136 



THE FLATWORMS— PLATYHELMINTHES 



your eyes closed. You can still detect changes in light intensity with 

 your eyes closed and, therefore, become aware of movements which cause 

 variations in this intensity. Down under the rocks at the bottom of a 

 stream there is not much light to see by anyway, so this lack of visual 

 acuity is of no great consequence in the life of planaria. 



If you watch one of the animals moving it will appear to be gliding 

 along over a surface without any noticeable effort, such as the body con- 



Photo by Winchester 



Fig 10 1. Planaria, a free-living flatworm. The individual on the left has the pro- 

 boscis retracted and it can be seen faintly near the center of the body. The individual 

 on the right has extended the proboscis. Some of the branches of the intestine show 

 near the anterior end of the animal on the left. 



tractions that characterize the crawling of a snake. This is possible be- 

 cause the body is covered by microscopic cilia which beat to propel the 

 animal forward in a thin layer of mucus which is secreted by the body 

 as it moves along. You will also notice other movements from time to 

 time as the body shape changes and the head is turned from side to side 

 as if it were feeling its way as it goes. Such movements are made by 

 the three types of muscles which are found in planaria. These consist 

 of longitudinal muscle fibers, which can contract to shorten the body and 

 make it fatter ; circular muscle fibers, which can contract to constrict the 



