196 



ZOOLOGY 



SECT. 



CN 



H.IYG 



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 hcemal spine (D, H. SP.). 



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

 entirely from the skeletogenous layer, and not from the sheath 

 of the notochord as in Elasmobranchs (see pp. 71 and 143). 



The posterior end of the caudal region is curiously modified 

 for the support of the tail-fin. The hindmost centra (Fig. 869, CN.) 

 have their axes not horizontal, but deflected upwards, and following 

 the last undoubted centrum is a rod-like structure, the urostyle 

 (UST.), consisting of the partly ossified end of the notochord, 

 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 

 UST 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 



FIG. 869. Salmo fario, caudal end of vertebral is formed, to the edge of 

 column. OX. centrum ; D. F. R. dermal fin-rays ; i i ,r j i 



H. SP. hfemal spine ; H. ZYG. haemal zyga- WlUCh the Caudal Iin-rayS 



pophysis ; N. S.P. neural spine ; N. ZYG. neural ( T) T? 7? Varo a+tanli^rl fan 



zygapophysis ; UST. urostyle. \ U ,'^ *) are ^ttaCJ 



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 rays are, in strictness, ventral. 



The skull (Fig. 870) is an extremely complex structure, composed 

 of mingled bone and cartilage. The cartilage has no superficial 

 mosaic of lime-salts such as we find in many Elasmobranchs, but 

 certain portions of it are replaced by bones, and there are in 

 addition numerous investing bones developed in the surrounding 

 connective- tissue. As in the Dog-fish, the skull may be divided 

 into cranium, upper and lower jaws, with their suspensory apparatus, 

 and hyoid and branchial arches. 



The cranium (Fig. 871) is a somewhat wedge-shaped structure, 

 its apex being directed forwards. At first sight the distinction 

 between replacing and investing bones is not obvious, but after 

 maceration or boiling certain flat bones (the paired parietals, PA., 

 frontals, FR., and nasals, NA., and the unpaired supra-ethmoid, 



HSP 



Zt.FH 



