496 ALFRED GIBBS BOURNE. 



very constant within a genus, differs in different genera. The 

 amount of coelom present, the amount of true vascular system 

 developed, differs very greatly among the genera inter se. 

 The funnel of the nephridium may open into very different 

 portions of the system of coelomic spaces. 



The alimentary canal always presenting the same regions, 

 may differ entirely in the presence or absence of ceecal diverti- 

 cula in those regions in either group (cp. Pontobdella with 

 Clepsine, Trocheta with Hirudo). 



The two genera (Clepsine and Pontobdella) most 

 closely allied in most respects present the widest difference 

 of structure in their nephridia. 



Lastly, although the groups of the Rhyncobdellidse and 

 Gnathobdellidse are so well separated by the characters of the 

 pharynx, the presence or absence of haemoglobin in the blood, 

 the vascularisation of the pigmented connective tissue, and 

 the difference in the manner of communication between sinus 

 and vessel, yet we may have such close correspondence between 

 a single genus of the one group and the genera of the other 

 group as that mentioned above as occurring in the arrange- 

 ment of the main duct of the nephridium in Clepsine on 

 the one hand and the Gnathobdellidae on the other. 



I may repeat that it is not the variability merely to which 

 I am drawing attention but the curious distribution in its 

 amount, with regard to systems of organs or portions of such, 

 among the various genera. 



What conclusions are to be drawn from these facts ? 



They point to the persistence of the genera of Leeches 

 existing at the present day over a very long period ; in other 

 words, they demonstrate the very archaic nature of the group. 

 In the existing genera there is no series of modifications, 

 it is not possible as it is in so many groups to point to a genus 

 or genera as being more archaic than the rest, these latter 

 exhibiting serial modifications with either a progressive or a 

 retrograde tendency. We find in one genus what is apparently 

 an archaic condition of a certain system of organs, but then we 

 have to look in a widely removed genus for the archaic condition 



