390 Animal Biology 



Valves in the hearts and dorsal vessel prevent the backward flow. Blood 

 is returned from the body wall to the lateral neural vessels, in which it 

 flows posteriorly, eventually re-entering the dorsal blood vessel. 



The blood of the earthworm consists of ( 1 ) liquid plasma with its 

 oxygen-carrying red pigment, hemoglobin, dissolved in it and (2) nu- 

 merous colorless white blood cells resemblins, those of human blood and 

 with possibly similar functions. The blood carries absorbed foods from 

 the digestive tract to all parts of the body, transports wastes rapidly from 

 the tissues to the organs of elimination, and exchanges oxygen and car- 

 bon dioxide by coming near the body surface. 



When the liquid portion of the plasma of the blood passes out through 

 the walls of the blood vessels into the tissues and organs, it is known as 

 lymph. The latter consists of (1) liquid plasma of the blood which has 

 osmosed from the blood vessels, (2) numerous colorless leucocytes, (3) 

 foods contained within the lymph plasma and secured from the blood, 

 (4) wastes secured from the tissues, which are either carried by the 

 lymph toward the organs of excretion or placed into the circulatory sys- 

 tem to be carried to the excretory organs, and (5) oxygen on its way 

 from the blood to the tissues and carbon dioxide on its way from the 

 tissues to be eliminated. 



The lymph fills the coelom (body cavity) and occupies smaller spaces 

 within the tissues and organs. It is from these cavities that the lymph 

 functions. Lymph is circulated by the muscular movements of the body 

 or of the internal organs. 



Respiration. — There is no respiratory system but oxygen is obtained 

 and carbon dioxide eliminated through the moist body surface. Many 

 thin-walled capillaries just beneath the cuticle make the exchange of 

 gases possible. Excess water around the animal interferes with respi- 

 ration, which partly explains why earthworms are "rained out" after a 

 rain. 



Excretion and Egestion. — A pair of nephridia (ne-frid'ia) (Gr. 

 nephroSj kidney) is present in each segment (metamere) except the first 

 three and the last one. The internal, free end of each is a ciliated, 

 funnel-shaped nephrostome (nef rostome) (Gr. nephros, kidney; stoma, 

 mouth) which selects the wastes from the coelomic fluid. The nephridia 

 select the wastes and pass them through the ciliated tubes to the exterior 

 through openings called nephridio pores. The latter are located on the 

 ventral side of the body just posterior to the segment in which their par- 

 ticular nephridia are located. Chlorogogen cells covering the intestinal 



