Cnidaria
Cnidaria
Fossil range: 580–0 Ma
PreЄ
Є
O
S
D
C
P
T
J
K
Pg
N
Ediacaran - Recent
Pacific sea nettles, Chrysaora fuscescens
Scientific classification
Domain:
Eukaryota
Kingdom:
Animalia
Phylum:
Cnidaria
Hatschek, 1888
Subphylum/Classes[3]
Anthozoa – corals and sea anemones
Medusozoa:[1]
Cubozoa – box jellyfish, sea wasps
Hydrozoa – hydroids, hydra-like animals
Scyphozoa – jellyfish
Staurozoa – stalked jellyfish
Unranked, may not be scyphozoans[2]
Myxozoa – parasites
Polypodiozoa – parasites
Cnidaria (pronounced /naɪˈdɛəriə/ with a silent c) is a
phylum containing over 9,000 species of animals found
exclusively in aquatic, mostly marine, environments.
Their distinguishing feature is cnidocytes, specialized
cells that they use mainly for capturing prey. Their bod-
ies consist of mesoglea, a non-living jelly-like substance,
sandwiched between two layers of epithelium that are
mostly one cell thick. They have two basic body forms:
swimming medusae and sessile polyps, both of which are
radially symmetrical with mouths surrounded by
tentacles that bear cnidocytes. Both forms have a single
orifice and body cavity that are used for digestion and
respiration. Many cnidarian species produce colonies
that are single organisms composed of medusa-like or
polyp-like zooids, or both. Cnidarians’ activities are co-
ordinated by a decentralized nerve net and simple re-
ceptors. Several free-swimming Cubozoa and Scyphozoa
possess balance-sensing statocysts, and some have
simple eyes. All cnidarians reproduce sexually. Many
have complex lifecycles with asexual polyp stages and
sexual medusae, but some omit either the polyp or the
medusa stage.
Cnidarians were for a long time grouped with Cteno-
phores in the phylum Coelenterata, but increasing
awareness of their differences caused them to be placed
in separate phyla. Cnidarians are classified into four
main groups: sessile Anthozoa (sea anemones, corals, sea
pens); swimming Scyphozoa (jellyfish); Cubozoa (box jel-
lies); and Hydrozoa, a diverse group that includes all the
fre