282 COMPARATIVE ANATOMY. 



elongated in Amphiuma, Proteus, and the Gymnophiona. It 

 is continued anteriorly into a conus arteriosus, as in Elasmo- 

 branchs, Ganoids, and Dipnoans; this has usually a slight 

 spiral twist, and possesses a transverse row of valves at either end, 

 as well as a spiral fold extending into its lumen. 1 This holds good 

 for the Axolotl, Amblystoma, Salamandra, Amphiuma, 

 and Siren. In others (e.g. Menobranchus, Proteus, Gymno- 

 phiona), retrogression is seen in a lengthening of the conus, the 

 disappearance of the spiral fold, and the presence of only a single 

 row of valves. 



In Anura, the fold lying within the conus extends so far 

 back that no undivided portion of the cavity is left. The conse- 

 quence of this is that the blood in one section of the branchial 



A 



FIG. 226. DIAGRAM SHOWING THE COURSE OF THE BLOOD THROUGH THE HEART 

 IN Urodela (A) AND Anura (B). 



A, right atrium ; A 1 , left atrium ; V, ventricle ; tr, conus arteriosus, divided in 

 Anura (B) into two portions, tr, tr 1 : through tr venous blood passes into the 

 pulmonary arteries, Ap 1 , Ap 1 , while through tr 1 mixed blood goes to the carotids, 

 ci ce, and to the roots of the aorta, KA ; lr, lv, pulmonary veins ; v, r, 

 pre- and postcavals (only one precaval is indicated) opening into the right 

 atrium. 



vessels that from which the pulmonary artery arises is mainly 

 venous, while the others contain mixed blood (Fig. 226, B) ; for, 

 owing to the spongy nature of the ventricle, there is no time for 

 its contained blood to get thoroughly mixed before it is forced into 

 the conus. 



As in the Dipnoi, four branchial arteries arise on either side 

 from the short conus in the Amphibia, which taking as a good 

 type the larva of Salarnandra have the following relations. 



The three anterior branchial arteries pass to numerous external 

 gill-tufts, where they break up into capillaries (Fig. 227, 1, 2, 3}. 

 From the latter three branchial veins (/to III] arise, which pass to 

 the dorsal side, and there unite on either side to form the aortic roots. 

 The fourth branchial artery, which is smaller than the others (4), 

 does not pass to a gill, but to the pulmonary artery, which arises 



1 This spiral fold is to be looked upon as derived from a series of fused valves. 



