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Phylum Annelida – The “Little Ring” Worms

 🪱 Phylum Annelida – The “Little Ring” Worms

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Introduction: Annelids, also known as segmented worms, are found in:

  • Marine water (e.g., Nereis)

  • Freshwater (e.g., Leech)

  • Damp soil (e.g., Earthworm)

Some annelids are ectoparasites (organisms living on the outside of a host), such as leeches.

🧬 Body Structure:

  • The body is transversely segmented into similar parts called segments.
  • Internally, segments are separated by septa (cross walls).
  • Each segment has its own circulatory, excretory, and neural systems – this is called metameric segmentation (true segmentation).

Annelids are:

  • Bilateral (symmetrical left and right sides),
  • Triploblastic (three germ layers),
  • Coelomates (have a true body cavity),
  • Protostomes (mouth develops first in the embryo).

🧷 Special Features:

  • They have setae (chitinous bristles for movement, present on each segment’s ventral side) – Absent in leeches.
  • Covered by a moist cuticle (non-cellular layer secreted by the epidermis).

🍽️ Digestive System:

  • Tube-like and complete digestive tract, divided into parts with specific functions.
  • In parasitic annelids, the digestive system is simplified.

🚰 Excretory System:

Made up of metanephridia (ciliated, funnel-shaped structures).

Each segment contains one pair of metanephridia.

❤️‍🔥 Circulatory System:

  • Closed circulatory system (blood remains within vessels).
  • Has pseudohearts (specialized pulsating vessels).
  • Blood contains haemoglobin (respiratory pigment) dissolved in plasma.
  • Gaseous exchange occurs through moist skin.

🧠 Nervous System:

  • Brain is a cerebral ganglion in the anterior segment.
  • Double ventral nerve cord runs along the body.
  • Ganglia (nerve cell clusters) are present in each segment.
  • Sense organs include:
  • Tactile receptors (touch),
  • Chemoreceptors (chemical detection),
  • Statoreceptors (balance),
  • Photoreceptors (light),
  • Some have eyes with lenses.

🧬 Reproduction:

  • Most are hermaphrodites (both sexes in one, e.g., earthworm, leech).
  • Some are unisexual (separate sexes, e.g., Nereis).

🧠 Fun Fact:

  • The word "Annelida" comes from Latin "annellus", meaning "little ring", referring to the ring-like body segmentation.

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