Annelid

The annelids are a large phylum of animals, comprising the segmented worms, with about 15000 modern species including the well-known earthworms and leeches. They are found in most wet environments, including many terrestrial, freshwater, and especially marine species, as well as some which are parasitic or mutualistic. They range in length from under a millimetre to over 3 metres.

Annelids are triploblastic protostomes. The body cavity is a coelom, which is divided into a sequence of compartments by walls called septae. In the most general forms each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment is marked externally by one or more rings, called annuli. In most forms they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.

Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is usually specialized. Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders which simply ingest the sediments, and blood-suckers.

Most polychaete worms have separate males and females. The earliest larval stage, which is lost in some groups, is a ciliated trocophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size. The oligochaetes and leeches tend to be hermaphroditic and lack free-living larvae of this sort. Many annelids have some regenerative abilities, often to the point where each half of an adult divided cross-wise will survive.

The annelids are typically classified as follows:

• Class Polychaeta
• Class Clitellata
  • Subclass Oligochaeta (e.g. earthworms)
  • Subclass Acanthobdellida
  • Subclass Branchiobdellida
  • Subclass Hirudinea (leeches)
• Class Myzostomida
• Class Archiannelida (polyphyletic)

The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian, and there are some signs they were around in the later Precambrian. Their closest living relatives appear to be two small phyla of unsegmented worms, the Pogonophora and Vestimentifera. It has been suggested that these may in fact be highly modified annelids.

After these, the most obvious relationship is between the Annelida and Arthropoda, both of which have a segmented structure. However, a number of other differences between the two groups have led some to suggest this similarity is the result of convergent evolution. The other major phylum related to the annelids is the Mollusca, which is united with them by the common presence of trocophore larvae in the two groups.