Annelid

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.

 Nervous system 
Annelid worms have a nervous system. The brain forms a ring round the pharynx (throat), consisting of a pair of ganglia (local control centers) above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it.Ruppert E.E; Fox R.S. and Barnes R.D. 2004. Annelida, in Invertebrate Zoology 7th ed, Brooks/Cole. pp. 414–420.  In some very mobile and active polychaetes the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections.Rouse G. 1998. The Annelida and their close relatives, in Anderson D.T. Invertebrate Zoology. Oxford University Press. pp. 183–196. 

The rest of the central nervous system is usually 'ladder-like', with a pair of nerve cords that run along the bottom part of the body cavity. In each segment there are paired ganglia linked by a cross-connection. From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body.  

 Main groups 
Polychaetes: about 12,000 species, free-living
Clitellates: about 5,000 species
Oligochaetes
Hirudinea
Echiura

 References