MORPHOLOGY 83 



Numerous minute granules, less than l^u in diameter, occur usually 

 abundantly suspended in the cytoplasm. They can most clearly be 

 noted under phase microscope. Mast named those found in Amoeba 

 "alpha granules." 



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, Mastigophora and Ciliophora. Ma- 

 rine or parasitic Sarcodina and Mastigophora do not ordinarily have 

 a contractile vacuole. This organelle is present with a few exceptions 

 in all marine and parasitic Ciliophora, while it is wholly absent in 

 Sporozoa. 



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

 is formed by accumulation of water 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- 

 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. 184, /). 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 Mastigophora, the contractile vacuole 

 appears to be located in the anterior region. 



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- 

 chophthirus, Kidder (1934) observed a narrow slit in the pellicle 

 just posterior to the vacuole on the dorsal surface (Fig. 27). The 

 margin of the slit is thickened and highly refractile. During diastole, 

 the slit is nearly closed and, at systole, the wall of the contractile 

 vacuole appears to break and the slit opens suddenly, the vacuolar 

 content pouring out slowly. When there is only one contractile 

 vacuole, it is usually located either near the cytopharynx or, more 

 often, in the posterior part of the body. When several to many 

 vacuoles are present, they may be distributed without apparent 

 order, in linear series, or along the body outline. When the contrac- 



