262 GENERAL FEATURES OF BACTERIA 



Bacteria are rod-shaped (Bacillus, Fig. 144, d, f, Bacterium), 

 spherical (Micrococcus) , or curved (Spirillum, Fig. 144, k). 

 The rod-shaped individuals may be joined end to end to form 

 long filaments (Fig. 144, d), whilst the Coccws-forms may occur 

 in chains (Streptococcus, Fig. 144, b), in cubical packets (Sarcina), 

 or in irregular masses (Staphylococcus, Fig. 144, a). The individuals 

 may be capable of movement owing to the possession of cilia, 

 which are only visible after careful staining, or they may be 

 devoid of these, and merely exhibit Brownian movement in corre- 

 spondence with their minute dimensions. In the elongated forms 

 the cilia are either situated in tufts at the two ends (Spirillum, 

 Fig. 144, k), or else arise all over the body (Bacillus, Fig. 144, e, /), 

 but there are some genera possessing only one or two cilia re- 

 stricted to one end of the cell (Pseudomonas, Fig. 144, /). 



Very little is known regarding the internal structure of the 

 cells, and it is a matter of doubt whether or not they possess 

 a true nucleus ; plastids are of course absent. The cell-wall 

 appears to consist of a protein, and may probably be regarded 

 merely as a differentiated outer layer of the cytoplasm. The 

 latter contains various substances, of which the commonest are 

 glycogen (cf. p. 231), granules of volutin (p. 255), and fat -globules 

 which appear as brighter specks in the cytoplasm. 



Multiplication of the cells by division, under normal circum- 

 stances, ensues with great rapidity, often taking place once 

 every hour, or even more frequently. In some of the elongated 

 forms the daughter-individuals are separated off by a mere 

 constriction of the cell (e.g. Bacterium spp.), whilst in other 

 cases the latter is subdivided by a transverse septum which sub- 

 sequently splits down the middle. In certain Bacteria inhabi- 

 ting water or other fluids, the daughter-individuals develop thick 

 mucilaginous walls, and in consequence adhere together in vast 

 numbers to form an often iridescent pellicle on the surface, or 

 thick pale-coloured Palmella-like floating masses (zooglcea-stages). 



Bacteria survive unfavourable conditions by the formation 

 of resting spores, produced within the cells by a localised con- 

 centration of the greater part of the protoplasm, which then 

 stains differently to the peripheral portion. This central region 

 eventually becomes surrounded by a thick wall, and at maturity 

 the remainder of the cell is often empty (Fig. 145, b, c). The 



