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The Phylum Mollusca
Members of this phylum include:
Clams
Snails
Octopus
Oysters
Slugs
Squid
Mussels
Nudibranchs
Chambered
nautilus
Scallops
This phylum is referred to as:
1. The “soft-bodied” invertebrates.
2. All members have a soft body.
3. Most mollusks have a hard shell that protects them.
This phylum has over 112,000 different species.
Only the phylum Arthropoda has more species.
Mollusks are “coelomates”.
Animals that are coelomates
have a true coelom.
A coelom is a hollow, fluid filled body cavity that
is completely lined with mesoderm tissue.
All organ systems are suspended within this
coelom.
The advantages of having a true
coelom are:
The muscles of the body wall are separated from
those of the gut. The body wall muscles can
contract without hindering the movement of food
through the gut.
A coelom provides a space where the circulatory
system can transport blood without interference
from the internal organs.
The gut and other internal organs are suspended
from the body wall and are cushioned by the fluid
within the coelom.
The phyla having a true coelom are:
Mollusca
The mollusks are the only
coelomates that do not have….
Annelida
Arthropoda
Echinodermata
Chordata
…..segmented bodies.
This is the larval stage
of development. It is a
common feature
shared by most aquatic
mollusks and annelids.
The trochophore larva
is a free swimming
larva. It has cilia that
are used for movement
and for drawing food
into the mouth.
Since this type of larva
is found in both
mollusks and annelids,
it suggests that these
two groups may have
evolved from a
common ancestor.
The Trochophore Larva
All Mollusks have bilateral
symmetry.
This term means “two sided” shape.
Organisms with bilateral symmetry
have a right and a left side. There is
only one way to divide the organism
and get two equal sides.
The mollusk body plan
consists of three regions:
Head:
Contains the
mouth and
sensory organs
Foot:
Large and
muscular and is
used for
movement
Visceral Mass:
Contains all of
the internal
organs
The mollusk body also has a
“mantle”.
The mantle is a layer of epidermal
tissue that covers the visceral mass.
The mantle secretes the shell.
The Mollusk Shell
The shell protects the
body of the mollusk.
This disadvantage
was overcome by the
evolution of gills.
The disadvantage of a shell
is that it reduces the
surface area available for
the exchange of gases.
Gills are specialized for the
exchange of gases.
Gills have a large surface area in contact with a rich supply of
blood vessels. Oxygen from the water diffuses into the blood
vessels of the gills. Carbon dioxide in the blood diffuses across
the surface of the gills into the external environment.
Aquatic mollusks possess gills. Land dwelling mollusks do not
have gills or lungs.
In land dwelling mollusks, oxygen diffuses across thin, moist
membranes to enter the body.
Mollusk Nervous System
1. Mollusks have ganglia, which
serve as a primitive brain.
2. The ganglia are connected to
two pairs of nerve cords.
3. The ganglia and nerve cords
control the muscles involved in
movement and feeding.
4. The nervous system allows the
mollusk to respond to simple
stimuli such as light, touch,
and chemicals in the
environment.
Radula
The radula is the main
feeding adaptation in many
mollusks.
It is used to cut food or to
scrape food from a surface.
It is a flexible, tongue-like
structure with abrasive
teeth.
Classes of the Phylum Mollusca
Class
Class
Gastropoda
Bivalvia
Class
Cephalopoda
Characteristic
Gastropoda
Bivalvia
Cephalopoda
Members
Include?
Slugs, Snails,
Nudibranchs
Clams
Oysters
Mussels
Octopus, Squid,
Cuttlefish,
Chambered Nautilus
External
Shell?
Most species have
one shell; some
species have none
Two
No shells
(exception: Nautilus)
Yes
No
Yes
Yes
No
Yes
Crawling
Sessile
Rapid
swimmers
Head?
Radula?
Locomotion?
Class Gastropoda
This is the largest and most diverse class of mollusks, consisting of
90,000 different species.
Members include the mollusks with one shell……
Snails
Conches
Abalones
…..and the mollusks with no shells.
Nudibranchs
Slugs
The Gastropods have an open circulatory system.
Instead of blood, the circulatory fluid is call hemolymph.
The hemolymph does not remain enclosed inside a
system of blood vessels.
Hemolymph is collected from the lungs or gills where it
is oxygenated, pumped through the heart, and released
into spaces within the tissues of the body.
These fluid filled spaces are known as
hemocoels, or blood cavities.
From the hemocoel, the hemolymph returns via the gills or
lungs to the heart.
Class Bivalvia
Includes: clams,
oysters, mussels,
and scallops.
They are known as bivalves
because their shell is divided into
two halves. A hinge connects the
two valves.
They are able to open and
close their shells by using
powerful adductor muscles.
External Structure of the Clam
The umbo is the
oldest part of the
clam shell.
1 - Umbo
2 - Hinge
3 – Growth
Rings
4 - Valve
5 - Foot
The valve (or shell) consists of three layers:
a)The thin outer layer protects the shell against acidic conditions in the water.
b)The thick middle layer is composed of calcium carbonate and strengthens the shell.
c)The smooth inner layer protects the soft body of the clam.
Internal Structure of the Clam
15 – Gills
16 – Mantle
17 - Valve
1 – Mouth
2 – Palps
3 – Stomach
4 – Digestive Gland
5 – Intestine
6 – Anus
7 – Umbo
8 – Hinge
9 – Anterior Adductor
Muscle
10 – Posterior Adductor
Muscle
11 – Heart
12 – Foot
13 – Dorsal Excurrent
Siphon
14 – Ventral Incurrent
Siphon
Movement of the Clam
Some species
Most bivalves
extend their
are sessile. This muscular foot
means that the into the sand.
organism is
The muscle
contracts,
attached to one
spot and not
pulling the clam
moving.
down into the
sand.
Adductor
muscles allow
the clam to
open and close
the shells.
Food Getting and Digestion
1. Bivalves are filter feeders.
They are the only mollusks without
a radula.
6. The digestive gland secretes
enzymes into the stomach and
digestion is completed.
2. Beating cilia causes water to enter
the clam through the incurrent siphon
and leave the clam through the
excurrent siphon.
7. In the intestine, the digested
food is absorbed into the
bloodstream.
3. The water is filtered for small organisms
and organic debris. The filtered material
becomes trapped on the sticky gills. Cilia
then push the food toward the mouth.
8. Wastes pass to the anus.
4. The palps help to direct the food
into the mouth.
5. Food enters the stomach
9. Waste, along with water, is
excreted through the excurrent
siphon.
Clam Nervous System
There are three pairs of ganglia:
one pair near the mouth, one
pair in the digestive system,
and one pair in the foot.
The ganglia are connected by
nerve cords.
Some bivalves have a row or
eyes along the edge of the
mantle.
Clam Reproductive System
Most clams have separate
sexes.
Sperm and eggs are released
into the water and
fertilization occurs
externally.
The fertilized egg develops
into the trochophore larva.
This larva is free swimming,
but eventually settles to the
bottom and develops into
the adult.
Clam Circulatory System
Like the gastropods,
the bivalves have an
open circulatory
system.
Octopus
Squid
Cuttlefish
Chambered Nautilus
All of these are specialized for a freeswimming, predatory lifestyle.
The Class Cephalopoda
Members of this class include:
#1 A circle of tentacles extends from the head region.
The tentacles have powerful suction cups to grasp
objects and capture prey.
#2 Cephalopods kill and eat their prey with
the help of a pair of jaws that looks like the
beak of a parrot.
#3 Many
cephalopods can
release a cloud of
ink into the water
to distract
predators.
#4 Cephalopods contains chromatophores . These
are pigment cells in the skin that change color,
allowing the cephalopod to blend in with the
surroundings.
The Cephalopod Circulatory System
Cephalopods have a closed circulatory system. The
circulation of blood is enclosed inside a system of
blood vessels.
What is the advantage
of a closed circulatory
system?
A closed circulatory system transports fluid
more quickly than an open circulatory system.
Therefore, nutrients, wastes, oxygen, and
carbon dioxide are carried more quickly to and
from the tissues of the body. This increases
the activity level of the organism.
The Cephalopod Nervous System
The cephalopod brain is the largest and most
advanced brain of any of the invertebrates.
The octopus can learn to solve simple problems, can
perform simple tasks, and can learn to differentiate
between objects.
Cephalopods have well developed eyes that
are capable of forming images of objects.
Squid are
cephalopods with
10 tentacles.
Squid
The two longest tentacles
are used for capturing
prey and the other 8 are
used to force the prey
into the mouth.
Most squid grow to about one foot in
length, but the giant squid may reach a
length of 60 feet!
Octopus
The octopus has 8
tentacles.
Most octopuses
grow to about one
foot in length. The
largest octopuses
may reach 8 feet.
Chambered
Nautilus
This is the only
cephalopod that has
retained its shell.
The shell is spiraled and is divided into a series of gas filled
chambers. The soft body of the nautilus is confined to the
outermost chamber. As it grows, it moves forward in its shell,
and fills the chamber left behind with gas. The gas makes the
nautilus buoyant.
What is the advantage of a true coelom
over a pseudocoelom?
A true coelom separates the
muscles of the body wall from
those of the internal organs. It
provides a space for the
circulatory system to transport
blood without interference.
A closed circulatory system is advantageous over an
open system. Explain why an open circulatory system is
adequate for the gastropods.
Since gastropods move so slowly, they
consume very little energy. This makes
their need for nutrients and gas exchange
low. An open circulatory system can
adequately transport the amount of
oxygen and nutrients needed for such a
low metabolism.
Many aquatic invertebrates reproduce by releasing their
sperm and egg cells in the water. What is the disadvantage of
this type of reproduction? What is the usual solution to this
problem?
Releasing sperm and eggs into the water
makes the probability of fertilization
extremely low. Invertebrates that
reproduce in this manner usually release
very large numbers of sperm and egg cells
, increasing the chance that a few of them
will be fertilized.
Land snails and earthworms are hermaphroditic. Of
what advantage is this to these land animals?
Being hermaphroditic allows these animals to
mate with any other member of their species. If
they are separate sexes they can only mate with
half of the members of their species. Since
snails and earthworms are slow moving and
rather inactive, this increases their chances of
reproducing.
What are two characteristics that mollusks
and annelids have in common?
They both have true coeloms,
and they both have the
trochophore larva.
List 6 characteristics found in the
Phylum Mollusca.
1.A Coelom
2.Bilateral Symmetry
3.Body plan of three parts: head, foot, and
visceral mass.
4.Complete organ systems
5.A shell
6.A radula
What are the major features that distinguish the
three classes of mollusks from one another?
Gastropods: Have a single shell and a single large foot.
They have eyes on the ends of stalks on their heads.
Bivalves: Have a two part, hinged shell. They lack a head
and radula. They possess siphons for filter feeding.
Cephalopods: The foot has been divided into tentacles.
There is a large head with well developed sense organs.
They are the most intelligent of all the invertebrates.
What are the potential dangers of using a filter feeding
organism such as a clam or oyster as a food source?
If there is a contaminant in the environment in
which the clam or oyster lives, that contaminant
will be filtered from the water by the bivalve. These
toxins can accumulate in the tissues of the
organism.
If the water is polluted by sewage, the
clam or oyster may harbor many bacteria
that thrive in the sewage filled waters.