J. Johnstone 
3(59 
eating some invertebrate which acts as the true larval liost and in which 
some larval form of TetrarhyncJms lives. What then is the place of the 
Plerocercoid worms inhabiting the Teleosts in the life-history? We 
notice that very many fishes may act as this host, and that in these 
animals we only find two conditions of the larva; (1) the Plerocercoids 
containing fully developed scolices, and (2) degenerate forms of larvae. 
The Teleosts are final, not intermediate hosts, and the larvae Tetrarhynchi 
inhabiting their tissues degenerate and carry the life-history no further. 
This is the conclusion come to by Southwell (1911) with regard to 
the Ceylon Tetrarhynchus imionifactor. The conjectural life-histor}' of 
this species is as follows :—(1) a larval host (a mollusc)-containing very 
young larvae ; (2) a Teleost fish which eats the mollusc, and in which 
the Plerocercoid larva is found ; (3) an Elasmobranch which eats the 
Teleost and in which the mature Tetrarhynchid is found. Now the 
difficulty of proving the transmission of the Tetrarhynchid from one to 
another of these animals has led Southwell to suggest that the Teleost 
is not a necessary host in the life-history of T. unionifactor, but only a 
“ cul-de-sac, or blind termination.” The Teleosts and Elasmobranchs 
alike become infected by eating the molluscs. 
Whatever may be the case in the Ceylon species I think that the life- 
history of Tetrarhynchus e^hnaceus is probably similar to that suggested 
by Southwell and that some invertebrate is the true larval host. The 
species of Teleosts mentioned in this paper are not therefore larval hosts 
in the sense that they are stages in a life-history. They are collateral 
hosts with the Elasmobranchs ; and both series of fishes are infected by 
eating the true larval hosts—whatever these may be. The mature 
Tetrarhynchi are never found in the Teleosts and larvae entering these 
fishes finally degenerated 
III. The Plerocercoid larvae. 
The cysts which contain the larvae are either spherical or oval bodies 
embedded in the outer wall of the alimentary canal and covered by 
peritoneum; or they are stalked, egg-shaped bodies attached to the 
1 The degeneration of the larvae is accompanied by the progressive accumulation of 
the calcareous corpuscles. In a Megrim {Arnoglossus megastoma) examined since writing 
the above, living Plerocercoids were present among the muscles of the host. But there 
were also many groups of prismatic crystals of calcium carbonate lying freely among the 
muscles, these crystals were in some cases 1 cm. long and about 0‘5 mm. in diameter. 
I think they were the result of the crystallisation of masses of calcareous corpuscles 
resulting from degeneration of the larvae. 
