42 PA THOGENIC BACTERIA. It has even been suggested to classify the bacteria by the size and number of their fiagella, of which so little is known. The most convenient classification, though it cannot be purely scientific, seems to be the morphological one * given by Cohn. Baumgarten, recognizing the relative pleomorphism of certain of the species, has modified it as follows, and thus made it answer all the needs of the pathologist at least: I. Cocci, \ II. Bacilli, I species relatively monomorphous. III. Spirilla, J IV. Spirulina, \ V. Leptothrix, > species relatively pleoinorplious. VI. Cladothrix, J The members of the first group, the cocci, bacilli, and spirilla, are practically the only ones which are of patho- logical significance.