ARTHROP'ODA (Gk. ἄρθρον, arthron, a joint + πους, pous, foot). One of the phyla of animals. They are bilaterally symmetrical, and the body is divided into segments, of which each typically carries a pair of jointed appendages. The brain lies dorsal to the food canal, and is connected with a ventral chain of ganglia. The body is covered with a fine chitinous cuticle, periodically molted. The Arthropoda are descended from the Annulata, from which they differ chiefly in the jointed appendages, which are used largely as walking, instead of merely swimming, appendages, and they have advanced beyond the annelids in the specialization of the segments and appendages, so that in general, head, thorax, and abdomen may he distinguished. The head always bears a pair of jaws and at least one pair of antennæ, except when, as in spiders, these are rudimentary or altogether gone. The abdomen is usually devoid of developed appendages; when present, these have other functions than locomotion. Sense organs are usually well developed, except in the parasitic forms. The eyes are either simple and placed directly above the brain, or compound, and are sometimes placed on movable stalks. The mouth is usually provided with one or more pairs of appendages; the intestine may be coiled, and excretory organs occur chiefly either as specialized tracts or as diverticula of the midgut, or as tubular special glands. An incomplete blood system is present, but veins are often lacking, and the blood returns through the general body spaces. The dorsal vessel functions as a heart, and may become very short and specialized. Reproduction is usually bisexual, but parthenogenesis occurs. During development repeated moltings (ecdyses) occur, often associated with profound changes in form; from molt to molt constitutes a larval ‘stage.’
Classification. Class I. Crustacea, divided into Entomostraca, of small size and a variable number of segments, and Malacostraca, usually of large size, with twenty segments in the trunk (excepting one small and rare group, Nebalia). Orders of Entomostraca: Phyllopoda (fairy-shrimps), Trilobita (fossil), Ostracoda, Copepoda (water-fleas), Cirripedia (barnacles). Chief orders of Malacostraca: Isopoda (sow-bugs), Amphipoda (beach-fleas), Cumacea, Stomatopoda (mantis shrimps), Schizopoda (shrimps and prawns), Decapoda (lobsters, crayfish, crabs).
Class II. Arachnida — Orders: Scorpionida (scorpions); Pseudoscorpionida (book scorpions); Pedipalpida (scorpion spiders); Solpugida (Galeodes); Phalangida (harvestmen); Araneida (spiders); Acarida (ticks and mites); Niphosura (king crab).
Class III. Onychophora, containing only Peripatus.
Class IV. Myriapoda (Centipeds, etc.).
Class V. Insecta — Orders: Aptera (spring-tails and silver-fish): Orthoptera (cockroaches, grasshoppers, etc); Neuroptera (dragon-flies, May-flies, caddis-flies, etc.); Hemiptera (bugs); Diptera (gnats and flies); Lepidoptera (butterflies and moths); Coleoptera (beetles); Hymenoptera (bees, wasps, etc.).
Bibliography. References to books will be found under various articles describing groups or species of Arthropoda. Consult as general works, particularly the series “Cambridge Natural History,” and Parker and Haswell, Textbook of Zoölogy (London and New York, 1897); Lankester, Hutton, and others, “Are the Arthropoda a Natural Group?” in Natural Science, Vol. XI. (London, 1897); Kingsley, “Classification of the Arthropoda,” in American Naturalist, Vol. XXVIII. (Philadelphia, 1894).