280 COMPARATIVE ANATOMY. 



of the gills (R), where it becomes oxygenated, to pass thence 

 into the branchial veins (Fig. 223, &). The manner in which 

 the aortic roots become formed from the latter has already been 

 described. 



Dipnoi. In the Dipnoi, as in Fishes proper, the heart lies far 

 forwards, near the head. In correspondence with the double mode 

 of respiration, by lungs as well as by gills, it reaches a stage of 

 development mid-way between that seen in Fishes and in Amphi- 

 bians. The atrium becomes divided into two chambers by a septum, 

 as does also the ventricle to some extent. The conus arteriosus is 



FIG. 224. DIAGRAM OF THE HEAUT AND BRANCHIAL VESSELS OF CERATODUS. 

 (Mainly after J. E. V. Boas. ) 



V, ventricle ; A, A 1 , atria ; Co, conus arteriosus ; Co, and Cp, anterior and posterior 

 cardinal veins ; DC, ductus Cuvieri ; /to/F, branchial arteries ; 1 to 4, branchial 

 veins ; Ca, carotid ; Ap, pulmonary artery ; RE, capillaries of lung ; Vp, pul- 

 monary vein ; RA, roots of the aorta, beginning at t ; Ao, dorsal aorta. 



twisted spirally on itself (Fig. 224, Co) : in Ceratodus it is pro- 

 vided with eight transverse rows of valves, and begins to be divided 

 into two chambers. InProtopterus this division is complete, so 

 that two currents of blood, an arterial and a venous, pass out 

 from the heart side by side (Fig. 225, a,&). The former comes 

 from the pulmonary vein, from which it passes into the left 

 atrium, thence into the left ventricle, and so to the two anterior 



