2l6 ANIMALS 



per cent, on the average. If only carbohydrate foods were 

 assimilated the percentage of these gases should be equal, but 

 the oxygen consumed with hydrocarbon and proteid foods in 

 part leaves the body by another path. 



490. The nitrogenous waste matters are not gaseous and, 

 therefore, cannot be eliminated by the lungs, and in fact, we 

 find in all the higher animals a special set of organs for this 

 function. The organs which presumably perform this function 



Fig. 120. — A section through the nephridium of nereis showing the funnel in 

 longitudinal section and the convoluted tubule cut across at many points. The 

 blood-vessels are also cut at several points (black). B.W., The body wall; 

 B.V., blood-vessel; c, coiled tubule; F, funnel. 



in Worms are pairs of tubules arranged segmentally, one pair 

 in each segment. They are called nephridia and consist of 

 slender, more or less coiled, tubes which open into the body 

 cavity by a ciliated funnel-like opening. The other end of the 

 tubule opens by a pore on the surface of the body. 



491. In the crayfish there are organs, the ''green glands," 

 which are probably homologous to nephridia, but there is only 

 a single pair, located at the base of the antennae. They are 

 comparatively large organs and more compHcated in structure. 



492. The single pair of kidneys of Vertebrates are much 

 more complicated excretory organs and yet the uriniferous 

 tubules of the kidney resemble the nephridia of the worm and 

 are probably homologous organs. The kidney tubule is a long, 



