MOVEMENTS, RTO., OF FRESM-WATRIl PLANARTANS. G75 



of tlie body of Plan aria has tlie form sliowii in Fig. 37. It 

 is convex in outline on the dorsal side, and nearly straight on 

 tlie ventral. As a consequence of this shape of the dorsal 

 surface the animal when placed in an inverted position very 

 seldom lies exactly on the mid-dorsal line, and if it does at 

 first it almost immediately tips over to one side or the other, 

 so that its cross-section has the relation to the bottom shown 

 in Fig. 37, B and C. It h then found that the side of the 

 body which is in contact with the bottom determines in 

 which direction the spiral shall be thrown. If the right 

 side of the dorsal surface is down the right side of the head 

 will turn nnder towards the left and the left side will be 

 raised up over towards the right, or, in other words, the 

 head as a whole will rotate from I'ight to left, i. e. in a 



Dorsal 



Vfntral 

 A 



B C 



Fig. 37-— Diagranimalic cross-section of Plaiiaria t,o sliow the contact 

 relations of the dorsal surface of the body to Uie substrate in llie 

 case of a specimen in an inverted position. 



counter-clockwise direction. If the left side of the dorsal 

 surface of the body is down at the beginning, the head will 

 rotate from left to right. This relation may bo made out 

 easily by direct observation in all cases where the reaction is 

 not too rapid. 



The righting reaction is a fnirly rapid one. The head is 

 turned over and the spiral thrown in the case of a normal in- 

 dividual almost immediately when the dorsal surface touches 

 the solid. The length of time which it takes a specimen to 

 get completely righted evidently depends on the length of 

 the body, because the longer spiral which must be unwound, 

 the more the time which must be taken. The following 

 figures will bring out this relation between the size of the in- 

 dividual and the time taken in righting. In ten trials with 



