🪱 Annelids — The Segmented Worms
Phylum Annelida: Masters of Segmentation
Big Idea: Annelids are the first animals we study that have true body segmentation, a true coelom (body cavity), and a closed circulatory system. These three innovations make them far more complex than flatworms or roundworms!
1. What Are Annelids?
The word Annelida comes from the Latin anellus, meaning "little ring" — a perfect description of their ring-like body segments. The phylum contains over 22,000 species, including earthworms, ragworms, and leeches.
🌊 Class Polychaeta
Marine worms ("many bristles")
Example: Ragworms, tube worms
🌍 Class Oligochaeta
Earthworms ("few bristles")
Example: Common earthworm
🩸 Class Hirudinea
Leeches (parasites/predators)
Example: Medicinal leech
2. Key Feature #1 — Segmentation (Metamerism)
The most defining characteristic of annelids is metamerism — true segmentation of the body both externally and internally. Each segment contains its own set of muscles, nerves, blood vessels, and excretory organs (nephridia).
Why does segmentation matter? It increases the efficiency of body movement by allowing muscle contraction to be extremely localized. Each segment can expand or contract independently, enabling the worm to burrow through soil!
🔬 Interactive 3D Model: Segmented Earthworm Body
Interactive visualization — available in the full lesson
🖱️ Drag to rotate, scroll to zoom. Notice the ring-like segments and the clitellum band.