296 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
into two tubes. This is continued in the anterior part of the truncus by a horizontal 
septum (short in urodeles, longer in anura) separating aortic and pulmonary trunks, 
the former subdivided in a similar way a little farther forward into carotid and aortic 
portions. 
In the early larve of the amphibia each fully developed aortic arch except the 
last extends into the gills, but as the branchie begin to be absorbed, a small vessel 
connecting the afferent and efferent arteries at the base of each gill enlarges and 
Fic. 300.—Heart and adjacent parts of Protopterus, after Rése a, atrium; aoe, 
cesophageal artery; J, air bladder (lung); c, conus; h, hepatic vein; ji, is, superior and 
inferior jugular veins; oe, cesophagus; pa, pulmonary artery; pc, postcardinal vein; ph, 
pharyngeal artery; s, sinus venosus; sc, subclavian vein; 1-4, afferent branchial (aortic) 
arteries. 
becomes the path of the main blood stream and a part of the arch of the adult (fig. 
304). Of these four arches—3, 4, 5, and 6 of the primitive scheme—the fifth is 
lost in the adults of all except a few urodeles and cecilians. The fourth connects 
with the dorsal aorta and the sixth with the pulmonary arteries. These last, which 
often have a ductus Botallii, are noticeable for the large cutaneus arteries—anterior 
and posterior—which arise from them and which play an important part in respira- 
