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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 ...
In the class Anthozoa, comprising the sea anemones and corals, the individual is always a polyp; in the class Hydrozoa, however, the individual may be either a polyp or a medusa, [1] with most species undergoing a life cycle with both a polyp stage and a medusa stage.
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.
Historically, cubozoans were classified as an order of Scyphozoa until 1973, when they were put in their own class due to their unique biological cycle (lack of strobilation) and morphology. [4] At least 51 species of box jellyfish were known as of 2018. [5] These are grouped into two orders and eight families. [6]
Unlike some other cnidarian groups, the lining of the central cavity lacks stinging nematocysts, which are found only on the tentacles and outer surface. All colonial hydrozoans also include some polyps specialized for reproduction. These lack tentacles and contain numerous buds from which the medusoid stage of the life cycle is produced.
The affinities of the class Polypodiozoa, containing the single species Polypodium hydriforme, have long been unclear. This species is an endoparasite of fish eggs and has a peculiar life cycle. It has traditionally been considered to be a cnidarian because of its possession of nematocysts, but molecular studies using 18S rDNA sequences have ...
Siphonophorae (A) Rhizophysa eysenhardtii scale bar = 1 cm, (B) Bathyphysa conifera 2 cm, (C) Hippopodius hippopus 5 mm, (D) Kephyes hiulcus 2 mm (E) Desmophyes haematogaster 5 mm (F) Sphaeronectes christiansonae 2 mm, (G) Praya dubia 40 m (130 ft), (H) Apolemia sp. 1 cm, (I) Lychnagalma utricularia 1 cm, (J) Nanomia sp. 1 cm, (K) Physophora hydrostatica 5 mm
Polypodium was long considered a unique intracellular parasite among cnidarians. [6] [7] Its hosts include 14 species of Acipenser, 2 species of Huso, Polyodon spathula [6] and Scaphirhynchus platorynchus. [2] Polypodium hydriforme is an endocellular parasite with an unusual life cycle, a peculiar morphology, and high rates of DNA evolution.