324 STEREOTROPISM 



As to the remaining phases, the stereotropic reaction of the ray is 

 referable to the high degree of surface sensitivity of the tube feet 

 disks and the propagation of excitatory and inhibitory impulses, re- 

 sulting from stimulation, along appropriate paths to the muscles of 

 the ray. In this respect the tube feet may be regarded as playing a 

 role similar to that of tropistic receptors in general. That is, un- 

 equal stimulation of the receptors causes a corresponding inequality 

 of tone or of contraction in the musculature which ultimately results 

 in an equihbrium of orientation to the factor involved. 



It is evident that as a mechanism for righting stereotropism differs 

 in one important respect from other tropisms. The latter depend 

 for their operation upon unilateral effects in organisms which are 

 dynamically bilaterally symmetrical. In stereotropism the sensitivity 

 of the organism is not distributed in bilaterally symmetrical fashion. 

 As a rule only the ventral side is stereosensitive; i.e., the sensitivity 

 is unilaterally distributed. There is a tropistic action in such cases 

 because of the fact that when the sensitive surface is only partially 

 stimulated an unequal contraction of the musculature follows and this 

 as a result brings the sensitive surface into such a position that it is 

 all equally stimulated. Equal muscle tone follows and the organism 

 is in equilibrium with its environment.'' 



•Jennings (Jennings, H. S., Univ. California Pub., Zoology, 1907-08, iv, 156) 

 believes that he has discovered in Asterias a capacity for learning in connection 

 with this reaction; that is, a ray ordinarily passive during righting could be in- 

 duced by repeated suppression of the other more active rays to undertake righting 

 movements. In describing his experiments Jennings states that the active 

 rays were prevented from taking part in the righting by "stimulating their 

 tube feet with a glass rod." In 1910 (Moore, A. R., Biol. Bull. 1910, xix, 

 235), I called attention to the fact that Jennings' result did not prove the use of 

 memory or hysteresis of any kind in this functioning of a previously passive ray, 

 because strong stimulation or a slight injury of any sort to the other rays renders 

 them inactive and forces the use of the ray in question. By "stimulating with a 

 glass rod," Jennings simply diminished the reacting capacity of the active rays, 

 or, in other words, raised their threshold either to a point equal to or above that 

 of the ray he was seeking to "teach." The experiment may be much simplified 

 by making a single stroke with a glass rod in the ambulacral groove of each of 

 the four active rays. Animals treated in this way have been kept under observa- 

 tion and found to resume the use of the treated rays only after 1 or 2 weeks. In 

 the interval righting is always performed by the previously inactive ray which, 

 however, had not been stroked along the groove with a glass rod. 



