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Amoeboid

The amoeboids comprise the protists that form pseudopods, temporary cytoplasmic projections that are involved in locomotion and ingesting food. Originally they were grouped together as the Sarcodina, but the different sorts of amoeboid forms have each evolved a number of times. These come in four main groups: those with lobose, filose, and reticulose pseudopods, which are unsupported, and those with axopods, which are rigid projections around microtubules.

Lobose pseudopods are blunt and filose pseudopods are tapering, occasionally branching, but both arise from the same sort of underlying pressure system. The best-known genus is Amoeba, and the name amoeba is often applied to all such forms, less often to amoeboids in general. In these, the outer cytoplasm forms a distinct layer called the ectoplasm, which is generally clear in contrast to the inner cytoplasm or endoplasm, which is generally granular. The main groups of such protists are:

Ramicristates - most lobose and filose forms, including most slime mold groups
Pelobionts - giant amoebae & kin
Entamoebae - mostly parasitic forms, including those which cause amoebic dysentry[?]
Heterolobosea - amoeboflagellates, including acrasid slime molds
Vampyrellids
Nucleariids
Chlorarachniophytes

Lobose and filose pseudopods also occur in numerous other groups, including some cells in multicellular organisms. In particular, leukocytes (white blood cells) are essentially lobose amoebae in form. Several different sorts of movement are found among amoebae. Most form one or more pseudopods along the anterior margin, and move by the body mass flowing into them. Some roll, the ectoplasm sliding around them like a tank tread, or crawl, using relatively permanent pseudopods to support the cell like limbs.

Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found most notably among the Granuloreticulosa[?], which includes primarily the Foraminifera[?] - marine amoeboids with multichambered shells. Other forms include the Gymnophrea[?], Komokiacea[?], and Biomyxa[?].

Protists with axopods are traditionally divided into the radiolaria and heliozoa. The radiolaria are primarily marine with intricate mineral skeletons, including a central capsule separating the endoplasm from the vacuolated ectoplasm. There are three groups: the Phaeodarea[?], the Polycystinea[?], and the Acantharea. Along with the Foraminifera, these are the primary protozoa found in the fossil record. The heliozoa lack central capsules, though they may have scales or spines. There are a number of separate lineages:

Actinodines - including ciliophryids and actinophryids
Centrohelids
Desmothoracids
Gymnosphaerids
Dimorphids
Sticholonche[?]

There is also a strange group of giant marine protists, the Xenophyophorea[?], that are clearly amoeboid but do not fit nicely into any of these categories.



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