116 



THE RISE OF ANIMAL LIFE 



mecia. By studying the region of the gullet, 

 one can see the cilia beat in such a way as to 

 pass the yeast particles along its oral groove 

 and down into the gullet. There they are 



Fig. 7-8. Path taken by Paramecium when moving 

 freely through the water. 



Fig. 7-9. Movement of Paramecium when attachecJ at 

 posterior end. 



rounded up into a mass which finally 

 pinches off into the cytoplasm as a food 

 vacuole. Once in the cytoplasm the yeast 

 cells remain deep red for a time, but gradu- 

 ally begin to turn blue as they approach the 

 anterior end. This means that the contents 

 of the food vacuole are alkaline at first 

 (Congo red is red in alkali, blue in acid), 

 just as is the case in the mouth of man. As 

 digestion proceeds, the vacuoles become 

 acid as indicated by the blue color, remi- 

 niscent of the condition found in the hu- 

 man stomach. The digested material passes 

 through the wall of the vacuole and out 

 into the protoplasm where it is metabolized, 

 the same process that was noted for 

 amoeba. Finally the undigested portions 

 left in the vacuole pass through a tiny 

 opening to the outside, the cytopyge, which 

 is equivalent to an anus in higher animals. 

 Some species of ciliates are able to re- 

 ceive nourishment from dissolved organic 

 matter in the medium. In fact, one species, 

 Tetrahijmena geleii (Fig. 7-1), a small par- 

 amecium-like ciliate, grows in a culture 

 medium entirely free from bacteria or other 

 microorganisms. This tiny animal has about 

 the same food requirements as higher ani- 

 mals, including man himself. Recent experi- 

 ments have shown that to grow it requires 

 a diet containing amino acids, sugar, salts, 

 and vitamins. This seems to indicate that 

 even a single-celled animal maintains met- 

 abolic processes almost as intricate as 

 man's. 



Respiration and excretion 



Respiration and the excretion of nitrog- 

 enous wastes take place in paramecium 

 much the same as in amoeba. The contrac- 

 tile or pulsating vacuoles which lie at either 

 end of the cell contract alternately at about 

 15-second intervals. Several radiating ca- 

 nals empty into each contractile vacviole, 

 being most obvious when the vacuole is 

 nearly empty. Each vacuole discharges its 

 contents to the outside through a minute 

 pore in the pellicle. The rate of contraction 



