antithetic generation Alternation of generations in which the alternates are very different in appearance and origin.
antitoxin n. [Gr. anti, against; toxikon, poison] Any substance that neutralizes a toxin.
antitype n. [Gr. anti, against; typos, type] 1. An opposite type; a countertype. 2. A corresponding specimen of a type species, obtained at the same time and location of the type. This definition has no standing in the ICZN.
antizoea n. [L. anti, against; zoe, life] (ARTHRO: Crustacea) The first larval stage of the large, carnivorous marine mantis shrimps of the superorder Hoplocarida, that lack the raptorial claws. see pseudozoea.
antlered larvae (ARTHRO: Insecta) In some newly hatched Lepidoptera larvae, antler-like horns on the first thoracic segment and other horns (scoli) on the abdominal segments.
antlia n. [Gr. antlia, pump] (ARTHRO: Insecta) The spiral, tubular proboscis.
antrorse a. [L. antero, anterior; versus, turned] Directed or leaning upward or forward. see detrorse, retrorse.
antrum n. [L. antrum, hollow] 1. A hollow space or cavity. 2. A
anucleate a. [Gr. an, without; L. nucleus, kernel] Lacking a nucleus.
anural a. [Gr. an, without; oura, tail] Lacking a tail.
anuria n. [Gr. an, without; ouron, urine] Absence of or inability to excrete urine.
anus n. [L. anus] The terminal orifice of the alimentary canal, through which unabsorbed food and waste products are voided; in some groups it is associated with the cloaca. see uropore.
aorta n. [Gr. aorte, the great artery] The main vessel carrying blood from the heart; the dorsal blood vessel.
aortal chamber The thoracic expansion of the aorta.
aortic valve The closure mechanism of the dorsal vessel, separating the aorta and the heart proper.
apatetic color Those colors that enable an organism to mimic either its environment or another species.
apertum n. [L. aperire, to open] (ARTHRO: Insecta) In Coleoptera, a basal cell opening on the hind wing.
apertural a. [L. aperire, to open] Pertaining to or on the same side as the aperture.
apertural muscle (BRYO: Gymnolaemata) One of either two pairs of muscles of cheilostomate autozooids, the occlusor muscle of the operculum or the diaphragmatic dilator muscle.
aperture n. [L. aperire, to open] 1. An opening or hole, cleft, or gap. 2. (ARTHRO: Crustacea) The postero-ventral opening into the mantle cavity of barnacles. see orifice. 3. (BRYO: Stenolaemata) The terminal skeletal opening of a zooid. 4. (MOLL) An opening at the last-formed margin of a shell, providing the outlet for the head-foot mass.
apex n.; pl. apexes, apices [L. apex, tip] 1. That portion of any structure opposite the base of attachment; the tip. 2. (ARTHRO: Crustacea) The upper angle of the scutum or ter-gum of certain barnacles. 3. (ARTHRO: Insecta) Wing tip. 4. (MOLL) a. In Polyplacophora, the beak or umbo of a valve. b. In Gastropoda, the first-formed end of a shell.
aphagia n. [Gr. a, not; phagein, to eat] Unable to ingest.
apharyngeate cercariae (PLATY: Trematoda) Larvae that develop in daughter sporocysts in pulmonate or prosobranch snails.
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