ition in the group. Rarely there is an
irregular disposition of the setae which are not paired, though the
total number is eight to a segment (fig. 10), e.g. _Pontoscolex._ The
varying forms of the setae are illustrated in fig. 11.

[Illustration: FIG. 11.--Setae of _Oligochaeta_.

a, Penial seta of _Perichaeta ceylonica._
b, Extremity of penial seta of _Acanthodrilus_ (after Horst).
c, Seta of _Urochaeta_ (Perier).
d, Seta of _Lumbricus._
e, Seta of _Criodrilus._
f, g, Setae of _Bohemilla comata._
h, i, j, Setae of _Psammoryctes barbatus_ (f to j after Vezhdovsky).]

_Structure._--The body wall consists of an epidermis which secretes a
delicate cuticle and is only ciliated in _Aeolosoma_, and in that
genus only on the under surface of the prostomium. The epidermis
contains numerous groups of sense cells; beneath the epidermis there
is rarely (_Kynotus_) an extensive connective tissue dermis. Usually
the epidermis is immediately followed by the circular layer of
muscles, and this by the longitudinal coat. Beneath this again is a
distinct peritoneum lining the coelom, which appears to be wanting as
a special layer in some Polychaetes (Benham, Gilson). The muscular
layers are thinner in the aquatic forms, which possess only a single
row of longitudinal fibres, or (_Enchytracidae_) two layers. In the
earthworms, on the other hand, this coat is thick and composed of many
layers.

The clitellum consists of a thickening of the epidermis, and is of two
forms among the Oligochaeta. In the aquatic genera the epidermis comes
to consist entirely of glandular cells, which are, however, arranged
in a single layer. In the earthworms, on the other hand, the epidermis
becomes specialized into several layers of cells, all of which are
glandular. It is therefore obviously much thicker than the clitellum
in the limicolous forms. The position of the clitellum, which is
universal in occurrence, varies much as does the number of component
segments. As a rule--to which, however, there are exceptions--the
clitellum consists of two or three segments only in the small aquatic
Oligochaeta, while in the terrestrial forms it is as a general rule,
to which again there are exceptions, a more extensive, sometimes much
more extensive, region.

In the Oligochaeta there is a closer correspondence between external
metamerism and the divisions of the coelom t