26 PROCEEDINGS OF THE 



an insect" in this passage Kelaart meant such an organism as the 

 Cestode larva, which we now find is the determining cause of such 

 pearl-formation. There are other passages on which there is not 

 time to comment which show that Kelaart was tackling the problem 

 in a scientific manner when his work on the subject was tragically 

 interrupted by his sudden death in the Eed Sea in 1859. 



Thurston, in 1894, confirmed Kelaart so far as regards finding 

 in the tissues and also in the alimentary canal of the Ceylon pearl- 

 oyster the " larvae of some Platyhelminthian (flat-worm)," but he 

 was able to add little beyond figuring a couple of parasites encysted 

 in the body. Here the matter practically rested so far as actual 

 investigation of the Ceylon pearl-oyster was concerned, until 

 Mr. Hornell and I found the Cestode larvae in association with 

 pearls in the liver and gonads during our cruises in the steamer 

 ' Lady Havelock ' in the Gulf of Manaar during February and 

 March 1902. It was about March 6th, when cutting up oysters on 

 the western part of the Cheval Paar, that we first became convinced 

 that the opaque white globular larvae we were finding encysted in 

 the liver belonged to Cestode worms. Subsequent work showed 

 us"t'hM"some of them at least were referable to the genus Tetra- 

 rhynclius, and the various stages that we were able to find up to 

 the spring of 1904 were described by Shipley and Hornell in the 

 second volume of the Ceylon Keport (p. 79). Since then large 

 numbers of pearl-oysters from various localities in the Gulf of 

 Manaar have been esamined by Mr. Hornell in the field, and many 

 parasites and small pearls from these oysters have been investi- 

 gated by myself and my assistants in the laboratory. Although the 

 work is by no means complete, we have come to certain definite 

 conclusions, which will be published in the forthcoming fifth and 

 last volume of the Ceylon Report (Eoyal Society, 1906). 



The youngest stages in the life-history of our Tetrarhynclms are 

 still unknown, and ir is still uncertain whether some free-swimming 

 larvae caught in the tow-net on the Muttuvaratu Paar just over the 

 infected oysters really belong to this life-history. They have cal- 

 careous corpuscles of the right appearance, and an indication of 

 the invagiuated anterior end which we find in our later stages, and 

 they are almost certainly young Cestodes. One cannot add 

 materially to the statement of Shipley and Hornell : " On the 

 whole we think it probable that this larva is the first stage in tlie 

 life-history of the pearl-forming organism." The next stages occur 

 freely in the body of the pearl-oyster, where we find more or less 

 globular larvte of various sizes encysted in various parts of the 

 body — the smaller ones mostly in the mantle and the gills, and 

 larger ones in the liver and gonads and also sometimes in the 

 mantle. In referring all these larvae to the Cestoda we rely upon 

 the following four characteristics : — 



1. The invagination to form the head of the adult worm. 



2. The hooks upon portions of the invaginated surface. 



