All these appendages are made up of segments, each of which, like those of the body itself, is surrounded by a ring of hardened skin, and connected with its neighbours by a flexible integument that allows perfect freedom of movement; while within are the muscles, often very powerful, by which the appendage is moved. In the arthropods we have a sub-kingdom of highly organised animals, with distinct, and often very complicated, systems of organs for digestion, circulation, and respiration; and the nervous system consists of a well-developed chain of ganglia, connected by nerve cords, and from which nerve fibres are distributed to the various parts of the body. It should be noted, however, that some members of the group have degenerated into parasites, and in these, as with all such degraded creatures, many of the organs have retrogressed to such an extent that they are quite functionless, or have even disappeared entirely. These parasitic forms, when very young, are really highly organised creatures, not unlike the young of their industrious and more noble relatives; but, as the natural result of their degraded mode of living, in which they find no use for their organs of locomotion, digestion, circulation and respiration, these eventually disappear, with the result that the organs of reproduction predominate to such an extent that they often fill the greater part of the cavity of the body. It should be noted, too, that the sense organs of arthropods are well developed, most of them being supplied with complex eyes, hearing organs, and highly sensitive feelers. This sub-kingdom consists of four classes--the _Crustacea_, including lobsters, crabs, shrimps, prawns, &c.; _Arachnoidea_, containing spiders, mites, and scorpions; _Myriopoda_--centipedes and millepedes; and _Insecta_. [Illustration: FIG. 188.--THE NERVE-CHAIN OF AN ARTHROPOD (LOBSTER) _o_, optic nerve; _c_, cerebral ganglion; _i_, large ganglion behind the œsophagus; _th_, ganglia of the thorax; _ab_, ganglia of the abdomen] The first of these classes consists mainly of marine animals, and will therefore occupy much of our attention, but the members of the other three are mostly terrestrial and aërial creatures that do not fall within the scope of this work, except in the case of a few species that are more or less decidedly marine in their tendencies. The aquatic members are generally provided with well-formed gills by means of which they are enabled to extract the dissolved oxygen from the water in which they live, while those of terrestrial and aërial habits breathe by means of a system of tracheæ or air-tubes that are open to the air and supply branches to all parts of the body.