The annelids (also called "ringed worms"), formally called Annelida (from Latinanellus "little ring"), are a large phylum of segmented worms, with over 22,000 modern species includingragworms, earthworms and leeches. They are found in marine environments from tidal zones tohydrothermal vents, in freshwater, and in moist terrestrial environments.They are bilaterally symmetrical,triploblastic,coelomate organisms.They have parapodia for locomotion. Although most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species, research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans.
The basic annelid form consists of multiple segments, each of which has the same sets of organs and, in most polychaetes, a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly-defined or absent in some, and Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles to act as hearts. The septa of these 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. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques – some burrowing species turn theirpharynges inside out to drag themselves through the sediment.
Although many species can reproduce asexually and use similar mechanisms to regenerate after severe injuries, sexual reproduction is the normal method in species whose reproduction has been studied. The minority of living polychaetes whose reproduction and lifecycles are known producetrochophorelarvae, which live as plankton and then sink and metamorphose into miniature adults. Oligochaetes are full hermaphrodites and produce a ring-like cocoon round their bodies, in which the eggs and hatchlings are nourished until they are ready to emerge.
Earthworms support terrestrial food chains both as prey and by aerating and enriching soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development ofecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and asbait. Scientists observe annelids to monitor the quality of marine and fresh water. Although blood-letting is no longer in favor with doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength.
Since annelids are soft-bodied, their fossils are rare – mostly jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years agoin the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago. Scientists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, and the earliest certain fossils of the group appear in the Tertiary period, which began 5 million years ago.
Classification and diversity
There are over 22,000 living annelid species, ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus (Cognetti, 1922), which can both grow up to 3 metres (9.8 ft) long. Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids, most textbooks use the traditional classification into the following sub-groups:
Polychaetes (about 12,000 species). As their name suggests, they have multiple chetae ("hairs") per segment. Polychaetes haveparapodia that function as limbs, and nuchal organs ("nuchal" means "on the neck") that are thought to be chemosensors. Most are marine animals, although a few species live in fresh water and even fewer on land.
The Archiannelida, minute annelids that live in the spaces between grains of marine sediment, were treated as a separate class because of their simple body structure, but are now regarded as polychaetes.Some other groups of animals have been classified in various ways, but are now widely regarded as annelids:Clitellates (about 10,000 species ). These have few or no chetae per segment, and no nuchal organsor parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum ("pack saddle") round their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch or, in moniligastrids, yolky eggs that provide nutrition for the embyros . The clitellates are sub-divided into:
Oligochaetes ("with few hairs"), which includes earthworms. Oligochaetes have a sticky pad in the roof of the mouth.Most are burrowers that feed on wholly or partly decomposed organic materials.
Hirudinea, whose name means "leech-shaped" and whose best known members are leeches.Marine species are mostly blood-sucking parasites, mainly on fish, while most freshwater species are predators.They have suckers at both ends of their bodies, and use these to move rather like inchworms.
Pogonophora /Siboglinidaewere first discovered in 1914, and their lack of a recognizable gut made it difficult to classify them. They have been classified as a separatephylum, Pogonophora, or as two phyla, Pogonophora andVestimentifera. More recently they have been re-classified as afamily, Siboglinidae, within the polychaetes.
TheEchiurahave a checkeredtaxonomichistory: in the 19th century they were assigned to the phylum "Gephyrea", which is now empty as its members have been assigned to other phyla; the Echiura were next regarded as annelids until the 1940s, when they were classified as a phylum in their own right; but amolecular phylogeneticsanalysis in 1997 concluded that Echaiurans are annelids.
Myzostomidalive oncrinoidsand otherechinoderms, mainly as parasites. In the past they have been regarded as close relatives of thetrematodeflatwormsor of thetardigrades, but in 1998 it was suggested that they are a sub-group of polychaetes.However, another analysis in 2002 suggested that myzostomids are more closely related toflatwormsor torotifersandacanthocephales.
