A. R. MOORE 165 



joining the two points. In this we have a tropistic reaction analogous 

 to that of heliotropic orientation to two sources of light.^ The same 

 principle may be illustrated in another way by gently pressing the 

 ray between two small bodies such as glass rods. When the exten- 

 sion reflex takes place it will be seen that only a few of the tube feet 

 bend laterally and they are in the immediate vicinity of the points 

 touched. All the other tube feet bend along the axis of the ray 

 toward the area of excitation, swaying, a little from side to side, but 

 neither markedly to the right nor to the left (Fig. 3). If the pres- 

 sure is increased the tube feet central to the point of stimulation 

 reverse their orientation and bend toward the center (Fig. 4). How- 

 ever, not every animal gave both phases of this reaction; i.e., distal 

 and central bending of the tube feet. In the main the results were 

 similar to those obtained by Maxwell with Mustelus in which he 

 found that weak mechanical stimulation caused bending toward the 

 point of contact while strong stimulation produced, the opposite re- 

 sult.^ It happens therefore that when acted upon by contact on 

 two opposite sides of the ray, i.e., by two equally balanced impulses, 

 the tube feet orient themselves along a line perpendicular to a line 

 joining the two loci stimulated. Here again is a case analogous to 

 that of heliotropic orientation to two sources of light, since the star- 

 j&sh ray like the heliotropic insect is bilaterally symmetrical with 

 reference to right and left. 



If the tube feet as a result of their extension in response to contact 

 touch a surface, they at once adhere by means of their sucking disks. 

 When a considerable number of tube feet have thus taken hold it is 

 difficult to pull the animal away from a surface. Even if one succeeds 

 in doing so some of the tube feet will be torn from the animal and 

 left sticking to the surface, so strong is the hold they have upon it. 

 It is, however, possible by means of the light reaction to cause 

 adhering starfish to release their hold. This reaction may be 

 demonstrated as follows: A starfish is placed ventral side up in 

 a dish of sea water in a dimly Hghted room. As soon as the tube 

 feet have been thrust out, a flash of sunlight is thrown across the 

 animal. As a result the tube feet withdraw and the ambulacral 



2 Loeb, J., Forced movements, Iropisms, and animal conduct, Philadelphia 

 and London, 1918, 75. Patten, B. M., J. Exp. Zool., 1914, xvii, 213. 



