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  Section: Zoological Terms used in General Zoology
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R - Zoological Terms

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radial canals

Canals along the ambulacra radiating from the ring canal of echinoderms; also choanocyte-lined canals in syconoid sponges.

radial cleavage

Embryonic development in which early cleavage planes are symmetrical to the polar axis, each blastomere of one tier lying directly above the corresponding blastomere of the next layer; indeterminate cleavage.

radial symmetry

A morphological condition in which the parts of an animal are arranged concentrically around an oralaboral axis, and more than one imaginary plane through this axis yields halves that are mirror images of each other.


(L. radius, ray, spoke of a wheel, + Lar, tutelary god of house and field). Members of the classes Acantharea, Phaeodarea, and Polycystinea (phylum Sarcomastigophora) with actinopodia and beautiful tests.


(L. radius, ray, spoke of a wheel). Featherlike processes from the head of many tubicolous polychaete worms (phylum Annelida), used primarily for feeding.


(L. scraper). Rasping tongue found in most molluscs.

Ras protein

A protein that initiates a cascade of reactions leading to cell division when a growth factor is bound to the cell surface. The gene encoding Ras becomes an oncogene when a mutation produces a form of Ras protein that initiates the cascade even in the absence of the growth factor.


(L. ratis, raft). Referring to birds having an unkeeled sternum; compares with carinate


Summing up or repeating; hypothesis that an individual repeats its phylogenetic history in its development.

receptor-mediated endocytosis

Endocytosis of large molecules, which are bound to surface receptors in clathrin-coated pits.


An allele that must be homozygous for the allele to be expressed.

recombinant DNA

DNA from two different species, such as a virus and a mammal, combined into a single molecule.


pl. rediae (re´de-e) (from Redi, Italian biologist). A larval stage in the life cycle of flukes; it is produced by a sporocyst larva, and in turn gives rise to many cercariae.


In chemistry, the gain of an electron by an atom or molecule of a substance; also the addition of hydrogen to, or the removal of oxygen from, a substance.

regulative development

Progressive determination and restriction of initially totipotent embryonic material.


(L. relaxare, to unloose). Simple stimulus that elicits an innate behavior pattern.


(L. ren, kidney). An enzyme produced by the kidney juxtaglomerular apparatus that initiates changes leading to increased blood pressure and increased sodium reabsorption.


(M.E. renne, to run). A milkclotting endopeptidase secreted by the stomach of some young mammals, including bovine calves and human infants.


(L. replicatio, a folding back). In genetics, the duplication of one or more DNA molecules from the preexisting molecule.

reproductive barrier

(L. re + producere, to lead forward; M.F. barriere, bar). The factors that prevent one sexually propagating population from interbreeding and exchanging genes with another population.

repugnatorial glands

(L. repugnare, to resist). Glands secreting a noxious substance for defense or offense, for example, as in the millipedes.


(L. respiratio, breathing). Gaseous interchange between an organism and its surrounding medium. In the cell, the release of energy by the oxidation of food molecules.

restriction endonuclease

An enzyme that cleaves a DNA molecule at a particular base sequence.



(L. wonderful net). A network of small blood vessels so arranged that the incoming blood runs countercurrent to the outgoing blood and thus makes possible efficient exchange between the two bloodstreams. Such a mechanism serves to maintain the high concentration of gases in the fish swim bladder.


(L. reticulum, small net). Resembling a net in appearance or structure.

reticuloendothelial system

(L. reticulum, dim. of net, + Gr. endon, within, + thele, nipple). The fixed phagocytic cells in the tissues, especially the liver, lymph nodes, spleen, and others; also called RE system.


(L. retiulum, dim. of rete, net, + podos, pous, foot). Pseudopodia that branch and rejoin extensively.


(L. rete, net). The posterior sensory membrane of the eye that receives images.


(Gr. rhabdos, rod). Rodlike structures in the cells of the epidermis or underlying parenchyma in certain turbellarians. They are discharged in mucous secretions.


(Gr. rheos, a flowing, + receptor). A sensory organ of aquatic animals that responds to water current.


(Gr. rhis, nose). Hairless area surrounding the nose of a mammal.


(Gr. rhis, nose, + pherein, to carry). Chemoreceptive tentacles in some molluscs (opisthobranch gastropods).


(N.L. from Gr. rhopalon, a club). One of the marginal, club-shaped sense organs of certain jellyfishes; tentaculocyst.


(Gr. rhopalon, club, + tryo, to rub, wear out). Club-shaped bodies in Apicomplexa composing one of the structures of the apical complex; open at anterior and apparently functioning in penetration of host cell.


(Gr. rhynchos, snout, + koilos, hollow). In nemerteans, the dorsal tubular cavity that contains the inverted proboscis. It has no opening to the outside.


Subcellular structure composed of protein and ribonucleic acid. May be free in the cytoplasm or attached to the membranes of the endoplasmic reticulum; functions in protein synthesis.


In ethology, the evolutionary modification, usually intensification, of a behavior pattern to serve communication.


Ribonucleic acid, of which there are several different kinds, such as messenger RNA, ribosomal RNA, and transfer RNA (mRNA, rRNA, tRNA).

RNA world

Hypothetical stage in the evolution of life on earth in which both catalysis and replication were performed by RNA, not protein enzymes and DNA.


(L. small beak). Projecting structure on scolex of tapeworm, often with hooks.


(L. ship's beak). A snoutlike projection on the head.


(L. cud). The large first compartment of the stomach of ruminant mammals.


(L. ruminare, to chew the cud). Cud-chewing artiodactyl mammals with a complex four-chambered stomach.

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