138 



COLLEGE ZOOLOGY 



Epidermis 



Nucleus 



Light-sensitive 

 cell 



Pigment cell 



Pigment granules 



Nerve to brain 



Figure 71. Section through eye of the planarian. The pigmented cells form a cup which 

 insulates the light-sensitive cells within it against all light except that which enters through the 

 open side of the cup, as shown by the arrow. Since the direction of the light determines which 

 light-sensitive cells arc stimulated, the planarian can determine the direction from which the 

 light comes. No visual image is possible with such an organ. 



Figure 72. Planaria. Diagrams illustrate stages in the process of regeneration. A, B, specimen 

 cut into two parts; the head (A) regenerates another tail (dotted) (Ai) and finally regains 

 its normal shape (A2); B regenerates another head (B^) and lengthens into a normal specimen 

 (B2); C, a spht head, regenerates two heads (Ci). 



OTHER PLATYHELMINTHES 



Flatworms differ greatly among them- 

 selves due largely to the fact that the Turbel- 

 laria are for the most part free-living, 

 whereas the Trematoda and Cestoda are all 

 parasitic in habit. Turbellarians probably 

 exhibit the typical organization of the phy- 



lum, the trematodes and cestodes being 

 modified considerably for a parasitic exist- 

 ence. The epidermis is ciliated in the turbel- 

 larians, but in the trematodes and cestodes 

 there is no epidermis; they are covered with 

 a thick cuticle. Sense organs are probably 

 present in all. 



In the Turbellaria and Trematoda there 



