v Earthworms belong to the class Oligochaeta of the phylum Annelida.
             v The annelid phylum contains over 1,000 worms.
             v Many species of earthworms grow to a length of only a few centimeters, but some tropical species reach a length of up to 3.3 meters!
             v An outer series of circular or transverse muscle fibers girdle the body and an inner series of longitudinal muscle fibers are used to move the earthworm. They are able to burrow with considerable speed, particularly in loose soil.
             v Earthworms have bilateral symmetry. They contain an external muscular system, a circulatory system complete with a dorsal blood vessel (otherwise known as the "heart"), a central nervous system, a reproductive, a respiratory, an excretory, and a digestive system.
             v Earthworms are hermaphrodites (each worm has both female and male reproductive organs). Mutual cross-fertilization usually occurs, and the eggs are buried in the earth in captions until they hatch as fully developed worms.
             v One unique characteristic of the Earthworm are its nephridia. Nephridia are ciliated tubes (tubes lined with hairs) that eliminate liquid wastes.
             v Earthworms lack gas-exchange organs and breathe through their skin. Therefore, they must remain in damp environments or in water because their entire body must stay moist.
             v As opposed to most invertebrates, earthworms contain a closed circulatory system.
             v Earthworms have a one-way digestive system. Their alimentary canals have two openings. (The mouth and the anus.) Food is broken down both mechanically and chemically in the digestive tract. Usable nutrients are then absorbed into the body cells.
             v Earthworms play an important role in soil ecology. They sir up and loosen soil that they burrow through, enabling it to be aerated by their actions. They are valuable to farmers because they help to make the soil more fertile and better able to fertilize plants.
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