Examples of “round” animals include members of the phylum Cnidaria (hydras, jellyfish, corals) and phylum Echinodermata (starfish, sea urchins, sand dollars). An animal with radial symmetry usually has a nerve net that only allows very simple types of behavior. Animals that are round are usually sessile (nonmoving). This is in contrast to animals that display bilateral symmetry, which have a distinct head/tail and in which the animals can be divided into different planes. Bilaterally symmetrical animals usually move in a specific direction. (For more about animal symmetry, see the chapter “Aquatic and Land Animal Diversity.”)