Factors of Form Regulation in Harenactis Attenuata 495 



length of the tentacles in the different series is much less than after 

 fourteen days: in Series I the tentacles have undergone reduction, 

 in Series II slight growth has occurred in the pieces with shortest 

 tentacles, and in Series III the tentacles have almost doubled their 

 length between fourteen and twenty-six days. In Series IV and 

 V most or all of the growth of the tentacles has occurred during 

 this period. The chief point of interest in this connection is that 

 after twenty-six days there is no great difference in the length of 

 the tentacles in Series I, II, and III, although the difference in 

 size of pieces in the three Series, especially I and III is consider- 

 able: moreover, in Series IV the length of the tentacles (10 mm.) is 

 almost as great as the minimal length of tentacles in Series I 

 (12 mm.), though the pieces of Series IV are less than half as long 

 as those of Series I (Fig. 2). In Series V, where the pieces include 

 only the proximal fifth or fourth of the body, the tentacles are only 

 half as long as those in Series IV. It should be noted in passing 

 that in most cases such pieces as those of Series V simply close and 

 do not form tentacles at all. Series V shows the greatest devel- 

 opment of any case observed. 



Evidently the length of tentacles produced is not proportional 

 to the size of the piece, though larger pieces do produce somewhat 

 longer tentacles. The smaller pieces, however, except in ex- 

 treme proximal regions produce relatively longer tentacles than 

 the larger. 



One other point remains to be considered in connection with 

 these series, viz: the much more rapid reduction in length of the 

 tentacles after they have reached their maximum length, in Series 

 I than in the other series. Between fourteen and twenty-six days 

 the tentacles in Series I decrease in some cases almost half their 

 length (from 20-25 mm. to 12-20 mm.), while in the other series 

 the decrease in length during four months is in most cases less 

 than this (see Table I, last two columns). In another paper 

 (Child '09) I have called attention to the change of shape and 

 behavior which Harenactis undergoes when forced to live without 

 sand or mud in w^hich to imbed itself. Decrease in the length 

 of the tentacles is one of the features of this regulation, the atro- 

 phied tips often being visible on the tentacles in such cases. In 



