Annelids are a phylum including earthworms, leeches, polychaete worms, and some other worm groups(3). Annelids are also called segmented worms because a defining feature they all share is a body most of which is divided into segments, each of which contains parts of the basic body systems (2). This segmentation, or metamerism, allows annelids to move more efficiently and provides other benefits as well(2). Inside all of these segments, in a cavity known as the coelom, is fluid that presses against the annelid’s body, keeping the body firm so that it can bend and extend without breaking (1,2,3). Although segmented worms may appear simple, the organs inside them are highly developed; various annelids have organs that can sense touch, balance, and chemicals in their environment, and some even have eyes with lenses (2,3). While all annelids share some important physical characteristics, they live all over the world in a huge range of habitats on land and in the water (both freshwater and marine), with varying diets, feeding habits, reproductive methods, and life cycles (2,3).
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