MOLLUSKS:MOLLUSKS:

• Section 27 – 1Section 27 – 1

• Slugs, snails, and animal that once Slugs, snails, and animal that once lived in shells in the ocean or on the lived in shells in the ocean or on the beach.beach.

• Phylum MolluscaPhylum Mollusca

• Oysters and mussels live firmly Oysters and mussels live firmly attached to the ocean floor or to the attached to the ocean floor or to the bases of docks or wooden boats.bases of docks or wooden boats.

• Squid and the octopus swim freelySquid and the octopus swim freely

• 100 000 species100 000 species

• Some have shells Some have shells

• Bilateral symmetryBilateral symmetry

• True coelomTrue coelom

• Two body openingsTwo body openings

• Muscular “foot” for movementMuscular “foot” for movement

How mollusks obtain food:How mollusks obtain food:

• Radula: located within the mouth of Radula: located within the mouth of a mollusk, is a tongue-like organ with a mollusk, is a tongue-like organ with rows of teeth.rows of teeth.

• The radula is used to drill, scrape, The radula is used to drill, scrape, grate or cut food. grate or cut food.

Reproduction in Mollusks:Reproduction in Mollusks:

• Most have separate sexes and Most have separate sexes and reproduce sexually.reproduce sexually.

• Eggs and sperm are released at the Eggs and sperm are released at the same time into the water where same time into the water where external fertilization takes place.external fertilization takes place.

Larval stages:Larval stages:

• Larval stages of all mollusks are Larval stages of all mollusks are similar.similar.

Nervous Control in Mollusks:Nervous Control in Mollusks:

• Simple nervous system with a brain Simple nervous system with a brain and associated nerves that and associated nerves that coordinate movement and behavior.coordinate movement and behavior.

Circulation in Mollusks:Circulation in Mollusks:

• Well-developed circulatory system Well-developed circulatory system with a three-chambered heart.with a three-chambered heart.

• Open circulatory: blood is pumped Open circulatory: blood is pumped through vessels and into open spaces through vessels and into open spaces surrounding organs.surrounding organs.

• Some mollusks (such as the octopus) Some mollusks (such as the octopus) has a closed circulatory system.has a closed circulatory system.

Respiration in Mollusks:Respiration in Mollusks:

• Most mollusks have respiratory Most mollusks have respiratory structures called gills.structures called gills.

• Gills increase the surface area Gills increase the surface area through which gases can diffuse. through which gases can diffuse. They are an extension of the mantle.They are an extension of the mantle.

Excretion in Mollusks:Excretion in Mollusks:

• Oldest known animals to have Oldest known animals to have evolved excretory structures called evolved excretory structures called nephridianephridia

• Nephridia: organs that remove Nephridia: organs that remove metabolic wastes from an animal’s metabolic wastes from an animal’s body.body.

Diversity of Mollusks:Diversity of Mollusks:

• Seven classesSeven classes– GastropodaGastropoda– BivalviaBivalvia

Gastropods:Gastropods:

• One-shelled mollusksOne-shelled mollusks

• Largest classLargest class

• Stomach-footed (named for the way Stomach-footed (named for the way that the large foot is positioned that the large foot is positioned under the body)under the body)

• May be found in freshwater, May be found in freshwater, saltwater, or moist terrestrial saltwater, or moist terrestrial environments.environments.

Slugs:Slugs:

• No shellNo shell

• Body is protected by a thick layer of Body is protected by a thick layer of mucus.mucus.

• Nudibranchs: colorful sea slugsNudibranchs: colorful sea slugs

Bivalves:Bivalves:

• Two-shelled mollusksTwo-shelled mollusks

• Clams, oysters and scallopsClams, oysters and scallops

• Figure 27-7Figure 27-7

• Most are marine, but a few a Most are marine, but a few a freshwater.freshwater.

• Range in size from 1 mm to 1.5 Range in size from 1 mm to 1.5 metersmeters

• A ligament like a hinge connects the A ligament like a hinge connects the two shells.two shells.

• Filter feedersFilter feeders

• Cilia beat to draw water into an Cilia beat to draw water into an incurrent siphon.incurrent siphon.

• The cilia also act as a sorting device.The cilia also act as a sorting device.

Cephalopods:Cephalopods:

• Head-footed mollusksHead-footed mollusks

• All marineAll marine

• The only cephalopod with a shell is The only cephalopod with a shell is the chambered nautilus.the chambered nautilus.

• The cuttlefish has a reduced internal The cuttlefish has a reduced internal shell.shell.

• Very complex structuresVery complex structures

• ..

• The foot has evolved into structures with The foot has evolved into structures with hooks, suckers or adhesive structures.hooks, suckers or adhesive structures.

• The tentacles bring the prey to the The tentacles bring the prey to the mouth where it is bitten by the beak-like mouth where it is bitten by the beak-like jaw.jaw.

• The food is then pulled into the mouth The food is then pulled into the mouth by the radula.by the radula.

• Figure 27-8Figure 27-8

• Possess siphons that expel water.Possess siphons that expel water.• By expelling water forcefully, these By expelling water forcefully, these

mollusks can move quickly by jet mollusks can move quickly by jet propulsion.propulsion.

• Squids can attain speeds of 20 Squids can attain speeds of 20 meters per second with this method.meters per second with this method.

• They also expel “ink” to confuse and They also expel “ink” to confuse and escape predators.escape predators.