-Members of the Sarcodina seem to possess either of two basic mechanisms for protoplasmic movement: 1) a contraction-hydraulic system in which protoplasmic flow 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 of sol and one of gel, move in relation to one another. Our observations and those of others recorded in the literature indicate that an organism which has and employs one of these basic mechanisms either does not have or does not ordinarily employ the other. If these two mechanisms are distinct, possession and locomotive employment of one or the other must be of great phylogenetic importance to the organisms and, therefore, are also important, or should be, in taxonomy. If so, the Sarcodina logically might be divided into two major groups on the basis of the functional presence of either of these two mechanisms. Current and past taxonomic class levels for the Sarcodina are discussed and shown to be illogical. They require revision, particularly in view of the upsurge in the use of logical computer systems in taxonomy. The logical employment of a dichotomy of Sarcodina based on these two fundamental mechanisms of movement is an initial step in this direction. We suggest abolishment of all previously proposed classes of Sarcodina and the establishment of: 1) the class Autotractea for organisms employing the active shearing mechanism; and 2) the class Hydraulea for those organisms employing the contraction-hydraulic mechanism. One of the goals which taxonomists hopefully set for themselves, besides that of constructing a filing system for distinguishing organisms, is the demonstration of phylogenetic relationships of organisms, past and present. This obviously cannot be done completely because so many extinct organisms have not provided the taxonomist with fossils; and of those which may have left fossils, probably only a comparative few are presently known. However, as previously mentioned (Jahn, 1962a, 1962b), it should be possible to discern major dichotomies on a logical basis; and indeed it is imperative to do so because of the already evident impact of cybernetic technology on biological systematics (Denmark et al., 1958; Ehrlich, 1962a, 1962b; Jahn loc. cit.; Seitner, 1960; Seitner et al., 1960; Sneath, 1957, 1960). Both layman and professional have long employed, or rather tried to employ empirically, such a procedure; but much that is illogical has crept into and, even worse, has been retained by, our present taxonomic systems. For example, the widely used distinction of kingdoms called plant and animal, once perhaps logical in respect 1 A revision of the paper presented at the 15th Annual Meeting of the Society of Protozoologists, August 29, 1960, as a contribution to a Round-Table Discussion on Problems in Systematics and Evolution of Protozoa. Research supported by USPHS Grant No. 6462 and NIH-E-1158.