S curve A curve that depicts logistic growth; called an S curve because of its shape.

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 associations.

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 salivary glands.

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 the next.

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 regions.

saltwater intrusion Movement of saltwater into freshwater aquifers in coastal areas where groundwater is withdrawn faster than it is replenished.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 size range.

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.

saprophyte 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 of Sarcocystis.

Sarcodina The protozoan subphylum where members have pseudopodia for movement and food gathering; naked or with shell or test; mostly free living.

sarcolemma The 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 the myofibril.

sarcoplasm The 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 in syzygy.

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 matter.

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. Musk gland.

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 cuticle.

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 (protostomes).

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 stock.

sclereid A sclerenchyma cell with a thick, lignified secondary wall having many pits; variable in form but not usually elongated.

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.

scleroblast 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 a chaparral.

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 the host.

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 or mite.

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 absent polyp.

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 polyp.

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 locomotion.

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 mid-ocean ridges.

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 consumers.

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 rock.

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 regeneration.

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.

seminiferous 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 follows.

sensory (afferent) neuron or nerve A neuron or nerve that conducts an impulse from a receptor organ to the central nervous system.

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 nervous system.

sensory deprivation Withholding all or a specified portion of the sensory input that an animal would normally be receiving.

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 cavities.

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 ecological succession.

sere An entire successional sequence in an ecosystem (e.g., the sequence of stages in the succession of a lake to a climax forest).

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 of locomotion.

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 reproduction.

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 finding mates.

sexual reproduction Reproduction involving fusion of gametes.

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 females).

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 invasions.

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 watering.

shattering A trait found in wild plants in which the fruiting head breaks apart to scatter the seeds over a wide area.

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 action patterns.

signal Any behavior pattern that conveys information. Patterns modified through evolution to convey information are called displays.

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 marine organisms.

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 range.

Silurian The geological period beginning about 435 million years ago during which time the first land plants occurred.

simian Pertaining to monkeys or apes.

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 ovary.

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, without layers.

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 cavity.

siphonogamy 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 colonies.

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 tail.

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 duplicated chromosome.

sister group. The relationship between a pair of species or higher taxa that are each other's closest phylogenetic relatives.

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, virus-induced encephalitis.

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 plant.

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 any kind.

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, and dispersal.

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 individuals.

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) of conifers.

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 Cnidaria).

solenocyte 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 be dissolved.

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 dissolved.

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).

somatocoel Posterior coelomic compartment of echinoderms; left somatocoel gives rise to oral coelom, and right somatocoel becomes aboral coelom.

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 (attenuation).

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 water.

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.

spermatheca 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.

spermatogenesis Formation and maturation of spermatozoa.

spermatogonium 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.

spongocoel 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 another cell.

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 oocyst.

sporophyll A 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 waters.

spring tides The tides with a large tidal range; they occur around the times of full or new moon. Compare neap tides.

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 of sharks.

squama Prominent lobe in the anal angle of a dipteran wing.

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 uses.

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 carpels.

staminate A unisexual flower having stamens but no pistil.

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 people.

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 most plants.

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.

statocyst Sense organ of equilibrium; a fluid-filled cellular cyst containing one or more granules (statoliths) used to sense direction of gravity.

statolith Small calcareous body resting on tufts of cilia in the statocyst.

statolith See statocyst.

statutory law Rules passed by a state or national legislature.

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.

stenohaline Pertaining to aquatic organisms that have restricted tolerance to changes in environmental saltwater concentration.

stenophagous Eating few kinds of foods.

stenotopic 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 of cnidarians.

stereom Meshwork 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 nature's resources.

stewardship A philosophy that holds that humans have a unique responsibility to manage, care for, and improve nature.

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 arthropods.

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.

stomochord 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.

Stratified In layers.

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 same species.

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 host.

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.

strobilation Repeated, 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 strobilation.

strobilus 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 zones.

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.

subnivean 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 shelf .

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 is translocated.

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 evansi.

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.

sympatric Having the same or overlapping regions of geographical distribution. Noun, sympatry.

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 group.

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 character.

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 cell.

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.

syndrome A 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 target tissues.

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.

syngamy Fertilization of one gamete with another individual gamete to form a zygote, found in most animals with sexual reproduction.

synkaryon Zygote 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 pressure reading.

syzygy Stage during sexual reproduction of some gregarines in which two or more sporadins connect end to end.

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