PHYLUM PLATHELMINTHES 



109 



forms. Among the rhabdocoels is found even greater diversity 

 of form. Some are spindle-shaped, while others are distinctly 

 flattened. The ectoderm is covered with ciHa (Fig. 62) which by 

 their movement produce a smooth gliding locomotion. Currents 

 produced by these cilia are responsible for the name of the class. 

 Turbellaria are mostly free-living, aquatic organisms, though 

 some have acquired the parasitic habit and others have become 

 adapted to living in or on moist soil. 



Fig. 61. — Axial gradient in Planaria. -4, Planaria showing location of incip- 

 ient heads; B, curve showing rise in metabolic gradient at levels marked in .4. 

 {Redrawn from. Child). 



Axial Gradient. — When two new individuals are formed from 

 parts resulting from either natural or accidental separation of the 

 body of a planarian in a transverse plane, there is always a 

 tendency for the front piece to produce a tail and the hind one a 

 head. In planarians which normally divide by fission, the 

 posterior end of the body represents additional potential individu- 

 als even before there is any evidence of separation. Professor 

 C. M. Child has given experimental proof of this. In his physio- 



