172 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Figure 4. — Reproductions of actual records of shell gape and pumping rates, demonstrating the basic components of 

 the shell movements of oysters. In each, A indicates the full amplitude of gape, B the opening movement of phase I, 

 C the closing movement, which is always done in "steps." Lower figure shows two additional types of closures: 

 X an abnormal one resulting from mechanical disturbance, Y a typical snap closure, probably to eliminate unwanted 

 substances. Paper speed 2 inches an hour; vertical lines represent quarter-hour intervals. 



