90 MANUAL OF ANIMAL BIOLOGY 



in it is due to the pressure induced by the contractions of the aor- 

 tic loops. As the blood moves posteriorly in the ventral vessel it 

 is forced out into the segmental vessels (ventro-intestinal and 

 ventro-parietal) which convey it to the various important organ 

 systems where connection is made with tiny connecting vessels, 

 the capillaries, leading to the dorsal vessel, as noted above. 



In the region of the aortic loops and anterior thereto, both the 

 dorsal and ventral vessels distribute blood to the various tissues. 

 This is collected and returned to the dorsal vessel by either the 

 extra-esophageal vessels or the subneural vessel in association 

 with the dorso-subneurals. Thus the circuit is complete. 



In the above description of the circulatory system, emphasis has 

 been placed upon the various closed vessels. As a matter of fact, 

 if the circulatory system consisted only of the closed tubes, it 

 would fail to function in supplying the needs of the cells. So all 

 through the tissues there are blood spaces in which this blood fluid, 

 finding its way through the thin-walled capillaries, actually comes 

 into contact with, and bathes, the cells. In this way the inter- 

 change of materials between the blood and the cells takes place. 



In addition to the ' closed type ' of circulation of blood by means 

 of the regular vascular system, the Earthworm has an 'open type' 

 of circulation in the coelom. The latter contains the coelomic 

 fluid, which is closely affiliated with the blood in its composition 

 and in its functions. The various movements of the worm bring 

 about contractions of the body, with the result that the coelomic 

 fluid, from time to time, is forced back and forth through open- 

 ings in the septa, thus bathing the tissues with which it comes 

 into contact. 



3. Respiratory System 



The blood carries to the cells not only the food absorbed from 

 the intestine, but also oxygen which, as we know, is absolutely 

 necessary for the metabolic activities. Also the blood carries the 

 metabolic wastes, both liquid and gaseous, away from the cells to 

 the proper excretory organs. The gaseous interchange, in which 

 carbon dioxide is given off and oxygen is obtained, occurs at the 

 surface of the body. The body wall, which is normally kept moist 

 through the mucus secreted by gland cells in the outer epidermis, 

 is also richly supplied with the capillaries, and the blood passing 

 into the body wall comes into close contact with the oxygen of 



