$ viii.] Aquatic Saprophytes. Beggiatoa. 79 



3. The species of Beggiatoa (Fig. 7) agree closely, according 

 to Zopf, with Crenothrix and Cladothrix in their pleomorphous 

 course of development. Straight and spiral filaments, abscised 

 straight and spiral rod-like portions of filaments, the latter pro- 

 vided with cilia and described under the name of Ophidomonas 

 (d), round Cocci or spores (e-k) and Zoogloea-aggregates of 

 these, make their appearance in just the same alternation as in 

 the two preceding genera, rods, Spirilla, and Cocci having in many 

 cases a swarming motion. The distinction between them and the 

 species of Crenothrix and Cladothrix lies chiefly in the presence 

 of sulphur in their structure, and in the motility of the filaments 

 which, like those of Crenothrix, are never branched. 



Beggiatoa alba, Vaucher, the most common species, has 

 colourless filaments, attached when quite intact to solid bodies 

 but easily breaking off from them and thus set free, and varying 

 in thickness from i to 5 p. The filaments consist of cells of 

 more or less elongate cylindrical to flat disk-like form, the latter 

 occurring especially in the thicker specimens. They have no 

 distinct sheath clothing the row of cells ; moreover, while the pro- 

 toplasm of Crenothrix and Cladothrix is uniformly clouded or 

 finely granular, in Beggiatoa alba it has disseminated through its 

 substance comparatively thick round highly refringent grains, 

 with a dark contour therefore, and composed of sulphur, as 

 Cramer has shown. Similar sulphur-grains are also present in 

 the non-filamentous states or forms assigned here by Zopf. 

 Their number is not the same in different filaments ; in some 

 filaments (c) but few are to be seen, and in parts of them 

 they may even be entirely wanting. In most filaments they are 

 present in large numbers, so large sometimes that they en- 

 tirely conceal the structure of the thread, which looks like a 

 rod having its uniformly clouded protoplasm traversed by a 

 dense mass of granules with a black outline. It is only by the 

 use of reagents which largely withdraw the water of the cells 

 that it is possible to distinguish them (). 



Again, the filaments usually exhibit active movements, such as 



