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ABCS 409
MARINE BIOLOGY
MAJOR GROUPS OF MARINE
INVERTEBRATES: MOLLUSCS
DR
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LAMPTEY
(amlamptey@ug
...
gh)
DEPT
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The scientific study of molluscs is called
Malacology
...

• Molluscs are the largest marine Phylum, comprising about
23% of all the named marine organisms
...

• They are highly diverse, not just in size and in anatomical
structure, but also in behaviour and in habitat
...


Taxonomic classes
1
...

3
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5
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7
...

9
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• The gastropods (snails and slugs) are by
far the most numerous molluscs in terms
of classified species, and account for
80% of the total
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Features (cont’d)
• Although molluscs are coelomates, the
coelom tends to be small, and the main body cavity
is a hemocoel through which blood circulates;
• Their circulatory systems are mainly open
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• The generalized mollusc has two paired nerve cords,
or three in bivalves
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• Most molluscs have eyes, and all have sensors to
detect chemicals, vibrations, and touch
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• All produce eggs, from which may emerge
trocophore larvae, more complex veliger larvae,
or miniature adults
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• For example, the heart and nephridia (kidneys)
are important parts of the reproductive system,
• as well as the circulatory and excretory systems;
in bivalves,
• The gills both "breathe" and produce a water
current in the mantle cavity, which is important
for excretion and reproduction
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Molluscs Anatomy

Eating, digestion & excretion

• Most molluscs have muscular mouths with radulae
"tongues", bearing many rows of chitinous teeth,
which are replaced from the rear as they wear out
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• The radula is unique to the molluscs and has no
equivalent in any other animal
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Beating
cilia (tiny "hairs") drive the mucus towards the
stomach, so the mucus forms a long string called a
"food string"
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• Before the mucus string reaches the prostyle, the acidity
of the stomach makes the mucus less sticky and frees
particles from it
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Digestion and excretion (cont’d)
• Periodically, circular muscles at the hindgut's
entrance pinch off and excrete a piece of the
prostyle, preventing the prostyle from growing too
large
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• Carnivorous molluscs usually have simpler digestive
systems
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Circulatory system
• Molluscs' circulatory systems are mainly open
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• The main body cavity is a hemocoel through which
blood and coelomic fluid circulate and which
encloses most of the other internal organs
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The blood contains the
respiratory pigment hemocyanin as an oxygencarrier
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• The atria of the heart also function as part of the
excretory system filtering waste products out of the
blood and dumping it into the coelom as urine
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Respiratory system
• Most molluscs have only one pair of gills, or even
only one gill
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• They divide the mantle cavity so water enters
near the bottom and exits near the top
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Respiratory system (cont’d)
• If the osphradia detect noxious chemicals or
possibly sediment entering the mantle cavity,
the gills' cilia may stop beating until the
unwelcome intrusions have ceased
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• Molluscs have, for centuries, also been the
source of important luxury goods, notably
pearls, dye, and sea silk
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Hazards
• Mollusc species can also represent hazards or
pests for human activities
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• Stings from a few species of large tropical cone
shells can also kill,
• but their sophisticated, though easily produced,
venoms have become important tools in
neurological research
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• Snails and slugs can also be serious agricultural
pests,
• and accidental or deliberate introduction of
some snail species into new environments has
seriously damaged some ecosystems

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