🪱 Phylum Annelida – The “Little Ring” Worms
PTB
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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