502 



PAIRED FINS OF ELASMOBRANCHII. 



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The development of the skeleton has unfortunately not been as 

 yet very fully studied. 1 have however made some investigations 

 on this subject on Scyllium, and 'Swirski has also made some on the 

 Pike. 



In Scyllium the development of both the pectoral and pelvic fins 

 is very similar. 



In both fins the skeleton in its earliest stage consists of a bar 

 springing from the posterior side of the pectoral or pelvic girdle, 

 and running backwards parallel to the long axis of the body. The 

 outer side of this bar is continued into a plate which extends into 

 the fin, and which becomes very early segmented into a series of 

 parallel rays at right angles to the longitudinal bar. 



In other words, the primitive skeleton of both the fins consists of 

 a longitudinal bar running along the base of the fin, and giving off at 

 right angles series of rays which pass into the fin. The longitudinal 

 bar, which may be called the basipterygium, is moreover continuous 



in front with the pectoral 

 or pelvic girdle as the case 

 may be. 



The primitive skeleton 

 of the pectoral fin is shewn 

 in longitudinal section in 

 fig. 344, and that of the 

 pelvic fin at a slightly later 

 stage in fig. 345. 



A transverse section 

 shewing the basipterygium 

 {mpt) of the pectoral fin, 

 and the plate passing from 

 it into the fin, is shewn in 

 fig. 346. 



Before proceeding to 

 describe the later history 

 of the two fins it may be 

 well to point out that their 

 embryonic structure corn- 

 argument is based on the fact that a variable but often considerable number of the 

 spinal nerves in front of the pelvic fin are united, by a longitudinal commissure, with 

 the true plexus of the nerves supplying the fin. From this he concludes that the pelvic 

 fin has shifted its position, and that it may once therefore have been situated close 

 behind the visceral arches. If this is the strongest argument which can be brought 

 against the theory advocated in the text, there is I trust a considerable chance of its 

 being generally accepted. For even granting that Davidoff's deduction from the 

 character of the pelvic plexus is correct, there is, so far as I see, no reason in the 

 nature of the lateral fin theor}' why the pelvic fins should not have shifted, and on the 

 other hand the longitudinal cord connecting some of the spinal nerves in front of the 

 pelvic fin may have another explanation. It might for instance be a remnant of the 

 time when the pelvic fin had a more elongated form than at present, and accordingly 

 extended further forwards. 



In any case our knowledge of the nature and origin of nervous plexuses is far too 

 imperfect to found upon their character such conclusions as those of Davidoff. 



-P:^- 



344. Pectoral fin of a young embryo of 

 Scyllium in longitudinal and horizontal sec- 

 tion. 



The skeleton of the fin was still in the condition 

 of embryonic cartilage. 



b.p. basipterygium (eventual metapterygium) ; 

 fr. fin rays ; p.g. pectoral girdle in transverse 

 section ; /. foramen in pectoral girdle ; pc. wall 

 of peritoneal cavity. 



