466 Causes and Course of Organic Evolution 



to distribute blood to the gill pouches. But a decided advance 

 is the division of the auricle into two chambers by a septum. 

 The septum, however, is still imperfect, being perforated by 

 apertures. 



In some of the simpler Urodela (Proteus, Nedurus, etc.) 

 the structure remains as in Apoda, except that the heart has 

 advanced slightly nearer the head. But in other of the simpler 

 genera (Siren, Amphiuma), and in the salamandroid forms, 

 the conus shows a decided spiral twist; internally it has valves 

 at its fore and hind ends also, as well as a spiral jflap projecting 

 into it. The entire heart further in the last assumes a more 

 condensed and rounded aspect. All of these details constitute 

 increasingly perfected mechanical devices for forcing the blood 

 flow, and for ensuring increasingly perfected separation of 

 the impure and the oxygenated blood. 



An equally noteworthy advance and condensation can be 

 traced in the blood vascular system. 



From our knowledge of the cyclostome Bdellostoma Stouti, 

 and to a minor degree of Amphioxus, it may safely be said 

 that in types between the nemerteans and cyclostomes — now 

 evidently lost to us — a series of twelve or more pairs of gill 

 pouches was formed. Into these as many afferent vessels 

 passed; and as many efferent ones departed. Though slight 

 rearrangements have taken place, we can still in the above 

 named species trace twelve to fourteen pairs of these (p. 448), 

 which in other Bdellostomata become reduced to six, the typ- 

 ical number for higher craniate embryos, and also for Myxine. 

 In Petromyzon, after four pairs of afferent vessels have spread 

 out from the ventral aorta, it splits and each branch gives 

 off four lateral vessels. These latter, in process of condensing 

 specialization, and substitution of aerial for aquatic respira- 

 tion, would tend to disappear. 



In some of the Apoda distinct indications of five sets of 

 gill pouches occur, but in others only three are functional in 

 the young. Brauer has shown (15 Jf,: 496) that Hypogeophis 

 and Ichthyophis form three pairs, but only the anterior two 

 of these later develop expanded external gills, the third set 

 of gills being rudimentary. 



The gill pouches produce the external gills, which in origin 

 represent lobe-like expansions of the membrane formed from 

 the outer edge of each pouch that originates them. The anter- 

 ior three pairs of afferent ventral vessels supply them, while 

 the fourth or posterior pair proceeds to and joins with the 



