68 University of California Publications in Zoology. [Vol. 4 



cellariae are so constituted as to attack, under natural conditions, 

 only living creatures; inanimate objects, keeping quiet, are left 

 alone. 



The attack of the pedicellariae may be brought on by stimuli 

 on any part of the dorsal surface of the starfish. The part 

 touched may be the bare tip of the spine, its fleshy sides, the 

 rosettes themselves, the large or small pedicellariae, or the gills; 

 the result is to cause the neighboring rosettes to attack. 



Varied Beactions. — The attack of the rosettes is not a me're 

 stereotyped reaction, bound always to occur completely and in 

 the same way in any given rosette. On the contrary, parts of a 

 rosette may react separately, or in different ways, and there is 

 an active seeking out of the stimulating object. This is shown in 

 a striking way in the following experiment. If after a rosette 

 has risen to the top of the spine, and all the pedicellariae are 

 reaching upward with gaping jaws, we stimulate with a needle 

 one side of the base of the spine, below the rosette, then we find 

 that this side of the rosette curves over and bends down, direct- 

 ing the pedicellariae downward, so that they are likely to seize 

 the offending object. This does not resemble at all a mere re- 

 traction, an undoing of the original extension, but is an active 

 additional reaction. In a similar way different parts of the 

 rosette often reach out in various directions, depending on their 

 relation to the region stimulated ; or individual pedicellariae 

 bend over to that point on the rosette where a localized stimulus 

 is given. We have already seen, in our general account, that the 

 spines bend with the entire rosette toward a spot on the body that 

 is disturbed ; they may thus bend in any direction. The large 

 solitarj^ pedicellariae likewise bend toward a stimulated spot, 

 though for these the stimulus must be near. The large pedi- 

 cellariae usually do not bend toward a stimulated spot that is 

 distant more than their own length. 



When there is a general stimulation of the starfish as a whole, 

 the spines and pedicellariae often wave about, the jaws snapping 

 repeatedly, in this way much increasing the chances of finding 

 and seizing the cause of the disturbance. Thus a separate ex- 

 ternal stimulus is not necessary for each opening or closing of a 

 pedicellaria ; there are changing internal states which have the 

 same effect. 



