CH. VI] THE FRESH-WATER MUSSEL 197 



preserving the species in many shallow ponds which are 

 frequently frozen to the bottom in severe weather. Ex- 

 posure to the air is tolerated for several hours provided 

 the animals are kept cool and not exposed to a hot sun. 

 Warming the water to a moderate degree will often cause 

 protrusion of the foot and promote general activity; 

 excessive warmth however has the reverse effect and 

 is speedily fatal. 



Enemies, Parasites, etc. Anodonta is a favourite 

 article of food among coot, several species of wild duck 

 and the heron ; the latter at times carries them away to 

 the heronry and there breaks open the shell against the 

 branches. Occasionally water-voles appear to eat them, 

 for empty shells are sometimes found in quantities near 

 their burrows. Otters too are recorded as preying upon 

 them, nor is the " bargee " above making a meal at their 

 expense. 



The corrosion which is so frequently to be noticed 

 upon the outer surfaces of the shells, especially in the 

 region of the umbones, is the result of the attacks of a 

 unicellular alga Micrococcus conchivorus with which are 

 often associated species of Batrachospermum and others. 



It is hardly possible to dissect an Anodonta without 

 finding on the surface of the mantle or foot or gills one 

 or more specimens of a small black and white acarid, 

 Atax bonzi 1 . This and another species, A. crassipes, that 

 is sometimes also found associated with the mussel, is a 

 commensal rather than a true parasite. A. bonzi passes 

 its entire life in connexion with Unionidce, leaving one 



1 Clapar&le, Zeit. wiss. Zool. xvm. 1868. 



