596 RAYMOND PEARL. 



cedure. — Evidently wlieu a plauarian is cut tlie cutting 

 induces a strong stimulation, wliicli is of the same kind as 

 that induced by ordinary mechanical stimuli, only much 

 more intense. The immediate effects of operations may then 

 be taken up in this section. 



If we take first the typical case given by cutting the 

 animal transversely in two in the region between the pos- 

 terior border of the head and the origin of the pharynx, and 

 make the cut by a single stroke of a sharp scalpel, we find 

 that the effect on the anterior piece is precisely the same as 

 that of an ordinary strong mechanical stimulation of the 

 same place. That is, this piece merely changes from the 

 gliding to the crawling movement, and after giving three or 

 four crawling coiitractions settles down again into the 

 glide. This is the same result essentially as that obtained 

 by Norman (: 00) and earlier by Loeb ('94 and : 00). In the 

 behaviour of the posterior piece in this experiment under 

 discussion there is a great deal of variation. In about 70 

 per cent, of all cases in which I have observed the results of 

 such an operation, the posterior piece crawled backwards 

 as a result of the cut. In the remainder of the cases the 

 piece either stayed in the same place and contracted 

 violently, or else glided ahead. The amount of the back- 

 ward crawling when this occurs varies greatly, from a 

 short distance involving only one longitudinal crawling con- 

 traction to several times the length of the worm, the move- 

 ment lasting in this latter case for over a minute. In order 

 that this backward crawling may appear in a well-marked 

 and distinct form it is necessary that the posterior piece be 

 above a certain size. Very small posterior pieces after 

 operation usually remain quiet. 



A cut so made as to split the anterior end of the body in 

 the middle line in most cases causes the worm to crawl back- 

 wards just as does a transverse cut. In some cases this, as 

 well as other operations, merely causes the animal to contract 

 violently and squirm about at the same place. Splitting the 

 posterior end of the body in the middle line causes the parts 



