THE CEYLON PEARL-OYSTEll. 327 



pearly mass, with a central cavity, showing at one side a plug of 

 c()n(!hyoliH-like substance. External to the normal central nacre 

 were some irregular layers, such as one gets on the inner surface 

 of the shell when a dark blotch or l)lister is caused by derange- 

 ment of the secreting epithelium (compare tlie " Olfiecken " in 

 Margarttaua, Rubljel, 34 a). The opacpie character of these 

 layers, some of which were brown through the immigration or 

 infiltration of what appeared to be cellular matter, others 

 distinctly columnar (repair-substance), rendei'ed the real nature 

 of the pseudo-nucleus obscure till sections were cut. Outside 

 tliese abnormal and pathological layers typical nacre was sub- 

 sequently produced, thus giving a normal pearl with a dark 

 centi-e. 



The same characters are shown on PI. XLIV. fig. 48, where 

 the centre of the pearl appears opaque and granular for a similar 

 reason. In this case the pseudo-nucleus measured about 5 nnn. 

 in diameter, and, examined entire, might have been taken for a 

 dead parasite. It was such a nucleus, coupled with the presence 

 of Trematodes, in-oh;ih]y Mitttua margaritifera' Shipley & llornell, 

 in the tissues of Dr. Kelaart's pearl-oysters, that, in 1901, led me 

 to tlie probably mistaken conclusion that a Trematode might be 

 one of the organisms which afford the stimulus for the formation 

 of the pearl-sac in Margaritifera vulgaris, as the Trematode 

 Gymno'phaUus does in Mytilus * (25, p. 162). 



But examination of sections (PI. XXXVII. fig. 16 and 

 PI. XXXVIII. fig. 17) showed that the opaque pseudo-nucleus 

 was due to a break in the continuity of the nacre ; a layer of 

 granular substance (r/r.), apparently dead cells (perhaps of the 

 nature of the " oil-spots " in Margaritana, or derived from an 

 immigration of leucocytes such as Moynier de Villepoix observed 

 (28, p. 112) or from Protozoan parasites), being followed by the 

 secretion of two horny layers of amorphous repair-substance 

 {am., am.'), after which normal nacre resumed its development. 

 The real centre of this pearl, as in the rest of Kelaart's material, 

 consists (PL XXXVIII. fig. 18) of shreds of conchyolin-like 

 material, and a few obscure granules, in a cavity which is sur- 

 rounded first by granular repair-nacre and then by ordinary 

 nacre. 



In the two unlabelled specimens in the British Museum, from 

 which Pis. XXXVIII.-XL. figs. 19-28 and PI. XLIV. fig. 49 

 are drawn, while the pearls agree with those described above in 

 their clustei'ed habit, occurrence in the muscular regions, and 

 association with hyjwstracum-pearls, we have more frequently as 

 nuclei either hypostraciim or special sphserocrystal-like bodies, 

 which I regard as formed of columnar repair-substance. 



* Fuller knowledoce and closer study lead me to doul)t the accuvacy of my own 

 observations as to the occurrence of the remains of Trematodes in the pearls produced 

 by the other species ot molluscs referred to on p. IG'2 of my 1902 paper with the 

 exception, of course, of Mijtilus edulis, in which the relation between Trematodes 

 and pearls is beyond question. 



