[7] 



THE EVOLUTION OP THE FINS OF FISHES. 



987 



cal stage of developmeut of the urosoine, according to the observations 

 of Kowalevsky on Phallusia mammillata., and of Kupft'er on Ascidia 

 mentula. In the conrse of further development the Ascidians never 

 seem to pass beyond what I have called the second or lophocercal 

 stage, when the tail is absorbed in the caducichordate forms, but per- 

 sists in the sa^ue stage in the perennichordate Appendicular ia . 



The Elasmobranchs seem to pass through an archicercal stage, while 

 the Amphibians do hot usually exhibit it in so pronounced a way, very 

 soon l)ecoming lophocercal, though the larva of Bactylethra has the an- 

 terior part of the urosome, with high median folds, while the termination 

 is somewhat like that of CJmncvra monstrosa (Fig. 2), but tapers more 



and is typically archicercal. After the absorption of the lophocercal 

 tail of an anurous amphibian larva has been in progress for some 

 time, it seems to tend to lose its median folds somewhat and revert to 

 the archicercal condition. This is also the case with the young of most 

 Urodcia as they approach maturity. 



(2.) Lophocercy. — The second stage of development of the median fin 

 system of Ichthyopsida is what I have called lophocercal (= protocercal, 

 Wyman ; = leptocardial, A. Agassiz) when it consists of continuous 

 (Amphibia, Elasmobranchs, Teleosts, &c.) or exceptionally of discon- 

 tinuous folds {^Siphostoma, Gambima), which do not include true per- 

 manent rays, but may at about the close of this phase contain the 

 numerous fine embryonic rays of what may be called the protopterygian 

 stage of developmeut of the permanent fin-rays. The continuity of the 

 median fin-fold in embryo fishes, as shown in Fig. 3 in the embryo cod, 



seems to depend somewhat upon the extent to which the permanent 

 fins are approximated so as to form a more or less completely continuous 

 system in the adult. Several forms amongst the Clui)eoids develop 

 an expanded eradiate caudal fold, with the chordal axis dividing it into 

 equal moieties, which thus anticipates the outwardly homocercal tail of 



