74 University of Calif ODiia Publications i)i Zoology. [Vol. 4 



close, and the closing' to open. This occurs usually only under the 

 influence of some general stimulus which has roused the pedi- 

 cellaria, — bringing- on a condition of unstable equilibrium. 



If the large pedicellariae (or the region about them) are re- 

 peatedly and strongly struck and abused, they close up tightly 

 and remain closed. Now the surrounding rosettes of small pedi- 

 cellariae rise and open their jaws. Thus the large pedicellariae 

 open in response to a w^eaker stimulus than do the small ones, 

 and withdraw when the stimulation becomes intense enough to 

 engage the attention of the latter. Von Uexkiill found analog- 

 ous differences among the different classes of pedicellariae in 

 the sea urchin. A chemical stimulus causes the "ophicephalous" 

 pedicellariae and the spines in the sea urchin to shrink away, 

 while the strong poisonous pedicellariae advance to the attack. 



Closing. — Most stimuli ^^■hich cause the rosettes to withdraw 

 and contract likewise cause the pedicellariae to close. But, as 

 we have already seen, this is not invariably the case, — picric 

 acid sometimes causing the rosettes to contract strongly, while 

 the pedicellariae open. This varies much in different cases. 

 Picric acid always causes complete contraction of the rosettes, 

 but this is sometimes accompanied by closure of all the pedi- 

 cellariae, sometimes by closure of part while part remains open, 

 sometimes by the opening of all the pedicellariae of the rosette. 



If the pedicellariae have been caused to close by a chemical 

 stimulus, they later refuse for some time to open even under the 

 usual stimuli that cause opening. If one touches the retracted 

 rosettes repeatedly, in the waj^ that commonly causes them to 

 extend, they remain obstinately contracted, while the pedicellariae 

 remain closed. 



The closing of the pedicellariae when objects are seized is 

 usually brought about by a touch on the inner surface of the 

 jaws. The latter then snap quickly, seizing the object which 

 touched them. This action is best studied in the large pedi- 

 cellariae, where the jaws are so extensive that their inner surface 

 can be surveyed and the effect of stimuli in different regions 

 determined. The inner surface is divided by a sharp transverse 

 ridge into two parts; below this ridge the surface is hollowed. 

 Touching: the inner surface of the pedicellariae above the ridge 



