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Molluscs
Snails, Slugs, Clams, Oysters,
Octopi, Squids, Cuttlefish, etc.
Mollusc Characteristics
• 1. Coelomates
• 2. Bilateral symmetry.
• 3. First appearance of a one way digestive tract,
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with both mouth and anus.
4. Have a visceral mass, which houses the organs of
digestion, circulation, reproduction, and excretion.
5. Have a mantle which secretes the shell.
6. Have a mantle cavity which is open to the outside
for gas exchange and elimination of waste.
7. Most also have a beak which is similar to a bird’s
beak, and a tongue-like radula used to scrape food.
Trochophore Larva
• Remember that a
larva is an immature
form that does not
resemble the adult,
but that eventually
develops into an
adult.
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Class Gastropoda – Snails & Slugs
• The soft body is protected by a
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hard shell, which the snail
retreats into when alarmed.
Found worldwide in the seas, in
fresh water, and in moist areas
on land.
Locomotion: Snails move by
crawling, swimming, or floating
with currents. A special gland in
the foot secretes mucus that
helps the snail move.
Snails have two pairs of
tentacles on the head. Land
snails have a light-sensitive
eyespot located on each of the
larger tentacles. The smaller
pair of tentacles is used for the
sense of smell and the sense of
touch.
Diet: Most snails eat living and
decaying plants. They eat using
a radula, a rough tongue-like
organ that has thousands of tiny
denticles (tooth-like
protrusions).
Snail Biology
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Radular Structure
Source: From A Life of Invertebrates, Copyright © 1979 W. D. Russell-Hunter.
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Class Bivalvia
Class Bivalvia: Clams, Oysters,
Scallops, & Mussels
• Bivalves (pelecypoda) are
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soft-bodied animals that
are protected by two hard
shells, hinged together.
Oyster – stationary
Scallop – moves by “jet
propulsion”
Clams – burrowing foot
Open circulatory system –
no veins or arteries, just
free flowing blood
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Clam anatomy
Pericardial cavity
Metanephridium
Heart
Intestine
Mouth
Excurrent
Stomach
Incurrent
Intestine
Gonad
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Pearl formation
Shell
Developing pearl
Epithelium
Irritant lodged between shell and mantle
Layers of nacre secreted around foreign material
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Class Cephalopoda: Octopi, Squid,
& Cuttlefish
• Cephalopod means "head-foot." The foot of the
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cephalopod is a cluster of tentacles that connects
directly to the head.
The brain, the eyes, and the other sensory organs are
well-developed, more than any other mollusc.
Cephalopods breathe using gills.
Closed circulatory system
They swim by jet-propulsion; they rapidly expel water
from the mantle cavity through a specialized, tube-like
organ called the hyponome. Many can also eject a cloud
of ink to confuse enemies.
Many can change color for camouflage or to attract
mates.
Squid
Tentacle
Arm
Funnel (siphon)
Collar
Eye
Fin
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Squid
Shell (Pen)
Systemic
heart
Brachial heart
Ctenidium (gills)
Funnel
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Squid Male
Testis
Penis
Hectocotylous
arm – transfers
sperm
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Squid Female
Ovary with eggs
Oviducal gland
Nidamental
glands –
(of or
pertaining
to eggs or
a nest)
Oviduct opening
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cephalopod Eye
(The most well-developed
eye of all the invertebrates!
They can see just as well as
we can, and in perfect
color!)
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