322 STEREOTROPISM 



with the bottom surface. Or if a specimen of Asterias is suspended in 

 the water by a thread tied around the central disk, so that the rays 

 cannot touch a surface, dorsal bending of the rays does not occur at 

 all. The animal hangs inert for an indefinite length of time with the 

 rays in a position of partial ventral flexure (Fig. 1). Now hold a glass 

 rod in contact with the back of the ray. The latter bends slowly a 

 little distance dorsalward (Fig. 2) . This proves that the sensory cells 

 of the dorsal integument, when stimulated, cause contraction of the 

 dorsoflexors of the sheath. Ordinarily this reflex plays a part in 

 initiating the righting by pressing the entire ray against the bottom. 

 This enables the sensitive "feelers" of the tip of the ray to touch 

 bottom immediately, and from the numerous sensory cells of the am- 



FiG. 1. Fig. 2. 



Fig. 1. Position taken by an individual Asterias during suspension in an 

 aquarium. The animal is not in contact with a surface, consequently the rays 

 are inert. 



Fig. 2. A glass rod has been brought into contact with the dorsal side of the 

 ray at the point indicated by 5. As a result the ray has responded by bending 

 dorsally. 



bulacral disks impulses go to the musculature facilitating flexure. As 

 the tube feet secure hold, that part of the ray straightens and actual 

 bending occurs only centralward to the point where the last tube feet 

 have touched bottom. 



That the muscular responses of the later phases of righting are due 

 to stimulation of the sensory cells of the tube feet may be shown in 

 the following way. Place a medium sized Asterias ventral side up on 

 a glass plate. Stimulate the sensory cells of the tube feet by touching 

 these organs at the tip of the ray with a loop of thread dipped in 0.05 

 nHCI. At once the tip bends dorsally (Fig. 3). Now touch the tube 



