Physiology 471 



tion could be eliminated by the contractile vacuole (566). Ammonia, 

 rather than urea, seems to be the nitrogenous waste product for a number 

 of species. 



The assumption that the contractile vacuole is a hydrostatic regulator 

 is based upon the fact that, in a system involving two fluids of different 

 densities separated by a semipermeable membrane, water should pass from 

 the less dense into the denser medium until equilibrium is reached. The 

 cytoplasm would represent the denser medium in fresh-water Protozoa, 

 and the occurrence of endosmosis would necessitate a mechanism for pre- 

 venting excessive dilution of the cytoplasm. The general occurrence of 

 contractile vacuoles in fresh-water Protozoa and the absence of such struc- 

 tures in many marine and parasitic species support this assumption. An 

 osmoregulatory function also is indicated by certain experimental data. 

 Injection of distilled water into Amoeba dubia increases rate of pulsation 

 and water output of the contractile vacuole (214). A decrease in frequency 

 of contraction with increasing salinity of the medium has been observed 

 in Amoeba verrucosa (582), species of Paramecium (144, 197), Gastro- 

 styla steinii (197) and Blepharisma undulans (144). In A. verrucosa, pul- 

 sation ceases at a salt concentration of 1.5-2.5 per cent; in G. steinii, at 

 1.25 per cent (197). Conversely, the rate of pulsation in certain marine 

 and parasitic species rises with decreasing salinity, as in Amphileptus 

 guttula (268), Nyctotherus cordiformis (197), and Balantidium, entozoon 

 (112). Under similar conditions, appearance de novo of contractile vacu- 

 oles has been described for Amoeba biddulphiae (583) and Vahlkampfia 

 calkinsi (203). However, Flabellula mira develops no contractile vacuoles 

 even in a 1:20 dilution of sea water. This species seems to eliminate water 

 by way of large food vacuoles which are emptied at intervals (209). 



The water eliminated by the contractile vacuole may be traced to sev- 

 eral sources. Endosmosis may account for much of it in fresh water species. 

 Such a process demands the maintenance of a difference in osmotic pres- 

 sure across a selectively permeable membrane. The internal electrolyte 

 concentration of Arnoeba proteus and various ciliates, determined with 

 microelectrodes for measurement of intracellular conductivity, is equiv- 

 alent to 0.01-0.068N KCl (148, 149). The internal osmotic pressure of 

 Spirostomum ambiguum, determined by the vapor pressure method, is 

 equivalent to that of 0.15 per cent NaCl (448), and the difference of 

 osmotic pressure across the body wall of fresh water peritrichs approxi- 

 mates that of a 0.05M sucrose solution (295). Formation of food vacuoles 

 is another source of water in holozoic Protozoa, although there is some 

 compensation in the evacuation of old vacuoles. This source accounts for 

 8-20 per cent of the water eliminated by contractile vacuoles of marine 

 ciliates (297). Another source of water is that arising in metabolism, but 

 the relative amount has not been estimated. 



