Till': c.i;yl(j\ imcaih.-ovstior. "27'.* 



But iliL- woi'in specifically described hy Sliipley and Hornell as 

 Tetrarhyiichits unionifactor on p. 88 of Part 11. of Prof. Herdnian's 

 Report (Parasites of the Pearl-Oyster) and figured in plate ii. 

 figs. 19 (k 20, is a. well -advanced Tetrarhi/nchits 6-5 to 7 mm. 

 long, which occurs in and around the intestine of the Pearl- 

 Ovster; and, to sa.y the least of it, it is doubtful whether this worm 

 is a later stage of the globular (;ysts, which Prof. Herd man identi- 

 fied as the nuclei of pearls, and not an entirely distinct organism. 

 In order to avoid confusion of terms I am therefore giving 

 separate names to the larger and smaller globular Uestode larvpe 

 which Herd man recognises, as it is calculated to lead to much 

 confusion of issues if these are i-eferred to by the name of TeAra- 

 rlu/iiclitis unionifactor before theii- identity with it can be demon- 

 strated more satisfactorily. The arguments for regarding the 

 supposed pearl-producing parasites as distinct from TeArarhynchnn. 

 luilonifactor are set forth below. In the absence of satisfactory 

 evidence of their relation to the genus Tetrarhynchus, I pi'opose, 

 following Seurat (36), to whose larval Ce.stode, mentioned above, 

 they bear a close resembhince, to refer them to the genus Tyloce- 

 phaluni* and to desciibe the larger and smaller forms respectively 

 as Tijlocephalum IvJiJicmis, sp. n., and Tj/locephalmn viinus, sp. n. 



Two well-marked sizes occur in these globular larvje and they 

 are regarded by Heidman as distinct organisms (Report V. jj. 21). 

 On the other hand, .Southwell considei-s that the asexual repro- 

 duction, which he has occasionally observed, accounts for the 

 vaiyiug sizes of the larvae in the Ceylon Pearl-Oyster, and says : 

 •' I am now convinced that these different sizes merely repi-esent the 

 same species in diftei-ent grades of development." 1 am inclined 

 to share Prof. Herdnian's view that these two sizes are distinct 

 oriranisins. It may even prove that there are more than two 

 species represented. Indeed, I should not be surprised if further 

 research on fresh material were to show that both Tylocephahmn 

 ludiflcans and 7'. minus ai-e composite species. 



Pi'ofessor Herdman regards 2\ ludijicavs as the earlier stage 

 of Tetrarhynchns anio/iifnctor, and calls attention to its re- 

 semV)la.nce to Van Beneden's ideal figures of the young of 

 TetrarhyDchus, while he treats T. minus as another species of 

 Tetrarhynchus in its earlier stages. Nevertheless, he seems to 

 have had suspicions that some, at least, of these larvfe might be 

 Tylocephula, though he appears in the end to have decided that 

 they — and presumably with them Seurat's larva — are a hitherto 

 unknown stage in the life-history of the genus TeirarhynclmH. 



In the Pi^eface to Part II. of his Report he says ([>. vi) : — 



'• It is possiVjle that some of our Ceylon Pearl -(.h'ster 

 parasites may also belong to the genus Acrohothriiiin " 

 [/. p. the genus to which Seurat's larva, was then referred], 

 "• altliough the more advanced ones are certainly Tetru- 

 rhynchids "' ; 



* l/iuloii (27 rt). pp. S().5-0. pi. i\. Iig>. o-'J. Tyi't' T. jiinf/uc. tVuiii >pii;il \;il\i 

 of Rliinni>(cra f/iiadrilob((. 



