BEHAVIOR IN LOWER METAZOA 



245 



back; it thus forms a spiral of two turns. Finally it behaves in a 

 manner somewhat similar to that shown when it dries. It rolls the 

 two ends under the body, arching the dorsal surface. In this position 

 the animal rolls over on its back and dies. 



Thus under a single unlocalized stimulus of gradually increasing in- 

 tensity, the behavior of the organism passes through a series of stages, 

 closely resembling the reactions given under most diverse conditions. 

 As Mast (1903), to whom these observations are due, expresses it, "the 

 general impression is given that as the thermal stimulus increases, the 

 animal tries, in a sort of 'hit-or-miss' way, every reaction which it has 

 at command in order to get rid of the stimulation." 



The "righting reaction" of the flat worm is another example of a 

 response that is not stereotyped in character, but varies greatly. If the 

 animal is turned on its back, it quickly rights itself again. This usually 

 occurs as follows. The animal twists itself into a spiral (Fig. 138, .4), 



FIG. 138. Righting reactions in the flatworm. After Pearl. A, reaction of entire worm. 

 B, righting reaction of short piece from anterior end of worm, a, b, c, d, e, f, successive steps in 

 the process. C, righting reaction of triangular pieces, a, manner in which the piece is cut. 

 b, a small portion of the thin edge turns so as to bring the ventral surface in contact with the 

 bottom, c, d, this turning increases; by a continuation of the process the whole piece is finally 

 righted, e, /, cross sections through the pieces while turning. 



thus causing the ventral surface of the head to face the bottom, where 

 it attaches itself. Then the worm creeps forward, bringing successively 

 more and more of its ventral surface in contact with the bottom, pro- 

 .ceeding toward the rear. Thus the spiral is unwound, so that after the 

 animal has traversed a short distance, the entire ventral surface is in 

 contact with the bottom, as usual. 



But the righting reaction may take place in quite a different way. 

 Pearl (1903) cut the planarian into pieces of such form that it could no 



