Annelid

annelids
Glycera sp.
Scientific classification
Kingdom:	Animalia
Superphylum:	Lophotrochozoa
Phylum:	Annelida Lamarck, 1809
Classes and subclasses
Class Polychaeta (paraphyletic?) Class Clitellata*    Oligochaeta – earthworms, etc.    Acanthobdellida    Branchiobdellida    Hirudinea – leeches Class Myzostomida Class Archiannelida (polyphyletic) Class Echiura? Class Machaeridia† *Some people think that the subclasses under Clitellata are actually classes

Annelids are a phylum of invertebrate worms. They are the segmented worms, with over 17,000 known species.

Well known species are earthworms and leeches. Annelids can be found in most wet environments. Some of these species are parasitic or 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 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.