EXCRETION IN ANIMALS 



177 



evolutionary history indeed the structure of the human 

 kidneys is intelligible only in the light of the relatively simple 

 excretory organs of Invertebrates, such as the Earthworm, 

 and of lower Vertebrates. (Figs. 66, 67, 96.) 



The chief excretory organs of the Earthworm consist of 

 pairs of coiled tubes, or NEPHRIDIA, segmentally arranged in 

 the coelom on either side of the alimentary canal. Each 

 nephridium begins as an open funnel in the coelom of one seg- 

 ment, passes through the 

 partition to the next posterior 

 segment and there, after coil- 

 ing, passes to the ventral sur- 

 face and opens to the exterior 

 by a pore. Thus, reduced to 

 its simplest terms, a nephri- 

 dium is a tube communicat- 

 ing between the coelom and 

 the outer world, and afford- 

 ing a path of egress for the 

 waste matter in the coelomic 

 fluid. But the closed blood 

 vascular system of the worm 

 collects various waste products in addition to the carbon 

 dioxide which it delivers to the skin. Nitrogenous waste, 

 inorganic salts, etc., are carried to the coiled part of the 

 nephridial tube where gland cells take them from the blood 

 and deliver them to the interior of the tube to be passed out 

 of the body. Now strange as it may seem, although the 

 primitive segmentation of the coelom has disappeared in the 

 Vertebrates, nevertheless there are good grounds for believ- 

 ing that the archaic, segmentally arranged nephridia have 

 been taken over, as it were, and made the basis of the essen- 

 tial excretory elements of the kidneys. 



FIG. 96. Diagram to show the gen- 

 eral structural plan of a nephridium of 

 an Earthworm, anterior end toward the 

 right, a, internal opening of nephridium; 

 b, external opening; c, capillary network 

 about the coiled, glandular portion. 



