^^*^^J J ciDiiugs.— Behavior of the Starfish. 69 



Effects of Different Stimuli.— ^ot all effective stimuli cause 

 the same response in the rosettes. Mechanical stimuli of suffi- 

 cient intensity, if repeated, always cause the rosettes to attack. 

 Cutting or crushing a part of the body causes the rosettes of the 

 entire surrounding region to attack tiercely. If for example 

 we thrust a needle through the body wall, the surrounding 

 rosettes rise and bend over toward the injured spot. Very light 

 mechanical stimuli, on the other hand, often produce no eft'ect. 

 even if repeated. Thus, certain small soft nudibranch mollusks 

 (species of Chromodoris) were seen at times to creep over the 

 surface of the starfish without causing the pedicellariae to rise. 

 But the larger specimens, causing slightly greater pressure, wer<) 

 usually attacked. A strong current of water injected against a 

 small area often causes the pedicellariae to rise; weaker currents 

 do not. 



The effects of chemical stimuli differ with different sub- 

 stances. Fresh water brought into contact with the body sur- 

 face produces only a languid response; the rosettes may slowly 

 rise part way, and a few of the pedicellariae open. Often there 

 is no response to this. Acids cause the rosettes to contract 

 strongly, shrinking away and hiding the pedicellariae. In the 

 case of picric acid the pedicellariae opened, though the rosettes 

 shrank away as far as possible. Crystalized salt from sea-water 

 caused strong contraction and closure. Alkalies (KOH, XaOH) 

 when weak cause the rosettes to extend and the pedicellariae to 

 open ; when strong they cause contraction and closing. Potassi- 

 um bichromate and various other chemicals have the same effect. 

 With potassium bichromate used in a rather weak solution, there 

 is for a time a maximal extension of the rosettes in all the affected 

 region. No other form of stimulation produced so strongly 

 marked and unvarying an effect. About two minutes after the 

 action of the chemical had ceased, the rosettes began to retract. 

 After this chemical stimulation there is a much readier response 

 to mechanical stimuli : a single touch with a needle or bristle 

 now brings up the sui'rounding rosettes in haste. Methyline blue 

 injected into the cavity of a ray through a small opening caused 

 the rosettes to rise over the entire surface of the region aft'ected. 

 though injected sea water did not have this effect. 



