156 Transactions of the Boyal Society of South Africa. 
and it is quite clear that the body with suckers is easily derivable from 
Chaetopodan stock. 
The important point is that in Semilageneta further abbreviation has 
taken place in the absorption of anterior somites. 
The writer ventures to suggest the evolution of the leech body from 
a more typical Annulate condition by a modification of anterior and pos- 
terior somites is intimately concerned with the origin of the Arthropodan 
ancestors. 
In both we find a body composed of 21 somites. The posterior sucker 
of the Hirudinea is composed unmistakably of 7 somites ; but this may 
reasonably correspond to the telson of the Arthropoda. The extra somites 
of the head end in Hirudinea are not present in Semilageneta, w^here we 
still find, however, the 21 somites anterior to the posterior sucker. 
It is interesting to note that the species of Peripatus with 14 pairs of legs, 
fall in line with the other Arthropoda in regard to the number of somites. 
The reasons for suggesting this common origin for Hirudinea and 
Anthropoda are : — 
1. The constancy and similarity in number of body somites in 
Hirudinea and Arthropoda, which fact must be more than a mere 
coincidence. 
2. Annulate origin of Arthropoda. 
3. The progoneate character of Hirudinea and of the aquatic Arthro- 
poda, and the great similarity in position of the pores. 
4. The absence of a *'capula" region in Semilageneta. 
5. Possible relationship of " coelome " remains in Hirudinea and 
haemacoele in Arthropoda. 
6. The Hirudinea is the only class of existing Annualata with a 
constant body constitution. 
It is of special interest to note that by the disappearance of segmenta- 
tion in the "head" region — which has actually taken place in Semila- 
geneta — we would get a body form corresponding somitically to that of the 
ancestral Arthropodan stock in which the telson would replace the 
posterior sucker of the Hirudinea. 
Such a wonderful similarity in regard to the number of the somites 
referred to, and the existence of such a form as Semilageneta cannot be 
easily dismissed as mere coincidences. 
7. Such a common origin for the Hirudinea and ancestral Arthropoda 
would explain the unique morphological differences betv/een the Hirudinea 
and other Annulata. Further, we might well expect some specialized 
structures and habit (such as parasitism) in the case of one (Hirudinea) of 
two groups which are the result of divergence from a common ancestral 
line, when the other (Arthropoda) has taken the lead and finally developed 
into such a wide and comprehensive phylum. 
