Zoology Chapter 12 Mollusk

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Zoology Chapter 12 Mollusk

• Ms.K.Cox

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Introduction

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

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Page 179-185, specifically from page 180

• Two groups of coelomates animals have been
  distinguished the protostomes and the
  deuterostomes.
• 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.

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Page 180

• Most protostomes form the coelom by splitting the
  mesoderm form the coelom by outpocketings of the
  gut
• 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
  mass.
• 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

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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
  food

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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
  move

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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

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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
  products.

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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
  squid.
• 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
183

• 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

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Gastropoda- Snails, limpets, slugs Page 184
Locomotion

• How do they get around?
• Almost all have a flattened foot that is often
  ciliated with gland cells. These are used to
  creep around.

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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
  in
• Enzymes in the stomach break it down.
• Waste comes out in the form of pellets

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oyster
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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.

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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
  slugs.
• Subclass Pulmonata contain 17,000 species.

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End of Day 1
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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
  stomach.

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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.

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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).

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• 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.

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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
  understood.

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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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