1902.] ORIGIN OF PEARLS. 161 



Scoter (or another final host), and the set of the cui'rents must 

 favour the transportation of the larvje 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 with- 

 out finding here and there among them an infected individual. 



Duration of Life of the Larva in Mytilus, and Rate of 

 Grototh of Pearls. 



1 am at present making experiments to test tiie longevity of 

 the resting larva. I have, however, three facts to record that 

 lead me to think it Ls 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. When I was at Piel, Mr. Scott 

 showed me a small patch of mussels on the pearl-bearing beds, and 

 told me that these molluscs had l^een biought in from the Roose- 

 beck by a fisherman abovit 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 Oercariee 

 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 pi-eserved in 

 spirit), and measured i-espectively 1'3 x 1 mm., 1'5 x 1 mm., 

 9 X '85 mm., 1-2 x "8 mm., and 2'1 X 1*15 mm. The last was 

 obviously, from its foi-m, 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 con- 

 tained 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 mm. long, which size is probably attained 

 when the mussel is in its third year. I find Cercarife, on the 

 other hand, in specimens only 20 mm. in diameter. 



The average size of the larger pearls found in old mussels at 

 Piel is about 2x2 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 

 difierent 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 



