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Phylum Mollusca
General Characteristics
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from the Latin mollis meaning soft
Largest Marine Phylum
over 150,000 diverse species
have a muscular foot used for locomotion
Have a mantle, an outgrowth that covers the
animal
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is also used in respiration, waste disposal, and sensory reception
Many mollusks have an external calcium carbonate
shell, hanging over the mantle, that is actually
produced by the mantle.
General Characteristics
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The mantle often creates a mantle cavity,
where the gill is housed
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The gill of a mollusk extracts oxygen from the water
and disposes of waste
Have a complete digestive tract – therefore
has an opening for the mouth and the anus
Many also have a radula, a unique organ
composed mostly of chitin, in the mouth
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The radula allows the animal to scrape food from
surfaces, especially the ocean floor, by sliding back
and forth.
General Characteristics
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Have a true coelom – body cavity
There are seven main classes of mollusks:
Class Polyplacophora - Chitons
 Class Aplacophora - Mollusks without shells
 Class Monoplacophora - Mollusks with one shell
 Class Schapoda - Mollusks with a tubular shell
 Class Gastropoda – Gastropods - snails and their
relatives
 Class Bivalvia - Bivalves – clams, oysters, and
scallops
 Class Cephalopoda – Cephalopods - squids and
octopuses
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Class Polyplacophora
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Also known as the chitons are a small class
consisting entirely of marine species
are highly adapted to life in the intertidal zone
range in size from 1-2cm to over 30cm
The shells of chitons are characteristically
divided into 8 transverse, overlapping shell
plates or valves
Class Polyplacophora
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feed on small particles of algae on the rock
surface which are scraped off using the radula.
Feeding normally takes place when covered by
the tide, and they tend to be most active when
immersed at night.
They are normally inactive animals and move
only to feed, but often show 'homing behaviour',
which enables them to return to the same area
of rock.
Chiton body structure
Class Aplacophora
Class Monoplacophora
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Prior to 1952, the monoplacophorans were
known only from fossil shells from the
Cambrian and Devonian. Then in that year the
'Galathea' expedition dredged up 10 living
specimens of Neopilina from deep water off the
Pacific coast of Mexico. A second species N.
ewingi was also discovered.
These two species are the only known living
representatives of this class.
Class Monoplacophora
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of 6 pairs of nephridia
The animals possess a large flattened foot
surrounded by the pallial groove, in which are
situated 5 or 6 pairs of gills.
Class Scaphopoda
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One of the smaller classes of molluscs
Also known as tusk shells
are burrowing, marine molluscs
having a tubular tusk or tooth-shaped
shell which is open at both ends
They are elongated along the anteriorposterior axis and live buried in the
sand, head downwards, with the body
steeply inclined
Class Scaphopoda
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The head is very reduced and
lacks eyes, but is surrounded by
numerous thread-like tentacles or
captacula
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The captacula possess an adhesive
knob at their tip and are used to gather
small particles of food present in the
sand and pass them to the mouth
Scaphopods are found mainly in
the deep sea, but a few species
occur in shallow water.