The animal commonly known under this name is no bug at all, but belongs to the class Crustacea, which includes the lobster, crab, etc. These differ from the class Insecta principally in the method of breathing, which is done by means of gills. The common sow-bugs belong to the order isopoda, family onis-cidae, genus parcellio, six species of which have been described. P. vittatus is probably the most common species, although some of the others are quite abundant. These crustaceans are found in all damp, dark places, and feed upon decaying animal and vegetable matter. They are perfectly harmless, and are much sought for by poultry, being with them a much more important article of diet than is commonly supposed. In former times the species of this family were much used in medicine, and one of the old books tells us that when dried and pulverized "they leave a faint disagreeable smell, and a somewhat pungent sweetish nauseous taste, and are highly celebrated in suppressions, in all kinds of obstructions of the bowels, in the jaundice, ague, weakness of sight, and a variety of other disorders".