986 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [6] 



exceptional form is developed in consequence of an extensive degenera- 

 tion of the chordal axis and hinder end of the nrosome, unaccompanied 

 by the uphending of the axis, as in the case of heterocercy. The most 

 archaic stages, or those foniul to appear during the younger ]>hases of 

 growth of fishes, are approximated by the structure of the fins of some 

 of the most ancient Devonian, Triassic, and Jurassic forms, and by such 

 living forms as CJiimara, the Dipnoans, Leptocardians, and Lampreys, 

 but the parallelism of the development of the tail of young fishes with 

 the successive modifications of caudal structure found in the forms of 

 successive geological periods is not exact, as we shall ])resently show. 



(1.) Archicercy. — The most primitive modification of the uiosome is 

 that which I will call archicercal^ and which is characterized by the 

 absence of any fin-fohl whatsoever. While it is true that only a few 

 degenerate or specialized forms of true fishes, such as Hippocampus, 

 Nerophis, &c., permanently aiii)roximate such a condition, it must be 

 admitied that the fins are acquired structures, and that the folds from 

 which they nre devcloi>ed have been acquired in the course of the evolu- 

 tion of the ancestry of the fishes. Th * prol>a})ility is that the history 

 of the lateral Jins is similar; that is to say, inasmuch as the ])aire(l fins 

 of the lower types ])ossess a greater number of rays (basalia) derived 

 from the ends of tlie buds thrown otf from the lateral somites than 

 those of the higher foruis, it would seem that the lateral folds which 

 led to the dilferentiatioii of the i)ectoral and pelvic limbs were at first 

 much longer than at present, jiossibly continuous with each other; 

 such a conclusion would be favored by the i)resence of the large homo- 

 dynamous series of cartilages which enter into the formation of these 

 tins ill Dipnoans and Elasmobrauchs. The archicercal condition justi- 

 fies such a conclusion, for during its persistence in normal forms there 

 is little development of the rudiments of the paired fins; notwithstand- 

 ing this, however, a noteworthy objection may be raised against this 

 conclusion, which I have stated elsewhere. 



When a young fish is developing in the egg its tail grows out at first 

 as a blunt prolongation backwards of the body, which is for a time 

 wholly without fin folds, and is cylindrical and vermiform in general 

 appearance, with the muscular somites clearly marked. 



The larva of BrancMostoma (Fig. 1) is without median fin-folds, and 

 that oiPetrom'jZon seems to be without them during the ver^' early stages, 

 and, while wo must make due allowatice in both these cases for the ef- 

 fects of degeneration, we may, I think it probable, look upon these types 

 as possessing at one stage a tyi)ically archicercal and vermiform tail. 

 The solitary urochordate forms or Ascidiaus pass through an archicer- 



