ntal septa lodge the developing ova
and sperm. Special gonad ducts always present. Male ducts often open on
to exterior through a terminal chamber which is variously specialized,
and sometimes with a penis.

[Illustration:

FIG. 10.--Diagrams of various Earthworms, to illustrate external
characters. A, B, C, anterior segments from the ventral surface; D,
hinder end of body of _Urochaeta_.

A, _Lumbricus_: 9, 10, segments containing spermathecae, the orifices
of which are indicated; 14, segment bearing oviducal pores; 15,
segment bearing male pores; 32, 37, first and last segments of
clitellum.

B, _Acanthodrilus_: cp, orifices of spermathecae; [Female], oviducal
pores; [Male], male pores; on 17th and 19th segments are the apertures
of the atria.

C, _Perichaeta_: the spermathecal pores are between segments 6 and 7,
7 and 8, 8 and 9, the oviducal pores upon the 14th and the male pores
upon the 18th segment.

In all the figures the nephridial pores are indicated by dots and the
setae by strokes.]

Generative pores usually paired, sometimes single and median.
Spermathecae nearly always present. Alimentary canal straight, often
with appended glands of complicated or simpler structure; no jaws. Eggs
deposited in a cocoon after copulation. Development direct. Reproduction
by budding also occurs. Fresh-water (rarely marine) and terrestrial.

The Oligochaeta show a greater variety of size than any other group of
the Chaetopoda. They range from a millimetre or so (smaller species of
_Aeolosoma_) to 6 ft. or even rather more (_Microchaeta rappi_, &c.) in
length.

_Setae._--The setae, which are always absent from the peristomial
segment, are also sometimes absent from a greater number of the
anterior segments of the body, and have completely disappeared in
_Achaeta cameranoi._ When present they are either arranged in four
bundles of from one to ten or even more setae, or are disposed in
continuous lines completely encircling each segment of the body. This
latter arrangement characterizes many genera of the family
_Megascolicidae_ and one genus (_Periscolex_) of the
_Glossoscolicidae._ It has been shown (Bourne) that the "perichaetous"
condition is probably secondary, inasmuch as in worms which are, when
adult, "perichaetous" the setae develop in pairs so that the embryo
passes through a stage in which it has four bundles of setae, two to
each bundle, the prevalent cond