The annelids, collectively called Annelida (from Latin anellus "little ring"), are a large phylum of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches. They are found in most wet environments, and include many terrestrial, freshwater, and especially marine species (such as the polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimeter to over 3 meters (the seep tube worm Lamellibrachia luymesi).
Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is quite variable but is usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (to increase surface area) along much of its length. Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders which simply ingest the sediments, and blood-suckers. Annelids can also grow up to six inches.
The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel which conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall.
The nervous system has a nerve cord from which lateral nerves come in contact with each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion. Growth in many groups occurs by replication of individual segmental units, in others the number of segments is fixed in early development.
Depending upon the species, annelids can reproduce both sexually and asexually.
Asexual reproduction by fission is a method used by some annelids and allows them to reproduce quickly. The posterior part of the body breaks off and forms a new identical worm. The position of the break is usually determined by an epidermal growth. Lumbriculus and Aulophorus, for example, are known to reproduce by the penis breaking into such fragments. This complete regeneration is noteworthy as these Annelid species are the most highly organized animals to have this capability. Many other taxa (such as most earthworms) cannot reproduce this way, though they have varying abilities to regrow amputated segments.
Most polychaete worms are gonochoristic, that is, they have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated trochophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size.
Earthworms and other oligochaetes, as well as the leeches, are hermaphroditic and mate periodically throughout the year in favored environmental conditions. They mate by copulation. Two worms which are attracted by each other's secretions lay their bodies together with their heads pointing opposite directions. The fluid is transferred from the male pore to the other worm. Different methods of sperm transference have been observed in different genera, and may involve internal spermathecae (sperm storing chambers) or spermatophores that are attached to the outside of the other worm's body. The clitella lack the free-living ciliated trochophore larvae present in the polychaetes, the embryonic worms developing in a fluid-filled "cocoon" secreted by the clitellum.
The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian. Because the creatures have soft bodies, fossilization of a body is an especially rare event. However, a few annelids, such as the living polychaetes in the Serpulidae, secrete calcareous tubes, and such tubes are fairly common as fossils (although these are not necessarily from annelida, as other animal phyla can also secrete tubes). A fossil group has recently been assigned to the annelids, the machaeridians. These forms were polychaetes with rows of dorsal overlapping shell plates. The hard jaws of certain polychaetes, known as scolecodonts, are known from the Ordovician onward, and are common enough to be used for stratigraphic correlation in some cases. The best-preserved and oldest annelid body fossils come from the Cambrian Lagerstätten such as the Burgess Shale of Canada, and the Middle Cambrian strata of the House Range in Utah. The Annelids are also diversely represented in the Pennsylvanian-age Mazon Creek fauna of Illinois. A few small groups have been treated as separate phyla: the Pogonophora and Vestimentifera, now included in the family Siboglinidae, and the Echiura.
Sipuncula, Echiura and Siboglinidae have traditionally been placed in their own phyla, while Clitellata has been considered separated from the polychaete annelids. But recent research indicates that all of them actually belongs within the Polychaete, even if some of these groups have lost their segmentation .