Pond Life 
weeds it will be necessary to change the 
water every few days. But as long as 
there areenough weeds growzng, the same 
water will last for a long time, because 
from the growing leaves oxygen is given 
off, which keeps the water fresh and 
enables the tmmates to live, “And to 
prevent the growth of the green mossy 
substance which forms on the glass, 
some water-snails should be introduced, 
which feed on itand so keep the glass 
clean and clear. If you bring to this 
globe some of the creatures you catch 
in the pond, you will get a rough idea of 
the amount of life there is, and what a 
number of events are happening below 
the surface of the water of which you 
knew nothing. You will find that your 
prisoners don't all agree with one another, 
and it will be as well to keep several 
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