Britannica
Sources used in this answer
What are the three major classes of annelids?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
annelid
The structure of polychaetes. (Left) Free-moving polychaetes. (A) Neanthes, (B) Nereis. (Right) Tube-dwelling (sedentary) polychaetes. (C) Amphitrite, (D) Sabella.
Most authorities classify the phylum Annelida into three major classes based on their morphological characteristics and ecological adaptations:12
- Polychaeta (marine worms): These are typically characterized by paired lateral appendages called parapodia that bear numerous chitinous bristles, or setae. They include both free-moving and sedentary, tube-dwelling forms.13
- Oligochaeta (earthworms): Primarily found in freshwater or terrestrial environments, these annelids have few setae per segment and lack the complex head appendages found in polychaetes.13
- Hirudinea (leeches): These organisms generally possess a fixed number of body segments (34) and are distinguished by suckers at both the anterior and posterior ends.13
While these three classes are the most widely accepted, classification systems can vary depending on the author, with some schemes proposing up to six classes by separating specific orders or groups.2