PHYLUM PLATYHELMINTHES 173 



orientation of the tissue is retained ; i.e., an anterior portion develops 

 in the position of the original anterior portion, and a posterior por- 

 tion at the original posterior position. This process is not funda- 

 mentally different from budding in Hydra or strobilization in the 

 Scyphomedusae. 



The retention of the axial orientation during fission has been 

 explained by Dr. Child of Chicago University. The animal pos- 

 sesses a well-defined axial organization in which the "head" por- 

 tion as usual has the highest metabolic activity of the body. Be- 

 ginning at the anterior there is a gradient of decreasing metabolic 

 activity until a level just posterior to the mouth is reached, and here 

 a sudden increase occurs. Posterior to this the decreasing gradient 

 again folloAvs 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 may 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 

 by Dr. Child. When the animal is young, it is 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 organism. 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" dominajice 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 

 piece will replace the missing parts about as the process is carried 

 out in fission. A piece from the middle of the animal will regener- 

 ate a head portion at the anterior margin and a tail portion at the 

 posterior margin. A more complete discussion of this phenomenon 

 will be found in a later chapter on Animal Regeneration. 



