Chapter contents:

Cnidaria ←– 1. Anthozoa–– 1.1 Scleractinia–– 1.2 Rugosa –– 1.3 Tabulata–– 1.4 Octocorallia– 2. Hydrozoa– 3. Cubozoa– 4. Scyphozoa


The very first thing to know about this phylum of animals is that the "c" that begins "Cnidaria" is silent. Thus, the proper phonetic together of the team is "ni-daria."

Modern cnidarians exhibition a great diversity that forms, every one of which room aquatic and most that which are marine. Cnidaria contains all ancient and contemporary corals, sea anemones, jellyfish, and a selection of less popular groups.

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Features the Cnidarians

Mouths, but no top or butts

Cnidarians lack heads, therefore the typical terms the we use to characterize the positions of attributes on most other pets (e.g., anterior and posterior) execute not work well because that them (see image further below). Lock do have actually mouths (but lack anuses), for this reason terminology of your body creates relates to whether a given function is dental (mouth side) or aboral (the various other side). For example, the tentacles that sea anemones, corals, and also jellyfish surround the mouth, so they room on the oral side. Sea anemones connect themselves to rocks on your aboral sides (opposite the mouth); the "bells" of jellyfish represent the aboral side.


Polyps and also Medusae

Cnidarians come in two general forms: polyps and medusae. Polyps normally look something favor the sea anemone shown in the the photograph above. They have cylinder bodies and also are fastened by your aboral surface to a difficult substrate, v their tentacles and also mouth pointing upwards (or, as the photo over shows, kind of sideways). They might be solitary (like a sea anemone) or early american (like a coral). Medusae look choose jellyfish (see photograph above). Lock are totally free swimming and also bell shaped, with their aboral surface ar pointing upwards and also their mouth and tentacles pointing downwards. Medusae are almost always solitary. Part cnidarian varieties have both polyp and medusa forms, v each serving a different lifecycle function.

Radial symmetry (sort of)

Another necessary characteristic of part cnidarians is the they have bodies with practically radial symmetry. Take into consideration the illustration the the sea anemone below. Imagine cut it native the optimal down, choose a date of birth cake (gross!): no matter how you part it with the middle, one half is nearly the mirror picture of the other.


Illustration of radial body symmetry in a sea anemone. Each plane of symmetry divides the sea anemone right into a half that is the mirror picture of the various other half. Every cnidarians have actually this form of symmetry.

This is not true of most other pet groups (sponges and also ctenophores gift the exceptions), which have actually bodies with solid bilateral the contrary (or some variation of it). In together bilaterians, there is only one plane of the contrary that will make one side the mirror photo of the various other (see outline in the animal Phylogeny chapter).


Left: illustration of bilateral human body symmetry in a youth snapping turtle. The plane of symmetry makes one next the mirror picture of the other. Right: positional terms for bilaterally symmetry animals. The head end is anterior, the tail finish is posterior, the earlier side is dorsal, and the belly side is ventral.

It is worth noting here, however, that numerous cnidarian do have body creates that technique bilateral symmetry. Zapata et al. (2015) declared that "it is regularly asserted that cnidarians are radially symmetrical, yet most have actually bilateral symmetry, some have actually directional asymmetry, and only a subset of varieties have radial symmetry." also so, their headless/buttless bodies every retain a radial aspect of company that is no observed at every in many bilaterian pets (which have definite anterior and also posterior ends). Also though the characterization of cnidarians as radially symmetry animals and bilaterians together bilaterally symmetrical animals is too many simplistic, these designations room still useful for thinking around the as whole organization and structure of the bodies of this animals.

Cnidae and also nematocysts

Cnidarians are called for specialized cellular-scale frameworks that castle bear referred to as cnidae. Cnidae perform a selection of functions. Several of these cnidae are called nematocysts, which function much prefer harpoons and are offered to supply venom, either to record prey or because that defense against enemies. Nematocysts cover the tentacles that cnidarians and are fired as soon as prey or predators brush versus a tiny trigger-like structure dubbed a cnidocil. Nematocysts provide jellyfish their sting and are the reason why that is finest to avoid contact with them when you are swimming in the ocean.

Cnidarian phylogeny

Recent molecular phylogenetic work-related by Zapata et al. (2015) support the traditional view that the phylum Cnidaria have to be divided between two major clades, Anthozoa and Medusozoa.


Anthozoa is stood for by part 7,500 extant species (Zapata et al., 2015) and is divided into two significant subgroupings, Octocorallia and Hexacorallia. Octocorallia is stood for today by sea pens, gorgonian corals, and soft corals; the group is completely soft-bodied and also consequently has actually a really poor fossil record. Hexacorallia, however, consists of several major groups of extinct and modern corals that developed calcium carbonate skeletons, resulting in a superb fossil record. Soft-bodied sea anemones and also several various other groups likewise belong come the Hexacorallia. Many of these teams have polyp-like adult forms.

There are approximately 3,700 extant varieties of Medusozoa (Zapata et al., 2015), which is separated into four significant groups. The true jellyfish space assigned to the Scyphozoa, while the deadly box jellyfish belong come the Cubozoa. The stalked jellyfish space assigned to the Staurozoa. Finally, Hydra and related creates are assigned come the Hydrozoa. As their surname implies, most Medusozoan have medusoid-like (or, jellyfish-like) adult forms, despite this is no universally true (e.g., the polyp-like Hydra is one exception).

Most that this chapter focuses on the superb fossil records of members the the Hexacorallia, in certain the extinction rugose and tabulate corals and also the extant scleractinian corals. Very brief overviews of the other major groups of cnidarians are also presented, though none have actually an especially far-reaching fossil document (though part extraordinary fossils room known).

References and further reading

Kozloff, E. N. 1990. Invertebrates. Saunders university Publishing, Philadelphia, 866 pp.

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Zapata, F., F. E. Goetz, S. A. Smith, M. Howison, S. Siebert, S. H. Church, S. M. Sanders, C. L. Ames, C. S. McFadden, S. C. France, M. Daly, A. G. Collins, S. H. D. Haddock, C. W. Dunn, and also P. Cartwright. 2015. Phylogenomic analyses support timeless relationships within Cnidaria. PLoS ONE 10(10): e0139068.