32 
Pearl Tnelucing Worm 
concolor, during the feeding experiment of 1909 and 1910, and described 
in Part TV and V, Ceylon Marine Biological Reports. 
This is a very remarkable fact, and is no doubt to be correlated with 
the absence of oysters from the banks during the past five 3 ’eai'S, and to 
the resulting general scantiness of fish over the plateau. Up to the 
present, about 105 species of Cestodes have been recorded from fish 
caught on the pearl banks, and as the adult of the pearl-inducing worm 
was never obtained, its rarity will be obvious. 
We have already noted that no larvae have ever been found in an 
earlier stage of development than the globular cyst; that no stage has 
been found intermediate between the globular cyst and the adult but 
young Tetrarhynchid both of which occur in the oyster; and finally we 
have noted how very rarely the adult worm itself is found. 
In 1909 experiments were initiated with a view to develop the adult 
worm in various Plagiostomous fish by feeding them with oysters 
containing the globular cysts. These experiments are fully described in 
the Reports above cited. A square area in the open sea was isolated by 
means of expanded metal jfiaced vertically. The fish deemed suitable 
for experiment were often trawled 10 to 12 miles away and were trans¬ 
ported to the enclosure in the ship’s water boat, which had a mid-ship 
tank section of 13x9x4-| feet which could be flooded with sea water, 
and used to contain the fish. The bottom of the enclosure was specially 
covered with oysters of various ages. Before the fish were placed here 
they were dosed with male-fern extract and ca.stor oil, in order to clear 
away, if possible, any parasites they might already have, before the 
experiment was begun. The general success of this procedure was 
indicated after the ultimate death of the fish by the large number 
of reddish spots in the spiral valves, indicating the positions previously 
occupied by Cestodes. The following fish were thus dosed and then 
placed in the enclosure: Trygon walga, Taeniura melanospilos, Gingly- 
mostoina concolor, Rhynchobatus djeddensis, Serranus undulosus (4 feet 
long) and specimens of Tetrodon stellatus measuring 20 inches long. 
The results were roughly as follows: 
( 1 ) Although several specimens of Rhynchobatus djeddensis were 
placed in the enclosure they all died within three days, showing that in 
spite of their massive teeth and strong jaws this species will starve to 
death rather than eat oysters. 
( 2 ) Specimens of Tetrodon stellatus and Serranus undulosus lived 
in a healthy condition, but on being killed after several weeks no adult 
Cestodes were found in them. 
