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Cnidaria (ni-DA-re-a) is the Latinized form of a Greek word (κνιδοσ) that means sting.  The reference is to the stinging cells called cnidae or nematocysts.

The Cnidaria is a natural group of diploblastic organisms with a mostly acellular mesogloea that is derived from the ectoderm.  They contain specialized cells, the cnidocytes, which produce a variety of adhesive and stinging structures collectively called cnidae, most of which are the stinging structures called nematocysts.  The phylum includes hydroids (A), jellyfish (B-C), hydroids and corals (D).  The calcareous corals are responsible for the occurrences of tropical reefs and whole tropical islands.  These frequently form the basis of very productive tropical and subtropical marine ecosystems.  Hinde (2001) says that the Cnidaria have a fossil history that goes back to the late Pre-Cambrian and are members of the Ediacaran fauna.  Corals and coral-like animals appear in the fossil record of the Ordovician Period and form reef systems like the modern corals.


1. The Anthozoan Clade

2. The Medusozoan Clade

3. The Scyphozoan Clade (includes Cubozoa)

4. The Hydrozoan Clade

The Major Clades of the Cnidaria.

Anthozoan Clade (1)

Anthozoans are sisters to all other cnidarians.  This group has polyps that produce dispersive planula larvae.  The anthozoans are the corals (hard and soft) and the anemones.  They are marked by having no medusoid stage. The polyp is the sexual stage which produces the dispersive planula larva.  Mature polyps are large with thick, cellular mesogloea. The coelenteron is partitioned by mesentaries which have glands, filaments and nematocysts. The oral disc is surrounded by tentacles and the stomodaeum extends from the mouth down into the enteron. They have one or two ciliated grooves that direct water into the coelenteron. They have both epidermal and gastrodermal nematocysts, but no operculum. Often, the animals are colonial.  ~6,000 extant species.

Corals (Figure A) and anemones are among the most important marine organisms.  The hard corals form the basis for whole marine systems in coastal tropical and subtropical seas.  The biodiversity of a coral reef can be astounding and rival that of a tropical rain forest, far greater than any other marine biome.  The extraction of the calcium carbonate that produces the coral skeleton is facilitated by the photosynthesis of symbionts called zooxanthellae, symbiotic dinoglagellates, which make the coral polyps brightly colored.  

Charles Darwin (1842) was the first to describe the formation of a fringing tropical reef relative to changes in sea level based on observations that he made while on the voyage of the H. M. S. Beagle.  The karst limestone region on the northern part of Puerto Rico is from a fringing reef made high and dry by tectonic uplift.  One collapsed cave provides the circular basin within which the Arecibo radio telescope was built.  Fossil coral reefs form major deposits of limestone in other areas of the world as well.  

Typically, there are two types of anthozoans.  The Hexacorallia (sea anemones and stony corals ) are animals that secrete calcium carbonate to the outside of the animal, which may be solitary or colonial.  They have six internal mesentaries and six or multiples of six tentacles.  The Octocorallia, (sea fans, organ-pipe corals, soft corals, sea pens, and sea pansies)  have eight internal mesentaries, polyps with eight tentacles, and calcareous mineral deposits are internal.  

Medusozoan Clade (2)

The medusozoans have a motile jellyfish or medusa as a sexual stage in most taxa.  The phylogeny presented here suggests that the polyp is the most primitive form of the cnidarians, and medusae were added to the life history in this clade.


Scyphozoan + Cubozoan Clade (3)

The scyphozoans and cubozoans are sister groups in which the medusa is the dominant life stage.  The polyp is either highly reduced, absent, or  unknown.  The orientation of the medusa is mouth down rather than mouth up as in the polyps.  Contractions of the bell of the medusa provide locomotion.

Scyphozoans (Figure B) and Cubozoans (Figure C) have medusae that are free living with gastrodermal gonads (8), four septa partioning the coelenteron, and with cellular mesogloea.  The bell of the Scyphozaons is relatively large, generally with many tentacles, and without a velum. Scyphozoans have several feeding strategies represented by the following taxa.

The Cubozoans, considered to be a separate class of jellyfish in most modern systems, are small and delicate medusae.  They have a bell that has a velum and four flattened sides like a box with tentacles emerging only from each of the four the marginal corners of the bell.  The small Cubozoan jellys are very toxic.  Chironex, the Sea Wasp (Figure C), is one of the most venomous animals in the oceans.  They are relatively common on the Great barrier reef where they can grow up to 30cm across and have tentacles up to 2m long.  The sting of the Sea Wasp is deadly and each year claims the lives of at least two swimmers, usually within minutes of being stung.

Hydrozoan Clade (4)

Hydrozoans are the sister group to the Scyphozoans.  Their life histories typically include both polyps and medusae, though the polyps tend to dominate.  The polyp of a hydrozoan is very simple in that it has a simple mouth and no internal septa.  Many taxa have colonial polyps in which individual animals are connected, and often individual polyps have different functions. The hydrozoan medusa is small with a simple mouth and a bell with a velum.  Various life history strategies can be seen in the Hydrozoa and represented by the following taxa.


FIGURE A. Living corals from the Great Barrier Reef.


FIGURE B. Cassiopeia, the Upside-Down Jellyfish.

FIGURE C. Chironex, the Sea Wasp.



Image A.
Image C.
Images B & D from the Systematics biodiversity image collection.


The following information came from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984a), Brusca and Brusca (2003), Hickman (1973), Hinde (2001), Storer and Usinger (1965), Ruppert et al. (2004), Pechenik (2005), and Tudge (2000).


I. SYNONYMS: Coelenterates

II. NUMBER: >10,000 species known.


A. Structure

Symmetry: Radial.

Body Cavity: Not present.

Body Covering: Epidermis.

Support: Hydrostatic "skeleton" some with external or internal skeletons of calcium carbonate or organic material.

Digestive System: Digestive or gastrovascular cavity open at one end. Animal is a carnivore. It captures animals with nematocysts on tentacles.

Circulatory System: None.

Locomotion: Medusoid adults swim by undulations of bell-like body. Hydroids and corals are sessile. Some with planula larvae that move by ciliated epithelium.

Excretory System: None.

Nervous System: Simple nerve net.

Endocrine System: None.

B. Reproduction:

Reproductive System: Specialized gonads. Eggs and sperm are produced. Asexual reproduction occurs.

Development: Some with planktonic planula larvae.

C. Ecology: Found mainly in marine environments. Some freshwater taxa.


A recent review of metazoan relationships by Collins et al. (2005) suggests that the Cnidaria are sisters to the Bilateria and more recently derived than the Ctenophora (see also Martindale et al. 2002; and Aleshin and Petrov 2002).  Furthermore, the basal nature of the Anthozoa (no medusae) is called into question such that both the Anthozoa and the medusoids (Scyphozoa + Cubozoa) are basal in the Cnidaria.  All seem to agree on the more derived position of the Hydrozoa (e.g. Schuchert 1996; and Collins et al. 2005).  


TAXONOMY OF THE CNIDARIA.  Taxonomy of the Phylum after the system of of Brusca and Brusca (2003).  Descriptions of the following taxa were taken from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984a), Brusca and Brusca (2003), Hickman (1973), Hinde (2001), Storer and Usinger (1965), and Tudge (2000).



Polyp and medusoid stages; either might be reduced. Mesogloea acellular; nematocysts only in epidermis; gametes develop in epidermis. Polyp radially symmetrical. Coelenteron not separated by septa. Typically colonial with individuals connected; often individual polyps with different functions. Medusoid form small, usually with mouth on hollow stalk. Shelf or velum extending inward from the margin of the bell.  >3,000 extant species.

Bouganvilla, Calycopsis, Eleutheria, Eudendrium, Hydra, Hydractina, Hydrocoryne, Janaria, Lar, Pennaria, Polyorchis, Sarsia, Staurocladia, Stylactis,Tubularia, Abietinaria, Aequoria, Aglaophenia, Bonneviella, Campanularia, Cuvieria, Obelia, Gonionemus, Gonothyrea, Lovenella, Plumularia, Sertularia, Aegina, Botrynema, Craspedacusta, Cunina, Hydroctena, Liriope, Polypodium, Rhopalonema, Solmissus, Agalma, Apolemia, Eudoxoides, Nectocarmen, Physalia, Rhyzophysa, Sphaeronectes, Porpita, Velella, Halammohydra, Otohydra.


Medusa dominant; polyp small or absent. Medusa large and free living. Gastrodermal gonads. Mesogloea cellular. No velum. Coelenteron partitioned by four septa.  ~200 extant species.

Aurelia, Cyanea, Haliclystus, Lucernaria, Atolla, Linuche, Naustihoe, Periphylla, Stephanoscyphus, Tetraplatia, Chrysaora, Pelagia, Sanderia, Stygiomedusa, Cassiopeia, Cephea, Eupilema, Mastigias, Rhizostoma, Stomolophus.


Medusa square with four flat sides. Margin of bell drawn in to form velum-like organ. Tentacle (or cluster) at each corner. Nematocysts very painful.  ~15 extant species.

Tripedalia, Carybdea, Chironex, Tamoya.


No medusoid stage. Polyp large with thick, cellular mesogloea. Coelenteron partitioned by mesentaries with glands, filaments and nematocysts. Stomodaeum extends from mouth down into enteron. One or two ciliated grooves directing water into coelenteron. Oral disc surrounded by tentacles. Epidermal and gastrodermal nematocysts. No operculum. Often colonial.  ~6,000 extant species.


Unspecialized musculature. Mesentaries (single, not paired and complete) and tentacles simple. Six primary mesenteries.

Cerianthus, Antipathes, Arachnanthus, Botruanthus, Ceriantheomorphe, Ceriantheopsis, Pachycerianthus.


Colonial with internal skeleton of calcium carbonate or horny material. Polyps with eight tentacles and coelenteron divided into eight compartments by mesenteries.

Alcyonium, Anthomastus, Ceratocaulon, Gersemia, Parerythropodium, Pseudogorgia, Acanthogorgia, Biareum, Corallium, Eugorgia, Eunicella, Isis, Leptogorgia, Muricea, Parisis, Psamogorgia, Swiftia, Epiphaxum, Heliopora, Anthoptilum, Balticina, Cavernularia, Funicularia, Ptilosarcus, Stylatula, Umbellula, Virgularia, Haimea, Hartea, Monoxenia, Psuchastes, Taiaroa, Clavularia, Cornularia, Sarcodictyon, Tubipora, Gorgonia, Renilla, Pennatula, Coelogorgia, Paratelesto, Telesto, Telestula.


Solitary or colonial anthozoans, most with secreted supporting material that is calcareous and external to the polyp. Polyps with six (or multiples thereof) simple tentacles. Coelenteron divided into many compartments by paired septa, also in multiples of six and with retractor muscles.

Adamsia, Aiptasia, Alicia, Anthopleura, Anthothoe, Bartholomea, Bunodactis, Calliactus, Condylanthus, Diadumene, Edwardsia, Epiactus, Halcampa, Haliplanella, Heteractis (Radianthus), Liponema, Metridium, Peachia, Phyllodiscus, Ptychodactis, Stichodactyla, Stomphia, Triactis, Agaricia, Astrangia, Balanophyllia, Dendrogyra, Flabellum, Actinia, Fungia, Goniopora, Letepsammia, Meandrina, Montipora, Oculina, Pachyseris, Porites, Psammocora, Siderastraea, Stylophora, Epizoanthus, Isaurus, Isozoanthus, Palythoa, Parazoanthus, Thoracactus, Zoanthus, Amplexidiscus, Corynactis, Rhodactis, Ricordea.


Aleshin, V. V. and N. B. Petrov. 2002. Molecular evidence of regression in evolution of metazoa. Zh. Obshch. Biol. 63(3):195-208.

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Barnes. R. S. K. 1984a. Kingdom Animalia. IN: R. S. K. Barnes, ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc., Sunderland, MA. pp. 129-257. 

Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago.

Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass. 

Collins, A.G., P. Cartwright, C.S. McFadden, and B. Schierwater. 2005. Phylogenetic context and basal metazoan model systems. Integrative and Comparative Biology. 45:585-594. 

Darwin, Charles. 1842. The Structure and Distribution of Coral Reefs.  Smith, Elder, and Co. London.

Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis

Hinde, R. T. 2001. The Cnidaria and Ctenophora. In: Anderson, D.T., ed. Invertebrate zoology. Oxford University Press. Oxford , UK . pp. 29-57. 

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company.  New York.

Martindale, M. Q., J. R. Finnerty, and J. Q. Henry. 2002. The Radiata and the evolutionary origins of the bilaterian body plan. Molecular Phylogenetics and Evolution. 24:358-365.

Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill. New York.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders. Ft Worth, TX. 

Schuchert, P. 1993. Trichoplax adhaerens (Phylum Placozoa) has cells that react with antibodies against the neuropeptide RFamide. Acta Zool. 74: 115117.

Storer, T. I. and R. L. Usinger. 1965. General Zoology. 4th Edition. McGraw-Hill Book Company. New York .

Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York.


By Jack R. Holt.  Last revised: 01/25/2009