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Phylum Cnidaria: Characteristics of Jellyfish, Portuguese Man of War and More!

Updated on November 15, 2011
Flick photo by TANAKA Juuyoh ()
Flick photo by TANAKA Juuyoh ()
Flickr photo by ^riza^
Flickr photo by ^riza^
Flickr photo by cursedthing
Flickr photo by cursedthing

Cnidarians - Jellyfish, Sea Anemones, Portuguese Man of War and More!

The phylum (a biological ranking of species below kingdom and above class) Cnidaria is extremely diverse. There are 9,000 different species in this phylum and they stretch from jellyfish to coral.

Defining Characteristics

If there are so many Cnidarians and if they are so diverse, what is it that makes them Cnidarians? There are certain defining characteristics that a creature must have to fit into this phylum.

Cnidocytes – Stinging Cells

All Cnidarians have what are called Cnidocytes (this is where Cnidarians get their name). Cnidocytes are small stinging cells. Cnidocytes are truly amazing. These cells contain nematocysts and nematocysts in turn contain hollow, spoiled thread that is meant to entangle prey and shoot from the nematocyst upon contact. At the base of these threads there are both barbs and spikes which release stinging (sometimes paralyzing) poisons.

Polyp and Medusa – Two Body-forms

Most Cnidarians have two different body-forms; the polyp and the medusa. The early stage of a Cnidarian is usually the polyp stage: In this stage, the mouth of the creature is directed upward and it is sessile (stationary, immobile) with its bottom side attached to the substratum.

This polyp stage of a Cnidarian often transforms into a medusa (a common jellyfish is a perfect example of a Medusa). A medusa has its mouth facing downward and is mobile. It either moves by wind or current (involuntary) or by oscillating (voluntary).

There are some exceptions to the “two body-forms” Cnidarian, some skip one of the body-forms all-together and some are only one of the forms for a very short time.

Radial Symmetry

Cnidarians also have radial symmetry. This is quite amazing. If looking at a jellyfish from the bottom, you will be able to see that it is usually perfectly symmetrical with each body part (the tentacles, gonads and oral arms) with the mouth as the axis.

Classes of Cnidarians

There are four main classes of Cnidarians, each of them being very diverse and extremely interesting.


The Anthozoa class is a form of Cnidarian that often stays in the polyp stage for its entire existence. The most common Anthozoans are Corals and Sea Anemones.


The class Cubozoa is an amazing class of Cnidarians. These are more commonly known as “Box Jellies” and are most noted for their complex eyes.


These are also often found to stay in the poly body-form for their entire existence. The most common Hydrozoa is the Hydra. These are amazing species that reproduce by “budding”, which is a form of asexual reproduction where a portion of the Hydra ‘buds’ off of the creature and eventually separates itself and becomes a complete organism.


The Scyphozoa is the most famous class of Cnidarians. These are the “True Jellyfish” and are extremely diverse.

Since there are so many amazing species in the Cnidarian phylum I decided to do a quick look at some of the more interesting ones.

More on each specific class of Cnidarians and on some specific Jellyfish coming soon!

Flickr photo by Winston & Michelle
Flickr photo by Winston & Michelle

Portuguese Man of War

Portuguese Man of War

The Portuguese Man of War is amazing! Although this creature appears to be one animal, it is not. It is a colony of four different types of polyps and medusas. Each of these different types (which are called Zooids) is highly specialized. None of them have the essential characteristics for life alone, but together they make an amazing creature.

The top of a Portuguese Man of War is a sail (1st Zooid) that is filled with gases. The sail can get up to a foot long and floats above the water. If the Portuguese Man of War is in danger of being attacked from above the surface of the water, it is able to release the gases and deflate; causing it to submerge for a time until the danger has left.

A Portuguese Man of War is very much like an iceberg: Although it may only rise inches above the water, its tentacles (2nd Zooid) can stretch up to 150 ft! The average Portuguese Man of War has tentacles that extend 30 ft. There is a mass of these tentacles that act to sift through the water and catch fish and shrimp. These tentacles have powerful Cnidocytes (stinging cells) that kill their prey and then the tentacles contract and move the dead prey to the digestive portion (3rd Zooid). The fourth Zooids are the reproductive portion of the Portuguese Man of War.

These amazing creatures are found in the Atlantic, Pacific and Indian oceans. If you stumble across one (a rather unusual occurrence that would be!) and you keep your distance, you will be alright. Remember though, their numerous tentacles can reach up to 150 ft – this can be quite the death trap for the unsuspecting!

Definition of Zooid: One of the distinct individuals forming a colonial animal such as a bryozoan or hydrozoan. (from

To look at the Phylum Porifera (Sea Sponges) follow this link:

Phylum Porifera by apStumbo

Keeping Jellyfish in Fish Aquariums!

Jellyfish are extremely beautiful and as of late, many people have decided to bring this beauty into their homes. There are many different ways of keeping jellyfish in fish aquariums. A good place to start would be to look at this book and others like it! For the truly adventurous, I have also provided a link to one popular jellyfish aquarium.


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      jill 4 years ago

      man of war is cool