16 GENERAL MORPHOLOGY AND BIOLOGY 



are to be looked on as bacteria at all, one view being that in, 

 it may be, many cases they represent a stage in the life history 

 of what are really protozoa. The question is an important one 

 as these forms include many pathogenic agents. The ultimate 

 classification of this group of bacteria must at present be left an 

 open question, and it is convenient to denominate the non-septate 

 spiral rods Spirochcetce, and those whose vital unit is a single 

 curved rod Spirilla. 



II. The Higher Bacteria. These show advance on the lower 

 in consisting of definite filaments branched or unbranched. In 

 most cases the filaments at more or less regular intervals are 

 cut by septa into short rod-shaped or curved elements. Such 

 elements are more or less interdependent on one another, and 

 special staining methods are often necessary to demonstrate the 

 septa which demarcate the individuals of a filament. There is 

 further often a definite membrane or sheath common to all the 

 elements in a filament. Not only, however, is there this close 

 organic relationship between the elements of the higher bacteria, 

 but there is also interdependence of function ; for example, one 

 end of a filament is frequently concerned merely in attaching 

 the organism to some other object. The greatest advance, how- 

 ever, consists in the setting apart among most of the higher 

 bacteria of the free terminations of the filaments for the produc- 

 tion of new individuals, as has been described (p. 5). There 

 are various classes under which the species of the higher bacteria 

 are grouped ; but our knowledge of them is still somewhat 

 limited, as many of the members have not yet been artificially 

 cultivated. The beggiatoa group consists of free swimming 

 forms, motile by undulating contractions of their protoplasm. 

 For the demonstration of the rod-like elements of the filaments 

 special staining is necessary. The filaments have no special 

 sheath, and the protoplasm contains sulphur granules. The 

 method of reproduction is doubtful. The thiothrix group re- 

 sembles the last in structure, and the protoplasm also contains 

 sulphur granules ; but the filaments are attached at one end, 

 and at the other form gonidia. A leptothrix group is usually 

 described which closely resembles the thiothrix group, except that 

 the protoplasm does not contain sulphur granules. It cannot, 

 however, be with certainty said whether such organisms can be 

 sufficiently differentiated from the bacilli to warrant their being 

 placed among the higher bacteria. In the cladothrix group 

 there is the appearance of branching, which, however, is of a 

 false kind. What happens is that a terminal cell divides, and 

 on dividing again, it pushes the product of its first division to 



