Mollusk

Burrowing Molluscs  (Gastrochæna Pholodia ).

The mollusks, or "shellfish" (phylum Mollusca) are a homogeneous group of soft-bodied, unsegmented, typically bilateral, elaborately organized animals, mainly aquatic and marine, whose origin—probably as a derivative from a wormlike stock—is lost in the mists of geologic prehistory. In most cases the mollusks secrete from a larval gland an external shell which serves as skeleton and defensive armor; are bisexual and produce eggs, or if monœcious are never self-fertilizing. They possess a heart, and blood circulation (usually colorless); breathe in the water by means of gills, or, in the air, by a primitive kind of lung; have a nervous system and senses in some cases of a high order; the organs are normally paired, and protected by a general covering integument called the "mantle"; and the creeping species move by a muscular, elastic, ventral organ styled the "foot," while the swimmers are provided with a variety of swimming organs. Mollusks vary in size from all but microscopic minuteness to a bivalve weighing 500 pounds or a squid half as big as a right whale. They occur in all seas at all depths, abound in fresh waters both swift and stagnant, and are scattered over the earth wherever vegetation flourishes.

The phylum Mollusca is divided into five classes, as follows, and it will be noticed that four of thenames refer to the locomotive organ or "foot" (Greek pous, "foot"):

I. Pelecypoda, the Mussels—mollusks inclosed in a bivalve shell fastened by a muscular hinge, the adjacent part of the valves being generally more or less toothed; the foot is as a rule roughly comparable to the shape of an ax head.

II. Amphineura, the Chitons—flattened, bisymmetrical mollusks whose shell consists of eight crosswise, overlapping plates.

III. Gastropoda, Snails, whelks, etc.—mollusks that crawl on the flat undersurface of the body, or distensible foot.

IV. Scaphopoda, Tusk shells—mollusks that possess a long tubular shell open at both ends; with their small and elongated foot they are supposed to dig  into the mud in which they live.

V. Cephalopoda, Cuttlefishes, and Octopods—mollusks with tentaclelike "arms" arranged about the mouth, and either an external or internal shell. These are the highest in rank.

The Oyster and Its Relatives

The lowest in rank of these classes is the Pelecypoda, containing the "bivalves"—mussels, clams, oysters, and the like, in which the shell is in two parts or valves hinged together over the "back" of the animal, and attached to it on each side by a powerful muscle, the "adductor," by the contraction of which the shell may be tightly shut. Within the shell the body is enveloped in a "mantle," or fleshy membrane falling like a cloak on each side; and from it is secreted the outer shell, which grows by additions to its ventral margin. These additions are in a general way annual, so that the concentric lines of growth on its exterior are an indication of the years of the mollusk's life, which is slow in growth, and long-lived. The interior of the shell is usually pearly, and marked with microscopic rugosities, which, by breaking up the light, as if by innumerable prisms, gives the iridescence so beautiful in the pearl oyster, the fresh-water unios and many others. These pearly layers are called "nacre."

Bivalves were formerly classified in conchology as Acephala, because they have no proper head, but at the posterior end are two openings of tubes, provided with cilia. In one, the cilia induce a constant current of water which after leaving the gills brings into the animal's stomach floating microscopic food, both plants and animals, including eggs and larvæ, where it is captured and assimilated while water is ejected through the other (dorsal) pipe. This food includes bacteria, and if the mollusk lives and feeds in water polluted by sewage, or otherwise containing germs of disease, it becomes dangerous as human food; hence oysters and clams exposed to such bad conditions ought never to be sent to market because of the disease germs remaining in them.

In bivalves such as the oysters, horse mussels, piddocks, and others that are sedentary, and often fixed in place, or that, like river mussels, scallops, etc., move about freely, the mouth tubes are short; but many bivalves, as the clams, pinnas, razor fish and so forth, bury themselves in the sand of the bottom, by means of the strong distensible foot protruding from the forward end of the shell. These are provided with a double-barreled tube, called the "siphon," which may be contracted within the protection of the closed shell, or may be stretched out several inches; the animal may thus sink its body deep in the sand while its siphon reaches to the surface and inhales food-bearing water. The little squirts of water often seen jetting out of the beach at low tide as one walks along it are from clams so buried, and which, alarmed by the vibration of one's footsteps, hastily eject the water and withdraw their siphons.

The old name for this class, Lamellibranchiata, referred to the gills, two of which, on each side, hang like curtains inside the mantle and between it and the saclike body containing the viscera; when the shell is open they are laved by the water, and extract from it, by some quality hardly understood, the oxygen necessary to regenerate the blood that flows through them; and, in addition, respiration is carried on through the skin.

The nervous system is very primitive, and the sense organs consist of an otocyst (a minute sac in which a hard particle floats in a liquid) in the foot, by which, it is believed, a sense of direction is had, and which also serves the purpose of an ear; an organ that tests the water; and in some, as the scallop, rudiments of eyes are situated on the margin of the mantle. Most pelecypods are of two sexes, but some, such as our American oysters, are hermaphrodite. Eggs in vast number, and a cloud of spermatozoa, are thrown out in midsummer, and a little of the latter succeeds in reaching and so fertilizing fortunate eggs, but almost all merely serve as food for the host of mollusks, worms, sea anemones and what not that subsist on such provender. The few fertilized larvæ drift about and happily escaping multiplied perils, presently settle to the bottom to attach themselves to some fixed object, or otherwise get a chance to grow big enough to defy ordinary enemies. Some interesting variations in this rather commonplace larval history occur, however, in certain families.

It will be possible to name only a few of the most useful or otherwise conspicuous bivalves, beginningwith the oyster, concerning which an immense amount of detailed information is accessible to the reader in the reports of the United States Government (Tenth Census, and documents issued by the Fisheries authorities) and in those of States, like Connecticut, New York, and Maryland, where oyster culture is an extensive industry, said to be worth in the aggregate about $20,000,000. The oyster of the eastern American coast is to be found in the Gulf of St. Lawrence, but not in considerable numbers between there and western Maine, whence it is present southward to the Gulf of Mexico, except on the shifting sands of the outer beaches. It seeks protected waters and a rocky or weedy bottom furnishing objects to which it may, when young, attach itself, and later will not be torn adrift by storms, for where an oyster establishes itself in infancy it means to stay all its life. Hence the sheltered waters of Buzzards and Narragansett Bays, Long Island Sound, and the lagoons and inlets that lie behind the outer line of sandy beaches from Long Island to Florida are the sources of our supply—especially Chesapeake Bay.

A full-grown oyster will produce about 9,000,000 eggs, each being about one five-hundredth of an inch in diameter. When the little oyster (spat) is about one-eighth inch wide shells begin to form on its sides, and it settles to the bottom with its left side down, usually where other oysters are; and hence extensive colonies, or "reefs," of these mollusks form, and "rise on their dead selves" to a level where they may be reached by the oysterman's rake. Many years ago, however, it was discovered that large, marketable oysters were becoming very scarce. Oystermen therefore sought favorable places, and raking the natural beds transplanted their catch, little and big, to new ground, where they were left to mature. This crude method was next improved on by sowing thickly over the new ground, just before spawning time in midsummer, a great quantity of empty oyster and other shells. These were favorable to the catching of "spat," and would result in a new bed that in about four years would furnish salable oysters; and annual plantings produced, after a time, an annual crop. These are the essential facts of oyster culture everywhere, although methods differ somewhat in other parts of the world—in France, for example, fascines of twigs are spread over tidal flats to catch the spat, instead of shells.

Our eastern American oysters are undoubtedly the largest and finest for the table of the many species that exist all round the globe. Those of the Pacific coast of the United States are excellent, but small; and the same is true of the European species; nor is the use of oysters abroad so general and extensive as in the United States.

The pearl-bearing oysters are somewhat distant relatives of the edible oyster (Ostræa), the thorny oysters (Spondylus), the hammer shell, the windowglass shell (Placuna) and others. The pearl oyster of commerce is named Meleagrina margaritifera  and is found in scattered localities within the tropics on both continents. The chief fisheries are in the Persian Gulf, around Ceylon, in Australia, among the Sulu Islands and on the west coast of Panama. The Pearl Islands, south of Panama, yielded to the early Spanish adventurers riches in gems that rivaled those their competitors obtained from gold mines; but now they are a field of small importance. In fact, the pearl fishery is carried on now far less in hope of a profitable collection of gems than for the profit in the shells, which have a nacreous interior of remarkable beauty—the mother-of-pearl—and the great advantage of offering this in almost flat surfaces, sometimes eight or nine inches broad, making it useful in the arts as well as in the more practical line of buttons, knife handles, etc. Sometimes the whole surface of a fine shell has been carved, cameowise, with cunning art and an exquisite effect.

Mussels, Scallops and Chitons

The familiar marine mussels of the family Mytilidæ will some day become of great importance in this country as a food supply, as now they are useful in resisting encroachment by the sea on certain parts of the coast. They exist in vast numbers on both our coasts, and elsewhere in the world, in two genera, Mytilus and Modiolus, which differ a little in form, but not in habits. They have acquired the stationary habit, and in place of a "foot" of serviceable size have developed a gland that secretes an exceedingly tough, fibrous bunch of threads known as a "byssus," by means of which the animal may not only attach itself firmly to any sort of object, but may actually move about. The common species of Modiolus, the "horse mussel," lives in great numbers north of Cape Hatteras at and below the line of low water, and is much larger than the edible mussel just described. A smaller species of Modiolus is extremely numerous on the New England coast, and down to the Carolinas, forming dense tangled beds on muddy patches as well as among rocks, and serving to bind the mud and plants together and hold them from disintegration by stormy waves, in spite of the thin and brittle character of their shells. A southern species is bright yellow, with dark rays; and the common modiola of the Pacific coast is dark, glossy brown. Such mussels are eaten regularly in Europe, and come to us in a pickled condition as a luxury. There is no reason why we should neglect to add our own to our long list of sea foods.

The next useful mollusk to be considered is the scallop, one of the many species of the family Pectinidæ, of which we eat only the adductor muscle. The commercial species is Pecten irradians, the name referring to the (nineteen) ridges that radiate from the flattened hinges to the scalloped margin of the shell, which is prettily colored. This species is common in sandy, shallow places from Cape Cod to Florida, but the fishery is most productive about the eastern end of Long Island and in Narragansett Bay. Farther north is a very much larger species (P. islandicus ) especially abundant on the Grand Banks, off Newfoundland, where it forms an important food of the cod and other fishes. It is well known to cooks, who use it in baking their fish confections en coquille. A large number of other species are distributed throughout the world, one (P. jacobæus ), inhabiting the Mediterranean having the name "pilgrim shell" in allusion to the fact that in the days of medieval religious pilgrimages, those who had visited the shrine of Saint James at Santiago de Compostela, Spain, to pay homage on July 25, were accustomed to wear a scallop shell in their hats in token of the fact—this mollusk being connected with traditions of that saint.

Turning to the fresh-water mussels, or naids, as some books call them, one is staggered to learn that more than 1,500 species have been named, a large proportion of which belong to the United States,which is peculiarly hospitable to them because of our many rivers and lakes, together with the prevalence of limestone rocks, whose constant dissolution in water supplies the store of calcareous matter that these thick-shelled mollusks require. All belong to the family Unionidæ, in which two divisions are noted—one (Anodon) in which the mussel has a comparatively elongated thin shell with no "teeth" in the hinges; and the other (Unio) in which the shell is thick, various in shape from an oval to a triangle, and has prominent umbones, beneath which the valves (which are always alike) are hinged together by interlocking teeth embedded in a somewhat elastic gristle. The interior of all these unios is richly nacreous, and consequently pearls are produced in the same way as in the marine pearl-bearing shells; and some of the finest known gems have been derived from them, in this country and abroad, as well as innumerable specimens of moderate value. These mollusks like clear streams or lakes with a sandy bottom, and are not to be looked for in stagnant weedy waters. They keep an erect position, the nibs of the shell half buried in the sand, and move slowly about, plowing a path and dragging themselves along by means of the powerful foot, but keeping the short siphons at the other (or longer) end of the shell well above the mud.

We come next to our market clams. These are of two distinct kinds—"hard" and "soft," or quahog and long clam, as they are distinctively called. The quahog is a thick-shelled, roundish mollusk with a distinctly heart-shaped outline when looked at endwise. It dwells in fairly deep water, standing on its nibs half buried in the sand, like a wedge, and moving slowly about. Young ones become the "little necks" of our summer tables.

The soft clam belongs to a different race. Its elongated shell is thin and chalky, is loosely hinged, and gapes widely at both ends, and although it is used much as food, especially in chowders, it is by no means as good as the hard clam. Its principal value, indeed, is as bait in the cod fisheries, and for this purpose enormous quantities are gathered. It lives in, rather than on, muddy beaches, sometimes in crowds of thousands, its shell deeply buried, and its long siphons reaching up to suck in water and food when the tide covers the flat. When the tide is out, a tiny hole in the sand and a spurt of water show the clammer where to dig, and his spade quickly unearths the clam.

The second class, Amphineura, contains the chitons and their relatives. These chitons are flattened mollusks protected by an armature of eight crosswise plates, overlapping like shingles, which creep about the rocks close to shore, and when lifted curl up like sowbugs. The most interesting thing about the chitons is the fact that they are provided with excellent visual organs, "the whole dorsal surface of some forms being studded with eyes, of which not less than 8,000 occasionally exist on a single specimen." Many of them are complete, with cornea, lens, and a pigment layer within the iris.