its long axis, so as to present the flat of the tail to the water. 

 The movements of the tail are slowed when the tail is carried 

 away from the line a b, and quickened when the tail is forced 

 towards it. Nor is this all. When the tail is moved slowly 

 away from the line a b, it draws a current after it which, 

 being met by the tail when it is urged with increased velocity 

 towards the line a b, enormously increases the hold which the 

 tail takes of the water, and consequently its propelling power. 

 The tail may be said to work without slip, and to produce 



