Phylum – Annelida

Annelids live in many settings: some are aquatic, in the sea or in fresh water, while others are terrestrial. Most are free-living, though a few are parasitic.

Body organisation, symmetry, layers and cavity

• They show an organ-system level of body organisation.
• They are bilaterally symmetrical.
• They are triploblastic, built from three germ layers.
• They are coelomate, possessing a true body cavity.

Metameric segmentation

The defining feature is that the body surface is clearly divided into a series of rings or segments called metameres, a repeated pattern known as metameric segmentation. This ringed appearance gives the phylum its name, which comes from the Latin word annulus, meaning little ring.

Locomotion

They have both longitudinal and circular muscles in the body wall, and the coordinated action of these muscles produces movement. Aquatic annelids such as Nereis also bear lateral projections called parapodia, which assist in swimming.

Circulation, excretion and nervous system

• A closed circulatory system is present, in which blood stays within vessels.
• Structures called nephridia carry out osmoregulation and excretion.
• The nervous system is made of paired ganglia joined by lateral nerves to a double ventral nerve cord that runs along the underside of the body.

Reproduction …