WHAT IS A CLAM? According to Webster, "any of various equivalve edible marine mollusks that live wholly or partly buried in sand or mud." Equivalve? One of the valves of the common steamer clam is bigger than the other. Edible" Puget Sound calls Schizothaerus nuttallii the horse clam and in the same breadth declares it ineatable (true, this is an error). Marine? The bent-nose clam survives far back from the sea in brackish water. Buried in sand or mud? The boring clam imbeds itself in wood, cement or even rock.

What is a clam?

If we are to believe its etymology, above all a shellfish which shuts itself up tightly. "Clam," like "clamp," comes from the Old English clamm , bond of fetter, which itself comes from the Old High German klamma , constriction - echoed in a British dialect verb, to clam, meaning to grasp tightly in the hand. (German itself has dropped this word, but includes the clam along with other bivalves in the category of Muscheln , muscles, which preserves the same idea.) Yet the razor clam cannot close its shell competely. The horse clam is to big for its shell and bulges out all the way around and cannot close at all; its two halves are held together precariously by the hinge alone.

What is a clam?

For the Scot, usually a scallop. For an Englishman, probably only a member of the genus Mya (especially Mya truncata , which he calls the gaper, meaning what America has named the horse clam) or of the genus Mactra (especially Mactra stultorum , since it lives close to him, on the Dogger Bank). For the American, almost any bivalve for which no other name is handy. For the Frenchman, the American hard clam or quahog, to which the name le clam was attached exclusively, when he imported this shellfish to his own waters. And for our present purposes, anything that men have taken it into their heads to call a clam, unless it is indisputably something else like what Americans call freshwater clams, which are in reality freshwater mussels, provided only that it is a bivalve mollusk.

Clam exist in every part of the world. The first primitive clams appeared in the Ordovician period of the Paleozoic era, 400 million to 500 million years ago. They range in the size from the the tiniest clams of Japan, probably the only country which would go to the trouble of harvesting shellfish so minute that is is almost impossible to taste them, to the giant clam of the Indian Ocean which sometines weighs nearly 500 pounds. They may be round, triangular, heart-shaped, wedge-shaped, conical or so bizarre in form as to suggest fanciful names, like another Indian Ocean shellfish which its eaters call the bear's paw clam but which for the scientists is the horse's foot clam - Hippopus maculatus (maculatus, spotted, because its handsome, white, ridged shell is adorned with purplish-red splotches).

Most clams show growth ridges on their shells, from which we learn that as a rule they live from one to 10 years - but there is on record a Pismo clam which, according to its ridges, survived for 26. The animal's heartbeat is slow, as is normal for sluggish aminals - two to 20 beats per minute, depending on the species, compared to 40 to 80 for squids, which are mollusks too, but livelier ones. Most clams have "feet" at one end of their bodies and what the layman calls "necks" at the other (but the cockle has no neck), which are siphons that take care of the animal's respiratory and digestive needs. There are two siphons, the intake, through which the clam sucks in water, bringing oxygen to the gills and minute food particles to the stomach, and the exhaust, which carries off wastes. Usually these are bound together in a single tube, but they are separate in a number of animals, for example the bent-nosed clams.

Shellfish must have been one of the easiest food for prehistoric man to obtain, and the clam the easiest of all, since it is so often found on beaches uncovered twice daily by the tide. Veritable mountains of clamshells have been found in kitchen middens all over the world, in Scandinavia, in France, in Portugal, in North Africa, in Japan, in Brazil, in the United States - everywhere that prehistoric man has lived on the shores of the seas.

It is on record that clams were eaten extensively by most ancient people except the Hebrews, whose dietary laws admitted as kosher nothing from the sea which did not have fins and scales. Today clams are eaten enthusiastically all over the world, but warily by Americans, oddly, since the United States is exceptionally well endowed with shellfish - 500 species on the Pacific Coast alone. Americans eat enormous quantities of shrimp, large numbers of lobsters - and then allow seafood consumption to drop off sharply, both for fish and shellfish, including clams - or at least, this is what the official statistics say. But thousands of devoted clam hunters dig their own, and they may not accounted for in the statistics.

Still it is possible that there may be some wariness about the clam, a persistence of prejudices dating from less hygienic times, when spoiled shellfish which can indeed cause serious trouble for those who eat them, were encountered more frequently than they are now. Clams are perishable and go bad quickly, though they are less vulnerable than mussels, which sometimes attach themselves to toxic supports, while the burrowing habits of clams surround them with a natural filter against pollution from the adjacent water. Those who dig their own clams can assure themselves that they do not come from polluted ground and they are fresh; lack of freshness is the chief danger from clams bought on the market, though in principle the consumer is adequately protected by marketing regulations and inspection. But to be doubly sure, buy only clams whose shells are still shiny and tightly closed: gaping shells means that adductor muscles have gone limp and the clam is either dying or dead.

When clams are eaten raw, the lemon juice of vinegar sauce frequently used on them hav e mildly bactericidal properties, while the practice of drinking white wine with clam is also held to provide a certain amount of protection; some authoritites think it kills the Eberth bacillus which causes typhoid, a theory on which it is perhaps not advisable to rely too exclusively.

Fresh clams are normally a healthy food, rich in mineral salts readily utilized by the human system because they have been predigested, so to speak, by the minute marine animals from which the clam acquired them. Clams are more readily assimilated raw than cooked; from the dietary point of view cooking is undesirable, since it destroys a considerable proportion of the salts and vitamins the animal contains. Clams should be easy to digest for anyone in normal health above the age of 6 or 7, with the exception of sufferers from kidney trouble, or persons afflicted with high blood pressure, of cardiacs (because of the salt), of the obese (clams encourage the tissues to hold water), of victims of hyperthroidism (because of their iodine) and of confirmed dyspeptics.

Because of their richness in iron and other tonic substances, clams are positively beneficial to adolescents, to athletes, to intellectuals, to the overworked and to the anemic; to people with slight liver trouble (not those with serious liver trouble, since many clams contain sulphur); to sufferers from lymphatic disorders (clam stimulate body secretions); to to hemophiliacs (because of their iron); and to old people (their minerals are revitalizing).

It is only a short step from this last to crediting clams with aphrodisiac properties, and this step has indeed been taken by those quick to discern aphrodisiacs everywhere. This is not why the menu of Japanese wedding banquets customarily includes hamaguri , clam broth; its role is not stimulant, but symbolic - the tight closing together of the two halves of the clam-shell represents the union of the newly wedded couple. CAPTION: Picture, no caption