A FRESHWATER MUSSEL 91 
just above each is the scar of a much smaller muscle, the 
retractor of the foot, and just posterior to the anterior adductor 
muscle is the scar of the protractor muscle of the foot. Note 
the broad line which joins the scars, running parallel with the 
edge of the shell. This is the pallial line; it is formed by the 
insertion in the shell of the delicate muscle fibers at the edge 
of the mantle. Do you find hinge teeth in the shell just beneath 
the hinge ligament? Unio has such teeth; Anodonta is without 
them and is also characterized by the thinness of its shell. 
Bxercise 3. Draw a view of the inner surface of the shell. 
Break the shell and examine the broken edge with'a hand 
lens. Study the structure of the shell. It is composed of 
three layers——the inner mother-of-pearl layer, which is secreted 
by the entire surface of the mantle, the prismatic layer, and 
the organic layer or periostracum on the outside. The two latter 
layers are secreted by the edge of the mantle; the periostracum 
is very thin and gives the color to the shell. Place a piece of 
the shell in a solution of hydrochloric acid; note the efferves- 
cence which results; also that an organic remnant, even of the 
two inner layers, is left. 
Exercise 4. Draw a view of the broken edge of the shell on a 
scale of 5. Show the prisms of the prismatic layer. 
Place the animal in water and study it as it lies in the right 
shell.1 The two halves of the mantle will be seen to envelop 
entirely the visceral mass and the foot. Over the entire dorsal 
portion of the visceral mass the mantle is fused with it and 
cannot be separated, but the lateral and ventral portions of the 
mantle lobes hang free, enclosing an extensive space, which is 
called the mantle cavity. In it, on each side of the visceral mass, 
1¥For the study of the soft parts of the mussel it is well to have at hand 
also a specimen which has been deprived of both valves of the shell. 
