Significance of the Somitic Constitution, etc., in the Hirudinea. 149 
The Hirudinea includes a large number of genera and species which 
show, on the whole, a marked homogeneity of plan. 
Perhaps the most noticeable of these common features concerns the 
constancy in the number of somites constituting the body and in the 
development of suckers, together with the absorption of a definite number 
of body somites into those suckers. The annulation of the somite seems to 
be most reasonably explained as the result of a localization of the "annuli" 
areas of the somite, bound up with the restriction of the ganglia of the 
ventral nerve chain, and the necessity for an extension of the body by 
some means other than the increase in the number of somites. 
However, the constancy in the number of body somites is so charac- 
teristic that we must regard the ancestral form as having been similarly 
constituted. 
As regards the ancestral form there can be little doubt that it was of a 
Chaetopodan nature and provided with 33 or 34 somites. 
Of these the posterior seven have throughout the group given rise to 
the posterior sucker. Eliminating Acanthobdella, the only form which has 
been suggested as being aberrant in this respect is Semilageneta ; but 
reasons are brought forward later to indicate that such aberration does not 
exist. 
Further, between the fused ganglionic masses of the anterior and 
posterior extremities there are 21 ventral ganglia, indicating a similar 
number of somites. 
In the Ichthyobdellidae the genital apertures are situated at the base 
of the neck. The constitution of the neck and body regions is well shown 
in such a form as Pontobclella macrotJiela : — 
Annuli. Somite. 
Neck I I-IS 
(16 } 
/17 -V" 
118 I 
Trunk ^19-51 " vii-xvii " 
1 52-53 " xviii " 
-54-55 " xix or xix, xx " 
These somites are reckoned independently of the capula. This latter 
structure certainly represents a number of fused somites, but the annula- 
tion is not distinct. At the present stage I am not prepared to accept any 
of the constitutions of this anterior region of the body as laid down by 
various authorities until I have again investigated the nervous system of 
these parts. At the same time I feel justified in venturing an opinion 
based on the somitic constitution of the genus Ozobranchus. In 0. bran- 
chiatus (Hiriido hranchiata, Menzies, 1791), which occurs as a parasite on 
Chelone midas, the neck region of young individuals consists of eight (8) 
