There is a great diversity of species (>16,700) in the phylum Annelidae, with species present in all seas and oceans, as well as a great variety of sizes, from a few millimeters to specimens reaching 3 meters (Amynthas mekongianus). Popularly known as worms, this term has no taxonomic validity due to the existence of so-called ‘worms’ in phyla other than annelids, such as flatworms or nematodes.
Etymologically, the word annelid comes from the Latin ‘annellum‘ meaning ring. Annelids are characterized by the repetitive presence of these rings (or scientifically known as metamers) throughout the organism. Each metamer contains all the organs and could therefore be considered as autonomous structures. Other characteristics of these animals are that they have a soft, segmented body, as previously mentioned, cylindrical in shape and with bilateral symmetry. Generally, the body of annelids is differentiated into an anterior region known as prostomium and a posterior region known as pygidium. Many annelid species have filaments called quetae, which are used to subclassify species. In the last decades there have been many changes in the classification of annelids, although in many works the following groups are still used:
- Clitellates (class Clitellata): this group of annelids possess a specialized body region in which the reproductive organs (clitellum) are located. Species as well known as earthworms are part of this group, although there are many marine species too.
- Echiurans: Group of annelids in which we find exclusively marine species, characterized by an absence of segmentation in their bodies.
- Myzostomids: This is a group of annelids consisting of species mostly parasitic on echinoderms.
- Bristle worms (class Polychaeta): This is the group that includes the largest and most diverse number of annelid species, being mostly marine species. They are characterized by having a multitude of quetae, which they use to swim, dig, breathe and/or capture food. There are both wandering polychaetes that move freely through the substrate, as well as sedentary polychaetes that live in galleries that they excavate in the substrate.
