218 MOVEMENT 



lines j) 1 ,jP...n l ,n 2 ... 9 which unite the ventral and nodal 

 portions of the same wave, express, by their degree of 

 obliquity with respect to oo, the velocity of each. 



Actual measurement shows that the velocity of the 

 waves is greater than the rate of progress of the 

 animal, and that they travel in an opposite direction. 

 There must be, therefore, a recoil, as in the case of 

 the screw of a steamer, and it is due to the mobility 

 of the resisting-point. 



Thus, the direction of movement of these undulations 

 in an eel, as it moves forward, are directed from the 

 head towards the tail. We believe that these fish, 

 when they want to move backwards, reverse the direction 

 of these undulatory movements, namely, that the wave 

 travels from the tail towards the head. But this 

 phenomenon is difficult to produce, and we have not 

 yet been able to prove it by chronophotography. 

 We studied in the same way the locomotion of various 

 kinds of snakes, both terrestrial and aquatic; the 

 crawling of the former and the swimming of the latter 

 are very similar to the movement of an eel, but we 

 could not observe the same regularity of motion. 



Locomotion by means of Vertical and Undulatory 

 Movements : the Skate. — The skate, like an eel, pro- 

 gresses by means of undulatory movements, but the 

 wave is produced symmetrically by the animal's tw r o 

 fins. The movement is in a vertical direction. To 

 photograph this movement the animal must be viewed 

 from the side ; an aquarium, as before described, is 

 suitable for this purpose. The difficulty which arises 

 in this experiment is that of keeping the fish in a 

 convenient position, so as to show 7 its movements clearly. 

 Left to itself in an aquarium, the skate remains 

 motionless at the bottom ; yet if disturbed it swims 

 to the surface, and causes a disturbance of the water 

 by flapping its fins, and it is but seldom that it swims 



