More than 100,000 species of the phylum Mol-lusca are known to exist, and scientists continue to find many new species every year. In addition, the fossils of about 100,000 extinct species have been found. The three largest molluskan classes are the Gastropoda, which consists of snails, slugs, and limpets; Bivalvia, which includes oysters, mussels, and clams; and Cephalopoda, containing squids, cuttlefish, and octopuses. Mollusk fossils have been found in rocks about 550 million years old and are often well preserved because of their hard shells. But despite their age and the intact fossils, scientists still do not fully understand the origin of mollusks and their relationship to other invertebrates.
The three main mollusk classes look very different, but all follow the same basic plan. The main bulk of the body contains the internal organs and is called the visceral mass. All mollusks have a skinlike organ called a mantle—a fleshy extension of the body wall—which hangs down on each side of the visceral mass. The space between the mantle and the visceral mass is called the mantle cavity. The mantle secretes the shell, which is present in most mollusks. In mollusks with no outside shell, the mantle forms a tough cover around the body organs.
Almost all aquatic mollusks breathe with the aid of comblike gills in the mantle cavity called ctenidia, which are covered with many small, hairlike structures called cilia, whose rhythmic movement draws water over the gill surface. Blood that flows through the gill filaments takes up oxygen from the water current, and carbon dioxide from the blood diffuses out of the gill filaments.
The kidneys and anus open into the mantle cavity, and waste is also carried away by the water the mollusk exhales. In some species, the edges of the mantle may be joined to form tubes called siphons, which create a one-way flow of water through the mantle cavity. Many mollusks also have sensory cells on the edge of their gill membrane. These organs, called osphradia, are thought to be sensitive to chemical signals and able to detect the juices of prey or other food sources. They also determine the level of sediment in the incoming water, too much of which would block the delicate gill filaments.
Ancient gastropods had a mouth and anus at opposite ends of the body, as do the larvae of present-day gastropods. But early in the development of larval gastropods a remarkable change takes place—the visceral mass twists through 180°. This process, known as torsion, is important because it brings the mantle cavity from a lower backside position to an upper frontal position. The shell then only needs one opening, and the mantle cavity provides a space into which the young gastropod can withdraw for protection.
The problem of fouling by waste released over the head is avoided by having holes in the shell that direct excrement away from the head (as is found in keyhole limpets and the abalone), or by shifting the anus to the side. A one-way water flow through the mantle cavity also helps avoid this problem.
The left and right sides of the gastropod body grow at different rates because of the spiral shape of the shell. The organs on one side do not develop, and the visceral mass, mantle, and shell become spirally coiled. This development also makes the long digestive system of the gastropod more compact.
Most aquatic gastropods breathe by means of gills, but land-dwelling snails and slugs (subclass Pulmonata) lack gills and have modified the mantle cavity into a lung. Air is moved in and out of the mantle cavity by raising and lowering the mantle, which is moist and has a rich blood supply—essential features for gaseous exchange to take place.
Pulmonates show another adaptation for a land-dwelling existence in that nitrogen-containing waste is excreted as uric acid crystals. Aquatic gastropods, in contrast, excrete nitrogen-containing waste as ammonia, which is toxic in concentrated form and therefore requires a large amount of water to dilute it.
Gastropods are generally considered to be inhabitants of damp places, but the terrestrial adaptations and, more importantly, the possession of a shell, into which the animal can withdraw and seal itself off from the environment, have enabled them to invade such unlikely regions as deserts. Some pulmonates have secondarily returned to an aquatic existence, but they still breathe by means of a lung.
Most gastropods move on a flat sole, which in many species has a large pedal gland, which secretes mucus onto the surface over which the sole moves. Locomotion is achieved by waves of muscular contractions that pass down the foot. In the sea hare, the foot is formed into folds, or parapodia, which may be used for swimming. In the sea butterflies, or Pteropoda, the parapodia are drawn out into winglike structures.
Most gastropods feed using a radula—a serrated band of teeth—to rasp off fine particles from their food. Many species, such as land snails and limpets, feed on vegetation. Others, such as whelks, are carnivorous. They detect their prey by means of a structure that is sensitive to both chemical signals and touch, called an osphradium. The mouth of carnivorous gastropods often lies on an extension of the head, called the proboscis. The radula of these species has fewer, larger teeth than the plantfeeding gastropods. In the cone shells it has become a stalklike stinger with which they stab and inject a poison into their prey. Some carnivorous gastropods eat oysters. They do so by secreting a chemical that softens the oyster shell, then using the radula to wear it away. By alternately softening and rasping the shell, the carnivore makes a hole in the shell, through which it can insert the proboscis to eat the oyster.
Aquatic gastropods have separate sexes, but land snails and slugs are hermaphroditic. Male gastropods either shed sperm into the water, where they fertilize eggs from the females, or deposit them into the female with a penis. The female gastropod lays eggs singly, in strings, or in a thin shell. The females of some species carry their eggs in their bodies until they hatch.
In some gastropods, the young pass through two stages: the first is the “trochophore” stage, in which the larva is roughly spherical with a band of cilia for movement. It later develops a shell to become the veliger larva. In other gastropods, the trochophore stage remains in the egg, which hatches to produce the veliger, and in some species, both the trochophore and veliger stages remain in the egg.
Some aquatic snails are hosts to human platyhelminth parasites, such as the liver fluke and the blood fluke, the cause of the disease bilharzia.
The class Bivalvia includes clams, oysters, and mussels, and is characterized by the possession of a shell that is divided into two halves joined by an elastic ligament.
Bivalves are mainly marine, although some live in fresh water. The foot is adapted for burrowing mainly in mud and sand, but some species can bore into hard material. The shipworm, for example, drills into wood using the roughened edges of its shell, and other species even bore into rocks.
The body of a bivalve is long and laterally flattened, surrounded on each side by two lobes of the mantle. Each lobe secretes a shell called a valve. The gills hang from the roof of the mantle cavity and lie on each side of the body. They are used for collecting food as well as for breathing. Bivalves have no head— it would be impossible to perceive stimuli while buried in the sand—so the sense organs occur on the edges of the mantle, and these detect light and test water currents.
Most bivalves are filter-feeders, straining very small food particles from the water. The gills are highly modified for this purpose and secrete mucus in which food particles are trapped. A complex arrangement of cilia draws water into the mantle cavity, sorts the particles, and carries food to the mouth. Flesh folds near the mouth (palps) sort the trapped particles, and nonfood material is passed out with water while food particles pass into the stomach. There, a gelatinous rod called the crystalline style is rotated by cilia and rubs against the stomach wall and style sac, releasing enzymes that partly digest the food particles. Further digestion occurs within the cells of the digestive glands that surround the stomach.
Most bivalves use their foot as a burrowing organ. The two valves close and water is pushed out of the mantle cavity, which loosens the mud and makes the movement of the foot easier. Some species have serrated shell edges that also break up the mud and facilitate burrowing. Blood is then pumped into irregular channels, called blood sinuses, in the tissue of the foot. This action makes the top of the foot swell and anchors the animal. Contractions of the foot muscles then pull the animal down.
Most bivalves are sluggish, but some, such as the razor clam, have a thin, streamlined shell and large foot and can burrow very rapidly. Scallops use jet propulsion to swim by clapping the two valves of the shell together. Other species, such as oysters, which cement themselves to rocks, and mussels, which attach themselves to surfaces by strong threads of organic material, are unable to move.
In most bivalves, males and females shed sperm and eggs into the water, where fertilization produces a trochophore larva, which develops two valves to form the veliger larva. The veliger swims around in the open sea for approximately two weeks before settling on the bottom to become an adult. In the freshwater mussel, however, the female carries eggs on her gills. Sperm enter the mantle cavity with the water currents and fertilize the eggs there. The larvae then leave the mantle cavity and live as parasites on the gills offish until they develop into adults.
The cephalopods represent the highest stage of molluskan evolution. The three main groups belong to the subclass Coleoidea and are the squids (order Teuthoidea), the cuttlefish (order Sepioidea), and the octopus (order Octopoda).
All cephalopods are adapted to a free-swimming life in the sea, but only one living species—the nautilus—has an outer shell. Most other cephalopods have an inner chambered shell, which varies from species to species. In the cuttlefish, it is called a cuttlebone, and in the squid, a pen. The shell lies along the dorsal parts of the animal’s body and is mostly filled with gas, which provides lift in water. Toward the tail end, however, the shell is filled with liquid, which adds weight to that part.
In the cuttlefish and the squid, the shell has been reduced to a small plate, and their bodies are elongated and streamlined. The octopus, however, is less streamlined and lacks a shell. The edges of its mantle are fused together, forming a bag around the body, from which the head protrudes, surrounded by tentacles. The octopus usually crawls around the seabed using the suckers on its tentacles, but it is capable of jet propulsion when fast movement is necessary. The octopus propels itself by pumping water into the mantle cavity and forcing it out through a funnel below the head.
AH cephalopods are carnivorous. Cuttlefish and squid catch their prey with two long tentacles with suckers at the tips, and with arms— 8 in the cuttlefish and 10 in the squid—that are equipped with suckers all along their length. The octopus has 8 tentacles of equal length, any one of which can be used to grab prey. The octopus bites its prey with its two horny jaws, and its poisonous saliva then enters the wound and paralyzes the prey. Large pieces of flesh are torn off, pulled into the mouth by the radula, and swallowed.
Cephalopods have the most highly developed brains of all invertebrates. Squid and octopus can learn and remember information. Most cephalopods rely on their good eyesight to detect food and enemies. In addition, sensory cells on the octopus’s tentacles detect minute concentrations of chemicals, which also alert it to the presence of prey or to possible danger.
Many cephalopods can camouflage themselves in seconds, changing color by the expansion or contraction of small bags of pigment in the skin. Some species also have an ink sac—a pouch off the intestine—which releases a cloud of black liquid through the anus when the animal is alarmed, behind which it can make its escape.
Male cephalopods produce sperm that are enclosed in a case called the spermatophore. One of the male’s arms is modified to transfer the spermatophore into the mantle cavity of the female, where the eggs are fertilized. The female may stick them onto rocks, where they develop into adults without passing through a trochophore or veliger stage.