This slide shows a medusa from the colonial cnidarian Obelia.  Although most of the life of this colonial hydrozoan is spent in the sessile polyp form, medusa buds produced within reproductive polyps called gonangia escape into the water column.  These free-swimming medusae produce either eggs or sperm, which unite to form ciliated planula larvae that swim for a while and then settle to transform into a new polyp, thus completing the cycle.