The beautiful objects which we know as shells, and which form ornaments in many a home far distant from the sea, are the coverings of a group of animals called mollusks. They are found in all seas, many upon land, and in fresh-water streams, and are among the most attractive of all natural objects, so much so that many persons devote their entire lives to their collection, and many others much time to the study of their habits. It is rare to find a new shell, so well have these shell hunters searched the waters of the world. Such collections, especially if complete, are very valuable, and many of the great museums have paid thousands of dollars for them.

The mollusks or shells present a striking contrast to the worms. They have no joints, are soft, seemingly without form, and are very helpless creatures. The body is enveloped in a muscular coat or mantle, as shown in the oyster (Fig. 79). They have a nervous system, and a heart (H) which pumps colorless blood. Some have a foot for locomotion and eyes more or less well developed. The oyster represents a large group which have two shells, called bivalves. In Figure 80 we see the animal portion of the common snail, which illustrates another group with but one shell. These are called univalves. To the bivalves belong the shells best known, the oysters, clams, scallops, pectens, pearl oyster, razor shell, and many others, of which the oyster is the most familiar. The mantle, the soft, delicate lining, is the shell maker, and not only forms it, but repairs damages to it, piling up layer after layer of pearly matter called nacre. As there is a mantle on each side, two values are secreted. The sharp portion of the oyster is called the beak. Here the growth of the shell begins, and here are the marvelous valves which fit with such accuracy. These complicated parts are easily seen in the clam (Fig. 81). The hinge is joined by teeth (c, d, d,) which fit into cavities on the opposite valve, while the valves are held together by a perfect hinge, a horny ligament (h) that tends to open the shell or throw the valves apart.

In the interior of all shells are seen certain scars; in others a purple mark. These marks (e, e,) indicate the location of a strong muscle by which the clam or oyster closes its shells and keeps them closed with such rigidity. In opening oysters the man severs this muscle and the shell opens, forced apart by its ligament. This explains why most shells found on the beach are wide open. The curious columnar objects in rows are the gills or breathing organs of the oyster, and are covered with little oars (Fig. 82), or cilia, which move to and fro, continually sweeping the currents of water along, bearing oxygen and food. The former is taken up by the gills to purify the blood, and the latter is swept into the mouth located near the lungs.

There is great variety in the hearts of shells. In the oyster (Fig. 79) it is composed of one auricle and one ventricle; but in other shells the heart may be three-chambered, or there may be two distinct hearts, each having two chambers. The eyes of the shells are very minute and are situated along the mantle. Those of the pecten are very beautiful and are distinctly visible, resembling gems or emeralds.

The clams (Fig. 83) differ from the oysters in having a pronounced foot (f) which protrudes from the large end of the shell; and with it the animal digs its burrow. It also hears indirectly by its foot, as its ears are in this organ, little transparent sacs containing a clear fluid in which floats a glassy globule. The clam also has a siphon (s), which in the common clam is very long. It has a black head or tip and the clam may rest some distance down in its hole and take in water through its siphon, which is double-barreled. One opening (in.) receives water containing food and oxygen; the other (ex.) expels the water. In strolling along the sands at low tide one often sees a spurt of water shoot out of a hole, and may assume that a clam has been alarmed and has retracted its siphon so suddenly that it has shot a stream of water above the surface. The shells increase by eggs, the oyster depositing a vast number, which at first are curious little free-swimming objects (Fig. 84) paddling by the aid of cilia or whips, but soon attaching themselves to the bottom and taking the oyster form.

The oyster is perhaps the most valuable bivalve to man, being a favorite article of food, for which $1,500,000 is paid annually in New York alone. Thousands of men find employment collecting them in various parts of the world. In this country the most valuable oyster beds are in the vicinity of New York, at the mouth of the Shrewsbury River, in the Chesapeake Bay, and at various points alongshore to Florida, where there are large banks at the mouths of the rivers. In watching the excavation of a cellar at the town of Mayport at the mouth of the St. John River I saw oyster shells thrown up as deep as the men went. The town is built on an ancient oyster bed. Among the old shells numerous pieces of pottery have been found, showing that the early natives frequented the spot. The living oyster bed here to-day is some distance out in the stream. When sailing up a small river in Maine some years ago, I found, about ten miles from its mouth, a mound of oyster shells thirty or forty feet high. The river appeared to have cut the bed in two, and out of the top of the mound, which was of solid shells, grew a tree which must have been a century old. I believe there are no oysters on the Maine coast to-day, and the great pile was accumulated ages ago when Maine had oyster beds and the Indians carried the oysters ten miles up the river to this spot which must have been the site of an ancient Indian town or city. The pearl oyster is another valuable shell (Fig. 85). It is common in warmer waters. Near La Paz in the Gulf of California is a famous fishery, which is owned by the government and farmed out. In Ceylon it is estimated that 17,000,000 oysters are destroyed to obtain $80,000 worth of pearls. The shells are also very valuable, being made into buttons and various other objects. Liverpool is the great receiving port for these, and many tons are used annually. In diving for pearls the Ceylonese, who are able to remain beneath the water several minutes, place as many shells as possible in a basket and then ascend, leaving the crew to haul the basket up. In Lower California many divers of to-day go down in armor.

The seed pearls are those in which some impurity has been covered but a few times, while the very large pearls are those which have been bathed in nacre time and again. If a large pearl is cut in halves, the various layers can be counted, the sections recalling the interior of an onion. The skillful native fakirs of the East take advantage of this industry of the pearl oyster to introduce metal beads and figures of the Buddha into shells, which are then marked. The objects finally become covered, when they are removed from the shells and sold to the unsuspecting natives as "miracles."

One of the interesting shells of the seashore is the Pinna. I have found the shores of the outside islands of Texas scattered with them. They are also called fan shells, and are attached to the bottom by a peculiar cable, or byssus, formed of a silklike substance which has been woven. Gloves and hose of pinna silk may be seen in the British Museum.

The pectens are common forms famous for the beautiful gemlike eyes seen along the edge of their mantles. I once kept a number of these shells in an aquarium, and they were a source of much amusement, from their habit of dancing (Fig. 86). Generally they lay in the sand in the bottom of the tank with their valves open an inch or more, their bright eyes gleaming. Without any warning, one would open and close its valves with great rapidity, which would cause the shell to take convulsive and bounding hops. Then another shell would follow, and soon all the pectens were leaping up and down in a most extraordinary dance. The pecten changes its position or travels, not by pushing itself along, but by a sudden and spasmodic hop, clearing a foot or more.

The locomotion of shells itself is a fascinating subject. An interesting instance is observed in the common mussel. This shell has a remarkable foot, a pointed, fleshy organ which can be protruded. With this organ the mussel bores holes in the sand, jerks itself along, or clears the surface with a bound. But its most remarkable service is in aiding the mussel to climb. In the foot, near its base, is a gland which secretes a peculiar substance, which when exposed to the water hardens and resembles silk. The resemblance is so perfect that the "silk" has been woven into various articles, and an attempt was made in France to raise mussels for this purpose. When the animal desires to climb, it reaches out its foot as high as it can (Fig. 87), and presses it upon the pile or rock, whereupon a delicate cord, one of the cables of its byssus, is seen. Again the foot is extended, again a cable is attached, the entire operation calling to mind the action of a spider. Each step raises the mussel a little higher, and as it moves on, the cables that would hold it back are broken off, and the mussel at length reaches the position it desires.

The fresh-water mussels found in the Ohio and other rivers and streams are pearl producers. Very valuable gems have been taken from them in various states, and the fresh-water pearl fishery of the United States is of considerable importance. A fresh-water pearl found in New Jersey was valued at $2000, and one taken from a stream in Scotland brought $50,000.

The vast number of shells and the varieties of each kind can hardly be realized by those who have not examined a well-equipped collection. Over four thousand species of the mussel are known, and hundreds of species of almost every shell exist in various streams and seas. The shells range from minute specimens hardly visible to giants weighing several hundred pounds, one of the latter being the huge clam, Tridacna (Fig. 88), found in the equatorial Pacific. There are several species, and in the largest each valve weighs about two hundred and fifty pounds. The animal itself weighs thirty pounds, and affords a meal to forty or fifty men. The shell, by means of its foot, buries itself in the soft rock of the regions in which it lives. With its valves partly open it resembles a huge sea anemone; but it closes them at the slightest alarm. Large fishes, and even natives, it is said, have been trapped by this giant, whose jawlike valves, with three huge teeth, grip the fin of a fish or the foot of an unfortunate wader with a vicelike grasp. The byssus or anchor of this huge shell is so thick and tenacious that it is severed only with great difficulty and labor. The shells are valuable as ornaments, large numbers being sent to various countries for this purpose. The giant never moves, and in this respect is a sharp contrast to the little donax, so common on our various shores and in France, which leaps along the muddy flats by convulsive movements of its fleshy foot.

The odd shapes assumed by many bivalves is well illustrated in the hammer oyster (Fig. 90) and the pholas. The latter illustrates the power of the most insignificant animals, as by means of its foot this little shell burrows into the hardest granite. It is invariably found there and imprisoned; for when it reaches the interior of a stone, it grows and enlarges, leaving but a small opening for the siphons. It is supposed by some that the pholas possesses some secretions by which it dissolves the stone, and by others that it wears away the rock by using its shell as a file. In any event the shell is known to contain aragonite, a very hard substance. In the pillar of the temple of Serapis, Italy, the holes made by this shell are seen.

The teredo or shipworm (Fig. 91) is called a worm because it secretes a limy shell, but it is really a bivalve shell open at both ends, a shell which with one exception causes more destruction than all other marine animals combined. Instinctively it bores into wood, forming an irregular tunnel and lining it with a delicate coating of carbonate of lime. Some years ago I visited on the outer Florida Reef, an old wreck which was newly buried in the sand and partly exposed at low tide. The timbers of the vessel looked strong and able to stand many a storm, yet with a blow of my hand I broke through the planking. The interior was completely honeycombed by the teredo, so that it was a maze of tubes. At this place the life of a pile of yellow pine was a year and a half; in other words, after being exposed to the teredo for this length of time, it was useless. On the Pacific, at Avalon Bay, the piles last about two years, being rapidly destroyed, even though soaked in various poisonous fluids and coated with tar. Many thousands of dollars have been expended in experiments with devices to outwit the teredo, but without avail, and they are the greatest menace to navigation and piers to-day, making their way into hulls, despite the copper sheathing. In the mud banks of the waters of Sumatra, teredos are found which attain a length of six feet, with tubes four inches in diameter.

The shells are famous for their beauty, the polished valves and their marvelous tints presenting attractive combinations. The common mactra (Fig. 92), the cockle (Fig. 93) with its deep radiations, the gorgeous pectens of the South, the splendid pearl-bearing shells, all tell a wonderful story of the resources of nature, and emphasize the fact that the smallest and most inconspicuous animals vie with the larger forms in beauty of shape and color.