THE CEYLON PEARL-OYSTER. 285 
the adult of 7'ylocephalum ludificans—in fact, the only difference 
appears to be the presence of the four lateral suckers, which are 
absent in the larva, but which may well not develop till its 
transference to the final host. 
Comparison of these figures makes one feel doubtful whether 
the generic distinction between 7'ylocephalum and Cephalobothrium 
is a valid one, or whether the conditions shown in figs. 58 & 65, 
and in figs. 59 & 66, are not merely the expression of a uniform 
type of myzorhynchus in different stages of contraction, as 
suggested diagrammatically in the following text-figure. 
Text-fig. 34. 

Diagram illustrating the relationship between the condition of the myzorhynchus in 
Tylocephalum and Cephalotothrium. A, a generalised scheme of a scolex 
such as the Tylocephala of the pearl-oyster. B, by protrusion of the 
myzorhynchus, the outer surface of the same being tense, the inner surface 
thrown into folds, the Tylocephalum-form may be produced (compare 
Pl. XLVII. fig. 65). C, the partial retraction of the myzorhynchus to form 
a sucker-like disk gives the Cephalobothrium condition (compare Pl. XLVI. 
fig. 59 and Pi. XLVII. fig. 66). D, the myzorhynchus retracted within 
its collar, with its outer surface thrown into folds, as is characteristic of 
many of the larve of Tylocephalwm ludificans found in the pearl-oyster, and 
of the adult worm figured on Pl. XLVII. figs. 61-62. 
It does not necessarily follow that any of the above mentioned 
worms actually represents the final stage of Z'ylocephalum ludificans 
or 7. minus, though I think there are considerable grounds for 
regarding the worm shown at figs. 60-64 as the former; but it 
certainly appears more probable that these final stages will be 
found among this class of parasites rather than among the 
Tetrarhynchi, and it is strange that the position that the Tetra- 
rhynchus unionifactor hypothesis may be wrong has never been 
seriously faced and a fresh start made on the above lines. 
It is not known how T'ylocephalum ludificans and Tylocephalum 
minus enter the Pearl-Oyster; but Hornell found, in tow-netting 
on the Muttuvarattu Paar on the 19th November, 1902, a free- 
swimming larva, 37 mm. long when extended, which is figured 
in Prof. Herdman’s Report (Part II. Parasites of the Pearl- 
Oyster, plate 1. fig. la-h). This larva certainly suggests an earlier 
stage of Z'ylocephalum ludificans, and one of the chief difficulties 
in the way of accepting it disappears if this worm is dissociated 
from the genus Jetrarhynchus, the normal habit of which is to 
enter the digestive canal with the food while still in the egg- 
[27] 
