T. Southwell 
35 
during the feeding experiments were developed from the globular cysts 
in the oyster, then the young adult also found in the oyster must be 
the same species, for it corresponds in every detail with the adult, 
except in size. The position of these cysts containing young adults 
of Tetrarhynchus anionifactor, found in Batistes and Serranus, with 
reference to the life history of the parasite is not quite easy to under¬ 
stand. No adult Cestodes have ever been obtained from any Teleost 
caught on the Pearl Banks, and although this fact is very surprising, it 
is not unique, but falls in line with the observations made by others in 
different parts of the world. These bony fish are to be regarded as 
collateral, but not as intermediate hosts. When oysters are eaten by 
them, the globular cysts derived from the oyster develop, in the bony fish, 
into young adults, but no further. The strobila is never developed, and 
the stage attained is no further advanced than (in fact it is the same as) 
the young adults occasionally found encysted in the gut of the oyster 
itself. The stage in Batistes and Serranus represents a cul-de-sac in 
the life history of the parasite, for most of the species of these genera 
of fish are of such large size, that it is difficult to postulate a Plagiostomous 
host large enough to devour fish of such dimensions. In these cases 
we can only assume that the life cycle of the parasite is never com¬ 
pleted, a circumstance homologous with the occurrence of hydatids in 
man, where the larvae giving rise to the disease have, owing to their 
capability of adaptation within various hosts, lost themselves in the 
maze of their own liberties, and where of course the life history is 
never completed. These genera of fishes cannot therefore be considered 
as intermediate hosts. They are collateral hosts to the larvae. 
One assumes that should one of these Teleosts be devoured by a 
Plagiostome the encysted form of Tetrarhynchus unionifactor would 
develop into adult, just in the same way and to the same extent that 
the two types of larvae in the oyster do. Unlike certain Plagiostomes, 
where the larvae develop into the adult and liberate myriads of eggs, 
these Teleosts merely devour oysters without extending the means 
of infection. For oyster culture these Teleosts are undesirable in every 
way. 
Although I believe tliat this represents the real state of affairs 
existing between the oyster, Batistes and Serranus, and the final host of 
the adult parasite, it may be that in the case of other parasites, bony fish 
really act as intermediate hosts, in which case the parasite in question 
would have three hosts. In other cases it may be found that the 
smaller Teleosts are initially infected, and that the adult occurs in the 
