"Little Rings"

Totally interesting facts about annelids:

• Body metamerically segmented; symmetry bilateral
• Body wall with outer circular and inner longitudinal muscle layers; outer transparent moist cuticle secreted by epithelium
• Chitinous setae (absent in leeches), often present on fleshy appendages called parapodia
• Coelom well developed and divided by septa (except in leeches); coelomic fluid supplies turgidity and acts as a hydrostatic skeleton.
• Blood system closed and segmentally arranged
• Digestive system complete and not metamerically arranged
• Respiratory gas exchange through skin, gills or parapodia
• Excretory system typically a pair of nephridia for each metamere
• Nervous and sensory systems present. Hermaphroditic or separate sexes; asexual reproduction by budding in some

Annelids can be grouped into three classes:

 

Class	Classification	Example
Hirundinea	Many setae	Leeches
Oligochaeta	No parapodia, few setae	Earthworms
Polychaeta	No setae or parapodia	Neresis Worms, Aphrodites

Click on the above classes or the pictures below for more information on each:
 
 





                            Polychaeta                                        Oligochaeta                                     Hirundinea
                                     Courtesy of FINS                                                 Courtesy of Mandaville                                         Courtesy of LOLBAIT

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    There are nearly 15,000 species in this phylum. They are characterized by being soft bodied, mostly round in cross section, and longer than wide. Unlike most other vermiform, or worm-like, invertebrates, the bodies of annelids consist of a series of repeating segments that results in a serial repetition of many of the internal organs including the muscular system, nervous system, and excretory system. This repetition is called metamerism and each segment a metamere. Each segment is generally separated from the next one by a septum. This allows each segment to contract independently using circular and longitudinal muscles without altering the hydrostatic pressure in other parts of the animal.
    The annelid worms are thought to have evolved from a primitive coelomate worm-like ancestor which developed metameric segmentation. The development of a coelom conferred many advantages, including acting as a hydrostatic means of locomotion. However, in the ancestral coelomate the force of muscle contractions in one area was carried throughout the body and so precise control of body movements was not possible.
    Fossil annelida are rare because their soft bodies do not preserve well. Existing fossils date from more than 500 million years ago. They may be descended from the flatworms and are thought to have given rise to the arthropods. Some aberrant marine annelids are often placed in separate classes, such as the Archiannelida, which lack parapodia; others may even be in separate phyla, such as the Myzostomaria, which are parasites of echinoderms, and the Echiuroidea, which are unsegmented.