MOVEMENTS, KTO., OF TUKsn-WATKl^ rLANATHANS. G97 



movement of a specimen in tlie nir. If a s])ecinien is placed 

 on very Avet filter-])a])er it is not ahle to jtroo-ress nnless 

 water is kept constantly clroppino- on it from above, so that it 

 is at any time snrronnded by a laj^er of water. On account 

 of this lack of ability to move when out of water, there is no 

 true hydrotaxis in the sense of movement towards Avater. 



As has been mentioned before, specimens placed on a dry 

 surface dorsal side down do not show the rightino' reaction. 



To sum up, it is found that planarians, when removed 

 from the water and subjected to a ])rocess of tlryiny, are 

 unable to make progressive movements. At a certain stage 

 in the drying* process they attempt to crawl backwards — a 

 form of movement which, under certain circumstances, might 

 get the animal back into water. On meeting a dry surface 

 with the anterior end the animals give a well-marked negative 

 reaction. The animal does not give the righting reaction on 

 being inverted on a dry surface. 



On the whole, the general behaviour when subjected to 

 drying is purposeful ; that is, it would tend to prevent the 

 animal ever becoming dried up under natural conditions. 

 There is nothing in the behaviour of planarians to indicate 

 how the change from aquatic to terrestrial life could be 

 brought about. T^he fresh-Avater Triclads, so far as I have 

 observed them, never leave the water and crawl u}) into the 

 air above the surface film as some other forms do. 



VI. Rheotaxis. 



A large number of experiments were performed early in 

 the course of the work with various sorts of devices to deter- 

 mine whether the animal shoAved any distinct reaction to 

 currents in the Avater, but without success. Streams of Avater 

 from a pipette, currents made by filling the tube of the 

 diffusion apparatus described above (pp. 661, 662) with water 

 and bloAving into it, and other methods gave no results. If the 

 currents Avere made Avith sufficient force to threaten dislods"- 

 ment of the animal from its hold on the bottom it Avould stop 

 movino- and contract longitudinal) v, and thus attach itself 



