283 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



swaying about upon their contractile stalks as if 

 in search for something to catch, will snap at an 

 object only if it touches the inner surface of one 

 or more of the expanded mandibles. Moreover, in 

 the larger pediccllariiie, a certain part of the inner 

 surface of the mandibles is much more sensitive to 

 contact than is the rest of that surface; this part is 

 a little pad about one-third of the way down the 

 mandible : a delicate touch w^ith a hair upon this 

 part of any of the three mandibles is certain to 

 determine an immediate closure of all the three. It 

 is obvious that there is an advantage in the sen- 

 sitive area, or zone, being placed thus low enough 

 down in the length of the mandibles to ensui-e that 

 the whole apparatus will not close upon an object 

 till the latter is far enough within the grasp of the 

 mechanism to give this mechanism the best possible 

 hold. If, for instance, the tips of the mandibles 

 were the most sensitive parts, or even if their whole 

 inner surfaces were uniformly sensitive, the ap- 

 paratus would be constantly closing upon objects 

 when these merely brushed past their tips, and 

 therefore closing prematurely for the purpose of 

 grasping. But, as it is, the appaiatus is admirably 

 adapted to waiting for the best possible chance of 

 getting a secure hold, and then snapping upon the 

 object witli all the quickness and tenacity of a 

 spring- trap. 



Another point w^orth mentioning is that if, after 

 closure, any one or more of the mandibles be gently 

 stroked on its outer surface near the base, all the 

 mandibles are by this stimulation usually, though 



