LESSON XXVIII 



THE FRESH-WATER MUSSEL 



In the mussel we meet with an entirely new type of 

 structure : the animal is bilaterally symmetrical, with no 

 trace of metameric segmentation ; the power of locomotion 

 is greatly restricted, and food is obtained passively by ciliary 

 action, as in Infusoria, not by the active movements of 

 definite seizing organs — tentacles, limbs, or protrusible 

 mouth — as in most of the higher animal forms. 



Fresh-water mussels are found in rivers and lakes in most 

 parts of the world. Anodonta cygnea, the swan-mussel, is 

 the commonest species in England ; but the pearl-mussel, 

 Unio margaritifer, is found in mountain streams, and other 

 species of the same genus are universally distributed. 



The mussel is enclosed in a brown shell formed of two 

 separate halves or valves hinged together along one edge. 

 It lies on the bottom, partly buried in the mud or sand, 

 with the valves slightly gaping, and in the narrow cleft thus 

 formed a delicate, semi-transparent substance is seen, the 

 edge of the mantle or pallium. The mantle really consists 

 of separate halves or lobes corresponding with the valves of 



