27-4 Mollusks
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27-4 Mollusks
What Is a Mollusk?
What Is a Mollusk?
Phylum Mollusca: named from the Latin word
molluscus, meaning soft.
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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27-4 Mollusks
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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27-4 Mollusks
Form and Function in Mollusks
Form and Function in Mollusks
Mollusks have true coeloms surrounded by
mesoderm tissue.
They have complex, interrelated organ systems
that function together to maintain the body as a
whole.
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27-4 Mollusks
Form and Function in Mollusks
Body Plan
The body plan of most mollusks has four parts:
foot, mantle, shell, and visceral mass.
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.
The shell has been reduced or lost in slugs and some
other mollusk groups.
Just beneath the mantle is the visceral mass, which
contains the internal organs.
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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
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, tongue-shaped
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 in
herbivorous species,
and to drill through shells of
other animals to tear up and
swallow soft tissue inside.
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Form and Function in Mollusks
Octopi and certain sea slugs use sharp jaws to eat
prey. Some octopi also produce poisons to subdue
prey.
Clams, oysters, and scallops use feathery gills to
filter feed.
Food is carried by water, which enters the incurrent
siphon.
A siphon is a tube like 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.
As water passes over the gills, plankton become stuck
in sticky mucous.
Cilia on the gills move the mix of mucous and food
into the mouth.
Excurrent
siphon
Incurrent
siphon
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Form and Function in Mollusks
Respiration
Aquatic mollusks breathe using feathery gills, called
ctenidia, 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.
Land snails and slugs respire using a mantle cavity
that has a large surface area lined with blood vessels.
Because this lining must be kept moist so that oxygen
and carbon dioxide can diffuse, land snails and slugs
tend to live in moist places.
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Form and Function in Mollusks
Circulation
Some mollusks have open circulatory systems; other
mollusks have closed circulatory systems.
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.
A sinus is a large, saclike space. They do not have
closed veins or arteries.
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, like snails and clams.
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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.
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
consisting of small ganglia near the mouth, a few
nerve cords, and simple sense organs, like
chemoreceptors and eyespots.
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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, and may be
more intelligent than some vertebrates.
Some octopi are capable of complex behavior and
have been trained to perform different tasks for
reward or to avoid punishment.
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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 snails and two-shelled mollusks reproduce
sexually by external fertilization:
They release enormous amounts of eggs and sperm
into open water where they are fertilized and develop
into free-swimming larvae.
In tentacled mollusks and other snails, 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
Groups of Mollusks
The four major classes of mollusks are
• chitons
• gastropods
• bivalves
• cephalopods
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Groups of Mollusks
Class Polyplacophora: Chitons
Chiton's body consists of 8 overlapping shell plates
(valves) bound together by a leathery girdle.
Lacking eyes or tentacles, chitons can never the less
sense the light level through light-sensitive organs in
its shell.
Scrapes food from rocks with its radula.
Separate sexes. Gametes released into the water
column where fertilization takes place.
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Groups of Mollusks
Class Gastropoda: Gastropods
Includes pond snails, land slugs, sea butterflies, sea
hares, limpets, and nudibranchs.
Shell-less or single-shelled mollusks that move by
using a muscular foot located on the ventral side.
When threatened, they can pull completely into their
coiled shells.
Some snails are also protected by a hard disk on the
foot that forms a solid “door” at the mouth of their shell
when they withdraw.
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Groups of Mollusks
Land slugs and nudibranchs have no shell, but
protect themselves in other ways.
Most land slugs spend daylight hours hiding under
rocks and logs, hidden from potential predators.
Some sea hares can squirt ink into surrounding water
producing a smoke screen to confuse predators.
Some nudibranchs have body chemicals that taste
bad or are poisonous.
Other nudibranchs are able to recycle the
nematocysts from cnidarians they eat, using them to
sting predators. They are usually brightly colored as a
warning to predators.
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Groups of Mollusks
Class Bivalvia: Bivalves
Bivalves have two shells that are held together by one
or two powerful muscles.
Common bivalves include: clams, oysters, mussels,
and scallops.
Most stay in one place much of the time.
Clams burrow into mud or sand and mussels use
sticky threads to attach to rocks.
Scallops are the least sedentary and can move
around rapidly by flapping their shells when
threatened.
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Groups of Mollusks
Class Cephalopoda: Cephalopods
Most active of all mollusks, including octopi, squids,
cuttlefishes, and nautiluses.
Cephalopods are typically soft-bodied mollusks in
which the head is attached to a single foot.
The foot is divided into eight or ten tentacles or arms
equipped with sucking disks that grab and hold prey.
Nautiluses have many more tentacles than other
cephalopods (up to 90) and lack suckers. They do
have a sticky mucous covering.
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Groups of Mollusks
Most modern cephalopods have only small internal
shells or no shells at all.
The only present-day cephalopods with external
shells are nautiluses.
They can control their depth in water by regulating
the amount of gas in their shells.
Cuttlefishes have small shells inside their bodies.
A squid’s internal shell has evolved into a think
supporting rod known as a pen.
Octopi have lost their shells completely.
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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.
They can be as large as a dinner plate and
distinguish objects as small as 0.5 cm from a meter
away, allowing them to locate a wide variety of prey.
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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
• can be used to monitor water quality
• rarely develop any form of cancer
Some mollusks are hosts to symbiotic algae or to
parasites; others are themselves parasites.
Mollusks are food for many organisms.
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