vorous.

In external characters the Hirudinea are unmistakable and not to be
confused with other Annelids, except perhaps with the
_Bdellodrilidae_, which resemble them in certain particulars. The
absence of setae--save in _Acanthobdella_, where five of the anterior
segments possess each four pairs of setae with reserve setae placed
close behind them (fig. 14), and the presence of an anterior and
posterior sucker, produce a looping mode of progression similar to
that of a Geometrid larva. The absence of setae and the great
secondary annulation render the mapping of the segments a subject of
some difficulty. The most reliable test appears to be the nerve
ganglia, which are more distinct from the intervening connectives than
in other Annelids.

[Illustration: FIG. 14.--_Acanthobdella_, from the ventral surface,
showing the five sets of setae (S1 to S5) and the replacing setae (Sr)
behind them. The three pairs of pigmented spots show the position of
the eyes on the dorsal surface. (After Kovalevsky.)]

In the middle of the body, where the limits of the somites can be
checked by a comparison with the arrangement of the nephridia and the
gonads, and where the ganglia are quite distinct and separated by long
connectives, each ganglion is seen to consist of six masses of cells
enclosed by capsules and to give off three nerves on each side. This
corresponds to the usual presence (in the _Rhynchobdellidae_) of three
annuli to each segment. Anteriorly and posteriorly separate ganglia
have fused. The brain consists not only of a group of six capsules
corresponding to the archicerebrum of the Oligochaeta, but of a
further mass of cells surrounding and existing below the alimentary
canal, which can be analysed into five or six more separate ganglia.
The whole mass lies in the seventh or eighth segment. At the posterior
end of the body there are likewise seven separate ganglia partially
fused to form a single ganglionic mass, which innervates the segments
lying behind the anus and corresponding to the posterior sucker. So
that a leech in which only twenty-seven segments are apparent by the
enumeration of the annuli, separate ganglia, nephridia, lines of
sensillae upon the body, really possesses an additional seven lying
behind that which is apparently the last of the series and crowded
together into a minute space. The annuli into which segments are
external