MOVEMENTS, ETC., OF FRESH-WATKll PLANAUIANS. 597 



on either side of tlio cut to give violent lougitudiual con- 

 tractions, while the worm as a whole starts crawling ahead ; 

 that is, it changes from the gliding to the crawling 

 movement. 



Oblique cuts produce essentially the same effects as would 

 trausvei'se cuts in the same part of the body, i. e. forward 

 crawling of the anterior piece, and usually backward crawling 

 of the posterior piece. This is true unless the cuts are very 

 oblique, so as to form very acute angles with the sagittal 

 plane of the body. In such cases the effects produced more 

 nearly resemble those obtained in complete longitudinal 

 splitting of the body. If the body is split completely into 

 two parts longitudinally, there is usually very little pro- 

 gressive movement of either piece afterwards. The pieces 

 contract strongly on the cut sides very soon after the opera- 

 tion is performed, so that they take on the form of a bow, 

 which in many instances becomes a nearly complete circle. 

 This being the case, any progressive movement, either by 

 gliding or crawling, is nearly or quite impossible. Cuts 

 involving only a small portion of one side of the body 

 produce, if in the anterior region, the characteristic negative 

 reaction given to other strong mechanical stimuli, while if in 

 the posterior region they cause the crawling ahead. 



Cuts made on the resting animal produce essentially the 

 same effects as on the gliding specimen. Unilateral cuts 

 have the same effect in producing the negative reaction. 



7. The Effect of Mechanical Hindrance to Move- 

 ment. — A series of experiments was performed on Dendro- 

 coclum, sp., with reference to the behaviour of the animal 

 when progressive movement was made impossible, and yet 

 the animal was stimulated strongly at the same time. These 

 conditions can be realised by thrusting a needle through the 

 centre of the body from above, and then holding it fixed in 

 position. The results of this procedure varied somewhat, 

 according to the portion of the body through which the 

 needle was thrust. In case the hindrance is in the posterior 

 region of the body, e. g. at a point just behind the posterior 



