GENERAL SUMMARY .VXD RECOMMENDATIONS. 127 



caused by boring sponges and burrowing worms. Minute grains of sand and other 

 foreign particles gaining access to the body inside the shell, which are popularly 

 supposed to form the nuclei of pearls, only do so, in our experience, under exceptional 

 circumstances. In the whole of our observations we have only records of three cases 

 in which a grain of sand undoubtedly formed the nucleus of a pearl. 



Pearls of another class are found in the muscular tissue of the animal, most 

 frequently in the levators, in the palpar region, and in the pallial insertions. These 

 muscle pearls have no visible foreign bodies as nuclei. They form around minute cal- 

 careous concretions the calcospherules which are sometimes very abundant in the 

 tissues. Yet the pearls are very irregularly distributed. Single oysters have been 

 known to contain from one to two hundred pearls, and, on the other hand, a hundred 

 oysters may be opened without finding a single pearl. 



The best pearls, however, the " fine " or " orient" pearls, lie in the pallial connec- 

 tive tissue at the sides of the body or in the tissues around the liver and kidney, or, 

 when large, they may be free in any cavity of the body. The majority of these fine 

 pearls contain as their nuclei the more or less easily recognisable remains of certain 

 Platyhelminthian parasites, which we identify as the larval condition of Cestodes 

 belonging to the genus Tetrarhynchus. The evidence for this will be found in 

 the section dealing with pearl-formation (this vol., p. 19), and a description of 

 the probable species of Tetrarhynchus in question will be found in Shipley and 

 Hornell's report on the parasites (p. 87). We have traced most stages in the 

 life-history of this pearl-producing Tetrarhynchus, and find that it passes, in the 

 adult condition, into the body of one of the larger carnivorous fishes Rhinoptera 

 javanica, a Kay. The adult parasitic worm in its last host must then set free its 

 numerous young embryos, which pass into the sea and so gain access to the gills, 

 liver, and mantle of the pearl oyster. But it is not sufficient for the oyster to be 

 infected by the Tetrarhynchus larva. It must also live, retaining its parasite until 

 such time as it can produce sufficient deposit of the calcareous secretion to entomb 

 the living source of irritation, which thus becomes the nucleus of a pearl. This 

 history is discussed more fully in another section of this volume (p. 14). 



The Cysticercoid cysts of the Tetrarhynchus larvae are frequently very abundant 

 in the liver of the pearl oyster. In the case of some paars, the Muttuvaratu 

 especially, scarcely any of the individuals examined are free ; we have counted eleven 

 encysted larva? in a single liver. In the gill filaments and in their membranous bases 

 also they are common, while in many cases the mantle is infested. The gonads, the 

 foot and the palps all occasionally harbour the parasite. The muscles are the only 

 large organs where the cysts are rarely found. In one individual oyster Mr. Hornell 

 maue out a total of 45 cysts for all the tissues. It may be well to repeat here that 

 the Cestode parasites are not only common, but are also apparently very wide spread 

 and generally distributed, and that the fish-host with its parasite occurs also generally 

 in the seas around the Island, as well as in the Gulf of Manaar ; and that, in short, 



