Annelids

Annelids are a phylum of segmented worms that includes polychaetes, earthworms, and leeches. They are a diverse group with thousands of described species that span a wide range of habitats, from deep oceans to moist soil.

A defining feature of annelids is metamerism: their bodies are divided into repeated segments separated by septa. They possess a true coelom, a closed circulatory system, and a complete digestive tract with regional specialization. The nervous system is developed, with a brain (pair of cerebral ganglia) and a ventral nerve cord with segmental ganglia. Excretory systems typically include paired nephridia in each segment. Most annelids have chaetae, bristle-like structures that aid in locomotion; in polychaetes these chaetae are often borne on parapodia, while in oligochaetes and leeches they are reduced or absent.

Annelids display a range of locomotory and morphological adaptations. Polychaetes usually have well-developed parapodia with numerous chaetae and are primarily marine. Oligochaetes, including earthworms, generally lack parapodia and have a simpler body plan. Leeches are often dorsoventrally flattened and typically lacking chaetae.

Ecology and life history vary widely. Annelids occupy marine, freshwater, and terrestrial environments and contribute to nutrient cycling, sediment mixing, and soil structure. Many are detritivores, scavengers, or filter feeders; some are predators.

Reproduction is predominantly sexual, with many earthworms and leeches being hermaphroditic and others dioecious. Cross-fertilization is common, and in clitellates the clitellum forms a cocoon for developing eggs. Some species can reproduce asexually by fragmentation or budding. Development ranges from direct development to trochophore-like larval stages in some polychaetes.