294 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



Circulation in the Separate Classes. 



CYCLOSTOMES present marked differences in the circulation of the two 

 groups, the petromyzons being nearly normal, the myxinoids- decidedly aberrant. 

 The aortic arches vary in number with the number of gill pouches (p. 239). In the 

 myxinoids the common carotid is connected with all of the efferent branchials by a 



FIG. 297. Oblique ventral view of venous system of Petromyzon, drawn from a corro- 

 sion preparation (Princeton, 669); ac, precardinal; c, caudal; gs, genital sinus; hv, hepatic 

 vein; ij, inferior jugular; pc, postcardinal; sv, sinus venosus; -va, ventral aorta. 



trunk running parallel to the body axis, just dorsal to the gill pouches. The inter- 

 segmental arteries of the dorsal region are irregular, sometimes alternating, some- 

 times appearing in pairs on the two sides of the median line. In the myxinoids 

 (fig. 297) the subcardinals are united behind, the postcardinals in front, these 

 latter uniting' with the single inferior jugular of the left side to form the unpaired 

 Cuverian duct, the presence of which renders the sinus venosus asymmetrical and 



FIG. 298. Anterior arterial vessels of the tile fish (Lopholatilus), after Silvester, a, 

 auricle; ab, to air bladder; am, to angle of mouth; c, coeliac axis; d, dorsal arteries; da, 

 dorsal aorta; ec, external carotid; g, genital artery; gs, gastrosplenic; h, hyoid artery; ha, 

 hepatic; I, lingual; Ig, left gastric; m, mesenteric; mh, middle hypobranchial; o, ophthalmic; 

 pa, parietal; po, postcfrbital; ps, pseudobranch; rg, right genital; so, supraorbital; v, ven- 

 tricle; va, ventral aorta. 



forces the hepatic veins to empty into the right side. The hepatic portal receives 

 a vein from the head, and then passes back to a contractile portal heart, just before 

 it enters the liver. 



FISHES. In the fishes, the dipnoi excepted, the circulation corresponds rather 

 closely in its main features with the primitive condition described above. The 



