130 



COMPARATIVE ANATOMY. 



Chimaerae special cartilages are intercalated, which aid the superior 

 arches, which are connected with the bodies of the vertebrae, in 

 closing 1 the vertebral canal. 



In the.Sturiones the skeletogenous sheath forms a considerable 

 tube, and the separation of the column into vertebras is only indicated 

 by the superjacent arches. The vertebral column of the other Ganoidei 

 is sharply marked off from this, its lowest form. Amia resembles 

 the Teleostei. A small portion of cartilage is retained at the point 

 where the arches are connected with the centra of the vertebras ; 

 but this is absent in Polypterus, so that in it the arches and the 

 centra are united together by bone. 



Lepidosteus is the most divergent form, for in it the cartilage 

 becomes constricted between the vertebras. In 

 the cartilage which forms the constrictions, an 

 intervertebral articular cavity is formed, so that 

 the opisthoccelous vertebras articulate with one 

 another. So far they resemble the Amphibia, 

 but, later on, the remnant of the vertebral por- 

 tion of the notochord disappears, and a bony 

 centrum is developed, which is connected, and 

 continuous, with the upper arches. 



The vertebral column of the Teleostei is cha- 

 racterised by the reduction of the cartilaginous 

 rudiment. This reduction may be seen to be 

 gradual, and may indeed be seen in one and the 

 same vertebral column in certain stages of de- 

 velopment ; where, that is, the cartilage may be 

 seen to diminish in quantity as we go from before 

 backwards. As a rule, four cartilaginous pieces, 

 belonging to the superior and inferior arches 

 (Fig. 221 b, hh 1 ), may be seen around the chord, 

 and these take a certain share in the formation 

 of the arches. They very rarely form complete 

 superior arches. When osseous substance is 

 developed, these cartilages are generally retained 

 in the middle of the centrum, so that on making 

 a vertical section through it we get an obliquely set cross (cf. 

 Fig. 223, Jc7c') } the arms of which are directed towards the bony 

 arches. The notochord is always well developed between the 

 vertebras, so that the centra are amphiccelous. 



Fig. 223. Vertical 

 section, through the 

 middle of a vertebra 

 of Esox lucius. 

 ch Notochord. cs 

 Chordal sheath. Iclc' 

 Cartilaginous cross. 

 A' Corresponds to the 

 tipper, and 7/ to the 

 lower arches (in rudi- 

 ment), h Osseous 

 transverse process. 

 n Spinal canal. 



330. 



The vertebral column of Fishes can only be divided into two 

 regions, the body and the tail. They are distinguished from each 

 other by the characters of the inferior processes of the vertebra?, 

 while the upper arches arc connected with the vertebras in the 

 same manner throughout; and are generally distinguished by the 

 possession of median (spinous) processes. In the region of the 



