THE CEYLON PEARL-OYSTER. 32 
‘pearly mass, with a central cavity, showing at one side a plug of 
conchyolin-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 blister is caused by derange- 
ment of the secreting epithelium (compare the “ Olflecken” in 
Margaritana, Rubbel, 34a). ‘The opaque 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), rendered the real nature 
of the pseudo-nucleus obscure till sections were cut, Outside 
these abnormal and pathological layers typical nacre was sub- 
sequently produced, thus giving a normal pearl with a dark 
centre. 
The same characters are shown on Pl. 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 min. 
in diameter, and, examined entire, might have been taken for a 
dead parasite. It was such a nucleus, coupled with the presence 
of Trematodes, probably Muttua margaritifere Shipley & Hornell, 
in the tissues of Dr. Kelaart’s pearl-oysters, that, in 1901, led me 
to the 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 
Gymnophallus does in Mytilus * (25, p. 162). 
But examination of sections (Pl. XXXVII. fig. 16 and 
Pl. XX XVIII. fig. 17) showed that the opaque pseudo-nucleus 
was due to a break in the continuity of the nacre; a layer of 
granular substance (g7.), 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. XX XVIII. 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 Pls. XX XVIII.-XL. figs. 19-28 and Pl. XLIV. fig. 49 
are drawn, while the pearls agree with those described above in 
their clustered habit, occurrence in the muscular regions, and 
association with hypostracum-pearls, we have more frequently as 
nuclei either hypostracum or special sphzrocrystal-like bodies, 
which I regard as formed of columnar repair-substance. 
* Fuller knowledge and closer study lead me to doubt the accuracy of my own 
observations as to the occurrence of the remains of Trematodes in the pearls produced 
by the other species of molluscs referred to on p. 162 of my 1902 paper with the 
exception, of course, of Mytilus edulis, in wnich the relation between Trematodes 
and pearls is beyond question, 
(09) 
