The Chitons, Gastropods, Bivalves and
Cephalopods
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The molluscs are a diverse group of soft
bodied invertebrates
Some varieties of molluscs produce an
external shell (“test” or “valve”)made of
calcium carbonate
All molluscs have a fold of skin associated
with the dorsal body wall called the Mantle
which encloses the Mantle Cavity. The mantle
cavity houses the gills, if present.
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Triploblastic (well developed mesoderm)
Organ System level of body organization
True body cavity (eucoelomate) – the coelom
is completely bound by mesodermal tissue
and lined with peritoneum.
Complete gastrulation – one way digestive
tract with a mouth and an anus
Protostomes – the first opening of the
gastrula forms the mouth
Bilateral symmetry
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Molluscs have a unique body plan with 2
general body regions – the Head-Foot and
the Visceral Mass
The Head-Foot, as the name implies, contains
the primary sense organs characteristic of
cephalization, the mouth and feeding
apparatus, and locomotor organs
The Visceral Mass contains the “guts” – the
organs associated with digestion, respiration,
reproduction, and circulation
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Varying degrees
of cephalization
occur in the
molluscs. Some
(particularly the
cephalopods,
which include
squid and
octopus) have
very well
developed eyes.
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Many molluscs
(notably the
gastropods and
cephalopods)
have tentacles
associated with
the Head-Foot
region
The tentacles
are used for
“touch” and
“taste”
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https://www.youtube.com/watch?feature=pla
yer_detailpage&v=yE0QqxwyL_8
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Most molluscs (all
gastropods,
chitons and
cephalopods) have
a specialized
rasplike feeding
organ called a
Radula. The radula
is a muscular
tongue covered
with teeth for
scraping off food
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Video of a snail eating
Note the antennae, eyes, and mouth
Note the texture and movement of the radula
http://www.youtube.com/watch?v=cEMK3VN
8pW8
http://www.youtube.com/watch?v=mLVDwlrS
q5U
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The foot portion of the head-foot region is
used for locomotion and varies greatly within
the phylum
Gastropods (“stomach foot”) and Chitons have
a broad, laterally flattened ventral foot
Pelecypods (“hatchet foot”) have a muscular
foot elongated along the dorso-ventral
midline
Cephalopods (“head foot”) have modified the
foot to form a siphon to eject water for
propulsion
Snails
 https://www.youtube.com/watch?v=pRmChG
3Jf_Q
 https://www.youtube.com/watch?v=utxH9M
H4tOI
Sea Slugs
 https://www.youtube.com/watch?NR=1&v=E
wpexfYnlDE
 https://www.youtube.com/watch?v=9VL_p2A
7DPg
Octopus
 https://www.youtube.com/watch?v=rOvhs5icTfg
 https://www.youtube.com/watch?v=tLTWFnGmeg&feature=player_detailpage
Squid
 https://www.youtube.com/watch?v=Nf2iRRWH5O
I
 https://www.youtube.com/watch?v=EeaMOOnm
HAA
 https://www.youtube.com/watch?v=5oOMa53F1s
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The Visceral Mass contains what we normally
think of as the “internal organs”
Aquatic forms have Gills for respiration
Gills are enclosed in the Mantle Cavity, and
covered by the Mantle. All molluscs have a
mantle and mantle cavity
Some terrestrial forms have diminished gills,
and perform gas exchange across the moist
membranes of the mantle cavity
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Many molluscs secrete protein and calcium
carbonate from the mantle to create a shell
The shell typically has 3 layers:
◦ The periostacum is the outer layer, made of protein
secreted by the edge of the mantle
◦ The prismatic layer is the middle layer, made of
densely packed prisms of calcium carbonate in a
protein matrix
◦ The nacreous layer is secreted continuously by the
surface of the mantle. This layer is what we call
Mother of Pearl
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The mechanism of blood circulation varies
Bivalves have an Open Circulatory System.
Blood pumped by the heart circulates through
blood vessels in the gills, but empties into
the coelom where the visceral organs are
bathed by blood
Cephalopods have a Closed Circulatory
System. Blood is circulated through blood
vessels, including capillaries, into all body
regions
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Metabolic wastes and excess water are
removed by the kidneys (metanephridia)
The kidneys open on the inside to the
coelom, communicating with the blood, and
discharge wastes into the mantle cavity
The ducts of the excretory system are shared
by the Reproductive system, providing
passageways for sperm and egg to the mantle
cavity
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The larval form of a mollusc is called a
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See diagrams and descriptions p. 330 in your
textbook
Trochophore
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Mollusca is one of the most diverse and
widespread phyla in the animal kingdom,
second only to the arthropods
We will focus on only a few of the major
classes of molluscs
◦ Polyplacophora (Chitons)
◦ Gastropods (snails and slugs)
◦ Pelecypods (Bivalves – Clams, Oysters, Mussels &
Scallops)
◦ Cephalopods (Squid, Nautilus and Octopus)
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Chitons are similar to gastropods in general
body form, but lack the degree of
cephalization that is characteristic of slugs
and snails
Chitons are flattened in shape, with multiple
prominent gills between the mantle and the
foot
As the name polyplacophora suggests, the
shell consists of many plates
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Foot is flattened and tight to the belly (thus
“gastro” “pod” – belly foot)
Definite cephalization with both eyes and
tentacles (for tactile sense and
chemoreception)
Slugs – no shell. Snails – single shell
Asymmetrical deposition of calcium
carbonate by the mantle causes the shell of
snails to coil (see page 335 in text)
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See page 334 in the textbook
During embryonic development, the mantle
cavity of gastropods undergoes torsion – the
mantle twists so that the posterior of the mantle
is relocated over the anterior of the head-foot
Torsion allows the snail to more readily retract
the head into the shell for protection
Torsion also positions the excretory openings
over the gills. This problem is resolved by
bringing water into the mantle cavity from the
left and expelling waste water from the right
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Due to shell coiling, most snails have the
right gill diminished or absent. The whorls of
the coiled shell compress the right side of the
mantle cavity and visceral mass
Terrestrial forms of both snails and slugs are
lacking gills entirely, using the moist skin of
the mantle cavity for gas exchange. These
forms are called Pulmonates
Nudibranchs are slugs with exposed gills
(nudi = naked, branch = gill)
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Pelecy = hatchet. The pelecypods are
molluscs with a hatched shaped foot
Pelecypods are bivalves (2 shells). The shell
has 2 distinct parts, attached by a hinge
ligament
Clams, oysters, and mussels are all
pelecypods
All are filter feeders and lack a radula
All have little or no cephalization (although
some have eyes at the rim of the mantle)
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Many bivalves use their hatchet shaped foot
to burrow into the soil
The mantle is organized to form an incurrent
siphon and an excurrent siphon – which may
extend significantly out of the shell
Water circulates through the mantle cavity,
and ciliary action of the gill surface channels
particles of food trapped in the gill mucus to
the mouth
See textbook pages 340-342
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Bivalve shells have a distinct Umbo, the oldest
part – representing the position of the
embryonic shell
The two valves of the shell are connected by a
protein hinge ligament. The elasticity of the
hinge ligament causes the shell to open
A pair of adductor muscles (one anterior and
one posterior) contract to close the shell
Sand particles trapped between the shell and
the mantle may be covered with secretions of
calcium carbonate, forming a pearl
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True Facts about the Land Snail
◦ https://www.youtube.com/watch?v=VTV23B5gBsQ
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True Facts about the Cuttlefish
◦ https://www.youtube.com/watch?v=GDwOi7HpHtQ
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True Facts about the Octopus
◦ https://www.youtube.com/watch?v=st8-EY71K84
https://www.
fossilera.com
/fossils-forsale/ammoni
te-pendants