HOW DO STARFISHES OPEN OYSTERS? 



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The points, recognisable by the injury just described, at which 

 Asterias opens the oysters, show a certain degree of definiteness in 

 position. They do not, however, as I at first suspected, exhibit a 

 perfectly regular relation to a line drawn through the hinge and the 

 muscle scar. In general, indeed, they lie on this line, and this can be 

 readily understood, since the two shells of the oyster represent, to 

 some extent, two levers, with a fulcrum at the hinge. The further the 

 point of seizure lies from the hinge, that is to say, the longer the arm 

 of the lever, the more effective will be the force applied. Precisely on 

 the longest shells, nevertheless (Fig. 7, Nos. 3, 6, and 10), we find the 

 point of seizure lies, not on the line mentioned, but displaced quite 



Fig. 7. 



to one- side. I can only explain this variation on the supposition that 

 these oysters were of too great length in the direction of the line 

 through the hinge and muscle for the arms of the starfish, and the 

 latter had to find positions in which their arms could reach further 

 over the shell. The point of seizure in these shells always lies on the 

 side which exhibits the less vigorous growth. This seems to be a 

 general rule in all cases in which the oysters show unequal growth 

 (compare also Nos. 7 and 11 in Fig. 7), and possibly depends on the 

 fact that on the side where there is less growth the shells are naturally 

 less laminated, and the starfish, therefore, has more chance of coming to 

 firm portions of the shell, upon which it can effectually fix its tube- 

 feet. In oysters from other localities, whose shells arc firm at the 



