PEARL PRODUCTION. 23 



Tetrarhynchids which we have met with either in the pearl oyster or in fishes which 

 we know to feed upon that mollusc (figs. 11, 12, 13, 14, 15, and 16). 



It is quite evident from the examination of a large series of sections, such as we 

 have worked through, that the majority of these encysted parasites, do not become 

 encased in pearls. Probably none of those in thick connective-tissue cysts are destined 

 to form nuclei. They are awaiting their legitimate further development in the next 

 host, after their sheltering mollusc has been devoured by a fish. In such cysts 

 and around such parasites we find no epithelial sac, and as a consequence there can 

 be no pearl. Whether or not it is the case that only dead parasites supply the 

 stimulus necessary to induce pearl-formation, and whether, as GIARD has suggested, 

 the parasites may be infested and killed by a species of Glugea, so that that Sporozoon 

 comes to be eventually responsible for the pearl, we are not prepared to say we 

 have found no fresh evidence in the Ceylon material bearing upon that point. It 

 seems clear to us, however, that the epithelium is always associated with pearl- 

 formation, and that in the absence of the epithelium only a thick-walled connective- 

 tissue cyst is produced. If we adopt the view (see below) that this epithelium 

 is genetically related to the ectoderm, then a possible explanation of the difference 

 in behaviour in the encysted condition would be that those larvae that carried in 

 ectodermal cells became covered (when dead or while still alive) by a pearl sac and 

 embedded in a pearl, while those that were free from ectoderm become surrounded 

 by the connective-tissue cyst. 



The larger globular larva (Tetrarhynchus unionif actor) is illustrated in Plate III. 

 Figs. 5 and 6 show common stages ; fig. 8 is more highly magnified, giving 

 histological details, and the spines at the anterior end are shown enlarged in fig. 9. 

 The sections represented by figs. 3 and 4 are probably oblique. Fig. 11 shows 

 a section of a young Tetr&rhynchus, such as we find in the wall of the intestine and 

 occasionally elsewhere in the tissues of the pearl oyster ; and after the finding of 

 the intermediate form shown in fig. 10, it can scarcely be doubted that these 

 Tetrarhynchids are a later stage of the pearl-inducing globular larvae. 



In our first account of these parasites we suggested that the next stage after that 

 found in the pearl oyster, occurred in a species of Batistes, which we showed was 

 sometimes found feeding on oysters, and that the adult worm inhabited one of 

 the large Elasmobranch fishes (Rays), which in their turn devour the Batistes. 

 SHIPLEY and HORNELL have now identified as the adult Tetrarhynchus unionifactor 

 a parasite that we found in Rhinoptera javanica* the " Walwadi tirikkai" of 



* SEURAT considers that the sting-ray Aetobatis narinari, EUPHRASEN, is the host of the pearl-inducing 

 Cestode which he investigated in the Pacific. He does not state what evidence he has of this, but 

 it is quite probable. We find the same species in Ceylon, where it is known as "Kuruvi tirikkai" by 

 the natives, and it has an evil reputation on the pearl banks and many Entozoa in its interior. Its main 

 food in Ceylon, as shown by the stomach contents, consists of sand-living Lamellibranchs, such as species 

 of Cardium and Venus. 



