The amoebae belong to the class Sarcodasida. Members of this class move by means of pseudopods. They have no cilia and, except in rare instances, no flagella. The group is named for sarcode, a term introduced by Dujardin for what was later called protoplasm. Most sarcodasids are holozoic, ingesting their prey by means of their pseudopods. Their cytoplasm is usually divided into endoplasm, containing the food vacuoles, nucleus, etc., and relatively clear ectoplasm. The fresh water forms contain one or more contractile vacuoles; these are absent in the salt water and parasitic species. With a few exceptions, reproduction is asexual, by binary fission or rarely by multiple fission, by budding or by plasmotomy. Most species form cysts.
The Sarcodasida originated from the Mastigasida. The group did not arise from a single progenitor, but is polyphyletic. One line, for example, passes from Tetramitus thru Naegleria to Vahlkampfia. In Tetramitus, which is usually classified among the flagellates, the life cycle includes flagellate and amoeboid stages, and the flagellate stage has a permanent cytostome. In Naegleria, which is usually classified among the amoebae, the life cycle also includes flagellate and amoeboid stages, but there is no permanent cytostome. In Vahlkampfia, there is no flagellate stage, but the amoebae are very similar to those of Naegleria. Another line passes from the amoeboid flagellate, Histomonas, to the very similar but non-flagellate amoeba, Dientamoeba.
Only a few of the Sarcodasida are parasitic. The free-living forms include the most beautiful protozoa of all, the pelagic Radiolaria with their delicate, latticework siliceous skeletons. One group of Radiolaria has skeletons of strontium sulfate - perhaps some day protozoologists will be asked to develop ways of using them to eliminate strontium 90 pollution. Another marine group, the Foraminifera, has calcareous shells. Their skeletons form our chalk, and they and the Radiolaria are of great geological interest. They are used, too, as indicators in oil well drilling. More species of Foraminifera have probably been named than of all the other protozoa put together; 493 new species and 53 new genera of Foraminifera were listed in the Zoological Record for 1956, and of these, 470 species and 48 genera were fossil. In contrast, 57 new species and 4 new genera of parasitic protozoa were listed. And this was not an exceptional year.
Members of this order have a single-chambered shell or test.