Mollusks are invertebrate animals belonging to the Phylum Mollusca. They have a soft body consisting of a visceral mass containing the internal organs, a muscular foot used for locomotion, and, in all mollusks except the bivalves, a head. In addition to the mouth, which may be located at the end of an elongated organ called a proboscis, the head often bears sensory tentacles and eyes. The mouth leads into a chamber known as the pharynx, which usually contains a radula, a flexible, ribbon-like structure consisting of numerous rows of minute teeth, used in obtaining food. In all mollusks the body wall surrounding the visceral mass is prolonged into a fleshy or sheet-like mantle, which usually contains glands that secrete the shell.
The Phylum Mollusca is divided into several classes that differ in the form of the shell, the structure and location of the foot, and the presence or absence of a head bearing sensory organs. Each class is divided into orders, and the orders are in turn subdivided into families. For example, the familiar marine snails known as periwinkles belong to the Class Gastropoda, which contains all the snails; to the Order Mesogastropoda, a very large order containing a variety of marine and freshwater snails; and to the Family Littorinidae, which includes the periwinkles and a few close relatives. Families are divided into genera (singular, genus), and the genera are divided into species. When people speak of a particular kind of mollusk, what they usually have in mind is a species, such as the Common Periwinkle (Littorina littorea), or the Atlantic Bay Scallop (Argopecten irradians). Members of a species have common characteristics, usually look alike, and are capable of interbreeding. All Common Periwinkles, for example, resemble one another. However, within any one area, members of one species may differ in color or shape; these different types, living together and interbreeding, are called forms or morphs. Within a particular species, populations from different geographical areas may vary slightly in color or shape. Such geographically different populations, called subspecies, can and do interbreed with other populations of the species where they come in contact with one another.
This guide covers four classes of mollusks with external shells: the Polyplacophora (chitons), Gastropoda (gastropods), Scaphopoda (tusk shells), and Bivalvia (bivalves). Squids and octopuses are mollusks too; however the North American species do not produce external shells and are therefore outside the scope of this guide. Although nudibranchs are members of the Class Gastropoda, they have no external shells and are also not included here. The animals in each of the four classes that we cover are quite different and produce distinct types of shells. Following is a brief description of anatomical and shell characteristics for each group.
Chitons:
These primitive mollusks are restricted to marine environments.
The Animal:
Chitons have a bilaterally symmetrical, elongately flattened body, with a broad foot. Between the mantle and foot there is a series of gills, which extract oxygen from the water. The head lacks eyes or tentacles, and the mouth has a well-developed radula.
The Shell:
The shell consists of eight flat, overlapping valves, which are usually arched. Between the valves there are joints, which permit the chiton to roll up like a pill bug. The valve at the front is called the head valve, and the hindmost one is called the tail valve; in between there are six intermediate valves. The intermediate valves are divided into several areas that usually have different sculpture. At their outer margin, all valves have projecting insertion plates that fit into the girdle, a band of muscular tissue that surrounds the valves. The insertion plates may be smooth or have slits or fine teeth. The girdle, which often extends over the valves, is either leathery or covered with scales, minute beads, or spines. In some species, there are bundles of bristles in the girdle at the joints between the valves.
Gastropods:
These mollusks have adapted to saltwater and freshwater habitats and to life on land.
The Animal:
Most gastropods possess a flattened, muscular, creeping foot. They usually have a well-developed head with tentacles, eyes, and a mouth, which may be located at the end of a long proboscis. There is usually a siphon present, used for taking in water; some gastropods possess two siphons, one for bringing water into the mantle cavity, and another for expelling wastes. The mouth leads into a pharynx, or threat, that generally has a radula. In adults the internal organs are arranged asymmetrically, and the gills and reproductive and excretory organs are located near the head.
The Shell:
In most gastropods, the shells consists of an elongated tube that increases in size as it winds in a spiral around a central axis, or columella. Each turn, or whorl, of the shell is separated from the next by a suture. At the apex there is often a tiny larval shell ("the nuclear whorls"). At the bottom of the shell there is a body whorl, which is the newest whorl and contains most of the animal's soft parts. The spire of the shell consists of all the whorls above the body whorl. The base of the shell is the bottom part of the body whorl. The body whorl ends in an opening, or aperture, through which the animal's foot and head can be extended or withdrawn. In many groups, the hind part of the animal's foot carries an operculum, a plate that fits into the aperture when the animal has withdrawn into its shell. Gastropods are usually dextral, or right-handed, with the aperture on the right side, but some species are sinistral, or left-handed, with the aperture on the left side.
The columella is either solid or hollow; if hollow, it sometimes opens at the base in a small depression known as an umbilicus. The margin of the aperture is called the lip; the side of the lip farthest away from the columella is called the outer lip and the side near the columella is called the inner lip. At the bottom of the aperture a basal or siphonal canal is sometimes present. It may be an enclosed tube or an open groove; the living animal's siphon lies within it. There may be a smaller canal or notch at the top of the aperture for a second siphons, which expels water and waste products.
The inner lip of the aperture consists of the wall of the body whorl, or parietal wall, and the columellar area near the siphonal canal. Sometimes the parietal wall and columellar areas are covered with a thick or thin calcareous layer, or callus, which often has spiral ridges or teeth.
In some shells the whorls are angled below the suture and there is a flattened area, or shoulder, between the angle and the suture. Or the whorls may be angled or rounded at the periphery, the widest part of the whorl.
Some gastropods are smooth; others have spiral or axial sculpture, or both. Many are covered with a horny layer called a periostracum.
Tusk Shells:
Most of these strictly marine mollusks live in very deep water.
The Animal:
These mollusks are elongate and symmetrical, with a long, conical foot. The head has no eyes or tentacles; the mouth has a radula.
The Shell:
Tusk shells are tubular, open at both ends. At the wide end of the slightly curved shell there is a round or oval opening, the aperture. The smaller opening, the apex, is either smooth or has one or more slits or notches. In some species the shell is swollen in the middle.
Bivalves:
These varied mollusks inhabit fresh water as well as brackish water and oceans.
The Animal:
Bivalves are compressed and are usually bilaterally symmetrical. The animal is enclosed by two typically shell-like valves. Bivalves have no head, eyes, tentacles, radula, or jaws. There is a well-developed foot at the front end. Each valve is lined by a mantle lobe and there may be sensory organs present along the mantle edges. In some groups, water and food particles are carried in and waste products are discharged through openings in the mantle edge; the openings may be in the form of tubular siphons.
The Shell:
The two valves are usually mirror images of one another, although in some species one valve differs noticeably from the other. The upper, earliest part of each valve is called the umbo (plural, umbones). The two valves are joined along the upper margin by a horny, elastic ligament that is usually located behind the umbones, and often by a hinge, which consists of interlocking teeth.
On the interior along the upper margin, most bivalves have teeth, which are very important in identifying many species. Often there are numerous small teeth located along the hinge line. Some groups have distinct central teeth, situated under the umbones; side teeth may be present on either or both sides of the central teeth. In some species the ligament is partly or entirely contained inside the shell, near the hinge line in a groove or on a shelf. Muscle scars are often present on the bivalve's concave inner surface. These are marks that indicate where the living animal's muscles were attached. There may be one near the middle of the shell, or one at each end; smaller scars may also be present. When there are two muscle scars, they are usually joined by a pallial line, where the mantle was attached to the shell; the pallial line may be curved inward in an embayment, or pallial sinus, near the hind muscle scar.
Many bivalves have a byssus, a bundle of fibers attached to the foot that anchors the shell to an external object. In some species there is an opening between the two valves, the byssal gap, through which the fibers pass from the animal to the object to which the animal is attached.
Bivalves may be smooth, or have concentric or radial sculpture, or both. Many species are covered with a horny skin, or periostracum.
In identifying bivalves, it is often necessary to distinguish the front and hind ends and the right and left valves. The front end can usually be identified be referring to the ligament, which generally lies behind the umbones, or by referring to the pallial sinus, which is at the hind end of the pallial line. In scallops, file shells, and wing shells, the byssal gap is located at the front end. Once you have located the front and hind ends of your shell, it is easy to determine which is the right valve and which is the left. Hold the shell upright, with the lower margin on a flat surface. Place the front end away from you. Viewed from this position, the right valve is on the right and the left valve is on the left.
How Mollusks Reproduce and Grow:
Reproduction varies greatly among the mollusks. Many are hermaphroditic, with both sexes united in a single animal. Some hermaphrodites, such as the Eastern Oyster, change their sex one or more times. During the female stage eggs are released and during the male stage sperm are discharged. Other mollusks have separate male and female sexes. Chitons, tusk shells, primitive gastropods, and many bivalves shed large numbers of sperm and eggs directly into the water. The quantity of sperm and eggs helps to assure that fertilization will take place and that young will develop and survive.
More advanced gastropods fertilize the eggs internally, usually after copulation. The female lays the fertilized eggs in gelatinous masses or in capsules. Some species produce capsules singly; they are either attached to a solid object or float freely. Others lay them in compact masses or strings. A capsule can contain just a single egg, or almost 1000. When the young emerge from the gray-green capsules, they either crawl or swim about. If they have a swimming stage, it may last for only a few hours or for as long as four weeks; in rare cases, it lasts a year.
In some groups, the female retains the eggs within the mantle cavity until the young escape as crawling larvae. While the growing larva is still within the egg, a shell gland begins to develop; eventually this gland becomes the edge of the animal's mantle. At first the gland secretes a brownish horny material called conchin; shelly layers are added later. When the young emerge from the egg capsule, they possess a tiny larval shell. This larval shell is usually retained in the adult shell forming the nuclear whorls, but in some species it is cast off. Most often it is sculptured differently than the succeeding whorls and there is a distinct line where the larval shell ends. The shell continues growing at a regular rate until the mollusk reaches sexual maturity; after that growth continues at a diminished pace. As the shell matures, glands at the edge of the mantle secrete shelly material. In tusks and gastropods this is added at the outer lip of the aperture; in bivalves and chitons it is added at the valve margins.
Young and immature shells may look quite different from mature shells. For instance, a young cowry is slender, with a pointed spire. As the cowry approaches maturity, the outer lip extends upward and becomes thickened and the upper ends of both lips eventually cover the spire. And unlike adult queen conchs, which have a large, flaring outer lip, the young have a high spire and a thin outer lip.
