Rhizopod sarcodines

 Domain Eukarya
Kingdom Protista
Section Rhizopod sarcodines

These images courtesy of Department of Parasitology, Faculty of Medicine, Chiang Mai University (Thailand)
A parasitic entamoeba, Entamoeba coli, stained with iodine

An encysted (immature) binucleated entamoeba, Entamoeba hystolica, stained with Trichrome

These images courtesy of Hosei Bio Lab Server
Perhaps the most famous Rhizopod, Amoeba proteus; this picture demonstrates its ability to change shape in order to move over varied terrain

Amoeba proteus with distinct pseudopodia

Amoeba proteus ingesting an actinopod, Actinosphaerium

A species of Filamoeba, a genus with oval, flattened cells and numerous thin, often spiny, pseudopodia

The rhizopods (Superclass Rhizopoda of Phylum Sarcodina) comprise many types of protozoans, including the rhizopod amoebas, the entamoebas, the foraminiforans, and the testaceans. Rhizopod cells are characterized by several types of pseudopodia, temporary extensions of the cytoplasm used in locomotion and digestion: reticulopodia, long, thin pseudopodia that fuse into a network; filopodia, like the reticulopodia but nonfusing; and lobopodia, the blunter pseudopodia of the amoebas. Most rhizopods have tests, or protective shells.

The amoebas are characterized by their pseudopodia. These are extended to encircle food (bacteria or smaller protists), then close around it to ingest it, forming a food vacuole. Digestive enzymes are secreted into this vacuole. Amoeboid cells also contain a jellylike cytoplasm differentiated into three parts: an thin, outer plasma membrane, an stiff ectoplasmic layer just within this membrane, and a central, granular endoplasm. This contains the nucleus, as well as food vacuoles and a contractile vacuole, not present in most marine or parasitic organisms, that helps remove excess water from the cell. Amoebas absorb water and oxygen across the whole of the plasma membrane. They reproduce by binary fission. During adverse conditions, the amoeba can secrete a protective test, eliminate most of its water, an enter a temporary encysted stage. Entamoebans are a parasitic amoeban genus, many living in the digestive tracts of vertebrates. They are transported in fly and roach feces while encysted, and break free of the cyst inside the vertebrate intestine.

Foraminiferans are marine rhizopods with many thin pseudopodia. The cytoplasm of foraminiferan cells is encased in a test, which can grow up to 5 cm in length. They live at all depths of ocean, and buildup of the tests of dead foraminiferans covers 30% of the ocean floor. Foraminiferans usually reproduce asexually, but most species have regular or intermittent sexual generations. Reproduction time varies with cell size, and the sexual generation may be completed in anywhere from a month to several years.

Testaceans are sarcodines that typically inhabit fresh water, and reproduce asexually. All species are encased in protective tests, but the composition of the test varies among genera. The membrane beneath the tests is made of a chitinous substance similar to the exoskeletons of insects.

The organisms of this group are believed to be the ancestors of the first animals. Though locomotion and ingestion are primitive, these cells are the simplest to show such hallmarks of modern animals.

Phyla of the Rhizopod sarcodines



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