ly divided are so deeply incised as to render it impossible to
distinguish, as can be readily done in the Oligochaeta as a rule, the
limits of an annulus from that of a true segment. As remarked, the
prevalent number of annuli to a segment is three in the
_Rhynchobdellidae_. But in that group (_Cystobranchus_) there may be
as many as eight annuli. In the _Gnathobdellidae_ the prevailing
number of annuli to a segment is five; but here again the number is
often increased, and _Trocheta_ has no less than eleven. The reason
for this excessive annulation has been seen in the limited number of
segments (thirty-four) of which the body is composed, which are laid
down early and do not increase. In the Oligochaeta, on the other hand,
there is growth of new segments. It is important to notice that the
metameric plan of growth of Chaetopods is still preserved.

The nephridia are like those of the Oligochaeta in general structure;
that is to say, they consist of drain-pipe cells which are placed end
to end and are perforated by their duct. The internal funnel varies in
the same way as in the Oligochaeta in the number of cells which form
it. In _Clepsine_ (_Glossiphonia_) there are only three cells, and in
_Nephelis_ five to eight cells. In _Hirudo_ the funnel is not pervious
and is composed of a large number of cells. Externally, the nephridium
opens by a vesicle, as in many Oligochaetes whose lumen is
intercellular. In _Pontobdella_ and _Branchellion_ the nephridia form
a network extending from segment to segment, but there is only one
pair of funnels in each segment. Slight differences in form have been
noted between nephridia of different segments; but the Hirudinea do
not show the marked differentiation that is to be seen in some other
Chaetopods; nor do the nephridia ever acquire any relations to the
alimentary canal.

[Illustration: FIG. 15.--Section of _Acanthobdella_ (after
Kovalevsky).

c, Coelom.
c.ch, Coelomic epithelium (yellow-cells).
cg, Glandular cells.
cl, Muscle cells of lateral line.
cp, Pigment cells.
ep, Ectoderm.
g, Nerve cord.
m, Intestine.
mc, Circular muscle.
ml, Longitudinal muscle.
vd, Dorsal vessel.
vv, Ventral vessel.]

[Illustration: FIG. 16.--Section of _Acanthobdella_ (after
Kovalevsky). Identical letters as in fig. 2; in addition, cn, nerve
cord; in, intestine; nf, parts of nephr