244 COMMON FRESHWATER SHELLS. 



Let us now turn our attention to two species of our freshwater 

 bivalve shells. Perhaps the best known of this class is that 

 extremely common, very prolific, and remarkably variable shell, 

 the common River Mussel, known as Anodonta cygnma. There 

 can be no wonder that this species is common when it is con- 

 sidered that a single individual has been estimated to produce 

 three hundred thousand young in one season. And it is no less a 

 wonder that it is so variable when, owing to its large numbers, it 

 lives under so many different conditions. 



It is found in rivers, canals, lakes, ponds, etc. This species is 

 easily kept in an aquarium, and when kept thus in captivity can 

 easily be made to demonstrate the interesting manner in which 

 these creatures breathe. If a very small quantity of carmine or 

 other suitable colouring matter be placed gently near the anodon> 

 the existence of two currents will be at once perceptible, one 

 current going to and the other from the animal. This is because* 

 in order to supply the animal with the necessary amount of oxygen, 

 the gills are kept continually bathed with water, which, entering by 

 one tube or " siphon," leaves the animal by another alongside the 

 first, the oxygen having been duly extracted from the water. Some 

 of these shells attain really large dimensions, measuring as much 

 as nine inches. 



From a utiUtarian point of view, the anodon is not of much 

 value, although I have heard it said that fresh-water mussels are 

 eaten by some people. This, however, may not refer to the 

 species in question. The practice is certainly not a fanciable one, 

 and, one would think, not very conducive to health. 



Pearls are occasionally found in these Swan mussels, and are 

 generally either a green or rose colour. The once-famous Pearl 

 mussel of this nation, however, belongs to another genus, all the 

 British members of which produce pearls. This is Unto marga- 

 ritifer. It is found among the gravel and small stones of quick- 

 running rivers or mountain torrents. It has also been known to 

 occur in several parts of a canal, where, however, the bottom is 

 gravelly. In this case the animal has been carried in by the 

 water-courses which supply the canal. This mollusc is, or was, 

 used in the Aberdeen fishery as bait. 



The Scottish Pearl Fishery continued till the end of the last 



