22 CEYLON PEARL OYSTER REPORT. 



various parts of the pearl oyster, we have been unable to find any stage intermediate 

 between that shown in figs. G and 8, on Plate III., and the young Tetrarhynchus with four 

 proboscides. It is probably therefore a rare occurence for the larva to advance further 

 in its development in this molluscan host ; but that it does occasionally happen is 

 shown by our finding a few young Tetrarhynchids in cysts on the wall of the pearl 

 oyster's intestine (see Plate III., fig. 11, for a section of this stage, and fig. 16 on 

 Plate II. for the general appearance of what is probably the same species). We have 

 found, in all, about six such Tetrarhynchids in company with over 200 of the globular 

 parasites. If the parasite normally does not go beyond the globular stage in the 

 body of the pearl oyster, but only occasionally advances a stage further and 

 acquires the tour proboscides, and then again remains quiescent in a cyst, it follows 

 that the transition form which we have looked for in vain may be passed over very 

 rapidly. In that case we should find the greater number of the parasites in the 

 younger globular stage, a very few in the more advanced Tetrarhynchid condition, 

 and practically none in an intermediate state. 



Since the above was printed, and the diagrams shown in text-fig. 4 (p. 21) were 

 drawn, we have found, encysted in the liver of the pearl oyster, a very young 

 Tetrarhynchid larva which possesses the characteristic four proboscides, but has not 

 yet become elongated. It is of ovate form (Plate III., fig. 10) and measures 

 0*53 millim. in length. It shows at the anterior end the lateral projections bounding 

 the central depression just as in earlier stages (see fig. 6), but the central papilla is 

 traversed by several openings which are clearly the tubular proboscides (fig. 10). In 

 fact it agrees so well in all other respects except the proboscides with the larger form 

 of globular larva that we can scarcely fail to recognise it as the later stage of the 

 same animal Rliynchobothrius unionifactor (SHIPLEY and HORNELL). 



SHIPLEY and HORNELL have described (this vol., p. 43, et seq.) several other species 

 of Tctrarhynchus from Ceylon, but none of them from the pearl oyster; so we are as 

 yet unable to refer to its species the smaller globular larvae which we find commonly 

 encysted, and which may occasionally form the nuclei of pearls. 



Both our larval Tetrarhynchids we believe to be pearl-inducing parasites in the 

 Ceylon pearl oyster. The figures on Plate II. show for the most part the appearances 

 presented by the smaller globular parasite in our specimens. Though small, they are 

 visible to the eye (figs. 1 and 2). Fig. 17 shows the relation in size between the two 

 kinds of larvae the larger (0'9 millim. in length) being about six times the size of 

 the smaller (0*14 millim. in length). SEURAT'S larvae are 0*25 millim. 



We give on Plate II. (figs. 3 to 16) some of the drawings made by one of us (J. H.) 

 in Ceylon, and which were used in SHIPLEY and HORNELL'S article upon the parasites 

 of the pearl oyster (this work, Part II, 1904). They show mode of occurrence in the 

 tissues (figs. 1, 2, 3, 17, 18, 19), relation to pearls (figs. 4, 5, 6, 7), stages in the 

 structure of the larva (figs. 8, 9, 10, 17, 20, 21, 22), differences in the amount of the 

 connective-tissue cyst (figs. 17, 18, 19, 21, 22), and finally some later stages of 



