1^07] Jennings. — Behavior of the Starfish. 91 



the starfish. Such fish are very abundant about the shore, and 

 many of thctn were kept in the tanks with the starfish. Such 

 fish fre(;u(nt!y came against the starfish, resting upon it or hid- 

 ing behind it as if it were a stone. But their movements were 

 so g'entle and deliberate that the pedicellariae were not roused, 

 and such fish were not seized. 



Nature of iStimnIi Causing the Food Reaction. — Why is it that 

 the starfish sometimes feeds upon the crabs which its pedicellariae 

 have captured, while in other cases it does not, the crabs being 

 merely held for hours? Without question there are ditferences 

 in the physiological condition of ditferent starfish, possibly cor- 

 responding to hunger and satiety, so that the individuals vary in 

 their behavior toward food. This matter was not studied thor- 

 oughly. But variations in behavior also depend partly of course 

 on difference in the stimulus coming from possible food bodies. 

 A crab certainly acts as a mechanical stimulus, and it may act 

 also as a chemical stimulus. What are the relative effects of these 

 two classes of stimuli in causing the starfish to feed upon a given 

 object ? 



The taking of food usually begins either with the bending of 

 the ray downward, so as to carry the food to the mouth, or with 

 the thrusting up of the tube feet from the under side, to seize 

 the food and carry it to the mouth. If neither of these things 

 occurs, the body merely remains where it is held by the pedi- 

 cellariae. 



The effects of chemical stimulation in causing' food to be 

 taken when mechanical stimuli do not have this effect, are readily 

 seen in the following way : A starfish holds several living crabs 

 on its back, by means of the pedicellariae, but does not attempt 

 to feed on them. Now one of the crabs is crushed with forceps, 

 so that its juices diffuse into the water. At once the tube feet 

 appear from beneath, reach upward, seize the body of the crab, 

 pull it downward to the mouth, and feed upon it. In one case 

 a starfish had held on its back several crabs for more than an 

 hour. One of these was then crushed, and inside of a minute 

 it had been carried to the ventral side by the tube feet, and was 

 moving toward the mouth. 



Chemical stimuli alone mav be shown to initiate the food 



