366 EDWIN S. GOODRICH. 



With regard to tlie musculature, we have already decided 

 that there is no definite evidence that the nietanierisni is lost 

 (p. 359). The skeleton is still obviously segmentally divided 

 in the dorsal fins of Scyllinm, in spite of the slight con- 

 crescence of some of the radials at their base. The radials 

 of the paired fins have undergone much greater fusion and 

 modification. But there is nothing in their structure or de- 

 velopment which precludes the idea that even the basals were 

 once metamerically seguiented. lu modern sharks, however, 

 this segmentation of the skeleton of the paired fins is to a 

 great extent lost. It is to the nerves that appeal is generally 

 made for evidence against metamerism (pp. 338-340) ; let us, 

 therefore, examine further the nerve-supply of the fins. 



On the real nature of the nerve -plexus. — Many 

 anatomists seem to consider that the nerve-plexus is formed by 

 a combination of several nerves, which lose their individuality, 

 and are then redistributed to the limb, somewhat as a number 

 of blood-vessels may anastomose and supply a gland. In such 

 a case the nerves would be so mixed in the plexus that even 

 their motor fibres might lose all trace of metamerism. 



But such is not really the case, even in the highest verte- 

 brates, as Herringham (21), Patterson (27, 28), and others 

 have shown. 



Now, we may well ask whether in the Elasmobranch there 

 is really any motor plexus at all, if by plexus is meant a 

 mixing of nerve-fibres bringing about a disturbance or 

 destruction of the original metamerism. If the nerves could 

 be traced to each radial muscle of a fin, it would be easy 

 enough to prove whether or not it is the case. Unfortunately, 

 dissection can help us but little in settling this point. Most 

 of the nerves to the paired fins pass directly to the fin-base ; 

 but as soon as they reach it they become joined together by a 

 complicated system of connecting nerves, even before they 

 enter the muscles. When they reach the latter they become 

 involved in such a complex network that it becomes impos- 

 sible to determine for certain whither the nerve-fibres lead. 

 That on the whole each nerve supplies two radial muscles in 



