annélides

Annelids, or annélides, are a diverse phylum of segmented worms characterized by metamerism (body segmentation), a coelom divided by septa, and a complete gut. Their bodies typically consist of a series of repeating units, often bearing chaetae (bristles) made of chitin, and many display specialized regions that aid locomotion and digestion.

The phylum is traditionally divided into Polychaeta (mostly marine) and Clitellata, which includes Oligochaeta (earthworms and relatives) and Hirudinea (leeches). Polychaetes usually have well-developed heads, parapodia bearing numerous bristles on many segments, and a wide range of feeding strategies. Oligochaetes lack parapodia and typically have fewer bristles; leeches lack chaetae and often possess a fixed posterior and anterior sucker. The diverse body forms reflect adaptations to aquatic, soil, and host-associated lifestyles.

Habitat ranges widely: polychaetes are predominantly marine and inhabit diverse substrates, while oligochaetes are common in freshwater and soil, and leeches occupy freshwater, marine, and moist terrestrial environments. Annelids play essential ecological roles in nutrient cycling and soil structure, serve as prey for many animals, and include species historically used in biomedical and ecological research.

Reproduction varies by group. Many earthworms are hermaphroditic and cross-fertilize, producing cocoons secreted by a clitellum. Polychaetes often exhibit planktonic trochophore or related larvae, while many clitellates have direct development without a free-swimming larval stage.

The annelid fossil record extends back to the Cambrian, illustrating a long, conserved body plan that remains productive in modern ecosystems.