  obelia is a genus in the class hydrozoa, which consists of mainly marine and some freshwater animal species and have both the polyp and medusa stages in their life cycle. the genus belongs to the phylum cnidaria, which are all aquatic and mainly marine organisms that are relatively simple in structure. ==habitat== obelia has a worldwide distribution except the high-arctic and antarctic seas.cornelius, p.f.s., 1990a. european obelia (cnidaria, hydroida): systematics and identification. journal of natural history 24: 535-578. the medusa stage of obelia species are common in coastal and offshore plankton around the world.cornelius, p.f.s., 1995b. north-west european thecate hydroids and their medusae. part 2. synopses of the british fauna (new series), no 50. obelia are usually found no deeper than  from the water's surface, growing in intertidal rockpools and at the extreme low water of spring tides. obelia is a genus in the class hydrozoa, which consists of mainly marine and some freshwater animal species and have both the polyp and medusa stages in their life cycle. the genus belongs to the phylum cnidaria, which are all aquatic and mainly marine organisms that are relatively simple in structure. ==life cycle== the polyp colony reproduces asexually. during this stage of life, obelia are confined to substrate surfaces. on this mature colony there are individual hydranths called gastrozooids, which can be found expanded or contracted, to aid in the growth of this organism by feeding; the reproductive polyp gonozooids has medusa buds. other hydranths are specialized for defense. the main stalky body of the colony is composed of a coenosarc, which is covered by a protective perisarc. the next generation of the life cycle begins when the medusae are released from these gonozooids, producing free swimming only male medusae velum with gonads, a mouth, and tentacles. the physical appearance of the male and female medusae velum, including their gonads, are indistinguishable, and the sex can only be determined by observing the inside of the gonads, which will either contain sperm or eggs. the medusae reproduce sexually, releasing sperm and eggs that fertilize to form a zygote, which later morphs into a blastula, then a ciliated swimming larva called a planula. the planulae live free-swimming for a while but eventually attach themselves to some solid surface, where they begin their reproductive phase of life. once attached to a substrate, a planula quickl
Leptomedusae