Zoology Chapter 12 Mollusk - PowerPoint PPT Presentation

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Zoology Chapter 12 Mollusk


Zoology Chapter 12 Mollusk Ms.K.Cox – PowerPoint PPT presentation

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Title: Zoology Chapter 12 Mollusk

Zoology Chapter 12 Mollusk
  • Ms.K.Cox

  • Molluscs are very successful
  • There are over 100,000 living species
  • Two main classes Gastropoda (snails, and slugs)
    and Bivalvia (clams, etc)

Page 179-185, specifically from page 180
  • Two groups of coelomates animals have been
    distinguished the protostomes and the
  • Protostomes include molluscs, annelids, and some
    lesser phyla both the annelids and the molluscs
    have a trochophore larval stage in development,
    suggesting an evolutionary relationship.
  • Because molluscs are the first group of
    coelomates to be covered here, a brief discussion
    of coelom formation is needed.

Page 180
  • Most protostomes form the coelom by splitting the
    mesoderm form the coelom by outpocketings of the
  • There has been some question as to which method
    of coelom formation was present in the ancestral
    coelomate, but given that both forms now exist,
    some people believe that the coelom evolved
    independently in each lineage.

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Page 180 Characteristics of a Mollusk
  • 1. Body of two parts head-foot and visceral
  • 2. Mantle that secretes a calcareous shell and
    covers the visceral mass
  • 3. mantle cavity functions in excretion, gas
    exchange, elimination of digestive wastes, and
    release of reproduction products.
  • 4. bilateral symmetry

Page 180 Characteristics of a Mollusk
  • 5. Have protostome characteristics, including
    trochophore larvae, spiral cleavage, and
    schizocoelous coelom formation
  • 6. coelom reduced to cavities surrounding the
    heart, nephridia, and gonads
  • 7. open circulatory system in all but one class
  • 8. radula usually present and used in scraping

Page 182 Common Body Structures
  • The body has three main regions head-foot,
    visceral mass and the mantle.
  • 1. head-foot, is elongated containing the head (
    mouth, senesory structures, nervous structures)
    and the foot used for attachment and locomotion.
  • Parts in this section
  • A. radula- mouth
  • B. odontophore- muscle that allows the tongue to

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Page 182 Common Body Structures
  • 2. Visceral mass- contains the digestion,
    circulation, reproduction, and excretion systems.
  • It is found dorsal to the head-foot. What does
    dorsal mean.
  • Above

Page 182 Common Body Structures
  • 3. Mantle- this attaches to the visceral mass,
    and enfolds most of the body, it may contain a
    shell over the mantle.
  • Shells are secreted in 3 layers.
  • Between the mantle and the foot, is the mantle
    cavity which is open to the outside and functions
    to exchange gas, excrete wastes, and reproductive

Side notes
  • Molluscs have been very successful in a myriad of
    habitats. Members of this very diverse group
    range in size from a few mm to the 18 meter giant
  • Bivalves and gastropods are the most successful
    of the 8 molluscan classes.
  • Molluscs have bilateral symmetry, a trochophore
    larva, and basic protostome characteristics

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Gastropoda- Snails, limpets, slugsTorsion Page
  • Torsion- a 180 degree counterclockwise twisting
    of the visceral mass, mantle, and mantle cavity.
  • Why? Not completely known but these are the ideas
    so far.
  • 1. so that the head can enter the shell first and
    be protected.
  • 2. allows only fresh water to enter
  • 3.causes the mollusk (snail) to be more sensitive
    to stimuli

Gastropoda- Snails, limpets, slugs Page 184
  • How do they get around?
  • Almost all have a flattened foot that is often
    ciliated with gland cells. These are used to
    creep around.

Gastropoda- Snails, limpets, slugsFeeding and
Digestion Page 184
  • Most eat by scraping algae
  • They use their radula
  • Some are herbivores and some are predators
  • Food is trapped in mucous strings, and then taken
  • Enzymes in the stomach break it down.
  • Waste comes out in the form of pellets

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Gastropoda- Snails, limpets, slugsFeeding and
Digestion Page 185
  • Reproduction
  • Can be either external or internal
  • External- also called dioecious. Gametes are
    discharged into the sea.
  • Internal- also called monoecious. Internally
    fertilized eggs are deposited in gelatinous
    strings or masses. Normally put in areas that are
    moist. Example forest floor, leaf litter.
  • Marine gastropods when emerging from the egg are
    called trochophore larva. Another type is a larva
    with foot, eyes, tentacles, and shell is called
    veliger larva. Torsion occurs during the veliger
    larva stage.

Gastropoda- Snails, limpets, slugsFeeding and
Digestion Page 185
  • Diversity
  • The largest group of Gastropoda is the
    Prosobranchia, 20, 000 species.
  • Subclass Opisthobranchia includes sea haries, sea
  • Subclass Pulmonata contain 17,000 species.

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Start of Day 2
  • Class Bivalvia includes clams, mussels, oysters
    and scallops, and is the second largest molluscan
    class. Bivalves are found in almost all aquatic
    habitats, buried, or attached to rock or man-made
    substrata by byssal threads. Bivalves are
    laterally compressed and covered by two valves,
    or shell halves. A pair of adductor muscles
    keeps the shell closed.

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  • Bivalves are typically sedentary filter feeders.
    The incurrent siphon is the conduit for providing
    the water current, and. Filter feeding is
    accomplished by the lamellae of the gills gills
    are used in both respiration and feeding. Once
    collected, food is directed to the labial palps,
    which surround the mouth, and then into the

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  • A crystalline style in the stomach aids in
    digestion, along with the gastric shield. Feces
    pass through the anus, and then out via the
    excurrent siphon.

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  • Circulation and respiration involve blood vessels
    in the heart, tissues, sinuses, and gills. The
    circulatory system is open. The nervous system
    is primitive, including several ganglia, sense
    organs, around the margin of the mantle, and may
    include complex eyes as seen scallops.
  • Most molluscs are dioecious, undergo external
    fertilization, with both trochophore and veliger
    stages of development.
  • Freshwater bivalves tend to brood their young in
    their gills the young may develop into a larval
    stage known as a glochidium. The glochidium may
    become parasitic in fish gills or other parts of
    a fish it uses the fish as a dispersal agent.

Class Cephalopoda (octopuses, squid)
  • Class Cephalopoda, including octopuses, squid,
    cuttlefish, and the nautilus, contains the most
    morphologically complex invertebrates,
    particularly with respect to the nervous system.
    Ancestral cephalopods were shelled most extant
    cephalopods have reduced or lost the shell (the
    nautiloids are the exception). The cephalopods
    move by contraction of longitudinal and circular
    muscles in the mantle which produces a rapid
    water jet from the funnel (a modified foot).

  • Their rapid locomotion aids in their predatory
    habits. This increased activity is supported by
    the unusual closed circulatory system allowing
    more efficient blood flow for excretory and
    respiratory functions. Carnivorous cephalopods
    feed using their jaws and radula digesting food
    is moved by peristalsis through the
    gastrointestinal tract wastes pass out the anus
    and exhalent water flushes it out of the mantle
    cavity. Discharge of ink from the mantle cavity
    may also deter predators.

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  • The cephalopod nervous system is the most
    advanced of any invertebrate. They have large
    brains tied to chromatophores, pigment sacs in
    the skin that allow cephalopods to change colors
    rapidly. The cephalopod eye is complex and
    image-forming it is convergent evolution with
    the vertebrate eye.
  • Cephalopods are dioecious, and typically males
    have a specialized tentacle (the hectocotylus)
    for transferring spermatophores to the female.
    Many cephalopods tend their eggs hatchlings are
    miniature adults.

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Class Polyplacophora
  • The class Polyplacophora contains the chitons.
    All chitons have 8 plates on their dorsum, and a
    ventral food to adhere to the substrate. They are
    mainly herbivorous grazers,
  • feeding on algae is accomplished by a radula.
    Gills in the mantle cavity provide for
    respiration. The nervous system is ladder-like
    with a nerve ring around the esophagus.
  • Chitons are dioecious.

Class Scaphopoda
  • The class Scaphopoda contains the tooth or tusk
    shells. Tusk shells are burrowers,
  • feeding on protists, while lying partially buried
    in the marine sediments.
  • The tubular or cone-shaped shell is open at both
    ends. Various sensory structures are found in
    many places on the body. These dioecious animals
    have both a trochophore and veliger larva in the
    life cycle.

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  • The class Monoplacophora contains primitive
    molluscs with an extensive fossil record living
    forms have been known only since 1952.
    Neopilina is the only extant genus. They have
    one limpet-like shell, in spite of the serially
    repeated retractor muscles and gills that are
    also present. They are dioecious.
  • The class Caudofoveata contains poorly known
    worm-like molluscs.
  • The class Aplacophora contains 2 subclasses of
    primitive shell-less molluscs. The subclass
    Neomeniomorpha houses the solenogasters (some
    with a radula). The subclass Chaetodermomorpha
    (previously the Caudofoveata) contains animals
    with scale-like spicules on the body surface.
    Most burrow or creep on the substrate. They have
    a nervous system similar to that of flatworms.
  • Phylogenetic studies of the Mollusca indicate
    that the group is more than 500 million years old
    and did not have a segmented ancestor. Many
    characters, such as segmentation, may be
    secondarily derived. The evolutionary
    relationships among the classes are not well

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  • Molluscs have long been used as ornamentation
    shell jewelry, buttons and other decorative and
    useful pieces are known from the beginning of our
    species. Mollusc shells have been used second
    only to metal as currency, as shells are small,
    uniform and easily carried. Shell currency was
    used in China as early as 700 B.C.
  • Pearls are formed when an irritant like sand
    lodges between the mantle and the shell. Since
    the mantle is not ciliated, any particles that
    work themselves between the mantle and the shell
    form an irritation, causing the mantle to produce
    shell material (a pearl) around them. Most
    bivalves form pearls, but the best come from the
    oyster Pinctada from the Pacific. Cultured pearls
    are formed when a piece of shell is intentionally
    inserted in the oyster. Therefore, there is no
    difference between natural and cultured pearls
    simply cultured pearls are grown in oysters in
    enclosures. The average cultured pearl is
    produced 3 years after introduction of the
    irritant. Pearls from marine oysters tend to form
    round pearls, as the ocean currents, tides, etc.
    produce relatively even forces during formation.
    Freshwater pearls are usually oval, due to the
    constant unidirectional flow in the stream where
    they are grown.

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  • Comparison of Body Parts
  • Shell Mantle Foot
  • Class
  • Polyplacophora
  • Gastropoda
  • Bivalvia
  • Cephalopoda

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