74 PAIRED FINS 



Since the girdles are ingrowths from the base of the fin-skeleton, 

 it is natural that some of the nerves should become surrounded by 

 them, and in the adult pass through foramina to supply the fin. 



It is difficult to find any facts which actually support the gill- 

 arch theory, and much evidence may be urged against it. If 

 developed from gill-septa, lying across the long axis of the body, 

 such transverse folds would rather hinder than favour progres- 

 sion. Moreover, the two pairs would presumably be at first close 

 together, behind the other gills in a position very disadvantageous 

 mechanically. Now, in ontogeny, a paired fin never makes its 

 appearance as a dorso-ventral fold ; but, on the contrary, always as 

 a more or less longitudinal ridge. 



The position of the pelvic fins is attributed (Gegenbaur) to 

 their backward migration. But neither in primitive fishes 

 generally nor in their early fossil representatives is there any 

 evidence of a more anterior pelvic fin. When, as in some 

 Teleostei (p. 425), the pelvic fins are far forwards, their position 

 is on good evidence considered to be secondary. 



The presence of rudimentary muscle-buds in front of the paired 

 fins is supposed to indicate backward migration. This, however, 

 can hardly be the case, since such buds are also found behind these 

 same fins. It has been urged that the presence of a ' nerve-plexus ' 

 or collector nerve (Davidoff [97-99]) at and in front of the base of 

 the pelvic fins, and that the greater extension of the collector in 

 the young than in the adult, are evidence of backward migration. 

 But, again, such a plexus and extension are found at the posterior 

 end of the fins. 



It is true that an attempt has been lately made (Fiirbringer 

 [143], Braus [48]) to account for the position of the pelvic fins by 

 the assumption that the primitive Gnathostomes had much more 

 numerous gill-arches extending much farther back than in known 

 forms ; but of this there is no evidence. That the paired limbs 

 occupy very different relative positions on the trunk is an obvious 

 and striking fact, which will be dealt with farther on (p. 79). 



The gill -arch theory gives no intelligible account of the 

 participation of a large number of segments in the formation of the 

 musculature and nerve-supply of the paired limbs. Yet it is always 

 the case that a considerable, and sometimes a very large number of 

 nerves and myotomes contribute towards them ; and the area from 

 which they are supplied is wider than the actual base of the fin 

 (Fig. 51). Speaking quite generally, the lower the class of verte- 

 brate concerned, the more segments take part in the formation of 

 the paired limbs (Braus [46]). 



If the skeleton of the paired fins were derived from gill-rays, we 

 should expect their muscle -supply to be drawn, not from the 

 myotomes at all, but from the ' lateral-plate ' musculature, inner- 



