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Glossary
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GLOSSARY
Abyssal: Oceanic depths from 2000 to 6000 m.
Carnivore: Flesh eating animal.
Adductor muscle: One of the muscles used to close
a Bivalvia shell. Most species have anterior and
posterior adductor muscles; others have just a
single adductor muscle.
Circumpharyngeal nerve ring: A ring of nerves and
ganglia (the brain) that form a circular structure
surrounding the pharynx.
Aestivation: A period of inactivity seen in some
land snails usually during prolonged dry or hot
periods; the aperture is often covered by dried
mucous, also known as the epiphram, to retard
the loss of moisture.
Aperture: An opening, such as the opening of a
gastropod shell.
Aragonite: The form of calcium carbonate most
commonly used by mollusks, with an unstable
crystal structure, and a specific gravity of 2.9.
Bathyal: Oceanic depths from 200 to 2000m.
Bauplan: An idealized and generalized body plan.
Benthos (adj.: benthic): The bottom of a body of
water; e.g. ocean floor, lakebed, river bottom.
Blastomeres: Cells formed by cleavage from a
fertilized egg.
Blastopore: An opening, in the gastrula stage of
the embryonic development of mollusks (and
many other animals) that is destined to become
the mouth.
Blastula: An early stage of embryonic development, usually consisting of a hollow ball of cells,
preceding the gastrula larval stage.
Calcite: A form of calcium carbonate with a specific
gravity of 2.7.
Coeloblastula: The hollow cavity found in the
blastula.
Commensalism: A form of symbiosis where individuals of two different taxa live together, one
deriving a benefit from the association (usually
food), while the other is not affected by the association.
Commissure: A seam, where the two valves of
Bivalvia come together.
Conchiolin: The organic substance that forms
the thin outer layer of mollusks known as the
periostracum.
Crystalline style: In the stomach, a structure that
rotates and rubs against the gastric shield aiding
in mixing and grinding food particles, and releasing digestive enzymes. Found in conchiferan
mollusks (i.e., not chitons or Aplacophora), but
not all members of Conchifera have a crystalline style.
Ctenidium (pl.: ctenidia): A respiratory organ (specialized gills) unique to mollusks; in Bivalvia it
also serves in food gathering.
Demibranch: When filaments of the gills have two
branches, each branch is a demibranch. Present
in gills of Bivalvia and Cephalopoda.
Detritus: Dead and decaying litter, which is derived
from plants, and covers the environmental surface
(e.g. ocean bottom, forest floor).
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Diploid: Having paired chromosomes.
Epifaunal: Animals that live upon (as opposed to
within) the substrate of a body of water. They may
be attached by a byssus or be free-living.
Escutcheon: An area of the posterior-dorsal bivalved shell associated with the ligament.
Euryhaline: A term applied to organisms that tolerate a wide range of salinity.
Foraminifera: A group of Protozoa that form
shells, usually of calcium carbonate. Though
most are microscopic, some are visible to the
naked eye.
Four d (4d): Mollusks and most other protostomes
have determined or fixed cell fates during early
embryo development. All mesoderm is derived
from the 4d (four little d) cell, formed at the sixth
division after fertilization. After the first two
cleavages, the more or less equally sized cells are
termed A, B, C, D. The third cleavage produces
unequal sized cells; the smaller cells are 1a, 1b,
1c, 1d. The fourth cleavage of the larger cells
produces the 2a, 2b, 2c, and 2d cells. The fifth
cleavage produces the 3a, 3b, 3c, and 3d cells.
The sixth cleavage of the larger cells produces
4a, 4b, 4c, and 4d.
Gamete: Specialized reproductive cells; eggs and
sperm.
Ganglion (pl.: ganglia): A structure composed of
nerve cell bodies.
Gastric shield: a hardened cuticular plate near the
anterior end of the stomach, made of protein and
chitin with a roughened, rasp-like surface. The
crystalline style rotates against the gastric shield
aiding in mixing and grinding food particles, and
releasing digestive enzymes.
Gastrula: The mass of embryonic cells following
the blastula. During the formation of the gastrula,
cells that are fairly superficial migrate to where
they will develop into future organs.
Glossary
Glochidium (pl.: glochidia): A modified larva of a
freshwater mussel (Unionidae)
Gonopore: Where the reproductive system opens
to the outside of the body.
Hadal: Oceanic depths greater than 6000 m.
Hemocoel circulation: The hemocoel is a coelom
(cavity) surrounding the heart. Mollusks do not
have a system of arteries, veins, and capillaries.
When the blood is pumped out of the heart, it
passes into sinuses that carry it to the various
organs. The blood collects in other sinuses and
is then returned to the heart.
Hermaphrodite: An individual that produces both
male and female gametes.
Homeobox Genes: genes that provide the identity of particular body regions in a developing
embryo by determining where body structures
will develop.
Infaunal: Animals that live within the sediment at
the bottom of a body of water.
Labial Palps: In Bivalvia, folds of tissue on either
side of the mouth that help to bring food to the
mouth.
Lecithotroph: An egg rich in yolk.
Lentic: A freshwater habitat that is characteristically calm, e.g. ponds, lakes, swamps.
Ligament: An elastic structure linking the two
valves in Bivalvia.
Littoral: The shoreline zone where the water meets
the land.
Lotic: A freshwater habitat that is characteristically
running, e.g. brooks, streams, and rivers.
Lunule: An area of the anterior-dorsal bivalved
shell that is circular to heart shaped and separated
from the rest of the shell by a ridge.
Glossary
Macrophyte: A large macroscopic plant; often used
in reference to kelp and other seaweeds.
Mantle: An organ that covers the body of a mollusk.
It functions in producing the shell and periostracum. In air-breathing gastropods (Pulmonata), it
serves in a respiratory function as well.
Mantle Cavity: The space within the mantle where
the gills, osphradia, excretory, and genital openings are found.
Mesoderm: Embryonic cells that develop into
muscle, blood, and connective tissue.
Metamerism: Repetition of body segments or
repeated organ systems. Seen in Annelida, but
mollusks do not have true metamerism. Those
mollusks having repeated organ systems, such as
the Polyplacophora, Monoplacophora, and some
Aplacophora, have pseudometamerism.
Microphagous: Feeding on small particles or small
prey (microorganisms).
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Osphradium (pl.: osphradia): Sense organs found
in some mollusks, usually in the mantle cavity.
These structures can detect certain chemicals in
the water surrounding the mollusk.
Ovoviviparous: A condition in which the female
holds the eggs in her uterus until they hatch, so
her offspring emerge as juveniles rather than as
eggs.
Pallial line: In Bivalvia, a fine linear impression,
on the inner surface of the shell, made by the
mantle edge. The line runs from one adductor
muscle to the other.
Pallial sinus: An invagination of the pallial line
where the siphons pass through. This is found in
the posterior end of the shell.
Palps: see labial palps
Parasitism: A form of symbiosis in which individuals of two different taxa live together, one
deriving a benefit and the other being harmed by
the association.
Molluscivore: Feeding on mollusks.
Mutualism: A form of symbiosis in which individuals of two different taxa live together, both
deriving a benefit from the association.
Nacre: The iridescent or shiny layer that lines the
inner surface of some mollusks such as Unionidae
(freshwater mussels) and Ostreidae (oysters).
Nephridium (pl.: nephridia): A renal organ in mollusks, a kidney.
Nidamental gland: Provides the outer coating for
eggs and sometimes the gelatinous part of the
egg mass.
Omnivore: An animal that feeds on both plants
and animals.
Operculum (pl.: opercula): A plate that wholly or
partly covers the aperture of some gastropods. It
may be horny or calcareous.
Parthenogenesis: Development of an organism
from an egg that did not require fertilization. The
offspring are genetically identical to the parent.
Pectinoid: A member of the superfamily Pectinoidea (Bivalvia): examples include Pecten,
Chlamys, Lima, and Spondylus.
Pelagic: Open water. It is the water column in the
marine environment extending from the air-sea
interface down to the benthos.
Pelecypoda: A synonym of the Class Bivalvia.
Pericalymmal larva: a barrel-shaped larva with
prominent apical tuft. It propels itself with transverse bands of cilia and a patch of accessory locomotory cilia. In the later stages, the larvae become
laterally compressed as the shell develops
Pericardium: A sac surrounding the heart. The molluscan pericardium is unique to mollusks.
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Periostracum: The proteinaceous outer shell layer.
Polyploidy: Having three or more of each type of
chromosome.
Prismatic Layer: A cross section of a molluscan
shell will usually reveal several layers. The prismatic layer is often the middle layer, between the
outer (periostracum) and inner (often nacreous)
layers.
Propodium: The leading edge of the gastropod foot.
Protostome: A major group of animals including
Mollusca in which, during development, the
mouth forms from the blastopore, and the anus
forms from a second opening.
Pseudofeces: After bivalves remove the desirable
food particles through suspension feeding, the
remaining material, bound by mucus, is ejected by
reverse flow. This material is called pseudofeces.
Pseudometamerism: The appearance of being segmented, when in fact true segmentation does not
occur, as in some mollusks.
Piscivorous: An animal that eats fish. Some Conidae are piscivorous.
Radula (pl.: radulae): A rasp-like structure, found in
the oral cavity of most mollusks (except Bivalvia)
that is used in feeding. It usually looks like a ribbon with small tooth-like structures attached to it
in rows and columns.
Refugium (pl.: refugia): A location where major
environmental changes have not occurred and
therefore the biota of an earlier period persists.
Resilifer: A recess on the hinge plate that contains
the resilium (internal portion of the ligament).
Glossary
(connective tissue). In Mollusks, as development
progresses, this coelom becomes reduced to just
cavities surrounding heart, kidneys, and gonads
(reproductive organs).
Semislug: A snail partially covered by an external
shell, into which the animal cannot retreat. A
condition of some land snails and some sea slugs
(e.g., Notaspidea).
Shell: A hard calcareous structure found encasing
mollusks. In some mollusks, such as Bivalvia,
this structure encloses most or all of the animal.
In other mollusks, such as some Gastropoda
(slugs and semislugs) the shell is greatly reduced
or vestigial.
Sipuncula: A phylum of marine worms, often called
peanut worms, considered to be one of the possible sister groups to mollusks.
Spicule: A small calcareous projection found in
some mollusks [Aplacophora and on the girdles
of some Polyplacophora (chitons)].
Stenocalymma: a specialized larval form of some
Aplacophora intermediate between pericalymma
and trochophore larvae
Symbiosis: Individuals of two different taxa living
in close association. Forms of symbiosis can be
described as commensalistic, mutualistic, and
parasitic.
Teeth: There are three uses for this term in malacology. 1) Projections from the hinge of a bivalve
shell that fit into sockets on the opposing shell.
2) Projections seen on the inner surface of the
aperture in some gastropods that might function
as anti-predator barriers. 3) Structures on the
radula that give it its rasp-like character.
Riffle: part of a stream with shallow, fast-flowing
water followed by deep, slow-flowing water.
Tetraneury: A nervous system, found in mollusks,
that is composed of two sets of paired nerves, one
dorsal and the other ventral.
Schizocoel coelomic cavities: A coelom is a cavity
formed by a splitting of the embryonic mesoderm
Torsion: A feature of all gastropods: an early developmental 180º twisting of the shell, mantle,
Glossary
and visceral mass with respect to the head and
foot. Torsion results in the nervous system being
twisted or crossed and in the products of excretion and reproduction being discharged from a
position above the animal’s head.
Trochophore: early larval stage of most mollusks.
The animal is more or less shaped like a toy top
and has an equatorial band of cilia and an apical
tuft that provides locomotion as well as help with
feeding. The trochophore often develops into a
veliger larva.
Uropod: A structure, found in Malacostraca
(shrimp, crabs, and lobsters) that forms part of the
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swimming apparatus of the tail. Some Bivalvia
attach to this structure.
Veliger: late larval stage of many mollusks, derived
from the trochophore. The more or less spherical
animal has a pair of large lobes, each being known
as a velum, which help with swimming and feeding. The larval shell, the protoconch, is formed at
this stage. Eventually the animal transforms into
a juvenile by metamorphosis.
Visceral Mass: The soft tissues of mollusks that
comprise the internal organs (digestive and reproductive) of a mollusk.