RESPIRATORY ORGANS OF PERENNIBRANCHIATA. 699 



dividing into four branchial arteries ; but as in the adult Menopoma 

 there are no branchiae, these vessels (o o o) wind round each side of the 

 neck, and again unite into two trunks (r r) which by their union form 

 the aorta (t t). It will easily be perceived that this arrangement is 

 precisely that met with in fishes ; only that, as there are here no gills 

 intervening between the terminations of the branchial arteries and the 

 commencements of the branchial veins, these vessels are immediately con- 

 tinuous with each other. Moreover, from the lowest branchial arch (o) a 

 pulmonary artery is given off, which ramifies over the surface of the as 

 yet rudimentary lung (e), and thus gives rise to a distinct pulmonary 

 circulation. 



(1957.) Having carefully considered the disposition of the vessels in 

 the Menopoma, above described, the reader will be able to appreciate 

 the arrangement of the vascular system in those Amphibia which, being 

 provided with both gills and lungs through the whole of their lives, 

 literally combine the blood-vessels of a fish with those of an air-breath- 

 ing reptile. 



(1958.) In the PERENNIBRANCHIATA, as, for example, in the Proteus, 

 instead of the bulbus arteriosus being immediately continuous with the 

 aorta (as it is in the Menopoma) through the interposition of the vessels 

 ooo (fig. 343), the blood derived from the heart is obliged to pass more 

 or less completely through gills appended to the sides of the neck before 

 it arrives in the vessels (r r) which may be said to represent the bran- 

 chial veins of fishes. 



(1959.) The branchiae are either vascular tufts or pectiniform organs 

 (fig. 344, b b), essentially ana- 

 logous in structure to those lg * 

 of a fish. The blood, however, 

 which is propelled from the 

 heart is not here entirely ve- *-i*y3|sP- 

 nous, but consists of a mixed "^ 

 fluid, partially derived from 

 the systemic and partially from 

 the pulmonary auricle, the two 

 having, of course, been mingled 



together in the common ventricle of the tripartite heart. The contrac- 

 tion of the heart forces the blood into the bulbus arteriosus, from which 

 it is in great part driven into the branchiae : arrived there, it passes 

 along the great branchial artery (fig. 344, a), is made to circulate over 

 the branchial fringes (b), and being again collected into the branchial 

 vein (c), in a purified condition, it is poured into those large trunks, the 

 representatives of the vessels r r (fig. 343), which form the aorta. 



(1960.) But, besides the branchial circulation, these creatures like- 

 wise possess lungs (fig. 340, z, t), and a pulmonary circulation of greater 

 or less importance in different genera. Nevertheless the pulmonary 



