Significance of the Somitic Constitution, etc., in the Hirudinea. 155 
Such a reduction of the coelome by a dilatation of the blood system was 
discovered by BenhacQ in the Polychaete genus Magelona. In the Hiru- 
dinia the so-called coelome is represented either by sacs lined by a de- 
grading epithelium as in Clepsine and Herpobdella, or by sinuses as in 
Gnathobdellida, and these spaces carry blood. The interesting feature in 
connection with these structures is that they communicate with the blood 
system — a unique relation of coelome and blood system. Although the 
evidence very strongly supports the idea that these sinuses are the 
remains of the coelome, and they are accepted by most workers as such, 
it is just possible, as suggested by Lankester, that they may not be of 
coelomic origin. 
Such a suggestion is due to the peculiar relations of the sinuses to the 
blood system. 
If not of coelomic origin they would then be explained as parts of the 
blood system itseif, and thus we would get an approach towards a haemo- 
coele in the Hirudinea. 
If the suggestions made later in regard to the somitic relations of the 
Arthropoda and Hirudinea be correct, then we may be sure that there is a 
close relationship between their haemocoele and sinuses respectively. 
If we now assume that the Arthropoda are descendants of an Annulate 
stock with a constant number of somites, can we find any existing Annu- 
lata manifesting the same somitic constitution ? 
The only division of the group having a regularly constituted body is 
the Hirudinea, and we now proceed to demonstrate an agreement between 
them and the Arthropodan stock. 
In the leech body 33 or 34 somites exist, and of these 21 are con- 
tained in a region lying between the head or capula and the posterior 
sucker, and are represented in the nervous system by the 21 ganglia lying 
between the complex anterior and posterior extreme ganglionic masses. 
In Semilageneta, as pointed out previously, only 21 segments are repre- 
sented altogether in front of the posterior sucker. 
In the Herpobdellida and Gnathobdellida the oval sucker is confluent 
with, and grades into, the body region, forming a "head end." In most 
Glossophoniida the same is the case, but in a few there is a slight expan- 
sion of the sucker region. 
In most Ichthyobdellida there is a distinct capula formed by the 
sucker, and between this and the posterior sucker 21 distinct somites are 
to be made out. 
It is apparent, then, that the region of the leech body between anterior 
and posterior suckers consists of 21 somites, the posterior sucker of 
7 somites, and the head or capula of 5 or 6 somites; further, that in 
Semilageneta the anterior 5 or 6 have been absorbed. 
The 33 or 34 somites of the leech body are early laid down in ontogeny, 
