INTRODUCTION 



such chains, then growth and division of the still-connected cells 

 in two and finally in three dimensions, producing first sheet-like 

 and finally more solid structures, the constituent cells of which 

 became variously differentiated and specialised. 



Those extremely minute, thread-like (Leptothrix, Spirillum), or 

 rod-like (Bacillus) plants devoid of chlorophyll, which often break 

 up without losing vitality into spherules or into granules of 

 even ultramicroscopic tenuity, known as the Schizomycetes (or 

 colloquially Bacteria), cannot be considered as primitive. Like the 

 Fungi and many of the most highly organised plants, they have 

 descended from chlorophyll-bearing forms, and have become adapted 

 to a parasitic or saprophytic mode of nutrition whilst retaining 

 the general characteristics of growth and form of their ancestors. 

 The intimate connection of the Schizomycetes with the Oscillatoriae 

 does not seem to admit of any doubt, and forms closely allied to them 

 develop chlorophyll as well as peculiar blue and red pigmentary 

 substances, the function of which is obscure but may be related 

 to their modified nutritional processes. We are thus led to regard 

 all the non-filamentous, non-chlorophylligerous microscopic forms 

 which are not referable to the Schizomycetes or to the simpler 

 Fungi as "Protozoa." The debatable ground is limited to the 

 chlorophyll-forming Flagellata, amongst which are some which, 

 being devoid of mouth and at all periods of their growth incapable 

 of zootrophic activity, are yet so closely allied in life-history and 

 structure with truly zootrophic species that it is not possible to 

 draw a sharp line and assign them definitely either to the Animal 

 or to the Plant series. Such are the Volvocineans, which zoologists 

 will probably for some time to come consider it desirable (as we do 

 in the present treatise) to treat of in the description of the Animal 

 series, whilst botanists will find it equally desirable to discuss 

 them in connection with closely allied minute Plants. 



In view of these considerations, we consider the following 

 groups of the simplest organisms as belonging to the Animal 

 series, and as constituting a lowest " grade " of animal organisa- 

 tion, to which the term Protozoa is applicable. The groups in 

 question are given the title of "classes," but it will readily be 

 understood that it is not intended to imply by that term that 

 they have any exact equivalence in the amount of divergence 

 from one another to that which is presented by the " classes " of 

 any one of the phyla of the Metazoa. 



PROTOZOA. Class 1, Proteomyxa; Class 2, Heliozoa; Class 3, 



Mycetozoa ; Class 4, Lobosa ; Class 5, Radiolaria ; Class 6, 



Mastigophora ; Class 7, Sporozoa; Class 8, Ciliata; Class 9, 

 Acinetaria. 



