IV 
ANIMAL LIFE IN ITS LOWEST FORMS 
HE biological division, or discrimination, between animal 
and vegetable life, is based on the manner of assimilating 
food. Plants feed upon mineral substances, or, in other words, 
assimilate inorganic matter, while animal life requires for its 
support vegetable or some other organic matter. 
Animal as well as vegetable life in its lowest forms begins 
with one-celled organisms, which are called respectively Protozoa 
(first animals) and Protophyta (first plants). Both of these divi- 
sions are composed mostly of microscopic objects, and, together 
with other minute forms of life of the marine species, constitute 
a great part of the plankton, or free-floating organisms of the sea. 
These minute organisms seem like connecting-links between the 
two kingdoms. They were claimed by both botanists and zodlo- 
gists until the use of the microscope made close observation of 
minute structure possible. 
Among the small animalcules of the phylum Protozoa are some 
which are familiar to all by name, such as the Infusoria, which 
are most interesting creatures to examine in a drop of water 
under the microscope. A more tangible example of the Protozoa 
are the Foraminifera. Foraminifera, like diatoms, have a shell- 
like covering, and these shells, among the most plentiful of which 
are those of the genus Globigerina, fall, as do those of diatoms, 
in immense numbers to the bottom of the ocean, and form re- 
spectively what are known as Globigerina and diatomaceous ooze. 
In course of time the sedimentary strata become fossilized ; thus, 
the stone of which the city of Paris is built consists of fossilized 
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