28 CEYLON PEARL OYSTER REPORT. 



CONCLUSIONS ON PEAKL-FORMATION. 



We may now sum up our views as to pearl-formation in the Ceylon pearl oyster 

 as follows (using with but slight changes and additions the wording of our 1903 note* 

 on the subject) : 



1. The majority of pearly excrescences on the interior of the shell are due to the 

 irritation caused by Clione, Leucodore, and other boring animals. In exceptional 

 cases a free pearl may be formed in this way. 



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

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

 e.g., 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. The ectoderm, 

 in such cases, would probably also be damaged, and cells may be carried in with the 

 inorganic particles. 



3. Many pearls are found in the muscles close to the surface, especially at the 

 levator and pallial insertions, and these are formed around minute calcareous concre- 

 tions, the " calcospherules," which are produced in the tissues and form centres of 

 irritation. These are, in all cases, close to the surface of the mantle, or even in 

 contact with the ectoderm. 



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

 remains of Cestode 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. Probably in all cases, whatever its nucleus may 

 be, the pearl, like the nacre, is deposited by an epithelial layer derived from the 

 ectoderm. 



These four categories are separated according to the cause of the stimulation. The 

 first set, however, can scarcely be considered as " pearls," and the others may be 

 conveniently classified under the following three names : 



I. Ampullar- pearls, where the nucleus and resulting pearl lie between the shell 

 and the body, or in a pouch (the ampulla) of the ectoderm projecting into the mantle. 

 The others lie in closed (ectodermal) sacs. 



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



III. Cyst pearls, formed around encysted parasites. The parasite in the case oi 

 the majority of the cyst pearls of Ceylon is the larva of one or more species ot 

 Cestodes, belonging to the genus Tetrarhynchus. 



It seems possible that in Placuna placenta, the " vitre chinoise " or window oyster 

 of Tampalakam Lake near Trincomalee, a Distomid parasite which we find in the 

 tissues both free and encysted, may also occasionally be a cause of pearl-formation. 



* ' British Association Report, Southport,' p. 695. 



