one system of cavities which consist
of four principal longitudinal trunks, of which the two lateral are
contractile, which communicate with a network ramifying everywhere,
even among the cells of the epidermis. The network is partly formed
out of pigmented cells which are excavated and join to form tubes, the
so-called botryoidal tissue, not found among the _Rhynchobdellidae_ at
all. It seems clear from the recent investigations of A.G. Bourne and
E.S. Goodrich that the vascular system and the coelom are in
communication (as in vertebrates by means of the lymph system). On the
other hand, it has been held that in these leeches there is no
vascular system at all and that the entire system of spaces is coelom.
In favour of regarding the vascular system as totally absent, is the
fact that the median coelomic channels contain no dorsal and ventral
vessel. In favour of seeing in the lateral trunks and their branches a
vascular system, is the contractility of the former, and the fact of
the intrusion of the latter into the epidermis, matched among the
Oligochaeta, where undoubted blood capillaries perforate the
epidermis. A further fact must be considered in deciding this
question, which is the discovery of ramifying coelomic tubes,
approaching close to, but not entering, the epidermis in the
Polychaete _Arenicola_. These tubes are lined by flattened epithelium
and often contain blood capillaries; they communicate with the coelom
and are to be regarded as prolongation of it into the thickness of the
body wall.

_Gonads and Gonad Ducts._--The gonads and their ducts in the Hirudinea
invariably form a closed system of cavities entirely shut off from the
coelom in which they lie. There is thus a broad resemblance to the
_Eudrilidae_, to which group of Oligochaeta the Hirudinea are further
akin by reason of the invariably unpaired condition of the generative
apertures, and the existence of a copulatory apparatus (both of which
characters, however, are present occasionally in other Oligochaeta).

The testes are more numerous than the ovaries, of which latter there
are never more than one pair. The testes vary in numbers of pairs.
Four (_Ozobranchus_) to six (_Glossiphonia_) or ten (_Philaemon_) are
common numbers. In _Acanthobdella_, however, the testes of each side
of the body have grown together to form a continuous band, which
extends in front of external