The septa of such species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis ("ripples" that pass along the body) or by undulations that improve the effectiveness of the parapodia.

The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor.

[33] Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and external chaetae (Greek χαιτη, meaning "hair") and, in some species, appendages.

[8] Hence annelids' chetae are structurally different from the setae ("bristles") of arthropods, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle.

[8] The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body.

Each septum forms a sandwich with connective tissue in the middle and mesothelium (membrane that serves as a lining) from the preceding and following segments on either side.

Each mesentery is similar except that the mesothelium is the lining of each of the pair of coelomata, and the blood vessels and, in polychaetes, the main nerve cords are embedded in it.

Two families, the Eunicidae and Phyllodocidae, have evolved jaws, which can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter.

Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths.

[23] The gut is generally an almost straight tube supported by the mesenteries (vertical partitions within segments), and ends with the anus on the underside of the pygidium.

[8] However, in members of the tube-dwelling family Siboglinidae the gut is blocked by a swollen lining that houses symbiotic bacteria, which can make up 15% of the worms' total weight.

[40] In annelids the points at which fluid enters the protonephridia or metanephridia are on the forward side of a septum while the second-stage filter and the nephridiopore (exit opening in the body wall) are in the following segment.

[46] Charles Darwin's book The Formation of Vegetable Mould Through the Action of Worms (1881) presented the first scientific analysis of earthworms' contributions to soil fertility.

[52] Ragworms are commercially important as bait and as food sources for aquaculture, and there have been proposals to farm them in order to reduce over-fishing of their natural populations.

In the 19th century medical demand for leeches was so high that some areas' stocks were exhausted and other regions imposed restrictions or bans on exports, and Hirudo medicinalis is treated as an endangered species by both IUCN and CITES.

More recently leeches have been used to assist in microsurgery, and their saliva has provided anti-inflammatory compounds and several important anticoagulants, one of which also prevents tumors from spreading.

[1] The small shelly fossil Cloudina, from 549 to 542 million years ago, has been classified by some authors as an annelid, but by others as a cnidarian (i.e. in the phylum to which jellyfish and sea anemones belong).

[56][57] Until 2008 the earliest fossils widely accepted as annelids were the polychaetes Canadia and Burgessochaeta, both from Canada's Burgess Shale, formed about 505 million years ago in the Middle Cambrian.

[58] Then Simon Conway Morris and John Peel reported Phragmochaeta from Sirius Passet, about 518 million years old, and concluded that it was the oldest annelid known to date.

[61] A trace fossil consisting of a convoluted burrow partly filled with small fecal pellets may be evidence that earthworms were present in the early Triassic period from 251 to 245 million years ago.

[61][62] Body fossils going back to the mid Ordovician, from 472 to 461 million years ago, have been tentatively classified as oligochaetes, but these identifications are uncertain and some have been disputed.

[68] In 2007 Torsten Struck and colleagues compared three genes in 81 taxa, of which nine were outgroups,[10] in other words not considered closely related to annelids but included to give an indication of where the organisms under study are placed on the larger tree of life.

It also concluded that the classification of polychaetes into Scolecida, Canalipalpata and Aciculata was useless, as the members of these alleged groups were scattered all over the family tree derived from comparing the 81 taxa.

It also placed sipunculans, generally regarded at the time as a separate phylum, on another branch of the polychaete tree, and concluded that leeches were a sub-group of oligochaetes rather than their sister-group among the clitellates.

[10] Rouse accepted the analyses based on molecular phylogenetics,[12] and their main conclusions are now the scientific consensus, although the details of the annelid family tree remain uncertain.

[10][b] The updated phylogenetic tree of the Annelid phylum is comprised by a grade of basal groups of polychaetes: Palaeoannelida, Chaetopteriformia and the Amphinomida/Sipuncula/Lobatocerebrum clade.

[70][71][72][73][74] Oweniidae Magelonidae Apistobranchus Psammodrilidae Chaetopteridae Amphinomida Lobatocerebrum Sipuncula Eunicida Phyllodocida Polygordiidae Protodrilida Myzostomida Orbiniida Cirratuliformia Siboglinidae (pogonophorans) Sabellida Spionida Opheliida Capitellidae Echiura Terebelliformia Maldanomorpha Clitellata (oligochaetes, leeches...) Annelids are members of the protostomes, one of the two major superphyla of bilaterian animals – the other is the deuterostomes, which includes vertebrates.

[68] The current view is that annelids are grouped with molluscs, brachiopods and several other phyla that have lophophores (fan-like feeding structures) and/or trochophore larvae as members of Lophotrochozoa.

Gnathifera Rouphozoa Mesozoa Cycliophora Annelida Mollusca Brachiopoda Phoronida Entoprocta Ectoprocta Nemertea The "Lophotrochozoa" hypothesis is also supported by the fact that many phyla within this group, including annelids, molluscs, nemerteans and flatworms, follow a similar pattern in the fertilized egg's development.

Both groups share in common: the presence of chaetae secreted by microvilli; paired, metameric coelomic compartments; and a similar metanephridial structure.