NOTES ON MEDIAN AND PAIRKD FINS OF FISIJ. 367 



regular order can be fairly well established ; but it cannot 

 be asserted that they do not also supply others. In fact, I 

 have found it impossible to prove by mere dissection that 

 these muscles are haploneurous. Nevertheless, it can be shown 

 that the muscles of the paired fins are innervated in regular 

 order from before backwards by the spinal nerves, each of 

 which supplies a pair above and below. 



Turning to the dorsal fins, we find that not only do the 

 rami pterygiales form a longitudinal collector, in which it is 

 impossible to follow out for certain the nerve-fibres from indi- 

 vidual segments, but also that the branches running to the 

 fin from the collector form a plexus of even more complicated 

 structure. Over and over again have I tried in vain to follow 

 the nerve-fibres from a spiual nerve to a radial muscle. It 

 must be remembered that the rami pterygiales are nerves of 

 mixed character, containing motor and sensory fibres. The 

 real difficulty is, not to trace a branch to a muscle, but to 

 make sure that no motor fibres from that nerve pass on 

 elsewhere to other muscles along the ramifying twigs of the 

 plexus. 



Having failed to analyse the plexus of the dorsal fin by 

 dissection, it remained to be seen whether any fin could be 

 found in which the motor fibres are distinguishable- from the 

 sensory. Such a condition I discovered in the ventral lobe 

 of the caudal fin of Scyllium. 



The small radial muscles with which this lobe is provided, 

 unfortunately, do not develop from regular muscle-buds, so 

 they cannot be traced in ontogeny to the myotomes. They 

 are subdivided into a large number of small bundles, much 

 more numerous than the segments, and are developed from 

 cells which come off from the proliferating lower edge of the 

 myotomes. The same thing occurs in the anal fin. 



Fortunately, in the tail of Scyllium the nerves from the 

 ventral motor roots do not combine into mixed trunks with 

 those from the dorsal sensory roots. Both the motor and the 

 sensory branches pass obliquely downwards to the base of 

 the fin. Here they form an elaborate plexus (fig. 24), in 



