COMPREHENSIVE KEY 993 



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. 



