IC 



fishery, corroborated all I have stated about the ovaria or 

 genital glands and their contents, and that he has discovered, 

 in addition to the filaria and cercaria, three other parasitical 

 ■worms infesting the viscera and other parts of the pearl 

 oyster. We both agree that these worms play an important 

 part in the formation of pearls, and it may yet be found 

 possible to infect pearls in other beds with these worms, and 

 thus increase the quantity of these gems. The nucleus of 

 an American pearl drawn by Mobius is nearly of the same 

 form as the cercaria found in the pearl oysters of Ceylon.'* 

 The " cercaria " referred to were, probably, Cestode worms 

 [Anfhocejihalus, &c.), which are found in the internal organs 

 of various fishes caught off the coast of Southern India, and 

 gave rise to a scare in the European fish-loving community 

 a few years ago. During the recent fishery in only a few 

 out of many hundreds of oysters which I examined did I 

 find small nemertine worms living on the mantle or gills of 

 the oyster, so that their presence cannot be regarded as a 

 common or essential occurrence. 



The Grulf of Manaar pearl oyster {Avicala fucata, Grould) 

 is represented in plate 1, as it appears after removal of one 

 valve of its shell, the " ovarium," mantle, gills, adductor 

 muscle, and byssus being exposed. The presence of a small 

 pearl imbedded within the substance and projecting from 

 the surface of the " ovarium " is indicated at A. The byssus 

 (B), of which the function has given rise to much discussion 

 and speculation, is made up of a bundle of tough, green- 

 coloured fibres, secreted by a gland in the foot, and is 

 capable of being protruded beyond, or retracted within the 

 shell. By its means the animal is enabled to anchor itself 

 on the sea-bottom to a neighbouring oyster or other mollusc 

 shell, coral-block, melobesian nodule, or other convenient 

 object; and it is said that the animal can, even in the adult 

 stage, voluntarily shift its position and migrate to a con- 

 siderable distance. That the young oyster can, during its 

 phase of existence as a minute, free-swimming organism 

 wander about and eventually settle down on some congenial 

 spot no one ^vn^U dispute ; but the evidence that the adult 

 oyster can, under natural conditions, migrate to any con- 

 siderable distance is wholly insufficient, even though it has 

 been demonstrated by experiments that a young pearl oyster 

 under unnatural conditi(;ms in a soda-water txmibler full of 

 sea-water can, though v/eighted with two other oysters of 

 nearly its own size, climb up a smooth perpendicular surface 



