CHORDATA. 



339 



350. In Figs. 164 to 169 will be found diagrams of the 

 circulation in the principal groups of vertebrates. It will be 

 seen that there is a progressive complication of the structure, 

 involving the heart, veins, and arteries, as we ascend the scale. 



FIG. 168. FIG. 169. 



-C. li c. 



MT.C 



O/. 



FIG. 1 68. Diagram of the heart and branchial arches in a Reptile. Position and 

 lettering as in preceding figures, l.v., left venticle; r.v., right venticle. 



Questions on the figure. Compare this with figures 164-167 and make 

 a note of the differences. How much communication is there between the 

 two sides of the heart? What tends to insure that the purest blood in the 

 heart shall go to the head? That the least pure goes to the lungs? 



FIG. 169. Diagram of the heart and .the branchial arqhes in Mammals. A dotted 

 outline of the arches of the Fish is drawn for ready comparison. The auricles are 

 represented in a posterior position, as in the preceding figures. 



Questions on the figure. What changes in the heart are shown in 

 this as compared with former figures? In the systemic branchial arch? 

 Remember that the heart is not represented in its normal position; the 

 auricles are really at the anterior of the heart (see Fig. 164). Compare 

 this condition with table, page 340. What are the grounds for believing 

 that the auricles are, morphologically, the posterior part of the heart? 



