234 COMPARATIVE ZOOLOGY. 
and the blocks in the Pyramids of Egypt are largely com- 
posed of extinct Foraminifers. 
A Polyeystine differs from a Foraminifer in secreting 
a siliceous, instead of a calcareous, shell, studded with 
spines; and the central part of the body is made up of 
many cells, and surrounded by a strong membrane. While 
Foraminifers live mostly at the bottom of the sea, Polycys- 
tines generally float on the surface. They are also more 
minute, but as widely diffused. They enter largely inte 
the formation of some strata of the earth’s crust, and 
abound especially in the rocks of Barbados and at Rich- 
mond, Va. 
Crass II1.—tInfusoria. 
This unassorted group of living particles derived its 
name from the fact that they 
were first discovered in vegeta- 
ble infusions. Every drop of a 
stagnant pool is crowded with 
them. They are all single and 
microscopic, 
Fig. 182.—A Compound Monad yet of various 
Ue Aum: sizes, the differ- 
ence between the smallest and largest 
being greater than the difference be- 
tween a Mouse and an Elephant. Some 
are fixed (as Vorticella); but the major- 
ity are free, and constantly in motion, 
propelled by countless cilia, as a galley 
by its oars. The delicate body consists 
of unorganized sarcode (¢. ¢., there are ,, 
1@. 183. — Infusorium 
no cellular tissues, but the whole body (Paramecium auretia), 
represents a single cell), covered by a santeaetie veatalede ey 
membrane, or skin, and containing a ™°"* 
semi- fluid substance, a contractile cavity, and several 
