organs are formed from patches of the lining of the perivisceral
cavity, and the reproductive elements, when fully developed, fall
into the perivisceral fluid and are carried out by nephridia, just
such as we found in the schematic worm. Beside the perivisceral
cavity and its fluid there is a special circulatory system. This
consists mainly of one long tube above the intestine and a second
below, with often several smaller parallel tubes. Transverse
vessels run from these to all parts of the body. The dorsal tube
pulsates and thus acts as a heart. The surface of the body no longer
suffices to gather oxygen, hence we find special feathery gills on
the parapodia. But these gills are merely expanded portions of the
body wall, arranged so as to offer the greatest possible amount of
surface where the capillaries of the blood system can be almost
immediately in contact with the surrounding water.

[Illustration: 8. CROSS-SECTION OF BODY SEGMENT OF ANNELID. LANG.
_dp_ and _vp_, dorsal and ventral halves of parapodia; _b_ and _ac_,
bristles; _k_, gill; _dc_ and _vc_, feelers; _rm_, lateral muscles;
_lm_, longitudinal muscles; _vd_, dorsal blood-vessel; _vo_, ventral
blood-vessel; _bm_, ventral ganglion; _ov_, ovary; _tr_, opening of
nephridium in the perivisceral cavity; _np_, tubular portion of
nephridium. The circles containing dots represent eggs floating in
the perivisceral fluid.]

The nervous system consists of a large supra-oesophageal ganglion
in the first segment; then of a chain of ganglia, one to each
segment, on the ventral side of the body. With one ganglion in each
segment there is far more controlling, perceptive, ganglionic
material than in lower worms. Furthermore the supra-oesophageal
ganglion is relieved of a large part of the direct control of the