Fig. 13.—A Centipede, Lithobius elongatus, from Tunis, slightly reduced in size.
What are theoretically regarded as legs, however, may practically be turned to many other uses, according to the position of the particular body-ring to which they are attached. Thus, in the case of a body-ring near the mouth, we find such things as "jaw-feet," maxillipedes—that is to say, legs used for jaws. It consequently results that zoologists are sometimes driven to speak of "walking legs," or, hiding the tautology under a Latin phrase, "ambulatory legs"; and absurd although this may seem, it is sometimes quite necessary for the sake of accuracy. It is therefore more convenient to speak of the "appendages" of a body-ring than of its legs. For this vague term can be applied equally to all the row, whatever their uses. Among the different forms taken by the "appendages" are those of "antennæ," long, hair-like feelers attached to the head; "chelæ," or claws, such as the large claws of the lobster; "cheliceræ," or "claw-horns," tearing appendages attached to the head; "mandibles," mouth appendages used for biting, etc., etc. The reader who wishes to attain a clear idea of the structure of a segmented animal, and of the ways in which its parts are modified, should consult Huxley's classical study of "The Crayfish" (International Science Series).
The Arthropoda include two main groups—the Crustacea, or Jointed Animals of the water, which breathe by gills; the Insects, or Jointed Animals of the land, which breathe through tubes in their sides, called tracheæ.
Fig. 14.—Shell of the Bell Barnacle, Balanus tintinnabulum, one-half the natural size. The figure shows several successive generations, perched one upon another.
The Crustacea include the familiar Crabs and Lobsters. These are among their highest forms as well as their largest, and if we begin at the beginning we must seek much smaller forms. The group called Entomostraca include the so-called Freshwater Flea, a very active little thing found in English ditches, and a great many other freshwater forms: also the little Cypris, which has a shield forming a sort of bivalve-shell, and is interesting from its wide occurrence as a fossil form. Most of the Entomostraca have a larval form called a Nauplius; but this larva refuses to tell us anything about the past history of the Arthropods. It is itself already a jointed animal with legs. So we see that the Arthropods, unlike the worms and the Chordata, have obliterated all record of their poor relations. The parasitic "fish-lice," so-called, are entomostracous Crustacea, often greatly degenerated in consequence of their habit of life. Some live in the gill-chambers of a fish, some on, or even embedded in the skin.
Among the most curiously modified forms of the Crustacea are the Barnacles or Cirripedia. These creatures, like the sponges, have a free-swimming larvæ, which eventually fixes itself by its anterior end, so that the adult animal passes its existence upside down. The young is an ordinary little creature with jointed legs, but the adult protects itself by a strange armour of shell. An intermediate stage exists in which the creature eats no food; it has therefore been compared with the chrysalis of insects. At the top of the adult shell two little valves open and shut, allowing the legs to dart out and seize upon prey. These legs, gathered into a bunch, and extended and retracted together, remind one of the fingers of a hand opening and closing. They are clothed with a fringe of "cirrhi" or small processes; hence the name of the group. The Common Barnacle of our own shores, sometimes called the Acorn-Shell, is found on shells and stones, and often on those that are left uncovered between tides. In places where the rocks of the coast are very steep, a belt of white, several feet or yards deep, may often be seen above low-water mark. This white zone, when examined more nearly, is found to consist of barnacles, so crowded together that they obscure the natural colour of the rock. The Common Barnacle is one of the smaller species of the genus: in warmer seas barnacles attain to a much greater size (Figs. [14] and [15]).
The higher Crustacea, Malacostraca, include the familiar Crabs and Lobsters, Decapoda. The lobsters receive the name of Macrura or Big-tails; associated with them are the Shrimps and the Hermit-Crabs ([Fig. 16]). The latter are therefore not crabs at all, but somewhat divergent lobsters. Their tails are soft, and they thus require protection: they choose the dried shell of some univalve mollusc and live in it ([Fig. 16]). How far the case is that they need a house because their tails are soft, and how far the contrary is true that their tails are soft because they live in a house, it would be difficult to say. Readers of another volume in this series, Professor Hickson's "Story of Animal Life in the Sea," will remember that the hermit-crab often offers a curious instance of "commensalism" or partnership with other animals. The hermit-crab was, in fact, one of the earliest instances in which such a partnership was observed, the companion being in this case a sea-anemone perched on the shell in which the crab lives.