336 



ZOOLOGY. 



matter (haemoglobin) has an affinity for oxygen and thus be- 

 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 



FIG. 163. 



S. V. 



FIG. 163. Diagrams of the structure of the heart in the lower Vertebrates. A, 

 primitive condition; B, the position of the parts in the fishes, a, artery; au., auricle; 

 c, conus arteriosus with valves; s.v., sinus venosus; v, valves; ve., vein; vent., 

 ventricle. The dorsal portion of the heart is toward the bottom of the figure. 



Questions on the figures. Which side of the figure represents the 

 anterior? Compare the walls of the vessels. Where are the valves lo- 

 cated? How is the term " sigmoid flexure" appropriate to the form in Bf 

 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. 



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 f rorn the veins, and 

 (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. 163, A ) , but in development the heart 

 has undergone an ^--shaped bending which has brought the 

 auricle in front of the ventricle (Fig. 163, B). The veins and 

 arteries are often specially enlarged and modified in the region 



