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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.
As with all Cnidarians, these organisms have a complex life cycle including a motile planktonic phase and a later characteristic sessile phase. Hexacorallia also include the significant extinct order of rugose corals .
The cnidae, the explosive cells characteristic of the Cnidaria and used in prey capture and defence, are of a single type, there being nematocysts but no spirocysts or ptychocysts. [4] In contrast, the anthozoan life cycle involves a planula larva which settles and becomes a sessile polyp, which is the adult or sexual phase. [11]
Unusual characteristics have led to much controversy regarding the phylogenetic position of Polypodium within metazoans. [2]Polypodium has traditionally been considered a cnidarian because it possesses nematocysts, the stinging structures characteristic of this phylum. [2]
Coelenterata is a term encompassing the animal phyla Cnidaria (corals, true jellies, sea anemones, sea pens, and their relatives) and Ctenophora (comb jellies). The name comes from Ancient Greek κοῖλος (koîlos) 'hollow' and ἔντερον (énteron) 'intestine', referring to the hollow body cavity common to these two phyla.
The Scyphozoa are an exclusively marine class of the phylum Cnidaria, [2] referred to as the true jellyfish (or "true jellies"). The class name Scyphozoa comes from the Greek word skyphos (σκύφος), denoting a kind of drinking cup and alluding to the cup shape of the organism. [3] Scyphozoans have existed from the earliest Cambrian to the ...
Hydrozoa (hydrozoans; from Ancient Greek ὕδωρ (húdōr) 'water' and ζῷον (zôion) 'animal') is a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water.
Hydra (/ ˈ h aɪ d r ə / HY-drə) is a genus of small freshwater hydrozoans of the phylum Cnidaria.They are native to the temperate and tropical regions. [2] [3] The genus was named by Linnaeus in 1758 after the Hydra, which was the many-headed beast of myth defeated by Heracles, as when the animal has a part severed, it will regenerate much like the mythical hydra's heads.