Class Bivalvia

Members of the class Bivalvia include aquatic mollusks, such as clams, oysters, and scallops. These mollusks are called bivalves because, as Figure 35-6 shows, their shell is divided into two halves, or valves, connected by a hinge. A bivalve can close its shell by contracting the powerful adductor muscles that are attached to the inside surface of each valve. When the adductor muscles relax, the valves open.

Each valve consists of three layers that are secreted by the mantle. The thin outer layer protects the shell against acidic conditions in the water. The thick middle layer of calcium carbonate strengthens the shell. The smooth, shiny inner layer protects the animal's soft body.

Shell

Heart Kidney

Adductor muscle

Mouth

Adductor muscle

Intestine

Mantle

Anus

Shell

Heart Kidney

Adductor muscle

Anus

Adductor muscle

Mouth

Excurrent siphon Incurrent siphon

Intestine

Mantle

Excurrent siphon Incurrent siphon

In contrast with gastropods, which move about in search of food, most bivalves are sessile. Some species extend their muscular foot into the sand and fill the foot with hemolymph to form a hatchet-shaped anchor. The muscles of the foot then contract, pulling the animal down into the sand. As an adaptation for a sessile existence, bivalves usually are filter feeders. They are the only mollusks without a radula.

Bivalves lack a distinct head. Their nervous system consists of three pairs of ganglia: one pair near the mouth, another pair in the digestive system, and the third pair in the foot. The ganglia are connected by nerve cords. Nerve cells in the ganglia receive information from sensory cells in the edge of the mantle that respond to touch or to chemicals in the water. Some bivalves also have a row of small eyes along each mantle edge. Stimuli detected by these sensory structures can trigger nerve impulses that cause the foot to withdraw and the shell to close.

Clams

Clams are bivalves that live buried in mud or sand. The mantle cavity of a clam is sealed except for a pair of hollow, fleshy tubes called siphons, which you can see in Figure 35-7. Cilia beating on the gills set up a current of water that enters through the incurrent siphon and leaves through the excurrent siphon. As the water circulates inside the clam, the gills filter small organisms and organic debris from the water. The filtered material becomes trapped on the gills in a sticky mucus that moves in a continuous stream toward the mouth. Water passing over the gills also exchanges oxygen and carbon dioxide with the hemolymph.

Most species of clams have separate sexes. Marine clams reproduce by shedding sperm and eggs into the water, and fertilization occurs externally. The fertilized egg becomes a trochophore that eventually settles to the bottom and develops into an adult.

figure 35-7

In this illustration, one valve has been omitted to show a clam's anatomy. The internal structure of a clam is typical of most bivalves.

Materials 2-3 bivalve shells, colored pencils, paper

Describing a Mollusk

Materials 2-3 bivalve shells, colored pencils, paper

Procedure

1. Using colored pencils, draw a bivalve shell on a sheet of paper.

2. Use Figure 35-7 to help you locate and label the adductor muscle scars, the mantle cavity, and the hinge area on the bivalve.

Analysis Describe the shell of the bivalve, including its color, its ridges, the appearance and texture of the mantle, and the location of the hinge area.

In some clam species, adults may weigh 200 kg (440 lb) and be more than 1 m (3.3 ft) across. In most freshwater clams, eggs are fertilized internally by sperm that enter through the incurrent siphon. The larvae that develop are discharged into the water through the excurrent siphon. If they contact a passing fish, they may live as parasites on its gills or skin for several weeks before settling to the bottom.

Other Bivalves

Oysters are bivalves that become permanently attached to a hard surface early in their development. Some are grown commercially as food or as sources of cultured pearls. Scallops can move through the water by repeatedly opening their valves and snapping them shut. This motion expels bursts of water, creating a form of jet propulsion. The teredo, or shipworm, is one of the few bivalves that does not filter-feed. Instead, it bores into driftwood or ship timbers and ingests the particles that are produced by the drilling. The wood cellulose is broken down by symbiotic bacteria that live in the shipworm's intestine.

figure 35-8

Most of the body of cephalopods is made up of a large head attached to tentacles. The tentacles of cephalopods, such as this cuttlefish, Sepia latimanus, surround their mouth. The streamlined body of many cephalopods enables them to swim rapidly in pursuit of prey.

figure 35-8

Most of the body of cephalopods is made up of a large head attached to tentacles. The tentacles of cephalopods, such as this cuttlefish, Sepia latimanus, surround their mouth. The streamlined body of many cephalopods enables them to swim rapidly in pursuit of prey.

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