Phylum-Annelida – (Name Annelida: Latin word, annulus: little ring)

The Annelids are also known as the ringed worms or segmented worms, are a large phylum. The phylum includes over 22,000 extant species. The variation includes ragworms, earthworms, and leeches. They can survive in marine environments, freshwater, and other terrestrial environments.

Characteristics features of Phylum Annelids
  • Habitats: The members of this phylum may be aquatic (i.e. marine and freshwater), or terrestrial, free-living, and in some members, parasitism is also found.
  • Level of organization: in annelids the organ-system level of organization.
  • Symmetry: Bilateral symmetry
  • germinal layer– These are triploblastic animals.
  • The annelids are coelomate organisms, and the body is metamerically segmented.
    • The body surface of annelids marked distinctly into segments or metameres, hence the name of phylum is Annelida.
  • Locomotion: These organisms having longitudinal, circular muscles for locomotor help.
    • Some aquatic annelids like Nereis having lateral appendages called parapodia that help in swimming.
  • Respiration through the body surface.
  • Closed circulatory system, Nephridia helps in osmoregulation and excretion.
  • well-developed circulatory and digestive system.
  • Neural system: A pair of ganglia (singular- ganglion) is connected by lateral nerves with a double ventral nerve cord.
  • Tactile receptors, gustratoreceptor, and photoreceptors are found in the members.
  • Regeneration is very common in these organisms.
  • Reproduction:  Mostly these are hermaphrodites, i.e. male and female organs in the same body. Sexual and asexual both types of reproduction in Annelids.
    • Earthworms and leeches are monoecious (sexual reproduction), and Neris is dioecious.
  • Some examples: Pheretima (Earthworm), Nereis, and Hirudinaria (Bloodsucking leech).

Annelida- earthworm

Classification of Annelids
  1. Polychaeta
  2. Oligochaeta
  3. Hirudinea
  4. Archiannelida

Polychaeta

  • Found in the marine environment.
  • Excretion through metanephridia and protonephridia
  • external fertilization
  • well-developed nervous system
  • Closed type circulatory system
  • These are Hermaphrodites
  • These organisms lack clitellum, and these are dioecious
  • Example: Nereis. Syllis

Oligochaeta

  • They are mostly freshwater and terrestrial organisms
  • Metamerically segmented body
  • In these organisms, the head, eyes, and tentacles are not distinct
  • Oligochaetes have a sticky pad in the roof of the mouth.
  • Fertilization is external
  • Cocoon formation takes place
  • Parapodia is not present but clitellum is present
  • These are monoecious organisms.
  • No free larval stage, development inside the cocoons.
  • example: Pheretima, Tubifex

Hirudinea

  • These organisms commonly found in freshwater, some are marine, terrestrial, and parasitic.
  • The tentacles, parapodia, and setae absent.
  • These are monoecious
  • Dorsoventrally flattened body
  • The larval stage is absent
  • Internal fertilization, hermaphrodites.
  • Example: Hirudinaria

Archiannelida

  • These organisms are exclusively marine.
  • Setae and parapodia are absent
  • Archineelids are unisexual or hermaphrodite.
  • tentacles are present on the prostomium.
  • Example: Dinophilus, Protodrilus.

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