GROUP I. THALLOPHYTA : FUNGI : MYXOMYCETES. 283 



forms of Bacterium which cause Phthisis, Cholera, Anthrax, and 

 other diseases. 



The particular form presented, and the degree of the physiolo- 

 gical activity manifested, at any given time, is determined by the 

 external conditions, such as the nature of the obtainable food, the 

 temperature, the presence or absence of oxygen, etc. ; important 

 variations in any of these conditions may induce change from one 

 form of the organism to another and may modify its physiological 

 activity. 



There is a general resemblance in organisation and reproduction 

 between the Schizomycetes and the Cyanophyceae, as well as a 

 remarkable correspondence between individual forms belonging to 

 the two groups. On this ground they are sometimes placed to- 

 gether in a distinct group, the Schizophyta. It is, however, prefer- 

 able to place them respectively in the classes Fungi and Algae as 

 corresponding sub-classes. 



Sub-Class II. MYXOMYCETES. These organisms are characteris- 

 tically saprophytic, living on decaying organic substances, such 

 as spent tan, decaying leaves, tree-stumps, etc. 



Their life-history is, in most cases (Endosporese), as follows : 

 On the germination of the spores, the contents of each spore escape 

 as a zoogonidium, a naked mass of protoplasm, enclosing a nucleus 

 and a contractile vacuole, provided with a single cilium ; this con- 

 stitutes the mastigopod stage, and in this stage the cells multiply 

 by division. After a period of active swimming, the zoogonidium 

 draws in its ciliuin, and now creeps about by means of temporary 

 protrusions of its protoplasm termed pseudopodia ; this is the 

 amoeboid or myxopod stage, and in this stage also multiplication by 

 division takes place. The amoebae then collect together, cohering 

 into a plasmodium ; the protoplasm of the amoebae in some cases 

 fuses completely so that the plasmodium presents no cellular 

 structure, whereas in others (pseudoplasmodium) the outlines of 

 the coherent amoebae persist ; but, in any case, there is no fusion 

 of the nuclei of the constituent amceba3, so that the plasmodium is 

 multinucleate and coenocytic. 



The plasmodium creeps about, like a gigantic amceba, by means 

 of pseudopodia, until spore-formation begins. At this time the 

 plasmodium conies to rest ; and it either forms a single sporangium, 

 or divides into several portions each of which forms a sporangium. 

 The mass of protoplasm then assumes the form of the future 

 sporangium ; the external portion of it hardens to form the wall, 



