248 PHYSIOLOGY OF THE INVERTEBRATA. 



organs of this organism. By the same reactions as those 

 described in connection with Amceha the tiuid of these 

 vacuoles jdelds crystals of murexide and uric acid. There is 

 little doubt that these vacuoles eliminate the waste nitro- 

 genous products during the systoles which take place 

 periodically. 



In these three primitive forms of the animal kingdom 

 there are the rudiments of a true renal system. The con- 

 tractile vacuoles perform the same function (among others) 

 as the kidney of higher forms, by yielding the same nitro- 

 genous substance which is present in the renal organs of 

 the highest Vertebrates. By the agency of living proto- 

 plasm, even these insignificant microscopic cells bring about 

 chemical metamorphoses in albumiiioid molecules, with the 

 production of uric acid and possibly other substances. In 

 these lowly creatures there is present the same power of 

 chemical metamorjjhosis as is present in the more complex 

 cells of the highest Vertebrate. But the contractile vacuoles 

 do not represent solely the renal organs in these forms, for 

 it is by their agency that the mechanisms of respiration and 

 nutrition are performed. The Protozoan cell performs many 

 functions, and in this respect it does not altogether differ 

 from specialised cells of the higher animals. It should be 

 borne in mind that "there is no perceptible relation between 

 the nature of the fluid and of the cell or gland secreting it ; 

 and secretions, as pus, for example, are formed by structures 

 where no such secretion previously existed ; they alter also 

 without any visible change in the structure of the gland or 

 cell " (Milne Edwards). 



The Ccelenterata. 



As the means of eliminating the waste products in the 

 Porifera and the Ccelenterata have already been referred to in 

 this chapter, we now proceed to describe the secretion of 

 carbonate of lime in the Ccelenterata. 



