212 TWE ANIMAL KINGDOM 



Figure 1 1 .9. Polarity and regeneration in Dugesia. Top left, each of five pieces re- 

 generates, but the rapidity with which the head develops depends upon the level of the 

 piece. Lower left, occasional errors that occur, and an example of changed polarity. 

 Lower right, preservation of polarity depends upon whether or not the piece bends. 

 Upper right, a two-headed form produced by repeated splitting of the anterior end. 



for the tendency to torm a tail. Such gradients predict that any piece 

 of the worm will regenerate so as to retain its original polarity. 



Polarity can be altered. If a triangular piece is cut from the side 

 of the body (Fig. 11.9), it usually regenerates a head at the inner end, 

 forming a tail from the lateral edge. A strip cut from the side of a 

 worm will regenerate normally if it remains straight, but if it bends 

 the head appears on the inner side. 



Monsters can also be produced. If a worm is partially split (Fig. 

 1 1.9), and the split is kept open by continual recutting, the worm will 

 eventually regenerate so as to produce some double structures. Many of 

 these monsters eventually solve their problems by splitting up and de- 

 veloping into several worms. If a two-headed worm is produced, for 

 example, the split gradually deepens until the worms separate as two 

 complete individuals. 



78. Class Turbellaria 



The platyhelminthes are divided into the three classes given at the 

 beginning of the chapter. The Turbellaria are characterized by the pres- 

 ence of a ciliated epiclermis, which is not found in any adults of the other 

 two classes. 



Turbellarians are divided into a number of orders (Fig. 11.10), 

 according to the branching of the intestine. Planarians belong to the 



