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Members of the Sarcodina seem to possess either of two basic mechanisms for protoplasmic movement: (1) a contraction-hydraulic system, in which flow of protoplasmic sol is caused by contraction of a tube of gel, and (2) an active shearing or active sliding mechanism in which two surfaces, usually both of gel but possibly one gel and one of sol, move in relation to each other. The first system occurs typically in Amoeba (Mast, 1926) and Physarum (Jahn, Rinaldi, and Brown, 1960; Jahn, 1960), and the second in Allogromia (Jahn and Rinaldi, 1959). Examination of older literature and of living specimens reveals that the contraction-hydraulic system is found in the Amoebida, Mycetozoida, Acrasinorida, and some the the Testacida (e.g., Arcella), and that the active sliding syste is found in the Foraminiferida, Radiolarida, Acantharida, Helozoida, Helioflagellorida, most Protemyxida, and some Testacida (e.g., Euglypha). No organism has been found which possesses both mechanisms.
If these two mechanisms are distinct, possession of either one or the other must be of great phylogenetic importance to the organisms, and therefore should be of taxonomic importance. If so, we should divide the Sarcodina into two major groups on the basis of possession of one or the other.
The morphological basis established by the French school (Grasse, 1948) for the rearrangement of the orders of Rhizopoda on the basis of the morphology of pseudopods is further emphasized by the existence of two basic mechanisms for pseudopod formation. However, use of these mechanisms as a basis of the major dichotomy combines the Actinopoda with the Filosa and the Granuloreticulosa into one of the two major groups.
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