The individual cells, generally not visible without staining, occur in trichomes; within the trichomes the cells are arranged in chains. The trichomes show a gliding motion when in contact with a substrate; they also show flexing movements. When grown in the presence of hydrogen sulfide, the trichomes contain sulfur globules. The structure of these organ- isms is very similar to that of the Oscillatoriaceae, but the cells are devoid of chlorophyll and phj^cocyanin. Special reproductive structures are unknown. In proposing the family name Beggiatoaceae for the two genera of this subgroup known in 1894, Migula {op. cit., 1894, 238) remarked that "it would be best to combine them with the Oscillatoriaceae and classify them among the Schizophyceae" . The same authority (in Engler and Prantl, Die natiirl. Pflanzenfam., 1, la, 1895, 41) has stated: "Also in view of their in- ternal structure the species of Beggiatoa are so similar to those in the genus Oscillaria that they can hardly be separated generically". Since then, the close relationship between the colorless sulfur bacteria occurring in trichomes and the blue-green algae of the family Os- cillatoriaceae has become even clearer. A particularly important line of evidence is supplied by the discovery of sulfur bacteria paralleling each of the major genera of the Oscillatoria- ceae. Taxonomically these organisms could readily be classified as colorless members of the class Schizophyceae, for many species of Oscillatoria live in the same environments as do those of Beggiatoa, and some grow in the presence of hydrogen sulfide and contain sulfur granules. However, the Beggiatoaceae are, in this Manual, retained with the bacteria (Schizomycetes) for practical reasons: they have been so included in the past, they are not included in modern treatises on the blue-green algae, they show close kinship to other or- ganisms commonly included in the bacteria, and they grow under natural conditions in close association with such organisms. Studies on the physiologies of the organisms of this family and of the related Oscilla- toriaceae are needed. Key to the genera of family Beggiatoaceae. I. Trichomes are free and motile and are not attached to a substrate. A. Trichomes occur singly and are not embedded in a common slime -sheath. 1. Trichomes straight or somewhat bent, not permanently coiled. Genus I. Beggiatoa, p. 838. 2. Trichomes coiled or spirally w^ound. Genus II. Thiospirillopsis, p. 840. B. Trichomes occur in bundles and are surrounded by a slime-sheath. Genus III. Thioploca, p. 841. II. Trichomes attached to substrate at one end; apical segments, when freed, are motile until attached. Genus IV. Thiothrix, p. 842. Genus I. Beggiatoa Trevisan, 1842. (Prospetto della Flora Euganea, 1842, 56.) Beg.gi.a.to'a. M.L. fem.n. Beggiatoa a genus of bacteria; named for F. S. Beggiatoa, a physician of Vicenza. Cells occur in unattached, motile, segmented trichomes; the trichomes occur singly or in white to creamy, felted masses in which the trichomes retain their individuality. Ex- istence of a sheath not definitely established. Movements of the trichomes are dependent upon contact with a solid substratum over which they glide in the same manner as do spe- cies of Oscillatoria. The gliding movements are often accompanied by a rotation of the tri- chomes around the long axis. Reproduction is by transverse fission of the cells; the trichomes may also break up into smaller units, each continuing a separate existence. The latter mode of multiplication corresponds to that of the so-called motile gonidia or segments in Thio-
