286 DR, HW. LYSTER JAMESON ON 
stage. (The fact that the undoubted Vetrarhynchi in the pearl- 
oyster occur in and around the digestive canal suggests that 
they follow the normal course and are swallowed in the ¢ egg-stage, 
and first hatch out in the intestine of the oyster. ) Mr. ‘South- 
well states (Ceylon Marine Biological Reports, Part IV. No. 6, 
p. 169, 1910) that this free-swimming larva has not been seen 
since it was first discovered (see also 42, p. 127). 
I may here mention that one of Prof. Herdman’s slides which 
I examined shows an interesting phase in the biology of these 
supposed pearl-inducing Cestodes which may have escaped him. 
It shows a small Cestode, 0-12 mm. long, with myzorhynchus and 
collar fully developed, clearly in the act of passing through the 
tissues. This may possibly represent the young of either form, 
when first entering the oyster, or it may be a case of voluntary 
or accidental migration by Z'ylocephalum minus (Pl. XXXII. 
fig. 1). Dr. Willey (48, p. 50) records a similarly free larva seen 
moving about in the liver of a species of Venus. 
To return to the life-history of the true Tetrarhynchus unioni- 
factor, Shipley and Hornell have shown without doubt that the 
adult sexually mature worm occurs in the Ray, Rhinoptera javanica 
(Herdman’s Report V. Cestodes, pp. 65-66). The identification 
of the final host as Rhinoptera javanica is announced by Mr. 
Hornell in a postscript to his Report on the November Inspection 
of the Pearl Banks, 1904 (20, p. 8). Mr. Southwell (42, p. 130) 
gives Teniura melanospilos as another host. 
Professor Herdman in his Royal Institution Lecture, and in 
Pt. I. p. 12 of his Report, claimed the File-fish, Lalistes, as an 
intermediate host ; but Shipley and Hornell, in Herdman’s Report, 
Part II. p. 83, say that “a more minute examination, however, 
renders the connection between the parasites of the pearl-oyster 
and those of the file-fish a doubtful one”; and the immature 
Tetrarhynchi found in this fish are described as distinct species 
under the names of 7'e trarhynchus balistidis and 7’. pinne. Prof. 
Herdman sums up the position in the article on Pearl-Production 
(Report V. p. 24) by saying :— 
“No fresh light has been thrown upon the _ possible 
occurrence of an immature stage in alistes (which is eaten 
by the large rays), and although that intermediate host may 
not be necessary to the life-history, since the rays also feed 
upon pearl-oysters, still there is nothing in the observed facts 
to forbid the existence of such a stage, and it is not unusual 
in Tetrarhynchids to have two fish-hosts, an intermediate 
Teleostean which is devoured by a final Elasmobranch.” 
Mr. Southwell’s subsequent investigations confirm Prof. Herd- 
man’s view that Balistes occurs as a collateral intermediate host 
or “earrier”; he says (42, p. 132):—“It is certain that my 
encysted Vetrarhynchus unionifactor from Balistes is not the 
same species as those described” (7. e. by Shipley and Hornell 
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