The phylum Mollusca is one of the largest and most diverse groups of invertebrate animals on Earth, second only to the arthropods in described species. Derived from the Latin word mollis meaning “soft,” this group includes over 85,000 recognized species inhabiting environments across the globe. Mollusks are the dominant invertebrate phylum in marine habitats, making up approximately 23% of all named marine species. They are also found in freshwater systems and have colonized terrestrial environments, primarily as snails and slugs, demonstrating adaptability to diverse ecological niches.
Shared Anatomical Characteristics
Mollusks possess a unique, soft, and unsegmented body plan, distinguishing them from segmented worms and arthropods. This structure is built upon three universal components: the muscular foot, the visceral mass, and the mantle.
The muscular foot is a highly adaptable structure located ventrally, primarily used for locomotion and anchorage. In shelled mollusks, the foot is retractable and extendable, allowing the animal to move or attach firmly to a substrate. The foot varies greatly in shape, ranging from a broad, flat surface for crawling in snails to a hatchet-shaped structure for burrowing in clams.
The visceral mass sits dorsally, containing most internal organs responsible for digestion, excretion, reproduction, and circulation. This centralized location holds the complex systems necessary for metabolic function, including the heart, digestive gland, and gonads.
The mantle is a specialized fold of tissue that covers the visceral organs and often secretes the shell, which is composed of calcium carbonate. The mantle creates a cavity housing the respiratory structures, known as ctenidia or gills, in aquatic species. Another defining characteristic is the radula, a specialized feeding organ found in most groups. This rasping structure, resembling a chitinous tongue with rows of teeth, is used to scrape or shred food particles, though it is absent in filter-feeders.
Major Classes of Mollusks
Gastropoda
The class Gastropoda, including snails, slugs, and limpets, is the most diverse group, accounting for approximately 80% of all known mollusk species. Gastropods are characterized by torsion, a developmental process where the visceral mass twists up to 180 degrees, positioning the anus and mantle cavity over the head. This unique anatomical arrangement is a defining feature of the group, which inhabits marine, freshwater, and terrestrial environments.
Most gastropods possess a single, coiled shell (univalve), which provides protection for the soft body. The muscular foot allows them to glide over surfaces using rhythmic contractions and secreted mucus. However, some gastropods, such as sea slugs and garden slugs, have lost the external shell or have a small, reduced shell buried within the mantle tissue.
Bivalvia
The class Bivalvia comprises clams, oysters, mussels, and scallops, recognized by their two-part shell. The name Bivalvia refers to these two hinged valves that enclose the laterally flattened body. The valves are held together dorsally by a ligament and powerful adductor muscles, allowing the animal to rapidly close its shell for defense.
Bivalves are predominantly aquatic organisms that employ suspension-feeding, drawing water into the mantle cavity through an incurrent siphon. They lack a radula, filtering small food particles like plankton from the water using enlarged gills. The foot is typically wedge-shaped and adapted for burrowing into the sediment, though some, like mussels, use strong threads to anchor themselves to hard surfaces.
Cephalopoda
Cephalopoda, meaning “head-foot,” includes octopuses, squids, cuttlefish, and the nautilus. These are the most complex and neurologically advanced mollusks. In these marine predators, the muscular foot is modified into grasping arms and tentacles surrounding the mouth. They are the only class of mollusks with a closed circulatory system, supporting their active, predatory lifestyle.
Most cephalopods have a reduced internal shell or have lost the shell entirely. The nautilus is the exception, retaining an external, chambered shell for buoyancy control. Cephalopods utilize a siphon for jet propulsion, rapidly expelling water from the mantle cavity for swift movement. Their complex nervous system, large eyes, and ability to rapidly change skin color for camouflage or communication make them effective hunters.
Other Significant Mollusk Classes
The class Polyplacophora consists of chitons, recognizable by their armor-like shell composed of eight overlapping plates. These marine animals have a broad foot for suction, allowing them to cling tightly to rocks in the intertidal zone. Chitons use their radula to graze algae off hard surfaces.
The tusk shells belong to the class Scaphopoda, named for their slender, conical shell that is open at both ends. These exclusively marine mollusks are burrowers, typically found buried in the sand with the narrow end of the shell exposed to the water. They possess a rudimentary head but lack eyes, utilizing specialized tentacles called captaculae to capture small prey.
The Monoplacophora are a small group of deep-sea mollusks that possess a single, cap-like shell, visually resembling a limpet. These animals were once thought extinct until living specimens were discovered in the 1950s, revealing a segmented arrangement of gills and muscle scars.
The class Aplacophora includes worm-like, shell-less mollusks that live exclusively in marine environments. These animals are characterized by a lack of a foot and a body covered in calcareous spicules rather than a shell.