CONTROL OF HEAD-FORM IN PLANARIA 



113 



TABLE 6 



Series 559. Worms 20-25 mm., in laboratory one week bejore experiment. I. Con- 

 trol in water; II-V . Whole worms in KCN m/100000 five minutes before sec- 

 tion, pieces cut in this solution and in same concentration after section. II. In 

 KCN three hours; III. In KCN sixteen hours; IV. In KCN forty-eight hours. 

 V. In KCN sixteen days 



and the first zooid had in many cases undergone a further physio- 

 logical division (Child, 'lid) so that the anterior end of the 

 new posterior zooid thus formed was almost at the level of 

 the mouth. In such animals the c-pieces, cut as in the other 

 series, usually include a part or nearly all of this short zooid 

 and we find correspondingly that the head-frequency shifts 

 upward instead of downward from h to c. A comparison of 

 I h and I c in table 6 shows this difference and a further compari- 

 son of these percentages with those of I fe and I c in tables 3, 

 4, and 5 shows the influence of this zooid in increasing head-fre- 

 quency. The c-pieces in tables 1 and 2 which often include a 

 portion of a posterior zooid also show the influence of this region 

 in a higher level of head-frequency as compared wdth the 6-pieces. 

 From table 6, I a-IV a, I 6-IV h, it is evident that periods of 

 3, 16, and 48 hours in KCN m/100000 have little or no effect 

 in shifting head-frequency in the a- and 6-pieces. In the c- 

 pieces, however, a period of three hours has no effect (II c), but 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 21, NO. 1 



