CHAPTER 1 
BRANCH I.—PROTOZOA. 
General Characters of Protozoans.—We can imagine no 
more elementary forms of life than certain members of this 
branch, whose bodies in the simplest forms are merely 
masses of albumen, without any distinct permanent organs, 
or portions set apart for the performance of any special 
function. Yet the primary acts of animal life. such as tak- 
ing food, its digestion and assimilation, and reproduction, 
are carried on as effectively by these lowest as by the high- 
est forms. The simplest Protozoans are like minute drops 
of protoplasm or albumen, having a gliding motion, and 
constantly changing their forms, throwing out temporarily 
root-like projections called 
pseudopodia, which serve to iy 
gather food-particles. Fig. 
7 illustrates a typical Proto- 
zoan. It is the common 
Amoeba of standing water. 
Most Protozoans are provid- 
ed with a central organ or 
nucleus, which corresponds 
to the reproductive organs of the many-celled animals. 
The Protozoa are one-celled in distinction from all other 
animals, from the sponges to man, which are many-celled, 
though it is claimed thai, a few shelled forms (Rhizopods) are 
composed of several indistinct cells. Thus a Protozoan cor- 
responds to an egg or to any one of the cells composing the 
bodies of higher animals. They may be naked, asin Prota- 
meba or Ameba, or may secrete a silicions or calcareous 
shell. The Infusoria, forming the highest class, are quite 
complicated, with permanent cilia, a mouth, throat, repro- 
Fig. 7. —Ameha, the nnclens not shown. 
