282 DR, H. LYSTER JAMESON ON 
Tetrarhynchus form, with its complex proboscides, cannot be said 
to have been satisfactorily bridged. 
Herdman found the Vetrarhynchus form much scarcer than 
the Tylocephalum form, and it appears from his Report, Part V. 
p- 22, that the ratio of the latter to the former is about 200: 6. 
Shipley and Hornell (Report, Part II. p. 79) give the ratio of 
globular larvee to undoubted Tetrarhynchi as 100: 1. 
Prof. Herdman’s suggested explanation of this, namely, that the 
globular parasite only occasionally advances to the Tetrarhynchus 
stage, requires, it seems to me, a greater effort of the imagination 
than the hypothesis that the two worms are distinct forms. 
It is hard to conceive of conditions that would lead a small 
minority of Z'ylocephalum ludificans or T. minus to leave their 
tough fibrous cysts in the peripheral tissues, and migrate to the 
intestine, there to take on the Zetrarhynchus form. It seems to 
me much easier to regard these as two (or three) distinct species, 
and their simultaneous presence in one host as a case of parallel 
infection. 
In his latest paper (42, p. 129), Southwell, speaking of these 
undoubted Tetrarhynchids, says :— 
“These are by no means rare, and are in almost every 
case confined to a particular part of the wail of the gut, about 
one inch from the anus and on the terminal part of the gut. 
They often occur in clusters of three or four. They are 
small (about 1 mm.), but appear to be adult im every way, 
save that strobilization has not commenced. This encysted 
young Tetrarhynchid is quite dissimilar to the globular cysts 
found in the same oyster. In the latter case, the larvee are 
so young that the Cestodian characters are but ill defined. 
In the former case, a normal and full-grown Tetrarhynchid 
head is present. No stage or stages have been found inter- 
mediate between them, and the evidence that they are both 
stages in the life-history of the same parasite rests on circum- 
stantial evidence and on evidence obtained by feeding 
experiments.” 
And with reference to these feeding experiments, which are 
referred to below (p. 287), and in which Zetrarhynchi were found 
in Sharks that had been fed on pearl-oysters, Mr. Southwell says 
(p. 130) :— 
“The mere fact that the adults were obtained by feeding 
is in itself almost sufficient to prove that they are the 
adult of the pearl-inducing worm, for it is difficult to believe 
that their occurrence in the Ginglymostoma was a mere 
coincidence each year.” 
I think there is very good reason to believe that Southwell 
did, in his feeding experiments, actually transmit Zetrarhynchus 
unionifactor from the Oyster to the Elasmobranch, but it 1s 
[24] 
