• Webster's Revised Unabridged Dictionary
    • n. pl Acephala (Zoöl) That division of the Mollusca which includes the bivalve shells, like the clams and oysters; -- so called because they have no evident head. Formerly the group included the Tunicata, Brachiopoda, and sometimes the Bryozoa. See Mollusca.
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Century Dictionary and Cyclopedia
    • acephala A term introduced by Cuvier into systematic zoölogy, and applied by him as a class name to a combination of the conchiferous lamellibranchiate mollusks and the tunicates. Later writers apply it to the lamellibranchiate mollusks alone, which constitute a natural class, distinguished by Lamarck as the Conchifera. All the ordinary bivalves belong to this class. The Acephala or Acéphales of Cuvier were at first (1789) the third order of Mollusca, and included cirripeds, tunicates, and brachiopods with ordinary bivalve mollusks, being thus equivalent to Cirripedia, Tunicata, and Conchifera of Lamarck. In 1804 Cuvier excluded the cirripeds and brachiopods, and made Acephala a class of Mollusca. In the “Règne Animal” (1817–1829) Acephala are Cuvier's fourth class of Mollusca, with two orders, Acephala testacea, or shelled acephals, the ordinary bivalve mollusks, and Acephala nuda, or shell-less acephals, the tunicates.
    • acephala Same as Acrania.
    • acephala In Latreille's system of classification (1795), one of seven orders of the Linnean Aptera, containing the spiders, etc., corresponding to the Arachnides palpistes of Lamarck, and synonymous with Arachnida.
    • acephala In Haeckel's classification, a group of Mollusca composed of the Spirobranchia, or Brachiopoda, and the Lamellibranchia.
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Webster's Revised Unabridged Dictionary
NL., fr. Gr. , adj. neut. pl., headless. See Acephal