SINGLE-CELLED ANIMALS AS ORGANISMS 167 



already learned. Such is a simple plant cell. Let us now 

 examine a simple animal cell in order to compare it with that 

 of a plant. 



Where to find Paramoecium. — If we examine very carefully 

 the surface of a hay infusion, we are likely to notice in addition to 

 the scum formed of bacteria, a mass of whitish tiny dots collected 

 along the edge of the jar close to the surface of the water. More 

 attentive observation shows us that these objects move, and that 

 they are never found far from the surface. 



The Life Habits of Paramoecium. — If we place on a slide a drop 

 of water containing some of these moving objects and examine 

 it under the compound microscope, we find each minute whitish 

 dot is a cell, elongated, oval, or elliptical in outline and somewhat 

 flattened. This is a one-celled animal known as the 'paramoecium 

 or the slipper animalcule (because of its shape) . 



Seen under the low power of the microscope, it appears to be 

 extremely active, rushing about now rapidly, now more slowly, 

 but seemingly always taking a definite course. The narrower end 

 of the body (the anterior) usually goes first. If it pushes its way 

 past any dense substance in the water, the cell body is seen to 

 change its shape temporarily as it squeezes through. 



Response to Stimuli. — Many of these little creatures may be 

 found collected around masses of food, showing that they are at- 

 tracted by it. In another part of the slide we may find a number 

 of the paramoecia lying close to the edge of an air bubble with 

 the greatest possible amount of their surface exposed to its 

 surface. These animals are evidently taking in oxygen by 

 osmosis. They are breathing. A careful inspection of the jar 

 containing paramoecia shows thousands of tiny whitish bodies 

 collected near the surface of the jar. In the paramoecium, as 

 in the one-celled plants, the protoplasm composing the cell 

 responds to certain agencies acting upon it, coming from ^vithout ; 

 these agencies we call stimuli. Such stimuli may be light, differ- 

 ences of temperature, presence of food, electricity, or other factors 

 of its surroundings. Plant and animal cells may react differently 

 to the same stimulus. In general, however, we know that proto- 

 plasm is irritable to some of these factors. To severe stimuli, 



