696 RAYMOND PEARL. 



the head from drying. At fairly frequent intervals the 

 animal straightens out and extends the head in front as far 

 as possible^ and makes "feeling'^ movements. It is then 

 withdrawn, and the animal curls up again. After the drying 

 has proceeded for some time the inost characteristic feature 

 of the whole reaction appears. This is a lengthening of the 

 posterior part of the body to its fullest extent. The posterior 

 end then attaches itself to the surface, and strong waves of 

 contraction, like those in the crawling movement, pass over 

 the body from the posterior end forward. No progressive 

 movement is made, but backward crawling is evidently 

 attempted, and is only prevented by the dry surface Avhich 

 the animal is on. There may be considerable variation in the 

 first part of the reaction with regard to the curling up ; this 

 may appear or may not, but the attempted backward crawl- 



¥iG. -t'J. — Diagram showing the reaction of Planaria to desiccation. 



ing movement of the posterior part of the body I have found 

 to be a constant feature in the experiments which I have 

 performed. When the dorsal surface of the worm becomes 

 dry all movement ceases. If quickly put back into the water 

 the worm will usually recover completely, even though all 

 movement has ceased in the air. 



If the Avorm is put on a slide in the centre of a small area 

 which has been wet, but on which there is no standing water, 

 it will squirm about and extend the head frequently, as in 

 the last experiment. If the head goes outside the wet area 

 it is very quickly jerked back, and the specimen gives the 

 negative reaction, i. e. turns away from the side stimulated. 

 The attempted backward crawling occurs in this case just as 

 in the others, a short time before the dorsal surface dries off. 



It is to be noted that there is never any actual progressive 



