Anatomy of the Food Mollusks 17 



animal is wedged open, and thin images, punched out of a 

 sheet of bell-metal, are inserted. The animal is then re- 

 turned to the water, and is left there until enough new 

 shell has been formed to cover them with a varnish of 

 pearl thick enough to cover them, and to hide the metal, 

 while permitting the raised outline to be seen." 



Pearl growth is really very common. Fresh water 

 clams, of which there are scores of varieties, exhibit it 

 with great frequency. But usually it will be found that 

 the pearl has become fastened to the shell. Even when 

 these growths are large, they cannot be removed and 

 ground into a symmetrical form, because abrasion of the 

 surface destroys their luster. It is only the large, sym- 

 metrical growths, which have not been glued to the shell 

 during their formation, that have great commercial value, 

 and these are relatively very rare. 



There is often considerable variation in the shapes of 

 shells, especially in oysters and long-neck clams. This is 

 often due, in the case of the former, to the close crowding 

 of individuals, and in the latter, to pressure against ob- 

 jects in the walls of the burrow. When young oysters 

 crowd each other closely, after their attachment, the di- 

 rect effect is that the shells grow narrow and become 

 greatly elongated. If a clam in its burrow presses against 

 an unyielding obstruction, the growing shell will be dis- 

 torted by conforming to the outline of the object. 



Right and left valves are normally symmetrical in most 

 bivalves, but in the oyster there is a great inequality. The 

 animal is attached by the left valve, which is very much 

 larger than the right. It forms the stony box in which 

 the soft parts of the body lie, and the right valve is little 

 more than a lid to the box. In the embryo, the valves are 

 of the same size, and are perfectly symmetrical. This 



