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    Annelida General Characteristics, Classification and Examples | Free Biology Notes

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    After reading this article you will learn about Annelida General Characteristics, Classification and Examples

    Annelida General Characteristics

    • Habit & habitat :- Terrestrial, fresh water or marine. Free living or parasitic
    • Lamarck coined the term annelida for ringed animals
    • Body is soft elongated, cylindrical and divided into segments or metameres
    • Body wall consist of Cuticle = Moist and elastic, Epidermis – Living layer that secretes cuticle, Muscle layer – Contains circular and longitudinal muscles
    • Anterior end has a distinct head with sense organ
    • Body cavity is true coelom lined by mesodermal epithelium
    • Digestive system is complete, straight and extends through entire body
    • Respiration is through moist skin, some have gills
    • Circulatory system is closed type and pulsatile heart present
    • Excretory organ is nephridia
    • Nervous system consist of nerve ring and double ventral nerve cord
    • All annelids except leeches also have chitinous hair­like structures, called setae
    • Classification of annelida is based on position & arrangement of setae
    • Reproduction is sexual
    • Annelids may be monoecious or dioecious
    • Clitellum present in bisexual annelids is useful in the formation of cocoon
    • They are oviparous and the eggs are laid in ootheca or cocoons or egg case
    • Development is direct or indirect with free swimming ciliated trochophore larva
    • Mostly cross fertilization takes place

    Classification of Annelida

    Class 1. Polychaeta

    • Polychaeta are marine and carnivorous
    • Setae or parapodia are numerous
    • Clitellum is absent
    • Sexes separate and fertilization external
    • E.g. Nereis, sabella

      Class 2. Oligochaeta

      • They are mostly terrestrial or some fresh water forms
      • Parapodia are absent and locomotory organs are setae
      • Hermaphrodite
      • E.g. Tubifex, pheretima

      Class 3. Hirudinea

      • They marine, freshwater or terrestrial
      • Parapodia and setae are absent
      • Suckers at both the ends
      • Clitellum is developed only breeding season
      • E.g. Hirudo, dina

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