An arthropod is a member of the phylum Arthropoda, the largest in the animal kingdom, which includes such familiar forms as lobsters, crabs, spiders, mites, insects, centipedes, and millipedes. About 84% of known species of animals are members of this phylum. Arthropods are represented in every habitat on Earth and show a great variety of adaptations. Several types live in aquatic environments, and others reside in terrestrial ones; some groups are even adapted for flight.
The distinguishing feature of arthropods is the presence of a jointed 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). Arthropods lack locomotory cilia, even in the larval stages, probably because of the presence of the exoskeleton. The body is usually segmented, and the segments bear paired, jointed appendages, from which the name arthropod (â€œjointed feetâ€) is derived. About one million 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 may also represent only a fraction of the existing number.
The phylum Arthropoda is commonly divided into four subphyla of extant forms: Chelicerata, Crustacea, Hexapoda, and Myriapoda. Some zoologists believe that arthropods possessing only single-branched appendages, particularly the insects, centipedes, and millipedes, evolved from a separate ancestor and therefore group them within a separate phylumâ€”the Uniramia, or Atelocerata; however, in this treatment these forms are dispersed among several subphyla. In addition, the phylum Arthropoda contains the extinct subphylum Trilobitomorpha. This group is made up of the trilobites, the dominant arthropods in the early Paleozoic seas (542 million to 251 million years ago). Trilobites became extinct during the Permian Period (299 million to 251 million years ago) at the end of the Paleozoic Era.
The myriapods (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.