THE CEYLON PEARL-OYSTER. 28) 
It is strange how the TZetrarhynchus unionifactor theory, 
once enunciated, has prevailed :—indeed nobody seems to have 
seriously followed up the obvious clue given by Seurat’s discovery 
of the supposed adult of his worm. 
Prof. Herdman says in his Report, Part V. pp. 20-21: 
“Shipley and Hornell in Part II. (p. 77) described and 
figured various stages of the Cestode larve both from the 
centre of decalcified pearls and also free in the tissues of the 
pearl-oyster, but left it an open question whether the sub- 
globular younger larvee |i. e. T'ylocephalum ludificans, n. sp.| 
belong to the same life-history as the elongated older forms, 
which are young Tetrarhynchids. If our arrangement of 
the stages observed in the tissues of the pearl-oyster is 
correct, and if all these larve belong to the same species, 
then the interpretation we have given above brings us to 
the conclusion that the larger of our two globular larve 
belongs to the worm which Shipley and Hornell described as 
Tetrarhynchus unionifactor in 1904.” 
And, referring to some figures of J'ylocephalum ludificans on 
pl. ui. (Report, Part V. Pearl-Production) figs. 1-8, he says 
(p. 21): 
“there can scarcely be any doubt (1) that they all belong 
to the same life-history, and (2) that they are young Tetra- 
rhynchids leading on to the stages shown in figs. 10 and 11.” 
Prof. Herdman gives, in support of his theory, a series of 
figures showing the hypothetical transition from 7T'ylocephalum 
ludificans to Tetrarhynchus unionifactor (Report, Part V. p. 21). 
Later, a younger Zetrarhynchus, 1 mm. in length, was found 
in the stomach and alimentary canal of the oyster, which Shipley 
and Hornell (Report V. pp. 87-88) regarded as probably an 
earlier stage of Tetrarhynchus unionifactor, though possibly a 
distinct form. <A single example of a still younger form, which 
is figured in the Report on Pearl-Production (V., pl. 111. fig. 10), 
and is described in the text (p. 22) as occurring ‘ encysted in the 
liver” and in the explanation of the plates as “ from cyst between 
stomach and liver,” appears to go still further towards linking 
the two forms. This larva is referred to in the text as °53 mm. 
long. Tylocephalum ludificans grows to a much greater size than 
this without changing its characters ; thus the one figured by 
Shipley and Hornell in the “ Parasites of the Pearl-Oyster” 
(Report, Part IT. pl. i. fig. 12), and described as x 40, appears 
from the size of the figure to be over 1°5 mm. long, and still 
shows no sign of becoming a TVetrarhynchus. And T. ludificans 
quite frequently measures | mm. in diameter. This discrepancy 
in dimensions makes it hard to believe that they are the same 
organism, and the gap between the Tylocephalum form, with its 
round Balanoglossus-like scolex or ‘“‘ myzorhynchus,”’ and the 
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