THE FUNCTION AND STRUCTURE OF FINS 453 



sudden stopping is not so great in their case as they do not 

 indulge in rapid darting. On the contrary they glide with snake- 

 like motion through the water. The front part of the body is 

 not swung round like the head of a boat turning in a confined 

 space, because they are so lithe that even this part of the body 

 participates in that U-shaped bend with which they glide round 

 a curve. Nevertheless these fins can be tilted upwards or 

 downwards and thus the fish is able to ascend or descend. The 

 execution of these movements is also helped by the vertical 

 flexibility of the body which is not possessed by the shorter 

 fishes. In the dogfish and skate the head can be inclined so as 

 to serve the same purpose. 



When turning sideways the various fish just mentioned often 

 roll partly over to the side and they are then able to use their 

 paired fins for horizontal as well as vertical curves. 



In the primitive fossil shark Cladoselache the paired fins 

 were very little more than horizontal lobes on the side of the 

 body. They possessed the tilting power, referred to in dealing 

 with the sturgeon, to only a very slight degree, if indeed they 

 had it at all. They served mainly as lateral keels. 



Looking at all these facts broadly it is possible to recognise a 

 progressive extension of the functions of the paired fins. Their 

 primary purpose was that of a keel, viz. to steady the fish as it 

 swam through the water ; to this was added that of a lateral 

 rudder directing the fish either towards the surface or to- 

 wards the bottom ; finally there was added the brake function, 

 enabling it to execute sharp turns or to come suddenly to a 

 standstill. 



It remains now merely to indicate the way in which the 

 internal structure seems to have been modified in association 

 with this extension of function. 



According to many the hypothetical starting point for the 

 paired fins was a continuous fin-fold running along either side 

 of the body from just behind the head to the base of the tail. 

 Here the two folds converged towards one another and united 

 in the median ventral fold, which passed backwards, round 

 the tip of the tail and then forwards along nearly the whole 

 length of the back. All these folds were supported internally 

 by a series of gristly or cartilaginous fin-rays (fig. ii). 



Possibly all these folds helped in propelling the body by 

 undulatory movements such as are now shown by the median 



