In almost every garden there may be found, under flower-pots and the like, a Woodlouse, about two-thirds of an inch long, of a brown colour, with yellowish blotches arranged in a row on each side of the back. This is _Oniscus asellus_, a species widely distributed in Europe and North America. It has the antennæ shorter than in _Ligia_, and the flagellum is composed of only three segments. The uropods are quite short. The endopodites of the pleopods are membranous gill-plates, which serve for respiration in the moist air in which these animals generally live. The exopodites are stiff plates which cover and protect the delicate endopodites; it is probable that they also aid in respiration, for they contain a system of minute channels, filled with air, where the cuticle is separated from the underlying cells. As these channels are nowhere open to the outside, the air must find its way in by diffusion through the cuticle. [Illustration: FIG. 64--STRUCTURE OF THE BREATHING ORGANS OF _Porcellio scaber_. (From Lankester's "Treatise on Zoology," after Stoller.) A, Exopodite of first pleopod, showing the tuft of air-tubes ("pseudo-tracheæ"), seen through the transparent cuticle; B, vertical section through same; C, part of section more highly magnified. _art_, Point of attachment of exopodite to peduncle; _c_, cuticle; _gr_, grooved area of cuticle; _hy_, hypodermis, or layer of cells under the cuticle; _n_, nucleus of hypodermis cell of air-tube; _o_, external opening; _tr_, air-tubes] Even more abundant than _Oniscus asellus_, and often found together with it, is _Porcellio scaber_ (see Fig. 20, p. 51). It is usually of a dark bluish-grey, but occasionally it is irregularly mottled with a lighter colour. The flagellum of the antenna has only two segments. The most interesting difference from _Oniscus_, however, is found in the pleopods. If the under-side of the living animal be examined with a pocket lens, a white spot will be seen on each exopodite of the first two pairs of pleopods. When the structure of the pleopods is investigated by means of microscopic sections (Fig. 64), it is found that the white spots are tufts of fine branching tubes radiating into the interior of the exopodite from a slit-like opening on the outer edge. These tubes arise by an in-pushing of the integument, and they are lined throughout by a delicate continuation of the external cuticle. During life they are filled with air, and they serve to aerate the blood circulating in the interior of the appendage. [Illustration: FIG. 65--_Armadillidium vulgare._ × 2-1/2. (After Sars.)]