MOVEMENTS, ETC., OF EUESH-WATER PLANAHIANS. 593 



as will be showily this caiiuot be done. On tin's account it 

 will not be necessary in the description of the reactions to 

 sharply distinguish between the effects of stimulation of the 

 margins and of the dorsal surface, as has been done in the 

 pi'evious cases. 



Strong mechanical stimulation of the posterior region of 

 the flat-worm produces as a specific reaction an immediate 

 change from the gliding to the crawling movement. The 

 direction of the crawling is the same as that of the gliding; 

 that is to say, the worm keeps on in a straight line, taking 

 itself directly and in the quickest possible way away from 

 the stimulus. The duration of the crawling movement 

 following stimulation of the posterior region varies with the 

 relative intensity of the stimulus and the physiological con- 

 dition of the specimen. The most usual number of the 

 strong, crawling contraction waves following strong stimula- 

 tion is three or four. We may get a smaller number than 

 this, and very frequently do, but in the species studied I 

 have very rarely seen more than four of the general con- 

 tractions following a single stimulus. This is evidently all 

 that would be necessary under normal circumstances, since 

 four o£ these strong contractions will carry the animal a 

 considerable distance ahead, and probably out of reach of 

 the stimulating agent. The weaker the stimulus is, the fewer 

 are the contractions and the shorter the distance crawled. 

 In some individuals it is at times almost impossible to induce 

 the crawling movement except by repeated stimulation. 

 Such specimens will merely draw up the posterior end in a 

 single crawling contraction after stimulation, and then im- 

 mediately relapse into the glide. If a strong stimulus is 

 repeatedly given at the posterior end the crawling is con- 

 tinued, becoming more and more rapid. This is the only 

 effect of continued stimulation in this region, there being no 

 summation effect corresponding to that produced by stimu- 

 lating the anterior end. No different effect is produced by 

 stimulating the margins of the posterior region of the body 

 from what takes place when the point stimulated lies near 



