THE FLATWORMS— PLATYHELMINTHES 135 



Most people use their right arm more than twice as much as their left ; 

 yet, by actual measure, there will be little difference between them be- 

 cause of this balancing- action. Otherwise, we would tend to develop in 

 a lop-sided manner so that we could not easily stand upright, not to men- 

 tion the disuption of symmetry of lines that makes a well-developed body 

 a pleasing sight aesthetically. Sometimes this action goes to undesira- 

 ble extremes ; one eye may be blinded and the other goes blind in "sym- 

 pathy" with the first ; and dentists frequently find that a decayed tooth 

 on one side of the head is duplicated by a similar decay on the opposite 

 side. 



The metazoan animals previously studied have symmetry also, but 

 of a different nature. They have radial symmetry in which the body 

 parts radiate out from a central point, such as the spokes of a wheel. 

 Such an animal can be cut in any longitudinal plane to give two halves 

 approximately alike. The cut must be longitudinal, because the two 

 ends differ and a transverse cut would yield two different halves. A 

 layer cake with icing on the top and none on the bottom would be 

 radially symmetrical and you could cut it in many ways to give two 

 halves alike as long as the cut went through the center and was in the 

 vertical plane. Hydra and jellyfish are good examples of radial sym- 

 metry, and such animals have only two directions in comparison with the 

 six mentioned for the bilateral animals. These two are usually referred 

 to as the oral and aboral surfaces which are the mouth surface and the 

 surface opposite the mouth, respectively. 



Free-living Flatworms — The Turbellaria 



This class includes the free-living flatworms which make up a com- 

 paratively small number of the species found in this phylum. We speak 

 of these as being free-living in contrast to the others which are parasites 

 in the bodies of other animals ; but. since parasitic forms tend to become 

 highly modified for their specialized existence, it is better to choose one 

 of this class as a type animal for the phylum. The little fresh-water 

 flatworm, Planaria, is a very good example. 



Planaria leads an existence that will probably seem rather dull to 

 most of you ; he spends his life crawling around under rocks and leaves 

 which lie at the bottom of ponds and streams. He has a body about a 

 half an inch long with a distinct head on one end which bears a pair of 

 eyes, but he seems to be cross-eyed because of pigment spots in the 

 inner side of both eyes. The planaria is not able to see definite images 

 with these eyes since there is no lens to focus an image, but they are light 

 sensitive. You might say that he can see about as well as you can with 



