A worm is a long, soft-bodied invertebrate that belongs to the phylum Annelida or Nematoda. They typically live in soil, freshwater, or marine environments and play a crucial role in ecosystems by breaking down organic matter and aerating the soil. Common types of worms include earthworms, which are beneficial for gardening, and roundworms, which can be found in various habitats. Worms have a simple body structure, lacking limbs and a hard skeleton. They move by contracting and relaxing their muscles, allowing them to burrow through the ground. Many species of worms are also important in scientific research, such as the Caenorhabditis elegans, a model organism used in genetics and developmental biology studies.