252 STUDIES IN PHYSIOLOGY 



of the excretory organs. Almost all the carbon dioxid 

 formed in oxidation is given off from the lungs, very little 

 being excreted by the kidneys and skin. Water is given 

 off by all three organs ; urea from two (kidneys and skin). 



The Liver as an Excretory Organ. We learned, on p. 100, 

 that the bile contains the wastes produced by the destruc- 

 tion of red corpuscles. The liver, then, besides its. func- 

 tions of storing sugar and of secreting a digestive juice, 

 must be regarded as an organ of excretion. 



The Kidneys of Vertebrates. Hitherto it has been our 

 plan to begin the comparative study of a given system of 

 organs with the lowest forms of animal life, and to follow 

 on up to the highest mammals. In the case of the kidneys 

 the opposite plan seems preferable. Among all mammals 

 the kidneys have a similar position and structure, and con- 

 sist of cortical and medullary regions, the latter being made 

 up of separate pyramids. Birds show the first striking 

 modification in kidney structure, for in this group the cor- 

 tex and medullary layer cannot be distinguished. Birds, 

 too, have no urinary bladder, and the ureters empty directly 

 into a cavity known as the do-a'ca (Latin cloaca = a sewer), 

 into which opens also the rectum. The urine is thus 

 mingled with waste food substances from the alimentary 

 canal. Among reptiles, amphibia, and fishes, there is so 

 much diversity of structure that any comparative study of 

 excretion is impossible within a limited space. 



The Kidneys of Invertebrates. In the lobster there are 

 two so-called green glands situated in the head region near 

 the base of the antennae, and these act as kidneys, since 

 they have been proved to excrete urea. Each segment of 

 an earthworm has a pair of twisted tubes that open upon 

 the surface of the body (Fig. 38, A). These are of use in 

 removing urea and other wastes. Hence an earthworm may 

 be said to have twice as many kidneys as it has segments. 

 In some other groups of invertebrates organs corresponding 

 to kidneys have not been identified. 



