Glossary of Invertebrate Terms


Term Definition
abductor muscle scar

The area or depression on the inside of a bivalve shell where the foot muscle was attached.

aboral side

The side of an organism opposite of where the mouth is.

accessory arm plate

In starfish, a small plate or scale on both sides of the central arm scale.

acontium (pl. acontia)

Filamentous defensive organs, composed largely of stinging cells thrown out of the mouth or special pores on the body walls of certain anemones when irritated.

air bladder

A gas filled sac used for floatation in the Portuguese man o war and some gastropods. Also called a pneumatophore.


Pertaining to the head side (top or front.); opposite of posterior.


The main opening in a gastropod shell where the foot and head extrude.


The part of gastropod shell that was formed first, typically pointed, at the posterior end of the shell.


A hard spoon-shaped protuberance extending from underneath the beak of a bivalve.

Aristotle's lantern

The chewing organism of sea urchins and sand dollars, made up of five teeth (forming a jaw) and a fleshy tongue-like structure.

arm plate

In starfish, one of the row of plates or scales that extend down the center of the arms.

arm spine

In starfish, a spine (usually in groups) found on each side of the arms, assists in locomotion and feeding.

arms (of a cephalopod)

Shorter, thicker (as opposed to tentacles), fleshy extensions hanging beneath the head of the cephalopod. Usually has suckers most of its length.


Running longitudinally across the whorls in gastropods.

basal disk

The base of a sea anemone that attaches to the substrate.


In bivalves, the pointed or rounded end of the shell in the hinge area. Also called apex or umbo. In cephalopods, the pair of curved sharp teeth at the opening of the mouth.


The gelatinous, radially symmetrical covering of a jellyfish. The mouth is on the underside of the bell.

bifid tooth

A bivalve tooth in the hinge that is split into a V-shape, still connected at one end.


Ending with two branches or tips.


A mollusk that is surrounded by 2 hinged shells. Includes clams, oysters, and mussels.

body whorl

The lowest and largest whorl of a gastropod shell

branchial lamellae

Folds of the gills in nudibranchs.

byssal threads (byssus)

A bundle of threads used by bivalves to attach to hard surfaces.

calcareous plates

A ring of plates surrounding the esophagus in a sea cucumber.


A raised hardened part on a shell.

calyx (pl. calyces)

The depression or extension in a coral skeleton that houses the polyp.


Lattice-like; e.g., radial ribs crossed by concentric ribs.

cardinal teeth

Teeth on the hinge line in bivalves.


A marine mollusk having bilateral symmetry, a well developed head and eyes, having arms and tentacles. Includes octopuses, squids, cuttlefishes and the nautilus.

ceras (pl cerata)

Dorsal and lateral outgrowths on the upper surfaces of the body of nudibranchs.


A tough, semitransparent substance (a nitrogen-containing polysaccharide) that forms a protective covering.


Made of a tough, semitransparent substance (a nitrogen-containing polysaccharide) that forms a protective covering.


A hard scooped-out projection or pocket on the hinge line of a bivalve.


A cell containing pigment.


Minute hair-like structures that can move rhythmically and are used for locomotion or moving fluids and particles along a structure.


The common tissue that connects colonial polyps.


Skeletal material between the walls of the corallites of a coral.

columella - coral

The central part of the calyx of a coral skeleton where the lower elements of the septa fuse.

columella - mollusk

The central pillar of a gastropod shell around which the whorls form. It is partially or totally hidden inside the shell.


Curving inward.


Ridges or lines that run parallel to the margin of a shell in bivalves.


Curving outward.


The part of a coral skeleton produced by a single polyp.

corallum (pl. coralla)

The entire coral skeleton.

costa (pl. costae) - coral

Extension of the septum to the outside wall of the corallite.

costa (pl. costae) - mollusk

Tubular or circular ridge on the surface of a shell.


Having a round-tooth or scalloped edge.


Having margins that are scalloped, indented, wrinkled or notched.


A thin plate of shell underneath the beak that connects the anterior and posterior margins of the shell.


With angular or tooth-like projections of a margin.


Right-handed. In gastropods, when the whorls wind clockwise.


Farthest from the center of the body.


Pertaining to the back side (top or rear); opposite of ventral.


A depression on the posterior dorsal margin of bivalve shells.


The muscular part of a mollusk's body used for locomotion.


A mollusk typically having a single, coiled shell (cap-shaped in limpets, absent in nudibranchs), a flattened muscular foot used for locomotion, and eyes and tentacles on a distinct head. Includes snails, whelks, limpets and slugs.


The internal pen (hard structure) of a squid.


The toothed area beneath the beak and ligament that helps hold the two shells together in bivalves.

hinge teeth

Small nodules or projections on the hinge line of a bivalve shell. Helps hold the shells together. Also called cardinal teeth.

hyponotal lamellae

Folds on the underside of a nudibranch that contain the branches of the digestive gland.


Not flat. The degree of plumpness of the bivalve shell.

lamella (pl. lamellae)

A thin plate or fold of hard or soft tissue.

lateral fins

Fleshy fin-like projections on the either side of the mantle in squids.

lateral teeth

Small nodules or projections on the inside lateral edges in bivalve shells. Keeps the shells from sliding anteriorly and posteriorly.

left valve

In bivalves with a pallial sinus, when looking at the underside of a shell, the pallial sinus will be on the left in the left valve. In shells with only 1 muscle scar (posterior), the scar favors the left side in a left valve.


An elastic band of connective tissue that connects and holds the shells together in bivalves.

ligature area

Area above the hinge, usually between or posterior to the beaks, where the ligament occurs in bivalves.

lunule (of bivalves)

A depression on the anterior dorsal margin of bivalve shells.

lunule (of sand dollars)

Oval holes in the body of some sand dollars. The purpose of lunules is not known for sure. It may help the sand dollar to burrow, right itself, find food or to prevent the waves from lifting it out of the sand.


The membranous covering in a bivalve shell that lies next to the shell.

mantle of a squid

The external soft body of a squid. Does not include internal organs, arms or tentacles.


An additional plate behind the beaks in some bivalves.


An additional plate above the beaks in some bivalves.


An invertebrate with a soft, unsegmented body, usually having a hard outer shell or a reduced internal shell.

muscle scar

The area or depression on the inside of a bivalve shell where the foot muscle was attached.

nacreous layer

Shiny, iridescent part of shell.


A marine shell-less gastropod with external gills and appendages, sometimes with brightly colored bodies. Often called a sea slug.


Angled, either upward or downward, usually refers to mouths, stripes or lines.


The external hard plate or door at the aperture opening in gastropod shells..

oral arms

Soft, sometimes convoluted, structures that hang beneath the bell of a jellyfish that are used to pass food to the mouth.

oral cavity


oral disk

The membranous disk that contains the mouth in sea anemones.

oral papilla (pl. papillae)

In starfish, one of several plates or scales that line the sides of the jaws.

oral side

The side of an organism that contains the mouth.


A microscopic, calcified skeletal element found in the body walls, tentacles and tube feet of a sea cucumber.

pallial line

A line on the inside of a bivalve shell that indicates the edge of where the mantle lay.

pallial sinus

An indentation in the pallial line of bivalve shells.

pedalia (sing. pedalium)

Gelatinous, muscular extensions of the bell of some jellyfish.


A fleshy stalk used to attach to a hard surface.

pen (of a squid)

A semi-hard, feather-shaped structure, inside a squid's mantle, used for support and muscle attachment. Also called the gladius.


A protective skin that covers the outside of shells.


Specialize tube feet, used as gills, arranged in a 5-petalled flower shape, on the upper surface of a sand dollar.


An individual organism, usually cylindrical in shape with an oral end surrounded by tentacles and a base for attachment. When in colonies, each polyp usually serves only one function, such as digestion or reproduction.


Pertaining to the tail side (bottom or rear); opposite of anterior.


An additional plate in front of the beaks in some bivalves.


Nearest the center of the body or point of attachment to body.


The stem or main shaft of a structure.


extending out from a common point

radial shield

One of two plates at the base of each arm on the disk of a brittlestar.


A pair of tentacles, on the head of nudibranchs, which contain scent or taste receptors.

right valve

In bivalves with a pallial sinus, when looking at the underside of a shell, the pallial sinus will be on the right in the right valve. In shells with only 1 muscle scar (posterior), the scar favors the right side of a right valve.


A stiff, upright structure on some hydroids that is used to catch the wind to enable movement.


With closely set or overlapping flat, scale-like projections.


A skeletal element in soft corals, composed of calcium spicules.

septum (pl. septa) - coral

Skeletal radiating plates inside the calyx of a coral.

septum (pl. septa) - mollusk

A hardened partition which subdivides a cavity in a shell.


Left-handed. In gastropods, when the whorls wind counterclockwise.


A tubular structure for bringing in or expelling fluids.

siphonal canal

A channeled extension in a gastropod shell from which the siphon extends.

siphonal notch

A notch at the base of the siphonal canal in gastropods.


Minute hard pointed projections.


The coiling whorls above the aperture of a gastropod.


In bryozoans, branch-like extensions (not considered "true" branches as in plants).

stromboid notch

A notch or indentation in the lower outer lip of conch gastropods, used to extend 1 of its stalked eyes out of the shell.


Junction between whorls in gastropods.

tentacle (cephalopods)

In cephalopods, an elongated arm use to capture prey (usually with suckers on the distal ends). Tentacles contain the stinging cells.

tentacle (gastropods)

An elongated projection of the body.

tentacle (jellyfishes)

One of several narrow elongated string-like processes projecting from the lower surface of the bell. Tentacles contain the stinging cells.

tentacle (sea anemones)

One of many motile and retractable arms surrounding the oral cavity. Tentacles contain the stinging cells.

terminal process

In bryozoans, extra extension at the end of each stolon (branch); may be single or branched.


The wall of a corallite of a coral.

tube feet

Small tube-like projections on the underside of a starfish's arm that are used for locomotion, feeding and respiration.


A calcified and hard protuberance.


A sunken or depressed area at the base of a gastropod.

umbo (pl. umbones)

The beak in a bivalve shell.


A shell of a bivalve.


The broad, lateral part on both sides of the rachis (central shaft) of a squid's pen (internal hard structure).

varix (pl. varices)

Longitudinal thickened ridge found in some gastropods.


Pertaining to the front or bottom side; opposite of dorsal.


A spiral of a gastropod shell.


An individual polyp in a colony, as in bryozoans