Pearls and Parasites

ILLUSTRATIONS

PREFACE

Most of the following essays have appeared in the pages of the Quarterly Review, and I am greatly indebted to the editor and to the proprietor of that periodical for permission to reprint them. The article on ‘The Infinite Torment of Flies’ is an address I delivered before the British Association at Pretoria in 1905, and the eighth essay appeared in Science Progress.

As far as possible I have tried to avoid the use of long words, and thus escape the censure of recent critics in the Times; but I fear I have not altogether succeeded, and my excuse must be that with new discoveries new conceptions arise, and these conceptions require new names, or we cannot talk or write about them with any precision.

The essay dealing with zebras and hybrids was the first to be written, and appeared before the rediscovery of Mendel’s remarkable work, and must be regarded as a pre-Mendelian contribution to a subject which has recently, in connexion with the Deceased Wife’s Sister Bill, again aroused attention. Had it been written later the language and the attitude taken would have been modified by recent research.

In the inquiry into the aims and finance of Cambridge University—the only essay which does not deal with questions of economic zoology—I have had the great advantage of the collaboration of Mr. H. A. Roberts, the Secretary of the Cambridge University Association. But for his help I fear I should have lost my way in the intricate mazes of the University accounts.

For the care he has taken in making the Index, I owe thanks to Mr. G. W. Webb, of the University Library.

A. E. S.

Christ’s College,

Cambridge.

March 10, 1908

BIBLIOGRAPHY

‘Report to the Government of Ceylon on the Pearl-Oyster Fisheries of the Gulf of Manaar.’ By W. A. Herdman, F.R.S. Parts I. and II. Published by the Royal Society. London, 1904.

‘On the Origin of Pearls.’ By H. Lyster Jameson. Proceedings of the Zoological Society of London, 1902.

‘Aus den Tiefen des Weltmeeres.’ By C. Chun. Jena: Gustav Fischer, 1900.

‘Tierleben der Tiefsee.’ By O. Seeliger. Leipzig: Wilhelm Engelmann, 1901.

‘Report of the Scientific Results of the Voyage of H.M.S. Challenger.’ Edited by the late Sir C. Wyville Thomson and John Murray. A Summary of the Scientific Results. Published by Order of Her Majesty’s Government, 1885.

‘La Vie au Fond des Mers.’ By H. Filhol. Paris: G. Masson, 1885.

‘The Fauna of the Deep Sea.’ By Sydney J. Hickson. London: Kegan Paul, Trench, Trübner and Co., 1894.

‘British Fisheries: their Administration and their Problems.’ By James Johnstone. London: Williams and Norgate, 1905.

‘An Examination of the Present State of the Grimsby Trawl Fishery, with Especial Reference to the Destruction of Immature Fish.’ By E. W. L. Holt. Journal of the Marine Biological Association, vol. iii. Plymouth, 1895.

Journals of the Marine Biological Association of the United Kingdom, vols. i.-vii. Plymouth.

‘Conseil Permanent International pour l’Exploration de la Mer. Rapports et Procès Verbaux,’ vol. iii. Copenhagen.

‘Fishery Board for Scotland. Report on the Fishery and Hydrographical Investigations in the North Sea and Adjacent Waters, 1902-1903.’ [Cd. 2612.] London, 1905.

‘Marine Biological Association. First Report on the Fishery and Hydrographical Investigations in the North Sea and Adjacent Waters (Southern Area), 1902-1903.’ [Cd. 2670.] London, 1905.

‘Annual Reports of the Inspectors of Sea-Fisheries for England and Wales.’ London, 1886-1905.

‘The Penycuik Experiments.’ By J. C. Ewart. London and Edinburgh: A. and C. Black, 1899.

‘Experimental Investigations on Telegony.’ A paper read before the Royal Society, London, June 1, 1899. By Professor J. C. Ewart.

‘La Vie de Pasteur.’ Par René Vallery-Radot. Paris: Hachette, 1900.

‘Pasteur.’ By Percy Frankland and Mrs. Percy Frankland. (Century Science Series.) London: Cassell, 1898.

‘The Soluble Ferments and Fermentation.’ By J. Reynolds Green. (Cambridge Natural Science Manuals.) Cambridge University Press, 1899.

‘Micro-organisms and Fermentation.’ By Alfred Jörgensen. Translated by A. K. Miller and A. E. Lennholm. Third edition. London: Macmillan, 1900.

‘Lectures on the Malarial Fevers.’ By William Sydney Thayer, M.D. London: Henry Kimpton, 1899.

‘On the Rôle of Insects, Arachnids, and Myriapods as Carriers in the Spread of Bacterial and Parasitic Diseases of Man and Animals. A Critical and Historical Study.’ By George H. F. Nuttall, M.D., Ph.D. ‘Johns Hopkins Hospital Reports,’ vol. viii.

‘Instructions for the Prevention of Malarial Fever.’ Liverpool School of Tropical Medicine. Memoir I. Liverpool: University Press, 1899.

‘Report of the Malaria Expedition of the Liverpool School of Tropical Medicine and Medical Parasitology.’ By Ronald Ross, D.P.H., M.R.C.S.; H. E. Annett, M.D., D.P.H.; and E. E. Austen. Liverpool School of Tropical Medicine. Memoir II. Liverpool: University Press, 1900.

‘A System of Medicine, by many Writers.’ Edited by Thomas Clifford Allbutt, M.A., M.D., LL.D., vol. ii., 1897; vol. iii., 1897. London: Macmillan and Co.

‘A Handbook of the Gnats and Mosquitoes.’ By Major George M. Giles, I.M.S., M.B., F.R.C.S. London: John Bale, Sons, and Danielsson, Limited, 1900.

‘Reports to the Malaria Committee, Royal Society, 1899 and 1900.’ By various authors. London: Harrison and Sons, 1900.

Proceedings of the Boston Society of Natural History, vol. xvi., 1874.

U.S.A. Department of Agriculture, Division of Entomology, Bulletin 4, new series.

‘Manchester Memoirs,’ vol. li., 1906, p. 1; Quarterly Journal of Microscopical Science, vol. li., 1907, p. 395.

‘Endowments of the University of Cambridge.’ Edited by John Willis Clark, M.A., Registrar of the University of Cambridge. Cambridge: University Press, 1904.

‘Report of a Meeting held at Devonshire House on January 31, 1899, to inaugurate the Cambridge University Association.’ Cambridge: University Press, 1899.

‘Statements of the Needs of the University.’ Cambridge: University Press, 1904.

‘University Accounts for the Year ended December 31, 1904.’ Cambridge University Reporter, March 17, 1905.

‘Abstracts of the Accounts of the Colleges.’ Cambridge University Reporter, February 10, 1905.

PEARLS AND PARASITES

Know you, perchance, how that poor formless wretch—

The Oyster—gems his shallow moon-lit chalice?

Sir Edwin Arnold.

Certain Eastern peoples believe that pearls are due to raindrops falling into the oyster-shells which conveniently gape to receive them.

‘Precious the tear as that rain from the sky

Which turns into pearls as it falls on the sea,’

as the poet Moore writes. This belief is of ancient origin, and is probably derived from classical sources, since Pliny tells us that the view prevalent in his time was that pearls arise from certain secretions formed by the oyster around drops of rain which have somehow effected an entrance into the mantle cavity of the mollusc. Probably this theory of the origin of pearls has ceased to be held for many centuries except in the East, where tradition has always received more credit than experiment. In the West it has long been known that pearls are formed as a pathological secretion of the mineral arragonite, combined with a certain amount of organic material, formed by the oyster or other mollusc around some foreign body, whose presence forms the irritant which stimulates the secretion. This secretion is of the same chemical and mineralogical nature as the mother-of-pearl which gives the inside of the shell of so many molluscs a beautiful iridescent sheen.

An oyster-shell consists of three layers, the outermost termed the periostracum, the middle the prismatic layer, and the innermost the nacreous layer. Everywhere the shell is lined by the mantle, consisting of a right and left fold or flap of the skin, which is in contact with the nacreous layer all over the inside of the shell. The edge of the mantle is thickened and forms a ridge or margin; and it is this edge which secretes the two outer layers. This permits the shell to grow at its edge whilst the rest of the mantle secretes all over its surface the nacreous or pearly layer. The relative thickness of these three layers varies very greatly. In the fresh-water mussel (Unio) the nacreous layer is many times thicker than the two outer layers put together; and such nacreous shells are usually associated with molluscs which are known to represent very ancient or ancestral species. It is also the layer which disappears most readily as the specimens become fossilized; and in fossil Mollusca it is often represented by mere casts, which fill the position it once occupied.

The fact that the nacre is deposited by the whole surface of the mantle has been appreciated by the Chinese. By inserting little flattened leaden images of Buddha between the mantle and the shell, and leaving the oyster at rest for some time, the image becomes coated with mother-of-pearl and incorporated in the substance of the shell; and in this way certain little joss figures are produced. This industry is said to support a large population in some coast districts of Siam.

The nacre, then, is produced by the outermost layer of the mantle or fleshy flap that lines the shell—the external epithelium; and, if a foreign body gets between this epithelium and the shell, the mantle will, in order to protect itself, secrete a pearly coat around it. But valuable pearls are not those which are partially or wholly fused with the shell, but those which lie deep in the tissues of the body; and they are probably formed in the following manner: The intrusive, irritant body forms a pit in the outer surface of the mantle; this pit deepens, and at first remains connected with the outside by a pore; ultimately the pore closes, and the bottom of the pit becomes separated as a small sac free from all connexion with the outside. The sac now sinks into the tissues of the oyster, enclosing in it the foreign body. It will be noticed that the inside of the sac is lined by and is derived from the same tissue or epithelium as covers the outside of the mantle. Now this epithelium continues to do what it has always been in the habit of doing; that is, it secretes a nacreous substance all round the intrusive particle. Layer after layer of this nacre is deposited, and thus a pearl is formed. At first the layers will conform roughly to the outline of the embedded body, but later layers will smooth over any irregularities of the nucleus around which they are deposited, and a spheroidal or spherical pearl is produced. If the irregularities are too pronounced, an irregular pearl is formed; and such pearls, on merely æsthetic grounds, command a lower price.

It is thus clear that pearls are formed around intrusive foreign bodies; and until comparatively recently these bodies were thought to be inorganic particles, such as grains of sand. Recent research has, however, shown that this is seldom the case, and that as a rule the nucleus, which must be present if a pearl is to be formed, is the larva of some highly-organized parasite whose life-history is certainly complicated but as yet is not completely known. The knowledge, however, which we already possess enables us to do much to ensure steady success in a very speculative industry; and with complete knowledge there is no reason why pearl fisheries should not be under as good control as oyster fisheries now are.

It was about fifty years ago (1857-1859) that the problem of the Ceylon pearl-oyster fishery was first attacked in a thoroughly scientific spirit by a certain Dr. Kelaart. His reports to the Government of the island contain the following suggestive sentences:

‘I shall merely mention here that M. Humbert, a Swiss zoologist, has, by his own observations at the last pearl fishery, corroborated all I have stated about the ovaria or genital glands and their contents; and that he has discovered, in addition to the Filaria and Circaria (sic), three other parasitical worms infesting the viscera and other parts of the pearl-oyster. We both agree that these worms play an important part in the formation of pearls; and it may be found possible to infect oysters in other beds with these worms, and thus increase the quantity of these gems. The nucleus of an American pearl drawn by Möbius is nearly of the same form as the Circaria found in the pearl-oysters of Ceylon. It will be curious to ascertain if the oysters in the Tinnevelly banks have the same species of worms as those found in the oysters on the banks off Arripo.’

Unfortunately Dr. Kelaart died shortly after making this report, leaving his investigations incomplete.

Some seven years before, in 1852, Filippi had shown that the pearls in our fresh-water mussel (Anodonta) were formed by the larvæ of a fluke (a trematode), to which he gave the name of Distomum duplicatum. Many students of elementary biology, as they painfully try to unravel the mystery of molluscan morphology, must have come across small pearls in the tissues of the fresh-water mussels (Unio or Anodonta); but these are said to have less lustre and to be more opaque than the sea pearl; so the pearl fisheries of the Welsh and Scotch rivers are falling into disuse. Our ancestors, however, thought otherwise. Less than fifty years ago the Scotch fisheries brought in some £12,000 a year; and a writer of the early part of the eighteenth century describes Scotch pearls as ‘finer, more hard and transparent than any Oriental.’ British pearls were highly thought of by the Romans. Pliny and Tacitus mention them; and Julius Caæsar is said to have dedicated a breastplate ornamented with British pearls to Venus Genitrix. Fresh-water pearls are still ‘fished’ with profit in Central Europe; but the Governments of Bavaria, Saxony, and Bohemia watch over the industry and only grant a licence to fish any stretch of water about once in twelve years—a restriction which, had it been imposed on our fisheries, might have saved a vanishing industry.

In 1871 Garner showed that the pearls in the edible mussel (Mytilus edulis), which is largely used for bait upon our coasts, were formed round the larvæ of a fluke, a remote ally of the liver-fluke that causes such loss to our sheep-breeders. This origin of pearls has been more completely followed out by Mr. Lyster Jameson. Nor must we forget to mention the researches of Giard (1897) and Dubois (1901) in the same subject. We know the life-history of the organism forming pearls in this edible mussel more completely than we do that of any other pearl-forming parasite; and, before returning to the Ceylon pearls, we will briefly consider it.

Mr. Lyster Jameson finds that the pearls of the Mytilus are formed around the cercaria or larval form of a fluke which, in its adult stages, resides in the intestine of the scoter (Œdemia nigra), and was originally described from the eider-duck (Somateria mollissima) in Greenland and named Leucithodendrium somateriæ, after its first known host. The cercaria larvæ of these flukes form the last stage in a complex series of larval forms which occur in the life-history of a trematode or fluke, and they differ from the adult in two points—their generative organs are not fully developed, and they usually have a tail; but this organ is wanting in our pearl-forming cercaria, called a cercariæum by Mr. Jameson. Such a larva has only to be swallowed by a scoter to grow up quickly into the adult trematode capable of laying eggs. Now this bird, called by the French fishermen the ‘cane moulière,’ is the greatest enemy to the mussel-beds; it is not only common around the French mussel-beds of Billiers (Morbihan), but occurs in numbers at the mouth of the Barrow channel, close to our English pearl-bearing mussel-beds. With its diving habits it destroys and eats large quantities of the mollusc. Those cercariæ which are already entombed in a pearl cannot, of course, grow up into adults, even if they gain entrance to the alimentary canal of the scoter; but those that are not ensheathed may do so. Further, the fluke may possibly live in other hosts where no pearl is formed. At any rate, there seems no lack of larvæ successful in their struggle to attain maturity, for it has been calculated that the alimentary canal of an apparently healthy scoter may harbour as many as six thousand adult flukes.

Thus there are two courses open to the cercaria when it has once found its way into the mussel; it either forms the nucleus of a pearl and perishes, or it is swallowed by a scoter, becomes adult, and prepares to carry on the race. But how do the cercariæ make their way into the mussel, and whence do they come? At present their birth, like that of Mr. Yellowplush, is ‘wrapped up in a mistry.’ We may presume that the eggs make their way out of the scoter into the sea-water, and that there they hatch out a free-swimming larva, which, after the manner of trematodes, swims about looking for a suitable host. Within this host it would come to rest and begin budding off numerous secondary larvæ, in which stage it may assume considerable size and becomes known as a sporocyst. No one, however, has seen the eggs hatch, or the free-swimming larva; but Mr. Jameson produces evidence to show that the sporocyst stage occurs in two other common molluscs—viz., in a clam (Tapes decussatus) and in the common cockle (Cardium edule). The former mollusc abounds in the black gravelly clay which forms the bottom of the mussel-beds at Billiers; and every specimen out of nearly two hundred examples investigated by Mr. Jameson was found to be infested with sporocysts containing larvæ closely resembling those which act as pearl-nuclei in the edible mussel. Exactly similar sporocysts were found in about fifty per cent. of the common cockles examined in the Barrow channel, where the species Tapes decussatus does not occur.

Within the sporocyst certain secondary larvæ are formed, as is habitual with the flukes. These secondary larvæ are the cercariæ; and it is in this stage that the animal makes its way into the pearl-mussel and ultimately forms the nucleus of a pearl. Precisely how it leaves the sporocyst and the first host—i.e., the Tapes or Cardium—is not known. Certain experiments made by Jameson, who placed mussels which he thought were free from parasites in a tank with some infected Tapes, are not quite conclusive, and have been ably criticized by Professor Herdman. It is true that, when examined later, the mussels were well infected; but it was not definitely shown that they were not infected at the start; and further, the numbers used were too small to justify a very positive conclusion. Still, on the whole, it may safely be said that life-history of the organism which forms the pearls in Mytilus edulis probably involves three hosts: the scoter, which contains the mature form; the Tapes or Cardium, which contains the first larval stage; and the mussel, which contains the second larval stage, which forms the pearl.

Recently Professor Dubois has been investigating the origin of pearls in another species of Mytilus (M. galloprovincialis) which lives on the French Mediterranean littoral. The nucleus of this pearl is also a trematode, but of a species different from that which infests the edible mussel. The interest of Professor Dubois’ work, however, lies in the fact that he claims to have infected true Oriental pearl-oysters by putting them to live with his Mediterranean mussels. He fetched his oysters, termed ‘Pintadin,’ from the Gulf of Gabes in Southern Tunis, where they are almost pearlless—one must open twelve to fifteen hundred of these to find a single pearl—and brought them up amongst the mussels. After some time had elapsed they became so infected that three oysters opened consecutively yielded a couple of pearls each. These observations, however, require confirmation, and have been adversely criticized by Professor Giard.

To return to the Ceylon pearls. The celebrated fisheries lie to the north-west of the island, where the shallow plateaux of the Gulf of Manaar afford a fine breeding-place for the pearl-oyster. The pearl-oyster is not really an oyster, but an allied mollusc known as Margaritifera vulgaris. It lives on rocky bottoms known locally as paars. The fisheries are very ancient and have been worked for at least 2,500, perhaps for 3,000 years. Pliny mentions them, but he is,