CEYLON PEARL-INDUCING WORM. , [32 
It is to be noted here that we have been referring above to the 
encysted Tetrarhynchid, and not to the globular cyst found in the 
oyster. 
What is the significance of the stage of T'etrarhynchus unionifactor 
found in these Teleosts ? 
There can be no shadow of doubt that they are derived from the 
oyster. But in no case are the cysts further developed than those 
normally found in the oyster. These fish are not intermediate hosts, 
but carriers, and they illustrate the fact that the larvee of 7'etra- 
rhynchus unionifactor can live in various hosts and in various organs, 
just as we have seen to be the case in the cysticercus of Tenia 
solium and other Cestodes. If oysters are eaten by Balistes (or 
Serranus and Lutjanus), two things happen to the cysts in the 
oyster :— 
(i.) The encysted Tetrarhynchids in the oyster are transferred 
to the Balistes, where they encyst in the mesenteries, 
without developing any further. 
(ii.) The globular cysts in the oyster are dissolved, and the larva 
is liberated; it migrates, develops into a young Tetra: 
rhynchid—the same stage is in (i.)—and encysts on the 
mesenteries. 
It might be argued from the preceding, that since the globular 
cysts develop into young Tetrarhynchids in Balistes, that therefore 
Balistes is a secondary host. 
We would point out, however, that the globular cysts often 
develop into young Tetrarhynchids in the oyster itself, and further, 
that if oysters are eaten by certain Elasmobranch fish, both the 
globular cyst and the young Tetrarhynchid become adult directly 
in the Elasmobranch. Similarly, it is almost certain that should 
Balisies be eaten by a suitable Elasmobranch, the young Tetra- 
rhynchid would become adult. The stages occurring in Balistes and 
in the oyster are the same. Balistes is not an intermediate host, but 
merely a carrier. In this way it may be useful in the life-history of 
the parasite, without being in the least necessary. If the species or 
specimen of Balistes is small, the encysted larva has a favourable 
chance of completing its life-cycle. 
In the case of Serranus undulosus, which likewise contains encysted 
and young Tetrarhynchids of many species, and including 'etra- 
rhynchus unionifactor (but more rarely than in Salistes), specimens 
often measure 43 feet in length and°10 to 12 inches in diameter. It 
is difficult in these cases to postulate a Plagiostomous host large 
enough to eat a fish of these dimensions. In these cases we can 
but logically assume that the life-cycle of the various Tetrarhynchids 
contained in these large Teleosts are never completed. They are 
culs-de-sac in the life-history of the parasite, a circumstance simu- 
lating the occurrence of hydatids in man, where the larve giving 
