Phylum Annelida.

1. They are triploblastic, coelomate organisms.
2. They have bilateral symmetry.
3. They are metamerically segmented.
4. They have a definite outer covering called cuticle.
5. They have chitinous hair-like structures called chaetae (except in leeches), the chaetae of polychaetes are found on structures known as parapodia.
6. They have a lip-like extension on the first segment above the mouth (prostomium).
7. They have nephridia as typical excretory organs.
8. They have central nervous system, with paired cerebral ganglia in which paired commissures around the gut lead to a double ventral nerve cord, expanded to form a ganglion in every segment.

The phylum has three classes namely: Class Polychaeta, which includes the marine bristle worms with many chaetae and parapodia, examples are Nereis (the ragworm) and Arenicola (the lugworm).

Class Oligochaeta, which includes the earthworms and freshwater worms, that have few chaetae without parapodia, for example Lumbricus (the common earthworm), Tubifex (freshwater earthworm sometimes called sludge worm, or sewage worm). Another class of Annelida is class Hirudinea which includes segmented worms with fixed number of segments, and have neither chaetae nor parapodia; an example is Hirudo (the leeches).

An earthworm is an elongated, tube-shaped, metamerically segmented organism. Its digestive system runs through the length of its body. It has a thin wet skin through which gaseous exchange takes place. There is a double transport system composed of coelomic fluid that moves within the fluid-filled coelom. The circulatory system is simple and closed.

The worm has a central and peripheral nervous systems. The central nervous system consists of two ganglia above the mouth, one on either side, connected to a nerve cord running back along its length to motor neurones and sensory cells in each segment. A large number of chemoreceptors are concentrated near its mouth. Circumferential and longitudinal muscles on the periphery of each segment enable the worm to move. Similar sets of muscles line the gut and their actions move the food towards the worm's anus.

1. They have ideal shapes with chaetae for burrowing.
2. They have gizzard for breaking up plant tissues.
3. They have clitellum, which holds together earthworms during copulation and forms an egg depositer structure called cocoon to ensure efficient provision of offspring during unfavourable conditions.
4. The mucus secreted helps to bind the walls of burrows and helps the worm to pass through smoothly.
5. They can thrust the earth aside when it is loose and consume it when it is tight. This ability ensures burrowing under both conditions.
6. They are omnivorous; as they feed on live plant matters, fungi, bacteria and microscopic animals. They also feed on dead organic matter from plants and animals. This feeding habit ensures sustainable food supply.
7. Under natural conditions, earthworms are less active; this limits oxygen consumption.
8. They have looped surface vessels, which facilitate absorption of oxygen over a short diffusion distance.
9. Their haemoglobin has high affinity to oxygen.
10. The head is less elaborate. The worm has tactile and photoreceptive cells for locating food at night and sensing light during the day respectively.
11. They have chaetae that are used for movement.