as representatives of the Schizomycetes on the basis of their vegetative morphology. Al- though the vegetative cells of mj^xobacters resemble those of rod-shaped true bacteria in size and general appearance, the}^ are distinguishable both by their gliding movement and by their lack of a rigid cell wall, and most later authors have agreed that the relationship between true bacteria and myxobacters is not close. The remarkable life cycle that char- acterizes the fruiting myxobacters is paralleled only among the amoeboid protists, in slime molds belonging to the order Acrasiales; however, the differences between the two groups with respect to vegetative morphology are so great that this developmental parallel evidently represents a case of evolutionary convergence. The postulation of a relationship between myxobacters and blue-green algae, put forward by Jahn (op. cit., 1924) and ac- cepted by subsequent writers (Stanier and van Niel, Jour. Bact., 4^, 1941, 437; Pringsheim, Bact. Rev., 13, 1949, 47) was based in the first instance on the similarity in the mechanism of locomotion. This phylogenetic assumption became more probable with the recognition of non-fruiting myxobacters, not characterized by the life cycle previously considered to be diagnostic of the group; physiology apart, there is little to distinguish the members of the genus Cytophaga from certain unicellular, rod-shaped, blue-green algae. In a recent anal- ysis of the relationships between bacteria and blue-green algae, Pringsheim {loc. cit.) has expressed the view that the mechanism of movement is a basic character, upon which can be based a separation of microorganisms at this level of organization into "gliding" forms and "swimming" {i.e., flagellated) forms. If this view is accepted, then the myxobacters, together with other non-photosynthetic gliding organisms, should be grouped taxonomically with the blue-green algae rather than with the true bacteria as constituting non-photosyn- thetic offshoots from various groups of blue-green algae. By long tradition, however, even colorless gliding organisms such as the Beggiatoaceae, which show very close morphological relationships to specific genera of blue-green algae, have been treated taxonomically as "bacteria," and for practical reasons a continuance of this policy may be deemed desirable, even if one accepts the plausibility of Pringsheim's phylogenetic views. In recent years, many new colorless, gliding organisms have been discovered, and their relationships to the Myxobacterales require discu.ssion. The non-fruiting myxobacters of the genera Cytophaga and Sporocytophaga show clear evidences of relationship to the fruiting forms, both in their vegetative morphologj' and in the nature of the life cj'cle evidenced by the members of the latter genus. When their inclusion in the order Myxobacterales was pro- posed (Stanier, Jour. Bact., 40, 1940, 619), the only other colorless, gliding organisms known were the chemoautotrophs of the family Beggiatoaceae, whose filamentous structure related them to certain genera of blue-green algae in the family Oscillatoriaceae. Thus a wide and clear-cut gap between the Beggiatoaceae and the non-fruiting myxobacters seemed to exist. Soriano (Revista Argentina de Agronomia, 12, 1945, 120; Antonie van Leeuwenhoek, 12, 1947, 215) has described a number of nutritionally unspecialized, gliding organisms which he places in a new genus, Flexibacfer. Some of the.se are morphologically very similar to the representatives of the genus Cytophaga while others have a filamentous structure. Further- more, Pringsheim (Jour. Gen. Microbiol., 5, 1951, 124) has described a wide variety of filamentous, gliding organisms, some of which closely resemble the Beggiatoaceae in structure but do not share the nutritional peculiarities of this family. It is accordingly apparent that an argument can be made for linking up the unicellular, non-fruiting my.xobacters with the Beggiatoaceae and other non-fruiting, colorless, gliding organisms which show a more or less pronouncedly filamentous vegetative organization. In fact, Soriano (op. cit., 12, 1945, 120) has suggested uniting all these forms in an order P'lexibacteriales and conserving the order Myxobacterales for organisms capable of forming fruiting bodies. This proposal has not been adopted in the present revision of the Manual; instead, unicellular, non-fruiting forms are retained in the order Myxobacterales, and the filamentous forms are placed in a new order, Beggiatoales. Families of the Order Myxobacterales. Five families are recognized, of which four (Archangiaceae, Sorangiaceae, Polyangiaceae and Myxococcaceae) were originally proposed
