Harper, Reactions of Perichceta. 58 1 



the anterior end. The retraction of the head of Serpula is ascribed 

 to the change produced by the stimulus and is a shock effect. The 

 turning toward the light is attributed to the constant action of the 

 stimulus, producing a differential tonus of the musculature of the 

 two sides either directly or by reflex action. 



The chemical stimuli described in this paper differ from light, of 

 course, in that they are transitory and decreasing in intensity. These 

 reactions have been described as consisting of two phases in types 

 II to V, an initial shrinking and a final negative turning. The 

 latter is a phase of the reaction that appears after the initial excite- 

 ment has worn off. 



It may appear then that in the two-phased reactions to chemical 

 stimuli the shock effect and the tonic effects are dissociated as suc- 

 cessive phases of the same reaction instead of separate reactions as 

 in the case of Serpula. Moreover it is seen from types I and II that 

 the threshold for shock effects is higher than for tonic effects. A 

 stimulus which is too weak to cause the worm to shrink back may be 

 able to produce a differential tonus of the two sides and lead to 

 negative turning. Also after the shock effect of a stronger stimulus 

 has worn off its continuing action may produce a tonic effect when 

 forw^ard crawling is resumed, after some intermediate forms of 

 movement. We have therefore the primary tropic reaction occurring 

 after a very weak stimulus, and a secondary one occurring as the 

 final phase of a reaction to a stronger stimulus after the shock has 

 subsided. 



Reactions to Light. 



Reactions to light are characterized by the relative absence of 

 shrinking movements and by the great length of the latent period. 

 To get well marked sudden effects worms must have been kept in the 

 dark. A comparison will be made here to see to what extent the analysis 

 of negative reactions already made will apply. Reactions correspond- 

 ing to the first three types in a general way were obtained by the use of 

 a fifty candle power incandescent reflecting bulb, with a cylinder of 

 black paper to transmit the rays. Different effects were obtained by 

 simply varying the distance. With weak light fifty trials showed 

 twenty-six negative turns (type I) six with initial retraction (type II) 



