
1922.] Pearl Formation in Indian Pearl Oyster. 215 
tods worm (Jameson and Boutan), and in certain fresh-water 
mussels in one locality this is replaced by a little commensal 
mite (Kiichenmeister). In the case of the Ceylon and Indian 
pearl oyster, Prof. Herdman and the author found it in many 
cases to consist of the dead body of a larval Cestode. To this 
we gave the name TJ'elrarhynchus unionifactor, and we correl- 
lated it with an advanced larval presence of typical 
form found commonly encysted in the walls of the oyster’s 
intestine. At a later date we pean that the adult of the 
latter worm is found in the sexually mature condition in the 
intestine of an oyster-eating ray, Rhinoptera javanica. At 
one time we intercalated an intermediate host, one of the file- 

Fic. 1.—Three cestode larvae extracted from i found i 
idee Ps ‘the pearl-oyster (Gulf of Mannar). A. is the youngest phiaee 
found; B. is an elongated form (older) occasionally foced. while C., seen 
of a vascular aga as also a terminal excretory pore. Note in all the 
minutely spinous of the collar sins the multitude of sis calcare- 
ous ated dennaes filling the body reg 
fishes (Balistidae) but eventually the species found in the file- 
fishes was found to be of a ——— species, not parasitic in 
the larval condition in pearl oyste 1 have, however, come 
now to the conclusion that the sphenionl cestode larva found in 
abundance in the tissues of the pearl oyster and ee. as 
a nucleus in cyst pearls from the same mollusc, is a 
younger stage of the undoubted Tetrarhynchid larvae pines 
n its intestine. Possibly it is the larva of some species of 
Velicehaians or other closely related genus, but this is a 
subject for further investigation. 
arl oysters are free from this parasite. Usually 
the gills contain hundreds, often very minute and never differ- 
ing in any appreciable degree from those shown in figure | 
