MORPHOLOGY OF ANIMALS 



Excretory Syste?n 



Excretion is the process of removing nitrogenous waste 

 products, particularly urea and allied compounds, from 

 the body. These waste substances are formed as the result 

 of protein combustion within the body, and as this form 

 of metabolism is universal among animals nitrogenous 

 waste substances are everywhere formed. With few excep- 

 tions all animals possess some form of excretory organ ; in 

 fact this is one of the distinguishing characteristics of 

 animals as contrasted with plants. In Protozoa the excre- 

 tory organ is a pulsatile vacuole which gradually fills with 

 fluid containing these waste products and then suddenly 

 contracts, forcing this fluid out of the body. In coelenter- 

 ates excretion is probably performed by isolated gland 

 cells, so that no specific organ exists for this function. Even 

 in higher animals excretion is performed to a limited ex- 

 tent by individual cells or small glands, for example the 

 chlorogogue cells of annelids, the dermal glands of Crus- 

 tacea, and the sweat glands of mammals. 



Nephridia. In all higher animals a special excretory 

 organ exists; this usually consists of minute tubules which 

 take up the waste substances and pass them through the 

 tubules to the exterior ; such an excretory tubule is known 



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