FUNCTIONAL ACTIVITIES 91 



Excretion 



The contractile vacuole in many protozoa also acts as an 

 organ of excretion, and in many forms it becomes quite compli- 

 cated. In some a central contractile vacuole is filled by material 

 coming through canals, which act as feeders, leading from various 

 parts of the organism. (See, for example, Frontonia leucas, 

 Fig. 46, page 81.) In Paramecium there are two vacuoles, one 

 in the anterior third of the body, the other in the posterior third. 

 Leading toward each of them are five or six canals which are 

 quite obvious when filled, but which are difficult to see when 

 emptied. The fluid collected by these canals is poured into the 

 vacuoles, which contract when full. At ordinary temperatures 

 these vacuoles contract about twice a minute. 



Opinions as to the function of the contractile vacuole are 

 not unanimous. To the German naturalist, Christian Gottfried 

 Ehrenberg (1795-1876), and other early students of the pro- 

 tozoa, the vacuole appeared as a beating heart and the canals 

 were interpreted as arteries or larger blood vessels. More re- 

 cently the vacuole has come to be regarded as a means of regu- 

 lating the water equilibrium between the protoplasm and the sur- 

 rounding medium. Thus a fresh-water form like Amoeba crystal- 

 ligera or Amoeba verrucosa has a contractile vacuole, but if one 

 of these individuals is transferred to sea water the vacuole is lost 

 and the protoplasm becomes much condensed. Conversely, if a 

 parasitic ameba of the oyster which is normally without vacuoles 

 be transferred to fresh water, it develops several of them. It 

 has also been demonstrated that by increasing the density of a 

 fresh-water medium the vacuole becomes smaller and at a cer- 

 tain density disappears. Such experiments indicate a definite 

 physical relationship between the organism and the environment 

 — a relationship which involves the density factors of both the 

 protoplasm and the medium, the permeability of membranes, and 

 the processes of endosmosis and exosmosis. 



There is, however, still another probable function that may 

 be attributed to vacuoles. A considerable volume of water 

 passes through the protoplasm of a Paramecium in an hour. 

 Here the canal distribution indicates that water is collected from 

 all parts and is ultimately discharged from the vacuole. Any 



