THE SIMPLEST ANIMALS 307 



in higher animals. For example, muscle cells have specialized 

 for movement ; but for them other cells must do the digest- 

 ing, absorb the oxygen, discharge the excretions, etc. In 

 short, the cells of every organ depend upon the cells of all 

 the other organs ; and all must work together harmoniously, 

 because there is mutual interdependence. And just as a man 

 specializing in one business becomes expert, so a cell specializ- 

 ing in movement, secreting, or in any other one necessary 

 function can do that work better than it is done in a parame- 

 cium, for that animal has so many things to do in its one cell 

 that it does none of them as well as they are done by the 

 specialized cells in higher forms of life. 



270. Amoeba. In many ways the most interesting of 

 one-celled animals is one known to biologists as amoeba, 

 which lives on the surface of submerged objects in ponds and 

 other bodies of water. 



(D) If one scrapes the ooze from the lower surface of lily leaves, 

 and from various other aquatic leaves and stems, or collects some of 

 the green felt-like growth which often forms a coating on the mud 

 in the bottom of shallow stagnant ponds or streams, it is probable 

 that some amoebas will be obtained. But they are rarely as abun- 

 dant as paramecia, and so it is usually necessary to spread the collected 

 material in soup-plates or other shallow dishes and allow a few days 

 in which the amoebas may crawl to the surface, and perhaps also 

 increase in numbers. From time to time collect, with a rubber- 

 bulbed pipette, some of the scum, especially near the edge of the 

 dish, spread on glass slides, and with a low-power objective look 

 for little specks having the appearance of ground glass, and irregu- 

 lar in form, as represented in Fig. 91. Sometimes it is necessary to 

 examine dozens of slides with material from different parts of the 

 dish before an amoeba is located. It is only in this way that amoebas 

 can be found, for they are rarely larger than T ^ inch in diameter 

 and usually very much smaller. 



Having located a good specimen, look at the cover-glass from the 

 side of the objective and estimate the approximate position of the 

 animal, so that if the slide is moved, it will be easy to find the 

 place again. Then carefully adjust a higher power objective, and 

 study according to the following description. 



