76 AUSTRALIAN HIRUDINEA, VI. 



that seen iu Phihemon, with its tetranuulate somite; or again, by 

 adding another annulus to the somite of Philcemon the pent- 

 annulate somite of Geobdella would be obtained; but the position 

 of the sensory annulus would not correspond in both. 



It must, of course, be borne in mind that although the number 

 of annuli in the whole body is different in the two genera — 79 in 

 Philcemon, 95 in Geohdella — and the peculiar positions of the 

 genital apertures in Geobdella are of great importance, tiieir 

 anatomy agrees very closely. 



The total number of annuli in the body is dependent on the 

 fact that one is pentannulate, and the other tetranuulate, and 

 this may explain to some extent also the peculiar relative- 

 positions of the genital apertures in Geobdella, which at first 

 would seem to be of such great importance. 



In view of what I have stated in connection with metamerism' 

 and the importance of the order of origin of the annuli in 

 discussing genetic relationships of leech-forms ; and taking into- 

 consideration what I have stated as conclusions to be drawn 

 from a study of the distribution of these two forms as a reflection 

 of their ai'chaic nature; seeing that the order of origin of the 

 annuli is so different in these two forms; I conclude that they 

 have been derived from a common ancestor which agreed very 

 closely with them in regard to the jaws, position of the eyes, and- 

 general anatomy, l)ut which, at the time these two geuera were 

 differentiated, had not developed a somite of more than two 

 annuli. 



In conclusion, I may state that the remarks which have been 

 made in this paper in regard to the conclusions to be drawn 

 from the distribution of our terrestrial Hirudinea in regard to 

 zoogeographical schemes, are in keeping with those which the 

 distribution of Monotremes, Marsupials, and Peripatus, etc., 

 has long since justified. In this direction I have, then, merely 

 added corroborative evidence from a study of the Hiriodinea 

 themselves, and have hopes that I have conclusively pointed out 

 that the terrestrial members in general of the Hirudinea serve- 

 as good types to l>e considered in connection with a study in- 

 zoogeography. 



