VERTEBRATA 



PHYLUM VIII 



ossification. In completely ossified vertebrae the anterior and posterior 

 edges of the amphicoelous centra 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 centrum, 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 



Fig. 16. 



Vertebrae from the abdominal region of the Sturgeon (I *g- 1') j in the anterior Or abdominal 

 (Acipenser), in side view (A) and I in vertical transverse re on n the lower arches proiect as short 

 section (B). Cartilage dotted, bone white, ch, Is oto- o J 



chord ;/, Nerve foramina ; ha, Haemal arches ; i, Inter- 

 calary pieces; m, Neural canal; n, Neural arches; 

 p, Transverse processes (parapophyses) ; r, Ribs ; s, 

 Sheath of notochord ; sp, Neural spines (after R. 

 Hertwig). 



C 



parapophyses, and serve as supports 

 for the movable ribs. True transverse 

 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 (Polypterus, Pleuronectes) 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 



muscles, and rest on 



the vertebral centra, 



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 diphycercal or heterocercal. In the first condition the vertebral column 



Fig. 17. 



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

 vertebrae. B, Detached vertebra, chiefly in side view. C, Last abdominal 

 and first caudal vertebra, c, Centrum ; eft, Notochord ; ha, Haemal arches ; 

 n, Neural arches ; sp.d, Neural spines ; za, Zygapophyses (after R. Hertwig). 



