What is Annelid?
{{Taxobox | name = annelids | image = Nerr0328.jpg | image_width = 230px | image_caption = ''Glycera sp.'' | regnum = [[Animal]]ia | superphylum = [[Lophotrochozoa]] | phylum = '''Annelida''' | phylum_authority = [[Jean-Baptiste Lamarck|Lamarck]], 1809 | subdivision_ranks = Classes and subclasses | subdivision = Class [[Polychaeta]] ([[paraphyly|paraphyletic]]?)<br> Class [[Clitellata]]*<br> &nbsp;&nbsp;&nbsp;[[Oligochaeta]] – [[earthworm]]s, etc.<br> &nbsp;&nbsp;&nbsp;[[Acanthobdellida]]<br> &nbsp;&nbsp;&nbsp;[[Branchiobdellida]]<br> &nbsp;&nbsp;&nbsp;[[Hirudinea]] – [[leech]]es<br> Class [[Myzostomida]]<br> Class [[Archiannelida]] ([[polyphyly|polyphyletic]])<br> Class [[Echiura]]?<br /> Class [[Machaeridia]]†<br> <small>*Some people think that the subclasses under Clitellata are actually classes</small> }} Annelids are a [[phylum]] of [[invertebrate]] worms. They are the [[segment]]ed [[worm]]s, with over 17,000 known [[species]]. Well known species are [[earthworm]]s and [[leech]]es. Annelids can be found in most wet environments. Some of these species are [[parasitic]] or [[Mutualism|mutualistic]]. This means they live together with (or inside) another organism. A mutualistic relationship is beneficial to both organisms. Their size varies from under a millimetre to about 3 metres. The largest known species is the [[seep tube worm]] (''Lamellibrachia lymnesi''), which is related to the [[giant tube worm]]. Annelids are made of one or more body segments. Each segment has one or more rings. These rings are called ''annuli'' (singular: 'annulus' = 'little ring'). Annelids reproduce sexually by [[Hermaphrodite|hermaphroditic]] cross-fertilization. They can regenerate cut off pieces of their body, unlike sponges, which can come back together if ripped apart. They have a hydrostatic [[skeleton]], a one-way digestion system, and [[bilateral symmetry]]. == Nervous system == Annelid worms have a nervous system. The [[brain]] forms a ring round the [[pharynx]] (throat), consisting of a pair of [[ganglia]] (local c