201] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 



INTRODUCTION 



Although commonly little known, the Turbellaria as a class present certain 

 characters of morphology and Ufe-history which make them of especial interest. 

 Furthermore, their biologic processes and Ufe habits in relation to other forms, 

 shed some light upon general problems of common existence, A knowledge 

 of any group of animals is of value from a phylogenetic standpoint and also 

 in the explanation of the interdependence among various types. So a study 

 of this group, which stands out as a connection between the very lowest phyla, 

 the aberrant parasitic worms, and the more speciahzed higher types, makes 

 possible an understanding of one of the steps in the evolution of the animal 

 kingdom, namely the transition from radial to bilateral symmetry. The 

 simplicity of anatomical structure and lack of conspicuous detail, thus especially 

 important in a comparative way, also enable these forms to carry on an exis- 

 tence almost unsuspected and generally entirely overlooked. The fact of 

 their widespread distribution in both salt and fresh water passes, for the most 

 part, unnoticed. 



While the fresh water Turbellaria of Europe have been recognized and care- 

 fully studied for nearly one hundred and fifty years, the American species have 

 received little attention. The earliest records for this country are those of 

 Leidy. In 1848, he gave the first brief description of the now well-known and 

 very common Planaria maculata. He also studied rather completely Phago- 

 cata gracilis, distinguishing the fundamental points in which it differed from 

 Planaria. Later (1852) he added several other species to the Ust. No otheir 

 studies were reported until a paper by Silliman appeared in 1885. This was 

 also descriptive and systematic in character. The first morphological work 

 was that by Ott (1892) who made the common Stenostoma leucops O. Sch. the 

 basis of a detailed histological study. Woodworth (1896, 1897) next pub- 

 lished results of collections made from rather widely separated localities, viz., 

 Massachusetts and Illinois. He named four new species and added full 

 descriptions of twelve others. 



Since 1900 there have appeared a number of papers, a few descriptive, but 

 by far the most the results of experimental work. Of the first type, the paper 

 by von Graff (1911) is noteworthy. In this, he gives descriptions and notes 

 of some seventy-five species. Several other writers have from time to time 

 added a few forms, but this is all. On the other hand, the more common species 

 have been used in a vast amount of experimental work. Child's work with 

 the common flatworm has explained a large number of facts of far-reaching 

 general biologic significance. Other investigations, some of them exhaustive, 

 have dealt with the more primitive rhabdocoels as well as with planarians. 

 Among the many writers along this line are Kepner (1911, 1915), Curtis (1905), 

 and Pearl (1903). • 



