Anatomy of the Food Mollusks 15 



spread over the entire outer surface (Figure 64). Each 

 represents what was, at one time, the edge of the shell. 

 A cross section of a tree trunk reveals similar concentric 

 growth lines. Each line in the section marks the cessa- 

 tion of growth in the fall and its resumption in the spring, 

 so that the years of a tree's life are recorded in them. 

 From such an analogy it might be assumed that the 

 growth lines on a bivalve shell indicate its age, also ; but 

 the analogy is misleading. Most shells, like that of 

 Venus, possess fine and closely crowded, as well as con- 

 spicuous lines, all irregularly arranged. While they rep- 

 resent successive deposits of lime, many are formed in a 

 summer, and no idea of age may be had from them. Dif- 

 ferences in their distinctness and size are probably due to 

 the irregular action of weather, tide, temperature, and the 

 abundance or scarcity of food. 



The mantle edge secretes a thin, dark colored, rubbery 

 coat that is applied to the outer surface of the shell. This 

 probably is produced to prevent the dissolution of lime, 

 especially in the young. In some bivalves, this cuticle, 

 as it is called, is so thick and tough that it performs its 

 function perfectly throughout life; but in most cases it 

 wears away, especially on the older part of the shell, and 

 the lime is slowly dissolved. The addition of new shell 

 to the inner surface may keep pace with this, but in the 

 case of clams living in foul mud, in which humus acids 

 are abundant, the shell is sometimes perforated and the 

 animal dies. Very little of the cuticle is to be found on 

 the shells of adult oysters, clams, or scallops. 



Closely connected with the addition of new layers to the 

 inner surface of the shell, is the formation of pearls. 

 These precious structures are merely shell formations un- 

 attached to the valve. Their shape has much to do with 



