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Pacific sea nettles, Chrysaora fuscescens. Cnidaria (/ n ɪ ˈ d ɛər i ə, n aɪ-/ nih-DAIR-ee-ə, NY-) [4] is a phylum under kingdom Animalia containing over 11,000 species [5] of aquatic invertebrates found both in fresh water and marine environments (predominantly the latter), including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites.
Anthozoa is a class of marine invertebrates which includes sessile cnidarians such as the sea anemones, stony corals, soft corals and sea pens.Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as planktons.
Siphonophorae (from Greek siphōn 'tube' + pherein 'to bear' [2]) is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria.According to the World Register of Marine Species, the order contains 175 species described thus far.
Sea pens are marine cnidarians belonging to the order Pennatulacea, which are colony-forming benthic filter feeders within the class Octocorallia (subphylum Anthozoa).There are 14 families within the order and 35 extant genera; it is estimated that of 450 described species, around 200 are valid.
Fossils of cnidarians that do not build mineralized structures are rare. Scientists currently think cnidarians, ctenophores and bilaterians are more closely related to calcareous sponges than these are to other sponges, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians.
Hence ctenophores and cnidarians have traditionally been labelled diploblastic. [18] [20] Both ctenophores and cnidarians have a type of muscle that, in more complex animals, arises from the middle cell layer, [21] and as a result some recent text books classify ctenophores as triploblastic, [22] while others still regard them as diploblastic. [18]
Cnidarians have huge numbers of cnidocytes on their tentacles. In medusoid form , the body floats on water so that the tentacles hang down in a ring around the mouth. In polyp form, such as sea anemone and coral, the body is below with the tentacles pointed upwards.
The man o' war is described as a colonial organism because the individual zooids in a colony are evolutionarily derived from either polyps or medusae, [15] i.e. the two basic body plans of cnidarians. [16] Both of these body plans comprise entire individuals in non-colonial cnidarians (for example, a jellyfish is a medusa, while a sea anemone ...