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How do earthworms, leeches, and polychaetes differ?
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annelid
The structure of polychaetes. (Left) Free-moving polychaetes. (A) Neanthes, (B) Nereis. (Right) Tube-dwelling (sedentary) polychaetes. (C) Amphitrite, (D) Sabella.
Earthworms, leeches, and polychaetes are all members of the phylum Annelida, characterized by segmented bodies and a coelom. They differ significantly in their anatomy, habitat, and behavior:
- Polychaetes: Primarily marine, these worms are known for their parapodia—fleshy outgrowths on each segment equipped with setae (bristles). They exhibit diverse lifestyles, including free-moving, burrowing, and tube-dwelling forms. Unlike the other groups, many polychaetes possess specialized gills for respiration.123
- Earthworms (Oligochaeta): These are mostly terrestrial or freshwater scavengers. They lack the complex appendages of polychaetes, having only simple setae embedded in their body walls. They are hermaphroditic and possess a clitellum, a glandular structure that secretes a cocoon for their eggs.143
- Leeches (Hirudinea): Typically found in freshwater or humid environments, leeches are distinguished by having a fixed number of 34 segments and prominent suckers at both ends. They lack the setae found in the other two groups and have a reduced coelom. Most are specialized bloodsuckers or carnivores.1253