Phylum Mollusca

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Phylum Mollusca

• Class Gastropoda

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Class Gastropoda

• Defining characteristics
• Visceral mass and nervous system become twisted
  90-180 during embryonic development
• Proteinaceous shield on the foot (operculum)

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Gastropod Morphology and Movement

• The typical snail consists of a visceral mass,
  which sits atop a muscular foot
• The visceral mass is protected by a univalve
  shell that is coiled

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Dextral and Sinistral Coiling

• The shell is usually carried on the left side of
  the body and coils to the right (dextral)
• Few species are sinistral
• Shell coils to the left (lightning whelk)

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Torsion
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Gastropod Movement

• Most move via cilia located on the ventral
  surface of the foot or through pedal waves

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Subclass Prosobranchia

• Defining characteristic
• Mantle cavity generally anterior due to torsion
• Usually have protective shells
• Includes most marine gastropods

Oyster Drills
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Prosobranchia External Anatomy
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Prosobranchia Anatomy

• Molluscan gill
• Ctenidium, consists of a series of flattened,
  triangular sheets
• Water is drawn into the animal by gill cilia
• Osphradium

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Reproduction

• Life begins as a trochophore larvae which then
  develops into a veliger larvae
• The veliger is where most of the organ systems
  develop
• During development the process of torsion begins

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Local Representatives

• Banded Tulip (Fasciolaria tulipa)
• Knobbed Whelk (Busycon carica)

True Banded Tulip
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Subclass Opisthobranchia

• Defining characteristic
• Mantle cavity located posteriorly due to
  detorsion
• Sea slugs and sea hares

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Gas exchange and defense
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Nudibranchs, Sea slugs, Sea hares

• A radula and foot for creeping is usually present
  (sea slugs)
• Some have parapodia used for swimming
• Well developed head and sensory tentacles

Anemone Sea Slug
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Lemon Nudibranch
Nudibranchs have no shell or mantle cavity Lack
gills, instead have respiratory projections on
the dorsal surface
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Subclass comparisons

• Different from prosobranchia
• Reduction or loss of the shell and operculum
• Limited torsion
• Reduction or loss of the mantle cavity
• Reduction or loss of ctenida
• Many have evolved different respiratory
  structures which are unrelated to ctenidia

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Local species

• Sooty sea hare (Aplysia brasiliana)
• Have large swimming parapodia and a greatly
  reduced shell on the back