8 ILLINOIS BIOLOGICAL MONOGRAPHS [202 



Although in general the Turbellaria are free-living forms, a few cases have 

 been reported in which they are parasitic on molluscs and it is in these species 

 that a beginning has been made in the study of the embryology and life-history. 

 Two writers, Linton (1910) and Stanley C. Ball (1916), have published such 

 investigations of a species living on Modiolus. 



The Turbellaria of the central and western states are almost unknown. 

 Woodworth (1897) and Stringer (1909, 1913) have published descriptions of 

 five new species from IlUnois and Nebraska. Eleven other forms, found also 

 in the eastern states, have been reported from a few localities in IlUnois, Neb- 

 raska, Michigan, and Wisconsin, but these are all. That some species, at 

 least, are present in great numbers is evident from the various collections for 

 experimental studies. The whole Mississippi valley region with its lakes and 

 almost numberless ponds furnishes conditions in many respects ideal for such 

 types, and their existence will sooner or later be demonstrated. 



In fact, both Planaria and Rhabdocoelida are of much more common occur- 

 rence than is generally supposed and for several reasons are especially valuable 

 for study. First, they are available as living reproducing animals thruout the 

 year. Then, they are good for experimental work since they illustrate a some- 

 what primitive phylum, one in which the body structure is very different from 

 that commonly encountered. Well adapted to such studies by their simplicity 

 and great vitality, they have come to be the classic specimens employed both 

 for elementary experiment and for complicated investigation. The free-living 

 method of life, together with the simple fundamental anatomy, when compared 

 with the specialized and much changed condition of the nearly related parasitic 

 worms, shows clearly the variations possible and actually brought about by a 

 different mode of life. As a whole, the group is of more than ordinary interest. 



Always considered of no importance economically, their possible relation 

 to other forms has been overlooked, so that only by chance have their habits of 

 parasitism and voracious feeding been discovered, and there is need of further 

 investigation along this line. 



In the present paper is reported a brief study of a few forms found in small 

 lakes and ponds in Illinois and Iowa. This work covers three phases: First, 

 a summary and comparison of the types of habitat which deals with such en- 

 vironmental factors as flora, whether algae or higher water plants, animal com- 

 munities, whether few or many, bottom, whether sandy or muddy, and also 

 the general water conditions; second, a few of the biological aspects, this out- 

 line comprising a study of the reactions to the various stimuli; third, a mor- 

 phological description of eight new species and additional data concerning 

 fifteen others. These descriptions embrace to some extent histological struc- 

 ture as well as gross anatomy, and a number of points regarding the distribu- 

 tion and characteristic variations of well-known species. In the case of one 

 form a brief resume of the embryological stages is also given. In the plates 

 are included, toto drawings of all the species, besides separate sketches of iso- 

 lated parts. 



