SECTION I 

 INTRODUCTION 



The present report is a review of the species and subspecies of the 

 North American freshwater triclads or planarians known at the present 

 time. It should not be considered to give a definitive picture of 

 the planarian fauna of the geographic area covered, north of Mexico, 

 as many parts of the continent are still little explored with regard 

 to lower aquatic invertebrates. This is particularly true for large 

 sections of the West and South of the United States and the greater 

 part of Canada. 



It was considered appropriate not to limit the review to a simple key. 

 When attempting to identify a given species through a key, one ends up 

 with a specific name which one must accept on good faith. Should one 

 happen to have an undescribed species at hand, it would, in most cases, 

 key out to a known species. In order to avoid this pitfall, every 

 species is treated separately, its distinguishing external appearance 

 and anatomical features are described, and the principal literature 

 sources which may lead to a more detailed description of its morphology 

 and ecology are listed. In this way it should be possible to avoid the 

 uncertainties inherent in the use of simple dichotomic keys and to have 

 a means of either confirming or questioning the correctness of the 

 identification. 



In addition to the valid names of the individual species, all synonyms 

 which have been used for the American planarians are listed in the 

 species descriptions, giving the name of the author and the date of 

 the first publication of each synonym. 



Planarians have played a certain role in the biological assessment of 

 water quality in Europe. Some species have been found to be very sen- 

 sitive to organic and inorganic pollution of their habitats while others 

 tolerate mild degrees of pollution. In general it may be said that 

 planarians are intolerant to the presence of heavy metal salts in the 

 water. Little has been done in the study of American turbellarians with 

 regard to their behavior toward pollutants. The American fauna of 

 planarians differs from that of Europe, and no native species is conmion 

 to both continents. One American species was introduced in Europe at the 

 beginning of this century, and one imported European species was dis- 

 covered recently in North America. It is, therefore, not possible to 

 apply the results obtained by European workers to the American planarian 

 fauna without detailed field observations. 



It is hoped that the present report will be useful in any future system- 

 atic investigation using the presence or absence of planarians in the 

 evaluation of water quality in North America. 



