CERTAIN GENERAL CHARACTERISTICS 



45 



in the posterior zooid; in most other species it increases to the posterior 

 end or as far posteriorly as head regeneration occurs, but even very small 

 fragments of Fonticola velata from any postcephalic level reconstitute com- 

 plete individuals, though only after encystment, practically complete de- 

 generation of internal organs, and reorganization of body wall and paren- 

 chyma." 



Marked differences in scale of organization of the preoral region occur in 

 planarian reconstitution in relation to level of origin of the piece. These 

 consist in a gradual decrease in relative length of the preoral region and 

 more anterior localization of the pharynx in successive pieces of equal 

 length from the most anterior level posteriorly to the fission zone, except 

 in pieces from pharyngeal levels; in those the pharyngeal region or some 



Fig. 21, A-G. — Differences in length of prepharyngeal region and in position of pharynx in 

 pieces from different body-levels of Dugesia tigrina. A-E, decrease in prepharyngeal length in 

 successive pieces from anterior to posterior end of anterior zooid; F, G, increase in prepharyn- 

 geal length in pieces from posterior zooid region. 



part of it is already present, but its level may undergo gradual change in 

 consequence of differential growth in relation to scale of organization 

 (Fig. 21, A-E). In the region of the posterior zooid length of preoral re- 

 gion is again greater (Fig. 21, F, G). In species without a posterior zooid 

 region decrease in preoral length continues to the posterior end. Pieces 

 from anterior and posterior levels of Curtisia foremanii (Fig. 22, A, B) and 

 similar pieces from young individuals of Fonticola velata reconstituting 

 without encystment show this difference in scale clearly." 



Scale of organization of the new head in earlier stages shows a similar 

 relation to level of origin of the piece. It decreases from the anterior level 

 to the fission zone and is again larger in the posterior zooid region in the 

 species with posterior zooids (Fig. 21). In those without such zooids the 



" For fragmentation, encystment, and reorganization in F. velata see Child, 1913c, igi^d; 

 W. A. Castle, 1928. 



'^ For further data see the following: Morgan, 1898, 1900a, 1901c; Child, 1906, 1911&, 

 1913c, igi^d; Sivickis, 1923; Buchanan, 1927. 



