1922] 



MADRAS PEARL FISHERIES 



131 



of pearls, differing in origin, and that in the case of cyst-pearls 

 the causative body may and usually does differ with the locality 

 and the species investigated. In the case of certain mussels 

 (Myiilus edulis) the causative nucleus has been found in certain beds 

 in France to be a larval trematode worm (Jameson andBoutan), and 

 in certain fresh-water mussels in one locality this is replaced by a 

 little commensal mite (Kuchenmeister). 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 ga^e the name Tetrarhynchus unionifactor, and we correllated 

 it with an advanced larval Tetrarhynchid of typical form found 

 commonly encysted in the walls of the oyster's intestine. At a 

 later date we discovered 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 inter- 

 mediate host, one of the file-fishes {Balistes mitis) but eventually 

 the species found in the file-fish was found to be of a distinct 

 species, not parasitic in the larval condition in pearl oysters. I 

 have however come now to the conclusion that the spherical cestode 





A 





^ZSlXDiZi' 1 



B 





^ 





f : 



Fig. 9. Three cestode larvae extracted from cysts found in the tissues of the pearl-oyster 

 (Gulf of Mannar). A. is the youngest stage found ; B. is an elongated form (older) 

 occasionally foand, while C, seen under higher magnification and slightly com- 

 pressed, shows the beginnings of a vascular system and also a terminal excretory pore. 

 Note in all the minutely spinous nature of the collar and the multitude of tiny 

 calcareous granules densely filling the body region. 



