Phylum: Annelida

• Includes segmented worms, e.g earthworms, sea worms and leeches
• Most members of this phylum live in water (fresh and marine), while some live on land in moist soil
• The bodies of worms are divided into similar segments/ metameres
• Externally the segmentation is visible as ring-shaped grooves and internally the body is divided into similar compartments by partitions (septa)
• Internal structures are segmented, except for the digestive tract, which stretches throughout the length of the body as a long tube
• Bilateral symmetry
• Cephalisation occurs
• A ventral nerve cord, with a ganglion (cluster of nerve cell bodies) in each segment, extends throughout the whole length of the body
• Bilateral symmetry, with cephalisation, is an ideal body plan for animals that move with their heads first
• Stimuli are detected more efficiently, and faster locomotion with better muscle coordination is possible
• Annelids are triploblastic, three germ layers in the embryo
• Annelida is the first phylum that has a coelom. They are known as coelomate animals
• Ectoderm gives rise to two outer layers of the body wall; glandular epidermis, cuticle, nervous system
• Endoderm forms the inner lining of the digestive tract
• Mesoderm gives rise to the muscles, blood vessels and excretory organs
• The coelom occurs in the mesoderm and therefore surrounded by mesoderm on both sides
• The outer mesodermal layer forms the circular and longitudinal muscles of the body wall, innermost layer forms the muscle wall of the digestive tract
• In earthworms the coelom is filled with fluid and it serves as a hydrostatic skeleton
• The muscle layers of the body wall contract and exert pressure on the coelomic fluid
• Annelids have a through gut that has a mouth on the one end and an anus at the other
• Food moves through the digestive tract in one direction by peristaltic wave-like movements
• Annelids do not have specialised gaseous exchange oragns. Gaseous exchange occurs by diffusion through the body wall
• Coelomic fluid is not an efficient transport system between the body wall and the digestive tract
• Therefore the mesoderm gives rise to a system of blood vessels
• Blood is restricted to the blood vessels, it is called a closed blood system
• Blood transports nutrients and respiratory gases (O2 and CO2) to and away from cells
• An excretory system, consisting of organs called nephridia, occurs in annelids, which excretes nitrogenous wastes