e leech the two branches are fused into one. We have here clearly a
case of a true coelomoduct performing the function of an oviduct in
both leeches and _Eudrilidae_. The facts just referred to suggest
further comparisons between the Hirudinea and _Eudrilidae_. The large
sacs which have been termed vagina are suggestive of the large
coelomic spermathecae in Eudrilids, a comparison which needs, however,
embryological data, not at present forthcoming, for its justification.
It is at least clear that in _Ozobranchus_ this comparison is
justifiable; but only probable, or perhaps possible, in the case of
_Philaemon_. In the former, the duct, leading from the ovarian sac,
and swelling along its course into the spherical sac, the
"spermatheca," is highly suggestive of the oviduct and receptaculum of
the _Eudrilidae_.

The testes during development become hollowed out and are prolonged
into the vasa efferentia. These ducts therefore have not their exact
counterparts in the Oligochaeta, unless we are to assume that they
collectively are represented by the seminal vesicles of earthworms and
the vasa deferentia. It is to be noted that the Hirudinea differ from
the Oligochaeta in that the male pore is in advance of the gonads
(except in _Acanthobdella_, which here, as in so many points,
approximates to the Oligochaeta), whereas in Oligochaeta that pore is
behind the gonads (again with an exception, _Allurus_).

_Classification_.--The Hirudinea may be divided into three families:--

(i.) _Rhynchobdellidae_.--A protrusible proboscis exists, but there
are no jaws. The blood is colourless. _Pontobdella_, _Glossiphonia_,
&c.

(ii.) _Gnathobdellidae_.--A proboscis absent, but jaws usually
present. Blood coloured red with haemoglobin. _Hirudo_, _Nephelis_,
&c.

(iii.) _Acanthobdellidae_.--Proboscis present, but short. Paired setae
of Oligochaetous pattern present in anterior segments. Blood red.
_Acanthobdella_.

LITERATURE.--A.O. Kovalevsky, _Bull. Imp. Sci._ (St Petersburg,
November 1896) (_Acanthobdella_); A.G. Bourne, _Quart. Journ. Micr.
Sci._, 1884; A. Oka, _Zeitschr. wiss. Zool._, 1894; E.S. Goodrich,
_Quart. Journ. Micr. Sci._, 1899; W.E. Castle, _Bull. Mus. Comp.
Zool._, 1900; A.M. Lambert, _Proc. Roy. Soc._ (Victoria, 1897); C.O.
Whitman, _Journ. Morph._, 1889 and 1891; O. Burger, _Zeitschr. wiss.
Zool._, 1902, and other memoirs by the above, and