Significance of the Somitic Constitution, etc., in the Hirudinea. 151 
and third annuli of somite xii. In regard to the male orifice, we can sub- 
divide the various representatives into three groups in which respectively 
this pore is placed {a) bet wee q somites xi and xii {b) between annuli 1 and 
2 of somite xii, and (c) between annuli 1 and 2 of somite xii and coincident 
with the female pore. In group [a) the male and female pores are in 
consecutive somites and separated by two annuli ; in group (b) they 
are situated in the same somite and separated by one annulus. From the 
fact that in practically all known members of the Hirudinea the orifices 
are situated in consecutive somites, we can safely conclude that the 
primitive condition in the Glossiphoniidae was that now indicated in 
group (a) ; that is, the male pore in somite xi and the female pore in 
somite xii. This conclusion is based to a large extent on the affinities of 
these three groups of triannulate genera, and receives special support from 
the fact that in the biannulate genus Microbdella the pores are similarly 
situated, and furthermore are separated by two annuli as in group (a). 
In the Herpobdellidae the genital apertures are very constant in 
position, lying in somites x and xi respectively. 
In the Gnathobdellidae they lie in a similar position. We have now 
to consider a few genera which show a marked contrast to the other 
Hirudinea in regard to the genital apertures. 
In the genus Semilageneta (mihi), which I originally placed pro- 
visionally in the Glossiphoniidae, there is a great " reduction " in the 
number of body somites. Unfortunately, owing to the loss of the specimens 
before any of the internal anatomy had been investigated, I had to limit 
my remarks on the somitic constitution of the body to the external 
morphological characters. 
Only one genital aperture was made out, namely, on the 21st annulus, 
that is on the last annulus of somite viii. At the time I placed this genus 
in the Glossiphoniidae I felt that the anterior region differed so much from 
that of all other members of the G., and further that the small number of 
somites visible externally was so unique that I anticipated the removal of 
the genus when it should have been investigated anatomically. 
Again, the somitic constitution was so plainly made out externally that 
I was able to show conclusively that, unlike all other members of the 
Glossiphoniidae, this genus had the sensory and primitive annulus of the 
somite anterior to the non-sensory annuli. This again is exceptional, 
since in most Hirudinea the sensory annulus is either the middle or 
potentially middle annulus of the somite. 
It was at first suggested that in this case, since only twenty or 
twenty-one somites were represented in the body region, the remaining 
somites were all to be found in the posterior sucker, that is, thirteen or 
fourteen instead of seven. Such is hardly likely, when it is borne in mind 
that the somitic constitution of the body region in Semilageneta corresponds 
