Mollusks and
Annelids
Biology
Mollusks and Annelids
Two phylums in Kingdom Animalia
 They share two main characteristics

– Coelomate body plan
– Develop from trochophores
Mollusks and Annelids

These shared characteristics indicated
that both phylums may have shared
common ancestry.
Phylum Mollusca
Mollusca contains
more than 112,000
species of
organisms.
It is a very diverse
phylum.
Mollusca
Known as coelomates because they
have a fluid filled cavity surrounded by
mesoderm
 Most have a larval stage of
development called a trochophore.

– Trochophores are larva that develop from
the fertilized eggs and free swims in
water using cilia
Mollusca

They also have muscles on their body
wall that are separated from those of
the gut.
– The body wall can contract without
hindering the movement of food.
Mollusks
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
Body Plan of Mollusks

Divided into two main regions:
– Head-foot
– Visceral mass
Head-foot
Consists of the head – which contains
the mouth and a variety of sensory
structures
 Foot- a large muscular organ usually
used for locomotion

Visceral Mass

Contains the heart and the organs of
digestion, excretion, and reproduction.
Mollusks

In most mollusks, the mantle secretes
one or more hard shells containing
Calcium Carbonate.
– Shells protect mollusks from predators
– Shells reduce the area available for gas
exchange
Mollusks
Gills are a structural adaptation of
mollusks (specialized for the exchange
of gases with water)
 Gills are protected within the mantle
cavity

Mollusks
Most mollusks are bilaterally
symmetrical
 This is apparent in the nervous system
which consists of paired clusters of
nerve cells called ganglia.

Mollusks

The ganglia are in both the head-foot
and visceral mass, and are connected
by two pairs of long nerve cords.
Mollusks
The nerve cells in the ganglia control
the muscles involved in locomotion
and feeding.
 They also process sensory
information.

Mollusks

Mollusks use a radula, or long flexible
tongue like strip of tissue with tough
abrasive teeth that point backward.
– Terrestrial snails use the radula to cut
through leaves
– Aquatic snails use the radula to scrape up
algae or even drill through the shells of
other mollusks
Mollusks
There are eight classes of Mollusks
 Three large classes of Mollusks are:

– Gastropoda
– Bivalvia
– Cephalopoda
Gastropoda
90,000 of the 112,000 species of
mollusks come from this class.
 Examples: Snails, slugs, conchs,
abalones, and nudibranchs

– Most have one external shell
– Head
– Radula
– Crawling for locomotion
Class Gastropoda
Bivalvia


Called bivalves because their shell is
divided into two halves connected by a
hinge.
Examples: clams, oysters, and scallops
–
–
–
–
Two external shells
No head
No radula
Sessile
Bivalvia


Oysters are a
member of the
Class Bivalvia.
They have two
shells attached by a
hinge.
Class Bivalvia


Scallops are a
member of the
Class Bivalvia.
They are often
eaten in seafood
restaurants.
Cephalopoda
Examples: Octopuses, squids,
cuttlefishes, and chambered nautilus
 Cephalopods (means head-foot)

– Most have no shell
– Head
– Radula
– Rapid Swimming
Cephalopods



Can release a
cloud of ink into the
water
Contain pigment
cells called
chromatophores
Chromatophores
are pigment cells
that allow for rapid
color change
Chambered Nautilus

The only
cephalopod with an
external shell.
Phylum Annelida
Annelid means little
rings.
This refers to the
many segments
that make up an
annelid.
Annelids
Most annelids have external bristles
called setae.
 Some have fleshy protrusions called
parapodia.
 The number of setae and the presence
or absence of parapodia provide the
basis of dividing annelids into three
classes.

Annelids

Three types of Annelids are:
– Oligochaeta
– Polychaeta
– Hirudinea
Oligochaeta
Oligochaeta (few bristles)
 Annelids of this class generally live in
the soil or fresh water and have no
parapodia.
 Example: Earthworm

Earth Worm
Class Polychaeta


Polychaeta (many
bristles)
Two thirds of all
annelids are members
of this class.
– Numerous setae
– Differ from other
annelids in that they
have antennae and
specialized mouthparts
– The only annelids that
have a trochophore
stage
– Most live in marine
habitats
Class Hirudinea


Smallest class of Annelids
Consists of about 300 species of leaches
– No setae
– No parapodia
– Many leeches are carnivores feeding on small
invertebrates
– Some species are parasites creating an
anasthetic so that hosts can’t feel their presence
– If undisturbed, a leech can ingest 10 times its
own weight in blood
Create a Brochure


Research a type of mollusk
Create a brochure with information on your
mollusk
–
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Where does it live?
What does it eat?
What is its class?
What is the population of the species?
Must have a picture of the species
Information about the body plan of your mollusk