207] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 13 



which project up from the bottom or out into the water or which lie against 

 each other so that the under side is not buried. The stones are more or less 

 rough, covered with tiny crevices which make good hiding places, the worm 

 being almost below the surface and thoroly protected but still able to make 

 use of the swift current of fresh water. Collections were made in several 

 other localities upstream and within the limits of the rocky bed, but no plan- 

 arians were discovered. 



A situation similar to the above is found in the Salt Fork of the Vermihon 

 river at Homer, Illinois, where for a distance of a few hundred yards there are 

 many rocks and stones in the stream bed. The water runs sluggishly every- 

 where except at one point, and here there is always a swift current from the 

 overflow of a dam. This is an ideal locality and living upon the rocks over 

 which the water is continually pouring are many planarians, the only ones 

 to be found either up or down stream. The Sangamon is another stream 

 of this same type, generally slow-flowing with few or no stones. Scattered 

 planarians were found clinging to rocks at one place where a Httle fall gave 

 rise to a swifter current for a short distance. 



The larger rivers in this region with sandy or muddy bottom have, as a 

 rule, a fairly even shore-line with few small coves or indentations, while a 

 strong current generally keeps the banks washed clean of any driftwood or 

 riff-raff which might lodge masses of organic debris and thus offer shelter to 

 microscopic forms. On the side of the river where the current is undermining 

 and changing the shore, no life of any sort is possible, but on the beach side, 

 many clams, snails, and insect larvae find a feeding ground. These, however, 

 are able to anchor themselves firmly in sand, while such t}^es as flatworms 

 and small crustaceans cannot exist. Altho there have been at different times 

 many collections made by various workers and at numerous points along the 

 Mississippi, rhabdocoels and planarians have not been reported. This absence 

 seems to be easily explained since the conditions in the river are very different 

 from those of any of its tributaries. Forms carrying on a thriving existence 

 a few yards up some smaller stream meet with entirely new enemies in the 

 main river and encounter there a powerful current which quickly tears them 

 away as soon as they enter this expanse of water. 



In the main branches of the Iowa a hke situation prevails, but here all along 

 the course there are numerous small ponds left by its periodic overflow. This 

 backwater is generally protected by growths of willow and swamp maple, 

 and may receive enough drainage from surrounding fields to last thruout the 

 season. The plant and animal life is often very profuse and many Stenostoma 

 species have been taken at different times from such ponds. 



Another type of stream whiich often becomes a suitable location for many 

 microscopic species as well as larger forms, is the artificial ditch or canal. Here 

 the water generally flows slowly and comparatively evenly, there is little danger 

 of flood, and many types gain a foothold. The Hennepin canal presents con- 

 ditions rather unique in these respects. The banks are riff-raffed with rough 



