Racemose Formed of a number of coalescing ducts draining to a central cavity or duct.
Radial canal One of five fluid filled channels of the echinoderm water vascular system that join the ring canal to the lateral canals.
Radial cleavage Type of cleavage pattern in which the cleavage spindles or cleavage planes are at right angles or parallel to the polar axis of the egg.
Radial symmetry The arrangement of similar parts around a central axis.
Radiole Each of the several pinnate tentacles on the head of a sabellid, serpulid, or spirorbid polychaete.
Radula (pl. Radulae) A belt of transverse rows of teeth supported by the odontophore.
Radula sac Pocket of the buccal cavity from which the molluscan radula arises.
Ramus A branch.
Raptorial Animals that capture prey.
Recent The current epoch of the Quaternary Period.
Redia (pl. Rediae) Stage in the digenean life cycle between the sporocyst and cercaria.
Regulated compartment A space, such as an organelle, gut region, or body cavity, in which the chemical environment can be controlled.
Regulative development Embryonic fate determination in which cell fates are determined by a network of cellular communication in the embryo.
Relictual A remnant of a once more widespread distribution.
Repugnatorial gland Arthropod gland producing repellent and toxic compounds for defense.
Reserve stylet One of several accessory reserve stylets present on each side of the nemertean central proboscis stylet.
Resilium The inner portion of the hinge ligament.
Respiratory tree One of two respiratory organs of most holothuroid echinoderms. Consists of a network of thin walled tubules in the perivisceral coelom that originates from the cloacal wall.
Reticulopodium (pl. Reticulopodia) ••A pseudopodium that forms a threadlike branched mesh and contains axial microtubules.
Retractor muscle Muscle that withdraws an eversible or protrusible body part.
Retrograde Passing in a direction opposite the direction of motion of the animal, ie anterior to posterior.
Retroperitoneal Outside, or behind, the peritoneum, i.e. outside the coelom but typically bulging into the coelom and covered by peritoneum.
Rhabdite Platyhelminth epidermal secretion droplets which are characterized microscopically by a specific, layered ultrastructure.
Rhagon Developmental stage immediately following the metamorphosis of a demosponge larva. Typically, it is asconoid or syconoid in structure.
Rhinophore One of the second pair of sensory tentacles.
Rhombogen A dicyemid rhombozoan similar to a nematogen but whose axial cell is in the process of forming an infusoriform larva. A sexually reproductive nematogen.
Rhopalium (pl. Rhopalia) A club shaped, marginal sensory organ of scyphozoans.
Rhopalial lappet One of two small, specialized flaps on a rhopalium.
Rhynchocoel A fluid filled coelomic cavity that houses the retracted nemertean proboscis.
Rhynchodeum In nemerteans, the short anterior canal that joins the proboscis pore to the proboscis.
Ring canal Part of the echinoderm water vascular system that joins the stone canal to the radial canals. The marginal canal of the gastrovascular system of some medusae.
Rostroconchida An extinct class of molluscs that may have been ancestral to modern bivalves.
Rostrum Middorsal projection in some rotifers that bears cilia and sensory bristles at its tip and is also adhesive. Median, anteriorly directed spine from the carapace and head of some crustaceans.


Saccate nephridium Excretory organ derived from a coelomic end sac and metanephridial tubule.
Sanguivorous Feeding on blood (= hematophagous).
Saltatory Jumping, leaping locomotion.
Scaphognathite Paddle like projection of the second maxilla that produces a ventilating current; gill bailer.
Schizocoel Coelomic cavity derived from the separation, or splitting apart, of a solid mass of mesodermal cells.
Schizogamy Apicomplexa. Multiple fission that produces merozoites.
Sclerite Thickened, tanned area of cuticle in the exoskeleton of arthropods.
Scleroseptum (pl. Sclerosepta) ••One of the many radiating calcareous partitions in the skeletal cup of stony corals.
Sclerotized Highly tanned (hardened), darkened, and thickened arthropod exoskeleton.
Scolex (pl. Scoleces) Anterior head region of tapeworms that is adapted for adhering to the host.
Scutum (pl. Scuta) One of the calcareous plates forming the barnacle operculum.
Scyphistoma (pl. Scyphistomae) A scyphozoan polyp.
Segmentation Body composed of a linear series of repeating units, or segments; metamerism.
Sediment Particles (clay, sand, detritus) deposited on the ocean or lake bottom.
Selective deposit feeding Feeding in which animals selectively remove organic detritus particles from the surrounding sand particles.
Seminal receptacle Chamber in the female gonoduct for the reception and storage of allosperm.
Seminal vesicle Part of the male gonoduct that functions in the storage of autosperm.
Sensillum (pl. Sensilla) Arthropod sense organ involving a specialized part of the exoskeleton.
Sensu lato (s.l.) In the broad sense.
Sensu stricto (s.s.) In the strict sense.
Septum (pl. Septa) A double walled tissue partition in the cross sectional plane of a bilaterian or a radial plane of a cnidarian.
Septal filament The free edge of an anthozoan septum that is trilobed.
Sessile In anatomy: attached directly to the body surface and not stalked, also flush with the body surface . In ecology: attached firmly to a substratum and not free to move.
Seta (pl. Setae) An exoskeletal bristle composed of a -chitin
Setiger A segment with setae.
Setose Searing setae.
Sexual dimorphism Male and female of a species with different shapes or appearances.
Shell A rigid skeleton on the outside of an organism. The calcified covering of molluscs and brachiopods.
Shield A small calcareous plate in certain echinoderms, especially ophiuroids.
Sieve tracheae Arthropod tracheal system in which the spiracle opens into an atrial or tubelike chamber from which a great bundle of tracheae arises.
Sigmoid S-shaped
Siliceous Composed of silica.
Simple epithelium Composed of a single layer of cells, ie monolayered.
Sinus Saclike space.
Siphon An accessory gut channel of echiurans and some echinoids. A tubular fold of the molluscan mantle used to direct water to or from the mantle cavity. Inhalant and exhalant apertures of urochordates.
Siphonoglyph Ciliated groove in the pharyngeal wall of some anthozoans that moves water into the coelenteron.
Siphonozooid A highly modified pennatulacean polyp that pumps water into, or allows it to escape from, the interconnected gastrovascular cavities of the colony.
Siphuncle A strand of tissue in a delicate calcareous tube functions in filling chambers with gas.
Slug Opisthobranch or pulmonate in which the shell is absent or reduced and buried in the mantle.
Solenocyte A protonephridial terminal cell with one flagellum and a long microvillar collar.
Somatic Pertaining to the body.
Somite A body segment or metamere.
Spasmin Ciliate contractile protein which requires ATP for extension.
Speciose Having many species.
Spermatheca (pl. Spermathecae) Another term for a seminal receptacle.
Spheridium (pl. Spheridia) An echinoid statocyst.
Spicule A small needle like or rodlike skeletal piece.
Spinneret Spinning organ of spiders.
Spiracle Slitlike external opening of the arthropod tracheal system.
Spiral cleavage Type of cleavage pattern in which the cleavage spindles or cleavage planes are oblique to the polar axis of the egg.
Spire All the whorls of a gastropod shell above the body whorl.
Spirocyst Cnida with a long adhesive thread that functions in capture of prey and in attachment to a substratum.
Spongin A large, collagenous, connective tissue fiber of sponges.
Spongiome System of small vesicles or tubules that surrounds the contractile vacuole in the contractile vacuole complex of ciliates.
Spongocoel Interior cavity of asconoid sponges. Atrium.
Sporocyst Nonciliated second stage in the life cycle of digeneans. Arises from a miracidium and gives rise to rediae.
Sporosac Incomplete gonophore (made up of only the gonadal tissue) that remains attached to the polypoid colony.
Sporozoite Apicomplexa. Infective sporelike stage that results from meiosis of the zygote.
Spring tides Tides occurring on new and full moons characterized by large tidal amplitude.
Spur A long, slender dorsal appendage of doliolids that trails behind the oozooid and bears buds. Cadophore.
Squamous epithelium Epithelium of flattened tile-like cells.
Statocyst A sense organ that can provide orientation to the pull of gravity. Typically composed of a chamber containing concretions (statoliths) in contact with receptor cells.
Stenohaline Restricted to a narrow range of environmental salinities.
Stenopod A narrow, cylindrical, leglike appendage.
Stereoblastula A solid blastula, lacking an internal cavity or blastocoel.
Stereogastrula A solid gastrula, lacking an archenteron cavity.
Sternite The ventral plate of the cuticle of each segment of an arthropod.
Sternum (pl. Sterna) The combined sternites.
Stolon Rootlike extension of the body that interconnects colonial zooids.
Stomodeum Invaginated embryonic ectoderm joining the mouth with the endodermal midgut.
Stone canal Part of the echinoderm water vascular system that joins the madreporite with the ring canal. Usually, but not always calcified.
Storage excretion Internal, indefinite retention of some excretory products, such as uric acid.
Stratified epithelium Composed of two or more layers of cells, only one of which rests on the basal lamina.
Streptoneury Gastropod nervous system twisted by torsion into an asymmetrical figure-8.
Stridulate To generate sound by rubbing body parts together.
Stridulate To produce sound by rubbing one body part against another.
Strobila (pl. Strobilae) A scyphozoan polyp that buds medusae; or the posterior part of a tapeworm that consists of proglottids.
Strobilation Process by which scyphomedusae arise as buds that are released by transverse fission of the oral end of the scyphistoma.
Stylet A dagger like structure associated with various systems of different animal groups.
Stygobiotic Living in caves.
Subchelate A pincer in which the movable finger closes against a flat palm.
Sublittoral The sea floor between the low tide line and the seaward edge of the continental shelf. Subradula organ Cushion shaped chemosensory structure of chitons.
Subumbrella Lower oral surface of a medusa.
Subterminal Located some distance from the end.
Subtidal The sea below the low tide line.
Supratidal Above the high tide line.
Sulcus A longitudinal groove of dinoflagellates that bears the posteriorly directed flagellum.
Suppinate Rotate the leading edge of a limb up.
Suspension feeding Feeding on organic particles (plankton and detritus) suspended in water.
Suture The junction between the septum and the wall of a cephalopod shell.
Syconoid sponge A radially symmetrical sponge that has a body wall folded into radially oriented canals.
Symbiosis An intimate, long-term, physical interraction between two species, in which at least one of the species is dependent, to various degrees, upon the other.
Symmetrogenic Producing mirror image daughter cells as a result of fission.
Synanthropic Living with humans.
Syncytium Tissue in which nuclei are not separated by cell membranes.
Synkaryon Zygotic nucleus of ciliates.
Systole The contraction phase of a heart beat.


Tagma (pl. Tagmata) An arthropod body region of arthropods (i.e., head, thorax, abdomen).
Tanned Stabilization of the arthropod exocuticle by the formation of cross linkages.
Tapetum A reflective layer within an eye.
Tan To increase the strength, and darken the color, of protein by establishing crosslinks between adjacent polypeptides.
Tarsal organ Cuplike spider chemoreceptor for detecting pheromones.
Taxodont Hinge dentition and consisting of uniform alternating teeth and sockets in a row.
Taxon A group of organisms with a common ancestor.
Tegmen Membranous oral wall of the crinoid disc.
Tegument The nonciliated outer syncytial layer of the body wall of parasitic platyhelminths and acanthocephalans.
Telolecithal Type of egg in which the yolk material is concentrated to one side (vegetal) of the egg.
Telopodite The movable part of an appendage extending outward from an immovable protopod. Telotroch A ring of cilia encircling the anus at the posterior end of a trochophore larva.
Tensilium The outer portion of a bivalve hinge ligament.
Tentacle Evagination of the body wall surrounding the mouth which aids in the capture and ingestion of food.
Tentacle sheath In Bryozoa, the part of the withdrawn body wall that encloses the withdrawn tentacles of the lophophore. See vestibula.
Tergite The dorsal, sclerite of each arthropod segment.
Tergum (pl. Terga) The combined tergites. A plate contributing to the barnacle operculum.
Terminal At the end.
Terminal anchor Anchor at the leading end of a burrowing animal.
Terminal cell Tubular flagellated cell attached to the inner end of the protonephridial tubule.
Test An encasing or shell like skeleton, typically covered externally by cytoplasm or living tissue.
Theca (pl. Thecae) ••The nonliving cuticle around the hydranths of thecate hydroids. Hydrotheca.
Thecate Refers to hydroids with a hydrotheca surrounding the polyp proper.
Tetramerous Radial symmetry in which a basic pattern is repeated in multiples of four.
Thigmotactic Responding to touch or surface contact.
Thoracopod Any thoracic appendage of an arthropod.
Tiedemann’s body One of the interradial outpockets of the ring canal of many echinoderms. Removes unwanted particulates from the water vascular system.
Tongue bar A downgrowth of pharyngeal tissue that divides a developing gill opening into two side-by-side slits.
Tornaria Transparent, long lived, planktotrophic larva of enteropneusts.
Torsion The counterclockwise twist of the gastropod visceral mass over the head and foot.
Toxicyst A vesicular organelle in the pellicle of gymnostome ciliates which discharges long threads with bulbous bases; used for defense or capturing prey.
Transmitted light In microscopy, light, from a source below the stage, which passes through the plane of the stage to reach and pass through the object.
Trichobranchiate Having filamentous gills.
Trichocyst A bottle shaped extrusible organelle of the ciliate pellicle.
Trilobite larva Horseshoe crab larva that superficially resembles trilobites.
Triploblastic Embryos possessing all three germ layers: ectoderm, endoderm, and mesoderm.
Triturate To grind or masticate.
Trochophore Type of larva found in molluscs, annelids, and other groups in which the larval body is ringed by a girdle of cilia, the prototroch.
Trochus The anterior band of cilia of the divided corona of some rotifers.
Troglobitic, troglodytic Dwelling in caves or otherwise underground.
Trophi Cuticular hard parts of the rotifer mastax.
Trophosome Central mass of tissue in the trunk of the pogonophore that is packed with symbiotic bacteria.
Trophozoite Apicomplexa. Feeding stage that occurs when the sporozoite invades the host.
Trophozooid Nutritive or feeding zooid of doliolid urochordates.
Tropic hormone A hormone whose target is an endocrine cell.
Tube tracheae Simple branched or unbranched trachea.
Tubicolous Tube dwelling.
Tubules of Cuvier Eversible toxic or sticky tubules associated with the bases of the respiratory trees of some holothuoid echinoderms.
Tubulus (pl. Tubuli) Sensory papilla on the trunk of some aschelminths.
Tunic Special cuticular covering of the body of ascidians.
Tunicate A urochordate.
Tunicin A kind of cellulose that forms structural fibers in ascidian tunics.
Typhlosole A ridge projecting internally from the intestinal wall to increase its surface area.


Ultrafiltration Passage of fluid across a fine-mesh filter to retain proteins and larger particles.
Umbo (pl. Umbos, Umbones) A dorsal protuberance of a bivalve valve rising above the hinge.
Uncinus (pl. Uncini) A minute seta modified into a hook.
Undulating membrane Type of ciliary organelle that is a row of adhering cilia forming a sheet.
Uniramous Having one branch.
Ureotelic Producing urea as the end product of nitrogen metabolism.
Uricotelic Producing uric acid as the end product of nitrogen metabolism.
Uropods Sixth abdominal appendages of most malacostracans but the 4 th, 5 th, and 6 th of amphipods.


Vanadocytes Yellowish green ascidian blood cells that contain high concentrations of vanadium.
Vascular plug Specialized nemertean exchange site across which an ultrafiltrate passes from the blood to the rhynchocoel.
Vegetative nucleus Macronucleus.
Velarium Velum like structure of cubozoans.
Veliger Planktotrophic molluscan larva that follows the trochophore.
Velum Shelf formed by the margin of the umbrella projected inward which is characteristic of most hydromedusae. One of the two ciliated flaps with which a veliger larva swims and feeds.
Vermiform Having the shape of a worm.
Vermiform embryo Asexually produced young of dicyemids that has the same form as the parent; formed within the axial cell of the parent.
Vessel A small tubular blood channel.
Vestibule Preoral chamber. In Bryozoa, a space enclosed by the withdrawn body wall of a retracted zooid distal to the withdrawn tentacles and tentacle sheath.
Vestigial Reduced to a non-functional remnant.
Vestimentum The collar like body region of a vestimentiferan that helps to secrete the animal’s tube.
Vibraculum (pl. Vibracula) Bristle like heterozooid found in some cheilostome bryozoans.
Visceral mass One of three primary parts of the molluscan body; contains the internal organs.
Viscous drag Friction that results from the tendency of the polar water molecules to stick to each other and to surfaces.
Vitellarium (pl. Vitellaria) Specialized part of the ovary for the production of yolk filled nurse cells. Nonfeeding barrel shaped larval stage of some echinoderms.
Viviparous Embryos gestated internally within the female where supplemental nutrition is supplied.


Whorl Any complete turn (360 ° ) of a coiled molluscan shell.


Xylophagous Feeding on wood.


Zoarium (pl. Zoaria) The form of a bryozoan colony.
Zoea (pl. Zoeae) Penultimate larval stage of many decapod crustaceans, preceding the postlarva.
Zoochlorella (pl. Zoochlorellae) Unicellular green algal symbiont of certain animals, especially freshwater sponges and freshwater and marine cnidarians and turbellarians.
Zooecium (pl. Zooecia) The cuticle, or exoskeleton, of a bryozoan zooid.
Zooflagellate A flagellate that has one to many flagella, lacks chloroplasts, and is heterotrophic.
Zooplankton Microscopic animals suspended in the water of oceans and freshwater lakes.
Zooplanktivore Feeding on zooplankton.
Zooxanthella (pl. Zooxanthellae) A golden brown alga, usually a dinoflagellate, symbiotic with various marine animals, especially cnidarians.