🪱 Phylum Annelida – The “Little Ring” Worms

PTB

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

• Marine water (e.g., Nereis)
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• Freshwater (e.g., Leech)
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• 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.

Related Topics

• Phylum Platyhelminthes
• Phylum Nematoda
• Phylum Molluska