376 THE EVOLUTION OF MAN. 



system. In Dipneusta, the auricle of the heart separates 

 into two halves by the formation of an incomplete partition. 

 Only the right auricle now absorbs the venous blood of the 

 body-veins. The left auricle, on the other hand, absorbs 

 the .arterial blood of the lung-veins; both auricles dis- 

 charge in common into the simple ventricle, in which the 

 two kinds of blood mingle, and are then propelled through 

 the arterial stalk into the arterial arches. From the last of 

 these latter spring the lung-arteries (Fig. 301, p) these 

 convey a part of the mixed blood into the lungs, while the 

 remainder is driven through the aorta into the body. 



From the Dipneusta upward, we trace a progressive 

 development of the vascular system, which finally leads, 

 with the loss of gill respiration, to a complete separation of 

 the two parts of the double circulatory system. In Am- 

 phibia, the partition between the two auricles becomes 

 complete. In their young form, these yet retain gill- 

 respiration and the circulatory system as in Fishes, and the 

 heart contains only venous blood; at a later period, the 

 lungs, with their vessels, are developed also, and the main 

 chamber of the heart then contains mixed blood. In Pro- 

 tamnia and Reptiles, the main chamber and the arterial 

 stalk belonging to it begin to separate, by the formation of 

 a longitudinal partition, into two halves, and this partition 

 becomes complete in the higher reptiles on the one side, in 

 the parent-form of Mammals on the other. The right half 

 of the heart alone now contains venous blood, the left half 

 only arterial, as in all Birds and Mammals. The right 

 auricle receives venous blood from the body-veins, and the 

 right ventricle propels this through the lung-arteries into 

 the lungs ; from there it returns as arterial blood through 



