ONE- CELLED ANIMALS AND SPONGES. 121 
When they die their shells fall to the bottom and, 
mingling there with other similar shells, form a soft 
white mud which may harden to form chalk. The 
chalk cliffs of England were doubtless produced in this 
way. 
Tripolite. Some of the Rhizopoda have shells made 
of the finest possible bits of glass or silica. Deposits 
of these shells with similar shells of one-celled plants 
form ¢ripolite, a substance used as a polishing powder. 
Infusoria. If hay be placed in warm water and 
allowed to stand in a warm place for a few days the 
Fic. 104.—Infusorial Earth Fic. 105. — Infusorians 
(magnified). * (magnified). 
water will be found to be filled with many minute, one- 
celled, rapidly moving animals. A look at these 
through the microscope shows that their movements 
are due to the motion of hair-like projections on the 
body. These hair-like bodies are called czla. One- 
celled animals which move by cilia are called /ufusoria 
because some of their kind appear when infusions of 
hay or other vegetable matter are allowed to stand. 
Some of the Infusoria are fixed by a stalk, or stem, like 
the bell-animalcule, or vorticella, shown in the figure, 
and some are rapidly moving free animals like those 
found in infusions of hay. 
