264 W. C. ALLEE 



initial shock. This might happen although no reaction was given 

 to the initial stimulus. 



In order to eliminate these rebounds, the apparatus was de- 

 vised as shown in figure 1. A short cylinder A was fastened 

 directly above the slanted portion of the steel block C. The 

 steel ball to be dropped rested upon a cardboard B which worked 

 in a slot in the cylinder. This was arranged 50 cm. above the 

 point of concussion D on the block C. The block C is made of 

 forged steel 10 cm. wide, 2.5 cm. high, and 20 cm. in greatest 

 length. The upper surface is 15 cm. long. The sloping part 

 measured 5.6 x 10 cm. and was machined to give a true sur- 

 face. The slope is such that the falling ball rebounds at an 

 angle and thus strikes the block but once. A steel ball weigh- 

 ing 11.927 grams was found to furnish the most satisfactory 

 stimulus when dropped 50 cm. At the moment of striking the 

 block, this had a momentum of 3734 C. G. S. units. 



The isopod to be tested was confined in a petri dish 3 cm. 

 in diameter, placed on the level surface 5 cm. from the point 

 of impact. The isopod was allowed to stand five minutes to 

 recover from the shock of handling. Then the responses were 

 taken to twenty successive concussions. A reading glass was 

 used in order to make out Slight movements. After each indi- 

 vidual trial the isopod was allowed to come to rest if a response 

 had occurred. In all cases the reactions were taken for the first 

 twenty trials and the lack of a reaction was considered to be quite 

 as significant as a definite movement. Whenever the animal 

 showed a tendency to crawl without the action of an external 

 stimulus, it was allowed to come to rest before further trials 

 were made. With practice the response to shock could be read- 

 ily distinguished from movement due to other causes. 



Numbers with the following values (cf. Banta '10, p. 453) 

 were used to record the strength of the reactions: 



no reaction 



1 movement of antennae or slight movement of other 



appendages 



2 extended movement of the antennae or strong enough 



movement of the legs to result in a slight stirring of 

 the body 



