Name of the phylum-Annelida was first coined by Lamarck. The body of annelids is elongated, bilaterally symmetrical, triploblastic, truely coelomate and metamerically segmented into similar metameres. The coelom is true, schizocoelous. Blood vascular system is closed. The nervous system is with a pair of cerebral ganglia and a double ventral nerve cord bearing ganglia and lateral nerves in each segment.
The blood vascular system consisting of blood vessels and capillaries. Blood is composed of fluid plasma and colourless corpuscles, physiologically comparable to the leucocytes of vertebrates.
Pseudocoelom is the body cavity of Aschelminthes.