134 



PRINCIPLES OF ANIMAL BIOLOGY 



the lungs. Considerable water (about one-tenth of the total water loss 

 of the human body at rest) is also there removed in the form of vapor. 

 Excretion through the lungs, therefore, involves only gaseous wastes. 



Water and Urea. — Urea is a solid substance ; hence by most organisms 

 it can be excreted only in solution. Many other substances besides urea 

 contain nitrogen and are produced by decomposition of proteins, but 



nearly all of them are solids that 

 require to be eliminated in dissolved 

 form. As just stated, only about a 

 tenth of the excess water taken in by 

 man can be removed as vapor, so that 

 the bulk must leave as a liquid. 

 These two groups of wastes may thus 

 be removed by a single operation. 

 The urea and the other nitrogen-con- 

 taining substances are dissolved in 

 water, and all are eliminated to- 

 gether. The amount of these wastes 

 is much greater than that of all other 

 wastes combined, and their removal 



Fig. 107. — Portion of a protonephridial sjstem from the tapeworm Taenia crassicollis. 

 f, flame cell; n, nucleus of excretory tubule; tu, excretory tubule. {From Hesse and Doflein 

 after Bugge.) 



Fig. 108. — Flame cell of a protonephridium of a flatworm: ci, cilia within funnel- 

 shaped cavity of flame cell; n, nucleus. {From Hesse and Doflein after Lang.) 



is the chief task of what is called the excretory system of the multicellular 

 animals. The excretory system is often aided by the skin, and there are 

 other minor ways of removing water. 



Excretory Systems of Invertebrate Animals. — The excretory system 

 varies considerably in different animals. In tlui flatworms and some 

 others it consists of protonephridia, which are fine tubes rising in flame 

 cells and discharging to the exterior. A portion of such a system is shown 

 in Fig. 107, and the structure of a flame cell in Fig. 108. The flame cell is 



