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Phylum Cnidaria

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Cnidaria is a phylum present under the kingdom Animalia that contains over 11,000 species of aquatic animals that are found both in freshwater and marine environments. Their distinguishing feature is cnidocytes, these are specialized cells that they use primarily for capturing prey. Their bodies consist of a non-living jelly-like substance called mesoglea, which is sandwiched between two layers of epithelium that are mostly one cell thick.

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They mostly have two basic body forms such as swimming medusae and sessile polyps. Both of these body forms are radially symmetrical whereas the mouth is surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for the process of digestion and respiration. Many cnidarian species produce colonies of single or unicellular organisms that are composed of Medusa-like or polyp-like zooids, or both such organisms are called trimorphic. Cnidarians activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts, and a few have simple eyes. Not all cnidarians reproduce sexually, many species that consist of complex life cycles such as asexual polyp stages and sexual medusae.

Cnidarians formerly belonged to Ctenophores under the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Many scleractinian corals which form the structural foundation for coral reefs possess polyps that are filled with symbiotic photosynthetic zooxanthellae. While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians are often found at great depths, in polar regions, and in freshwater. 

Phylum Cnidaria Classification

Cnidaria belongs to the kingdom Animalia and phylum Coelenterata. It is classified into three main groups:

  1. Anthozoa:  Phylum cnidaria examples under this classification are sea anemones, corals, sea pens. Some of the features of this class include: 

  • All the animals of this class are marine

  • They may be solitary or colonial

  • All are sedentary polyploid forms. The medusa stage is absent

  • The mouth is oval and is surrounded by a whorl of tentacles resembling a flower-like structure. Hence the name of the class.

  • The mouth leads into a tubular pharynx called the stomodaeum that in turn opens into coelenteron. Coelenteron is divided into radial compartments by vertical septa called mesenteries.

  • Cnidocytes occur in the epidermal as well as gastrodermal region

  • Gonads occur in the gastrodermis.

  1. Scyphozoa: Cnidaria examples under this class include jellyfish. Features of the organism include:

  • All the animals belonging to this class are marine in nature

  • The Medusa stage is predominant in this class. Medusa is acraspedote.

  • The mouth is surrounded by four oral arms.

  • Mesoglea is cellular and contains amoebocytes

  • Cnidocytes occur in the epidermis and also in the gastrodermis region

  • Gonads occur in the gastrodermis region.

  • Polyps are solitary or may also exist in colonies. The polyp stage is scyphistoma where the body is divided by septa. This scyphistoma produces juvenile medusa called ephyrae by the process of strobilation. Finally, this ephyra grows into the sexual adult medusa.

  1. Cubozoa: Phylum cnidaria examples under this class include box jellies. Features of this class are:

  • It is classified most recently.

  • They exist both as polyp and medusa.

  • These are voracious predators and stronger in swimming.

  • The base of each tentacle present forms a pendulum.

  1. Hydrozoa: Features of this class include:

  • These are mostly marine animals but some may also live in freshwater. 

  • They are chiefly colonial. Some forms may also appear solitary.

  • Medusa stage is absent in few animals. Sometimes both polyp and medusa stages are present in few animals of this class. Medusa is craspedote, which means the presence of velum.

  • Coelenteron of the polyps divided under this class is undivided.

  • Mesoglea is acellular.

  • Cnidocytes are restricted to the epidermis

  • Gonads also occur in the epidermal region

  • Their colonies are polymorphic with different types of zooids like gastrozooids that are feeding type, dactylozooids that are defensive type, and gonozooids that are reproductive type.

Cnidaria Description

Basic Body Forms

Most of the adult cnidarians appear as either sessile polyps or free-swimming medusae. Most of the cnidarians have fringes of tentacles that are equipped with cnidocytes, and medusae generally have an inner ring of tentacles that are present around the mouth. Some hydroids may contain colonies of zooids that serve different purposes, like reproduction, defense, and catching prey. The mesoglea of polyps is typically thin and soft in some cases, but that of medusae is typically thick. To return to its original shape the muscles around the edge get contracted to squeeze the water out, enabling the medusae to swim by a kind of reaction propulsion.


In medusae, the mesoglea is the sole structure. Hydra and most of the sea anemones close their mouths if they are not feeding, and therefore the water within the digestive cavity then acts as a hydrostatic skeleton. Other polyps like Tubularia use columns that are water-filled cells for the support.

In some of the colonial polyps, a chitinous periderm provides support to the connecting sections and to the lower parts of individual polyps. Stony corals secrete massive calcium carbonate exoskeletons. A few polyps collect materials like sand grains and shell fragments, which they attach to their outsides. Some colonial sea anemones stiffen the mesoglea with sediment particles.

Cell Layers

The following types of cell layers are found in the cnidarians:

  • Epitheliomuscular cells are the cells whose bodies are formed from part of the epithelium but their bases extend to form muscle fibers in a parallel row. The fibers of the outward-facing cell layers run at the right angles to the fibers that are facing inward. In the classes of Anthozoa and Scyphozoa, the mesoglea also contains some muscle cells.

  • Cnidocytes are the  "nettle cells"; they are like harpoon cells. These appear between or on top of the muscle cells in some cases. 

  • Sensory cells appear between or on top of the muscle cells and communicate through the synapses that take place with the nerve cells, these mostly lie in between the bases of the muscle cells.

  • Interstitial cells are the unspecialized cells and they can replace lost or damaged cells by transforming into the appropriate types. These are found between the bases of muscle cells.

  • Along with epitheliomuscular, interstitial, and nerve cells, the inward-facing gastrodermis contains gland cells that secrete digestive enzymes. In some species, it also contains low concentrations of cnidocytes, which are wont to subdue prey that's still struggling.

  • The mesoglea contains small numbers of amoeba-like cells and muscle cells in some species. However, the amount of middle-layer cells and kinds are much less than in sponges.


Polymorphism is a process that refers to the occurrence of structurally and functionally quite two differing types of people within an equivalent organism. It is a characteristic feature of Cnidarians, particularly in the case of polyp and medusa forms, or of zooids that are found within colonial organisms such as Hydrozoa. In Hydrozoans, colonial individuals arise from individuals such as zooids who will take on separate tasks. 


The main components of a cnidocyte are:

  • A cilium that projects above the surface and acts as a trigger.

  • A tough capsule, called cnida that is a house of the thread, acts as a payload and consists of a mix of chemicals which will include venom or adhesives, or both.

  • A tube-like extension found on the wall of the cnida, points into the cnida, just like the finger of a rubber glove pushed inwards. When a cnidocyte fires, the finger pops out.

  • The extension of a finger coils around it until the cnidocyte fires. The thread is usually hollow in nature and delivers the chemicals from the cnida to the respective target.

  • An operculum is present at the end of the cnida. The lid may be a single hinged flap or three flaps that are arranged like slices of pie.

  • It is difficult to review the firing mechanisms that take place in the cnidocytes as these structures are small but very complex. At least four hypotheses have been proposed, they are:

  • Rapid contraction of fibers that takes place around the cnida may increase its internal pressure.

  • The thread could also be sort of a coiled spring that extends rapidly when released.

  • In the case of Chironex, chemical changes in the cnida's contents may cause them to expand rapidly by polymerization.


Medusae swim by a form of jet propulsion: In this process muscles that are especially present inside the rim of the bell squeezes the water out of the cavity, and the springiness of the mesoglea gets powered up with the recovery stroke. Since the tissue layers are very thin, they supply insufficient power to swim against currents and only enough to regulate movement within currents. Hydras and a few sea anemones can crawl over rocks and sea or stream beds by various means such as crawling like inchworms, creeping like snails, or by somersaulting. A few can swim clumsily by waggling their bases.

Nervous System and Senses

Cnidarians are generally thought to have no brains or even central nervous systems. However, they have integrative areas of neural tissue that are considered as some sort of centralization. Most of their bodies are innervated by decentralized nerve nets that control their swimming musculature. These nerve nets connect with sensory structures, even though each clade consists slightly of different structures. These sensory structures are usually known as rhopalia. These can generate the signals in response to varied sorts of stimuli such as light, pressure, etc. 

In scyphozoans, the diffuse nerve net has modulatory effects on the nervous system. As well as it forms the "signal cables'' between sensory neurons and motor neurons. The intermediate neurons that are present within the nerve net can form the ganglia that act as local coordination centres. Communication that happens in between the nerve cells can occur by chemical synapses or gap junctions in the class hydrozoans. 

Feeding and Excretion

Cnidarians feed in several ways such as predation, absorbing the dissolved organic chemicals, filtering the food particles that are present in the water, obtaining nutrients from the symbiotic algae within their cells, and parasitism. By the process of predation, most of the cnidarians obtain a majority of their food. But some, including the corals such as Hetroxenia and Leptogorgia, depend completely on their endosymbionts and on absorbing the dissolved nutrients. Indigestible remains of prey are expelled through the mouth. The main waste of cells that takes place during the internal processes is ammonia, which is removed by the external and internal water currents.


There are no specialized respiratory organs, and both of the cell layers absorb oxygen and expel Carbon dioxide into the encompassing water. When the water within the digestive cavity becomes stale it must get replaced, and nutrients that haven't been absorbed are going to be expelled with it. Some Anthozoa has ciliated grooves on their tentacles that allow them to pump water out of and into the digestive cavity without opening the mouth. This improves the process of respiration after feeding and the animals which use the cavity as a hydrostatic skeleton, in order to regulate the water pressure within the cavity without expelling undigested food. The animals produce large quantities of antioxidants to neutralize the surplus oxygen.


All cnidarians can regenerate, allowing them to get over injury and to breed asexually. Medusae have limited ability to regenerate, but polyps can do so from small pieces or maybe collections of separated cells. This enables corals to recover even after apparently being destroyed by predators.

Cnidaria Reproduction

Cnidarian amphimixis often involves a posh life cycle with both in the case of polyp and medusa stages. This grows normally, on the other hand, these organisms absorb their tentacles and split them horizontally into a series of disks that become juvenile medusae, through a process called strobilation. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and should continue strobilation periodically. The adults have gonads that are found within the gastrodermis, and these are involved in the release of ova and sperm into the water within the breeding season. This phenomenon of succession is organized differently among generations is sometimes called "alternation of asexual and sexual phases" or it is also known as "metagenesis". But these alteration processes should not be confused with the alternation of generations as found in plants as both of them are different in nature.

The shortened forms of this life cycle are commonly found. Hydrozoa has a variety of life cycles. Some do not have any polyp stages and a few do not include any medusae. In some species, the medusae remain attached to the polyp and are liable for sexual reproduction. In the case of extreme conditions, these reproductive zooids might not look very similar to medusae. Meanwhile, the life cycle is a reversal in nature in which the polyps are formed directly from the medusae without the involvement of the sexual reproduction process; this was observed in both Hydrozoa and Scyphozoa. Anthozoa has no medusa stage at all and thus only polyps are responsible for sexual reproduction.

Spawning is usually driven by environmental conditions like changes within the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or the phase of the moon. Many species of Cnidaria may spawn simultaneously within the same location. There are too many ova and sperm for predators to eat quite a small percentage. One of the famous examples is the Great coral reef, where a minimum of 110 corals and a couple of non-cnidarian invertebrates have produced enough gametes to make the water cloudy. These mass spawnings may produce a hybrid. In some species, the ova releases the chemicals that attract the sperm of an equivalent species.

The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere and then a depression forms at one end and eventually becomes the digestive cavity. However, in Cnidarians, the depression formed at the end is from the yolk, while in bilaterians it forms at the other end. The larvae are called planulae, which can swim or crawl by means of cilia. They are cigar-shaped but slightly broader at the "front" end, which is that the aboral, vegetal-pole end and eventually attaches to a substrate if the species features a polyp stage.

Anthozoan larvae either have large yolks or are capable of feeding on plankton, and a few have already got endosymbiotic algae that help to feed them. Since the parents are immobile, these feeding capabilities extend the range of larvae and avoids the overcrowding of the sites. Scyphozoan and hydrozoan larvae have little yolk and most lack endosymbiotic algae and thus need to settle quickly and metamorphose into polyps. Instead, these species believe their medusae to increase their ranges.

Asexual: All the known cnidaria can reproduce asexually by various means, additionally to regenerating, after being fragmented. Hydrozoan polyps can produce only buds, while the medusae of some hydrozoans can divide themselves into halves. Scyphozoan polyps can produce both buds and can split down the middle to form halves. In addition to both of these methods, Anthozoa can split horizontally just above the base. Asexual reproduction makes the daughter cnidarian just like the adult.

Difference Between Polyp and Medusa



These have a sessile or sedentary lifestyle.

These are adapted for the floating type of lifestyle.

The body is tubular in nature.

The body is umbrella or bell-shaped in nature.

These have simple body structures.

These have complex body structures.

Velum is absent.

Velum is present.

The mouth is circular in form.

The mouth is rectangular in form.

They have poorly developed mesoglea.

Mesoglea is developed completely.

Cnidaria Facts

  • Depending on the situation, cnidarians can live for about a few days to 4000 years. 

  • Most of the organisms that belong to this phylum cnidaria do not contain eyes and not even any developed organ.

  • It is dangerous to touch the small jellyfishes or any of the jellyfishes as they are predators and produce certain kinds of chemicals that are capable of killing a human.

  • These jellyfishes are made up of water and they do not contain the brain, nervous system, or blood.

  • The cnidaria meaning is as follows, it is obtained from the Greek language where it means stinging nettle.

  • Most of the corals live in colonies whereas the mushroom corals make only a single polyp. 

  • Sea anemones are found majorly in rock coastal rock pools. 

FAQ (Frequently Asked Questions)

Q1. What are Cnidarians?

Ans: Cnidaria is a phylum that consists of organisms such as aquatic invertebrates. Where the term is obtained from the Greek word “Cnidos” which means stinging nettle.

Q2. Mention the Basic Characteristics of Phylum Cnidaria.

Ans: The characteristics of the organisms that belong to the phylum cnidaria are:

  • All of the organisms are aquatic where most of them are marine. 

  • They consist of stinging cells called tentacles that help to capture the food.

  • These are made up of only two body layers. These layers are ectoderm and endoderm where these two layers are separated by a separate jelly-like layer called mesoglea. 

  • These are radially symmetrical.

  • The reproductive phase is very complicated and they can reproduce by both asexual and sexual means.

  • Some of the cnidarians are sessile in nature where some of them are sedentary in nature.

Q3. What are the Four Major Groups of Cnidaria?

Ans: The four major groups of cnidaria are as follows:

  • Anthozoa includes examples such as true corals, anemones, and sea pens.

  • Cubozoa consists of amazing box jellies with complex eyes and potent toxins.

  • Hydrozoa consists of most of the diverse groups with siphonophores, fire corals, and many medusae.

  • Scyphozoa includes examples such as true jellyfish.