Mollusca , taxonomic name for the one of the largest phyla of invertebrate animals (Arthropoda is the largest) comprising more than 50,000 living mollusk species and about 35,000 fossil species dating back to the Cambrian period. Mollusks are soft-bodied, and most have a prominent shell. The members of this highly successful and diverse phylum are mostly aquatic and include the familiar scallop , clam , oyster , mussel , snail , slug , squid , cuttlefish , octopus , chiton , and a variety of others. Mollusks occupy habitats ranging from the deep ocean to shallow waters to moist terrestrial niches. Certain mollusks, such as clams, squids, and scallops, constitute important food staples, and molluskan shells are highly valued by collectors. In times past these shells were used as money and today are used ornamentally for such items as buttons and jewelry. There are seven classes of mollusks.

Anatomical Features

Although highly diverse, all members of the phylum share certain general features. Most have a well-developed head, which may bear sensory tentacles; in some, like the clam, the head is very reduced.

The Body Wall

All mollusks possess a flexible body wall, which surrounds a body cavity containing the internal organs. The wall, which varies greatly in shape in different species, is usually folded to form a structure called the mantle, which is attached at the top of the body and surrounds it like a tent; the shell is formed on the outside of the mantle. On the underside of the body the wall is usually stretched out to form a thickened mass called the foot. The wall is covered by an outer epidermis and an underlying dermis. The epidermis usually contains gland cells that secrete mucus, which in mollusks has a variety of important uses, such as locomotion, food entrapment, and prevention of water loss. Muscle tissue is found in the body wall, and is particularly plentiful in the foot, which is used for locomotion in most mollusks (although some swim and some are sedentary), and in the mantle in species with reduced shells.

The Shell

The shell is formed by secretions of glandular cells in the mantle. Except in the chitons, the shells of all mollusks are basically similar, differing only in certain mineralogical details. The shell is composed of an outer, prismatic layer containing densely packed cells of calcareous material secreted by the edge of the mantle; and an inner, nacreous layer of thin, laminated plates of calcareous material laid down by the entire mantle surface. When very thin, the nacreous lining of the shell is pearly and iridescent. Layers of this material may form around a grain of sand or other irritant that lodges between the mantle and the shell; this process eventually forms a pearl. Pearl oysters of the genus Pinctada are the most commercially important pearl formers.

The Digestive Tract

The digestive tract of the Mollusca is complex. The foregut region consists of an esophagus and a mouth cavity, which contains a toothed belt called the radula, found in almost all mollusks and peculiar to the phylum. The radula is usually used for scraping food, such as algae, from surfaces. The number and form of radula teeth are highly variable; some species have a single radula tooth while others may have several hundred thousand. In some the teeth are hollow and poison-containing and are used as weapons; other radula modifications exist. The stomachs of mollusks are generally complex, and these, too, differ with the species and according to the feeding habits of the animal.

Respiration

Respiration is through gills called ctenidia (sing. ctenidium), located in the mantle cavity (the space between the mantle and the body wall proper) and varies with the species and with the type of habitat. For example, intertidal marine mollusks are exposed to air and water alternately and must be able to respire in both conditions; terrestrial species have lost their ctenidia, replacing them with lungs that can function in both water and air. Excretion of wastes is through structures called metanephridia and through the body and gill surfaces.

Circulatory and Nervous Systems

The blood circulates through the gill filaments, where exchange of carbon dioxide and oxygen occurs between the blood and the water flowing over the gill surface. Most molluskan blood contains a respiratory pigment called hemocyanin, a copper compound. When oxygenated such blood is bluish in color; when deoxygenated the blood is colorless. Only a few mollusks have hemoglobin in their blood. Blood circulation is variable within the phylum, but is generally mediated by a muscular heart, which distributes the blood to the tissues. Most mollusks possess well-developed sensory organs. The highest degree of development of the nervous system is found in the class Cephalopoda (octopuses, squids, and nautiluses).

Reproduction

Reproduction is sexual and may be simple or highly complex. The fertilized egg develops into a swimming form called a trochophore larva, which is seen also in the development of annelids; this then elongates to become a veliger larva, characteristic of mollusks, and differing in form in the different classes.

Class Aplacophora

The class Aplacophora contains about 300 species of wormlike, deepwater marine mollusks formerly classified with the sea cucumbers in Echinodermata . These mollusks lack shells and in many cases a foot as well. Generally small, they burrow in the upper layer of the sea bottom.

Class Polyplacophora

This class contains about 600 species of sedentary animals commonly known as chitons, marine forms found from shallow waters to depths of about 1,300 ft (400 m). A chiton has a broad foot and a shell consisting of eight overlapping plates.

Class Monoplacophora

This class was created for the genus Neopilina, a mollusk discovered in 1952, when specimens were dredged from a deep trench off the Pacific coast of Central America. Neopilina displays primitive molluskan characteristics; it is the only mollusk with a segmented internal structure and is thought to show a relationship between mollusks and annelids. The animal is about 1 in. (2.5 cm) long and has characteristics of both chitons and gastropods, but does not quite fit into either class. A number of other species in several families have since been discovered or identified as belonging to this class.

Class Gastropoda

This class, containing over 35,000 living and 15,000 fossil gastropod species, comprises the largest class of Mollusca, and includes the limpets , top shells, periwinkles , slipper shells, snails, slugs, sea hares, abalones , nudibranches, or sea slugs , and sea butterflies. Gastropods are primarily marine, but freshwater and terrestrial forms occur. When present, the typical gastropod shell is a three-layered, spiral whorl of calcium carbonate, which varies in color, shape, ornamentation, and size according to the species. Within this shell is the tall, coiled body mass. Some forms, such as slugs, are shell-less and do not have a tall body mass. Gastropod larvae undergo a twisting, or torsion, that brings the rear of the body (mantle cavity, gills, and anus) to a position near the head and results in the twisting of internal organ systems. In many this twisted form is retained by the adult; in others it is partially lost.

There are three subclasses: the Prosobranchia, which contains the majority of gastropods; the Pulmonata, which contains the land snails; and the Opisthobranchia, which includes the sea hares and sea slugs. The latter subclass consists of animals with reduced shells or none at all. Most gastropods are motile, but some, e.g., the slipper shell ( Crepidula ), are sedentary. Some, such as the sea butterflies, swim, and others, including the terrestrial snails, move by means of a well-developed foot.

Many gastropods are herbivores, or plant eaters, with multitoothed radulas for scraping algae from various substrata. Among the carnivorous, or animal-eating, species is the conch , which feeds on smaller mollusks, and the cone shells ( Conus ), which feed on fish and annelid worms that they first paralyze with poison contained in their hollow radula teeth. The poison is also toxic to humans, causing paralysis and sometimes death. Gastropods have a complex nervous system with ganglia.

Reproduction is variable, but most gastropods have separate sexes. Fertilization of the egg occurs in seawater. Some gastropods are hermaphrodites (having both sexes in the same individual) and some are protandric hermaphrodites, i.e., they are male first and become female as they age.

Gastropods are economically valuable as food for many animals, including humans. Some gastropods are serious pests; the common slug, for example, causes much garden damage.

Class Bivalvia

This class, formerly known as Pelecypoda, contains the mollusks known as bivalves, including the mussels, oysters, scallops, and clams. All have shells composed of two pieces known as valves. In most, the valves are of similar size, but in some sedentary species, such as the oysters, the upper valve, which covers the left side of the body, is larger than the lower valve, which covers the right side and is attached to the substratum. Two large muscles, called adductors, hold the valves together at the top of the body. Bivalve shells vary greatly in size, color, and ornamentation. The freshwater seed shells are among the smallest known, being less than .1 in. (c.2 mm) in length, while the shell of the giant clam may exceed 4 ft (120.4 cm) in length.

The foot of bivalves is adapted for burrowing in all species except the sedentary ones, where it is reduced in size. Some species, e.g., the cockles , use the foot to hop about from place to place. Bivalves have a greatly reduced head and no radula. Most have a single pair of large gills used for respiration and for trapping minute food particles. Members of the order Protobranchia use another structure, the proboscis, to feed on bottom detritus. The order Septibranchia contains animals that have lost their gills; they are carnivores or scavengers. Bivalves have a relatively simple nervous system with three pairs of ganglia and two pairs of long nerve cords. An organ of equilibrium, called a statocyst, is present in most. Fertilization normally occurs in surrounding seawater, and most bivalves have separate sexes. All are aquatic, and they constitute an important food source for many animals, including humans.

Class Scaphopoda

This small class of marine mollusks includes 200 species of burrowing animals commonly known as the tusk, or tooth, shells. The shell is long, cylindrical and tooth- or tusk-shaped, and open at both ends. The foot and the small head project from the larger end. Threadlike tentacles hang from the head and are used for gathering the microscopic organisms on which tusk shells feed. Most scaphopods are tiny, usually only several inches (about 6 cm) long. They are found in both shallow and deep waters; they burrow into the bottom, with only the upper opening protruding.

Class Cephalopoda

This class contains the cephalopods , animals commonly known as squid, cuttlefish, octopus, and nautilus . The giant squid is the largest of all mollusks. Most cephalopods are highly adapted for swimming. The body mass is very tall. There is no foot; the lower part of the body wall is drawn out to form a ring of arms, or tentacles, around the head. Among living cephalopods, only the nautilus (subclass Nautiloidea) has a complete external shell; extinct members of the subclass and the extinct ammonites (subclass Ammonoidea) had similar spiral shells. Members of the subclass Coleoidea (the squid, cuttlefish, and octopus), have an internal shell or no shell at all.

All cephalopods are carnivorous and possess a radula and powerful beaks. The nervous system and the sense of vision are highly developed. In most cephalopods the sexes are separate and reproduction requires copulation. Fertilization may occur inside or outside the mantle cavity. Cephalopods are worldwide in distribution and are found in all depths of the ocean. They are an important food staple for many animals, including humans.

Mollusks

Mollusks belong to the phylum Mollusca and make up the second largest group of invertebrates (animals lacking backbones) after the arthropods. Over 100,000 species of mollusks have been identified. Restaurant menus often include a variety of mollusk dishes, such as oysters on the half-shell, steamed mussels, fried clams, fried squid, or escargots.

Mollusks have certain characteristic features, including a head with sense organs and a mouth, a muscular foot, a hump containing the digestive and reproductive organs, and an envelope of tissue (called the mantle) that usually secretes a hard, protective shell. Practically all of the shells found on beaches and prized by collectors belong to mollusks. Among the more familiar mollusks are snails, whelks, conchs, clams, mussels, scallops, oysters, squid, and octopuses. Less noticeable, but also common, are chitons, cuttlefish, limpets, nudibranchs, and slugs.

Classes of mollusks

The largest number of species of mollusks are in the class Gastropoda, which includes snails with a coiled shell and others lacking a shell. The next largest group are the bivalves (class Bivalvia), the chitons (class Amphineura), and octopus and squid, (class Cephalopoda). Other classes of mollusks are the class Scaphopoda, consisting of a few species of small mollusks with a tapered, tubular shell, and the class Monoplacophora. The last of these classes was once thought to be extinct, but a few living species have been found in the ocean depths. Some fossil shells recognizable as gastropods and bivalves have been found in rocks 570 million years old.

Evolutionary patterns

Mollusks provide a clear example of adaptive radiation. Adaptive radiation is the process by which closely related organisms gradually evolve in different directions in order to take advantage of specialized parts of the environment. The gastropods and bivalves were originally marine organisms, living in salt water. They subsequently evolved to take advantage of freshwater habitats. Without much change in their outward appearance, these animals developed physiological mechanisms to retain salts within their cells, a problem they did not face as marine organisms. This new development prevented excessive swelling of their bodies from intake of freshwater.

Several groups of freshwater snails then produced species adapted to life on land. The gills they originally used for the extraction of oxygen

from water were transformed in land snails into lungs, which extract oxygen from air. Similarly, the excretion of ammonia typical of aquatic mollusks evolved into uric acid excretion typical of birds and reptiles.

molluscs, the phylum of invertebrate animals that includes snails, bivalves, and squid. Some are pelagic but most are bottom-living. Many have shells of calcium carbonate (lime). The edible ones, like oysters (Ostraea spp.) and scallops (Pecten spp.), are known generically as shellfish.

Sea butterflies (Pteropoda) are members of the plankton and have very light shells. Their foot (which corresponds to the flat process on which snails crawl) is developed into large swimming flaps that are covered in tiny hairs (cilia). These move across the foot a sheet of mucus that traps even the smallest phytoplankton cells. On the seabed the molluscs are either snail-like or have two shells (bivalves). The snails (Gastropoda) crawl over the bottom on a flattened foot. Many are grazers like the winkles (Littorina spp.), feeding by scraping algae off the rocks with a toothed tongue called a radula. Whelks (Buccinidae) are predatory snails that either bore holes in the shells of bivalves, or dissolve holes by secreting strong acids. Some cone shells (Conidae), which inhabit the coral reefs including the Great Barrier Reef, feed on fish, killing them by shooting poisoned barbs into them; handle these at your peril! Bivalves are mostly sedentary bottom-dwellers either living burrowed in sand or mud like clams (Bivalvia), or anchored to rocks like mussels (e.g. Mytilus spp.) and oysters. They feed on particles they extract from the water by passing it through their large gills across which flow sheets of mucus. Scallops and queens (Chlamys spp.) are free living and can escape danger by swimming by rapidly opening and closing their shells. There are many commercially important species of mollusc, and oysters and mussels are farmed. Scallops are targeted by dredgers and squid are caught in purse seine nets for both food and bait.

Bibliography

Yonge, C., and and Thompson T. , Living Marine Molluscs (1976).www.manandmollusc.net/

M. V. Angel

Mollusk

The Mollusca (mollusks) are a large phylum of animals that includes the snails, slugs, clams, squids, and octopi, among others. Most are marine, many are freshwater, and some snails and slugs are terrestrial. The phylum name refers to their soft, pulpy bodies (mollis means "soft"). In many cases, the body is protected by a hard shell of calcium carbonate—the seashells familiar to beachcombers and "half shells" familiar to oyster lovers.

The shell is secreted by a membrane called the mantle that envelops the body like a cloak. In species without an external shell, such as octopi, the mantle forms an outermost skinlike body covering. The mantle encloses a space, the mantle cavity, which usually contains comblike gills for respiration. In some seemingly shell-less species—squids and cuttlefish—the shell is embedded in the mantle and can be found only by dissection. Most mollusks also have a radula. In snails, this is a tonguelike belt equipped with a few hundred to thousands of chitinous teeth, used to scrape food from surfaces such as rocks.

The most behaviorally sophisticated of all invertebrate animals are the cephalopod mollusks: the octopi, squids, cuttlefish, and nautilus. Cephalopods have long, flexible arms, equipped in most cases with suckers for prey capture. They are active swimmers; some have eyes remarkably similar to human eyes; they have more complex brains than any other invertebrate; and, correspondingly, they exhibit remarkably subtle social behaviors and learning capabilities.

see also Animalia; Ocean Ecosystems: Hard Bottoms; Ocean Ecosystems: Open Ocean; Ocean Ecosystems: Soft Bottoms; Vision

Kenneth S. Saladin

Bibliography

Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.

Rupert, E. E., and R. D. Barnes. Invertebrate Zoology, 6th ed. Philadelphia, PA: W. B. Saunders, Co., 1994.

molluscs The molluscs are an extremely diverse and numerous, mostly marine, invertebrate phylum, second only to arthropods in numbers of species. They include a range of organisms, both living and fossil, that seem at first sight to be so different as to be unrelated. These range from slow-moving herbivores, scavengers, and predators, such as chitons and slugs and snails, to suspension-feeders such as tusk-shells and bivalves, and their extinct ancestors the rostroconchs, and the most complex and highly organized group the cephalopods, which includes squids, octopus, and the fossil ammonoids and belemnoids.

Although no one character is unique to every mollusc, there is a suite of characters that serves to unite the disparate members of this phylum. Characteristically, molluscs are free-living organisms with a calcareous shell secreted by a sheet of tissue termed the mantle, which in turn encloses a space termed the mantle cavity. The mantle cavity surrounds the viscera of the animal and opens externally, allowing the discharge of waste products and the free circulation of water across the gills. It is the variable development of these characters in the different classes that is used to separate them.

Molluscs are biostratigraphically important animals and are widely used for zoning marine rocks from the Upper Palaeozoic to the Cenozoic.

David K. Elliott

Mollusca (molluscs) A phylum of coelomate (see COELOM) invertebrates comprising classes which are morphologically quite diverse, including the Amphineura, Bivalvia, Cephalopoda, Gastropoda, Monoplacophora, and Scaphopoda. They are fundamentally bilaterally symmetrical, with metameric segmentation almost or completely absent. Shell material is secreted by the mantle covering the visceral hump. Calcium carbonate shells may be univalve, bivalve, or plated. Some groups have shells modified to serve as internal skeletons. Most have a well-differentiated head, with radula and salivary glands. A ventral muscular foot is very common. The anus and kidneys open into the mantle cavity. The alimentary canal usually has a buccal mass. A heart and an arterial and venous system are present, and there are paired ctenidia for respiration. The nervous system is ganglionic (see GANGLION). Most Mollusca are aquatic. They first appeared in the Lower Cambrian. There are six extant and three extinct classes, with more than 80 000 species.

Mollusca (molluscs) A very diverse phylum of invertebrates which have a common body plan modified in various ways. There is usually a shell, secreted by a series of tissues (called the mantle), and beneath this there is the body, which contains a space (mantle cavity) in which lie the gills. The body plan is modified to give the major classes of the phylum. One class, the Monoplacophora, shows signs of internal segmentation. The Amphineura (chitons) have a bilaterally symmetrical shell of eight plates. Scaphopoda have tapering, curving shells which are open at both ends. Bivalvia have the soft parts enclosed between two shells. Gastropoda usually possess a coiled univalve shell. The most advanced of the molluscs are the Cephalopoda, which possess either internal or external, chambered shells. Other groups are also assigned to the Mollusca, but only cephalopods, bivalves, and gastropods have a good geologic record.

mollusc Any of more than 80,000 species of invertebrate animals in the phylum Mollusca. They include the snails, clams and squids, and a host of less well-known forms. Originally marine, members of the group are now found in the oceans, in freshwater and on land. There are six classes: the gastropods, chitons, univalves (slugs and snails), bivalves, tusk shells, and cephalopod. The mollusc body divides into three: the head, the foot, and the visceral mass. Associated with the body is a fold of skin (the mantle) that secretes the limy shell typical of most molluscs. The head is well developed only in snails and in the cephalopods. The visceral mass contains the internal organs. The sexes are usually separate but there are many hermaphroditic species.

mollusc A member of a phylum (Mollusca) of invertebrate animals, most of them aquatic, comprising classes which are morphologically quite diverse. Molluscs are fundamentally bilaterally symmetrical. Calcium carbonate shell material is secreted by the mantle covering the visceral hump and shells may be univalve, bivalve, plated, or, in some groups, modified to serve as internal skeletons. Most have a well differentiated head, with radula and salivary glands. A ventral muscular foot is very common. Molluscs first appeared in the Lower Cambrian. There are six extant and three extinct classes, with more than 80 000 species.

Mollusca A phylum of soft-bodied invertebrates characterized by an unsegmented body differentiated into a head, a ventral muscular foot used in locomotion, and a dorsal visceral hump covered by a fold of skin – the mantle – which secretes a protective shell in many species. Respiration is by means of gills (ctenidia) or a lunglike organ and the feeding organ is a radula. Molluscs occur in marine, freshwater, and terrestrial habitats and there are six classes, including the Gastropoda (snails, slugs, limpets, etc.), Bivalvia (bivalves, e.g. mussels, oysters), and Cephalopoda (squids and octopuses).

mol·lusk / ˈmäləsk/ (chiefly Brit. also mollusc) • n. an invertebrate of a large phylum (Mollusca) that includes snails, slugs, mussels, and octopuses. They have a soft, unsegmented body and live in aquatic or damp habitats, and most kinds have an external calcareous shell. DERIVATIVES: mol·lus·kan / məˈləsˌkən/ (or mol·lus·can) adj.

molluscs Marine bivalve shellfish with soft unsegmented bodies; most are enclosed in a hard shell; they include abalone, clams, cockles, mussels, oysters, scallops, whelks, winkles.

mollusc XVIII. — F. mollusque, f. modL. mollusca, n. pl. of L. molluscus soft, f. mollis soft (see prec.).

mollusk see Mollusca .

molluscs See MOLLUSCA.

molluscs See MOLLUSCA.

mollusc •Basque, Monégasque •ask, bask, cask, flask, Krasnoyarsk, mask, masque, task •facemask •arabesque, burlesque, Dantesque, desk, grotesque, humoresque, Junoesque, Kafkaesque, Moresque, picaresque, picturesque, plateresque, Pythonesque, Romanesque, sculpturesque, statuesque •bisque, brisk, disc, disk, fisc, frisk, risk, whisk •laserdisc • obelisk • basilisk •odalisque • tamarisk • asterisk •mosque, Tosk •kiosk • Nynorsk • brusque •busk, dusk, husk, musk, rusk, tusk •subfusc • Novosibirsk •mollusc (US mollusk) • damask •Vitebsk •Aleksandrovsk, Sverdlovsk •Khabarovsk • Komsomolsk •Omsk, Tomsk •Gdansk, Murmansk, Saransk •Smolensk •Chelyabinsk, Minsk •Donetsk, Novokuznetsk •Irkutsk, Yakutsk

mollusk •Basque, Monégasque •ask, bask, cask, flask, Krasnoyarsk, mask, masque, task •facemask •arabesque, burlesque, Dantesque, desk, grotesque, humoresque, Junoesque, Kafkaesque, Moresque, picaresque, picturesque, plateresque, Pythonesque, Romanesque, sculpturesque, statuesque •bisque, brisk, disc, disk, fisc, frisk, risk, whisk •laserdisc • obelisk • basilisk •odalisque • tamarisk • asterisk •mosque, Tosk •kiosk • Nynorsk • brusque •busk, dusk, husk, musk, rusk, tusk •subfusc • Novosibirsk •mollusc (US mollusk) • damask •Vitebsk •Aleksandrovsk, Sverdlovsk •Khabarovsk • Komsomolsk •Omsk, Tomsk •Gdansk, Murmansk, Saransk •Smolensk •Chelyabinsk, Minsk •Donetsk, Novokuznetsk •Irkutsk, Yakutsk