Rhizopods (Class Rhizopoda) Examples, Characteristics and Orders

Rhizopods (Class Rhizopoda)

This class includes the various species of amoebas. Typically amoebas crawl on the surface of the mud, submerged plants, in the spaces between soil particles, or inside animals as parasites. Locomotion is by means of pseudopodia, which may be long and thin but usually unbranched (filopodia), long, thin, and branched, uniting with one another to form a network (rhizopodia) or shorter, broader, and usually blunt (lobopodia). A generalized cell contains all the usual complement of organelles and, if found in freshwater, a contractile vacuole as well. The shape is highly plastic. There are few or no reference points on the surface or within the cell.

Order Rhizomastigina

These are naked amoebas with a flagellate stage in their life-history. Perhaps the most studied species is Naegleria gruberi which lives in organically rich soils. It is a small limax-type amoeba (see below) in dry conditions or in high osmotic pressure liquids. When placed in distilled water a rapid transformation occurs: first the posterior (uroid) develops long filaments of cytoplasm which move about; the thickest of these become flagella and as soon as they are fully formed the earlier processes are withdrawn; finally, the two to four flagella migrate to the anterior end and the flagella beat more vigorously until the body breaks its attachment with the substrate.

The flagellate swims with rotatory movement. The reverse process has been shown recently to be very sudden: the flagellate sticks to a surface, the flagellum stops, and after a few minutes is quickly withdrawn and the amoeba crawls away. Dieniamotba is a small (5-18μ) limax-type amoeba sometimes found in the intestine of man. It has been shown recently to have a flagellate phase. Apart from having two nuclei, it is very similar to other entamoebas. Hislomonas is an amoeba with one to four flagella.

The comb and wattles of poultry infected by this parasite, known as ‘blackhead’, turn black. The parasite is flagellated in the lumen of the intestine ‘ but becomes amoeboid upon invading the liver. Masligamoeba is large (150-200μ long) and has both pseudopodia and a flagellum at the same time. The pseudopodia are radial or terminal (posterior). Some species of the genus are free-living, others are parasitic, especially in the gut of the amphibians.

Amoeba is an example of Class Rhizopoda

The living cell has pseudopodia with clear expanding tips, flowing central cytoplasm, and numerous food vacuoles. In living cells the nucleus is not readily seen, but when fixed and stained it becomes conspicuous.

Arcella dentata is an example of Class Rhizopoda

Arcella dentata is a testate amoeba. The shell or test is secreted around the cell. Frequently the amoeba dies and the permanent shell is left behind as in the photograph. Among the genera and species a wide variety of shapes and sculpturing of the shells is found. Testate amoebas are common in freshwater and feed crawling over submerged vegetation.


Typical amoebas (Order Amoebina)

These are amoebas in which a flagellate stage is not known. The size range is great, from very small soil species of 3- 15μ to large, sometimes multinucleate, freshwater species up to 3 mm. in length. Reproduc-tion is believed to be asexual and may be by promitosis (binary fission) or true mitosis.

The habits are both free-living and parasitic. Amoeba rains is probably the most commonly studied organism in schools. It is also known as Chaos &films. But it is doubtful whether the various organisms from all over the world which roughly fit the pictures in textbooks arc the species proteus. This type of amoeba has several large pseudo-podia which contain ectoplasm and endoplasm.

It is usually seen to move in one direction at once and the spent pseudopodia collect as small bumps in the uroid. Also in or near the uroid is the contractile vacuole, while the nucleus may be close but somewhat anterior. In the endoplasm of many species of free-living amoeba are bipyramidal crystals of uncertain function. Few pseudopodia reveal much of taxonomic value because the shape is ever-changing. The genera are distinguished principally by nuclear and cyst structure. Therefore identification is possible only by the specialist. The limax-amoebas are the small amoebas which abound in soil.

They are distinguished from the other free-living forms by their single pseudo-podium which contains relatively more ectoplasm. The smallest are only 3μ across and the majority are under 20μ, though up to 40μ has been reported. They are extremely simple in structure. They feed on bacteria and round up into resistant cysts when food is in short supply or conditions are otherwise unfavorable. The Hartmanellidae are amoebas in this group that show true mitosis.

The parasitic amoebas are typically found in the gut of vertebrates and invertebrates. Frequently there is little morphological distinction between the species found in different hosts. They are mostly 20-40μ in diameter. Amoebic dysentery is caused by Entamoeba hystolytica, which invades the walls of the large intestine and causes loss of blood and excessive secretion of mucus. An almost identical species, Entamoeba colt, is found in the same part of the gut but is non-pathogenic.

The precise condi-tions in which one species gains a numerical advantage over the other are difficult to determine as many apparently healthy humans are ‘cyst-. passers’ and clearly infected carriers. The cysts are most resistant to chemical treatment and even to gentle drying. Amoebic dysentery is therefore easily spread by contamination of food. A related parasite, Entamoeba invadens, causes great damage in snakes and other reptiles. It does not infect man.

Testaceans (Order Testacea)

This order contains amoebas that build a shell or test in which they live. They are distinguished from the foraminifers by having only one test. The type of test determines classification within the order. In some the test is simple, made of secreted substances and not complicated by plates. The genus Arcella belongs here. Its test is pale brown and shaped like the cap of a young mushroom. The organism lives outside the test and extends blunt pseudopodia through a hole where, in the mushroom, the stalk would join the cap. These organisms are 25-100μ and are found on surfaces of decaying plants in freshwater.

Foraminifera Elphidium and Plectofrondicularia are examples of Class Rhizopoda

These photographs illustrate the range of forms found in the group. Only shells are shown. The smallest chamber at the center of spiral types or the thin end of elongate shells is the proloculum.

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