212 MOVEMENT 



animals. If the rotatory motion of the screw plays 

 no part in organic nature, there are at least certain 

 undulatory movements of the body or tail of certain 

 animals, which, from a functional point of view, are 

 entirely analogous to those of a screw. 



In addition, aquatic animals have other means of 

 propulsion, the like of which men have never made 

 use of, and which might perhaps be tried with 

 advantage. 



Without attempting to offer a complete list of the 

 various modes of progression represented among aquatic 

 animals, the following may be enumerated. 



Progression by the force of reaction — animals which 

 project a stream of water : jelly-fish, octopuses, larvae 

 of certain insects, bivalve molluscs. 



Progression by means of certain organs which meet 

 with unequal resistance in the two phases of movement : 

 comatulae, crustaceans, etc. 



Progression by means of an undulatory movement, 

 propagated along the body in a direction opposed to 

 that pursued by the animal : eels, long-bodied fish. 



Progression by means of alternate shocks from a 

 flexible paddle : aplysia, carinaria, and most fishes 

 possessed of a caudal fin. 



The possession of an aquarium facilitates the study 

 of aquatic locomotion. But, as in the case of all other 

 animal movements, the eye is frequently unable to 

 follow manoeuvres so rapid and complicated. 



The following are the fruits of our first attempt to 

 apply chronophotography to the elucidation of this 

 subject, concerning which at present so little is known. 

 Methol of taking Photographs of Aquatic Animals. — 

 The methods vary very much according to circum- 

 stances. In the simplest cases, the object-glass is 

 directed towards an aquarium provided with glass sides, 

 and let into the outside wall of a room (Fig. 149). 



