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Show 1902.] ORIGIN OF PEARLS. 153 seminal vesicle receives the vasa deferentia (PL X V . fig. 6, v.d.), 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 granules 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 (PL XIV. fig. 2; PL X V . 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-fluke and so many other Cercariae encyst. The dark colour of the epithelial sac, which can often be isolated with the worm, suggests, on casual observation, 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 Sporocyst Stage. After a laborious examination of most of the organisms which inhabit Billiers Harbour, I was rewarded by finding, in Tapes decussatus GmeL, Sporocysts containing tailless larvae, almost identical with those which occur in Mytilus (PL X V I . figs. 9, 10, and text-fig. 24). They differed from the latter only in their smaller size, paler colour, more distended excretory 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 gravelly 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 Villaine, it is nowhere found in sufficient 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 hundred. They occur in the muscular or PROC. ZOOL. Soc-1902, VOL. I. No. X L 11 |
