A. R. MOORE 



323 



feet near the middle of the ray with the acid thread. Two results 

 appear; (a) the tip returns to its former position, and (b) the ray 

 bends dorsally immediately central to the point of the last application 

 of the acid (Fig. 4). Stimulation of the sensory cells of the tube feet, 

 therefore, results in dorsal flexure of the ray central to the point of 

 stimulation, and inhibition distal to this point. It was found as a 

 rule that strychnine abolished this phase of inhibition, so that in a 

 thoroughly strychninized ray stimulation of the tube feet at any 

 point resulted in dorsal flexure of the entire ray. 



Fig. 3. 



Fig. 4. 



Fig. 3. The specimen lies ventral side up on a glass plate. A thread 

 previously dipped in 0.05 n HCl has touched the extended ray (indicated by the 

 dotted outline) near the tip as at S, with the result that the distal end of the ray 

 bends sharply dorsalward. 



Fig. 4. With the ray in the position of Fig. 3, shown here by dotted outline, 

 the acid stimidus is applied to the tube feet near the center of the ray. As a result 

 the distal half is extended to the normal position but dorsal flexure occurs central 

 to the point of stimulation. 



CONCLUSIONS. 



' From what has been said concerning the neuromuscular organiza- 

 tion of the Asterias ray, it is possible to arrive at certain definite con- 

 clusions regarding the nature of the stereotropism of that organ. 

 The righting movements from the start are not haphazard; i.e., they 

 cannot be explained on any hj^othesis of "trial and error." The exci- 

 tation of the sensory cells of the dorsal sheath initiates dorsal flexure 

 of the ray. This movement makes it possible for the tube feet of the 

 tip to touch bottom and start the vigorous action which foUows and 

 completes the righting If the "feelers" by chance touch bottom they 

 initiate the reaction de novo. 



