1902.] ORIGIN OF PEARLS. 153 



seminal vesicle receives the vasa deferentia (PI. XY, fig. 6, v.cZ.), 

 which can be traced back into the spherical testes (te.). 



Biology. 



When the larva first enters Mytilus it is somewhat smaller 

 than the resting specimens and more transparent. The excretory 

 organs, which are laden with gTanules while in the Sporocyst, are 

 comparatively empty, and the gut is not yet distended with food. 

 As it grows older both the excretory and digestive systems become 

 more and more laden, so that they form the great mass of the 

 body (PI. XIV. fig. 2 ; PL XY. fig. 5). It is largely to the 

 contents of the latter that the parasite owes its dark yellow 

 colour, the cuticle being pale golden or straw-coloured. 



The worm often excretes some granular substance, which may 

 almost surround it in the sac. It is this stuff that serves as 

 " nucleus" for a pearl, if the Trematode migrates to another part 

 of its host. 



It is interesting to note that at no period is this worm encysted, 

 in the sense in which the Liver-fiuke and so many other 

 Cercarife encyst. The dark colour of the epithelial sac, which 

 can often be isolated with the worm, suggests, on casual obser- 

 vation, that the latter is encysted, but I have determined by 

 sections that this is never the case. 



In a certain sense it is a resting stage, but the distension of 

 the alimentary system makes it obvious that it is also a highly 

 assimilative phase in the life of the worm, which is storing up 

 energy for the maturation of the gonads, on reaching the final 

 host. 



Life-history of the Parasite. The Sjiorocyst Stage. 



After a laborious examination of most of the organisms which 

 inhabit Billiers Harbour, I was rewarded by finding, in 2^«/>es 

 decussatus Gmel., Sporocysts containing tailless larvte, almost 

 identical with those which occur in Mytilus (PI. XYI. figs. 9, 10, 

 and text-fig. 24). They differed from the latter only in their 

 smaller size, paler colour, more distended exci-etory organs and 

 empty gut, and in the possession of special sense-organs and eyes. 

 Tapes decussatus is extremely abundant in Billiers Harbour, 

 living in burrows about six inches deep in the black gi-avelly 

 clay that forms the bottom. The local name of this mollusc is 

 Palourde, and it is extensively collected for food. I am informed 

 that although Tapes occurs in small numbers all along the shores 

 of the Yillaine, it is nowhere found in sufiicient quantities to be 

 worth fishing, except in Billiers Harbour. Indeed, when I visited 

 Billiers in December 1901, a lugger from the other side of the 

 estuary came over specially to collect this shell and Mya arenaria 

 for the markets, there being no supply on the south shore. 



I found Sporocysts in every specimen of Tapes that I examined, 

 numbering nearly two hundi'ed. They occur in the muscular or 



Proc. ZooL. See— 1902, YoL. I. No. XI. 11 



