Mollusca is a large and diverse phylum of invertebrate animals known as molluscs. This group includes a wide range of organisms, many of which have soft bodies and, in many cases, are protected by a hard external shell.
1.0Introduction
Molluscs inhabit a wide range of environments, from tropical to polar regions, and even at altitudes up to 7,000 meters.
They thrive in nearly all aquatic habitats—from high mountain lakes to the deepest parts of the ocean.
Ecologically, molluscs occupy diverse niches, including Bottom feeders, Herbivorous grazers, Predatory carnivores, and Filter feeders.
Many molluscs are important to humans: they are consumed as food, and some, like oysters, are cultivated for pearls.
2.0Characteristics of Phylum Mollusca
Symmetry & Development: - Molluscs are bilaterally symmetrical, coelomates (having a true body cavity), and exhibit protostome development.
Locomotion: They possess a muscular foot, typically used for movement or burrowing.
Mantle & Shell: The dorsal body wall forms a mantle, a sheath of tissue that covers the internal organs and often secretes a protective shell—though the shell is absent in some species.
Feeding: Most molluscs use a specialised, rasping tongue-like structure called a radula for feeding.
Circulatory System: Most molluscs have an open circulatory system, in which blood drains into body cavities.
Cephalopods (e.g., octopuses and squids) are an exception, having a closed circulatory system for more efficient oxygen transport.
Respiration: Gas exchange occurs via gills, lungs, mantle cavity, or directly through the skin.
Excretion: Typically, one or two kidneys (metanephridia) excrete waste into the mantle cavity.
Nervous System: Consists of paired cerebral ganglia, forming a relatively well-organised nervous system.
Organ Systems: Molluscs have highly developed organ systems that support complex functions.
3.0General Anatomy of Mollusca
4.0Body Structure of Molluscs
The body of a mollusc is typically divided into two main regions:
Head-Foot Region: This region includes the head, which contains the mouth with a radula, eyes, and tentacles, as well as the muscular foot, which serves as the primary organ of movement.
Visceral Mass: This region houses the internal organs, including the digestive, circulatory, respiratory, and reproductive systems.
5.0Mantle, Circulation, and Larval Stages in Molluscs
The visceral mass is enclosed within the mantle, a protective tissue layer that plays a key role in many physiological functions.
The outer surface of the mantle secretes a calcium carbonate and protein-based shell in most molluscs (though it may be reduced or absent in some species).
The mantle cavity contains respiratory structures such as gills or lungs, and it may also function directly in gas exchange.
Most molluscs possess an open circulatory system that includes a heart, blood vessels, and blood sinuses.
Cephalopods, such as squids and octopuses, are an exception—they have a closed circulatory system with a heart, blood vessels, and capillaries for more efficient circulation.
Many aquatic molluscs undergo a free-swimming larval stage known as the trochophore.
In certain molluscs—such as marine snails, bivalves, and tusk shells—the trochophore further develops into a more advanced larval form called a veliger.
6.0Taxonomy of Mollusca
Monoplacophora-single-shelled molluscs
Polyplacophora-chitons
Scaphopods-tusk shells
Gastropods-snails, slugs
Bivalves-clams, oysters
Cephalopods-octopus, squid, nautilus, cuttlefish
On this page
1.0Introduction
2.0Characteristics of Phylum Mollusca
3.0General Anatomy of Mollusca
4.0Body Structure of Molluscs
5.0Mantle, Circulation, and Larval Stages in Molluscs
6.0Taxonomy of Mollusca
Frequently Asked Questions
Molluscs are soft-bodied invertebrate animals, many of which have a hard external shell. They belong to the phylum Mollusca, which includes snails, clams, squids, and octopuses.
Molluscs live in a variety of habitats, including marine, freshwater, and terrestrial environments—from deep ocean floors to mountain lakes and even land.
A mollusc’s body has three main parts: head-foot region, visceral mass, and mantle.
No. While many molluscs, like snails and clams, have external shells, others, like octopuses and slugs, either have internal shells or none at all.
The radula is a specialised, rasping, tongue-like organ used by most molluscs (except bivalves) to scrape or cut food before digestion.
Molluscs breathe through gills, lungs, or sometimes the mantle cavity/skin.
Most molluscs have an open circulatory system, but cephalopods (such as squid and octopuses) have a closed circulatory system, which allows more efficient oxygen transport.
Molluscs usually reproduce sexually. Many have separate sexes, while some are hermaphrodites. Aquatic species often have larval stages such as trochophore and veliger.
Molluscs are important for food, pearls, and research, though some can be harmful pests.
Examples include snails, slugs, clams, oysters, squid, and octopuses.