CHAPTER XI. 
THE ACTIVITIES OF ONE-CELLED ANIMALS AND 
SPONGES. 
THUS far we have been dealing with animals having 
more or less complicated machinery for carrying on the 
activities of life. We know that an egg is a single cell 
and that as it grows it changes by increasing the num¬ 
ber of cells and by setting apart groups of these cells 
to perform different duties. In a single cell life is 
reduced to its lowest terms. In an egg we may see 
the process of unfolding and get greater insight into 
the workings of living machinery. So in studying 
living animals which never develop beyond the single¬ 
cell stage of existence we may gain a knowledge of 
animal activities not otherwise obtainable. 
These animals are so small that a complete study of 
them makes necessary the use of the compound micro¬ 
scope, but as it is not proposed to burden this course 
with the details of microscopic manipulation, we must 
content ourselves with verbal descriptions for the 
present. 
The one-celled animals are put in a sub-kingdom by 
themselves, called Protozoa. They are very small and 
for the most part inhabit the water. There are many 
of them, but their ways may be very well understood 
by studying descriptions of a few forms. 
The Amoeba. This minute animal has been much 
studied and its modes of life are well known. It may 
be found in stagnant water in small shallow pools. It 
is less than a hundredth of an inch in diameter and 
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