21. Rigid cells 6 to 50 microns long; actively motile by means of polar flagella; deposit sul- fur internally when growing in waters containing hydrogen sulfide Thiospira p. 82 Flexible cells; not flagellated; do not deposit sulfur internally 22 22. Large, spiral cells, with tapered ends, up to 100 microns long; protoplast wound spirally around a well defined axial filament; no cross striations; motile by means of a flexu- ous movement Spirochaeta p. 893 Spiral cells with a round cross section and blunt ends; up to 60 microns long; cells have a ridge or crista composed of numerous fibrils running along one side of the spiral, cross striations distinct; found in the intestinal tract of molluscs Cristispira p. 896 23. Stalked cells ; aquatic in habit 24 Cells not borne on stalks 25 24. Cells rod-shaped; 2 by 6 to 12 microns; single cells attached terminally and at right angles to branches of a lobose, dichotomously branched stalk; form globular bush- like or plate-like growths on the surface of waters Nevskia p. 216 Cells pear-shaped to spherical; multiply by budding; cells attached by a long slender stalk to a holdfast, several stalks frequently arising from one holdfast. (This organ- ism has so far been found only in lake waters where temperature does not exceed 23°C.) Blastocaulis p. 279 Cells pear-shaped; borne on a very short stalk; cells grow attached to each other in a cauliflower-like mass and reproduce by longitudinal division and budding. Colonies break up at intervals, and liberated cells start new colonies. Cells and methods of reproduction resemble those found in Chaemosiphon , a blue-green alga; discovered in the body cavity of a fresh-water crustacean Pasteuria p. 279 25. Endospores produced 26 No endospores produced 27 26. Spherical cells in cubical packets Sarcina p. 467 Rod-shaped cells Clostridium p. 634 27. Cells contain bacteriochlorophyll and carotenoid pigments; cell masses are various shades of red and purple; proceed to Section J p. 1031 Not as above 28 28. Iron deposited on the cells or in capsules 29 Note: In the absence of further information, these organisms are identified on their iron-depositing characteristics. Most iron organisms studied in pure culture metabolize the organic compound which forms the iron chelate, and the liberated iron chelates with some cell component. Citrate-utilizing organisms will, for example, release iron from ferric ammonium citrate. Accumulation of the iron in or on the cell may depend only on the nature of the cell sub- stance. Pure-culture studies may place these organisms in more commonly recognized genera. Many more organisms, if tested, may fall into the follow- ing genera. They should also be treated as non-iron-depositing cells and should be followed through the key. Not as above 32 29. Iron deposited as a torus, a solid ring partially or completely around the cell in one area only giving the cells the appearance of open or closed links of a chain 30 Iron deposited uniformly over the cells or capsules 31 30. Cells completely surrounded by a torus Naunianniella p. 223 Cells only partially enclosed, appearing like a horseshoe. Flagella of unequal length borne at the open end Ochrobium p. 225 Note: The type of flagellation suggests that this may be an alga.
