Part 7

Many of these dead pearls are formed throughout of this material. Others, perfectly spherical, are simply successive layers of prism groups like the conchiolin plates of the shell. Upon cutting these through the centre the skins are shown by the concentric rings marking their divisions and the prismatic formation appears as glistening lines radiating from the nucleus to the surface. Under the microscope these layers, which are thicker than the nacreous skins of true pearls, appear identical with the epidermis plates, except that they are concentric instead of flat, and are free from the coarse, rough, conchiolin deposit which forms the extreme outer coating of the shells. This deposit is also found, however, in some pearl formations, as many of the abalone baroques, especially when they are somewhat flat in shape, are like two pearl blisters joined, with the shell-building process reversed, the rough, black conchiolin being inside, and the nacre outside. Undoubtedly pearls containing hidden qualities which made them once gems are thrown away as valueless, while others found just as nature had covered their earlier coarseness with a coat of beauty, are worn and excite much admiration for their skin-deep beauty.

Though the successive skins of a pearl do not usually vary much in color, except in abalone pearls, it does happen occasionally, for the removal of dark yellow skins sometimes discloses another of better color—a good pink for instance. From the sectional appearance of pearls it seems probable, that in the majority of cases the color of yellow pearls would be improved by the removal of the outer waves of the outer skin.

Changes in shape sometimes occur during the growth of the pearl, the tendency being always toward the rounding of the surface. If the nucleus is fast to the shell, a dome is built over and around it. If the nucleus permits, the nacre is deposited not only over but under its edges to the point of contact with the shell, so that a button pearl connected with the shell at the centre only, results. Two pearls held against the shell and growing side by side are separately enveloped until they touch each other, after which they are included in single deposits of nacre and the depression between their domes becomes less distinct with each successive coating. Similarly, a cluster of small pearls lying together often forms the nucleus of a large rounded baroque or button pearl. Examination of such formations shows, that up to a certain period the pearls have a separate existence and growth. They then become joined in an irregular mass of twinned pearls, and finally, if allowed to remain in the oyster long enough, all individuality is lost in the tendency to round over. The same thing occurs when grains of sand or other intrusions become attached to a growing pearl. They are quite prominent when first included in the nacreous deposit and can be easily detached from the under pearl by breaking through the layer which binds them on; but they are soon obliterated by succeeding deposits. This filling-in process is sometimes accomplished by additional layers in the depression, sometimes by thicker layers. It happens occasionally, when skinning a round pearl, that one of these fillings is uncovered and flakes out, leaving the pearl irregular, as it was in a former stage of its growth.

Although pearls naturally grow spherically, many free pearls are more or less buttoned, that is, have a flat place from which the pearl rises like a dome, high or low. This happens when the pearl is held during growth by the fish against the shell with a part of its body intervening. According to circumstances, the pearl varies in form from slightly button, to a low dome, rising from a plane at its greatest diameter. Should a pearl of this description become dislodged, the rounding action of the mollusk would begin at once to obliterate the plane.

If undisturbed, the process would result eventually in changing the button to a round or nearly round pearl, but should the pearl be taken from the fish before the metamorphosis is completed, a depression, or pit, would mar its contour. When borers intrude through the shell, the presentation is at once covered with nacre, and successive deposits are built up around it resulting in the nacreous wart known as a baroque. The rounding action of the mollusk is clearly shown in these excrescences, as the borer is not simply covered and levelled with the shell, but the slight elevation above the level of the lining receives a continuity of concentric deposits which finally raise it very considerably above the surface and separate it in construction from the lining to which it is attached. The shell herewith reproduced illustrates the result. Borers pierced it at the thick part of the hinge, and burrowing down, entered the interior at the point where the baroque is shown. In rare instances, pearls attached to the shell do escape the concentric deposition, for they have been found buried under even layers of nacre, when the mother-of-pearl was cut up in the process of manufacture.

From the appearance of the striæ when they are divided lengthwise, pear-shaped pearls appear to have been spherical at one time. During a stage in the growth, the forming layer has curved away from the centre at one section of the sphere to a point. Succeeding layers, following the innovation, are deposited around the extension until it becomes sufficiently elongated to give the pearl the obovoid form.

Many pearls are shaped like a capsule. The ends of most are rounded up to a full dome; some have somewhat flatter ends; many are long and cylindrical like an ordinary capsule; others are short and appear in shape like two high button pearls joined at their bases; while some resemble a cartridge, one end being almost flat and the other a somewhat pointed dome. It is noticeable that such pearls have a chalky line around the middle, and sometimes there is a lustrous band between two. These chalky lines are found, on peeling such a pearl, to extend through all the interior layers. Similarly, a high button joined at its entire circumference to the shell, if the junction is abrupt, has an intersecting chalky line, marking the juncture of the two, between the luster of the pearl and the shell lining. If the base of the pearl and the shell form a curve there is no chalky line of demarcation.

This suggests that whenever the animal is unable to envelop the thing upon which the mantle deposits its secretions completely or is not in touch with every part of it, there is at the extremity of its action, an unnacreous deposit, corresponding to the deposit of conchiolin or calcite, at the extreme edge of the shell which precedes the nacreous layers following within and slightly back of it. As the luster of the pearl arises from the transparency of the calcium carbonate modified by the undulating lines formed by the edges of the wave-plates, it may be that the lapping action of the mantle is necessary for the regular formation and crystallization of these plates, and that at points beyond the reach of this action, the depositions of the mantle are therefore not pearly.

Much is necessarily conjectural as to the modus operandi by which the shell and the pearl are formed but the invariable tendency toward sphericity suggests that the nucleus of a pearl, when free within the mollusk's mantle, is not only enveloped in its exudations, but is either kept constantly moving with a rolling motion or lapped on all sides by the membrane which exudes upon it the nacreous material.

The instances cited of the short capsule shaped pearl and the high button joined to the shell, which seem to escape the nacreous deposit at the basis of the domes, favor the lapping or licking method of depositing the nacreous solution and this action by the mollusk would result in a constant rolling or turning motion imparted to the object if it were free within the creature's body. The licking and rolling action of the mollusk, modified by the conceivable influences of position in the shell, would account for the spherical form with all the various modifications in which the pearl is found.

To account for the variation of quality which undoubtedly exists in the successive skins of some pearls, and the imperfections in the nacre of the same skin, the theory has been advanced that the secretions for the lining, the shell proper, and the epidermis, are exuded by different parts of the mantle; the pearl traverses during growth these different bands and its skins are modified by the secretions, as they come within the various zones of influence. But there are several facts which seem to oppose the theory.

In the first place all these parts of the mantle which supply the material for the epidermis, the middle shell, and the lining, are enclosed within the shell and in touch with the lining yet each receives the exudations of that part of the mantle which supplies the material suitable for it, the mantle invariably pushing the coarser excretions outwardly to the shell's exterior. Again, whatever the quality of the skin of the pearl may be, it is never of conchiolin like the outer epidermis and though sometimes similar to the plates, of which the conchiolin is the exposed fringe, it always contains sufficient nacre to render the surface smooth. The fact that the skins of a pearl do sometimes correspond with the different parts of the shell, and that the same skin on the surface is occasionally partly nacreous and unnacreous, in connection with the variation of quality which exists in the internal composition of the skin, favors an idea that the mixed and variable quantity of nacre in the skins may be caused by the abnormal position of the mantle wrapped about the growing pearl which would thereby come more or less under the influence of the calcite and conchiolin zones distorted from their normal extension and action.

It has also been suggested that the oyster deposits the nacreous layer in a fluid state and then rests until the deposit hardens, when the process is repeated. To a certain extent this may be true though apparently it could not be a yearly process as pearls found in the small varieties of the avicula which mature in four to six years and die out in seven years, often contain a greater number of layers than the years of the mollusk's life, and no pearl is ever found with a soft exterior, though it seems possible that pearls with a dead white chalky exterior are taken from the oyster at a period when the crystallization of the outer skin has not been perfected, or that they have escaped some action, chemical or of the animal, necessary for the formation of the lustrous waves of nacre. Mr. Ludwig Stross, who has had much experience at the pearl fisheries, says that he has frequently found pearls of fair size in shells of the Lingah type which could not be over twelve to fifteen months old. Some of these pearls weighed fully three grains. As there are many apparent skins in a pearl of that size, the divisions could not mark either years, seasons, or breeding periods. In some experiments made by Mr. Stross, he found that borings made to the interior of a living mollusk's shell were closed by a film of hard nacre in two days.

The known facts about a pearl are these. It is composed of about ninety-two per cent. carbonate of lime, about six per cent. organic matter and a little over two per cent. water in combination almost identical with the lining of the shell in which it grows and similar to the mineral aragonite. In construction it is usually a series of layers, which can sometimes be peeled off entirely, each one successively enveloping its predecessors apparently as an independent structure though itself composed of a number of thin lapping waves. Upon cutting through these layers the divisions appear as a series of rings and the intervals, though composed of many thin waves, appear compact. It grows spherically or with such modifications as the exigencies of position in the shell would reasonably account for. These facts seem to justify the hypothesis that a foreign substance upon entering the shell of a pearl oyster is at once enveloped or washed in the creature's exudations; that the organic matter of the secretions forms a filmy envelope in which the mineral contained in them is precipitated or crystallizes in wave-like layers of crystals of great tenuity, and that as these layers harden the process is repeated, and that during the process the creature either revolves the object, or about it, as it is free, or fastened to the shell. It is also possible that changes in the organic matter interwoven with the calcium carbonate may produce some chemical action resulting in the crystallization of the lime, and the crystallization in turn be provocative of another deposit, each process in turn being almost simultaneous and that the process is continued until a paucity of mineral in the exudations induces a rest for recuperation, after which the process is repeated, the result being a succession of composite skins as we find them. Whatever the cause, it is evident in all parts of the shell and in the pearl that continuity of construction is periodically arrested to be resumed upon exactly the same plan, except that the material used in the succeeding layer of the pearl may be formed occasionally like another of the shell sections though usually it is like the preceding one.

Marked differences in the same skin occur more frequently in the pearl formations of univalves. The skins of the abalone pearl especially, are frequently nacreous in part only.

Pearl oysters are found in immense numbers on banks having a calcareous foundation. They are extraordinarily prolific, the spat of one oyster being estimated at upwards of several hundred thousands to millions, so that were it not for the natural enemies of their young and the liability of being swept away and scattered by storms before they have anchored, the banks would be over-crowded with the myriads produced. Some idea of the numbers may be gained from the fact that during the fishing season the Ceylon divers raise about one million each day.

The oysters are seldom found in water with a temperature below 75 degrees and they seem to thrive best in warm sheltered bays and inlets, especially when the banks are situated far from the equator. They attach themselves to the beds by a bunch of tough threads which pass out through an aperture in the shells, near the hinge, and fasten on the rocks and stones; consequently the oysters do not lie flat, as might be supposed, but maintain an upright position, hinge down, lip end up, and the shell slightly open for the passage of the food-laden water, as the fresh-water mussels do. These threads are called the beard or byssus, and are composed of material similar to the epidermis of the shell.

The abalone, which is a univalve, holds on to the rocks by the foot, a flat muscular appendage used for locomotion and also as an anchor on the principle of the leather toy known to boys as a sucker.

Although pearls of value are found only in shells containing mother-of-pearl, a small proportion only of the mother-of-pearl shells contains pearls, and many varieties in which pearls are found do not yield enough nacre to make the shells valuable. The size of the meleagrina in some seas is remarkable. That at page 127, photographed from a Tuamotu shell, measures 8-7/8 inches by 6-7/8 inches and weighs twenty-eight ounces troy.

It is of the black-edge variety, contains a large quantity of fine quality mother-of-pearl, and has a beautiful small pearl attached to the lining near the center of the shell. Though large, it is not full grown. It is probably twelve to fourteen years old and would continue to lay on mother-of-pearl and so grow thicker and heavier until sixteen to eighteen years of age, when the oyster would reach maturity. The Australian white shell at page 129 is a young shell—that is, it has not attained the full thickness and weight of a mature shell. The shells at pages 131 and 161 are from the coast of Venezuela; they measure 2-1/4 by 2-1/4 inches and weigh seven pennyweights each.

The common form of the pearl-bearing fresh-water mussel unio (nigger-head) is illustrated at page 146. This shell measures 3-3/4 by 2-3/4 inches and weighs 3-1/2 ounces. It is from the Middle West of the United States. In construction it resembles the meleagrina, the epidermis being dark, though not as rough as that of the oyster, and the lining white, showing slight iridescence around the lip-edge and to a greater degree on the adductor muscle scar. The mother-of-pearl under the epidermis at the thick or hinge end is quite iridescent, and the lines which make the color play are plainly discernible under the loup.

The largest and finest pearls, also the greatest number, are found usually in distorted shells. This has given rise to the idea that they are a symptom of disease in the fish, but having in mind the functions of the three zones of the creature's mantle by which they supply separately material for the epidermis, middle shell and lining, one may conceive that if, by some extraordinary cause, the secretions of one of these is largely withdrawn from the natural channel, the losing part of the shell would warp the normal growth of the others to its own dwarfage.

When the nacre grows to a pearl, contrary to the intent of nature, instead of a lining for the shell endeavoring to keep pace with the growing oyster, the full-growing exterior is distorted in accommodating itself to the undersized lining. In view of the fact that an oyster sometimes contains a large number of pearls (one shell in New Caledonia contained 256) the diversion of nacre sufficient to cover them, or to produce one large pearl, might reasonably be expected to result in a considerable distortion of the shell. It may also be that the displacement of the mantle, caused by the wrapping of itself about the growing pearl, interferes with the even deposit of shell material about the edges of the shell and so distorts it.

Because deformed shells are more fruitful of pearls some have advocated the practice of throwing perfectly-formed shells back into the sea unopened, but, inasmuch as the mother-of-pearl of the shells often exceeds in value the pearls found in them, this is not likely to happen. Few fisheries could be made to pay if they were fished for the pearls alone. In many of them the shells yield 90 per cent. of the total value and are in fact the sole incentive for the investment of the necessary capital.

Luckily for the world's supply of pearls, however, the disturbers of the mollusk which cause these gems by their intrusions appear to be more abundant in waters where the shell is valueless, the banks about Ceylon especially being infested with the cestodes which are commonly the nuclei of Indian pearls. It is interesting also to learn that Mr. James Hornell (inspector of the pearl banks) finds these worms in another stage in the file-fish, which frequents the banks to prey upon the oysters, and confidently expects to find them in the adult stage in the shark, which in turn devours the file-fish.

It is the opinion of Jameson of London and others, that the parasite which causes the formation of pearls in the mussels of Europe is frequently the larva of distomum somaterœ, from the eider-duck and scoter, and that the larva first inhabits Tapes, or the cockle, before getting into the mussel.

Generally the nuclei appear to be the bodies or eggs of minute parasites—distoma, filaria, bucephalus, etc., and they vary in different localities according to the animal life of the neighborhood. In the still parts of the river Elster, where water-mites (Limnochares anodontœ) were abundant, Kuchenmeister found that the mollusks contained more pearls.

METHODS OF FISHING

The beds of the marine shell-fish from which pearls are taken lie always under water. Unlike others which are sometimes left exposed by the tides, to be gathered by man without difficulty, the pearl oyster is never left uncovered by the sea. It is found usually on shoals some distance from shore, sometimes but five to seven feet from the surface; more frequently fifteen to forty feet deep, and often one hundred to one hundred and twenty-five and even one hundred and fifty feet deep.

Everywhere, then, man's quest for pearls is confronted by the heaving, restless waters of the sea, for the greater part of the year rough and turbulent, frequently lashed to furious racing by tropic tempests but through which he must in any case go to get them. In a few places where the beds lie in shallow inlets and sheltered bays they can be dredged, but almost universally the oysters are gathered by divers. During the greater part of the year, when storms rage, diving is very dangerous if not quite impossible; but when the song of the sea is hushed to low crooning, and the gentle roll of the waves does no more than playfully slap the boats in passing, then in the seas where men dive for pearls they gather to the harvest of gems.

There are two ways of diving—naked, and with dress. The former is the common method throughout the Orient and is practised to-day after the same manner that it was in the days of the Pharaohs and the Cæsars, for the primitive method survives with few variations wherever eastern people control the fisheries.

In the fishing season one sees now in the Red Sea and the Persian Gulf and about Ceylon, the same scenes as they were enacted there before Rome was a city, or France a nation, or the Macedonians overran Egypt. Naked divers, diving into fifteen to forty feet of water, use few aids. They grease their bodies, put greased cotton in the ears and a forked stick, or tortoise-shell clip, upon the nostrils to compress them, hang a wide-mouthed wicker basket or net at the waist, and they are ready.

There are several methods of naked diving: head-first from a spring-board attached to the side of the boat, as the Malabar coast Hindus and some of the Egyptians do; swimming to the bottom, as practised in the deep waters of the South Sea; and dropping to the oyster bed with a stone. The latter is the most common way in Indian, Egyptian, and Arabian waters, especially where the banks lie in forty to fifty feet of water.

Standing on the spring-board a few seconds to fill his lungs, the head-first diver suddenly plunges overboard and passes smoothly and rapidly through the water straight to the shoal below. Gathering quickly as many oysters as possible while his breath lasts, he places them in the net at his waist, attaches them to a convenient rope hanging from the boat's side and shoots to the surface. There he recuperates by lazily floating about if the water is shallow, if deeper, by climbing back into the boat for his next plunge. If diving in pairs, one rests while his partner dives.