Annelida General Characteristics, Classification and Examples | Free Biology Notes

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