Gastropoda is the largest class of Mollusca, encompassing diverse species such as snails, slugs, and sea slugs, known for their characteristic "stomach foot" and a muscular foot used for locomotion. Most gastropods possess a single, often coiled shell for protection, though some, like slugs, have lost or reduced their shells. These animals exhibit remarkable adaptability, thriving in marine, freshwater, and terrestrial environments worldwide.
Bivalves are soft-bodied marine or freshwater invertebrates characterized by their two hinged shells, called valves, joined by a ligament. They lack a distinct head, possess a large muscular foot for movement or burrowing, and are filter feeders that use their gills to strain food particles and oxygen from water. Prominent examples include clams, mussels, oysters, and scallops.
Cephalopods are marine mollusks, like octopuses and squid, that have a prominent head, a mantle, and at least eight arms or tentacles with suction cups. They possess large, sophisticated brains and well-developed, vertebrate-like eyes, making them highly intelligent. Key characteristics also include a jet-propulsion system for movement, the ability to change color and texture, and, for most, a reduced or absent shell.
Polyplacophora, or chitons, are marine mollusks characterized by a flattened, oval body with a dorsal shell composed of eight interlocking plates, a thick, muscular girdle that surrounds the shell, and a broad ventral foot for attachment to hard surfaces. They are herbivores that use a hardened, tooth-covered radula to scrape algae from rocks and possess a poorly developed head with no eyes or tentacles.