PAIRED FINS 



79 



(Goodrich [176]), and there is evidence that in Eaja, for instance, the 

 radial muscles of the pectoral fin retain the primitive metamerism. 



We have now to account for the apparent migration of the limbs. 

 Even if the ancestral Gnathostome had possessed continuous paired 

 fin-folds, the position of the paired limbs cannot be accounted for 

 in phylogeny merely by their persistence in certain regions and 

 suppression in others. In all classes there has been perpetual 



-TIC. 



.cr. 



FIG. 52. 



Transverse section through the centre of A, a trunk vertebra of Raja. B, a caudal vertebra 

 of Rhiiiabatus granulatus, Cuv. C, a trunk vertebra of Rhina squat ina, L. D, left-side view 

 of a portion of the vertebral column, and of the skeleton of the first dorsal tin of Rhina sryuolina, 

 L. bp, basal ; bi>, basiventral (haemal arch) ; c, centrum ; o.r, calcareous ring ; d.f, dorsal tin ; 

 h.a, haomal arch ; in, interdorsal ; n.a, basidorsal (neural arch) ; n.c, neural canal ; n.sj>, neural 

 spino (or anterior radial); r, distal end of radial ; r.c and v.c, radiating calcification (black); 

 sd, supradorsal. 



alteration of the position occupied by the paired limbs, just as in 

 the case of the unpaired fins of fish (Figs. 47, 51). It has been 

 held (Gegenbaur) that these changes of position are due to the actual 

 migration of the paired limbs from one place to another. Now 

 embryology affords no evidence for this view. In ontogeny there is 

 little or no migration of the whole fin. Considerable apparent 

 motion is brought about by processes of concentration, growth, and 

 reduction. It has already been mentioned that any trunk segment 



