426 MB. R. GAENEU ON BRITISH PEARLS. 



On the Formation of British Pearls and their possible Improve- 

 ment. By Egbert G-arner, F.L.S. 



[Read December 7, 1871.] 



The formation of these beautiful ornaments has been more 

 written about than really investigated, — written about, indeed, 

 from Pliny downwards, and with a misajiprehension of their real 

 origin which is extraordinary, when it might have been easily ex- 

 plained by a little closer observation. Confining ourselves to 

 British pearls, some of which (from the Alasmodon of our moun- 

 tain-rivers) nearly equal the oriental ones in beauty, but including 

 the very inferior ones I'rom the Mytilus or edible mussel of our 

 shores (the oi'igin of which has been more especially studied), it is 

 not difficult to convince ourselves tliat they are due to the exist- 

 ence, in the shell-secreting mantle of the animal, of minute pa- 

 rasitical entozoa, fully developed distoraes, against which the 

 natural protection which the mollusks have is a calcifying process 

 around the parasites ; and as after this they still act as foreign 

 bodies, a continuation of the same process leads to the formation 

 of pearls. It is fair to conclude that oriental pearls are formed in 

 an analogous way ; and in many other mollusks similar distomes, 

 which are sometimes calcified, are seen. Tiiese distomes are so 

 small as to be only just visible to the naked eye ; and they always 

 abound in the mussel which contains pearls. They may be found 

 covered with a shelly layer so thinly as to appear quite inu-lianged 

 in appeai'ance, and to be known as incipient pearls only by pres- 

 sure revealing the shelly envelope ; they may sometimes be found 

 still living, but partly covered with calcareous deposit ; and, lastly, 

 by the use of dilute acid, the young pearls may be seen to have 

 each one or ni-tre di:^tomes, or their remains, as a nucleus. The 

 above was made out some years since indejKmdently ; but from a 

 hmchure since obtained, written by Signer Antonio Villa, it would 

 appear tluit others in Italy have suspected the same thing. Such 

 is the common origin of the pearls which are found free in the 

 mantle of the mollusks ; but pearl}'^ excrescences attached to tlie 

 inner surface of the shell may very likely arise from other irri- 

 tants. Thus in the common Anodon the presence of a minute 

 AcarittS (Atax), at least the half-developed animal, appears to pro- 

 duce little pearly elevations on the retral side of the inner sui"- 

 face. In Alasmodon there is oFUmi a cluster of small pearls in 

 the mantle just behind the antcn-ior adductor muscle; but avc can- 



