18 CEYLON PEARL OYSTER REPORT. 



Before leaving the Mytilus material, we may add two further points of interest. 

 The first is that some pearls have no trace of a nucleus. Plate I., fig. 5, shows a 

 case where a careful search through all the sections (serial) showed no internal cavity 

 and no imhedded foreign structures We have similar cases also in our Ceylon 

 material. The second point is that in some places the pearl sac shows a mass of 

 enlarged and proliferating epithelial cells which are generally adherent to the pearl 

 at points where there is a depression and a marked irregularity in the deposition of 

 the layers (Plate I., figs. 10 and 11). Some of the Mytilus pearls are exceedingly 

 irregular in form, projections being given off which appear like separate pearls in 

 some of the sections (fig. 11). In addition to such cases, there are sometimes two or 

 more pearls in the same sac (fig. 2), and in figs. 6 and 7 we find a pearl and a 

 parasite enclosed together by the one layer of epithelium. In some places small blood 

 sinuses adjoin the pearl-sac for portions of its extent, but these are not larger than 

 those seen elsewhere in the mantle of Mytilus. We do not find that the pearl-sac is 

 surrounded by a blood sinus, as Boutan states is the case. 



Encysted Cestodes. 



The smallest and simplest cysts we have seen in the Ceylon pearl oyster are in the 

 mantle (Plate II., fig. 1). They have no pearl and no pearl-secreting epithelial sac, 

 and the connective-tissue cyst contains an embryo which shows only an outer wall 

 and some irregularly scattered internal cells. It is presumably an onchosphere or 

 pro-scolex stage in which the hooks have been lost and invagination to form the scolex 

 has not yet taken place. 



Similar early stages are found also in the gills, either in the principal gill 

 filaments (see Plate II., fig. 3), or, more usually, alongside the great blood-vessels in 

 the axis of the gills where they adjoin the body. 



The majority of the cysts, however, contain later stages (text-fig. 3) where more or 

 less invagination to produce the scolex has taken place. These measure from - 07 millim. 

 to - 16 millim. in longest diameter, most of them are about - 14 millim. A number 

 may be present in the same host ; we have frequently seen two close together in the 

 same section, under the microscope. Figs. 17 to 22 on Plate II. show several of these 

 stages, from the liver, the gonads, and the mantle. One end of the globular or ovate 

 parasite forms a cup-like invagination with a central boss or papilla rising from the 

 bottom of the cup. In some cases the margin of the cup is turned in so as almost to 

 close the aperture. Bound the outside of the invagination there may be more or less 

 of a projecting pad in the form of a collar or annular thickening. This form (seen 

 typically in Plate II., fig. 17) agrees very closely with the figures given by Seurat 

 for the pearl-causing Cestode parasite of the Gambier Islands,* and with the figures 

 given by Shipley and Hornell of the nuclei of pearls in Part II. of this Report 



* See GlARD, " L'Origiue Parasitaire des Perles," ' Comptes Rendus Sue. Biol.,' lv., p. 1223. 



