CHAPTER IV 



FINS 



Different kinds of fins. Their origin and evolution. Structure of fins: 

 dorsal and anal, pectorals and pelvics, caudal. Types of tail. Develop- 

 ment of tail. Modifications of dorsal and anal in different fishes, of 

 caudal, pectoral and pelvic fins. 



The nature and functions of the fins have been indicated in an 

 earUer chapter [cf. p. 13). Briefly, they are of two kinds: 

 (i) median or unpaired, sometimes described as vertical fins; 

 and (2) paired fins. The median fins include a dorsal in the 

 middle line of the back, an anal along the belly behind the vent, 

 and a caudal at the hinder end of the fish. The paired fins are 

 of two kinds only, pectorals and pelvics^ corresponding respec- 

 tively to the fore and hind limbs of land vertebrates (Fig. 7). 

 The pectorals, sometimes referred to as the breast-fins, are 

 always placed close behind the head, but the position of the 

 pelvics varies in different groups of fishes. 



Before considering the structure of the fins, it will be as well 

 to discuss the manner in which they have arisen in course of 

 evolution. It is generally agreed that the earliest fishes possessed 

 no true fins, but swam entirely by undulations of the body, 

 and the fins were probably first developed as stabilising keels 

 to counteract the tendency of the body to roll over sideways 

 when in motion. In this connection some ingenious experiments 

 carried out by Professor Cunningham may be of interest. 

 Having coated an ordinary penholder thinly and evenly with 

 wax, he held this firmly at one end, and, keeping it in a hori- 

 zontal position, moved it rapidly to and fro in a vessel con- 

 taining warm water. He found that very soon a vertical ridge 

 made its appearance above and below the pen in exactly the 

 same position as that occupied by the dorsal and anal fins of a 

 fish, and that this gradually increased in height until, after five 

 minutes' movement, an upper and lower ridge about half an 

 inch high had developed. Experiments made by a French in- 

 vestigator on plastic bodies {cf. p. 13) produced similar results. 



During the embryonic or larval stages of almost any fish the 

 development of the ordinary fins is preceded by a stage at 



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