A HAED-SHELL CLAM 95 



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. 1 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. 



