76 PROCEEDINGS OP TUE AMERICAN ACADEMY 



V. 



CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY 



OF THE MUSEUM OF COMPARATIVE ZOOLOGY 



AT HARVARD COLLEGE. 



No. IV.-THE EXTERNAL MORPHOLOGY OF THE LEECH. 



By C. 0. Whitman. 



Communicated June 11th, 1884, by Alexander Agassiz. 



There is no invertebrate animal about which more has been written 

 than about the Medicinal Leech ; but, as Dalyell loug ago remarked, 

 " it does not appear that the history of the leech has advanced in pro- 

 portion to the number of the literati who have rendered it the subject 

 of discussion." As a considerable share of the work done in this 

 direction is purely systematic, it is somewhat surprising that not a 

 single description of any Hirudo has been given with sufficient accu- 

 racy and completeness for a close comparison of even its more impor- 

 tant external characters witli those of other species. More tliau this, 

 it would be impossible, from the many monographs, memoirs, and 

 stray papers devoted to this subject, to patch up a description that 

 would fully meet the requirements for a critical comparison of any 

 two species. By far the greater number of species-diagnoses that 

 have been showered upon us from time to time have been so super- 

 ficially and slovenly done, that it would probably puzzle the perpe- 

 trators to identify the species they profess to have described. Some 

 of the more important diagnostic characters have been either entirely 

 ignored, or given with such vagueness that they are of little service 

 in identification, and absolutely worthless for comparative purposes. 

 No uniform mode of counting the rings has been adopted ; and, judg- 

 ing from the descriptions themselves, systematic writers have, for the 

 most part, failed to place much value on the rings in the determina- 

 tion of species and genera. No one appears to have suspected the 

 existence of segmental sense-orijans in the leech ; much less the serial 

 homology of the eyes with such organs. It is tlie object of this paper 

 to make clear both these facts ; and, at the same time, to show that 

 the rings and somites form the oidy proper basis of classification. 

 The latter fact will be brought out by a comparison of a few well- 

 marked irenera. 



