latter the oesophagus
is thickened through its extent with muscular fibres. The oesophagus
is often furnished with glandular diverticula, the "glands of Morren,"
which are often of complex structure through the folding of their
walls. Among the purely aquatic families such structures are very
rare, and are represented by two caeca in the genus _Limnodriloides_.
It is a remarkable fact, not yet understood, that in certain
_Enchytraeidae_ and _Lumbriculidae_ the spermathecae open into the
oesophagus as well as on to the exterior. The only comparable fact
among other worms is the Laurer's canal or genito-intestinal canal in
the Trematoda. The intestine is usually in the higher forms provided
with a typhlosole, in which, in _Pontoscolex_, runs a ciliated canal
or canals communicating with the intestine. It is possible that this
represents the syphon or supplementary intestine of _Capitellidae_,
which has been shown to develop as a grooving of the intestine
ultimately cut off from it. The intestine has a pair of caeca or two
or three pairs (but all lie in one segment) in the genus _Pheretima_
and in one species of _Rhinodrilus_. In _Typhoeus_ and _Megascolex_
there are complex glands appended to the intestine.

[Illustration: FIG. 13.--Female reproductive system of
_Hyperiodrilus_.--XIII, XIV, thirteenth and fourteenth segments.

sp, Spermatheca.
sp', Spermathecal sac involving the last.
ov, Ovary.
r.o, Egg sac.
od, Oviduct.]

In _Benhamia caecifera_ and at least one other earthworm there are
numerous caeca, one pair to each segment.

_Classification._--The classifications of Adolf Eduard, Grube and
Claparede separated into two subdivisions the aquatic and the
terrestrial forms. This scheme, opposed by many, has been reinstated
by Sedgwick. The chief difficulty in this scheme is offered by the
Moniligastridae, which in some degree combine the characters of both
the suborders, into neither of which will they fit accurately. The
following arrangement is a compromise:--

Group I. _Aphaneura._--This group is referred by A. Sedgwick to the
Archiannelida. It is, however, though doubtless near to the base of
the Oligochaetous series, most nearly allied in the reproductive
system to the Oligochaeta. It contains but one family,
_Aeolosomatidae_. There are three pairs of spermathecae situated in
segments III-V, a testis in V and an ovary