MORPHOLOGY 73 



The contractile and other vacuoles 



The majority of Protozoa possess one or more vacuoles known 

 as pulsating or contractile vacuoles. They occur regularly in all 

 freshwater inhabiting Sarcodina and Mastigophora, and in Cilio- 

 phora regardless of habitat. In the Sporozoa, which are all parasitic, 

 and the Sarcodina and Mastigophora, which live either in salt water 

 or in the body of other animals, there is no contractile vacuole. 



In various species of free-living amoebae, the contractile vacuole 

 is formed by accumulation of wate-r in one or more droplets which 

 finally fuse into one. It enlarges itself continuously until it reaches 

 a maximum size (diastole) and suddenly bursts through the thin 

 cytoplasmic layer above it (systole), discharging its content to out- 



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Fig. 27. Diagrams showing the contractile vacuole, the accessory vacu- 

 oles and the aperture, during diastole and systole in Conchophthirus 

 (Kidder). 



side. The location of the vacuole is not definite in such forms and, 

 therefore, it moves about with the cytoplasmic movements; and, as 

 a rule, it is confined to the temporary posterior region of the body. 

 Although almost spherical in form, it may occasionally be irregular 

 in shape, as in Amoeba striata (Fig. 161, /). In many testaceans and 

 heliozoans, the contractile vacuoles which are variable in number, 

 are formed in the ectoplasm and the body surface bulges out above 

 the vacuoles at diastole. 



In the Mastigophora, the contractile vacuole appears to be more 

 or less constant in position. In Phytomastigina, they are usually 

 located in the anterior region and, in Zoomastigina, as a rule, in the 

 posterior half of the body. The number of the vacuoles present in 

 an individual varies from one to several. 



In the Ciliophora, except Protociliata, there occur one to many 

 contractile vacuoles, which seem to be located in the deepest part 

 of the ectoplasm and therefore constant in position. Directly above 

 each vacuole is found a pore in the pellicle, through which the con- 

 tent of the vacuole is discharged to outside. In the species of Con- 



