CEYLON PEARL-INDUCING WORM. tou 
are the adult of the pearl-inducing worm, for it is difficult to believe 
that their occurrence in the Ginglymostoma was a mere coincidence 
each year. 
The line sketch given of this worm in Vol. V., ‘ Ceylon Reports,” 
gives no details of the adult structure, which is somewhat unfortunate. 
A figure is, however, given in Part V., ‘‘ Reports from the Ceylon 
Marine Biological Laboratory.” 
The absence of the adult worm in the fish caught during the 
last five years is doubtless incident on the fact that oysters have 
practically been absent from the banks over this period. 
My own observations point to the fact that the fish found on the 
banks have steadily decreased in number during the last few years. 
It seems probable that the fish have migrated to other feeding 
grounds, particularly those species which feed on molluscs. Whereas 
the mollusean fauna of the banks is usually abundant, I have not 
found more than six specimens during the last two years, even 
though diving, trawling, and dredging has been carried on almost 
daily. This fact serves to show that other molluscs suffer equally 
with the oyster, and it seems natural to explain the absence of 
predatory fish as due to the lack of food over the plateau. 
We have seen that the adult pearl-inducing worm has up to the 
present been found in three species of fish, viz., Rhinoptera javanica, 
Hornell; Taniura melanospilos, Hornell; and Ginglymostoma 
concolor, Southwell. 
There seems to be no reason for associating the adult worm 
exclusively with Rhinoptera javanica. In fact, it is somewhat 
surprising to find that the adult worm has been found in this species, 
since the fish has only as yet been caught on the muddy basins of 
Dutch Bay, Portugal Bay, and near the Mannar channel. It seems 
likely that the adult worm occurs in all Plagiostomes which eat 
oysters, and I should not be surprised to find that subsequent 
research proved this to be the case. 
It is now well known that very many species of Cestodes occurring 
in marine fishes in Ceylon have several hosts, and there is every 
reason for believing that the same is true of T'etrarhynchus union- 
factor. 
Some species of fish, such as Rhynchobatus djeddensis, possess 
tremendously powerful jaws with undulating, continuous. plate-like 
rows of teeth. It seems natural to suppose that fish possessing a 
powerful apparatus of this kind should feed on oysters and other 
molluscs. I have, however. had numerous proofs that they do not. 
Of ten specimens of Rhynchobatus djeddensis placed in the nursery 
for feeding experiments, not one survived the third day. Their 
normal habitat is on muddy and weedy basin in two to four fathoms, 
where they feed almost exclusively on crabs. They will die of 
starvation with oysters under their nose, and it may be taken as a 
general rule that fish normally living on a muddy basin of this kind 
