46 THE SIMPLEST ANIMALS — PROTOZOA 



25. The Protozoan Cell. — The protozoan cell is a single 

 mass of living matter, called protoplasm. In a general way 

 it carries on the same life processes as the grasshopper; or 

 any other animal. When this living cell comes in con- 

 tact with heat, cold, electricity, chemicals, or other stimuli, 

 it moves, and we say that it is irritable. The term irrita- 

 bility, used with a scientific meaning, is defined as the power 

 of being aware of a stimulus. When this living cell is 

 brought into contact with cold, for example, it makes a 

 definite movement. It is aware of the cold stimulus. 



The . living cell grows by using food. It takes in oxy- 

 gen from the water or from the air, according to where it 

 happens to live. It gives off waste substances. It can 

 grow or reproduce other cells of the same kind. 



Many protozoan cells have no limiting wall between 

 the living substance and the water in which they live. 

 Yet the protoplasm and the water do not mix, though we 

 do not understand why. Other Protozoa living in the 

 ocean are surrounded by extremely thin skeletons of lime, 

 and when the animals die their skeletons sink to the bot- 

 tom and become massed in a sort of rock. The famous 

 chalk cliffs of England were formed in this way. 



26. Habitat. — The habitat of any animal is the place 

 where it lives. The Protozoa are small, usually micro- 

 scopic, animals common in stagnant pools and in swamp 

 water. They are also common in salt water. In fact, 

 Protozoa are likely to be found in nearly all ponds of 

 water that contain food for them. Often, in the summer 

 time, our attention is called to the activities of Protozoa 

 when the water from lakes or reservoirs has a fishy taste. 

 This peculiar taste may be due either to animals or plants, 

 or to both. When it is due to animals, it is caused by a 

 disagreeable oil formed by a certain kind of Protozoa. 



By far the greater number of Protozoa are harmless, 



