THE FLATWORMS— PLATYHELMINTHES 141 



like the cilia, lining the oral groove of Paramecium, sucked food down 

 into the mouth. When seen under the microscope these beating cilia re- 

 semble the flickering of a flame — hence the name flame cells. After 

 being sucked into these cells, the waste passes into one of two excretory 

 tubes located on either side of the body. These, in turn, empty it into 

 the surrounding water through excretory pores which are very small 

 holes opening on the dorsal surface with the first pair lying directly 

 posterior to the eyes and several other pairs farther down the body. 



The extensive branching of the digestive and excretory system is nec- 

 essary in the planaria in order to carry the food to all parts of the body 

 and to bring back the excretory wastes from all regions, since there is 

 no circulatory system to take care of transportation of materials from 

 one part of the body to another. More advanced animals with blood 

 coursing all over their bodies have the digestive and excretory systems 

 localized and depend on the blood to distribute the food to the more dis- 

 tant parts of the bodies and to bring back the wastes of metabolism to 

 the excretory organs for removal and elimination. 



In hydra we found our first nervous system consisting of a network 

 of nerves without any brain, but with a concentration of nerves at the 

 oral end. In planaria this concentration of nerve tissue forms a brain 

 in the anterior end, which exercises a degree of control over the entire 

 body, thus giving a better coordination than was possible in hydra. The 

 brain has two distinct lobes with a nerve cord from each running the 

 length of the body on either side. Large nerves run across to connect 

 these two nerve cords at regular intervals and give a structure resem- 

 bling a ladder, which is frequently called a ladder-like nervous system. 

 Many smaller nerves branch off from these and run to all parts of the 

 body to pick up sensations resulting from various stimuli and to transfer 

 impulses to the muscles for the proper reaction to these stimuli. 



For instance, if we stimulate the sense organs of sight, the eyes, with 

 a strong light, the muscles respond and take the animal in the opposite 

 direction ; if we stimulate the sense organs of touch by sticking a part 

 of the body with a needle, the muscles will move it away in the opposite 

 direction ; if we stimulate the sense organs of taste by placing some food 

 in the water, the muscles will bring it to the food. 



Thus we see that some of the nerves are sensory and pick up sensa- 

 tions from sense organs to carry them to the main nerve cords and brain, 

 while others are motor and carry impulses from the brain and main 

 nerves to the muscles in order to produce the proper response to the 

 stimulation. 



When we come to study reproduction, we find that planaria has a very 

 well-developed reproductive system with many of the organs similar in 



