The Life History of the Soft Clam 297 



meters in length removed from one of its earliest bur- 

 rows. The outline of the shell is now quite as different 

 from that of an earlier period as from that of the adult 

 condition. An idea of the actual size of the clam may be 

 had by comparing it with the bodies s g, which represent 

 sand grains of average size. Byssus threads continue to 

 be secreted until the clam is at least half an inch long. 

 It is probably at about this time that a final descent is 

 made. The byssus gland then atrophies and entirely dis- 

 appears, and the animal never comes to the surface again 

 through its own effort. 



