Irreversibility rule see Dollos rule

irritability n. [L. irritare, to provoke] Ability to receive external impressions and the power to react to them.

irritant n. [L. irritare, to provoke] Any external stimulus that can provoke a response.

irrorate a. [L. in, not; roris, dew] Covered with minute marks, colors, or minute grains or specks of color.

isauxesis n. [Gr. isos, equal; auxesis, growth] Equality in growth; isometry. see bradyauxesis, heterauxesis, tachyauxesis.

ischia pl. of ischium ischiocerite n. [Gr. ischion, hip; keras, horn] (ARTHRO: Crustacea) Third segment of an antennal peduncle.

ischiomerus a. [Gr. ischion, hip; meros, part] (ARTHRO: Crustacea) Refers to the third (ischium) and fourth (merus) segments of subchelate anterior appendages.

ischiopod(ite) n. [Gr. ischion, hip; pous, foot] (ARTHRO) The third segment of a generalized limb; the second trochanter, or second segment of the telopodite; prefemur. see ischium.

ischium n.; pl. ischia [Gr. ischion, hip] (ARTHRO: Crustacea) The third segment of a pereopod, or first segment of an endopod articulating with the basis; an ischiopodite.

islet n. [L. dim. insula, an island] A spot in a plaga differing in color.

isoallele n. [Gr. isos, equal; allelon, one another] An allele whose effect can only be distinguished from that of a normal allele by special techniques.

isobilateral a. [Gr. isos, equal; L. bis, twice; latus, side] Having bilateral symmetry where a structure can be divisible in two planes at right angles.

isobrachial a. [Gr. isos, equal; brachion, arm] A chromosome in which the centromere occupies the median position.

isochela n. [Gr. isos, equal; chele, claw] 1. A chela with two like parts. 2. (PORIf) A diactinal microsclere with like recurved hooks, plates, flukes or anchor shaped at each end. see anisochela.

isochromosome a. [Gr. isos, equal; chromos, color; soma, body] Monocentric or dicentric chromosome with equal and genetically identical arms which are mirror images.

isocies n.pl. [Gr. isos, equal; L. socius, companion] A group of associated organisms with differing taxonomic affinities, at times used merely in the sense of habitat groups. see associes, consocies, subsocies.

isocytous a. [Gr. isos, equal; kytos, container] Having cells of equal size or height.

isodactylous a. [Gr. isos, equal; daktylos, finger] Bearing digits of equal size.

isodiametric a. [Gr. isos, equal; dia, through; metron, measure] Having equal diameters or axes.

isodictyal a. [Gr. isos, equal; dictyon, net] (PORIF) Pertaining to a type of skeletal construction with spicules and/or fibers interlocking in a regular triangular pattern.

isodont a. [Gr. isos, equal; odous, tooth] (MOLL: Bivalvia) With hinge teeth arranged symmetrically; homodont.

isoenzyme n. [Gr. isos, equal; en, in; zyme, yeast] An enzyme differing in polymorphic states and isoelectric point, but having the same function; an isozyme.

isogametes n. [Gr. isos, equal; gamete, spouse] Outwardly similar male and female gametes.

isogamy n. [Gr. isos, equal; gamos, union] The mutual fertilization process of isogametes.

isogenes n. [Gr. isos, equal; genos, race] Lines on a gene map that connect points of identical gene frequency.

isogenic a. [Gr. isos, equal; genos, race] A group of individuals that have the same genotype.

isoglottid a. [Gr. isos, equal; glottis, mouth of the windpipe] (nemata) Having metarhabdions situated at the same level. see anisoglottid.

isograft n. [Gr. isos, equal; graphion, stylus] Tissue graft between animals of the same genotype.

isolate n. [L. insula, island] A breeding population or group of populations isolated from other populations by physiological, behavioral, or geographic barriers.

isolation n. [L. insula, island] Separation from similar forms.

isolecithal egg An ova with yolk granules randomly distributed through the cell; a small amount of yolk; an oligolecithal egg. see centrolecithal egg.

isomer n. [Gr. isos, equal; meros, part] Compounds of the same chemical composition but with different structures.

isomerases n.pl. [Gr. isos, equal; meros, part; -asis, enzyme] Enzymes which convert one chemical compound to another; said to be isomeric compounds.

isomeric a. [Gr. isos, equal; meros, part] Equivalent genes which can each produce the same phenotype.

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