168 SINGLE-CELLED ANIMALS AS ORGANISMS 



protoplasm usually responds by contracting, another power which 

 it possesses. We know, too, that plant and animal cells take in 

 food and change the food to protoplasm, that is, that they assimi 

 late food ; and that they may waste away and repair themselves. 

 Finally, we know that new plant and animal cells are reproduced 

 from the original bit of protoplasm, a single cell. 



The Structure of Paramoecium. The cell body is almost 

 transparent, and consists of semifluid protoplasm which has a 



granular grayish appearance under the 

 microscope. This protoplasm appears to 

 be bounded by a very delicate membrane 

 through which project numerous delicate 

 threads of protoplasm called cilia. (These 

 are usually invisible under the micro 

 scope). 



The locomotion of the paramoecium is 

 caused by the movement of these cilia, 

 which lash the water like a multitude of 

 tiny oars. The cilia also send particles 

 of food into a funnel-like opening, the 

 gullet, on one side of the cell. Once in- 

 A paramoecium. c.v.,contrac- side the cell body, the particles of food 



tile vacuole; f.v., food , ,-, -, . , ,., ,, , 



vacuole; m, mouth; ma.n., materials are gathered into little balls 

 macronucleus; mi.n., mi- within the almost transparent proto- 



cronucleus; w.v., water i mi r , 



vacuole. plasm. These masses of food seem to be 



inclosed within a little area containing 



fluid, called a vacuole. Other vacuoles appear to be clear ; these 

 are spaces in which food has been digested. One or two larger 

 vacuoles may be found; these are the contractile vacuoles; their 

 purpose seems to be to pass off waste material from the cell 

 body. This is done by pulsation of the vacuole, which ultimately 

 bursts, passing fluid waste to the outside. Solid wastes are passed 

 out of the cell in somewhat the same manner. No breathing 

 organs are seen, because osmosis of oxygen and carbon dioxide 

 may take place anywhere through the cell membrane. The 

 nucleus of the cell is not easily visible in living specimens. 

 In a cell that has been stained it has been found to be a double 



