The annelids constitute a large phylum of animals that comprises 16,500 species and that includes the segmented worms such as the well-known earthworms and leeches. The name Annelida comes from the Latin work “anellum” that means little rings and was given by Lamarck in 1802 because of the different segments that are seen on these animals.
Annelids are bilaterally symmetric and possess two key morphological features: segmentation and the chaetae. Annelids have three body regions (prosomium, trunk and pygidium) and the majority of the body is divided in units called segments. Each segment is separated by a barrier called septa and contains a coelom (fluid-filled cavity) and also different structures such as execratory, respiratory and locomotory organs. Each segment also has an outer layer of circular muscle underneath an epidermis and a system of longitudinal muscles. In earthworms the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. The parts of the body that are not segmented are the head and the pygidium, a terminal post-segmental region. Annelids also have a closed circulatory system that contains a complex network of blood vessels.
Within the phylum Annelida, some species can reproduce sexually or asexually. The asexual reproduction is undergone by fission, a phenomenon where the posterior part of the body breaks off in order to form a new animal identical.
Among the annelids that undergo sexual reproduction, some are hermaphrodite and others have distinct sexes. For example earthworms and leeches are hermaphrodite and will mate by copulation when the environmental conditions are favorable.
Most species prefer to live in marine environments but some species are also terrestrial (earthworms) and live where there are sufficient amount of water. Many species of annelids play an important role in the environment: earthworms are very important for soil creation and marine worms are essential food sources for fish in the maintenance of the marine food chain. There are species that are parasitic, mutualistic and commensal.
The phylum Annelida is divided into three classes: Polychaeta, Clitellata and Aelosomatida. The class Clitellata contains around 3,100 species that are divided in 3 subclasses: Oligochaeta (earthworms and marine and terrestrial worms), Branchiobdella(marine) and Hirundinea (leeches). Polychaeta is the largest class of Annelida with over 5.500 species and are mostly marine animals. Aelosomatida is the smallest class with approximately 25 species living mostly in intertidal zones.