Cnidarians are simple animals that split off from other phyla earlier than sponges. They have two germ layers and lack true organs and a body cavity. They use specialized cells called cnidocytes to feed and defend themselves. Cnidarians are stationary or mobile and have two main body shapes: the polyp and the medusa. They can reproduce asexually or sexually. Corals are among the most recognizable anthozoans and can create huge reef structures.
Cnidarians, members of the phylum Cnidaria, are among the simplest and most basal animals, splitting off from other phyla even earlier than sponges, as was discovered by molecular studies in 2008. They are among the earliest animal fossils with recognizable affinity to modern groups. The four main groups (or classes) of cnidarians are Anthozoa (anemones, corals, etc.), Scyphozoa (jellyfish), Staurozoa (stem jellyfish), Cubozoa (box jellies), Hydrozoa (Obelia, Portuguese Man o’ War, Aequorea, etc. .), and Polypodiozoa, which includes only one species, Polypodium hydriforme, a small parasite.
Like organisms of the phylum Ctenophora, cnidarians are diploblastic, meaning that their body plan consists of only two germ layers, in contrast to all other animal phyla except sponges. This means they lack true organs and a coelom, or body cavity. While superficially simple, modern members of this phyla are actually genetically sophisticated and biochemically complex.
The phylum gets its name from the Greek word for sting because its members feed and defend themselves using specialized cells called cnidocytes, which contain tiny, harpoon-like stingers. The release of these “harpoons” is among the fastest processes known in nature, it takes place in just 600 nanoseconds and reaches an acceleration of about five million G.
Cnidarians are stationary, like the Anthozoans, or mobile, like all other groups. Corals, among the most recognizable anthozoans, are known for the huge reef structures they create, which can be 328m tall and cover areas as large as 100 square km by 133,000 square miles, as is the case on the Great Barrier Reef off the northeastern coast of Australia. Like other members of this phyla, corals can reproduce asexually, by budding, or sexually, by releasing sperm into the water.
These organisms have two main body shapes: the polyp, which is a stalk with cnidocytes at the top, or the medusa, which is the iconic shape of the medusa. While anthozoans remain in the polyp stage throughout their lives, hydrozoans generally have life stages that go through both, while among medusae, the medusa body plan is dominant. A polyp can transform into a jellyfish by detaching itself from its substrate and developing specialized tissues in a process called strobilation, which allows for high productivity of offspring.
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