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Annelida




True worms, a phylum containing worms that have segmented bodies, coelomic body cavities, a central nervous system composed of cerebral ganglia from which branches extend along the lower part of the body and, typically, bristles called chaetae that project from the body wall. All the annelids have encircling grroves usually corresponding to internal partitions of the body. A digestive tube, nerves, and blood vessels run through the entire body, but each segment has its own set of internal organs.

Annelids are grouped into three classes, the Polychaeta, Oligochaeta, and Hirudinea. The polychaetes include the ringworms and are marine and have groups of long chaetae extending in bundles from extensions of the body called parapodia. The oligochaetes include the earthworms, which are terrestrial forms, and some small aquatic species. They have relatively few chaetae per segment and lack parapodia – features that distinguish them from the polychaetes. Members of the Hirudinea, otherwise known as leeces, are found mainly in freshwater. They lack chaetae and parapodia, but possess suckers at each end of the body.

Annelids form an important part of the diets of many animals.


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