Page:The Book of the Aquarium and Water Cabinet.djvu/55

Rh snails breed rapidly in tanks, but the carp devour the young as fast as they appear; hence it is advisable to remove the spawn into jars containing healthy plants, such as Callitriche, in which they may remain for observation of growth, till stout enough to be committed to the tank.

Among the bivalves, the fresh-water swan mussel, Anodon cygneus, and the Duck mussel, Unio pictorum, are interesting burrowers, and perform a great service in the tank. They act as scavengers, not by the process of eating off objectionable growths, as in the case of univalves, but by the straining off of matters held in