Every mollusk shares a basic body plan built around four key features: a mantle (a sheet of tissue covering the body), a muscular foot, a visceral mass containing the organs, and in most cases a radula for feeding. These structures unite an enormous group of roughly 86,600 described living species, from garden snails to giant squid, making mollusks the second-largest animal phylum after arthropods. What’s remarkable is how wildly different mollusks can look from one another while still sharing this underlying blueprint.
The Mantle: The One Universal Feature
If there’s a single feature that every mollusk possesses, it’s the mantle. This sheet of tissue covers the body’s internal organs and gills, and in most species it secretes a shell. The mantle is so fundamental that biologists consider it the true defining trait of the phylum. Bivalves like clams have it. Shell-less slugs have it. Octopuses, which look nothing like a snail, have it.
The mantle encloses a space called the mantle cavity, which houses the gills (called ctenidia) and serves as the main site for gas exchange. In bivalves, the ctenidia have evolved beyond simple breathing organs into complex feeding structures. Tiny hair-like cilia on the gills beat rhythmically to pull water through the mantle cavity, filtering out food particles suspended in the water while simultaneously absorbing oxygen.
How the Shell Forms
Most mollusks produce a shell made primarily of calcium carbonate. The mantle’s outer layer of cells secretes this shell in stages. First, it lays down a thin organic membrane on the outside. Then, in the fluid-filled space between that membrane and the mantle itself, calcium and carbonate ions crystallize into the hard shell material. Proteins, chitin, and other organic molecules act as scaffolding, guiding where and how the mineral crystals form. This process produces structures as varied as a clam’s paired valves, a snail’s spiral, and a chiton’s eight overlapping plates.
Not every mollusk keeps its shell. Slugs have reduced theirs to a small internal remnant or lost it entirely. Octopuses and most squid have no external shell at all. The nautilus is the only living cephalopod that still carries a full external shell.
The Muscular Foot
The foot is the mollusk’s primary means of getting around, though its shape varies dramatically depending on the lifestyle. In snails, it’s a broad, flat surface that glides along on a trail of mucus. In bivalves, it’s a wedge-shaped muscle that extends between the two shell halves to burrow into sand or mud. In cephalopods, the foot has been radically reshaped into the arms and tentacles used for grasping prey and the funnel used for jet propulsion.
Beyond locomotion, the foot serves hydraulic roles in many species. Gastropods (snails and slugs) use their circulatory system to inflate and extend the foot, making it function partly as a hydraulic tool for movement and anchoring.
The Radula: A Ribbon of Teeth
Most mollusks feed using a radula, a flexible ribbon made of chitin and studded with rows of tiny teeth. The animal moves this ribbon back and forth across food surfaces, scraping, rasping, or cutting depending on the species. A snail grazing algae off a rock is dragging its radula across the surface like a miniature cheese grater.
The radula’s structure varies enormously to match different diets. Species that feed on hard surfaces like rock tend to have teeth with a gradient of mechanical properties, harder at the tips and more flexible at the base, which reduces breakage. Chitons and certain limpets even reinforce their radula teeth with iron-based minerals for extra durability. Soft-substrate feeders, by contrast, have teeth that are more uniform in their mechanical properties since they don’t need the same wear resistance.
The major exception is bivalves: clams, mussels, oysters, and scallops have no radula at all. They’re filter feeders that rely entirely on their gills to capture food from the water, so a scraping tongue would serve no purpose. They’ve also lost their head entirely, along with the tentacles and sensory organs that come with it.
The Visceral Mass
The visceral mass is the compact bundle of internal organs (digestive system, reproductive organs, kidneys) sitting above the foot and typically enclosed by the mantle. It’s not a flashy feature, but it’s part of the basic body plan that distinguishes mollusks from other invertebrates. In snails, the visceral mass is coiled up inside the spiral shell. In bivalves, it sits between the two valves. In cephalopods, it forms the main body (what most people would call the “head” of a squid, though it’s technically the visceral mass and mantle wrapped together).
An Open Circulatory System, With One Exception
Most mollusks have an open circulatory system. Blood is pumped by the heart into arteries, but instead of flowing through a network of tiny capillaries, it empties into open spaces called sinuses and lacunae, bathing the tissues directly before draining back to the heart. This system works well enough for slow-moving animals like snails and clams, and it doubles as a hydraulic system for inflating the foot and other soft tissues.
Cephalopods are the exception. They’ve evolved a fully closed circulatory system where blood stays inside vessels the entire time, flowing through capillaries that allow efficient oxygen and nutrient delivery. This is a major reason cephalopods can be such fast, active predators. Their hearts generate at least a hundred times more power than a gastropod heart of comparable size. The trade-off is that they’ve given up the hydraulic functions that other mollusks rely on, but with their muscular arms and jet-powered movement, they don’t need them.
Nervous Systems From Simple to Extraordinary
The range of nervous system complexity across mollusks is staggering. At the simplest end, chitons and worm-like aplacophorans have a ladder-shaped arrangement of nerve cords with neurons distributed along their length. Snails and clams have clusters of nerve cells (ganglia) positioned near the organs they control, connected by long nerve fibers.
Cephalopods have taken this basic plan and expanded it by four to five orders of magnitude. An octopus has roughly half a billion neurons, organized into a brain of about 40 interconnected lobes arranged in a ring around the esophagus. Two-thirds of those neurons aren’t even in the brain itself; they’re distributed through the arms, giving each arm a degree of independent control. This distributed nervous system lets an octopus coordinate the complex, fluid movements of eight boneless arms while simultaneously processing visual information and solving problems.
Eight Living Classes
The phylum Mollusca contains eight recognized classes of living animals, each representing a different variation on the shared body plan:
- Gastropoda (snails, slugs, nudibranchs) is the largest class, covering everything from land snails to sea slugs. Most have a single coiled shell, though many lineages have reduced or lost it.
- Bivalvia (clams, mussels, oysters, scallops) have two hinged shells, no head, and no radula. They’re mostly filter feeders.
- Cephalopoda (octopuses, squid, cuttlefish, nautiluses) are active predators with large brains, camera-like eyes, and arms derived from the ancestral foot.
- Polyplacophora (chitons) have eight overlapping shell plates and cling to rocks in intertidal zones.
- Scaphopoda (tusk shells) have a single tube-shaped shell open at both ends and live buried in marine sediment.
- Monoplacophora are rare deep-sea animals with a single cap-shaped shell, once thought to be extinct until living specimens were found in the 1950s.
- Caudofoveata and Solenogastres are small, worm-like, shell-less marine animals that burrow in or glide across the ocean floor.
Why Such Different Animals Share a Phylum
It can seem strange that an oyster cemented to a rock and an octopus solving a puzzle box belong to the same phylum. The connection becomes clearer when you look at what they share beneath the surface: a mantle that defines the body cavity, a visceral mass of organs, a foot (however modified), and developmental patterns that link them together. Bivalves lack a radula and a head, but they still build their shells using the same mantle-driven calcium carbonate process as every other shelled mollusk. Octopuses have lost their shell, but their mantle cavity, their gill structure, and even the nerve cord organization in their arms echo the anatomy of far simpler relatives like chitons.
The roughly 86,600 known living species likely represent only a fraction of actual mollusk diversity, with estimates reaching as high as 200,000 species. Around 800 to 1,000 new species are still described every year. What holds them all together is that same fundamental body plan: a soft body, a mantle, and a set of organ systems that have been stretched, compressed, duplicated, or lost across half a billion years of evolution, but never fully reinvented.