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. The presence of air-tubes in the pleopods of many Woodlice raises some questions which are of importance with reference to the classification of the Arthropoda as a whole. The Six-legged Insects, most Spiders and many of their allies, the Centipedes and Millipedes, and the worm-like _Peripatus_, all breathe air by means of fine tubes which penetrate throughout the body, and bring the air into close contact with the tissues. These tubes, which are known as "tracheæ," arise as ingrowths of the outer layer of the embryo, and are lined by a delicate continuation of the external cuticle. It has been held by some zoologists that so peculiar a system of breathing organs must indicate a common descent of the animals that possess them, and accordingly it has been proposed to separate the Insects, Arachnids, Myriopods, and _Peripatus_, as a group, Tracheata, from the Crustacea and some other Arthropods which have no tracheæ. The air-tubes of the Woodlice, however, are precisely like tracheæ in structure and function, and only differ from the tracheæ of the other groups in the fact that they are confined to the appendages, and do not penetrate into the body. Since the Woodlice are a small and highly specialized branch of the Crustacea, we can hardly suppose that they derive their tracheæ from any ancestral type which they had in common with the widely different Arachnids, for example; and if tracheæ have been evolved independently in these two groups, there seems no reason why those of the Insects may not have arisen independently of either. This is only one example out of many which go to show that, in attempting to reconstruct the genealogy, or phylogeny, as it is called, of the animal kingdom, we must constantly admit the possibility of "convergent evolution."