ARTHROP'ODA (Gk. api9pny, arthran, joint + uric, pans, 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 append ages. The brain lies dorsal to the food canal, and is eonneeted with a ventral chain of ganglia. The body is covered with a fine ehitinous cuticle, periodically molted. The Arthropoda are de scended 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 antennae, except when, as in spiders. these are rudimentary or altogether gone. The abdomen is usually devoid of developed ap pendages: when present, these have other func tions than locomotion. Sense organs are usu ally 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 oecur chiefly either as special ized tracts or as divertieula 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, hut partheno genesis occurs. During development repeated moltings (eedyses) occur, often associated with profound changes in form; from molt to limit constitutes a larval 'stage.'
CLAssi•e.vrfox. Class I. Crustacca, divided into L'ntounoatrora, of small size and a variable number of segments, and 3/0/acostroco. usually of large size, with twenty segments in the trunk (excepting one small and rare group. Nebalia). (Inters of Entomostraca: l'hyllopoda, (fairy shrimps), Trilobita ( fossil), Gstracoda, Cope poda ( wa ter-fleas), Cirripedia ( barnacles ) . Chief orders of ,Ilalacostraca : Isopoda (sow-hugs), Amphipoda (beach-fleas). Stomato poda ( mantis shrimps ) Seh izopoda ( shrimps and prawns), De•apoda (lobsters, crayfish, crabs).
Class I I. .4 rod( n ido —Orders : Seorpionida (scorpions) ; I'sendoscorpionida ( hook seor pious) : Pedipalpida (scorpion spiders) : (Galeodes) ; Phalangida (harvestmen) ; Araneida (spiders) : Aearida (ticks and unites) : Xiphosura (king crab).
Class III. Onychopho•o, containing only Peri pattlA.
Class IV. Ilyrio porta (('entipeds, Class V. Insectr--nrdcrs Aptera (spring tails and silver-fish) ; Grthopte•a (cockroaches, grasshoppers. etc i : Neuropte•a (dragon-tlies, May-flies, caddis-flies. etc.); Hemiptera (hugs); Diptera (gnats and flies) ; Lepidoptera (butter flies and moths) ; Coleoptera (beetles); Hytne nolitera (bees, Wit ps. ) .
BIBLIOGRAPHY. Ilefercuces to hooks will beBibliography. Ilefercuces to hooks will be found under various articles deseribing ,groups or species of Arthropoda. Consult as general works, particula•ly the series "Cambridge Natu ral Ilisfory," and Parker and Haswell, of Zoology (London and New York, Iso7) ; Lam. kester, Hutton. and others. "Are the Arthropoda, a Natural Group?" in :Volum/ Science, Vol. XI. (London, ISII7) ; Kingsley. "Classification of the Arth•opoda." in American :Vat ra is t, Vol. XXVIII. ( Philadelphia. IS94 ).