No. 399.] FRESH-WATER AQUARIA. 205 
largest animal representative is a so-called “ bull-head ” (one of 
the species of Uranidze), about four inches long, kept princi- 
pally for the balance, and because he needs no further attention 
than a few earthworms every few days. In this aquarium fresh- 
water polyps, Polyzoa, and other interesting forms appear in 
their season. 
In speaking of the time these aquaria have been kept, it is 
to be understood without changing the water but covered in 
such a way as to require very little or no additions of water to 
supply evaporation. 
The aération of the water for the breathing of animals will 
be readily seen to come from the oxygen given out by the plants 
while they feed (during photo-syntax) on the carbonic acid gas 
given out by the animals. The only thing needing attention 
is the feeding of such animals as cannot find their source of 
food in the aquaria themselves, and this should be done so 
carefully that no food is left to decay. 
For aquaria almost any kind of glass vessel that can be 
securely covered against dust and bacteria may be used — jam 
jars, battery jars, culture dishes and globes for the smaller win- 
dow aquaria; window glass, properly cemented into wooden 
frames that are kept coated with paraffin or asphalt varnish, 
for a medium size; and slate bottom and ends, with plate-glass 
sides, for the larger though more expensive ones. 
In setting up the aquaria it is better to begin with water from 
some clean pond containing considerable plant and animal life. 
Fill up to within a few inches of the top with water, and then 
add about j. its bulk of plants and animals; or hydrant water 
may be used, adding some plants, and later the animals desired. 
Where there is no choice, snails and Crustacez are the most 
convenient for use, unless they are hostile to the organisms 
desired for experiment. The snails feed on the plants, giving 
these carbonic acid in turn for food, while the Crustacez feed 
mostly on the débris from other organisms. 
If the animals keep near the surface, too many are prob- 
ably present, and some must be removed or more plants added. 
When the plants become yellow they are too abundant or have 
not had light enough. In some cases the water becomes foul 
