THE FLATWORMS— PLATYHELMINTHES 



137 



body to make it thinner and longer ; and oblique muscle fibers, which 

 can contract to shorten the body obliquely and thus turn it to the side. 

 Thus, planaria is the first type animal to have a muscular system ; and, 

 since we learned in the last chapter that the muscles are formed from 

 mesoderm, this also indicates that planaria has a mesoderm, or middle 

 laver of cells, in addition to the ectoderm and endoderm. With three 

 body layers, planaria is a triploblastic animal. 



If you put a little piece of liver or other fresh meat in shallow water 

 at the edge of a pond in which there are planaria, you are likely to find 

 it covered with a mass of these animals within a few hours. If you re- 

 move them to a dish of clear water you will be able to observe digestion 

 and ingestion. Since the piece of liver is too large for ingestion as it is, 

 the planaria throws out the proboscis from its mouth, which is located 



Proboscis 

 Pharyngeal cavity 

 Epidermis 



Lumen 



Intestine 



Diverticulum 



Ectoderm 



Mesoderm 



Endoderm 



Fig. 10.2. Cross-section of planaria through region of proboscis showing the body 



parts formed by the three germ layers. 



about a third of the body length from the posterior end on the ventral 

 surface, and attaches it to the meat. The proboscis is somewhat similar 

 to the trunk of an elephant in structure and function except that it is 

 normally kept retracted inside the mouth. Digestive juice is poured onto 

 the meat through the proboscis, and the bits of partially digested food 

 are sucked up as they break off. These food particles then enter the 

 intestine which has three major branches, one anterior and two posterior, 

 with many smaller branches called diverticula. These branches carry 

 the food to all parts of the body where the cells engulf the still incom- 

 pletely digested food particles and complete digestion inside the cells, 

 thus combining extracellular and intracellular digestion as was done in 

 hydra. Small animals and smaller bits of food will be taken directly 

 into the intestine through the proboscis without this preliminary diges- 

 tion outside the body, but the planaria will usually take time to secrete 

 mucus on the food to insure its smooth passage into the intestine. 



