SWIJVUVIING ANIMALS 93 



the point at which they meet, is a spindle shaped mass of 

 water full of eddies. If the block be built up behind with 

 wood of such a shap? as just to fill this eddying area, then 

 it will be found that the shape produced sets up the least 

 possible resistance in its passage through the water. Its 

 form is exactly the same as the outline traced by the water 

 flowing round an object and hence the term " stream- 

 line." It is just this shape that is typical of most 

 fishes. 



At the same time it can be shown that for different speeds 

 slight variations are required in this outline in order to 

 obtain the greatest efficiency. This probably accounts for 

 the obvious differences in outline between slow-swimming 

 fishes, such as the cod, and rapid swimmers such as the 

 mackerel. A close examination of the mackerel shows that 

 besides having this definite shape designed for high speed, 

 there is a smoothing off of all excrescences that may set up 

 resistance. The bones around the mouth and jaws, that 

 in many slow-moving fish are somewhat projecting, are 

 here inset absolutely flush with the smooth outline of the 

 fish. 



Very different in appearance are the fishes that live a 

 great part of their time actually on the sea bottom. Many 

 of them are quite flattened, so that they have a very large 

 surface on which to lie, and their fins are so arranged that 

 by a slight flapping they can throw up sand and pebbles, 

 which settle down on the broad surface of the fish's back 

 and eventually completely bury it from view. The common 

 sole and the plaice are typical examples of flat fishes. 

 They are remarkable in that in reality they are lying on 

 their sides. As has been mentioned above, in their very 

 youngest stages thay are quite normally symmetrical like 

 any other fishes, but at a certain staga, when they are 

 between half and three-quarters of an inch in length, a 



