These segments are very much alike except the first two and the last.  If we examine one from the middle of the body we shall find its structure very much like that of our schematic worm.  Outside we find a very thin, horny cuticle, secreted by the layer of cells just beneath it, the hypodermis.  Beneath the skin we find a thin layer of transverse muscles, and then four heavy bands of longitudinal muscles.  These latter have been grouped in the four quadrants, a much more effective arrangement than the cylindrical layer of the schematic worm.  Furthermore, the animal has on each segment a pair of fin-like projections, stiffened with bristles, the parapodia.  These are moved by special muscles and form effective organs of creeping.

[Illustration:  7.  EUNICE LIMOSA (ANNELID).  LANG, FROM EHLERS.  Front and hind end seen from dorsal surface. fa, fp, fc, feelers; a, eye; k, gill; p, parapodia; ac, anal cirri.]

Within the muscles is the perivisceral cavity, and in its central axis the intestine, segmented like the body-wall.  The reproductive 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 muscles of each segment, and is becoming more a centre of control and perception for the body as a whole.  It is more like our brain, commander-in-chief, the other ganglia constituting its staff.  The sense-organs have improved greatly.  There are tentacles and otolith vesicles as very delicate organs of feeling, or possibly of hearing also.