THE PRTXCIPLES OF CLASSIFICATION 75 



(8) Intimate Structure of the Cell. — The structure of the cell 

 is simple and no nucleus is differentiated. In addition, the 

 membrane in the majority of the sulphur bacteria is not well 

 marked (see Chap. X.). So far as is known at present, the 

 presence of sulphur inclusions in the cell does not imply the 

 constant presence of any other solid substance in the cell. 

 The intimate structure of the cell is too uniform in the sulphur 

 bacteria to permit of use being made of it in classification. 



(9) Colour. — The primary division in all classifications of the 

 sulphur bacteria is based on the presence or absence of colour. 

 The distinction of colour is most useful from the practical point 

 of view, and it is probable also that the differences between 

 the coloured and the uncoloured sulphur bacteria are deep 

 seated. The coloured forms probably utilize the energy of 

 light, as is done by green plants ; and if this be the case 

 their metabolism is essentially different from that of the 

 uncoloured forms. To some extent the distinctive features 

 of the coloured and the uncoloured forms are different, but 

 they are not sufficiently well marked to justify their use as 

 definite connotative marks to distinguish one kind from the 

 other. 



The terms Leuco and Rhodo are already in use as prefixes 

 for the colourless and the coloured forms respectively, and it 

 is proposed to retain them, so that the first division in the 

 author's revised system will be 



1. Leuco-thiohacteria. 



2. Rhodo-thiohacteria. 



A difficulty has arisen in taking this step. The name 

 Thiospirilhmi was originally used by Omelianski for all sul- 

 phur spirilla, both coloured and uncoloured. If this primary 

 division on the basis of colour is to be retained, it will be 

 necessary to make a distinction between the coloured and the 

 uncoloured spirilla. To preserve uniformity of nomenclature 

 it is proposed to make the following distinction : — 



Thiospirilhmi— \Jnco\oured sulphur spirilla. 

 Rhodothiospir ilium — Coloured ,, ,, 



