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Far from being lowly worms, annelids are impressively powerful and capable animals. Annelids are distinguished by ring-like external bands along their muscular body wall that coincide with internal partitions dividing their bodies into segments. With a circulatory system to distribute blood and oxygen and a one-way gut, their bodies are enormously more complex than modern flatworms. Developing a gut that runs from one end of the body to the other was a major evolutionary step. With such a digestive tract, food can be continuously taken in by a mouth, processed as it passes through the body and released as waste at the other end. Not only can worms continually digest their food, but they can squirm, crawl and slither as they do it. Annelids creep along or burrow by coordinating two sets of muscles. One set allows them to expand and anchor one part of their body while the other set contracts and pushes the rest of the body forward into the sediment. By alternating these two muscle sets, the worm can powerfully inch forward.
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