Phylum Mollusca

Phylum Mollusca

• Soft Bodied
• Shelled
• Some without
• Bilateral Symmetry
• Cephalization
• Sensory organs

Tonicella lineata

• Body Divisions
• Visceral Mass
• Head-Foot
• Mantle
• Secretes Shell

Shell Structure & Formation

Pearl Formation

Digestive System�

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• Complete digestive tract
• Digestive glands
• Specialized feeding �structures

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Octopus briareus

Gastropod Feeding Apparatus�

• Circulatory system
• Heart (2 chambers)
• Open system in most �groups
• Closed in cephalopods
• Coelom restricted to pericardium in most cases

Respiration: Gills

Phylum Mollusca

• Advanced nervous system
• Brain
• Sensory organs
• Eyes
• Sensory papillae
• Excretory system
• Kidney-like nephridia

• Reproduction
• Monoecious and dioecious
• External & internal fertilization
• External development

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Generalized Mollusc Anatomy

Classification of Mollusks

Mollusks are classified intro three common groups based on shell presence, shell type, and foot type.

Mollusc Taxonomy

• Major classes
• Gastropoda
• Largest group of mollusks
• Usually have a single shell
• Use a radula (a tongue-like organ with rows of teeth) to get food
• Have foot glands that secrete a layer of mucus for sliding
• Includes snails, conchs, and garden slugs

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Mollusc Taxonomy

• Major classes
• Gastropoda
• Bivalvia
• Have a hinged, two-part shell
• To open or close their shell they either contract or relax their muscles
• Includes clams, oysters, and scallops

Bay scallop (Aequipecten irradians)

Edible mussels �(Mytilus edulis)

Giant Clam & Burrowing Clam

Siphon

Mollusc Taxonomy

• Major classes
• Gastropoda
• Bivalvia
• Cephalopoda
• Octopus, squid, nautilus

Giant cuttlefish (Sepia latimanus)

Nautilus sp

Octopus sp

Class Cephalopoda

• Most specialized and complex mollusks.
• Include squid, octopuses, and chambered nautiluses.
• Have a well developed head and many tentacles for capturing prey.
• Closed circulatory system
• Moves blood through the body in a series of closed vessels like humans.
• Use jet propulsion to move at speeds of 6 m/s.

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Mollusc Reproduction & Development

• Indirect development
• Larval stages
• Trochophore larva
• Free swimming, ciliated, shell formation begins
• Veliger larva
• Free swimming, ciliated velum forms
• Shell/body torsion occurs
• Spat
• Metamorphic form between veliger and juvenile
• Shell elaborates

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Trochophore Larva

Gastropod Veliger Larva

Velia

Mouth

Shell

Zygote

Trochophore

Veliger

Spat

Juvenile

Adult

Adult Abalone

Class �Gastropoda

The Shell Game

Class Gastropoda - Torsion

Generalized Gastropod Anatomy

Marine Gastropods

Cowries

Marine Gastropods

Sea Hare – Aplysia sp

Marine Gastropods - Nudibranch

Terrestrial Gastropods �- Pulmonata

• The mantle cavity serves as a “lung”
• Glandular epidermis
• Secrete mucus (slime) upon which gastropod glides

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Terrestrial Gastropod - Pulmonata

• Slugs lack �shells
• Mantle thickened
• Pneumostome
• Air intake into mantle cavity

Pneumostome

Phylum Mollusca, Class Bilvalia

• Two shells
• Incurrent & excurrent siphons
• No cephalization

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Generalized Bivalve Anatomy

Generalized Bivalve Anatomy (XS)

Bivalve Anatomy (Sagittal)

Bivalves are Typically Filter Feeders

Bivalve Life Cycle

Glochidia – Unique Larval Stage of Freshwater Bivalves

• Internal fertilization
• Release of glochidia veliger larvae
• Glochidia attach to gills of fish and are distributed

Phylum Mollusca, Class Cephalopoda

• Closed circulatory system
• Shell: lost in Octopus, extremely reduced and enclosed in mantle in squid, planar spiral in Nautilus
• Cephalization
• eye
• Beak around �mouth
• Tentacles/arms

Cephalopod Anatomy

Squid are the largest Cephalopods

Nautilus Anatomy

Cephalopod Eye

Direct Development in Cephalopods

Color & Morphology Changes for Camouflage