THE CONTRACTILE VACUOLE 429 



temperature increases movement and pulsation frequency. 



Kitching (1934) found that the rate of output of fluid from the 

 contractile vacuole of a fresh-water peritrich ciliate is decreased to a 

 nev^ steady value immediately, when the organism is placed in a mixture 

 of tap water and sea water. The rate of output returns to its original 

 value immediately, when the organism is replaced in tap water. Pulsa- 

 tion is stopped when the medium contains more than 12 percent of sea 

 water. Transference of marine peritrich ciliates from sea water to 

 mixtures of sea water and tap water leads to an immediate increase in 

 body volume, to a new and generally steady value. Return of the organ- 

 isms to pure sea water results in an immediate return of body volume 

 to normal, or less. When the concentration of sea water is less than 

 75 percent, the pulsation rate increases, and then generally falls off 

 slightly to a new steady value which is still considerably above the normal 

 in sea water. The maximum sustained increase in rate observed by Kitch- 

 ing was 80-fold. From these observations it is concluded that the vacuole 

 is probably a regulator of hydrostatic pressure in the fresh-water Protozoa, 

 but in those marine Protozoa which possess vacuoles the functions remain 

 obscure. 



Hyman (1936) believes that the vacuole in Amoeba verspertilio serves 

 to discharge water which has necessarily entered the cell from a hypotonic 

 medium. 



One of the most remarkable instances of adjustment of a protozoan to 

 abnormal media is shown in the experiments of Hopkins (1938) on the 

 marine amoeba, Flabelhda mira. He found that this amoeba can be 

 cultured in any concentration, from sea water diluted twenty times with 

 fresh water to sea water concentrated ten times by evaporation. It never 

 forms contractile vacuoles such as are typical for fresh-water Protozoa. 

 The food vacuoles, when extruded from the cell, contain large quantities 

 of water as well as fecal material. The rate of elimination of fluid by 

 means of these vacuoles is inversely proportional to the concentration 

 of the medium, and directly proportional to the volume of the amoeba. 

 When the concentration of the medium is decreased, the organism swells 

 at first, and then shrinks to its original volume. During shrinkage, 

 elimination of fluid by food vacuoles does not nearly account for the 

 volume loss. If the concentration of the medium is increased, the amoeba 

 shrinks at first, and then swells to its original volume. Only a small 



