MOLLUSCS

Common snail

The large group of animals called molluscs live on land and in both fresh and salt water. They constitute the phylum Mollusca, a major group of animals known to have as many as 100,000 living species and more than 50,000 fossil forms. Most molluscs, including snails, clams, oysters, and mussels, have shells. A major group called the cephalopods, however, including octopuses, squids, and cuttlefish, have shells that are either greatly diminished or absent. One of the most distinctive anatomical features characteristic of molluscs is a true coelom, a body cavity that contains most of the vital organs. The digestive tract, heart, liver, and reproductive organs are all housed inside the coelom. Many of the lower invertebrates, in contrast, have no coelom.

Mantle, in biology, outer fold of tissue that envelops the body and lines the shell of a mollusc; the shell is produced by secreting glands in the mantle.

Two other features that are characteristic of the molluscs and absent in most other groups of animals are the visceral, or buccal, mass and the mantle. The visceral mass is the main body of the animal and contains all the vital organs. The name mollusc, which is derived from a Latin word meaning "soft," refers to this large, soft body mass. The mantle is a thick covering of tissue that surrounds the visceral mass and has glands that secrete the shell, if the animal has one. Many of the aquatic molluscs also have another distinctive feature gills that are enclosed within a cavity formed by the mantle.

Giant squid (also called devil fish), a mollusc of the class Cephalopoda and the genus Architeuthis.

Most molluscs are only a few inches in length and weigh less than a quarter of an ounce (7 grams). However, they can vary considerably in size: some are minute and others enormous. The giant clam (genus Tridacna) may reach 3 feet (1 meter) in length and weigh more than 440 pounds (200 kilograms). Giant squids (genus Architeuthis) found in the North Atlantic off the coast of Newfoundland have been reported to be longer than 50 feet (15 meters) and weigh more than 4,400 pounds (2,000 kilograms).

Habitat and Locomotion

Most molluscs are marine animals; some are found in shallow coastal areas and others live in the deepest parts of the ocean. Most live in the bottom sediments, though the cephalopods are primarily free-swimming species. Some molluscs have been found at depths of 2,200 feet (670 meters) or more, in regions where molten volcanic sediments come in contact with the cold ocean waters. Molluscs also inhabit most freshwater and terrestrial habitats on Earth. The majority are free-living species that feed on algae that they scrape from underwater surfaces, food particles collected from the water, or larger prey that they have captured. Some species are parasitic.

Shelled molluscs move around by means of a foot that they extend from the shell to contact the substrate, or the surface on which they live. The foot is a muscular extension of the ventral, or lower, side of the animal, and in many species it can be completely withdrawn into the shell. Octopuses and squids, on the other hand, are open-water molluscs that propel themselves by forcing water out of a funnel-shaped apparatus located at the front end of the body. The mantle of squids is flattened into fins that are used to propel the animal slowly backward or forward.

Reproduction

Trochophore, free swimming ciliate larvae typical of molluscs and worms.

Mollusc reproduction is highly varied and can be very complex. Some species lay eggs, whereas others are live-bearers. Some of the marine species produce enormous numbers of eggs that develop into larvae known as trochophores. Most terrestrial and freshwater snails hatch from eggs as tiny replicas of the adult, without passing through a free-swimming stage. Some molluscs are parthenogenetic; that is, the female produces eggs or young directly without the need for the eggs to be fertilized by a male. Thus, in some species, males are unknown. Parental care has been observed in some groups of molluscs, such as the whelks, which deposit their eggs in grooves on the shells of both the female and male so that the eggs are protected from predators.

Body Structures and Functions

The basic body structure, consisting of a foot, visceral mass, and mantle, is similar in most molluscs The foot is located below the visceral mass. The external shell consists of three layers. The thin outer layer, called the periostracum, is made of a tough hornlike material and serves to protect the lower layers. The thicker underlying layer, called the prismatic layer, is composed of calcium carbonate. In some species the inner surface of the shell, called the nacreous layer, is composed of very thin, alternate layers of calcium carbonate and hornlike material. Its translucent surface in some species is referred to as mother-of-pearl.

The organ systems of molluscs are highly developed and include a complex nervous system. Their circulatory system is open; that is, blood is pumped from the heart into large sinuses, where it supplies oxygen to the various tissues. From the sinuses the blood passes to the gills or pulmonary tissues to be resupplied with oxygen, then returns to the heart. The heart consists of a ventricle, which pumps blood to the body, and auricles, which receive the blood before it returns to the ventricle. The digestive system consists of a mouth, oesophagus, stomach, intestines, and anus. A large gland connected to the stomach contains digestive enzymes and functions like a liver. Most molluscs have a mouth structure called a radula. The radula has numerous tiny teeth along the sides and can be extended from the mouth to scrape algae from rocks. Excretion of nitrogenous waste materials is accomplished by means of organs called nephridia or similar structures that function as kidneys.

Classes of Molluscs

The phylum Mollusca is divided into six living classes. These classes are defined primarily on the basis of body plan, type of shell, and modifications of the internal organs. The following classes are discussed in order from simplest to most complex.

Monoplacophorans. The class Monoplacophora was first known from fossil records, and all species were assumed to have been extinct for the past 350 million years. In 1952, however, marine biologists discovered ten live specimens at a depth of almost 13,000 feet (4,000 meters) in the Pacific Ocean off Costa Rica. These animals, placed by zoologists in the genus Neopilina, are segmented, suggesting that the molluscs may be closely related to the segmented annelid worms.

Amphineurans. The class Amphineura comprises about 850 species of marine organisms including the chitins, or "coat-of-mail shells." Chitins have a slender, ovoid shell consisting of eight plates arranged in a series. They have bilateral symmetry divided longitudinally, the two halves of the body are similar. The mouth is located at one end and the anus at the other. Chitins are usually found in shallow coastal waters, though some species live at great depths in the ocean. The foot is located on the lower surface and is used to carry the animal over rocks in search of algae, a primary source of food. The foot is also used as a suction device to help prevent predators from dislodging the animal from the surface of a rock. Included in this class of molluscs are the worm like solenogasters (subclass Aplacophora). These are among the most primitive molluscs and are the only ones that completely lack a mantle, head, foot, and nephridia. Chitins and other amphineurans are of little commercial value because of their small size and relative scarcity in most regions.

Gastropods, molluscs of the class Gastropoda, including snails and slugs.

Gastropods. The snails and slugs belong to the largest class of molluscs, Gastropoda, which contains more than 40,000 species. Most gastropods have a single shell that covers the viscera and is coiled in some manner. Some gastropod shells are extraordinarily elaborate and ornamental. Other members of the class, however, such as the terrestrial slugs and the strikingly coloured sea slugs, are without a shell entirely. Although during their larval stages the gastropods are bilaterally symmetrical, they normally become spiral-shaped in their later development. Gastropods have a complete digestive system, and the head and sense organs are well developed.

Most gastropods live in the ocean and are a major source of many of the seashells collected on coastal shores. In addition, freshwater snails are found worldwide, and an array of terrestrial species can be found throughout the world. Some of the terrestrial forms are arboreal, or tree climbing. Many gastropods are parasites that attach themselves to other animals, such as fish.

Some land snails have completely lost their gills, and the mantle cavity serves to exchange oxygen and carbon dioxide, similar to a lung. These air-breathing forms are known as pulmonate snails. Land snails and slugs have a slime gland in the forward end of the foot that secretes a mucus over which the animal glides as it moves. A silvery trail of dried mucus marks the progress of these creatures. Most pulmonates have a spiral shell that may be attenuated or flattened. Also included in the class Gastropoda are the whelks, conchs, periwinkles, and abalones.

Members of the gastropod class are of extensive economic value. Seashells from the shores and coastal waters of most oceans of the world are prized by collectors and are sold commercially in many regions. Some snails, particularly those in Europe, are gourmet delicacies known as escargots.

Unfortunately many of the freshwater species of snails act as intermediate hosts for parasites that infect vertebrates, including humans. The disease schistosomiasis is caused by a parasitic flatworm that spends a portion of its life cycle in a snail. The larvae of the flatworm leave the snail and enter the water. They then penetrate the skin of any mammal, including humans, that they encounter. Attempts to control schistosomiasis in tropical regions are often directed toward eliminating the freshwater snails.

Scaphopods. Members of the class Scaphopoda are called tusk shells because their soft bodies are enclosed in slender, tapering shells. The shell is open at both ends, but because one end is larger than the other, the shell has the appearance of a long tooth. Tusk shells live in the ocean sediments of both coastal and deep water areas. They move around very little. There are only about 350 species of tusk shells.

Pelecypods. There are more than 7,000 species of bivalve molluscs in the class Pelecypoda. These animals have two shells, or valves, that are usually similar in shape and size and are hinged along one side. The two shell halves can close up completely as a means of protection. Two large muscles called adductors connect the shells and make it difficult to pry the shells open. In some species, when the shells are open, the foot can be extended so that the animal can move slowly over the rocks or sediments. Many species, however, are quite sedentary and move little during their adult lives. Pelecypods are the only major group of molluscs that do not have a radula for scraping algae from rocks. Instead, they gather food particles from the water by straining the water across their gills. In their adult stage the pelecypods also lack a head, though one may be present in earlier stages.

Cockle, bivalved mollusc, especially common in tropical waters; the European cockle (Cardium edule) is a valuable shellfish; related to the oyster.

Scallops and cockles belong to this class, as do clams, oysters, and mussels, which make up a major portion of the world's seafood. Some pelecypods, particularly oysters, occur in very high concentrations, and millions are harvested annually in many parts of the world. The crushed shells of oysters and clams are the primary road-building material in many coastal regions. Oysters are also of value in the production of pearls, and freshwater mussels were once a major source of material for making buttons.

These creatures are not available in unlimited supply, however. Oyster drills, small gastropods that burrow into live oyster shells and kill the oyster, pose a serious threat to the oyster industry in some areas. In addition several species of freshwater clams in the United States are endangered and can be found only in certain stream and river systems.

Cuttlebone, dietary supplement for caged birds providing calcium; comes from the internal shell of the cuttlefish.

Cephalopods. The approximately 1,000 species in the class Cephalopoda differ in general appearance and behaviour from most other molluscs Generally the shell is lacking or greatly diminished. In many species a small remnant of the shell is embedded in the mantle. This internal shell is called the pen in squids and the cuttle in cuttlefish. Cephalopods are highly mobile marine animals that actively seek large prey. Some cephalopods rank as the most active and largest invertebrate animals in existence. Various fishes and crustaceans are their chief sources of food.

Sepia ink, drawing ink obtained from the ink sac of cuttlefish and squid.

Because cephalopods lack a protective shell, the visceral mass of the smaller animals is quite vulnerable to attack. For protection, they rely on speed and mobility to help them flee from their predators. In addition some cephalopods produce an inky substance called sepia that they squirt into the surrounding water when confronted by an enemy. The clouded water serves as a distraction or cover that allows the cephalopod to escape. At other times many cephalopods may rapidly change colour in order to camouflage themselves.

Cephalopods have a distinctive head region usually surrounded by eight or ten large tentacles. The mouth is located in the centre of the disk from which the tentacles radiate. Octopus tentacles bear large suckers that are used to grasp prey or to attach the animal firmly to a rock. (The popular tales of giant cephalopods that use their tentacles to prey on humans is a gross exaggeration.) The internal anatomy contains cartilaginous support structures, including a brain case, or skull. They also have large brains and complex nervous systems.

Octopuses and squids are major commercial food molluscs in many parts of the world. Small squids are popular as fish bait. The calcareous cuttle of cuttlefish is placed in cages of pet birds as a dietary supplement and bill sharpener. The cuttle can also be ground into a powder that is used as a polishing agent. In addition cuttlefish ink has been used by artists to produce the brownish pigment known as sepia.

Nautilus, cephalopod mollusc of genus Nautilus; adult lives in innermost chamber of approximately 36-chambered, coiled shell, 10 in. (25 cm) in diameter; found in seas surrounding East Indies and Fiji Islands; last surviving genus of ancient order Nautiloidea.

The chambered nautilus is unusual among the living species of cephalopods because it has a shell, though a number of extinct species are known to have had shells as well. The shell of the nautilus is divided into compartments. As the animal grows, a new compartment is formed and the older, smaller chamber is sealed off. The nautilus has a large number of tentacles extending from the head.

Assisted by J. Whitfield Gibbons, Senior Research Ecologist and Professor of Zoology, Savannah River Ecology Laboratory, University of Georgia.