The inner plate (or endopodite) is in structure a gill, but the blood that passes through it, is enabled to take up oxygen from moist air, while the outer division (or exopodite) acts as a protecting cover (fig. 14). In Porcellio, air-tubes (tracheae) may be present (see below).
In the male, the first two pairs of abdominal appendages are specially modified, the inner divisions (endopodites) being long and pointed (figs. 15 and 16). The last pair, or tail appendages, in the male are often considerably larger than in the female, and the form of these structures is sometimes of value in classification.
Alimentary canal.—The main portion of the alimentary system is, practically speaking, a straight tube (fig. 17). Its first part (not shown in the figure) is a narrow gullet, which after passing through the nerve collar dilates to form a sort of stomach. Into this the secretion of four digestive glands is poured by two ducts. These glands have a somewhat striking appearance, being yellow tubes spirally coiled, and they end blindly. From the stomach the intestine runs to the hinder end of the body and passes under the heart.
Circulatory system.—The blood being aërated in the abdominal appendages, we find that the heart is situated towards the hinder end of the body (fig. 18). Three main arteries supply the thorax and head, while the blood is brought from the gills to the heart.
