COLOUR, FORM, AND MOVEMENT. 71 



we see that they are raised and lowered at will. These 

 fins have been removed from fishes, when it has been found 

 that a straight course could not then be followed. In 

 some fishes, e.g. pipe fishes, sea-horses, and sticklebacks, 

 the dorsal fin by means of rapid undulations along it is 

 used to propel the fish. 



Tail Fin. The action of this fin should be closely studied. 

 It should not be difficult to note that this is the propelling 

 organ. Two principal movements can be observed the 

 bending of the tail to the side preparatory to the making 

 of the stroke, and the backward effective stroke. In fishes 

 with a large lobed tail fin, one can readily see that in the 

 bending preparatory stroke the fish by a graceful undula- 

 tory movement of the fin presents its edge to the water 

 suggestive of the feathering of an oar. In the extending 

 stroke, on the other hand, the flat surface is presented to 

 the water and effective propulsion is the result. In the 

 two movements there is in effect a slipping through and a 

 gripping of the water. 



An excellent illustration of the force there is in the 

 stroke of a fish's tail is seen when the fish in a nearly 

 vertical position by a single movement of this organ throws 

 itself out of the water, e.g. the trout after fly, flying fish 

 when chased by their enemies, or salmon in climbing weirs, 

 etc. No better testimony to the effectiveness of the stroke 

 of a fish's tail can be got than this last. Salmon of very 

 large size are known to readily effect a six-foot leap. 



Besides propulsion the tail fin is on occasion, by bending 

 to the side, made to act as a rudder. 



It is of interest here to note two common recognisably 

 different tail forms.* There is the type seen in bony 

 fishes in which the upper and lower lobes are of the same 

 size, and that occurring in sharks and dogfishes in which 

 the upper lobe is much the larger. This* latter type of tail 

 is the more ancient. 



* This does not exhaust the list of tail types, of which there are 

 four amongst living fishes. 



