338 



ZOOLOGY 



comes a vehicle for transporting it. The colored corpuscles 

 have no motion of their own, but are carried by the blood cur- 

 rents. The colorless cells are much less numerous than the red 

 and have power of independent motion (amoeboid). The fluid 

 in which the cells float is called the plasma and carries the 

 food and waste materials of the body in solution. 



The muscular heart always consists of at least two cham- 

 bers, (i) an auricle which receives blood from the veins, and 



S. V. 



au. 



FIG. 165. Diagrams of the structure of the heart in the lower Vertebrates. A t primitive con - 

 dition; B, the position of the parts in the fishes, a, artery; au., auricle; c, conus arteriosus with 

 valves; s.v., sinus venosus; t, valves; ve., vein; vent., ventricle. The dorsal portion of the heart is 

 toward the bottom of the figure. 



Questions on the figure. Which side of the figure represents the anterior? 

 Compare the walls of the vessels. Where are the valves located? How is the 

 term "sigmoid flexure" appropriate to the form in B? Notice how it results in 

 what is morphologically the posterior portion of the heart becoming anterior. 

 Trace the course of the flow of the blood. 



(2) a ventricle which has thick walls and propels the blood 

 into the arteries. Morphologically the auricle is the posterior 

 portion of the heart (Fig. 165, A), but in development the heart 

 has undergone an s-shaped bending which has brought the 

 auricle in front of the ventricle (Fig. 165, B). The veins and 

 arteries are often specially enlarged and modified in the region 

 of the heart. The main trunk leaving the heart is called the 

 aorta. As the vessels are followed from the heart they branch 

 and become smaller and the walls become thinner. The final 



