36 
Pearl hidueing Worm 
Plagiostome devouring them, in which case only two hosts would be 
present. In view of what appears to be the case with Tetrarhynchus 
unionifactor, it seems more likely that Cestodes in general have only 
two real and necessary hosts, and that the stages found in these bony 
fish merely extend the distribution of the larvae without being essential 
—or in many cases useful—for the fulfilment of the life history. 
It is astonishing how little is known of the life history of Cestodes 
in general. Of the 105 species collected from the Pearl Banks the life 
history of Tetrarhynchus unionifactor alone is known, and this not 
completely. 
Moreover, innumerable encysted forms of Cestode larvae are to be 
found in the molluscs, crabs and bony fish inhabiting the Pearl Banks, 
but the adults are not known. 
In the elucidation of these problems alone, there lies ample scope 
for future work. 
REFERENCES. 
Haswell and Hill (1894). On Polycercus, a proliferating Cystic Parasite of the 
Earthworms, Proc. Lin. Soc. JV.S. Wales (2) Vol. vii. 
.Shipley and Hornell (1904, 1905, 1906). Herdman’s Ceylon Reports, Vols. ii, in 
and V. 
Southwell (1909, 1911). Ceylon Marine Biological Reports, Parts iv and v. 
Willey (1907). Spolia Zeylanica. Report on the window-pane oyster of the 
Eastern Province, June 1907. 
