CIRCULATORY ORGANS. 



295 



heart is purely venous and the only peculiarities to be mentioned are the following: 

 In the elasmobranchs and ganoids the valves of the conus are arranged in several 

 (3-8) rows, but in the teleosts (Butyrinus excepted) they are reduced to a single row, 

 apparently corresponding to the first of the lower forms. In the latter group the 

 bulbus is especially well developed. The aortic arches correspond in number to 

 the functional gill slits six or seven in the notidanid sharks, five in other elasmo- 

 branchs and at most four in ganoids and teleosts. Paired inferior jugulars are 

 usually present, but they are lacking in Polypterus, while in Lepidosteus and many 

 teleosts they are united into a single trunk emptying directly into the sinus venosus. 

 Epigastric veins are usually present and paired but are absent from many bony 

 fishes. 



FIG. 299. Anterior venous system and heart oiLopholatilus, after Silvester, a, auricle 

 ab, veins from air bladder; b, bulbus; bv, brachial vein; c, cerebral vein; cd, Cuvierian duct; 

 cv, caudal vein; d, dorsal branches of parietal veins;/, facial vein; g, gastric veins; hp, 

 hepatic portal; hv, hepatic veins; /'_/, inferior jugular; in, is, veins from intestine and spleen; 

 I, liver; pc, postcardinal; pd, postcloacal; per, peritoneal; ph, pharyngeal; po, postorbital; 

 re, anterior revehentes; s, sinus venosus; si, veins from stomach and intestine; th, thyreoid; 

 tm, thymus; v, ventricle; va, ventral aorta; vf, vein from ventral fin; iu, outline of Wolffian 

 body. 



DIPNOI. In this group the atrium, in correlation with the development of 

 lungs, becomes partially divided as described above. No true atrio-ventricular 

 valves occur, their place being taken by a strong ridge which, in systole, closes the 

 canal and at the same time partially divides the ventricle into arterial and venous 

 halves. The conus has eight rows of valves and in Ceratodus the truncus shows the 

 beginning of a division (completed in Protopterus) separating the arterial from the 

 venous arches. For veins, see fig. 296. 



AMPHIBIA. In the amphibia the division of the atrium by a septum atriorum 

 into right (venous) and left (arterial) halves is carried farther. This septum is 

 fenestrate in urodeles and gymnophiones, entire in anura, but in none is it carried 

 clear to the atrio-ventricular wall. In systole the edge of the septum is forced for- 

 ward, completely separating the two atria. No corresponding septum is developed 

 in the ventricle, but numerous muscular bands extending through its cavity tend 

 to prevent the mingling of arterial and venous blood. In Proteus, Cryptobranchus 

 and the caecilians the bulbus is simple but in the other urodeles and the anura a 

 spiral septum (possibly representing fused valves) is developed in it, separating it 



