THE FREE-LIVING FLATWORMS (TURBELLARIA) 329 



If the worm is in a normal condition a delicate mechanical 

 stimulus induces a positive reaction, i.e., the animal pauses mo- 

 mentarily, then turns towards the source of the stimulus and 

 glides forward in that direction. A negative reaction is usually 

 given in response to a strong mechanical stimulus. In this case 

 the animal turns away from the source of the stimulus. The 

 positive and negative reactions are given not only in response to 

 weak and strong mechanical stimuli but to changes in tempera- 

 ture and to various chemical stimuli. The food reaction is essen- 

 tially a positive one. If food is placed in a dish where planarians 

 are gHding about, as they pass near enough to receive the stimulus 

 supplied by the juices of the tissues, they give a positive reaction 

 similar to that following delicate mechanical stimuli. This reac- 

 tion brings them to the food and as they pass over it the anterior 

 end closes over the food as if testing it. This process completed, 

 the animal moves ahead sufficiently to bring the mouth opening 

 over the food. The pharynx is extruded and the feeding process 

 begins. An interesting reaction is given where a planarian falls 

 dorsal side down, as it rights itself by forming a more or less 

 complete spiral. 



There is a constant secretion of slime over the entire body and 

 especially on the ventral surface. Irritation causes an increase in the 

 quantity discharged. The sKme layer and rhabdites probably serve 

 the purpose of protection to some extent and aid in holding the prey. 



Some Turbellaria occur in shallow quiet pools only; others 

 in larger ponds, lakes, or rivers, while a few species seem to prefer 

 swiftly flowing spring-fed brooks and streams. They are found 

 not only in all kinds of water but under varying temperature 

 conditions as well, since they may be collected during the winter 

 from beneath the ice and also are found in hot springs with a 

 temperature of 47° C. They collect on the under side of stones, 

 sticks, and leaves, conceal themselves among algae and in (lel)ris, 

 or cHng to the stems of Chara, CeratophyUunu and other h>-dro- 

 phytic plants. Certain forms are found near the surface in com- 

 paratively open water, and others in the mud or sediment at 

 the bottom of ponds or lakes. Peat bogs and swampy places 

 often furnish a large number of forms. 



