THE FRESH-WATER AQUARIUM. 25 
unwelcome visitor. Of the mussels, those found in ponds 
with their many rayed shells, and those river mussels with 
their thick, unattraetive coverings, are alike useful; they 
move from one side of the tank to the other with ease, and 
we must not expect to find them always in one position; the 
number of snails which may be kept to advantage in a tank 
is very large; they are so apt to perish during the winter 
that it will be well to begin the season with as large a stock 
as two hundred for a medium sized tank; a dozen mussels 
of a size proportioned to the tank will be sufficient. 
There are many specimens, such as fishes at the time of 
spawning, or those particularly fierce, or certain larvze, which 
would either be destroyed or seen to disadvantage in the: 
general collection. For each of these a separate tank is in- 
dispensable ; some glass jars of strong clear material holding 
about two quarts, will answer every purpose, and the contents 
ean be arranged precisely as if they were large aquaria. After 
one has had an aquarium in operation for some time extra 
tanks of this sort will be found very useful and necessary ; 
for if a specimen gets injured or is in poor condition, a few 
weeks recruiting in a separate tank will often save its life ; or, 
if we have a larger stock of plants than the large tank will 
: accommodate at the time, when later in the winter the plants 
die off, then we shall wish to replace them from specimens 
in the reserve stock. 
The instruments used for aquarial purposes are, few in 
number and simple. We need a good net a foot or two in 
diameter, with very fine meshes, and a flat basket so par- 
titioned off that it will hold four good sized jars; these jars 
may be of earthen-ware or of strong glass, the latter mate- 
rial being perhaps better, as we can then see how many 
specimens each jar contains without trouble. Most of the 
plants ean be taken home (if the distance is not too great) 
rolled up in the net, while the mussels can occupy the room 
between the jars. It is very necessary to keep the plants 
moist, as they are much blighted if allowed to dry; if 
AMER. NATURALIST, VOL. IV. 4 
