40 COMPARATIVE ANATOMY 



intervertehrcdly it remains expanded and so persists as a kind of 

 connecting- or packing-substance between contiguous centra, which 

 are consequently of a deeply biconcave or ampMccelous form (Figs. 

 29a and 29b). 



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

 tion to other Fishes as regards its vertebral column, inasmuch as 

 definite ai-ticidations are formed between the centra. A con- 

 cavity is formed at the hinder end of each centrum (Fig. 28), which 

 articulates with a convexity on the next vertebra behind 

 {opisthocodous form). The notochord (except in the caudal region) 

 entirely disappears in the adult ; in the larva it is seen to be ex- 

 panded intravertehraUy , and constricted intervertebrally , a condition 

 of things which appears again in the higher types — as, for instance. 



wK 'e 



Fll!. 29a. — DIA0RA5I SHOWIXCJ THE INTERVERTEBRAL ReMAIK.S OF THE 



Notochord. 



C, C, expanded and constricted portions of notochord ; WA', centra ; Li, inter- 

 vertebral ligaments. 



Fig. 29b. — Portion of the Vertebral Column of a Young Dogfish 

 (Sryllinm canicuta). After Cartier. 



aotoeliord ; Kn, outer, and Kii^, inner, zone of cartilage ; FK, the libro-carti- 

 laginous mass lying between these zones, which is undergoing calcification ; 

 Li, invertebral ligament. 



in Eeptiles. In a still earlier larval stage, however, the constric- 

 tions are intravertebral, as in other Fishes. 



The vertebral column of Fishes is characterised by a very 

 uniform character of its elements, so that a distinction can only be 

 seen between the trunk and caudal vertebras. Its primitive 

 character is shown by the fact that the neural arches are usually 

 incomplete dorsally. As a rule, the closing in of the arch is 

 effected by special pieces of cartilage (comp. p. 38) and by an 

 elastic longitudinal band (Figs. 24, 28) which is always present : 

 this also applies to the haemal arches. Articidctr processes between 

 the arches {zyrjapophyses) are usually present in Fishes which 

 possess bony vertebrte; in Rays and Chimeeioids only amongst 

 Fishes are definite articulations formed between the skull and 



