288 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 pedicellariz, a certain part of the imner 
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 with 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 ensure 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 evenif 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 apparatus is admirably 
adapted to waiting for the best possible chance of 
getting a secure hold, and then snapping upon the 
object with all the quickness and tenacity of a 
spring-trap. 
Another point worth 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. 
