ORIGINE REELLE DES PERLES FINES. 63 



4. Some pearls or peaiiy excrescences on the inlerior of the shell 

 are due to the irritation caused by Clione, Leucodore, and other 

 boring animais. 



2. Minute grains ofsand and other inorganic particles only form 

 the nuclei of pearls under exceptional circumstances. Probably it is 

 only when the shell is injured, e.(j, 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 palliai insertions, and thèse are formed around minute calca- 

 reous concrétions, the 'calcospherules', which are produced in the 

 tissues and form centres of irritation. 



4. Most of the Une pearls found free in the body of the Geylon 

 oyster contain the remains of Platyhelminthian parasites, so that 

 the stimulation which leads to the formation of an 'Orient' pearl is, 

 as bas been suggested by various writers in the past, due to the 

 présence of a minute parasitic worm. In ail cases, what ever its 

 nucleus may be, the pearl, like the nacre, is deposited by an epithe- 

 lial layer. 



Thèse pearls may be convenienly classified as — 



I. Ampullar pearls, where the nucleus and resulting pearl lie in 

 a pouch, or ampulla, of the ectoderm projecting into the mantle. 

 The others lie in closed sacs. 



.11, Muscle-pearls, formed around calcospherules near the inser- 

 tions of muscles. 



m. Cyst-pearls, formed around encysted parasites. The parasite 

 in the case of the majority of the cyst-pearls of Ceylon is the larva 

 of a Gestode which appears to be new, and will be described under 

 the name Tetrarhynchus unionifactor. The younger larval stages 

 hâve 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 Tetrarhynchus is found in the file 

 fishes. Batistes mitis and B. stellatus, which feed upon the 



