What Are Cnidarians? Facts About 4 Amazing Species

what are cnidarians
By Kevin Gessner, Flickr, copyright 2021

While at times contributed by guest authors, our content is medically reviewed periodically by professionals for accuracy and relevance. We pride ourselves on our high-quality content and strive towards offering expertise while being authoritative. Our reviewers include doctors, nurses, mental health professionals, and even medical students.


Cnidaria is a phylum under Kingdom Animalia. Hence, the word “Cnidarians” gives a clear-cut answer to what are cnidarians.

This article will give you in-depth knowledge and answer all your questions related to “What are Cnidarians?”

These invertebrates are aquatic animals (marine Cnidarians or freshwater Cnidarians). Cnidarian is also popularly known as Coelenterata.

At first, coelenterates included the cnidarians and sponges (phylum Porifera) and comb jellies (phylum Ctenophora). Now, “coelenterate” generally refers only to cnidarians. But Cnidarians are often used for animals falling under this phylum to eliminate ambiguity.

The phylum is so-called Cnidaria because it has Cnidoblast cells or stinging cells. The phylum is also called Coelenterata because it has a coelenteron or gastrovascular cavity (body cavity), which is meant for digestion and circulation (gastro- digestion, vascular- circulation).

Cnidarians range in size from Hydra being just 5–20 millimetres to Lion’s mane jellyfish, which may exceed 2 meters in diameter and 75 meters in length.

Cnidarians evolved in the Precambrian Era, but what type of organism they are is unknown. Primarily, Cnidarians include jellyfishes, Hydra, sea anemones, and corals.

What are Cnidarians?: Salient Features 

Phylum Cnidaria is the first phylum to have nerve cells that are apolar or non-polar neurons (in apolar neurons, there is no axon, only dendrites are present. Apolar neurons have a web-like structure). The impulse is received and transmitted in any direction.

Phylum Cnidaria has diffused nervous system (No brain formation observed). The animals of phylum Cnidaria are diploblastic (Diploblastic animal has only two germinal layers: outer ectoderm and inner endoderm, between them, a non-cellular undifferentiated layer is present called Mesoglea.

The mesoglea is just one cell thick, non-living, and looks like jelly). They are, as a fact, the first diploblastic animals.

Cnidarians are mostly marine ( like sea anemones, coral reefs, sea pens, moon jellyfish, and many more), but a few are freshwater Cnidarians (like Hydra). Cnidarians can be solitary (present/ existing alone) like Hydra or colonial (present/ existing in a colony) like Obelia.

Animals of phylum Cnidaria are radially symmetrical (When any plane passing through the center along an oral-aboral axis that is longitudinal axis divides the animal into equal parts, it is called radial symmetry).

Cnidarians have the tissue grade of the organization. Tissue grade organization means the formation of tissues formed by a group of cells that work together to accomplish one or more specific functions. These tissues perform all the functions.

Digestion is through the body surface or cell membrane in animals of phylum Cnidaria. Excretion through body surface and excretory material is ammonia; that is, they are ammonotelic.

Despite their diversity, Cnidarians share several essential characteristics like- they all are radially Symmetrical: Cnidarian body parts are arranged around a central point. They have two Layers of Cells: Cnidarians have an epidermis, or outer layer, and a gastrodermis (also called the endodermis), which lines the gut.

Cnidarians’ similarities support the theory that they had a single origin.

What are Cnidarians: About Their Body Wall

Cnidarian’s body wall consists of two layers of cells:

1. Epidermis-

Outer layer derived from ectoderm and consists of the following cells-

a) Epitheliomuscular cells-

These calls have contractile fibrils called myonemes arranged along the longitudinal axis of the body, and upon contraction, they change the length of the body or tentacles.

b) Epithelioglandular cells-

They secrete a slimy or sticky substance present mainly in the basal region.

c) Cnidoblast-

They are stinging cells. They contain poison.

d) Interstitial cells-

They are totipotent cells (totipotent cells are the cells that can make any cells).

e) Sensory cells-

Receives the stimuli.

f) Nerve cells-

It is an apolar neuron forming a nerve net.

g) Germ cells (sex cells)-

They are either sperm or ova.

2. Gastrodermis-

The other layer out of two layers of cells are the inner endodermal layer, and it has the following gastrodermis cells:

a) Endotheliomuscular cells-

These cells have fibrils called myonemes. Myonemes are perpendicular to the longitudinal axis, and upon contraction, they change the body’s width. They are also called healthy muscle cells and have pseudopodia and flagella.

They capture food and ingest the food particles through phagocytosis. Nutrients are circulated throughout the body by either the process of diffusion or some mobile cells.

b) Endothelioglandular cells-

They secrete digestive enzymes for extracellular digestion in coelenteron.

c) Interstitial cells-

They are totipotent cells (totipotent cells are the cells that can make any cells).

d) Sensory cells-

Receives the stimuli.

e) Nerve cells-

It is an apolar neuron forming a nerve net.

NOTE: In the gastrodermis of Hydra (cnidarians), cnidoblast and germ cells are absent. However, in jellyfish and corals, the gastrodermis contain germ cells, and therefore, the gametes are released in the digestive cavity/ gastrovascular cavity.

There are no respiratory organs present in Cnidarians. Respiration and digestion take place through one orifice and a body cavity.

The cell layers remove carbon dioxide while absorbing oxygen from the water by simple diffusion without particular respiratory organs.

What are Cnidarians?: About Cnidoblast/Nematoblast/ Nematocyte/ Stinging cells!

These cells are present on tentacles (majorly) and the body.

Most Cnidarians have these stinging cells. The stinging cells have “hypnotoxin” (proteins and phenols), which paralyzes/ subdue prey when injected.

Once cnidoblast is used, it is never used again. A fresh cnidoblast is formed from interstitial cells. It moves to the gastrovascular cavity/ digestive cavity and gets digested.

Most species have cnidoblast.

Not all cnidarians have stings that are painful to humans, but some do, and some may even be fatal. People even eat jellyfish and some other Cnidarians, but mostly jellyfish.

Different cnidarian species may also be collected for trade for aquariums and jewelry.

What are Cnidarians?: Types of Cnidoblast

There are four types of cnidoblast:

1. Penetrant/stenotele- It is the largest cnidoblast.

2. Desmoneme/Volvent

3. Stereoline glutinant or small Glutinants or atrichous isorhiza

4. Streptoline glutinant or large glutinant or holotrichous isorhiza

All four types of cnidoblast help capture prey, while glutinants also help adhesion.

DID YOU KNOW? Hydra can walk on tentacles.

What are Cnidarians?: About Life cycle

33433736 freshwater polyps under the microscope 100x
by Dr-Lange / unlimphotos

Cnidarians reproduce asexually and sexually as they have a complex life cycle. A phenomenon called polymorphism (the presence of two or more morphological forms in a species) is observed in Cnidarians.

The two primary body forms are polyp and medusa. Sea anemones and corals have the polyp form, while jellyfish are typical medusae.

Polyp stages reproduce asexually to give the medusa stage, and the medusa phase does sexual reproduction to give polyp stages. This process in Cnidarians is called metagenesis.

Metagenesis is the alternation of generations in which polyp form produces medusa stage asexually, and medusa stage produces polyp form from sexual reproduction.

What are Cnidarians?: Classification

There are about 10 000 species of Cnidarians in total number. Classification of Cnidarians is based on the dominance of polyp form, medusa phase, or both in life.

Cnidarians are classified into four main groups, which simplify the question: what are Cnidarians?-

  1. Non-moving (sessile) Anthozoa– These include sea anemones and corals (sea anemones-Adamsia, Metridium. Corals- Heliopora, Meandrina), sea pens (a coral scientifically so-called Pennatula).
  2. Cubozoa– These include box jellies. They are named box jellyfish because they have a cube-like shape of the bell. (Examples of box jellies are Carukia barnesiMalo king, Chironex fleckeri– Sea wasp).
  3. Swimming Scyphozoa– These include jellyfish (Aurelia, Rhizostoma).
  4. Hydrozoa– A broad group that includes all the freshwater Cnidarians and many marine forms. Includes Hydra (freshwater Cnidarian), Obelia (also called Sea fur), Physalia (so-called Portuguese man of war).

Most species of cubozoans, hydrozoans, and scyphozoans pass through the medusa phase and polypoid body forms. The medusae give rise sexually to larvae that change into polyps, while polyps produce medusae asexually.

Hence, the polyp is essentially a juvenile form, while the medusa is an adult.

As we already know, the lifecycle of Cnidarians is very complex. Let’s look deeper into two different lifecycles present in the phylum Cnidaria.

What are Cnidarians: Lifecycle of Obelia (Sea fur)-

Polyp stages reproduce asexually (via budding) to give the medusa phase. These medusae can either produce sperms or ova.

Adults release sperms and ova into the water from their gonads during the breeding season. The sperms and ova from such medusae undergo external fertilization to give zygote.

The zygote undergoes multiple divisions in short periods (a process called cleavage) to give ciliated planula larva. Planula larva settles and grows to form polyp stages again.

What are Cnidarians?: Lifecycle of Aurelia (Jellyfish)-

4220684 jellyfish
by jeffbanke / unlimphotos

Only medusa form is present in jellyfish, and polyp stages are absent. The medusa phase is unisexual hence produces either sperm or ova.

Adults release sperms and ova into the water from their gonads during the breeding season.

The sperm goes in the medusa with ova and does internal fertilization in the body cavity. The zygote formed goes out and, after successful cleavages, turns into a ciliated planula larva. The larva settles and forms the Scyphistoma stage.

Scyphistoma performs modified budding called strobilation in which the body forms several segments, and each component is a future individual or ephyra. The ephyra grows into an accessible and mobile jellyfish. The juveniles swim off.

The Scyphistoma stage can’t be considered a polyp because each ephyra gets detached from the body and becomes jellyfish after some time.

What are Cnidarians?: Facts About Different Species From This Phylum: 

Here are some examples of Cnidarians and some amazing facts about them that shed more light on “what are Cnidarians?”


Hydra is a freshwater polyp and has 6-10 tentacles. In a bisexual hydra, the testis is a conical structure, 1-8 in number. The conical elevation on which the mouth is present is called hypostome.

Hydra has a growth zone, which is present below the hypostome. It contains interstitial cells (totipotent cells), moving to other parts of the body and replacing the worn-out cells. Therefore, Hydra renews/ polyp regrows itself every 45 days.

Hydra cannot digest starch but can digest proteins, fat, and small carbohydrates. The favorite food of Hydra is Cyclops and Daphnia (also called water fleas, they are crustaceans- arthropods).

Hydra is carnivorous, and the chemical favored for consumption is Glutathione which Cyclops and Daphnia secrete. Most of the Hydra are unisexual. However, bisexual Hydra is also present. Usually, Hydra is sessile but can walk on tentacles.

In green Hydra, a symbiotic algae (green algae) Zoochlorella (Chlorella Vulgaris) is present in the gastrodermal cells. The symbiotic algae have chloroplast, which gives green color.

However, the Brown Hydra contains brown symbiotic algae called Zooxanthellae.

Physalia (Portuguese Man-of-War)-

21011048 portuguese man o war physalia physalis
by aldorado / unlimphotos

Physalia is named so because it resembles a Portuguese warship as it has everything it needs for survival. It was observed that the venom of Physalia is lethal to man. It can also produce cutaneous stings of varying severity.

It also is a tertramorphic colony because it is a collection of 4 main zooids living together.

Zooids are minute, highly specialized individuals attached to one another. They are members of the colony and cannot exist individually.

Adamsia (sea anemone)-

18215934 a starfish in a tank with stones
by Wavebreakmedia / unlimphotos

Sea anemones show protocooperation (two animals living together, but separation does not affect any one of these) with Eupagurus (hermit crab, which is a crustacean).

Hermit crabs enter into empty gastropod shells (mollusks) upon which sea anemones attach themselves.

Coral Reefs

25150206 aquarium coral
by leungchopan / unlimphotos

Coral reefs are a ridge (shallow ridge) in a topical sea that forms a stable marine ecosystem. Madreporarian corals chiefly contribute to it.

Coral reefs formed from the remains of some Cnidarians are of immense economic value to coastal communities in terms of tourism, building materials, and fishing grounds. They are also protectors of shore buildings against currents and tides.

Types of Coral Reefs-

  1. Fringing coral reefs are present along the shore of the island—for example- Hawaii island of the U.S. in the Pacific.
  2. Barrier coral reefs- The reef is at a distance from the island. For example, Australia’s great barrier reef is 900 nautical miles long (9- miles away from Australian island).
  3. Atoll/ Coral island- Only reefs are present, and the island is absent, for example- Lakshadweep and Maldives.

    28623858 closeup of a tropical stony coral specie popular decorative animals for the aquarium marine life background
    by charlottebleijenberg / unlimphotos

This was all about “What are Cnidarians?” I hope this article on “What Are Cnidarians? Facts About 4 Amazing Species” fascinated you enough and kept you engaged in the world of Cnidarians.

Mother nature is breathtaking, isn’t it? Now you can spread the scientific knowledge gained through this article among your peers with the same taste and walk out as a hero of the Cnidarian world.

While at times contributed by guest authors, our content is medically reviewed periodically by professionals for accuracy and relevance. We pride ourselves on our high-quality content and strive towards offering expertise while being authoritative. Our reviewers include doctors, nurses, mental health professionals, and even medical students. -----------------------------------

Any information found on the site does not constitute legal or medical advice. Should you face health issues, please visit your doctor to get yourself diagnosed. Icy Health offers expert opinions and advice for informational purposes only. This is not a substitute for professional medical advice.


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