nal orifices.
It is possible, however, that those oviducts belong to a separate
morphological category, more comparable to the dorsal pores and to
abdominal pores in some fishes. The sperm ducts are usually longer
than the oviducts; but in Limicolae both series of tubes opening by
the funnel into one segment and on to the exterior in the following
segment. While the oviducts always open directly on to the exterior,
it is the rule for the sperm ducts to open on to the exterior near to
or through certain terminal chambers, which have been variously termed
atrium and prostate, or spermiducal gland. The distal extremity of
this apparatus is sometimes eversible as a penis. Associated with
these glands are frequently to be found bundles or pairs of long and
variously modified setae which are termed penial setae, to distinguish
them from other setae sometimes but not always associated with rather
similar glands which are found anteriorly to these, and often in the
immediate neighbourhood of the spermathecae; the latter are spoken of
as genital setae.

[Illustration: FIG. 12.--Female reproductive system of
_Heliodrilus_.--XI-XIV, eleventh to fourteenth segments, sperm,
spermatheca; sp.o, its external orifice; sp.sac, spermathecal sac; ov,
sac containing ovary; r.o, egg sac; od, oviduct.]

_Spermathecae._--These structures appear to be absolutely distinctive
of the Oligochaeta, unless the sacs which contain sperm and open in
common with the nephridia of _Saccocirrus_ (see HAPLODRILI) are
similar. Spermathecae are generally present in the Oligochaeta and are
absent only in comparatively few genera and species. Their position
varies, but is constant for the species, and they are rarely found
behind the gonads. They are essentially spherical, pear-shaped or oval
sacs opening on to the exterior but closed at the coelomic end. In a
few _Enchytraeidae_ and _Lumbriculidae_ the spermathecae open at the
distal extremity into the oesophagus, which is a fact difficult of
explanation. Among the aquatic Oligochaeta and many earthworms (the
families _Lunibricidae_, _Geoscolicidae_ and a few other genera) the
spermathecae are simple structures, as has been described. In the
majority of the _Megascolicidae_ each sac is provided with one or more
diverticula, tubular or oval in form, of a slightly different
histological character in the lining epithelium, and in them is
i