CLASSIFICATION AND CREATION. 55 



surface of which arises a double row of gills, 

 forming two pendent folds on the sides of the 

 body. At one end of the body these folds do 

 not meet, but leave an open space, where is the 

 aperture we ^all the mouth. This is the only 

 indication of an anterior extremity ; but it is 

 enough to establish a difference between the 

 front and hind ends of the body, and to serve as 

 a guide in distinguishing the right and left sides. 

 If now we lift the mantle and gills, we find the 

 principal organs beneath : the stomach, with a 



Common Fresh-water Mussel, Unio, cut transversely : o, foot ; b &, gills ; 

 c, mantle ; d, shell ; e, heart ; /, main cavity, with intestines. 



winding alimentary canal ; the heart and liver ; 

 the bloodvessels, branching from either side of 

 the heart to join the gills ; and a fleshy muscle 

 passing from one valve of the shell to the other, 

 enabling the animal by its dilation or contraction 

 to open and close its shell at will. A cut across 



