scular system and
nephridia as in Terricolae. Only one family, _Moniligastridae_.

Group IV. _Terricolae._--Earthworms, rarely aquatic in habit. Of small
to very large size. Clitellum commonly extensive and more posterior in
position than in other groups. Vascular system complicated without
regular connexion between dorsal and ventral vessels, except in
anterior segments. Nephridia as a rule with abundant vascular supply.
Testes, and occasionally ovaries, enclosed in sacs. Sperm sacs
generally limited to one or two segments with interior subdivided by
trabeculae. Sperm ducts traverse several segments on their way to
exterior. They open in common with, or near to, or, more rarely, into,
glands which are not certainly comparable to the atria of the
Limicolae. Egg sacs minute and functionless(?). Eggs minute with
little yolk. Nephridia sometimes very numerous in each segment.
Spermathecae often with diverticula.

Earthworms are divided into the following families, viz.
_Megascolicidae_, _Geoscolicidae_, _Eudrilidae_, _Lumbricidae_.

As an appendix to the Oligochaeta, and possibly referable to that
group, though their systematic position cannot at present be
determined with certainty, are to be placed the _Bdellodrilidae_
(_Discodrilidae_ auct.), which are small parasites upon crayfish.
These worms lay cocoons like the Oligochaeta and leeches, and where
they depart from the structure of the Oligochaeta agree with that of
leeches. The body is composed of a small and limited number of
segments (not more than fourteen), and there is a sucker at each end
of the body. There are no setae and apparently only two pairs of
nephridia, of which the anterior pair open commonly by a common pore
on the third segment after the head, whose segments have not been
accurately enumerated. The intervening segments contain the genitalia,
which are on the Oligochaeta plan in that the gonads are independent
of their ducts and that there are special spermathecae, one pair. The
male ducts are either one pair or two pairs, which open by a common
and complicated efferent terminal apparatus furnished with a
protrusible penis. The ganglia are crowded at the posterior end of the
body as in leeches, and there is much tendency to the obliteration of
the coelom as in that group. _Pterodrilus_ and _Cirrodrilus_ bear a
few, or circles of, external processes which may be branchiae;
_B