98 Drs. Kelaart and Mobius on the Natural History of 



s^owa-nucleus in many hundred small free pearls from the fresh- 

 water mussels, and in " about 40,000 specimens of Unio marga- 

 ritifer, opened partly by Von Hessling- and partly by the fisher- 

 men, no trace of a parasite or of an egg could be found." No 

 traces of eggs or parasites were detected in hundreds of oriental, 

 Scotch, and Bavarian pearls, which he opened with chisel and 

 saw, and treated with organic and inorganic acids. 



Dr. Mobius, however, has arrived at a directly opposite result, 

 by following a rather more cautious method of investigation, and 

 grinding down the pearls with a fine file and stone. His results 

 are therefore far more to be depended upon than those of Von 

 Hessling, who seems to have carried on his researches in rather 

 too wholesale a manner. In eight pearls from the west coast of 

 America he found remains of Entozoa forming the nucleus, and 

 one of those figured in his plate is evidently a Trematode larva. 

 In other curious specimens the nucleus is covered with layers of 

 epidermis, which appear to have been drawn asunder by the 

 movements of the living nucleus. As a general rule, the organic 

 nucleus seems to shrink more or less after its enclosure, and the 

 space thus left is usually tilled up in part by crystalline fibres, 

 probably consisting of carbonate of lime infiltrated through the 

 first layers. The pearls already referi'ed to as containing a 

 nucleus of crystalline carbonate of lime are also probably caused 

 originally by the access of young parasites, which, retaining 

 their vitality, subsequently break through the first layers of the 

 pearl, and thus leave a hollow space in its centre. The great 

 weight of evidence, therefore, is in favour of the organic origin 

 of the nuclei of pearls, partly from the eggs and encysted young 

 of parasitic animals, and partly, according to Kelaart, from the 

 escape of the eggs into the interstices of the mantle, on the 

 accidental rupture of an overcharged ovary. Dr. Kelaart and 

 Von Hessling also admit the intrusion from without of foreign 

 inorganic matters as one of the causes of pearl-formation ; and 

 the former states that the oysters, when removed into an aqua- 

 rium, retract their mantle considerably, and retain it in this 

 condition even for some days, — a condition which would be 

 very favoui*able to the access of extraneous matters to the in- 

 terstices between the mantle and the shell. It is quite pos- 

 sible that a similar condition may prevail amongst the Pearl 

 Oysters residing in the open sea, for Dr. Kelaart says, " Most 

 of the oysters in which I have found pearls had external marks 

 of having been retarded in their lateral growth, and displaced in 

 early life from their fixed position on a bank. I am inclined to 

 believe that oysters which have abundance of food, and are not 

 disturbed, remain fixed for the last two or three years of their 

 growth to one spot. These are less likely to have a large pro- 



