saccule Small chamber
of the membranous labyrinth of the inner ear.
sacrum Bone formed
by fused vertebrae to which pelvic girdle is attached; pertaining
to the sacrum.
Saefftigen's pouch Internal, muscular sac near
the posterior end of a male acanthocephalan. It contains fluid
that aids in manipulating the copulatory bursa.
sagittal Pertaining to the
median anteroposterior plane that divides a bilaterally symmetrical
organism into right and left halves.
salicylic acid A compound with pain-relieving
characteristics, found in willow bark and other plants; the basic
ingredient of aspirin.
salience In the context of classical conditioning,
the fact that animals can learn to make certain associations,
depending upon which stimuli are involved, more rapidly than other
salinity Amount of dissolved salts (especially
sodium chloride) in a given volume of water.
salinization A process in which mineral salts accumulate
in the soil, killing plants; occurs when soils in dry climates
are irrigated profusely.
saliva The enzyme-containing secretion of the
salivarium Chamber in buccal cone of acarines into
which salivary ducts open.
salivary gland Any of the glands in molluscs, vertebrates,
and other animals that release digestive enzymes into the mouth.
salmon ranching Practice in which cultured juvenile
salmon are released into fresh water and allowed to migrate to
sea so they can be harvested later when they return as adults.
salps Pelagic tunicates with a transparent, cylindrical body, sometimes forming long colonies.
salt The reaction product of an
acid and a base; dissociates in water solution to negative and
positive ions, but not H+ or OH_.
saltatory conduction type of nerve impulse conduction
in which the impulse seems to jump from one neurofibril node to
salt gland An orbital gland of many reptiles and
birds that secretes a hyperosmotic NaCl or KCl solution an important
osmoregulatory organ, especially for marine species.
salt marsh A grassy area that extends along the
shores of estuaries and sheltered coasts in temperate and subpolar
saltwater intrusion Movement of saltwater into
freshwater aquifers in coastal areas where groundwater is withdrawn
faster than it is replenished.
salt wedge A layer of denser, saltier seawater
that flows along the bottom in estuaries .
samara A simple, dry indehiscent fruit with the
pericarp bearing winglike outgrowths; winged fruit of maple.
sand Soil inorganic particles in the .02--2.0 mm
sand dollars Sea urchins with a flat, round test
and short spines that live partly buried in soft sediments.
sand fly Member of the dipteran subfamily Phlebotominae,
family Psychodidae; sometimes also applied to Simuliidae (New
Zealand) and Ceratopogonidae (Caribbean).
sandstone A sedimentary rock composed of sand.
Saphenous saph = clear, apparent. A large superficial
vein of the leg.
saponin A glycoside with a steroid molecule as
the active component, such as diosgenin from yams.
saprobe An organism deriving its food from the
dead body or nonliving products of another organism.
saprophagous Feeding on decaying matter; saprobic; saprozoic.
A plant living on dead or decaying organic matter.
saprophytic Plant living on dead organic matter.
saprozoic nutrition Nutrition of an animal by absorption
of dissolved salts and simple organic nutrients from surrounding
medium. Also refers to feeding on decaying organic matter.
sapwood The functional secondary xylem found between
the vascular cambium and the nonfunctional heartwood in the center
of the trunk or branch.
sarcocystin Powerful toxin produced by zoitocysts
Sarcodina The protozoan subphylum where members
have pseudopodia for movement and food gathering; naked or with
shell or test; mostly free living.
thin, noncellular sheath that encloses a striated muscle fiber.
Sarcomastigophora The protozoan phylum where members
possess flagella, pseudopodia, or both for locomotion and feeding;
single type of nucleus.
sarcomere The contractile unit of a myofibril. The
repeating units, delimited by the Z bands along the length of
clear, semifluid cytoplasm between the fibrils of muscle fibers.
sarcoptic mange Disease caused by mites of the
genus Sarcoptes. Also called scabies.
Sargasso Sea The region of the Atlantic Ocean north
of the West Indies that is characterized by floating masses of
Sargasso weed, a brown alga.
satellite Posterior member of a pair of gregarines
saturated fat A fat in which all the carbons in
the fatty acids are connected by single bonds, thereby having
the maximum number of hygrogen atoms.
saturation light intensity The light intensity
that maximizes the photosynthetic rate.
saturation point The maximum concentration of water
vapor the air can hold at a given temperature.
sauropterygians Mesozoic marine reptiles.
savanna Found between the tropics and deserts,
this vegetative community is composed of extensive grasslands
with scattered deciduous trees.
saxitoxin a paralytic toxic produced by dinophytes that accumulates in the butter clam (Saxadoma)
scabies Disease caused by mites of the genus Sarcoptes.
Also called sarcoptic mange.
scale A thin, compacted, flaky fragment.
scalid A set of complex spines found on the kinorhynchs,
loriciferans, priapulans, and larval nematomorphs, with sensory,
locomotor, food capture, or penetrant function.
scan sampling A method for recording behavior observations
that involves looking briefly at each animal at prescribed time
intervals (e.g., every minute or every five minutes) to record
their activity at the time of the sample.
scanning electron microscope The type of microscope
in which an electron beam, instead of light, forms a three-dimensional
image for viewing, allowing much greater magnification and resolution.
scanning tunneling microscope The type of microscope
that uses a needle probe and electrons to determine the surface
features of specimens.
scaphoid scaph = a boat or a bowl.
Scaphopoda A class of molluscs whose members have
a tubular shell that is open at both ends. Possess tentacles but
no head. Dentalium.
scaphopods See tusk shells.
Scapula The shoulder blade.
scarabaeiform Grublike larvae with lightly sclerotized
cuticle; found in some coleopteran families.
scarification A mechanical or chemical degradation
of a hard surface, such as seed coats, so that oxygen and water
can penetrate the hard layers.
scavenger An animal that feeds on dead organic
scent gland A gland located around the feet, face,
or anus of many mammals; secretes pheromones, which may be involved
with defense, species and sex recognition, and territorial behavior.
schistosomule Juvenile stage of a blood fluke,
between a cercaria and an adult; a migrating form taking the place
of a metacercaria in the life cycle.
schizeckenosy System of waste elimination found
in some mites with a blindly ending midgut; the lobe breaks free
from the ventriculus and is expelled through a split in the posterodorsal
schizocarp A dry indehiscent fruit that splits
into two one-seeded halves at maturity.
schizocoel A coelom formed by the splitting of embryonic
mesoderm. Noun, schizocoelomate, an animal with a schizocoel,
such as an arthropod or mollusc. Adj., schizocoelous.
schizocoelous mesoderm formation Embryonic formation
of the mesoderm as cords of cells between ectoderm and endoderm;
splitting of these cords results in the coelomic space.
schizocoely Coelom formation accomplished by a
split in the mesoderm during embryonic development of some animals
schizogony Form of asexual reproduction in which
multiple mitoses take place, followed by simultaneous cytokineses,
resulting in many daughter cells at once.
schizont Cell undergoing schizogony, in which nuclear
divisions have occurred but cytokinesis is not completed; in its
late phase sometimes called a segmenter.
school a well-defined social organization
of marine animals consisting of a single species with all members
of a similar size
Schüffner's dots Small surface invaginations
that appear as stippling on the membrane of an erythrocyte infected
with Plasmodium vivax after Romanovsky staining.
Schwann cell See neurolemmocyte.
sciatic The hip.
science A methodical, precise, objective way to
study the natural world.
scientific method The objective process of approaching
a problem. Involves hypothesis establishment, testing and observing
the results, reevaluation of the hypothesis in light of new knowledge,
and retesting to seek repeatability and thus validity of the hypothesis.
scientific name A Latinized binomial (genus and
species) unique to each identified organism.
scion A small twig or bud that is grafted to a
sclereid A sclerenchyma cell with a thick, lignified
secondary wall having many pits; variable in form but not usually
sclereids Stone cells found in tissues varying
from pear fruits to the hard shell of nuts.
sclerenchyma The tissue type characterized by thick, sclerified cell walls; includes both fibers and sclereids (stone cells).
sclerenchyma Tissue composed of cells with thick
secondary walls; functioning in support or protection.
sclerite A hard chitinous
or calcareous plate or spicule; one of the plates making up the
exoskeleton of arthropods, especially insects.
An amebocyte specialized to secrete a spicule, found in sponges.
sclerocyte An amebocyte in sponges that secretes spicules.
sclerophyllous Vegetation characterized by thick
leathery leaves with abundant sclerenchyma cells; vegetation of
sclerosponges Sponges with
a massive calcareous skeleton.
sclerotic Pertaining to the
tough outer coat of the eyeball.
sclerotin Highly resistant and insoluble protein
occurring in the cuticle of arthropods; also thought to occur
in structures secreted by various other animals, such as in the
eggshells of some trematodes, in which stabilization of the protein
is achieved by orthoquinone crosslinks between free imino or amino
groups of the protein molecules.
sclerotium A fungal resting body
resistant to unfavorable conditions; a firm, hardened mass of
hyphae (or a hardened plasmodium of a slime mold) that will germinate
on the return of favorable conditions.
sclerotization Process of hardening of the cuticle
of arthropods by the formation of stabilizing cross linkages between
peptide chains of adjacent protein molecules.
scolex The attachment or holdfast organ of a tapeworm,
generally considered the anterior end; it is used to adhere to
scoliosis Lateral curvature of the spine.
scramble polygyny A mating system in which males
actively search for mates without overt competition.
scrotum The pouch that contains the testes
in most mammals.
scrub typhus Rickettsial disease transmitted by
certain chigger mites.
scuba (self-contained underwater breathing apparatus) The use of tanks of compressed air for breathing underwater.
scutellum The single cotyledon in grass seeds.
scutum Large, anteriodorsal sclerite on a tick
scyphistoma The polyp stage of a scyphozoan (phylum
Cnidaria); develops from a planula and produces ephyrae by budding.
Scyphozoa A class of cnidarians whose members have
prominent medusae. Gametes are gastrodermal in origin and are
released to the gastrovascular cavity. Nematocysts are present
in the gastrodermis. Polyps are small. Aurelia.
scyphozoans Cnidarians whose
life cycle includes a conspicuous medusa and a much reduced or
sea A wave that has a sharp peak and a relatively
flat trough. Seas are found in areas where waves are generated
by the wind.
sea anemones Anthozoans that consist of one large
sea cows See sirenians.
sea cucumbers Echinoderms
with a soft, elongate body that lacks spines.
sea lilies See crinoids.
sea slugs See nudibranchs.
sea spiders Arthropods that
have a reduced body and four pair of legs .
sea squirts Tunicates
with a sac-like, attached body as adults.
sea stars Echinoderms
with five or more radiating arms and tube feet that are used in
sea urchins Echinoderms with
a round or flattened test and movable spines.
sea-floor spreading The process by which new
sea floor is formed as it moves away from spreading centers in
seagrasses Grass-like flowering plants such
as eelgrass that are adapted to live at sea.
seamount A submarine volcano in the abyssal plain.
search image The hypothetical mental image of a
prey species that a predator forms as it improves in its ability
to capture that particular species.
seasonal Fluctuations in environmental factors
such as temperature over an annual cycle.
seaweed A common name for various macroalgae.
sebaceous (oil) gland Gland of the skin that secretes
sebum; oil gland.
sebaceous A type
of mammalian epidermal gland that produces a fatty substance.
sebum Oily secretion from the sebaceous gland.
Secernentea The class of nematodes formerly called
Phasmidea. Examples: Ascaris, Enterobius, Necator, Wuchereria.
second law of thermodynamics States that, with
each successive energy transfer or transformation in a system,
less energy is available to do work.
Second World The industrialized, socialist, centrally
planned economy nations of Eastern Europe and the former Soviet
Union and its allies.
secondary cell wall A cellulosic wall, often impregnated
secondary compounds Organic molecules synthesized
by certain species of plants and not thought to be directly involved
in essential metabolism.
secondary consumer An animal that feeds on other
secondary growth Growth derived from secondary
or lateral meristems, the vascular and cork cambiums; secondary
growth results in an increase in diameter or circumference; contrasted
with primary growth, which results in an increase in length.
secondary immune response The immune response that
follows a second exposure to a specific antigen.
secondary lamella a small extension of a gill filament
containing blood capillaries for gas exchange
secondary palate A plate of bone that separates
the nasal and oral cavities of mammals and some reptiles.
secondary phloem All phloem tissue formed by the
vascular cambium in woody tissues.
secondary pollutants Chemicals modified to a hazardous
form after entering the air or that are formed by chemical reactions
as components of the air mix and interact.
secondary product Chemical compounds synthesized
by plants or fungi but not critical for the basic metabolic functions
of that organism; often functioning to deter predators or attract
pollinators; a secondary metabolite.
secondary recovery technique Pumping pressurized
gas, steam, or chemical- containing water into a well to
squeeze more oil from a reservoir.
secondary standards Regulations of the 1970 Clean
Air Act intended to protect materials, crops, visibility, climate,
and personal comfort.
secondary succession Revegetation of cleared land;
return to previous community structure.
secondary treatment Bacterial decomposition of
suspended particulates and dissolved organic compounds that remain
after primary sewage treatment.
secondary wall The innermost layer of a cell wall
formed after cell elongation has ceased; often characterized by
the deposition of lignin.
secondary xylem All xylem tissue formed by the
vascular cambium in woody plants.
secure landfill A solid waste disposal site lined
and capped with an impermeable barrier to prevent leakage or leaching.
Drain tiles, sampling wells, and vent systems provide monitoring
and pollution control.
sedatives Compounds that produce a relaxed, euphoric
state--opium and its derivatives, morphine and heroin, are sedatives.
sedentary Stationary, sitting, inactive; staying
in one place.
sediment Loose material such as sand and mud that
settles on the bottom. Also see biogenous and lithogenous sediments.
sedimentary rock Deposited material that remains
in place long enough or is covered with enough material to compact
into stone; examples include shale, sandstone, breccia, and conglomerates.
sedimentation The deposition of organic materials
or minerals by chemical, physical, or biological processes. Sediments
can be transported from their source to their place of deposition
by gravity, wind, water, or ice. If subjected to sufficient heat,
pressure, or chemical reactions, sediments can solidify into sedimentary
seed A matured ovule containing an embryo and food
supply and covered by a seed coat.
seed bank Storage facilities for seeds of domesticated
plants and wild relatives; facilities for preserving genetic diversity.
seed coat The outer layer of a seed that is developed
from the integuments of the ovule; the testa.
seed ferns Fossil plants with fernlike foliage
but producing seeds belonging to the division Pteridospermophyta.
seeding In mariculture, the release of cultured
juvenile individuals to enrich a natural stock.
seedling The embryonic product of the germination
of a seed; the young shoot and root axis.
seed plant Common term for gymnosperms and angiosperms.
seed-scale complex The spirally arranged scales
on a female strobilus in gymnosperms.
segmentation (1) In many animal species, a series
of body units that may be externally similar to, or quite different
from, one another. (2) The oscillating back-and-forth
movement in the small intestine that mixes food with digestive
secretions and increases the efficiency of absorption.
segmented worms Invertebrates
that display an elongate body with distinct segmentation and a
digestive tract that lies in a coelom.
seismic sea waves See tsunami.
Seisonidea A class of rotifers containing members
that are commensals of crustaceans; large and elongate body with
rounded corona. Example: Seison.
seive cell The organic solute-conducting cell
of the phloem in gymnosperms.
selection differential (S) The difference in mean
phenotypic value for a selected group of breeding animals and
the mean value in the population from which they are drawn.
selection pressure The tendency for natural selection
to occur; natural selection occurs whenever some genotypes are
more fit than other genotypes.
selective cutting Harvesting only mature trees
of certain species and size; usually more expensive than clear-cutting,
but it is less disruptive for wildlife and often better for forest
selective permeability The ability of the plasma
membrane to let some substances in and keep others out.
selectively permeable membrane a membrane that
is permeable to small molecules, usually H2O, O2, and CO2, but
not permeable to larger molecules or ions.
selectively permeable. Permeable to small particles,
such as water and certain inorganic ions, but not to larger molecules.
selectively permeable membrane Living membranes
that allow only certain elements to pass through.
self-incompatibility Condition of a flower
that cannot successfully complete the reproductive process with
pollen produced by its own stamens.
self-pollination Transfer of pollen from stamen
to stigma within the same plant.
self-pollinating plant A plant that has its
own pollen fall on its own stigma.
self-propagating A description of the events
occurring during an action potential, with each regional depolarization
by sodium voltage-gated channels causing a similar event
at an adjacent area downstream.
self-regulating population A population with
a growth rate that is dependent on its own numbers.
self-shading Reduction in the amount of light
available to phytoplankton that live below other phytoplankton.
self-stimulation A situation in which a subject
animal is able to introduce stimuli to its nervous system.
semantic communication Of or relating to meaning
of signals; specifically used to denote the use of different alarm
signals to warn about different predators.
semelparity The production of offspring by an organism
once in its life.
semen The thick, whitish secretion of the reproductive
organs in the male; composed of sperm and secretions from the
prostate, seminal vesicles, and various other glands and ducts.
semicircular canals Bony canals in the vertebrate
inner ear; the hair cells within the canals detect changes in
the direction of movement of the head.
semidiurnal tide A tidal pattern with two high
and two low tides each day.
seminal receptacle A structure in the female reproductive
system that stores sperm received during copulation (e.g., many
insects and annelids).
seminal vesicle 1. One of the paired accessory glands
of the reproductive tract of male mammals. It secretes the fluid
medium for sperm ejaculation (phylum Chordata). 2. A structure
associated with the male reproductive tract that stores sperm
prior to its release (e.g., earthworms_phylum Annelida).
seminiferous tubule The male duct that conveys semen.
Pertains to the tubules that produce or carry semen in the testes.
semipermeable Permeable to small particles, such as
water and certain inorganic ions, but not to larger molecules.
senescence The aging process, usually characterized
by the loss of some functional capacity, including reproduction.
sensation The process of transducing environmental
stimuli or energy into action potentials.
sensilla Modifications of the exoskeleton of an
arthropod that, along with nerve cells, form sensory receptors.
sensillum, A small
sense organ, especially in the arthropods.
sensitive period The time interval when an animal
can develop an imprinting attachment. More broadly, in behavior
development, this refers to time intervals when particular events
must occur for proper ontogenetic sequencing.
sensitization (1) Enhanced responsiveness to a
repeated stimulus. (2) Strengthening of a response that was initially
produced via a CS resulting from a pairing with a US and UR. (3)
A stimulus priming the animal to pay particular attention to what
sensory (afferent) neuron or nerve A neuron or nerve
that conducts an impulse from a receptor organ to the central
sensory adaptation A process that occurs at the
level of the sensory receptors and that consists of a slowing
down or cessation of nerve impulses transmitted to the central
sensory deprivation Withholding all or a specified
portion of the sensory input that an animal would normally be
sensory filter Neural circuits that selectively
transmit some features of a sensory input and ignore other features.
sensory neuron A neuron that is modified to respond
to a particular set of stimuli.
sepal The flower part attached outside the others,
enclosing the flower when in bud.
septa Peritoneal (mesodermal) sheets separating
adjacent segments, as in annelids, or body divisions, as in chaetognaths.
septate Divided by crosswalls into cells.
septicemia Systemic infection where a pathogen
is present in the circulating blood.
septum, A wall between two
sequential hermaphroditism The type of hermaphroditism
that occurs when an animal is one sex during one phase of its
life cycle and an opposite sex during another phase.
seral stage One stage of the communities in an
sere An entire successional sequence in an ecosystem
(e.g., the sequence of stages in the succession of a lake to a
serial homolog Series of segments in which each
repeats the genetic expression of the genes in the segment (somite)
serially homologous Metameric structures that have
evolved from a common form; the biramous appendages of crustaceans
are serially homologous.
seriously undernourished Those who receive less
than 80 percent of their minimum daily caloric requirements; are
likely to suffer permanently stunted growth, mental retardation,
and other social and developmental disorders.
serosa The outer embryonic
membrane of birds and reptiles; chorion. Also, the peritoneal
lining of the body cavity.
serotonin A phenolic amine,
found in the serum of clotted blood and in many other tissues,
that possesses several poorly understood metabolic, vascular,
and neural functions; 5-hydroxytryptamine.
serous Watery, resembling serum;
applied to glands, tissue, cells, fluid.
serum The liquid that separates
from the blood after coagulation; blood plasma from which fibrinogen
has been removed. Also, the clear portion of a biological fluid
separated from its particulate elements.
Sesamoid Resembling a sesame grain. A small
bone formed in tendons.
sessile Bottom-dwelling and generally incapable
sessile Attached at
the base; fixed to one spot, not able to move about.
seta Hairlike modifications of an arthropod's exoskeleton
that may be set into a membranous socket. Displacement of a seta
initiates a nerve impulse in an associated cell.
sex chromosome one of a pair of chromosomes whose
composition determines gender
sex hormone A hormone that controls the timing
of reproduction and sexual characteristics in vertebrates.
sex-influenced traits Traits that behave as if they are dominant in one sex but recessive in the other sex.
sexual Pertaining to the fusion of gametes; sexual
sexual imprinting The process by which young of
many species establish an attachment to opposite sex conspecifics;
the effect manifests itself later in life during the process of
sexual reproduction Reproduction involving fusion
sexual selection Selection in relation to mating.
It is composed of intrasexual competition among members of one
sex (usually males) for access to the other sex and intersexual
choice of members of one sex by members of the other sex (usually
shale A sedimentary rock composed of mud.
shallow ecology A critical term applied to superficial
environmentalists who claim to be green but are quick to compromise
and who do little to bring about fundamental change. Many of those
accused of being shallow ecologists counter that they are merely
pragmatic, progressive reformers who prefer to work within the
established social contract rather than try to overthrow it.
shantytowns Settlements created when people move
onto undeveloped lands and build their own shelter with cheap
or discarded materials; some are simply illegal subdivisions where
a landowner rents land without city approval; others are land
shaping The procedure used in operant conditioning
wherein the experimenter can reinforce particular responses or
behavioral actions by the subject and not others, thus enhancing
the frequency of some actions and extinguishing other actions.
shard A piece of broken clay or ceramic pot placed
over the drainage hole of a pot to prevent the loss of soil during
shattering A trait found in wild plants in which
the fruiting head breaks apart to scatter the seeds over a wide
shear boundary The boundary between two plates
that move past each other on the earth's surface. Also see fault.
sheath The base of a leaf in monocots, usually
wrapping around the stem.
sheet erosion Peeling off thin layers of soil from
the land surface; accomplished primarily by wind and water.
shelf break The section of the continental shelf
where the slope abruptly becomes steeper, usually at a depth of
120 to 200 m (400 to 600 ft).
shell The calcium carbonate outer layer of cnidarians,
molluscs, and other animals. Produced by mucous glands.
shivering thermogenesis The generation of heat by
shivering, especially within the animal body.
shoot system Collective term for the stem and its
leaves; generally the aboveground portion of the plant.
short grass prairie A grassland biome characterized
by short grass and low rainfall; also known as the plains.
short-day plant A plant that flowers when
the length of day is shorter than some critical value.
shrub(by) A plant that is shrublike in habitat;
usually a short, perennial plant without strong apical dominance.
siblicide The killing of one sibling by another.
sibling species Reproductively isolated species
that are so similar morphologically that they are difficult or
impossible to distinguish using morphological characters.
sickle cell anemia A condition that causes the
red blood cells to collapse (sickle) under oxygen stress. The
condition becomes manifest when an individual is homozygous for
the gene for hemoglobin-S (HbS).
sieve plate The part of the wall of sieve-tube
members bearing one or more highly differentiated sieve areas
or modified primary pit fields.
sieve pores Holes or modified plasmodesmata found
within a sieve area either on a sieve plate or sieve cell.
sieve tube A long tube specialized for the conduction of food materials (products of photosynthesis) and consisting of several-
sieve-tube member One of the cells of a sieve
tube; found primarily in flowering plants and typically associated
with a companion cell; also called sieve-tube element.
sign stimuli Specific external stimuli that trigger
stereotyped responses from conspecifics, usually called fixed
signal Any behavior pattern that conveys information.
Patterns modified through evolution to convey information are
signaling pheromone A chemical signal that is quickly
perceived and causes an immediate response. Also called releasing
pheromone. Contrast with priming pheromone.
signs See sign stimuli.
silent mutation A permanent genetic change, but
one that is never expressed by the phenotype.
silica (SiO3) A mineral similar to glass that is
the major component of the cell wall, shell, or skeleton of many
siliceous ooze A type of biogenous sediment that
consists mostly of the silica shell and skeletons of marine organisms.
Also see diatomaceous ooze and radiolarian ooze.
siliceous Containing silica.
silicoflagellates Unicellular and eukaryotic members
of the phytoplankton that have a star-shaped silica skeleton.
silique A simple fruit that develops from a two-carpeled
ovary; at maturity the two halves fall away, leaving the seeds
attached to the persistent central wall.
silt Soil inorganic particles in the .002_.02 size
Silurian The geological period beginning about
435 million years ago during which time the first land plants
simian Pertaining to monkeys or
Simphyla A class of arthropods whose members are
characterized by having long antennae, 10 to 12 pairs of legs,
and centipedelike bodies. Occupy soil and leaf mold.
simple diffusion The process of molecules spreading
out randomly from where they are more concentrated to where they
are less concentrated until they are evenly distributed.
simple fruit A fruit that develops from a single
simple leaf A leaf having a single blade portion;
may be highly lobed or dissected.
simple pistil A pistil that contains a single carpel.
simple sugar A sugar, such as glucose, that cannot
be broken down into simpler sugar molecules.
simpl e Without complexity. In the case of a tissue,
sinistral Pertaining to the
left; in gastropods, shell is sinistral if opening is to left
of columella when held with spire up and facing observer.
sink A site of collection of metabolites, such
as sugar; metabolic sinks may exist anywhere in the plant where
organic solutes are being transported by the phloem and stored.
sinkholes A large surface crater caused by the
collapse of an underground channel or cavern; often triggered
by groundwater withdrawal.
sinus A cavity or space in tissues
or in bone.
siphon A tubular structure through which fluid flows;
siphons of some molluscs allow water to enter and leave the mantle
A reproductive process in seed plants in which a pollen tube carries
the sperm cells to the egg located within the integumented megasporangium.
siphonoglyph Ciliated furrow in the gullet of sea anemones.
siphonophores Hydrozoans that exist as drifting
siphuncle Cord of
tissue running through the shell of a nautiloid, connecting all
chambers with body of animal.
Sipuncula A phylum of protostomate worms whose members
burrow in soft marine substrates throughout the world's oceans.
Range in length from 2 mm to 75 cm. Peanut worms.
sipunculans See peanut worms.
sirenians (sea cows; order Sirenia)Marine mammals
with anterior flippers, no rear limbs, and a paddle-shaped
sister chromatid One of the two identical parts
of a duplicated chromosome in a eukaryotic cell. Sister chromatids
consist of exact copies of a long coiled DNA molecule with associated
proteins. Sister chromatids are joined at the centromere of a
sister group. The relationship between a pair of
species or higher taxa that are each other's closest phylogenetic
skeletal muscle Type of muscle tissue found in muscles
attached to skeletal parts.
skin The outer integument or covering of an animal
body, consisting of the dermis and the epidermis and resting on
the subcutaneous tissues.
sleeping sickness African trypanosomiasis and mosquito-borne,
slime ball Mass of mucus-covered cercariae
of dicrocoeliid flukes, released from land snails. Also a term
of derogation applied to really disgusting persons.
slime mold Members of the Myxomycetes, characterized
by a creeping, plasmodial stage.
sludge Semisolid mixture of organic and inorganic
materials that settles out of wastewater at a sewage treatment
slums Legal but inadequate multifamily tenements
or rooming houses; some are custom built for rent to poor people,
others are converted from some other use.
small intestine The part of the digestive system consisting of the duodenum, jejunum, and ileum.
smog The term used to describe the combination
of smoke and fog in the stagnant air of London; now often applied
to photochemical pollution products or urban air pollution of
smolt Ayoung salmon just before it migrates downstream
and out to sea.
smooth ER The portion of endoplasmic reticulum
that lacks ribosomes.
smooth muscle Type of muscle tissue found in the
walls of the hollow organs; visceral muscle.
social deprivation Withholding or removal of contact
with any form of stimulation from conspecifics.
social ecology A socialist/humanist philosophy
based on the communitarian anarchism of the Russian geographer
Peter Kropotkin. It shares much with deep ecology except that
it is more humanist in its outlook.
social justice Equitable access to resources and
the benefits derived from them; a system that recognizes inalienable
rights and adheres to what is fair, honest, and moral.
social organization The species-typical pattern
of relationships among all members of a group. This would include
spatial distribution patterns, interindividual relationships involving
dominance hierarchies or territoriality, mating systems, parenting,
society A group of individuals belonging to the
same species and organized in a cooperative manner. Usually assumed
to extend beyond sexual behavior and parental care of offspring.
sociobiology A study that involves the application
of the principles of evolution to the study of the social behavior
and social systems of animals.
sociopolitical The awareness by society of how
the political process can be influenced by well-informed
sodium-potassium ATPase pump The active transport
mechanism that functions to concentrate sodium ions on the outside
of a plasma membrane and potassium ions on the inside of the membrane.
soft corals Colonial anthozoans
with no hard skeleton.
softwood General term for the wood (secondary xylem)
soil horizons Horizontal layers that reveal a soil's
history, characteristics, and usefulness.
soil profile A vertical section of soil showing
the zones of particle sizes from surface down to bedrock.
soil A complex mixture of weathered mineral materials
from rocks, partially decomposed organic molecules, and a host
of living organisms.
solenia Channels through the
coenenchyme connecting the polyps in an alcyonarian colony (phylum
Special type of flame bulb in which the bulb bears a flagellum
instead of a tuft of flagella. See flame cell, protonephridium.
solenophage Blood-feeding arthropod that introduces
its mouthparts directly into a blood vessel to feed.
soleus The sole of the foot.
solubility The relative ability of a solute to
solute potential The water potential component
caused by the presence of solutes in water.
solute Any material dissolved in a solution.
solvent The liquid matrix in which a solute is
soma The whole of an organism except
the germ cells (germ plasm).
somaclonal variant A plant showing a mutation that
developed asexually during the tissue culture of a single callus.
somatic cell Ordinary body cell; pertaining to or
characteristic of a body cell. Any cell other than a germ cell
or germ-cell precursor.
somatic mutation A mutation that occurs in cells
of leaves, stems, or roots; a mutation occurring in any cells
that are not involved in gamete formation.
Somatic Either pertaining to body cells (as
opposed to gametes) or to the body wall (as opposed to the viscera).
Posterior coelomic compartment of echinoderms; left somatocoel
gives rise to oral coelom, and right somatocoel becomes aboral
somatoplasm The living matter that makes up the mass of the body
as distinguished from germ plasm, which makes up the reproductive
cells. The protoplasm of body cells.
somite One of the blocklike masses
of mesoderm arranged segmentally (metamerically) in a longitudinal
series beside the neural tube of the embryo; metamere.
sonar A technique or
equipment used to locate objects underwater by the detection of echoes.
sonar or biosonar. A system that uses sound at sonic
or ultrasonic frequencies to detect and locate objects.
sorting. Differential survival and reproduction
among varying individuals; often confused with natural selection
which is one possible cause of sorting.
sorus A cluster of sporangia found on
a fern leaf.
sound window The use of frequencies for communication that are
transmitted through the environment with little loss of strength
southern pine forest United States coniferous forest
ecosystem characterized by a warm, moist climate.
Southern Oscillation See El Nino--Southern Oscillation.
SPAC SoilPlantAtmosphere Continuum referring
to water movement in plants.
sparganum Cestode plerocercoid of unknown identity.
spawning The release of gametes or eggs into the
specialists Species that occupy a narrow range
of habitats and eat a narrow range of foods. Contrast with generalists.
specialized niche A species occupying a niche with
a narrow range of tolerance.
speciation The process by which two or more species
are formed from a single ancestral stock.
species diversity The number and relative abundance
of species present in a community.
species recovery plan A plan for restoration of
an endangered species through protection, habitat management,
captive breeding, disease control, or other techniques that increase
populations and encourage survival.
species A group
of interbreeding individuals of common ancestry that are reproductively
isolated from all other such groups; a taxonomic unit ranking
below a genus and designated by a binomen consisting of its genus
and the species name.
species-typical behavior Actions and displays
that are broadly characteristic of a species.
specific heat The amount of heat required to raise
the temperature of 1 gm of any substance 1 degree Celsius.
Specific-Mate Recognition System (SMRS) Signals
that increase the likelihood that members of the same population
will mate with one another. Contrast with species isolating mechanisms.
sperm A male gamete.
spermaceti organ a large organ in the forehead
of sperm whales that is filled with a fine-quality liquid
or waxy spermaceti oil.
spermagonium A structure that
produces spermatia in the rust fungi.
spermalege Organ that receives the sperm in the
female cimicid bug during copulation.
A sac in the female reproductive organs for the reception and
storage of sperm.
spermatid A growth stage of a male reproductive cell arising by division of a secondary spermatocyte; gives rise to a spermatozoon.
spermatium Minute, nonmotile male
gametes that occur in the rust fungi.
spermatocyte A growth stage of a male reproductive cell; gives rise
to a spermatid.
spermatodactyl Modification in some Acari of chelicera,
which functions in transfer of sperm from male's gonopore to copulatory
receptacles between third and fourth coxae of female.
Formation and maturation of spermatozoa.
Precursor of mature male reproductive cell; gives rise directly
to a spermatocyte.
spermatophore Formed "container" or packet
of sperm that is placed in or on the body of a female, in contrast
to the sperm in copulation which are conducted directly from male
reproductive structures into the female's body.
sperm competition A situation in which one male's
sperm fertilize a disproportionate number of eggs when a female
copulates with more than one male.
sperm nuclei Each pollen grain produces two sperm
nuclei, which effect double fertilization in angiosperms.
sphenoid The sphenoid bone.
sphenophyta The vascular plant division commonly
termed the horsetails to which the genus Equisetum belongs.
sphincter A ringlike band of muscle fibers that
constricts a passage or closes a natural orifice.
spice A pungent, aromatic plant product derived
from plants native to tropical regions and used to flavor foods.
spicule One of the minute
calcareous or siliceous skeletal bodies found in sponges, radiolarians,
soft corals, and sea cucumbers.
spike An inflorescence in which the main axis is
elongated and the flowers are sessile.
spikelet A small group of grass flowers; a unit
of the inflorescence in grasses.
spindle fibers The protein fibers formed during
prophase of nuclear division; chromosomes attach to these fibers
at the centromere.
spindle The aggregation of microtubules that is
involved in the movement and separation of chromosomes during
mitosis and meiosis.
spine A sharp projection from the body
or main part of a bone.
spiracle An opening for ventilation. The opening(s)
of the tracheal system of an arthropod or an opening posterior
to the eye of a shark, skate, or ray.
spiral cleavage. A type of embryonic cleavage in
which cleavage planes are diagonal to the polar axis and unequal
cells are produced by the alternate clockwise and counterclockwise
cleavage around the axis of polarity; determinate cleavage.
spiral valve A spiral portion in the intestine
of cartilaginous fishes.
spirillum A spirally coiled bacterium.
spondylosis Degeneration of a vertebra.
sponges (phylum Porifera)Invertebrates that consist
of a complex aggregation of cells, including collar cells, and
have a skeleton of fibers and/or spicules.
spongin Fibrous, collagenous
material making up the skeletal network of horny sponges.
spongioblast Cell in a sponge that secretes spongin, a protein.
Central cavity in sponges.
spongocyte A cell in sponges that secretes spongin.
spongy parenchyma A tissue composed of loosely
packed, irregular parenchymatous cells containing chloroplasts;
commonly found in leaves.
spontaneous ovulators Species in which females
release eggs whether they have copulated or not.
spontaneous recovery A process in which a conditioned
response has been extinguished and then is followed by some time
interval (generally one minute up to one day or more depending
upon the species and experimental conditions), after which the
animal may immediately exhibit a nearly normal correct response
rate upon reintroduction to the test situation.
sporadin Mature trophozoite of a gregarine protozoan.
sporangiospore A spore that develops within a sporangium.
sporangium A structure in which
spores are produced.
spore A reproductive unit (often unicellular) that
is capable of developing into a new organism without fusion with
sporoblast Cell mass that will differentiate into
a sporocyst within an oocyst.
sporocyst (1) Stage of development of a sporozoan
protozoan, usually with an enclosing membrane, the oocyst. (2)
An asexual stage of development in some digenean trematodes that
arises from a miracidium and gives rise to rediae.
sporocyst residuum Cytoplasmic material "left
over" within a sporocyst after sporozoite formation; seen
as an amorphous mass.
sporogony Multiple fission that produces sporozoites
after zygote formation. Occurs in the class Sporozoea.
sporont Undifferentiated cell mass within an unsporulated
leaf that bears sporangia.
sporophyte The diploid, spore-producing plant
in the alternation of generations; undergoes meiosis to produce
the haploid spores.
sporoplasm Amebalike portion of a microsporan or
myxosporan cyst that is infective to the next host.
sporozoite A stage in the life history of many
sporozoan protozoa; released from oocysts.
spout (blow)The water vapor and seawater that
is observed when whales surface and exhale.
spring overturn Springtime lake phenomenon that
occurs when the surface ice melts and the surface water temperature
warms to its greatest density at 4C and then sinks, creating
a convection current that displaces nutrient-rich bottom
spring tides The tides with a large tidal range;
they occur around the times of full or new moon. Compare neap
springwood The cells in the secondary xylem that
are formed early in the season, usually with wide vessels (angiosperms)
or wide tracheids (gymnosperms); also called
squalene A liquid
acyclic triterpene hydrocarbon found especially in the liver oil
squama Prominent lobe in the anal angle of a dipteran
squamous epithelium Simple epithelium of flat, nucleated cells.
Squamous Flat or scalelike.
squatter towns Shantytowns that occupy land without
owner's permission; some are highly organized movements in defiance
of authorities; others grow gradually.
stability In ecological terms, a dynamic equilibrium
among the physical and biological factors in an ecosystem or a
community; relative homeostasis.
stabilizing selection Natural selection that results
in the decline of both extremes in a phenotypic range; results
in a narrowing of the phenotypic range.
stable runoff The fraction of water available year-round;
usually more important than total runoff when determining human
stalk-and-rush Predators that approach
prey as closely as possible, then close with a sudden burst of
speed. Contrast with coursers.
stamen The floral organ that produces pollen; consisting
of an anther and filament.
staminate flower A flower having stamens but no
staminate A unisexual flower having stamens but
staminodium A sterile stamen; nonfunctional anthers
and often with petaloid filaments.
standard deviation A statistical measure of the
degree of variation from the mean value among the individual measurements
in a series of values.
standard error of the mean (SEM) A statistical
measure of variation most properly restricted to use with a group
of means, though often reported as a measure of variation around
the mean value in a series of individual measurements.
Standard Metropolitan Statistical Area (SMSA) An
urbanized region with at least 100,000 inhabitants with strong
economic and social ties to a central city of at least 50,000
standing crop total amount of plant or animal material
in an area at any one time
standing stock The total amount, or biomass, of
an organism at a given time.
stapes Stirrup-shaped innermost
bone of the middle ear.
starch A polysaccharide composed of a thousand
or more glucose molecules; the chief food storage material of
statoblast Biconvex capsule containing germinative cells and produced
by most freshwater ectoprocts by asexual budding. Under favorable
conditions it germinates to give rise to new zooid.
Sense organ of equilibrium; a fluid-filled cellular cyst
containing one or more granules (statoliths) used to sense direction
Small calcareous body resting on tufts of cilia in the statocyst.
statolith See statocyst.
statutory law Rules passed by a state or national
steady-state economy Characterized by low
birth and death rates, use of renewable energy sources, recycling
of materials, and emphasis on durability, efficiency, and stability.
stele The central vascular cylinder of roots and
stems of vascular plants.
Pertaining to aquatic organisms that have restricted tolerance
to changes in environmental saltwater concentration.
stenophagous Eating few kinds of foods.
Refers to an organism with
stereocilia The short, modified cilia at the apex
of a hair cell.
stereogastrula A solid type of gastrula, such as the planula
structure of endoskeletal ossicles of echinoderms.
stereotyped behavior A pattern of behavior repeated
with little variation in performance.
sternite Main ventral sclerite of a somite of an arthropod.
sternum Ventral plate of an arthropod
body segment; breastbone of vertebrates.
steroid An organic substance whose molecules include
four complex rings of carbon and hydrogen atoms. Examples are
estrogen, cholesterol, and testosterone.
sterol One of a class of organic compounds containing a molecular
skeleton of four fused carbon rings; it includes cholesterol,
sex hormones, adrenocortical hormones, and vitamin D.
sterols Complex alcohols (steroid alcohols) that
are important in animals as hormones, coenzymes, and precursors
for vitamin D.
stewards Protectors, careful managers of land and
stewardship A philosophy that holds that humans
have a unique responsibility to manage, care for, and improve
stichosome Column of large, rectangular cells called
stichocytes, supporting and secreting into the esophagus of most
nematodes of the family Trichuridae.
Stieda body Plug in the inner wall of one end of
a coccidian oocyst.
stigma The mass of bright red photoreceptor granules
found in certain flagellated protozoa (Euglena) that serves as
a shield for the photoreceptor. Also the spiracle of certain terrestrial
stimulant A psychoactive compound that excites
and enhances mental alertness and physical activity; often reduces
fatigue and suppresses hunger.
stimulus filter The ability of the nervous system
to block incoming stimuli that are unimportant for the animal.
stimulus Any form of energy an animal is able to
detect with its receptors.
stipe A supporting stalk; such as those in mushrooms
and brown algae.
stipule A small appendage found in pairs at the
base of leaves.
stock The size of a population.
stolon A rootlike extension of the body wall giving rise
to buds that may develop into new zooids, thus forming a compound
animal in which the zooids remain united by the stolon. Found
in some colonial anthozoans, hydrozoans, ectoprocts, and ascidians.
stoma The epidermal complex consisting
of two guard cells and the pore between them.
stomach The expansion of the alimentary canal between
the esophagus and duodenum.
stomates The small openings in leaves, herbaceous
stems, and fruits through which gases and water vapor pass.
Anterior evagination of the dorsal wall of the buccal cavity into
the proboscis of hemichordates; the buccal diverticulum.
stony corals Anthozoans, often colonial, that secrete
a calcareous skeleton.
strata Layers of sedimentary rock, the oldest rocks
occurring at the bottom.
strategic minerals Materials a country cannot produce
itself but that it uses for essential materials or processes.
stratification The separation of the water column
into layers, with the densest at the bottom and the least dense
at the surface. A stratified water column is said to be stable.
An unstable column results when the surface water becomes more
dense than the water below.
stratosphere The zone in the atmosphere extending
from the tropopause to about 50 km (30 mi) above the earth's surface;
temperatures are stable or rise slightly with altitude; has very
little water vapor but is rich in ozone.
streambank erosion Washing away of soil from banks
of established streams, creeks, or rivers, often as a result of
the removal of trees and brush along streambanks or cattle damage
to the banks.
stress shock A loose set of physical, psychological,
and/or behavioral changes thought to result from the stress of
excess competition and extreme closeness to other members of the
stress Physical, chemical, or emotional factors
that place a strain on an animal. Plants also experience physiological
stress under adverse environmental conditions.
stress-related disease See stress shock.
stretch receptor Sensory receptor that responds
to stretch; found in muscle tissue, lungs, and other organs that
undergo changes in position or size.
stridulation The production of sound by an insect
rubbing one body part against another (e.g., in male crickets).
strike Deposition of fly eggs or larvae on a living
strip-farming Planting different kinds of
crops in alternating strips along land contours; when one crop
is harvested, the other crop remains to protect the soil and prevent
water from running straight down a hill.
strip-mining Removing surface layers over
coal seams using giant, earth-moving equipment; creates a
huge open-pit from which coal is scooped by enormous surface-operated
machines and transported by trucks; an alternative to deep mines.
strobila A stage in the development of the scyphozoan jellyfish.
Also, the chain of proglottids of a tapeworm.
linear budding of individuals, as in scyphozoans (phylum Cnidaria),
or sets of reproductive organs, as in tapeworms (phylum Platyhelminthes).
strobilocercoid Cysticercoid that undergoes some
strobilation; found only in Schistotaenia.
strobilocercus Simple cysticercus with some evident
A number of sporangia- or ovule-bearing stuctures (sporophylls,
sporangiophores, scales, etc.) grouped together on an axis.
stroma thylakoid A thylakoid that does not occur
in a granum; connects separate grana.
stroma The matrix between the grana in chloroplasts
and site of the dark reactions of photosynthesis.
stromatolites Massive calcareous skeletons formed
by cyanobacteria (blue-green algae) .
structural color A color that results when light
is reflected by a particular surface.
structural genes Genes that code for enzymes or
structural proteins rather than proteins that control the function
of other genes.
structural molecule A complex molecule, such as
cellulose, that provides support and protection.
structure Patterns of organization,
both spatial and functional, in a community.
style Terminal segment of the antenna of a brachyceran
dipteran. It is drawn into a sharp point.
Styloid stylos = pillar or stalk.
stylops Member of the insect order Strepsiptera.
stylostome Hardened, tubelike structure secreted
by a feeding chigger mite.
subchelate Condition of an arthropod appendage
in which the terminal podomere can fold back like a pincer against
the subterminal podomere.
subduction The downward movement of a plate into
the mantle that occurs in trenches, which are also known as subduction
suberin A fatty material found in the cell walls
of cork cells and the Casparian strip of the endodermis.
sublimation The process by which water can move
between solid and gaseous states without ever becoming liquid.
sublittoral zone See subtidal zone.
submarine canyon A narrow, deep depression in the
continental shelf formed by the erosion of rivers or glaciers
before the shelf was submerged.
submergent plant community a marine plant community
restricted to subtidal environments
submetacentric Pertaining to a chromosome with
a centromere located between the center and one end of the chromosome.
Applied to environments beneath snow, in which snow insulates
against a colder atmospheric temperature.
subsidence A settling of the ground surface caused
by the collapse of porous formations that result from withdrawal
of large amounts of groundwater, oil, or other underground materials.
subsoil A layer of soil beneath the topsoil that
has lower organic content and higher concentrations of fine mineral
particles; often contains soluble compounds and clay particles
carried down by percolating water.
substiedal body Additional plug material underlying
a Stieda body.
substrate The substance acted on by an enzyme;
the surface on which a plant or fungus grows or is attached.
substrate-level phosphorylation The generation
of ATP by coupling strongly exergonic reactions with the synthesis
of ATP from ADP and phosphate.
subtidal zone The bottom above the continental
succession the gradual replacement, through time,
of one group of species in a community by other groups
succulent A fleshy plant that accumulates water.
sucrose A disaccharide (glucose + fructose) found
in many plants; the primary form in which sugar produced by photosynthesis
sudoriferous gland A sweat gland.
sugar A monosaccharide; a carbohydrate with the
general formula CnH2nOn.
Sulcus A furrow or groove.
sulfide One of the minerals that is abundant in
the hot water that seeps through hydrothermal vents.
sulfur dioxide A colorless, corrosive gas directly
damaging to both plants and animals.
sulfur oxide Produced from burning coal, this gas
is a pollutant that can produce sulfuric acid in rain droplets.
summerwood The cells in the secondary xylem that
are formed late in the season, usually with few vessels (angiosperms)
or narrow tracheids (gymnosperms); also called late wood.
sun compass The mechanism in which animals use
the sun and an internal clock that allows them to adjust for movement
in the sun as they navigate.
superior ovary An ovary located above the sepals,
petals, and stamens.
superior ovary Attachment of the ovary to the receptacle
above and free from the attachment of the other floral parts.
superior Above a point of reference (e.g., the neck
is superior to the chest of humans).
supply The quantity of that product being offered
for sale at various prices, other things being equal.
suprachiasmtic nucleus (SCN) A brain nucleus involved in the visual pathway that has been clearly identified for its involvement in biological rhythms mediated by photoperiod.
suprapopulation of parasites All individuals of
a single parasite species at all stages in the life cycle in all
hosts in an ecosystem.
surf A wave that becomes so high and steep as it
approaches the shoreline that it breaks.
surface (mixed) layer The upper layer of water
that is mixed by wind, waves, and currents.
surface current long-term directional flow
of water at the sea surface
surface mining Some minerals are also mined from
surface pits. See strip-mining.
surface of a cell. Eukaryotic flagella are longer
versions of cilia. Flagellar undulations drive a cell through
a fluid medium.
surface tension the mutual attraction of water
molecules at the surface of a water mass that creates a flexible
molecular "skin" over the water surface
surface-to-volume ratio (S/V ratio) The
amount of surface area relative to the total volume of an organism.
surra Disease of large mammals caused by Trypanosoma
survivorship The percentage of a population reaching
a given age or the proportion of the maximum life span of the
species reached by any individual.
suspension feeder The type of feeding whereby an
animal obtains its nutrients by the removal of suspended food
particles from the surrounding water by some sort of capturing,
trapping, or filtering structure.
sustainable agriculture An ecologically sound,
economically viable, socially just, and humane agricultural system.
Stewardship, soil conservation, and integrated pest management
are essential for sustainability.
sustainable development A real increase in well-being
and standard of life for the average person that can be maintained
over the long-term without degrading the environment or compromising
the ability of future generations to meet their own needs.
sustained yield Utilization of a renewable resource
at a rate that does not impair or damage its ability to be fully
renewed on a long-term basis.
suture sutur = a seam. An immovable joint.
swamp Wetland with trees, such as the extensive
swamp forests of the southern United States.
swarmer Daughter trophozoites resulting from multiple
fissions of Ichythophthirius multifiliis and a few other protozoa.
swarming The aggregation of individuals for spawning
or other purposes.
sweeper tentacle A type of tentacle in corals that
is used to sting neighboring colonies.
swell A wave with a flatter, rounded crest and
trough. Swells are found away from the area where waves are generated
by the wind.
swidden agriculture See milpa agriculture.
swim bladder A gas-filled sac, which is usually
located along the dorsal body wall of bony fishes; it is an outgrowth
of the digestive tract and regulates buoyancy of a fish.
sycon A type of canal system in
certain sponges. Sometimes called syconoid.
sylvatic Existing normally in the wild, not in
the human environment.
symbiology Study of symbioses.
symbiont Any organism involved in a symbiotic relationship
with another organism, the host.
symbiosis The living
together of two different species in an intimate relationship.
Symbiont always benefits; host may benefit, may be unaffected,
or may be harmed (mutualism, commensalism, and parasitism).
symbiotic An association of two or more different
organisms in which both benefit.
symmetrogenic fission Mitotic fission between the
rows of flagella of protozoa.
symmetry A balanced arrangement of similar parts
on either side of a common point or axis.
sympathetic nervous system Portion of the autonomic nervous system that arises from the thoracic and lumbar regions of the spinal cord; also called thoracolumbar division.
Having the same or overlapping regions of geographical distribution.
sympatric speciation Speciation that occurs in populations
that have overlapping ranges.
sympatry Populations or species with overlapping
geographic distributions. Contrast with allopatry.
symplast The interconnected protoplasm of all cells
in a plant.
symplastic Pertaining to the movement of water
and solutes through tissues by passing through biological membranes.
symplesiomorphies Taxonomic characters that are
common to all members of a group of organisms. These characters
indicate common ancestry but cannot be used to describe relationships
within the group.
symplesiomorphy Sharing among species of ancestral
characteristics, not indicative that the species comprise a monophyletic
synangium Fused sporangia.
synanthropism Habit of an organism of living in
or around human dwellings.
synapomorphic Uniquely shared, derived (modified)
character states; describes a group of species that can be defined
as different from all others because they share some unique homologous
synapomorphies Characters that have arisen within
a group since it diverged from a common ancestor. Synapomorphies
are used to indicate degrees of relatedness within a group. Also
called shared, derived characters.
synapomorphy Shared, evolutionarily derived character states
that are used to recover patterns of common descent among two
or more species.
synapse The place
at which a nerve impulse passes between neuron processes, typically
from an axon of one nerve cell to a dendrite of another nerve
synapsids An amniote
lineage comprising the mammals and the ancestral mammal-like
reptiles, having a skull with a single pair of temporal openings.
synapsis The pairing of homologous chromosomes
that occurs in Prophase I of meiosis.
synaptic cleft The narrow space between the terminal
ending of a neuron and the receptor site of the postsynaptic cell.
synaptic processes The threadlike extensions at
the end of an axon.
synaptonemal complex The structure that holds homologous chromosomes
together during synapsis in prophase of meiosis I.
synchronous flight See direct flight.
syncytial hypothesis The idea that multicellular
organisms could have arisen by the formation of cell boundaries
within a large multinucleate protist.
syncytium A multinucleated cell.
group of symptoms characteristic of a particular disease or abnormality.
synergid Two haploid cells on either side of the
egg cell at the micropylar end of the embryo sac.
synergism The interaction of two or more agents
or forces so that their combined effect is greater than the sum
of their individual effects, as when two hormones combine to affect
Synergist syzyg = yoked together. Acting together.
synergistic effects When an injury caused by exposure
to two environmental factors together is greater than the sum
of exposure to each factor individually.
of one gamete with another individual gamete to form a zygote,
found in most animals with sexual reproduction.
nucleus resulting from fusion of pronuclei.
synsacrum The fused posterior thoracic vertebrae,
all lumbar and sacral vertebrae, and anterior caudal vertebrae
of a bird; helps maintain proper flight posture.
syntax Information provided by the sequence in
which signals are transmitted.
syrinx The vocal organ of
birds located at the base of the trachea.
system A group of organs specialized
for one function.
systematic variation In learning, a procedure that
involves examining qualitative differences in learning by investigating
a wide variety of species from diverse taxonomic groups.
systematics The study of the classification and
phylogeny of organisms. See taxonomy.
systemic A condition or process that affects the
whole body; many metabolic poisons are systemic.
systemic circuit The portion of the circulatory
system concerned with blood flow from the left ventricle of the
heart to the entire body and back to the heart via the right atrium.
systole Phase of the cardiac cycle during which a heart chamber wall is contracted; also systolic pressure.
systolic pressure The portion of blood pressure
measurement that represents the highest pressure reached during
ventricular ejection; it is the first number shown in a blood
syzygy Stage during sexual reproduction of some
gregarines in which two or more sporadins connect end to end.