Bacillary layer see brush border

bacilliform a. [L. bacillum, little stick; forma, shape] 1. Rod-shaped. 2. (ARTHRO: Insecta) In certain male Diptera, refers to a pair of sclerites in the postabdomen joining the hypandrium and epandrium.

back n. [A.S. baec, the rear or dorsal part] The dorsal or upper surface.

back-cross A cross between a heterozygote and one of its parents.

back-mutation The reversion by mutation of a mutant gene to the form from which it was derived.

bacteremia n. [Gr. bakterion, small rod; haima, blood] The presence of bacteria in the hemolymph or blood of invertebrates, without production of harmful toxins or other deleterious effects.

bacteriophagous a. [Gr. bakterion, small rod; phagein, to eat] Feeding on bacteria.

baculiform a. [L. baculum, stick or rod; forma, shape] Appearing rod- or staff-like.

baculite n. [L. baculum, stick or rod] (MOLL: Cephalopoda) A straight shelled ammonite, extinct at the end of the Cretaceous Period.

baenomere n. [Gr. bainein, to walk; meros, part] (ARTHRO: Insecta) The leg-bearing thoracic segment.

baenopoda n. [Gr. bainein, to walk; pous, foot] (ARTHRO: Insecta) The thoracic legs.

baenosome n. [Gr. bainein, to walk; soma, body] (ARTHRO: Insecta) The thorax.

Baer's disc (platy: Trematoda) In Aspidogastrea, the large, ventral sucker. see opisthaptor.

bailer n. [F. baille, a bucket] (ARTHRO: Crustacea) An exopod of the maxilla of crayfish and lobsters that functions in regulating the flow of water in the gill chamber; scaphog-nathite.

balanced lethals Heterozygotes in which a gene mutation or chromosome structural change occurs that blocks normal development and is fatal before sexual maturity. see lethal factor.

balanced load A term describing a decrease in the overall fitness of a population due to the component genes that are maintained in the population because they add to fitness in different combinations, i.e., heterozygotes.

balanced polymorphism A polymorphism maintained in the same breeding population by a selective superiority of the heterozygotes over either type of homozygotes. see transient polymorphism.

balancers n.pl. [L. bis, two; lanx, plate, pan of a balance] (ARTHRO: Insecta) The halteres of Diptera.

balanoid a. [Gr. balanos, acorn; eidos, like] Acorn shaped.

balantin, balantine n. [Gr. balantidion, little bag] (ANN: Oli-gochaeta) Male and prostatic pores in segment xix.

Balbiani's body/nucleus The yolk nucleus.

Balbiani ring (ARTHRO: Insecta) In chironomid Diptera, a large RNA puff on chromosome IV of the salivary glands during larval development.

bald a. [ME. balled, equivalent to ball, white spot] Lacking specific hair or other surface covering.

Baldwin effect The condition in which an organism can stay in a favorable environment, with modification of the pheno-type by mutation and selection, until genetic assimilation has been achieved.

ballonets n.pl. [OHG. balla, a spherical body] (nemata: Se-cernentea) Four inflated areas in the cephalic region, im mediately posterior to the lips, that form a collarette in Gnathostomatidae; head bulb of some authors.

ballooning n. [OHG. balla, a spherical body] (ARTHRO: Cheli-cerata) Flying through the air on silken lines spun by spiders.

band n. [A.S. bindan, band] 1. A transverse marking broader than a line. 2. (ARTHRO: Insecta) a. In Diptera, the chromosome pairing of like chromomeres that produce the giant chromosomes. b. In male Lepidoptera, a descriptive term of a linear series of cornuti.

barb n. [L. barba, beard] 1. Any spine or hair-like bristle with a process projecting obliquely or crosswise from the surface. 2. A spine with teeth pointing backward.

barbate, barbatus a. [L. barba, beard] Tufts or fascicles of hair or short bristles; bearded; having tufts of hair.

barbula n.; pl. -lae [L. dim. barba, beard] (ARTHRO: Insecta) In Coleoptera, a tuft of hairs or short bristles at the sides of the abdomen near the anal region of scarabaeoid larvae.

bark n. [Sw. bark, rind] (PORIF) The outer casing of spongin of those having discontinuous fiber structure.

baroceptor, baroreceptor n. [Gr. baros, weight; L. capere, to take] An organ perceiving changes in pressure.

barotropism n. [Gr. baros, weight; tropein, to turn] A response to pressure stimuli.

barrier n. [OF. barre, bar] An obstacle or obstruction that limits the spread or distribution of animals.

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