Phylum Rhizopoda

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The phylum Rhizopoda includes a large group of protists that move by means of pseudopods, extensions of the cytoplasm. The diagram to the left shows an ameba, perhaps the most familiar Rhizopod.

Most Rhizopodians use their pseudopods to capture prey and also for locomotion. Rhizopods are heterotrophic, feeding on a wide range of bacteria, algae and other protists.

Some Rhizopodians are important parasites. Entamoeba histolytica causes amebic dysentary and another Entamoeba is a common symbiont in the human mouth where it feeds on bacteria and human macrophages. This Entamoeba has been implicated as a contributing factor to periodontal disease.

While many Rhizopodian have the "simple" cell structure illustrated for ameba, some forms do construct elaborate skeletal structures or tests. For example the ameba Difflugia has a test or shell composed of small pebbles in a protein based gel. The ameba constructs the test by ingesting small pebbles, particles of sand etc and extruding them into the protein gel.

Some Rhizopodians are among the largest protists known. For example, Pelomyxa palustrus, easily reaches 5mm in length. While still others apprach one cm in length. Pelomxya is sometimes put in its own phylum because it lacks mitochondria for cellular respiration and relies on other types of bacterial derived symbionts to perform this task.

Related organisms that are often grouped with the Rhizooda are the Foramnifera ,the Radiolarians and the Heliozoa. All three of these groups are extremely abundant and have mineralized skeletons of great beauty.

pgd revised 2/17/01

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