560 



RAYMOND PEARL. 



There is a "well-marked tendency for the planarians studied 

 to come to rest in such a way that the long* axis of the body 

 forms a right angle, or nearly a right angle, with the lines 

 of the force of gravitation. The cases in which the organisms 

 come to rest with the long axis forming an angle of less than 

 thirty degrees with the line of gravitation are rather few. 

 Of course, when they come to rest on the bottom, the angle 

 formed is approximately ninety degrees. A large number of 

 observations on individuals which came to rest on the sides 

 of dishes of various shapes have given the general result 

 stated above. There are, of course, exceptions, but there is, 



Fig. 7- — Diagram showing (lie positions taken by planarians coming to 

 rest iu a dish (see text). 



after making due allowance for these, a tendency to a hori- 

 zontal position as the position of rest. This is the only 

 behaviour of the organism which bears any resemblance to 

 a geotactic reaction. Another tendency, less marked than 

 the former, is for the animals to come to rest in the angle 

 formed by the sides and bottom of the dish. Not only do 

 specimens come to rest lying directly in the angle, as shown 

 at A in fig. 7, but also, and more frequently, they lie in such 

 a position that a part of the body is on the side of the dish 

 and a part on the bottom, as shown at B, fig. 7. In this 

 position the anininl usuall}' lios obliquely rather than at right 



