220 



MOVEMENT 



this liberty, and only moves them when irritated. We 

 have found the most successful way of doing this is to 

 scratch it beneath the tail with a piece of stick. A 



curious result fol- 

 lows : an undulatory 

 movement of the fins 

 is propagated down 

 the length of the 

 body, taking a direc- 

 tion from the head 

 to the tail. The 

 movement can hardly 

 be seen with the eye, 

 although it is con- 

 tinued perhaps for 

 some minutes. The 

 chr on ophotographic 

 apparatus should be 

 brought to bear dur- 

 ing such a period of 

 movement. 



Undulatory Move- 

 ments of the Skate as 

 seen from the Side. — 

 When the fish is 

 viewed from the side, 

 a series of photo- 

 graphs may be ob- 

 tained such as ap- 

 pears in Fig. 154. 

 The undulatory 

 movements com- 



& — _ . .^^. . - . 



Fig. 154.— Undulations of the tins of a skat?, 

 viewed from the side. 



mence at the anterior end of each fin, and are propagated 

 in a posterior direction, increasing in amplitude as they 

 proceed. As fresh portions of the fins are raised, those 

 behind are lowered, so that the centre of the wave, 



