A HARD-SHELL CLAM 105 
muscle, the anterior retractor of the foot. Note the broad line 
which joins the scars and runs parallel with the edge of the 
shell except near the posterior muscle scar, where it bends for- 
ward, forming a triangular indentation. This is the pallial line; 
it is formed by the insertion in the shell of the delicate muscle 
fibers near the edge of the mantle. The indentation is the 
pallial sinus. Note the hinge teeth just beneath the umbo. 
Exercise 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; note also that an inorganic 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! The two halves of the mantle will be seen to envelop 
entirely the visceral mass of the foot. Over the dorsal portion 
of the visceral mass the mantle is fused with it and cannot 
be separated, but the lateral and the ventral portions of the 
mantle lobes hang free, enclosing an extensive space, which is 
called the mantle cavity. In this cavity, on each side of the 
visceral mass, lie the two leaf-like gills. Observe the edges of 
the mantle. They are fused forward of the anterior adductor 
1 For the study of the soft parts of the clam it is well to have also at hand a 
specimen which has been deprived of both valves of the shell. 
