h the
simplest animals, the Protozoans. These are the cause of relapsing
fevers in man and of several diseases of domestic animals. It is
believed by certain eminent zooelogists that when the germ that causes
yellow fever is discovered it will be found to belong to this group.

The members of the class Infusoria, so called because they were early
found to be abundant in various infusions, are characterized by numerous
fine cilia or hair-like organs by means of which the organism moves
about and procures its food. The well-known "slipper animalcule"
(_Paramoecium_) (Fig. 11), and the "bell-animalcule" (_Vorticella_)
(Fig. 12) are two common representatives. The _Paramoecia_ were the
animals mostly used by Jennings in his wonderfully interesting
experiments on the behavior of these lowly forms of life. He showed that
they always reacted in a certain definite way in response to particular
stimuli, and he was led to believe that he could see "what must be
considered the beginnings of intelligence and of many other qualities
found in the higher animals." A species of _Vorticella_ was probably the
first Protozoan that was ever observed. An old Dutch microscopist, Anton
von Leeuwenhoek, in 1675, while studying with lenses of his own
manufacture, discovered and described forms which undoubtedly belong to
this genus. Few if any of the Infusoria are pathogenic, although some
are said to be associated with certain intestinal diseases both in man
and the lower animals (Fig. 13).

[Illustration: FIG. 11--_Paramoecium._ (From Kellogg's Elementary
Zooel.)]

[Illustration: FIG. 12--_Vorticella_, one individual with the stalk
coiled, the other with the stalk extended. (From Kellogg's Elementary
Zooel.)]

[Illustration: FIG. 13--Pathogenic Protozoa; a group of intestinal
parasites. _A_, _B_, _Megastoma entericum_, _C_, _Balantidum entozoon_.
(After Calkins.)]

The last class, the Sporozoa, or the spore-forming animals, while small
in the number of known species, only about three hundred kinds being
known, is extremely important. A number of diseases in man and other
animals are due to the presence of these Sporozoans, for they are all
parasitic. Few if any animals are exempt from their attacks. They even
attack other minute Protozoa. One hundred and fifty-seven species have
been recorded as attacking insects, one hundred species attack birds,
fifty-two reptiles, eighty crustaceans, twenty-two fish, and so through
the list. Ten hav