Major Groups of Mollusks The mollusks represent a diverse group of marine, freshwater, and terrestrial invertebrates, including such varied forms as snails, chitons, limpets, clams, mussels, oysters, octopuses, squid, cuttlefish, tusk shells, slugs, nudibranchs, and several highly modified deep-sea forms. They all have one anatomical feature in common, the presence of a shell at some stage in the life cycle. Although most mollusks have a shell as adults, the octopus, squid, and deep-sea forms do not. They do however have a small, shell-like structure, called a shell gland, present for a short time during embryonic development

Chiton Chitons range in length from less than 2.5 to 30 cm (1 to 12 in), but all sport a coat of mail made of eight jointed, symmetrical shell plates. This flexible structure enables the animal to roll into a ball if its muscular foot becomes detached from the rock to which it clings.

Gastropod Anatomy The development of a coiled shell and the process of torsion, or the systematic twisting of the body over time, led gastropods to evolve from bilateral to assymetrical body structures. This diagram shows the generalized internal anatomy of a gastropod.

Snail Anatomy The development of a coiled shell and the process of torsion, or the systematic twisting of the body over time, led snails to evolve from bilateral to assymetrical body structures. This diagram shows the generalized internal anatomy of a snail.

Snail Some groups of snails have shells that coil to the left (left-handed), while others have shells coiling to the right (right-handed). In some groups both right- and left-handed members are present. The shell affords protection from predators and the rigors of the environment. When threatened, the snail pulls its entire body inside. A small, circular structure called the operculum closes securely behind the snail to form a tight seal that keeps out predators and keeps in moisture.

Tortoiseshell Limpet The tortoiseshell limpet is a species of limpet common to waters off the northeastern coast of the United States. Limpets, single-shell marine gastropods, live along rocky coasts all over the world. Most limpets cling to rocks while grazing for food, usually algae. Some limpet species are consumed by humans while others are used as fishing bait.

Cone Shell The cone shell is one of 400 to 500 cone species of mollusks that can be harmful to humans. Cones are carnivores, equipped with powerful nerve poisons. They search out prey with a muscular, retractable proboscis that carries a mouth, a salivary gland, and teeth.

Sea Slug Lacking the external shell that typifies other gastropods, sea slugs such as the lavender species pictured here have evolved other mechanisms of defense. The coloration of some matches their environment to camouflage the animal from predators. The tissues of others contain foul-tasting substances, prompting any inexperienced captor to spit them out quickly and avoid their often bright pigmentation the next time

Sea Slug Sea slugs are marine gastropods that have adapted without a main feature of the gastropod family, a shell. Most sea slugs move with their single foot by using waves of muscular contraction. They are the largest species of the gastropod family, weighing up to 13 kg (about 29 lb).

California Sea Hare Sea Hares are mollusks that inhabit shallow ocean waters around the world. Their prominent tentacles are sensitive to touch and smell. When threatened, certain species squirt purple ink to form a protective cloud.

Bubble shells are mollusks with a thin, sometimes colorful shell. They live in shallow marine waters in temperate zones

Generalized Anatomy of a MolluskThere are around 50,000 species of mollusks, ranging from tiny snails less than 1 cm (0.25 in) in length to giant squids, which reach a length of 18 m (60 ft). Despite this great variation in size, most mollusks have the same basic body structure. Most mollusks have a glandular body covering, called the mantle. In some mollusks, such as clams and snails, the mantle secretes a hard shell. Most mollusks have a large muscular organ, called the foot, which is used for burrowing or for moving over the ground or sea bottom. Many mollusks feed by means of the radula, a flexible organ that bears many sharp teeth. Mollusks use gills to absorb nutrients from water and release waste products from cells.

Mussel A view of the interior of a mussel shell reveals the mussel’s visceral mass containing the gills and internal organs. The circular white structure at the upper right hand corner of the visceral mass is the adductor muscle, which helps to keep the shell tightly sealed against predators or during low

The prominent growth rings found on the outer surface of the clam shell are useful in determining the clam’s age.

Scallop Scallops are bivalve mollusks related to oysters, clams, and mussels. The circular white muscle tissue dominating the interior of the shell has a flavor and texture that makes the scallop one of the most prized of all seafoods.

Generalized Anatomy of an OysterEnclosed within a thick, sturdy shell, the soft body of an oyster is adapted for filtering minute planktonic organisms from the surrounding water. The gills filter and collect food that the stomach then digests. The mantle is a thin membrane that covers the body and lines the inside of the shell. The adductor muscles and the hinge between the two halves of the external shell help keep the shell closed

Close-up of a Mussel This close-up shot of a mussel shows its mouth and the muscle tissue surrounding it..

Giant Clam The giant clam, which lives on coral reefs in the Indian and Pacific oceans, is the second-largest extant mollusk, weighing up to 227 kg (about 500 lb) with the shell. The giant clam feeds on marine bacteria. The bright-blue giant clam here has burrowed into the ground, as many clams do.

Pearls in Shell Pearls are valued as gemstones although they are not actually minerals. They are made from the same material that covers the inside of mollusk shells, an iridescent substance called nacre, more commonly known as mother-of-pearl. When an irritant such as a parasite enters its shell, a mollusk secretes a nacre coating to make them less dangerous to its soft tissues. Eventually, the nacre coatings transform the irritant into a pearl.

Zebra Mussel Accidentally introduced from Europe in the mid-1980s, zebra mussels have spread throughout many North American lakes and rivers. These hardy freshwater mussels grow and reproduce quickly. They attach to almost any submerged surface, frequently coating or clogging boat hulls, fishing equipmen

Generalized Anatomy of a SquidThe squid, a representative cephalopod, shows several variations on the mollusk body plan. With a name meaning “head-footed,” the cephalopod’s “foot” is divided into sucker-bearing arms, or tentacles, specialized for drawing food into the animal’s beaklike jaws. Another cephalopod adaptation is the mantle. Highly muscular, it forces water from the cavity through the tubular siphon to propel the animal quickly through the water. Most notable perhaps is the internal shell, greatly reduced from the large external shells of other mollusks. This shell is entirely absent in octopuses.

Cuttlefish The cuttlefish is an open water, or pelagic, species that swims using its marginal fin. Cuttlefish feed on fish and invertebrates and are themselves the object of commercial fisheries in many parts of the world. Cuttlebone, a supporting rod of calcium carbonate found only in cuttlefish, is used as a polishing agent as well as a calcium and salt supplement for captive birds and other animals

Chambered Nautilus A cutaway view of the shell of the chambered nautilus reveals the compartments that housed the nautilus when it was smaller. These smaller chambers, now connected together by a small calcified tube, regulate the buoyancy of the nautilus as it swims along. The compartments are filled with nitrogen gas, which is produced by the nautilus.