Annelida General Characteristics, Classification and Examples | Free Biology Notes

Annelida General Characteristics, Classification and Examples

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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