Chapter contents:

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


The an initial thing to know around this phylum of animals is that the "c" that begins "Cnidaria" is silent. Thus, the appropriate phonetic joint of the group is "ni-daria."

Modern cnidarians exhibition a good diversity that forms, every one of which room aquatic and most of which are marine. Cnidaria includes all old and modern-day corals, sea anemones, jellyfish, as well as a selection of less renowned groups.

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Features of Cnidarians

Mouths, yet no top or butts

Cnidarians lack heads, therefore the normal terms the we usage to characterize the location of functions on most other animals (e.g., anterior and also posterior) do not occupational well for them (see image further below). Castle do have actually mouths (but absence anuses), for this reason terminology of their body develops relates to whether a given feature is oral (mouth side) or aboral (the other side). For example, the tentacles that sea anemones, corals, and jellyfish surround the mouth, so they are on the oral side. Sea anemones connect themselves to rocks on your aboral political parties (opposite the mouth); the "bells" of jellyfish represent the aboral side.


Polyps and also Medusae

Cnidarians come in two general forms: polyps and also medusae. Polyps normally look something choose the sea anemone shown in the the picture above. They have cylinder bodies and are attached by their aboral surface to a tough substrate, through their tentacles and also mouth pointing upwards (or, together the photo over shows, sort of sideways). They might be solitary (like a sea anemone) or colonial (like a coral). Medusae look favor jellyfish (see picture above). Lock are totally free swimming and bell shaped, with their aboral surface ar pointing upwards and their mouth and also tentacles pointing downwards. Medusae are practically always solitary. Some cnidarian varieties have both polyp and also medusa forms, with each serving a various lifecycle function.

Radial the contrary (sort of)

Another necessary characteristic of some cnidarians is the they have actually bodies with practically radial symmetry. Think about the illustration of the sea anemone below. Imagine cutting it native the height down, favor a date of birth cake (gross!): no matter just how you slice it through the middle, one half is nearly the mirror picture of the other.


Illustration the radial human body symmetry in a sea anemone. Each airplane of the opposite divides the sea anemone right into a half that is the mirror image of the various other half. All cnidarians have this kind of symmetry.

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


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

It is precious noting here, however, that numerous cnidarian do have body creates that strategy bilateral symmetry. Zapata et al. (2015) proclaimed that "it is regularly asserted the cnidarians are radially symmetrical, but most have bilateral symmetry, some have directional asymmetry, and only a subset of species have radial symmetry." even so, their headless/buttless bodies every retain a radial facet of organization that is not observed at all in many bilaterian animals (which have actually definite anterior and also posterior ends). Also though the characterization that cnidarians together radially symmetry animals and bilaterians as bilaterally symmetrical pets is overly simplistic, this designations are still beneficial for thinking about the all at once organization and structure that the body of this animals.

Cnidae and also nematocysts

Cnidarians are named for dedicated cellular-scale frameworks that lock bear dubbed cnidae. Cnidae perform a variety of functions. Some of these cnidae are dubbed nematocysts, which duty much like harpoons and are used to deliver venom, one of two people to capture prey or because that defense against enemies. Nematocysts cover the tentacles of cnidarians and also are fired as soon as prey or predators brush against a small trigger-like structure referred to as a cnidocil. Nematocysts give jellyfish your sting and also are the reason why the is finest to avoid call with them when you are swimming in the ocean.

Cnidarian phylogeny

Recent molecule phylogenetic job-related by Zapata et al. (2015) supports the timeless view the the phylum Cnidaria need to be divided between two significant clades, Anthozoa and Medusozoa.


Anthozoa is represented by some 7,500 extant varieties (Zapata et al., 2015) and also is split into two major subgroupings, Octocorallia and Hexacorallia. Octocorallia is represented today by sea pens, gorgonian corals, and soft corals; the team is completely soft-bodied and consequently has a really poor fossil record. Hexacorallia, however, has several major groups of extinct and contemporary corals that created calcium carbonate skeletons, leading to a superb fossil record. Soft-bodied sea anemones and also several various other groups also belong to the Hexacorallia. Most of these teams have polyp-like adult forms.

There are roughly 3,700 extant types of Medusozoa (Zapata et al., 2015), i m sorry is divided into four significant groups. The true jellyfish are assigned to the Scyphozoa, while the deadly crate jellyfish belong come the Cubozoa. The stalked jellyfish room assigned to the Staurozoa. Finally, Hydra and related forms are assigned to the Hydrozoa. As their surname implies, many Medusozoan have medusoid-like (or, jellyfish-like) adult forms, despite this is not universally true (e.g., the polyp-like Hydra is one exception).

Most that this chapter concentrates on the superb fossil documents of members that the Hexacorallia, in certain the extinction rugose and tabulate corals and also the extant scleractinian corals. An extremely brief overviews the the other major groups of cnidarians are additionally presented, though none have an especially far-ranging fossil document (though part extraordinary fossils are known).

References and also 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 classic relationships within Cnidaria. PLoS ONE 10(10): e0139068.