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CHAPTER VII. 



ORGANS OF NITROGENOUS EXCRETION. 



VERY much doubt hangs over the function of the 

 organs which are said to have a renal function in the 

 lower animals, owing to the great discrepancies be- 

 tween the results attained to by those who have inves- 

 tigated the excreta of, or concretions in, these organs, 

 and the very great difficulties which lie in the way of 

 such chemical inquiries. 



Nothing can be certainly said as to the renal 

 organs of the Protozoa, if we may use the term 

 renal organs in a general way, as applying to such 

 parts of the organism as purify the body of its nitro- 

 genous waste ; this, however, is certain, that in the 

 course of the molecular activity of a mass of proto- 

 plasm, nitrogenous products are formed which are in 

 the nature of waste products, and which are injurious 

 to the organism if not speedily removed from it. We 

 know that in most Protozoa there are one or more 

 spaces which, expanding, take up, and, contracting, 

 drive out, water. Knowing, as we do, from our own 

 experience, the value of water as a diuretic agent, it 

 seems almost justifiable to suppose that, while this 

 water has no doubt a respiratory function in the 

 Protozoa, it acts also as an a^ent for removing waste. 



* o o 



The supposition that the office of the contractile vesicle 

 is to drive fluid out of the body is supported by the 

 discovery of Vorticellids, in which the contractile 

 vesicle is connected by a canal with the " vestibule " 

 which lies beneath the mouth opening ; on the con- 

 traction of the vesicle the contained water passes into 

 the mouth-opening, and so, of course, makes its way 



