Amoeba
[8][9] In older classification systems, most amoebae were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow.
Amoeba move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell.
Amoebozoan species, such as those in the genus Amoeba, typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section.
The shells of testate amoebae may be composed of various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.
Marine amoebae do not usually possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water.
Amoebae typically ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material.
[32] Studies of Entamoeba invadens found that, during the conversion from the tetraploid uninucleate trophozoite to the tetranucleate cyst, homologous recombination is enhanced.
[43] Microorganisms that can overcome the defenses of one-celled organisms can shelter and multiply inside them, where they are shielded from unfriendly outside conditions by their hosts.
[46] In 1822, the genus Amiba (from the Greek ἀμοιβή amoibe, meaning "change") was erected by the French naturalist Bory de Saint-Vincent.
[49] In 1841, Félix Dujardin coined the term "sarcode" (from Greek σάρξ sarx, "flesh," and εἶδος eidos, "form") for the "thick, glutinous, homogeneous substance" which fills protozoan cell bodies.
[51]: 156 Later workers, including the influential taxonomist Otto Bütschli, amended this group to create the class Sarcodina,[52]: 1 a taxon that remained in wide use throughout most of the 20th century.
[53] For convenience, all amoebae were grouped as Sarcodina and generally divided into morphological categories, on the basis of the form and structure of their pseudopods.
During the 1980s, taxonomists reached the following classification, based exclusively on morphological comparisons:[55][53] Archezoa Percolozoa (Heterolobosea) other excavates Eosarcodina Neosarcodina Apusozoa → Choanozoa → Animals, Fungi Actinopoda Alveolata → Plants, Chromista In the final decades of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not a monophyletic group, and that amoebae evolved from flagellate ancestors.
[10] The protozoologist Thomas Cavalier-Smith proposed that the ancestor of most eukaryotes was an amoeboflagellate much like modern heteroloboseans, which in turn gave rise to a paraphyletic Sarcodina from which other groups (e.g., alveolates, animals, plants) evolved by a secondary loss of the amoeboid phase.
[57] Shortly after, phylogenetic analyses disproved this hypothesis, as non-amoeboid zooflagellates and amoeboflagellates were found to be completely intermingled with amoebae.
[10][59] The following cladogram shows the sparse positions of amoeboid groups (in bold), based on molecular phylogenetic analyses:[66] Stramenopiles alveolates Rhizaria haptophytes Centroplasthelida plants, etc.
In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.
