24 CEYLON PEARL OYSTER REPORT. 



the Tamils (see this vol., p. GO, and Part III., Preface, p. viii). No fresh light has 

 been thrown upon the possible occurrence of an immature stage in Balistes (which 

 is eaten by the large rays), and although that intermediate host may not be 

 necessary to the life-history, since the rays also feed upon pearl oysters, still there 

 is nothing in the observed facts to forbid the existence of such a stage, and it is 

 not unusual in Tetrarhynchids to have two fish-hosts, an intermediate Teleosteau 

 which is devoured by a final Elasmobranch. 



Cyst and Pearl-Sac. 



We now turn from the larvae to the cysts which enclose them. In the youngest 

 stages of both species these are merely thickenings of the connective tissue of the 

 mantle (Plate II., figs. 17 and 18), or the mesoderm around the tubules of the liver, 

 gonads, and other viscera. The thickening is laminated (Plate II., fig. 21), and the 

 fibres, when fibres are visible in the lamellae, run concentrically around the more or 

 less spherical body of the larva. In the thicker cysts the outer layers may contain 

 many blood spaces (lacunas), and sometimes the thickening becomes quite spongy 

 or oedematous (fig. 19). In some cases a considerable increase in the number of 

 connective-tissue corpuscles or leucocytes is evident and in later stages (Plate III., 

 figs. 7 and 10) cells are sometimes seen to accumulate, and probably proliferate, along 

 the inner surface of the fibrous cyst. It is just remotely possible that it is in this 

 way that the pearl-producing epithelial sac is formed, from apparent mesoblast cells, 

 inside the connective-tissue cyst. The other and more probable view that may be 

 held is that these cells proliferating on the inner surface of the connective-tissue 

 cyst are ectodermal in origin, and produce the pearl-sac. 



As our specimens do not give conclusive evidence as to the stages in the formation 

 of the epithelial sac, and as previous observers seem to have left this matter in some 

 doubt, we think it advisable to state here fully the two possible alternative views that 

 have been and may be held. 



The first of these views is that the epithelial sac which surrounds the parasite and 

 secretes the pearl is derived directly or indirectly from the ectoderm on the outer 

 surface of the mantle the layer which normally secretes the nacreous layer of the 

 shell. By " directly " we mean where the sac as a continuous layer is formed by a 

 pouching inwards of the ectoderm, the pouch being then cut off from the surface to 

 form a closed sac. We should call "indirect" such cases as those where isolated 

 ectoderm cells wandered into the mesoderm or were carried in by a moving parasite 

 (the "processus ccenogenetique " of Giard) ; these ectoderm cells proliferating, it may 

 be supposed, around the parasite to form the sac which then secretes the pearl. 



In favour of this ectodermal origin may be stated : 



1. The very close resemblance between the epithelium of the pearl-sac and that 

 of the outer surface of the mantle, amounting to identity in staining reaction. 



