PROTOPLASM OF PROTOZOA 69 



seems to have been moved out of its position in contact with the mem- 

 brane closing the pore. In spite of this, the feeding canals continue to 

 form other main vacuoles, each of which expels its contents through 

 the pore in normal fashion. The displaced vacuole is free in the endo- 

 plasm, and, if not too large, may move about in the protoplasmic stream. 

 Such vacuoles have been observed to be present in the cytoplasm twenty- 

 four hours after centrifuging. These observations were interpreted in 

 further support of the view that the main contractile vacuole and its 

 membranes are purely temporary, forming anew before each systole by 

 the fusion of feeding vesicles formed at the vacuolar ends of the feeding 

 canals. The feeding canals are markedly osmiophilic, and for this reason 

 they have been described by Nassonov (1924) as homologous to the 

 Golgi apparatus. However, this view is not supported by Beams and 

 King (1932) and King (1935). Gelei (1928) thinks of the cytoplasm 

 surrounding the feeding canals as "nephridial-plasma" and suggests a 

 parallelism between the nephridial system in Paramecium and that of 

 higher organisms. Metcalf (1910) observed, in the cytoplasm surround- 

 ing the contractile vacuole of A. proteus, small round granules which 

 he termed "excretory granules." These bodies, which are permanent 

 structures, are thought by Metcalf to be functionally connected with 

 secretion. However, Mast (1938a) has been unable to substantiate this 

 view. 



It is interesting to note that the permeability of the cell membrane and 

 that of the contractile vacuole system are in some ways similar. For in- 

 stance, Morita and Chambers (1929) report that in A. duh'ia both the 

 surface membrane and the contractile vacuole membrane are imperme- 

 able to HCl. Kitching (1936) reports that the vacuolar surface, like 

 the cell surface, is relatively impermeable to salts in the peritrich ciliates. 

 He further presents arguments to show that the vacuolar system in these 

 forms actively secretes water. 



4. Vood vacuoles. — In many free-living Protozoa the ingested food 

 particles are surrounded by a distinct vacuole. It is here that the digestive 

 enzymes collect and act upon the solid food particles, converting them 

 into a dissolved form suitable for use by the organism. The so-called food 

 vacuolar membranes, in forms such as Amoeba and ParajJiecium, must 

 perform much the same function as the cell membranes surrounding the 

 cells in the intestine of higher organisms. In other words, the semiperme- 



