XIII 



PHYLUM CHORDATA 



187 



UST 



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 

 vertebra bear no ribs. In the last three the neural spines (N. SP.) 

 are single. 



In the caudal vertebrae the outgrowths corresponding to the 

 parapophyses are fused with the centrum and unite in the middle 

 ventral line, forming a hcemal arch (C, H. A.), through which the 

 caudal artery and vein run. In the first six caudals each haemal 

 arch bears a pair of ribs (R.), in the rest the arch is produced 

 downwards and backwards into a hceinal spine (D, H. SP.). 



The centra as well as the arches of the vertebrae are formed 

 entirely from the skeleto- 

 genous layer, and not 

 from the sheath of the 

 notochord as in Elasmo- 

 branchs (see pp. 66 and 

 137). 



The posterior end of 

 the caudal region is curi- 

 ously modified for the 

 support of the tail-fin. 

 The hindmost centra 

 (Fig. 810, CN.) have their 

 axes -not horizontal but 

 deflected upwards, and 

 following the last un- 

 doubted centrum is a 

 rod-like structure, the 



urostyle (UST), consisting of the partly ossified end of the noto- 

 chord, which has thus precisely the same upward flexure as in the 

 Dog-fish. The neural and haemal spines (N.SP., H.SP.) of the last five 

 vertebrae are very broad and closely connected with one another, 

 and are more numerous than the centra ; and three or four haemal 

 arches are attached to the urostyle. In this way a firm vertical 

 plate of bone is formed, to the edge of which the caudal fin-rays 

 (D.F.It.) are attached fan-wise in a symmetrical manner. It will 

 be obvious, however, that this homocercal tail-fin is really quite as 

 unsymmetrical as the heterocercal fin of the Dog-fish, since, its 

 morphological axis being constituted by the notochord, nearly the 

 whole of its ra} T s are, in strictness, ventral. 



The skull (Fig. 811) is an extremely complex structure, com- 

 posed of mingled bone and cartilage. The cartilage has no super- 

 ficial mosaic of lime-salts such as w r e find in Elasmobranchs, but 



HSP 



FIG. 810. Salmo fario, caudal end of vertebral 

 column. CN. centrum; D. F. R. dermal fin-rays; 

 H. SP. haemal spine ; H. ZYG. haemal zyga- 

 pophysis ; N. SP, neural spine ; N. ZYG. neural 

 zygapophysis ; UST. urostyle. 



