52 



COMPARATIVE ANATOMY 



XnZtf 



and so persists as a kind of connecting or packing substance 

 between contiguous centra, which are consequently of a deeply 

 biconcave or amphiccclous 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 (opisthocwlous 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- 

 tervertebrally, a condition of things 

 which appears again in the higher 

 types, as, for instance, in Reptiles. 



C, notochord; FK 9 the fibro-car- In a still earlier larval stage, how- 

 tllaginous mass lying between ever, the constrictions are intra- 

 the cartilaginous zones which vertebral as in other Fishes. 

 L;t^d C t^; The 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 Dipnoaiis, 



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

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



FIG. 41. PORTION OF THE VERTE- 

 BRAL COLUMN OF A YOUNG 

 DOGFISH (Scyllium canicula). 

 (After Cartier. ) 



cn' 



c.n 



FIG. 42. 



-PORTION OF THE VERTEBRAL COLUMN OF Lepidosteus. (After Balfour 

 and Parker.) 



A , vertebra from anterior surface ; B, two vertebrae from the side, cn, anterior 

 convex face, and cn', posterior concave face of centrum; h.a, transverse 

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

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



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

 therefore spoken of as diphycercal : this condition is also met 



