OF ARTS AND SCIENCES. 



85 



genera. The abbreviation at the anterior end is the same in both, 

 except that the 4th ring of the aqnatic leech has been dropped in tlie 

 huul leech. The loss of this ring brings the fourth pair of eyes into 

 close order with the first three pairs, and the nephridial pores and sex- 

 ual orifices one ring farther forward. At the posterior end (Fig. 3), 

 we find only four rings at most to offset the last nine rings in Hirudo. 

 The occurrence of segmental papillai on each of these four rings, with 

 jierhaps the exception of the last, enables us to identify them, and to 

 say precisely which rings have been lost since this leech abandoned its 

 acjuatic life. The rings may be identified as follows : — 



93d ring (IIa;madi[)sa) = Oith ring (Hirudo). 

 94th " " = 97th " 



95th " « = 99th " 



9Gth " " = 101st " 





The lost rings are the 4th, 95th, 9Gth, 98th, 100th, and 102d. 



Ilasmopis is almost an exact copy of Hirudo iu all the details of 

 abbreviation ; and scarcely deserves to be called a distinct genus. 

 Aulostoma differs from Hirudo by a slightly less advanced abbrevia- 

 tion of its 23d somite, and in some other particulars that need not be 

 mentioned here. 



The degree of abbreviation is a key to the phylogenetic relationship 

 of species and genera. The ten -eyed leeches have descended from a 

 common form, having twenty-six somites, of which at least eighteen 

 were complete. Of the genei-a mentioned in this paper, three may be 

 said to liave branched from the archaic form, Hirudo representing the 

 main branch, and Macrobdella and Microstoma diverging branches. 

 From Hirudo have arisen the three secondary branches, represented 

 by Hiemadipsa, Ha^mopis, and Aulostoma. 



Hteniadipsa. 



Aulostoma. 



Haemoiiis 



Macrobdella 



Microstoma. 



Archaic form. 



