Phylum Annelida

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Topics: Annelid
Phylum Annelida
L. annelleus = little ring
Annelida have bodies consisting of many essentially similar ringlike segments (somites or metameres).This segmentation usually shows in both external and internal features, including muscles, nerves, and circulatory, excretory and reproductive organs.

Phylum Annelida
This phylum is divided into three classes:
Oligochaeta: Earthworms - Most earthworms and their equals are inhabitants of damp soil and fresh waters.
Hirudinea: Leeches - The leeches are found mainly in fresh water or on moist ground.
Polychaeta: Bristle-worms - The marine worms are found chiefly in close vicinity of the shore.
• OLIGOCHAETA = Earthworms
• HIRUDINEA = Leeches
• POLYCHAETA = Bristle-worms

Some annelids are free-living, many inhabit burrows or dwell in tubes, some are commensals on other aquatic animals (few are ecto- or endoparasites), and many of the leeches attach to vertebrates.
• Symmetry bilateral. Triploblastic. The body is elongate and usually conspicuously segmented both internally and externally.
• Appendages are minute rodlike chitinous setae, little to many per somite.
– Polychaeta have fleshy tentacles on its head and has the setae situated on the lateral fleshy parapodia.
Most species of the class Hirudinea lack setae.
• The Body is covered by a thin moist cuticle over columnar epithelium containing unicellular gland cells and sensory cells.
Both the body wall and the digestive canal has layers of circular and longitudinal muscles. The body cavity (coelom) is well developed (except in the leeches) and is divided by septa in the Oligochaeta and Polychaeta.
• A complete digestive canal is present in a tubular shape, extending the whole length of the body.
• A closed circulatory system of longitudinal blood vessels with branches extending laterally in each segment. Dissolved hemoglobin and free amoebocytes are usually present in the blood plasma.
Respiration occurs via the epidermis or through gills in some

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