Nettle cells built jellyfish, corals, and anemones
Cnidaria holds more than eleven thousand aquatic species—mostly marine—including jellyfish, hydroids, anemones, and corals. Radial medusae and polyps share tentacles armed with cnidocytes, stinging cells named from the Greek for nettle. Fossils reach about 580 million years into the Ediacaran.
Four main groups dominate: the almost entirely stationary Anthozoa of anemones, corals and sea pens; swimming Scyphozoa, the true jellyfish; Cubozoa, the box jellies; and Hydrozoa, which includes every freshwater cnidarian along with Hydra and the colonial Portuguese man o' war. Staurozoa recently earned class status, and the tiny parasitic Myxozoa were only confirmed as cnidarians in 2007. Comb jellies, once lumped in with them as Coelenterata, now form their own phylum.
The body is two cell layers, mostly one cell thick, sandwiching a non-living jelly called mesoglea, with a decentralised nerve net rather than a brain. A single opening serves as both mouth and anus. Medusae swim by jet propulsion, springy mesoglea snapping back after rim muscles squeeze water out, while anemones and Hydra close their mouths and use trapped water as a hydrostatic skeleton. Many hydrozoans alternate between polyp and medusa, and colonies such as Obelia divide labour among feeding, budding and sexual individuals.
The defining cnidocyte is a capsule holding a coiled, inside-out thread fired by a hair-like trigger. Venomous nematocysts pierce and anchor with barbs, spirocysts entangle prey with sticky hairs, and ptychocysts, found only in tube anemones, spin the threads of their homes. Prey ranges from plankton to animals several times the hunter's size, though many species live largely on sugars from symbiotic algae; reef-building stony corals pack their polyps with them.
Fossils date to about 580 million years ago, before the Cambrian explosion, and molecular clocks push the group's origin back to roughly 741 million years. Genetic analyses place cnidarians as the sister group of bilaterian animals.
Source: Cnidaria
