What are Annelids?

Annelids, members of phylum Annelida, are a group of worms with about 18,000 species, including the familiar earthworm. Annelids have a fossil record stretching back to the Cambrian, and are distinguished from other organisms called "worms" by the presence of a body cavity (coelom) and true segmentation, both of which give them an evolutionary advantage. Annelids are the most complex organisms capable of full regeneration if they are cut in half, and indeed are able to reproduce asexually by releasing an end of their tail, which subsequently grows into a complete organism. Annelids can also reproduce sexually.

There are four classes of annelids: polychaete worms, marine annelids which make up the majority of all species (more than 10,000), clitellates, a large group which includes leeches and earthworms, haplodrils, simple marine worms, and myzostomids, small parasites of sea lilies. Polychaetes are characterized by lateral outgrowths called parapodia, which are covered in chitinous bristles called chaetae, giving them their name. The parapodia are used for respiration, locomotion, burrowing, and to create a feeding current.

On land, annelids perform a critical role in breaking down organic matter to create rich soil. As such, earthworms are highly valued by farmers, and there are even industrial worm farms used to break down organic material en masse. Unique among organisms, earthworms have a mouth that connects directly to the anus without an intermediate stomach. This allows them to continually eat and excrete waste as they burrow through the soil. As such, they may be considered terrestrial filter feeders.

Though earthworms are more familiar to us among the annelids, some of the most impressive members of the group are polychaetes, found in the ocean. One polychaete, the Pompeii worm (Alvinella pompejana) lives in hydrothermal vents in the Pacific ocean, where it is exposed to temperatures of up to 80°C (176°F). As such, the Pompeii worm is the most heat-tolerant macroscopic organism known. It is, however, difficult to study, because the organism lives at such depths and is so fragile that it does not survive the decompression from being brought to the surface.

Another interesting polychaete is Hesiocaeca methanicola, the only known animal that can inhabit methane clathrates, extensive methane deposits locked in ice on certain parts of the ocean floor. It is thought that these worms consume bacteria which feed on the methane.