Phylum Annelida ("a little ring")
Description
Phylum Annelida is made up of the segmented worms. There are about 11,000 species belonging to this phylum, including Earthworms, leaches and marine worms. They live either in the water or underground. They are very closely related to the mollusks, and in the future might be grouped with them. They reproduce hermaphroditially. Those in Phylum Annelida are coelomates, with a two opening digestive tract. Their digestive system has some specialized organs to support their unusual diet. Their bodies are bilaterally symetrical. They have a hydrostatic skeleton, which is a structure made of muscles surrounding the coelom, used to change the shape of the organism for movement. While they don't have a brain, they do have a cerebral ganglia, which is the beginnings of one. Their muscles are both circular and lengthwise, and they have a closed circulatory system. They respire across their skin, or their gills if they are aquatic.