In Hirudo, Leuckart has described three similar pairs of organs, the structure of which he has fully elucidated.
They may either be provided with a peritoneal funnel (Nephelis, Clepsine) or have no internal opening (Hirudo).
In Hirudo the posterior sacculation of the stomach is primitively unpaired.
They were well known to the ancients under the name of hirudo.
In Hirudo and Nephelis the mesenteron has from the first a sack-like form.
