VERTEBRATA 



PHYLUM VIII 



Fig. ig. 



ossification. In completely ossified vertebrae the anterior and posterior 

 edges of the amphicoelous ceutra are in close contact and also bound 



together by ligaments : the notochord 

 persists only in the intervertebral 

 Spaces, and does not form a continuous 

 thread. The arches are firmly fused 

 with the centrnni, and on the anterior 

 margin of the upper arches there 

 usually projects a short process 

 (zygapophysis), which overlaps a 

 similar posterior process of the centrum 

 on each side, and thus strengthens the 

 Union of contiguous vertebrae (Fig. 17). 

 In the hinder or caudal region the lower 

 arches form a ventral spinous process 



Vertebrae froni the abdominal region of the Sturgeon (Fig. 17); in the anterior Or abdominal 



iAcipense72,mHide View (A) and i^^ reorion the lower arches project as short 



section (B). Cartilage dotted, bone white., ch, Noto- ö r J 



chord;/, Nerve foramina; ;ia,Haemal arches ; i, In ter- parapophvses, and Serve as SUpportS 



calary pieces ; m, Neural canal ; n, Neural arcjhes ; 7 ^1 ^ ^ • 1 m 



p, Transverse processes (parapophyses) ; r, Ribs ; s, lOY the mOVable I'lbs. i rue tranSVerse 



Hertwigf "^^^'^^''^ = '^'' ^^'"'^^ "P"'"' ^''^^'■' ^- processes (diapophyses, processus trans- 



versi), to which the ribs are attached in 

 the higher Vertebrata, and which arise from the upper arches, occur only 

 exceptionally (Poh/pterus, Pleuroneäes) besides the parapophyses. In the 

 Cyclostomi and Chimaeras ribs are entirely absent ; in 

 the Selachii and cartilaginous ganoids they are usually 

 feebly developed or rudimentary. The ribs of the two 

 sides are never united 

 with each other at the 

 ventral border either 

 directly or through the 

 Intervention of a 

 sternum. The inter- 

 muscular bones, which 

 arise in the membrane 

 between the muscle 

 plates, must not be 

 confounded with ribs ; 

 they are delicate bony 

 filaments, often forked 

 at one end, which are 

 disposed between the ^ ^, _^ ^ ,.v ^ . ,. ^. , , •. ^- , 



^ - Caudal vertebrae of the Carp. A, Vertical longitudinal section of three 



muscles, and rest on vertebrae. B, Detached vertebra, chiefly in side view. C, Last abdominal 



tVip vprf a1 1 + ^^^^ ^^^^ caudal vertebra. c, Centrum; ch, Notochord; ha, Haemal arches; 



tue verieDrai centra, », Neural arches ; sp.d, Neural spines ; za, Zygapophyses (after K. Hertwig). 



arches, or ribs, 



The number of the vertebrae in fishes varies most remarkably in the 

 different groups. In certain bony fishes (Ostracion) only 15 are to be counted, 

 while in others there are 70 to 80, in the eel about 200, in many sharks from 

 350 to 400. 



The hinder end of the vertebral column in the embryonic stage of all 

 fishes is (Uph?jcerml or heterocercal. In the first condition the vertebral column 



Fig. 17. 



