114 University of California Publications in Zoology. [Vol. 4 



Factors Determining the Nature and Variations op the 

 Reactions to Given External Stimuli. 



We have now seen the typical negative and positive reactions 

 to certain classes of stimuli. Bnt the reactions to a given stimu- 

 lus or class of stimuli are by no means invariable : on the contrary 

 they vary much, depending on many factors, internal and ex- 

 ternal. It is not possible to classify these different factors in any 

 precise way, since they interact in a complicated manner, so that 

 the effect of each varies in different cases. But we shall attempt 

 to deal with some of the more important of these factors, under 

 the following heads : 



1. Interaction and Interference of Stimuli. — As we have seen, 

 the starfish when illuminated from one side usually moves away 

 from the source of light. But if under these conditions the tips 

 of the tube feet of a ray come in contact with some object, as a 

 pebble or the side of the vessel in which the starfish is situated, 

 the animal at once begins to move toward this object. It may 

 then move at any angle with the light, or even directly toward 

 the source of light. The starfish may usually be induced to move 

 directly toward the sun by placing it in such a position that the 

 tip of a ray comes in contact with the side of the glass vessel (or 

 with any other object) on the side directed toward the sun. 



This reaction is closely related to that described previously, 

 in which the starfish when subjected to strong light moves to- 

 ward an object near by, even though it may not be in contact 

 with that object nor shaded by it (pp. 110-113). By thus moving 

 toward any object, the starfish is of course likely to find pro- 

 tection from the light more readily than by simply fleeing 

 straight away from the sun, so that the reaction has an evident 

 regulatory value. 



When the animal reaches the object toward which it has 

 moved, it usually explores its surface, or if it is large, follows 

 along it, until it comes to a region where it is protected from the 

 light. If it thus comes in contact with the sides of a glass vessel, 

 it may thus follow the wall completely around the vessel, moving 

 part of the time away from the light, part of the time obliquely, 

 part of the time transversely, and part of the time directly to- 



