132 



MADRAS FISHERIES BULLETIN 



[VOL. XVI, 





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larva (Fig 9), found in abundance in the tissues of the pearl-oyster 

 and frequently as a nucleus in cyst-pearls from the same mollusc, is 

 not a younger stage of the undoubted Tetrarhynchid larvae 



encysted in its intestine (Fig. 10). 

 Possibly it isthe larva of a species 

 belonging to the cestode genus 

 Tylocephalum but this is a subject 

 for further investigation. 



Few pearl-oysters are free 

 from this parasite. Usually the 

 gills contain hundreds, often very 

 minute and never differing in any 

 appreciable degree from those 

 shown in Fig. 9. The digestive 

 gland is another favourite loca- 

 tion for these cysts, opalescent 

 white spheres conspicuous in the 

 dark green of the gland. In 

 figure 11 are drawn two nuclei 

 which I obtained by decalcifica- 

 tion of small Orient pearls ; there 

 can be no question as to their 

 identity with the spherical larvse 

 found alive in the tissues. 

 Neither Prof. Herdman nor I 

 ever claimed that all cyst-pearls 

 have such nuclei ; we recognized 

 that other foreign bodies, notably 

 grains of sand, occasionally 

 function as the intrusive irritating 

 factor and become pearl nuclei. 

 We have also even found a small 

 nematode worm coiled upon 

 itself, forming the nucleus. So 

 far we went, over 16 years ago. 

 A. view of the entire worm. B. Fn- Subsequent investigation shows 



larked view of the fore end, to show , - . ,. c ,. _ 



.. s . . , ,,, • „ me that a further qualification 



the 4 proboscides and their facs. l " * 



is necessary whereby cyst-pearls 

 may be divided into two sections, the one comprising pearls 

 induced by the irritation of foreign bodies and the other those with 



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Fig. 10. 1't'trarhynchits uniotiif actor. 

 Larval form found encysted in the 

 wall of ihe pearl-oyster's intestine. 



