Structure of bivalvesThe bulk of the bivalve body lies toward the hinge and is covered by a structure called the mantle. This grows outward on either side of the body and forms a lining to the inside of both shell halves. Indeed, it is the tissue lining or epithelium of the mantle that is responsible for secreting the shell. A muscular foot – flattened from side to side, extensible, and often used in burrowing – projects from the body into the cavity separating the halves of the mantle.
If one shell and half the mantle of a bivalve are removed the most obvious structures exposed are the gills which, besides serving as organs of respiration, are used in feeding. Bivalves are filter feeders. Tubular extensions of the mantle form a pair of siphons, one for the entry of a current of water, the other for its exit. This current is maintained by the beating action of hair-like cilia with which the gills are liberally covered. Water is drawn in through tiny pores in the gills and eventually ejected by way of the discharge siphon. Particles of food are strained out and trapped in a layer of sticky mucus, a substance similar in consistency to human saliva, that covers the gill surfaces.
From there, trapped food passes to the edge of the gills farthest away from the hinge, and larger particles, such as silt, fall off into the mantle cavity. The remainder, still entangled in a string of mucus, moves forward to the palps surrounding the mouth. Further sorting then takes place and selected particles enter the mouth and pass into the stomach for digestion.
Digestion and movement of bivalvesA gland surrounding the stomach is the chief organ of digestion in bivalves, but projecting into the stomach, from an intestinal pouch at one end of it, is the crystalline style, a gelatinous rod containing carbohydrate-splitting enzymes. Cilia in the pouch make this rod rotate continually and its free end rubs against an area of the stomach known as the gastric shield. As the crystalline style is worn away so the enzymes it contains are released into the stomach contents. Digestion is completed in the intestine, which follows a tortuous course to the anus. This opens into the mantle cavity near the discharge siphon, thus ensuring the removal of feces in the outgoing water.
Paired excretory organs lie on either side of the heart and a "kidney" in each extracts waste products and discharges them, via a bladder, into the gill passages and thence to the exterior. Bivalves have a three-chambered heart whose single muscular ventricle pumps blood into arteries that run to the foot, mantle, and the body organs. Many of the tissues of the body are permeated not by capillaries, as in higher animals, but by blood spaces or sinuses. Sinuses in the foot enabling sand-burrowing – the foot swells and elongates as extra blood is pumped into them. Blood then returns via a series of veins to the paired atrial chambers of the heart.
Bivalves have simple reproductive systems with paired gonads and no glands. The sexes are separated and most allow the spermatozoa and ova to escape into the water where, in marine species, they form part of the plankton. Some freshwater bivalves brood their young, while others release a larval stage called a glochidium that passes through a brief parasitic phase before assuming an independent adult existence.
Methods of movement used by bivalvesSedentary bivalves, such as mussels, spend their lives attached to a form base such as a rock by a bundle of threads called the byssus. These threads are secreted by a gland in the foot and anchor their owner with surprising strength – a necessity if the animal is to avoid being swept away by the force of the waves. A more common mode of life, found in bivalves such as the cockles and razor shells, is sand-burrowing by means of a mobile foot. The depth of burrowing varies among species but the deeper penetrating forms often have long siphons for obtaining surface water for respiration and feeding.
In contrast to the sand-burrowing species, the shipworm burrows into wood by back and forth rotation of its shells – it literally drills its way forward. Some piddocks can even burrow into rock. The piddock must periodically cease borrowing and project the edge of the mantle on to the shell surface to deposit a fresh layer of calcium carbonate.
In most bivalves the head is reduced or absent and sensory organs are generally insignificant. But the animals do react to waterborne chemicals and to light. Some of the razor shells in particular are extremely sensitive to vibration and will vanish rapidly down their burrows if they are disturbed.
Related category ZOOLOGY
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