122 



CEYLON PEARL OYSTER REPORT. 



mind that, from the pearl-fisheries point of view, their influence is not wholly evil, as 

 their ravages are closely associated with pearl-production. Although these fishes 

 doubtless devour many of the oysters, at the same time they receive and pass on 

 the parasite which leads to the production of pearls in others. One of the largest 

 and most voracious of rays, Rhinoptera javanica, the " Valvadi tirikkai " or 

 gregarious ray of the divers, we have found to be the host of the adult Tetra- 

 rhynchus unionif actor, which in its larval stages causes pearl-production in the 

 oyster. The loss of some individuals from a bed is in that case a toll that we may 

 willingly pay, and no one could advocate the extermination of that particular enemy, 

 although we may desire to restrain his ravages within limits. During the fishery of 

 1889, the gregarious ray was present in such abundance that about 7,000 were caught 

 in a single haul of a net, near Dutch Bay (see this vol., p. 61). 



The Mollusca, which bore into shells by means of their radula, a toothed band 

 lying in the floor of the mouth, are for the most part small Gastropods, and they 

 are collectively known as "uri" by the divers. Fig. 10 shows a group of " uri " 

 such as we have frequently caught in the act of penetrating the valve of a pearl 

 oyster. It is chiefly young shells that are attacked, and amongst a large number 

 of dead valves, about an inch in diameter, examined on one occasion, we found 

 60 per cent, were perforated by the neat circular hole which clearly indicated the 

 cause of death. Probably adult pearl oysters are rarely killed by these small enemies. 



i% i # 



Fig. 10. "Uri," small Gastropods 

 that destroy young pearl oysters 

 by boring through the shells and 

 sucking out the soft body. 



Fig. 11. Inside of pearl-oyster 

 shell, showing adductor im- 

 pression affected by Glioma 

 borings on the outside, x A. 



During our work on the pearl banks we have not found a single full-grown shell 

 perforated by a Gastropod. If such do occur, the enemy cannot be the small <: uri'' 

 figured above, but must be more powerful animals, such as the larger species of 

 Murex and the ('hanks Turbinella pyrum and Fasciolaria trapezoides. 



The boring Sponge, Cli<m<< margaritiferce, Dendy (see Part III., p. 128), may be 

 considered damaging from two points of view first, as causing thickened deposits of 

 nacre and other irregularities, and hence disturbance of function, at the attachment 

 of the great adductor muscle (see fig. 11); and secondly, as honeycombing the shell 



