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Green Hydra: Example of Hydrozoan

What they are - biological definition

Hydrozoans are aquatic living beings belonging to the Cnidarian phylum. Hydrozoans can be found in both salt and fresh water. About 3,000 species of hydrozoans are known, with hydra being one of the best known.

Main characteristics of hydrozoans:

- Presence of gastrovascular cavity, without pharynx and without mesentery.

- Gonads of ectodermal origin.

- Life system in colonies.

- Specific functions within the colony.

- Presence of cnidoblasts (urticant cells) only in the epidermis.


In hydrozoans, reproduction can be asexual or sexual depending on the species.

In some species, such as hydra, reproduction is asexual, occurring through lateral budding. A bud sprouts on the animal's side. When it reaches a certain size, it comes off, thus giving rise to a new being.

Sexual reproduction is also present in some species of hydrozoans. In this case, sperm and eggs can be produced by the same or different beings.

Life cycles

- Through the development of polyps only. This is the case with hydras.

- By developing only jellyfish. This is the case of liriope tetraphyla.

- Through the development of jellyfish and polyps with metagenesis (alternation from generation to generation). This is the case with obelia.

Examples of hydrozoans:

- Hydra

- Portuguese caravel

- Fire Coral

- Obelia

- Antomedusa

- Liriope

Photo of a Portuguese Caravel (Physalia physalis), hydrozoan animal.