176 GENERAL ZOOLOGY 



therefore, unnecessary to have a separate circulatory system, 

 and none is present. Planaria does have a very elaborate 

 excretory system, however, which consists of a branching 

 tube on either side of the body. The branches end in 

 "flame cells;" so named because each is hollow and has a 

 bunch of cilia within which resemble the flickering of a 

 candle, when moving. The flame cells force waste prod- 

 ucts down the tubes and ultimately out of the body through 

 two little pores on the dorsal surface. 



Self -protection. Planaria escapes most of its enemies by 

 hiding. Its eyes are of value in enabling it to shun light 

 and seek dark crevices, but the whole surface of the body 

 is also sensitive and a planarian can respond nearly as well 

 after the eyes have been removed. 



If Planaria is captured and half eaten by some predaceous 

 enemy, it is not a very serious matter, for either half of the 

 body is able to regenerate what it may lack and after a 

 time becomes a complete individual again. During such 

 reconstruction no food is taken as a rule, but the body is 

 reorganized and forms a complete new worm from the sub- 

 stance already within it; the size, of course, being smaller. 

 Some species of planarians can regenerate a complete body 

 from about one twenty-fifth of the original, but there is a 

 definite limit and most species show polarity in their bodies. 

 Polarity in this sense means that certain regions are special- 

 ized and can regenerate only particular parts. For ex- 

 ample, if the head of Planaria is cut off just behind the eyes, 

 the anterior piece will not grow a new body but grows 

 another head and becomes a two-headed monster (Fig. 73). 

 A small portion from the opposite end will likewise form a 

 double-tailed individual. Such behavior shows that the two 

 ends of the body are so specialized that they can only form 

 parts like those they already possess. 



Race Preservation. Planaria takes advantage of its 

 remarkable powers of regeneration to produce new individ- 

 uals. A pair of eyes, a proboscis, and other necessary 

 organs are often formed in the posterior half of body, which 



