2 68 BEHAVIOR OF THE LOWER ORGANISMS 



pulled. The animal therefore occupies successively the positions 1,2,3, 

 and 4. In the position 4 both sides are equally affected by the stimulus, 

 so that there is no cause for further turning. The animal has become 

 oriented and its usual forward movements now take it away from the 

 source of stimulation. We have here a case of negative tropism or 

 taxis. 



Figure 143 illustrates the conditions producing positive tropism or 

 taxis. The stimulus, coming from the right side, is supposed to cause 

 the cilia of that side to beat less strongly backward, or to beat forward. 

 As a result the organism is turned to the right, through the positions 

 1, 2, 3, 4, till its anterior end is directed toward the source of stimulation. 

 Both sides are now affected alike, and there is no cause for further turn- 

 ing. The animal now moving forward in the usual way of course travels 

 toward the source of stimulation. 



As an example of the application of the tropism scheme to a mus- 

 cular organism, we may take Davenport's exposition of the action of 

 light in determining the direction of locomotion of the earthworm. 

 "Represent the worm by an arrow whose head indicates the head end 

 (Fig. 144, A ). Let solar rays SS fall upon it horizontally and perpendicu- 

 larly to its axis. Then the impinging ray strikes it laterally, or, in other 

 words, it is illuminated on one side and not on the other. Since, now, 

 the protoplasm of both sides is attuned to an equal intensity of light, 



that which is the less 

 S illuminated is nearer its 

 optimum intensity. Its 

 protoplasm is in a photO- 

 Low light attuncment tonic conc li t ion. That 



■A " T ,. , „ ., . ' which is strongly illumin- 



Low light attunement o J 



■c -.. , . ated has lost its phototonic 



Fig. 144. — Diagram to explain a tropism in a muscu- _ | r 



lar organism, such as the earthworm. After Davenport, condition. Only the dark- 



See text * ened muscles, then, are 



capable of normal contraction; the brightly illuminated ones are re- 

 laxed. Under these conditions the organism curves toward the darker 

 side; and since its head region is the most sensitive, response begins 

 there. Owing to a continuance of the causes, the organism will con- 

 tinue to turn from the light until both sides are equally illuminated, 

 i.e. until it is in the light ray. Subsequent locomotion will carry the 

 organism in a straight line, since the muscles of the two sides now act 

 similarly. Thus orientation of the organism is effected. The same 

 explanation, which is modified from one of Loeb ('93, p. 86), will ac- 

 count, mutatis mutandis, for positive phototaxis " (Davenport, 1897, 

 p. 209). 



