THE CEYLON PEARL-OYSTER. 279 
But the worm specifically described by Shipley and Hornell as 
Tetrarhynchus unionifactor on p. 88 of Part I. of Prof. Herdman’s 
Report (Parasites of the Pearl-Oyster) and figured in plate il. 
figs. 19 & 20, is a well-advanced TYetrarhynchus 6:5 to 7 mm. 
long, which oceurs in and around the intestine of the Pearl- 
Oyster; and, to say the least of it, it is doubtful whether this worm 
isa later stage of the globular cysts, which Prof. Herdinan identi- 
fied as the nuclei of pearls, and not an entirely distinct organism. 
In order to avoid confusion of terms I am therefore giving 
separate names to the larger and smaller globular Cestode larvee 
which Herdman recognises, as it is calculated to lead to much 
confusion of issues if these are referred to by the name of Tetra- 
rhynchus unionifactor before their identity with it can be demon- 
strated more satisfactorily. The arguments for regarding the 
supposed pearl-producing parasites as distinct from Tetrarhynchus 
unionifactor are set forth below. In the absence of satisfactory 
evidence of their relation to the genus Z'etrarhynchus, I propose, 
following Seurat (36), to whose larval Cestode, mentioned above, 
they bear a close resemblance, to refer them to the genus Tyloce- 
phalum* and to describe the larger and smaller forms respectively 
as T'ylocephalum ludificans, sp. n., and T'ylocephalum minus, sp. 0. 
Two well-marked sizes occur in these globular larve and they 
are regarded by Herdman as distinct organisms (Report V. p. 21). 
On the other hand, Southwell considers that the asexual repro- 
duction, which he has occasionally observed, accounts for the 
varying sizes of the larvee in the Ceylon Pearl-Oyster, and says : 
‘Tam now convinced that these different sizes merely represent the 
same species in different grades of development.” 1 am inclined 
to share Prof. Herdman’s view that these two sizes are distinct 
organisms. It may even prove that there are more than two 
species represented. Indeed, I should not be surprised if further 
research on fresh material were to show that both Z'ylocephalum 
ludificans and 7’. minus are composite species. 
Professor Herdman regards 7. ludificans as the earlier stage 
of Tetrarhynchus unionifactor, and calls attention to its re- 
semblance to Van Beneden’s ideal figures of the young of 
Tetrarhynchus, while he treats 7’. minws as another species of 
Tetrarhynchus in its earlier stages. Nevertheless, he seems to 
have had suspicions that some, ‘at least, of these larve might be 
Tylocephala, though he appears in the end to have decided that 
they—and presumably with them Seurat’s larva—are a hitherto 
unknown stage in the life-history of the genus Zetrarhynchus. 
In the Preface to Part IL. of his Report he says (p. vi) :— 

“Tt is possible that some of our Ceylon Pearl-Oyster 
parasites may also belong to the genus Acrobothrium’ 
|7.e. the genus to which Seurat’s larva was then referred |, 
“although poe more advanced ones are certainly Tetra- 
rhynchids ” 
* Linton (27 a), pp. 805-9, pl. ix. figs. 5-9. Type T. pingue, from spiral valve 
of Rhinoptera quadriloba. 
