Phylum Coelenterata/Cnidaria

The word Cnidaria is derived from Latin which means “the sting“.

The name Cnidaria has been given to this group of animals due to the presence of special cells called cnidocytes. These cells give rise to nematocysts-the stinging cells, characteristics of this group.

General Characteristics of Phylum Cnidaria

Grade of organization: They are multicellular organisms and are diploblastic i.e. they contain ectoderm and endoderm.

Shape and Symmetry: The cnidarians have radial symmetry and these organisms have no heads so their ends are called oral (proximal to mouth) and aboral (distal from the mouth).

Habitat: They are aquatic animals and found in marine and freshwater.

Body cells and body cavity:

Cnidocytes: These are the specialized stinging cells found in the cnidarians.

Nematocysts: Cnidocytes contain special stinging capsular organelles called nematocysts around the mouth or tentacles of cnidarians.


Enteron: The ectoderm forms the outer layer and gives rise to the nematocysts while the endoderm becomes specialized for digestion. Between ectoderm and endoderm, there is a jelly-like layer called mesoglea. These organisms have only one cavity which is called Enteron.

Sac like digestive system: Due to only one mouth opening they have sac like digestive system.

Body Forms: There are two basic body forms in Phylum cnidaria

Medusae: These are umbrella-like and are free swimming animals which are also involved in sexual reproduction as they have gonads. for example jellyfish.

Polyps: These are cylindrical or stalk-like and in most cases, they are nutritive in function. They are sessile. For example hydra.


Organs and Organ systems: They are complex then Porifera but less complex than bilaterians. So they have tissue organization and contain organs.

Nervous system: There is no proper nervous system but an irregular form of neurons that form net or plexus in the body wall.

Tentacles: Mouths of cnidarians are surrounded by tentacles which contain nematocysts- stinging cells that act as defense or offense organ.



Mode of nutrition: The coelenterates are carnivores and feed upon small organisms that develop contact with them or come in their way. These organisms are trapped by nematocysts and ingest into the digestive tract as meals at which it is digested and distributed by diffusion.

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Skeleton in Phylum cnidaria: A lot of colonial coelenterates such as corals generate a tough exoskeleton made up of calcium carbonate (CaC03). It is secreted by epidermal cells which simply take lime from seawater. The skeleton of coral is liable for the creation of coral islands or even coral reefs.

Reproduction and life cycle of Phylum Coelenterata:

In Coelenterates reproduction occurs both by asexual as well as sexual methods.

Asexual reproduction:

They reproduce asexually by the formation of buds on its surface. Afterward, it separates from the parent and grows into a new individual. As an instance in hydra.

Sexual reproduction:

Like in Obelia there is asexual reproduction in addition to sexual reproduction. It is some type of zooid Called blastostyle that gives rise to zooids known as medusae by the asexual method. The medusae when free in water develop reproductive organs that produce gametes that combine to create zygote where the Obelia colony is formed.

Life-cycle: The entire life span of coelenterates is characterized by the existence of the alternation of generations. There are just two generations, one reproduces by sexual means, and the other one by asexual means. Both generations are diploid. Frequently both generations contain just one free-living plus yet another sessile generation. For that reason, asexual and sexual generations alternate each other. That is referred to as alternation of generations e.g., Obelia.

Polymorphism – A Characteristic of Coelenterates (Cnidaria)

The phenomenon of functionally and structurally two or more than two different kinds of individuals, within the same organism, is called polymorphism. These are called zooids.

As an instance, in Obelia there are feeding individuals, the gastrozooids; the individuals with the capacity of asexual reproduction only, the gonozooids, blastostyles along with free living or sexually reproducing individuals, the medusae.

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Classification of Phylum Coelenterata / Cnidaria

Anthozoa: These including true corals, sea- anemones; found just in marines and could be colonial or solitary.

Scyphozoa: This class holds true jellyfish. They are only seen in marine habitats.

Cubozoa: All these are the remarkable box jellies with eyes that are elaborate along with powerful toxins.

Hydrozoa: probably the very diverse collection of siphonophores, hydroids, fire corals, and lots of medusae; and found within both marine and fresh water habitat.

Examples of phylum Coelenterata:

Hydra: A freshwater – coelenterate.


Obelia: A marine- coelenterate.


Aurelia: A jellyfish.


Economic Importance:

Corals are formed by the secretions produced by polyps which can be found in some coelenterates. All these polyps are covered by stony cups due to these secretions. By the mouth of this stony cup, the polyp can pass its tentacle for the feeding and capturing prey. Most such Coelenterates are colonial. The stony network or bulk of such Coelenterates are called Corals. Living polyps are observed on the surface of corals whereas under the bulk are dead mass with not any polyps inside. The stony masses which are formed in this manner are called coral reefs. All these are made up of calcium carbonates (limestone). Coral reefs are located in the coastal waters of Florida, West Indies, East Coast of Africa, Australia.


Healthy coral reefs are one of the most highly diverse and efficiently valuable ecosystems on the planet, providing valuable and vital ecosystem services. Coral ecosystems provide food to countless organisms; protect coastlines from erosion and storms; provide habitat, spawning, and nursery grounds for many fish species and are home to a variety of sea creatures. They provide income and jobs to local markets from recreation, fishing, and tourism. They are also an important source of new medicines, and also, therefore, are hotspots of marine biodiversity.

Concerning biodiversity, the number of species residing a coral reef is much more than any other shallow-water marine ecosystems and is still among the very diverse ecosystem on Earth. Coral reefs cover only one percent of oceans, but Coral reefs support over 800 hard coral species along with more than 4,000 species of fish.

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Do you know??

The largest coral reef which is known as The Great Barrier Reef is situated on the north-eastern coast of Australia. Spread over 1,429 miles over an area of approximately 133,000 square miles.



The organisms having specialized cells ‘cnidocytes’ are classified in the phylum Coelenterata or sometimes called phylum Cnidaria. These cnidocytes give rise to specialized stinging cells called nematocysts which are the characteristic of this phylum.

These are the aquatic animals with multicellular organization, radial symmetry, and are diploblastic. The body cavity of these organisms is called enteron. They have a sac-like digestive system. The two basic body forms in cnidaria are medusae – the umbrella-like free-swimming organisms involved in sexual reproduction like jellyfish and polyps – the cylindrical sessile organisms of nutritive importance like the hydra.

They have no proper nervous system but the plexus formed by neurons is present for coordination of the body. They have tentacles around their mouth which contain nematocysts – stinging cells for prey capture as well as defense organ. They sexually produce as medusae and asexually by the formation of buds. The life cycle is characterized by the alternation of generations. Hydra, obelia, and jellyfish are common but important organisms of phylum cnidaria.