Morphology of the Fins 81 



when adult, by an undulatory motion, the paired fins tend to 

 disappear, as in the eel and in all eel-like fishes, as blennies 

 and eel-pouts. 



The form of the tail at the base of the caudal fin varies in 

 the different groups. In most primitive types, as in most 

 embryonic fishes, the vertebrae grow smaller to the last (diphy- 

 cercal) . In others, also primitive, the end of the tail is directed 

 upward, and the most of the caudal fin is below it. Such a 

 tail is seen in most sharks, in the sturgeon, garpike, bowfin, 

 and in the Ganoid fishes. It is known as heterocercal, and 

 finally in ordinary fishes the tail becomes homocercal or fan- 

 shaped, although usually some trace of the heterocercal condi- 

 tion is traceable, gradually growing less with the process of 

 development. 



Since Professor Agassiz first recognized, in 1833, the dis- 

 tinction between the heterocercal and homocercal tail, this 

 matter has been the subject of elaborate investigation and a 

 number of additional terms have been proposed, some of which 

 are in common use. 



A detailed discussion of these is found in a paper by Dr. John 

 A. Ryder "On the Origin of Heterocercy" in the Report of 

 the U. S. Fish Commissioner for 1884. In this paper a dynamic 

 or mechanical theory of the causes of change of form is set forth, 

 parts of this having a hypothetical and somewhat uncertain 

 basis. 



Dr. Ryder proposes the name archicercal to denote the cylin- 

 droidal worm-like caudal end of the larva of fishes and amphibi- 

 ans before they acquire median fin- folds. The term lophocercal 

 is proposed by Ryder for the form of caudal fin which consists of 

 a rayless fold of skin continuous with the skin of the tail, the 

 inner surfaces of this fold being more or less nearly in contact. 

 To the same type of tail Dr. Jeffries Wyman in 1864 gave the 

 name protocercal. This name was used for the tail of the larval 

 ray when it acquires median fin-folds. The term implies, what 

 cannot be far from true, that this form of tail is the first in 

 the stages of evolution of the caudal fin. 



To the same type of tail Mr. Alexander Agassiz gave, in 

 1877, the name of lepto car dial, on the supposition that it repre- 

 sented the adult condition of the lancelet. In this creature, 



