The Phylum Annelida was created by Lamarck in 1801.
It is a phylum of triploblastic, coelomates worms which have bilateral symmetry and metameric segmentation, a preoral prostomium and a post-anal pygidium.
Characteristics of Phylum Annelida:
The Phylum Annelida contains both free-living and parasitic segmented worms. The free-living forms occur in moist soil, freshwater, brackish water and sea.
The body is externally divided into segments called metameres or somites by ring-like grooves called annuli, so-called ringworms or segmented worms.
Organ system level of body organisation and tube-within-a-tube body plan.
The organs of locomotion are segmented arranged paired setae (oligochaetes) and parapodia (polychaetes).
The alimentary canal is straight from mouth to anus. Digestion is extracellular.
These are the first animals to have a generally closed circulatory system. The Colour of blood is red due to the presence of erythrocruorin in plasma. In leeches, there is no true blood vascular system.
Excretory organs are commonly nephridia. Parapodia and botryoidal tissue also take part in excretion. Excretory product is ammonia in aquatic forms and urea in terrestrial forms.
The nervous system consists of a dorsal brain and double ventral nerve cord with ganglia and nerves in each segment.
Sense Organs- These are photoreceptors, chemoreceptors, tactile receptors, gustatory and olfactory receptors.
Asexual reproduction occurs in some annelids through fission and budding. Regeneration is quite common in earthworm and Nereis.
Animals may be unisexual (example- Nereis) or hermaphrodite (example- Earthworm, Leech). Fertilization is often external. Females are oviparous.
Development is direct in hermaphroditic annelids or indirect in unisexual annelids and includes a free-swimming trochophore larva. Cleavage is holoblastic spiral and determinate.