52 



COMPARATIVE ANATOMY 



XnKn 1 Li 



and so persists as a kind of connecting or packing substance 

 between contiguous centra, which are consequently of a deeply 

 biconcave or amphicalous form (Fig. 41). 



One of the Bony Ganoids, Lepidosteus, forms a marked excep- 

 tion to other Fishes as regards its 

 vertebral column, inasmuch as de- 

 finite articulations are formed be- 

 tween the centra (Fig. 42). A con- 

 cavity is formed at the hinder end of 

 each centrum which articulates with 

 a convexity on the vertebra next be- 

 hind (opisthoccelous form). The noto- 

 chord (except in the caudal region) 

 entirely disappears in the adult; in 

 the larva it is seen to be expanded 

 intravertebrally and constricted in- 



FIG. 41. -PORTION OF THE VERTE- tervertebrallv, a condition of things 



BRAL COLUMN OF A YOUNG ... . 



DOGFISH (Scyllwm caiw-iila). whl ch appears again in the higher 



(After Cartier.) types, as, for instance, in Reptiles. 



C, notochord; FK, the fibro-car- In a sti11 earlier larval stage, how- 



tilaginous mass lying between ever, the constrictions are intra- 



the cartilaginous zones which ver tebral, as in other Fishes, 

 is undergoing calcification ; FTM i i ^ r .1.1 i 



Kn, oute?, and Kn', inner, ^he skeleton of the posterior end 



zone of cartilage ; Li, inverte- of the tail in Fishes requires special 

 bral ligament. notice, and the condition in Amphi- 



oxus, Cyclostomes, and Dipnoans, 



may be taken as a starting-point. In these, the notochord extends 

 straight backwards to the hinder end of the body and is sur- 



f.S. 



i.e. 



A 



-li.tt. 



Fia. 42. PORTION OF THE VERTEBRAL COLUMN OF Lepidoxtvus. (After Balfour 



and Parker.) 



A, vertebra from anterior surface ; B, two vertebra from the side, en, anterior 

 convex face, and en', posterior concave face of centrum ; h.n, transverse 

 process; i.c, intercalary cartilages (fused supra-interdorsals) ; i.x, inter- 

 spinous bone ; /, /, longitudinal ligament ; n.a, upper arch (basi -dorsal). 



rounded quite symmetrically by the tail-fin, and the tail is 

 therefore spoken of as diphyccreal : this condition is also met 



