T. Southwell 
29 
Under these circumstances it is easy to imagine that the ripe pro¬ 
glottides drop in the close vicinity of the oystei’s on which the fish 
concerned normally feed. But we do not yet know whether the larvae 
are free-swimming, and bore their way into the oyster, or whether they 
are passive, and depend for their development on being ingested as food. 
So far as is known only the larvae of the Bothrioce'phalidae are ciliated 
and free-swimming, although it may be possible that many Cestode 
larvae are free-swimming without being ciliated. 
Our knowledge however of the earliest stages of the vast majority of 
Cestodes is practically nil. As illustrating this point I may here note 
that some time ago, whilst examining the ripe proglottides of a specimen 
of TetrarhynchiLS 7'uhromaoulatus, I found that the segmenting eggs, 
issuing in immense numbers from a ruptured proglottid, were ciliated, a 
phenomenon I have not seen noted elsewhere. 
In view of the microscopic nature of the eggs of Tetrarhynchus 
unionifactor, the chances of ever discovering the actual method of entry 
of the larvae into the oyster itself, are remote indeed. Our first 
knowledge then of the pearl-inducing worm begins with the larvae 
found in the tissues of the oyster. These larvae are globidar, and may 
be found in any part of the tissues. Figures of this larva will be found 
in Herdman’s Ceylon Reports (Shipley and Hornell). They vary in 
size from 1 mm. to tiny larvae which can only be seen under a magni¬ 
fication of 120. All sizes are to be found between these extremes. 
The presence of these very varying sizes of larvae in the oyster at fii’st 
gave rise to the impression that they represented larvae of different 
species, possibly of the same genus. Subsequent investigation however 
brought to light a different explanation (Southwell 1909-1910). It was 
observed that the larvae multiplied endogenously by the production of 
daughter cysts internally which escape by the temporary rupture of the 
parental wall. Thus although the initial infection of the oyster may 
be only slight, it usually becomes extensive merely by this method of 
endogenous reproduction, independent of a further direct infection. 
This method of multiplication fully and naturally accounts for the very 
varying sizes of larvae found in the oyster. At present this form of 
reproduction has only been observed to be monogenetic, that is one 
daughter cyst only is born at a time, but it may be found later to be 
digenetic, or polygenetic. Willey (1907) observed a similar phenomenon 
in the larvae inhabiting Placuna placenta (the window oyster) and in 
this case the reproduction of daughter cysts was polygenetic. It is 
now tolerably certain that the Cestode larvae found in the tissues 
