FLAT WORM ( PLAN ARIA) 



338 



again follows to the posterior tip of the body. The level where the 

 metabolic rate suddenly rises represents the point of fission or the 

 anterior end of the second individual. This seems to indicate a 

 kind of zooid organization in the animal. In larger, older individ- 

 uals there inay be other such points of increased metabolism pos- 

 terior to this first one. Such zooids are the result of successive 

 functional isolations of the basal structure accompanying growth 

 in length. This graduation of the rate of metabolism along the 

 principal axis of an axiate animal has been called an axial gradient 

 (metabolic gradient) by Dr. Child. When the animal is young, it is 



Fig. 121. — Fission in Planaria dorotocephala. 



relatively short and the entire body, but particularly the ''head," 

 carries on a high rate of metabolism. The head at this time holds a 

 dominance over the length of the t)rganism. As the animal grows 

 older, it becomes longer, and the entire metabolic rate decreases. 

 This means that the head loses its dominance over the entire length. 

 A new center of dominance and increased metabolism is established 

 just posterior to the point where this ''head" dominance fades out. 



Regeneration 



This group shows remarkable powers of replacing lost or muti- 

 lated parts of the body. It can be cut into several pieces, and each 



