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
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Fig. 65—Armadillidium vulgare. × 2-½. (After Sars.)
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.
Another Woodlouse common in England is Armadillidium vulgare ([Fig. 65]), a little slaty-grey species with a very convex body, which rolls itself into a ball when touched. Like the last-mentioned species, it has two segments in the flagellum of its short antennæ, and it has tufted air-tubes in the exopodites of the first two pairs of pleopods. It is often mistaken for an animal of widely different structure, which it superficially resembles—the Pill Millipede (Glomeris marginata). The latter, however, may easily be recognized by having either seventeen or nineteen pairs of walking legs (instead of seven pairs), set close together in the middle line of the body, and by lacking the plate-like pleopods. The resemblance between the two animals can hardly be regarded as a case of "mimicry," since there is no reason to believe that either benefits by its likeness to the other. As in so many other cases of "convergent resemblance" between animals of different structure, it does not seem possible to get beyond the vague suggestion that a similarity in habits may have led, in some way that we do not understand, to a similarity in appearance.