THE CEYLON PEARL-OYSTER. 281 



It is strange how the TeArarhyncJius unlonifactor theory, 

 once enunciated, has prevailed : — indeed nobody seems to have 

 seriously folloAved up the obvious clue given by Seurat's discovery 

 of the supposed adult of his worm. 



Prof. Herdman says in his Report, Part V. pp. 20-21 : 



" Shipley and Hornell in Part II. (p. 77) described and 

 figured various stages of the Cestode larvae both from the 

 centre of decalcified pearls and also free in the tissues of the 

 pearl-oyster, but left it an open question whether the sub- 

 globulai- younger larvse [i. e. Tylocephalam. ludijicans, n. sp.] 

 belong to the same life-history as the elongated older forms, 

 which are young Tetrarhynchids. If our arrangement of 

 the stages obsei-ved in the tissues of the pearl-oyster is 

 correct, and if all these larvae belong to the same species, 

 then the interpretation we have given above brings us to 

 the conclusion that the larger of our two globular larvte 

 belongs to the worm which Shipley and Hornell described as 

 Tet7-arhyuchus tmionif actor in 1904." 



And, referring to some figures of Tylocephahvni ludijicans on 

 pi. iii. (Report, Part V. Pearl-Production) figs. 1-8, he says 

 (p. 21): 



" there can scarcely be any doubt (1) that they all belong 

 to the same life-history, and (2) that they are yoimg Tetra- 

 rhynchids leading on to the stages shown in figs. 10 and 11." 



Prof. Herdman gives, in support of his theory, a seiies of 

 figures showing the hypothetical transition from I'ylocephalum 

 ludijicans to Teirarhynclius unlonifactor (Report, Part V. p. 21). 



Later, a younger Tetrarhynchus, 1 mm. in length, was found 

 in the stomach and alimentary canal of the o' ster, which Shipley 

 and Hornell (Report V. pp. 87-88) regarded as probably an 

 earlier stage of Tetrarhyndms imionifaC^or ^ though possibly a 

 distinct form. A single example of a still younger form, which 

 is figured in the Report on Pearl-Production (V., pi. iii. fig. 10), 

 and is described in the text (p. 22) as occurring " encysted in the 

 liver" and in the explanation of the plates as " from cyst between 

 stomach and liver," appears to go still further towards linking 

 the two forms. This larva is referred to in the text as '53 mm. 

 long. Tylocejihalum hidijicans grows to a much greater size than 

 this without changing its characters ; thus the one figured by 

 Shipley and Hornell in the " Parasites of the Pearl-Oyster " 

 (Report, Part II. pi. i. fig. 12), and described as x 40, appears 

 from the size of the figure to be over 1-5 mm. long, and still 

 shows no sign of becoming a Tetrarhynchus. And T. ludijicans 

 quite frequently measiares 1 mm. in diameter. This discrepancy 

 in dimensions makes it hard to believe that they are the same 

 organism, and the gap between the Tylocephahmi form, with its 

 round Balanoglossus-\\kQ scolex or " myzorhynchus,"' and the 



