§ VIII.] Aquatic Saprophytes. Crenothrix. 75 



under the name of Bacillus subtilis. Such collective names — col- 

 lective species, as we may shortly say — have occurred in all 

 branches of natural history and have been gradually dis- 

 entangled ; here, too, they will ultimately be cleared up. We 

 have only to keep an eye upon them, and not be induced by 

 names to adopt premature conclusions respecting them (35). 



We will now proceed to give some examples. 



The comparatively large arthrosporous forms, which are 

 described under the names of Crenothrix, Cladothrix, and 

 Beggiatoa, are found often in injurious, or at least in very dis- 

 agreeable quantities, in waters containing organic substances in 

 solution (36). 



I. Crenothrix Kiihniana, Rabenhorst (Fig. 5), in the most highly 

 differentiated stage of its development, forms filaments, accord- 

 ing to Zopf, 1-6 y. thick and about i cm. long, attached at one 

 end to fixed bodies, entirely unbranched, straight or less often 

 slightly spirally twisted. The filament consists of a row of 

 cylindrical cells, which are half to about one and a half 

 times as long as broad. The outer layers of their lateral 

 walls coalesce and form a delicate sheath surrounding the whole 

 filament, which is colourless when young, but at a later period 

 is often coloured from yellowish to dark brown or brownish 

 green by salts of iron. The filaments not unfrequently break up 

 transversely into pieces, which float free in the water and collect 

 into flocculent masses. The segments of the filaments may pass 

 by repeated bipartitions into the form of isodiametric cells which 

 then round themselves off. In this way the cells of thicker 

 filaments first take the shape of flattish disks, and then divide 

 one or more times in the longitudinal direction of the filament 

 into small roundish cells {b, c). These ultimately escape from 

 the sheath, either because the sheath swells up along its whole 

 length, or because it swells up and opens at the apex only and 

 allows the small cells to escape at that point ; the cells are either 

 passive and are thrust forth by the continued growth in length 

 of the lower portions of the filament, or have a slow movement 



