Chapter contents:

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


The first thing come know around this phylum of pets is the the "c" that starts "Cnidaria" is silent. Thus, the appropriate phonetic pronunciation of the team is "ni-daria."

Modern cnidarians exhibition a good diversity the forms, all of which are aquatic and most that which are marine. Cnidaria consists of all old and contemporary corals, sea anemones, jellyfish, as well as a selection of less popular groups.

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Features the Cnidarians

Mouths, yet no heads or butts

Cnidarians absence heads, so the common terms that we usage to characterize the location of attributes on most other pets (e.g., anterior and posterior) carry out not work well because that them (see image more below). Lock do have mouths (but absence anuses), so terminology of your body creates relates to whether a given attribute is dental (mouth side) or aboral (the various other side). For example, the tentacles of 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 Medusae

Cnidarians come in two general forms: polyps and also medusae. Polyps typically look something like the sea anemone displayed in the the photograph above. They have cylinder bodies and are attached by your aboral surface ar to a tough substrate, v their tentacles and also mouth pointing upwards (or, as the photo over shows, sort of sideways). They might be solitary (like a sea anemone) or colonial (like a coral). Medusae look prefer jellyfish (see picture above). Lock are cost-free swimming and bell shaped, v their aboral surface ar pointing upwards and also their mouth and also tentacles pointing downwards. Medusae are virtually always solitary. Some cnidarian varieties have both polyp and also medusa forms, with each offer a various lifecycle function.

Radial the contrary (sort of)

Another vital characteristic of part cnidarians is the they have bodies with practically radial symmetry. Consider the illustration that the sea anemone below. Imagine cut it indigenous the optimal down, like a birthday cake (gross!): no matter exactly how you part it through the middle, one half is nearly the mirror photo of the other.


Illustration that radial human body symmetry in a sea anemone. Each plane of the opposite divides the sea anemone into a fifty percent that is the mirror image of the various other half. Every cnidarians have this type of symmetry.

This is no true of many other pet groups (sponges and also ctenophores gift the exceptions), which have bodies with solid bilateral the opposite (or some version of it). In such bilaterians, over there is only one airplane of the opposite that will make one next the mirror picture of the various other (see outline in the animal Phylogeny chapter).


Left: illustration of bilateral body symmetry in a youth snapping turtle. The plane of symmetry renders one side the mirror picture of the other. Right: positional terms for bilaterally symmetry animals. The head finish is anterior, the tail finish is posterior, the ago side is dorsal, and also the ship side is ventral.

It is precious noting here, however, that countless cnidarian do have body creates that method bilateral symmetry. Zapata et al. (2015) declared that "it is frequently asserted that cnidarians are radially symmetrical, but most have bilateral symmetry, some have actually directional asymmetry, and only a subset of species have radial symmetry." also so, their headless/buttless bodies every retain a radial element of organization that is no observed at all in many bilaterian animals (which have actually definite anterior and posterior ends). Even though the characterization the cnidarians together radially symmetry animals and bilaterians as bilaterally symmetrical animals is overly simplistic, this designations are still useful for thinking around the overall organization and structure that the bodies of these animals.

Cnidae and also nematocysts

Cnidarians are called for devoted cellular-scale frameworks that they bear dubbed cnidae. Cnidae perform a variety of functions. Some of these cnidae are called nematocysts, which role much like harpoons and also are offered to supply venom, either to capture prey or because that defense against enemies. Nematocysts sheathe the tentacles that cnidarians and are fired as soon as prey or predators brush against a tiny trigger-like structure called a cnidocil. Nematocysts give jellyfish your sting and are the factor why it is finest to avoid call with them once you space swimming in the ocean.

Cnidarian phylogeny

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


Anthozoa is represented by part 7,500 extant types (Zapata et al., 2015) and also is divided into two significant subgroupings, Octocorallia and Hexacorallia. Octocorallia is represented today through sea pens, gorgonian corals, and soft corals; the team is totally soft-bodied and consequently has actually a really poor fossil record. Hexacorallia, however, has several major groups of extinct and modern-day corals that developed calcium carbonate skeletons, leading to a superb fossil record. Soft-bodied sea anemones and several other groups additionally belong come the Hexacorallia. Many of these groups have polyp-like adult forms.

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

Most of this chapter focuses on the superb fossil documents of members that the Hexacorallia, in details the extinction rugose and also tabulate corals and the extant scleractinian corals. Very brief overviews that the other significant groups that cnidarians are additionally presented, despite none have actually an especially significant fossil document (though some extraordinary fossils are 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 P. Cartwright. 2015. Phylogenomic analyses support timeless relationships within Cnidaria. PLoS ONE 10(10): e0139068.