🪱 Phylum Annelida – The “Little Ring” Worms

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.

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🧬 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).

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🧷 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).

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🍽️ Digestive System:

Tube-like and complete digestive tract, divided into parts with specific functions.

In parasitic annelids, the digestive system is simplified.

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🚰 Excretory System:

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

Each segment contains one pair of metanephridia.

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❤️‍🔥 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.

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🧠 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.

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🧬 Reproduction:

Most are hermaphrodites (both sexes in one, e.g., earthworm, leech).

Some are unisexual (separate sexes, e.g., Nereis).

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🧠 Fun Fact:

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