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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.
Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mouth is fringed with tentacles. Although some species are solitary, most are colonial.
Below is a phylogenetic tree of the phylum Chordata. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some groups in this tree (lancelets and tunicates) are invertebrates. The positions (relationships) of the lancelet, tunicate, and craniate clades are as reported.
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]
Some animals, like cnidarians, produce two germ layers (the ectoderm and endoderm) making them diploblastic. Other animals such as bilaterians produce a third layer (the mesoderm) between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal's tissues and organs through the process of organogenesis.
Definition of Phylum based on body plan – High level taxonomic rank for organisms sharing a similar body plan; Ediacaran biota – Life of the Ediacaran period; Macroevolution – Evolution on a scale at or above the level of species; Precambrian body plans – Structure and development of early multicellular organisms
Ctenophores resemble cnidarians in relying on water flow through the body cavity for both digestion and respiration, as well as in having a decentralized nerve net rather than a brain. Also like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ...
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