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This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (March 2011) Acantharea Acantharea species Scientific classification Domain: Eukaryota (unranked): Rhizaria Phylum: Radiolaria Class: Acantharea Haeckel 1881 emend. The Acantharea are a group of radiolarian[1] protozoa, distinguished mainly by their skeletons. Contents 1 Structure 2 Classification by spine arrangement 3 Life cycle 4 References Structure These are composed of strontium sulfate crystals, which do not fossilize, and take the form of either ten diametric or twenty radial spines. The central capsule is made up of microfibrils arranged into twenty plates, each with a hole through which one spine projects, and there is also a microfibrillar cortex linked to the spines by myonemes. These assist in flotation, together with the vacuoles in the ectoplasm, which often contain zooxanthellae. Classification by spine arrangement The arrangement of the spines is very precise, and is described by what is called the Müllerian law. This is easiest to describe in terms of lines of latitude and longitude - the spines lie on the intersections between five of the former, symmetric about an equator, and eight of the latter, spaced uniformly. Each line of longitude carries either two tropical spines or one equatorial and two polar spines, in alternation. The way that the spines are joined together at the center of the cell varies and is one of the primary characteristics by which acanthareans are classified. Holacanthida - diametric spines, simply crossed Symphyacanthida - radial spines, with free bases Chaunacanthida - radial spines, with articulated bases Arthracanthida - radial spines, with pyramidal bases packed together The axopods are fixed in number. Life cycle Adults are usually multinucleated. Reproduction is thought to take place by formation of swarmer cells (formerly referred to as "spores"), which may be flagellate. Not all life cycle stages have been observed, and study of these organisms has been hampered mainly by an inability to maintain these organisms in culture through successive generations. References ^ Polet, S; Berney; Fahrni; Pawlowski (2004). "Small-subunit ribosomal RNA gene sequences of Phaeodarea challenge the monophyly of Haeckel's Radiolaria". Protist 155 (1): 53–63. doi:10.1078/1434461000164. PMID 15144058.  edit v · d · eRhizaria Cercozoa Filosa Monadofilosa Imbricatea Euglyphida: Cyphoderiidae  · Euglyphidae  · Paulinellidae  · Trinematidae  · Thaumatomonadida: Thaumatomastigidae Thecofilosea Cryomonad  · Ebriidae  · Phaeodarea  · Tectofilosid  · Cryptodifflugia Spongomonadea Spongomonas, Rhipidodendron Cercomonadida Cercomonadidae  · Heteromitidae  · Sainouridae Reticulofilosa Proteomyxidea Gymnophryidae  · Heliomorphidae  · Desmothoracida  · Gymnosphaerida Chlorarachniophyta Chlorarachnion, Gymnochlora, Lotharella, Cryptochlora, Bigelowiella Endomyxa Phytomyxea, Vampyrellidae, Pseudospora, Ascetosporea: Paramyxea  · Haplosporidia, Gromiidae, Reticulosida Retaria Foraminifera Allogromiida  · Miliolid  · Textulariida Radiolaria Polycystinea  · Acantharea  · Taxopodida