The fundamental characteristic of the phylum is the division of the body into a linear series of cylindrical segments, or metameres. Each metamere consists of a section of the body wall and a compartment of the body cavity with its internal organs. The external divisions, which may be seen in the common earthworm, correspond to the internal divisions. The annelid body consists of a head region; a trunk, made up of metameres; and an unsegmented terminal region called the pygidium. In some primitive members of the phylum the metameres are identical, or very similar to one another, each containing the same structures; in more advanced forms there is a tendency toward a consolidation of some segments and a restriction of certain organs to particular segments. Because of the soft nature of the annelid body, fossils are not common. Fossils of tube-dwelling polychaetes have been found, but there is scarcely any fossil record for earthworms and none for leeches.
The body wall is covered with epidermis overlaid with a thin, pliant cuticle secreted by the epidermal cells. The body wall consists of well-developed, segmentally arranged muscles used for crawling and swimming movements. Most annelids possess short external bristles called setae, or chaetae, composed of chitin. Setae are used to grip the soil, to hold the animal in a tube, or to increase the surface areas of appendages for swimming.
The digestive system of annelids consists of an unsegmented gut that runs through the middle of the body from the mouth, located on the underside of the head, to the anus, which is on the pygidium. The gut is separated from the body wall by the body cavity, called the coelom. The segmented compartments of the coelom are usually separated from each other by thin sheets of tissue, called septa, which are perforated by the gut and by blood vessels. Except in the leeches, the coelom is fluid filled and functions as a skeleton, providing the animal with rigidity and the resistance necessary for muscular movement. If the worm is punctured, it loses its ability to move properly, since functioning of the body muscles is dependent on the maintenance of the fluid volume in the coelom. In primitive annelids each compartment of the coelom is connected to the outside by ducts for the release of sex cells, and by paired excretory organs, or nephridia. These openings are closed except when functioning, thus preventing the loss of coelomic fluid. In more advanced species both excretory and reproductive functions are sometimes served by a single type of duct, and ducts may be absent in certain segments.
Characteristics of the circulatory system vary within the phylum. The blood usually contains hemoglobin, a red oxygen-carrying pigment; some annelids have a green oxygen-carrying pigment, and others have unpigmented blood. The circulatory system is usually closed, i.e., confined within well-developed blood vessels; in some polychaetes and leeches the circulatory system is partly open, with blood and coelomic fluid mixing directly in the sinuses of the body cavity. Blood flows toward the head through a contractile vessel above the gut and returns to the terminal region through vessels below the gut; it is distributed to each body compartment by lateral vessels. Some of the lateral vessels are contractile and serve as hearts, i.e., pumping organs for driving the blood.
Some aquatic annelids have thin-walled, feathery gills through which gases are exchanged between the blood and the environment. However, most annelids have no special organs for gas exchange, and respiration occurs directly through the body wall.
The nervous system typically consists of a primitive brain, or ganglionic mass, located in the head region, connected by a ring of nerves to a ventral nerve cord that runs the length of the body; the cord gives rise to lateral nerves and ganglia in each segment. Sense organs of annelids generally include eyes, taste buds, tactile tentacles, and organs of equilibrium called statocysts.
Reproduction is sexual or asexual. Asexual reproduction is by fragmentation, budding, or fission. Among sexually reproducing annelids hermaphrodites are common, but most species have separate sexes. Fertilized eggs of marine annelids usually develop into free-swimming larvae. Eggs of terrestrial forms are enclosed in cocoons and hatch as miniature versions of the adults. The ability to regenerate lost body parts is highly developed in many polychaetes and digochaetes.