[15] THE EVOLUTION OF THE FINS OF FISHES. 995 



Stages of the higher Teleostean forms possessing a shorter somatic and 

 caudal region with fewer somites and a much modified heterocercal tail.* 



Such forms as SfylepJiorus, Ghimcera, Gastrostomns, and Eurypliarynx 

 would throw a great deal of light upon this subject, if we knew tlieir 

 embryonic history as well as we know that of Fieranferixwl Echiodon so 

 ably worked out by Carlo Emery.t The morphological and embry ological 

 evidence of caudal degeneration in F. acus is most complete, for the 

 chorda with a blunt rounded posterior end (Fig. 4, Plate VII) abuts 

 directly against the integument which covers the end of the tail. lu 

 F. acus- the terminal caudal vertebrae are most imperfectly d<n^eloped, 

 while in Fchiodon dentatus (Fig, 3, Plate VII) there is a more complete 

 development of the last vertebral body, that vertebra manifestly not 

 beiug the last one which would have been developed had the chorda 

 persisted. In the first form there are no interneural cartilages devel- 

 oped as in the latter; these facts, therefore, taken together with the 

 BQore pronounced persistence of the chorda in F. acus, show that the 

 latter has retained a more embryonic condition of the tail than F. denta- 

 tus. E. dentatus is typically gephyrocercal, while F. amis has not de- 

 veloped the tail so completely as even to attain the condition of gephy- 

 rocercy, so that it is absolutely without a true caudal Jin. On these and 

 other grounds. Dr. Gill thinks it proper to distinguish these forms as 

 separate genera. The young of F. dentatus has a very long and flagel- 

 liform tail which seems to be more i)rolonged than that of F. acus in the 

 relatively younger ^'■VexiWfer^^ stage, and which is either absorbed or 

 even possibly lost in some other way, but it does not matter from what 

 cause the tail is lost, the fact remains that in Echiodon the caudal, or 

 what represents that fin in other fishes, is formed by the coalescence of 

 a short posterior section of the dorsal and ventral series of rays into a 

 terminal fin having the same function as that found in heterocercal types. 



If both the heterocercal and homocercal types of the tail show in a 

 great number of instances that the caudal extremity has been at one 

 time provided with a more or less well- developed opisthure, and if such 

 an opisthure does not in some cases even bear a fin-fold, but is merely 

 the terminal part of the original urosome of the larva, it would seem 

 that such forms had descended from types at one time joossessed of 

 longer tails with well-developed myotomes extending to their very ends, 

 and that they were at first lophocercal, as are the larvte of existing 

 fishes. The enfeebled or degenerated i^osterior part of the urosome, 

 "which we designate as the opisthure, may not have contained suflScient 

 muscular tissue to flex it from side to side, and that condition may 

 have been brought about during development. In fact, less meso- 



* See my paper on the Development of the Silver Gar {Belone Jongirostris), BuU. U. S. 

 Fish Commission, I, 1881, p. '293, Fig. 11, PL XX. 



tAtti K. Accad. dei Lincei, VII, 1879-80. Fierasfer. Studi intoruo alia sistema- 

 tica, Fanatomia e la biologia delle specie mediterrauee di qnesto genere pel dott. 

 C. Emery. 



