364 EDWIN S. GOODIJTCH. 



ploxns supplying the paired fins is stronger and much less 

 difficult to expose. The pectoral and pelvic plexus have been 

 admirably described nnd figured by Brans (3, 6) in a 

 large number of Elasmobranclis, while Pnnnitt (29, 30) 

 has studied the pelvic plexus in Mustelus and Acanthias. 



The nerve-plexus of the paired fins is ver}^ variable, both 

 as reg'ards the number of nerves which contribute towards it 

 and the exact course of its secondary branches. Such is also 

 the character of the nerve-plexus of the dorsal fins; but here 

 it is less easy to decide as to the exact number of nerves which 

 enter into its composition. In minor details no two specimens 

 correspond, and even the two sides of the same individual 

 may differ considerably. However, on the whole, there is 

 great constancy in the character and mctameric value of the 

 plexus of the median fins, as is shown by comparing a large 

 number of specimens. Unfortunately, it is so complex, and 

 the nerve branches are so fine, that I have not found it pos- 

 sible to trace out its formation in ontogeny. 



The plexus of the dorsal fins. — We have seen above 

 that a dorsal fin contains some fourteen muscle segments. 

 We should, therefore, expect at least fourteen spinal nerves 

 to join in its formation. Moreover, since coireentration takes 

 place to a much greater extent in front than behind, we should 

 expect the longitudinal collector, formed by the gathering 

 together of various nerve components, to be situated cliiefly in 

 front of the fin-base. Now this is just what dissection reveals. 



Figs. 21 and 22 show the general nerve-supply of the two 

 dorsal fins. A comparatively stout collector is seen to run 

 forwards from the base of each fin {ale). It is composed of a 

 number of twigs derived from the rami dorsales of some dozen 

 segments. The collector increases in bulk as it passes back- 

 wards, and more nerves enter into it. AVhere the collector 

 beo-ins is often very difficult to determine, in the case of the 

 first dorsal especially ; for its first components are so extremely 

 slender that they arc very hard to discriminate from the inter- 

 crossing plexus of nerve-twigs which are present all nlong t1u> 

 median dorsal septum. 



