TUE CEYLON PEARL-OYSTER. 321 



I adopt Professor Herdman's term "Muscle-Pearls"' for the 

 former class, while for the latter category I pi'opose the name 

 " Parenchyma-Pearls " *, be(!ause they occur typically in the 

 parenchymatous subepidermal tissues of the non-muscular parts 

 of the body-wall and mantle, or, by secondaiy 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-productio)i, 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 parenchyma-pearls 

 arise from causes different from those that lead to the formation 

 of muscle-pearls, and, indeed, it is quite possible that parenchyma- 

 pearls 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 tliey originate, 

 and it is certainly quite impossible, in many cases, to saj* from 

 the structure of a pearl and of its nucleus anfl pseudo-nucleus, 

 whether it is a " muscle-pearl " or a " parenchyma-pearl." AVitli 

 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 absorption of the tissues intervening between the pearl 

 and the shell, and to the eijithelium 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 

 t)rigin or derived from the pearl-oyster otherwise than through 

 the agency of the shell-secreting mechanism. l^o the bodfes 

 formed by the shell- and pearl-secreting mechanism, composed, 

 as a 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-nncleus." 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 tiine.s a 

 difficulty in defining a sharp boundary -line between the ol)jects 

 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 



* RuLbd (34rt) applies the term " Mantelperlen " to th se bodies, a term whicb 

 1 prefer to ntiue, though it is too late to alter the nomencla nve in this jiajier. 



Pboc. Zool. Soc— 1912, No. XXI. 21 



