44 Comparative Animal Physiology 



and loses by its contractile vacuoles 3.8 per cent of its body volume per hour. 

 Much of the water given off by c. vacuoles has entered with food. If marine 

 Protozoa are isotonic, no water can enter by osmotic gradient. In Amoeba viira, 

 for example, clear vacuoles appear while food is being digested; later the 

 vacuoles expel their contents. ^'^ No large vacuoles appear when the amoeba 

 is not feeding (Fig. 18). In fresh-water Peritricha ^-^^ the rate of uptake of 

 water by the food vacuoles is 8 to 20 per cent of the rate of fluid output by the 

 c. vacuoles. In Paramecium the uptake by the food vacnoles may account for 

 30 per cent of the excreted water. *^^ There is a good correlation between rate 

 of food vacuole formation and rate of c. vacuole elimination.^ i- ^^ 



A third route of water intake, utiHzed by ciliates, is through the pharynx 

 surface. Many parasitic ciliates which have no mouths also have no c. vacuoles, 

 although this is not a universal rule. ^°^ In Paramecium c. vacuoles are most 

 active while the animals are at rest and pumping water toward the mouth, as 

 judged by currents of India ink particles. ■^*^' '^^ The vacuoles are less active 

 while the Protozoa are swimming and the peristome is partly closed. A group 

 of parasitic ciliates, the Ophryoscolecidae, from the stomach of cattle are 

 able to close the oral passage; when this is done the pulsation of their c. 

 vacuoles is very greatly slowed. ^^'^ 



Protozoa differ in the way they take in water. All the water to be excreted 

 must have entered through the pellicle in mouthless and nonfeeding animals. 

 All of it must enter through the food vacuoles in animals like Amoeba mira, 

 which have no vacuoles when not feeding. Much water enters ciliates by way 

 of the pharynx. 



Mechanism of Filling of the Contractile Vacuole. There are three theories 

 of the filling of c. vacuoles: (1) hydrostatic pressure, (2) secretion of water, 

 and (3) secretion of solute with subsequent diffusion of water. These could 

 be resolved if a method were available for removing the contents of a vacuole 

 and measuring its osmotic concentration. 



A hydrostatic filtration theory has been proposed. -^^ The colloid osmotic 

 pressure of the cytoplasm of Spirostomum is 2 cm. HoO and the hydrostatic 

 pressure is 4 cm. H2O. That protozoan cells have some turgidity is indicated 

 by the variety of their shapes. Hence the filtration pressure across a membrane 

 which would not pass colloids would be the difference between the hydro- 

 static and the colloid osmotic pressure. However, as Kitching points out, the 

 hydrostatic pressure would be no less if water is squeezed into a vacuole con- 

 tained inside the cell. Also, vacuoles are often not spherical. This theory, 

 therefore, cannot apply. 



Secretory work is done in c. vacuole production. Evidence for this comes 

 from the use of respiratory inhibitors.^^^- ^^^' ^^^ When the marine peritrich 

 Cothurnia was transferred to 12.5 per cent sea water the volume increased 

 and vacuolar output also rose; when cyanide was added the vacuolar output 

 declined nearly to zero and the cell volume increased (Fig. 19). A fresh-water 

 species reacted to cyanide by similar near-cessation of vacuolar pulsation and 

 increase of volume, but when 0.05 M sucrose was added, in addition to the 

 cyanide, the volume remained constant even though no vacuolar pulsations 

 occurred. Other respiratory inhibitors such as HjS and urethane had similar 

 effects. 



Secretion is further evidenced by the association of granules with vacuole 



