34 
Pearl Inducing Worm 
Trygon spp. which I obtained from local fishermen, and in one species 
found forthwith a few specimens of Tetrarhynchus unionifactor. This 
circumstance lends support to the belief that the pearl-inducing larvae 
inhabiting these far removed genera of pearl bearers are the same 
as those found in the pearl oyster. In view of the close proximity of 
these oysters this fact is not surprising. The astonishing fact is that 
the adult worm should apparently be so common at Tarablegam and so 
rare on the Pearl Banks. 
Tliere seems to be no reason for presuming that the adult of the 
pearl-inducing worm in the pearl oyster occurs exclusively in Rhinoptera 
javanica. We have already shown that it also occurs in Ginglymostoma 
concolor and in Trygon sp. and there seems every reason to believe 
that future investigations will prove its presence in all Plagiostomes 
which feed on oysters. I know of no Cestode obtained from the Pearl 
Banks which does not occur in more than one host, and in many 
instances the adult worms are found in as many as seven species of fish. 
Moreover it is natural to presume that the adult worm will occur in 
species which normally eat oysters. 
It has already been pointed out that many Teleosts feed on oysters, 
such as members of the genera Balistes, Serranus, Lutjaniis, Tetrodon, 
and genera allied to the latter’. Except in the genera Tetrodon, 
Tetrarhynchid cysts have been found on the mesenteries of nearly all 
the specimens examined of the above genera, and this fact originally 
led to the idea that possibly members of the genus Balistes (which were 
better known at that time than the other genera) formed an inter¬ 
mediate host for the pearl-inducing parasite. Shipley and Hornell 
however pointed out that the young adult larvae fouird encysted in the 
gut of the oyster were different from those obtained from Balistes. 
This fact has been amply corroborated since, but careful search has 
revealed the fact that encysted forms of Tetrarhynchus unionifactor do 
occur both in certain species of Balistes and Serranus, and the young 
adult found encysted in the mesenteries of certain of these genera is 
identical in every way to the young adult found encysted in the gut of 
the oyster. It is difficult to understand why it is that in the oyster 
itself two stages occur in the development of the parasite which are so 
widely separated. True, the young adults are very rare as compared 
with the globular cysts. We are not absolutely certain that these tw’o 
larvae belong to the same parasite, although the circumstantial evidence 
afforded by the feeding experiment seems to indicate that they are. 
Moreover, if the adult specimens of Tetrarhynchus unionifactor obtained 
