MOVEMENTS, ETC., OF FRESH-WATEK rLANAIilANS. G97 



movement of a specimen in the air. If a specimen is placed 

 on very wet filtei'-paper it is not able to prog-ress nnless 

 water is kept constantly dropping on it from above, so that it 

 is at any time surrounded by a layer 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 water. 



As has been mentioned before, specimens placed on a dry 

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



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

 from the water and subjected to a process of dr3'ing, 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, mis'ht 

 get the animal l)ack 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. The fresh-water Triclads, so far as I have 

 observed them, never leave the water and crawl up into the 

 air above the surface film as some other forms do. 



VI. Rheotaxis. 



A large number of experiments were performed eai-ly in 

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

 mine whether the animal showed any distinct reaction to 

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

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

 difi^usion apparatus described above (pp. GOl, G62) with water 

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

 currents were made with sufiicient force to threaten dislodo-- 

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

 moving and contract longitudinal) v, :uid thus attach itself 



