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.
Acoelomate Body organization lacking a fluid filled cavity between epidermis and gastrodermis; compact.
Acron The anteriormost region, preceeding the first segment of the arthropod body.
Adductor A muscle that moves a structure toward the middle of the body.
Agamete Nucleus within the plasmodium of an orthonectid “mesozoan” that divides mitotically and eventually gives rise to a sexual adult.
Ambulacrum (pl. Ambulacra) Groove, ridge, or double band of tube feet, radial canal, and associated body wall of echinoderms.
Amphiblastula Sponge larva which is hollow. One hemisphere is composed of small flagellated cells and the other is composed of large nonflagellated macromeres.
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.
Asconoid A sponge body that is a simple cylinder and always small.
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
Autogamy Nuclear reorganization without conjugation or exchange of micronuclear material between two protozoans.
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.
Axopodium (pl. Axopodia) Fine, needle like pseudopodium that contains a central bundle of microtubules.


Barrier reef Reef whose platform is separated from the adjacent land mass by a lagoon.
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.
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.
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.
Buccal cavity Cavity just inside the mouth opening. The first region of the gut.
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. Cephalization Tendency to coalesce the segmental ganglia into a large anterior neural center.
Cephalothorax The combined head and thorax.
Cercaria (pl. Cercariae) Free swimming developmental stage of digenean trematodes.
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 cellA 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.


Facies A characteristic shape or appearance.
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.
GravidBearing developmental stages, such as eggs or embryos, internally.
Gross Large scale, not fine or delicate, i.e., not microscopic or ultrastructural.
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 Translucent 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