- Name Meaning: Little ring
- English Common Name: Segmented worms
- Major distinguishing characteristics: Multiple circular segments
- Approximate number of species described: over 22,000 living
The life history of the Annelida is diverse. They live in terrestrial and aquatic environments, including around deep sea hydrothermal vents. They are common, yet rarely seen because most Annelids are burrowing. Some are highly mobile, some seldom move once established.
Some, such as earthworms, are omnivorous, eating any organic matter in their path. Others, such as the leaches, are specialist in what they will eat. Leaches, for example, are specialist blood eaters.
In all environments where they occur, Annelids are very important in moving and mixing the soil, and breaking up and distributing organic material. This distribution helps plants get better access to nutrients.
The Phylum Annelida contains 7 Classes, 6 of which have living representatives:
- Oligochaeta (earthworms)
- Hirudinea (leeches)
- Machaeridia (extinct)
- Echiura is a former Phylum that has been demoted to Class rank within the Annelida.
Annelids have long bodies. They are generally segmented, the segments usually being visible externally. Each segment generally contains a full set of organs. The segments share a common digestive system, circulatory system, and nervous system.
Three segments, the prostomium (1st segment), peristomium, (2nd segment), and pygidium (last segment), are specialized. The prostomium contains the brain and sense organs. The peristomium contains the mouth. The pygidium is the location of the anus.
Annelids grow from behind. The growth region is just in front of the pygidium. The peristomium is the oldest segment.
The Fossil Record
Annelids are soft bodied and so fossils are rare. The first Annelids probably evolved during the Precambrian, before 541 million years ago. They are present in the Burgess Shale (about 505 million years old), though there may be some Australian fossil annelids about 518 million years old. There is a better fossil record of Annelids by 65 million years ago.