TROPICAL AQUARIUM FISHES 13 
in an aimless manner, keeping the water stirred up and the natural sedi- 
ment agitated. Furthermore, they soon learn to eat fish food and, after 
that step in education, they refuse to consume the less desirable particles 
found in an aquarium occupied by fish. 
Another scavenger is the fresh water mussel. The chief value of the 
mussel is to keep down the vegetable growth which causes aquarium water 
to turn green. Mussels are equipped with a sort of siphon arrangement, 
Fic. 5. FRESHWATER MussEL, SHOWING Water INTAKE AND OuTLeT; ALso “Foot” 
Wits Wuicu Tuey Burrow AND TRAVEL 
by which they suck in water in one opening and eject it from another. 
In the few moments which they hold the water they extract from it the 
floating vegetable organisms. Two or three mussels should keep a ten- 
gallon aquarium free of green water. Care should be taken to occas- 
ionally see whether the mussels are living, as they decompose very rapidly 
and spoil the aquarium water. This can be done by tapping lightly on the 
shell and seeing whether they respond by closing. 
A curious but useful scavenger is an eel-like fish called the Weather- 
fish. Varieties are native to Europe and Asia. They are freely imported 
Fic. 6. THe WEATHERFISH (Cobitis fossilis) 
and inexpensive. When not scouring the aquarium bottom for bits of 
decomposing matter, they sometimes burrow into the sand, leaving only 
the head exposed, producing a quaint appearance. In their special occu- 
pation as scavengers they employ a method which is both effectual and 
interesting. The dirt and top sand are taken in the mouth and rapidly 
shot out through the gills. Any particle of food considered edible is 
automatically separated from the bulk of the dirt and swallowed. 
