Label the Life Cycle of Obelia – Colony, Medusa and Planula

All 11 stages and parts on the diagram

Show the list

The cycle 7

Colony The fixed stage: a branching stem with many polyps. It grows by budding and makes no eggs or sperm.
Medusa The free-swimming stage, shaped like a small umbrella. Each one makes either eggs or sperm.
Sperm Tiny swimming cells shed into the water by a male medusa. Currents carry them to the eggs.
Egg A single large cell made by a female medusa. A sperm from another medusa joins it in the water.
Fertilized egg The egg after a sperm has joined it. It divides again and again into a ball of cells.
Planula A small oval larva covered with fine hairs called cilia. It swims for a time, then settles and fixes itself by one end.
Young polyp The planula after it has settled, with a mouth and tentacles. It sends out buds and grows into a new colony.

The colony, enlarged 4

Hydranth (feeding polyp) A feeding polyp, with a mouth and a ring of tentacles. It sits in a clear cup at the end of a ringed stalk.
Blastostyle (reproductive polyp) The middle stalk of a reproductive polyp, in a clear case. Medusa buds grow along it.
Medusa bud A young medusa still fixed to the blastostyle. When it is ready it breaks off and leaves by the open end of the case.
Hydrorhiza (root-like base) The root-like base of the colony. It creeps over a rock or a seaweed and holds the colony down.

How this list was compiled: Every name checked on 2026-10-01 in two sources. R. W. Hegner, College Zoology (1912, public domain), a colonial hydrozoon, Obelia, and metagenesis, https://archive.org/details/collegezoology00hegn. T. J. Parker and W. A. Haswell, A Text-book of Zoology, volume 1 (1910, public domain), Obelia: the colony, reproduction and development, https://archive.org/details/textbookofzoolog00park. Animal Diversity Web, University of Michigan Museum of Zoology, Hydrozoa: the planula and the polyp it grows into, https://animaldiversity.org/accounts/Hydrozoa/. Wikipedia, Planula, https://en.wikipedia.org/wiki/Planula. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

Label the life cycle of Obelia, a small sea animal with two body forms. A fixed colony of polyps buds off swimming medusae, and their eggs grow into new colonies. The first view shows the cycle, the second a piece of the colony.

Read the full guide

The cycle

Read the ring clockwise from the colony. The colony buds off medusae. One medusa sheds sperm and another eggs. The fertilized egg divides and becomes a planula, a larva that swims by its cilia. The planula settles, fixes itself by one end and becomes a young polyp, which buds into a new colony.

The colony

Switch to Colony. The hydrorhiza creeps over the surface and the stem rises from it. Each hydranth is a feeding polyp in a clear cup. Each blastostyle stands in a clear case and carries medusa buds, which break off and swim away. Press Quiz me to name each stage yourself.

The colony makes no eggs or sperm; only the medusae do. Taking turns like this is called alternation of generations.

Questions and answers

What are the stages in the life cycle of Obelia?

A colony of polyps, medusa buds, free-swimming medusae, sperm and eggs, a fertilized egg, a swimming planula larva and a young polyp that buds into a new colony.

What is alternation of generations in Obelia?

A colony of polyps that only buds takes turns with medusae that make eggs and sperm. The colony is the asexual generation and the medusa the sexual one. It is also called metagenesis.

What is a planula?

A small oval larva whose outer cells carry cilia. It swims freely for a time, then settles on a hard surface, fixes itself by one end and becomes a polyp.

Where does Obelia live?

In the sea. The colonies grow fixed to rocks, wharves and seaweeds, from the low-water line down into deeper water.