CONTROL OF HEAD-FORM IN PLANARIA 107 



shows a downward change from a to h which are both within the 

 first zooid, while in c there is a slight further shift downward, 

 but at the same time an increase in normal heads. 



These characteristics of I c as compared with I h indicate, as 

 will appear later, that c includes in many cases a portion of the 

 second zooid. The approximate range in position of the anterior 

 end of the second zooid is indicated in figure 8 by the bracket 

 m. In I d which represents part of the posterior zooids the 

 head-frequenc}^ shifts upwards to a marked degree as compared 

 with I c,. in I e it is also high and in I /, the posterior tip of the 

 body it is highest of all. 



The effect of KCN m/ 200000, a very low concentration, for 

 twenty-four hours is slight in II a, while in // h the shift is some- 

 what upward, in II c still more upward and in II d and II e also 

 upward to a lesser degree, while in II / no effect is apparent. 

 Comparing these changes with those produced in III by KCX 

 m/50000, a concentration four times as high as in II, but used 

 for the same length of time, we find that in III a the head-fre- 

 quency is shifted downward to a marked degree in III h also 

 downward — while in III c -III / no change is produced. In 

 short, the lower concentration has little effect on anterior pieces 

 but shifts head-frequency upward in the more posterior pieces 

 except the last, while the high concentration decreases head- 

 frequency in the more anterior pieces and has little effect on 

 the more posterior. 



Series 494, table 2, is a second series of \ pieces (fig. 8) hke 

 Series 512, but showing the effect of forty-eight hours in KCX 

 m/200000. The cyanide lots, II a-f, show as compared with 

 the control, I a-/, a shift downward in a, a slight shift upward 

 if anything in 6, a very great shift upward in c, less in d and e 

 and no change in /. 



In both these series in which the animals were cut as nearly 

 as possible into equal sixths excluding the head (fig. 8) the c 

 pieces often include a part of the second zooid, somewhat more 

 often in table 2 than in table 1 , as the higher frequency in the 

 former indicates. The high level of head-frequency in the c 



