THE CEYLON PEARL-OYSTER., 283 
difficult to escape the conclusion that the worms found in Gingly- 
mostoma were derived from the Tetrarhynchus larvee in or around 
the alimentary canal of the Oysters, and not from the globular 
Tylocephala in the other tissues, to which Southwell refers when 
he speaks of the “ pearl-inducing worm.” To dispute this view, 
it would be necessary to demonstrate that the Zetrarhynchus-stage 
was not present in the Oysters used. 
Shipley remained throughout sceptical about the identity of 
the supposed pearl-forming larvee with Tetrarhynchus unionifactor. 
In Part Il. of Herdman’s Report, p. 86, he says it is most 
improbable that the young larve grow into the Tetrarhynchus 
larva. In their report on the Cestode and Nematode Parasites 
from the Marine Fishes of Ceylon, Shipley and Hornell say 
(Pt. V. p. 66) :— 
“Tt seems increasingly probable that the pearl-forming 
Cestode is a 7. unionifactor, but this has not yet been 
proved.” 
Shipley and Hornell, in Herdman’s Report, Part V. p. 98, offer 
the following hypothetical life-history :— 
“Of the given number of larvee which enter at a very 
early stage into the body of the Oyster a certain number arrive 
in the mantle and other tissues, acquire an ectodermal sac and 
there encyst, and find a costly grave in the developing pearl.” 
|The ectodermal sac around these parasites is so far purely 
hypothetical and has never been demonstrated.—H. L. J.| 
“The remainder, however, réach the alimentary canal and 
grow and flourish there. When they attain the dimensions of 
thestages described in Part II. they leave the alimentary canal 
and encyst, usually upon the outer surface of the intestine. 
Now they are too big for enclosure in a pearl, and they can 
wait without anxiety for the advent of their second host 
(Rhinoptera javanica), within whose intestine they rapidly 
become sexually mature.” 
It would seem to the present writer much simpler to set aside, 
for a while, the hypothesis that Tylocephalum ludificans and 
7’. minus are younger stages of a Tetrarhynchus, and to seek for 
their adult stages among the members of the genus Tylocephalum, 
or allied types described as new genera, occurring in oyster-eating 
Elasmobranchs. Shipley and Hornell have already described a 
number of these, which I give below :— 
*+Tylocephalum (Tetragonocephalum) trygonis (Report, Part 
Jil. p. 51 and Part V. pp. 48 & 83). Habitat: intestine of 
Trygon walga and Aétobatis narinari. Diameter of head 
0-03 mm. 
*Tylocephalum (Tetragonocephalum) aétobatidis (Report, Part 
III. p. 52 and Part V. p. 48). Intestine of Aétobatis 
narinart. Diameter of head 1-5 mm. 
