CEYLON PEARL-INDUCING WORM. 129 
This fact was emphasized by Shipley and Hoarnell,.who_ remarked : 
“Under slight pressure, as first seen it (the pearl-inducing larva) 
exhibited:a striking resemblance to a tiny Trematode, or it might be 
mistaken fora large Gregarine.” * The figure nearest approaching 
that of the larva found in the pearl oyster is that of the onchosphere 
of Tenia cucumerina given by Gamble in Vel. IT. ,‘‘ Cambridge Natural 
History ” (after Grassi and Rovelli).t There can be no doubt, 
however, that the larva is a Cestode. The possession of calcareous 
corpuscles, of spines on the collar, and of the protrusible proscolex- 
like head are all essentially Cestode characters, and doubt only arose 
on account of the isolated and more or less unconnected state of 
development of the globular cyst. 
(3) Encysted Tetrarkynchids on the Wall of the Gut in the 
Pearl Oyster. 
These are by no means rare, and are in almost every case confined 
to a particular part of the wall 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 in every way, save that strobilization has not commenced. 
This eneysted 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 intermediate between 
them, and the evidence that they are both stages in the life-history 
of the same parasite rests on circumstantial evidence and on evidence 
obtained by feeding experiments. We shall, however, refer to this 
matter again later. 
(4) The Adult Pearl-inducing Worm, ‘‘ Tetrarhynchus 
umonifactor.” 
‘The adult stage of the pearl-inducing worm was obtained by 
Hornell from the stomach of Rhinoptera javanica—a gregarious ray— 
and also later from the intestine of Ta@niura melanospilos. In spite 
of the fact that hundreds of fish, including at least fifty large rays 
of various genera and species, and also a large number of Carchariide, 
have been repeatedly and carefully examined during the last five 
years, the adult has never since been found, except in Ginglymostoma 
concolor, during the feeding experiments of 1909 and 1910, described 
in Parts IV. and V., ‘‘ Ceylon Marine Biological Reports.” This 
is a most remarkable fact, especially as the research, having been 
repeatedly fruitless, was carried on with increasing energy. 

* Shipley and Hornell. Herdman’s ‘‘ Ceylon Reports,”’ Vol. II., p. 20. 
+ And Herdman notes the resemblance in many ways to the larva of 
Acrobothrium figured by Giard and to the “ figures idéales ”’ of early stages of 
Tetrarhyuchids given by van Beneden. 
8 6(2) 11 
