LOCOMOTION IN WATER 



221 



namely, the most elevated part, travels rapidly from 



the head towards the tail. Having rim its course, the 



wave elevates the posterior extremity of the fin, and 



then disappears. But another 



wave is already commencing 



at the anterior end, growing 



larger, and travelling along in 



the same way as the one which 



preceded it, and so on ad 



infinitum. 



It would be interesting to 

 observe what disturbances are 

 caused in the water by these 

 undulatory movements. This 

 could be ascertained, we think, 

 by introducing into the aqua- 

 rium some of those little bright 

 beads which served to show all 

 the movements in liquids men- 

 tioned in Chapter VI. We 

 by no means despair of obtain- 

 ing photographs of a skate 

 swimming in the normal free 

 state — it is only a matter of 

 time and patience. 



Undulatory Movements of the 

 Skate as seen from the Front. — 

 With the express object of 

 studying the movement of the 

 fins from another point of view, 

 we fixed the animal in a new 

 position, by giving half a turn 

 to the iron framework. 



In this new arrangement the axis of the fish ran 

 in an antero-posterior direction, the head facing the 

 photographic apparatus. The series of photographs 

 16 



Fig. 155. — Undulations uf the fins of 

 a skate, viewed from in iront. 



