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.
radiolarian
(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.
radioles
(L. radius, ray, spoke of a
wheel). Featherlike processes from the head
of many tubicolous polychaete worms
(phylum Annelida), used primarily for
feeding.
radula
(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.
ratite
(L. ratis, raft). Referring to birds
having an unkeeled sternum; compares with carinate
recapitulation
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.
recessive
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.
redia
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.
reduction
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.
releaser
(L. relaxare, to unloose). Simple stimulus
that elicits an innate behavior pattern.
renin
(L. ren, kidney). An enzyme
produced by the kidney juxtaglomerular
apparatus that initiates changes leading to
increased blood pressure and increased
sodium reabsorption.
rennin
(M.E. renne, to run). A milkclotting
endopeptidase secreted by the
stomach of some young mammals, including
bovine calves and human infants.
replication
(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.
respiration
(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.
rete
mirabile
(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.
reticular
(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.
reticulopodia
(L. retiulum, dim. of rete, net, + podos, pous,
foot). Pseudopodia that branch and rejoin
extensively.
retina
(L. rete, net). The
posterior sensory membrane of the eye that
receives images.
rhabdite
(Gr. rhabdos, rod). Rodlike
structures in the cells of the epidermis or
underlying parenchyma in certain
turbellarians. They are discharged in
mucous secretions.
rheoreceptor
(Gr. rheos, a
flowing, + receptor). A sensory organ of aquatic animals that responds to water
current.
rhinarium
(Gr. rhis, nose).
Hairless area surrounding the nose of a
mammal.
rhinophore
(Gr. rhis, nose, + pherein, to carry). Chemoreceptive
tentacles in some molluscs (opisthobranch
gastropods).
rhopalium
(N.L. from Gr. rhopalon, a club). One of the marginal,
club-shaped sense organs of certain
jellyfishes; tentaculocyst.
rhoptries
(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.
rhynchocoel
(Gr. rhynchos,
snout, + koilos, hollow). In nemerteans,
the dorsal tubular cavity that contains the
inverted proboscis. It has no opening to
the outside.
ribosome
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.
ritualization
In ethology, the evolutionary
modification, usually intensification, of a
behavior pattern to serve communication.
RNA
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.
rostellum
(L. small beak).
Projecting structure on scolex of tapeworm,
often with hooks.
rostrum
(L. ship's beak). A
snoutlike projection on the head.
rumen
(L. cud). The large first
compartment of the stomach of ruminant
mammals.
ruminant
(L. ruminare, to chew
the cud). Cud-chewing artiodactyl mammals
with a complex four-chambered stomach.
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