434 ALFRED O. GROSS 
The average reaction time was very much lengthened in the 
case of the twelve individuals from which the appendages were 
removed, but it remained practically unchanged in the unoper- 
ated animals used as a control. After these determinations 
were made, the worms were put aside until the appendages 
were completely regenerated. With the regrowth of the appen- 
dages the sensitiveness of the worms to chemicals was restored, 
as evidenced by the reaction time which became about equal to 
that of the control animals and to that of the same worms before 
the operations were made. The results of this preliminary 
experiment indicate that certain of the appendages of the head 
region are more sensitive to chemical stimulation than the gen- 
eral integument of the worm. In order to determine whether 
the chemical sense is shared equally by all the appendages or 
more strongly developed in some than in others, the experiments 
were repeated, but. with this difference, only one of the three 
pairs of cephahc appendages was removed from the worms of 
any one series. The anal cirri were Ukewise tested for their 
sensitiveness to chemical stimulation. The following tables 
contain the results of the tests made upon Nereis virens under 
the various conditions indicated. In each case the number of 
animals used, the number of readings made, and the mean of 
the reaction times with the probable error is given. It is deemed 
impracticable to pubhsh the individual readings which, with the 
preliminary tests, involve more than 2000 determinations. For 
convenience in comparing the reaction times of the worms under 
two conditions, the difference of the means of the reaction times 
and the 'significance factor' are also given in the tables. 
The significance factor involves a comparison of the means of 
the reaction times including their probable errors. The value of 
this factor is obtained by dividing the difference of the means of 
the reaction times of the normal and operated animals by the 
square root of the sums of the squares of the probable errors of 
these two reaction times. As an example, take the case of the 
palps in table 3 in which the worms were tested with KOH. 
The reaction time of the normal animals is 9.44 seconds with a 
probable error of 0.479, and the reaction time of the same set of 
