Page:Popular Science Monthly Volume 63.djvu/241

Rh (his own words are 'there is hardly any doubt') that the parasite causing the pearl-formation in our common mussel (not in the Ceylon 'pearl oyster') is the larva of Distomum somateriæ, from the eider duck and the scoter. He also believes that the larva inhabits Tapes or the cockle as a first host before getting into the mussel.

We have found, as Kelaart did, that in the Ceylon pearl oyster there are several different kinds of worms commonly occurring as parasites, and we shall I think be able to show in our final report that Cestodes, Trematodes and Nematodes are all concerned in pearl formation. Unlike the case of the European mussels, however, we find so far that in Ceylon the most important cause is a larval Cestode of the Tetrarhynchus form. Mr. Hornell has traced a considerable part of the life history of this parasite, from an early free-swimming stage to a late larval condition in the file fish (Batistes mitis) which frequents the pearl banks and preys upon the oysters. We have not yet succeeded in finding the adult, but it will probably prove to infest the sharks or other large Elasmobranchs which devour Balistes.

It is only due to my excellent assistant, Mr. James Hornell, to state that our observations on pearl formation are mainly due to him. During the comparatively limited time (under three months) that I had on the banks I was mainly occupied with what seemed the more important question of the life-conditions of the oyster, in view of the frequent depletion of particular grounds.

It is important to note that these interesting pearl-formation parasites are not only widely distributed over the Manaar banks, but also on other parts of the coast of Ceylon, Mr, Hornell has found Balistes with its Cestode parasite both at Trincomalie and at Galle, and the sharks also occur all round the island, so that there can be no question as to the probable infection of oysters grown at these or any other suitable localities.

There is still, however, much to find out in regard to all these points, and other details affecting the life of the oyster and the prosperity of the pearl fisheries, Mr, Hornell and I are still in the middle of our investigations, and this must be regarded as only a preliminary statement of results which may have to be corrected, and I hope will be considerably extended in our final report.

It is interesting to note that the Ceylon Government Gazette, of December 22 last, announced a pearl fishery, to commence on February 22, during which the following banks would be fished:

The southeast Cheval Paar, estimated to have 49 million oysters.

The East Cheval Paar, with 11 millions.

The Northeast Cheval Paar, with 13 millions.

The Periya Paar Kerrai, with 8 million—making in all over 80 million oysters.