246 BEHAVIOR OF THE LOWER ORGANISMS 



longer twist itself into a spiral. Then some portion of the ventral sur- 

 face was brought by other means into contact with the bottom, and 

 from this point the remainder of the surface was pulled into contact. In 

 small strips from the head region, the posterior ends are turned under, 

 bringing the ventral side at this point against the bottom, then by pull- 

 ing from this point, the entire piece was turned over endwise (Fig. 138, 

 B). In tria,ngular pieces from the middle of the animal, one edge was 

 turned under, then the remainder righted from this region, by pulling 

 the rest of the piece over (Fig. 138, C). These modifications bring out 

 the essentially adaptive character of the behavior. The essential point 

 seems to be, to get some portion of the ventral surface, by any means 

 whatever, into contact with the substratum, then by working out from 

 this point, to bring the whole ventral surface into attachment. 



From certain points of view the whole behavior of the flatworm may 

 be considered a process of testing all sorts of conditions, retaining some 

 and rejecting others. As we have seen in the section which precedes 

 the present one, the positive reactions of this animal are not due to any 

 specific quahties of stimulation. On the contrary, the animal turns 

 toward weak stimuU of all sorts. Sohd bodies, whether fit for food or 

 not, chemicals of all sorts, including the injurious as well as the bene- 

 ficial, heat, and cold, all induce, when acting but' slightly on one side, 

 a turning toward the source of stimulation. The flatworm may thus 

 be said to investigate every sKght change occurring in its surroundings. 

 On reaching a region where the agent in question acts more intensely, 

 the positive reaction may either continue or be transformed into a nega- 

 tive one. Thus the turning toward food is not due to the specific quali- 

 ties which make the substance in question fit for food, but is the result 

 only of this general tendency to move toward all sources of weak stimu- 

 lation. The flatworm proves all things, holding fast only to that which 

 is good. 



In most if not all other invertebrates there occur many "trial move- 

 ments" similar to those already described. In many recent accounts 

 of the behavior of other invertebrates little mention, it is true, will be 

 found of such movements. This is apparently because attention has 

 been directed by current theories to other features of the behavior, and 

 the trial movements have been considered of no consequence. Often 

 an attentive reading of papers on "tropisms," etc., will reveal paren- 

 thetical mention of various "disordered" movements, turnings to one 

 side and the other, and other irregularities, which disturb the even tenor 

 of the "tropism," and are looked upon for some reason as without sig- 

 nificance and not requiring explanation. Further, one often finds in 

 such papers accounts of movements which are clearly of the "trial" 



