The term Annelida coined by Lamarck in 1809. Annelida are triploblastic, bilaterally symmetrical, coelomate and segmented Metazoans. They show dermo-muscular body wall. The body shows metameric segmentation. The coelom is filled with coelomic fluid. It acts as a hidraulic 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 Annelida means "little ring". It is derived from Latin "annelus" (ring) or from French "anneles" (arranged in rings) and (ireek "eidos" (form). Grube coined the terms "Polychaeta and Oligochaeta". Ehleres worked hard to develop this Phylum:
Annelids are triploblastic, bilaterally symmetrical, metamerically segmented and true coelomate worms. They show dermo-muscular body wall. The epidermis secretes and outer cuticle.
The body cavity is a true perivisceral coelom lying between the layers of 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 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 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 upto 1 m.m. in length. The gaint earthworms of Ecuador called Rhinordrilus will grow upto 21.1 metres 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 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.