CHAPTER VII. 

 THE POND MUSSEL (ANODONTA CYGNJEA). 



The common Fresh-water Mussels really belong to two genera ; 

 but those referable to Anodonta are most usually made use of 

 for purposes of dissection, and most current descriptions refer 

 to that genus. It contains two species that are found in this 

 country, A. cygruea and A. anatiiia. They both occur in 

 lakes, ponds, and canals, where the water is sluggish. The 

 animal lies buried in the mud at the bottom of such a piece 

 of water, with the narrower end imbedded in the mud and the 

 broader end freely emerging. A current of water is kept up 

 by the action of cilia through the posterior end of the shell, so 

 that minute organisms are brought towards the mouth at the 

 opposite extremity. At the same time the current of water 

 serves for the aeration of the gills. The anodon is of two 

 sexes, and the males cannot be distinguished externally from 

 the females, as has been often asserted to be possible. So 

 rare, too, are the males, that the females may possibly also 

 develop male products ; but there is no positive evidence 

 upon the point. Out of fifty anodons dredged, Mr. Latter 

 found only two males. The life-history of the anodon is 

 interesting. The eggs are shed into the external, not into the 

 internal, gill ; the walls of the gill secrete a mucous matter, in 

 ■which the developing eggs are held. The young are hatched 

 from the eggs in a stage which has been termed Glochidium, 

 on account of the fact that these young organisms were regarded 

 originally as independent organisms. The glochidium has the 

 valves of its shell armed each with a long curved spine, 

 and the foot secretes a bunch of filaments, the byssus. This 



