VERTEBRAL COLUMN 



205 



In the dominant group of existing Fishes, the Teleostei, the 

 centra are almost invariably biconcave, although in the Eels they 

 may be flat or even slightly convex in front. Eibs are absent in 

 the Syngaathidae and in the Plectognathi. In addition to the 

 usual articulation between the centra, the vertebrae often articu- 

 late with one another by means of paired processes arising from 

 the anterior margin of each neural arch, or from the centrum 

 at the base of the arch (pre-zygapophyses), and meeting similar 

 processes which project either from the hinder margin of the arch 

 of the vertebra in front, or from the adjacent portion of its centrum 

 (post-zygapophyses). The haemal arches may have similar pro- 



FiG. 119. — A, side" view of pi-ecaudal vertebrae of a Cod (Gadus vim-rhua) without the 

 lib.s ; B, similar view of caudal vertebrae of the same Teleost. c, Centrum ; h.a, 

 haemal arch ; h.sp, haemal spine ; n.a, neural arch ; n.sp, neural spine ; 2h para- 

 pophysis ; ^J.z, pre-zygapophysis ; pt.z, post-zygapojihysis. 



cesses (Fig. 119). One, two, or in some Teleosts, three pairs of 

 slender intermuscular bones radiate outwards from the centra 

 into the myocommata (epicentrals), or from the neural arch 

 (epineurals), or from the ribs (epipleurals). 



The Ribs. — It is doubtful if the structures termed "ribs" 

 are homologous in the different groups of Fishes. There appear 

 to be two kinds, distinguishable as dorsal and ventral ribs (Fig. 

 15 6). Dorsal ribs are situated in the fibrous tissue separating 

 the epiaxial from the hypaxial muscles of the body wall, and they 

 take no part in forming the haemal arches of the caudal region. 

 Ventral ribs, on the other hand, always lie internal to the 

 hypaxial muscles, and directly external to the peritoneal lining 

 of the coelom, and they usually contribute to the formation of 



