MEDIAN FINS 



69 



is more than doubtful (pp. 101 and 105). The haemal arches are 

 similarly completed below by haemal spines. 



In the lower Gnathostomes (Pisces) the median dorsal and 

 ventral fins are provided with endoskeletal supports, generally in 

 the shape of rods known as pterygiophores, radials, or somactidia 

 (Lankester). Now the question arises as to the morphological 



sa 



nc- 



fSf 



fr- 



et/ 



Fig. 45. 



n 



av. 



Seyllium canicvla, L. Left-side view, enlarged, of a portion of the vertebral column and 

 ribs with the connective tissue septa, to show their relation to the blood-vessels and nerves. 

 ao, dorsal aorta ; as, abdominal wall ; at, artery ; av, vein ; c, centrum ; d.n, dorsal nerve ; d.r, 

 ganglion of dorsal root ; in, interdorsal ; 11, dorsal ligament ; m.s, median dorsal septum ; v, 

 ventral nerve ; na, basidorsal ; n.c, nerve-cord ; r, rib ; s.a, segmental dorsal artery ; sd, supra- 

 dorsal ; spt, vertical transverse septum passing between successive myotomes ; s.v, segmental 

 dorsal vein ; t.s, chief transverse horizontal septum in which lie the ribs ; o.s, oblique upper 

 longitudinal septum, a similar lower septum occurs between it and the transverse horizontal 

 septum ; v, posterior cardinal vein ; v.r, ventral spinal root ; w, intervertebral ligament. 



significance of this supporting skeleton. Is it a special appendicular 

 skeleton, separate from the axial skeleton (skull and vertebral 

 column) as held by Cuvier and Huxley 1 Or is it derived from 

 the axial skeleton itself (Gegenbaur) 1 In the Cyclostomes, the 

 cartilaginous rays of the median fins reach down to the neural 

 tube, and may even fuse to a continuous cartilage passing down 

 each side, so as to enclose the nerve-cord and notochord (p. 32 

 and Fig. 19). In most of the Teleostomi the dorsal fin skeleton 



