April 30, 1903] 



NA TURE 



62 1 



paar," under the impression that the young oysters that 

 come and go in fabulous numbers migrate or are carried 

 inwards and supply the inshore paars with their popula- 

 tions. During a careful investigation of the Periya Paar 

 and its surroundings, we satisfied ourselves that there is no 

 basis of fact for this belief ; and it became clear to us that 

 the successive broods of young oysters on the Periya Paar, 

 amounting probably within the last quarter century alone 

 to many millions of millions of oysters, which if they had 

 been saved would have constituted enormous fisheries, have 

 all been overwhelmed by natural causes, due mainly to the 

 configuration of the ground and its exposure to the south- 

 west monsoon. 



A -tudv of the history of the Periya Paar for the last 

 twenty-four years [given more fully in the lecture] shows 

 that since 1880 the bank has been naturally restocked with 

 young ovsters at least eleven times without yielding a 

 fisherw 



The 10-fathom line skirts the western edge of the paar, 

 and the ioo-fathom line is not far outside it. An examin- 

 ation of the great slope outside is sufficient to show that 

 the south-west monsoon running up towards the Bay of 

 Bengal for six months in the year must batter with full 

 force on the exposed seaward edge of the bank and cause 

 great disturbance of the bottom. We made a careful survey 

 of the Periya Paar in March, 1902, and found it covered 

 with young oysters a few months old. In my preliminary 

 report I estimated these young oysters at not less than a 

 hundred thousand millions, and stated my belief that these 

 were doomed to destruction, and ought to be removed at 

 the earliest opportunity to a safer locality further inshore. 

 Mr. Hornell was authorised to carrv out this recommenda- 

 tion, and went to the Periya Paar early in November with 

 boats and appliances suitable for the work, but found he 

 had arrived too late. The south-west monsoon had inter- 

 vened, the bed had apparently been swept clean, and the 

 enormous population of young oysters, which we had seen 

 in March, and which might have been used to stock many 

 of the smaller inshore paars, was now in all probability 

 either buried in sand or carried down the steep declivity 

 into the deep water outside. This experience, taken along 

 with what we know of the past history of the bank as 

 revealed by the inspectors' reports, shows that whenever 

 young oysters are found on the Periya Paar, they ought, 

 without delay, to be dredged up in bulk and transplanted 

 to suitable ground in the Cheval district — the region where 

 the most trustworthy paars are placed. 



From this example of the Periya Paar it is clear that in 

 considering the vicissitudes of the pearl oyster banks we 

 have to deal with great natural causes which cannot be 

 removed, but which may to some extent be avoided, and 

 that consequently it is necessary to introduce large measures 

 of cultivation and regulation in order to increase the adult 

 population on the grounds, give greater constancy to the 

 supply, and remove the disappointing fluctuations in the 

 fishery. 



There are in addition, however, various minor causes of 

 failure of the fisheries, some of which we were able in 

 investigate. The pearl oyster has manv enemies, such as 

 star-fishes, boring sponges which destroy the shell, boring 

 Molluscs which suck out the animal, internal Protozoan ami 

 Yermean parasites and- carnivorous fishes, all of which cause 

 some destruction, and which may conspire on occasions to 

 ruin a bed and change the prospects of a fishery. But in 

 connection with such zoological enemies, it is necessary to 

 bear in mind that from the fisheries point of view their 

 influence is not wholly evil, as some of them are closely 

 associated with pearl production in the oyster. One enemy 

 (a Plectognathid fish) which doubtless devours many of the 

 oysters, at the same time receives and passes on the parasite 

 which leads to the production of pearls in others. The loss 

 of some individuals is in that case a toll that we very 

 willingly pay, and no one would advocate the extermination 

 of that particular enemy. 



In fact the oyster can probably cope well enough with 

 its animate environment if not too recklessly decimated at 

 the fisheries, and if man will only compensate to some ex- 

 tent for the damage he does by giving some attention to the 

 breeding stock and " spat," and by transplanting when 

 required the growing young from unsuitable ground to 

 known and trustworthy " paars." 



NO. 1748, VOL. 67] 



Those were the main considerations that impressed me 

 during our work on the banks, and were, therefore, the 

 leading points dealt with in the conclusions given in my 

 preliminary report (July, 1902), which ended as follows : — 



" To the biologist two dangers are, however, evident, 

 and, paradoxical as it may seem, these are overcrowding 

 and overfishing. But the superabundance and the risk of 

 depletion are at the opposite ends of the life cycle, and there- 

 fore both are possible at once on the same ground — and 

 either is sufficient to cause locally and temporarily a failure 

 of the pearl oyster fishery. What is required to obviate 

 these two dangers ahead, and ensure more constancy in the 

 fisheries, is careful supervision of the banks by someone 

 who has had sufficient biological training to understand the 

 life-problems of the animal, and who will therefore know 

 when to carry out simple measures of farming, such as 

 thinning and transplanting, and when to advise as to the 

 regulation of the fisheries." 



In connection with cultivation and transplantation, there 

 are various points in structure, reproduction, life-history, 

 growth and habits of the oyster which we had to deal with, 

 and some of which we were able to determine on the banks, 

 while others have been the subject of Mr. Hornell 's work 

 since, in the little marine laboratory we established at 

 Galle. [Discussed and illustrated by lantern slides in the 

 lecture.] 



Turning now from the health of the oyster population on 

 the " paars " to the subject of pearl formation, which is 

 evidently an unhealthy and abnormal process, we find that 

 in the Ceylon oyster there are several distinct causes that 

 lead to the production of pearls. Some pearls or pearly 

 excrescences on the interior of the shell are due to the 

 irritation caused by boring sponges and burrowing worms. 

 Minute grains of sand and other foreign bodies gaining 

 access to the body inside the shell, which are popularly sup- 

 posed to form the nuclei of pearls, only do so, in our 

 experience, in exceptional circumstances. Out of the 

 many pearls I have decalcified, only one contained in its 

 centre what was undoubtedly a grain of sand ; and from 

 Mr. Hornell's notes, taken since I left Ceylon, I quote the 

 following passage, showing that he has had a similar ex- 

 perience : — 



" February 16, 1903 — Ear-pearls. Of two decalcified, 

 one from the anterior ear (No. 148), proved to have a 

 minute quartz grain (micro, preparation 25) as nucleus." 



It seems probable that it is only when the shell is injured, 

 as, for example, by the breaking off or crushing of the 

 projecting " ears," thereby enabling some fine sand to gain 



access to the interior, that such inorg 



particles supply 



the irritation which gives rise to pearl formation. 



The majority of the pearls found free in the tissues of the 

 body of the Ceylon oyster contain, in our experience, the 

 more or less easily recognisable remains of Platyelmian 

 parasites ; so that the stimulation which causes eventually 

 the formation of an " orient " pearl is, as has been sug- 

 gested by various writers in the past, due to infection by 

 a minute lowly worm, which becomes encased and dies, 

 thus justifying, in a sense, Dubois's statement that — " La 

 plus belle perle n'est done, en definitive, que le brillant 

 sarcophage d'un ver " (Comptes rendus, October 14, 1901.) 



[The lecturer then dealt with the work of Dr. Kelaart 

 (1859), to whom belongs the honour of having first con- 

 nected the formation of pearls in the Ceylon oyster with 

 the presence of Vermean parasites, Filippi, Kukenmeister, 

 Moebius, Humbert, Garner, Thurston, Giard, Seurat, 

 i Jameson, and finally Dubois — bringing the record up to 

 January, 1903.] 



We have found, as Kelaart did half a century ago, that 

 in the Ceylon pearl oyster there are several different kinds 

 of worms commonly occurring as parasites, and we shall, I 

 think, be able to show in our final report that Cestodes, 

 Trematodes, and Nematodes are all concerned in pearl 

 formation. Unlike the case of the European mussels, how- 

 ever, we find, so far, that in Ceylon the most important 

 cause is a larval Cestode of the Tetrarhynchus form. Mr. 

 Hornell has traced a considerable part of the life-history of 

 this parasite, from an early free-swimming stage to a late 

 larval condition in the file fish (Bcllslcs milis) which 

 frequents the pearl banks and prevs upon the ovsters. We 

 have not yet succeeded in finding the adult, but it will 

 probably prove to infest the sharks or other large Elasmo- 



