Cnidaria have radial symmetry and tissues that are made of specialized cells. Cnidaria are named after cnidocytes, the stinging cells which all members have.
Flatworms have bilateral symmetry and they are the first animal group with a head.
They also have organs and systems including testes, ovaries, and kidney-like structures. Some examples of flatworms are planaria, fludeworms, and tapeworms. Roundworms have a body cavity filled with fluid that houses the organs. It serves as a circulatory system and also makes the worm rigid, like a hydrostatic skeleton. Roundworms are parasitic to both animals and plants.