These animals have radial symmetry. The one opening serves as both a mouth to ingest food and for expulsion of any wastes. The bodies of cnidarians are described in one of two ways, depending on whether the opening of the gastrovascular cavity is ventral or dorsal. Cnidarian tentacles serve to capture prey. These tentacles bear stinging cells, called cnidocytes , to subdue their prey, and some jellyfish cnidocytes irritating or toxic to humans.
Cnidarians have both sexual and asexual forms of reproduction.
Hydra , for example, will periodically develop lumps on the sides of their bodies that are either testes or ovaries. 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.