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Mollusks
Section 27.1
Animal Classification
Invertebrates
Sponges
Cnidarians
Worms
Mollusks
Arthropods
Echinoderms
- Class Gastropoda
- Class Bivalvia
- Class Cephalopoda
Animals
(cold-blooded)
Fishes
Amphibians
Reptiles
Endotherms
Birds
Mammals
Ectotherms
Vertebrates
(warm-blooded)
Characteristics
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Has at least 110,000 different species
Molluscus – Latin word meaning “soft”
All mollusks have
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bilateral symmetry (excluding octopi and squids)
a coelom
a digestive tract with two openings
a muscular foot
a mantle (a membrane that surrounds the internal
organs of the mollusk)
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The mantle produces
the shell
Mantle
Foot
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Snails have external mantles while squids
have internal mantles.
How Mollusks Obtain Food?
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Radula – a tongue-like organ with rows of
teeth
The radula is used to drill, scrape, grate, or
cut food
Octopi and squids use their radulas to tear up
the food that they capture with their tentacles
Bivalves do not have radulas; they filter feed
Reproduction in Mollusks
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Mollusks reproduce sexually and most have
separate sexes
Many terrestrial gastropods, and a few
bivalves, are hermaphrodites; they undergo
internal fertilization.
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Terrestrial – organisms that live on land
Most aquatic species reproduce via external
fertilization
Nervous Control in Mollusks
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Mollusks have simple nervous systems that
coordinate their movement and behavior
Octopi and squids have brains
Most mollusks have paired eyes that range
from simple eyes (detecting light) to complex
eyes (having irises, pupils, and retinas)
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Octopi have complex eyes
Circulation in Mollusks
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Mollusks have a well-developed circulatory
system that usually includes a two- or threechambered heart
Circulation in Mollusks
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Open circulatory system is used in simpler
mollusks, and the closed circulatory system is
used in complex mollusks
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A clam has an open system
Respiration in Mollusks
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Aquatic mollusks have respiratory structures
called gills
Respiration in Mollusks
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In land snails and slugs, the mantle cavity
appears to have evolved into a primitive lung
Excretion in Mollusks
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Nephridia (nephridium is singular) – organs
that remove metabolic wastes from an
animal’s body
Excretion in Mollusks
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Wastes are discharged into the mantle cavity
and expelled from the body by the pumping
of the gills
Diversity of Mollusks: Gastropods
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The largest class of mollusks is Gastropoda,
or the stomach-footed mollusks
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Gastro – stomach
Pod – foot
Most species of gastropods have a shell.
Other gastropod species, such as slugs, have
no shell
Diversity of Mollusks: Gastropods
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They can be found in freshwater, marine
water, or moist terrestrial habitats
Instead of being protected by a shell, the
body of a slug is protected by a thick layer of
mucus
Diversity of Mollusks: Gastropods
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Sea slugs are brightly colored and protected
by poisonous nematocysts
Diversity of Mollusks: Bivalves
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Two-shelled mollusks such as clams, oysters,
and scallops belong to the class Bivalvia
Most bivalves are marine, but a few species
live in freshwater habitats
Diversity of Mollusks: Bivalves
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Bivalves have no distinct head or radula
Most use their large, muscular foot for
burrowing in the mud or sand
Foot
Diversity of Mollusks: Bivalves
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A ligament, like a hinge, connects their two
shells, called valves
Strong muscles allow the valves to open and
close over the soft body
Diversity of Mollusks: Bivalves
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Bivalves are filter feeders
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As water moves over the gills, food and sediments
become trapped in mucus
Food enters
siphon
Diversity of Mollusks: Cephalopods
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The head-footed mollusks are marine
organisms in the class Cephalopoda
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Cephalo – head
Pod – foot
This class includes the octopus, squid, and
chambered nautilus
Diversity of Mollusks: Cephalopods
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The foot has evolved into tentacles with
suckers, hooks, or adhesive structures
Once tentacles have captured prey, it is
brought to the mouth and bitten with beaklike
jaws
Then the food is torn and pulled into the
mouth by the radula
Diversity of Mollusks: Cephalopods
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Like bivalves, cephalopods have siphons that
expel water which help them to move (jet
propulsion)
Squid can move up to 65 ft per second
or 24 mi/hr (44 km/hr)
Diversity of Mollusks: Cephalopods
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Squids and octopi also can release a dark
fluid to cloud the water
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This “ink” helps to confuse their predators so they
can make a quick escape
The “ink”
is made
from
melanin