The Obelia colony grows as a branched stem sheathed in a chitinous covering, anchored by root-like stolons. Feeding polyps, or gastrozooids, sit in protective cups along the branches and catch small plankton. Reproductive polyps, gonozooids, are shaped quite differently and bud off medusae rather than feeding at all.
The released medusa is around a millimetre or two across, flattened, with many short tentacles around the margin and gonads on the radial canals. It swims, feeds, and produces eggs and sperm. Fertilisation yields a ciliated planula larva that settles on a surface and grows into a new colony, closing a cycle in which a settled asexual generation alternates with a free-swimming sexual one.
What the textbook diagram tends to flatten is that colony and medusa are ecologically separate animals occupying different habitats and different food webs. The medusa also differs from typical hydromedusae in lacking a well-developed velum, the muscular shelf that most hydrozoan medusae use to jet, and it is a comparatively feeble swimmer that drifts more than it propels.