Annelida

Annelida:

  • Annelids may be aquatic (marine and freshwater) or terrestrial.
  • They are free living or parasitic.
  • Nephridia (sing.: nephridium) help in osmoregulation and excretion.
  • Neural system contains paired ganglia (sing. ganglion) connected through lateral nerves to a double ventral nerve cord.

 

Characteristics-

  • Body Organisation:
    • Annelids are triploblastic organisms, meaning they have three germ layers: ectoderm, mesoderm, and endoderm.
    • They exhibit organ-system level of body organisation and show bilateral symmetry.
    • They exhibit a segmented body plan, with each segment called a metamere.
    • Annelids have a coelom (body cavity) that provides space for organ systems.
    • They possess a closed circulatory system, with blood contained within vessels.
  • Locomotion:
    • Annelids exhibit diverse modes of locomotion.
    • Many annelids, such as earthworms, move by peristaltic contractions of their longitudinal and circular muscles, allowing them to burrow through soil or crawl on surfaces.
    • Some marine annelids, such as polychaetes, possess bristle-like structures called setae that aid in movement and provide traction.
    • Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
  • Reproduction:
    • Annelids can reproduce both sexually and asexually.
    • Asexual reproduction occurs through fragmentation, where a fragment of the worm can regenerate into a complete organism.
    • Sexual reproduction involves the presence of separate sexes (male and female).
      • Nereis is dioecious (having the male and female reproductive organs in separate individuals).
      • Earthworms and leeches are monoecious (having both the male and female reproductive organs in the same individual).
    • Most annelids have internal fertilisation, with sperm transferred to the female reproductive organs.
  • Feeding Strategies:
    • Annelids exhibit various feeding strategies depending on their ecological niche.
    • Many annelids are detritivores, feeding on decomposing organic matter in soil or sediment.
    • Some annelids are filter feeders, using specialised structures like tentacles or ciliated structures to capture small particles from water.
    • Predatory annelids may have specialised mouthparts or jaws to capture and consume prey.
  • Examples:

Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech).