250 



DIPNOI 



twisted conus of Ceratodus, one is so enlarged as to longitudinally 

 divide the cavity into two channels. In the Dipneumones, and 



especially in Lepidosiren, 

 where this subdivision 

 of the cavities of the 

 heart is more completely 

 carried out, these valves 

 fuse to a longitudinal 

 fold. The pulmonary 

 blood entering the left 

 side of the atrium is 

 thus borne forward to 

 the first two aortic 

 arches, and the venous 

 blood to the last two 

 (Figs. 218, 221). 



From the truncus 

 arteriosus, the short 

 remnant of the ventral 

 aorta, spring four pairs 

 of afferent branchial 

 vessels, passing up the 

 first four branchial 

 arches. Four corre- 

 sponding pairs of epi- 

 branchial arteries are 

 formed 'b}?" the junction 

 of two efferent vessels 

 in each arch in Ceratodus 

 (Fig. 2 20). They join the 

 dorsal aorta. From the 

 posterior epibranchial, 

 the sixth aortic arch, 

 counting the mandibular 

 as the first, is given off 

 a pulmonary artery to 



Forsteri, Kreftt. Ventral view of the heart **' 



dissected so as to expose the inside of the ventricle and presence OI tWO efferent 

 conus, and the disposition of the aortic arches, a 1 - 4 , four i v i U,*ot 



aortic arches, a dotted line passes up the base of the 1st vessels in each branchial 



and combined 3rd and 4th; at. atrium; c, cut wall of Knr in Vnrh thp "Dinnm 



conus ; /,<\>lug filling the atrio-ventricular opening ; Iv, Dar 



small posterior valves ; p, portion of wall of pericardium ; and the Selachll IS prob- 

 ti.o, specialised row of enlarged valves; t, truncus; v, , , ,. , , ,. 



anterior valve, also cut wall of ventricle ; w and z, dotted ably 01 no phylOgenetlC 



line^passing into the sinus venosus. 'Compare Fig. 218, significance . in the re l a - 



tion of the epibranchial 



arches to the bars the Dipnoi are the more normal (p. 111). On 

 the other hand, the reduction of the ventral aorta, the gathering 



Fio. 210. 



