MOVI'IMENTS, KIT'., OF FIfKSIT-WATKT? I'LAN AKIANS. C21 



appeared wlien two ?-periin(Mis o-liditiQ* alono-, wifli the nnterior 

 ends slightly r:iised in tlu^ iioi'inal inaiiiicv, met liead-on. 

 Both were siinnltaiieonsly stimulated to the positive reaction 

 and raised the anterior ends, and then let them di'op ai>-ain. 

 As they came down the two ventral surfaces were l)roni;ht 

 squarely together in the way shown in Fig. 2t; then each 

 started gliding up the ventral surface of the other. In a 

 movement ns a result of the constantly changing form of the 

 body, the ventral surfaces slipjiod off fi*om one another and 

 the two worms went on their way. When the obstruction is 

 a hard body, as a piece of glass, the specimen meeting it 

 obliquely usually turns the head away slightly at the first 

 contact (negative reaction), and then glides along parallel to 

 the edge of the body for a distance. If it happens to touch 

 it again with the side of the head, it frequently gives the 

 negative reaction and turns away again. After the solid body 



Fig. 2i: — Side view of two plaiiariaiis starting: to glide up on Uie 

 ventral surfaces of cacli other. 



has been touched several times, however, the positive reaction 

 is usually given, and the worm passes at once up on to the 

 solid body. This behaviour is shown in Fig. 2h. The precise 

 form of the behaviour on meeting obliquely a solid body in 

 the path varies considerably with the general physiological 

 condition of the individual. In case it is much excited, the 

 first touch will induce a strong negative reaction, and tlie 

 individual will turn away and pass out of the neighbourhood. 

 In the cases whei'e the final positive reaction is preceded by 

 two or three negative ones, it would seem as if repetition of 

 what must be an almost identical stimulus causes it to l)e- 

 come in effect weaker. Leaving aside all variations in the 

 exact character of the behaviour on meeting a solid, the 

 important point to be brought out is that all this behaviour is 

 based on the simple reactions to mechanical stimuli. The 



VOL. 46, PART 4. NEW SERIES. S S 



