Description of Volvocida

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Solitary or colonial; flagellated; most freshwater, but also inhabit soil, snow and ice; marine genera and species are rare; a few colourless genera; radial or near-radial external symmetry; glycoprotein-type cell wall present or absent; flagella inserted apically, and without a scaly covering; basal bodies carrying microtubular flagellar roots are either directly opposed (12/6 o'clock) or displaced into the 1/7 o'clock direction; two system II fibres connect the basal bodies with the nucleus and terminate at the nuclear envelope (nucleus-basal body connectors); mitosis is closed, interzonal mitotic spindle collapses at telophase and a phycoplast (an assembly of microtubules oriented in the plane of cytokinesis) develops; sexual reproduction nearly always involves the formation of a dormant zygote and zygotic meiosis; chloroplasts/leucoplasts present, with chlorophylls a and b, chloroplast/leucoplast envelope consists of two membranes, starch (a-1,4 glucan) synthesized in the stroma of the chloroplast/leucoplast; flagella with a stellate structure in the flagellar transitional region; flagella 2 or 4 of similar length and behaviour; the cell wall may become impregnated with iron, manganese or calcium salts; plastid single, mostly cup-shaped; a single pyrenoid is often present located mostly in a basal position (several pyrenoids may occur or pyrenoids may be absent); an eyespot occurs in most species and consists of one to several layers of eyespot lipid globules located inside the chloroplast; nutrition phototrophic or chemo-organotrophic; phagotrophy absent, contractile vacuoles (2-several, rarely 1) usually present; asexual reproduction either by longitudinal division or by zoospores formed through successive divisions of the protoplast within the parental cell wall; sexual reproduction common, isogamous, anisogamous (heterogamous) or oogamous; typically flagellate gametes agglutinate with their flagella which triggers a sequence of events resulting in cell wall lysis, mating structure activation, flagellar deagglutination and sexual fusion; zygote a thick-walled resting stage (hypnozygote), zygote germination involves meiosis and formation of flagellate meiospores.
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