In the case of the fresh water mussel Filippi of Turin showed in 1852 that the species of Trematode (fluke) Distomum dupli catum was the cause of a pearl formation in the fresh-water mussel Anodonta. Kuchenmeister subsequently investigated the question at Elster in Saxony and came to a different conclusion, namely that the central body of the pearl was a small specimen of a species of water mite which is a very common parasite of Anodonta. Filippi however states that the mite is only rarely found within a pearl, the Trematode occurring in the great ma jority of cases. R. Dubois and Dr. H. Lyster Jameson have made special investigations of the process in the common mussel Mytilus edulis. The sac or cyst is formed by the larva of a species of Trematode belonging to the genus Lecithodendrium, a species closely resembling and probably identical with L. somateriae, which lives in the adult state in the eider duck. At Billiers, Mor bihan, in France, the host of the adult Trematode is another spe cies of duck, namely the common scoter, Oidemia nigra, which is notorious in the locality for its avidity for mussels. Trematodes of the family Distomidae, to which the parasite under considera tion belongs, usually have three hosts in each of which they pass different stages of the life history. In this case the first host at Billiers is a species of bivalve called Tapes decussatus, but at Piel in Lancashire there are no Tapes and the first stages of the para site are found in the common cockle. The Trematode enters the
first host as a minute newly hatched embryo and leaves it in the form called cercaria (see TREMATODA). The cercaria makes its way into the tissues of a mussel and there becomes enclosed in the cyst previously described. If the mussel is then swallowed by the duck the cercariae develop into adult Trematodes or flukes in the liver or intestines of the bird. In the mussels which escape being devoured the parasites cannot develop further, and they die and become embedded in the nacreous deposit which forms a pearl. Dr. Jameson points out that, as in other cases, pearls in Mytilus are common in certain special localities and rare elsewhere, and that the said localities are those where the parasite and its hosts are plentiful.
In the case of the Ceylon pearl oysters, Prof. Herdman con cluded that the exciting cause of pearl formation was the larva of a cestode or tapeworm (q.v.). The cestode theory has, however, been somewhat discredited in recent years; it is by no means cer tain that the pearl nucleus is always a parasite; many different exciting causes may lead to pearl formation.