184 REPRESENTATIVE SINGLE-CELLED ANIMALS 



along a definite course (Fig. 95 E). As with amoeba, it is presumed 

 that enzymes are secreted into the vacuoles, and that the products 

 of digestion undergo assimilation by the surrounding cytoplasm. 

 Finally, egestion of the indigestible remnants contained within the 

 vacuoles takes place at the anal spot, or cytopyge. As in the 

 many-celled body, the assimilated material becomes a part of the 

 protoplasm. Dissimilation is, of course, always occurring; and 

 excretion of the end products of metabolism may take place by 

 diffusion over the entire surface of the cell or by means of the 

 contractile vacuoles. While the latter have been commonly 

 regarded as having an excretory function, it is possible that they 

 are primarily for the purpose of regulating the water content of 

 the cell. Even so, we should expect the extruded water to con- 

 tain any soluble wastes that might be in solution in the water 

 within the protoplasm. Under suitable conditions the storage of 

 nutrient materials, such as starches and fats, may take place in 

 the cytoplasm. Respiration, as in other protozoa, is the pas- 

 sage into the cell of oxygen from the surrounding water and is 

 comparable with internal respiration in the cells of metazoa. 



Reproduction and Life Cycle. — As with amoeba and euglena, 

 the life cycle of paramcecium consists principally of asexual repro- 

 duction by cell division (Fig. 99, E and F). Sexual reproduction 

 occurs by conjugation (Fig. 100). The animal seldom, if ever, 

 encysts under laboratory conditions, but encystment has been 

 observed (Fig. 99 A to D). Perhaps it takes place more frequently 

 in nature, since it is difficult to understand how any protozoan can 

 be so universally distributed in fresh water without undergoing at 

 least occasional encystment as a means of tiding over adverse con- 

 ditions. It is true, however, that encysted stages are rarely 

 observed, and some investigators have doubted whether encyst- 

 ment ever occurs. As seen in the laboratory, the life cycle consists 

 apparently of an endless active stage with frequent cell division 

 and occasional conjugation. There also occurs a process of internal 

 nuclear reorganization, termed endomixis. 



Aside from the problem of encystment, the life cycle is very 

 well known, because of the ease with which the common species of 

 paramcecium can be reared under laboratory conditions. Wood- 

 ruff, for example, has maintained P. aurelia in the laboratory, 

 without conjugation, from May 1, 1907, until the present time, 

 and this race can no doubt be thus maintained indefinitely with- 



