250 



DIPNOI 



-a2 



- a3 



twisted conns 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 by 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 

 the air-bladder. The 



eraot's orsteri, ret. enra vew o e ear , , 



dissected so as to expose the inside of the ventricle and presence 01 tWO eiterent 

 conns, and the disposition of the aortic arches, ai-4, four . _ p i_ i n ,.}, Kranphial 

 aortic arches, a dotted line passes up the base of the 1st ' es 



and combined 3rd and 4th ; at, atrium ; c, cut wall of Ij^r in both the Dipnoi 

 conus ; /, plug tilling the atrio-ventricular opening ; Iv, ..... 



small posterior valves ; p, portion of wall of pericardium ; and the Selachii is prob- 

 s.v, specialised row of enlarged valves; t, truncus; r, > , c 1,1 



anterior valve, also cut wall of ventricle ; TO and z, dotted ably OI no pnylOgenetlC 

 lines^passing into the sinus venosus. (Compare Fig. 218, s ig n jfi cance . j n the rela- 

 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 



4)'' 



Fio. 219. 

 CeraMt'x Forsterl, Kreflt. Ventral view of the heart 



