Phylum Annelida

General Characters

The organisms are triploblastic, bilaterally symmetrical, coelomate, organ-system level of body organization & metamerically segmented.

Body wall with an epidermis of columnar epithelium, coated externally by moist albuminous cuticle & with circular & longitudinal muscle fibres.

Chitinous setae, aiding in locomotion, may or may not be on fleshy parapodia; absent in leech.

A true coelom is present. Annelids are first animals to have a true schizocoelic coelom. Coelom is divided by septa into compartments.

The coelomic fluid acts as a hydrostatic skeleton.

Digestive system is complete & digestion is extracellular.

Respiration by moist skin (cutaneous respiration) or through gills (branchial respiration).

Blood-vascular system is usually closed. Respiratory pigments either haemoglobin or erythrocruorin dissolved in blood plasma. Free amoeboid blood corpuscles are present, but there are no RBC’s. In leech,

there is no true blood-vascular system.

Nephridia is the excretory organ. Ammonia is chief excretory waste.

The nervous system consists of a nerve ring & a solid, double, mid-ventral nerve cord with ganglia & lateral nerves in each segment.

Sensory organs include tactile organs, taste-buds, statocysts, photoreceptor cells & eyes with lenses.

The sexes may be separate (e.g., Nereis) or united (e.g., earthworm, leech).

Development is mostly direct (e.g., earthworm). There is indirect development in Nereis. Larva, when present is trochophore.

Classification : The phylum Annelida is divided into three classes; Polychaeta, Oligochaeta, Hirudinea.