130 SPOLIA ZEYLANICA. 
Trawling has heen almost continuous during every season. The 
fish caught have all been carefully examined, and although not less 
than 8,000 Cestodes, distributed over 24 genera and 77 species, have 
been collected, the adult T'etrarhynchus unionifactor has never been 
obtained. 
It would almost appear that this fact in itself is sufficient proof 
that the adult of the pearl-inducing worm is not Tetrarhynchus 
unionifactor. We have noted that (1) no larve have been found in 
an earlier stage of development than the globular cysts found in 
the oyster, (2) that although encysted Tetrarhynchids occur in the 
oyster, no stage intermediate between the globular cyst and the 
young Tetrarhynchid has been found to prove that both these 
stages in the development belong to the same worm. Finally, 
we have seen that the adult has never been found in any of the 
Plagiostomi trawled on the banks during the last five years. 
The evidence afforded by the feeding experiments, described in 
Part IV., ‘‘ Ceylon Marine Biological Reports,” is important. An area 
was isolated in the open sea by means of expanded metal having a 
4-inch mesh. Into this area large specimens of the following fish 
were placed: T'rygon walga, Teniura melanospilos, Ginglymostoma 
concolor, Rhynchobatus djeddensis, Serranus undulosus (4 feet), 
Tetrodon stellatus. 
These fish were first medicated with male fern extract and castor 
oil, and then fed exclusively on oysters for several weeks. 
The results were roughly as follows :— 
Tetrodon stellatus and Serranus undulosus lived in a healthy 
state, but no adult Cestodes were found in them. 
Rhynchobatus djeddensis.—These specimens all died within three 
days. They are dwellers on mud, and I attribute their death to the 
fact that they do not feed on oysters. No Cestodes found. 
Ginglymostoma concolor.—Adductor muscle of oyster found in 
stomach. Thirty-eight specimens of T'etrarhynchus unionifactor in 
spiral valve (other Cestodes also found), and fifty-one T'etrarhynchus 
unionifactor in another specimen. 
Teniura melanospilos.—Tetrarhynchus herdmani only in spiral 
valve. 
These results are described fully elsewhere,* and the point that 
immediately concerns us is the fact that Tetrarhynchus unioncfactor 
was obtained in numbers on two separate occasions. 
Since the adult worm was never obtained by me on any other 
occasion, even though numbers of the same species of fish were 
examined, it seems almost certain that these specimens were 
developed from the larvz in the oysters eaten, and there are many 
points which favour this view. ‘The mere fact that the adults were 
obtained by feeding is in itself almost sufficient to prove that they 

* «* Ceylon Marine Biological Reports,’ Parts IV. and V. 
