Cnidaria (Coelenterates)


Cnidaria (Gr., cnidae, nettle) is a phylum of mostly marine Metazoa distinguished by cnidocysts, subcellular capsules containing an inverted tubule capable of everting and, in some cases, discharging venom. Sexual reproduction results in solid planula embryos that develop into columnar polyps. Asexual reproduction results in clones of polyps, polypoidal colonies or disk‐ to bell‐shaped sexual medusas (jellyfish).

Keywords: cnidocyst; corals; Hydra; jellyfish; sea anemone

Figure 1.

Cnidarian life cycles. Anthozoa (dashed circle on left) consists exclusively of polyps, while Medusozoa (dashed circle on right) consists of both medusas and polyps. The sexual phase, in which eggs and spermatozoa are produced, occurs in anthozoan polyps and medusozoan medusas or reduced medusoids attached to polyps. Polyps alone are produced by sexual reproduction (single arrow). Medusas are produced by the asexual reproduction of polyps (double arrow).

Figure 2.

Light micrograph of section through wall of young sea anemone Bunodactis verrucosa at the origin of a mesentery. Two cellular layers are separated by a dense mesoglea.

Figure 3.

Phase‐contrast micrograph of macerated Hydra viridis cells spread on a haemocytometer grid. Large endodermal cells containing refractive endosymbiotic algae have basal muscular extensions. Smaller cnidoblasts in nests of 2–8 cells contain differentiating cnidocysts.

Figure 4.

Cnidocysts (penetrating nematocysts known as euryteles) from the jellyfish Cyanea capillata. (a) Encapsulated cynidocysts and (b) evacuated cnidocysts with everted tubules are differential interference contrast micrographs, and (c) is a scanning electron micrograph. The cnidocyst capsules measure 8 μm×14 μm, and the discharged tubule is 310 μm long. Micrographs by Professor Carina Östman, (Uppsala University, Sweden).


Further Reading

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Bouillon J (1985) Essai de classification des Hydropolypes–Hydromeduses (Hydrozoa–Cnidaria). Indo–Malayan Zoology 2: 29–243.

Bridge D, Cunningham CW, DeSalle R and Buss W (1995) Class‐level relationships in the phylum Cnidaria: molecular and morphological evidence. Molecular Biology and Evolution 12: 679–689.

Cavalier‐Smith T (1998) A revised six‐kingdom system of life. Biological Reviews 73: 203–266.

Fang Y, Huang CC, Kain SR and Li X (1999) Use of coexpressed enhanced green fluorescent protein as a marker for identifying transfected cells. Methods in Enzymology 302: 207–212.

Fautin DG and Mariscal RN (1991) Cnidaria: Anthozoa. In: Harrison FW and Westfall JA (eds) Microscopic Anatomy of Invertebrates, vol. 2 Placozoa, Porifera, Cnidaria, and Ctenophora, pp. 267–358. New York: Wiley–Liss

Fukami H, Budd AF, Paulay G, et al. (2004) Conventional taxonomy obscures deep divergence between Pacific and Atlantic corals. Nature 427: 832–835.

Hessinger DA (1988) Nematocyst venom and toxins. In: Hessinger DA and Lenhoff HM (eds) The Biology of Nematocysts, pp. 33–368. San Diego: Academic Press

Kortschak RD, Samuel G, Saint R and Miller DJ (2003) EST analysis of the cnidarian Acropora millepora reveals extensive gene loss and rapid sequence divergence in the model invertebrates. Current Biology 13: 2190–2195.

Lesh‐Laurie GE and Suchy PE (1991) Cnidaria: Scyphozoa and Cubozoa. In: Harrison FW and Westfall JA (eds) Microscopic Anatomy of Invertebrates, vol. 2 Placozoa, Porifera, Cnidaria, and Ctenophora, pp. 185–266. New York: Wiley–Liss

Petersen KW (1979) Development of coloniality in Hydrozoa. In: Larwood G and Rosen BR (eds) Biology and Systematics of Colonial Organisms, Systematics Association Special Volume No. 11, pp. 105–139. London: Academic Press

Russell FS (1970) The Medusae of the British Isles. II. Pelagic Scyphozoa with a Supplement to the First Volume on Hydromedusae. Cambridge: Cambridge University Press.

Schlegel M, Lom J, Stechmann A, et al. (1996) Phylogenetic analysis of complete small subunit ribosomal RNA coding regions of Myxidium lieberkuehni: evidence that Myxozoa are Metazoa and related to the bilateria. Archiv für Protistenkunde 147: 1–9.

Schmidt H (1974) On evolution in the Anthozoa. Proceedings of the 2nd International Coral Reef Symposium, pp. 533–560. Brisbane, Australia: Great Barrier Reef Committee.

Schuchert P (1993) Phylogenetic analysis of the Cnidaria. Zeitschrift für zoologische Systematik‐ und Evolutionsforschung 31: 161–173.

Shick JM (1991) A Functional Biology of Sea Anemones. London: Chapman & Hall.

Shostak S (1993) Cnidaria. In: Adiyodi KG and Adiyodi RG (eds) Reproductive Biology of Invertebrates, vol. VI, part A: Asexual Propagation and Reproductive Strategies, pp. 45–105. New Delhi, India: Oxford and IBH

Shostak S and Kolluri V (1995) Symbiogenetic origins of cnidarian cnidocysts. Symbiosis 19: 1–29.

Siddall ME, Martin DS, Bridge D, Dessert SS and Cone DK (1995) The demise of a phylum of protists: phylogeny of Myxozoa and other parasitic cnidaria. Journal of Parasitology 81: 961–967.

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Stephenson TA (1927, 1935) The British Sea Anemones, vols 1 and 2. London: The Ray Society.

Thomas MB and Edwards NC (1991) Cnidaria: Hydrozoa. In: Harrison FW and Westfall JA (eds) Microscopic Anatomy of Invertebrates, vol. 2 Placozoa, Porifera, Cnidaria, and Ctenophora, pp. 91–183. New York: Wiley–Liss

Veron JEN (1995) Corals in Space and Time: The Biogeography and Evolution of the Scleractinia. Ithaca, NY: Cornell University Press.

Werner B (1973) New investigations on systematics and evolution of the class Scyphozoa and the phylum Cnidaria. Publication Seto Marine Biological Laboratory 20: 35–61.

Wilt C, Huettel M, Klueter A, et al. (2004) Coral mucus functions as an energy carrier and particle trap in the reef ecosystem. Nature 428: 66–70.

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How to Cite close
Shostak, Stanley(Sep 2005) Cnidaria (Coelenterates). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0004117]