ine annelids, of which nereis,
or a clam-worm, is a good example, are more typical. They are often
quite large, a foot or even more in length. They are composed of
many, often several hundred, rings or segments. Between these the
body-wall is thin, so that the segments move easily upon each other,
and thus the animal can creep or writhe.

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_,
brist