Mollusca (redirected from Mollusks)

Mollusca (məlŭs`kə), 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 six 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 Amphineura

This class contains two very different kinds of mollusk. The subclass Polyplacophora 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. The subclass Aplacophora contains about 100 species of wormlike, deepwater marine mollusks.

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.

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 Pelecypoda (Bivalvia)

This class 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. Pelecypod 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 pelecypods 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. Pelecypods 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. Pelecypods 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 pelecypods 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.

---

Mollusca

A major phylum of the animal kingdom comprising an extreme diversity of external body forms (oysters, clams, chitons, snails, slugs, squid, and octopuses among others), all based on a remarkably uniform basic plan of structure and function. The phylum name is derived from mollis, meaning soft, referring to the soft body within a hard calcareous shell, which is usually diagnostic. Soft-bodied mollusks make extensive use of ciliary and mucous mechanisms in feeding, locomotion, and reproduction. Most molluscan species are readily recognizable as such.

The Mollusca constitute a successful phylum; there are probably over 110,000 living species of mollusks, a number second only to that of the phylum Arthropoda, and more than double the number of vertebrate species. More than 99% of living molluscan species belong to two classes: Gastropoda (snails) and Bivalvia. Ecologically, these two classes can make up a dominant fraction of the animal biomass in many natural communities, both marine and fresh-water.

Classification

The phylum Mollusca is divided into seven distinct extant classes, three of which (Gastropoda, Bivalvia, and Cephalopoda) are of major significance in terms both of species numbers and of ecological bioenergetics, and one extinct class. An outline of their classification follows.

• Class Monoplacophora (mainly fossil; but
• one living genus Neopilina)
• Class Aplacophora
• • Subclass Neomeniomorpha
  • Subclass Chaetodermomorpha
• Class Polyplacophora
• Class Scaphopoda
• Class Rostroconchia (fossil only)
• Class Gastropoda
• • Subclass Prosobranchia
  • • Order: Archaeogastropoda
    • Mesogastropoda
    • Neogastropoda
  • Subclass Opisthobranchia
  • • Order: Bullomorpha (or Cephalaspidea)
    • Aplysiomorpha (or Anaspidea)
    • Thecosomata
    • Gymnosomata
    • Pleurobranchomorpha
    • (or Notaspidea)
    • Acochlidiacea
    • Sacoglossa
    • Nudibranchia (or Acoela)
  • Subclass Pulmonata
  • • Order: Systellommatophora
    • Basommatophora
    • Stylommatophora
• Class Bivalvia (or Pelecypoda)
• • Subclass Protobranchia
  • Subclass Lamellibranchia
  • • Order: Taxodonta
    • Anisomyaria
    • Heterodonta
    • Schizodonta
    • Adapedonta
    • Anomalodesmata
  • Subclass Septibranchia
• Class Cephalopoda
• • Subclass Nautiloidea
  • Subclass Ammonoidea (fossil only)
  • Subclass Coleoidea
  • • Order: Belemnoidea
    • Sepioidea
    • Teuthoidea
    • Vampyromorpha
    • Octopoda

Functional morphology

The unique basic plan of the Mollusca involves the different modes of growth and of functioning of the three distinct regions of the molluscan body (see illustration). These are the head-foot with some nerve concentrations, most of the sense organs, and all the locomotory organs; the visceral mass (or hump) containing organs of digestion, reproduction, and excretion; and the mantle (or pallium) hanging from the visceral mass and enfolding it and secreting the shell. In its development and growth, the head-foot shows a bilateral symmetry with an anterioposterior axis of growth. Over and around the visceral mass, however, the mantle-shell shows a biradial symmetry, and always grows by marginal increment around a dorsoventral axis. It is of considerable functional importance that a space is left between the mantle-shell and the visceral mass forming a semi-internal cavity; this is the mantle cavity or pallial chamber within which the typical gills of the mollusk, the ctenidia, develop. This mantle cavity is almost diagnostic of the phylum; it is primarily a respiratory chamber housing the ctenidia, but with alimentary, excretory, and genital systems all discharging into it.

Generalized model of a stem mollusk (or archetype) in side view

In looking at any mollusk, it is important to realize that whatever the shape of the shell, it is always underlain by the mantle, a fleshy fold of tissues which has secreted it. The detailed structure of the shell and of the mantle edge (with three functionally distinct lobes) is also consistent throughout the Mollusca. The shell is made up of calcium carbonate crystals enclosed in a meshwork of tanned proteins. It is always in three layers: the outer periostracum, the prismatic layer and the innermost or nacreous layer.

Each of the eight classes of the Mollusca has a characteristic body form and shell shape. Two classes are enormous (Gastropoda and Bivalvia), one of moderate extent (Cephalopoda), the others being minor by comparison. The Gastropoda constitute a diverse group with the shell usually in one piece. This shell may be coiled as in typical snails—that is, helicoid or turbinate—or it may form a flattened spiral, or a short cone as in the limpets, or it may be secondarily absent as in the slugs. Most gastropods are marine, but many are found in fresh waters and on land; in fact, they are the only successful nonmarine mollusks.

The Bivalvia are a more uniform group, with the shell in the form of two calcareous valves united by an elastic hinge ligament. Mussels, clams, and oysters are familiar bivalves. The group is mainly marine with a few genera in estuaries and in fresh waters. There can be no land bivalves since their basic functional organization is as filter feeders. The third major group, the Cephalopoda, includes the most active and most specialized mollusks. There is a chambered, coiled shell in Nautilus and in many fossil forms; this becomes an internal structure in cuttlefish and squids, and is usually entirely absent in octopods.

A diversity of gill patterns have evolved in the major molluscan groups, paralleling the evolution of the mantle-shell patterns. The more advanced gastropods show reduction from a pair of aspidobranch ctenidia to a single one, and from that to a one-sided pectinibranch ctenidium (or comb gill), and subsequently to no gill at all in the pulmonate snails. The bivalves show enlargement of gill leaflets to longer filaments and their subsequent folding into the true lamellibranch condition, used in filter feeding. The gills in the cephalopods, while still structurally homologous, are modified with new skeletal elements to resist the stresses of water pumping by muscles.

Besides the gills, the other organs of the mantle cavity (termed collectively the pallial complex) again show morphological and functional consistency throughout the main groups of the Mollusca. The ctenidia form a curtain functionally dividing the mantle cavity into an inhalant part (usually ventral) containing the osphradia (pallial sense organs which sample the incoming water), and an exhalant part (usually dorsal) containing hypobranchial glands and both the anus and the openings of the kidney and genital ducts.

The cardiac structures of mollusks are also closely linked to the pallial complex. If there is a symmetrical pair of ctenidia, there will be a symmetrical pair of auricles on either side of the muscular ventricle of the heart; if one ctenidium, one auricle; if four ctenidia, four auricles. Note that body fluids in mollusks are almost all blood, just as body cavities are almost all hemocoel.

The respiratory pigment is usually hemocyanin in solution, so that neither circulatory efficiency nor blood oxygen-carrying capacity is high. However, mollusks are mostly sluggish animals with low metabolic (and hence respiratory) rates.

Uniquely molluscan is the use of cilia in “sorting surfaces,” which can segregate particles into different size categories and send them to be disposed of in different ways in several parts of the organism. In a simpler type of sorting surface, the epithelium is thrown into a series of ridges and grooves, the cilia in the grooves beating along them and the cilia on the crests of the ridges beating across them. Thus, fine particles impinging on the surface can be carried in the direction of the grooves, while larger particles are carried at right angles. Such sorting surfaces occur both externally on the feeding organs and internally in the gut of many mollusks. For example, on the labial palps of bivalves, they are used to separate the larger sand grains (which are rejected) from the smaller microorganisms which then pass to the mouth.

The range in levels of complexity of molluscan nervous systems is comparable to that found in the phylum Chordata. The four-strand nervous system with one pair of tiny ganglia found in chitons is not dissimilar to the neural plan in turbellarian flatworms. In contrast, the nervous system and sense organs of a cephalopod like an octopus are equaled and exceeded only by those of some birds and mammals. In the majority of mollusks the nervous system is in an intermediate condition. In mollusks other than cephalopods, the main effectors controlled by the nervous system are cilia and mucous glands. In fact, apart from the muscles which withdraw it into its shell, the typical mollusk is a slow-working animal with little fast nervous control or quick reflexes. In the brain of modern cephalopods, paired ganglia have been fused into a massive structure, with over 300 million neurons and extensive “association” centers providing considerable mnemic and learning capacities.

In all primitive mollusks, the sexes are separate, and external fertilization follows the spawning of eggs and sperm into the sea.

In more advanced mollusks, eggs are larger (and fewer), fertilization may become internal (with complex courtship and copulatory procedures), and larval stages may be sequentially suppressed. A remarkably large number of mollusks (including many higher snails) are hermaphroditic. Although some are truly simultaneous hermaphrodites, many more show various kinds of consecutive sexuality. Most often the male phase occurs first, and these species are said to show protandric hermaphroditism.

Distributional ecology

Mollusks are largely marine. The extensive use of ciliary and mucous mechanisms in feeding, locomotion, reproduction, and other functions demands a marine environment for the majority of molluscan stocks. Apart from a small number of bivalve genera living in brackish and fresh waters, all nonmarine mollusks are gastropods.

Despite the soft, hydraulically moved bodies and relatively permeable skins typical of all mollusks, some snails are relatively successful as land animals, although they are largely limited to more humid habitats. The primary physiological requirements for life on land concern water control, conversion to air breathing, and temperature regulation.

In the sea, all classes of mollusks are found, and all habitats have mollusks. Protobranchiate bivalves are found at depths of over 30,000 ft (9000 m). Although ecologically cephalopod mollusks are limited to the sea, there are sound reasons for claiming modern cephalopods as the most highly organized invertebrate animals. The functional efficiencies of jet propulsion and of massive brains in squid, cuttlefish, and octopuses have not been paralleled in their other physiological systems.

In addition to the extreme diversity of external body form exhibited by different mollusks, they show a remarkable diversity in their ecological distribution and life styles. However, the basic molluscan plan of structure and function always remains recognizable. See Bivalvia, Cephalopoda, Gastropoda