THE CEYLON PEARL-OYSTER,. 287 
from Lalistes); and he claims that they are “ exactly similar to 
the encysted Tetrarhynchid found in the oyster ”—i. e. the true 
Tetrarhynchus unionifactor, except that they are slightly larger. 
Mr. Southwell is satisfied that they are derived from the oysters 
eaten by Balistes, and thinks that they are derived from 
both the genuine Tetrarhynchids in the Oyster’s intestine and 
from the globular cysts in its tissues. He considers that if 
- Balastes is eaten by an Elasmobranch, the young worms become 
adult ; but Balistes is not a necessary host, it is merely a “ carrier.” 
Johnstone (26 b) confirms the view that the Teleostean host is 
a collateral one, and not a normal stage in the life-history, by his 
recent researches on the European Tetrarhynchus erinaceus. 
The adult stage of this worm occurs in various species of Skates 
and Rays, and the first host is probably some Invertebrate. 
Johnstone regards the frequent occurrence of a larval form of 
this worm in Teleosts as a “ cul-de-sac” stage, due to the first 
host being eaten by the wrong fish ; and as normally leading no 
farther, but ending in degeneration. 
Mr. Southwell has further shown (38) that the adult Zetra- 
rhynchus unionifactor occurs also in the Shark, Ginglymostoma 
concolor. ‘This fish was doctored with Male-fern and castor-oil, 
and subsequently fed on pearl-oysters ; but Mr. Southwell does not 
claim that the Zetrarhynchi were actually derived from these 
pearl-oysters, though he is inclined to think they were. The 
same experiment was subsequently repeated [Southwell (41)] 
and Tetrarhynchus unionifactor was again found in Ginglymostoma 
concolor ; and while Mr. Southwell admits that his results are not 
altogether conclusive, it seems highly probable that the infection 
was in fact induced by his feeding experiment. There is nothing, 
however, to show that the adult tapeworms in Ginglymostoma 
were derived from the globular cysts in the oysters; it seems 
more probable that they were derived from the Yetrarhynchi 
in the oysters’ intestines. 
These elaborate experiments are, of course, chiefly of academic 
interest, in the absence of proof (1) that the Vetrarhynchi are a 
later stage of the globular cysts, and (2) that the latter are 
concerned in pearl-formation. 
To sum up, then, the gap between (a) the resting scolex 
enclosed in its tough fibrous cyst in the connective tissues of the 
Pearl-Oyster and strongly suggesting by analogy with other 
forms—notably Seurat’s larva—a young Z'ylocephalum, awaiting 
and ready for its final host to devour the tissues which contain it, 
and (b) the equally expectant, but much larger, Vetrarhynchus 
wutonifactor in the wall of the oyster’s intestine, has not yet been 
bridged. 
I set out below diagrammatically the conclusions of Herdman, 
Hornell, Shipley, and Southwell, as to the probable life-history 
of these parasites, and also, for purposes of comparison, my own 
attempt at an interpretation of the facts. 
[29] 

