368 EDWIN S. GOODRICH. 



which can be distiiiguished a large, longitudinal '^collector" 

 and twigs running outwards to the radial muscles and skin of 

 the fin. Now, by careful dissection, under the high powers 

 of the binocular microscope of Zeiss, one can follow out the 

 motor and the sensory fibres to their destination. It soon 

 becomes evident that, while the latter combine to form the 

 longitudinal trunk and the plexus of anastomosing nerves, 

 which send branches at intervals to the skin (fig. 24), the 

 motor fibres pass through the plexus without really becoming 

 involved in it. Each spinal nerve sends down motor branches 

 supplying a considerable number of radial muscles. It is, of 

 course, by no means easy to follow out every twig to its 

 ending ; but from a careful and minute study of several tails 

 I have satisfied myself that the motor branches of one seg- 

 ment do not anastomose or mix with those of another segment 

 — the area supplied by one motor root begins where that of 

 another ends. In the specimen figured there is one twig (marked 

 with an *), in two segments, Avhich seems to join one segment 

 to the next behind; but I am inclined to believe that the 

 fibres do not mix peripherally. In other tails investigated 

 since, I have found no such junction. At all events, the 

 facts are quite compatible with the view that no mixture 

 takes place. 



There appears, then, to be no such thing as a real motor 

 plexus in the caudal fin. Whether there is a true sensory 

 plexus, or Avhether it is more apparent than real, I am unable 

 to determine for certain, as the fibres cannot be disentangled. 



Seeing that the so-called motor " plexus " in the caudal is 

 probably only apparent, we may well ask whether a true 

 motor plexus exists in any of the dorsal or paired fins. May 

 it not be that here also the motor fibres pass through a sen- 

 sory network and do not lose their original metameric order ? 



I am strongly of opinion that this is the case, and that the 

 radial muscles arc haploneurous, the original metamerism of 

 the fin being preserved in the adult. Since this question 

 cannot be answered by anatomy, we must aj)peal to experi- 

 ments on the living tissues. 



