These make and release sperm or eggs, which then join to form a zygote, which eventually grows into a new hydra. Hydra also reproduce asexually by a process known as budding in which a new, small Hydra begins to grow from the side of a large Hydra , eventually pinching off when it is fully formed. These are the flatworms because their bodies are flat and ribbonlike. They have bilateral symmetry, but are acoelomates. They have a gastrovascular cavity with one opening.
Planaria have eyespots to detect light.
Some interesting experiments have been done to study their learning ability. They are hermaphroditic and reproduce sexually. They are also capable of asexual reproduction, and many experiments have been done to study the ability of cut planarians to regenerate new planaria or parts of planaria. Genus Schistosoma is a parasite whose life cycle involves both humans and snails, hence the caution not to wade in water in tropical areas where this fluke occurs. Their bodies are made of a number of subsequent segments, each of which is pretty much on its own and separate from the rest.