TRANSACTIONS OF SECTION D. 695 



TUESDAY, SEPTEMBER 15. 



The following Papers were read : — 



1. Note on Pearl-formation in the Ceylon Pearl Oyster. 

 By Professor W. A. Herdman, D.Sc, F.P.S., and James Hornell. 



Professor Ilerdman and Mr. Hornell have had two cruises of several weeks 

 each amongst the pearl-oyster banks in the Gulf of Manaar, and have had the 

 experience of the three consecutive inspections of March and November 1902 and 

 March 1903, and also the successful fishery of 1903, from which to draw conclu- 

 sions. Many hundreds of oysters have been examined, and large numbers of 

 pearls have been decalcified. As a result of this work they have come to the 

 conclusion that there are several distinct causes that lead to the production of 

 pearls in the Ceylon pearl ' oyster' {Margaritifera vulgaris, Sebum.). 



1. Sorae pearls or pearly excrescences on the interior of the shell are due to 

 the irritation caused by CHone, Leucodore, and other boring animals. 



2. Minute grains of sand and other inorganic particles only form the nuclei 

 of pearls under exceptional circumstances. Probably it is only when the shell is 

 injured, e.^. by the breaking of the 'ears,' thus enabling sand to get into the 

 interior, that such particles supply the irritation that gives rise to pearl- 

 formation. 



3. Many pearls are found in the muscles, especially at the levator and pallial 

 insertions, and these are formed around minute calcareous concretions, the 

 ' calcospherules,' which are produced in the tissues and form centres of irritation. 



4. Most of the fine pearls found free in the body of the Ceylon oyster contain 

 the remains of Platyhelminthian parasites, so that the stimulation which leads 

 to the formation of an ' Orient ' pearl is, as has been suggested by various writers 

 in the past, due to the presence of a minute parasitic worm. In all cases, what- 

 ever its nucleus may be, the pearl, like the nacre, is deposited by an epithelial 

 layer. 



These pearls may be conveniently classified as — 



T. Ampullar pearls, wh-BTQ the nucleus and resulting pearl lie in a pouch, or 

 ampulla, of the ectoderm projecting into the mantle. The others lie in closed sacs. 



II. Muscle-pearls, formed around calcospherules near the insertions of muscles. 



III. Cyst-pearls, formed around encysted parasites. Tlie parasite in the case 

 of the majority of the cyst-pearls of Ceylon is the larva of a Cestode which 

 appears to be new, and will be described under the name TetrarJtynclms unioni- 

 factor. The younger larval stages have been found free-swimming in the Gulf 

 of Manaar and on the gills of the oyster ; later stages are common in the liver, 

 mantle, and gills ; and a more advanced Tetrarhymhus is found in the file fishes. 

 Batistes mitis and B. stellatus, which feed upon the oysters. The sexually mature 

 Cestode has not yet been found, but we may expect it to occur either in one of 

 the large Elasmobrancbs (such as Trygon uarnak) which abound on the pearl 

 banks, or possibly in one of the smaller cetaceans, which may also feed upon such 

 fishes as Batistes. 



2. Oil a Phosphorescence Plicnomenoji in the Indian Ocean. 

 By Professor W. A. Herdman, D.Sc, F.R.S. 



Professor Herdman described how during hia recent expedition to Ceylon, 

 as they lay at anchor in the Gulf of Manaar, on March 13, 1902, about 9 p.m., 

 the sea was seen to be dotted with bright phosphorescent lights, of considerable 

 size, singly placed at some distance apart. These for over an hour continued to 

 glow with a pulsating appearance in harmony, all shining brightly at the same 

 moment, and then all flickering out together, to reappear simultaneously a few 



