126 SPOLIA ZEYLANICA. 
The hydatids thus produced represent culs-de-sac in the life-history 
of the parasite. It is to be noted that whilst the life-history of 
a Cestode is usually—almost always—completed in two hosts, the 
parasites may vary their hosts and occur adult in many genera. 
Instances of this kind are too numerous to mention here. The 
cysticercoid stages are in many instances equally adaptable with 
reference to their hosts, but it is to be noted that should the 
cysticercoid enter a primary host which is not eaten, or does not 
form the food of the second host, the life-history is never completed, 
and the larve eventually die. 
The preceding details have been given in order to elucidate more 
fully the life-history of the pearl-inducing parasite, and to facilitate 
the understanding of what are obviously abnormal conditions and 
situations which occasionally befall the larve found in the pearl 
oyster. 
Professor Herdman found that the globular cysts which normally 
occur in the tissues of the oyster were the larve of a Cestode, which 
was named T'etrarhynchus unionifactor, Shipley and Hornell. When 
the infected oysters’ were eaten by the ray, Rhinoptera javanica, the 
worm became adult in the latter fish. A further stage also occurred 
in the oyster in the form of an encysted but young Tetrarhynchid 
occurring on the intestinal wall, but no stage was found strictly 
intermediate in development between the widely different globular 
cyst in the oyster and the encysted but young Tetrarhynchid 
occurring on the wall of the oyster’s gut. 
Free-swimming Cestode larvee were found in the tow-net taken on 
the banks, but, as Herdman says, “it is still uncertain whether the 
free-swimming larve found on the Muttuvaratu Paar really belong 
to the life-history.’’* 
In addition to the two stages found in the oyster, and the adult 
found originally in Rhinoptera javanica, and later in Taeniura 
melanospilos (large rays), other megacestoid stages were found in 
Balistes (a small file-fish). Since these file-fish were known to feed 
on oysters, it was pointed out that, although the life-history was 
probably direct from oyster to Rhinoptera, it might be found that 
Balistes formed an intermediate host. “‘ A more minute examination. 
however, renders the connection between the parasites of the pearl 
oyster and those of the file-fish a doubtful one,’’+ and, again, * the 
more advanced larve from the pearl oyster have arrived at a later 
stage in development than the larvee found in Balistes.” ¢ 
The life-history of this interesting parasite was worked out thus 
far as a result of Professor Herdman’s investigation. Since that 
time the life-history has been further studied as time and material 


* <¢ Ceylon Reports,” Vol. V. es 
+ Shipley and Hornell. Vol. II., Herdman’s ‘‘ Ceylon Reports. 
t Loc. cit. 
