THE CEYLON PEARL-OYSTER. 273 
below) that this epithelium is genetically related to the 
ectoderm, then a possible explanation of the difference in 
behaviour in the encysted condition would be that those larvee 
that carried in ectodermal cells become covered (when dead or 
while still alive) by a pearl-sac and embedded in a pearl, 
while those that were free from ectoderm become surrounded 
by the connective-tissue cyst.” 
No satisfactory instance, however, is recorded of the Cestode 
parasite being observed surrounded by an epidermal sac. 
Again in Part III. of the Report, p. 32, Professor Herdman, 
quoting a Report furnished to him by Mr. Hornell, says the 
abundance or otherwise of cyst-pearls “ is connected with the 
factors which control the relative abundance of the pearl-inducing 
Cestode and those which conduce to its death during encystment 
in suitable localities within the tissues—problems as yet obscure ” ; 
while in Part V.(Pearl-Production), p. 15, he says “it is appar- 
ently very difficult indeed to hit upon a stage showing the 
commencement of the pearl-formation.” 
And again, in an address delivered at the Anniversary Meeting 
of the Linnean Society of London in 1906 (17), Prof. Herdman, 
speaking of his Ceylon work, says that it is probably only those 
Cestodes that are provided with an ectodermal covering forming 
a pearl-sac that become the nuclei of pearls. But, as stated above, 
such an ectodermal pearl-sac has not yet been found to occur 
around the parasite, and its occurrence is purely theoretical. 
The investigations made subsequently to the publication of 
Prof. Herdman’s Reports have added little to our knowledge of 
the subject. In 1905 Mr. Hornell published a Report on the 
Placuna placenta pearl-fishery of Lake Tampalakamam, dated 
June 15th, 1905 (21). In this he states (p. 5) that he dissolved 
two Placuna pearls (out of five in his possession) and found that 
“in each case the nucleus proved to be the dead remains of a 
minute Platyhelminthian larva of the same stage and species as 
that which forms the nucleus of cyst-pearls in Margaritifera vul- 
garis.” Further study revealed the presence of Cestode larve in 
the dorsal portion of the visceral mass. He considers that these 
are identical in details of form and structure with those of Mar- 
garitifera vulgaris, but expresses some doubt as to their specific 
identity, and adds that if they prove distinct the Placuna 
parasite will need afresh name. Mr. Hornell further observed 
that these larve multiply asexually by a process of endogenous 
budding, which he wrongly refers to as parthenogenesis. 
Mr. Hornell then goes on to make the following extraordinary 
statement, which I quote as giving some indication of the con- 
fusion of ideas which existed as to the identity of the supposed 
pearl-producing larvee even in 1905, 7. e. nearly three and a half 
years after the beginning of the observations :— 
“The discovery of a stage in the life-history of this 
parasite, which I am confident is homologous with the 
Proc. Zoou, Soc.—1912, No. XVIII. 18 
