18 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



I. INTRODUCTION. 



In the fall of 1897 a small leech, which is very abundant in the ponds 

 about Cambridge, Massachusetts, was selected as an object for study in 

 the class in Microscopical Anatomy in Harvard University. This selec- 

 tion brought under my observation a rather large number of leeches 

 living or prepared in one of various ways, and gave occasion to the 

 studies out of which this paper has grown. The kindness of friends has 

 greatly aided me in obtaining material. In this connection my thanks 

 are due to Mr. G. M. Allen, who sent me living leeches from the White 

 Mountains in New Hampshire and also collected for me much valuable 

 material in Massachusetts ; to the Museum of Comparative Zoology for 

 the privilege of studying its collection of leeches ; to Professor James G. 

 Needham, who sent me collections made in New York and Illinois, and 

 also loaned me for study the collection of leeches belonging to Lake 

 Forest University ; to Dr. C. A. Kofoid, who obtained for me leeches 

 from Havana, Illinois ; to Mr. R. H. Johnson, for specimens collected in 

 Lake Chautauqua, N.Y. ; and last but not least, to Professor E. L. Mark 

 and Dr. Otto Zur Strassen, who collected and pi'eserved for me indi- 

 viduals of several European species. 



Professor Whitman, who has given so much attention to the study 

 of leeches, several years ago ('91*) pointed out the inadequacy of all 

 descriptions then existing of our North American species of "Clepsine," 

 showing that the descriptions in question were based on characters alto- 

 gether too superficial and unreliable. Whitman himself presented a 

 model in his description of " Clepsine plana ; " but as this has not been 

 followed by any similar account of our other species, I have thought it 

 worth while to record in this paper some observations of my own, to- 

 gether with the views regarding the external morphology and relation- 

 ships of our common species, to which studies, chiefly anatomical, have 

 led me. 



II. METHODS. 



For the study of the general anatomy of a leech and particularly for 

 the study of its external morphology, it is important to have both living 

 animals and those which have been killed in a good state of extension. 

 Of the former I have been fortunate enough to obtain an abundance ; in 

 preparing the latter I have found very serviceable the method recom- 



