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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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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

Trochophore

Veliger

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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

Lemon Nudibranch

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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