202 



ZOOLOGY 



In the caudal vertebrae the outgrowths correspouding to the 

 parapophyses are fused with the centrum and unite in the middle 

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

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

 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. 73 and 147). 



The posterior end of the caudal region is curiously modified 

 for the support of the tail-fin. The hindmost centra (Fig. 858, CN.) 

 have their axes not horizontal, but deflected upwards, and 

 following the last iTndoubted 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 

 hsemal spines (n. SP., 

 H. SP.) of the last five 

 vertebrse are very broad 

 and closely connected 

 with one another, and are 

 more numerous than the 

 centra ; and three or four 

 haemal arches are at- 

 tached 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.B.) 

 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 rays are, in strictness, ventral. 



The s^itZ^ (Fig. 859) 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. 860) 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,, 



.hm. 



Fig. S5S.— Salmo fario, caudal end of vei-tebral 

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

 U. SP. hairaal spine ; H. ZVGr. haiinal zyga- 

 pophysls ; N. SP. neural spine ; N. ZYG. neural 

 zygapophysis ; UST. urostyle. 



