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See references at bottom for sources of many of these definitions.  I have modified some definitions slightly, and many others are entirely in my own words.



Abactinal: Aboral (in an echinoderm such as a seastar)

Abdomen:  The posterior part of the body of some species, usually contains digestive organs.  In crustaceans, the posteriormost region of the body which includes several segments, including the tail.  Often the abdominal segments have appendages for swimming, breathing, or holding eggs.  The abdomen of most true crabs is held tightly folded under the thorax, while that of shrimp and lobsters is stretched out behind the thorax.  In some polychaete worms, the body behind the prostomium and peristomium is divided into two dissimilar regions, an anterior thorax and a posterior abdomen.  Some may also have a tail, behind the abdomen, which has small segments with no appendages.

Aboral:  Opposite the end or side on which the mouth is located.  Used especially for echinoderms for which the terms "dorsal" and "ventral" are not used.

Acanthostyle:  A type of spicule in sponges.  Acanthostyles are spiny styles.  The prefix Acantho- for any spicule means "spiny".

Accessory flagellum:  A second, short flagellum branching from the base or peduncle, such as in the antennae of some amphipods.

Accessory Plate:  In some boring bivalves, one of 5 extra plates or extensions to the valves which may be present.  See hypoplax, mesoplax, metaplax, protoplax, siphonoplax

Aciculum (pl. Aciculae):  In polychaetes, a thick internal seta which supports either the notopodium or the neuropodium.  It may project.

Acontium (pl Acontia):  A thread-like extension of the lower end of a septal filament into the gastrovascular cavity of some Anthozoan Cnidarians, especially sea anemones.  It bears nematocysts and may be protruded through cinclides of the body wall (column) to exterior (photo).

Actinal: Oral (in an echinoderm) such as a seastar

Acute:  Sharply pointed

Adambulacral:  In seastars, the adambulacral ossicles are the row of ossicles lining the edges of the ambulacral groove (photo).

Adductor Muscle:  Large muscle spanning between valves of a bivalve; one may typically distinguish an anterior and posterior adductor muscle, each leaving a muscle scar (picture picture). Anterior muscle often reduced, posterior one rarely reduced.  Serves to draw valves together and funtions as antagonist to ligament.

Adnate:  Attached to the side of something

Ambulacral Furrow:  Endoskeleton structure; in irregular sea urchin, one in a series of narrow grooves radiating from mouth.  Located in ambulacra and typically restricted to underside of test. Bears specialized tube feet.

Ambulacral Groove:  In seastars, furrow-like depression along oral surface of each ray from which tube feet (podia) project.  Formed by inverted V-shaped arrangement of skeletal elements.

Ambulacral Plate:  In echinoids such as sea urchins, one in a series of plates along each ambulacrum.  The plates are typically paired, in two rows from the oral to the aboral side, and have prominent pores for passage of the water-vascular system into the tube feet. (photo)

Ambulacral row:  In many Holothuroids (sea cucumbers), a distinct row of tube feet which overlies the radial canal of the water vascular system.  Sea cucumbers with ambulacral rows usually have 5 of them.

Ambulacrum:  In echinoderms, a region along which the tube feet (podia) are arranged (in asteroids, each ambulacrum is a prominent ambulacral groove bordered by tube feet).

Anal notch:  In some shelled gastropods, a notch in the posterior margin of the aperture, which would be opposite any siphonal notch, if present (picture).

Anastomose:  To form a network of branches which diverge and then reconverge

Anatriaene:  A type of megascleretriaenespicule in sponges which has one long ray and three short rays on one end which are reflected back toward the direction of the long ray, similar to the flukes of an anchor.  Compare with protriaene and orthrtriaene.

Ancestrula:  First bryozoan individual (zooid) of a colony; typically differs in size and shape from subsequently formed zooids.   Arises from metamorphosis of free swimming larva or, in freshwater forms from a statoblast.

Anchor:  In sessile scyphozoans, a well developed, cushion-like rhopalioid containing mucus glands.

Anisochela:  A type of spicule in sponges.  Anisochelas are microscleres, a chela with the ends differing in size and shape (photo). Contrast with isochela

Antenna (pl antennae):  A sensory appendage on the head of some species.  Crustaceans have two pair of antennae, the first of which are called antennules and the second of which are called the antennae or second antennae.

Antennal flagellum:  The distal, flexible portion of crustacean antennae.

Antennal scale (scaphocerite):  In crustaceans, a flattened, leaflike extension formed by the exopodite of antenna 2.  Also called a scaphocerite.

Antennal spine:  An anteriorly-directed spine at the anterior edge of the carapace of crustaceans such as shrimp, below the suborbital spine and near the antennae.  If both spines are present, is ventral to the suborbital spine. and dorsal to the branchiostegal and pterygostomial spines.

Antennule:  The first antenna of crustaceans.

Anterior:  Toward the front (head) of an animal

  Anterior for bivalves:  The end of the shell where the head and foot lie; opposite the end where the siphons are located (photo).  Often most easily identified by the posterior location of the pallial sinus, or by the fact that the  umbo is usually anterior to the hinge ligament.

Anterolateral:  Located toward the sides of the front of the animal, or pointed in that direction

Anteroventral:  Located ventrally on the front of the animal, or pointed in that direction

Aperture:  Orifice, used especially for a gastropod shell (drawing) or for the orifice in a bryozoan from which the lophophore protrudes.

Apex:  Part of gastropod shell that was formed first; typically pointed, at the posterior end of the shell, consisting of the protoconch or smallest whorls of the spire. (drawing).  Apical = having to do with the apex.

Apophysis (pl. Apophyses):  An internal projection of the shell, often spoonlike, in some bivalves such as family Pholadidae, which serves for the attachment point for the foot (photo).  Also called a myophore.

Appendix interna (stamblys):  A projection from the medial branch (endopod) of the pleopods of Decapod Crustaceans.  May serve to link the endopod of one leg to the endopod on the opposite leg if the pleopods are used for swimming.

Appendix masculina:  A median projection from the endopod of the second pleopod in male shrimp.  Used for copulation and transfer of the spermatophore.

Aristotle's lantern:  The mouthparts of urchins and sand dollars.  Composed of a set of 5 movable teeth in 5 movable jaws set in a flexible sling.  All together the parts form a structure reminiscent of a fancy medieval lantern. Used for eating algae.

Arm:  In asteroids and ophiuroids, the ray, a lateral extension of the main body or central disk, typically 5 or multiples of 5.  Contains extensions of the water vascular system, ossicles, nerves, the hemal system, and in some contains branches of the reproductive system and gut.  In cephalopod mollusks, one of the circle of 8 or 10 muscular projections surrounding the mouth; usually with suckers.  Developmentally they correspond to perioral tentacles found in other mollusks.

Arm Comb:  At junction of arm (ray) and disk of brittle stars, comb-like series of papillae surrounding arm laterally and aborally.

Article:  In a crustacean appendage, a segment of the appendage which articulates with other segments.  Also, some segments of appendages such as legs or the flagellum of antennae may be subdivided by circular constrictions along with thinning of the expskeleton so that the segment is composed of a string of beadlike subsections called articles.

Articulation:  A joint, or a place where a stiff object can bend

Ascopore:  In Bryozoans with rigid frontals, a pore in the frontal which allows the ascus to expand and contract

Ascus:  In Bryozoans with rigid frontals, a water-filled sacklike organ behind the frontal wall.  The ascus expands when the lophophore is drawn in and contracts when the lophophore is extended, thus compensating for the motion of the lophophore.  Also called a compensation sack.

Athecate:  A type of hydroid in which the polyps which are not covered with a cuplike extension of the perisarc, which in thecatehydroids is called the theca.  The athecate hydroids form a suborder Athecata of Order Hydroida

Atrial aperture:  In ascidians, the opening through which water is pumped out of the pharyngeal basket.

Atrial siphon:  In ascidians, an atrial aperture which is shaped like a tube.  Exhalent siphon, Cloacal siphon

Atrium:  An inner chamber.  In sponges, the spongocoel.  In ascidians, the chamber inside the tunic which contains the pharyngeal basket.

Auricularia:  Early, free-swimming, larval stage of holothuroids.  Characterized by continuous curving, flagellated band.

Autotomy (verb:  autotomize):  The forceful release of a limb at a predetermined spot by special muscles or tissues in animals such as crabs or seastars.  Often used for predator avoidance.

Avicular:  (Root:  Pertaining to birds):  Hooklike or beaklike

Avicular uncinus:  In polychaetes, a short, stout seta that is hooklike in form, with one large tooth and several small teeth distal to it

Avicularium:  In a Bryozoan colony (zoarium), a modified individual (zooid) with reduced internal organs and in which orifice region and operculum are well developed (the latter into the mandible).  Serves to protect colony.

Axial rib:  In gastropod shells, a ridge running longitudinally on the shell (across the whorls). (photo).  Is nearly perpendicular to the whorls and to spiral ridges, if present.


Basis (basipodite):  The second segment of a crustacean leg.  Articulates proximally with the coxa and distally with the ischium. (photo).  In barnacles, the basis is the underside of the barnacle, which is attached to the substrate.  It may be either calcareous or membranous.

Bathyal:  Benthic, at depths greater than 200 m.

Bathypelagic:  Living in deep midwater.  In our area, bathypelagic species usually are not usually found shallower than 500 m.  Bathypelagic species are usually not vertical migrators.  Species living deeper than 4000 m are called abyssalpelagic.  Shallower-living species are mesopelagic or epipelagic.

Beak:  In a bivalve, the umbo

Bell:  The main, gelatinous body mass of a medusa, or jellyfish.  The upper surface of the bell is called the exumbrella and the lower surface, where the mouth and often the tentacles are, is called the subumbrella.

Benthic:  Living on, in, or attached to the bottom.

Benthopelagic: Pelagic just above the bottom.

Bifid:  Divided into two parts or branches, often at the apex (photo)

Bifurcated:  Splitting from one branch into two

Bipectinate:  The form of gills in protobranch bivalves:  Having two comblike rows of plates, attached to one another at the base to make a structure V-shaped in cross-section

Bipinnate:  Twice pinnate--the featherlike side branches of a pinnate structure are themselves divided into a pinnate structure.

Bipocillum (Bipocillon):  A type of spicule in sponges.  A bipocillum or bipocillon is a microsclere, a highly modified, irregular chela.

Biramous:  Having two branches or rami.  For example, most legs of crustaceans are biramous because the gill forms the outer branch or exopod (photo)

Biserial:  Arranged in two rows

Body whorl:  In a gastropod shell, the anterior, usually largest whorl which ends at the aperture. (drawing)

Branchia:  Gills.  Any highly vascularized projection of the body wall which is used for gas exchange.

Branchial chamber:  The gill chamber; in crustaceans such as shrimp and crabs, the portion of the carapace that folds around the gills forming a more or less enclosed chamber for the gills.

Branchial region: In the carapace of Decapods such as crabs and shrimp, the lateral areas of the carapace which overlie the gills. The anterior branchial region is the epibranchial, the middle is the mesobranchial, and the posterior is the metabranchial region.

Branchiostegal spine:  An anteriorly-directed spine in the branchial region on or near the anterior margin of the carapace of crustaceans such as shrimp.  The branchiostegal spine is ventral or ventral to and behind the antennal spine but dorsal or dorsal and anterior to the pterygostomial spine.

Branchiostegite:  In crustaceans such as shrimp, the lateral or dorsolateral carapace region overlying the gills.

Brood pouch:  A chamber for brooding eggs or young.  In peracarid crustaceans, the marsupium.

Buccal:  Refers to the mouth or cheek

Buccal Shield:  Endoskeleton structure of an urchin: one in a series of five plates on the oral surface of central disc.  Located interradially, flanked by adoral shields, and more or less covering jaws.  One or more buccal shields may serve as madreporite.

Buccal siphon:  In an ascidian, the siphon through which water is drawn into the pharyngeal basket.  Incurrent siphon.

Buccal Tentacles:  Tentacles surrounding the mouth of a Holothuroidean.

Byssal threads:  The threads some bivalves such as mussels and scallops secrete with their foot to attach the shell to a rock or other substrate.

Byssus:  Bundle of strong, hair-like threads originating from posterior end of (often reduced) foot.  Produced by byssal gland in conjunction with byssal groove and byssal disc pit and extended from shell to attach to hard substrates.


Caecum (pl caeca):  A blind-ended pocket

Callum:  A calcareous anterior plate or portion of the shell which grows over the anteriorventral gape of the shell when some boring bivalves such as family Pholadidae reach maturity (photo).  The animal can then no longer extend its foot to bore further into the rock.

Callus:  An unusually hardened or thickened area.  In gastropods, a shelly deposit (usually smooth) called inductra which may be found near the inner lip of the aperture and spreading over nearby parts of the shell.

Calyx:  In Entroprocta, the cuplike portion of the body in which the gut lies and from which the tentacles arise.  In attached jellyfish (Stauromedusae), the expanded, umbrellalike portion of the bell (contrasted with the narrow stalk by which they are attached)

Capillary seta:  A harilike seta, or a long, slender seta tapering to a fine point.

Capitate:  With a knob or swelling at the tip, for example of a tentacle (photo) (photo)

Capitulum:  In stalked barnacles, the armored portion within which the appendages and most of the viscera are located (photo).

Carapace:  In some malacostracan Crustacea, a continuous covering (often hardened to at least some extent )over the head and thorax; in barnacles, the aggregation of plates that protects the rest of the body

Cardelle (Cardella): In Bryozoans, one of two hardened, lateral projections (condyles) from the zooecium at the edge of the aperture. They serve as the pivot point for the operculum or for the mandible if the zooid is an avicularium.

Cardiac region: In the carapace of decapods such as crabs and shrimp, the posterior mid-dorsal region of the carapace which overlies the heart.

Cardinal tooth:  On a bivalve shell, a (usually relatively short) tooth below the beak of the umbo, and usually aligned with the beak (photo).  Contrasted with lateral tooth.  See also pseudocardinal tooth.

Carina:  A ridge.  In acorn (sessile) barnacles (suborder Balanomorpha), it is the most posterior of the six primary fixed plates (carina, two carinolaterals, two rostrolaterals, and rostrum) on the outer surface of the barnacle (photo).   The median part of the carina is called the paries and the lateral parts form triangular winglike alae which overlap the adjacent plate.  In stalked (goose) barnacles (suborder Lepadomorpha), the carina is the most posterior of the five primary plates (carina, two terga, two scuta) on the outer surface of the capitulum.

Carinolateral:  In acorn (sessile) barnacles (suborder Balanomorpha), the paired fixed plates on either side of the carina, near the posterior end of the barnacle (photo).  They are usually just posterior and attached to the two rostrolateral plates.

Carpus (carpopod, carpopodite, wrist): Fifth segment of thoracopod of a decapod such as a shrimp or crab; located between merus and propodus. (picture)

Caruncle:  A posterior extension of the prostomium in polychaetes, occasionally covering several segments.  Has a sensory function.

Cataleptic posture:  A defense posture assumed by some shrimp, especially spiny ones, in which the abdomen is arched backwards and held rigidly in an extreme position and body spines are exposed to a maximum extent.  The posture is thought to aid in defense by making the animal as spiny, rigid, and awkward to swallow as possible.

Caudal: Pertaining to the tail or posterior end.

Caudal cirrus:  A thin, tail-like posterior projection found in some Nemertean worms of Class Heteronemertea.

Caudal fan:  In many crustaceans, especially Malacostracans, a posterior combination of a central telson and two lateral uropods which together comprise a flattened structure at the tail which can be spread out like a fan by abducting the uropods. (photo).  Also known as a tailfan.

Caudal ramus (pl Caudal rami): One of two branches (forks) of the "forked tail" of some crustaceans such as branchiopodans and copepods which have caudal rami (forked tail).

Central disk:  In seastars, the central part of the body which contains the mouth, anus, madreporite, and gonopores and from which the rays radiate.

Centripetal:  Directed radially toward the center or axis of an object

Centripetal canal:  In medusae, a canal which attaches to the outer ring canal and proceeds toward the apex of the bell but does not continue all the way to the central stomach.

Centrotylote:  A term used to describe a sponge spicule such as a tylostyle in which the head is central instead of terminal.

Cephalic:  Referring to the head

Cephalic furrow:  See cephalic groove

Cephalic groove:  In Nemertean worms, one of a pair (sometimes two pairs) of transverse furrows in the epidermis across the anterior end of the body near the head.  The left and right cephalic grooves usually don't meet either mid-dorsally or mid-ventrally.

Cephalic shield:  In some species such as nudibranchs, a flaplike extension of the mantle which extends over the head

Cephalic slit:  In nemertean worms, one of a left and right pair of deep, longitudinal grooves in the epidermis along the sides of the head (photo)

Cephalon:  Anterior division of arthropod body bearing eyes, mouth, two antennules, two antennae, and three pairs of mouthparts; fused with first thoracic segment, in certain cases also with second and therefore occasionally referred to as a cephalothorax.  This term is used in isopods (photo) and amphipods

Cephalothorax:  In some arthropods such as crabs, a situation in which the cephalon (head) and thorax segments are united into one structure.

Ceras (pl. Cerata):  One in a series or bundle of repiratory projections on the dorsal surface of a snail with reduced or no shell.  Usually contain outpocketings of the digestive gland.  (picture)

Cerebral sense organ:  In Nemertean worms, a pair of ciliated glandular and sensory canals or pits which open into the cephalic slits or elsewhere on the head.

Chaeta (pl Chetae):  Small, bristle-like projection.  This term refers to the same type of structure as a seta but is sometimes used when describing annelid worms such as polychaetes to distinguish those bristles from the similar-appearing ones on arthropods such as crustaceans.  The bristles on arthropods and annelids are both referred to as setae by some, while others call those on arthropods setae and those on annelids chetae.  Chetae are especially abundant and prominent on the parapodia of polychaete annelids.

Chaetigerous:  Bearing chaetae (in Annelids).  Segments which are chaetigerous are often called Setigers.

Chela (pl Chelae):  In Crustacea, a pincer in which the dactyl serves as the movable claw and the propodus serves as the stationary claw (picture, picture, picture).  In sponges, a type of spicule.  Chelae in sponges are microscleres which have shovel-like structures on the ends.

Chelate: Provided with a chela.  More specifically, a chelate cheliped is one in which part of the 6th element (propodus) is elongated into a fixed "finger" against which the 7th, movable element (dactyl) clamps as the movable "finger" (picture, picture).  Contrasted with subchelate.

Chelicera (pl Chelicerae):  The sucking mouthparts that Arthropods of the subphylum Chelicerata have instead of jaws.  Chelicerates include spiders, scorpions and, in the ocean, Pycnogonids (sea spiders) and Horseshoe crabs.

Chelifore:  The chelicerae of pycnogonids, which are the anteriormost of 3 sets of appendages which arise from the pycnogonidcephalon (photo).  Usually chelate, but the chelae on some species are not developed.

Cheliped: Any chela (claw)-bearing thoracopod; typically refers to first pair(s) of pereopods. (picture)

Cheta (pl. chetae): A bristly, usually flexible #Cheta hairlike projection. Often used for polychaetes to emphasize that, though they may look similar they are not the same as the setae found in arthropods.

Chiton granules:  Very small, rounded structures, smaller than scales, on the surface of a chiton.  Generally must be viewed with magnification.

Chiton hairs:  Relative long, erect structures on the girdle or between the plates of a chiton.  May be branched.

Chiton scales:  Small, relatively flat, usually oval, structures on the girdle of a chiton, secreted by the mantle epidermis.  They usually overlap each other.

Chiton spicules:  Relatively small projection on the girdle of a chiton, secreted by the mantle epidermis and usually surrounded at the base by a cuticularized sheath.  May be like a stiff, hard hair.

Chomata:  In oysters, tiny folds along the inside of the margin of the shell.  In rock oysters (genus Saccostrea) the chomata run along the entire margin of the shell.  In flat oysters (genus Ostrea) there are only a few small chomata near and just beyond both ends of the hinge ligament. Cup oysters (genus Crassostrea) do not have chomata.

Chondrophore:  In the valve of a bivalve such as a clam, a shelly projection with a hollowed-out surface on hinge plate; serves for attachement of internal ligament. (picture)

Chromatophore:  A pigment-filled cell, usually near the body surface.  Usually chromatophores can be expanded or contracted by nervous control, causing a change of color or reflectivity of the animal's surface.  Common in cephalopods and crustaceans.

Cilium (pl Cilia):  Tiny cell organelles which protrude through the outer membrane of many animal cells.  The base of the cilium has a cellular motor which allows the cell to swing the cilium back and forth like a paddle.  Cilia are used for circulating fluids past a surface, such as on the gills of bivalves, or for propelling the cell or animal through the water, such as in ctenophores.

Cinclide:  One of usually many tiny holes on the side of the column of sea anemones.  The holes may be located on the top of tubercles.  Some anemones can eject threadline acontia through the cinclides, in what is thought to be a defense reaction.

Cirrus (pl Cirri):  "Cirrus" in Latin means "hair".  In barnacles, the cirri are the feathery feet.  In polychaete worms, a thin projection which may be of various shapes but is often thin (photo) (photo).

Clavus:  The distal portion of the rhinophore of nudibranchs, often club-shaped and may be branched resembling a gill (photo).

Chromatophore:  A pigmented cell, especially in the outer body surface (integument).  Often visible as a colored spot on the surface of the animal.  May be fixed in size (as in some crustaceans) or may be expanded and contracted (as in cephalopods) to cause a change in the animal's overall color.

Cirrus (pl Cirri):  In general, a soft appendage, usually fingerlike or tentaclelike; in barnacles, one of the biramous thoracic appendages ("feet") used for collecting food

Clavus:  The distal portion of the rhinophore in nudibranchs.  It is often club-shaped and may resemble a gill.

Cnida (Pl Cnidae): A unique organelle found within members of phylum Cnidaria, within Cnidocytes. Cnidae fire stinging and/or sticky projectiles when disturbed. The most commonly known type of cnida is a nematocyst.

Cnidocyte (pl. Cnidocytes):  Specialized cell type in ectoderm of Cnidarians.  Consists of basal nucleus and often a distal cnidocil, and contains a cnida organelle internally.  Typically concentrated on tentacles, on the mesenteries, and/or on the oral arms.

Colloblast cell:  A type of cell found in ctenophores.  The cell produces sticky material which helps to snare prey.

Columella:  In a gastropod shell, the axis around which the shell coils.  Usually is exposed at the inner lip of the aperture (drawing), where it may have several small columellar folds (picture, picture).

Column:  In an anemone, the cylindrical outer wall above the pedal disk and below the tentacles and oral disk.

Commensal:  Commensalism is a type of symbiosis in which one species (the commensal) benefits while the other (the host) is neither benefited nor harmed.  Possible examples of commensalism are the clams, crabs, and fish that inhabit the burrows of some burrowing worms

Compound seta: In polychaete worms, a seta which is composed of more than one segment, end-to-end, with a joint between them (photo)

Condyle: A rounded projection which is part of a joint

Continental shelf:  The nearshore, relatively shallow land under the ocean out to the edges of the continental plates.  Typically slopes gradually deeper offshore down to a depth of approximately 200 m; at which point the slope becomes much steeper on the continental slope.

Continental slope:  The relatively steeply-sloping land at the edges of the continental plates that plunges fairly steeply down from the shallow continental shelf to the abyssal plain.  Typically the upper edge starts at about 200 m depth and the lower edge is whatever depth the abyssal plain is (typically 4000-6000 m) in that section of the ocean.

Cornea:  In crustaceans, the portion of the compound eye which actually has ommatidia and is directly involved with vision, as opposed to the eyestalk.  The cornea is usually pigmented, often black.

Corbula (pl Corbulae):  In colonial hydroids, a basketlike structure containing the gonozooids

Corneous:  Horny, made of a horny  material.

Coronal muscle:  In Scyphozoan jellyfish, the muscle that encircles the margin of the bell.

Coronate:  Shaped like a crown

Costa (pl Costae): In Bryozoans, one of a series of spines along the sides of the frontal membrane and extending over it. Typically the spines on the two sides of the plate meet and fuse over the membrane to form a shield over tit

Coxa (Coxopodite, pl Coxae):  In Crustacea, the first article of an appendage (if an epipodite is present, it is borne on the coxa; in amphipods, the coxa is expanded into a broad plate).  Articulates with the body and with the basis. (picture).  Pycnogonids have 3 coxae (photo)

Coxal Plate:  Lateral extension of proximal segment (coxa) of pereopod; may be fused to body wall.  Seen especially on amphipods and isopods (photo).

Crenulate:  Having a scalloped margin.  Wrinkled along the margin

Crepuscular:  Taking place or being active near dawn and dusk

Cryptocyst:  Skeletal structure, horizontal calcareous shelf parallel to and below frontal membrane.  Formed by ingrowth of vertical walls of zooecium and penetrated by pores.

Ctene row:  One of 8 rows of fused cilia seen in ctenophores.  The cilia of adjacent cells are fused together into a comblike structure (ctene means "comb").  Eight series of these ctene rows extends from the oral to the aboral pole of the ctenophore and are used for swimming (photo).

Ctenidium (pl. Ctenidia):  A gill, especially a gill that projects from the body wall in a comblike fashion.

Cuticle:  An outer layer or covering of some material, usually noncellular

Cypris: The second, freeswimming larval stage of some crustaceans, isuch as barnacles. The structure is somewhat like an ostracod. It has a large,  carapace which attaches to the dorsal side but has two large valves which enclose its entire body, making its overall appearance something like a seed. The abdomen is very small. The head has cement glands at the base of its first antennae. The cypris swims with its six pairs of swimming legs until it encounters a suitable substrate to settle on, which may be detected by chemical cues. It then settles onto the substrate and glues its head down. It may detach and move several times, but eventually permanently attaches and metamorphoses into the adult barnacle form.


Dactylozooid:  In colonial hydroids, a polyp specialized for defense.  Has well-developed nematocysts but tentacles, mouth, and gastrovascular cavity are not well developed.

Dactylus (dactyl, dactylopodite) :  Seventh and terminal segment of appendage (thoracopod).   Follows propodus and may serve as distal element in subchela or as movable finger in chela. (picture)

Demibranch:  In the gills of mollusks such as bivalves and cephalopods, one of the two main branches of each gill filament.

Dentate:  Toothed, having a series of pointed projections like teeth

Denticulate:  Having a series of small toothlike projections (smaller projections than dentate)

Dextral:   A "right-handed" coil of a gastropod shell as opposed to the less common (usually rare) sinestral coiling.  To tell whether a shell coils dextrally, hold it with the spire up and the aperture facing you.  If the aperture is on the right side, the coil is dextral.

Dichotomous (adv Dichotomously):  Branched into twos

Distal:  Beyond, toward the end of.  In a structure such as a limb attached to another larger structure such as the body, the part of the limb farthest from the body.  Opposed to proximal.

Diverticulum (pl diverticula):  A side channel or side pocket, usually blind-ended.

Dorsal:  For most animals, the side of the body held upright, away from the substrate.
  Dorsal for bivalves:  Toward the hinge and umbo (photo)

Dorsal cirrus:  In polychaetes, a projection from the dorsal side of a uniramousparapodium, or from the dorsal ramus (notopodium) of a biramousparapodium,  The dorsal cirrus may be modified into branchia, and in shape may be annulated, conical, filiform, leaf-shaped, lobe-like, or spherical

Dorsal lamina:  In tunicates, a narrow membrane on the mid-dorsal side of the pharyngeal basket.  It is a ridge curved to form a ciliated gutter which carries food from the pharynx to the esophagus.

Dorsolateral:  Toward or along the lateral edge of the dorsal side.

Dorsomedial:  In the middle of the dorsal side, or toward the middle of the dorsal side

Dorsoventrally flattened:  With a flattened body that is wider than high, as seen for example in isopods.

Druse: An aggregation of bivalves such as mussels, attached to one another by byssal threads, that can be dislodged free of the substrate by wave action and washed ashore (Photo)

Dorsum:  The dorsal side of an organism, especially the dorsal side of a nudibranch, opposite the foot.

Dusky:  Somewhat dark in color, shadowy


Elytron (pl. Elytra):  In polychaetes, a scalelike modification of the distal part of the dorsal cirrus, forms a scale-like plate that covers the dorsum of that segment of the worm

Emersion:  Being exposed to the air, outside the water such as occurs for many intertidal organisms at low tide..

Endopod:  Inner branch of biramous appendage.  In crustacean walking leg it consists of the last 5 segments and branches from the basis.

Endoskeleton: An internal skeleton such as in vertebrates

Endostyle:  In tunicates, a grooved band of tissue along the mid-ventral line of the pharynx.  Bears cilia and glands.  It produces mucus which is swept dorsally along the pharyngeal basket to trap food.  The mucus with trapped particles is eventually swept into the dorsal lamina and carried to the digestive system.

Entire:  When referring to an object with an edge (such as a leaf or an antennal scale), the edge is smooth, with no spines or serrations

Epibenthic:  Living at or close to the surface of the sediment

Epibiont:  An organism that lives on the surface of another organism

Epimeron (Epimeral plate):  In crustacea, a flattened extension of the lateral plates of a segment; extends ventrally or ventrolaterally.

Epipelagic:  Living in midwater, near the surface.  Epipelagic species are usually found in the upper 200 m of the ocean.  Deeper-living species are mesopelagic or bathypelagic.

Epipodite:  In certain Crustacea, a lateral extension of the coxa of a leg (pereopod), usually functioning as a gill

Epitoke:  A specialized reproductive individual in some polychaete annelids.  An epitoke may be a special individual formed by budding off from some body part, or it may be an entire individual which has been modified from its non-reproductive form.  In either case, the epitoke is packed with eggs or sperm and specialized for reproduction.  Epitokes often engage in synchronous swarming, often coordinated with phases of the moon.  Polychaetes with live within tubes often produce pelagic epitokes by budding them off from some body part, usually from the back of the animal.  In this case the epitoke usually has no head or eyes but can swim well.  I in polychaetes which are already active, such as those in Family Nereidae, the entire worm often swarms as an epitoke.

Esophagus:  The tube that carries food from the mouth to the stomach.

Evert:  To project or extend in an outward motion, usually by turning inside out, as in the everted pharynx of polychaetes

Euaster:  A class of microsclerespicules in sponges.  Star-shaped with multiple rays projecting from a common center.  Examples:  oxyaster, spheraster

Excurrent siphon:  In bivalves, the mantle fold or siphon through which water is pumped back out of the shell after passing across the gills.  The excurrent siphon is dorsal to the incurrent siphon.

Exopod (exopodite):  Outer branch of biramous appendage.  In crustacean leg it branches from the basis

Exoskeleton: An external skeleton such as in Arthropods

Externa:  In female parasitic rhizocephalan barnacles, the reproductive sac or brood chamber which protrudes from the host.  Distinguished from the interna.

External hinge ligament:  In bivalves, a hinge ligament which is external to the shell when the valves are closed; so that it can still be seen from the outside with the valves closed (photo).

Exumbrella:  The aboral (upper) surface of the bell of a medusa (jellyfish).  Contrasted with subumbrella

Exuvium (pl. exuviae):  The molted exoskeleton of an arthropod such as a crustacean (photo)

Eyescale:  In decapod crustaceans, a flattened projection near the base of the eyestalk.


Falciger (adj: Falcate):  In polychates, a seta (falcigerous seta) which is typically compound and whose distal part is stout, hook-shaped, and usually blunt.

Femur:  One of the segments of a leg; the 4th segment of a pycnogonid leg, often elongated, which is located after the three coxae and precedes tibia 1 (photo)

Fenestrated:  Having one or more openings or pores

Filiform:  Slender, threadlike or hairlike in structure

Flagellum:  Distal, flexible division of antennule or antenna; composed of a varying number of articles.

Flute: A rounded channel groove, or furrow, especially one parallel to the long axis of the object it is on.

Fluting: A series of parallel channels or grooves parallel to the long axis of an object such as a shell.

Foramen:  A hole or passage

Forceps:   A type of spicule in sponges.  A microsclere folded over on itself so that it is shaped like forceps or a bobby pin

Frontal:  In bryozoans, the relatively thin, membranous zooecium wall in front of (ventral to) the zooid

Frontal lamina:  In isopods, a plate-like structure anterior to the mouthparts, usually between the antennae.  It may be pentagonal, quadrate, spiniform, or triangular.

Funnel:  In cephalopod mollusks, the muscular, constricted, tubelike or folded excurrent siphon, which is found on the ventral side and directs jets of water for swimming by jet propulsion.  Developmentally, it corresponds to what would become the foot in other mollusks.

Furrow spine: In a seastar, a spine which projects over the ambulacral groove (photo)


Ganglion: A cluster of nerve cells

Gastrovascular Cavity:  In Cnidarians, a spacious cavity or gastrovascular system within the column; divided into interseptal spaces by longitudinal partitions.

Gastrozooid:  In colonial hydroids, a polyp specialized for feeding.  Has tentacles and a mouth.  Also called a trophozooid.

Geniculate:  Bent abruptly at a sharp angle, like a knee

Genital plate:  In Echinoids such as urchins, a skeletal plate which contains a gonopore (photo).

Gill: A thin-walled structure specialized for oxygen exchange between the external environment and the internal environment of an animal. Often is elongated or flattened to increase surface area for exchange and typically contains blood or coelomic fluid.

Girdle: In chitons, the dorsal portion of the mantle that border the 8 shell plates.

Gonophore:  In colonial hydroids, a reproductive bud which asexually produces a medusa or sex cells.  May be formed from a gonozoid or from some other part of the colony.

Gonopore:  An opening of the reproductive system to the exterior.  In male malacostracan crustaceans the pore is on the basal segment (coxa) of the last (eighth) thoracopods; in female the pore is on the coxa of the sixth thoracopods.  In urchins the 5 gonopores are found in a ring of genital plates around the anus.

Gonozooid:  In colonial hydroids, a polyp specialized for reproduction.  Produces gonophores.

Gnathopod:  In amphipods and certain other Crustacea, one or both of the first 2 legs (pereopods), usually differrering from the remaining legs in being chelate or subchelate.  (Photo)

Gravid:  Having eggs

Gymnocyst:  Skeletal structure: calcified part of frontal wall between frontal membrane and vertical walls.  May give rise to series of spines extending over membrane.


Hectocotylus:  A special spermatophore-transfer structure in male cephalopods (eg octopus).  The male's (usually third right) arm is modified into a sperm transfer organ, and is used to pass the spermatophore into the mantle cavity of the female.  The degree of modification of the hectocotylus varies by species.  In some, it breaks off inside the female mantle cavity during copulation.  In Octopus, the tip of the hectocotylus contains a spoon-like depression, unlike other arms.

  Bivalve shell height:  The maximum distance between the dorsal and ventral edges of the shell (photo).

Hepatic diverticulum:  In some mollusks such as Aeolid nudibranchs, an extension of a pocket from the hepatopancreas of the gut up into the dorsal cerata.  Materials such as nematocysts from the nudibranch's food can be stored in these pockets within the cerata.

Hepatic spine:  In shrimplike crustaceans, an anteriorly-directed spine on the hepatic region of the carapace, which is toward the front of the carapace and near the midline in the anteroventral direction.

Hepatopancreas:  A secretory and absorptive digestive organ in mollusks.  Usually found near the stomach.

Hermaphrodite:  An individual with both a male reproductive system (testes, produce sperm) and female reproductive system (ovaries, produce eggs).  Simultaneous hermaphrodites have fully developed ovaries and testes at the same time.  Protandrous species develop the male organs first, then at some later time the male organs regress and female organs develop.  Protogynous hermaphrodites develop first as a female, then later as a male.

Heterogomph:  In polychaetes, referring to a compound seta in which the distal segment is cradled in a socket on the end of the proximal portion, and in which one of the socket walls is higher than the other so that the distal setal segment is further enclosed in the socket on one side than on the other.  See homogomph.

Hinge:  In the valve, of bivalve molluscs, collective term for all structures along the dorsal margin of the shell which function during opening and closing of shell.

Hinge ligament:  In bivalves, the proteinaceous connector on the dorsal side near the hinge which holds the two valves together and provides tension to open the valves when the adductor muscles are relaxed.  An internal hinge ligament is mainly inside the shell and not readily visible from the outside, while an external hinge ligament is mainly outside the shell and readily apparent from the outside.

Hinge plate:  The flattened portion of the bivalve shell adjacent to the hinge.  Contains the hinge teeth.

Hinge teeth: Elevated ridges on the dorsal margins of the shell of a bivalve near the umbo and hinge  ligament (photo). Used to keep the two valves in alignment when the bivalve closes. Has two possible types: Cardinal teeth and lateral teeth.

Holotype:  The individual specimen studied and described by the taxonomist who first described the species, and which serves as the 'type specimen' or official example of the characteristics of the species.

Homogomph:  In polychaetes, referring to a compound seta in which the distal segment is cradled in a socket on the end of the proximal portion, and both walls of the socket are of similar height.  See heterogomph

Hood:  In scyphozoan cnidarians, a small, hood-like extension of exumbrella over rhopalium; may bear outer and inner sensory pits basally on outer and inner sides.

Hydranth:  The distal end of a polyp, consisting of the ring of tentacles and mouth.

Hydroid:  The polyp form of Cnidarian Class Hydrozoa, which usually forms a colony of multiple individuals. (photo) (photo)

Hydromedusa (pl hydromedusae):  A medusa from Class Hydrozoa.  Usually are small, colorless, and have a velum but no lappets along the bell margin.  Mouth is often extended into a manubrium.

Hydrotheca:  In a colonial hydroid, the portion of the perisarc which surrounds the polyp(hydranth).

Hypoplax:  In some boring bivalves, a ventral flap which grows along the posteroventral margin of the shell.  One of five possible accessory plates, it partly covers a region of the clam which would otherwise be exposed because of the gape in the valves when the valves are closed.  See also protoplax, mesoplax, metaplax, siphonoplax

Hypostome:  The region below the mouth or oral disk.


Incurrent siphon:  In bivalves, the mantle fold or siphon through which water is drawn into the shell and across the gills.  The incurrent siphon is ventral to the excurrent siphon.

Infauna (adj. infaunal): An animal that lives buried within the substrate such as sediment

Inferomarginal plate: In echinoderms, a marginal plate which is on the oral side, and thus (in a seastar) is "on the bottom" when the animal is in its normal position

Integument:  The outer surface, often a protective layer.

Interambulacral plate:  In echinoids such as urchins, the plates in the interambulacrum area between the ambulacral plates (photo).  The interambulacral plates do not have pores through which the tube feet project.  Typically there are two rows of interambulacral plates from the oral to the aboral side between each set of ambulacral plates.

Interambulacrum:  In echinoderms, the region between the ambulacra

Interna:  In a female parasitic rhizocephalan barnacle, the rootlike part of the barnacle's body that extends through the host tissues.  In Order Kentrogonida the interna is formed by growth of the kentrogon.  Distnguished from the externa.

Internal hinge ligament:  In bivalves, a hinge ligament which is enclosed within the shell when the valves are closed (photo); so that it cannot be seen from the outside.

Interramal cirrus:  In polychaetes, a variously shaped cirrus which projects between the notopodium (dorsal branch) and the neuropodium (ventral branch) of the parapodium.

Interstitial:  An adjective which means small organisms which live between sand grains.  Also called meiofauna (a noun).

Intertidal:  At a depth between low tide and high tide; sometimes exposed to air at low tide.

Introvert:  In sipunculans, an anterior portion of the body that can be retracted by being pulled into itself. (picture).  In some Holothurians (sea cucumbers), a smooth, collar-like portion of the body wall posterior to the buccal tentacles.  It may be retracted into the body.

Invaginate: To fold or collapse inward upon itself, such as the finger of a glove being turned inside-out

Ischium:  Third segment of appendage (thoracopod), positioned between basis (last segment of protopod) and merus and thus representing the first segment of the endopod (picture). May be fused with basis to form ischiobais or with merus to form ischiomerus.

Isoanchor:  A type of spicule (microsclere) in sponges.  An isoanchor is a C-shaped spicule with palmate ends whose branches extend toward the opposite end like teeth on a pair of jaws (photo).

Isochela:  A type of spicule in sponges.  An isochela is a chela in which the two ends are similar (contrast with anisochela)


Keel:  A strongly developed ridge, such as those on the carapace of some shrimp and lophogastrids.

Kentrogon:  In a female parasitic rhizocephalan barnacle of Order Kentrogonida, the simple, the dedifferentiated stage which follows the cyprid larva.  The cyprid attaches to the host, molts, loses its antennae, carapace, and appendages, and injects itself as a kentrogon into the host tissue.  The kentrogon grows up into the interna.


Lamella (pl lamellae):  Thin plate, typically referring to a series of projections in the aperture of certain terrestrial snails.  According to position one may distinguish columellar, infraparietal, interparietal, and parietal lamellae.  When referring to the antennal scale of a crustacean, the lamella is the broad, flattened, leaflike portion of the scale.

Lamellar:  Formed of lamellae

Lanceolate:  Shaped like a lance head:  Tapering to a point at the tip and sometimes also at the base.

Lappet (marginal lappet):  In scyphozoan cnidarians, one in a series of lobe-like extensions around umbrellar margin; according to whether sensory structures (rhopalia) or tentacles are located between lappets one may distinguish rhopalial and tentacular lappets.

Lateral: Having to do with the side

Lateral tooth:  On bivalve shells, a usually relatively long tooth extending more or less parallel to the dorsal shell margin; separated from cardinal teeth by a space.  Are called anterior lateral teeth if anterior to the beak of the umbo; posterior lateral teeth if posterior to the beak.  Contrasted with cardinal tooth.  See also pseudolateral tooth.

Laterally compressed:  Having a body which is narrower from left to right sides than it is from the dorsal to ventral side, as seen in amphipods.

  Shell length for bivalve shells:  the greatest distance from the anterior to the posterior end of the shell (photo)

Ligament:  In bivalves, an elastic, multilayered structure joining two valves of shell dorsally, typically posterior to beak.  According to its position either dorsal to or within hinge.  Serves as antagonist to adductor muscles.

Ligule:  In polychaetes, a major process from either the notopodium or the neuropodium.  May be flaplike, leaflike, rounded, straplike (photo), triangular, or conical

Limbate:  Long and flattened like an oar (often used to describe polychaete setae)

Lophophore:  A specialized feeding structure found in phyla such as Bryozoa (photo), Brachiopoda, Phoronida, and Entoprocta.  It consists of a ridge from which ciliated tentacles project.  The beating of the cilia circulates water past the lophophore.  Small particles are captured from the water and carried down to the mouth near the base of the ridge.

Lunule:  In some bivalves, a flattened feature on the dorsal surface anterior to the hinge and beak.  It is formed by a pattern of growth of the hinge structures.  Viewed from above, the lunule may form a heart-shaped depression with one-half of the heart on each valve.

Lyrula (lyrule): In bryozoans, a median tooth projecting from the zooecium on the proximal rim of the aperture


Maculation:  A spot, usually fairly large (not a pinpoint size)

Madreporite:  A perforated calcareous ossicle ("seive plate") on the aboral surface of echinoid and asteroid echinoderms, permitting water to enter the water-vascular system (a simplified madreporite is found on the oral surface of some ophiuroids)

Mandible:  In Crustacea, one of the third pair of appendages of the head (the first pair of appendages associated with the mouth), used for chewing. In bryozoan avicularia, the movable, jawlike part of the pincher mechanism which pinches against the rostrum.

Mantle:  In molluscs, a muscular sac or tube-shaped organ around the outer part of body; encloses cavity containing gills and in cephalopods by contracting expels water through funnel.  In shelled molluscs the mantle secretes the shell.  In bivalves the mantle is a layer of tissue next to the shell.  In gastropods the mantle is on the dorsal side and attached to the shell.  In cephalopods the mantle enfolds much of the visceral mass behind the eyes, encloses the mantle chamber, and is contracted for jet propulsion.

Mantle cavity:  In bivalves, the open space enclosed by the mantle, which contains the visceral mass, foot, and gills.

Manubrium:  The mouth of medusae (jellyfish), in the center of the subumbrella.  In Hydromedusae it is often a long, tubular structure which may have frills or lobes on the lips

Marginal plate:  In seastars, the ossicles along the sides of the ray or lining the edges of the ambulacral groove.  In some species these are much larger and differently shaped (platelike) than are the other ossicles.

Marsupium:  A pouch for eggs and young; such as the pouch formed on the underside of the thorax on female peracarid crustaceans such as isopods, amphipods, mysids, and lophogastrids by overlapping oostegites.

Maxilla (pl Maxillae):  In crustaceans, one of the pairs of leglike mouthparts posterior to the mandibles and anterior to the maxillipeds.  They are usually smaller than maxillipeds and are used for manipulating food.  They are appendages from the head

Maxilliped:   One of three pairs of mouthparts posterior to maxillae on underside of head.  These appendages are actually derived from the thorax which is fused to the head, rather than from the head itself.  That is, they are a type of leg that comes from the thorax but are used for feeding rather than for walking.  The maxillipeds usually fold forward and lie flat against the maxillae and mouth on the underside of the cephalothorax. (photo)

Medial:  Toward the middle (usually of the body)

Median:  In the middle, down the middle

Medusa (pl Medusae):  In Cnidarians, the gelatinous, bell or umbrella-shaped, usually pelagic form.  A "jellyfish".  Alternate form = a "polyp"

Medusoid:  A medusa formed by some Hydroids (hydrozoans) which is not actually released from the polyp and so does not become a free-living medusa.  Some medusoids are nearly fully-formed medusae while others may be degraded to not much more than gonads.

Megalops (pl Megalopae):  The postlarva of a crab, which follows the zoea.  Megalops have all the adult appendages and a widened carapace, but their abdomen still extends behind them like a shrimp.  Megalops swim as plankton at first but then settle to the bottom.

Megasclere (pl megascleres):  A type of spicule in sponges.  Megascleres are larger spicules (usually 60-2000 microns), which often function as the chief support elements in sponges.  Contrasted with microscleres.

Merus:  Fourth segment of crustacean thoracic appendage (thoracopod); positioned between ischium and carpus (picture).  May be fused to ischium to form ischiomerus.

Mesentery (pl mesenteries): In members of phylum Cnidaria, internal membranes which subdivide the gastrovascular cavity in a radial fashion, Mesenteries may be complete (with no free edges) or incomplete (with a free edge).

Mesopelagic:  Living in midwater, mostly below the surface (especially during the day).  Off our coast the mesopelagic species are usually found above the oxygen minimum layer and range down to 500 or a maximum of 700 m depth.  Many of them are vertical migrators and come up near the surface at night.  Shallower-living species are epipelagic, while deeper-living species are bathypelagic.

Mesoplax:  In some boring bivalves, a dorsal flap which grows near the hinge of the shell.  One of five possible accessory plates, it partly covers the hinge region of the clam which would otherwise be exposed because of the gape in the valves when the valves are closed. See also protoplax, metaplax, hypoplax, siphonoplax

Metachronous: Proceeding in a wave of sequential movements down the length of the body. The wave may be prograde (front to back) or retrograde (back to front).

Metaplax:  In some boring bivalves, a dorsal flap which grows from the posterior region of the shell (photo).  One of five possible accessory plates, it partly covers the posterodorsal portion of the clam which would otherwise be exposed because of the gape in the valves when the valves are closed.  See also protoplax, mesoplax, hypoplax, siphonoplax

Microsclere:  A type of spicule in sponges.  Microscleres are small spicules (usually 10-60 microns), which are usually more or less randomly distributed and do not function as a major support element.  Contrasted with megascleres.

Microstrongyle:  A type ofspicule in sponges.  Microstrongyles are microscleres shaped like a strongyle

Molar Process:  Proximal grinding process (projecting part) of each mandible.

Monaxon:  A class of spicule structure in sponges.  A monaxon has a single straight or curved axis.

Multiarticulated:  Divided by a series of constrictions into a series of sections strung together in a row.  The sections are often called articles.  Often seen in the carpus of the second pereopod in several families of shrimp, especially Pandalidae.  Multiarticulated segments tend to be flexible and easy to bend in multiple directions.  (Here is a photo of a 7-article multiarticulated carpus in family Hippolytidae)

Multiserial:  Arranged in multiple rows

Myophore:  In bivalves, an internal projection from the shell for attachment of the foot muscle (photo).  Also called an apophysis.  Not to be confused with a chondrophore, which is a socketlike, spoonlike, or shelflike structure to which the internal hinge ligament of some bivalves is attached


Nacre:  The shiny, pearly lining on the inside of some bivalve and gastropod shells.  Sometimes called "mother of pearl".

Nannoplankton (Nanoplankton):  Very tiny planktonic organisms (typically 2-20 microns), such as diatoms, dinoflagellates, and perhaps even bacteria which are so small that they pass through the finest-mesh of plankton nets

Nauplius (pl Nauplii):  The first larva after hatching in most crustaceans.  Swims with three appendages which in later stages will become the first and second antennae and mandibles. Often has only one anterior (naupliar) eye.

Nektonic:  Living suspended in the water, and such a strong swimmer that most movement is due to active swimming rather than to water currents

Nematocyst:  One of the types of organelles contained in cnidocytes and used for predation in Cnidarians.  Consists of thick, double-walled capsule with coiled tubule and often a distal operculum.  Generally used for prey capture.

Nematophore:  In colonial hydroids, a small dactylozooid, typically occurring in threes surrounded by perisarc and closely clustered around a gastrozooid.

Neotype:  A 'type specimen' designated by a taxonomist as the official example of a species if the holotype and any existing paratypes have been lost.

Nephridiopore:  A pore or opening in the skin through which the kidney (nephron) empties waste to the outside.  In annelids, the nephridiopores are located in the side of most of the segments.

Neritic:  Living relatively near the shore, over the continental shelf.  Contrasted with oceanic.

Neuropodium:  In polychaetes, the ventral extension of the parapodium

Neurosetae:  In polychaetes, setae that arise from the ventral, neuropodial extension of the parapodium

Notopodium:  In polychaetes, the dorsal extension of the parapodium

Notosetae:  In polychaetes, setae that arise from the dorsal, notopodial extension of the parapodium

Nuchal epaulette:  In some polychaetes, a nuchal organ which projects from the posterior margin of the prostomium

Nuchal organ:  In polychaetes, a sensory organ (ciliated pit, groove, fold, or slit) which is located at the posterior end of the prostomium

Nuchal Tentacle:  A filiform or thick, fleshy tentacle situated on the anterior parts of worms (such as annelids or flatworms) or on the back of the prothorax of some insects. (picture)

Nuclear whorl:  In a gastropod shell, one of the smallest, earliest formed whorls at the posterior end (apex) of the shell (drawing).  The nucleus is also called the protoconch


Oblate:  Flattened or depressed at the poles.

Oceanic:  Living far offshore, beyond the edge of the continental shelf.  Contrasted with neritic.

Ocellus (pl Ocelli):  An eyespot, or light-sensitive spot, usually pigmented

Ochre or ocher:  An earthy reddish or yellowish ore used as a pigment

Ocular tubercle:  The projection on the dorsal side of the cephalon of a pycnogonid, on which the eyes are located.

Ommatidium (pl Ommatidia):  In arthropods with compound eyes, one of the individual eye units behind one of the facets of the compound eye.

Oocyte:  A developing egg

Ooeciostome:  In cyclostome bryozoans, the aperture of a communal ovicell

Oostegite:  In Peracarid crustaceans such as amphipods and isopods, a flaplike inner extension of the thoracic legs (pereopods) used to form a pouch (the marsupium) on the ventral surface of adult females in which the eggs and young are brooded (photo).  Also called a "brood plate".  The oostegites branch from the inner surface of the coxae.

Operculum:  A covering.  In some gastropods, a calcified or horny shield attached to the foot which seals the aperture of the shell when the animal is withdrawn inside (picture) (photo).  In bryozoans, a skeletal (zooecium) structure lidlike structure closing the orifice when the lophophore is retracted.   May also refer to a lidlike covering the opening of the nematocyst in cnidarians.  In unstalked barnacles, the set of four movable plates (two scuta and two terga) which cover the body when closed but can be folded aside to allow extension of the cirri and feeding.  In tubedwelling polychaetes, a round lid, usually made of modified setae, which caps the tube when the animal withdraws.

Oral:  Of or pertaining to the mouth.

Oral aperture:  In ascidians, the inhalent opening (opening through which water enters into the atrial chamber)

Oral Arms:  In Scyphozoanmedusae (jellyfish), the long lobes, frills, or tentacle-like extensions around the mouth

Oral Disc:  In cnidarians, a somewhat expanded, flat distal division of the body.  Bears a central mouth and one or more rings of tentacles.

Oral papilla:  In ophiuroids, one of a series of small calcareous projections from each jaw, along the fringe of the jaw margin which is closest to the substrate.  May be conical, flattened, papilliform, scale-shaped, serrate, spiniform (spine-shaped), or simple.

Oral Shield:  Buccal Shield in an urchin.  In a brittle star, a large central disk plate lateral to the mouth and between the bases of the legs, on the oral side of the brittle star (photo)

Oral Side:  In echinoderms, the side which the mouth is on.  Usually the underside.

Oral Siphon:  In ascidians, an Oral aperture which is shaped like a tube

Oral Veil:  In nudibranchs, A dorsal flap overhanging the head and mouth.  Often the veil is bordered with papillae

Orbit:  On the anterior margin of a shrimp or crabcarapace, a circular opening enclosing the eyestalk.

Orthotriaene:  A type of megasclerespicule in sponges.  A triaene with one long ray and three short rays directed at right angles to it.  Compare with anatriaene and protriaene.

Osculum (pl Osculi):  The excurrent pore from the atrial chamber of a sponge; is often a much larger and more conspicuous hole than the incurrent pores.

Ossicle:  In echinoderms a calcareous skeletal structure, often platelike or spinelike.

Ovate:  Egg shaped in outline

Ovicell:  In a bryozoan, a globular skeletal (zooecial) structure enclosing a brood chamber.  In some it is a modified zooid (gonozooid)

Ovigerous:  Bearing or carrying eggs.

Ovigerous leg:  In Pycnogonids, a special leg possessed by the male on which the eggs are carried.  A reduced ovigerous leg may be present in females of some species.

Oviparous:  A life cycle in which the parents release the young as eggs.   Compare Ovoviviparous and Viviparous

Ovoviviparous:  A life cycle in which eggs are released from the ovary but are then brooded internally until they hatch; the larvae are released at birth.  Compare Oviparous and Viviparous

Ovum (pl Ova):  An egg

Ovigerous:  Bearing or carrying eggs, e.g., as in certain appendages of crustacean.

Oxea:  A type of spicule in sponges.  A megasclere, monaxon with both ends tapering to a point.

Oxyaster:  A type of microsclerespicule in sponges.  A euaster with thin, pointed rays.(photo)

Oxyspheraster:  A type of microsclereeuasterspicule in sponges with thin, tapering rays. (photo)


Palea (pl Paleae):  In polychaetes, a strong, broad, or flattened seta, usually associated with the operculum or the anterior part of the body.

Pallial groove:  In chitons and some gastropods such as limpets, the groove-shaped mantle cavity where the gills are located

Pallial line:  In bivalves, the sinuous linear scar left on the inside of each valve where the mantle attached to the valve. (picture)

Pallial sinus (Pallial notch):  In bivalves, a U-shaped "dip" in the pallial line near the posteroventral margin of the shell (picture).  Seen in bivalves with long siphons.  Represents the attachment scar of the siphonal retractor muscles.  Allows the siphons to be withdrawn into the shell before closing the shell.

Palmate:  Having several branches or lobes spreading from a common base, such as the palm of the hand

Palp:  In certain polychaete annelids, a sensory outgrowth (sometimes consisting of 2 units) of the ventral or frontal side of the prostomium (photo); in some other polychaetes, a long sensory outgrowth of the first true segment; often shaped like a finger.  In Crustacea and other arthropods such as insects, the slender, projecting distal portion of a mandible or maxilla, which functions as a sensory structure.  Often shaped like a several-jointed finger.  In Pycnogonids the palp (pedipalp) (photo) is not an extension of the mandible since pycnogonids have no mandibles.

Papilla: (pl Papillae) A small, fleshy projection of the body wall or of some other structure.

Papula (pl Papulae):  A coelomic pouch which extends through the integument of seastars and forms a saclike projection into the water.  Can often be seen as a thick fuzzy layer on the surface of the seastar when it is underwater.  The papulae are used for gas exchange and for excretion (photo)(photo).

Paragnath:  In polychaetes, a tiny, hard tooth-like projection (denticle) on the pharynx.  Typically found in groups on or rings around the evertedpharynx.

Parapodium (pl Parapodia):  In polychaetes, the paddlelike extension on either side of most segments.  Is often divided into a dorsal notopodium and a ventral neuropodium and bears tufts of setae.

Paratype:  Other individuals designated by the taxonomist who first described the species as examples of the species, in addition to the holotype.

Paxilla (pl Paxillae):  In some asteroid echinoderms, a columnar calcareous ossicle on the body surface (the top of a paxilla, usually nearly flat, is typically covered by small ossicles)

Pectinate:  Comblike

Pedal disk:  In an anemone, the base which attaches to the substrate.

Pedal laceration:  A form of asexual reproduction in some anemones, in which fragments of the pedal disk break off from the anemone and grow into new, small anemones which are a clone of the parent.  This may be associated with crawling of the parent across the substrate, so that the offspring are left in a line behind the parent.

Pedal tentacle:  In gastropods such as nudibranchs, a tentacle-like extension of the foot, usually at the anterolateral corners. (Photo)

Pedalium (pl Pedalia):  A gelatinous thickening at the base of the tentacles in some Scyphozoan medusae (Order Coronatae).  Tentacles, rhopalia, and lappets arise from the pedalia.

Pedicellaria (pl Pedicellariae):  In asteroid and echinoid echinoderms, a pincerlike structure (usually stalked) on the body surface (photo) (in asteroids, the pedicellariae commonly have 2 jaws, but sometimes are formed by a cluster of ossicles; in echinoids of our region, the pedicellariae have 3 jaws)

Pedicle:  The fleshy stalk by which brachiopods attach to the substrate.

Pedicle Valve:  Ventralvalve. In brachiopods, this valve is often attached to the substrate or overlaps the dorsal valve and is attached to the substrate by a tough fleshy stalk (pedicle)

Pedipalp:  In Chelicerates such as Arachnids and Pycnogonids (sea spiders), a cephalic appendage behind the chelicerae but anterior to the oviger and to the legs (photo).  The structure is quite variable.  Often the pedipalps are used for manipulating food.  May be chelate.  Sometimes they are simply called palps.

Peduncle (Pedicle, Stalk):  In stalked barnacles, one of two external divisions of the body.  Attached to substratum by cement glands: contains ovaries.

Pelagic:  Living suspended in the water, either drifting (planktonic) or strongly swimming (nektonic)

Pentactula:  Late larval stage of a holothuroid (sea cucumber).  Characterized by five primary tentacles (equivalent to buccal podia). Other podia may not yet be developed.

Pereomere: A body segment which is part of the pereon. A pereonite.

Pereon:  In Crustacea, the complex of thoracic segments from which the legs (pereopods, gnathopods) originate (it does not include segments that are fused to the head, and whose appendages [maxillipeds] function as mouthparts).  The part of the body between the head (cephalon) and the abdomen (pleon).  The term is especially used in groups such as amphipods.

Pereonite:  A segment of the pereon

Pereopod: An appendage (leg) arising from a segment of the pereon (thorax).  Usually used as walking legs.  Decapods have 5 pairs of pereopods.

Perfoliate:  Refers to a stalked structure in which the stalk is surrounded by a series or stack of continuous flanges

Periostracum:  Outermost, relatively thin layer of shell.  Composed of horny organic material termed conchiolin and secreted by periostracal glands in mantle folds. Often gives a dark or brown coloration to the shell (picture)

Periproct:  The opening in an urchin test (shell) which contains the anus (photo).  The opening is covered by the periproctal membrane and contains a number of small, embedded periproctal plates.

Perisarc:  A chitinous outer layer surrounding much of a colonial hydroid.  If the perisarc forms a cup around the individual polyps, into which the polyps can retract, the colony is thecate.  If not, the colony is athecate.

Peristome:  In Bryozoa, the part of a tubular zooecium that projects out from the general surface of the colony; or in Cheilostome Bryozoa, an elevated rim around the aperture of the zooecium

Peristomium:  In polychaetes, the second body segment, behind the prostomium, which contains the mouth.

Petalidium:  In irregular echinoids such as sand dollars and sea biscuits, a flower-shaped pattern of ambulacral plates on the aboral side (photo). Tube feet, used for respiration, project through pores in these plates.

Pharyngeal basket:  In ascidians, the internal basket-like structure within the tunic used for feeding.  Water is brought into the pharyngeal basket through the buccal (incurrent) siphon and is pumped out through the atrial (excurrent) siphon.

Pharynx: Tubular section of digestive system extending back from mouth.  In Cnidarians, the pharynx extends into the gastrovascular cavity; may be attached to column by septa and typically bears one or two flagellated grooves.

Pinnate:  Feather shaped, i.e., with main stem (usually called a rachis) and a series of branches (usually called pinnules) along each side.

Planispiral:  Spiraling all in the same plane (i.e., in gastropods, the spire of the shell is completely enclosed by the encircling body whorl)

Planktonic:  Living suspended in the water and drifting with the currents.  May be able to swim but either swims so slowly or is so small that overall movement is primarily due to currents

Planula:  An oval, ciliated larva of Cnidarians.

Pleomere: A segment of the pleon. A pleonite.

Pleon:  In crustaceans, the posterior division of the body (cephalon, pereon, pleon) consisting basically of six segments (pleonites).  Also called the abdomen.  Appendages from the pleon are pleopods (or uropods).  Used especially in isopods and amphipods.

Pleonite:  In crustaceans such as amphipods and isopods, one of basically six segments of the pleon.  Appendages of the pleonite are called pleopods.  Pleonites are often considerably narrower than pereonites.  In isopods the last (=sixth) pleonite is typically fused to the telson to form the pleotelson.

Pleopod:  One of two appendages of each abdominal somite;  typically consists of base (protopod) and two branches (endopod, exopod).  Serves in swimming or variously modified as copulatory structures in male and egg brooding structures in female.

Pleosome:  In some crustaceans such as some amphipods, the anterior portion of the pleon, which is different from the posterior portion of the pleon, which is called the urosome.

Pleotelson:  Terminal structure of a crustacean body, especially in isopods, formed by fusion of one or more abdominal segments (pleonites) with the telson.

Pleuron:  (Pleurite, pleural lobe, epimeron, pleurepimere, tergal fold) (pl Pleura).  The lateral part of the integument (exoskeleton) of a body segment on shrimp-like crustaceans.  May form prominent lateral or ventral extensions or plates.  Attaches dorsally to the tergite and ventrally to the sternite.

Plicate:  With pleated folds like a handheld fan.

Pneumostome:  In Pulmonate and Gymnomorph snails, the external hole or opening in the mantle which leads to the internal vascularized lung (photo).

Podium (pl Podia):  In echinoderms, A "tube foot", part of the water vascular system.  General term for several types of projections from body wall to exterior.  Associated with ampullae and supplied by channels (podial canals) branching from radial dorsal canals.  Locomotory podia are termed pedicels; nonlocomotory dorsal types are termed papillate podia, papillae, or warts, while enlarged anterior podia are termed buccal podia or tentacles.

Podobranch:  In some crustaceans such as shrimp, a type of gill attached to the first segment (coxa) or to an epipod of thoracopods 2-7 (photo)

Polyp:  An individual, usually attached and cuplike form of Cnidarians, such as an anemone.  Typically has a ring of tentacles, an oral disk, and a central gastrovascular cavity.  May be solitary or part of a colony of many polyps. Exist in several forms; one may distinguish autozooid, siphonozooid, and mesozooids.  Alternate form = a medusa

Posterior:  For most animals, the end opposite the head

  Posterior for bivalves:  The end of the shell where the siphons are located, opposite the end with the head and foot (photo).  Often most easily identified by the posterior location of the pallial sinus, or by the fact that the hinge ligament is usually posterior to the umbo.

Posterodorsal: Located in the posterior portion of or posteriorly to the dorsal part.

Posterolateral:  Located toward the side of the posterior (rear) of an animal

Posteromesial:  On the dorsal midline at the posterior end.  Usually used when referring to posteromesial spines on shrimp abdominal segments.  These spines are on the dorsal midline at the posterior end of the segment and point backwards toward the telson.

Prehensile:  Used for grasping

Primary Rib:  On a mollusc shell, one of the main ridges that run along the shell.  Primary ribs may or may not have secondary ribs interspersed between them

Proboscis:  An anterior, often tubelike or troughlike extension, coming from and extending anterior to the head region.  The proboscis is usually not a part of the digestive system.

Prograde: In reference to a metachronous wave of movement, proceeding from the front to the back of the animal

Propodus:  Sixth segment of appendage, between carpus and dactylus (picture).  May serve as proximal element of subchela or be divided into proximal manus and distal (fixed) finger of chela.

Prostomium:  The first body segment in polychaetes.  It is in front of but does not include the mouth , and is often a small shelflike extension over the dorsal side of the mouth.  The prostomium often has extensions such as palps, tentacles, or cirri.

Protandry:  A form of hermaphrodism in which the individual is first a male, then later becomes a female.  Contrasted with protogyny.

Protoconch:  In gastropod shells, the smallest, most posterior whorl or two at the apex of the shell in which there is no obvious sculpturing (drawing) (photo). Whorls of the protoconch are the nuclear whorls.  Later whorls of the spire comprise the teloconch.

Protogyny:  A form of hermaphrodism in which the individual is first a female, then later becomes a male.  Contrasted with protandry.

Protoplax:  In some boring bivalves, a dorsal flap which grows from the anterior region of the shell.  One of four possible accessory plates, It partly covers the anterodorsal portion of the clam which would otherwise be exposed because of the gape in the valves when the valves are closed (photophoto).  See also mesoplax, metaplax, hypoplax, siphonoplax

Protopod (Protopodite):  The basal segment of a crustacean leg, usually includes the coxa and the basis

Protriaene:  A type of megascleretriaenespicule in sponges which has one long ray and three short rays on one end which are directed away from the long ray (photo).  Compare with anatriaene and orthotriaene.

Proximal:  In a structure such as a limb which projects from another structure such as the body, the basal part which is closest to the connection to the main structure.  Opposed to distal.

Pseudocardinal tooth:  In bivalve shells, a cardinal tooth that is not separated from the lateral teeth on the hinge (pseudolateral teeth extend up to or overlap pseudocardinal teeth).

Pseudofeces:  Feces-like deposits released by bivalves, which have not passed through the gut (photo).  As the bivalve filter feeds, rejected materials are gathered by cilia and mucus on the gills and swept out as pseudofeces, while food particles are transported to the mouth.

Pseudolateral tooth:  In bivalve shells, a lateral tooth that is not separated from the cardinal tooth on the hinge (pseudolateral teeth extend up to or overlap pseudocardinal teeth)

Pseudopaxilla (Pseudopaxillae):  In sea stars, a type of ossicle which resembles a paxilla.  It is shaped like an erect column with spinelets on the top.

Pterygostomial (Pterygostomian) spine:  In shrimplike crustaceans, an anteriorly-directed spine on the anterolateral corner of the carapace, near the mouth (photo).  If both spines are present, is found ventral or ventral and posterior to the branchiostegal spine.

Pubescent:  Hairy

Pygidium:  In polychaetes, the most posterior part of the body (not actually a segment) where the anus is located (photo).



Rachis:  A central shaft, as of a feather or a pinnate leaf

Radial canal:  In Hydrozoan or Scyphozoan medusae, a gastrodermis-lined canal which runs through the mesoglea from the gut out to the ring canal.

Radial shield:  Paired large plates on the central disk near where the arms (rays) of a brittle star attach (photo) (photo)

Radial rib, radial ridge:
  In a bivalve, a ridge extending from near the umbo toward the ventral margin of the shell.
  In a coiled gastropod, a ridge oriented along the length of a whorl so that it spirals around the shell with the whorl.
  In a caplike shell such as a limpet, a ridge from the apex out to the margin of the shell

Radiole:  In tubedwelling polychaetes such as  sabellids, serpulids, and spirorbids, one of several to many featherlike structures forming a crown at the anterior end of the body,  attached to the peristomium.  Typically the radioles are bipinnate, with cilia both on the pinnules and on the centrally located longitudinal groove.

Radula:  Specialized, protrusile armature of anterior region of digestive tract in a mollusk.  Refers to full complement of chitinized teeth along radular membrane.

Ramus (pl Rami):  Branch of an appendage, typically referring to inner (endopod) and outer (exopod) branches of a biramous crustacean appendage.

Ray:  In a seastar, one of the "arms" which radiate out from the central disk

Recumbent:  Lying down

Recurved:  Curved back

Reticulation: A web-like network of lines across a surface

Retractile: Capable of being retracted (drawn back or inward), often into a sheath

Retrograde: In reference to a metachronous wave of movement, proceeding from the back to the front of the animal

Rhinophore:  Pair of tentacles posterior to cephalic tentacles;  forms sole pair of projections on head of certain shell-less snails (nudibranchs).  Innervated by cerebral ganglion and may be retractable into sheath.

Rhopalioid:  In sessile scyphozoans, short perradial and interradial projection (reduced tentacle) of exumbrellar margin; flanked by rhopalial lappet and covered by hood.  Contains statoliths distally.

Rhopalium:  In scyphozoan medusae, a structure concerned with equilibrium, consisting of a fleshy outgrowth of the margin of the bell, weighted by a mass of crystals, that contacts a sensory lobe

Ring canal:  In hydrozoan medusae, a ring-shaped canal lined with gastrodermis which runs around the margin of the bell.  Connected to the gut via the radial canals.

Rostrolateral:  In unstalked (acorn) barnacles (suborder Balanomorpha), the two paired fixed plates on either side of the rostrum near the anterior end of the barnacle (photo).  They are usually just anterior and attached to the two carinolateral plates.

Rostrum:   In shrimp and crabs, an unpaired anterior extension of carapace which projects between the eyestalks and represents an extension of the frontal region of the carapace.  Often appears as a hornlike extension between the eyes of shrimp and crabs. (picture)  In unstalked (acorn or sessile) barnacles (suborder Balanomorpha), the rostrum is the unpaired, anterior plate, next to the scutum and opposite the end from which the cirri (feet) protrude and from the carina and terga(photo).  In stalked (goose) barnacles (suborder Lepadomorpha), a rostrum may not be found but if so it is an unpaired anteroventral plate on the capitulum. In bryozoans, the rostrum is part of the pinching mechanism on aviculariumzooids, which protect the colony. The rostrum is a thickened, nonmoving structure against which the movable mandible pinches.

Rudimentary:  Reduced in size and usually in function.  In a nearly undeveloped state.


Scaphognathite: In Crustaceans (especially Decapoda) a large outer branch of a maxilla, covered with setae, which projects up into the anterior portion of the gill chamber and circulates water through the gills by beating. Also called a "gill bailer".

Scutum:  (pl Scuta)  In unstalked (acorn or sessile) barnacles, one of the four paired, movable plates (two anterior scuta and two posterior terga) which cover the exposed part of the body within the wall plates and form the operculum which can be opened so the barnacle can feed (photo).   In unstalked barnacles, the two scuta form the anterior end of the operculum and lie on the left and right sides of the aperture.  In stalked (goose) barnacles (suborder Lepadomorpha), the two scuta are a pair of plates found on the anterior end of the capitulum.  Along with the two terga they can be closed together to protect the barnacle, or they can be pulled apart to open the aperture and allow extension of the cirri and feeding.

Scyphistoma (pl Scyphistomae):  In Cnidarian Class Scyphozoa, the asexual, attached polyp stage.  It usually reproduces by strobilization to produce free-swimming ephyra larvae which grow up to become medusae.

Scyphomedusa (pl Scyphomedusae):  A medusa from Class Scyphozoa.  May be large and colorful, have no velum, but may have lappets around the margin of the bell.  Mouth typically has long oral arms but is not extended into a manubrium.

Secondary Rib:  On a mollusc shell, a small ridge which is between larger primary ribs

Sensory Pit (olfactory pit):  Small sensory depression associated with base of rhopalium; according to position one may distinguish an outer and inner sensory pit.

Septal filament:  In Anthozoan Cnidarians, a stringlike process which extends from the inner end of some of the septae down into the gastrovascular cavity.  The septal filaments contain secretory cells and cnidocytes.  In some species the septal filaments can be ejected through the body wall as acontia.

Septum:  A membrane, wall, or partition which subdivides a cavity.  In octamerous anthozoans, eight longitudinal partitions dividing gastrovascular cavity into interseptal spaces; in hexamerous forms, there are typically a multiple of six.  Extends from wall of column to pharynx orally; free inner edge below pharynx bears thickening filament.  Each septum bears a longitudinal muscle.

Serrated:  Refers to an edge covered with a series of teeth or spines, giving an appearance like a saw blade

Sessile:  Not moving, set in place.  For many animals sessile is used in the sense of not being on a stalk; for example, sessile eyes are attached directly to the head instead of being on eyestalks.  Acorn barnacles are sessile while gooseneck barnacles are stalked.

Seta (pl setae):  Small, usually flexible, bristle-like or hairlike projection articulating with or extending through cuticle (exoskeleton) (photo). Setose or Setiferous means having setae.  Note:  These types of structures are common on several phyla, especially on arthropods and on annelid worms.  To point out that their derivation is different than in arthropods, sometimes setae on annelid worms are referred to as chaetae.

Setiger:  In polychaetes, a segment which has setae (which are also called chaetae).  Sometimes called a chaetigerous segment.

Shield:  In anomuran hermit crabs, the dorsal, calcified portion of the carapace.  The rest of the carapace is soft.

Sigma:  A type of spicule in sponges.  Sigmas are C- or S-shaped microscleres. (photo)

Sinestral:  A "left-handed" coil of a gastropod shell as opposed to the more common dextral coiling.  To tell whether a shell coils sinestrally, hold it with the spire up and the aperture facing you.  If the aperture is on the left side, the coil is sinestral.

Sinus:  A depression, cavity, or hole, which usually does not penetrate all the way through the object it is within.

Siphon:  Tube-like, muscular extension of the mantle in some mollusks formed by fusion of the mantle margins.  In bivalves, typically present in the form of two posteriorly directed tubes, the lower (more ventral) siphon for inhalent current and the upper one for exhalent current.  In gastropods, an anteriorly-directed inhalant tube.  In ascidians, one of two openings (buccal or incurrent and atrial or excurrent siphon) for intake and exit of water from the pharyngeal basket.

Siphonal Canal:  In some gastropod shells, an anteriorly directed tubular, trough-like or notch-like extension of the aperture.  Serves as a channel for protrusion of the siphon. (drawing)(picture)

Siphonal notch:  In a gastropod shell, a notch at the anterior end of the aperture through which the siphon can be extended.

Siphonoglyph (sulcus):  In Cnidarians, a flagellum-bearing longitudinal groove extending from mouth into gastrovascular cavity along pharynx; typically located at narrow end of oval pharynx; defines ventral side of directive axis.

Siphonoplax:  In some boring bivalves, One or more flaps which grow near the posterior end of the shell.  One of five possible accessory plates, it partly covers the siphon.  See also protoplax, mesoplax, metaplax, hypoplax

Spatulate:  Shaped like a spatula

Spermatheca: A socket or pouch in females of some species which stores a spermatophore or sperm after mating until ovulation and fertilization.

Spermatophore:  A package containing sperm, usually with a hardened outer covering. Produced by males.

Spheraster:  A type of microsclereeuasterspicule in sponges, in which tapering, conical rays project from a spherical center.

Spicule:  A tiny, slender, usually pointed hard object  such as the spicules in sponge and holothuroid skeletons.  Usually microscopic.

Spine:  A stiff, sharp projection, usually with a solid, inflexible base

Spinule:  A stiff, sharp spinelike projection.  Sometimes used to distinguished spines which have a flexible base so they can bend.

Spiral ridge:  In gastropod shells, a ridge spiraling around the shell around the whorls.  Is perpendicular to axial ribs. (drawing)

Spire:  In a gastropod shell, the (usually smaller) whorls posterior to the body whorl (drawing)

Spirocyst:  A nematocyst-like organelle in Cnidarians that fires a sticky tubule instead of a sharp barb.  After discharge the sticky material spirals around the tubule.  May be used to capture prey or to attach to the substrate.

Spongocoel:  In sponges, an inner chamber or atrium, lined with choanocyte cells.  Water is pumped into the spongocoel through porocyte cells and exits through the large osculum

Statocyst:  An organ used for balance, usually consisting of an internal space lined with sensory hairs or cilia and a dense pellet or statolith in the center.  The sensory hairs that the statolith is resting on indicate serve to indicate which direction is up.

Statolith:  A dense stone or pellet used within organs of balance, especially in statocysts.  In Scyphozoans, a minute, solid, movable body within one of numerous cells at distal end of a rhopalium.  Used for sensing gravity.

Sternite:  In shrimplike crustaceans, the ventral plate of integument (exoskeleton) covering each segment.  Attaches on the sides to the pleurite.

Sterraster:  A type of microsclerespicule in sponges composed of a large central sphere with many short rays emanating from it.  Starlike in form. (photo)

Stigmata:  In Ascidians, gill-like perforations in the pharynx.

Stolon:   A slender tube or column, usually horizontal.  In bryozoan Colony (zoarium) structure a slender tube of modified individuals (kenozooids) bearing feeding individuals (autozooids) along its length.

Strand line: On a sandy beach, the line of water-carried debris left at a previous high tide mark on the beach

Strigilis:  The distal several segments of the oviger of pycnogonids, curved and often with setae, which is used for grooming the legs (photo)

Strobilization:  The process seen in some animals such as many Scyphozoan jellyfish.  In strobilization the Scyphistoma polyp fragments into a series of ringlike or platelike layers at the oral end, sometimes looking like a stack of plates.  Each fragment becomes an ephyra larva which grows into an adult medusa.

Strongyle:  A type of spicule in sponges.  Strongyles are megascleres, a monaxon with both ends rounded or blunt.

Style:  A type of spicule in sponges.  Styles are megascleres, a monaxon with one end rounded and the other end pointed (photophoto).

Stylet:  In nemertean worms, a barb which projects from the tip of the evertedproboscis

Subchelate:  In Crustacea, the condition of a gnathopod , maxilliped, or pereopod in which the dactyl (article 7) folds back against the palm of the propodus (article 6) (picture) to form a pincer or chela (differing from chelate in that the propodus does not have a distal prolongation or "finger" against which the dactyl closes).  A subchelate chela is a claw in which the axis of closure is at nearly a right angle with the long axis of the leg (as in the picture) rather than parallel to the long axis of the leg.  Contrast this term with chelate.  Here is a photo of a subchelate chela of a shrimp.

Suborbital spine:  In crustaceans such as shrimp, an anteriorly directed, spine-like projection on the orbital region of the carapace; located at the anterior edge, below the orbit.

Subtidal:  Informally, occurring at deeper depths than the intertidal zone.  Formally, a designation for benthic species which inhabit a depth zone from 0 to 200 m depth, typically on the continental shelf.

Subtylostyle:  A type of spicule in sponges.  A subtylostyle is a megasclere, a monaxon resembling a tylostyle but with only a small head.

Subtylote:  A type of spicule in sponges.  A subtylostyle is a megasclere, a monaxon resembling a tylote but with only small heads.

Subumbrella:  The undersida (mouth) side of a jellyfish medusa.  Contrasted with exumbrella.

Sulcus (sulci):  A groove

Superomarginal plate: In echinoderms, a marginal plate which is on the aboral side, and thus (in a seastar) is "on top" when the animal is in its normal position

Supraorbital spine:  In crustaceans such as shrimp, an anteriorly directed, spine-like projection on the orbital region of the carapace; located at the anterior edge, above and behind the orbit.(photo)

Suture:  In a gastropod shell, the joints where the whorls are joined to one another (drawing)

Swash zone:  The portion of a beach on which the waves are running up the beach as each breaks, then running back down the beach with the backwash.  The region moves up and down the beach as the tide goes in and out.

Swimmeret:  The pleopods, or abdominal legs of decapods such as shrimp, lobsters, and crabs. The swimmerets are not used for walking.

Synapomorphy:  In Cladistics, a characteristics shared by members of a clade (taxonomic group) but not possessed by their immediate ancestor.  May be used as a defining characteristic of a group

System:  In compound tunicates, a group of individual zooids which have separate oral siphons but whose atrial siphons all empty into a common atrial cavity (photo).


Tarsus:  One of the sections of a leg, for example in pycnogonids the 7th segment of the leg, following tibia 2 and preceding the propodus. (photo)

Taxodont dentition:  In bivalves, a pattern of hinge teeth in which there are many small parallel or nearly parallel teeth along the hinge area and at right angles to the hinge ligament (photo) (photo).

Teloconch:  In gastropods (such as whelks), the portion of the spire formed after the protoconch (photo). May have more sculpture, less erosion, or a different shape than the protoconch does.

Telson:  Most posterior segment of the body of a crustacean.   May form tailfan (caudal fan) together with uropods. (photo)

Tentacle:1. In Anthozoa one in a series of variously arranged, motile and contractile, nematocyst-bearing processes around oral disc.   2.  In Scyphozoa, one in a series of elongate processes around umbrellar margin, either on or between lappets or projecting from subumbrellar surface.  In a sessile scyphozoan, one of numerous minute, capitate tentacles at tip of each arm.   3.  In Holothuroidea, part of the water vascular system; variously modified anterior tube feet (podia); highly retractile, near the mouth, supplied by radial canals or directly by ring canal.  4.  In Cephalopoda, a term occasionally used as synonym for arm, yet more precisely applied to one of the two elongate arms used by squid and sepiolids to capture prey (photo).  Consists of a more slender stalk and a terminal sucker bearing an expansion (tentacular club or pad) (photo).  5.  In Gastropoda, one of several types of elongate projections of the body.  One may distinguish the typically paired cephalic tentacles and rhinophoral tentacles on head, epipodial tentacles along foot, mantle tentacles along mantle margin, pedal tentacles of sessile, tube-dwelling snails, or labial tentacles around mouth.

Tentacular bulb:  A bulbous swelling at the base of the tentacles of hydromedusae (photo)

Tentacular cirrus:  In polychaetes, an elongated cirrus (a parapodial remnant?) which projects from the body segment fused to the peristomium (photo)

Tentilla (Tentillae):  Small side branches on tentacles (photo)

Tergite:  In shrimplike crustaceans, the plate of integument (exoskeleton) covering the dorsal part of each segment.  Attaches on each side to the pleurite.

Tergum: (plural: terga) In kinorhynchs and arthropods, the dorsal portion of an interrupted ring of cuticle that encircles a segment.  In unstalked (acorn or sessile) barnacles (suborder Balanomorpha), the two terga are a pair of movable plates that form the left and right sides of the posterior portion of the operculum (photo). When pulled together the terga and scuta protect the animal.  When pulled apart they allow the aperture to open so that the cirri can project and the animal can feed.  In some unstalked barnacles the terga are long and sharp and form a movable "beak".  In stalked (goose) barnacles (Suborder Lepadomorpha) the terga are two of the five primary plates (carina, two scuta, two terga) on the outer surface.  They border the posterior end of the aperture and, along with the two scuta, can be pulled together to cover the animal (forming the operculum) or can be pulled apart, opening the aperture and allowing extension of the cirri and feeding.

Terminal:  Occurring at the very end.

Test (Shell):  Endoskeleton structure; rigid shell determining shape of body and enclosing internal organs.  Consists of a series of fused plates.

Tetraxon (Tetract):  A class of spicule in sponges.  Tetraxons or Tetracts are (always megasclere?) spicules with 4 diverging axes

Theca:  A cup, such as the cuplike extension of perisarc that surrounds the polyp in thecate colonial hydroids.

Thecate:  A type of hydroidpolyp in which the perisarc extends into a cuplike theca into which the polyp can withdraw.  Forms a suborder Thecata of Order Hydroida.

Thoracopod:  A leg coming from the thorax.  Includes pereopods (walking legs) and maxillipeds or gnathopods, which are used for feeding.

Thorax:  In crustaceans, a section of the body which consists of multiple segments.  Segments comprising the thorax are found behind the head (but united with the head if the species has a head and thorax united into a cephalothorax) and anterior to the abdomen.  For malacostracan crustaceans all walking legs (pereopods) originate from the thorax.  Often the gills are also located in the thorax, and for many species a carapace may extend back from the head and cover most or all the thoracic segments dorsally.  In some polychaete worms, the body behind the prostomium and peristomium is divided into two dissimilar regions, an anterior thorax and a posterior abdomen.  Some polychaetes with a thorax also have a posterior subregion of the thorax, called the parathorax, which has 3-4 segments with biramousparapodia.  Some may also have a tail, behind the abdomen, which has small segments with no appendages.

Tibia:  One of the segments of a leg, for example, tibia 1 and tibia 2 are the 5th and 6th segments of a pycnogonid leg, following the femur and preceding the tarsus (photo).

Tooth:  One in a series of elements of a radula.  May also refer to tooth-like structure on columella.

Tornote:  A type of spicule in sponges.  Tornotes are megascleres, a monaxon with both ends shaped like a lance.

Torus (pl Tori):  In polychaetes, an uncigerous ridge.  On the parapodium, a modification of the neuropodium into a low glandular ridge from which setae arise

Transverse:  Oriented crossways, perpendicular to the anterior-posterior axis of the body or structure.

Triaene:  A type of megasclerespicule in sponges which has 4 rays:  One long ray and three short, equal rays.

Triaxon (Triact):  A class of spicule in sponges.  Triaxons or Triacts are (always megasclere?) spiculeswith 3 diverging axes.  A simple triaxon has 3 axes (rays), while a hexactinial spicule is a triaxon with 6 rays.

Trichogon:  In the parasitic Rhizocephalan barnacles, Order Kentrogonida, Famly Sacculinidae, a larval stage of the male.

Trifid:  Divided into three branches

Truncate:  Blunt, as if cut off.

Tube Foot: Podium

Tubercle:  A small, usually rounded projection (picture)

Tunic:  The outer covering, often rather thick, of a solitary or social ascidian; also the matrix in which the several to many zooids of a compound ascidian are embedded

Tylostrongyle:  A type of spicule in sponges.  Tylostrongyles are megascleres, a monaxon with one end rounded or blunt and a knob or head on the other end

Tylostyle:  A type of spicule in sponges.  Tylostyles are megascleres, a monaxon with one end pointed and a knob or head on the other end.

Tylote:  A type of spicule in sponges.  A tylote is a megasclere, a monaxon with knobs or heads on both ends.


Umbilicus:  In gastropod shells, a hollow tube along the axis of the shell around which the whorls coil; occupies the position of the columella.  May be only a small hole at the anterior end of the shell or may extend back several whorls.  (picture)

Umbo (Umbone, pl Umbones):  In the valve of a bivalve, a strongly curving dorsal region following the beak.  Represents the early growth stages of the shell. (picture).  In bryozoans, a blunt projection from the frontal wall.

Umbrella (Bell):   Main body or bell of medusa; upper surface termed exumbrella, lower surface subumbrella.  Typically with thick mesoglea and marginal lappets.

Uncinus (pl Uncini):  In polychaetes, a type of seta which is a modified hook with a dentate tip, either S- or Z-shaped (avicular uncini), gently curved (acicular uncini), or plate-like with several teeth.  Long-handled uncini have a long, straight base before the hook while short-handled uncini have only a short straight base below the hook.  This type of seta is common in tube dwellers.

Uniramous:  Having only one main ramus (stalk); not branching into two or more branches.

Uropod:   One of two appendages of last (sixth) abdominal segment (pleonite) of a crustacean.   Typically flattened and consisting of basal protopod and two branches (endopod, exopod.)  May form tailfan (caudal fan) together with telson. (photo).  In amphipods, the last 3 segments may each bear a uropod.

Urosome:  In some crustaceans such as some amphipods, the posterior portion of the pleon, which is different from the anterior portion of the pleon which is called the pleosome.  Contains 3 somites, each of which bears a uropod.


Valve:  In bivalves, one of the two calcareous shell elements secreted by the mantle and more or less covering left and right sides of the soft body.  The two valves are joined dorsally by a hinge and ligament.

Varice (pl Varices):  A high, thin, winglike ridge running longitudinally on some gastropod shells.  Plural = varices.

Velum:  In Hydroid medusae (jellyfish), an inward-directed shelf around the inner margin of the bell (photo).  Often helps to generate more thrust on contraction of the bell.

Ventral:   For most animals, the side oriented downwards, toward the substrate.
  Ventral for bivalves:  The side farthest from the hinge and umbo (photo)

Ventral cirrus:  In polychaetes, a projection from the lower side of a neuropodium

Verruca (wart):  In hexamerous anthozoan (anemone or coral), one in a series of muscular, hollow adhesive structures lined with gland cells; often in longitudinal rows along outer surface of column between lines of septum attachment. (picture)

Vestigial:   Comparatively imperfect in size or shape.  Not fully developed or functional.

Vibraculum (pl. Vibracula):  In some Bryozoa of Order Cheilostomata, a specialized zooid whose body is mainly a long, thin, vibrating projection.  Functions to keep the colony free of settling organisms.

Villous:  Covered with tiny, soft, long hairs

Visceral mass:  In mollusks, the mass of internal organs such as the stomach, intestine, excretory system, and heart, typically dorsal to the foot.

Viviparous:  A life cycle in which the young are born as larvae or small individuals rather than as eggs.  Compare Oviparous and Ovoviviparous


Web:  In cephalopod mollusks, a thin skinlike membrane stretching between the bases of the arms.  In some species it may be extensive enough to form an umbrellalike structure.

Whorl:  In shell, any complete (360o) coil or exposed surface of a complete coil (drawing).  First-formed and smallest whorls begin at apex and increase in size until last whorl (body whorl).   The line forming border between two adjoining whorls is termed a suture.

  Shell width for bivalve shells:  In the closed shell, the distance between the furthest expansion of the left and right valves.

Wrack line:  The line of kelp and other debris deposited on a beach, especially sandy beaches, at high tide.




Zoea (zoeae):  A freeswimming crab larval stage occurring after the nauplius and before the megalops (postlarva).  Zoeae typically are shrimplike in shape and have a long rostral spine, and often other spines as well.

Zooecium:  Skeletal outer structure encasing single bryozoan individual (zooid).

Zooid:  A single individual of a bryozoan colony (photo)

Zooxanthellae:  Symbiotic dinoflagellates of genus Symbiodinium.  Found primarily in Cnidarians.

Kozloff, 1987, 1996
Stachowitsch, 1992
Morris, 1992

This page created and edited by Dave Cowles, 2002-2013
Also edited by: Melissa McFadden, 08-2002
                Kelly Williams, 08-2002
                Christina Smith, 08-2002
                Janisse Maxwell, 08-2002
                Hans Helmstetler 10-2002 - 1-2003

Salish Sea Invertebrates web site provided courtesy of Walla Walla University