MOLLUSKS Section 27 1 Slugs snails and animal

MOLLUSKS: • Section 27 – 1 • Slugs, snails, and animal that once lived in shells in the ocean or on the beach. • Phylum Mollusca

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

• 100 000 species • Some have shells • Bilateral symmetry • True coelom • Two body openings • Muscular “foot” for movement

How mollusks obtain food: • Radula: located within the mouth of a mollusk, is a tongue-like organ with rows of teeth. • The radula is used to drill, scrape, grate or cut food.

Reproduction in Mollusks: • Most have separate sexes and reproduce sexually. • Eggs and sperm are released at the same time into the water where external fertilization takes place.

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

Nervous Control in Mollusks: • Simple nervous system with a brain and associated nerves that coordinate movement and behavior.

Circulation in Mollusks: • Well-developed circulatory system with a three-chambered heart. • Open circulatory: blood is pumped through vessels and into open spaces surrounding organs.

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

Respiration in Mollusks: • Most mollusks have respiratory structures called gills. • Gills increase the surface area through which gases can diffuse. They are an extension of the mantle.

Excretion in Mollusks: • Oldest known animals to have evolved excretory structures called nephridia • Nephridia: organs that remove metabolic wastes from an animal’s body.

Diversity of Mollusks: • Seven classes – Gastropoda – Bivalvia

Gastropods: • One-shelled mollusks • Largest class • Stomach-footed (named for the way that the large foot is positioned under the body)

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

Slugs: • No shell • Body is protected by a thick layer of mucus. • Nudibranchs: colorful sea slugs

Bivalves: • Two-shelled mollusks • Clams, oysters and scallops • Figure 27 -7 • Most are marine, but a few a freshwater. • Range in size from 1 mm to 1. 5 meters

• A ligament like a hinge connects the two shells. • Filter feeders • Cilia beat to draw water into an incurrent siphon.

• The cilia also act as a sorting device.

Cephalopods: • Head-footed mollusks • All marine

• The only cephalopod with a shell is the chambered nautilus. • The cuttlefish has a reduced internal shell. • Very complex structures • .

• The foot has evolved into structures with hooks, suckers or adhesive structures. • The tentacles bring the prey to the mouth where it is bitten by the beak-like jaw. • The food is then pulled into the mouth by the radula. • Figure 27 -8

• Possess siphons that expel water. • By expelling water forcefully, these mollusks can move quickly by jet propulsion. • Squids can attain speeds of 20 meters per second with this method. • They also expel “ink” to confuse and escape predators.