PROTOZOA. 
243 
Fig. 187.— A Compound Monad 
( Uvella ), X 1000. 
Class IV. —Infusoria. 
This unassorted group of living particles derived its 
name from the fact that they were first discovered in veg¬ 
etable infusions. Every drop of 
a stagnant pool is crowded with 
them. They are all single and 
microscopic, yet of various -sizes, 
the difference between the small¬ 
est and largest being greater than 
the difference between a Mouse 
and an Elephant. Some are fixed 
(as Vorticella ), but the majority are free, and constantly 
in motion, propelled by countless cilia, as a galley by its 
oars. The delicate body consists of two 
layers of sarcode (there are no cellular 
tissues, but the whole body represents a 
single cell), covered by a membrane, or 
skin, having one or two contractile cavi¬ 
ties, and a nucleus. Food-granules can 
often be seen. On one side is a slight 
depression, or “ mouth,” leading to a 
short, funnel-shaped throat. A mouth 
and a rudimentary digestive cavity are 
among the distinctive features of these 
Protozoans. Some have a pigment-speck 
—the simplest sense organ—and in the 
stem of Vorticella the first rudiments of 
muscle may be found. They multiply so rapidly (chiefly 
by self-division), that a Paramecium , the most common 
form, may become the parent of 1,364,000 in forty-two days. 
There are two main groups: Flagellata , or Monads, 
provided with one or two flagella, or long, bristle-like 
cilia; and Ciliata , which are furnished with numerous 
vibratile cilia. 
(Paramecium, aurelia), 
X 300: m , mouth ; v, 
contractile vesicles; n, 
nucleus. 
