Mollusks - Banning High School

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Biology
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27-4 Mollusks
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27-4 Mollusks
What Is a Mollusk?
What are the defining features of
mollusks?
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What Is a Mollusk?
What Is a Mollusk?
Mollusks are soft-bodied animals that
usually have an internal or external shell.
Mollusks include snails, slugs, clams, squids,
and octopi.
Many mollusks share similar developmental
stages.
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What Is a Mollusk?
Many aquatic mollusks
have a free-swimming
larval stage called a
trochophore.
The trochophore larva is
also characteristic of
annelids, indicating that
these two groups may
be closely related.
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Form and Function in Mollusks
Form and Function in Mollusks
Mollusks have complex, interrelated organ
systems that function together to maintain the body
as a whole.
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Form and Function in Mollusks
What is the basic body plan of mollusks?
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Form and Function in Mollusks
Body Plan
The body plan of most mollusks has four
parts: foot, mantle, shell, and visceral
mass.
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Form and Function in Mollusks
The muscular foot takes many forms
• flat structures for crawling
• spade-shaped structures for
burrowing
• tentacles for capturing prey
Squid
Snail
Clam
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Form and Function in Mollusks
The mantle is a thin layer of tissue that covers most
of the mollusk's body.
The shell is made by glands in the mantle that
secrete calcium carbonate.
Just beneath the mantle is the visceral mass, which
contains the internal organs.
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Form and Function in Mollusks
Feeding
Mollusks can be herbivores, carnivores, filter
feeders, detritivores, or parasites.
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Form and Function in Mollusks
Snails and slugs feed
using a flexible, tongueshaped structure known
as a radula.
Hundreds of tiny teeth are
attached to the radula.
The radula is used to
scrape algae off rocks or
to eat the soft tissues of
plants.
Teeth
Radula
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Form and Function in Mollusks
Clams, oysters, and scallops use gills.
Food is carried by water, which enters the incurrent
siphon.
A siphon is a tubelike structure through which water
enters and leaves the body.
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Form and Function in Mollusks
The water flows over the gills and leaves by the
excurrent siphon.
Excurrent
siphon
Incurrent
siphon
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Form and Function in Mollusks
Respiration
Aquatic mollusks breathe using
gills inside their mantle cavity.
Gills
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Form and Function in Mollusks
As water passes through the mantle cavity,
oxygen in the water moves into blood flowing
through the gills.
At the same time, carbon dioxide moves in the
opposite direction.
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Form and Function in Mollusks
Land snails and slugs respire using a mantle cavity
that has a large surface area lined with blood
vessels.
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Form and Function in Mollusks
Circulation
Some mollusks have open circulatory systems;
other mollusks have closed circulatory systems.
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Form and Function in Mollusks
In an open circulatory system, blood is pumped
through vessels by a simple heart.
Heart
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Form and Function in Mollusks
Blood leaves the vessels and works its way through
different sinuses.
Blood passes from the sinuses to the gills, where
oxygen and carbon dioxide are exchanged. Blood is
then pumped back to the heart.
Slow-moving mollusks often have open circulatory
systems.
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Form and Function in Mollusks
Faster-moving mollusks have a closed circulatory
system.
A closed circulatory system can transport blood
through an animal’s body much more quickly than an
open circulatory system.
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Form and Function in Mollusks
Excretion
Cells of the body release nitrogen-containing
waste into the blood in the form of ammonia.
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Form and Function in Mollusks
Nephridia remove ammonia from the blood and
release it outside the body.
Nephridium
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Form and Function in Mollusks
Response
The complexity of the nervous system and the
ability to respond to environmental conditions
varies among mollusks.
Two-shelled mollusks have a simple nervous
system.
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Form and Function in Mollusks
Octopi and their relatives have the most highly
developed nervous system of all invertebrates.
Well-developed brains in these animals allows them
to remember things for long periods.
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Form and Function in Mollusks
Movement
Snails secrete mucus along the base of the foot,
and then move over surfaces using a rippling
motion of the foot.
The octopus draws water into the mantle cavity
and then forces the water out through a siphon.
Water leaving the body propels the octopus in the
opposite direction.
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Form and Function in Mollusks
Reproduction
Some mollusks reproduce sexually by external
fertilization.
In other mollusks, fertilization takes place inside
the body of the female.
Some mollusks are hermaphrodites and usually
fertilize eggs from another individual.
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Groups of Mollusks
What are the characteristics of the three
main classes of mollusks?
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Groups of Mollusks
Groups of Mollusks
The three major classes of mollusks are
• gastropods
• bivalves
• cephalopods
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Groups of Mollusks
Gastropods
Gastropods are shell-less or singleshelled mollusks that move by using a
muscular foot located on the ventral
side.
Many gastropods have a single shell that
protects their bodies.
When threatened, they can pull completely
into their shells.
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Groups of Mollusks
Gastropods include
•
•
•
•
•
•
pond snails
land slugs
sea butterflies
sea hares
limpets
nudibranchs
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Groups of Mollusks
Bivalves
Bivalves have two shells that are held
together by one or two powerful muscles.
Common bivalves include:
• clams
• oysters
• mussels
• scallops
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Groups of Mollusks
Cephalopods
Cephalopods are typically soft-bodied
mollusks in which the head is attached to a
single foot. The foot is divided into
tentacles or arms.
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Groups of Mollusks
Cephalopods have eight or more tentacles equipped
with sucking disks that grab and hold prey.
Most modern cephalopods have only small internal
shells or no shells at all.
The only present-day cephalopods with external
shells are nautiluses.
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Groups of Mollusks
Cephalopods have complex sense organs that help
them detect and respond to external stimuli.
Cephalopods distinguish shapes by sight and texture
by touch.
The eyes of many cephalopods are complex.
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Ecology of Mollusks
Ecology of Mollusks
Mollusks
• feed on plants
• prey on animals
• filter algae out of the water
• eat detritus
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Ecology of Mollusks
Some mollusks are hosts to symbiotic algae or to
parasites; others are themselves parasites.
Mollusks are food for many organisms.
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The trochophore larva is a characteristic that
mollusks share with
a. flatworms.
b. roundworms.
c. annelids.
d. flukes.
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Water enters and leaves the body of a bivalve
through
a. a siphon.
b. cilia.
c. a coelom.
d. a nephridium.
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The most active group of mollusks is the
a. gastropods.
b. bivalves.
c. cephalopods.
d. shell-less gastropods.
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Unlike the other mollusks, cephalopods have
a(an)
a. closed circulatory system.
b. ventral blood vessel.
c. open circulatory system.
d. dorsal blood vessel.
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The body plan of most mollusks includes all of
the following EXCEPT a
a. foot.
b. mantle.
c. shell.
d. radula.
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