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Polychaetes are a taxanomic class of segmented worms. Annelida is the phylum for segmented worms and polychaetes are the marine varieties of segmented worms. These worms were created on the fifth day of creation.
Some polychaetes might not even look like worms but they come in all kinds of different shapes and colors and sizes. Polychaetes can inhabit mostly all marine habitats including on the shoreline and in the deep ocean.
Polychaetes have segmented bodies and each segment has parapodia, which are flat lateral outgrowths from the body wall. These parapodia have supporting structures called acicula that help the polychaete with movement. In some polychaetes the parapodia are modified to serve as gills for respiration. digestive system in polychaetes is considered a "tube within a tube" because it is a long intestine that runs the length of the body. Polychaetes feed using a pharynx, or mouthpart, that pumps food through the esophogus (throat), then to the crop to be stored. The gizzard then grinds the food into small pieces so that the intestine can finish digesting the food. (Miller/Levine pg.695)
Most polychaetes have a closed circulatory system. They have two main blood vessels, dorsal (topside) and ventral (bottomside), that run the length of the body. Smaller blood vessels take the blood to each segment. (Miller/Levine pg.695)
Polychaetes reproduce both sexually and asexually. The reproductive organ in polychaetes and all annelids is called the clitellum. Eggs are fertilized externally and the larvae are ciliates (surrounded by hairlike cilia for movement). These larvae are called trochopores. 
Some polychaetes can also reproduce asexually by budding, which is the growth of a new worm on itself then dropping the new worm off. 
Polychaetes in the particular genus Eunice reproduce all at the same time in concordance to a phase of the moon. The females emit their own light to attract males, then the posterior (rear) end of both sexes falls off with the sex organs. The pieces of worm release clouds of eggs and sperm. In some species, the fallen off end can grow a new worm.
Not every type of polychaete is motile (actively moves), some are sessile (settle in one spot and stays). The motile types of polychaetes have an active lifestyle being grazers, scavengers, or predators. Some have even developed sharp, jagged jaws and suckers to inhabit parasitic habitats.
A lot of sessile types live in a tube that they make for themselves. When many of these polychaetes live together they can form a reef. These polychaetes often grow feathery tentacles to extend for filter feeding.
- Polychaete Wikipedia
- Polychaete UCMP
- Polychaete Zoology
- Biology, Miller/Levine, copyright 2008, published by Pearson Prentice Hall