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Show 1902.] ORIGIN OF PEARLS. 161 Scoter (or another final host), and the set of the currents must favour the transportation of the larvae or eggs (whichever it may be) to the beds where Tapes occurs. Although it is only on certain beds that these conditions exist, infection takes place to a small extent on very many beds. I have hardly ever examined a sample of mussels from any locality without finding here and there among them an infected individual. Duration of Life of the Larva in Mytilus, and Rate of Growth of Pearls. I a m at present making experiments to test the longevity of the resting larva. I have, however, three facts to record that lead m e to think it is less than two years. While the mussels on the foreshore opposite Piel Fish-Hatchery are highly infected, those on the Roosebeck Scar, outside the Barrow Channel, are not so. W h e n I was at Piel, Mr. Scott showed m e a small patch of mussels on the pearl-bearing beds, and told m e that these molluscs had been brought in from the Roosebeck by a fisherman about two years previously and thrown down there. I examined a number of these mussels, and each of them contained several small pearls. Some, indeed, had as many as ten, and all were infested with the Trematode. From the presence of pearls in these specimens, it is probable that the first Cercariae to enter them had been dead some time. The dimensions of these pearls throw some light on the time required to produce pearls of a certain size. The five largest specimens weighed together 6'9 mg. (dried on filter-paper after being preserved in spirit), and measured respectively L3 x 1 mm., 15 x 1 mm., 9 x '85 mm., L 2 x '8 mm., and 2*1 x 1"15 m m . The last was obviously, from its form, a double pearl. Again, as four out of the six specimens that I dissected after they had been about two years in the Brighton Aquarium contained each a small pearl, but no live Trematodes, it is probable that the latter were unable to survive two years in the Aquarium. Moreover, at Piel and Billiers pearls are very seldom found in mussels less than 40 m m . long, which size is probably attained when the mussel is in its third year. I find Cercariae, on the other hand, in specimens only 20 m m . in diameter. The average size of the larger pearls found in old mussels at Piel is about 2 x 2 mm., but all sizes, from the dimensions of the parasite to 3*35 X 3-2 mm., were found. At Billiers they are usually smaller, as the mussels are regularly fished there and seldom reach a great age. The pearl-bearing beds at Piel are not fished, as the infected mussels are not marketable. The sizes to which pearls grow in other molluscs differ very greatly for the several species and for the same species in different localities. Their growth is, in fact, regulated by the causes which control the thickening of the shell. Hence the white porcellaneous pearls of Tridacna gigas and Hippopus |