152 Transactions of the Boyal Society of South, Africa. 
exactly to that portion of the IchthyobdeUid leech lying between the 
capula and posterior sucker. It seems, then, more reasonable to conclude 
that the capula region has been absorbed to form the prostomium of 
Semilageneta in which a postomial region is quite distinct anterior to the 
first somite. 
In Acanthohdella ijeledina, which has been thoroughly investigated by 
Livanow, the apertures are found between somites ix and x, and in x 
respectively. Further, the number of somites visible externally is 
reckoned as twenty-nine potentially by Livanow, and the eyes and setae, 
with the exception of the last groups of the latter, are placed on the first 
annulus of the somite. The last group of setae and the sensory organs of 
the body region, that is that portion lying behind the fifth somite, are 
situated on the second annulus of the somite. 
It would, then, seem that in Arhynchobdellidae the male and female 
pores are situated in somites x and xi respectively, whereas in the 
Ehynchobdellidae they are situated in xi and xii, that is one somite behind 
the corresponding pore in Arhynchobdellidae. 
All workers on the Hirudinea concede that the number of somites is 
constant in all representatives of the group, although they fall into one or 
the other school of thought as to whether that number is thirty-three or 
thirty-four, the main point of disagreement being the number of somites 
represented in the head region. 
Now, it is interesting to note that the somites visible externally, that 
is, those anterior to the posterior sucker, are readily reckoned as twenty- 
seven (27) in Glossiphoniidae, and as twenty-six (26) in Arhynchobdellidae. 
This would seem to indicate that the difference by one somite in the 
position of the genital apertures is bound up with the difference by one of 
the number of somites visible externally, especially so as all workers will 
concede that seven (7) somites are potentially represented in the posterior 
sucker of the Glossiphoniidae and Arhynchobdellidae. This being the 
case, we may suggest that the position of the pores are identical in these 
groups, and that the pores are situated in somites x and xi or in somites 
xi and xii respectively. 
Again the genital apertures, whether they be regarded as situated in x 
and xi or in xi and xii, are always to be found in the seventeenth and 
sixteenth somites respectively in front of the posterior sucker in 
Gnathobdellida, Herpobdellida, Glossiphoniidae (other than Semilageneta) 
and Ichthyobdellida. This certainly suggests a common position for the 
genital apertures. 
Before passing on to consider the significance of the preceding 
remarks, it is to be noted that the Hirudinea show a remarkably 
constant progoneate character and appear to be an ancient compact 
group. 
