organs of Tomosvary (ARTHRO) A pair of sensory organs present on the head at the base of the antennae in Lithobio-morpha, Scutigeromorpha and some Insecta, consisting of a disc with a central pore into which the endings of subcuticular sensory cells converge; temporal organs.
organs of Valenciennes (MOLL: Cephalopoda) Paired lamel-lated organs in female nautiloids.
orichalceous a. [L. aurum, gold; Gr. chalkos, copper] A color or luster between gold and brass.
Oriental Realm A zoogeographical region including Asia east of the Indus River, south of the Himalayas and the Yang-tse-kiang watershed, Ceylon, Sumatra, Java and the Philippines.
orientation n. [L. oriens, the rising sun] Sense of direction; a change in position.
orifice n. [L. os, mouth; facere, to make] 1. An opening into a cavity; a mouth-like opening. 2. (ARTHRO: Crustacea) In sessile Cirripedia, the opening in the wall occupied by the operculum. see aperture. 3. (BRYO) The opening on the margin of the orificial wall through which the lophophore passes.
orificial wall (BRYO) 1. In Gymnolaemata, an exterior zooidal wall that bears or defines the orifice through which the lophophore passes. 2. In Stenolaemata, an orifice through which the tentacles protrude.
original description A statement of characters along with the proposal of a name for a new taxon.
orismology n. [Gr. horos, boundary; logos, discourse] The science of defining technical or scientific words of a particular subject or field of study.
ornamentation n. [L. ornare, to adorn] Sculpturing on the body of an animal or shell.
ornate cercaria (PLATY: Trematoda) A larval form in the Xiphidiocercaria group, with a tail fin fold; cercariae orna-tae.
orphan nest (ARTHRO: Insecta) In social insects, a nest containing offspring without adults.
orthocerous condition (ARTHRO: Insecta) In some adult Coleoptera, antennae showing no sign of geniculation, the scape being longer than succeeding segments, and the club loose and three-segmented. see gonatocerous condition.
orthochromatic a. [Gr. orthos, straight; chromos, color] With normal staining characteristics.
orthochromatin n. [Gr. orthos, straight; chromos, color] Stable chromatin.
orthocline a. [Gr. orthos, straight; clinein, to lean] (MOLL) At right angles to the growth direction of the helicocone, especially in oysters; growth lines.
orthodont hinge (MOLL: Bivalvia) A hinge in which the teeth approximate the direction of the cardinal margin.
orthodromic a. [Gr. orthos, straight; dromos, running] Moving in a normal direction. see antidromic.
orthogenesis n. [Gr. orthos, straight; genesis, beginning] Evolution following a predetermined rectilinear pathway, independent of natural selection.
orthognathous a. [Gr. orthos, straight; gnathos, jaw] Having straight mouth parts; not projecting.
orthogyral, orthogyrate a. [Gr. orthos, straight; gyrate, revolve] (MOLL: Bivalvia) Having the beak point at right angles to the hinge axis, especially oysters.
orthokinesis n. [Gr. orthos, straight; kinesis, movement] A non-directional response in which the speed or frequency of activity depends on the intensity of stimulation. see kli-nokinesis.
orthometaneme n. [Gr. orthos, straight; meta, after; nema, thread] (NEMATA) A metaneme parallel to the longitudinal body line; found at the dorsal or ventral border of the lateral epidermal cords.
Orthonectida, orthonectids n.; n.pl. [Gr. orthos, straight; nekton, swimming] A class of Mesozoa with an asexual parasitic plasmodial generation in many marine invertebrates and a sexual free-swimming generation.
orthoneury n. [Gr. orthos, straight; neuron, nerve] (MOLL: Gastropoda) In forms with bilateral zygoneury, the condition of the visceral loop ganglia and crossed connectives indicating earlier streptoneury are still evident though sometimes reduced. see detorsion.
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