70 ZOOLOGY 



the case of other necessary substances, the blood is the vehicle 

 by which oxygen is distributed from the gills or lungs to the parts 

 of the body needing it. The student will realize that this is 

 only the first step in respiration. The real respiration takes 

 place in the protoplasm of the individual cells. 



95. The Circulatory System and Function. In such con- 

 ditions as are shown in Fig. 81, there is no circulatory system. 

 The digested food is merely distributed from cell to cell. In 

 animals in which the digestive apparatus is well developed, 



FIG. 34- 



FIG. 34. A scheme to represent the circulation of the blood, in its essential features. The 

 arrows indicate the course of the blood, a, arteries; aur., auricle or receiving portion of the heart; 

 d, digestive tract; c. d., capillaries of the digestive tract; c.r., capillaries of the respiratory organs; 

 c.s., capillaries of the system; va., valves; ve, veins; vt. t ventricle. 



Questions on the figure. What portions of the apparatus are necessary to 

 secure circulation? Which secure the real objects for which the circulation 

 exists? Why are valves essential ? What common work occurs in the three classes 

 of capillaries figured above? What special type of work is characteristic of each of 

 the three? 



some device becomes necessary for the distribution of the food. 

 The body cavity with its contained fluids may do this work as 

 in Fig. 31. Usually, however, when the mesodermal layers be- 

 come well developed, there arises in connection with it a series 

 of branching tubes containing special fluids, blood or lymph. 

 These tubes by their ramifications connect the digestive sur- 

 faces with the various parts of the body. Some branches like- 

 wise extend to those special surfaces where the oxygen of the 

 external medium may be had. Naturally then the complexity 



