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Phylum Mollusca Fact Sheet
Phylum Mollusca constitutes as one of the largest phyla of animals:
# of living species
# of individuals
Significant characteristic is the possession of a coelom (a fluid-filled cavity that develops within
the mesoderm):
Functions as hydrostatic skeleton
Provides space within which internal organs can be suspended
All mollusks have a soft body:
Usually protected by a hard, calcium-containing shell
In some cases shell has been lost over the course of evolution (slugs, octopuses) or
greatly reduced in size and internalized (squid)
All mollusks have the same fundamental body plan:
Head-foot: contains both sensory and motor organs
Visceral mass: contains well developed organs of digestion, excretion, and reproduction
Mantle: specialized tissue that enfolds the visceral mass and secretes the shell
Excluding cephalopods, mollusks have an open circulatory system. Cephalopods have a closed
circulatory system because they complete vigorous activities that require the cells to be
supplied with large quantities of oxygen and food particles.
All members except for bivalves have a radula for feeding. A radula is a ribbon-like set of
hooked teeth.
Class Bivalvia
7,500 living species
Derive name from the two parts the shell is divided into
Adductor muscles are used close the shell swiftly and tightly in times of danger
Lack defined heads, no radula, foot is laterally compressed, large mantle cavity with
gills, eyes may be present elsewhere on body
Have open circulatory system, poor sensory organs, simple nervous system
Separate sexes, fertilization can be internal or external depending on the water
Most adults are sedentary, herbivorous filter feeders, cilia on gills bring in food particles,
mostly microscopic algae
Class Gastropoda
Largest group of mollusks
Have either a single shell or no shell
Common in both freshwater and saltwater
Land-dwelling snails do not have gills, but the area in their mantle cavities once
occupied by gills is rich in blood vessels, and the snail's blood is oxygenated there
More mobile life than bivalves
Have a ganglionated nervous system with as many as six pairs of ganglia connected by
nerve cords, concentration of nerve cells in the tentacles, at the anterior end
In some, eyes are highly developed
Some have separate sexes but most are hermaphrodites
Have an operculum to protect their soft bodies from environmental hazards or
Class Cepholapoda
Most evolutionarily advanced animals to be found among the invertebrates
600 living species, bilaterally symmetrical
Rival the vertebrates in complexity and, in some cases, in intelligence
Only modern shelled cephalopod, Nautilus
Mantle has more flexibility, due to freedom from shell
Effect of this is the jet propulsion by which cephalopods dart through the water
Have sacs from which they can release a dark fluid that forms a cloud, concealing their
retreat and confusing their enemies
Have well-developed brains, composed of many groups of ganglia, in keeping with their
highly developed sensory systems and their lively, predatory behavior
Separate sexes, internal fertilization, courtship behaviors are complex, males fight for
access to females
Class Polyplacophora
900 living species
Aka chitons
Shell consisting of eight overlapping plates
Bilaterally symmetrical
Well-developed foot surrounded by a groove for gills
Head lacks eyes and tentacles
Has light sensitive areas and chemical receptors for finding food and heading in the right
Make a living by grazing algae from rocks
Many live in shallow waters but few occur in depths greater than 5,000 meters