176 BIOLOGY OF THE PROTOZOA 



ditions, such as decrease in temperature of the medium in which a 

 protozoon lives, means decreased oxidation, retarded movements, 

 less food and a lower division rate. Increase in temperature involves 

 a speeding up of all activities and, if food is abundant, a higher 

 division rate. External conditions involving absence of food lead 

 to starvation and death of the cell through uncompensated loss by 

 oxidation. In short, interference with any one of the fundamental 

 functions leads to disturbance of them all, and the various phases 

 of vitality of the protolasm during a typical life cycle may be due 

 to inadequate functioning of one or another or all of these activities. 



B. Excretion of Metabolic Waste.— The waste matters of oxida- 

 tion and continued metabolism are frequently voided in the same 

 manner that water and oxygen are taken in, namely, by osmosis. 

 In such cases there is no physiological need of specialized excretory 

 organs. It is possible that all Protozoa excrete in this way, although 

 the majority of fresh water Protozoa possess contractile vacuoles 

 which are generally regarded as excretory organs. In marine forms 

 and in parasites they are generally absent. If the latter forms, 

 and these are in the majority of Protozoa, are able to dispose of 

 the products of destructive metabolism without definite organs for 

 the purpose, why are the latter necessary in fresh water forms? 

 Hartog (1888) has long maintained that contractile vacuoles are 

 not obligatory excretory organs, but are primarily hydrostatic 

 organs for the purpose of maintaining a pressure equilibrium between 

 the fluids within the cell and those in the surrounding water. Degen 

 (1905) interprets the vacuole in a similar way, its variations in size 

 and pulse depending upon permeability of the membrane which 

 varies with the environmental salts. Here difference in density of^ 

 the surrounding medium is largely responsible for loss of the organ 

 characteristic of fresh water forms, but changes in permeability of 

 the cell membrane due to salts in the new medium undoubtedly 

 play an important part. Other experiments by different observers 

 bear out the same principle. Thus dilution of the normal neutral 

 salts in the medium causes enlargement of the contractile vacuoles 

 in ciliates according to Massart (1891), while increased concentra- 

 tion leads to reduction in size, retardation in rate of contraction, 

 or total disappearance of the vacuole. 



While there is justification for Hartog's view of the purely physical 

 significance of the vacuole, there is every reason for believing that 

 water in protoplasm picks up any soluble waste matter that may 

 be present, and holds it in solution. Early experiments to prove 

 this, by Brandt (1885), Griffiths (1889) and others using chemical 

 indicators, or the murexid test for uric acid, were not convincing, 

 and the function of the contractile vacuole as a primitive type of 

 excretory organ remained an hypothesis. 



Not only water, C0 2 (see Lund, 1918) and urea, but other prod- 



