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 call 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 thé 
principal organs beneath: the stomach, with a 
Common Fresh-water Mussel, Unio, cut transversely: a, foot ; bb, gills; 
c, mantle ; d, shell; e, heart ; f, 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 
