275] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 81 



CONCLUSIONS AND SUMMARY 



Of the sixteen families which make up the Rhabdocoela, representatives 

 of five have been found in the United States, Up to the present time, about 

 eighty species have been identified from the eastern states but only sixteen 

 from the central region. To this list of sixteen can be added, as a result of this 

 study, eight new species and new locahties for fifteen known forms. From the 

 collections just made it appears that a great many forms found along the sea- 

 coast and in the border regions are also present far inland. The biologic condi- 

 tions in the lakes and streams of the Mississippi Valley are not widely different 

 from those of the ponds and rivers of the eastern slope and evidently the same 

 species are to be looked for in both regions. When, however, such a form as 

 Microstoma sensitivum, which is present in the brackish water of the open shore in 

 Massachusetts, appears again in the small ponds of New York and then is found 

 in isolated puddles in Ilhnois, some note must be taken of its power of adapta- 

 bihty, and some consideration must be given to the fact that a species so fra- 

 gile and dehcate may floiuish in situations far removed from each other and of 

 such different conditions. Other examples of a like nature are many, and it 

 seems not too presumptuous to conclude that while few North American spe- 

 cies are identical with those of Europe, the forms within the United States 

 not separated by more than a few thousand miles will prove to be identical. 



Another point of importance in connection with the distribution is the 

 amount of variation found in individuals of the same species living under 

 different habitat conditions. This was noted long ago, in the difference in 

 average size among specimens of Mesostoma ehrenbergii from Europe and from 

 the United States and in the differences in both size and shape of Rhyncho- 

 mesostoma rostratum from the two coimtries. Then, too, many minor variations 

 are exhibited among individuals evidently of the same species. This possibility 

 of change is apparent in nearly every one of the old species found for the first 

 time in the central localities. These variations are not of either histological 

 or of gross anatomical structure, but are rather in the relative and comparative 

 development of the several body organs. For example, the eyes may be larger 

 or smaller or different in position in specimens from one pond than in those 

 from another. Prorhynchus applanatus and Typhloplana viridata are other 

 striking instances of minor changes. The specimens of the former from Ur- 

 bana show a very much shorter posterior portion with a more rounded tail than 

 is given in the original description; also the intestinal diverticula are not 

 regular and straight but pointed and curved. In Typhloplana viridata 

 the differences are very marked. The shape of the head region instead of being 

 pointed is nearly as broad as any part of the body. Then, the green color is 

 diffused thru the parench5Tna fluid and not held in the color bodies alone. 

 These examples are sufficient to show the kind of variations common among the 

 forms of the inland ponds. 



