<P> Obelia is a genus of hydrozoans, which consists mainly of 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 . It is also called sea fur . </P> <P> Obelia has a worldwide distribution except the high - arctic and Antarctic seas . The medusa stage of Obelia species are common in coastal and offshore plankton around the world . Obelia are usually found no deeper than 200 metres (660 ft) from the water's surface, growing in intertidal rock pools and at the extreme low water of spring tides . </P> <P> 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 . </P> <P> 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 . </P>

Various types of zooids found in obelia colony