The term Annelida was coined by Lamarck in 1809. Annelida is triploblastic, bilaterally symmetrical, coelomate, and segmented Metazoans. They show a dermo-muscular body wall. The body shows metameric segmentation. The coelom is filled with coelomic fluid. It acts as a hydraulic skeleton. The phylum contains over 8,600 species in the present-day world.
Phylum Vermes represented a heterogeneous assemblage of organisms. In 1798 Cuvier first pointed out the differences between unsegmented worms and segmented worms. Lamarck coined the name Annelida. He included segmented worms in this group. The name is Annelida means "little ring". It is derived from the Latin "annulus" (ring) or the French "Annales" (arranged in rings) and (greek "eidos" (form). Grube coined the terms "Polychaeta and Oligochaeta". Ehlers worked hard to develop this Phylum.
Annelids are triploblastic, bilaterally symmetrical, metamerically segmented, and true coelomate worms. They show a dermo-muscular body wall. The epidermis secretes an outer cuticle.
The body cavity is a true perivisceral coelom lying between the layers of the mesoderm. In Leeches it is small and in other annelids, it is a big cavity. In these animals, the blood vascular system is well developed. It is a closed type The blood is colored. In the blood, hemoglobin is dissolved. The excretory system contains segmentally arranged nephridia. The nervous system is well developed they may be hermaphrodites or unisexual animals. In unisexual animals, the development includes a larval form, which is typically Trochophore.
Annelids are of worldwide distribution. They are abundant in the present time. They might have started during the Precambrian period. The annelids inhabit all parts of the temperate and tropical regions of the earth. They are aquatic, terrestrial, free-living, and parasitic forms. Among annelids "oligochetes" will show diversity in size. The smallest earthworms are Chaetogaster, Aeoiosoma. They grow up to 1 m.m. in length. The giant earthworms of Ecuador called Rhinordrilus will grow up to 21.1 meters in length. The leaches are generally small but range from 10 to 200 m.m. in length.
The important feature of annelids is metamerism. All the segments of the body will show a repetition of similar body parts. These segments are called metameres.
It is generally believed within Annelida Polychaeta gave rise to Oligochaeta and Archiannelida. Oligochaeta gave rise to Hirudinea. Polychaeta is represented by marine bristle worms. Oligochaeta is represented by earthworms. Hirudinea is represented by leeches.