MOVEMENTS, ETC., OF FIil!]SH-WATEli PLANARIANS. 5G1 



angles to the line of the junction of the side and bottom of 

 tlio dish. The animals usually come to vest in this position 

 after they have been gliding on the side of the dish. When 

 they come to rest from movement on the bottom of the dish, 

 a position frequently taken is that shown in C, fig. 7, where 

 only the very anterior end is in contact with the side. This 

 coming to rest in the angle of a dish is apparently a reaction 

 which agrees with those usually called thigmotactic reactions. 

 But it is not, as has been stated by several writers, due to a 

 tendency to get more of the body in contact with something 

 solid^ than is in such contact under usual conditions; for in 

 the case of an organism like a flatworm, it is impossible for 

 any more of the surface of the body to be in contact with a 

 solid when it is bent, as shown in Fig. 8, A, than when it is 

 flat, as in B. There is the same amount of surface in contact 

 in either case. The ventral surface of the flat-worm is 



L 



A B 



Fig. 8. — Diagrammatic cross-sect ion of a ])laiiaiiaii at rest — A, in 

 an angle ; and B, on a i)!ane surface. 



strongly positively thigmotactic under all circumstances, and 

 the dorsal surface negatively thigmotactic, but this does not 

 help us understand why the animal comes to rest frequently 

 in angles. This behaviour of the flat-worm in dishes is due to 

 the same sort of reaction as that which causes them to come 

 to rest on unevennosses on rocks, and also causes the same 

 phenomenon in a more marked degree in the case of Lit- 

 torina, as recently described by Mitsukuri (:01). The 

 common factor in the reaction in all cases is that difl^erent 

 parts of the body are brought into such positions that they 

 form unusual angles with each other. Since this phenomenon 

 is distinctly different from any embraced by the term thig- 

 motaxis as used in its true sense, it seems desirable that it be 



