142 THE SIMPLEST ANIMALS — PROTOZOA 
any other animal. When this living cell comes in contact 
with heat, cold, electricity, chemicals, or other stimuli, it 
moves, and we say that it is irritable. The term irritability, 
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 oxygen 
from the water or from the air, according to where it happens 
to live. It gives off waste 
substances. It can grow 
and reproduce other cells 
of the same kind. 
Many protozoan cells 
have no limiting wall be¬ 
tween the living substance 
and the water in which 
they live. Yet the proto¬ 
plasm 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. When the animals die their skeletons sink to the 
bottom and become massed in a sort of rock. The famous 
chalk cliffs of England were formed in this way. 
111. 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 
