HEAD-FREQUENCY IN PLAN ARIA 417 



though the anterior ends of both are at the same level of the body. 

 The tables also show, as do the earlier data, that in pieces of 

 equal length head-frequency decreases with increasing distance 

 from the head of the animal, back to the level of the posterior 

 zooid (Child, '11 d). Series 201 and 277 of table 1, the only series 

 of this paper in which the posterior zooid is included, show an 

 increase in head-frequency in the region of the posterior zooid. 

 These relations between head-frequency and relative length of 

 piece and region of body have been discussed and interpreted in 

 the same terms as the relations between head-frequency and 

 physiological condition considered in this paper (Child, 'lib, 

 '14 b, '14 d) and recent work on carbon-dioxide production in 

 pieces (Robbins and Child, '20) has added further evidence in 

 support of the conclusions drawn from the earlier investigations. 



SUMMARY 



1. Head-frequency in the regeneration of pieces is lower in 

 physiologically younger (smaller) than in physiologically older 

 (larger) animals. 



2. Head-f-requency is lower in pieces from starved than in 

 pieces from well-fed animals, even when the two are of the same 

 size. 



3. Head-frequency is higher in pieces which are frequently 

 stimulated to motor activity during at least several hours after 

 section than in pieces remaining undisturbed. 



4. The range of head forms is the same in relation both to 

 physiological conditions and to external chemical and physical 

 agents, and the changes produced are changes in the frequency of 

 the different forms. This non-specific effect of both physiologi- 

 cal and external factors indicates that the action of these factors 

 is essentially quantitative. It is shown that the quantitative 

 interpretation of changes in head-frequency previously advanced 

 serves for the facts presented in this paper. 



