Updated by: Arabinda Naik
(Phylum—Annelida — ring; — form) includes segmented worms. The term 'Annelida' was first coined by Lamarck (1809). It includes about 12,000 species of animals.
Habit and Habitat: They may be aquatic, terrestrial, and free-living or parasitic. Body wall: The outermost covering of body thin and moist cuticle secreted by the epidermis.
Metamerism: The body is divided into Segments or metameres by ring like grooves—the annuli. It is called metameric segmentation. The segmentation is external as well as internal.
Symmetry Annelids are bilaterally symmetrical.
Germ Layers They are triploblastic animals.
Organisation and Body Plan: They show organ system level of organisation and tube within a tube body plan.
Cephalisation: These show true cephalisation.
Body cavity: Annelids have true coelom. It is formed by splitting of mesodermal epithelium and such a coelom is called schizocoelom.
Skeleton: The coelomic fluid in the body cavity acts as hydrostatic skeleton for the animal.
Locomotion: Annelids by paired. lateral. hollow, fleshy. appendages in each segment called parapodia chitinous Setae. annelids like Nereis possess lateral appendages that help in swimming and have longitudinal and circular muscle for locomotion.
Digestives system: The digestive tract is straight and complete. Starting from mouth and ending at anus.
Respiration It occurs through skin, gills or parapodia.
Circulatory System They are first animals that have a closed circulatory system. the blood flows through a System Of blood vessel.
Excretory System: The excretory organs are nephridia in each Segment, which helps excretion and osmoregulation.
Nervous System: It contains a nerve ring around the pharynx and a double ventral nerve cord with ganglia.
Reproduction: Annelids reproduce sexually. They are unisexual. Nereis or bisexual (hermaphrodite). e.g. Leech, earthworm