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aeolid – A nudibranch sea slug of the suborder Aeolidina.

annulate – A form that carries rings-shaped segments.

anterior – The front or towards the front.

anus – The external opening at the end of the alimentary canal through which solid waste matter (faeces) is expelled.

aperture – Relating to a shell it is the single large terminal orifice through which the head, foot and mantle is extruded.

apex – The first part of the shell formed, typically bearing a protoconch (the tip of the spire).

apical – The region at a tip or apex.

arborescent – repeatedly branched, bushy or tree-like.

axial – Refers to the direction of shell sculpture that is roughly parallel to the coiling axis.

autotomy – A process by which an animal deliberately casts off part or parts of its body as a defensive action – cerata or sections of the mantle.

benthic – Relating to the bottom of a body of water. A benthic organism lives on or in the substrate.

bifid – Split or cleft into two parts – forked.

bilobed – Having two lobes.

bipinnate – With reference to the gill – having small side branches arising on two sides of the main stem (like a feather).

bivalves – An aquatic class of molluscs (Bivalvia) having two shells (valves) that are typically symmetrical along the hinge line and feed by siphoning and filtering particles from the water.

body whorl – One complete spiral turn of a univalve shell.

branchia – Refers to the gill.

branchial pocket – A cavity on the dorsal midline of a dorid’s body into which the gills can be retracted. May be situated from halfway back to very close to the posterior end depending on species.

buccal – Referring to the mouth or mouth cavity.

carnivorous – Refers to an organism that eats organisms of the animal kingdom.

caryophyllidia – Microscopic spicule bearing tubercles on the mantle surface of some discodorids. Of unknown function but postulated to be sensory.

cerata – (singular ceras) Finger-like processes arising in groups from the notum of aeolid and some armininid nudibranch sea slugs and containing diverticula of the digestive gland. They function in respiration, digestion and (in aeolids) defence. The term is also used for the similar-looking processes in some sacoglossans but they are not morphological related.

caruncle – Elevated ridge or lobe between or anterior to the rhinophores in some armininid nudibranchs. Has a presumed sensory function.

caudal – Of or like or relating to the tail.

cerebral ganglion – A ganglion is group or cluster of nerve cell bodies. In sea slugs the cerebral ganglion in the head region is the closest structure they have to a brain.

chemosensory – able to detect chemicals.

chloroplasts – The plastid organelles within a plant cell that contain chlorophyll and conducts the process of photosynthesis.
cilia – (singular cilium) Fine hair-like organelles. There are two types; motile, that beat with a lash-like stroke and non-motile, that serve in a sensory capacity.

clavus – The distal portion of the nudibranch rhinophore, above the stalk. It is often enlarged, carrying lamellae, papillae or other embellishments to increase surface area.

club – As for clavus.

cnidosac – A small sac at the apex of each ceras of most aeolids in which functional undischarged nematocysts, acquired from the cnidarian prey, are stored for defensive use.

columella – The pillar forming the central axis of the shell around which the whorls spiral.

columellar lip – Inner lip of the shell aperture.

crenulated – Finely notched or corrugated – relating to the edge of a shell.

cryptic – Body camouflaged to resemble part of the natural microhabitat; or possessing secretive behaviour. May also refer to species being thus far unrecognised as a separate species, being hidden and undescribed amongst others.

cryptobranchs – An unnatural grouping of dorid nudibranchs that can completely withdraw their gill into a branchial pocket below the notum – crypto = hidden.

dendritic – branching, tree-like

detorsion – A developmental process whereby the sea slugs reverse much of the torsion that most gastropods undergo to adapt to living in a shell with a single aperture.

diffusion – A passive process of movement of a substance (including gases) from an area of high concentration to an area of low concentration – e.g. through the thin cell walls of a gill.

distal – The part which is situated away from the point of attachment or origin or a central point – opposite to proximal.

digestive diverticulum – A branch or lobe of the digestive gland usually located within a ceras.

digitiform – Of finger-like appearance

dorid – Nudibranchs with a circle of gills surrounding the anus, or an arc of gills just anterior to the anus, in the midline located in the posterior-half of the body. They are grouped in the suborder Doridoidea.

dorsal – The upper surface or back of an animal.

dorsolateral – Referring to a region that is at the sides or edges of the top surface.

dorsum – Refers to the upper (top) surface of the sea slug mantle = notum

endemic – Referring to a species that is native and restricted to a certain locality in its geographic distribution.

enrolled – Rolled up like a cylinder or tube but with a lateral groove.

epiphytic – A term borrowed from botany meaning, in this context only, plant or sessile animal growth attached and living on animal or plant but not as a parasite.

epithelium – Most often used to describe the skin tissue of an animal but also refers to the outer lining of organs and the also the lining of internal cavities.

filament – A slender thread-like process.

foot – The large ventral muscle mass being the organ of locomotion for gastropods.

fusiform – Spindle-shaped.

gastropod – A taxonomic class of molluscs including the sea slugs that if there is a shell present is most always only one.

gill – (= branchia) The respiratory organ enabling gaseous exchange with the water environment. The term may also be applied to a wide variety of different structures performing the function. These are referred to as secondary gills.

gizzard – A modified muscular region of the gut used for masticating food, sometimes with grinding plates.

habitat – The type of natural environment in which an organism lives and is characterised by both physical and biological aspects.

haemocoel – the primary body cavity of molluscs in which the internal organs are bathed in haemolymph (= blood) – an open circulation.

haemolymph – The circulatory fluid of the sea slugs that contains blood and lymph fluid combined. There are no blood cells.

Hancock’s organ – A bilateral chemosensory organ located anteriorly between the headships and foot. Found in the majority of the shelled sea slugs.

headshield – An adaption of the head where it is widened to assist the sea slug to push through the substrate but preventing

herbivorous – Describing an animal that feeds on plant material such as algae.

hermaphrodite – Describes an organism possessing both male and female functional reproductive organs and capable of producing both sperm and eggs. All sea slugs are hermaphrodite.

homologous – Usually referring to organs of different taxa that are similar in position, structure and evolutionary origin but not necessarily in function.

hyponotum – The under surface of the mantle. Sometimes referring to the area between the underside of the mantle and the foot (the hyponotum groove).

in situ – Refers to when the animal is situated in its natural environment.

incised – A deep etching or mark on the surface of a shell.

infaunal – Refers to an animal that lives within the substrate e.g. burrowing.

infrapallial lobe – A posterior fold of the body enclosing the posterior end of the shell in cephalaspideans.

intertidal – The area of foreshore lying between high tide (when it is covered by water) and low tide (when it is exposed to the air).

intraspecific variation – Refers to the natural variation found in one or more features between individuals of the same species.

invertebrate – Animals without and do not develop a vertebral column.

involute – Describes the spire of a shell that is sunk below the level of the apex.

jaws – In sea slugs refers to the thickened, horny plates within the buccal cavity for gripping prey.

kleptoplasts – Chloroplasts that are sequestered through the agal diet of certain sacoglossans and then maintained by the host to temporarily continue photosynthesis for its benefit.

lamellate – (= perfoliate) A term used to describe the structure of some rhinophores that bear plates or leaflets on the clavus that are either horizontal or vertical.

lappets – The muscular plates or fleshy extensions associated with the mouth. Often sensory for close proximity food location.

last whorl – The body or last, most recently formed whorl of the gastropod shell terminating in the aperture.

lateral – Positional term relating to of, at or towards the side.

larvae – (singular larva) The juvenile stage of an animal prior to its undergoing metamorphosis into an adult. Its form can be completely different to the adult.

lecithotrophic – Refers to the type of development in which larvae utilise only the reserves of yolk in the egg to nourish themselves Generally a free-swimming, non-feeding, short duration larval stage.

limaciform – slug-shaped.

mantle – The organ surrounding the viscera of molluscs and secreting the shell. In those sea slugs without a shell it develops into the layer that covers the body – the notum.

mantle skirt – The expanded edges of the mantle in the shell-less sea slugs.

mantle margin – The outermost edge of the mantle.

medial – Positional term relating to of, at or towards the middle or midline. Opposite to lateral.

metamorphosis – A biological process whereby an animal develops by undergoing a significant physical and abrupt change in body structure often accompanied by a change in habitat, diet and behaviour.

metapodium – A posterior lobe or extension of the foot. It bears the operculum when present.

nematocyst – The specialised stinging cell of the Cnidaria (a minute capsule with a coiled thread that rapidly unwinds on discharge) to capture prey or act in defence. Present in hydroids, sea anemones, corals, etc. the food of aeolid nudibranchs. They separate the nematocysts from the remainder of the hydroid tissues and pass them, undischarged, through the digestive diverticulum in the cerata to the cnidosac at the tip where they are stored to be used for the aeolid’s own defence.

nocturnal – Active at night only.

notum – Refers to the upper (top) surface of the sea slug mantle = dorsum

noxious – Usually used to describe something not as dangerous as toxic but still harmful and often distasteful as well.

nudibranch – Correctly used only for those sea slugs that are members of the order Nudibranchia. Incorrectly used in much popular literature to refer to all the sea slugs.

operculum – A horny or calciferous plate that partially or completely closes the aperture on retraction into the shell of most (not all) shelled molluscs.

opisthobranch – No longer a valid term although sometimes now used informally to refer to all the sea slug types. It was previously used to describe members of the subclass Opisthobranchia, now an invalid taxon being non-monophyletic.

oral tentacles – Pair of tactile and chemosensory tentacles (or flaps) around the mouth; used for close proximity food location and recognition.

oral veil – A membranous extension of the head above the mouth sometimes with well-developed sensory projections.

osphradium – A chemosensory organ of some molluscs associated with the respiratory ctenidium and believed to monitor the water quality within the mantle cavity. Of the sea slugs only the primitive shelled species have an osphradium located in the mantle cavity anterior to the gill.

outer lip – Outer edge of aperture of a shell.

palmate – Shaped like a hand with processes that are extended like fingers.

papilla – (plural papillae) A small, delicate, finger-like projection.

papillate – bearing papillae

parapodia – (singular parapodium) Thin flap-like lateral extensions of the foot in Cephalaspidea, Sacoglossa, Anaspidea and Thecosomata sea slugs that wrap up over the sides of the shell or body that may or may not meet in the midline. They may be used for body protection, respiration, plastid “farming” or swimming.

pelagic – Passively drifting or floating in the sea including on the sea surface, with currents or winds.

perfoliate – = lamellate

pericardium – Sac-like structure enclosing the heart.

periostracum – The protective outermost layer of gastropod and bivalve shells. It may vary from thin and transparent to thick and horny or fibrous.

phanerobranch – An unnatural grouping of dorid nudibranchs that can contract their gill but not withdraw it into a pocket below the notum so that it is always visible – phanero = visible or to show.

pinnate – Having side branches – resembling a feather.

phylogenetic – The evolutionary development and therefore the history of a species or a species trait. Also applied to higher taxons.

planktotrophic – Refers to the type of development in which free-swimming larvae spend a considerable time feeding and growing in the plankton before settlement and metamorphosis into the adult.

plicate – Folded, corrugated or twisted – producing a larger surface area.

posterior – The rear or towards the rear.

propodial – Referring to features of the anterior part of the foot.

propodium – The anterior part of the foot.

protoconch – Larval shell of a gastropod. Smallest spiral tip of a shell, if not lost through damage.

proximal – The part which is situated closer to the point of attachment or centre of the body – opposite to distal.

pustule – A small, conical or rounded swelling on the notum. Technically, should describe such a protuberance that is filled with a fluid or thicker compound. While pustule should not = tubercle the terms are often used interchangeably.

radula – A flexible, chitinous ribbon of small teeth contained within the buccal cavity used for feeding by, rasping at, cutting away pieces of, puncturing or pulling in prey. Combination, arrangement and shape of the teeth is useful in delineating species etc.

remnant – A part that is left of the original whole. Refers to organs or structures that are reduced through evolutionary development.

respiration – Process of gaseous exchange between an organism and the environment.

reticulum – Intersection of lines or structural parts forming at network-like pattern.

rhinophores – Paired chemo-sensory tentacles located anteriorly on the mantle or on the head, of most sea slugs.

rhinophoral pocket – A cavity into which the rhinophore of a dorid nudibranch can be withdrawn.

rhinophoral sheath – A sleeve, raised above the mantle surface of dendronotoid nudibranchs, surrounding the base of a rhinophore and into which the rhinophore can be contracted for protection.

sculpture – Texture of and any embellishment on the surface of a shell.

sessile – Refers to animals that are predominantly immobile, living a fixed existence. Some have a motile larval stage.

siphon – A tubular extension of the mantle to facilitate water flow into or out of the mantle cavity. Also a verb to describe the action of that water flow.

shell – A calcareous structure that acts as an exoskeleton (in most cases) to support and protect the soft vulnerable body parts. In sea slugs the shell may be external, reduced, internal, a remnant or completely lost in the adult.

spatulate – Refers to the wide flattened shape of the anterior portion of the body of some sea slugs.

spawn – Refers to the grouping of laid eggs as a – string, coil or mass fixed in a transparent jell.

spicule – A minute calcareous rod or other shaped body in the mantle to give a protective or supportive texture to the mantle.

spire – The visible part of a shell – all the whorls excepting the last or body whorl.

stalk – Usually a term used to describe the base portion of a rhinophore that supports the sensory clavus.

striate – Marked with fine grooves.

subapical – Referring to the portion below the apex or tip.

substrate – The surface upon which an organism is attached or moves across I.e. lives upon.

symbiosis – Describes a close and often long term interaction between two or more different species usually where both organisms benefit.

taxonomy – Concerned with the classification of organisms into a hierarchy or scheme of classification.

tentacle – Protruding sensory appendage most often on the head or around the mouth but also of the anterior portion of the foot – usually paired – left and right.

torsion – The rotation by 180 degrees of the visceral mass in an anticlockwise direction during gastropod development to suit living in a shell with a single aperture.

toxic – containing or being poisonous – capable of causing serious debilitation or even death.

tripinnate – Refers to a gill that has a main stem and primary and secondary branches.

tubercle – Short rounded projections above the surface usually on the mantle but can also refer to similar projections on rhinophores and cerata.

truncate – Having the appearance of being shortened by removal of the end thus looking blunt or cut off.

uniseriate – Describing an arrangement of teeth in the radula where there is only a single tooth in each row.

vascularised – refers to a tissue or structure that has circulatory vessels throughout.

velar tentacles – A term used to describe the processes projecting from the oral veil in some sea slugs. These have a sensory ability.

veliger – Larva of molluscs. Named derived from the characteristic ciliated, bilobed velum used for swimming and feeding.

ventral – The lower surface or underside of an animal.

viscera – The soft internal organs of the animals body.

whorl – One complete spiral turn of a univalve shell.

zooxanthellae – Symbiotic single-celled dinoflagellates, that photosynthesize, found in many cnidarian animals. Taken up and utilised symbiotically by some aeolid nudibranchs that prey on the cnidarians.