295 



^ ^ 



\ L 



Fig. 140 



A chain-forming flatworm, Stenostonnmi grandis. Physiological dominance of 

 zooids with older head-regions over those anterior to them which are headless 

 or have less advanced head-regions. A, Chain of five zooids, showing piece 

 isolated, between X-X and Y-Y. B, The headless anterior zooid-fragment is 

 partly resorbed. C, It is further resorbed but is attempting to differentiate a 

 head. D, It has been totally resorbed, and the next zooid is undergoing resorp- 

 tion. E, F, The original posterior zooid, with the oldest head in the fragment, 

 has resorbed all the material anterior to it, and has divided to form a younger, more 

 posterior zooid. (From Child, Arch. Entzomech. cxvii, 1929.) 



