CHAPTER IV 
THE SPONGES =~ ° 
22. Their relation to the Protozoa,—While the greater 
number of one-celled forms are not united with their fel- 
lows, there are several species where the reverse is true. In 
Fig. 9, for example, a fresh-water form known as Pandorina 
is represented, consisting of sixteen cells embedded in a 
spherical, jelly-like substance, 
each one of which is precisely 
like its companions in form 
and activity. The aggregation 
may be looked upon as a colo- 
ny of sixteen Protozoa united 
together to derive the benefit 
of increased locomotion and 
a larger amount of food in 
consequence. As a result of 
such a union they have not 
lost their independence, for if 
one be separated from the main Fie. 9.—Pandorina (from Nature). 
company it’ continues to exist. Highly magnified. 
From such a simple colonial 
type we may pass through a series of several more complex 
forms which reach their highest development in the beau- 
tiful organism, Volvoz (Fig. 10). In this form the indi- 
vidual members, to the number of many thousand, are ar- 
ranged in the shape of a hollow sphere. The united efforts 
of the greater number, which bear on their outer surfaces 
two flagella, drive the colony with the rolling movement 
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