402 



THE MUSSEL 



and a left inner gill (Fig. 103, ext. gl, inf. gl.}. Seen from 

 the surface (Fig. 101), each gill presents a delicate double 

 striation, being marked by faint lines running parallel with, 



> 13 



2 . 



and by more pronounced lines running at right angles to, 

 the long axis of the organ. Moreover, each gill is double, 

 being formed of two* similar plates, the inner and outer 



