THE CEYLON PEARL-OYSTER. 



285 



the adult of Ti/locephalavi ludificans^in fact, the only difference 

 appears to l)e the presence of the four lateral suckers, which are 

 absent in the larva, hut which may well not develop till its 

 transference to the final host. 



C(Mnparison of these figures makes one feel douT)tful whether 

 the genei'ic distinction between Ti/locephalum und Cephalobothriuni 

 is a valid one, or whether the conditions shown in figs. 58 ife 65, 

 and in figs. 59 it 66, are not merely the expression of a uniform 

 type of my/.oi-hynchus in different stages of contraction, as 

 suggested diagram matically in the following text-figure. 



Text fi". ?A. 



Dingrnm illustvatiii<? the relationsliip between the condition of the niyzorli.viiclius in 

 Tf/Incepludum an<l Crphalnlothriuni. A, a generalised scheme of a scolex 

 siKrh as the Ti/li>replu(la of the pearl-oyster. H, by protrusion of the 

 inyzorhynchns, the outer surface of the same being tense, the inner surface 

 ttirown into folds, the Ti/Iornphalum-fovm may be produced (compare 

 I'l. XL VII. tig. 65). C, the partial retraction of *the myzorhynchus to form 

 a sncker-like disk gives tlu! Ceplialolothrium condition (compare PI. XLVI. 

 tig. 59 and Pi. XliVII. tig. H6). D, the myzorhynchus retracted within 

 its collar, with its outer surf;ice thrown into folds, as is characteristic of 

 many of the larv;e of TyJncephahim ludijicans found in the pearl-oj'ster, and 

 of the adult worm figured on PI. XLYII. figs. 61-62. 



It does not necessarily follow that any of the above mentioned 

 worms actuall}' repi-esents the final stage of Tylocephalam ludijicans 

 or T. minus, though I think there are considerable grounds for 

 regai'ding the worm shown at figs. 60-64 as the former ; but it 

 certainly appears more probable that these final stages will be 

 found among this cla,ss of parasites rather than among the 

 TeirarhyncM, and it is strange that the position that the Tetra- 

 rhynchus unionifactor hypothesis may be wrong has never been 

 seriously faced and a fresh start made on the above lines. 



It is not known how TyJocephalum ludijicans and Tyloceq^hahna 

 miiius enter the Pearl-Oyster ; but Hornell found, in tow-netting 

 on the Muttuvarattu Paar on the 19th November, 1902, a free- 

 swimming larva, "37 mm. long when extended, which is figured 

 in Prof. Herdman's Pteport (Part II. Parasites of the Pearl- 

 Oy.ster, jjlate i. fig. 1 a-h). This larva certainly suggests an earlier 

 stage of Tylocej)halum ludijicans, and one of the chief difiiculties 

 in the way of accepting it di.sappears if this worm is dissociated 

 from the genus Tetrarhynehus, the normal habit of which is to 

 enter the digestive canal with the food while still in the eg'g- 



