CHAPTER XIII 

 EXCRETION IN ANIMALS 



The ihathematically accurate end-reaction of a chain of 

 known and unknown causes and effects. Noyes. 



PROVISIONS for eliminating from the organism the waste 

 products of metabolism are only second in importance to 

 those for supplying the matter and energy by which the vital 

 processes are carried on. Accordingly we find the kidneys 

 devoted solely to excretion; the gills or the lungs, largely to 

 excretion ; and the skin and liver acting in subsidiary capaci- 

 ties. In nearly every case the essential parts of the excretory 

 organ are gland cells which select from the blood supply at 

 their disposal one or another waste product. This material 

 they secrete in more or less changed form so that it eventually 

 leaves the body as an excretion. There is therefore an essen- 

 tial distinction between an EXCRETION, which represents 

 chemical waste from the vital processes, and the major part 

 of the material which is eliminated from the digestive tract 

 as FAECES. The latter is almost entirely indigestible material 

 taken in with the food which has not directly contributed to 

 the metabolic processes of the organism. Accordingly the 

 digestive tract is not included in the list of excretory organs, 

 though as a matter of fact certain waste products excreted by 

 the liver reach the outside world with the faeces. 



We have already emphasized the elimination of carbon 

 dioxide by the GILLS or the LUNGS. Here the cells of the RES- 

 PIRATORY MEMBRANES play essentially a passive role in excre- 

 tion, since the carbon dioxide, which is under higher tension 



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