Phylum Coelenterata (Cnidaria): Characteristics, Classification, Examples

Phylum Coelenterata (Cnidaria) Definition:

Phylum Coelenterata (Cnidaria) are defined as diploblastic metazoa having a nematocyst and a single gastrovascular cavity, or coelenteron, as the tissue construction grade.

Phylum Coelenterata (Cnidaria) Characteristics

They are aquatic, except for a few freshwater species, such as the hydra.

  1. They are multicellular and organized at the tissue level.
  2. They are either single or conical. Inactive or free swimming.
  3. Individuals are radially or biradially symmetrical about a longitudinal oral-aboral axis.
  4. The majority of cells are dispersed and specialized for distinct purposes. Some cells form tissues such as nerve nets or nervous tissues.
  5. The exoskeleton is chitinous (perisarc) or calcareous(corals).
  6. They are diploblastic organisms with two cellular layers, an epidermis, and a gastrodermis, separated by an acellular gelatinous mesoglea.
  7. Acoelomate organisms lack a second bodily cavity known as the coelom.
  8. One or two whorls of short and delicate tentacles encircle the mouth.
  9. The tentacles are equipped with nematocysts; they are used for capturing food, ingesting it, adhering to surfaces, and defense.
  10. Attached sessile and asexual zooid (polyps) and free-swimming and sexual zooid are the two sorts of people (medusae). Some species are distinguished for their polymorphism or diversity of morphologies.
  11. They are typically carnivorous, with both extracellular and intracellular digestion.
  12. There is no anus.
  13. Coelom and deficiencies in the respiratory, circulatory, and excretory systems.
  14. The primitive nervous system is composed of a diffuse nerve network. Central nervous system missing.
  15. Epithelia-muscle cells and endothelial-muscle cells create longitudinal and circular muscle fibers in the muscular system.
  16. The mouth opens into a single gastrodermis-lined chamber termed the gastrovascular cavity or coelenteron.
  17. Sensory organs are composed of ocelli and statocysts.
  18. Reproduction occurs both asexually and sexually.
  19. The process of asexual reproduction is budding, while sexual reproduction is the generation of gametes.
  20. The development comprises a free-swimming planula larva with cilia.
  21. Life history is characterized by the phenomena of alternation of generation or metagenesis, in which the asexual, sessile polypoid generation alternates with the sexual, free-swimming medusoid generation.

Phylum Coelenterata (Cnidaria) Classification

  • There are almost 11,000 recognized species in the phylum Coelenterata, half of which are extinct. This categorization was established by Hyman, L.H. (1940).
  • According to the Hyman phylum, Coelenterata is divided into three groups.

Class 1. Hydrozoa (Gr., hydra=water + zoios=animal)

  • Freshwater or saltwater? Unique or conical. Sessile or swimming freely.
  • They display radial tetramerous and polymerous symmetry.
  • The body wall is composed of the outer ectoderm, inner endoderm, and noncellular mesoglea.
  • The gastrointestinal cavity is devoid of stomodaeum, septa, or nematocysts containing gastric filament.
  • In some forms, the skeleton or horny structure is horny perisarc, but in other forms, coenosarc produces a calcium carbonate skeleton, forming a massive stony structure or coral.
  • There is polymorphism present. The asexual polyp and the sexual medusa are the two basic forms of zooids.
  • Lack of stomodaeum and septa in a polyp (mesentery).
  • Medusa with authentic velum (Craspedote).
  • Mesoglea non-cellular.
  • Many of them display generational alternation.
  • Gonads are epidermal. Sex cells are expelled externally.
  • Holoblastic cleavage, ciliated embryo in planula.

Order 1: Hydroida

  • Unique or conical.
  • The polypoid phase predominates.
  • Medusae are transient or nonexistent.
  • The sense organs of medusae are ocelli and statocysts of ectodermal origin only.

Suborder 1. Anthomedusae or Athecata

  • unique, or conical.
  • Polyps and blastostyles athecate, meaning that the perisarc does not produce hydrothecae and gonothecae.
  • Medusae are tall, bell-shaped creatures with gonads on the manubrium and an umbrella with a strong arch.
  • Medusae have eyespots, or ocelli, at the base of their tentacles.
  • Statocysts are not present.
  • Examples: Hydra, Ceratella, Tubularia, Clava, Eudendrium.

Suborder 2. Leptomedusae or Thecata

  • Conical Hydrozoa
  • Polyps are surrounded by hydrothecae, whereas medusae are covered by gonothecae.
  • Flattened, bowl-or saucer-shaped medusae with gonads on the radial canal.
  • Medusa’s gonads are located in radial canals.
  • Typically, medusae have statocysts.
  • Eyespots, or ocelli, are missing.
  • Examples: Obelia, Sertularia, Plumularia, Aglaophenia.

Order 2: Milleporina

  • Conical Hydrozoa lacks a perisarc, resembling coral.
  • The ectoderm produces an enormous calcareous skeleton with holes from which polyps emerge.
  • There are two types of zooids in a colony: the gastrozooid and the dactylozooid.
  • Gastrozooids (nutritional zooids) have small mouths and tentacles.
  • Dactylozooids are hollow, elongated, and thin, with tentacles but no mouth.
  • In tiny chambers, medusae grow into free creatures devoid of mouths, radial canals, and tentacles.
  • Example: Millepora.

Order 3: Stylasterina

  • There are two types of zooids seen in coral-like Hydrozoa colonies: dactylozooids and gastrozooids.
  • Dactylozooids are tiny, solid, and limbless.
  • Gastrozooids have a cup-shaped body with a sharp spine.
  • Reduced gonophores to sporosacs. Medusae are captive.
  • Planula are released as larvae.
  • Stylaster, for example.

Order 4. Trachylina

  • Polypoid stage is diminished or missing.
  • Medusae are enormous, dominating, free-swimming creatures that can evolve from fertilized eggs.
  • Statocysts or marginal sense organs with endodermal statoliths,

Suborder 1: Trachymedusae

  • Tentacles positioned over the bell edge
  • The umbrella’s border is smooth.
  • Manubrium is lengthy.
  • Gonads arise in radial canals.
  • Example: Geryonia.

Suborder 2. Narcomedusae

  • Tentacles emerge between the exumbrella’s bell border and apex.
  • The manubrium is short in length.
  • The presence of gonads on the manubrium or stomach floor.
  • Examples: Cunina, Solmaris.

Order 5. Siphonophora

  • They are colonies of polymorphic, free-swimming, or floating Hydrozoa.
  • Several forms of polypod and medusoid individuals are linked to the colony’s stem or disc.
  • Oral polyps are devoid of oral tentacles.
  • Medusae are fragmentary and hardly released.

Suborder 1. Calycophora

  • The uppermost portion of the colony is equipped with at least one swimming bell (nectophores).
  • Apical float or absence of Pneumatophore.
  • Examples: Diphyes, Praya, Abyla.

Suborder 2. Physophorida

  • The colony’s upper section produces a vast, gas-filled float (pneumatophore).
  • Examples: Physalia, Halistemma, Stephalia.

Class 2. Scyphozoa (Gr., skyphos=cup +zoios=animal)

  • It contains big, marine-exclusive medusae, or real jellyfish.
  • Large, bell- or umbrella-shaped, without real velum, free-swimming, or linked by an aboral stalk, medusae lack genuine velum and have no true velum.
  • Reduced polyp stage or absence.
  • Tentaculocysts with endodermal statoliths compose marginal sense organs.
  • Gastrovascular canal with endodermal gastric filaments and gastric pouches.
  • No stomodaeum.
  • Extensive, gelatinous, and composed of fibers and cells, Mesoglea.
  • Gonads are composed of gastrodermal tissue. Sexually active cells are discharged into the digestive tract.

Order 1. Stauromedusae or Lucernaridae

  • Goblet or trumpet-shaped body.
  • Sessile, with an aboral stalk for attachment.
  • The mouth is cruciform with modest oral lobes and a short quadrilateral manubrium.
  • Four inter-radial septa separate the gastrovascular system into the core stomach and four per-radial pouches.
  • On the faces of septa, gonads appear as elongated bands.
  • There are no peripheral sensory organs or tentacles.
  • Fertilization from outside.
  • The larva is a cilia-less planula.
  • Examples: Lucernaria, Haliclystus.

Order 2. Cubomedusae or Carybdeida

  • Cubic body with four flattened sides.
  • Free-swimming Scyphozoa that inhabit warm, shallow tropical and subtropical seas.
  • Four interradial tentacles that are hollow are carried along the edge of the sub-umbrella.
  • 4 per-radial tentaculocysts, or rhopalia, are found.
  • Each tentaculocyst contains a lithocyst and at least one ocelli.
  • There are cruciform gastric pouches present in the mouth.
  • Leaf-like gonads.
  • For example, Charybdis and Tamoya.

Order 3. Coronate

  • Conical body separated by a deep, circular coronary groove
  • Scyphomedusae inhabits the ocean’s deep waters and swim freely.
  • A coronal groove (horizontal furrow) divides the umbrella into an upper cone and a lower crown.
  • The crown is composed of pedalia, or pedal lobes.
  • The pedalia have robust tentacles.
  • The border of the bell is scalloped into alternate lappets and pedalia.
  • Cruciform oral shape
  • Four to sixteen tentaculocysts are found.
  • Examples: Pericolpa, Periphylla.

Order 4: Semaeostomeae

  • The most prevalent free-swimming medusae are found in the coastal waters of all seas.
  • The form of the umbrella is flat, saucer, or bowl.
  • A square-shaped mouth with four long oral limbs.
  • The umbrella’s edging is fringed with hollow tentacles.
  • Eight tentaculocysts are present.
  • The absence of gastric pouches and filaments
  • Examples: Aurelia, Cynaea.

Order 5. Rhizostomae

  • Typically, the body is a hemisphere without peripheral tentacles.
  • Free-swimming Scyphozoa are found in tropical and subtropical shallow seas.
  • The top of the umbrella is saucer- or bowl-shaped, flattened, or even concave.
  • The mouth is encompassed by eight oral arms with multiple funnel-shaped mouths along their edges.
  • Typically at least eight tentaculocysts.
  • Four subgenital pits are frequently present.
  • Examples: Rhizostoma or Pilema, Cassiopeia.

Class 3. Anthozoa (Gr., anthos= flower+ zoios= animal)

  • Exclusively aquatic Unique, or conical.
  • Absolutely polypoid
  • No medusoid stage.
  • Typically, the body has hexamerous, octamerous, or polymerous biradial or radobilateral symmetry.
  • The oral end of the animal is radially extended into an oral disc with tentacles encircling the mouth’s center.
  • The stomodaeum is often equipped with one or more siphonoglyphs, which are ciliated grooves.
  • The gastrovascular cavity is subdivided by at least eight septa or mesenterie
  • The inner free borders of the mesentery contain nematocysts.
  • Mesoglea has fibrous connective tissue and amoeboid cells.
  • Either an exterior or an interior skeleton.
  • The exoskeleton is composed of calcium carbonate, which frequently creates coral reefs.
  • The nervous system resembles a conventional nerve network but lacks a central nervous system.
  • The endodermal gonads form in the mesentery.
  • The mature sexual products are released into the coelenteron.
  • External fertilisation
  • The fertilized egg becomes a planula larva, which, after a brief period of freedom, settles down and matures into an adult.

Subclass 1. Alcyonaria or Octocorallia

  • Unquestionably colonial
  • Polyps are long or short cylinders that end orally in a flat, circular oral disc with an oval or elongated mouth in the middle.
  • A polyp has eight tentacles and eight septa.
  • Eight complete mesenteries are present.
  • Single ventral siphonoglyph is present.
  • Endoskeleton is composed of calcareous or horny spicules and is produced by mesogleal cells.
  • Polyps are in some way dimorphic.

Order 1. Stolonifera

  • Tropical and temperate area inhabitants of shallow water.
  • Polyps that develop independently from a mat or stolon.
  • Calcareous tubes, spicules, or absence of a calcareous skeleton
  • Examples: Tubipora, Clavularia.

Order 2. Telestacea

  • The colony consists of unbranched or branched branches that emerge from a creeping base.
  • Each stem is a polyp with lateral polyps that are extremely elongated.
  • The skeleton is made up of calcareous spicules.
  • Example: Telesto.

Order 3, Alcyonacea

  • Colonies formed like mushrooms or branched into robust, blunt processes.
  • One oral end protrudes from the fleshy mass, or coenenchyma, formed by the fusion of the polyp’s lower halves.
  • In some forms, polyps are dimorphic and contain both autozooids and siphonozooids.
  • The skeleton is composed of distinct clarous spicules, not axial bones.
  • Examples: soft corals. Alcyonium, Xenia.

Order 4. Coenothecalia

  • Not fused spicules, crystalline calcareous fibers of calcium carbonate make up the majority of the skeleton.
  • Embedded polyps are linked by solenial tubes.
  • Blue corals are corals found in Indo-Pacific coral reefs and are commonly referred to by this name.
  • Example: Heliopora (blue coral).

Order 5. Gorgonacae

  • The colony is often plant-like, with the main stem originating from the basal plate or tuft of stolon and a number of polyp-bearing branches.
  • An axial skeleton is comprised of gorgonin, separate or fused spicules, or both.
  • Typically referred to as sea fans, sea feathers, or sea whips.
  • on the tropical and subtropical coastlines.
  • Examples: Gorgonia, Corallium.

Order 6. Pennatulacea

  • Colony stretched and separated into a proximal stalk or peduncle and a distal rachis.
  • Their peduncles were immersed in dirt and sand.
  • The upper portion (rachis) is comprised of a very long axial polyp with lateral branches bearing a polyp that is dimorphic.
  • The main stem is supported by a calcareous or horny skeleton.
  • Examples: Pennatula, Renilla, Cavernularia, Pteroides.

Subclass 2. Zoantharia or Hexacorallia

  • Individual or colonial?
  • The marine form
  • Tentacles are typically simple and unbranched, organized into multiples of five and six, but seldom eight.
  • Numerous mesenteries are organized into multiples of 5 or 6, and maybe complete or insufficient.
  • Typically, the gullet has two siphonoglyphs.
  • When present, the endoskeleton is calcareous and derived from the ectoderm.
  • Typically, a monomorphic polyp.

Order 1. Actiniaria

  • Individual or colonial?
  • Typically, simple things are enormous in size.
  • No skeleton.
  • Muscular body, frequently with an aboral pedal disc.
  • Numerous tentacles and mesentery are present.
  • Multiple siphonoglyphs
  • Examples: Actinia, Metridium, Adamsia, Edwardsia.

Order 2. Madreporaria

  • Rarely solitary and predominantly colonial
  • The exoskeleton is typically rigid, compact, and massively calcified.
  • Small polyps inhabit cup-like spaces on the exoskeleton.
  • No siphonoglyphs and weak muscles.
  • Examples: genuine or stony coral. Astraea (star coral), Fungia, Favia, Madrepora (staghorn coral), and Meandrina (brain coral).

Order 3: Zoanthidea

  • Typically colony, occasionally solitary types.
  • There is neither a skeleton nor a pedal disc, but the body wall includes calcareous bodies.
  • mostly epizoic
  • Typically, small polyps are joined by basal stolons.
  • matched mesentery A pair consisting of a full mesentery and an incomplete mesentery
  • Exists only one ventral siphonoglyph.
  • Example: Zoanthus.

Order 4: Antipatharia

  • Colonial and arboreal.
  • Located in the ocean’s deep waters.
  • Typically, the base of the colony consists of a basal plate for the attachment of various things.
  • Tentacles and mesentery numbers are rather low (6-24).
  • The skeleton is a branching, ectoderm-derived chitinoid axis.
  • The axial backbone carries dioecious polyps, yet the colony may be hermaphrodite.
  • Two siphonoglyphs exist.
  • Examples: Black corals. Antipathes.

Order 5. Ceriantharia

  • Long, solitary, anemone-like organisms inhabit the vertical cylinder chambers of the ocean floor.
  • No pedal disc or skeleton.
  • With a smooth, cylindrical, elongated body and an oral disc.
  • Simple, many tentacles organized into two whorls-oral and marginal.
  • Individual and dorsal siphonoglyphs
  • Multiple, solitary, and full mesenteries exist.
  • Instance: Cerianthus.


  • Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  • Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.
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