STUDY OF FISHES 137 



must be unimpeded, and to choose a small fish is obviously a 

 less expensive way of securing this than buying a large tank. 

 A globular vessel is altogether unsatisfactory, because of the 

 distorted image. 



MOVEMENTS. (A) Starting from observed facts of rapid 

 and graceful swimming, one may first inquire into the way in 

 which the body is adapted for cleaving the water. It is spindle- 

 like, wedge-like, torpedo-like, boat-like, but different from all 

 these. It is thicker in front than behind, broader above than 

 below ; where is its centre of gravity ? In many ways there is 

 reduction of friction, e.g. in the gently curved contour, the absence 

 of laterally projecting pieces, the pointed head, the tightly fitting 

 jaws, the sunken lidless eyes, the absence of external ear. In the 

 great majority of cases there is beneath the back bone a " swim- 

 bladder " or " air-bladder " containing gas, which makes the 

 fish very buoyant. 



The teacher who understands about the lines of boats and 

 can make the matter plain, should follow up this question of the 

 fitness of the fish-shape for cleaving the water. " In acquiring 

 this perfect outward symmetry it is interesting to note that the 

 forms of fishes may be said to have actually evolved the practical 

 solution of the most theoretical problems of curves and displace- 

 ment in relation to submarine motion. A study of the ' lines ' of 

 typical fishes by naval engineers has led to some most interesting 

 results." l 



But without entering upon abstruse questions, it is feasible 

 to get the pupils to make observations on the fitness of shape 

 for use as illustrated in typical fishes. This would be a study 

 in adaptation. The flatness of the skate should be contrasted 

 with the flatness of the sole, flounder, and plaice ; and the eel at 

 one extreme should be contrasted with the globe-fish at the other. 



() By watching a typical fish swimming slowly, it will be 

 possible to verify the following facts : 



(i) The chief organ of locomotion is the muscular posterior 

 region of the body the " tail," which is bent first to one side and 

 then to the other. The muscles on one side contract, and the tail 



1 Bashford Dean, see references, p. 150. 



