Annelida
Annelida, commonly known as segmented worms, are a phylum of bilaterally symmetric, triploblastic invertebrates. They are characterized by their segmented bodies, which are made up of repeating units called metameres. Each metamere contains its own set of organs, including a nervous system, circulatory system, and excretory system. This segmentation allows for increased flexibility and mobility, as well as specialized functions for different parts of the body.
Common Names
Annelids are commonly referred to as segmented worms, but some specific groups have their own names. For example, earthworms belong to the class Oligochaeta, while leeches belong to the class Hirudinea. Marine annelids include a diverse range of species, often grouped under the common name polychaetes.
Characteristics
Key characteristics of Annelida include:
- Segmentation: The body is divided into segments, each with its own set of organs.
- Coelom: A fluid-filled body cavity that provides hydrostatic support and allows for organ development.
- Closed circulatory system: Blood is contained within vessels and circulates throughout the body.
- Nervous system: A well-developed nervous system with a brain and ventral nerve cord.
- Excretory system: Nephridia or metanephridia remove waste products from the body.
Environments
Annelids are found in a wide range of habitats, including:
- Marine: Many species of polychaetes live in the ocean, from the intertidal zone to the deep sea.
- Freshwater: Some species of annelids live in freshwater lakes and rivers.
- Terrestrial: Earthworms are a common example of terrestrial annelids.
References
- Wikipedia: Annelida
- Britannica: Annelid
- National Geographic: Annelids