Phylum Mollusca1

Phylum Mollusca

Polyplacophora

Gastropoda

Bivalvia

Scaphopoda

Cephalopoda

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Molluscs

One of the largest of all phyla Have adapted to a wide variety

of habitats– Terrestrial, marine, benthic,

and accomplished swimmers.

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The Molluscan Body Plan

Despite the external differences between snails, clams, and squids the body plan is similar and distinct from all other phyla

The Mollusca body plan includes:– A large muscular foot– A radula– Mantle and mantle cavity– Usually a small head– Soft unsegmented body– A hard non-living calcareous shell

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The Foot

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The Radula

Radula

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The Mantle

The mantle is the body wall that enclose the body cavity

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The Mantle Cavity

Houses the visceral mass and comb-like gills– Gills are respiratory in function and can collect food particles

Mantle cavity also is the site for reproductive, excretory, and digestive systems

The molluscan coelom is very small; being restricted to the area surrounding the heart and gonads

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Taxonomic Summary

Phylum Mollusca– Class Polyplacophora– Class Gastropoda – Class Bivalvia– Class Scaphopoda – Class Cephalopoda

Systems

SkeletalSkeletal- Mantle may secretes a shell. Use hydrostatic pressure for ventral muscular foot.

MusclesMuscles -Ventral muscular foot and other muscles present.

DigestiveDigestive- complete complex with salivary glands, digestive gland and Rasping tongue (Radula).

Systems

CirculatoryCirculatory - Open except for Cephalopoda. Dorsal heart, usually in a pericardial cavity.

RespiratoryRespiratory - Ctenidia (gills) in mantle cavity, respiratory pigment is copper.

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Systems

ExcretoryExcretory- by nephridia usually connecting to the pericardial cavity,

– the coelom is usually reduced to the cavities of the nephridia, gonads and pericardium.

NervousNervous - Nerve ring with various pairs of ganglia—two pairs of nerve cords, one innervating the foot, the other the visceral mass (modified ventral ladder-like system)

Systems

IntegumentaryIntegumentary – Mantle

EndocrineEndocrine - nervous systems produces hormones.

ReproductiveReproductive - varied- monoecious, protandric, or dioecious. Larva in marine = trochophore and veliger, in freshwater clam is glochidium.

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Phylum Mollusca

Class Polyplacophora

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Class Polyplacophora

Chitons

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Chiton Lifestyles

Found close to shore mainly in the intertidal where they live on hard substrates

– Strong foot and low profile help from being swept away

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Ingestion and Digestion

Radula is used to scrape algae from the rocks

Mouth is anterior and anus is posterior; linear digestive tract

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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

Largest and most varied group of the phylum Mollusca

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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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Prosobranchia Anatomy

Molluscan gill – Consists of a series of

flattened, triangular sheets

– Water is drawn into the animal by gill cilia

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Class Bivalvia

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Class Bivalvia

Defining characteristics– Two-valved shell– Body flattened laterally

This class contains clams, oysters, mussels, scallops, and shipworms

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Bivalves and You

Many species are edible– In today’s waters this can cause certain problems

since bivalves are filter feeders

Also, an important food item for our commercially important fish

Important to humans in their ability to strain harmful bacteria out of polluted waters– Some can pump up to 40 liters per hour

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Bivalve Locomotion?

Most bivalves move by expansions and contractions of the foot

Mussels usually are sessile and attach to substrate with threads

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Bivalve shells

Protein hinge ligament stores energy required to open the shell

Adductor muscles is responsible for closing the valves

The umbo is dorsal and the valves are addressed as left and right

Siphons are posterior and the excurrent siphons are always dorsal

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The Mantle

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Bivalve Feeding

Bivalves are filter feeders– Ctenidia, gills, are

used for feeding & respiration and are enlarged and folded to increase surface area

– Cilia on the gills create a current to bring food-laden water into the mantle cavity

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Bivalve Reproduction

Dioecious The male sheds sperm into

the water column– Sperm can enter the

female or fertilization can occur in the water column

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Class Scaphopoda

Class Cephalopoda

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Class Scaphopoda

Defining characteristics– Tusk shaped conical

shells, open at both ends– Development of anterior

thread-like adhesive feeding tentacles

Elongated tapering tubular shells that burrow in sandy bottoms

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Class Cephalopoda

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Class Cephalopoda

Defining characteristics– Closed circulatory

system– Foot modified to form

flexible arms and siphons

– Ganglia fused to form a large brain encased in a cartilaginous cranium

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Cephalopod Characteristics

The most highly organized of all the molluscs, the foot is closely associated with the head

Nautilus, octopus, cuttlefish, and squids Shell size varies from a full shell (nautilus) to a

vestige of a shell in the squid and octopus Can be as large as 1,000kg. and 18m.

Squids 10 tentacles. Propels by pumping jets of water with the mantle through an excurrent siphon.

Octopuses 8 tentacles They often crawl along the ocean bottom or lie in wait for prey.

Chambered Nautiluses The only existing cephalopod with external shell. Shell is coiled & divided into chambers. The body is confined to the outermost chamber.

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Defensive Mechanisms

Since they are susceptible to predation they have evolved chromatophores

– May posses hundreds or thousands, all controlled by the brain

An ink sac that is associated with the digestive system and discharged out the anus

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Sensory & Nervous System

Eyes– All but the nautilus have

image forming eyes that are like mammals

Convergent evolution

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Reproduction

Sexes are separate– Males have one modified

arm (hectocotylus) for sperm transfer

Sperm are enclosed in packets called spermatophores

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Squid

10 sucker bearing arms surround the mouth– 2 longer tentacular arms can shoot forward and capture prey

Large bites of food are removed and swallowed quickly Form large schools and feed on crustaceans, and fishes The shell is reduced and lies under the mantle, pen Large folded gills and a circulatory system distribute

oxygen quickly High oxygen demands are meet by the contraction of the

mantle cavity– This method also removes waste products

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Squid Internal Anatomy

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Squid Locomotion

The foot forms the funnel The upper end of the mantle is

extended into a pair of triangular fins

– Water enters the mantle cavity at the free end

– When the mantle contracts the edge is tightly sealed and water is forced out the funnel

– Can move very quickly 5-10 m/s

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Octopods

Have 8 arms all equal in length– Rows of suckers from base to tip

Compact body with a greatly reduced shell The body is extremely flexible and assumes a streamlined shape

when swimming with squid-like jets through the funnel

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Learning and Memory

Benthic feeders on bivalves, crabs and fishes

– Some are pelagic and spend their entire life swimming, usually at great depths

Amazing ability to learn quickly and remember for several weeks