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Rh leaves it in the form called Cercaria, which is really an immature condition of the adult. The Cercaria makes its way into the tissues of a mussel and there becomes enclosed in the cyst previously described. If the mussel is then swallowed by the duck the Cercariae develop into adult Trematodes or flukes in the liver or intestines of the bird. In the mussels which escape being devoured the parasites cannot develop further, and they die and become embedded in the nacreous deposit which forms a pearl. Dr Jameson points out that, as in other cases, pearls in Mytilus are common in certain special localities and rare elsewhere, and that the said localities are those where the parasite and its hosts are plentiful.

The first suggestion that the most valuable pearls obtained from pearl oysters in tropical oceans might be due to parasites was made by Kelaart in reports to the government of Ceylon in 1857–1859. Recently a special investigation of the Ceylon pearl fishery has been organized by Professor Herdman. Herdman and Hornell find that in the pearl oyster of Ceylon Margaritifera vulgaris, Schum, the nucleus of the pearl is, in all specimens examined, the larva of a Cestode or tapeworm. This larva is of globular form and is of the type known as a cysticercus. As in the case of the mussel the larva dies in its cyst and its remains are enshrined in nacreous deposit, so that, as a French writer has said, the ornament associated in all ages with beauty and riches is nothing but the brilliant sarcophagus of a worm.

The cysticercus described by Herdman and Hornell has on the surface a muscular zone within which is a depression containing a papilla which can be protruded. It was at first identified as the larva of a tapeworm called Tetrarhynchus, and Professor Herdman concluded that the life-history of the pearl parasite consisted of four stages, the first being exhibited by free larvae which were taken at the surface of the sea, the second that in the pearl oyster, the third a form found in the bodies of file-fishes which feed on the oysters, and the fourth or adult stage living in some species of large ray, It has not however been proved that the pearl parasite is a Tetrarhynchus, nor that it is connected with the free larva or the form found in the file-fish, Batistes; nor has the adult form been identified. All that is certain is that the pearls are due to the presence of a parasite which is the larva of a Cestode, all the rest is probability or possibility. A French naturalist, M. Seurat, studying the pearl oyster of the Gambier Archipelago in the Pacific, found that pearl formation was due to a parasite quite similar to that described by Herdman and Hornell. This parasite was described by Professor Giard as characterized by a rostrum armed with a single terminal sucker and he did not identify it with Tetrarhynchus.

Genuine precious pearls and the most valuable mother-of-pearl are produced) by various species and varieties of the genus Meleagrina of Lamarck, for which Dr Jameson in his recent revision of the species prefers the name Margaritifera. The genus is represented in tropical regions in all parts of the world It belongs to the family Aviculidae, which is allied to the Pectens or scallop shells. In this family the hinge border is straight and prolonged into two auricular; the foot has a very stout byssus Meleagrina is distinguished by the small size or complete absence of the posterior auricula. The species are as follows, The type species is Meleagrina margaritifera, which has no teeth on the hinge. Geographical races are distinguished by different names in the trade. Specimens from the Malay Archipelago have a dark band along the margin of the nacre and are known as black-edged Banda shell; those from Australia and New Guinea and the neighbouring islands of the western Pacific are called Australian and New Guinea black-lip. Another variety occurs in Tahiti, Gambier Islands and Eastern Polynesia generally, yielding both pearls and shell. It occurs also in China, Ceylon, the Andaman Islands and the Maldives. Another form is taken at Zanzibar, Madagascar, and the neighbouring islands, and is called Zanzibar and Madagascar shell. Bombay shell is another local form fished in the Persian Gulf and shipped via Bombay. The Red Sea variety is known as Egyptian shell. Another variety occurs along the west coast of America and from Panama to Vancouver, and supplies Panama shell and some pearls. A larger form, attaining a foot in diameter and a weight 10 ℔ per pair of shells, is considered as a distinct species by Dr Jameson and named Margaritifera maxima. It is found along the north coast of Australia and New Guinea and the Malay Archipelago. The nacreous surface of this shell is white, without the black or dark margin of the common species; it is known in the trade as the silver-lip, gold-lip and by other names. It is the most valuable species of mother-of-pearl oyster. Dr Jameson distinguishes in addition to the above thirty-two species of Margaritifera or Meleagrina; all these have rudimentary teeth on the hinge. The most important species is Meleagrina vulgaris, to which belong the pearl oyster of Ceylon and southern India, the lingah shell of the Persian Gulf and the pearl oyster of the Red Sea. Since the opening of the Suez Canal the latter form has invaded the Mediterranean, specimens having been taken at Alexandria and at Malta, and attempts have been made to cultivate it on the French coast. The species occurs also on the coasts of the Malay Peninsula, Australia and New Guinea, where it is fished both for its shells (Australian lingah) and for pearls. Two species occur on the coasts of South Africa but have no market value. Meleagrina carchariarum is the Shark’s Bay shell of the London market. It is taken in large quantities at Shark’s Bay, Western Australia, and is of rather small value; it also yields pearls of inferior quality. The pearl oyster of Japan, known as Japan lingah, is probably a variety of Meleagrina vulgaris. Meleagrina radiata is the West Indian pearl oyster.

The largest and steadiest consumption of mother-of- earl is in the button trade, and much is also consumed by cutlers for handles of fruit and dessert knives and forks, pocket-knives, &c. It is also used in the inlaying of Japanese and Chinese lacquers, European lacquered papier-mâché work, trays, &c., and as an ornamental inlay generally. The carving of pilgrim shells and the elaboration of crucifixes and ornamental work in mother-of-pearl is a distinctive industry of the monks and other inhabitants of Bethlehem. Among the South Sea Islands the shell is largely fashioned into fishing-hooks. Among shells other than those of Meleagrina margaritifera used as mother-of-pearl may be mentioned the Green Ear or Ormer shell (Haliotis tnberculata) and several other species of Haliotis, besides various species of Turbo.

Artificial pearls were first made in western Europe in 1680 by Jacquin, a rosary-maker in Paris, and the trade is now largely carried on in France, Germany and Italy, Spheres of thin glass are filled with a preparation known as “essence d'orient,” made from the silvery scales of the bleak or “ ablette," which is caused to adhere to the inner wall of the globe, and the cavity is then filled with white wax. Many imitation pearls are now formed of an opaline glass of nacreous lustre, and the soft appearance of the pearl obtained by the judicious use of hydrofluoric acid. An excellent substitute for black pearl is found in the so-called “ironstone jewelry,” and consists of close-grained haematite, not too highly polished; but the great density of the haematite immediately destroys the illusion. Pink pearls are imitated by turning small spheres out of the rosy part of the conch shell, or even out of pink coral.

See Clements R. Markham, “The Tinnevelly Pearl Fishery,” in ''Journ. Soc. Arts (1867), xv., 256; D. T. Macgowan, “Pearls and Pearl-making in China,” ibid.'' (1854), ii. 72;  Hague, “On the Natural and Artificial Production of Pearls in China,” in ''Journ. Roy. Asiatic Soc. (1856), vol. xvi; H. J. Le Beck, “Pearl Fishery in the Gulf of Manar,” in Asiatic Researches (1798), v. 393; K. Mobius, Die echten Perlen (Hamburg, 1857); H. Lyster Jameson, “Formation of Pearls,” Proc. Zool. Soc (1902), pl. 1; idem, “On the Identity and Distribution of Mother-of-Pearl Oysters,” Proc. Zool. Soc. (1901), pl, 1, pp. 372–394; Herdman and Hornell, Rep. Ceylon Pearl Fisheries (London, Royal Soc., 1903); and Kunz and Stevenson, Book of the Pearl'' (New York, 1908), with bibliography.

 PEARL, THE. The Middle-English poem known as Pearl, or The Pearl, is preserved in the unique manuscript Cotton Nero Ax at the British Museum; in this volume are contained also the poems Cleanness, Patience, and Sir Gawayne and the Green Knight. All the pieces are in the same handwriting, and from internal evidences of dialect, style and parallel references, it is now generally accepted that the poems are all by the same author. The MS., which is quaintly illustrated, belongs to the end of the 14th or the beginning of the 15th century, and appears to be but little later than the date of composition; no line of Pearl or of the other poems is elsewhere to be found.

Pearl is a poet’s lament for the loss of a girl-child, “who lived not upon earth two years”—the poet is evidently the child’s father. In grief he visits the little grave, and there in a vision beholds his Pearl, now transfigured as a queen of heaven—he sees her beneath “a crystal rock,” beyond a stream; the dreamer would fain cross over, but cannot. From the opposite bank Pearl, grown in wisdom as in stature, instructs him in lessons of faith and resignation, expounds to him the mystery of her transfiguration, and leads him to a glimpse of the New Jerusalem. Suddenly the city is filled with glorious maidens, who in long procession glide towards the throne, all of them clad in white, pearl-bedecked robes as Pearl herself. And there he sees, too, “his little queen.” A great love-longing possesses him to be by her. He must needs plunge