THE CEYLOX PEARL-OYSTER. 331 



Cestodes may possibly occur in the centres of these pearls at 

 times, as Professor Herdman maintains, just as grains of sand 

 and other organic and inorganic particles certainly do ; but this 

 remains to be proved, and my material has afforded no evidence 

 in favour of this much-quoted theory. And, even if it should 

 l)e proved that Cestodes do occasionally occur as the " nuclei" of 

 Ceylon pearls, the real causes which underlie the development 

 of the pearl-sac (which is the essential factor in pearl-production) 

 have still to be discovered. The characters of the centres of 

 parenchyma-pearls can best be understood by reference to the 

 next section of the paper, which is largely desci'iptive of them, 

 though many of the pearls referred to therein are unquestionably 

 " muscle-pearls." 



(14) Descriptions of the Centres of Individual Pearls. 



In the absence of evidence connecting them with muscle- 

 pearls I propose to treat the four pearls from the Persian Gulf, 

 and the twenty-one peai'ls purchased for me in Ceylon by the 

 Ceylon Company of Pearl Fishers, Ltd., as parenchyma-peai-ls, 

 and to describe along with them eight of the pearls in Prof. 

 Herdman's collection of slides which are not obviously muscle- 

 pearls. These descriptions will be followed by an account of a 

 number of samples and collections of pearls from M. vulgaris 

 from Ceylon and other localities. 



A. Material from the Persian, Gulf. 



It may be useful to begin with the material from the Persian 

 Gulf. This, so far as it has been examined — for some of my 

 specimens still remain unopened, — comprised three pearl-bearing 

 examples, containing four small pearls. All these pearls 

 appeared to be of the kind classified by Prof. Herdman as " cyst- 

 pearls" and by the present Avriter as "parenchyma-pearls," as 

 they seem to have arisen quite independently of, and away from, 

 the muscle-insertions. 



I append a short description of the conditions in each of these 

 cases. 



Preparation LXIV A contained a solitary peai'l, about 2 mm. 

 in diameter and slightly lenticular in shape, situate in the left 

 mantle-lobe, on a level with the anterior end of the attachment 

 of the outer gill-lamella. Unfortunately, owing to imperfect 

 penetration in the embedding process, this pearl did not present 

 a com])lete series in section. It lay wholly in the subejiidermal 

 parenchyma, which was sharply cut off from the underlying 

 A'isceral connective-tissue by a layer of musculature, under which 

 were the tubules of the ovary. The pseudo-nucleus was a radially 

 calcified concentrically laminated body, about 3 mm. in diameter, 

 with a few darkly staining granules in the centre. 



The second specimen, Preparation LXIV B, contained another 



