idium; on, external opening of
nephridium; ov, ova; t, testis.]

_Coelom._--The coelom of the Hirudinea differs in most genera from
that of the Oligochaeta and Polychaeta. The difference is that it is
broken up into a complex sinus system. The least modified type is
shown by _Acanthobdella_, a leech, parasitic upon fishes, in which
transverse sections (see figs. 15 and 16) show the gut, the nervous
system, &c., lying in a spacious chamber which is the coelom. This
coelom is lined by peritoneal cells and is divided into a series of
metameres by septa which correspond to the segmentation of the body,
the arrangement being thus precisely like that of typical Chaetopoda.
Moreover, upon the intestine the coelomic cells are modified into
chloragogen cells. In _Acanthobdella_ the testes are, however, not
contained in the general coelom, and the nephridia lie in the septa.
It is remarkable, in view of the spaciousness of the coelom, that the
funnels of the latter have not been seen. _Ozobranchus_ possesses a
coelom which is less typically chaetopodous than that of
_Acanthobdella_, but more so than in other leeches. There is a
spacious cavity surrounding the gut and containing also blood-vessels,
and to some extent the generative organs, and the nervous cord.
Furthermore, in the mid region of the body this coelom is broken up by
metamerically arranged septa, as in _Acanthobdella_. These septa are,
however, rather incomplete and are not fastened to the gut; and, as in
_Acanthobdella_, the nephridia are embedded in them. In addition to
the median lacuna there are two lateral lacunae, one upon each side.
These regions of the coelom end at the ends of the body and
communicate with each other by means of a branched system of coelomic
sinuses, which are in places very fine tubes. Neither in this genus
nor in the last is there any communication between coelom and vascular
system. In _Clepsine_ (_Glossiphonia_) there is a further breaking up
of the coelom. The median lacuna no longer exists, but is represented
by a dorsal and ventral sinus. The former lodges the dorsal, the
latter the ventral, blood-vessel. The gut has no coelomic space
surrounding it. A complex network places these sinuses and the
lateral sinuses in communication. Here also the blood system has no
communication with the sinus system of the coelom. In _Hirudo_ and the
_Gnathobdellidae_ there is only