36 HANDBOOK OF PROTOZOOLOGY 



pie Entamoeba histolytica, may be holozoic, since they ingest the 

 host tissue cells. 



Certain ciliates seem to be able to live in water rich in 

 sulphurous substances produced by putrefying vegetable and 

 animal matter. Lauterborn called them sapropelic forms. 



Many Protozoa apparently nourish themselves by more than 

 one method at the same or different times. Many chromato- 

 phore-bearing Protozoa are known to nourish themselves by 

 holophytic as well as saprozoic methods under changed en- 

 vironmental conditions. This mode is sometimes known as 

 mixotrophic (Pfeiffer). 



In the major groups of the Protozoa except the Sporozoa, 

 there occur one or more vacuoles known as contractile, or 

 pulsating, vacuoles. As a rule, the fresh-water inhabiting Pro- 

 tozoa contain contractile vacuoles, and those of the salt water or 

 of parasitic habitat do not, although exceptions are noted here 

 and there. In the Protozoa which do not possess a pellicle, the 

 vacuole is formed by accumulation of watery substance in one 

 or more droplets, which increase in size and unite into one. 

 Thus the vacuole enlarges slowly but continuously, until it 

 reaches a certain size, which may vary even within one indivi- 

 dual, and finally bursts through the thin ectoplasm to the 

 outside. Aside from Protociliata, contractile vacuoles occur gen- 

 erally in the ciliates even among the parasitic forms. They show 

 in many cases a more conspicuous and complicated structure 

 than those found in other groups. In them the vacuoles are 

 more or less definitely located in the superficial portion of the 

 endoplasm, although there is no delimiting membrane around 

 them. In a number of species there are accessory canals known 

 as radiating canals. These canals, which are easily seen in 

 Paramecium, are spaces in the endoplasm through which fluid 

 matter becomes collected and flows toward the central vacuole. 

 When the vacuole is fully formed, its contents are discharged 

 to the exterior through the ectoplasm and pellicle. In forms 

 such as Stentor or Spirostomum, the contractile vacuole is fed 

 by a long longitudinal canal. In some forms there is a definite 

 permanent pore in the pellicle through which the expulsion 

 takes place (Figs. 156 and 160). 



In many Mastigophora such as Euglena, there occur several 



