1909.] 



The Caudal Fin of Fishes. 



135 



The whole of my work has inchided the examination of well over a hundred 

 vditterent species of fishes representative of nearly all the sub-groups of 

 Teleosts, several Ganoids, and a few Elasmobranchs. Probably the most 

 interesting results have been obtained from the Gadidoc and Apodes and the 

 frequent misinterpretations to be read on the caudal fin of these groups 

 has convinced me that caudal fins have not been subjected to a sufficiently 

 •close structural examination ; for this reason I have made a point of 

 ■examining microscopically every tail fin, and have invariably subjected each 

 one to clearing agents, a process which has revealed many features it is 

 impossible to recognise by mere dissection. 



Since this must be considered only a preliminary communication, it should 

 Jje remarked that the statements here made will be discussed in detail and 

 substantiated by examples in the near future. The work was suggested by, 

 .and carried out under the supervision of, the late Professor Bridge, F.R.S. ; I 

 would like to take this opportunity of expressing my deepest thanks to 

 anany friends who have supplied me with material and given valuable 

 ■criticisms. 



II. — Types of Cavdal Fin. 



It is now generally admitted that the specialised caudal fin of fishes has 

 been evolved from a common type called the protocercal, and that this 

 primitive type was originally part of a continuous median fin. By the 

 •npturning of the end of the axis, heterocercy was estabUshed, the form 

 ■exemplified by modern Elasmobranchs and Ganoids. Specialisation of the 

 iieterocercal form produced the homocercal, as seen in the adult Teleost. 



1. Protoceraj. — A protocercal fin may be defined as one which is primitively 

 symmetrical externally and internally. This implies that the tail has not 

 undergone any reduction or modification of the original form. 



It is probably correct to say that no true fishes possess a protocercal tail in 

 the adult condition ; the Cyclostomes are generally credited with having 

 protocercal tails, but it is Hkely that even here a degenerate condition is 

 ipresent. It will be well to state clearly what is meant by an internally and 

 •externally symmetrical caudal fin. Internal symmetry depends on the endo- 

 •skeleton ; the chorda must end in a perfectly straight line, and the supports 

 ■of the fin-rays must be equal in number and kind dorsally and ventrally ; 

 thus the dorsal and ventral fin-rays contributing to the caudal fin must be 

 equal in number and similarly supported. External symmetry obviously 

 •depends on the equal size of the dorsal and ventral lobes of the fin. 



Unfortunately, the terms protocercal and diphycercal have come to be 

 regarded by authors as synonymous ; strictly speaking, the term diphycercal 

 iipplies to a caudal fin which is quite symmetrical externally and internally, 



