222 MOVEMENT 



(Fig. 155) shows how the skate raises and lowers the 

 flexible edges of its fins, or, rather, how the resistance 

 of the water elevates them when the base of the fin is 

 lowered. When we come, later on, to demonstrate the 

 appearance of a bird's wing as it strikes the air the 

 same appearance will be noticed. In fact, the two 

 movements have the same effect, both propel a fluid 

 by an oblique movement of an inclined plane. 



The undulatory movements of the skate which we 

 have just described, and which are apparently due to 

 the co-ordinated action of successive portions of the 



Fig. 156.— Sea-horse, showing the successive and ascending phases of the undulations 

 of the dorsal fin as the animal descends through the water. 



fins, is similarly to be found in other aquatic species. 

 Cuttle-fish move their lateral fins with a very similar 

 motion, as far as one can judge by simple observation, 

 for we have not yet had an opportunity of photo- 

 graphing the movements of these molluscs. It is a 

 remarkable fact that cuttle-fish can alter at will the 

 direction in which the movements of their fins are 

 propagated. They can be seen swimming in an 

 aquarium either to the right or to the left without 

 turning round. If they advance, the undulating move- 

 ment of their fin passes from the head-end towards the 

 tail ; if they go backwards, the wave passes in the con- 

 trarv direction. It must, however, be understood that 



