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Many species live most of their lives as single cells or are filamentous, while others form colonies made up from long chains of cells, or are highly differentiated macroscopic seaweeds. Red algae , a (disputed) phylum contains about 7,000 recognised species, [ 50 ] mostly multicellular and including many notable seaweeds.
A third lineage of algae, the glaucophytes (25 species), [47] contains rare and obscure species found in surfaces of freshwater and terrestrial habitats. [90] The red algae or Rhodophyta (>7,100 species) are a group of diverse morphologies, ranging from single cells to multicellular filaments to giant pseudoparenchymatous thalli, all
The 16-cell Gonium colony shown in the diagram on the right is organized into two concentric squares of respectively 4 and 12 cells, each biflagellated, held together by an extracellular matrix. [51] All flagella point out on the same side: It exhibits a much lower symmetry than Volvox, lacking anterior-posterior symmetry.
Its field of study therefore overlaps with the more traditional disciplines of phycology, mycology, and protozoology, just as protists embrace mostly unicellular organisms described as algae, some organisms regarded previously as primitive fungi, and protozoa ("animal" motile protists lacking chloroplasts).
Unlike plants, fungi and most types of algae, most protozoa do not have a rigid external cell wall but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoa, such as the ciliates and euglenozoans , the outer membrane of the cell is supported by a cytoskeletal infrastructure, which may be ...
A protist (/ ˈ p r oʊ t ɪ s t /) is any eukaryotic organism (one with cells containing a nucleus) that is not an animal, plant, or fungus.The protists do not form a natural group, or clade, since they exclude certain eukaryotes with whom they share a common ancestor; [a] but, like algae or invertebrates, the grouping is used for convenience.
The eyespot apparatus (or stigma) is a photoreceptive organelle found in the flagellate or (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids. It allows the cells to sense light direction and intensity and respond to it, prompting the organism to either swim towards the light (positive phototaxis ...
Microorganisms make up about 70% of the marine biomass. [4] A microorganism, or microbe, is a microscopic organism too small to be recognised adequately with the naked eye. In practice, that includes organisms smaller than about 0.1 mm. [12]: 13