25. — THE FRESH-WATER AQUARIUM. 
other; and now the newly clad cray-fish appears in a coat 
of a pinker hue than before, and tries to keep under the 
plants and conceal itself, until accustomed to its new gar- 
ment it can venture forth once more into its little world. 
Cray-fishes eat small pieces of raw beef eagerly. We shall 
have to be careful that they do not crawl out of the tank, 
for if even a tassel of a curtain is left so near the water 
that it can be reached, we shall find our much prized spec- 
imen some morning dried up and lifeless in a corner of the 
room upon the floor. 
Frogs are interesting objects of study, and to many are 
great favorites; they are best kept in a tank with an inch or 
two of water, with a number of islands or resting-places 
above the water for them. A wire screen over the top of 
the tank will be necessary to keep the specimens together. 
Two of the most useful and instructive sets of specimens 
which the aquarium contains are its snails and mussels; use- 
ful, because they act as the scavengers of the tank, and from 
what would otherwise be the refuse matter make their living 
from day to day; instructive, because they serve to illustrate 
in a small way the great principle by which the health and 
purity of all our larger ponds and lakes is maintained. The 
snails live upon the bits of decayed plants and the confervoid . 
growths in the tank, and the mussels by filtering the water 
act as constant purifiers. There are three kinds of snails 
common in our ponds and streams, the Planorbis trivolvis 
the Paludina decisa, and the Lymnea desidiosa. Of 
these the best is the Planorbis, a snail with a shell coiled like 
a modern chignon; it is hardy and of clean habits, and does 
almost as much work as its neighbor, the Paludina; it is 
found chiefly in ponds or large streams, while the Paludina 
can be obtained in great numbers in small brooks or pond 
holes. The Lymn:a is found near the gravelly beaches of 
the larger ponds; it is a beautiful snail, but does not confine 
itself to the refuse matter, and is apt to eat eagerly the most 
delicate plants in the tank; it is, therefore, generally an 
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PSAL ONITE ace N 
