Mollusca Overview and Classes

Overview

The lecture explores the diverse phylum Mollusca, highlighting its major classes—gastropods, bivalves, and cephalopods—and their unique anatomical and ecological adaptations.

Introduction to Molluscs

• Molluscs are a diverse phylum with about 85,000 known species including snails, clams, mussels, squid, and octopods.
• All molluscs share a common ancestor and have similar internal structures.

General Mollusc Anatomy

• Most molluscs have three main body regions: head (contains sense organs and brain), visceral mass (internal organs), and foot (muscular movement).
• The mantle is an extension of the body wall that secretes the shell.
• Many molluscs have a radula, a chitinous, sandpaper-like tongue for feeding.

Gastropods ("Stomach-foot" Molluscs)

• Gastropods make up about 70% of mollusc species, including snails, limpets, slugs, and nudibranchs.
• Snails and shelled gastropods are protected by hard, portable shells.
• Some snails are scavengers, while others, like cone shells, are venomous predators.
• Nudibranchs lack shells and use cerata for defense and respiration.

Bivalves ("Two-shell" Molluscs)

• Bivalves include clams, scallops, mussels, and oysters, living mainly in aquatic environments.
• They have two shells and may burrow or attach with byssal threads.
• Most bivalves feed by filtering organic particles from water.
• Giant clams harbor symbiotic algae for photosynthesis.

Cephalopods ("Head-foot" Molluscs)

• Cephalopods include squid, octopods, cuttlefish, and nautiluses.
• Cephalopods usually lack external shells, except nautiluses.
• They possess advanced nervous systems and sophisticated eyes (convergent evolution with mammal eyes).
• Arms surround the mouth and are used for capturing and eating prey; some arms are specialized as tentacles.
• Octopods can secrete venom; blue-ringed octopus venom is lethal to humans.

Key Terms & Definitions

• Molluscs — A diverse phylum of animals including snails, clams, and squid.
• Mantle — Body wall extension that secretes the mollusc shell.
• Radula — Chitinous, sandpaper-like feeding organ found in many molluscs.
• Gastropoda — Class of molluscs with "stomach-foot" anatomy, such as snails.
• Bivalve — Mollusc with two shells joined together, mainly aquatic.
• Byssal threads — Glue-like strings some bivalves use for attachment.
• Cephalopoda — Class of molluscs with "head-foot" anatomy, including squid and octopods.
• Cerata — Fleshy projections on nudibranchs for defense and respiration.

Action Items / Next Steps

• Review key differences among mollusc classes.
• Memorize anatomical terms and class characteristics.
• Prepare for possible quiz on mollusc diversity and adaptations.