214 MOVEMENT 



Sometimes a white cloth, set at an oblique angle, serves 

 as a reflector and luminous background, against which 

 the animals are silhouetted. 



A series of photographs are taken on moving films, 

 to show the successive attitudes corresponding to the 

 phases of movement. The chief difficulty consists in 

 compelling the animal to move in a limited space, so 

 as not to leave the prescribed area of the photographic 

 plate. 



Four lines are traced on the walls of the aquarium, 

 so as to form a rectangular space on which to focus. 

 The observer then watches till the animal crosses this 

 space, and although the transit may occupy only a 

 fraction of a second, a series of from ten to twenty 

 photographs can easily be taken in the time. This 

 will be quite sufficient to show the j^hases of the 

 movement.* 



Jelly-fish are fairly easy to study, owing to the 

 transparency of their tissues ; and some of the details 

 of their internal structure can be seen silhouetted in 

 the photograph (Fig. 150). 



By means of a rod introduced into the aquarium, 

 a jelly-fish can be brought into the field of the object- 

 glass. The alternate contractions and relaxations of 

 its bell may be noticed, each of which operations 

 displaces a certain amount of water, and, by means of 

 the reactionary impulse, the animal is propelled in an 

 opposite direction. If a jelly-fish takes up a vertical 

 position, the direction of progression is from below 

 upwards, and the animal rises in the water ; if it is 

 horizontally inclined, the direction of progression is in 

 a corresponding direction. 



The locomotion of a comatula is exceedingly curious. 

 It is usually found fixed on some solid support like 



* The size of the page being too small for such a long series we 

 can only give incomplete specimens of these photographs. 



