XVI. THE EVOLUTION OF THE CAUDAL FINS O^ 



FISHES. 



By R. H. Whitehouse, M.Sc, Professor of Zoology, 

 Government College, Lahore. 



(With text-figures 1—3.) 



There is ample justification for the assumption that the most spe- 

 cialized of caudal fins among fishes have been evolved from a simple 

 type which formed a part only of a once continuous median fin-system, 

 extending from immediately behind the head on the dorsal side round 

 the posterior end of the body to the vent on the ventral side ; thus there 

 was no caudal fin as a differentiated structure. The whole of this pri- 

 mitive median fin- system was almost certainly provided with similar 

 skeletal elements throughout, probably of the nature of interspinous 

 bones or radials, not unlike those now found to support the dorsal and 

 anal fins of modern fishes. The caudal extremity was therefore per- 

 fectly symmetrical both externally and internally, the dorsal contri- 

 bution meeting the ventral in a line continuous with the chordal axis. 

 Such a type of caudal fin is referred to as ' protocercal.' 



Embryology is not of much assistance in verifying the exact details 

 of skeletal structure in the primitive caudal fin, for modifications oi 

 this primitively symmetrical type set in very early before skeletal 

 elements are properly laid down. Yet it is reasonable to suppose that 

 no specializations were present in the primitive caudal fin ; the early 

 fishes undoubtedly moved by serpentine action, undulations of the whole 

 body producing a forward motion resembling the progress of the modern 

 eel. Thus the posterior extremity of the body had no special demands 

 made upon it as a propulsive organ beyond those shared by the rest 

 of the body. 



Before proceeding further it will be advisable to describe the types 

 of caudal fin now met with among fishes. It is probably quite safe to 

 say that no fish at the present time possesses a protocercal caudal fin ; 

 hence any tail fins which shew perfect symmetry both externally and as 

 regards internal skeletal supports will be secondarily symmetrical, that 

 is gephyrocercal. When, however, it would not be safe dogmatically 

 to assert the primary or secondary nature of caudal symmetry, it is con- 

 venient to employ a non-committal term signifying symmetry only ; 

 diphy cereal is the term which conveys such a meaning, thus protocercv 

 will be synonymous with primitive diphycercy and gephyrocercy with 

 secondary diphycercy. 



Now undoubtedly the protocercal fin is the earliest in evolutionary 

 order ; heterocercal forms no doubt succeeded the jirotocerca in the 

 ascending scale towards the highly specialized tail fin of the Teleosts. 

 Heterocercy is characteristic of the Elasmobranchs and the Ganoids ; 

 it differs from protocercy in one important feature, viz., that synnnetry 

 has been disturbed. In general, the asymmetry affecta l)oth t he external 



