Most roundworms and flatworms are found in all parts of the Earth such as in soil, salt flats, and aquatic sediments.
Adaptation to their environment
Unlike sponges or cnidarians, unsegmented worms have well-developed senses and nerve systems to live inside a host. Unsegmented worms also have specialized cells such as muscles cells which help worms to contract and twist their bodies. However, some of unsegmented worms are parasitic and may cause terrible damages and diseases to living organism.
Comparisons to other phyla
Phylum porifera, also known as sponge, does not have well-developed nerve systems, and gastro vascular cavity. Unlike sponges and cnidarians, unsegmented worms are mostly parasitic and have structure that help them to absorb food and harm their hosts.
All highlighted words are important to know and defined in the paragraphs.