38 TAXONOMY 



The bluish-green colouring matter is borne by a special organ, the chromato- 

 phore (Fig. 15, a: ch), generally having the shape of a hollow cylinder or 

 sphere, forming the outermost layer of the cell-body and enclosing the 

 remaining cell contents (Fig. 15, a, c). The central mass stains deeply, and 

 thus has somewhat the appearance of a nucleus. We are, however, no more 

 justified in describing it as a nucleus than in calling the strongly-stained 

 granules in the protoplasm 'nuclear chromatin.' As in the case of the 

 'chromatin grains' of bacteria, the nature of these substances is unknown. 

 In no bacteria, not even in the chromogenic species, is such a differentiation 

 of the cell contents to be observed. 



If we now turn to the Flagellata we find among them also many points 

 of similarity to the bacteria. In a species like Polytoma uvella (Fig. 15, b), 

 for instance, often found in stagnant waters, we have an egg-shaped cell 

 enclosed in a definite membrane (h), and provided with a pair of permanent 

 polar cilia or flagella. Other forms, such as Monas, are ' monotrichous,' others 

 again, like Tetramitus, have a brush or tuft of cilia at the anterior end of the 

 cell. The process of ' encystment,' too, closely resembles the formation of 

 endospores, the cell-contents contracting and secreting a new membrane, 

 the cyst or spore thus formed being set free by the disintegration of the 

 membrane of the mother cell just as are the endospores of bacteria. But 

 notwithstanding these points of likeness the Flagellata differ from the bac- 

 teria in one fundamental feature : they possess a definite nucleus (Fig. 15, b : k) 

 like that of the cells of higher organisms. It is therefore evident that we 

 have as little right to assume a close genetic relationship between bacteria 

 and Flagellata as between bacteria and Cyanophyceae. The view most 

 in harmony with observed facts is that which looks upon the Bacteria as 

 a distinct group of protista, simpler in structure than the rest, and showing 

 affinities to both Cyanophyceae and Flagellata. We might regard the bac- 

 teria as the more direct descendants of the ancestral stock from which these 

 arose, the differentiation of a chromatophore leading to the Cyanophyceae 

 and the development of a nucleus as well as the general attainment of motile 

 power giving rise to the flagellate phylum. Among the arche-bacterial 

 types there may well have been both motile and non-motile forms which 

 would have been respectively the starting-points of the two branches, whilst 

 capsulation, the production of cell-chains or filaments, and other colonial 

 or growth forms were primitive acquirements which have reappeared 

 again among the Cyanophyceae and Flagellata and undergone greater 

 specialization. 



The protista {micro-organisms), the simplest living things, among which 

 we have tried to indicate the place of the bacteria, are not only manifold 

 in form, but have very different modes of life and exercise a great variety of 

 different functions in the economy of nature. As a general rule, these physio- 

 logical features are not so conspicuous among the other groups as they are 



