PHYSIOLOGICAL DOMINANCE 



95 



The act of fission in this animal consists of an inde- 

 pendent motor reaction of the posterior zooid or group. 

 When the worm is creeping quietly, 

 the posterior zooid or the zooid 

 group suddenly attaches itself to the 

 surface on which the animal is 

 creeping, while the whole anterior 

 individual endeavors to advance 

 and the body in front of the attached 

 region becomes greatly stretched 

 (Fig. 46) and finally ruptures. The 

 occurrence of fission can often be 

 controlled experimentally in a way 

 that shows the variable range of 

 dominance very clearly. If an ani- 

 mal is very slightly stimulated, e.g., 

 by a shght jarring of the aquarium, 

 the posterior zooid will often attach 

 itself, and fission will occur, while 

 with stronger stimulation the animal 

 is able to control this region and it 

 does not become attached but ad- 

 vances with the rest of the body. 

 Evidently when the animal is only 

 moderately active the posterior re- 

 gion is physiologically isolated, but 

 when it is intensely active the range 

 of dominance of the anterior indi- 

 vidual extends to this posterior 

 region and determines its subordina- 

 tion in behavior. Similarly, in very old animals which 

 have been prevented from undergoing fission by keeping 



Fig. 46. — Planaria 

 dorotocephala in the act 

 of fission. 



