Annelida, commonly known as segmented worms, is a phylum of invertebrates that includes earthworms, leeches, and polychaetes. These creatures are characterized by their segmented bodies, which allow for greater flexibility and movement in their environments. Annelids play vital roles in ecosystems, such as soil aeration and nutrient cycling, making them essential for healthy habitats. Annelids exhibit a wide range of habitats, from freshwater to marine environments. They possess a coelom, a fluid-filled body cavity that aids in movement and organ function. Some species, like Hirudo medicinalis, are even used in medicine for their blood-sucking abilities and therapeutic properties.