Biology and Ecology of Cnidarians [Phylum Cnidaria (Coelenterata)]

The chief characteristics of the phylum Cnidaria are: radial or biradial symmetry, diploblastic, presence of gastrovascular cavity, a single opening functioning as both mouth and anus, and the presence of nematocysts. The majority of cnidarians live in seawater habitats at different water depths. The true characteristic of hydrozoans is that their gonads for sexual reproduction are derived from epidermal tissue, whereas in all other cnidarians, they are derived from gastrodermal tissue. Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves. Some cnidarians have been reported to absorb dissolved organic matter directly from seawater. The hydrozoans have a worldwide range, and about 40 species, including Hydra, live in freshwater. Among the marine hydrozoan species, Obelia is seen in the coastal waters of all the oceans, and Liriope may form large shoals near the surface in mid-ocean.