DOMINANCE AND PHYSIOLOGICAL ISOLATION 321 



mordia show a high degree of regularity. In the Tubularia stem and prob- 

 ably in Corymorpha circulation is in both directions. Why does not the 

 circulating factor inhibit the dominant hydranth? Also, how is it possible 

 for hydranths to develop simultaneously at both ends of a short piece 

 without greatly inhibiting each other if each produces an inhibiting sub- 

 stance? 



FISSION AND BUDDING IN FLATWORMS AND ANNELIDS 

 PLANARIANS 



In certain species of the triclad genera Planaria and Dugesia ( = Euplan- 

 aria) fission usually occurs in individuals above a certain length at a more 

 or less definite body -level posterior to the mouth. Morphological evidence 

 that a new individual is developing in the posterior region is usually lack- 

 ing in planarians, but repetition of the copulatory organs in the posterior 

 zooid region of D. dorotocephala has been reported (Kenk, 1935a). Ani- 

 mals showing this dupHcation were prevented from undergoing fission 

 by maintaining them at low temperature. According to Kennel (1883, 

 1888), cephahc ganglia, eyes, and a new pharynx develop before separa- 

 tion in the posterior zooid of a planarian from Trinidad. In D. paramensis, 

 which divides anterior to the pharynx, a new pharynx develops in the 

 anterior zooid preceding fission, suggesting some degree of physiological 

 isolation of the posterior zooid. 



In D. dorotocephala, the material for most of the experimental work on 

 fission, difference in susceptibility, respiration, dye reduction, head fre- 

 quency, and motor reactions indicate that the region posterior to the 

 fission zone represents one or more zooids, partially isolated physiologi- 

 cally from the dominance of anterior regions.^ There is no evidence of a 

 posterior zooid in very small animals soon after hatching or in individuals 

 reconstituted from short pieces, but with increase in length the posterior 

 zooid region becomes distinguishable by various methods, though fission 

 does not usually occur until animals are 15 mm. or more in length. With 

 rapid growth in length the posterior zooid appears and fission takes place 

 in shorter animals than when growth is slow, and with increasing degrees 

 of inhibition of head development the length attained before fission de- 

 creases (Child, 191 It/). Evidently the range of effective dominance in- 

 creases with increase in length of the animal, but in general less rapidly 

 than the length, so that progressive physiological isolation of the posterior 

 region results. 



^ See pp. 42, 43, 45, 109, 112, 181. 



