The second cell form is that of a cylinder or rod typified by a section of a lead-pencil. The name bacillus, plural bacilli, is applied to this type ([Fig. 34]). The bacillus may be short ([Fig. 35]), 1µ or less in length, or long, up to 40µ in rare cases. Most bacilli are from 2µ to 5µ or 6µ long. The ends of the rod are usually rounded, occasionally square and very rarely pointed. It is evident that a very short rod with rounded ends approaches a coccus in form and it is not always easy to differentiate in such cases. Most bacilli are straight, but some are slightly curved ([Fig. 36]).
The third cell form is the spiral, typified by a section of a cork-screw and named spirillum, plural spirilla ([Fig. 37]). A very short spiral consisting of only a portion of a turn is sometimes called vibrio ([Fig. 38]). Vibrios when seen under the microscope look like short curved rods. The distinction between the two can be made only by examining the organism alive and moving in a liquid. The vibrio shows a characteristic spiral twisting motion. Very long, flexible spirals are usually named spirochetes ([Fig. 39]). The spirochetes are motile but flagella have not been shown to be present.
Fig. 33.—Cocci.
Fig. 34.—Bacilli.
Fig. 35.—Short bacilli.
Fig. 36.—Curved bacilli. Only the one in the center of the field is in focus. The others curve out of focus.