Section 3: Mollusks

Section 3: Mollusks

Essential Questions

• What is the function of the mantle and what are its adaptive advantages to mollusks?
• What is the importance of mucus and the muscular foot to mollusks?

Mollusks

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Vocabulary

New

• mantle
• radula
• gill
• open circulatory system
• closed circulatory system
• nephridium
• siphon

Review

• herbivore

Mollusks

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Body Structure

• Part of phylum Mollusca, include slugs, snails, scallops, and squid.
• May be the first coelomates

Mollusks

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Body Plan : All mollusks share a basic body structure with three main parts:

✅ Mantle – A tissue layer that secretes calcium carbonate to form shells in many mollusks. ✅ Foot – A muscular structure used for movement or attachment (varies across classes). ✅ Visceral Mass – Contains internal organs such as the heart, digestive organs, and reproductive structures. Example: Even though clams, snails, and squids look very different, they all have a mantle, foot, and visceral mass, linking them evolutionarily.

Mollusks

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Body Structure

• Have bilateral symmetry, a soft internal body, a digestive tract with two openings, a muscular foot, and a mantle – a membrane that surrounds the internal organs

Reduced internal shell

Head

Squid

Mantle

Circulatory system

Radula

Digestive system

Shell

Arm

Mantle

Tentacle

Digestive system

Head

SNAIL

Circulatory system

Radula

Foot

Mollusks

Body Structure

Feeding and digestion

• Many mollusks use a radula, a tonguelike organ with rows of teeth, to scrape food into their mouths.
• Other mollusks, such as clams, filter feed and do not have radulas.
• Have a complete gut with digestive glands, stomach, and intestines

Mollusks

Respiration

• Most mollusks have gills, respiratory structures that consist of filamentous projections with lots of surface area for gas exchange.
• Land snails and slugs remove oxygen from the air using the lining of their mantle cavities.
• Gills also function in filter feeding.

Circulation

• Most mollusks have an open circulatory system, where blood is pumped out of vessels into open spaces surrounding the body organs.

• Some mollusks have a closed circulatory system, where blood is confined to vessels as it moves through the body.

Mollusks

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Circulation

Most mollusks have an open circulatory system, where blood is pumped out of vessels into open spaces surrounding the body organs.

Definition: An open circulatory system is a type of blood circulation where blood is not fully contained within vessels but instead flows freely through body cavities, bathing organs directly.

Mollusks

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Circulation

Some mollusks have a closed circulatory system, where blood is confined to vessels as it moves through the body.

A closed circulatory system is a type of blood circulation where blood is completely enclosed within vessels as it moves through the body.

Mollusks

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Body Structure Excretion

• Mollusks get rid of metabolic wastes from cellular processes through structures called nephridia.
• After nephridia filter the blood, waste is passed out through the mantle cavity.

Mollusks

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Response to stimuli

• Mollusks have a nervous system that coordinates movement and behavior.
• More highly evolved mollusks, such as octopuses, have brains.
• Most mollusks have simple eyes.

Mollusks

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Visceral Mass – Contains internal organs such as the heart, digestive organs, and reproductive structures.

Mollusks

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Movement

• Mollusks with two shells can clap their shells together for rapid bursts of swimming.

• Snails and slugs move along a trail of mucus produced by their muscular foot.

• Octopuses and squids take water into the mantle cavity and expel it through a tube called a siphon.

Mollusks

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Body Structure Reproduction

• Mollusks reproduce sexually.
• All mollusks share similar developmental patterns.

Mollusks

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Diversity of Mollusks Gastropods

• Gastropoda, or stomach-footed, is the largest class of mollusks.
• Most species of gastropods have a single shell, such as snails and limpets.
• Other gastropods have no shell, such as slugs and nudibranchs.

Mollusks

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Diversity of Mollusks Bivalves

• Bivalves are two-shelled mollusks, such as clams and oysters.
• All aquatic, most marine
• Filter-feeders
• Burrow or attach to hard surfaces such as rocks

Mollusks

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Diversity of Mollusks Cephalopods

• Cephalopods are the head-footed mollusks, such as octopuses and squid.
• Most cephalopods have an internal shell, with the exception of the chambered nautilus.
• The cephalopod foot is divided into arms and tentacles with suckers.
• Octopuses are considered to be the most intelligent mollusk, are capable of complex learning

Mollusks

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Ecology of Mollusks

• Important to marine and terrestrial food webs as predators, herbivores, scavengers, and filter feeders
• Can be keystone species, a species who’s health influences the health of the entire ecosystem
• Filter feeders are important for cleaning aquatic ecosystems
• Filter feeders can also serve as environmental monitors as they accumulate toxins in their tissues
• Some snails produce toxins with pharmaceutical promise

Mollusks

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Review Essential Questions

• What is the importance of the coelom to mollusks?

• What is the importance of mucus and the muscular foot to mollusks?

Vocabulary

• nephridium
• siphon

• open circulatory system
• closed circulatory system

• mantle
• radula
• gill

Mollusks

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Gastropods

Slower circulation

Bivalves

cephalopods

Faster movement and higher energy levels.