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).
ARTHUR, Prince, Duke of Connaught. See Connaught, Arthur William Patrick Albert, Duke of.
ARTHUR. A half-legendary king of the
Britons, supposed to have reigned in the Sixth
Century. He was the great national hero of the
British Celts, and became the central figure of
one of the principal cycles of mediæval romance.
Nothing is absolutely known of his history, and
his existence has sometimes been denied
altogether. The more usual view, however, recognizes
at least an historic starting-point for the
great body of tradition that centres in his name.
In accordance with a favorite theory of modern
mythologists, Arthur is often regarded as a
combination of an actual British hero with one or
more ancient Celtic gods. Thus, Professor Rhys,
who connects Arthur's name with an Aryan
root which means ‘to plough,’ thinks that some
elements in his legend belonged originally to a
culture-god described on Continental inscriptions
as Mercurius Artaois, or Mercurius Cultor, while
perhaps other elements were derived from an
old sky-god, a kind of Celtic Zeus.
The usual account of Arthur is briefly as follows: He was the son of Uther Pendragon, King of Britain, and Igerne, the wife of the Duke of Cornwall, whose union was effected by a device of Merlin the Wizard. After the death of the Duke, Uther made Igerne his queen, and in due time Arthur succeeded to the throne. Upon becoming King, Arthur at once took the lead of his people in their wars with the Saxons, and defeated the invaders on every hand. Encouraged by victory, he extended his conquests to Ireland, the Orkneys, and even to Norway and Gaul. Meanwhile he established a great court at Cærleon-on-Usk, where, with his Queen Guinevere (or Guanhumara), he was surrounded by a grand assemblage of knights and kings. After a time a message came from the Emperor of Rome demanding tribute, and Arthur, ably supported by Gawain, conducted a successful expedition against the forces of the Empire. In the midst of his victories on the Continent Arthur was recalled to defend his kingdom and queen from the traitorous Modred, who had seized upon both in his absence. Arthur undertook to put down the rebellion, and in the first battle his forces were victorious, but Gawain was slain. Then, in the battle of Camlan, Modred was defeated and killed, but Arthur himself was grievously wounded and carried off to the Island of Avalon to be healed. The hope was long cherished by the British people that Arthur would some day return and restore them to power.
Much of this narrative is obviously unhistoric, and very little of it can be traced with certainty to sources older than the Ninth Century. The first recorded mention of Arthur is in the Historia Britonum of Nennius, a work which assumed its present shape about 850. He is there described as a dux bellorum, who, along with other leaders of the Britons, fought twelve battles against the Saxons. The Annales Cambriæ (probably written in the Tenth Century) also mention him, giving the year 537 as the date of his death. The Vita Gildæ (usually ascribed to the Twelfth Century) speaks of him as a king of all Britain. But not till we come to the Historia Regum Britanniæ of Geoffrey of Monmouth (written about 1136) do we find the more fully developed story of which an outline has just been given. The question with regard to Geoffrey's sources is very difficult, and may never be fully settled. The later chroniclers
