134 SPOLIA ZEYLANICA. 
rise to the disease have, owing to their adaptability within various 
hosts, lost themselves in the maze of their own liberties, and where 
the life-history is, of course, never completed. 
It is a significant fact that in Ceylon no adult Cestodes have ever 
been found in any Teleosts, even though larve are numerously 
distributed within the order. This fact is most peculiar, but so far 
as I know it is a usual and well-known phenomenon, except amongst 
the family of Cestodes named Bothriocephalide, adult forms of 
which occur in the salmon and in Gadus. 
Possibly adult forms of Cestodes may be found later in Ceylon 
Teleosts, but up to the present a most careful scrutiny has been 
fruitless. . 
Conclusion.—It will be obvious from the preceding that there still 
remains much to be done before all the stages in the life-history 
of the pearl-inducing worm are fully known. Work on the elucida- 
tion of this problem has been seriously hampered during the last 
few years by the lack of material. 
There can be little doubt, I think, that the life-history of this 
parasite is direct from the oyster to such fish of the group Plagiastomi 
as feed on them, and that the stage found in various Teleosts is 
accidental, not necessary, and may be useful or otherwise. It would 
be interesting (1) to discover undoubted larve prior to their enter- 
ing the oyster ; (2) to ascertain the exact way in which they enter the 
oyster ; (3) to ascertain why certain cysts produce pearls and the 
vast majority do not; (4) to find stages between the globular cyst 
and the young Tetrarhynchid. These details are necessary to round 
off our knowledge of this worm. 
Although these questions remain unsolved, infection of the oyster 
continues, and is never found faulty, except in such reef forms as 
occur in very shallow water where one supposes that the necessary 
fish seldom approach. 

