NOTES ON MEDIAN AND PAIRED FINS OF FISH. 365 



The collector gives off branches to the fin as soon as it 

 reaches its base, and often ceases about half way down the 

 fin. Then come one or two nerves which give off branches 

 independently to the fin. In many fins all the nerves passing 

 to the fin are joined together by communicating branches, 

 continuations of the collector (fig. 26). 



At the hind end of the fin are one or two nerves with a 

 short, and often ill-defined, posterior collector {pic). 



When pterygial nerves, passing out from the collectors, 

 reach the base of the radial muscles, they run in amongst 

 them and branch repeatedly. A plexus of extraordinary 

 complexity is thus formed round and through the muscles 

 and along the cartilages outwards to the web of the fin. 



We find, then, that some fourteen to sixteen spinal nerves 

 undoubtedly contribute to the innervation of the first dorsal 

 fin, and that the rami pterygiales of those situated in front of 

 the middle of the fin-base always combine to a longitudinal 

 collector. The collector clearly shows that the radial muscles 

 derived from these segments have been displaced backward. 

 The one or two rami pterygiales gathered into a posterior 

 collector indicate a similar but very much less extensive con- 

 centration forwards. 



An examination of the nerve-supply of the second dorsal 

 fin yields the same results. The anterior collector of this fin 

 begins immediately behind the posterior collector of the first 

 dorsal. 



The anatomy of the adult fully bears out the conclusion 

 arrived at from a study of the development ; the dorsal fins 

 are made up of a large number of greatly concentrated 

 segmental elements — muscular, nervous, and skeletal. The 

 lateral fold theory is, then, strongly supported by our know- 

 ledge of the structure and development of the median and 

 paired fins, since the paired fins have long been known to be 

 constructed and developed on exactly the same principle. 



But there remains one important, though not essential, 

 question to discuss : How far is the original metameric struc- 

 ture preserved in the adult ? 



VOL. 50, PART 2. — NEW SEEIES. ' 26 



