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CHAPTER 16
Molluscs
16-1
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16-2
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Characteristics
Phylum Mollusca
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16-3
Over 90,000 living species and 70,000 fossil
species
Soft body and belong to the lophotrochozoa
protostomes
Include chitons, tusk shells, snails, slugs,
nudibranchs, sea butterflies, clams, mussels,
oysters, squids, octopuses, and nautiluses
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16-4
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Characteristics
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16-5
May weigh up to 900 kg and grow to nearly
20 m long, but 80% are under 10 cm in size
Herbivorous grazers, predaceous carnivores,
filter feeders, and parasites
Most are marine, but some are terrestrial or
freshwater aquatic
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16-6
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Characteristics
Evolution
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Fossil evidence
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Some bivalves and gastropods
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Evolved to become relatively intelligent
Coelom limited to a chamber around the heart
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16-7
Limited to moist, sheltered habitats with calcium in
the soil
Cephalopods
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Moved to brackish and freshwater
Snails (gastropods) successfully invaded land
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Indicates molluscs evolved in the sea
Most have remained marine
Some believe molluscs arose separately from
annelids and coeloms are not homologous
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Characteristics
Economics
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16-8
Many are used as food
Culturing of pearls and pearl buttons is an
important industry
Burrowing shipworms destroy wooden ships
and wharfs
Snails and slugs are garden pests
Some snails are intermediate hosts for
parasites
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Form and Function
Mollusc Body Plan: Head-Foot and
Visceral Mass Portions
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Head-foot region contains feeding, cephalic
sensory, and locomotor organs
Visceral mass contains digestive, circulatory,
respiratory, and reproductive organs
Mantle Cavity
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16-9
Two folds of skin form protective mantle or pallium
Space between mantle and body wall is the mantle
cavity
Mantle cavity houses the gills (ctenidia) or a lung
In most molluscs
 Mantle secretes a shell over the visceral mass
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16-10
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Form and Function
Head-Foot
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Most have well-developed head bearing
mouth and some sensory organs
Photosensory receptors range from simple
to complex eyes
Tentacles may be present
Posterior to mouth is the chief locomotor
organ, the foot
16-11
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Form and Function
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Radula
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16-12
Unique to molluscs
Found in all except bivalves and some solenogasters
Protruding, rasping, tongue-like organ
Ribbon-like membrane has rows of tiny teeth (up to
250,000) pointed backward
Radula rasps off particles of food from surfaces
Serves as a conveyor belt to move particles to
digestive tract
New rows of teeth replace those that wear away
Pattern and number of teeth are used in
classification of molluscs
 Some specialized to bore through hard material or
harpoon prey
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16-13
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Form and Function
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Foot
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Usually ventral
Functions in attachment to substratum or for
locomotion
Modifications include
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16-14
Attachment disc of limpets
Hatchet foot of clams
Siphon jet of squids
Secreted mucus aids in adhesion or helps
molluscs glide on cilia
Snails and bivalves extend the foot hydraulically
by engorgement with blood
Burrowers extend the foot into mud or sand,
enlarge the tip as an anchor, and draw forward
Free-swimming forms have modified the foot into
wing or fin-like swimming agents
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Form and Function
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Mantle and Mantle Cavity
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16-15
Mantle is a sheath tissue on each side of the body
 Secretes the shell when present
Mantle cavity
 Houses the gills or lungs that develop from the
mantle
Exposed surface of the mantle also functions in
gaseous exchange
In aquatic molluscs
 Continuous flow of water brings in oxygen and
food, and flushes out wastes
Products of digestive, excretory and reproductive
systems empty into the mantle cavity
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Form and Function
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16-16
Cephalopods
 Use head and mantle cavity to create jet
propulsion
Mollusc gill has leaf-like filaments
 Cilia propel water across the surface
Countercurrent blood movement in gill absorbs
oxygen efficiently
In most molluscs, two ctenidia on opposite sides
 Form an incurrent and an excurrent chamber
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16-17
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Form and Function
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Shell
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If present, secreted by the mantle and lined by it
Periostracum
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Middle prismatic layer
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16-18
Closely packed prisms of calcium carbonate
Inner nacreous layer
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Outer horny layer
Composed of conchiolin, a tanned protein
Next to the mantle; the nacre is laid down in thin layers
Thick periostracum of freshwater molluscs
protects against acid from leaf decay in streams
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16-19
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Form and Function
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Internal Structure and Function
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Open circulatory system
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16-20
Pumping heart, blood vessels, and blood sinuses
Most cephalopods have a closed system with a
heart, vessels, and capillaries
Most molluscs have a pair of kidneys or
metanephridia
Kidney ducts also discharge sperm and eggs
Nervous system
 Pairs of ganglia but generally simpler than in
annelids
In air-breathing snails, nervous system produces
growth hormones
Sense organs vary and may be highly specialized
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Form and Function
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Reproduction and Life History
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16-21
Most dioecious, some hermaphroditic
Egg hatches and produces a free-swimming
trochophore larva
Larva undergoes direct metamorphosis into a
small juvenile in chitons
In many gastropods and bivalves
 Intermediate larval stage, the veliger, is a
derived state
Trochophore larvae are considered by some to
unite molluscs with annelids, marine turbellarians,
nemertines, phoronids, etc. in a taxon called
Trochozoa within superphylum photrochozoa
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16-22
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16-23
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Classes of Molluscs
Class Caudofoveata
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Wormlike, marine organisms ranging from two
to 140 mm in length
Most burrow
 Terminal mantle cavity and gills are near
entrance
Feed primarily on microorganisms and detritus
Have no shell but body is covered with
calcareous scales
Radula present but may be reduced
Fewer than 120 species
16-24
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Classes of Molluscs
Class Solenogasters
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Formerly grouped with caudofoveates in
Aplacophora
Resemble caudofoveates but have no radula
or gills
Foot has a midventral, narrow furrow called
the pedal groove
Do not burrow but are bottom dwellers and
feed on cnidarians
Approximately 250 species
Class is sometimes called Neomeniomorpha
16-25
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Classes of Molluscs
Class Monoplacophora
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Previously considered extinct
Living specimen was discovered in 1952
About 25 extant species now known
Small molluscs with a rounded shell,
resemble limpets
Some organs are repeated: 3–6 pairs of gills,
two pairs of auricles, 3–7 pairs of
metanephridia, one or two pairs of gonads
and 10 pairs of pedal nerves
Radula characteristic of many other molluscs
16-26
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16-27
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Classes of Molluscs
Class Polyplacophora: Chitons
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Chitons are somewhat flattened with 7or 8
dorsal plates
About 1000 currently described species
Head and cephalic organs are reduced
Photosensitive structures (esthetes) similar
to eyes pierce the plates
Most prefer rocky intertidal surfaces
Chiton radula is reinforced with iron mineral
 Scrapes algae from the rocks
Mantle extends around margin
16-28
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16-29
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16-30
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Classes of Molluscs
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Gills suspended from roof of mantle cavity
and grooves
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Pair of osphradia serves as sense organ
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Sample water in mantle groove near anus
Blood pumped by a three-chambered heart
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Form a closed chamber
Water flows from anterior to posterior
Travels through aorta and sinuses to gills
Pair of metanephridia
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Carries wastes from pericardial cavity to exterior
Sexes are separate
 Trochophore larvae metamorphose into
juveniles without a veliger stage
16-31
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Classes of Molluscs
Class Scaphopoda
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Tusk or tooth shells
Live on the ocean bottom from subtidal zone
to 6000 m depth
About 900 living species
Slender body covered with a mantle
Tubular shell is open at both ends
Unique body plan
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16-32
Mantle is wrapped around the viscera and fused to
form a tube
Foot protrudes from larger end to burrow into mud
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Classes of Molluscs
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Foot and ciliary action moves water through
mantle cavity
Gills are absent and gaseous exchange
occurs via the mantle
Detritus and protozoa are caught on cilia on
foot or mucus-covered knobs of tentacles
Radula carries food to a crushing gizzard
Head or captacula lacks eyes, tentacles or
osphradia
16-33
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16-34
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Classes of Molluscs
Class Gastropoda
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Most diverse class
Over 70,000 living and more than 15,000 fossil
species
Snails, limpets, slugs, whelks, conches,
periwinkles, sea slugs, sea hares, and sea
butterflies
Forms range from marine forms to airbreathing terrestrial snails and slugs
Typically sluggish, sedentary animals
Shells are chief defense
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16-35
Some produce distasteful or toxic secretions
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Classes of Molluscs
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Gastropod Shells
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One-piece univalve, coiled or uncoiled
Apex is smallest and oldest whorl
Whorls become larger and spiral around central
axis or columella
Many snails have an operculum covering shell
aperture
Giant marine gastropods have shell up to 60 cm
long
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16-36
Some fossil forms are 2 meters long
Terrestrial gastropods are restricted by soil
mineral content, temperature, dryness, and acidity
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16-37
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Classes of Molluscs
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16-38
Snails serve as intermediate hosts to many
parasites and are often harmed by larval stages
3 gastropod subclasses
 Prosobranchia
 Opisthobranchia
 Pulmonata
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Classes of Molluscs
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Form and Function
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16-39
Torsion
 Developmental process that changes the
relative position of the shell, digestive tract and
anus, nerves that lie on both sides of the
digestive tract, and the mantle cavity
containing the gills
Contraction of a foot retractor muscle pulls shell
and viscera 90 counterclockwise
Moves anus to the right side of the body
Recent studies have shown that shell movement
is independent of visceral movement
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Classes of Molluscs
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16-40
1st movements of shell rotate it between 90 and
180 degrees into a permanent position
Mantle cavity develops on the right side of the
body near the anus, but is initially separate from it
Anus and mantle cavity usually move further to
the right and the mantle cavity is remodeled to
encompass the anus
Digestive tract moves both laterally and dorsally
so that anus lies above head within mantle cavity
After torsion, anus and mantle cavity open above
mouth and head
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16-41
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Classes of Molluscs
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16-42
Certain viscera on the left are now on the right
side and vice versa
Nerve cords form a figure eight
Varying degrees of detorsion in opistobranchs
and pulmonates have been observed
This arrangement resulting from torsion creates
fouling
 Wastes being washed back over the gills
Developmental sequence is called ontogenetic
torsion
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Classes of Molluscs
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Coiling
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16-43
Coiling or spiral winding of the shell and visceral
mass not the same as torsion
Occurs at same larval stage as torsion but had a
separate, earlier evolutionary origin
All living gastropods descended from coiled,
torted ancestors
Planospiral shell has all whorls in a single plane
 Primitive state
Conispiral shape provides more compactness
 Each whorl is to the side of the previous one
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16-44
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Classes of Molluscs
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16-45
Shifting the shell upward and back helped
balance uneven weight distribution
Gill, auricle and kidney of right side are lost in
most species
Loss of the right gill provides one solution to the
problem of fouling
 Wastes expel to the right
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Classes of Molluscs
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Feeding Habits
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16-46
Adaptation of the radula provides much variation
in gastropod feeding habits
Many are herbivorous and graze, browse or feed
on plankton
Some scavenge decaying flesh
Others carnivores that tear prey using radula
Oyster borer alternates rasping with chemical
softening of the shell to bore a hole
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16-47
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16-48
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Classes of Molluscs
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Species of Conus deliver a lethal sting to secure
prey
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16-49
Venom is a conotoxin
Specific for the neuroreceptors of its preferred prey
Some collect debris as a mucus ball to ingest it
Sea butterflies secrete a mucus net
Digestion usually extracellular in lumen of
stomach
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Classes of Molluscs
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Internal Form and Function
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16-50
Respiration in many performed by ctenidia in
mantle cavity
Derived prosobranchs lost one gill and half of
remaining gill
Resulting attachment to wall of mantle cavity
provided respiratory efficiency
Pulmonates lack gills
Have a highly vascular area in mantle that serves
as lung
Lung opens to outside by small opening, the
pneumostome
Aquatic pulmonates surface to expel a gas bubble
and inhale by curling, thus forming a siphon
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16-51
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16-52
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Classes of Molluscs
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16-53
Most have a single nephridium and welldeveloped circulatory and nervous systems
Sense organs include eyes, statocysts, tactile
organs, and chemoreceptors
Eyes vary from simple cups holding
photoreceptors to a complex eye with a lens and
cornea.
Sensory osphradium at base of the incurrent
siphon may be chemosensory or
mechanoreceptive
Monoecious and dioecious species
Copulation in monoecious species may involve
exchange of spermatozoa or spermatophores
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Classes of Molluscs
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16-54
Many terrestrial species inject a dart to heighten
arousal before copulation
Primitive gastropods discharge ova and sperm
into water and fertilization is external
Eggs emitted singly or in clusters, and may be
transparent or in tough egg capsules
Young may emerge as veliger larvae or pass this
stage inside the egg
Some species, including most freshwater snails,
are ovoviviparous
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16-55
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Classes of Molluscs
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Major Groups of Gastropods
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Traditional classification has recognized
three subclasses of Gastropoda
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Prosobranchia, Opisthobranchia, and
Pulmonata
Recent evidence suggests the
Prosobranchia is paraphyletic
 Opisthobranchia may or may not be
paraphyletic
 Opisthobranchia and Pulmonata together
form a monophyletic grouping
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16-56
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Classes of Molluscs
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Prosobranchs
Includes most marine snails and some
freshwater and terrestrial gastropods
 Mantle cavity is anterior due to torsion
 Gills are in front of heart
 Water enters the left side and exits from the
right side
 Long siphons may separate incurrent and
excurrent flow
 Have one pair of tentacles, separate sexes, and
usually an operculum
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16-57
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16-58
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Classes of Molluscs
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Opisthobranchs
Includes sea slugs, sea hares, sea butterflies, and
canoe shells
 Most are marine, shallow-water and often hide
under stones and seaweed
 Partial to complete detorsion
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Anus and gill(s) are displaced to right side
Two pairs of tentacles, one pair modified to
increase chemo-absorption
 Shell is reduced or absent
 Monoecious
 Sea hare Aplysia
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16-59
Large anterior tentacles and a vestigial shell
Foot of pteropods
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Modified into fins for swimming
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16-60
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16-61
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Classes of Molluscs
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Pulmonates
Includes all land and most freshwater snails
and slugs
 Ancestral ctenidia have been lost and the
vascularized mantle wall is now a lung
 Air fills lung by contraction of mantle floor
 Anus and nephridiopore open near the
pneumostome
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Waste is forcibly expelled
Monoecious
 Aquatic species have one pair of tentacles
 Landforms have two pair of tentacles and the
posterior pair has eyes
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16-62
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16-63
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Classes of Molluscs
Class Bivalvia
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Mussels, clams, scallops, oysters, and
shipworms
Range in size from 1–2 mm in length to the
giant South Pacific clams
Most are sedentary filter feeders
Bivalves lack a head, radula, or other aspects
of cephalization
Most are marine
16-64
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16-65
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16-66
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16-67
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16-68
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Classes of Molluscs
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Native freshwater clams in the U.S. are the
most jeopardized animal group
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Of more than 300 species once present, 12 are
extinct, 42 are threatened or endangered and 88
more are of concern
Sensitive to water quality changes, including
pollution and sedimentation
Zebra mussels are a serious exotic invader
into the Great Lakes Region
16-69
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Classes of Molluscs
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Form and Function
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16-70
2 shells or valves are held together by a hinge
ligament
Valves are drawn together by strong adductor
muscles
Umbo is the oldest part of the shell with growth
occurring outward in rings
Pearls are produced when an irritant is lodged
between the shell and mantle
 Layers of nacre are secreted around the
foreign material
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Classes of Molluscs
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Body and Mantle
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16-71
Visceral mass is suspended from the dorsal
midline
Foot is attached anteroventrally
Ctenidia hang down on each side, each covered
by a fold of the mantle
Posterior edges of the mantle folds form
excurrent and incurrent openings
In burrowing clams, mantle forms long siphons to
reach the water above
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Classes of Molluscs
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Locomotion
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16-72
Foot is extended out from between the valves
Blood is pumped into the foot
Foot swells and anchors the bivalve in the mud
Shortening of the foot pulls the clam forward
Scallops clap valves to create a jet propulsion
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16-73
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Classes of Molluscs
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Gills
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16-74
Both mantle and gills perform gaseous exchange
Gills are derived from primitive ctenidia by
lengthening the filaments to each side
Filaments fused to form plate-like lamellae with
vertical water tubes inside
 Water enters incurrent siphon
 Passes into water tubes through pores
 Proceeds dorsally to suprabranchial chamber
 Exits through the excurrent siphon
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16-75
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Classes of Molluscs
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Feeding
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16-76
Suspended organic matter enters incurrent
siphon
Gland cells on gills and labial palps secrete
mucus to entangle particles
Food in mucous masses slides to food grooves at
lower edge of gills
Cilia and grooves on the labial palps direct the
mucous mass into mouth
Some bivalves feed on deposits in sand
Shipworms excavate particles of wood
Septibranchs draw in crustaceans by creating a
sudden inflow of water
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Classes of Molluscs

Internal Structure and Function
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Stomach is folded into ciliary tracts for sorting
particles
Style sac secretes a crystalline style which is kept
whirling by cilia in style sac
Rotating style helps free digestive enzymes and
roll up a mucous food mass
Dislodged particles are directed to a digestive
gland or are engulfed by amebocytes
3 chambered heart has two atria and one ventricle
Some blood is oxygenated in mantle
Returns to the ventricle through the auricles
Remainder circulates through sinuses, the
kidneys, the gills, and then back to the auricles
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Classes of Molluscs
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Pair of U-shaped kidneys is ventral and posterior to
heart
Nervous system has three pairs of widely separated
ganglia connected together
Sense organs are poorly developed
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Statocysts in the foot
Osphradia in the mantle cavity
Pigment cells on the mantle
Some mantle eyes have a cornea, lens, retina and
pigmented layer
Tentacles may have tactile and chemoreceptor cells
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Classes of Molluscs
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Reproduction and Development
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Sexes usually separate
Gametes discharged in suprabranchial chamber
are carried out in excurrent flow
Fertilization usually external
Embryos develop as trochophore, veliger, and spat
larval stages
Freshwater clams have internal fertilization
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Larvae develop into a bivalved glochidia stage
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Sperm enter the incurrent siphon to fertilize eggs in
water tubes of the gills
Attaches to gills of passing fish where they live briefly as
parasites
Eventually sink to begin independent life on the
streambed
“Hitchhiking” having helped distribute the species
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Classes of Molluscs
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Boring
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Burrowing has led some to evolve a mechanism
for boring into harder surfaces
Shipworms are destructive to ships and wharfs
Radula functions as a wood rasp
 Symbiotic bacteria produce cellulase, which
helps digest wood
 Bacteria also fix nitrogen
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Diet is high in carbon but deficient in nitrogen
Some clams bore into rock and produce long
burrows
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Classes of Molluscs
Class Cephalopoda
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Squids, octopuses, nautiluses, devilfish, and
cuttlefish
All marine predators
Foot is in the head region
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Range from 2 cm to the giant squid
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Modified for expelling water from mantle cavity
Largest invertebrate
Cephalopod fossil record goes back to the
Cambrian
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Earliest shells were straight
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Classes of Molluscs
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Nautilus
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Octopuses and squids apparently evolved
from early straight-shelled ancestors
Ammonoids
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Culmination of shell coiling
Remaining survivor of nautiloids
Series of gas chambers in shell helps maintain
neutral buoyancy
Extinct but had elaborate shells
Cephalopods
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Mostly marine
Octopuses mostly intertidal
Squids are deep-sea animals
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Classes of Molluscs

Form and Function
 Shell
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Nautiloid and ammonoid shells had gas
chambers allowing them to swim
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Gas pressure in the nautilus chambers is only one
atmosphere compared to the
41 atmospheres of
pressure in the surrounding deep ocean
Nautilus shell is divided into chambers
Living animal only inhabits last chamber
Cord of living tissue, the siphuncle, connects chambers
to visceral mass
Cuttlefish shell is enclosed in mantle
 Squid shell is a thin strip called the pen, enclosed
in mantle
 Octopus has completely lost the shell
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Classes of Molluscs

Locomotion
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Cephalopods swim by forcefully expelling water
through a ventral funnel or siphon
Control direction and force of the water, thus
determining its speed
Lateral fins of squids and cuttlefishes are
stabilizers
Nautilus swims mainly at night
Octopuses mainly crawl on the bottom but can
swim backward by spurting jets of water
 Some with webbing between their arms swim
with a medusa-like action
Active life of cephalopods is reflected in their
respiratory, circulatory and nervous systems.
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Classes of Molluscs

Respiration and Circulation
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Except for nautiloids, cephalopods have one pair
of gills
With higher oxygen demands, cephalopods have a
muscular pumping system to keep water flowing
through the mantle cavity
Circulatory system has a network of vessels
conducting blood through gill filaments
Blood goes to the systemic circulation before the
gills
Accessory or branchial hearts at the base of each
gill increase pressure to blood going through gill
capillaries
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Classes of Molluscs

Nervous and Sensory Systems
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Cephalopod brain is the largest of any
invertebrate
Squids have giant nerve fibers
Sense organs are well-developed
 Eyes are complex, complete with cornea, lens,
and retina
Can learn by reward and punishment, and by
observation of others
Cephalopods lack a sense of hearing but have
tactile and chemoreceptor cells in their arms
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Classes of Molluscs
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Communication
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Use chemical and visual signals to communicate
Chromatophores are cells in the skin that contain
pigment granules
Contractions of the muscle fibers attached to the
cell boundary causes the cell to expand and
change the color pattern
Color patterns can be changed rapidly
Deep-water cephalopods have elaborate
luminescent organs
Ink sac empties into rectum;
 Contains ink gland that secretes sepia when
animal alarmed
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Classes of Molluscs
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Reproduction
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Sexes are separate
In male seminal vesicle, spermatozoa are
packaged in spermatophores and stored
One arm of male is modified as an intromittent
organ, the hectocotylus
 Removes a spermatophore from mantle cavity
and inserts it into female
Fertilized eggs leave oviduct and are attached to
stones, etc.
Large, yolky eggs undergo meroblastic cleavage
 Hatch into juveniles with no free-swimming
larval stage
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Classes of Molluscs
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Major Groups of Cephalopods
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3 subclasses of cephalopods
 Nautiloidea
 Ammonoidea
 Coleoidea
Nautiloidea have two pairs of gills
Ammonoidea are extinct
Coleoidea have one pair of gills
Nautilus is the only surviving genus in Nautiloidea
that populated the Paleozoic seas
 5 or 6 living species
 Head has 60–90 tentacles that can extend from
the opening of the shell
 Tentacles lack suckers
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Classes of Molluscs
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Ammonoids became extinct at end of the
Cretaceous
 Chambers resembled those of the nautiloids
but septa were more complex and frilled.
 Reason for extinction is unknown
4 orders of Coleoidea include all living
cephalopods except Nautilus
 3 subclasses of cephalopods
 Nautiloidea
 Ammonidea (extinct)
 Coleoidea (extinct)
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Classes of Molluscs
Only one genus of Nautiloidea remains
 Subclass Coleoidea includes all living
cephalopods except Nautilus
 Order Sepioidea includes cuttlefishes with a
round body, eight arms and two tentacles
 Orders Myopsida and Oegopsida are squids
with a more cylindrical body, eight arms and
two tentacles.
 The single species of deepwater vampire squid
is in the order Vampyromorpha
 The Octopoda have eight arms and no
tentacles
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Phylogeny and Adaptive Diversification
Phylogeny
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First mollusc probably arose in Precambrian times
Due to spiral cleavage, mesoderm from the 4d
blastomere, and a trochophore larva, many
zoologists consider the Mollusca as protostomes
allied with annelids in Lophotrochozoa
Opinions differ about the exact nature of the
relationship among lophotrochozoans
Until lophotrochozoan phylogeny is better resolved,
not possible to determine whether mollusks and
annelids shared a coelomate ancestor
A new cladogram places monoplacophorans as the
sister taxa to chitons
 Unites the two taxa with repeated body parts in a
clade called Serialia
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Phylogeny and Adaptive Diversification
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The question remains, did segmentation originate
independently within the three metameric taxa?
Ongoing studies suggest that differences in
biochemical pathways and developmental steps
that produce segmented bodies across taxa,
support the hypothesis that segmentation arose
several times independently
“Hypothetical Ancestral Mollusc”
 Probably lacked a shell or crawling foot
 Probably small (about 1 mm)
 Likely was a worm-like organism with a ventral
gliding surface
 Probably possessed a dorsal mantle, a
chitinous cuticle and calcareous scales
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Phylogeny and Adaptive Diversification
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Caudofoveates and solenogasters both probably
branched off before development of a solid shell
Polyplacophorans then branched off before the
veliger was established as a larva
Gastropoda and Cephalopoda appear to form a
sister group with Monoplacophora
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Phylogeny and Adaptive Diversification
Classification
 Class
Caudofoveata
 Class Solenogastres
 Class Monoplacophora
 Class Polyplacophora
 Class Scaphopoda
 Class Gastropoda
 Class Bivalvia
 Class Cephalopoda
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