CHAPTEE IX. 



SECRETION AND EXCRETION IN THE INVERTEBRATA. 



All living organisms assimilate and disassimilate incessantly, 

 but the conditions of assimilation become more complex as 

 we ascend in the zoological scale. For the accomplishment 

 of the function of assimilation in the higher forms, auxiliary- 

 apparatus, such as those of digestion, circulation, and respira- 

 tion, are needed. The last three mentioned functions have 

 already been described, but in addition to these there is 

 another, for the waste products of the cells, tissues, food, &c., 

 have to be eliminated from the system. It does not matter 

 how low the organism is in the scale of animal life, there 

 are always waste products formed, and these have to be 

 eliminated, or they act as poisons. The function of elimina- 

 tion of matters hurtful to the animal organism is spoken of 

 as excretion. Excretion is partly mechanical — as in the 

 evacuation of the fseces — ^partly dependent upon the physical 

 process of pressure forcing fluid matters through thin-walled 

 tubes, and partly upon cell development and growth, as in 

 secretion. In the higher animals secretion is performed by 

 five separate mechanisms — the kidneys, the intestines, the 

 lungs or branchise, the liver, and the skin. The excretion 

 of carbonic anhydride and other gaseous products by respira- 

 tion has already been alluded to in the last chapter. The 

 function of excretion by means of a liver does not occur in 

 the Invertebrata, as a true liver (similar to that occurring in 

 the Vertebrata) is absent in these animals. From these 

 remarks it will be seen that, so far as excretion is concerned 



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