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Biology
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27-4 Mollusks
Photo Credit: © Jeffrey L. Rotman
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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.
Mollusks are an incredibly diverse group, including clams, octopi, and snails, as well as the lesser known cuttlefish, shown here. Many mollusks have a larval stage called a trochophore), shown here, which has at least one band of cilia encircling its body. All mollusks have a soft body, and most mollusks have an internal or external shell.
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6. 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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7. Form and Function in Mollusks
What is the basic body plan of mollusks?
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8. 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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9. Form and Function in Mollusks
The muscular foot takes many forms
flat structures for crawling
spade-shaped structures for burrowing
tentacles for capturing prey
The body plan of most mollusks includes a foot, mantle, shell, and visceral mass. Early mollusks may have looked like the animal shown here. As they evolved, their body parts became adapted for different functions.
Squid
Snail
Clam
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10. 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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11. Form and Function in Mollusks
Feeding 
Mollusks can be herbivores, carnivores, filter feeders, detritivores, or parasites.
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12. 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.
Snails use a radula for feeding. The teeth of a radula give it the look and feel of sandpaper. Beneath the radula is a stiff supporting rod of cartilage. When the mollusk feeds, it places the tip of the radula on its food and pulls the sandpapery layer back and forth.
Teeth
Radula
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13. 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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14. Form and Function in Mollusks
The water flows over the gills and leaves by the excurrent siphon.
Excurrent siphon
The anatomy of a clam is typical of bivalves, or two-shelled mollusks. The mantle and part of the foot have been cut away to show internal organs. The adductor muscles are used to open and shut the two exterior shells. The gills exchange oxygen and carbon dioxide between the body and the surrounding water. The arrows show the path of water over the gills.
Incurrent siphon
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15. Form and Function in Mollusks
Respiration 
Aquatic mollusks breathe using gills inside their mantle cavity.
Gills
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16. 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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17. 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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18. Form and Function in Mollusks
Circulation 
Some mollusks have open circulatory systems; other mollusks have closed circulatory systems.
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19. Form and Function in Mollusks
In an open circulatory system, blood is pumped through vessels by a simple heart.
Heart
The anatomy of a clam is typical of bivalves, or two-shelled mollusks.
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20. 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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21. 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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22. 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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23. Form and Function in Mollusks
Nephridia remove ammonia from the blood and release it outside the body.
Nephridium
The anatomy of a clam is typical of bivalves, or two-shelled mollusks.
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24. 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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25. 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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26. 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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27. 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 single- shelled 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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27-4
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27-4
The trochophore larva is a characteristic that mollusks share with
flatworms.
roundworms.
annelids.
flukes.
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27-4
Water enters and leaves the body of a bivalve through
a siphon.
cilia.
a coelom.
a nephridium.
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27-4
The most active group of mollusks is the
gastropods.
bivalves.
cephalopods.
shell-less gastropods.
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27-4
Unlike the other mollusks, cephalopods have a(an)
closed circulatory system.
ventral blood vessel.
open circulatory system.
dorsal blood vessel.
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27-4
The body plan of most mollusks includes all of the following EXCEPT a
foot.
mantle.
shell.
radula.
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44. END OF SECTION