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 Zodlogy 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 preserved 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- 
