CH. Xl] BACTERIA IN THE SEA 255 



divisions in which the original bacterial cell, which gave rise to the 

 colony, cleaved. (2) The Bacillaceae; these are the typical 

 bacteria : they are rod-shaped bodies, usually cylindrical, but 

 sometimes egg-shaped and difficult to distinguish from cocci. 

 (3) The Spirallaceae : in this family the vegetative cells are 

 always cylindrical, but they are spirally twisted rods, or bent, or 

 slightly twisted "commas," or long and slender spirals or Spiro- 

 chaetes. Soi^ie of these latter organisms belong to other 

 groups of protista. Finally there are the (4) Trichobacteriaceae, 

 among which the vegetative form is a long branched or unbranched 

 filament, sometimes rigid and enclosed in a sheath, and sometimes 

 motile with undulating or gliding movements. Individual cells 

 break off from these filaments and each of these, the swarm-spores, 

 or gonidia, gives rise to a new colony. They are the most highly 

 organised of the bacteria. In Fig. 31 I give an illustration of 

 some of the typical forms of bacteria. The reader must note that 

 these morphological characters carry us only a little way in the 

 determination of the species to which a bacterium belongs. The 

 biology of the organisms ; their effect on different food media ; the 

 nature of their sporulating process; their reaction towards oxygen, 

 temperature, &c. ; their pathogenic effects : these and many other 

 properties all are made use of in the identification of the species. 

 Most bacteria are motile, that is they can execute movements, and 

 these are caused by cilia or flagella. Many, or most, produce 

 spores, which consist of the protoplasm of the cells concentrated 

 to form small rounded masses possessing a thick wall or capsule- 

 They constitute the resting stage of the organism and they are 

 always highly resistant to agencies which destroy the bacterial 

 vegetative cell. Some bacteria — anaerobes — can live in the total 

 absence of oxygen, while others live preferably in the presence of 

 this gas, and may become attenuated or enfeebled if the amount 

 of oxygen in their atmosphere diminishes — between aerobic and 

 anaerobic bacteria there are many gradations. For every bacterium 

 there is an " optimum " temperature at which the organism grows 

 best ; a maximum temperature at which it is killed ; and a 

 minimum temperature at which it may survive, but at which it 

 cannot grow. Usually the multiplication of a bacterium is arrested 

 at a temperature of 0° C. ; and at a temperature above 42° C. 



