Phylum annelida
• The term Annelida was first coined by Lamarck for the higher segmented worms.
• The annelid worms were thought to be evolved from coelomate worm like ancestors
which developed metameric segmentation.
general CharaCters
• Mostly aquatic, some terrestrial, burrowing or tubicolous. Some commensal and
parasitic.
• Body elongated, bilaterally symmetrical, triploblastic, truely coelomate and metamerically
segmented into similar metameres.
• Epidermis of a single layer of columnar epithelial cells, covered externally by a thin
cuticle not made of chitin.
• Body wall dermo-muscular. Outer muscle fibres circular, inner longitudinal.
• Locomotory organs are segmentally repeated chitinous bristles, called setae or chaetae,
embedded in skin. May be borne by lateral fleshy appendages or parapodia.
• Coelom, true, schizocoelous. Mostly well-developed except in leeches. Usually divided
into compartments by transverse septa. Coelomic fluid with cells or corpuscles.
• Digestive system straight and complete. Digestion entirely extracellular.
• Blood vascular system closed. Respiratory pigments either haemoglobin or erythrocruorin
dissolved in blood plasma.
• Respiration by moist skin or gills of parapodia and head.
• Excretory system consisting of metamerically disposed coiled tubes, called nephridia.
• Nervous system with a pair of cerebral ganglia (brain) and a double ventral nerve cord
bearing ganglia and lateral nerves in each segment.
• Sensory organs include tactile organs, taste buds, statocysts, photoreceptor cells and
sometimes eyes with lenses in some.
• Both unisexual (e.g., Neries) and bisexual (e.g., Earthworm, Leech) forms are found.
Cleavage spiral and determinate. Larva, when present, is a trochophore. Regeneration
common.
Animal Kingdom-III
Analysis of various PMTs from 2009-2014200920102011201220132014AIPMT/NEET244384AIIMS1231AMU32541Kerala28454K.CET2421J & K6641BHU37Class XI