CHAPTER X 

 THE SIMPLEST ANIMALS: THE PROTOZOA 



266. One-Celled Animals. The simplest animals con- 

 sist of one cell; that is, they consist of a small mass of pro- 

 toplasm with a nucleus. Within this one cell must be carried 

 on all the processes connected with the fundamental life- 

 activities (feeding, breathing, excreting, reproducing), for 

 each of which processes an animal like a frog has special 

 organs with thousands of cells. In order to understand how 

 an animal with one cell can carry on the same life-activities 

 as does an animal with thousands of cells, it is necessary to 

 study some examples of the simplest animals. 



267. Paramecium. (D or L) If some chopped hay be placed in 

 water in a fruit-jar or other convenient vessel, and then to this be 

 added some decaying sticks, leaves, and other objects taken from 

 a pond where aquatic plants are'growing, there will probably develop 

 within a few weeks large numbers of transparent animals appearing 

 to the naked eye as minute whitish specks. With a rubber-bulbed 

 pipette take a drop of water from the surface and near the edge of 

 the vessel, and place on an object-slide. Then place a few shreds of 

 cotton on the slide, and put on the cover-glass. 



Place the slide on a piece of black cloth or paper, and notice the 

 moving white specks. Use a hand-lens. Now examine with a mi- 

 croscope, using first the low-power objective. Look for rapidly 

 moving objects having the form shown in Fig. 88, A. These are 

 specimens of paramecium, sometimes called the "slipper-animal- 

 cule." In size, they are about T fo- inch long. Several other 

 similar animals are often abundant in the same water. 



Study the following points : form of body, movements, response 

 to stimuli (e.g., when the animals bump against obstructions). Is 

 there evidence that one end is anterior (i.e., goes forward in loco- 

 motion) ? 



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