Invertebrate Anatomy OnLine
Glossary ©
Copyright 2003 by
Richard Fox , Lander University
Edward Ruppert, Clemson University
            This is the glossary for Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology. Additional exercises can be accessed by clicking on the links to the left. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises. 
            This glossary is adapted from those of earlier editions of Invertebrate Anatomy OnLine and the sixth edition of Ruppert and Barnes (1994). It has been modified to support the accompanying OnLineLab exercises as well as the seventh edition of Invertebrate Zoology (Ruppert, Fox, & Barnes, 2004).
Abductor A muscle that moves a structure away from the middle of the body.
Abyssal The ocean bottom between 4000 and 6000 m.
Abyssobenthic The ocean bottom at depths of 4000-6000 m.
Abyssopelagic The region of the ocean's pelagic zone deeper than 4000 m.
Acanthella Acanthocephalan larva, following the acanthor and preceding the cystacanth.
Acanthor First acanthocephalan larval stage.
Aciculum (pl. Acicula) Chitinous rod that internally supports the divisions of the parapodium.
Acinus A small sac.
Acoelomate Body organization lacking a fluid‑filled cavity between epidermis and gastrodermis; compact.
Acontium (pl. Acontia) A thread originating from the middle lobe of an anthozoan septal filament that projects freely into the gastrovascular cavity.
Acron The anteriormost region, preceeding the first segment of the arthropod body.
Acrorhagus (pl. Acrorhagi) Cnidocyte‑covered elevations on specialized sweeper tentacles or on the column of certain anthozoans.
Actinotrocha  Tentaculate ciliated larva of phoronids.
Actinula A polyp‑like larva of certain hydrozoans that resembles a short stemless hydranth.
Action the function of a muscle; the result accomplished by its contraction.
Adductor A muscle that moves a structure toward the middle of the body.
Adoral zone Region within the buccal cavity of certain ciliates.
Aesthetasc Chemoreceptive sensilla of crustaceans, usually on the first antenna.  
Agamete Nucleus within the plasmodium of an orthonectid “mesozoan” that divides mitotically and eventually gives rise to a sexual adult.
Allochthonous arising outside the organism or entity.
Allosperm Sperm received from a sexual partner. 
Alveolus (pl. Alveoli) One of many flattened vesicles that form a more or less continuous layer beneath the cell membrane of ciliates and a few other protozoans.
Ambulacrum (pl. Ambulacra) Groove, ridge, or double band of tube feet, radial canal, and associated body wall of echinoderms.
Ametabolous Insect development in which the young are identical to adults except for size and sexual maturity. No instar has wings and there is no metamorphosis.
Amictic egg The thin-shelled, parthenogenetic, diploid, rotifer egg that cannot be fertilized and develops into amictic females. Also known as subitaneous or summer eggs.
Ammonotelic Producing ammonia as the end product of nitrogen metabolism. 
Amphiblastula Sponge larva which is hollow. One hemisphere is composed of small flagellated cells and the other is composed of large nonflagellated macromeres.
Amphid Paired, anterior chemo- and mechanosensory organs of many nematodes.
Amphidectic In bivalves; the hinge ligament extending anterior and posterior to the umbo.
Ampulla (pl. Ampullae) Small, muscular sac attached to an echinoderm tube foot that bulges into the perivisceral coelom. The posterior, usually expanded, end of the phoronid body.
Analogy Similarity resulting from evolutionary convergence rather than common ancestry.
Anamorphic Development in which the young at hatching, have only a part of the adult complement of segments, i.e. indirect development.
Anastomose Branching and rejoining in a complex pattern.
Ancestrula (pl. Ancestrulae) The bryozoan zooid that develops from the egg and which produces, by cloning, all subsequent zooids of the colony.
Anisomyarian Unequal adductor muscles; resulting from reduction of the anterior adductor.
Antenniform Shaped like an antenna, i.e. whiplike and composed of a series of small articles.
Anoxic Without oxygen; less than 1 mg/L dissolved oxygen.
Aphotic zone Region of the ocean or a lake in which, due to insufficient light, respiration exceeds photosynthesis.
Apical field The anterior cilia-free area surrounded by the circumapical band of rotifers.
Apodeme An internal projection of the arthropod cuticle to which muscles are attached.
Apomorphic Refers to an evolutionarily derived state of a homolog.
Apophysis A projection, either internal or external, of the arthropod exoskeleton.
Apopyle Outlet from a flagellated chamber to an excurrent canal in leuconoid sponges.
Aposematic Warning coloration typical of toxic, noxious, or otherwise dangerous species.
Arborescent Branching in a tree- or bushlike pattern.
Archenteron The embryonic gut formed during gastrulation.
Architomy Form of fission in which some planarians simultaneously fragment the body into several pieces.
Aristotle’s lantern Highly developed chewing apparatus used for feeding by sea urchins.
Article A section of an arthropod appendage between successive joints.
Articulate To connect by means of a joint.
Artifact Anything introduced by the process of observation and that is not a natural part of the living organism. Also, an external product of the organism.
Asconoid A sponge body that is a simple cylinder and always small.
Ascus Internal pressure‑regulating sac of some cheilostome bryozoans.
Astaxanthin The red pigment in some crustaceans.
Athecate Refers to those hydroids that lack a hydrotheca.
Atoke In polychaetes showing epitoky, the non‑reproductive, benthic individual.
Atoll Reef that rests on the summit of a submerged volcano.
Atrium (pl. Atria) Internal cavity through which water flows in asconoid sponges (spongocoel). The internal cavity that receives the outflow of water from the pharynx in hemichordates and chordates.  In molluscs, the heart chamber(s) receiving oxygenated blood from the gills; also auricle
Auricularia Primary larval stage in holothuroid development.
Autochthonous Arising within the organism or other entity.
Autogamy Nuclear reorganization without conjugation or exchange of micronuclear material between two protozoans.
Autosperm Sperm produced by an individual.
Autotomy Self amputation. Deliberate loss of appendages, typically at specialized fracture zones.
Autotrophic Type of nutrition in which organic compounds are obtained by reduction of CO 2.
Autozooid Typical feeding zooids of bryozoans and some colonial anthozoans.
Avicularium (pl. Avicularia) Jawed heterozooid found in many cheilostome bryozoans.
Axial rod Tough, collagenous endoskeleton of gorgonians.
Axil The angle between a branch or appendage and the body from which it arises.
Axopodium (pl. Axopodia) Fine, needle‑like pseudopodium that contains a central bundle of microtubules.
Axoneme  Microtubules and other proteins composing the core of flagella and cilia.
Barrier reef Reef whose platform is separated from the adjacent land mass by a lagoon.
Basal body An organelle equivalent to a centriole at the base of flagellum or a cilium.
Basal lamina Thin, collagenous, fibrous sheet secreted by epithelial cells and on which they rest.
Basement membrane The layer of fibrous connective tissue under the epidermis consisting of the basal lamina plus additional connective tissue. 
Basis A bulbous, secreted structure that supports the hoplonemertean proboscis stylet. The attached calcified floor of a sessile barnacle. The second of two basal articles of the crustacean limb.
Bathyal The ocean bottom between 200 and 4000 m, roughly equivalent to the continental slope.
Bathypelagic The subdivision of the pelagic zone of the ocean between 1000-4000 m.
Benthic The bottom of a body of water. Organisms living on or in the bottom.
Benthos Community of organisms that lives on or in the bottom of a water body.
Bilateral symmetry  Body plan in which there is a single plane of symmetry.
Binary fission Asexual division that produces two similar individuals.
Bipectinate A gill in which the filaments arise on both sides of the axis.
Biphasic A life cycle with benthic and pelagic phases.
Bipinnaria Primary free‑swimming larval stage of asteroids.
Biradial symmetry Body plan with two planes of symmetry. 
Biramous An annelid or arthropod appendage with two branches.
Blastaea Hypothetical ancestor that is suggested by the blastula stage which occurs in the development of all animals.
Blastema Dome‑shaped mass of unspecialized cells that forms beneath the epidermis prior to healing and regeneration and is the source of new cells.
Blastocoel The fluid or gel‑filled embryonic cavity beneath the germ layers. The embryonic connective‑tissue compartment.
Blastomere A cell resulting from the cleavage divisions of the zygote.
Blastopore Primary opening of the archenteron to the exterior of the embryo.
Blastostyle A reduced, finger‑like gonozooid that bears gonophores.
Blastozooid A tunicate bud that arises from the body of the oozooid.
Blastula (pl. Blastulae) A sphere of blastomeres created by repeated cleavage divisions of the zygote.
Blood‑vascular system Circulatory system that develops within the connective tissue.
Body ciliature Cilia distributed over the general body surface of ciliates.
Body whorl The last and largest whorl of the gastropod shell.
Bonellin Echiuran dermal pigment that may have antibiotic properties.
Brachiolaria Second asteroid larva, following the bipinnaria, marked by the appearance of three adhesive arms at the anterior end.
Brachiole Slender, pinnule‑like projection of fossil echinoderms.
Brackish  Diluted sea water intermediate in salinity between sea water and fresh water.
Branchium  A gill.
Brood To care for developing eggs outside the body.
Brown body  A dark sphere of waste-containing cells that remains lodged in the coelom following regression by bryozoan polypides.
Buccal cavity Cavity just inside the mouth opening. The first region of the gut. 
Buccal field  A large ventral ciliated area which surrounds the mouth of some rotifers.
Bud Protozoans: The smaller of two progeny cells resulting from fission. Metazoans: Asexually‑produced progeny that either remains attached to the parent as a colonial zooid or undergoes differentiation before being released as a separate individual.
Bulbous pharynx Platyhelminth pharynx characterized by a sucking muscular bulb.
Bursa (pl. Bursae) A pouchlike structure. Commonly refers to a female reproductive chamber for the reception and temporary storage of sperm received at copulation. The ten respiratory invaginations are at the bases of the arms of many ophiuroids.
Byssus  A bundle of secreted protein threads used to attach some bivalves to the substratum.
Calcareous  Composed of calcium carbonate.
Calymma A broad vacuolated cortex formed by extracapsular cytoplasm that surrounds the central capsule of radiolarians.
Calyx Skeletal cup of a crinoid disc. The body and tentacles of an entoproct.
Campanulate Bell-shaped
Capitulum  The body of stalked barnacles exclusive of the stalk.
Captaculum (pl. Captacula) A threadlike feeding tentacle of scaphopod molluscs.
Carapace  The fold of body wall that extends posteriorly from the arthropod head to cover some or all of the trunk.
Carina  Posterior median plate of the barnacle exoskeleton. One of the five primary plates.
Casting Continuous pile of defecated organic and mineral matter.
Caudal gland  A posterior spinneret typical of many free‑living nematodes.
Central capsule The membrane‑enclosed, innermost cytoplasm of a radiolarian cell.
Centriole Microscopic cylindrical structure, composed of microtubules, which is situated at each pole of the mitotic spindle and is distributed to daughter cells during mitosis. There it may function as a basal body and give rise to a flagellum or cilium.
Centrolecithal  Refers to an arthropod egg which gives rise to a blastula in which the yolk is central and surrounded by peripheral cytoplasm.
Centrosome Structure from which bundles of microtubules radiate outwards.
Cephalic gland Slime secreting gland of nemerteans.
Cephalization Tendency to coalesce the segmental ganglia into a large anterior neural center.
Cephalothorax  The combined head and thorax.
Ceras (pl. Cerata) Projection from the dorsal body surface of many nudibranchs.
Cercaria (pl. Cercariae) Free‑swimming developmental stage of digenean trematodes.
Cerebral organ One of a pair of ciliated sensory canals associated with the nemertean brain.
Cetacea  Order of marine mammals containing whales and porpoises.
Chaeta A cuticular bristle composed of b -chitin. 
Chain A free‑swimming aggregate of sexual zooids in salps.
Chelate  Refers to appendages that are pincer‑like consisting of movable and immovable fingers.
Chelicera (pl. Chelicerae) The anteriormost appendages of chelicerates.
Cheliped A chelate thoracic appendage of decapod crustaceans.
Chilarium (pl. Chilaria) The appendage of the first abdominal segment of horseshoe crabs. Chitin A polysaccharide of polymerized N-Acetylglucosamine residues.
Chlorocruorin Type of polychaete hemoglobin that is green in color.
Chondrophore A depression in the hinge housing the inner ligament of some bivalves.
Chorion The shell‑like membrane secreted by ovarian follicle cells that surrounds the eggs when they reach the oviduct.
Chromatophore  A cell or organ that expands or contracts to alter the color of the organism.
Cilium (pl. Cilia) Characteristic of many protozoan and metazoan cells, a motile outgrowth of the cell surface that is typically short and its effective stroke is stiff and oarlike.
Cingulum  Dinoflagellates: horizontal or transverse groove that bears the transverse flagellum. Rotifers: posterior (postoral) band of cilia of the divided corona.
Circumapical band  A ribbon of cilia encircling the anterior end of the rotifer head.
Cirrus (pl. Cirri) Name given to various appendages, usually tentacle‑like and curled.
Cirrus sac Contains the internal seminal vesicle, prostate glands, and cirrus of some platyhelminths.
Cnida (pl. Cnidae) An eversible cnidarian organelle that occurs in a cnidocyte.
Cnidocil A short, stiff, bristle‑like cilium that is borne on a cnidocyte.
Cnidocyte A cnidarian cell that contains an eversible cnida.
Cnidosac Distal tip of a ceras of cnidarian‑eating nudibranchs. The sac is an extension of the gut and contains undischarged nematocysts acquired from the prey.
Coenecium A branching tubular network inhabited by pterobranch colonies that is secreted from glands in the oral shields of the zooids.
Coelenteron The body cavity and gut of cnidarians and ctenophores. Gastrovascular cavity.
Coeloblastula Blastula having a well developed blastocoel.
Coelom Body cavity lined by a mesodermally derived epithelium.
Coelomate An animal having a coelom.
Coelomocyte A circulating coelomic cell which may or may not contain a respiratory protein.
Coelomoduct A mesodermally derived duct leading from a coelom to the exterior. Usually a gonoduct.
Coenenchyme All of the tissue situated between polyps in anthozoan colonies.
Coenosarc Ther living tissue underlying the cuticular perisarc of hydroids.
Collagen Common animal fibrous protein that forms extracellular skeletal materials.
Collar Anthozoans: Circular fold at the junction of the column and the oral disc. Enteropneusts: The second of three body divisions.
Collencyte A fixed cell of sponges that is anchored by long, cytoplasmic strands and secretes dispersed collagen fibers (not spongin).
Colloblast An adhesive cell situated on the tentacles of ctenophores.
Collum The first anterior, legless segment of millipedes that forms a collar behind the head.
Colony Body composed of structurally joined zooids that share resources.
Columella Central axis of asymmetrical shells around which whorls are coiled.
Columnar epithelium  Epithelium of elongated cells.
Comb A flat paddle of fused cilia in ctenophores.
Comb row One of eight ciliary bands of ctenophores, each composed of a series of combs.
Commensalism A type of symbiotic relationship in which one species benefits from the relationship and the other species (host) is neither benefited nor harmed.
Commissure A more or less transverse nerve that joins the two ganglia of a pair.
Compact Body without a large fluid filled space: acoelomate.
Complemental male A male barnacle that develops attached to a hermaphrodite individual.
Complete cleavage Cleavage furrows extend completely through the egg mass; holoblastic.
Compound eye An arthropod eye composed of multiple ommatidia.
Compressed Flattened laterally, from side to side.
Conchiferan “Shell‑bearers”, includes monoplacophoran, gastropod, bivalve, scaphopod, and cephalopod molluscs.
Conchiolin The secreted molluscan protein of which the periostracum, byssus, and operculum are composed. 
Confamilial Belonging to the same family.
Congeneric Belonging to the same genus.
Conjugant One of a pair of fused ciliates in the process of exchanging genetic material.
Connective A more or less longitudinal nerve that connects two ganglia of different pairs.
Connective tissue Body layer between epithelia, composed of a fluid or gel extracellular matrix with or without cells.
Connective tissue compartment ··Body layer occupied by connective tissue.
Conspecific  Belonging to the same species. 
Contractile vacuole Large spherical vesicle responsible for osmoregulation in protozoans and some sponge cells.
Contractile vacuole complex Protozoan system of water and ion pumping organelles.
Convergence Independent evolution of similar structures.
Copraphagy Ingestion of feces. 
Coracidium A ciliated free‑swimming developmental stage of cestodes.
Cordate Heart-shaped.
Corona Ciliated organ at anterior end of rotifers used for feeding and swimming.
Cortex An outer ectoplasmic layer.
Cosmopolitan Worldwide distribution. 
Coxa (pl. Coxae) The proximal article of an arthropod appendage.
Cryptobiosis A desiccated, metabolically inactive, resistant condition. 
Ctenidium (pl. Ctenidia) A molluscan gill.
Cuboidal epithelium Epithelium in which the cells are roughly cubical in shape.
Cursorial Running. 
Cuticle Protective or supportive, nonliving, external, layer secreted by the epidermis.
Cyclomorphosis Seasonal changes in body shape or proportions.
Cydippid A free‑swimming ctenophore larva having an ovoid or spherical body.
Cyphonautes Planktotrophic larva of some species of nonbrooding gymnolaemate bryozoans.
Cypris An ostracod-like, settling larval stage of barnacles.
Cysticercus Developmental stage of certain tapeworms, following the oncosphere, and characterized by a fluid‑filled oval body with an invaginated scolex.
Cystid The exoskeleton and body wall of bryozoans.
Cytopharynx Permanent oral canal, or passageway, of ciliates that is separated from the cytoplasm by the cell membrane.
Cytoproct Permanent cellular anus of some ciliates.
Cytostome Cell mouth.
Dactylozooid A finger‑shaped, defensive, hydrozoan polyp.
Dedifferentiation Loss of specialized cellular features returning to a more generalized condition. Characteristic of certain aspects of development, especially regeneration.
Definitive host The host for the adult stage of a parasite.
Dendritic Treelike
Dendrobranchiate Having bushy, branching gills. 
Deploying point A site of separation of an asexually‑produced group of salp blastozooids from other such groups.
Deposit feeding Feeding upon detritus that has settled to the bottom of aquatic environments.
Depressed Flattened dorsoventrally. 
Derived Changed evolutionarily from the ancestral condition.
Determinate cleavage Developmental process during which the fates of the blastomeres are fixed early in cleavage; mosaic development.
Detritus Fragments of dead plants or animals.
Deuterostome Member of a major branch of the animal kingdom in which the site of the blastopore is posterior—far from the mouth, which forms as a new opening at the anterior end.
Diapause A period of arrested metabolism to survive adverse environmental conditions.
Diastole The relaxation, or dilation, phase of a heart beat.
Dicondylic Articulated by two movable hinges, or condyles.
Dioecious Having separate sexes; gonochoric.
Digitiform Finger-shaped.
Dimorphism Exhibiting two shapes or appearances.
Diotocardian Heart with two atria. 
Diploblastic With only two embryonic germ layers.
Diplosegment Double trunk segments derived from the fusion of two separate segments.
Direct deposit feeding (non-selective deposit feeding) Indiscriminate ingestion of mixed organic and mineral particles with no selection or sorting prior to entry into the mouth.
Direct development Lacking a larval stage. On hatching the young have the adult body form.
Directive Either of two pairs of septa at each edge of the compressed anthozoan pharynx.
Distal  Distant from the center, origin, or midline. 
Diurnal  Active during the day. 
Diverticulum An outpocketing or pouch, cecum.
Doliolaria Barrel‑shaped larval stage, following the auricularia, of holothuroids.
Dormant egg  Anegg capable of adverse conditions for long periods before hatching.
Dorsal lamina Longitudinal tissue fold along the inner dorsal pharyngeal wall of some ascidians. Gathers mucous net and food and conveys them into the esophagus.
Duogland Secretory system consisting of cells producing an adhesive and others producing a compound to inactivate the adhesive. 
Dwarf male A male reduced in size through degeneracy or loss of structures.
Ecdysis The periodic loss of the exoskeleton; molting.
Ecdysone Hormone that promotes molting.
Echinopluteus Planktotrophic larva of echinoid echinoderms that bears six pairs of long larval arms.
Eclosion Emergence of the imago from the pupa or last nymphal cuticle but sometimes, confusingly used as a synonym for hatching from the egg.
Ectoderm Embryonic germ layer composing the outer wall of the gastrula.
Ectoparasite Parasite that lives on the outside of its host.
Ectosymbiont An symbiont living outside its host.
Electron dense  Appearing dark in electron photomicrographs.
Electron lucid  Appearing clear in electron photomicrographs.
Elytrum (pl. Elytra) Platelike scale, modified from a dorsal cirrus, that is borne on a short stalk on the dorsal side of the body of scaleworm polychaetes.
Embryonated egg  An embryo, rather than an ovum, enclosed in an egg shell.
Encystment Forming resistant cysts in response to unfavorable conditions such as lack of food or desiccation.
Endemic Species found in a restricted geographic area and nowhere else.
Endocytosis Process in which some extracellular materials enter a cell in minute pits on the cell’s membrane that later pinch off internally.
Endoderm Embryonic germ layer composing the archenteron wall.
Endogastric coiling The shell coils posteriorly, over the foot.
Endoparasite Parasite that lives inside its host.
Endosymbiont A symbiont living inside its host.
Endoral membrane Ciliate undulating membrane that runs transversely along the right wall and marks the junction of the vestibule and buccal cavity.
En face  Head on.
Enterocoel Coelomic cavity formed from an outpocketing of the embryonic archenteron.
Enteronephric Refers to either typical or modified nephridia that open into various parts of the digestive tract of earthworms.
Enzymatic‑gland cell Cell responsible for the secretion of digestive enzymes into the cnidarian coelenteron.
Ephemeral  Short-lived, brief. 
Ephippium (pl. Ephippia) A resistant egg capsule formed in the cladoceran brood chamber.
Ephyra (pl. Ephyrae) An immature scyphomedusa.
Epiathroid Nervous system in which the cerebral and pleural ganglia are contiguous.
Epibenthic Living on or just above the bottom of an aquatic habitat.
Epiboly Type of morphogenetic movement in gastrulation in which ectodermal cells overgrow the inner germ layers.
Epicuticle Thin, outer, proteinaceous layer of the arthropod skeleton.
Epidermal replacement cell A platyhelminth parenchymal cell that migrates from the parenchyma to the body surface and replaces a damaged or destroyed epidermal cell.
Epidermis Outer epithelial layer of the body.
Epifauna The animals that live on the surface of ocean, lake, and stream bottoms.
Epigastric coiling The shell coils anteriorly, over the head. 
Epigean  Above ground. 
Epimorphic Development in which the young hatch with the full complement of segments, i.e. direct development.
Epipelagic The uppermost layer, to a depth of 200 m, of the pelagic zone, roughly equivalent to the euphotic zone.
Epiphytic Living on the surface of a plant.
Epiplasm Dense supportive mesh formed by filamentous proteins in the cortical cytoplasm.
Epitheliomuscular cell A cnidarian contractile cell that has characteristics of both epithelial and muscular cells.
Epitoky Reproductive phenomenon in some polychaetes: the production, either by transformation or budding, of a reproductive individual (epitoke) adapted for a pelagic existence from a nonreproductive individual adapted for a benthic existence.
Epizoic Living on the surface of an animal.
Equilateral Anterior and posterior ends of a bivalve valve are of similar shape and size. 
Equivalve The two valves of a clam being the same size and shape. 
Esthete A sensory organ in a minute vertical canal in the upper layer of the chiton shell plate.
Estivation (= aestivation)A dormant state in which some animals pass hot, dry seasons.
Estuary Embayment at the junction of a river with the sea, typically with brackish water.
Eukaryotic A cell with membrane‑bound organelles including nucleus and mitochondria.
Eulamellibranch gill Bivalve gill with filaments joined together by continuous sheets of tissue.
Euphotic zone Upper layer of water, 0-100 m depending on turbidity, in which there is sufficient light to support photosynthesis in excess of respiratory needs.
Eutely Having an invariant, species-specific, and genetically‑fixed number of cells or nuclei.
Euthyneury Symmetrical, untwisted, detorted, gastropod nervous system.
Euryhaline Tolerant of a wide range of environmental salinities.
Evert Protrusion by turning inside out.
Evisceration When the anterior or posterior end of a species ruptures and parts of the gut and associated organs are expelled.
Exconjugant Ciliates that have separated after sexual reproduction.
Exocytosis Process in which indigestible material is released from a cell to the exterior by fusion of the residual vesicle with the cell membrane.
Extrinsic A muscle extending from one structure to another.
Exumbrella Aboral, upper surface of the bell of a medusa.
Fasciole One of several ciliated spines of certain echinoids that together form a siphon.
Filibranch gill Bivalve gill in which filaments are held together by tufts of cilia.
Filiform Having the shape of a filament or thread.
Filopodium (pl. Filopodia) Pseudopodium that is slender, clear, and sometimes branched.
Filter feeding A type of suspension feeding in which organic particles (plankton and detritus) are removed from a water current by a filter.
Fin box One in a longitudinal series of small, median, unpaired coelomic cavities that form and help to support the dorsal and ventral fins of cephalochordates.
Fin ray Any of several stiff, slender structures that support a fin.
Fission Asexual division of an organism into two or more progeny.
Fixed parenchymal cell A large, branched, mesodermal cell of platyhelminths that makes contact with and interjoins other cells and tissues.
Flagellum (pl. Flagella) A characteristic of many protozoan and metazoan cells; it is typically long and its motion is a complex whip‑like undulation.
Flame cell A protonephridial terminal cell that has many flagella, which beat synchronously and resemble a minute flickering flame; its nucleus is at the base of the flame. Flame bulb. 
Flosculi Cuticular sensory structures consisting of monociliated cells with a collar of microvilli. 
Foliaceous Erect, leaflike, bryozoan colony composed of one or two sheets of zooids.
Food vacuole Cellular vesicle containing ingested food.
Foot Muscular, flattened, ventral surface of a mollusc, forming a creeping sole.
Forcipule Appendage of the first centipede trunk segment; poison claw.
Fossorial Adapted for digging. 
Free living 1. Not parasitic. 2. Not permanently attached to a substratum. 
Fringing reef Reef that extends seaward directly from the shore.
Frontal gland Anterior aggregation of secretory cells in platyhelminths.
Fruticose Erect bushlike bryozoan colony.
Funiculus (pl. Funiculi) A mesothelial cord extending across the bryozoan coelom.
Fusiform Spindle- or cigar-shaped, i.e. thick in the middle and tapered bluntly at both ends.
Gamogony Multiple fission that forms gametes that fuse to form a zygote.
Ganglion (pl. Ganglia) An aggregation of neuronal cell bodies.
Gap junction Intercellular junction that allows for intercellular communication, such as electrical coupling of muscle cells.
Gastric filament One of several cnidocyte‑bearing threads that extend into the scyphozoan stomach from the septa between gastric pockets.
Gastric mill Part of the malacostracan cardiac stomach where food is triturated by internal teeth.
Gastric pouch or pocket One of four pockets in the wall of the scyphozoan stomach.
Gastrodermis Cellular epithelial lining of the gastrovascular cavity of cnidarians and ctenophores and the midgut lining of bilaterally symmetrical animals.
Gastrolith A calcareous concretion in the stomach of some crustaceans for calcium storage.
Gastrovascular cavity Internal extracellular cavity of cnidarians and ctenophores lined by gastrodermis.
Gastrozooid Nutritive or feeding polyp of cnidarians which is similar to a short hydra.
Gastrula A two‑layered embryo.
Gastrulation The developmental establishment of germ layers.
Geniculate Bent at a sharp angle, like an elbow. 
Genital atrium A small chamber in parasitic platyhelminths that receives the openings of both the male and female reproductive systems.
Gestate To care for developing eggs inside the maternal body.
Gill An outward expansion of the body surface for the purpose of gas exchange in water.
Girdle The thick, stiff, peripheral area of the chiton mantle laterally beyond the shell plates.
Glycocalyx (pl. Glycocalyces) The carbohydrate and protein surface coat of eukaryotic cells.
Gnathobase Spiny medial surface of the basal articles of many arthropod limbs.
Gnathochilarium A broad flattened plate formed of a fused pair of maxillae in millipedes.
Gnathopod Each of the second and third thoracic appendages of amphipods.
Gonangium (pl. Gonangia) Type of gonozooid that consists of a central blastostyle bearing gonophores and is surrounded by an extension of the perisarc (gonotheca).
Gonochoric Separate sexes (dioecious).
Gonoduct Principal duct providing for the transport of sperm or eggs in any reproductive system.
Gonophore A hydroid reproductive bud that bears the germ cells and may become a free‑swimming medusa or a variously modified sessile medusa. Medusoid.
Gonopore External opening of any reproductive system.
Gonotheca (pl. Gonothecae) An extension of the perisarc around a gonozooid.
Gonozooid A hydrozoan reproductive polyp which is often reduced, lacking mouth and tentacles, and bears gonophores. A sexually reproductive zooid of thaliaceans.
Gorgonin A tanned collagen.
Gravid Bearing developmental stages, such as eggs or embryos, internally.
Gross Large scale, not fine or delicate, i.e., not microscopic or ultrastructural.
Facies A characteristic shape or appearance. 
Growth zone Region that includes all of the larva between the mouth and telotroch on the fully developed trochophore larva.
Hadal The deep oceanic trenches at depths greater than 6000 m.
Halteres Reduced second pair of dipteran (fly) wings, functions as a gyroscope to maintain stability in flight.
Haptocyst Special adhesive organelle borne on the tentacles of suctorians.
Haptor Attachment organ that bears hooks and suckers.
Hatschek’s groove A shallow ciliated invagination of the dorsal wall of the vestibule of cephalochordates.
Hemal system Blood‑vascular system.
Hemimetabolous Insectdevelopment characterized by nymphs that do not closely resemble adults but that do not undergo a radical metamorphosis.
Hemocoel A voluminous, blood‑filled cavity, occupying much of the body.
Hermaphroditic Having both male and female reproductive systems in the same individual. When both systems are present at the same time, the hermaphroditism is said to be simultaneous; when the male system appears and functions first and is followed by the female system, the hermaphroditism is said to be protandric.
Heterogony Alternating sexual and asexual phases in a life cycle.
Heteronomy Segments and appendages regionally specialized.
Heterotrophic Nutrition in which organic compounds are obtained by consuming other organisms.
Heterozooid Modified bryozoan zooids that have functions other than feeding.
Higgins larva Larval stage of loriciferans.
Hinge ligament A noncalcified, elastic, proteinaceous band joining the two valves of a bivalve.
Holoblastic cleavage Cleavage furrows extend completely cut through the egg mass.
Holometabolous Insect development in which larvae and adults are distinctly different and a major metamorphosis is required to transform the juvenile into the adult.
Holonephridium (pl. Holonephridia) A typical, segmental metanephridial duct of an oligochaete.
Holoplankton Plankters that spend the entire life cycle in the plankton.
Holothurin Toxic substance released in the Cuvierian tubules of certain holothuroids.
Homolecithal egg Egg in which the yolk is uniformly distributed. Isolecithal.
Homology Similarity of structure attributable to common ancestry in two or more species.
Homolog A characteristic a species that shares a common genetic, evolutionary, and developmental origin with a characteristic in another species.
Homonomy All segments and appendages alike, without regional specialization. 
Hyaline T ranslucent or transparent, clear.
Hydranth The oral end of a hydroid polyp bearing the mouth and the tentacles.
Hydrocaulus The stalk of a hydroid polyp.
Hydrocoral Colonial, calcified polypoid hydrozoan with either an encrusting or an upright growth form.
Hydroid colony A collection of polyps in which each polyp is connected to the others.
Hydromedusa (pl. Hydromedusae)··Hydrozoan medusa.
Hydrorhiza (pl. Hydrorhizae) Horizontal rootlike stolon of a hydroid colony that grows over the substratum.
Hydrotheca (pl. Hydrothecae) A cuticle that encloses the hydranth. Theca.
Hyperparasitism A parasite parasitized by another parasite.
Hyperstrophic coiling Larval protoconch is coiled at right angles to the post-larval teloconch.
Hypoathroid Nervous system with the pedal and pleural ganglia contiguous. 
Hypobranchial gland Mucus‑secreting epithelium on the molluscan mantle roof.
Hypogean Subterranean, below ground.
Hypognathus Insect head orientation that causes the mouthparts to be directed downward.
Hypostome A mound or cone that bears the mouth of hydropolyps. Manubrium.
Hypoxia Less than 2 mg/L of dissolved oxygen
Imago The final instar, or adult stage, of an insect life cycle.
Incident light In microscopy, light striking the object from above the stage.
Incomplete cleavage Cleavage furrows do not completely cut through the egg mass; meroblastic.
Incurrent canal Tubular invagination of the sponge pinacoderm that leads into the flagellated chambers.
Incurrent pore Small opening on the surface of sponges that leads into an incurrent canal. Ostium.
Indeterminate cleavage Fate of the blastomeres is fixed relatively late in development. Regulative development.
Indirect deposit feeding (selective deposit feeding)Use of appendages, tentacles, or cilia to select organic particles for ingestion.
Indirect development Having a larval stage(s) between egg and adult.
Inequilateral anterior and posterior ends of a bivalve valve are dissimilar. 
Inequivalve The two valves of a clam of different sizes.
Infauna Animals that live within bottom sediments.
Infraciliary system The entire assemblage of ciliary basal bodies, or kinetosomes, and the fibers that link them together in the cell cortex of ciliates.
Infusoriform larva The final free‑swimming larval stage of rhombozoans.
Ingression Mode of gastrulation in which cells of the blastula wall proliferate cells into the blastocoel.
Insertion One of the two attached ends of a muscle. Of the two, the insertion is usually distal and moves when the muscle contracts.
Instar Each of the several stages between successive ecdysozoan molts.    
Integument The outer layers of the body wall. Usually comprising the epidermis and underlying connective tissue (dermis) plus any secreted cuticle or exoskeleton.
Intercellular junction Membrane specialization that binds cells together, promotes communication between cells, or helps to regulate transport across an epithelium.
Intermediate host The host for larval stages of a parasite.
Interstitial cell A small, rounded totipotent cnidarian cell, sandwiched between cells of the epidermis and gastrodermis.
Interstitial fauna Animals that live in the spaces between sand grains.
Intertidal The coastline between the low and high tide levels, also known as the littoral zone.
Intrinsic Confined within a structure; not extending to neighboring structures.
Introvert Eversible part of the bryozoan, priapulid, or sipunculan body.
Invagination Infolding. In gastrulation, this refers to a type of morphogenetic movement in which the cells of the vegetal hemisphere fold into the interior to form the archenteron.
Isolecithal egg Egg in which the yolk is uniformly distributed. Homolecithal.
Isomyarian Anterior and posterior adductor muscles approximately equal in size..
Jellyfish A cnidarian medusa.
Kairomone A substance released by a predator to which the prey may respond defensively.
Kenozooid A bryozoan heterozooid modified to form a stolon.
Kinetodesma (pl. Kinetodesmata) A fine striated fiber that connects kinetosomes (basal bodies) of ciliates.
Kinetoplast Conspicuous mass of DNA that is situated within the single, large, elongated mitochondrion of kinetoplastid (trypanosome) protozoans.
Kinetosome A ciliary or flagellar basal body.
Kinety (pl. Kineties) One row of cilia, kinetosomes, and kinetodesmata of ciliates.
Lacustrine  Pertaining to lakes. 
Lamella (pl. Lamellae) A sheet or flat plate of tissue. In bivalves, each of the gill surfaces.
Languet One of several folds of tissue along the dorsal pharyngeal wall of some ascidians which together convey food to the esophagus. A discontinuous dorsal lamina.
Lappet Lobe formed by the margin of the scalloped scyphozoan bell. Movable flaps that can expose or cover the ambulacral groove of crinoids.
Larva (pl. Larvae) An independent, motile, developmental stage that does not resemble the adult.
Larviparous Eggs brooded internally within the female that are later released as larvae.
Lateral canal Part of the echinoderm water‑vascular system that joins the radial canal and tube feet.
Laurer’s canal Short, inconspicuous canal that extends from the seminal receptacle of trematodes to the dorsal surface, where it may open at a minute pore.
Lecithotrophic brooding Viviparous development in which the embryo is nourished by yolk.
Lecithotrophic larva A nonfeeding larva that utilizes yolk as a source of nutrition.
Lemniscus (pl. Lemnisci) Fluid filled invaginations of unknown function in acanthocephalans. 
Leuconoid Refers to a type of sponge body organization built around flagellated chambers and an extensive system of canals.
Ligament sac A connective‑tissue sac in the acanthocephalan hemocoel containing the gonads.
Littoral In the sea; synonymous with intertidal. In lakes; the nearshore zone in which light sufficient to support rooted vegetation reaches the bottom.
Lobopodium (pl. Lobopodia) A pseudopodium that is rather wide with rounded or blunt tips, is commonly tubular, and is composed of both ectoplasm and endoplasm.
Longitudinal cord Ridge that entends the length of the body created by the inward expansion of the epidermis in nematodes and some gastrotrichs.
Lophophoral organ An area on the phoronid lophophore where spermatophores are formed.
Lophophore A circular or horseshoe‑shaped fold of the mesosomal body wall encircling the mouth, bearing hollow ciliated tentacles, and excluding the anus.
Lorica In rotifers; an intracytoplasmic skeleton.
Luciferase Enzyme that catalyzes the bioluminescence reaction.
Luciferin The substrate of luciferin capable of bioluminescence.
Lunule One of the large, elongated notches or openings in the bodies of some clypeasteroids (sand dollars).
Lyriform organ Group of slit sense organs found on some arachnids.
Macerate To soften and separate the parts of a solid object.
Macromere One of several large blastomeres located in the yolky vegetal hemisphere of early embryos.
Macronucleus (pl. Macronuclei Large, usually polyploid, ciliate nucleus concerned with the synthesis of RNA, as well as DNA, and therefore directly responsible for the phenotype of the cell.
Macrophagous Collecting and ingesting large food particles.
Madreporite Pore or sieve plate of the echinoderm water‑vascular system that connects the stone canal to the exterior seawater (most echinoderms) or to the perivisceral coelomic fluid (crinoids and holothuroids).
Malpighian tubule Excretory system consisting of a blind, tubular, contractile, excretory evagination of the arthropod midgut.
Manca larva A peracarid postlarva that has all appendages except the eighth thoracopods. 
Mangrove A small tropical tree or large shrub adapted for living in the intertidal zone.
Mantle A body wall fold that secretes a shell, as in molluscs, barnacles, and brachiopods.   The body wall beneath the ascidian tunic.
Mantle cavity Protective chamber created by the overhang of a mantle; pallial cavity.
Manubrium (pl. Manubria)··Tubelike extension, bearing the mouth, that hangs down from the center of the subumbrella of cnidarian medusae. Hypostome of hydroid polyps.
Marsupium (pl. Marsupia) Brood pouch outside the body.
Mastax The cuticular pharyngeal jaw apparatus of a rotifer.
Mastigoneme One of the many fine, lateral branches of some flagella.
Mastigont system Complex formed by groups of flagella and several microtubular and fibrillar organelles.
Matrotrophic brooding Viviparous development in which the embryo is nourished by the mother. 
Medulla Central part of the heliozoan cell that is composed of dense endoplasm, containing one to many nuclei and the bases of the axial rods.
Medusa (pl. Medusae) Form of cnidarian that has a well developed, gelatinous mesoglea and is generally free‑swimming.
Megalops Crab postlarva with a large abdomen and full complement of appendages.
Megasclere A large spicule forming one of the chief supporting elements in the skeleton of sponges.
Mehlis’s gland Conspicuous unicellular gland cells associated with the reproductive system of trematodes which play a role in egg capsule formation.
Meiofauna Small metazoans that pass through a 1 mm sieve but are retained by a mesh of 42 µm; usually referring to those living in small confined spaces.
Membranelle Type of ciliary organelle derived from two or three short rows of cilia, all of which adhere to form a more or less triangular or fan‑shaped plate that beats as a unit.        
Meroblastic Cleavage furrows do not completely cut through the egg mass.
Meroplankton Plankters that spend only part of the life cycle in the plankton.
Merozoites Individuals produced by multiple fission of sporozoan trophozoites.
Mesenchyme A network of loosely associated, often motile, embryonic cells, that are usually, but not always, of mesodermal origin. The term is still applied to adult connective tissues of some groups of animals.
Mesentery (pl. Mesenteries) A longitudinal sheet of tissue that divides the body cavity of bilaterally‑symmetrical animals.
Mesentoblast Blastomere associated with spirally cleaving zygotes that contains an unidentified cytoplasmic factor which causes the cell and its progeny to form mesoderm.
Mesoderm Embryonic germ layer that forms the tissues situated between ectoderm and endoderm.
Mesoglea Connective‑tissue layer between the epidermis and gastrodermis of cnidarians and ctenophores.
Mesohyl Sponge connective tissue. Lies beneath the pinacoderm and consists of a gelatinous proteinaceous matrix containing skeletal material and ameboid cells.
Mesopelagic Subdivision of the pelagic zone, 200-1000 m.
Mesothelium (pl. Mesothelia) Single, nonstratified epithelium lining the coelom.
Metacercaria (pl. Metacercariae) Encysted final stage of digenean development.
Metachrony Wave pattern that results from the sequential coordinated action of cilia or flagella over the surface of a cell or organism.
Metamere A body segment or somite.
Lacunar canal system  Acanthocephalan circulatory system within the syncytial epidermis.Metamerism Segmentation;division of the body into a linear series of similar modules.
Metamorphosis (pl. Metamorphoses)··Transformation from a larva into an adult.
Metanauplius (pl. Metanauplii) One of several instars following the crustacean nauplius.
Metanephridial system Excretory system composed of a vascular ultrafiltration site, a coelomic space, and a metanephridium tubule.
Metanephridium (pl. Metanephridia) An excretory tubule that opens into the coelom by a ciliated funnel and to the exterior by a nephridiopore.
Metatroch A second girdle of cilia that develops posterior to the prototroch of a trochophore.
Micromere One of many small blastomeres located in the animal hemisphere of the cleaving zygote.
Micronucleus (pl. Micronuclei)··Small, usually diploid, ciliate nucleus concerned primarily with the synthesis of DNA. It undergoes meiosis before functioning in sexual reproduction.
Microphagous Specialized for feeding on small food particles.
Micropyle An opening in the eggshell or resting stage from which the primordium eventually emerges.
Microsclere A tiny sponge spicule.
Microtrich Type of microvillus found on the tegument of tapeworms.
Microtubule organizing center (MTOC) A region around basal bodies and centrioles that controls the organized assembly of microtubules.
Mictic egg Type of fertilized rotifer egg that is thin‑shelled, haploid, and can be fertilized.
Milieu  Environment. 
Miracidium (pl. Miracidia) Ciliated, free‑swimming, first larva of digenean trematodes.
Molt To shed the old cuticle as a new cuticle is being secreted. Ecdysis.
Monocondylic Articulated by one movable hinge (condyle).
Monolayered epithelium Consisting of a single layer of cells resting on a basal lamina (= simple epithelium).
Monomyarian Bivalve condition in which the anterior adductor muscle is lost.
Monopectinate Refers to a gill in which the filaments occur on only one side of the axis.
Monophyletic group All species descended from a common ancestor.
Monospecific A taxon consisting of a single species. 
Monotocardian Heart with one atrium.
Monotypic A taxon consisting of a single species.
Mosaic development Embryonic fate determination in which cell fate is determined early in development and is the result of the action of specific factors that are unevenly distributed, like pieces of a mosaic, in the cytoplasm of the uncleaved egg.
Mucocysts Mucigenic bodies that are arranged in rows, similar to ciliate trichocysts, and discharge a mucoid material.
Mucus Animal secretion utilized in a variety of ways as an adhesive, protective cover, or lubricant.
Mutualism A symbiotic relationship in which both species benefit.
Myocyte Type of sponge mesohyl cell which displays some similarities to a smooth muscle cell in shape and contractility. A muscle cell.
Myoepithelial cell A muscle cell that is part of an epithelium.
Myogenic Originating in a muscle cell. 
Myoneme A bundle of contractile filaments that lies in the pellicle of some protozoans.
Nacre The smooth, lustrous, usually innermost, shell layer of some molluscs; mother of pearl.
Natatory Adapted for swimming.
Naupliar eye Median crustacean eye composed of three or four ocelli.
Nauplius Earliest hatching stage and basic crustacean larva; has three pairs of appendages.
Neap tides Tides occurring on quarter moons characterized by modest tidal amplitudes. 
Nectophore Mouthless, pulsating swimming bell of siphonophores.
Nematocyst Stinging cnida of cnidarians.
Nematodesma (pl. Nematodesmata) One of several microtubular rods that line and support the wall of the ciliate cytopharynx and assist in the inward transport of food vacuoles.
Nematogen Adult dicyemid.
Neoblast A totipotent cell that is important in wound healing and regeneration.
Nephridium (pl. Nephridia) An excretory tubule usually opening to the exterior via a nephridiopore. See protonephridium, metanephridium.
Nephrocyte A large phagocytic cell, alone or in clusters, in the hemocoel of many arthropods.
Nephromyces A unicellular fungus that occurs in the renal sacs or the pericardium of some ascidians.
Nephrostome An open ciliated funnel at the inner, coelomic, end of a metanephridium.
Neritic zone The water above the continental shelves.
Nerve net Plexus.
Neurogenic Originating with a neuron. 
Neuropil  A concentration of axons and synapses in a ganglion. 
Neuropodium (pl. Neuropodia) The ventral branch of a polychaete parapodium.
Niche An organism's role in its ecosystem.
Nocturnal Circadian behavior characterized by activity at night. 
Non-selective deposit feeding See direct deposit feeding.
Notopodium (pl. Notopodia) The dorsal branch of a polychaete parapodium.
Nuchal organ One of a pair of ciliated chemosensory pits or slits that are often eversible and are situated in the head region of most polychaetes.
Nutritive‑muscle cell A muscle cell in the cnidarian gastrodermis that usually bears a cilium and is responsible for intracellular digestion.
Obturaculum (pl. Obturacula) One of two elongated, medially fused structures which arise anteriorly from the head of vestimentiferan pogonophores and bear and support the gills.
Occluding junction Sealing junction between cells.
Oceanic zone The division of the pelagic realm seaward of the continental shelf.
Ocellus (pl. Ocelli) A simple eye.
Odontophore A muscular and cartilaginous mass in the buccal cavity of many molluscs, it supports the radula.
Oligomery Division of the body into three linear regions, characteristic of many deuterostome animals. Tricoelomate.
Oncomiracidium (pl. Oncomiracidia) The ciliated larva of monogeneans.
Oncosphere Encapsulated first stage in the life cycle of certain tapeworms that bears six hooks and cilia. Typically referred to as a coracidium when released into the water.
Ontogeny The development of the individual from fertilization to death.
Oostegite A large medial platelike process of a thoracic coxae that contributes to a marsupium.
Ootype Small, centrally‑positioned sac within the female reproductive system of most parasitic platyhelminths.
Oozooid The zooid developing from the fertilized egg of urochordates.
Operculum (pl. Opercula) A lid or covering of an opening or chamber.
Ophiopluteus (pl. Ophioplutei) Planktotrophic larva of many species of ophiuroids.
Opisthodetic In bivalves; the hinge ligament situated posterior to the umbo.
Opisthognathus Posteriorly directed position of insect head.
Opisthosoma The posterior end of the pogonophore body which is composed of numerous (up to 95) segments. The posterior chelicerate tagma, also called the abdomen.
Oral arm One of the four, often frilly extensions of the scyphozoan manubrium.
Oral ciliature Cilia that are associated with the mouth region of ciliates.
Oral disc Area around the mouth of an anthozoan polyp which bears eight to several hundred hollow tentacles.
Oral shield One of a series of large plates that frame the ophiuroid mouth and also form a chewing apparatus with five triangular, interradial jaws at the center.
Oral sucker Organ that surrounds the trematode mouth, prevents dislodgement and aids in feeding.
Organ of Tömösvary Hygroreceptive or chemoreceptive organs on the tracheate head.
Origin One of the two attached ends of a muscle.  Of the two, the origin is usually proximal and remains stationary when the muscle contracts.
Osculum (pl. Oscula) The excurrent opening of the water circulation system of the sponge.
Osmoconformation Internal osmolarity is allowed to vary with external osmolarity.
Osmoregulation The maintenance of an internal osmolarity unlike the external. 
Ossicle An internal skeletal piece, commonly calcareous as in echinoderms.
Ostium (pl. Ostia) A small incurrent opening or pore on the surface of a body, gill, or heart.
Ovigerous Bearing eggs externally.
Ovigerous leg The third appendage of pycnogonids, used by the male to brood the fertilized eggs.
Oviparous Egg‑laying.                                                   
Paedogenesis  Achievement of sexual maturity as a larva, without attaining adult morphology. 
Pallial line The line of mantle attachment impressed on the inner surface of the shell as a scar.
Pallium Mantle.
Palmella Nonflagellated stage of flagellated protozoans.
Papula (pl. Papulae) Finger‑like, respiratory evagination of the aboral body wall of some asteroids.
Paramylon Photosynthetic storage product of euglenoids.
Paraphyletic A taxon containing some, but not all, of the descendants of an ancestor.
Parapodium (pl. Parapodia)··Lateral, fleshy, paddle‑like appendage on polychaete annelids.
Parasitism A symbiotic relationship in which one species (parasite) benefits from the relationship and the other species (host) is harmed but usually not killed.
Parasitoidism A prolonged intimate symbiosis in which one member eventually kills the other.
Paratomy The phenomenon of linear budding in some turbellarians and annelids.
Parenchyma Connective tissue compartment between the body wall musculature and gut of platyhelminths.
Parenchymula A sponge larva that lacks an internal cavity and bears flagellated cells over all of its outer surface except, often, the posterior pole. Parenchymella.
Parthenogenesis Unisexual reproduction with unfertilized eggs and no contribution by males.
Parturial molt The molt that results in the appearance of complete, functional oöstegites. 
Patch reef A small circular or irregular reef that rises from the floor of a lagoon behind a barrier reef or within an atoll.
Paurometabolous Insect development in which nymphs closely resemble adults but lack wings and are sexually immature.  There is no radical metamorphosis.
Paxilla (pl. Paxillae) An echinoderm ossicle crowned with small, movable spines.
Pectinate Having teeth or side branches arranged like a comb.
Pectine A comb-like sensory appendages unique to scorpions.
Pedal disc In some sea anemones, a flattened disc at the aboral end of the column for attachment.
Pedal laceration Method of asexual reproduction in some anemones in which parts of the pedal disc are left behind as the animal moves.
Pedicellaria (pl. Pedicellariae) A small, specialized jawlike appendage of asteroids and echinoids which is used for protection and feeding.
Pedicle Muscular, flexible stalk that attaches articulate brachiopods to the substratum.
Pedipalp The second chelicerate appendage, it is modified for a variety of functions.
Peduncle Muscular, flexible attachment stalk of goose barnacles.
Pelagic The water of the open ocean, including the neritic and oceanic zones. Also, organisms living in the water independent of the bottom.
Pelagosphera Secondary planktotrophic larva of sipunculans.
Pellicle Protozoan “body wall” composed of cell membrane, cytoskeleton, and other organelles.
Pellucid Clear, transparent.
Penetration anchor An anchor that holds one part of a burrowing animal’s body in place as another part penetrates and advances into the sediment.
Peniculus (pl. Peniculi) A modified membranelle that is greatly lengthened and thus tends to be similar to an undulating membrane in function.
Pentactula Metamorphosing stage of holothuroid development that bears five primary tentacles.
Pentamerous Divided into five parts, characteristic of the body of echinoderms.
Periostracum The outer proteinaceous layer of a molluscan shell, composed conchiolin.
Periproct The membranous area, often bearing ossicles, around the anus of echinoids.
Perisarc A supporting, nonliving chitinous cuticle secreted by the epidermis surrounding most hydroids.
Peristalsis A wave of muscular contraction moving along a body or internal tube or vessel. Peristome Buccal cavity of ciliates. The membranous area around the mouth of some echinoderms, i.e., sea urchins.
Peristomium The first true segment, immediately posterior to the prostomium, of an annelid. Usually lacks locomotory appendages.
Peritoneum The innermost, noncontractile layer of a stratified coelomic epithelium; separates the coelomic fluid from the musculature.
Petaloid One of five petal‑shaped areas on the aboral surface of irregular urchins that bear specialized respiratory podia.
Phagocytosis The engulfment of large particles, such as bacteria and protozoans, by evagination of the cell surface.
Pharynx (pl. Pharynges) An anterior gut region, often heavily muscularized.
Phorozooid A locomotory zooid of doliolids that has a short posterior spur upon which buds differentiate into gonozooids.
Photocyte Specialized cell within which light is produced.
Photophore A light‑producing organ.
Photosynthate The organic carbon fixed by the photosynthetic pathway.
Phyllobranch Having flat, leaflike gills. 
Phyllode Each of five oral ambulacral areas of irregular echinoids that contains specialized podia for obtaining food particles.
Phyllopod Flattened, leaflike appendage.
Phytoflagellate A photosynthetic flagellate.
Phytophagous Plant eating.
Phytoplankton Microscopic algae suspended in the water column of lakes and seas.
Pilidium A free‑swimming and planktotrophic larva of many heteronemerteans which is characterized by an apical tuft of cilia and is somewhat helmet‑shaped.
Pinacocyte One of the epithelial‑like flattened cells which together make up the sponge pinacoderm.
Pinnate Having side branches, like a feather.
Pinnule Side branch of an appendage, i.e., on octocoral tentacles, crinoid arms.
Pinocytosis A nonspecific form of endocytosis in which the rate of uptake is in simple proportion to the external concentration of the material being absorbed.
Planispiral All whorls of a coiled molluscan shell lying in a single plane. 
Plankton Organisms suspended in the water column and unable to move independently of water current because of small size or insufficient motility.
Planktotrophic larva A planktonic larva that feeds on other planktonic organisms.
Planula (pl. Planulae) A cnidarian larva that is elongated and radially symmetrical but with anterior and posterior ends.
Plasmodium (pl. Plasmodia) Amoeboid syncytial mass.
Pleopod The anterior abdominal appendages of malacostracans.
Plerocercoid The final stage in the life cycle of certain tapeworms.
Plesiomorphic Refers to an evolutionarily‑primitive state of a homolog.
Pleurite (pl. Pleura) Either of the two primary, lateral, exoskeletal plates of each segment of an arthropod; also pleuron.
Plicate Folded or ridged.
Podocyst A foot extension of some pulmonate embryos for excretion and absorption.
Podocyte Cell with branching interdigitating toelike processes, usually over the surface of a blood vessel. An adaptation for ultrafiltration.
Polyembryony Development of multiple embryos from a single cell mass.
Polymorphism Two or more individuals or zooids of a species modified for different functions.
Polyp Form of cnidarian that has a thin layer of mesoglea and is generally sessile.
Polyphyletic A taxon that includes the descendants of more than one ancestor.
Polypide The innermost parts of a bryozoan zooid, including the introvert, lophophore, and viscera but not the body wall or zooecium. 
Polypide regression Degeneration and replacement of bryozoan polypide from the cystid.
Porocyte A sponge cell that surrounds an opening which extends from the external surface to the spongocoel.
Preoral pit The developmental precursor of the wheel organ and Hatschek’s groove that opens on the left side of the head of larval cephalochordates.
Pressure drag The difference in pressure at the front end (higher pressure) of a forward‑moving organism as compared to the rear end (lower pressure).
Pretrochal region Apical plate, prototroch, and mouth region of a trochophore larva.
Primary host See definitive host.
Proboscis (pl. Proboscides) Any tubular process of the head or anterior part of the gut, usually used in feeding and often extensible.
Proboscis apparatus The complex, eversible, prey‑capturing organ of nemerteans.
Proboscis pore The opening of the proboscis apparatus at or near the anterior tip of a nemertean.
Procercoid Developmental stage of certain tapeworms between oncosphere and plerocercoid.
Proctodeum Invaginated embryonic ectoderm joining the anus with the endodermal midgut. Procuticle Thick, inner layer of the arthropod exoskeleton.
Proglottid One of the linearly arranged segment‑like sections that make up the strobila of a tapeworm.
Prognathus Anteriorly directed position of insect head.
Prograde Propagating in the direction in which the animal is moving, ie posterior to anterior (= direct propagation).
Pronate Rotation of the leading edge down. 
Propodium (pl. Propodia) The front of a gastropod foot which acts like a plough and anchor.
Prosopyle Internal opening of a sponge through which water flows from the incurrent canal into a radial canal or flagellated chambers.
Protandry Type of hermaphroditism in which the individual is first a male and then a female.
Protoconch The shell of the veliger which may remain at the apex of the adult shell.
Protogyny Type of consecutive hermaphroditism in which the individual is first female then male.
Protonephridium (pl. Protonephridia) A ciliated excretory tubule capped internally by one or more terminal cells specialized for ultrafiltration.
Protopod The basal part of a crustacean appendage, consisting of the combined coxa and basis.
Protostome Member of a major branch of the Animal Kingdom, in which the blastopore contributes to the formation of the mouth.
Prototroch Preoral ring of cilia of a trochophore larva.
Protozoea Third larval stage of a decapod (shrimp); after the metanauplius and before the zoea.
Proximal Close to the origin, center, or midline. 
Pseudocoel Fluid‑filled body cavity that occupies the connective tissue compartment. Differs from the hemocoel only in the absence of a heart.
Pseudofeces In filter feeders such as bivalves, material removed from the water flow, aggregated, and rejected before entering the gut.
Pseudolamellibranch gill Bivalve gill with filaments bound together with small tissue junctions.
Pseudopodium (pl. Pseudopodia) A flowing extension of a cell.
Ptychocyst A cnida that discharges a thread used to weave a tube.
Pygidium (pl. Pygidia) The terminal, nonsegmental part of the body of a segmented animal. Typically bears the anus. Telson.
Pyriform  Pear-shaped.
Pupa (pl. Pupae) In holometabolous insects, the stage between the last larval instar and the adult.
Pyramid Large calcareous plate that composes Aristotle’s lantern; shaped somewhat like an arrowhead with the point projected toward the mouth.
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.
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