Anellidi

Anellidi, or Annelida in English, is a diverse phylum of segmented worms that inhabit marine, freshwater, and terrestrial environments. The group includes familiar forms such as earthworms, leeches, and many polychaete worms. Annelids range from a few millimeters to several meters long and possess a true coelom and a closed circulatory system.

A defining feature is metamerism: the body is divided into a series of repeating segments that carry repeated organ systems. Most annelids bear bristles called setae; in many polychaetes these are paired with lateral appendages called parapodia. They typically have a complete digestive tract, a nerve ring and a ventral nerve cord, and a well-developed musculature that drives locomotion. The prostomium at the anterior end and a segmented trunk are characteristic.

Taxonomically, annelids are divided into two main groups: Polychaeta, largely marine; and Clitellata, which groups Oligochaeta (earthworms and relatives) and Hirudinea (leeches). Reproduction varies: many Clitellata are hermaphroditic and form a clitellum that secretes a cocoon for eggs; fertilization is usually cross-fertilization. Polychaetes show a range of reproductive modes, including free-living larvae.

Ecology and importance: annelids are key players in nutrient cycling and soil formation; earthworms mix and aerate soil, while marine polychaetes and oligochaetes contribute to sediment turnover and organic matter breakdown. They serve as model organisms in biology and are commonly used as bioindicators of environmental quality.

Evolution and classification: annelids have a rich fossil record dating to the Cambrian; they are part of the Lophotrochozoa, a superphylum that also includes mollusks and related groups. Trochozoan larval forms are present in many lineages, particularly among polychaetes.