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 of Lopholatilus, after Silvester. @, 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; f, facial vein; g, gastric veins; hp, 
hepatic portal; hv, hepatic veins; 7j, inferior jugular; im, 7s, veins from intestine and spleen; 
1, liver; pc, postcardinal; pd, postcloacal; per, peritoneal; ph, pharyngeal; po, postorbital; 
re, anterior revehentes; s, sinus venosus; s¢, veins from stomach and intestine; th, thyreoid; 
tm, thymus; v, ventricle; va, ventral aorta; vf, vein from ventral fin; w, 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. 206. 
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 cecilians 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 
