MOVEMENTS, ETC., OP PRESH-WATKR PLANARIANS. 559 



rest is not in form or rate distinguishable from the other 

 slow gliding motion of the worm which is not followed by 

 rest. In other words, a specimen may glide slowly for a long 

 time without stopping, so that one cannot prophesy with 

 certainty from the rate of movement whether the specimen is 

 soon coming to rest or not. The coming to rest is practically 

 always preceded by a period of slower gliding, but all slow 

 gliding is not immediately followed by rest. After a brief 

 period of this slower gliding the worm suddenly stops, and 

 the posterior half of the body remains fixed in precisely the 

 same position. The anterior half of the body is slowly moved 

 about over the bottom from side to side, the head being 

 touched frequently to the bottom or any other solid object in 

 the neighbourhood. The anterior part in this "feeling" 

 movement moves about the posterior part as a fixed point, 

 the latter very rarely changing its position after it has once 

 stopped. The thoroughness of this "testing" of the sur- 

 roundings by the sensory anterior end varies much in different 

 cases, but in practically all cases one can see some indication 

 of it. I have in some instances seen it done very thoroughly, 

 so that the whole surroundings within a radius of 3 mm. 

 were gone over. Finally, when this is done the animal comes 

 to complete rest, and assumes the typical relaxed condition 

 shown in Fig. 6. The apparent significance of the " testing " 

 movements at the time of stopping is that it is apiece of pro- 

 tective behaviour. The worm examines the surroundings 

 before coming to rest, to see if there is anything dangerous 

 (either of a solid nature or a harmful chemical) in the 

 immediate neighbourhood. Whether or not this explanation 

 is the true one, and further, whether natural selection 

 developed this reaction for protective purposes, seems to me 

 to be very doubtful, for reasons brought out in anotlier place 

 (cf. pp. 542 and 543). In some cases I have seen the Avorms 

 come to rest by simply stopping without any appreciable trace 

 of the " feeling " movements, but this is not the usual pro- 

 cedure. In coming to rest in one of the collections already men- 

 tioned, the '^feeling" movements are usually very well marked. 



