^^^^J Jennings.— Behavior of the Starfish. 75 



usually causes the jaws to snap together, while a touch in the 

 throat below the ridge has no effect, even though the touch is 

 roughly given. Often, also, a touch on the inner surface of the 

 jaws at the very edge, on the teeth, does not cause closure. 



The effects of stimuli on the inner surface are not invariable, 

 but depend, like all of the reactions of the pedicellariae, on 

 various conditions. Thus, if we cause the pedicellaria to open 

 by a stroke on the outside of its jaws, then, while it is opening, 

 we stimulate the inner surface in the way that usually causes 

 closure, we find that this has now no effect ; the jaws continue to 

 open. When once a certain action is set in progress, this pro- 

 ceeds with something like inertia, so that it is not easily stopped. 

 We shall meet with many exaiuples of this in the behavior of 

 the starfish. The phenomena just described are evidently analog- 

 ous to the refractory period seen in muscles of higher animals. 



In the small pedicellariae, as in the large ones, the usual 

 stimulus to closure is the contact of some object with the internal 

 surface of the jaws, resulting in the seizure of that object. 



But in both the large and the small pedicellariae, closure 

 often results from internal changes. When a general stimulus is 

 given to the starfish as a whole, often the rosettes rise, the pedi- 

 cellariae reach out and snap frequently, opening and closing 

 several times without change in the external conditions. The 

 large solitary pedicellariae after bending over toward a stimu- 

 lated spot often snap repeatedly. The performance of one action 

 results in so changing the internal conditions as to cause the per- 

 formance of another. But in the pedicellariae of the rosettes 

 this occurs only in certain general physiological states of "ex- 

 citement," due to a preceding marked general stimulus. 



In such cases, as soon as the pedicellaria snaps upon any 

 object, so as to hold it, the jaws do not open again, but hold for 

 a time. If while the jaws are holding, this pedicellaria or the 

 neighboring region is stimulated in the w^ay that usually causes 

 opening, the pedicellaria with an object in its jaws usually neither 

 extends nor opens, though all of its fellows rise fiercely to the 

 attack and open widely. If this region is abused roughly, the 

 jaws only close the more tightly. But there are some curious 

 variations in this. Thus, in one case a pedicellaria had held a 



