cni·​dar·​i·​an nī-ˈder-ē-ən How to pronounce cnidarian (audio)
: any of a phylum (Cnidaria) of radially symmetrical, aquatic, invertebrate animals that have a hollow digestive cavity opening to the outside by a single opening surrounded by one or more nematocyst-studded whorls of tentacles, that occur as single or colonial sessile, typically columnar polyps or usually free-swimming, bell-shaped medusae, and that include the corals, sea anemones, jellyfishes, hydras, and Portuguese man-of-wars
Welcome to the world of cnidarians—a family of sea anemones, jellyfish and other marine invertebrates that kill their enemies and prey by firing poisonous, microscopic projectiles called nematocysts.R. Weiss

called also coelenterate

cnidarian adjective

Examples of cnidarian in a Sentence

Recent Examples on the Web An arm and a leg Jellyfish are cnidarians, a phylum of soft-bodied invertebrates. Elizabeth Rayne, Ars Technica, 3 Jan. 2024 Jellyfish and their relatives, collectively known as cnidarians, are considered to be the earliest living animals to develop nervous systems. Melissa Rudy, Fox News, 24 Sep. 2023 What’s more, similar genes and molecules seem to control sleep in cnidarians, as compared with other animals and humans. Stephanie Pappas, Scientific American, 16 June 2023 Is our cnidarian learning? John Timmer, Ars Technica, 22 Mar. 2023 If these typical cnidarians strike you as a bit bizarre, the myxozoans are even further out past the borders of the familiar. John Timmer, Ars Technica, 26 Feb. 2020 In 1995, however, Mark Siddall, then at the Virginia Institute of Marine Science, and his colleagues argued that myxosporeans are weird members of the cnidarians, the group that includes jellyfish and corals. Quanta Magazine, 19 Aug. 2019 The fork leading to cnidarians represents the final turnoff before animals become bilaterally symmetric, which makes them an interesting group to study because of the greater complexity that came with that later innovation. Quanta Magazine, 8 Jan. 2019 To find out, the researchers decided to sequence the genome of Aurelia, the moon jellyfish, and then compare it to those of cnidarians without medusas. Quanta Magazine, 8 Jan. 2019 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'cnidarian.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


New Latin Cnidaria, phylum name (from Greek knī́dē "nettle, sea nettle"—of uncertain origin— + Latin -āria, neuter plural of -ārius -ary entry 2) + -an entry 1

Note: The taxon Cnidaria is based ultimately on French cnidaire, a vernacular designation for a sub-class used by H. Milne Edwards in Histoire naturelle des coralliaires ou polypes proprements dits, tome 1 (Paris, 1857), p. 95. Milne Edwards alludes to use of Greek knī́dē for a sea nettle by Aristotle in Historia animalium, Book 5. The Greek noun has been associated with the verb knízein "to scratch, chop up, provoke" (perhaps from an Indo-European base *knid-), but the vowel length in knī́dē has no good explanation.

First Known Use

1894, in the meaning defined above

Time Traveler
The first known use of cnidarian was in 1894

Dictionary Entries Near cnidarian

Cite this Entry

“Cnidarian.” Dictionary, Merriam-Webster, Accessed 4 Mar. 2024.

Kids Definition


cni·​dar·​i·​an ni-ˈder-ē-ən How to pronounce cnidarian (audio)
cnidarian adjective

Medical Definition


cni·​dar·​i·​an -ē-ən How to pronounce cnidarian (audio)
cnidarian adjective

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