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."
Although Woodlice are very common animals, comparatively little is known of their habits. They seem to live chiefly on vegetable food, and sometimes damage seedlings and tender plants in gardens and greenhouses, but occasionally they are carnivorous, and even cannibalistic, in their habits. A few species live as "guests" in ants' nests, and one of these, the little blind white Platyarthrus hoffmannseggii, is common in many localities in this country. Why the ants tolerate their presence we do not know, for they do not seem to render any service to their hosts, as do the plant-lice and some other insects that are kept by the ants for the sake of the secretions which they yield.
The Woodlice, like some other Isopoda, have a peculiar method of moulting. Instead of the whole exoskeleton being cast off at one time, as in other Crustacea, that of the hinder half of the body is moulted first, and it is only after two or three days, when the new cuticle has hardened, that the exoskeleton of the anterior half follows. As a result of this arrangement, it occasionally happens that specimens are found with the fore part of the body differing in colour from the hind part, owing to the one having been moulted more recently than the other.
Woodlice occur in most regions of the globe, and one of the most remarkable features of their geographical distribution is the extremely wide range of certain species. This is probably due, at least in many cases, to their accidental transport by human agency. Thus, Porcellio scaber, so common in this country, is also found in great abundance in New Zealand; but Professor Chilton notes that it is usually found near buildings, and only rarely in the native bush, so that there can be no doubt that it has been introduced by artificial means.
[CHAPTER X]
CRUSTACEA AS PARASITES AND MESSMATES
The life of every animal is in more or less intimate relation with that of all the living creatures which surround it. Some serve for its food, or supply it with shelter or foothold; others prey upon it, or compete with it for the necessaries of life; and others, again, influence it for good or evil in countless ways more subtle than these, but equally important. There are some associations of a closer and more enduring nature, to which the names of Symbiosis, Commensalism, and Parasitism, are applied, and it is with examples of these that the present chapter is concerned.
The term Symbiosis is strictly applied to an intimate physiological partnership, such as we find in some of the lower animals and plants, and in this sense there are no truly symbiotic Crustacea. The word, however, is sometimes used, in its literal sense of a "living together," to embrace all cases of animals living together for mutual advantage. Commensalism means, literally, "sitting at the same table," and ought to be applied only to cases where two or more animals, living together as "messmates," partake of the same food; but it is sometimes used more loosely to include instances where one of the animals does not actually share in the food-supply of the other. Parasitism, again, implies that the parasite lives permanently at the expense of its host, by sucking its juices or otherwise, and in this case also there are innumerable degrees and varieties of dependence, which defy inclusion in a strictly logical scheme of classification. Even such typical parasites as Tape-worms, for example, might strictly be regarded as commensals, sharing in the host's food only after it has entered the alimentary canal. Finally, in all these kinds of interrelation, we find cases where the association is temporary, intermittent, or almost accidental, and where there are no perceptible adaptations of structure directed to its maintenance in either of the partners. From these we may trace a series of gradations leading to cases where the associated organisms are never found apart, and where the structure of both is profoundly modified in adaptation to the particular form of association.