Morphological Structure of Annelids

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Topics: Annelid
Introduction The annelids (also called "ringed worms"), formally called Annelida (from Latin anellus "little ring"), are a large phylum of segmented worms, with over 2,000 modern species including ragworms, earthworms and leeches. Annelids are abundance in freshwater, brackish and marine sediments. They are bilaterally symmetrical, triploblastic, coelomate organisms. These invertebrates can be recognized by their segmented bodies. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. They have parapodia for locomotion. Annelids are schizocoelous and with a large and well-developed true coelom (i.e., one that is lined with mesoderm). Except in leeches, the coelom is partially subdivided by septa. Hydrostatic pressure is maintained across segments and helps maintain body rigidity, allowing muscle contractions to bend the body without collapsing it. The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Gases are exchanged through the skin, or sometimes through specialized gills or modified parapodia. Each segment typically contains a pair of nephridia. The nervous system includes a pair of cephalic ganglia attached to double nerve cords that run the length of the animal along the ventral body wall, with ganglia and branches in each segment. Annelids have some combination of tactile organs, chemoreceptors, balance receptors, and photoreceptors; some forms have fairly well developed eyes, including lenses. The annelids number more than 13,000 species distributed among three classes: the marine worm (Polychaeta), which are divided into free moving and sedentary or tube-dwelling forms; the earthworms (Oligochaeta); and the leeches, (Hirudenia). Polychaeta can be found in the intertidal zone up to ~5000 meters of the ocean depth. Oligochaeta and Hirudinea can be found in land.

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References: 1. Fleming, T. P. (1981),The ultrastructure of the sperm funnel and non-glandular vas deferens of Tubifex tubifex (Annelida: Oligochaeta). Journal of Zoology, 194: 505–523. doi: 10.1111/j.1469-7998.1981.tb04598.x 2. http://www.pbs.org/kcet/shapeoflife/animals/annelids.html 3. http://tolweb.org/Annelida 4. http://animaldiversity.ummz.umich.edu/accounts/Annelida/ 5. Fauchald & Rouse (1997) , Tatiana Menchini Steiner; A. Cecilia Z. Amaral: The family Histriobdellidae (Annelida, Polychaeta) including descriptions of two new species from Brazil and a new genus, Contributions to Zoology, 68 (2) 95-108 (1999) To refer to this article use this url: http://dpc.uba.uva.nl/ctz/vol68/nr02/art03

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