162 REPRESENTATIVE SINGLE-CELLED ANIMALS 



and gives off the waste materials that are produced by dissimila- 

 tion. The contractile vacuole is commonly described as effecting 

 excretion by receiving water containing in solution the chemical 

 products of metabolism and discharging these to the outside, some- 

 what as does the kidney in a vertebrate animal. However, many 

 protozoa are without contractile vacuoles, and some investigators 

 regard this structure as serving to regulate the water content of the 

 cell and only incidentally for purposes of excretion. In protozoa 

 that do not possess contractile vacuoles, like the majority of both 

 marine and parasitic species, excretion must be supposed to take 

 place in a manner similar to the passage of waste products of metab- 

 olism from the cells of a many-celled animal with a well-devel- 

 oped circulatory system (cf. p. 104). Like a cell in the body of a 

 vertebrate (Fig. 60, p. 104), the protozoan cell is surrounded by fluid. 

 If the outer portions of the cell are permeable to substances in solu- 

 tion, an accumulation of such substances within the cell will result 

 in a diffusion outward, into the intercellular lymph in the one case, 

 and into the surrounding water in the other. The process will 

 continue as long as the outer cell region remains permeable and the 

 strength of solution is greater within the cell than on the outside. 

 Thus the mechanism for the continual removal of the waste prod- 

 ucts of dissimilation is provided alike in protozoa and in metazoa. 

 Even in protozoa having contractile vacuoles, such diffusion of 

 excretory material from the entire surface may be the more impor- 

 tant means of excretion. 



Respiration in amoeba is similar to internal respiration in the 

 cells of higher animals {cf. p. 93). Oxygen consumption can be 

 demonstrated in such protozoa by means of delicate micro-chem- 

 ical tests. As with the cells of a man or a frog, which are sur- 

 rounded by their intercellular lymph (Fig. 60, p. 104), so with the 

 amoeba surrounded by water (Fig. 86), it may be supposed that 

 oxygen diffuses into the cell as fast as it is consumed by chemical 

 combinations within the cytoplasm. The amoeba will die if 

 deprived of oxygen, as will the cells of higher organisms. 



When one recalls the various processes of ingestion and egestion, 

 digestion, absorption, circulation, assimilation, dissimilation, and 

 excretion, as described for the vertebrate, it becomes apparent that 

 the amoeba lacks only those steps of the process that are necessi- 

 tated by the size and complexity of the multicellular organism. 

 The essential steps in metabolism occur in amoeba in the same 



