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CHAPTER VI. 

 THE FBESHWATEK MUSSEL. Anodonta cygnea. 



THE body of the mussel is enclosed in a shell, the two halves 

 or valves of which cover the right and left sides of the body 

 respectively. These valves, which are hinged together along 

 the median dorsal line, can be opened to a slight extent by 

 means of an elastic ligament along the hinge-line, and closed 

 by powerful adductor muscles. When the valves are closed 

 they completely conceal all parts of the body. Mussels vary a 

 good deal in size according to their age ; a fair-sized specimen 

 is about four and a half inches long, one and a half thick, and 

 two and a half from the dorsal to the ventral edge. 



The mussel lives in freshwater streams, with its anterior 

 end buried somewhat obliquely in the mud of the bottom, 

 and the posterior end projecting up into the water. By 

 means of a muscular foot it can plough its way slowly 

 along the bed of the stream. Its food consists of minute 

 organisms, both animal and vegetable, which are swept in at 

 the hinder end of the shell by ciliary currents and carried 

 forwards to the mouth, the stream of water so kept up serving 

 for respiration as well as nutrition. 



The sexes are distinct. The young embryos are retained 

 within the mother during their early stages of development ; 

 they then hatch and attach themselves to fish. While 

 attached they increase considerably in size, and undergo a 

 distinct metamorphosis. Finally they detach themselves, and 

 adopt the mode of life of the adult. 



