Aequoreidae, Bougainvilliidae, Porpitidae, Physaliidae - Hydroids

Hydroids are colonial, plant-like organisms and are closely related to jellyfishes. Most hydroid life cycles consist of three stages: a flat, ciliated free-swimming larva (called a planula larva) that grows into a plant-like colony of polyps which, in turn, produces many free-swimming gelatinous medusae (jellyfish type body) by budding off the polyps. The polyp colony is considered to be in between the larval stage and the adult (medusa) stage. Some hydroids skip the polyp colony stage, and the larvae develop straight into medusa. Others reduce the medusa stage, and the colony continues to develop into the main adult form. Some of the colony forms are so small that it is extremely difficult to find them (as in the hydromedusa Nemopsis bachei whose colony stage reaches a mere 0.6 mm high). The colony stage of the many-ribbed jelly (Rhacostoma atlantica) is so rare it is yet to be described. The freshwater jellyfish, Craspedacusta sowerbyi, is a hydroid.

The colonies contain different types of polyps, each with a specific function. Some catch prey. Some eat and digest the prey. Some defend the colony, and some reproduce. Most have many small tentacles to catch and move food particles. The polyps are attached to a base stalk (stolen) which may be branched or unbranched. The polyp stages usually attach to hard substrates where as the medusa are mostly free-swimming. Most hydroid colonies are carnivorous as are the medusa. Both the polyps and the medusa can inflict sever stings in human.

Hydroid medusae are called hydromedusae to distinguish them from the true jellyfishes. The hydromedusae have a velum, which is a thin shelf of tissue encircling the underside of the medusa bell. The true jellyfishes lack a velum. Also, true jellyfishes do not go through a colonial polyp stage.