Phylum Cnidaria – General characteristics and classification

Cnidaria comes from the Greek word cnidos, which means stinging thread. The existence of cnidae characterises these creatures. Cnidaria is a phylum within the Kingdom Animalia that contains 10,000 known species, including corals, sea anemones, jellyfishes, and hydras. These organisms are found in both marine and freshwater environments.

It is an additional ancient group with several fossil specimens. These are the most basic living organisms with real tissues. It was believed that these were the first animals in evolutionary history to have a distinct form. This group is characterised by gigantic medusae and corals, colonial siphonophores, feathery hydroids, and box jellyfish with complex eyes, all of which have highly diverse morphologies. Yet, these animals possess stinging cells known as nematocytes.


Formerly, Cnidarians were classified in the phylum Coelenterata alongside ctenophores. With a growing understanding of their peculiarities, however, they were put in a separate phylum.

Definition of Phylum Cnidaria

Phylum Cnidaria consists of jellyfish, corals, sea anemones, and hydroids. They are distinguished by their stinging cells, known as cnidocytes, which they employ for defence and prey capture. The central body cavity of cnidarians functions as both the digestive and circulatory system. They possess radial symmetry, which indicates that they are symmetrical on a central axis. From little, basic polyps to big, complicated jellyfish, cnidarians exist in a broad variety of shapes and sizes. Important roles are played by these organisms in marine ecosystems throughout the world.


History of Phylum Cnidaria

Jellyfish, coral, sea anemones, and hydras are members of the phylum Cnidaria, which consists of aquatic organisms. These creatures have existed for more than 600 million years and have played a crucial role in the evolution of marine ecosystems.

The earliest known cnidarian fossils date back around 600 million years to the Ediacaran epoch. These early cnidarians likely had basic, fragile bodies and lived on the ocean floor. Over time, cnidarians have evolved a wide range of body types, including free-swimming medusae (jellyfish) and sessile polyps (corals and anemones).


Around 540 million years ago, during the Cambrian explosion, cnidarians evolved into a vast variety of forms, including several that are no longer seen in modern waters. Some of these ancient cnidarians are believed to have been the ancestors of other animal phyla, including the bilaterians, due to their sophisticated body designs (animals with bilateral symmetry).

Throughout Earth’s history, cnidarians have been essential components of marine ecosystems. For instance, coral reefs consist of the skeletons of small coral polyps, which provide habitat and shelter for countless different species. Jellyfish and other cnidarians are significant prey for numerous marine predators, and their stinging cells defend them from predators and enable them to capture their own food.


Cnidarians are currently found in all oceans, from the tropics to the arctic regions. While many species are numerous and common, some are threatened by habitat degradation, overfishing, and other factors. To better comprehend their evolution, ecology, and significance in marine ecosystems, scientists continue to examine these interesting organisms.

Characteristics of Phylum Cnidaria

  • Phylum Cnidaria contains organisms with radial or biradial symmetry that emerge from two embryonic layers, or are diploblastic. Around 99 percent of all cnidarians are marine species.
  • Cnidarians include specialised cells called cnidocytes (“stinging cells”) that contain nematocysts (stingers). The presence of these cells around the mouth and tentacles serves to paralyse prey with the toxins contained within.
  • Nematocysts are comprised of coiled threads that may be armed with barbs. The outer cell wall features hairlike projections called cnidocils that are touch sensitive.
Members of the phylum Cnidocytes are stinging cells found in cnidarians. Cnidocytes have enormous organelles known as (a) nematocysts, which store a thread and barb. When hairlike projections on the cell surface are contacted, (b) a thread, barb, and poison are released from the organelle.
Members of the phylum Cnidocytes are stinging cells found in cnidarians. Cnidocytes have enormous organelles known as (a) nematocysts, which store a thread and barb. When hairlike projections on the cell surface are contacted, (b) a thread, barb, and poison are released from the organelle. | Image Source:
  • On contact, the cells are known to release coiling threads that can either pierce or entangle cnidarian prey or predators. These coiled threads deliver toxins into the target, immobilising prey or frightening off predators.
  • This phylum contains organisms with two unique morphological body plans: the polyp or “stalk” and the medusa or “bell”. Hydra spp. is an example of the polyp form; jellies are the most well-known medusoid animals (jellyfish).
  • Adult polyps are sessile, with a single digestive system entrance (the mouth) facing upward and tentacles around it. The mouth and tentacles of a Medusa form are suspended from an umbrella-shaped bell.
  • Certain cnidarians are polymorphic, meaning they have two distinct body plans during their lifetime. Obelia, a colonial hydroid, serves as an example. The sessile polyp form has two types of polyps.
  • The first form of polyp is the gastrozooid, which is specialised for prey capture and feeding; the second type is the gonozooid, which is adapted for the asexual reproduction of medusa.
The medusa (a) and the polyp (b) are the two unique body designs of cnidarians (b). All cnidarians have two membrane layers separated by a jelly-like mesoglea.
The medusa (a) and the polyp (b) are the two unique body designs of cnidarians (b). All cnidarians have two membrane layers separated by a jelly-like mesoglea. | Image Source:
  • When the reproductive buds mature, they separate and transform into male or female medusa that swim freely (dioecious). The male medusa produces sperm while the female produces eggs. Upon fertilisation, the zygote transforms into a blastula, which then transforms into a planula larva.
  • After a period of free swimming, the larva attaches and a new colonial reproductive polyp is created.
  • All cnidarians possess two membrane layers generated from the embryonic endoderm and ectoderm. The outer layer (from ectoderm) is known as the epidermis and lines the animal’s outside, while the inner layer (from endoderm) is known as the gastrodermis and lines the digestive cavity.
  • Between these two membrane layers is a connective layer of nonliving, gelatinous mesoglea. Cnidarians exhibit distinct cell types in each tissue layer, including nerve cells, contractile epithelium cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as intercellular connections. Nevertheless, organ or organ system development is not advanced in this phylum.
  • The nerve cells of the primitive nervous system are dispersed throughout the body. This nerve network may contain cell clusters in the form of nerve plexi (singular: plexus) or nerve cords.
  • The nerve cells exhibit characteristics of both sensory and motor neurons. Chemical peptides, which perform both excitatory and inhibitory roles, are the major signalling molecules in these primitive neural systems.
  • Despite its simplicity, the nervous system regulates the movement of tentacles, the digesting of food, and the outflow of waste.
  • Cnidarians undertake extracellular digestion in which food is ingested into the gastrovascular cavity, enzymes are produced into the cavity, and the cells lining the gastrovascular cavity absorb nutrients. It is known as an incomplete digestive system because the gastrovascular cavity has a single aperture that functions as both the mouth and the anus.
  • Cnidarian cells exchange oxygen and carbon dioxide through diffusion between cells in the epidermis and water in the surrounding environment, and between cells in the gastrodermis and water within the gastrovascular cavity.
  • The absence of a circulatory system to transport dissolved gases restricts the thickness of the body wall and demands the presence of a nonliving mesoglea between the layers. There is no excretory system or organs, thus nitrogenous wastes simply permeate from the cells into the surrounding water or the gastrovascular cavity. There is also no circulatory system, so nutrients must travel through the mesoglea from the cells that receive them in the lining of the gastrovascular canal to other cells.
  • There are around 10,000 documented species under the phylum Cnidaria, which are classified into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa. Anthozoans, including sea anemones and corals, are sessile forms, whereas scyphozoans (jellyfish) and cubozoans (box jellies) are motile. The hydrozoans contain both sessile and colonial swimming species, such as the Portuguese Man-of-War.
The sessile form of Obelia geniculate has two types of polyps: gastrozooids, which are adapted for capturing prey, and gonozooids, which bud to produce medusae asexually.
The sessile form of Obelia geniculate has two types of polyps: gastrozooids, which are adapted for capturing prey, and gonozooids, which bud to produce medusae asexually. | Image Source:

Cnidarians Body Forms

  • They are distinguished by stinging cells known as Cnidoblast and a cavity known as coelenterates, hence the names Cnidaria and Coelenterata.
  • They are aquatic and marine solely.
  • These creatures are radially symmetrical and diploblastic.
  • The ectoderm of Coelenterates has a specialised cell type known as cnidoblasts or stinging cells. These cells are employed for protection and paralysing prey by hypnotising them. They are abundant near the mouth and on top of the tentacles.
  • They demonstrate both intracellular and extracellular digestion within the gastrovascular cavity.
  • Cnidarians lack the circulatory, respiratory, and excretory systems. Gaseous exchange and excretion occur at the surface of the organism.
  • Cnidaria possess simple or complex sensing organs such as statocysts and tentaculocysts to maintain equilibrium.
  • Their nervous system is underdeveloped and is present in the form of a nerve net in the body walls and tentacles.
  • They exhibit polymorphism. There are two bodily forms: the asexual polyp and the sexual medusa stage.
  • The Hydra Polyp has a cylindrical form.
  • Jellyfish Medusae are umbrella-shaped.
  • The way of asexual reproduction is budding or spores.
  • Sexual reproduction happens when gametes are produced. Sexes are not separate.
  • The embryo exhibits indirect growth.
  • They demonstrate external fertilisation.
  • Some exhibit alternation of generation, also known as metagenesis, a phenomena in which the asexual generation (polyp) and the sexual generation alternate (medusa).
  • Examples include Hydra, Aurelia (JellyFish-Medusae form), Adamsia (Sea anemone-Polyp form), Pennatula (Sea pen), and Gorgonia (Sea Fan). Meandrina (Brain coral), Physalia (Portuguese man-of-war), and Obelia are species of coral (Both polyp and medusa form).

Classification of Phylum Cnidaria/Classes in the Phylum Cnidaria

Phylum Cnidaria divided into these following Classes;

  1. Class Anthozoa
  2. Class Scyphozoa
  3. Class Cubozoa
  4. Class Hydrozoa

1. Class Hydrozoa (Gr. Hydros=water, zoon=animal)

  • Hydrozoa is a varied group with over 3,200 species; the majority are marine, but some freshwater species are recognised.
  • In their lifecycles, most species exhibit both polypoid and medusoid forms, while the familiar Hydra solely has the polyp form.
  • Hydromedusa is the name given to the medusoid shape because it bears a muscular veil or velum beneath the border of the bell. In contrast, the medusoid form of Scyphozoa is called a scyphomedusa and lacks a velum.
  • The polyp form of these creatures is frequently cylindrical with a gastrovascular canal lined by the gastrodermis in its centre. The epidermis and gastrodermis are separated by a single layer of mesoglea.
  • There is a mouth hole ringed by tentacles at the animal’s oral end. Many hydrozoans, such as the colonial hydroid Obelia, form sessile, branched colonies of specialised polyps that share a shared, branching gastrovascular canal (coenosarc).
  • Siphonophores are free-floating colonial species that contain medusoid and polypoid individuals that are specialised for eating, defence, or reproduction.
  • The characteristic rainbow-colored float of the Portuguese man-of-war (Physalia physalis) is created by a pneumatophore that fills and expels carbon monoxide gas to regulate buoyancy.
  • At first glance, these sophisticated superorganisms appear to be a single organism; nevertheless, even the tentacles are formed of zooids that are loaded with nematocysts.
  • Hence, although it superficially resembles a normal medusozoan jellyfish, P. physalis is a free-floating hydrozoan colony; each specimen consists of many hundreds of organisms, each specialised for a particular function, such as movement and buoyancy, eating, reproduction, and defence.
  • Although they are carnivorous and feed on numerous soft-bodied marine species, P. physalis lack stomachs and instead use polyps known as gastrozooids to digest their prey in open water.
  • Male and female Physalia colonies release their gametes into the water. The zygote matures into a single individual, which then forms a new colony through asexual reproduction.
  • Siphonophores include the largest known colonies of floating cnidarians, such as Praya dubia, whose chain of zoids can reach lengths of up to 50 metres (165 feet).
  • Additional species of hydrozoans are solitary polyps (Hydra) and hydromedusae (Gonionemus). The gonads of hydrozoans are derived from epidermal tissue, whereas they are derived from gastrodermal tissue in all other cnidarians.
Hydrozoans. The polyp colony Obelia (a), siphonophore colonies Physalia (b) physalis, also known as the Portuguese man o’ war and Velella bae (c), and the single polyp Hydra (d) all have distinct body forms, yet they are all members of the family Hydrozoa. (credit b: adaptation of work by NOAA; scale bar data contributed by Matt Russell)
Hydrozoans. The polyp colony Obelia (a), siphonophore colonies Physalia (b) physalis, also known as the Portuguese man o’ war and Velella bae (c), and the single polyp Hydra (d) all have distinct body forms, yet they are all members of the family Hydrozoa. (credit b: adaptation of work by NOAA; scale bar data contributed by Matt Russell)

Order 1 : Hydroidea : Solitary or colonial forms. Polyp well developed. Sense organs or medusa are statocysts.

  • Sub-Order 1 : Anthomedusae. Ex : Hydra, Bougainvillea.
  • Sub-Order 2. Leptomedusae. Ex : Obelia.

Order 2 : Trachylina: No fixed stage exists. All of them are mobile medusae. The marginal organs of sensation are modified tentacles.

  • Sub-Order I : Trachymedusae Ex : Petasus.
  • Sub-Order II : Narcomedusae. Ex : Polycolpa.

Order 3: Hydrocorallina : It includes coral like hydrozoans. CaC03 skeleton is secreted by coenosarc. Polyps are dimorphic. 1)Millipora (Hydrozoans coral) 2)Stylaster (Hydrozoan corals)

Order 4 : Chondrophora : It includes organisms with big floats. Ex: l)Velella 2)Porpita.

Order 5: Pteromedusae: Pelagic hydrozoans. Ex : Tetraplatia.

Order 6: Siphonophora: They show highest polymorphic tendency.

2. Class Anthozoa (Gr. anthos=flower, zoon=animal)

  • The class Anthozoa (“flower creatures”) contains an estimated 6,100 known species, including sea anemones, sea pens, and corals. Sea anemones are typically vividly coloured and can reach a diameter of 1.8 to 10 cm. Individual organisms are cylindrical and immediately linked to a substrate.
  • A sea anemone’s mouth is ringed by tentacles that contain cnidocytes. The ectoderm lines the slit-like mouth entrance and flattened throat. This anatomy of the pharynx imparts bilateral symmetry to anemones.
  • A siphonoglyph is a ciliated groove situated on two opposite sides of the pharynx that directs water into it. The pharynx is the muscular component of the digestive system that serves to both ingest and egest food.
  • It can stretch for up to two-thirds of the body’s length before opening into the gastrovascular cavity. This cavity is separated into multiple chambers by mesentery-like longitudinal septa. Each mesentery is made up of a fold of gastrodermal tissue sandwiching a layer of mesoglea.
  • The mesentery does not entirely partition the gastrovascular cavity, and the smaller cavities merge at the pharyngeal entrance.
  • The adaptive advantage of the mesenteries appears to be an increase in surface area for food absorption and gas exchange, as well as more mechanical support for the anemone’s body.
  • Typically, sea anemones consume small fish and shrimp by immobilising them with nematocysts. Certain sea anemones and hermit crabs form a mutualistic relationship when the crab attaches itself to their shell.
  • In this interaction, the anemone obtains food particles from the crab’s captured prey, while the crab is protected from predators by the anemone’s stinging cells.
  • Several species of anemone fish, also known as clownfish, are able to coexist with sea anemones because they develop an immunity to the nematocysts’ toxins and generate a mucus that keeps them from getting stung.
  • The structure of coral polyps resembles that of anemones, except the individual polyps are typically smaller and are part of a colony, some of which are enormous and the size of small buildings.
  • Coral polyps consume smaller planktonic creatures, such as algae, bacteria, and larvae of invertebrates. Certain anthozoans develop symbiotic relationships with zooxanthellae, a type of dinoflagellate algae.
  • The symbiotic association between zooxanthellae and modern corals, which provides refuge for the algae, gives coral reefs their colour and provides nutrition to both animals. According to a new study by an international group of scientists, this complex mutualistic link originated more than 210 million years ago.
  • The fact that this symbiotic association developed during a period of huge global coral reef development shows that the connection between algae and coral is essential for the health of coral reefs, which offer home for approximately one-quarter of all marine life.
  • Warming oceans have caused corals to eject their zooxanthellae algae and turn white, a phenomenon known as coral bleaching, which threatens reefs.
  • Anthozoans are polypoid throughout their lifetimes (note that this term is commonly mistaken with “polyploid”) and can reproduce asexually via budding or fragmentation, or sexually by creating gametes.
  • The fusion of male or female gametes produced by a polyp produces a free-swimming planula larva. On an appropriate substratum, the larva develops into a sessile polyp.
The sea anemone is shown (a) photographed and (b) in a diagram illustrating its morphology. (credit a: modification of work by “Dancing With Ghosts”/Flickr; credit b: modification of work by NOAA)
The sea anemone is shown (a) photographed and (b) in a diagram illustrating its morphology. (credit a: modification of work by “Dancing With Ghosts”/Flickr; credit b: modification of work by NOAA)

Order 2: Madreporaria : (True corals) Stony corals are present Polyps are small. Siphonoglyphs absent. Ex: 1) Meandrina (brain coral) 2) Fungia.

Order 3 : Zoanthidea : Solitaryor colonial organisms. Polyps are united by basal stolons. Only ventral siphonoglyphis present. Ex: 1) Zoanthus.

Order 4 : Antipatharia : Includes black corals. Two siphonoglyphs are present. Ex: Antipathes (Black coral)

Order 5: Ceriantharia : Solitary structure. Tentacles many, arranged into two whorls. Only single siphonoglyph occurs. Ex .- Cerianthus.

Order 6: Corallimorpharia : Solitary or aggregate, anemone like polyps. Ex : Corynactis.

Order 7: Ptychodactaria : Includes animals which are anemone like polyps. Ex: Ptychodactis.

Sub class : Octocoralia (Alcyonaria) : The tentacles and mesentries of these anthozoans are in multiples of eight. On the stomodium, there will never be more than one siphonoglyph. It is in the ventral position.

Order 1: Stolonifera : Polyps are connected by creeping stolon. Ex: Tubipora (orange pipe coral).

Order 2 : Telestacea : The colonies contain simple or branched stem which bears lateral polyps. Ex: Telesto.

Order3: Alcyonacea : These are soft corals. Polyps may be dimorphic. Ex: 1) Alcyonium (dead man’s fingers).

Order 4 : Coenothecalia : It includes a single genus. Ex: Heliopora (Blue coral)

Order 5: Gorgonacea : It is a compound tree like coral. Ex: 1) Gorgonia (seafan) 2) Corallium (red coral)

Order 6: Pennatulacea: These are elongated members. Emended in the mud, and sea bottom. Ex : Pennatula (Sea pen)

3. Class Scyphozoa (Gr. skyphos=cup, zoon=animal)

  • Class Scyphozoa (“cup animals”) contains all (and only) the approximately 200 known species of marine jellies. Although there is a polyp stage in the life cycle of most species, the medusa is the most conspicuous stage.
  • The length of most jellies ranges from 2 to 40 centimetres, although the largest scyphozoan species, Cyanea capillata, can reach a diameter of two metres. Scyphozoans have a distinctive bell-like shape .
  • On the underside of the sea jellies is a mouth opening surrounded by hollow tentacles containing nematocysts.
A jelly is shown (a) photographed and (b) in a diagram illustrating its morphology. (credit a: modification of work by “Jimg944″/Flickr; credit b: modification of work by Mariana Ruiz Villareal)
A jelly is shown (a) photographed and (b) in a diagram illustrating its morphology. (credit a: modification of work by “Jimg944″/Flickr; credit b: modification of work by Mariana Ruiz Villareal)
  • Scyphozoans spend the majority of their lives as solitary, free-swimming carnivores. The mouth leads to the gastrovascular cavity, which consists of four interconnected sacs known as diverticuli. In certain species, the digestive system may branch into radial canals.
  • Similar to the septa in anthozoans, the branched gastrovascular cells have two purposes: to enhance the surface area for nutrition absorption and diffusion and to maintain the animal’s body.
  • Nerve cells of scyphozoans are arranged in a neural net that spans the entire body, with a nerve ring surrounding the edge of the bell.
  • Rhopalia are clusters of sensory organs that may be present in pockets along the edge of the bell. Jellies have a ring of muscles surrounding the body’s dome, which generates the contractile power necessary for swimming through water and for ingesting food while swimming.
  • Scyphozoans have distinct genders. The gastrodermis gives rise to the gonads, and gametes are discharged by the mouth.
  • External fertilisation produces planula larvae, which settle on a substratum in polypoid form. These polyps may create more polyps or begin producing medusa buds immediately.
  • In a few species, planula larvae can transform directly into medusa larvae. The life cycle of the majority of scyphozoans comprises both sexual and asexual body forms.
Scyphozoan life cycle. The lifecycle of most jellyfish includes two stages: the medusa stage and the polyp stage. The polyp reproduces asexually by budding, and the medusa reproduces sexually. (credit “medusa”: modification of work by Francesco Crippa)
Scyphozoan life cycle. The lifecycle of most jellyfish includes two stages: the medusa stage and the polyp stage. The polyp reproduces asexually by budding, and the medusa reproduces sexually. (credit “medusa”: modification of work by Francesco Crippa)

Order 1 : Stauromedusae : (Lucernarida) Sense organs absent. Medusa is pyramidal shaped. Sedentary. Ex : 1) Lucernaria 2) Haliclystus.

Order 2 : Coronatae : The umbrella shows coronary grooves. 4 to 16 tentaclocysts are present.Ex: 1) Periphylla. 2) Nausithoe (It lives inside Porifera animals (sponges).

Order 3 : Cubotnedusae : Medusa is cubical 4 perradial tentacalocysts are present. Free swimming. Ex : 1) Choropsaimum (free medusa) 2)Chatybdaea.

Order 4 : Semeastomeae (or) Discomedusae : Most common medusae. Medusa is disc shaped, 4 perradial and 4 interradial tentaculocysts are present. Ex : 1) Aurelia -Jelly fish 2)’Rhopilema3)Pelagia (Luminescent Jelly fish)

Order 5 : Rhyzostomeae : Free swimming medusa. The oral arms are branched. Tentacles absent. 8 or more tentaculocysts are present. Ex : 1) Pilema, 2) Rhizostoma.

4. Class Cubozoa

  • This class includes jellyfish with a box-shaped medusa or a bell with a square cross-section, commonly referred to as “box jellyfish.”
  • These species may attain lengths of 15 to 25 centimetres, however Cubozoa members are typically smaller than Scyphozoa. In contrast, cubozoans share similar morphological and anatomical traits with scyphozoans.
  • Tentacle arrangement is a significant distinction between the two types. In the corners of the cubozoan’s square bell canopy are muscular pads called pedalia, with one or more tentacles attached to each pedalium.
  • Occasionally, the digestive system might expand into the pedalia. Nematocysts may be organised in a spiral orientation along the tentacles; this helps to sedate and capture prey successfully. Cubozoans consist of the most deadly cnidarians (Figure 4).
  • These animals have image-forming eyes, which consist of a cornea, lens, and retina. Due to the fact that these structures are composed of multiple interacting tissues, they can be considered real organs.
  • There are four clusters of eyes between each pair of pedalia. Each cluster is made up of four simple eye spots and two image-forming eyeballs orientated in opposite directions.
  • How images created by these extremely complex eyes are processed remains a mystery, as cubozoans lack a specialised brain but possess enormous nerve networks.
  • The existence of eyes enables cubozoans to be aggressive and successful hunters of small aquatic organisms such as worms, arthropods, and fish.
  • Cubozoans have distinct sexes, and fertilisation takes place within the female. Depending on the species, planula larvae may develop within the female or be expelled.
  • Every planula transforms into a polyp. These polyps may produce other polyps in order to establish a colony; each polyp then converts into a medusa.
The (a) tiny cubazoan jelly Malo kingi is thimble shaped and, like all cubozoan jellies, (b) has four muscular pedalia to which the tentacles attach. M. kingi is one of two species of jellies known to cause Irukandji syndrome, a condition characterized by excruciating muscle pain, vomiting, increased heart rate, and psychological symptoms. Two people in Australia, where Irukandji jellies are most commonly found, are believed to have died from Irukandji stings. (c) A sign on a beach in northern Australia warns swimmers of the danger. (credit c: modification of work by Peter Shanks)
The (a) tiny cubazoan jelly Malo kingi is thimble shaped and, like all cubozoan jellies, (b) has four muscular pedalia to which the tentacles attach. M. kingi is one of two species of jellies known to cause Irukandji syndrome, a condition characterized by excruciating muscle pain, vomiting, increased heart rate, and psychological symptoms. Two people in Australia, where Irukandji jellies are most commonly found, are believed to have died from Irukandji stings. (c) A sign on a beach in northern Australia warns swimmers of the danger. (credit c: modification of work by Peter Shanks)

Skeleton of Phylum Cnidaria

  • The mesoglea is the only structure in medusae. Hydra and the majority of sea anemones close their mouths when they are not feeding; hence, the water within their digestive cavity functions as a hydrostatic skeleton. Some polyps, such as Tubularia, utilise water-filled cell columns for support.
  • In certain colonial polyps, a chitinous periderm supports the connecting sections and the lower portions of the individual polyps. Stony corals secrete huge calcium carbonate exoskeletons. A few polyps acquire and adhere things such as sand grains and shell fragments to their exteriors. Some colonial sea anemones reinforce the mesoglea with particles of silt.

Cell Layers of Phylum Cnidaria

In cnidarians, the following types of cell layers are present:

  • Epitheliomuscular cells are cells whose bodies are composed of epithelial material, but whose bases stretch to create parallel muscle fibres. The fibres of the cell layers pointing outward are perpendicular to the fibres facing inward. In the Anthozoa and Scyphozoa classes, the mesoglea contains muscle cells as well.
  • Cnidocytes, which resemble harpoon cells, are the “nettle cells.” In certain instances, they form between or on top of the muscle cells.
  • Sensory cells arise between or on top of muscle cells and communicate with nerve cells through synapses, which are primarily located between the muscle cell bases.
  • Interstitial cells are unspecialized cells that can convert into the proper types to replace lost or damaged cells. They are located between the muscle cell bases.
  • Inward-facing gastrodermis comprises gland cells that emit digesting enzymes in addition to epitheliomuscular, interstitial, and nerve cells. In some species, it contains low amounts of cnidocytes, which are known to subdue struggling prey.
  • In some species, the mesoglea comprises a limited number of amoeba-like cells and muscle cells. Nonetheless, there are significantly fewer middle-layer cells and types than in sponges.

Polymorphism of Phylum Cnidaria

  • Polymorphism is the occurrence of at least two structurally and functionally distinct types of individuals inside the same organism.
  • It is a distinguishing feature of Cnidarians, especially polyp and medusa forms, as well as zooids seen in colonial organisms such as Hydrozoa.
  • In Hydrozoans, colonial individuals originate from individuals such as zooids that perform distinct functions.


These are the primary components of a cnidocyte:

  • A trigger-like cilium that projects above the surface.
  • As the payload, a strong capsule called cnida, which is the thread’s home, contains a mixture of compounds that may comprise venom, glue, or both.
  • A tube-like extension seen on the cnida’s wall points into the cnida, similar to how the finger of a rubber glove points inwards. When a cnidocyte is activated, the finger protrudes.
  • The extension of a finger coils around the cnidocyte until it activates. The thread is typically hollow and transports the cnida’s chemicals to the intended target.
  • There is an operculum at the end of the cnida. The lid may consist of a single hinged flap or three flaps arranged like pie pieces.
  • Because to the small size and complexity of cnidocytes, it is difficult to summarise the firing mechanisms that occur in these structures. Minimum of four hypotheses have been offered; they are as follows:
    • Fast fibre contraction occurring around the cnida may raise its internal pressure.
    • The thread may also resemble a quickly expanding spring when it is released.
    • In the case of Chironex, chemical changes in the cnida’s contents may result in fast polymerization-induced expansion.

Nervous System and Senses

  • Cnidarians are commonly believed to lack both brains and central nervous systems. Yet, they possess integrative regions of brain tissue that are regarded as a type of centralization.
  • Their swimming muscles are controlled by decentralised neural networks that innervate the majority of their bodies.
  • These neural networks connect to sensory structures, despite the fact that each clade contains slightly distinct structures. Typically, these sensory structures are referred to as rhopalia. They can generate signals in response to diverse stimuli, like light, pressure, etc.
  • The diffuse nerve network has modulatory effects on the neurological system of scyphozoans. In addition, it forms “signal cables” between sensory and motor neurons.
  • Within the nervous system, intermediary neurons can form the ganglia that serve as local coordination centres. In the class hydrozoans, communication between nerve cells can occur via chemical synapses or gap junctions.

Feeding and Excretion

Cnidarians receive nutrition in a variety of ways, including predation, absorption of dissolved organic compounds, filtration of food particles existing in the water, symbiotic algae within their cells, and parasitism. The majority of cnidarians receive the majority of their food through predation. Yet, other organisms, such as the corals Hetroxenia and Leptogorgia, rely solely on their endosymbionts and the absorption of liquid nutrients. Prey remains that are indigestible are ejected by the mouth. Ammonia is the primary byproduct of cellular internal operations, and it is eliminated by both exterior and internal water currents.


There are no respiratory organs, and both cell layers absorb oxygen and release carbon dioxide into the surrounding water. When the water in the digestive tract becomes stale, it must be replaced, and with it will be evacuated any nutrients that have not been absorbed. Certain Anthozoa have tentacles with ciliated grooves that enable them to pump water out of and into the digestive cavity without opening their mouths. In order to regulate the water pressure within the cavity without discharging undigested food, this enhances the process of respiration following feeding in animals that use the cavity as a hydrostatic skeleton. Large quantities of antioxidants are produced by the animals to counteract the excess oxygen.


  • Medusae swim using a form of jet propulsion: In this process, muscles located primarily within the rim of the bell squeeze the water out of the cavity, and the springiness of the mesoglea is energised by the recovery stroke.
  • Because to the thinness of the tissue layers, they provide insufficient force to swim against currents and only adequate power to govern movement within currents.
  • Hydras and a few sea anemones may crawl over rocks and sea or stream beds in a variety of ways, including inchworm-like crawling, snail-like creeping, and somersaulting. Few are capable of clumsy swimming by waggling their bases.

Cnidaria Reproduction

In the case of polyps and medusas, cnidarian amphimixis frequently entails a sophisticated life cycle. In contrast, in a process known as strobilation, these creatures absorb their tentacles and divide them horizontally into a series of discs that form juvenile medusae. The juveniles swim away and mature slowly, while the polyp re-grows and should regularly repeat strobilation. During the breeding season, the adults’ gonads, which are located in the gastrodermis, discharge oocytes and sperm into the water. This phenomenon in which succession is structured differently between generations is frequently referred to as “alternation of asexual and sexual phases” or “metagenesis.” However, these alterations should not be confused with the alternation of generations observed in plants, as the two are distinct in nature.

The abbreviated versions of this life cycle are prevalent. Hydrozoa have numerous life cycles. Some species lack polyp phases, while a few are devoid of medusae. In certain species, the medusae remain connected to the polyp and are capable of reproducing sexually. In severe situations, these reproductive zooids may not resemble medusae very closely. This was found in both Hydrozoa and Scyphozoa, where the polyps are generated straight from the medusae without the participation of sexual reproduction. Anthozoa do not have a medusa stage, hence only polyps are responsible for sexual reproduction.

Their release is typically triggered by environmental factors such as changes in water temperature, and by illumination conditions such as sunrise, sunset, or the phase of the moon. Several Cnidaria species may spawn simultaneously in the same area. Too many eggs and sperm exist for predators to consume a significant proportion. A notable example is the Great coral reef, where at least 110 corals and a few non-cnidarian animals have produced sufficient gametes to cloud the water. These mass reproductions could yield a hybrid. In certain species, the eggs secrete compounds that attract sperm of the same species.

The fertilised eggs transform into larvae by dividing until there are enough cells to form a hollow spherical, after which a depression develops at one end, which becomes the digestive cavity. In Cnidarians, however, the dip at the end is caused by the yolk, but in bilaterians it originates at the other end. Planulae are the larvae that can swim or crawl due to their cilia. They are cigar-shaped, but significantly broader at the “front” end, which is the aboral, vegetal-pole end, and connect to a substrate if the species has a polyp stage.

A small number of anthozoan larvae have endosymbiotic algae that aid in their nutrition. Anthozoan larvae either have huge yolks or can feed on plankton. Since the parents are immobile, the larvae’s ability to eat extends their range and prevents places from becoming overcrowded. Scyphozoan and hydrozoan larvae have minimal yolk and lack endosymbiotic algae; hence, they must settle and transform into polyps rapidly. These species believe that their medusae expand their ranges.

Asexual: All known cnidaria are able to reproduce asexually by a variety of ways, in addition to regenerating after fragmentation. The polyps of hydrozoans can only form buds, whereas the medusae of some hydrozoans can divide in half. Scyphozoan polyps can produce both buds and halves by splitting in the middle. In addition to these two techniques, Anthozoa are also capable of horizontal division immediately above the base. Asexual reproduction renders the offspring identical to the adult cnidarian.

Economic Importance of Phylum Cnidaria

Cnidaria is both ecologically and economically significant. Here are some of the economic benefits these animals provide:

  • Fisheries: Certain cnidarians, such as jellyfish, are collected and sold for human consumption. In some regions of the world, jellyfish are considered a delicacy and are used in recipes such as salad and soup. Other cnidarians, such as sea anemones, are utilised in the aquarium industry.
  • Tourism: Cnidarian-formed coral reefs are among the most popular tourist destinations in the world. They provide habitat for several types of fish and marine life, making them popular snorkelling and scuba diving locations.
  • Medical research: Cnidarians are a source of physiologically active chemicals with potential medical applications, according to medical studies. Several researchers are examining the use of cnidarian toxins for pain relief and as treatments for specific types of cancer, for instance.
  • Biotechnology: Cnidarians are being explored for their potential in biotechnology, which includes the creation of new medications, biomaterials, and sensors.

It is crucial to note, however, that many cnidarians are regarded as pests or nuisances. Jellyfish blooms, for instance, can damage fishing nets, jam the cooling water intakes of power plants, and even result in beach closures. In addition, cnidarians that feed on coral polyps, such as the crown-of-thorns starfish, can have deleterious effects on coral reefs. Ultimately, it is crucial to control and monitor the harvesting and utilisation of cnidarians so that their economic benefits do not come at the expense of their ecological health and biodiversity.

Ecological Importance of Phylum Cnidaria

Phylum Cnidaria has tremendous ecological value. Here are some of the contributions that cnidarians make to marine ecosystems:

  • Habitat creation: Coral reefs, which are created by cnidarians, provide refuge and habitat for a broad range of marine species. They are frequently referred to as “sea jungles” due to their remarkable variety.
  • Food source: A range of marine predators, including fish, sea turtles, and seabirds, rely on numerous kinds of cnidarians as an important food supply. Cnidarians also play a significant role in the energy transfer within marine food webs.
  • Nutrient cycling: Cnidarians, like all other living species, contribute to the nutrient cycle process in marine habitats. They absorb nutrients such as nitrogen and phosphorus from the water, which are then returned to the ecosystem upon their demise.
  • Symbiotic relationships: Certain cnidarians have symbiotic interactions with algae, fish, and crustaceans, among other creatures. Several types of coral, for instance, have a symbiotic connection with zooxanthellae algae, which offer nutrients to the coral and give it its unique colour.
  • Bioluminescence: Several species of cnidarians, such as jellyfish, are bioluminescent, meaning they generate their own light. This can be used as a method of communication or to lure prey.

Cnidarians are crucial components of marine ecosystems, supporting biodiversity, nutrient cycling, and the overall health of our oceans. Yet, similar to numerous other marine organisms, they are threatened by climate change, overfishing, and pollution. Preserving and conserving cnidarian populations is crucial for maintaining the health and diversity of our oceans.

Facts About Phylum Cnidaria

  • The majority of these are marine, however a few, such as hydra, inhabit fresh water.
  • Many are colonial (Eg: Corals). Some are single (Eg: sea anemone).
  • They are diploblastic and exhibit tissue organisation grade.
  • The body is radially symmetric, whereas sea anemones demonstrate biradial symmetry.
  • Polyp and medusa are the two distinct cnidarian forms. The polyp is a sessile hydroid with a mouth-up configuration. Medusa has an umbrella or bell form with her mouth facing downward. By limiting the bell, it swims.
  • The body wall consists of an outside epithelium known as the epidermis, an inner epithelium known as the gastrodermis, and a gelatinous mesoglea between the outer and inner epithelium. Mesoglea is composed of amoeboid cells that are derived from ectoderm. Mesoglea has few polyps. It is vital to the buoyancy of medusa since it is dense.
  • The body wall is composed of stinging cells known as cnidocytes. Therefore the term cnidaria. Each cnidocyte cell has a fluid filled membranous capsule called cnida. Cnidocytes aid in defence and prey capture.
  • The centre blind sac-like hollow is known as coelenterons or the gastrovascular cavity. Thus, Coelenterata’s name. It reveals its mouth, which is ringed by tentacles. The mouth serves both for intake and egestion.
  • In the form of a medusa, the coelenterons are differentiated into the stomach, radial canals, and ring canal. Coelenterons are beneficial for digestion and circulation.
  • The digestive process begins extracellularly in the coelenterons and then moves into the nutritive muscle cells of the gastrodermis.
  • Exchange of breathing gases and elimination of excretory wates happens by diffusion across the body wall.
  • Neurons in the epidermis and gastrodermis are coupled to form a pair of neural networks. The mesoglea is traversed by neurons that connect the two nerve networks. Conduction of nerve impulses is diffuse conduction. Impulses of nerve can go in any direction. In addition to nerve nets, medusae have nerve rings and ganglia along the bell border.
  • Sensory structures, such as statocysts, exist in medusoid form.
  • Asexual reproduction is carried out through budding, fission, and fragmentation.
  • The majority of cnidarians are unisexual, but some are bisexual. External fertilisation is used. There is holoblastic cleavage. Indirect development includes planula, a free-swimming, ciliated larval stage.
  • Metagenesis is the alternating of asexually reproducing polyp form and sexually reproducing medusa form in species with polyp and medusa phases.
  • Cnidarians have tremendous regenerative capacity.

Difference Between Polyp and Medusa

  • Cnidarians, a group of creatures that includes jellyfish, corals, and sea anemones, have two unique body structures: polyps and medusas.
  • A polyp is a sessile, cylindrical or conical body form with a mouth encircled by tentacles and a disc or foot at the other end. It adheres to an immobile substrate (such as a rock or the ocean floor). Cnidarians with a polyp body structure include sea anemones and coral.
  • A medusa, on the other hand, is a free-swimming, umbrella-shaped body with tentacles and a bell-shaped body. Its mouth is positioned on the underside and is surrounded by tentacles. It propels itself through the water by contracting and relaxing its bell-shaped body as it swims. Jellyfish are examples of cnidarians with a medusa body shape.
  • Polyps are stationary and have a cylindrical or conical body with a basal disc or foot, whereas medusas are free-swimming and have an umbrella-shaped body with a bell-shaped body.

Phylum Cnidaria Examples

A few examples of members belonging to Phylum Cnidaria are –

Jellyfish – Phyllorhiza punctata (Scyphozoa)

  • The jellyfish species Phyllorhiza punctata belongs to the class Scyphozoa. It is endemic to the western Pacific Ocean and is also known as the white-spotted jellyfish. But, it has been brought to other parts of the world, including the Gulf of Mexico and the Caribbean Sea.
  • Phyllorhiza punctata has a characteristic white hue with brownish patches and a maximum diameter of 50 cm. It features a bell-shaped body with long, trailing tentacles that are employed to capture prey, similar to other jellyfish.
  • This jellyfish is renowned for its adaptability to a variety of habitats, from estuaries and mangroves to the open ocean. It feeds on small fish, plankton, and other species of jellyfish.
  • In some regions, Phyllorhiza punctata has become an invasive species that can outcompete native species and damage local ecosystems. It is challenging to regulate due to its capacity to multiply rapidly and endure a range of salinities and temperatures.

Box jellyfish – Carybdea branchi (Cubozoa)

  • There appears to be an error in the reported species name, as “Carybdea branchi” is not a recognised species of box jellyfish. But, I can share some general information regarding the properties of box jellyfish (class Cubozoa).
  • Box jellyfish are a species of cnidarians distinguished by their cube-shaped bell and long, slender tentacles that can reach a maximum length of 10 feet (3 metres). Mostly found in the warm coastal waters of the Pacific and Indian Oceans, as well as certain regions of the Atlantic.
  • Box jellyfish are among the world’s most poisonous animals, and their stings can be fatal to humans. Their tentacles are lined with thousands of specialised cells known as nematocysts, which can produce potent toxins when touched. In certain situations, box jellyfish stings can cause intense pain, heart failure, and even death.
  • Certain species of box jellyfish, such as the sea wasp (Chironex fleckeri), are especially hazardous and are responsible for a large number of human deaths each year. Others, such as the “four-sided” box jellyfish (Chiropsalmus quadrumanus), are less poisonous and rarely pose a significant threat to human health.
  • It is essential to use caution when swimming or snorkelling in locations where box jellyfish are known to be present and to seek emergency medical attention if stung.

Siphonophore – Physalia physalis (Hydrozoa)

  • Physalia physalis is a siphonophore species belonging to the Hydrozoa class. Although it is not a genuine jellyfish, it is usually referred to as the Portuguese man o’ war.
  • Physalia physalis is a colonial organism composed of several polyps that collaborate to build a pneumatophore or “sail” Long, trailing tentacles stretch beneath the water’s surface to collect prey, while the sail is a gas-filled bladder that permits the siphonophore to float on the surface.
  • The tentacles of Physalia physalis are lined with stinging cells known as nematocysts, which can deliver a painful and perhaps lethal sting to humans. Contrary to its name, the Portuguese man o’ war is not a single creature, but rather a colony of specialised individuals that perform numerous duties, such as eating and breeding, in concert.
  • Physalia physalis is primarily found in warm, tropical waters throughout the globe and is frequently transported by ocean currents. It is an essential component of numerous marine ecosystems, acting as a food supply for numerous predators. Nonetheless, its deadly sting and propensity to wash up on beaches make it an annoyance and a threat to humans.

Polypodium hydriforme (Polypodiozoa)

  • Polypodium hydriforme is a species of parasite that belongs to the phylum Polypodiozoa. It is also called as “Hydra illness” or “Hydra cancer” because it can generate abnormal growths that resemble the tentacles of a hydra in freshwater fish.
  • The life cycle of Polypodium hydriforme is complicated, involving both sexual and asexual reproduction. It typically infects the eggs of freshwater fish and can remain latent within the fish until certain environmental cues activate it to initiate asexual reproduction.
  • Once triggered, the parasite can create massive, unnatural growths on the fish that resemble the tentacles of a hydra. These growths, which may be benign or cancerous, can cause a variety of health issues in fish, including reduced mobility and feeding.
  • Polypodium hydriforme is a fascinating and peculiar parasite that scientists do not fully comprehend. It is not known to infect humans or represent a hazard to human health, but it can have a considerable influence on populations of freshwater fish and is the topic of continuing research.

Cerianthus filiformis (Ceriantharia)

  • The tube-dwelling anemone Cerianthus filiformis belongs to the class Ceriantharia. These anemones are sometimes referred to as tube-dwelling or burrowing anemones because they inhabit a long, tube-like structure constructed of chitin, a tough, fibrous material.
  • From the Gulf of Mexico to Nova Scotia, Cerianthus filiformis is found in shallow waters throughout the eastern coast of North America. They inhabit sandy or muddy bottoms, with the top of their tube protruding from the soil. The tentacles of the anemone collect small prey such as plankton, and the anemone can retract inside its tube for protection.
  • Cerianthus filiformis can reach a maximum length of 30 centimetres (12 inches), while its tentacles can reach a maximum length of 20 centimetres (8 inches). Often, the tentacles are brown, white, or green in hue. Because of their distinct appearance and engaging behaviour, these anemones are frequently employed in home aquariums.

Sea anemones (Actinaria, part of Hexacorallia)

  • The predatory sea anemones belong to the class Actinaria, which is part of the larger group Hexacorallia. Due to their colourful and often delicate look, they are called after the anemone flower.
  • Sea anemones inhabit both shallow and deep water and can be found in both tropical and temperate environments worldwide. A basal disc, which also serves as their foot, is used to adhere them to rocks, coral reefs, and other hard surfaces.
  • Long, cylindrical bodies generally decorated with a ring of tentacles around the mouth characterise sea anemones. The tentacles contain stinging cells known as nematocysts, which the anemone employs to capture and immobilise its prey, typically small fish or crustaceans. Once captured, the anemone will use its tentacles to drag the victim to its mouth, where it will be eaten in its entirety.
  • The diameter of a sea anemone can range from a few millimetres to more than a metre. Several species have symbiotic relationships with other organisms, such as clownfish, which inhabit the tentacles of certain anemone species.
  • Sea anemones are intriguing organisms that are frequently kept in aquariums. However, it is essential to keep in mind that they are predators and should not be housed with smaller or more fragile fish, as they may swallow them.

Sea fan Gorgonia ventalina (Alcyonacea, part of Octocorallia)

  • Gorgonia ventalina, often known as the sea fan, is a type of soft coral belonging to the Alcyonacea order, which is a subgroup of the Octocorallia. The Atlantic Ocean contains sea fans, from Bermuda and the Bahamas to the coast of Brazil.
  • Sea fans have a delicate and intricate look, with a colony structure that resembles a fan and is composed of thin branches united by a central skeleton. The size of the colony can range from a few millimetres to over a metre in height and width. The coloration of the sea fan can range from pale yellow to orange or purple.
  • Sea fans are filter feeders, using their branches to catch plankton and other small animals from the water column. In addition, they have a symbiotic relationship with zooxanthellae, which are photosynthetic algae that reside within the coral’s tissue and provide it with energy.
  • Sea fans serve a crucial part in the ocean’s ecosystem, providing habitat and protection for a range of marine animals, including fish, crabs, and shrimp. Yet, issues such as pollution, overfishing, and climate change can cause coral bleaching and death.
  • Sea fans are utilised frequently in home aquariums due to their unique and attractive appearance. Nonetheless, it is essential to ensure that they are procured responsibly and stored in suitable circumstances, as they are fragile and require special attention.

Coral Acropora muricata (Scleractinia, part of Hexacorallia)

  • Acropora muricata is a species of coral belonging to the Scleractinia order, which is part of the larger Hexacorallia group. It is popularly referred to as staghorn coral due to its branching growth style.
  • The staghorn coral is found in tropical waters across the world, particularly in the Indo-Pacific. It is capable of forming enormous, densely-packed colonies that provide habitat and protection for a variety of marine species.
  • Like with all corals, staghorn coral is a filter feeder that captures plankton and other microscopic creatures from the water column using its polyps. It also has a symbiotic relationship with zooxanthellae, which are photosynthetic algae that reside within the tissue of the coral and provide it with energy.
  • Important component of coral reefs, staghorn coral is also imperilled by pollution, overfishing, and climate change. When coral is under stress, it may expel its zooxanthellae, a process known as bleaching, which can ultimately result in coral mortality.
  • There are efforts to preserve and restore staghorn coral populations, including coral gardening and outplanting projects. To ensure the long-term survival of these vital ecosystems, it is essential to address the core causes of coral decline, such as reducing greenhouse gas emissions and supporting sustainable fishing methods.


Which of the following is not a characteristic of the phylum Cnidaria?
a) Radial symmetry
b) Presence of nematocysts
c) Diploblastic body plan
d) Bilateral symmetry
Answer: d) Bilateral symmetry

Which of the following is a class of Cnidaria?
a) Crustacea
b) Gastropoda
c) Scyphozoa
d) Osteichthyes
Answer: c) Scyphozoa

Cnidocytes are specialized cells that contain:
a) Nematocysts
b) Stingers
c) Both a and b
d) None of the above
Answer: c) Both a and b

The body cavity of a Cnidarian is called the:
a) Coelom
b) Blastocoel
c) Pseudocoelom
d) Gastrovascular cavity
Answer: d) Gastrovascular cavity

The medusa form of a Cnidarian is:
a) A polyp-like structure
b) A free-swimming, bell-shaped structure
c) A filter-feeding structure
d) A photosynthetic structure
Answer: b) A free-swimming, bell-shaped structure

Coral reefs are made up of which class of Cnidarians?
a) Hydrozoa
b) Scyphozoa
c) Anthozoa
d) Cubozoa
Answer: c) Anthozoa

The common name for members of the class Hydrozoa is:
a) Jellyfish
b) Sea anemones
c) Corals
d) Hydra
Answer: a) Jellyfish

Cnidarians are found in which aquatic environments?
a) Freshwater only
b) Saltwater only
c) Both freshwater and saltwater
d) None of the above
Answer: c) Both freshwater and saltwater

Which of the following is not a function of nematocysts in Cnidarians?
a) Prey capture
b) Defense
c) Movement
d) Reproduction
Answer: d) Reproduction

Which of the following is not a type of Cnidarian symmetry?
a) Radial
b) Bilateral
c) Asymmetrical
d) Spiral
Answer: d) Spiral


What organisms are present in the Phylum Cnidaria?

The Phylum Cnidaria includes a diverse group of animals that are found in marine and freshwater environments. Examples of organisms in this phylum include jellyfish, sea anemones, corals, hydras, and sea pens.
Cnidarians are characterized by the presence of specialized cells called cnidocytes, which contain stinging organelles called nematocysts. These cells are used for defense and prey capture. Cnidarians also have a simple nervous system, with a nerve net that allows them to sense and respond to their environment.
Cnidarians are diploblastic, which means they have two layers of cells: an outer layer called the epidermis and an inner layer called the gastrodermis. In between these layers is a jelly-like substance called mesoglea, which can vary in thickness depending on the species.
Overall, the Phylum Cnidaria is an important group of animals in marine and freshwater ecosystems, playing important roles as predators, prey, and ecosystem engineers.

What are the different classes of the Phylum Cnidaria?

The Phylum Cnidaria is divided into four main classes based on their body forms and characteristics:
Anthozoa: This class includes sea anemones, corals, and sea pens. They have a polyp body form and lack a medusa stage. Anthozoans are mostly sessile, with the exception of a few species that can move slowly.
Scyphozoa: This class includes jellyfish, which have a medusa body form and a polyp stage in their life cycle. Scyphozoans are mostly free-swimming, and their bell-shaped bodies are usually equipped with tentacles for capturing prey.
Cubozoa: This class includes box jellyfish, which have a cube-shaped medusa body form with tentacles at each corner. They are typically found in tropical waters and are known for their potent venom.
Hydrozoa: This class includes a diverse range of organisms, including hydras, Portuguese man o’ war, and fire corals. Hydrozoans have both a polyp and a medusa stage in their life cycle, and can exhibit a wide range of body forms.
Each class of cnidarians has unique characteristics that set them apart from each other, but they all share the defining features of the phylum, such as the presence of cnidocytes and two cell layers.

What is the meaning of the term Cnidaria?

The term “Cnidaria” comes from the Greek word “knidē,” which means “nettle.” This refers to the stinging cells, called cnidocytes, that are found on the tentacles and other parts of cnidarians. These cells contain tiny harpoon-like structures called nematocysts, which are used for defense and capturing prey. The term “Cnidaria” is therefore used to describe the phylum of animals that possess these specialized stinging cells.

What are stinging cells?

Stinging cells, also known as cnidocytes, are specialized cells found in cnidarians (such as jellyfish, sea anemones, and corals) and some other animals. These cells contain tiny harpoon-like structures called nematocysts, which can be used for defense and capturing prey.
When a cnidarian comes into contact with an object, the cnidocyte is triggered to rapidly discharge the nematocyst. The nematocyst then injects a toxin into the target, which can paralyze or kill smaller prey, or deter larger predators.
Different types of cnidocytes can be used for different purposes, such as defense, feeding, or anchoring the animal to a substrate. The stinging cells of cnidarians are a unique adaptation that has allowed them to thrive in a variety of marine environments.


  • Phylum Cnidaria. (2021, December 4).

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