Hydrozoans are a carnivorous class in the Phylum Cnidaria. They are typically very small, and form colonies to support each other. Some cannot survive on their own. Excluding Siphonophores, Hydrozoan colonies are usually smaller than those of other Cnidarians. These colonies can appear to be coral, but unlike coral, Hydrozoans do not have a symbiotic relationship with zooxanthellae and rely only on catching prey to survive. This means they can live at greater depths than coral, since they do not need sunlight. The aforementioned Siphonophores are small organisms that form sometimes very large colonies that take the appearance of one animal, such as the Portuguese man o' war. Hydrozoans mostly live in the ocean, but a few such as freshwater jellies can live in lakes and rivers.
Physical Features:
Hydrozoans, like all Cnidarians, possess radial symmetry. Most go through a polyp stage and a medusa stage in their lives, but there are some exceptions. The polypoid stage is usually longer. Most species practice sexual reproduction