ARTHROPODS

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    The distinguishing feature of arthropods is the presence of a skeletal covering composed of chitin (a complex sugar) bound to protein. This nonliving exoskeleton is secreted by the underlying epidermis (which corresponds to the skin of other animals). The body is usually segmented, and the segments bear paired, jointed appendages, from which the name arthropod ("jointed feet") is derived. More than 879,000 arthropod species have been described, of which most are insects. This number, however, may be only a fraction of the total. Based on the number of undescribed species collected from the treetops of tropical forests, zoologists have estimated the total number of insect species alone to be as high as 10,000,000. The 30,000 described species of mites, another group of arthropods, may also represent only a fraction of the existing number.
    The phylum Arthropoda may be divided into four subphyla: Trilobita, Chelicerata, Crustacea, and Uniramia. The subphylum Trilobita contains only the trilobites, which were the dominant arthropods in the early Paleozoic seas (540 to 245 million years ago) but became extinct during the Permian Period (286 to 245 million years ago), at the end of the Paleozoic Era.
Most members of the subphylum Chelicerata belong to the class Arachnida, containing the spiders, scorpions, ticks, and mites. They are largely terrestrial arthropods, living beneath stones and logs, in leaf mold, and in vegetation, but there are some aquatic mites that live in fresh water and in the sea. There are also many parasitic mites. Two small classes of chelicerates, the Merostomata, containing the horseshoe crabs, and the Pycnogonida, containing the sea spiders, are entirely marine. The merostomes are an ancient group and probably gave rise to the arachnids. Indeed, the earliest known fossil scorpions were aquatic.
    The subphylum Crustacea contains mostly marine arthropods though many of its members, such as the crayfish, have invaded fresh water, and one group, the pill bugs (sow bugs) has become terrestrial, living beneath stones and logs and in leaf mold. In the sea, large crustaceans such as crabs and shrimps are common bottom-dwelling arthropods. Many minute species of crustaceans are an important component of the zooplankton (floating or weakly swimming animals) and serve as food for other invertebrates, fishes, and even whales.
    Uniramia is the largest of the arthropod subphyla. It contains not only the class Insecta but also four closely related classes of long-bodied arthropods collectively known as myriapods: class Chilopoda (centipedes), class Symphyla (symphylans), class Diplopoda (millipedes), and class Pauropoda (pauropods). They are mostly terrestrial and, in contrast to the other arthropod subphyla, the uniramians are believed to have had a terrestrial origin. Centipedes, symphylans, millipedes, and pauropods live beneath stones and logs and in leaf mold; insects are found in all types of terrestrial habitats and some have invaded fresh water. The sea has remained the domain of the crustaceans, however, and only at its very edges are insects found.