THE CEYLON PEARL-OYSTER. Bp 
T adopt Professor Herdman’s term ‘ Muscle-Pearls” for the 
former class, while for the latter category I propose the name 
‘“* Parenchyma-Pearls ”*, because they occur typically in the 
parenchymatous subepidermal tissues of the non-muscular parts 
otf the body-wall and mantle, or, by secondary displacement, in 
the more deeply seated soft tissues. 
This group corresponds, I think, to Herdman’s “ Cyst-Pearls,” 
but I prefer not to adopt the latter name, as, if the word “ cyst” 
refers to the encysted Cestode, which Herdman associated with 
pearl-production, I have been unable to trace the connection 
between it and the pearl; while if it refers to the pearl-sac or 
“cyst,” this is found around all pearls, including muscle-pearls. 
There is some reason for believing that some par renchyma- -pearls 
arise from causes different from those that lead to the formation 
of muscle-pearls, and, indeed, it is quite possible that parenchyma- 
pearis have several modes of origin, as Herdman believes ; but, 
on the other hand, their differences may be due in great measure 
to the different parts of the tissues in which they originate, 
and it is certainly quite impossible, in many cases, to say, from 
the structure of a pearl and of its nucleus and pseudo- nucleus, 
whether it is a “ muscle-pearl” or a “ parenchyma- -pearl.” W ith 
regard to Herdman’s “ Ampullar pearls,” I cannot regard this 
group as of equal value to the above two classes, as, in my 
experience, so far as it goes, the ““Ampulla” is of secondary origin, 
due to the absor ption of the tissues intervening between the pearl 
and the shell, and to the epithelium of the pearl-sac and that of 
the outer face of the mantle thus becoming continuous. 
Before going further I had better explain a term that I am 
introducing into this paper. I am_ restricting the word 
‘* Nucleus,” as applied to the body found in the centre of a 
pearl, to those bodies which appear to be either of foreign 
origin or derived from the pearl-oyster otherwise than through 
the agency of the shell-secreting mechanism. To the bodies 
formed by the shell- and _pearl- -secreting mechanism, composed, 
asa rule, of different kinds of repair- -substance (bodies which 
have no doubt often been wrongly mistaken for objects of foreign 
origin), I propose to apply the name “ Pseudo-nucleus.” I have 
endeavoured to be consistent in the use of these two terms, but, 
as is so often the case in biological matters, there is at times a 
difficulty in defining a sharp boundary-line between the objects 
to which they are respectively applied. 
A. Muscle-Pearls. 
I have set out above (p. 267) Professor Herdman’s views on 
the nature and origin of these. Briefly recapitulated, they are 
the following. From some unknown cause, minute calcareous 
* Rubbel (34 a) applies the term “ Mantelperlen” to these bodies, a term which 
I prefer to mine, though it is too late to alter the nomenclature in this paper. 
Proc. Zoou. Soc.—1912, No. X XI. 2) 
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