90 THE SMALLEST LIVING THINGS 



With other ciliates the effect of starvation may be simply a 

 reduction in size without the appearance of vacuolization 

 (Fig. 53). 



Similarly in all protozoa, as a result of continued movements, 

 the protoplasmic substances are burned up, the first to go being 

 the protein food reserves, after which the basic substances of 

 the protoplasm are used as fuel, a process which, in Paramecium, 

 results in great spaces being formed in the organism. Up to a 

 certain stage recovery of the animal is possible by the introduc- 

 tion of food, but beyond this point continued starvation ends in 

 death. 



Respiration 



Not much is definitely known about the process of protoplas- 

 mic oxidation,* but attempts have recently been made to deter- 

 mine the intensity of oxidation at any given time by injecting into 

 the protoplasm graded chemical indicators which change color 

 on undergoing oxidation. The depth of color indicates the ex- 

 tent of oxidation undergone. 



There are no special respiratory organs in protozoa. Oxy- 

 gen is taken into the cell with water or possibly by osmosis, t 

 and carbon dioxide (C0 2 ), is given off in the same way. In 

 all fresh-water protozoa, but not in marine forms, there is a 

 specialized organoid called the contractile vacuole which may 

 function in the disposal of carbon dioxide. This organoid is so 

 named from its rhythmic contractions whereby fluids in the vacu- 

 ole are expelled from the organism. In some forms this vacuole 

 is a simple spherical cavity which grows steadily in diameter by 

 increments of fluids from the protoplasm until it reaches a maxi- 

 mum size. It is connected with the outside by a fine and generally 

 short canal, which is closed by a delicate protoplasmic film. 

 When the internal pressure of the contained fluid wastes over- 

 comes the resistance of the surrounding film, the latter breaks 

 and the fluid escapes, a new film being formed by the protoplas- 

 mic wall of the vacuole. 



* Oxidation, the process of adding oxygen to other chemical elements or to 

 existing chemical combinations. 



t Osmosis, the passing of a fluid through a porous partition, such as a membrane, 

 to mix with another fluid on the other side. The inward flow is called aidosmosis, 

 and the outward flow, cxosmosis. 



