20 ANIMAL LIFE 
as polishing powder, are composed almost exclusively of 
the siliceous shells of ancient and long-extinct Radiolaria. 
10. Antiquity of the Protozoa.—All the animals of the 
ocean depend upon the marine Protozoa (and the marine 
Protophyta, or one-celled plants) for food. Either they 
prey upon these one-celled organisms directly, or they prey 
upon animals which do prey on these simplest animals. 
The great zodlogist. already quoted says: “The food sup- 
ply of marine animals consists of a few species of micro- 
scopic organisms which are inexhaustible and the only 
source of food for all the inhabitants of the ocean. The 
supply is primeval as well as inexhaustible, and all the life 
of the ocean has gradually taken shape in direct depénd- 
ence upon it.” That is, the marine simplest animals are 
the only marine animals which live independently; they 
alone can live or could have lived in earlier ages without 
depending on other animals. They must therefore be the 
oldest of marine animals. By oldest we mean that their 
kind appeared earliest in the history of the world. As it 
is certain that marine life is older than terrestrial life—that 
is, that the first animals lived in the ocean—it is obvious 
that the marine Protozoa are the most ancient of animals. 
This is an important and interesting fact. Zodlogists try 
to find out the relationships and the degrees of antiquity 
or modernness of the various kinds of animals. We have 
seen that the Protozoa, those animals which have the sim- 
plest body structure and perform the necessary life pro- 
cesses in the simplest way, are the oldest, the first animals. 
This is just what we would expect. 
11. The primitive form.—We find among the simplest 
animals a considerable variety of shape and some manifest 
variation in habit. But the points of resemblance are far 
more pronounced than the points of difference, and are of 
fundamental importance. The composition of the body of 
one cell, as opposed to the many-celled structure of the 
bodies of all other animals, is the fact to be most distinctly 
