division.
Two amoeba flow together and become one. It seems to rejuvenate the
organism so that it is able to go on with its division and thus fulfil
its life-mission which is the same for these lowly animals as with the
higher, that of perpetuating the species.

_Classes of Protozoa._ The group or Phylum Protozoa is divided into four
smaller groups or classes. The amoeba belongs to the lowest of these,
the Rhizopoda. Rhizopoda means "root-footed," and the name is applied to
these animals because most of them move about by means of root-like
processes known as pseudopodia or "false feet." This is by far the
largest class and contains thousands of forms, mostly living in salt
water but there are many fresh-water species. They are non-parasitic,
but some of them by their presence in the body may cause such diseases
as dysentery, etc.

[Illustration: FIG. 7--Typhoid Fever bacilli. (After Muir and
Ritchie.)]

[Illustration: FIG. 8--_Amoeba_, showing the forms assumed by a single
individual in four successive changes. (From Kellogg's Elementary
Zooel.)]

[Illustration: FIG. 9--_Euglina virdis._ (After Saville Kent.)]

[Illustration: FIG. 10--_Spirocheta duttoni_, x 4500. (After Breinl and
Carter.)]

The next class which may be known as the whip-bearers (_Mastigophora_)
includes those Protozoa that move by fine undulating processes called
flagella. One of the common representatives of this class is the little
green _Euglena_ (Fig. 9), whose presence in standing ponds and puddles
often imparts a greenish color to the water. Then in the salt water near
the surface there are often myriads of minute _Noctiluca_ whose
wonderfully phosphorescent little bodies glow like coals of fire when
the water is disturbed at night. Although this class contains fewer
forms than the preceding some of these have within recent years been
found to be of great importance because they live as parasites on man
and other animals. The trypanosome whose presence in the blood and
tissues of the patient causes that dreadful disease which ends in
sleeping sickness belongs here as well as do several other similar kinds
that produce serious troubles for various mammals and birds. The
Spirochaeta, about which there has been so much recent discussion, also
belong here. These are simple spiral-like forms (Fig. 10), that are
sometimes classed with the simple plants, bacteria, but Nuttall and
others have shown very definitely that they should be classed wit