-XIII 



PHYLUM CHORDATA 



201 



the rest (a) is hidden beneath the scales immediately anterior 

 to it. Besides the scales, the fin-rays belong to the exoskeleton, 

 but will be most conveniently considered in connection with the 

 endoskeleton. 



Endoskeleton. The vertebral column shows a great advance 

 on that of the two previous classes in being thoroughly 

 differentiated into distinct bony 

 vertebrae. It is divisible into 

 an anterior or abdominal region 

 and a posterior or caudal region, 

 each containing about twenty- 

 eight vertebrae. 



A typical abdominal vertebra 

 consists of a dice-box-shaped 

 centrum (Fig. 857, ON.) with 

 deeply concave anterior and pos- 

 terior faces, and perforated in 

 the centre by a small hole. The 

 edges of the centra are united 

 by ligament and the biconvex 

 spaces between them are filled 

 by the remains of the notochord ; 

 there are also articulations be- 

 tween the arches by means of 

 little bony processes, the zygapo- 

 physes (N. ZYG., H. ZYG.). To the 

 dorsal surface of the centrum is 

 attached, by ligaments in the 

 anterior vertebrae, by ankylosis 

 or actual bony union in the pos- 

 terior, a low neural arch (N. A.), 

 which consist in the anterior 

 vertebrae of distinct right and left moieties, and is continued above 

 into a long, slender, double neural spine (N. SP.), directed upwards 

 and backwards. To the ventro-lateral region of the vertebra 

 are attached by ligament a pair of long, slender pleural ribs (R.) 

 with dilated heads, which curve downwards and backwards 

 between the muscles and the peritoneum, thus encircling the 

 abdominal cavity. In the first two vertebrae they are attached 

 directly to the centrum, in the rest to short downwardly directed 

 bones, the parapophyses (PA. PH.) immovably articulated by broad 

 surfaces to the centrum. At the junction of the neural arch 

 with the centrum are attached, . also by fibrous union, a pair 

 of delicate inter-muscular bones (i. M. B.), which extend outwards 

 and backwards in the fibrous septa between the myomeres. 

 The first and second abdominal vertebrae bear no ribs. In the 

 last three the neural spines (B ; N. SP.) are single. i 



Fio. 857. Salmo fario. A, one of the 

 anterior, and B, one of the posterior ab- 

 dominal vertebrae ; C, one of the anterior, 

 and D, one of the posterior caudal vertebrae. 

 CN. centrum ; 1MB. intermuscular 

 bone ; HA. haemal arch ; H. SP. haemal 

 spine ; H. ZYG. haemal zygapophysis ; 

 N. A. neural arch ; BT. SP. neural spine ; 

 N. ZYG. neural zygapophysis ; FA. PH. 

 parapophysis ; R. pleural rib. 



