LOCOMOTION IN WATER 219 



quietly in a forward direction. To keep a skate within 

 the field of the object-glass, and to make it execute 

 its proper movements of natation, we finally, after 

 various attempts, settled on a method which answered 

 admirably. 



Special Arrangement for studying the Vertical Un- 

 dulations from Different Points of View. — Fig. 153 shows 

 an apparatus for holding the animal. A flat strip of 

 iron has its two ends bent at right angles ; holes are 

 bored in corresponding positions in the two uprights, 

 and two iron wires are passed through them, and 

 tightlv stretched. On these two wires two akss tubes 



Fig. 153.— The skate. Method of fixing the animal when observing the movements 

 of its fins. 



are threaded, and united by cross-bars. The latter are 

 provided with clips for holding the fish. One of the 

 tubes is fitted with a toothed forceps for holding the 

 front part of the fish ; the other is provided with a plate 

 on which the tail end rests, and to which it is fastened 

 by means of a ligature. 



The fish is then held immovable between these two 

 points of attachment ; the latter are more or less 

 widely separated, according to the length of the fish. 

 The iron plate rests at the bottom of the aquarium, 

 and the object-glass is focussed on the fish. 



The skate thus held in position can neither advance 

 nor recede, but it can use its lateral fins as much as 

 it chooses ; nevertheless, it seldom takes advantage of 



