218 Journal of the Asiatic Society of Bengal. [N.S., XVIII, 
lime salts, but continue the secretion of conchyolin thus 
giving a periostracal appearance to what would normally be a 
nacreous layer (conchyolin + carbonate of lime). 
Another deduction which I have made from the investiga- 
tion, is that only dead or dying parasites excite an irritation 
of the character necessary to induce pearl formation. A living 
parasite does not irritate the tissues in the same way ; indeed 
it merely induces the formation of a tough connective tissue 
sheath or cyst enveloping it wherein it lies quiescent and 
harmless, giving no further irritation. But in the case of a 
parasitic larva that arrives in the epithelium in a dying con- 
dition, exhausted or perhaps smothered in the secreted fluid 
poured out by the epithelial cells, a different situation is found. 
Instead of being within a layer of connective tissue, it lies in a 
depression of the epithelial layer of cells and these act different- 
y from connective tissue cells—with a correspondingly diver- 
gent result. 
In regard to the second and more numerous class of cyst 
pearls usually however much smaller in size than those of the 
first class, decalcification shows no definite nucleus other than 
a tiny amorphous scrap of brownish refractive substance, simi- 
lar apparently to periostracum. Rubbel of Marburg has in- 
vestigated the origin of pearls with a similar form of nucleus 
obtained from freshwater mussels. He showed that granules 
of the same appearance not infrequently appear in the secret- 
ing epithelial layer of the mantle. These at times appear to 
cause an irritation that induces the adjacent cells forthwith 
to begin the deposit of nacre upon these refractive bodies; 
later by radial division and multiplication these cells form a 
minute pearl-sac around each nuclear body, which continues 
the deposit of concentric layers of nacre and thereby produces 
a pearl. The same sequence of events occurs in the Indian 
pearl oyster eventuating as above stated in the production of 
the majority of cyst pearls found in the mantle. The irrita- 
tion produced is so slight that no shock is experienced and 
therefore no periostracal repair substance is deposited prior to 
the first nacreous layer. 
e third class, muscle pearls, remain for consideration. 
From their place of origin being invariably close to the inser- 
tion of muscles attached to the shell and from the columnar 
foreign body or an unwanted particle of periostracum does and 
with similar effects : &@ minute pearl sac is formed, enveloping 
the particle, which however in this case begins by secreting 
