MORPHOLOGY AND PHYSIOLOGY 37 



minute contractile vacuoles around the reservoir into which 

 their contents are discharged. The reservoir apparently gets 

 rid of the waste matter through a canal, commonly known as the 

 cytopharynx, which leads to the outside. Somewhat similar to 

 this is the contractile vacuole of Vorticellidae. Here a contrac- 

 tile vacuole discharges its contents by a short canal. In Nycto- 

 therus, Balantidium, etc., the contractile vacuole voids its con- 

 tents through a special canal at the posterior end of the body. 

 In the ciliates, the number, appearance and location of the 

 contractile vacuoles are usually constant and highly character- 

 istic, so that they have a taxonomic value. 



As to the function of the vacuole, it seems probable that it 

 adjusts the water contents of the cytoplasm by throwing out 

 from time to time a certain amount of water from the body, 

 and that in doing so it carries out dissolved waste products. 

 The pulsation of the contractile vacuole is, according to Biit- 

 schli, not due to the active contraction of the cytoplasm, but to 

 the physical attraction of the small droplet of fluid by the water 

 which surrounds the organism. 



Other cell-organs present in the cytoplasm vary a great 

 deal in different groups. Food vacuoles are conspicuously pres- 

 ent in holozoic Protozoa. These are droplets of fluid, usually 

 water in which are suspended food particles such as Protophyta, 

 other Protozoa or small Metazoa ingested as food. In the amoe- 

 boid Protozoa which do not possess a cytostome, the food par- 

 ticles are of variable dimensions, and when the particles are 

 large it is difficult to make out the thin film of water which sur- 

 rounds them. When minute food-particles are taken in through 

 a cytostome and cytopharynx, the food vacuoles are usually of 

 the same size. In saprozoic Protozoa and the majority of para- 

 sitic forms, in which dissolved food is absorbed by osmosis 

 through the body surface, food vacuoles containing solid particles 

 do not occur. 



Holophytic Protozoa possess in their endoplasm chromato- 

 phores, or chromoplasts (Fig. 15), which are made up of chloro- 

 phyll. The color of the chromatophores may be yellow, orange, 

 brown, grass-green, blue-green, or even red, due to the pigments 

 which envelop them. Ordinarily they have definite shape, e.g., 

 band-form, stellate, ovoid, ring-form, discoidal, or cup-like; but 



