Mollusca Revision Sheet

1. 📌 Essentials

• Mollusca are invertebrates with soft, unsegmented bodies, often with a calcium carbonate shell.
• Main body parts: foot, visceral, mantle, and mantle cavity.
• Radula: rasping feeding organ present in most classes except bivalves.
• Circulatory system: open in gastropods and bivalves; closed in cephalopods.
• Nervous system varies from simple ganglia to complex brains in cephalopods.
• Reproduction: mostly dioecious; some hermaphroditic; larval stages include trochophore and veliger.
• Shell is secreted by the; composed of calcium carbonate.
• Major classes: Polyplacophora, Gastropoda, Bivalvia, Cephalopoda.
• Excretion via nephridia; respiration through gills or lungs.
• Fossil record highlights their evolutionary significance.

2. 🧩 Key Structures & Components

• Foot — muscular organ for movement and attachment.
• Visceral mass — contains most internal organs.
• Mantle — secretes shell; forms mantle cavity.
• Mantle cavity — houses gills or lungs, involved in respiration and excretion.
• Radula — chitinous rasping tongue (absent in bivalves).
• Gills (ctenidia) — respiratory organs in aquatic species.
• Nephridia — excretory organs removing nitrogenous wastes.
• Nervous system — paired ganglia, nerve cords; advanced in cephalopods.
• Shell — calcium carbonate structure providing protection.
• Larval stages — trochophore and veliger.

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