168 BIOLOGY OF THE PROTOZOA 



In one way or another it is ever present to initiate the round of vital 

 functions. 



The energy of combination, released by oxidation, is paid for by 

 loss in the chemical compound oxidized. Other compounds may 

 be formed with lessened energy of combination, and end-products, 

 notably CO2 and urea (NH2)2CO are not only useless to the organ- 

 ism but positi^'ely harmful unless voided. Excretion, therefore, 

 must follow oxidation. To make good the loss of substance new 

 food materials must be taken in, digested and assimilated, but this 

 is possible only through movement, and movement in turn is an 

 expression of irritability. Hence a second consequence of oxidation 

 is movement and the latter is made possible })ecause of the energy 

 transformed by oxidation. P]xcretion and irritability thus are 

 fundamental vital functions, while a third, nutrition, is closely cor- 

 related. Excess of food intake over waste by oxidation leads to 

 growth of the diverse protoplasmic substances and to their redupli- 

 cation hy division, while the aggregate of such divisions, expressed 

 visibly by division of the cell, constitutes reproduction. The funda- 

 mental vital functions are thus intimately bound together; external 

 conditions 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 protoplasm during a typical life cycle may be due 

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



A. 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 such forms, and these 

 are 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 ec|uilibrium 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 



