PHYLUM PLATYHELMINTIIES. SIMPLE ORGAN-SYSTEM ANIMALS 



137 



Pharyngeal chamber 

 Pharynx 



Lunnen 



— Epidermis 



Parenchyma 



Intestinal epithelium 



Longitudinal muscle fibers 



Circular muscle fibers 



Epidermis 



Cilia 



Histological detail of a 

 portion of cross section 



Figure 70. Cross section through the pharjngeal region of a planarian. (This cross section 

 was drawn from a histological preparation provided through the courtesy of L.H. Hyman.) 



primary axis or axis of polarity is an imag- 

 inary line extending from the anterior to 

 the posterior end of the body. In the plana- 

 rian the head has a relatively high rate of 

 metabolism and dominates the rest of the 

 body. Experiments have shown that a gra- 

 dient of metabolic activity proceeds from 

 the anterior to the posterior end. For exam- 

 ple, if planarians are cut into 4 pieces, the 



anterior piece will be found to use up more 

 oxygen and give off more carbon dioxide 

 than any of the others; the second piece 

 comes next in its rate of metabolism; the 

 third piece next; and the tail piece gives the 

 lowest rate of all. Thus an axial gradient is 

 demonstrated in the metabolism of the 

 animal from the anterior to the posterior 

 end; its significance is controversial. 



