172 WET GROUND COMMUNITIES 



but the water of such marshes, however, gets very warm in summer, 

 while the spring-fed marsh water is usually cool. It is the subaquatic 

 stratum which differs most. 



One of our best examples of spring-fed marsh is at Gary, 111. (Fig. 

 120). This contains watercress, which is usually associated with springs. 

 The most characteristic animals are the flatworms or planarians. Pla- 

 naria dorotocephala (Fig. 121) is common on the under sides of leaves, etc. ; 



SPRING MARSH AND IXHABITAXTS 

 FIG. 1 20. A spring marsh at Gary, III. 



FIG. 121. Planaria dorotocephala; i^ times natural size (original). 

 FIG. 122. The brook amphipod (Gammanis fascialus) ; twice natural size. 



if one puts a piece of meat into the water it will be covered with worms 

 within a short time. The worms follow the diffusing meet juices, often 

 passing through the direct sunlight, which they usually avoid. When 

 they reach the piece of meat they crawl to the under side. 



Associated with this planarian is Dendrocoelum (144), a larger, light- 

 colored species, which does not come to the meat but is found with the 

 former, under boards, chips, leaves of plants, etc. The brook amphipod 

 (Gammarus fasciatus) (Fig. 122) occurs here also. The animals of the 



