Mollusks with Two Shells
Clams, oysters, and scallops belong to a class of mollusks that have two shells hinged together. These are called valves. The animals themselves are referred to as bivalves. A characteristic of bivalves is the shape of their muscular foot. For this reason, the class is called Pelecypoda which means ‘hatchet-foot’.
Most clams live in salt water, buried in the sand or mud at the sea bottom. Some clams, however, live in fresh water. Clams range in size from the tiny Condylocardia 0.1 mm across to South Pacific giants 1.2 m across.
A clam’s shell is usually off-white and consists of three layers: a tough horny outer layer, a smooth shiny inner layer – the pearly layer, and a middle prismatic layer. The two shells are held together by ligaments. Two adductor muscles open and close the shell.
Like all bivalves, clams are entirely encased in their shells. As a result, they have no real head and no radula. Sense organs are poorly developed, though sensory cells along the edge of the mantle do respond to light and touch. Two long pairs of nerve cords connect the sets of nerve cells, which are found above the mouth, in the digestive system, and in the foot.
Clams obtain both food and oxygen from the water that flows through their bodies. Water enters the clam through the incurrent siphon. Cilia move the water across the respiratory organs, called gills, in the mantle cavity. Gills have a large surface area and an abundant supply of blood to allow for the exchange of gases. Water drawn into the clam by the incurrent siphon has more oxygen than the blood supply of the gills has. For this reason oxygen diffuses from the water to the blood, and carbon dioxide diffuses from the blood to the water. The clam then expels the water through the excurrent siphon.
Clams are filter-feeders. They live on microscopic organic matter carried in the water that flows through their siphons. Mucus on the gills traps the food matter, and cilia push the food-laden mucus on the clam’s mouth. From there the food passes into the stomach. Undigested food particles leave the clam through the anus.
Clams have an open circulatory system. This means that the blood flows through large open spaces, or sinuses, rather than through a system of blood vessels. A three-chambered heart pumps the blood through the clam.
Most clam species have separate sexes. The sperm and eggs are shed into the water, where fertilization takes place. The fertilized egg becomes a trochophore larva that settles on the bottom and develops into an adult clam.
Scallops live in all oceans and range in diameter from 2.5 cm to 15 cm. They have a fan-shaped shell (smooth or sculptured). Scallops have a single large adductor muscle. The scallop’s eyes are located along the edge of the mantle. One scallop may have as many as 100 eyes. Although the eyes cannot focus, they can distinguish between light and dark and can sense passing shadows.
Unlike scallops, oysters cannot move about. Early in its life, an oyster permanently attaches its flat lower shell to a hard surface.
The outer shell is rough in texture, while the inner surface of the shell is smooth and white. If an irritant such as a grain of sand enters an oyster shell, the oyster protects itself by covering the foreign matter with several layers of calcium carbonate. This process forms a pearl.
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